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Self-Assembled Monolayers on Gold Substrates made from Functionalized Thiols and Dithiols Dissertation zur Erlangung des Grades eines Doktors der Naturwissenschaften der Fakultät für Chemie der Ruhr-Univeristät Bochum Vorgelegt von Mihaela Georgeta Badin Aus Brasov/Rumänien Bochum 2007

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Self-Assembled Monolayers on Gold Substrates made

from Functionalized Thiols and Dithiols

Dissertationzur Erlangung des Grades

eines Doktors der Naturwissenschaftender Fakultät für Chemie

der Ruhr-Univeristät Bochum

Vorgelegt von

Mihaela Georgeta Badin

Aus Brasov/Rumänien

Bochum 2007

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Self-Assembled Monolayers on Gold Substrates made

from Functionalized Thiols and Dithiols

Tag der mündlichen Prüfung:

23.11.2007

Prüfungskommission:

Referent: Prof. Dr. Ch. Wöll

Korreferent: Prof. Dr. R. Fischer

Vorsitzender: Prof. Dr. R. Heumann

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Die vorliegende Arbeit wurde im Zeitraum von Dezember 2003 bis März 2007 am Lehrstuhl

für Physikalische Chemie I der Fakultät für Chemie der Ruhr-Universität Bochum unter

Anleitung von Herrn Prof. Dr. Christof Wöll angefertigt.

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Dedicatie

A plecat de langa noi si s-a dus printre straini,

Sa patrunda-n miezul cartii, in adancul ei cuvant.

S-a luptat sa faca totul ca sa fie la-naltime,

Sa se multumeasca-n sine si pe noi cei dragi parinti,

Ca asa e bine-n viata, sa le faci cum se cuvine.

A fost munca si sudoare si o grija permanenta,

Ca sa faca totul bine si sa fie foarte-atenta.

A fost greu, dar a trecut si-a ajuns unde-a dorit.

Acum drumul e deschis, ca sa faca fapte mari,

Si sa fie printre lume, cu dorinte tot mai tari.

Dupa-atata straduinta si cu un bagaj frumos,

O sa mearga inainte, ca sa-i fie de folos.

Dar sa fie foarte-atenta, ca sfarsitu-abia incepe,

Si ca viata ii ofera multe griji si bucurii,

Ca sa fie foarte tare si sa urce, fara nici-o ezitare.

Stelian Badin

Dedicated to my parentsAurica and Stelian Badin

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Table of Contents

I

Table of contents

Table of contents ___________________________________________________________ I

Chapter One _______________________________________________________________ 1

Introduction _______________________________________________________________ 1

1. Motivation and Objective of the Work _______________________________________ 1

2. Outline of the Thesis _____________________________________________________ 3

Chapter Two _______________________________________________________________ 5

Basics and Techniques Used in This Work_______________________________________ 5

2.1. Infrared Spectroscopy __________________________________________________ 5

2.1.1. The Electromagnetic Spectrum 5

2.1.2. Molecular vibrations 7

2.1.3. Stretching vibrations 9

2.1.4. Infrared Spectrometer 12

2.1.4.1. Principle of Operation of FTIR- Spectrometer __________________________ 12

2.1.4.2. Rinsing Gas Supply and its Influence on the Measurement ________________ 14

2.1.4.3. RAIRS- Setup ____________________________________________________ 15

2.2. X-ray Photoelectron Spectroscopy (XPS) __________________________________ 17

2.3. Near Edge X-ray Absorption Fine Structure (NEXAFS)_______________________ 20

2.4. Scanning Tunneling Microscopy (STM) ___________________________________ 23

2.5. Ellipsometry (SE) ____________________________________________________ 25

2.6. Water contact angle (CA) ______________________________________________ 26

2.7. UV-VIS Spectroscopy _________________________________________________ 28

Chapter Three_____________________________________________________________ 30

Self-Assembled Monolayers and Sample Preparation _____________________________ 30

3.1. Self-Assembled Monolayers ____________________________________________ 30

3.1.1. Introduction_______________________________________________________ 30

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Table of Contents

II

3.1.2. Self-Assembly Kinetics and Mechanism _________________________________ 31

3.1.3. Self-Assembled Monolayer Structure ___________________________________ 32

3.2. Preparation and Characterization of Gold Substrates ________________________ 35

3.2.1. Introduction_______________________________________________________ 35

3.2.2. Preparation and Characterization of Gold on Silicon Wafers ________________ 35

3.2.3. Preparation and Characterization of Gold on Mica Substrates_______________ 36

3.3. Preparation of Self-Assembled Monolayers Films ___________________________ 36

3.4. Preparation of Bulk Pellets for the IR measurements_________________________ 37

3.5. Chemicals used in this work ____________________________________________ 37

3.6. Laboratory Equipment ________________________________________________ 38

Chapter Four _____________________________________________________________ 39

Triptycenethiol-based Self-Assembled Monolayers _______________________________ 39

4.1. Introduction and Objective of the Work Presented in this Chapter ______________ 39

4.2. Self-Assembly Process of Triptycenethiol on Au(111) ________________________ 40

4.2.1. Introduction_______________________________________________________ 40

4.2.2. Results ___________________________________________________________ 41

4.2.2.1. XPS and Ellipsometry _____________________________________________ 41

4.2.2.2. IRRAS _________________________________________________________ 43

4.2.2.3. NEXAFS _______________________________________________________ 46

4.2.2.4. STM ___________________________________________________________ 48

4.2.3. Discussion ________________________________________________________ 49

Chapter Five ______________________________________________________________ 52

Influence of the Leaving Group in case of Triarylaminethiols ______________________ 52

5.1. Introduction and Objective of the Work Presented in this Chapter ______________ 52

5.2. Self-Assembly Process of the Triarylaminethiols on Au(111)___________________ 54

5.2.1. Introduction_______________________________________________________ 54

5.2.2. Results ___________________________________________________________ 54

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Table of Contents

III

5.2.2.1. IRRAS _________________________________________________________ 54

5.2.2.2. XPS ________________________________________________________ 57

5.2.2.3. NEXAFS _______________________________________________________ 58

5.2.2.4. Deprotection Process _____________________________________________ 60

5.2.2.5. STM ___________________________________________________________ 62

5.2.3. Disscussion _______________________________________________________ 64

Chapter Six _______________________________________________________________ 66

Formation of Self-Assembled Monolayers from Alkane Thioacetates ________________ 66

6.1. Introduction and Objective of the Work Presented in this Chapter ______________ 66

6.2. Preparation of the SAMs of C12SAc ______________________________________ 67

6.2.1. Introduction_______________________________________________________ 67

6.2.2. Results ___________________________________________________________ 67

6.2.2.1. IRRAS _________________________________________________________ 67

6.2.2.2. Water contact angle ______________________________________________ 71

6.2.2.3. Ellipsometry ____________________________________________________ 71

6.2.2.4. XPS ___________________________________________________________ 71

6.2.2.5. NEXAFS _______________________________________________________ 73

6.2.2.6. STM ___________________________________________________________ 74

6.2.2.7. Re-immersion of C12SAc-SAMs into thiol solutions ______________________ 78

6.2.3. Discussion ________________________________________________________ 80

Chapter Seven_____________________________________________________________ 83

Determination of Trans/Cis Isomerization of Azobenzene Molecules_________________ 83

7.1. Introduction and Objective of the Work Presented in this Chapter ______________ 83

7.2. Preparation of the solutions containing the azobenzene molecules ______________ 85

7.2.1. Results ___________________________________________________________ 85

7.2.1.1. UV/VIS Spectroscopy _____________________________________________ 85

7.3. Preparation of the SAMs on gold surfaces (111) ____________________________ 86

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Table of Contents

IV

7.3.1. Results ___________________________________________________________ 87

7.3.1.1. IR spectroscopy __________________________________________________ 87

7.3.1.2. Water contact angle (CA) __________________________________________ 91

7.3.1.3. STM ___________________________________________________________ 93

7.4. Discussion __________________________________________________________ 95

Chapter Eight _____________________________________________________________ 98

Summary and Conclusions __________________________________________________ 98

Chapter Nine_____________________________________________________________ 102

Appendix 103

9.1. List of figures 103

9.2. List of tables 106

9.3. List of references 110

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Chapter One Introduction

1

Chapter One

Introduction

1. Motivation and Objective of the Work

Self-assembled monolayers (SAMs) consist of densely packed long-chain organic molecules,

which are chemisorbed on metal substrates through a sulfur head group, which has a specific

affinity for these surfaces [1]. The gold surface has been used in this work as a substrate,

because the gold surface is inert to oxidation and can be handled with few precautions and

does not require specialized facilities. For STM measurements the Au/mica substrate is very

adequate, because of the atomically flat and large terraces consisted by this gold surface.

These monolayers are closely related to many important technological applications such as

corrosion inhibition, adhesion promotion/inhibition [2], nano-fabrication and supra-molecular

assembly, molecular recognition [3], and bio-sensors [4], resulting in a broad interest in

SAMs from many subjects. Self-assembled monolayers are very complex, because of the

various interactions in the process of including the interactions between headgroup and

substrate, the interaction endgroup-endgroup, endgroup and substrate and the chain-chain

interactions.

Techniques employed in the characterization of the SAMs include ellipsometry [5, 6],

reflectance infrared spectroscopy (IRRAS) [5, 7], x-ray photoelectron spectroscopy (XPS),

near edge x-ray absorption fine structure [8], and scanning probe microscopy [9, 10] etc.

The purpose of this study was to investigate the arrangement and the structure of self-

assembled monolayers containing aromatic moieties, which have been given less attention

due to the synthetic difficulties and poor solubility and also to understand how the molecular

backbone will affect the structure of the monolayer. A lot of interest has been given by the

molecules protected with an acetyl group, to understand how the leaving of this group will

affect the kinetic of monolayer formation [11].

Thus, different adsorbates have been investigated in this work, including aromatic thiols,

aromatic thiols protected with acetyl groups, aliphatic thiol protected with acetyl groups and

aromatic dithiols SAMs on gold substrate, as shown in Figure 1. The molecular structure of

the first group contains triptycene moiety as a rigid molecule and also with a methylene

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Chapter One Introduction

spacer between the aromatic moiety and the sulfur group. The second group of thiols contains

triarylamine groups and and all these molecules are protected by acetyl groups as a

termination of the sulfur group. The third group contains an aliphatic moiety protected by

acetyl group and the last group of investigated molecules contains an azobenzene

photoresponsive moiety and is terminated by dithiols.

Apart from the investigation of the films also an evaluation of applicability of the methods

used for the investigation is necessary.

2

Figure 1: Structure of thiols used in this study.

In this study, to permit an efficient ordering within the monolayers, the molecules chosen

were derived from triptycenes, which have not only an axis of higher symmetry (C3) but also

permit a coaxial attachement of both, the anchoring group (in our case a sulfur atom) and the

headgroup [12]. It is very interesting to understand how the nature of the triptycene molecular

backbone influences the structure of these monolayers.

The triarylaminethiols protected with the acetyl groups have been selected to achieve a better

understanding about the kinetic stabilization which is formed by the leaving of the thioacetate

group during the deprotection process. These SAM structures can then serve as excellent

model systems for studying bridge mediated electron transfer (ET) [13].

To achieve a better understanding about this deprotection reaction, we continue with the

investigation of a simple aliphatic thioacetates like C12SAC compared with C12SH. The

adsorption of C12SAc on gold substrate (111) is investigated using a broad set of experimental

Triptycenethiol

Triarylaminethiol

Dodecyl thioacetate

Azobenzene dithiol-carboxylate

Triptycenethiol

Triarylaminethiol

Dodecyl thioacetate

Azobenzene dithiol-carboxylate

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Chapter One Introduction

3

techniques likewise in all chapters of this study, gracing incidence infrared spectroscopy, x-

ray photoelectron spectroscopy, contact angle measurements, XPS, and STM. Altogether the

experimental results allow for a very consistent description of the adsorption behavior. The

objective of this work was to give an explanation for the kinetic stabilization of flat lying

molecules or so-called striped phase, which is formed by the loss of the acetyl group.

Dithiols containing an azobenzene moiety have attracted considerable attention as a

possibility for creating photoresponsive materials [14-16]. The azobenzene molecules have

been selected because they are photoisomerizable from trans- to cis isomers and reversible

from cis to trans- isomers due to alternating the irradiation of UV light and blue light. The aim

was to understand how the structure of these SAMs will change by the irradiation, if this

process is really reversible.

2. Outline of the Thesis

To achieve a better understanding of the basic experimental findings described in this work,

the fundamental principles of the applied techniques will be given in chapter two. The

operation principle of reflection-absorption infrared spectroscopy (RAIRS) or FT-IR will be

presented first followed by the principles of x-ray photoelectron spectroscopy (XPS), near

edge x-ray adsorption fin structure (NEXAFS), scanning tunneling microscopy (STM),

ellipsometry (SE), water contact angle (CA) and finally UV/VIS spectroscopy.

In chapter three a small introduction into self-assembled monolayers is presented, their

kinetics and adsorption mechanism followed by the preparation of the gold substrates used in

this work, likewise gold deposited on silicon wafer used in case of RAIRS measurements,

contact angle, ellipsometry, XPS and NEXAFS. For measurements in the STM gold

evaporated on mica is suitable, because of the large flat (111) terraces separated by

monoatomically height steps (2.4 Å). This chapter highlights also the preparation of the

SAMs films, chemicals, technical equipment used in this study.

Chapter four reports the study of triptycene films like C0T, C1T and C3T using ellipsometry

(SE), reflection absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy

(XPS) near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and scanning

tunnelling microscopy (STM) techniques. Chapter five describes the formation of (SAMs)

from triarylaminethiols onto gold (111) substrates studied by using IR, XPS, NEXAFS and

STM techniques.

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Chapter One Introduction

4

In chapter six it has been found that the monolayers derived from the thioacetate (C12SAc)

have a significantly different structure and are compared to the ones obtained from the

corresponding alkanethiol (C12SH).

Finally the chapter seven gives the results for azobenzene molecules under alternating the

irradiation with UV and blue light using the UV/VIS spectroscopy measured in case of

prepared solutions of these molecules and the investigations by the contact angle and STM on

prepared SAMs from azobenzene molecules.

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Chapter Two Basics and Techniques Used in This Work

5

Chapter Two

Basics and Techniques Used in This Work

2.1. Infrared Spectroscopy

An important tool of the organic and inorganic chemist which was frequently used in this

work is Infrared Spectroscopy, or IR. This covers a range of techniques, the most common

type is absorption spectroscopy. IR is used to get information of the structure of a compound

and to determine the purity or composition of the sample [17, 18].

2.1.1. The Electromagnetic Spectrum

Infrared refers to that part of the electromagnetic spectrum between the visible and microwave

regions [19].

The radiation is characterized by its frequency or wavelength, which are connected through eq.

1. Frequency, ν is measured in Hz, where 1 Hz = 1/sec. Wavelength, λ is the length of one

complete wave period. It is often measured in cm (centimetres).

Eq. 1

c

and

c

,

where c is the speed of light, 103 10 cm/sec.

Energy is related to wavelength and frequency by the following formulas:

Eq. 2

hc

hE ,

where h=Planck’s constant, 6.6 10-34 joules-sec.

Note that energy is directly proportional to the frequency and inversely proportional to

wavelength.

The IR region is divided into three regions: the near, mid, and the far IR (see Figure 2). The

mid IR region contains wavelengths between 3 × 10-4 and 3 × 10-3 cm.

A wavenumber is the inverse of the wavelength in cm:

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Chapter Two Basics and Techniques Used in This Work

6

Eq. 3

1

,

where is in units of cm-1 and is in units cm and now:

Eq. 4 hcE

The mid IR range corresponds to 4000-400 cm-1 (wavenumber). Infrared radiation is absorbed

by molecules and converted into energy of molecular vibration, when the radiant energy

matches the energy of a specific molecular vibration.

Figure 2: The IR regions of the electromagnetic spectrum.

The adsorption of the light to the properties of the material through which the light is

travelling is called Lambert Beer law [20, 21]:

cdA , where A is the absorbance, is the molar

extinction coefficient, c the concentration and d the distance that the light travels through the

material. Band intensities can also be expressed as absorbance (A), which is the logarithm, to

the base 10, of the reciprocal of the transmittance (Figure 3):

)1(log10 TA

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Chapter Two Basics and Techniques Used in This Work

7

Certain groups of atoms absorb energy and therefore, give rise to bands at approximately the

same frequencies. The chemist analyzes a spectrum with the help of tables which correlate

frequencies with functional groups. The theory behind this relationship is discussed in the

next section on molecular vibrations.

2.1.2. Molecular vibrations

There are two types of molecular vibrations, stretching and bending. A molecule consisting of

n atoms has a total of 3 n degrees of freedom, corresponding to the Cartesian coordinates of

each atom in the molecule. In a nonlinear molecule, 3 of these degrees are rotational and 3 are

translational and the remaining corresponds to fundamental vibrations; in a linear molecule, 2

degrees are rotational and 3 are translational [22]. The net number of fundamental vibrations

for nonlinear and linear molecules is consequently:

Table 1: Number of vibrational degrees of freedom of nonlinear and linear molecules.

3000 2900 280070

75

80

85

OctadecanethiolSAM

Tra

nsm

itta

nce

/%

Wavenumber (cm-1)

3000 2900 2800

0.012

0.013

0.014 OctadecanethiolSAM

Ab

so

rban

ce

Wavenumber (cm-1)

Figure 3: The IR spectrum of octadecanethiol, plotted as transmission (left) and

absorbance (right).

Molecule Vibrational

nonlinear 3n - 6

linear 3n - 5

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Chapter Two Basics and Techniques Used in This Work

The fundamental vibrations for H2O are given in Figure 4. Water, which is nonlinear, has

three fundamental vibrations, symmetrical and asymmetrical stretching modes and one of

scissoring (bending) vibration.

Carbon dioxide, CO2 is a linear molecule and exhibits four fundamental vibrations. The

asymmetrical stretch of CO2 gives a strong band in the IR at 2350 cm-1. Since CO2 is present

in the atmosphere this band will appears in all IR spectra, if the IR light passes through air.

The two scissoring or bending vibrations have the same frequency and are degenerate,

appearing in the IR spectrum both at 666 cm-1.

Th

no

sp

th

Symmetrical stretching Asymmetrical stretching Scissoring (bending)

Figure 4: Stretching and bending vibrational modes for H2O.

Figure 5: Stretching and bending vibrational modes for CO2.

8

e symmetrical stretch of CO2 (Figure 5) is inactive in the IR because this vibration does

t produce a change in the dipole moment of the molecule. The selection rules of the IR

ectroscopy are that to be IR active, a vibration must cause a change in the dipole moment of

e corresponding molecule. In general, if the dipole moment change is larger, the intensity of

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Chapter Two Basics and Techniques Used in This Work

9

the band will be stronger in the IR spectrum [22].

An oscillating electric or magnetic moment can be induced in an atom or molecular entity by

an electromagnetic wave. Its interaction with the electromagnetic field is resonant if the

frequency of the latter corresponds to the energy difference between the initial and final states

of a transition (DE = hυ). The amplitude of this moment is referred to as the transition

moment.

The stretching and bending vibrations of the important organic group -CH2 are illustrated in

Figure 6 as follows:

Twisting, out-of-plane Rocking, in-plane Wagging, out-of-plane

1350-1150 cm-1 720 cm-1 1350-1150 cm-1

Figure 6: Stretching and bending vibrational modes for a –CH2 group.

The 3n – 6 rule does not apply since the –CH2 group represent only a portion of a molecule.

The bending vibrations occur at lower frequencies than corresponding stretching vibrations.

2.1.3. Stretching Vibrations

The stretching frequency of a bond can be approximated by Hooke’s Law. Two atoms are

treated as a simple harmonic oscillator composed of two masses joined by a spring:

Symmetrical stretching Asymetrical stretching Sccisoring, in-plane

2850 cm-1 2925 cm-1 1465 cm-1

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Chapter Two Basics and Techniques Used in This Work

According to Hooke’s

Eq. 5

where k is the force co

In the classical harmon

of the spring. If this mo

Figure 7: Harmonic a

The vibrational energy

Eq. 6

where is the frequen

the force constant of th

be than E1 = 3/2h .

According to the selec

anharmonic potentials

10

m1 m2

law, the frequency of the vibration is related by the formula:

m

k

2

1 ,

nstant, m is the mass and is the frequency of the vibration.

ic oscillator the energy ishkxE 2

2

1

, where x is the oscillation

del was true, a molecule could absorb energy of any wavelength [22].

pproximation via potential of the oscillator V(r) [23].

is quantized and only the transitions must fit the formula:

20 )(

2

1)()()

2

1( rrkrVnEhn ,

cy of the vibration, n is the quantum number (0, 1, 2, 3, ….) and k is

e bond. The lowest energy level is E0 = 1/2h and the next level will

tion rule, only transitions to the next energy level are permitted. For

also transitions like 2 h , 3 h can be observed. These are called

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Chapter Two Basics and Techniques Used in This Work

11

overtones in an IR spectrum and they are usually of lower intensity than the fundamental

vibration bands.

A record consequence of the molecule being an anharmonic oscillator is that the energy levels

become more closely with increasing interatomic distance and dissociation is possible.

If for a molecule the mass m is replaced by the reduced mass (for two atomic

molecules21

21

mm

mm

) eq. 5 turns into:

Eq. 721

21 )(

2

1

mm

mmf

c

,

where is the vibrational frequency (cm-1)

m1 and m2 are the mass of the atoms (g)

c is the velocity of the light (cm/s)

f is the force constant of the bond (dyne/cm)

As the force constant increases, the vibrational frequency (wavenumber) also increases.

Examples of the forces constant: single bond 5 × 105 dyne/cm

double bond 10 × 105 dyne/cm

triple bond 15 × 105 dyne/cm

As the mass of the atoms increases, the vibrational frequency decreases.

The regions of an IR spectrum where stretching vibrations are seen, depend on whether the

bonds are single, double, triple or bonds to hydrogen.

Table 2: Absorption by single, double and triple bonds observed in an IR spectrum.

Bond Absorption region, cm-1

C-C, C-O, C-N 800-1300

C=C, C=O, C=N, N=O 1500-1900

C≡C, C≡N 2000-2300

C-H, N-H, O-H 2700-3800

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Chapter Two Basics and Techniques Used in This Work

12

2.1.4. Infrared Spectrometer

In the following the equipment to measure IR spectra will be presented:

Since the measured radiation contains the optical functions of all components of the

spectrometer, first of all a spectrum of I0 ( ) without sample is taken, which is called

reference or background. Subsequently (or parallel), a spectrum I ( ) but this time with

assigned sample is recorded with otherwise identical configuration of the spectrometer. By

computation of the absorbance

Eq. 8)(

)(lg)( 0

I

IA

one receives an appropriate spectrum, which corresponds to the Lambert-Beer law [21] and its

band intensities are proportional to the concentration and to the layer thickness. If sample and

reference are identical, then one receives a “zero-line” over the entire spectral region. It

defines the baseline of the spectrum.

2.1.4.1. Principle of Operation of FTIR- Spectrometer

In contrast to the classical spectrometers, where the spectral absorption of a sample is being

scanned, Fourier-Transform Infrared (FTIR) spectroscopy is an interferometric method. An

FTIR spectrometer consists in principle of an infrared source, an interferometer, the sample,

and the infrared detector.

The interferometer is the heart of the spectrometer and consists in its simplest form of a beam

splitter, a fixed mirror, and a moving mirror scanning back and forth. Therefore, the spectrum

is not directly measured but its interferogram, i.e. the IR intensity reaching the detector as a

function of the mirror position. The spectrum is subsequently obtained by Fourier

transformation of the interferogram from the time domain into the frequency domain [23].

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Chapter Two Basics and Techniques Used in This Work

13

Figure 8: Schematic setup of FTIR- Spectrometer [23].

The major advantages of FTIR spectroscopy, as compared to conventional dispersive IR

spectroscopy, are the so-called multiplexing advantage (Felgett advantage: all the wavelength

are simultaneously collected) and the high energy flux reaching the detector (Jacquinot

advantage), therefore the FTIR spectrometers have a stronger intensity of light, which leads to

an improved signal to noise ratio and permits faster measurements. The schematic setup of the

FTIR spectrometer is shown in the Figure 8. The light which is leaving from the light source 1

is focussed with an ellipsoid mirror 2 into an aperture 3. With the help of the aperture 3 the

extension of the light source can be adapted. Afterwards the light will be reflected from the

paraboloic mirror 4 into the Michelson-Interferometer, where the KBr beam splitter will split

up the collimated light in two beams. After reflection 6 will comes the moveable mirror 7 and

the flat mirror 8. It is used only for the redirection of the light. On the other hand the

paraboloic mirror 9 will focus the light to the plane of the sample 10. Afterwards the ellipsoid

mirror 11 reflects the light to one of the two detectors 12. One of the detector is DTGS

(Deuteriertes Triglycinsulfat, D2N-(CD2-CO-ND)2-CH2-COOD) and the other one MCT

(Mercury-Cadmium-Telluride). The better signal to noise ratio of some orders of magnitude is

necessary for the measurement of self-assembled monolayers.

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Chapter Two Basics and Techniques Used in This Work

14

In addition the expanding of a Monomode HeNe laser 13 (15803 cm-1) with the help of a

plane mirror 14 is linked into the polychromatically bundle and go together through the

interferometer. Mirror 8 possesses an opening and lets so the laser light to the three detectors

15 arranged in the triangle perpendicularly to the laser bundle.

The source of light is constructed to give large intensity and the adjustment takes place via

attenuation on the necessary intensity [24].

2.1.4.2. Rinsing Gas Supply and its Influence on the Measurement

Figure 9: Set up of rinsing gas supply.

Air consists to 79% of nitrogen (N2) and 21% of oxygen (O2) and these molecules are not IR

active. Therefore air would be a good environment for the IR measurements if the small

portions of water vapour and carbon dioxide would be absent. These two substances because

of their high polarity show a very high absorption in the IR spectrum. Therefore it is

necessary to use dry air in the spectrometer, to remove C02 and water as far as possible from

the air. Compressed air made available from the central supply 1 with an operating pressure of

0.5 MPa is supplied after passing a stop valve 2 to the rinsing gas producer. The rinsing gas

producer contains two adsorption columns, which are alternately and automatically switched

on in the air flow. The manometer 4 serves for the monitoring of the minimum pressure

necessary for the operation of the generator of 0.4 MPa. The air regulator 5 limits the flow to

12 l/min, which is controlled by the flow meter 6. The main compartiment of the IR

spectrometer is dried with a blue gel cartridge and the flowing air will be held under a small

pressure (Figure 9). Therefore the major part of the flowing air serves for the air interchange

in the sample area.

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Chapter Two Basics and Techniques Used in This Work

15

3900 3600 2400 2100 1800 1500 1200

0.00

0.05

0.10

0.15

H2O asym/sym

3657/3756 cm-1 H

2O Sccis.

1595 cm-1

CO2asym.

2349 cm-1

Ab

sorb

an

ce[A

U]

Wavenumber[cm-1]

Figure 10: Spectrum of air in the sample area.

Figure 10 shows the regions of the water vapour and carbon dioxide in the sample area. After

an initial rinsing time of approximately 300 s an almost stationary condition is reached. It is

therefore reasonable not to start immediately a measurement after inserting the sample into

the spectrometer, but to wait until the IR spectrometer is purged and most of the H2O and CO2

are removed. Therefore was selected for the initial of all admission IR spectrums a waiting

period of 5 min [23].

2.1.4.3. RAIRS- Setup

Measurements of thin films on metal surfaces must take place in reflection so called Infrared

Reflection Absorption Spectroscopy (RAIRS) or External Reflection Spectroscopy (ERS)

[25]. Additional on metal substrates these techniques are limited by the surface selection rules.

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Chapter Two Basics and Techniques Used in This Work

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Figure 11: Deduction of the surface selection rules at metallic surfaces [23].

The most frequently used technique for the determination of the orientation is the

determination of the dichroism of the vibrations by means of IR spectroscopy. This means

that on electrically conductive surfaces all electrical field components (E-field) parallel to the

surface are repressed (Figure 11). In the result only normal E-field components are available

and this means that only molecule vibrations with the TDM (Transition Dipole Moment) Δp

perpendicular to the surface are IR active and for the band intensities A it follows:

Eq. 9 2

max

)(p

p

A

Az

Amax – means that the TDM orientation is perpendicular and the band intensities are

measurable. By the surface selection rules, the direct analysis of the orientation becomes

impossible from the IR spectra.

The setup (Figure 12) which is used into the sample area of the spectrometer follows:

Figure 12: Measuring accessories of the “Uniflex” of the Bio-Rad FTS-3000 [23].

The incident light is reflected by means of the plane mirrors 1 and 2 as well as the ellipsoid

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Chapter Two Basics and Techniques Used in This Work

17

mirror 3 on the sample lying from the sample table 4. After reflection at the sample the light

leaves over the ellipsoid mirror and the plane mirrors 6 and 7 the sample area toward detector.

All optical elements are installed on the two base plates 8 and 9. They are tiltable around the

optical axis of the light in the sample area, thus the angle of incidence can be varied (Figure

12, right) continuously. Since only the normal component of the E-field can contribute to the

spectrum, the incident light is polarized by the polarizer 10 parallel to the plane of incidence

(p-polarization). The aperture 11 limits the opening angle of the incident light beam. All the

IR spectra were measured in this work on SAMs with an angle of incidence of 80° with

respect to the surface normal. The diameter of the spots of the sample with perpendicular

incidence was measured with a heat sensitive foil. It amounts to about 10 mm. The field

aperture 12 reduces the diameter of the spots to half, which is covered by a diameter of a

sample of (2 cm × 4 cm). The measurements were started 5 min after mounting a new sample.

The resolution used was set to 2 cm-1 and 2000 scans were accumulated, averaged, and

transformed by using triangular apodization. For the bulk samples one has to use the DTGS

detector, averaged over 100 scans, and also transformed by using a triangular apodization [24].

2.2. X-ray Photoelectron Spectroscopy (XPS)

X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical

Analysis (ESCA) is a widely used technique to investigate the chemical composition of

surfaces based on the photoelectric effect [26]. XPS is a surface sensitive method with typical

information depth of 1-5 nm, determined by the mean free path of electrons in the solid state.

This technique was developed in the mid 1960s by K. Siegbahn and his research group. He

was awarded for his work in XPS with Nobel Prize for physics in 1981.

The energy of the incident radiation used in XPS is usually more than 1000 eV. For XPS, Al

Kα (1486.6 eV) or Mg Kα (1253.6 eV) are often the photon energies of choice [27]. The XPS

technique is highly surface specific due to the short range of the photoelectrons that are

excited from the solid. When a sample in UHV is bombarded with x-rays of characteristics

energy, electrons from the core levels of the sample are emitted. The kinetic energy

distribution of the emitted photoelectrons (i.e. the number of emitted photoelectrons as a

function of their kinetic energy) can be measured using an appropriate electron energy

analyser and a photoelectron spectrum can thus be recorded.

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Chapter Two Basics and Techniques Used in This Work

18

Figure 13: Energy pattern of x-ray photoelectron spectroscopy.

The process of photoionization can be considered in several ways: one way is to look at the

overall process as follows:

Eq. 10 eAhA

Conservation of energy:

Eq. 11 )()()( eEAEhAE

Since the electron's energy is present solely as kinetic energy (KE) this can be rearranged to

give the following expression for the KE of the photoelectron:

Eq. 12 )]()([ AEAEhKE

The final term in brackets, representing the difference in energy between the ionized and

neutral atoms is generally called the binding energy (BE) of the electron (as shown in the

Figure 14) - this then leads to the following commonly quoted equation:

Eq. 13 BEhKE

The binding energy (BE) of the photoelectron is a “fingerprint” of the elements in the material

and their chemical environment, the peaks appear in an XPS spectrum at distinct values of BE.

CCoonndduuccttiioonn BBaanndd

VVaalleennccee BBaanndd

LL22,,LL33

LL11

KK

FFeerrmmii

LLeevveell

FFrreeee

EElleeccttrroonn

LLeevveell

IInncciiddeenntt XX--rraayyEEjjeecctteedd PPhhoottooeelleeccttrroonn

11ss

22ss

22pp

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Chapter Two Basics and Techniques Used in This Work

19

XPS is a quantitative chemical spectroscopy, because the area of a photoemission peak is

proportional to the number of emitters in the analysis volume.

Figure 14: Schematic diagram of the photoelectronic spectroscopy.

The BE is now taken to be a direct measure of the energy required to just remove the electron

from its initial level to the vacuum level and the KE of the photoelectron is again given by :

Eq. 14 BEhKE

The binding energies (BE) of energy levels in solids are conventionally measured with respect

to the Fermi-level of the solid, rather to the vacuum level. This involves a small correction to

the equation given above in order to account for the work function ( ) of the solid. In this

work XPS was used to identify the elemental composition of the SAMs and to determine the

thickness of the SAMs. The film thickness can be calculate by the using of the relative

intensities of the Au 4f7/2 and C 1s peaks and by using a thiol with known thickness on Au as

a reference system (Ex: Au =36.5 Å at a kinetic photoelectron energy of 1169 eV for MgKα)

and carbon (Ex: c =30 Å at 996 eV for MgKα) [28].

By applying the equation 15, one can get the film thickness (d sample):

Eq. 15

)(exp1

)(exp

)(exp

)(exp1

)(

)(

cc

reference

Auc

reference

Auc

sample

cc

sample

Au

c

Au

c

E

d

E

d

E

d

E

d

referenceI

I

sampleI

I

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Chapter Two Basics and Techniques Used in This Work

20

The XPS spectrum of C18 thiol (Figure 15) is presented as follows, where one can observe

the core level electron energy states of the Au and peaks of C and S can be seen as well:

800 700 600 500 400 300 200 100 00

50

100

150

200

250

Au

4f

7/2

Au

4f

5/2

S2p

C1s

Au

4d

5/2

Au

4d

3/2

Au

4p

1/2

Au

4s

Au

4p

3/2

C18 on Au(111)

Inte

nsit

y[k

cp

s]

Binding energy [eV]

Figure 15: XPS overview spectrum of octadecanethiol (C18) on Au(111).

2.3. Near Edge X-ray Absorption Fine Structure (NEXAFS)

A further important technology for surface analysis is near edge x-ray absorption fine

structure or NEXAFS, which refers to the absorption fine structure close to an absorption

edge, about the first 30eV above the actual edge. This region usually shows the largest

variations in the x-ray absorption coefficient and is often dominated by intense, narrow

resonances. NEXAFS is also called X-Ray Absorption Near Edge Structure, XANES [29]. In

our days NEXAFS is typically used for soft x-ray absorption spectra and XANES for hard x-

ray absorption spectra.

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Chapter Two Basics and Techniques Used in This Work

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Figure 16: Energy pattern of NEXAFS spectroscopy.

The measurement principle is illustrated in Figure 16. This method uses monochromatic,

linear polarized x-rays of a synchrotron, which are absorbed in the material by excitation of

core electrons into unoccupied molecular orbitals. Each excited electron leaves a core hole

into which another electron relaxes. The relaxation energy is transferred thereby in the form

of fluorescence or transferred to a further electron, which is emitted as Auger electron. Since

the Auger process dominates for light elements, it is used as indirect proof of the excitation.

In NEXAFS the x-ray energy is scanned and the absorbed x-ray intensity is measured. As

illustrated in the Figure 17 below, NEXAFS spectra can be recorded in different ways. The

creation of secondary electrons is the principle of the electron yield measurements.

Figure 17: Different methods of recording x-ray absorption spectra.

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Chapter Two Basics and Techniques Used in This Work

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The detection of the entire swarm of electrons (TEY, Totally Electron Yield) contains more

frequently diffuse electrons of deeper layers of the sample. It is not very surface sensitive. A

good signal/noise relationship and sufficient sensitivity for the monomolecular adsorbate

layers can be reached by measuring in the "Partial Electron Yield" mode, whereby only

electrons emitted near to the surface can be acquired. All spectra presented here are measured

with a backlash potential of 150 V in the PEY mode, due to ensure of a good surface

sensitivity by the mean of the free path of the electrons of only unite nanometers. Regarding

by the measurements of the carbon edge, a gold lattice is used, the organic impurities, of

which give rise to π*-resonances.

In this work NEXAFS was used to determine the orientation of the molecules of the SAMs as

shown in the Figure 18.

The intensities I of the suggestions of resonances are described by transition probabilities P by

the Fermi’s Golden rule:

Eq. 162

2

1~ ipEf

EI

where i is the initial state (one C 1s orbital) and f the final state (can be σ* or π* molecule

orbital) of the electrical field E

of the stimulatory radiation and of the dipole operator p̂ .

Under definition of the TDM of the ipfp arises:

Eq. 17 ),(cos2 TDMEIpEI

NEXAFS spectra of a particular film are measured for at least two different angles of the

Figure 18: Definition and orientation of the angles in the surface coordinate system of

the NEXAFS experiment. E║ and E┴ are the p and the s-polarized portions of the

incident light, and TDM is the situation of the dipole transition moment of the excited

transition.

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Chapter Two Basics and Techniques Used in This Work

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incident e-field to the surface normal of the sample, so one can determine the middle

orientation of the TDMs from the observed dichroism of the spectra. In the case of triple

substrate symmetry (gold substrates) all orientations are averaged around the surface

normal, one receives only the middle tilting angle of the TDM. In the context of this work

for the determination of the middle tilt angles, one has to considered the π*-resonances. It is

incidental angle dependence in the case of two fold substrate symmetry:

Eq. 18 )sin)(sin1()cossincos(cos 222222

* PPI , the intensity

of the NEXAFS resonances depends on the angle of incidence , the adsorption angle of

the adsorbed molecules and on the angle of the adsorbed molecule for the azimuth direction

of the substrate.

The angle dependence in the case of three fold substrate symmetry:

Eq. 19 22222

* sin)1()sinsin2

1cos(cos PPI ,

the intensity of the NEXAFS resonances depends in the case of three fold substrate symmetry

on the angle of incidence , the adsorption angle of the adsorbed molecules.

In this work are presented NEXAFS and XPS spectra, which were measured at the HE-SGM

beam-line (resolution: E = 0.4 eV at 300 eV) of the BESSY II synchrotron in Berlin.

2.4. Scanning Tunneling Microscopy (STM)

Scanning tunnelling microscopy (STM) is a scanning probe technique, based on the quantum-

mechanical effect of the electron tunnelling. It has become an important instrument for real

space analysis in surface science. The Scanning Tunneling Microscope (STM) was introduced

by G. Binnig and W. Rohrer at the IBM Research Laboratory in 1982 which was honoured by

the Noble Prize in 1986.

The importance of the STM was realized in 1982 when images for the (7×7) reconstructed

structure of Si(111) were acquired [30]. The STM is used for the investigations of clean metal

surfaces, the atomic resolution imaging of these surfaces and surface defects [31].

The basic idea is to bring a fine metalic tip in close proximity (a few Å) to a conductive

sample. By applying a voltage (U~4V) between the tip and the sample a small electric current

(0.01nA-50nA) can flow from the sample to the tip or reverse, although the tip is not in

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Chapter Two Basics and Techniques Used in This Work

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physical contact with the sample. This phenomenon is called electron tunneling.

Figure 19: Principle of the imaging process by the STM. The lower part shows a band

structure diagram for a tunnel contact between the tip and the sample.

In scanning tunnneling microscopy a small bias voltage V is applied so that due to the electric

field the tunneling of electrons results in a tunneling current I. The height of the barrier can

roughly be approximated by the average workfunction of sample and tip.

Eq. 20 )(2

1tipsample

The electrons can never leave the metal unless they are given the necessary energy to go over

this potential barrier, if they tunnel. In the example presented in Figure 19, the electrons

tunnel from occupied states of the sample into empty states of the tip (in case Pt-Ir). The

system used in STM measurements is a piezolelectronic drive system and a feedback loop

with those can be obtained a topography map of the corresponding surface.

The relation of the tunnelling current to the gap distance is shown in the following expression:

Eq. 21 )exp( 21

0 dkI st

where, Φ is the average work function of the sample and the tip, d is the distance between the

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Chapter Two Basics and Techniques Used in This Work

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sample and the tip, ρt and ρs are the densities of the states of tip and sample, respectively, and

κ and k are constants. The tunnelling current decreases exponentially with the separation

between the tip and the sample. Changes in the separation such as 1Å result in measurable

changes in the tunnelling current [32, 33]. Alternatively the height of the tip over the sample

can be kept constant and the tunnelling current can be used as topography information

(constant height mode). More resuming details about the STM can be found in the technical

literature [34-37]. The STM measurements in this work were carried out in air, using a Jeol

JSPM 4210 microscope. The tips were prepared mechanically by cutting a 0.25mm Pt0.8Ir0.2

wire (Goodfellow). All data were collected in a constant-current mode with a tunneling

current of (60-90 pA) and a sample bias of (450-500mV). For these tunnelling conditions no

tip-induced changes were observed.

2.5. Ellipsometry (SE)

The characterization of the optical constants and thickness of self-assembled monolayers

(SAMs) is a part of our research, and ellipsometry is the best method to determine these

quantities. There are three types of data typically acquired with the ellipsometer, transmission

and reflection intensity and of course change of polarization [38].

Ellipsometry measures the change in polarisation state of light reflected from the surface of a

sample, expressed as Ψ and Δ. These values are related to the ratio of Fresnel reflection

coefficients, Rp and Rs. for p and s-polarized light, respectively [39].

Eq. 22s

pi

R

Re )tan(

Figure 20: Schematic of the geometry of an ellipsometry experiment.

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Chapter Two Basics and Techniques Used in This Work

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Because ellipsometry measures the ratio of two values, it can be very accurate and nicely

reproducible. The s-direction is taken to be perpendicular to the direction of propagation and

parallel to the sample surface. The p-direction is taken to be perpendicular to the direction of

propagation and contained in the plane of incidence (shown in the Figure 20).

Optical constants:

The optical constants define how light interacts with a material. The index of refraction, n,

defines the phase velocity of light in material.

Eq. 23n

c

where, υ is the speed of the light in the material and c is the speed of light in vacuum. The

extinction coefficient, k, determines how fast the amplitude of the wave decreases. The

extinction coefficient is directly related to the absorption of a material and is related to the

absorption coefficient by:

Eq. 24

k4

where, α is the adsorption coefficient and λ is the wavelength of light [40].

Ellipsometric measurements were performed using an ellipsometer SE 400 (Sentech

Instruments GmbH) under an incidence angle of 70° at a wavelength of 633 nm. While the

substrate parameters for each spot were determined before immersion, a refractive index of n=

1.45 was assumed for the organic layers.

2.6. Water contact angle (CA)

Contact angle measurement is a well-known technique, which is being used to control

adhesion, surface treatments, and polymer film modification. The wetting of solid substrates

by liquids is a basic element in many natural and commercial processes.

The contact angle between a liquid and a solid is a measure of the energetic interaction

between the solid and the liquid. This is usually determined using the sessile droplet method

[41].

In this case, a tangent is drawn and the angle between the solid and the surface is called as the

contact angle. The contact angle can be determined from the droplet contour by means of

calculational methods (for example using the Young-Laplace equation).

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Chapter Two Basics and Techniques Used in This Work

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As shown in Figure 21, when a liquid drop is settled on a solid surface, the contact angle of

the drop on the solid surface is defined by the mechanical equilibrium, of the drop under the

action of three interfacial tensions (solid, vapour and liquid). The equilibrium relation is

known as Young’s equation:

Eq. 25 YVLSLSV cos

where, θY is the Young contact angle, which is suited in Young’s equation; SV = contact

angle between the interfacial tensions of solid and vapour; SL = contact angle between solid

and liquid and VL = contact angle between vapour and liquid interfaces.

In practice, the contact angle measured in the laboratory could not be exactly the Young

contact angle because of the following experimental errors:

1. Any gravitational effect during the drop is set on the solid surface

2. Any volume reduction of the drop when the syringe is detached from the liquid drop

after the drop is set on the solid surface

3. Heterogeneity of the solid surface

4. Any absorption of the liquid phase of the drop by the solid

5. Any reaction between the liquid phase and the solid phase or any variation of the

liquid surface tension due to the adsorption of a surface active materials or impurities

when the liquid phase is not pure

6. Effect of the line tension produced at the solid/liquid/gas contact line (drop size effect)

7. Any experimental errors on the drop profile observation during the measurement

The line tension is defined as the specific free energy of the three-phase contact line as a force

operating in the three-phase line. The mechanical equilibrium condition for any point at the

Figure 21: Schematic of a sessile-drop contact angle system.

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Chapter Two Basics and Techniques Used in This Work

28

three-phase contact line can be given as:

Eq. 26 Rlvslsv cos

where, R is the radius of the three-phase contact circle [41].

Sessile water-drop contact angle (CA) measurements were obtained by using a video camera-

based commercial apparatus (Surface & Optics Co., Ltd, Korea; Phoenix 150). The reported

value is the average of three measurements with deionized water on each substrate of the

SAM.

2.7. UV-VIS Spectroscopy

UV-VIS spectroscopy is the measurement of the wavelength and intensity of absorption of

near-ultraviolet and visible light by a sample. Ultraviolet and visible light are energetic

enough to promote outer electrons to higher energy levels. UV-vis spectroscopy is usually

applied to molecules and inorganic ions or complexes in solution. The concentration of an

analyte in solution can be determined by measuring the absorbance at some wavelength and

applying the Beer-Lambert Law [21].

The photon energies are sufficient to promote or excite a molecular electron to a higher

energy orbital. Consequently, absorption spectroscopy carried out in this region is sometimes

called "electronic spectroscopy".

Figure 22: Electronic excitations for an organic molecule.

A diagram showing the various kinds of electronic excitation that may occur in organic

molecules is shown in Figure 22. Of the six transitions outlined, only the two lowest energy

ones (left-most, colored blue) are achieved by the energies available in the 200 to 800 nm

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Chapter Two Basics and Techniques Used in This Work

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spectrum.

Molar absorptivities from n* transitions are relatively low, and range from 10 to100 liter

mol-1 cm-1. * transitions normally give molar absorbtivities between 1000 and 10,000

liter mol-1 cm-1. The solvent in which the absorbing species is dissolved also has an effect on

the spectrum of the species. Peaks resulting from n* transitions are shifted to shorter

wavelengths (blue shift) with increasing solvent polarity. Often (but not always), the reverse

(i.e. red shift) is seen for * transitions. This is caused by attractive polarisation forces

between the solvent and the absorber, which lower the energy levels of both the excited and

unexcited states. This effect is greater for the excited state, and so the energy difference

between the excited and unexcited states is slightly reduced - resulting in a small red shift.

This effect also influences n* transitions but is overshadowed by the blue shift resulting

from solvation of lone pairs.

An optical spectrometer records the wavelengths at which absorption occurs, together with the

degree of absorption at each wavelength. The resulting spectrum is presented as a graph of

absorbance (A) versus wavelength analogue to infrared spectra presented in sec.2.1.

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Chapter Three Self-Assembled Monolayers and Sample Preparation

30

Chapter Three

Self-Assembled Monolayers and Sample Preparation

3.1. Self-Assembled Monolayers

3.1.1. Introduction

Organic films for instance the deposition of long chain carboxylic acids were first studied by

K. Blodgett and I. Langmuir [42, 43]. At that time, the amphiphilic monolayers were already

used to control the wetting behaviour of metal condenser plates in steam machineries [44-46].

Further research on systems related to self-assembled monolayers were done later by Zisman

et al. [47], see also the history of organic thin films summarized by [48].

Besides applications in the classical field of technology, organic thin films can play an

important role in interfacing bio-technological devices [13].

In comparison to the Langmuir-Blodgett films, which are formed by a mechanical process,

SAMs are formed spontaneously by the immersion of an appropriate substrate into a solution

of an active surfactant in an organic solvent [47, 49].

As shown in the Figure 23, Langmuir films consist of amphiphilic molecules spread on a

liquid surface like water [50, 51]. The hydrophilic headgroup has an affinity to the water

Figure 23: Compariosn between LB films and SAMs films.

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Chapter Three Self-Assembled Monolayers and Sample Preparation

31

whereas the hydrophobic endgroup sticks out on the other side. These Langmuir films are

transferred onto a solid substrate and they are called Langmuir-Blodgett films (LB) [48].

The self-assembled monolayers are strongly bound on solid substrates due to specific affinity

of the headgroup to the solid surfaces.

Since their discovery in 1983 by Nuzzo and Allara [52], self-assembled monolayers of thiols

and disulfides on gold (111) have been studied for their potential applications in molecular

technologies like chemical sensors, nonlinear optical materials [48], microelectronics, and

computer technology [53-56]. One can stabilize the surface with a desired functionality and

this idea is also used in wetting studies of SAMs [57] which also serve as a model for polymer

films. Furthermore chemical reactions at surfaces can be studied under controlled conditions

[58].

3.1.2. Self-Assembly Kinetics and Mechanism

Much of the recent work on self-assembling organic monolayers at metal surfaces has focused

on the adsorption kinetics of alkanethiolate/gold system, where alkanethiols adsorb

spontaneously onto the metal surface to form a highly ordered array [5, 7, 59-63].

For the examination of these monolayers little effort has been spent on understanding the

elementary steps in the formation of the monolayers. Most of these studies have suggested a

two-step kinetic model for alkanethiolate on gold surfaces, a fast initial adsorption step in

some minutes and a slow adsorption step with a time scale of hours or days [9, 64-66].

All spectroscopic methods used to characterize these self-assembled monolayers provide only

spatially averaged information about the adsorption process, which lead to a request for

molecular level information during the self-assembly process. Using the scanning probe

microscopy (STM/AFM) the process of adsorption kinetics of self-assembled monolayers of

thiols deposited on gold surfaces (111) has been studied and well understood [67-70].

Figure 24 shows that the thiol molecules will adsorb on the gold surface and they first will

form a so called striped-phase with their molecular axis parallel to the surface. The adsorption

process of thiols onto gold can be divided into two or three steps, the first is fast, and the

following steps are much slower [71]. The first step takes less than 10 s to finish (sulfur

adsorption) and 10 h to complete the second step (orientation ordering). The orientation

ordering step is governed by the interchain interactions [72, 73].

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Chapter Three Self-Assembled Monolayers and Sample Preparation

32

3.1.3. Self-Assembled Monolayer Structure

Figure 25: Schematic diagram of a SAM. Shaded circle indicates adsorbed or

chemisorbed headgroup and open circle endgroup, which can be chosen from variety of

chemical functionalities.

Figure 24: Schematic mechanism diagram for the self-assembly of thiols on Au(111): a)

Initial adsorption. b) Striped phase or lying-down phase. c) 2D phase with a transition

from lying-down to standing-up phase. d) Formation of a complete SAM [74].

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Chapter Three Self-Assembled Monolayers and Sample Preparation

33

Figure 26: Schematic diagram of different energies in the adsorbed SAM [71]

Self-assembled monolayers (SAMs) are ordered, chemically and thermally stable two-

dimensional aggregates that are formed spontaneously by the adsorption of surface active

molecules onto a solid.

The surface active molecule should feature a head group, suitable for strong interactions or

even chemical bonding with the surface, a molecular backbone moiety responsible for the 2D

packing and favorable lateral interactions with the next neigbors and a tail or end group

determining the properties of the newly formed solid surface as shown in the Figure 25. The

SA of a film is a concerted interplay of various forces. The overall stability of a SAM is

determined by all inter- and intramolecular forces in the film. (as shown in the Figure 26

ΔEads stands for adsorption energy, ΔEcorr corrugation of substrate potential experienced by

molecule, ΔEhyd van der Waals interaction of (hydrocarbon) tails, and ΔEg energy of gauche

defect or deviation from the fully stretched backbone [71]).

A gold surface can simply be coated with an SAM by immersing the substrate into a dilute

solution of thiol or disulphide [75] The formation of the SAMs may be considered formally as

an oxidative addition of the S-H bond to the gold surface, followed by a reductive elimination

of hydrogen. X-ray photoelectron spectroscopy experiments suggest that chemisorption of

alkanethiols on gold(0) surfaces yields the gold(I) thiolate (R-S-) species [76]. The adsorption

chemistry is:

R-SH + Aun0 → R-S- - Au+ + ½ H2 + Aun-1

0

The bonding of the thiolate group to the gold surface is very strong (the homolytic bond

strength is approximately 160 kJ/mol) [1].

A schematic model as in Figure 27 of the (√3×√3)R30° overlayer structure formed by

alkanethiolate SAMs on Au(111) shows that in S…S distance are in the order of 4.99 Å,

which is a result from their tilt to reestablish the vdW interchain interactions [53, 68]. This tilt

angle is found to be ~30° with respect to the surface normal towards the nearest neighbour

direction [71].The angle can be deduced using FTIR spectroscopy (shown in the Figure 28 ).

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Chapter Three Self-Assembled Monolayers and Sample Preparation

34

Figure 27: Constant-current STM topograph of an octanethiol monolayer on Au(111)

which shows a c(4 x 2) superlattice of a R3033 )( overlay structure [68].

From the theoretical calculations for alkanethiolate on the Au(111) [77-80], it has been

concluded that the angle between gold and the S-C bond is about 180° if the the S atom is sp

hybridized and about 104° if the S atom is sp3 hybridized [81].

Z

Y

X

t

Figure 28: The tilt angle Θt of the alkanethiol chain relative to the surface normal [71].

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Chapter Three Self-Assembled Monolayers and Sample Preparation

35

3.2. Preparation and Characterization of Gold Substrates

3.2.1. Introduction

The self-assembled monolayers formed from organic molecules like thiols, have a very high

affinity to gold substrates [53] These films of SAMs deposited on the Au (111) are the

principal topic of this work. Gold substrates have some better properties in comparison with

others substrates like glass [82, 83], copper [84], silver [85] and mercury [86] because upon

the exposure to the air they are inert to oxidation. For the sensitive measurements of IRRAS

or XPS, these substrates have to be contamination free. Therefore, gold substrates must be

handled using some preventive measures to keep them clean and without surface impurities.

3.2.2. Preparation and Characterization of Gold on Silicon Wafers

IR-, NEXAFS-, XPS and contact angle measurements were possible on evaporated gold

surfaces. These gold surfaces show some closed layers [87, 88], which are showing the

orientation (111) [89].

As substrates were used Si(100)-Wafer (Fa.Wacker). Au was evaporated at 10-7 mbar in a

commercial equipment (Leybold Univex 300 with two thermal evaporators): after twelve

hours of annealing the wafers at 300°C in vacuum, they were coated with 50 Å titanium

(Chempur, 99,8%) with a rate of 5 Å/s and subsequently with 1000 Å gold (Chempur,

99,995%) with a rate of 20 Å/s. Afterwards the measurement of the layer thickness took place

with a quartz balance ( Leybold, Inficon XTM/2). All the silicon wafers were prepared in the

same way and before use they were kept in desiccator under argon.

Figure 29: AFM (atomic force microscopy) image of gold evaporate on Si(100)-Wafer

(1μm × 1μm).

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Chapter Three Self-Assembled Monolayers and Sample Preparation

36

3.2.3. Preparation and Characterization of Gold on Mica Substrates

For STM studies, the substrate has to be atomically flat to distinguish and characterize any

deposited species [90] and this can not be achieved by using the silicon wafers substrates.

Therefore gold substrates were produced on mica as reported in [91, 92].

Figure 30: STM images of Au/Mica (111) and Au/Si (111).

A freshly cleaved sheet of mica was deposited in the evaporation chamber (Leybold) for 72h

and was heated up to 300°C and then 1000 Å gold was deposited at a pressure of

approximately 10-7 mbar. These substrates were flame annealed in a butane/oxygen gas flame

for some seconds. They reveal large terraces of about 0, 5 μm to 1 μm width and orientation

Au (111). Afterwards the samples were cooled down in air before immersing into ethanolic

solution. Also the (2√3×3) reconstruction of the gold surface [93] can be observed [94].

3.3. Preparation of Self-Assembled Monolayers Films

Films are prepared by immersion into thiol solutions. The gold substrates have to be handled

with preventive measure to keep them clean and without contaminations.

The dimensions of substrate pieces have to be: 0.5 cm × 0.5 cm for the STM, 3 cm × 2 cm for

the IR, contact angle and ellipsometry and 1 cm × 1 cm for XPS and NEXAFS and these

small pieces have to be immersed into appropriate thiol solution. The immersion time depends

on the thiol and the goal of the study, but normally for a good quality of the SAMs, the small

pieces of gold are immersed into the solution for 24 h.

The concentrations of these solutions, used in this work, vary from 10-20 μM to 0.1 mM

depending on the solubility of each thiol substances. After approximately 24 h the gold

substrates were removed from the incubation solutions and rinsed carefully with absolute

ethanol or dichloromethane and then again with ethanol to remove the physisorbed overlayers.

Afterwards the substrates were dried in a nitrogen stream.

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Chapter Three Self-Assembled Monolayers and Sample Preparation

37

3.4. Preparation of Bulk Pellets for the IR measurements

The pellets which are used for the IR measurements are made by grinding approximately 0.3

g of pure KBr, and 2 mg of the corresponding thiol as a powder into the capsule. For the

grinding, one can use the vibrating mill (Perkin-Elmer) for 30 s or also hand mortar in order

to obtain homogeneous dispersion. Afterwards the pellet is produced with a diameter of 13

mm [95].

3.5. Chemicals used in this work

In this work, the following solvents with the given purities for the preparation of the self-

assembled monolayers films:

Chemical Source Purity

Ethanol absolute, p.a.,

Reag. ISO.

Riedel-de Haen 99.8%

Dichloromethane,

analytical reagent

(stabilized with 0.1%

Ethanol)

VWR 99.84%

Diethylamine, for synthesis Merck 99%

KBr (for Pellets) Aldrich 98.3%

The thiols used in this work are listed in the following (they were already presented in

Chapter one):

Substance Substance

1. C0T 9-triptycenethiol 7. AH-4

2. C1T (9-triptycenyl)-methanethiol 8. C12SAc dodecyl thioacetate

3. C3T 3-(9-triptycenyl)propane-1-thiol 9. C12SH dodecanethiol

4. AH-10 10. Azo 1 azobenzene

5. VK-55 11. Azo 2 azobenzene

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Chapter Three Self-Assembled Monolayers and Sample Preparation

38

3.6. Laboratory Equipment

A very important step is to clean all the items, which are used in the laboratory. After purging

with first bath of KOH/H2O2/2-propanol (15/15/1) and finally a second H2O/HCl (50/1) bath

have to be used. The items are immersed in each bath for 24 h. Between the two steps of

immersion, the items were precleaned with water provided from the filter system and with

ethanol (for the cleaning one can use technical ethanol). Afterwards the equipment is stored in

the oven at 60° for drying and then the bottles have to be rinsed with ethanol or another

solvent and then filled with the solvent needed for preparing the samples.

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

39

Chapter Four

Triptycenethiol-based Self-Assembled Monolayers

In this chapter, the SAMs formed from triptycenethiol (C0T), mercaptomethyltriptycenethiol

(C1T), and mercaptopropyltriptycenethiol (C3T) on polycristalline Au/Si and Au/mica are

characterized by ellipsometry (SE), infrared reflection absorption spectroscopy (IR), X-ray

photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy

(NEXAFS) and scanning tunnelling microscopy (STM) and the results are following

described.

4.1. Introduction and Objective of the Work Presented in this Chapter

Self-assembled monolayers (SAMs) in particular of thiolates on gold have frequently been

used to tailor surface properties [53, 96, 97]. Of particular interest are surfaces with sensing

properties, usually attained by directed substitution at the exposed part of the molecules (the

so called end group) with a suitable recognition group [98]. A major problem arises from the

fact that most recognition groups are relatively bulky compared to the carbon-backbone of the

SAM-forming molecules: e.g. the frequently used, comparatively small biotinyl-group still

has a cross section about threefold as the one of the frequently used alkanethiole chains [1].

The problem becomes even more significant if one considers the size of the typical analytes,

biomolecules, which easily have diameters of 10 nm or more. The usual approach for the

design of monolayers suitable for the efficient accommodation of such bulky end groups is

the production of so called diluted monolayers, in which the molecules carrying the

recognition groups are singled out within the monolayers by a huge excess of inert, diluting

molecules. In the best cases, this approach results in a statistical distribution of the relevant

molecules, while in worse cases either a preferential deposition of the minority species at the

grain boundaries within the SAM [99] or a complete phase segregation of the two kinds of

molecules occurs [100, 101].

In contrast we suggest the use of molecules with a bulky backbone for the generation of self-

assembled monolayers consisting of only one species, which allow for the accommodation of

bulky headgroups in a defined, periodic manner. To permit an efficient ordering within the

monolayers, the molecules chosen were derived from triptycenes, which have not only an axis

of higher symmetry (C3) but also permit a coaxial attachement of both, the anchoring group

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

40

(in our case a sulfur atom) and the headgroup [12].

This coaxial arrangement allows for an unambiguous positioning of the molecules within the

SAMs as shown in Figure 31 (all three possible rotamers are equivalent) and also fits well

with the threefold symmetry of the commonly used Au(111) surface [99].

The aim was to understand better the intermolecular interactions and their effect on the

morphology and also the molecular orientation of these rigid aromatic thiols. We expected

that these rigid SAMs molecules will form high degree of order and packing structures. The

influence of the methylene spacer employed between the trip- unit and the sulfur atom, will

play also a very important role in their adsorption process on account of this we examined the

influence of this methylene spacer on the molecular arrangement. In the following the

formation of the triptycene SAMs are presented.

4.2. Self-Assembly Process of Triptycenethiol on Au(111)

4.2.1. Introduction

The aromatic rings and the rigidity of the aromatic system determine the molecular orientation

and orientational order of the adsorbed thioaromatic molecules. In C1T and C3T the insertion

of the methylene group creates a conformative flexibility which is important to yield high

quality films.

Measurements using IR, XPS and NEXAFS confirm that these molecules form SAMs on the

substrate surface. C1T and C3T molecules show a tilted orientation with respect to the surface

normal.

Figure 31: Molecular structures of C0T, C1T and C3T (triptycene molecules).

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

41

4.2.2. Results

4.2.2.1. XPS and Ellipsometry

The thicknesses of the films were determined using an ellipsometer SE 400 (Sentech

Instruments GmbH) under an incidence angle of 70° at a wavelength of 633 nm. A refractive

index of n= 1,45 was assumed for the organic layers, while the refractive index of the

substrate was determined before film formation on each spot separately [71].

C0T C1T C3T

XPS 7.4 7.8 10.3

Ellipsometry 4.4 8.0 9.2

Calculated 6.1 7.5 10.0

Photo-electron spectroscopy (XPS) is a suitable method to determine the chemical species

within self-assembled monolayers. The energy scales of all spectra were referenced to the

Au4f7/2 peak located at the binding energy 84.0 eV. If the reaction of the triptycene thiols

with the gold surface proceeds as reported in the literature for aliphatic and aromatic thiols,

the carbon signals (C 1s at 284 eV) would show an unchanged carbon backbone [99], while

the shift of the sulphur signals (S 2p of 162 eV) should indicate the formation of a thiolate

species.

From the intensities of the Au 4f and the C 1s signals the thickness of the organic layers could

be determined (see Eq.15 from sec. 2.2) using a mean free path of 27 Å for the C 1s electrons

and 35Å for the Au 4f electrons [102]. The results are presented in Table 3 together with the

values determined by ellipsometry and the calculated thicknesses. While the values for C1T

and C3T agree well, a significant deviation larger than the typical error (±1Å) can be seen for

C0T. C0T molecule shows a discrepance not only in ellipsometry, but likewise in XPS shown

in the Figure 32. Here in the C 1s region the spectra of C0T, C1T and C3T are compared with

alkanethiol ODT (octadecanethiol). The C 1s peak in the case of aromatic thiols like ours

thiols is at ~284 eV and for the alkanethiol located at 285 eV.

Table 3: The layer thicknesses (in Å) as determined by XPS and ellipsometry as well as

the expected values for molecules standing upright on the substrate.

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

42

Figure 32: XP spectra of C 1s and S 2p regions of the C0T, C1T and C3T molecules in

comparison with an alkanethiol ODT.

Examination of the spectrum in the S 2p region showed an extremely broad and asymmetric

peak with a binding energy maximum at 162.3 eV. Only the C0T molecule is different and

shows an easily deviation from this binding energy to higher binding energies. The chemical

shift of C0T spectrum is shown in Figure 33, whereas the doublet at 161.4 eV is assigned to a

metal thiolate species and the second doublet at ~163 eV is related to unbound thiol molecules

or the thiols are showing oxidized sulphur [103].

ODT

ODT

]

ODT

ODT

]

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

In

gr

m

co

re

pe

ar

pr

ob

tri

do

or

Th

Th

Figu

area

re 33: XPS S 2p spectrum of C0T adsorbed onto gold/Si. Two S 2p doublets with 2:1

43

4.2.2.2. IRRAS

frared spectroscopy at interfaces not only provides insight into the presence of functional

oups but – in case of metallic substrates - also permits to determine the orientation of the

olecules on the surface. Due to the shielding by the electron gas in the metal, the

mponents of the transition dipole moment parallel to the surface become invisible in the

spective thin-film spectrum. For comparison, the bulk spectra of the triptycene thiols in KBr

llets were recorded. As can be seen in Figure 34, the bands appearing in the surface spectra

e also visible in the bulk spectrum (in the case of C3T and C0T), suggesting an adsorption

ocess without decomposition, although some bands are somewhat shifted or are not

served in the SAM spectra. This small shift might result from a different packing of the

ptycene moieties in the film as compared to the bulk. In fact, the bulk structure of triptycene

es not show any coaxial alignment of the units [12, 104] as would be expected in an

dered film.

e peak assignment of the bands in the case of triptycene SAMs (C3T) is shown in Table 4.

e production of the preparation solutions was done as follows: 2 mg of the respective

ratios and splittings of 1.2 eV were used to fit experimental spectrum.

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

44

Triptycenethiols was dissolved in 100 ml Et-OH after 5 min. under ultrasonic effect. The

saturated solutions were cooled down in the refrigerator, then filtered and diluted 1:1 with the

ethanol solvent. Then the immersion of the substrates took place for 24 hours, then rinsing,

drying and measuring the samples. The positions and intensities of the measured oscillation

transitions are likewise in the tables of the appendix [23].

For the C0T there are problems in the reproducibility by the IR measurements. Because of the

rigidity of this molecule (inflexibility) differences in the orientation of the C0T on the gold

surface occur as shown in the Figure 35 [105].

Figure 34: Infrared spectra of KBr and SAM of C0T, C1T and C3T in ethanolic

solution (concentration for the solutions: 70 M).

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

Figure 35: Infrared spectra of C0T molecule showing the problem of the

reproducibility on the gold substrates.

(C-H)

(C-H)

(C=C

(CH2

s(CH2

as(CH

as(CH

s(CH2

Table 4: The assignm

Mode assignment SAM(cm-1)

, trip vibration op ~756

, trip vibration , op ~947

), trip vibration ip// axis 1150

sci) and trip vibration 1458

) 2908

2) 2961

2)-20a ~3052

)-20b ~3071

45

ent of the peaks in the SAM of C3T molecule.

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

46

4.2.2.3. NEXAFS

To obtain a more quantitative measure for the average molecular tilt of the molecules,

NEXAFS data were recorded. Briefly, if the degree of polarization P of the incident

synchrotron light is known, the intensity Iif of a NEXAFS resonance on substrates with 3-fold

symmetry can be described by sec. 2.3 [29].

Iif P(1-cos2 cos2 + ½ sin2 sin2 ) + (1- P) 1/2(1 + cos2 )

Here, denotes the angle of the parallel electrical field vector component relative to the

surface normal and represents the angle of the transition dipole moment for the transition in

question relative to the surface normal. Thus, the variation of the intensity upon change of the

angle of the incident light with respect to the surface normal is characteristic for different

orientations of the transition dipole moment.

Figure 36 shows three representative C 1s NEXAFS spectra for each of the triptycene thiol

films on gold (111). The spectra were normalized to the absorption step height of the C 1s

edge and referenced to the *-resonances at 285.1 eV and 288.4 eV and σ*- resonances at

292.1 eV and 298 eV. The anisotropy of the intensity of these *-resonances was used for the

determination of the tilt angles of the respective transition dipole moments and therefore of

the molecular orientation. It should be mentioned that the transition dipole moments of all

three rings become averaged by the C3-symmetry of the triptycene moiety. From the intensity

modulation, the C0T and C3T show “no dichroism”, in contrast to the others spectra from

C1T.

280 290 300 310 320 330

0

1

2

3

4

C0T

1

*1

*

Photon Energy[eV]

Inte

nsity

[Un

its

ofe

dg

e-j

um

p] 30°

55°90°

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

47

280 290 300 310 320 3300

1

2

3

4

C1T

1

*1

Inte

nsity

[Units

ofed

ge-j

um

p]

Photon Energy[eV]

*

30°55°90°

280 290 300 310 320 330

0

1

2

3

4

C3T

Inte

nsity

[Un

its

ofe

dg

e-j

um

p]

Photon Energy[eV]

1

*1

*30°55°90°

Figure 36: NEXAFS spectra of C0T, C1T and C3T thiolates for different angles of

incidence of the synchrotron light. The solvent used for the solutions was EtOH.

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

48

4.2.2.4. STM

Figure 37: STM image of C0T molecule on Au(111) at room temperature.

For the STM studies gold on mica, which was described in chapter 3.2.3, was prepared and

then immersed in ethanolic solution of triptycene thiol. Figure 37 shows the STM

measurement of C0T molecule, which is a very rigid molecule and Figure 38 and Figure 39

show the measurement of C1T molecule with a rate of flexibility due to the inserted

methylene group as shown in Figure 31. All the images are showing highly packed films on

the surface, but one can not observe specific structures as observed at other thiol SAMs on Au

(111) like a significant amount of holes in the morphology of the substrate [9].

As we can observe in the Figure 38 the morphology of the C1T film at room temperature is

characterized by a lot of islands (the small brighter points) and their height correspond to an

atomic step of the gold substrate. After annealing of the sample at 60°C one can observe that

these small islands are now bigger and wider.

Figure 38: STM image of C1T

molecule on gold (111) at room

temperature

Figure 39: STM image of C1T molecule

on gold (111) at high temperature

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

49

From the imaging with the STM it is apparent that all three molecules are forming densely

packed films on the gold surface. One can not determine the molecular orientation of the

molecules due to formation of the islands and the impossibility to get high molecular

resolution. C1T shows very rough surfaces and because of the insertion of the methylene

group one can expected that C1T and C3T are tilted away from the surface normal.

4.2.3. Discussion

Between IR measurements of pellets and SAMs on gold substrates there is an agreement

between the bands. Therefore one can deduce that the triptycenethiols are adsorbed on the

surface, without destruction of the molecules. Some bands like in the case of the C0T

molecule, the trip- band from ~950 cm-1 show a clear shift. These are suggesting different

crystal configurations of the SAMs compared with those of the pellets and the Figure 40

shows the crystal configurations of the triptycenes [12]. The individual molecules within the

unit cell do not exhibit a preferential plane, according to which the molecules of the

triptycenethiols can orient them self at the favourably energetically surface [23].

The measured vibrational transitions are done on basis of the assignment of the bands shown

in the Appendix. The calculations were made for respectively one (isolated) molecule,

therefore they stand for gaseous phase spectra.

As shown in the Figure 35 the reproducibility of the C0T spectra is not obtained, some bands

reveal a representative shift in comparison with others spectra of the same molecule and the

same concentration of the solution. Definitively C0T molecules because of the pronounced

rigidity of the trip- frame have a different orientation of the molecule on the Au (111)

substrates. The bands from 1450 cm-1 are showing a good agreement, the vibrations between

2900 cm-1 and 3100 cm-1 are somewhat deviated because of the limitations of the calculations.

In the finger print we have big deviations between the calculated spectra and the measured

spectra, which could be explained by the effect of crystallization.

The determination of the molecular orientation is only possible on basis of the more intensive

bands of the spectrum, because only these bands can be measured with a reasonable intensity

and quality on the surface. The most intensive bands however are consisting of an overlap of

several vibrational transitions with differently oriented transition dipole moments.

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50

Figure 40: The crystal structure of the triptycene [12, 23].

DFT calculations of the IR spectra were done to identify better the bands and to assign the

directions of the transition dipole moments (TDM). For the calculation of the spectra of the

triptycene molecules B3LYP/6-31G(d) functionals were used and the calculated spectra are

compared with Ref. [106]. The comparison shows a good agreement of the bands only with

small deviations in the case of C1T and C3T molecules. By C3T molecule exist a strong

interconnection between aromatic vibrations of triptycene with the propylene chain, therefore

one can not allocate the orientations (TDMs) to the vibrations of the benzene rings. On the

basis of IR measurement it is not possible to determine the molecular orientation of these

triptycene molecules.

The determination of the molecular orientation, therefore, was done with the help of NEXAFS

measurements. The π*-resonances show in the case of C0T molecule a very small dichroism,

which means that C0T show a disorder of the molecules on the surface. In case of C1T and

C3T one can observe differences in intensity because of the dependence in angle of incidence.

From these measurements result a tilt angle of 42 ± 2° in case of C1T molecule and 46 ± 2° in

case of C3T shown in Table 5. The angles refer to a tilting of the triple symmetry axis of the

triptycene frame against to the surface normal [107].

The confirmation of the chemisorption and the determination of the thickness take place with

C0T C1T C3T

Au(111) Very small dichroism 42 ± 2° 46 ± 2°

Table 5:Tilt angles of the triptycene units with respect to the surface normal in the

triptycene thiol films on gold

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Chapter Four Triptycenethiol-based Self-Assembled Monolayers

51

XPS (see Table 3). The S 2p region are showing the formation of the thiolates with a position

of the gold-sulfur-bond specific at 161.4 eV [108, 109] and the second doublet at ~163 eV is

related to unbound thiol molecules or the thiols are showing oxidized sulphur shown in Figure

33. In comparison with an alkanethiol like ODT taken as a reference, demonstrate that only

C1T and C3T form thiolates as ODT molecules and only C0T molecule exhibit two different

S 2p doublets. One can not clamp if the C0T molecule remains thiol or not. In C 1s region

with the exception of C0T molecule, the positions of the aromatic carbon species agree with

the literature. The value of C0T thickness is too high, in case of C3T the value agrees and for

C1T they were found some discrepancies. From XPS one can conclude with exception of C0T

molecule, that monolayers are formed on the substrates.

The STM images (Figure 37, Figure 38 and Figure 39) confirm the formation of the densely

packed films on the Au (111). The determination of the molecular orientation it is not possible

without molecular resolution. One can say about the C0T molecule that it does not form

laterally arranged structure, and perhaps they are tilted away on the surface with an angle of

55° (magical angle) because of the discrepancies resulted from IR, XPS and NEXAFS

measurements. Possible are additional arrangements with different tilting, whose average

value corresponds to the orientation measured at 55°. In case of C1T and C3T because of the

inserted methylene between the trip- frame and the sulphur group the distance between the

molecule and surface is bigger and the molecules are more flexible on the surface. These

densely packed films exhibit well ordered structure based also on the LEED (low energie

electron diffraction) data. They form (m3 x n3) R30° structure, where m and n can be

possibly 2 [23].

Figure 41: Model for the arrangement of the C1T on gold surface.

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

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Chapter Five

Influence of the Leaving Group in case of Triarylaminethiols

In this chapter, the formation of self-assembled monolayers (SAMs) from triarylaminethiols

onto gold (111) substrates has been studied by using gracing incidence infrared spectroscopy,

X-ray photoelectron spectroscopy, near-edge X-ray absorption spectroscopy, and scanning

tunneling microscopy with or without deprotection of these triarylaminethiols molecules. We

found by using gracing incidence infrared spectroscopy that the monolayers derived from the

deprotected thioacetate have a similar structure compared to the ones obtained from the

corresponding triarylaminethiols without deprotection.

5.1. Introduction and Objective of the Work Presented in this Chapter

The research topic of self-assembled monolayers (SAMs) has witnessed tremendous growth

in synthetic sophistication and depth of characterisation over the past 20 years [48]. The

reasons for the large interest in SAMs are their great advantages compared to Langmuir-

Blodgett (LB) films [43, 110]. Chemisorption involves relatively large heats of bond

formation (40 – 160 kJ/mol) and has two advantages: firstly, the chemical reaction displaces

any previously formed physically attached adsorbates or impurities from the surface.

Secondly, the adsorbed species, once bonded, is difficult to remove from the surface.

However there are three disadvantages of chemisorption: the uncertain degree of coverage,

the possibility of further chemical reactions, e. g. thiolates on gold slowly oxidise to

sulfoxides in high-humidity environment, and the formation of surface dipoles [111]. While

the early days of SAM research were almost exclusively devoted to the investigation of the

formation, structure and physical properties of alkanethiols on gold, in the past decade the

function of such SAMs moved into the foreground.

Of the numerous kinds of monolayer systems, the one of organothiols on gold have been

proved to be the most popular ones due to their reliable formation and almost predictable

structure. While for monolayer formation most often organothiols or organodithiols are being

used, in a number of cases also organothioacetates or organodithioacetates have been utilized

[62, 98]. For example most studies on so-called break junctions, where single molecules are

attached in between two gold electrodes in order to directly measure the electrical

characteristics of these molecules, dithioacetates have been used as precursors for the

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

53

resulting dithiolates [112, 113]. Nevertheless, the reported results on the adsorption chemistry

of thioacetates on gold surfaces are somewhat controversially discussed [114]. An early paper

[115], reported that organothiolate adlayers formed from thioacetates have a similar quality as

the ones obtained from the respective thiols. The authors speculated that some of the

thioacetate molecules are hydrolysed in solution followed by deposition onto the gold. In

contrast to this, a more recent paper [114] finds that it is not possible to grow well-defined

self-assembled monolayers of a biphenyl-based organodithioacetate by immersing gold

samples in the corresponding solution. Since thioacetates are used in many papers [116-118]

for the preparation of monolayers on metals, we decided to carry out a detailed study on the

formation of self-assembled monolayers from aromatic thioacetates in particular on gold. The

widespread interest in triphenylamine (TPA) derivatives is due to their actual use as hole

transporting materials in electroluminescent multilayer light emitting devices based on

molecular organic compounds [119-122]. In their pioneering work on triphenylamine based

compounds Sakanoue et.al [123] pointed out that the reorganization energy is one of the most

important factors to determine the hole transport mobility, and a good hole transporting

material must have a small reorganization energy in an ionization process. For investigation

we chose substituted triarylamines- shown in the Figure 42 which have the great advantage

that the distance of the redox centres from the electrode, the chemical surrounding of the

redox centres as well as the type of the molecular bridges can be varied systematically.

These SAM structures can then serve as excellent model systems for studying bridge

AH-10 VK-55 TS-10 AH-4

Figure 42: Molecular structures of substituted triarylamine- AH-10, VK-55, TS-10 and AH-4.

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

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mediated ET (electron transfer processes). This was the aim of the project [13] to synthese

these substituted triarylaminethiols to use them as redox centres connected to donor- or

acceptor-substituted aromatic bridge units with saturated or unsaturated spacers. On the other

hand the important characterization of the SAMs was done using a broad set of experimental

techniques, gracing incidence infrared spectroscopy, x-ray photoelectron spectroscopy,

contact angle measurements, XPS, and STM. Altogether the experimental results allow for a

very consistent description of the adsorption behavior.

5.2. Self-Assembly Process of the Triarylaminethiols on Au(111)

5.2.1. Introduction

The experimental conditions of self-assembly were adjusted to optimize the formation of the

triarylaminethiols SAMs with or without deprotection. The adsorption of the substituted

triarylaminethiols layer(s) with deprotection was carried out by immersing the gold substrates

into 50 M solution of the compounds in dichloromethane at room temperature together with

10 mM of diethylamine. Acetyl protected thiols were deprotected during the formation of the

SAMs. Self-Assembled Monolayers formed on the gold surface in a solution of 50 M

without using the deprotection of the acetyl protected are further described.

After one day of immersing, the samples were efficiently rinsed with the solvent used for

deposition and dried in a stream of nitrogen to remove physisorbed overlayers. The

measurements were then carried out within the next 18- 24h.

5.2.2. Results

5.2.2.1. IRRAS

IRRAS spectra of substituted triarylaminethiols AH-10, TS-10, AH-4 and VK-55 have been

recorded for polycrystalline gold substrates prepared by immersion into ethanolic solution at

room temperature. In Figure 43, the low frequency regions of the IRRAS spectra are shown

together with the corresponding bulk spectra recorded using KBr pellets. All peaks in these

spectra can be assigned using data in the literature [17, 23], and the results are summarized in

Table 6.

Due to the selection rules on metallic substrates, only the vibrations with a transition dipole

moment component (TDM) perpendicular to the surface plane can be observed in the spectra.

As can be seen in Figure 43, the bands appearing in the surface spectra are also visible in the

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

55

bulk spectrum, suggesting an adsorption process without decomposition, although some bands

are somewhat shifted. This shift might result from a different packing of the substituted

triarylamine- moieties in the film as compared to the bulk spectra. The compounds of

triarylaminethioacetate were deprotected during the formation of the SAMs using 10mM

(C2H5)2NH [13] and the process was successful, because in all four spectra of the monolayers

the vibration υ(C=O) around 1700 cm-1 disappeared.

While an exact tilt angle determination for the thiol-derived films was not possible with this

method, one can say with the help of selection rules of the IR that substituted triarlyamine-

molecules are not oriented perpendicular on the surfaces of gold.

Vibration band Literature(cm-1) Pellet(cm-1) SAM(cm-1)

υ (C≡C) 2100-2250 ~2199 ~2210

υ (C=O) 1710-1720 ~1707 -

υ (C=C)in ring 1500-1600 1504-1600 1510-1600

υ (C-N) 1200-1350 ~1240 ~1247

υ (C-N) 1200-1350 ~1289 ~1289

δ(C-H)oop 800-860 ~826 ~804

Table 6: Positions and assignment of the IR-modes In case of Au surface and KBr pellet.

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

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Figure 43: Infrared spectra of substituted triarylaminethiols AH-10, TS-10, AH-4 and

VK-55. The red trace displays are IRRAS spectra of the monolayer and the black trace

measured for a KBr pellet.

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

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5.2.2.2. XPS

Figure 44: XP spectra recorded for polycristalline gold substrates showing the carbon 1s,

nitrogen 1s, sulfur 2p and oxygen 1s region for the triarylaminethiols AH-10, TS-10,

VK-55 and AH-4.

X-ray photoelectron spectroscopy was used to confirm the adsorption of the triarylamienthiols

molecules on the Au (111) surfaces. The samples which are used for XPS measurements are

exactly prepared as those used for the IR measurements, the four compounds are deprotected

during the formation of the SAMs using 10mM (C2H5)2NH [13].

In the Figure 44, one can find the XPS spectra of the four SAMs in different regions measured

after 24 h of immersion in the ethanolic solution. The energy scales of all spectra were

referenced to the Au 4f7/2 peak located at the binding energy 84.0 eV. In case of S 2p region

only the spectrum of VK-55 shows a strange behaviour due to this shift to higher binding

energy. Examination of the spectrum S 2p region of the AH-10 SAM (see Figure 45) showed

168 166 164 162 160 158

0.0

0.1

0.2

S 2p regionah 10ts 10vk 55ah 4

Inte

nsit

y[c

ps]

Binding energie [eV]

406 404 402 400 398 396 394

0.00

0.04

0.08

0.12

Inte

nsit

y[c

ps]

N 1s regionah 10ts 10vk 55ah 4

290 288 286 284 282 2800.0

0.5

1.0

1.5

C 1s regionah 10ts 10vk 55ah 4

Inte

nsit

y[c

ps]

538 536 534 532 530 528

0.0

0.1

0.2

0.3

O 1s regionah 10ts 10vk 55ah 4

Inte

nsit

y[c

ps]

Binding energy [eV]

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

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two chemically different species S 2p(1) at 161.7 eV and S 2p(2) at ~163 eV. The two

doublets are having ratios of 2:1 and splittings of 1.2 eV, predetermined intensity ratio.

Figure 45: XPS S 2p spectrum of AH-10 adsorbed onto gold/Si. Two S 2p doublets with

2:1 area ratios and splittings of 1,2 eV were used to peak fit experimental spectrum.

Whereas the doublet at 161.7 eV is assigned to a metal thiolate species and the second doublet

at ~163 eV is related to unbound thiol molecules or the thiols are showing oxidized sulphur

[103].

The C 1s region for AH-10, TS-10 and AH-4 showed a single symmetric peak centred at

almost 284 eV and only the sample VK-55 shows a strange behaviour due to this shift of the

C 1s peak to the higher binding energy of ~ 285 eV.

The oxygen 1s region showed a broader peak with a binding energy of ~ 533.6 eV and only

for the VK-55 film exist a shift of the peak to lower binding energy. In case of the nitrogen 1s

region the AH-10 film shows a strange behavior due to a shift of the peak to lower binding

energy of ~ 398 eV comparisons to the others spectra, which are having their peak around 399

eV.

5.2.2.3. NEXAFS

To obtain information about the orientation of the molecules on the Au/Si surface, NEXAFS

spectra were recorded (see Figure 46). The samples which are used for NEXAFS

measurements are prepared as those used for IR and XPS measurements. If the degree of

polarization P of the incident synchrotron light is known, the intensity Iif of a NEXAFS

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

resonance on substrates with 3-fold symmetry, see sec. 2.3 [29].

Figure 46: NEXAFS C 1s spectra of TPA (triphenylaminethiols) AH-10, AH-4, TS-10 and

59

The spectra were normalized to the “edge jump” height of the C 1s edge and we observe the

*-resonances at 285.1 eV, Rydberg resonances (R*) at ~ 287.7 eV and σ*- resonances at

292.1 eV. The anisotropy of the intensity of these *-resonances was used for the

determination of the tilt angles of the respective transition dipole moments and therefore of

the molecular orientation. For the phenyl ring, the symbol “*C=C” is used to denote the

delocalized * structure in the phenyl ring for typographical convenience [124]. The origin of

the energy shift of core→ * transitions in different functional groups is due to differences in

the unoccupied orbital energy as well as that of the C 1s core level [29].

The C 1s→ *C=C transitions are dominating and the C 1s peak is assigned at 285 eV,

indicating that the E vector of the X-rays is parallel to the macromolecular backbone, which is

observed for these four SAMs.

VK-55 at different angles of incidence of the synchrotron light.

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

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One can observe in case of these four TPA compounds that “no dichroism” is present in the

NEXAFS spectra and therefore one can not analyze the orientation of the TDM and

simultaneously the orientation of these four molecules on the surface.

5.2.2.4. Deprotection Process

In chapter 5.2.2.1 presented above, IR measurements for the four compounds of

triphenylaminethiols (TPA) using the deprotection process were shown [13]. The process was

effective, because in all four spectra of the monolayers the vibration corresponding to υ(C=O)

around 1700 cm-1 disappeared. Now there are surface bound via an aromatic thiol unit, thus

there is no potentially insulating alkyl fragment between the aromatic moieties and the gold

[115].

In this chapter IR data of the same samples of triphenylamine thiols AH-10, TS-10 and AH-4

without using the deprotection process during the formation of the SAMs will be presented.

The monothioacetyl moiety could even be used, without deprotection by using the 10mM

(C2H5)2NH, to generate the SAM directly.

In case of the direct adsorption or amine-promoted adsorption ((C2H5)2NH)) of the thioacetyl-

terminated systems, the IR results confirmed that the SAMs were similar in their composition

to the SAMs generated by the using of 10mM (C2H5)2NH.

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

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Figure 47: IRRAS spectra of AH-10, TS-10 and AH-4. The red lines are showing

the spectra without deprotection process and the black lines the spectra with

deproctection (used 10mM (C2H5)2NH) [13].

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

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From Figure 47 it is obvious that there are no differences between the IR spectra measured

after deprotection during the formation of the SAMs and the spectra measured without using

the 10mM (C2H5)2NH agent. The most important and surprising effect was that the vibration

corresponding to υ(C=O) around 1700 cm-1 disappeared in both cases.

In the following chapter STM measurements on these triarlyaminethioacetates are presented,

which will show the influence of the deprotection process.

5.2.2.5. STM

For these measurements gold on mica was used, prepared as descriebed in the chapter 3.2.3.,

and then immersed in the ethanolic thiol (AH-4-conformative flexibility) solution for 24 h.

The measurements were done at room and elevate temperature on samples prepared with and

without using the deprotection process (the amine-promoted adsorption).

Figure 48: STM measurements on the AH-4 molecule. Conditions: with deprotection,

10-20 µM DCM solution; Ut= 600 mV, It= 70 pA.

Figure 48 shows measurements of AH-4 molecule done at the room temperature and using the

deprotection conditions. This molecule is rather flexible due to the inserted methylene group

and the monolayer shows the typical atomically flat terraces of the Au substrate, which has a

height of 2.4 Å [9]. One can observe specific structures as observed for other thiol SAMs on

Au (111) like holes in the morphology of the substrate [9] only in a lower number of them.

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

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Figure 49: STM measurements on the AH-4 molecule. Conditions: with deprotection at 60°,

10-20 µM DCM solution; Ut= 650 mV, It= 65 pA

Figure 49 shows STM measurements on the same SAM and in the same condition of

deprotection but an elevated temperature of 60°. In the large-scale image one can observe

many terraces separated by monoatomic steps characteristic to the thiols on Au (111). The

morphology of the structure slightly changed and now the SAMs are denser on the gold (111)

substrate. It was impossible to record images, showing AH-4 compounds in the molecular

resolution.

Figure 50: STM measurements on AH-4 molecule. Conditions: without

deprotection, 10-20 µM DCM solution; Ut= 600 mV, It= 70 pA.

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

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Figure 50 shows STM measurements of AH-4 SAMS formed without the deprotection

process. The deprotection has been done by the molecules themselves during the bond of the

sulfur to the gold surface, without the amine-promoted adsorption.

In the large-scale image one can observe many terraces separated by the steps and more holes,

which are characteristics for other thiols on Au (111). By zooming in the region indicated in

Figure 50 one can not observe many differences, in detail there are the same lower number of

the so-called edge pits in comparison with the images shown in Figure 48 where the

deprotection process was used. It was impossible to reach high resolution, however, and one

can say that these molecules are not forming highly ordered monolayers.

5.2.3. Disscussion

From the IR measurements between the bands of the pellets spectra and the SAM spectra

measured on the gold substrate there is good agreement. By the comparison between SAM

spectra measured after deprotection and SAM spectra measured without deprotection one can

observe that these spectra are almost similar and in very good agreement. The peak around

~1700 cm-1 disappeared in both cases, which means that the protecting acetyl group was

easily removed [125]. It is deduced that triphenylaminethiols (TPA) adsorbed on the surface

in both cases if the deprotection process was used or not. These molecules are able to

deprotect themselves with a longer and slower reaction by the bounding of the sulfur

compound to the gold (111) surface compared to the normal adsorption process of

alkanethiols [115].

From the STM measurements it is concluded that these molecules are not forming highly

ordered SAMs, perhaps the molecules are lying down (striped phase) on the surface or they

are tilted from the surface normal, but the determination of the molecular orientation it is not

possible without molecular resolution and with these variations arise from the measurements.

Also from the NEXAFS measurements (Figure 46) it is difficult to understand the orientation

of TPA molecules. The π*-resonances do not show any dichroism, which means that the

triphenylaminethiols molecules do not show any angle dependence, perhaps they exhibit a

disorder on the surface. The confirmation of the chemisorption takes place with XPS shown in

Figure 44 and Figure 45.

Examination of the spectrum S 2p region of AH-10 molecule showed two chemically

different species S 2p(1) at 161.7 eV [108, 109] and S 2p(2) at ~163 eV. The peak at 161.7 eV

is assigned to a metal thiolate species and the second peak at ~ 163 eV is related to unbound

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Chapter Five Influence of the Leaving Group in Case of Triarylaminethiols

65

thiol molecules or the thiols contain oxidized sulphur [103].

The spectrum of VK-55 shows a shift and one can not say if this molecule remains thiol or not.

In C 1s region three samples showed a single symmetric peak centred at almost 284 eV in

agreement with the literature. Only the VK-55 SAM shows a strange behaviour with the C 1s

peak shifted to the higher binding energy of ~ 285 eV.

From the XPS measurements one can conclude that monolayers are formed on the gold

substrates, with exception of the VK-55 molecule.

Because it is difficult to characterize the structure of these SAMs as we observed in STM

measurements and from NEXAFS measurements we can not determine the orientation of the

molecules on the gold surface, in the following an eventual model of the arrangement of one

molecule of AH-4 on the Au (111) will be presented.

Figure 51: Model of an AH-4 molecule on the Au (111) surface flat lying or tilted away

from the surface.

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Chapter Six Formation of Self-Assembled Monolayers from Alkane Thioacetates

66

Chapter Six

Formation of Self-Assembled Monolayers from Alkane Thioacetates

In this chapter, the growth of self-assembled monolayers (SAMs) from dodecyl thioacetate

C12SAc onto gold (111) substrates has been studied by using gracing incidence infrared

spectroscopy, X-ray photoelectron spectroscopy, contact angle measurements, ellipsometry,

near-edge X-ray absorption spectroscopy, and scanning tunneling microscopy. We found that

the monolayers derived from the thioacetate have a significantly different structure compared

to the ones obtained from the corresponding alkanethiol (C12SH) and are further described.

6.1. Introduction and Objective of the Work Presented in this Chapter

Although self-assembled monolayers are already known for a couple of decades, they still

receive a lot of attention, since they slowly start making their way into a variety of technical

applications. Of the numerous kinds of monolayer systems, the one of organothiols on gold

have been proved to be the most popular ones due to their reliable formation and almost

predictable structure. While for monolayer formation most often organothiols or

organodithiols are being used, in a number of cases also organothioacetates or

organodithioacetates have been utilized [62, 98]. For example most studies on so-called break

junctions, where single molecules are attached in between two gold electrodes in order to

directly measure the electrical characteristics of these molecules, dithioacetates have been

used as precursors for the resulting dithiolates [112, 113]. Nevertheless, the opinion on the

adsorption chemistry of thioacetates on gold surfaces is somewhat controversial [114].

An early paper [115], reported that organothiolate adlayers formed from thioacetates have a

similar quality as the ones obtained from the respective thiols. The authors speculated that

some of the thioacetate molecules are hydrolysed in solution followed by deposition onto the

gold. In contrast to this, a more recent research work [114] finds that it is not possible to grow

well-defined self-assembled monolayers of a biphenyl-based organodithioacetate by

immersing gold samples in the corresponding solution. Since thioacetates are used in several

works [116-118] for the preparation of monolayers on metals, we decided to carry out a

detailed study on the formation of self-assembled monolayers from thioacetates in particular

on gold. As a model substance we chose dodecyl thioacetate, CH3(CH2)11SCOCH3 (C12SAc),

since the monolayers of the corresponding thiol are well known and thoroughly characterized

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[126, 127]. In order to rule out any interference from small contaminations by free thiols, we

synthesized a high purity thioacetate virtually free of any thiol. Note that in the past, trace

amounts of thiols in the starting materials lead to wrong conclusions about the monolayer-

forming properties of dialkylsulfides [115]. The adsorption of C12SAc is investigated using a

broad set of experimental techniques, gracing incidence infrared spectroscopy, x-ray

photoelectron spectroscopy, contact angle measurements, XPS, and STM. Altogether the

experimental results allow for a very consistent description of the adsorption behavior.

We found that the monolayers derived from the thioacetate have a significantly different

structure compared to the ones obtained from the corresponding alkanethiol (C12SH). The

objective of this work was to understand better why this difference between thioacetate and

the corresponding thiol occurs, and to find the explanation for a kinetic stabilization of the flat

lying phase the structure, which is formed by the separation of the acetyl group.

6.2. Preparation of the SAMs of C12SAc

6.2.1. Introduction

The experimental conditions of self-assembly were adjusted to optimize the formation of

C12SAc-SAMs. The adsorption of the dodecanethiolate and dodecyl thioacetate layer(s) was

carried out by immersing the substrates (gold substrates) into 10-20 M solution of the

compounds in ethanol or dichloromethane at room temperature. Self-assembled monolayers

of dodecanethiol (C12SH) were grown as reference.

After one day of immersing, the samples were efficiently rinsed with the solvent used for

deposition (ethanol or dichloromethane, respectively) to remove physisorbed overlayers and

dried in a stream of nitrogen. The measurements were then carried out within the next 18- 24h.

All the measurements confirm that the thioacetate C12SAc adsorb on the surface as a striped

phase in comparison with the normal thiol of C12SH.

6.2.2. Results

6.2.2.1. IRRAS

Figure 52 displays gracing incidence IR spectra recorded from films obtained by immersion

of gold substrates in solutions of C12SH and C12SAc, respectively. The spectra obtained of

similarly prepared samples of these two substances are superimposed for comparison. The

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Chapter Six Formation of Self-Assembled Monolayers from Alkane Thioacetates

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spectra for C12SH films resemble closely those reported earlier for the same system [126]. The

spectra recorded for C12SAc are clearly different from those obtained for the C12SH and

indicate that much less material has adsorbed on the gold surface. To make sure that this is

not an effect for significant slower film formation kinetics for C12SAc, the experiments were

repeated with very long immersion times (120 h), resulting in virtually unchanged spectra for

both the thiol and the thioacetate (Figure 52c).

Due to the dipole selection rules on metallic substrates, only the vibrations with a transition

dipole moment component (TDM) perpendicular to the surface plane absorb the IR radiation

and can be seen in the spectra. As described in detail in previous publications [95, 128] the

order and – if applicable – the tilt angle of the molecules in the films can be determined from

the relative intensity of the CH3 and CH2 bands because of their differently oriented TDMs. In

Table 7, we assign the guide frequencies together with the data fitted for the C12SH spectra

using Gaussian curves as shown in the Figure 53.

Using these data, it can be concluded that the films deposited from dichloromethane (DCM)

consist either of much less material (C12SAc) or are more disordered (C12SH) than the ones

generated from ethanolic solution. We therefore focus on films deposited from ethanol in the

following.

While a tilt angle determination for the thiol-derived films was possible, an obvious change in

the TDMs of the vibrations for the thioacetate made this impossible. This kind of changes in

the vibrational spectra has been described earlier for molecules lying flat on metal surfaces

and is caused by the electronic coupling to the electron gas of the metal [129].

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Figure 52: Infrared spectra of C12SH and C12SAc a) in ethanolic solution, b) in ethanolic

solution for five days and c) in dichloromethane solution.

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Stretching mode Max [cm-1] Fit [cm-1] Lit. [cm-1]

CH2 sym (d+) 2850.5 2850.5 2850

CH3 sym (r+) 2878.8 2879.1 2876

CH2 sym FR(d+FR) 2899.5 2900.8 2895

CH2 asym (d-) 2918.8 2918.7 2917

CH3 sym FR(r+FR) 2937.4 2937.7 2932

CH3 asym ip (r-) 2964.2 2964.3 2968

Table 7: The assigment of the peaks of C-H stretch mode vibrations from Figure 53.

The peaks were fited by Gaussian (Origin 7) [23].

3000 2900 2800

0

1

2

3

296

4.3

293

7.7

2918.7

29

00

.4

28

79.1

28

50.5

C12

SH

Ab

so

rban

ce

[10

3 AU

]

Wavenum ber (cm-1

)

Figure 53: RAIRS spectra of C12SH on the gold substrate (111) showing the region

of C-H stretch mode vibrations and the approximation of these peaks.

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6.2.2.2. Water contact angle

Contact angle measurements are an effective and relatively effortless means to gain

information about macroscopic surface properties of molecular films [130]. Since these

measurements are extremely surface sensitive, they allow to distinguish between surfaces

exposing alkyl chains in different orientations [84, 131]. The advancing contact angle for

water on dense self-assembled monolayers of C12SH on gold have been reported to be larger

than 103° [132]. This corresponds very well with the value found for the C12SH monolayers

deposited from ethanol (104°), while deposition from DCM results in layers with contact

angles of 97°, hinting on a diminished homogeneity of the surface endgroups.

The monolayers of thioacetate were found to be much more hydrophilic, exhibiting contact

angles of about 65° independently from the immersion time in the ethanolic solution. A

similar value has been found for methylene-group terminated SAMs, suggesting that the

layers generated from C12SAc do not expose CH3 groups, but their central chains [84]. This

could either be the effect of strong disorder or of flat-lying molecules.

6.2.2.3. Ellipsometry

One of the most convenient possibilities to characterize thin films is the determination of the

film thickness by optical ellipsometry. This very fast method effectively gives an idea about

the properties of the respective systems. Within minutes the layers derived from the reference

system C12SH reach a final thickness (6 Å vs 18 Å) as has been reported in the literature for

closely packed films with almost upright molecules [61]. Although the films derived from

C12SAc also reach their final thickness within minutes, which suggests the formation of

incomplete SAMs from C12SAc with a thickness of 30% relative to the thickness of the C12SH.

This situation remains unchanged even after 120 h of immersion.

6.2.2.4. XPS

In Figure 54 we show results obtained by x-ray photoelectron spectroscopy for the C 1s, S 2p,

Au 4f and O 1s regions. The recording time for the spectra was as short as possible. As seen

in the S 2p data, even under these precautions a certain amount of beam damage already

occurred what has to be considered when analyzing the data. Besides this damage effect, only

one sulfur species with a binding energy of 161.8 eV can be found, being consistent with the

formation of thiolate upon adsorption for both films [133]. The data recorded in the Au 4f

region show Au core-level lines which are much more intense in the case of the C12SAc than

for C12SH, revealing that the adlayers of the thioacetate are considerably thinner. This is

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supported by the C 1s spectra showing much lower signal intensity for the monolayers

deposited from the thioacetate, hinting on highly tilted molecules. In the case of the C 1s

spectra one also notes a significant difference in the energy of the core-level lines: The

observed C 1s binding energy for the monolayers derived from the thioacetate is about 0.4 eV

lower than for the thiol-derived monolayers. This effect has been observed before for

molecules lying flat on metal surfaces [129].

The reason for this shift is probably final state effects, lowering the binding energy of the C 1s

core hole in case of flat-lying molecules because of the better screening by the metal

electrons [134]. Consistent with this are the observations in the O 1s regime: while in the

monolayer generated from the thiol only trace amounts of oxygen can be found, the layers

made from C12SAc contain about one oxygen atom for every three molecules. Since this

Figure 54: XP spectra recorded for polycristalline gold substrates showing the carbon

1s, gold 4f, oxygen 1s region and sulfur 2p region for thioacetate (C12SAc) and thiol (C12SH)

immersed in ethanol solution.

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cannot result from the adsorption of intact thioacetate molecules (for which a ratio of 1:1

would be expected), we believe that the oxygen atoms rather stem from the oxidation of the

monolayer after formation during manipulation and transfer into the XPS set-up. It has been

reported in the past that the sulfur atoms of flat-lying thiolates are rather prone to oxidation by

atmospheric oxygen [135].

6.2.2.5. NEXAFS

To obtain more detailed information about the orientation of the thiolate molecules in the two

kinds of monolayers, NEXAFS spectra were recorded (Figure 55). The spectra of the C12SH

SAM very closely resemble earlier NEXAFS results reported for alkanethiolate-based SAMs

[136].

In the spectrum for the C12SH the resonance denoted R* has been shown to result from an

excitation into unoccupied orbitals with stronger R character [137, 138]. These signals show a

pronounced variation of the resonance intensity with the angle of incidence. A quantitative

analysis of the linear dichroism reveals a tilt angle of θ = (63 ± 5)°. For the C12SAc-based

SAM, the NEXAFS spectra are quite different. While the R* resonance is much less

pronounced, an additional resonance labeled as * can be seen at energy of 285 eV. This

resonance is attributed to a small amount of C=C double bonds, possibly generated by some

beam damage. Since the spectra as well as their angle dependency are significantly different

compared to the ones for the thiol-derived monolayers, the orientation of the alkyl chains in

the adlayers obtained by immersion in C12SAc must be considerably different. Note, that in

previous work it has been found that if alkane chains are in direct contact with a metals

surface, the R* resonance is modified in a way similar to that seen in the figure obtained here

for the C12SAc layers [139].

Therefore, the NEXAFS results are consistent with a monolayer in which the axes of the

hydrocarbon chains are parallel to the gold surface.

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Figure 55: NEXAFS spectra of a) C12SAc and b) C12SH thiol for different angles of

incidence of the synchrotron light. The solvent used for the solutions was EtOH.

6.2.2.6. STM

Since the presence of an alkanethiolate-based SAM with the alkyl chains orientated parallel to

the substrate not as an intermediate [68, 140] but as the final state appears to be in conflict

with previous reports (see above) additional experiments were carried out using high-

resolution scanning tunneling microscopy (STM).

The STM data reproduced in Figure 56 clearly demonstrate the presence of a striped phase, a

typical signature of low-density, intermediate SAM phases. In Figure 56 (a and b) we show

large scan area STM micrographs recorded for Au substrates covered with monolayers of

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C12SH and C12SAc, respectively, formed after 48 hours from 10-20 M solutions in ethanol.

At this lengths scale no major differences are seen, both images are dominated by the terraces

of the gold substrate and small circular depressions, which in earlier work have been

identified as vacancy islands [9].

The only difference might be a somewhat lower number of depressions for the film derived

from the thioacetate, although we have not quantified this amount.

While the high resolution STM for the monolayers derived from the thiol solution show the

same patterns of a densely packed SAM as reported earlier (not shown here), those of the

thioacetate clearly show the presence of two different phases. The dominant one is a striped

phase exposing three different orientations on the gold substrates Figure 56 (c), exhibiting a

close similarity to the results reported in previous papers [68, 140-145]. A high resolution

micrograph like the one shown in Figure 56 (d) allows to determine the distance between

adjacent rows, resulting in 15-16 Å. This value closely agrees with the length of extended

dodecanethiolate molecules, suggesting straight, flat-lying molecules in a parallel order. The

distance between the parallel molecules, as shown by the line scan in Figure 56 (d), is 5 Å,

suggesting a rectangular (p×2√3) unit cell containing two molecules, with the larger value

being predominantly determined by the length of the molecule. It should be mentioned that in

case of an orientation of the molecules parallel to the longer vector (thus being oriented

perpendicular to the rows of neighboring sulfur atoms) the value of p should be 6 resulting in

a calculated unit-cell length of 17.3 Å.

Although a number of different flat-lying phases are already described [68, 140-145] and even

a phase diagram has been established by Poirier [68], this particular phase has, to the best of

our knowledge, not been observed before. All previously known striped phases either show a

head-to-head arrangement with a stripe width significantly larger than the alkanethiolate

length (up to twice) or an alternating (intercalating) ordering with a pairing of the sulfur atoms

in form of double rows, resulting in a typical elongated ‘plaid’ pattern.

The second structure to be found in the C12SAc films is shown in Figure 57. This structure is

basically identical with those found for high-quality SAMs prepared by immersion of Au(111)

surfaces into alkanethiol solutions. As in previous publications on dodecanethiol monolayers

[68], we find a unit cell (Fig. 7b-d) described by a rectangular c(4×2) superlattice. It is

interesting to note that these areas of apparently upright orientated molecules are always

located at domain boundaries or other defects of the striped phase. For prolonged immersion

times the relative area of this phase was observed to slowly increase, but even after two days

the relative area of this upright orientated phase did not exceed 40 % of a monolayer.

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Figure 56: (a) Constant-current STM micrographs showing the gold substrate after

immersion into a 10-20 µM ethanolic solution of C12SH at 273 K for 48 h. Tunneling

parameters: (a) Ut= 600 mV, It= 70 pA. (b) Constant-current STM micrographs showing

the gold substrate after immersion into a 10-20 µM ethanolic solution of C12SAc at 273 K

for 48 h. Tunneling parameters: (b) Ut= 650 mV, It= 80 pA.(c) Constant-current STM

micrographs showing the stripe phase with the three different orientational domains.

Tunneling parameters: (c) Ut= 1000 mV, It= 65 pA. High resolution STM micrographs

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Chapter Six Formation of Self-Assembled Monolayers from Alkane Thioacetates

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showing the stripe phase on one of orientational domains. Tunneling parameters:

(d) Ut= 1000 mV, It= 65 pA.

Figure 57: Constant-current STM micrographs showing the gold surface after immersion

into a 10-20µM ethanolic solution of C12SAc at 273 K for 48 h. In (b), the unit cell of the

(2√3×4)R30° structure is marked by the rectangular box. Tunneling parameters: (a) Ut=800

mV, It= 95 pA; (b) Ut= 1000 mV, It= 75 pA.

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Chapter Six Formation of Self-Assembled Monolayers from Alkane Thioacetates

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6.2.2.7. Re-immersion of C12SAc-SAMs into thiol solutions

Since the layers derived from the thioacetate remain virtually unchanged even after prolonged

immersion times in the C12SAc solution, we wanted to understand whether inhibition of the

addition of further molecules was a general phenomenon or was specifically for the

thioacetate molecules. For this, we re-immersed the monolayers formed from C12SAc into a

solution of the corresponding thiol, C12SH (for 24h). The IRRAS measurements are shown in

Figure 58.

The IR spectra reveal a close similarity to the ones obtained for dense thiolate SAMs, which

also indicate the presence of a saturated hydrocarbon chain with its C-C-C backbone

orientated parallel to the substrate (Figure 58). The XP spectra obtained after this re-

immersion step are presented in Figure 59. The monolayers now contain much more material

and are now very similar to the ones obtained by immersion of the plain gold substrates into

Figure 58: C-H vibrational area of the IRRA spectra of the monolayers formed from C12SAc,

C12SH, as well as of the C12SAc-derived monolayer after re-immersion into C12SH solution.

The significantly lower intensity of the signals in the C12SAc-derived monolayer hints on a

much lower coverage, while re-immersion of this layer forms monolayers similar to the ones

obtained directly from C12SH.

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Chapter Six Formation of Self-Assembled Monolayers from Alkane Thioacetates

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the C12SH solution, as demonstrated with the C 1s signal. In this particular experiment the

amount of beam damage, as can be seen in the S 2p area, is much more pronounced due to

experimental reasons. It nevertheless becomes clear that the re-immersed samples are densely

packed monolayers since they show an O 1s signal much smaller than even the monolayers

directly prepared from C12SH.

Figure 59: XP spectra recorded for polycristalline gold substrates showing the carbon

1s, oxygen 1s and sulfur 2p region for thioacetate (C12SAc) reimmersed for 24 h in the

thiol (C12SH) solution (solvent:EtOH).

Also the macroscopic properties of these re-immersed samples become similar to the ones of

C12SH monolayers: the contact angle of 102° indicates the formation of a well ordered SAM

with almost upright molecules.

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6.2.3. Discussion

All together the results obtained by the different techniques allow to put forward a structure

model for the C12SAc films as presented in Figure 60. On the basis of our results we can

conclude, that in contrast to C12SH SAMs, (upright molecules), the C12SAc do not form a

SAM within almost upright orientation of the alkanethiolate films but that the adsorption

stops at an earlier state. Essentially the surface is covered by the so called flat lying phase.

This phase has first been identified in early STM measurements by Poirier et al [144]. The

STM data reported here are in full agreement with the striped phase consisting of lying-down

molecules. Obviously the adsorption of further molecules must be kinetically strongly

hindered when this first layer is present on the surface. The structure model proposed in Fig.9

for the alkanethiolate SAMs derived from C12SAc is fully consistent with the fact that in the

XPS data the C 1s core-level is shifted by 1.6 eV to lower binding energy. In earlier work this

effect has been demonstrated in case of mono- and multilayers of alkanes adsorbed on Cu

surface and have been explained by final state screening effects, which are stronger for carbon

atoms in close proximity to metal surface. Already for a bilayer of saturated hydrocarbon on a

Cu(100) surface the C1s binding energy is reduced by almost 1 eV [146]. Also the spectral

features in the NEXFAS data are fully consistent with such a phase of flat lying molecules.

Since in earlier work it has been shown that in particular the prominent R* resonance

dominating the NEXFAS spectra of alkanethiolate based SAMs is strongly affected if the

hydrocarbon chain is in direct contact with the metal surface. It should be noted that in the

spectra recorded for the C12SAc there is some similarity with the SAMs obtained from C12SH

solutions after short immersion times in very diluted solutions [147]. Probably the most

conclusive evidence for the presence of such striped phase comes from the STM data where

clearly stripes with an average distance of 15-16 Å can be seen. This value corresponds very

well to the lengths of the hydrocarbon chain in the C12SAc molecule and also corresponds

very well to the results obtained by Poirier et al obtained for decanethiol on Au(111). The

fact that the (2√3×3)-islands (appearing higher in the STM micrographs) are always localized

at the domain-boundaries of the striped phase (Figure 60) or near surface defects (in particular

step edges) suggests that the transformation of the striped-phase containing flat-lying

molecules into the dense, upright (2√3×3) phase is significantly hindered. Since the latter

phase should be the thermodynamically more stable, we conclude that there must be a kinetic

limitation hindering the formation of adsorbed thiolates from the thioacetate.

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Figure 60: Model of the surface layer formed upon adsorption of C12SAc (top view). The

surface is dominated by the flat-lying phase with the molecules lying next to each other

with alternating orientation. At domain boundaries or defects the (2√3×3) structure with

upright molecules can be formed due to the accessibility of the gold surface for further

C12SAc molecules.

After having established the structure model for the C12SAc based SAMs we can now turn to

the discussion of the reasons why the adsorption apparently stops at a coverage, which is

significantly lower than that of a high quality SAM with almost upright molecules as obtained

by immersing the gold films in a solution of the alkanethiol, C12SH. The flat lying phase

sketched in Fig.9 corresponds to the lowest coverage where the gold surface is completely

covered by molecules. When this coverage is reached, molecules interacting with the surface

will first have to find small areas of the bare gold surface where the sulfur of the thioacetate

group can get closely enough to the gold surface to form a chemical bond to the substrate and

to break the chemical bond to the acetate group. We propose that this adsorption on the

surface is strongly hindered after the striped phase has fully developed. Such a kinetic

limitation is strongly supported by a closer investigation of the STM data [148]. These data

show that for prolonged immersions in the C12SAc solution small areas developed which can

be identified as regions with upright orientated molecules. Although the area of these small

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82

patches is to small to allow a spectroscopic characterization with IR, NEXAFS or XPS, the

high resolution STM data clearly allow an assignment to areas with upright ordered

molecules. It is very interesting to note that these small patches of upright orientated

molecules are only observed at defects, e.g at domain boundaries between perfect areas of the

striped phase. This is in full accord with the kinetic limitations proposed above. After the

striped phase has formed the solvated C12SAc molecules can only react with the gold surface

at defects like etch pits, domain boundaries and possibly step edges. This explanation based

on a kinetic stabilization of the flat lying phase is strongly supported by the experiments

where C12SAc based SAMs were immersed in a solution of the corresponding alkanethiol,

C12SH. IR spectra and XPS spectra as well as contact angle measurements recorded after this

second immersion are basically indistinguishable from the corresponding data recorded for

C12SH based SAMs. This result is expected since for the corresponding alkanethiol much less

space is needed for getting the sulfur in contact with the gold. Although in this case a two step

adsorption process has been observed, where first a highly disordered film is formed and this

is followed by significantly slower second step where the orientational order is introduced,

immersion in the solutions of alkanethiols generally results in the formation of upright

oriented films [136].

We have obtained experimental data for gold substrates immersed into solutions of C12SAc

with high purity which demonstrate that alkanethiolate SAMs obtained by immersion into

organo-thioacetate has a significantly different structure from SAMs obtained by immersion

into alkanethiol solutions. The SAMs obtained by immersion into the thioacetates correspond

to a low coverage phase, which has also been observed previously for the alkane thiols.

Further adsorption after the completion of the low coverage phase is strongly hindered by

kinetic effects. Re-immersion into alkane thiol solutions leads to the formation of SAMs,

which are indistinguishable from those obtained after immersion into thiols. We speculate that

small modifications of the thioacetate group, e.g adding a more bulky hydrocarbon group,

may show further strong effects on the SAM formation process.

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

83

Chapter Seven

Determination of Trans/Cis Isomerization of Azobenzene Molecules

Photo-induced trans–cis isomerization of N- [4-(Phenylazo)phenyl]-1,2-dithiolan-4-

carboxamid (Azo 1) and 4-(Phenylazo)phenyl]-1,2-dithiolan-4-carboxylat (Azo 2) were

investigated in this work in ethanol. The results of infrared spectroscopy, contact angle

measurements and scanning tunnelling microscopy obtained by the investigation of the SAMs

of these azobenzene molecules, were cross-correlated with the trans–cis isomerization

determined from UV–visible spectra in solution. The kinetics of the photoisomerization

reaction of the trans- species under UV light irradiation is described by a simple first order

exchange between the trans and cis forms of the molecules. The cis-to-trans reversion in the

absence of irradiation is about 3% of the back reaction under irradiation [149].

7.1. Introduction and Objective of the Work Presented in this Chapter

The azobenzene chromophore continues to attract considerable interest as it offers the

potential for creating photoresponsive materials [14-16]. The properties of these molecules

may be altered reversibly by irradiation at selected wavelengths or by the temperature.

Recently, the introduction of azobenzene into SAMs has been examinated, resulting in unique

SAM structures [150]. It is known that azobenzene is photoisomerizable from trans to cis

isomers and from cis to trans isomers upon irradiation with UV light (366 nm) and with vis

light (436 nm), respectively. SAMs containing a azobenzene moiety could have the potential

for photoresponse(shown Figure 61) [151]. This isomerization results not only in a change of

geometry and dipole moment but also in a change in its optical properties.

Nevertheless, the introduction of the azobenzene in the SAMs could not produce a

photoisomerizable monolayer because there is not enough space to photoisomerize in close-

packed molecular films. This means that a passage volume is necessary for the isomerization

of the azobenzenes, even in a monolayer [152] and a suitable spacer in order to reduce the

metal substrate-azobenzene interaction. In the cis configuration the footprint area is twice as

large as for trans configuration, that means for a efficient photoisomerization in the film one

needs larger free volume [153].

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

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UV light

Visible light

Trans form Cis form

Figure 61: Conformational change of azobenzene [151].

In this work, the photoresponse of the solutions Azo 1 and Azo 2 (Figure 62) was investigated

by using UV/VIS spectroscopy. The photoreactivity and the effects on the structure of the

SAMs formed from the same molecules has been observed on the basis of the measurements

of static contact angles [154], IRRA spectroscopy and scanning tunneling microscopy.

The purpose of this work was to develop further our studies on structural characterization of

these SAMs and to evaluate the possibility to credible distinguish cis and trans azobenzene

isomers due to the reversibility of the irradiation process.

NN

OO

S S

NN

NHO

S S

Azo 2 Azo 1

Figure 62: Azo 1 and Azo 2 molecules.

NN

NN

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

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7.2. Preparation of the solutions containing the azobenzene molecules

The experimental conditions of the solutions were adjusted to optimize the reversibility of the

irradiation process and to distinguish the trans and cis isomers in both solutions, Azo 1 and

Azo 2. The preparation of the solution takes place in a dark room, in ethanol and the

concentration of the solutions is about 0.1 mM. There the solutions were irradiated with UV

light of 366 nm and to bring the molecules back in the ground state, they were irradiated with

Vis light of 440 nm. UV–visible spectra were recorded using a SPECORD 200 UV-visible

spectrometer. Samples were contained in quartz cuvettes and spectra were collected in single

beam mode over a wavelength range of 200 nm to 600 nm.

7.2.1. Results

7.2.1.1. UV/VIS Spectroscopy

As outlined in the previous section, the reversible photo-induced trans– cis isomerization of

solutions Azo 1 and Azo 2 were investigated by UV–visible spectroscopy as shown in Figure

63.

Figure 63 shows the UV-vis spectra of the both solutions against the irradiation process. The

unirradiated sample solutions show two important absorption bands, an intense one at 345 nm

in the case of Azo 1 and for Azo 2 at 320 nm attributed to the π- π* electronic transition of the

trans-azobenzene moiety and the second band very weak at 442 nm for Azo 1 and at 435 nm

for Azo 2, which are assigned to the forbidden n- π* electronic transition [149].

After the irradiation with UV light (the red curves), one can observe that the bands

corresponding to the trans-azobenzene moiety are decreased for both solutions. This means

that the cis form of the isomers in the solutions are formed. To demonstrate that the irradiation

process is reversible, the solutions were then irradiated with the vis light for 1h and

subsequently again irradiated with UV light. In the Figure 63 one can observe that the second

cis-spectrum (the green curve) is very similar to the first one, which means that the irradiation

process is reversible and the reproducibility of the measurements was successfully

demonstrated.

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

86

Figure 63: UV-VIS spectra of Azo 1 and Azo 2 solutions in the ground state of trans

isomers and after irradiation with the UV light.

7.3. Preparation of the SAMs on gold surfaces (111)

The ordered monolayers of azobenzene molecules (Azo 1 and Azo 2) on gold have been

prepared by the self-assembling technique [155]. The molecules are bound covalently to the

gold through the disulfide group, which will break upon adsorption of the molecules on the

gold surfaces. Figure 64 shows the azobenzene chains on the gold substrates in the case of

Azo 1 and Azo 2 in the trans and cis form. It is important to note that the orientation of the

azobenzene unit is almost perpendicular to the substrate normal [150, 153, 156].

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

The freshly prep

azobenzene molec

in a nitrogen atmo

The samples were

controlled by sca

confirmed by the

7.3.1. Result

7.3.1.1. IR

IR spectra for KB

observe that the in

peaks located at

azobenzene grou

antisymmetric str

respectively. Tab

SAM.

Figure 64: Trans

87

ared gold substrates were dipped into 0.1 mM ethanol solutions of

ules for 24 h. Finally, the substrates were rinsed with pure ethanol and dried

sphere.

investigated by infrared spectroscopy [157] and the sample quality was also

nning tunnelling microscopy. The switching ability of the molecules was

contact angle change between the trans and cis configuration.

s

spectroscopy

r, calculated, and SAM of Azo 1 are shown below in Figure 65. One can

tensity of the peaks appearing below 1400 cm-1 is very low compared to the

1600 cm-1. The low-frequency region is dominated by modes of the

p and the high-energy is mainly governed by the symmetric and

etching modes of the CH2 group at 2855, 2927, 2960 and 3075 cm-1,

le 8 contains the assignment of the more intense peaks in the case of Azo 1

- and cis-Azo 2 on the gold substrate.

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

F

[

88

igure 65: KBr, calculated and SAM IR spectra in low and high frequency regions

157].

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

89

Mode assignment SAM(cm-1)

(C-H), ring op 756

(C-H), Ring , op 17b 812

(C-H), Ring , op 10a 845

(C=C), ip// axis 1002

(18)a ring, // axis 1022

=N-Ph 1156

(C-N)+Ring, ip 1,4-axis 1262

-N=N- + Ring, ip 1,4-axis 1409

Ring, ip// 1,4-axis 1478

(C-N) 1509

(N-H)+ (C-N) 1543

Benzene stre. // ring (8a) 1597

s (CH2) 2855

as(CH2) 2927

as(CH2) 2960

s (CH2) 3075

Table 8: The assignment of the more intense peaks in the SAM of Azo 1.

The broad peak around 1466-1525 cm-1 correspond to the overlap between the vibrations of

(N-H)+ (C-N), the peak at 1408 cm-1 is due to the –N=N- sym and the peak around 1156

cm-1 is due to =N-Ph. These peaks are showing that the SAM is consisting of N-H, –N=N-, C-

N, and =N-Ph. This rule out the possibility of the reduction of –N=N- to –NH-NH-.

For determination of the molecular orientation we are interested in the peaks, located firstly at

845, 812, 762 cm-1, these peaks having transition dipole moment (TDM) out of plane

perpendicular to the molecular axis and to the benzene plane. Secondly, the peaks located at

1022 and 1002 which having TDM parallel to the molecular axis. By comparing the op/ip

peak intensities (take 845/1020 cm-1) in KBr and SAM spectra, a large reduction in the ratio is

observed in the SAM spectrum. The latter suggest that the azobenzene moiety is tilted by a

certain degree from the surface normal. The C=O vibration around 1700 cm-1 is absent in the

SAM spectrum of Azo 1, which exhibit a parallel orientation of the C=O vibration to surface

[157].

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

Figure 6

region.

The pea

(C-O-C)

SAM an

The N-H

2 SAM.

90

6: Comparison between Azo 1 and Azo 2 SAMs in low and the high frequency

ks which appear in Azo 2 SAM at about 1200 and 1225 and 1252 cm-1 are due to as

and (Ph-O-C) vibrations. The (C=O) peak located at 1750cm-1 appears in Azo 2

d this means that the orientation of the C=O group is not parallel to the surface.

peak located at 1540 cm-1 which appears in Azo 1 SAM is not observed for the Azo

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

91

7.3.1.2. Water contact angle (CA)

A solid surface modified with photochromic azobenzenes is a promising material to control

the surface free energy with light, because trans–cis photoisomerization of azobenzenes is

accompanied by reversible changes in the physicochemical properties of the chromophores

such as dipole moment and geometrical molecular structure [158, 159].

The action of liquid droplets of water was examined by placing them on the photoresponsive

surface, alternating irradiation with homogenous UV and blue light to push the isomerization

of the azobenzene moieties assembled on the surface. The sessile contact angle of the droplets

(shown in Table 9) was changed in the case of Azo 1 from 72° in trans form to 71° after 1 h of

irradiation with UV light and then the sample was irradiated again with the blue light to show

the reversibility of this process. After 1 h of irradiation with the blue light the sessile contact

angle was 77°. By the Azo 2 SAM, the static contact angle in trans form was changed from

72° to the cis form of 69° after 1 h of irradiation with UV light and by the irradiation with the

blue light the contact angle increased again to 72°. The reversible changes of sessile contact

angles were maintained within experimental error after 3-4 cycles of alternating

photoirradiation.

Azo 1 Trans 1h irradiated UV light

(Cis)

1h irradiated VIS

light (Trans)

Sample 1 72; 71; 72. (72) 71; 70; 72 (71) 77; 76; 77. (77)

Sample 2 74; 74; 73. (74) 72; 74; 74. (73) 75; 75; 73. (74)

Azo 2 Trans 1h irradiated UV light

(Cis)

1h irradiated VIS light

(Trans)

Sample 1 72; 73; 74. (72) 70; 70; 68 (69) 73; 71; 71. (72)

Sample 2 74; 74; 75. (74) 69; 69; 70. (69) 75; 74; 73. (74)

The values were within an experimental error of ± 2°.

Table 9: Contact angles of Azo 1 and Azo 2 before and after irradiation (liquid: water).

Figure 67 shows the reversible process of a droplet of the water on the Azo 2 SAM upon

alternating irradiation with UV and blue light. The sessile contact angle of the droplet was

changed from 80° to 65° after UV irradiation (a difference of 15° between trans- and cis- was

not always observed, the measurements were not always reproducible).

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

F

U

T

p

th

ir

F

th

th

T

T

ir

In

c

UV light

Vis light

trans

igure 67: Droplet of water on Azo 2 SAM before

V light.

his discrepance between the measurements presen

resented in the Table 10 might be interpreted by th

e mobility of the molecular tails possibly makes th

regular [160].

urther, we continue to prepare samples of Azo 1 a

e contact angle was measured again in the stable

e UV light (cis form).

he values were within an experimental error of ± 2°

Azo 1 Trans

Sample 1 57

Sample 2 54

Azo 2 Trans

Sample 1 59

Sample 2 58

able 10: Contact angle of annealed Azo 1 and Az

radiation with the UV light.

case of annealing the samples at 60 °C we obse

ompounds in trans- form, from 72° (see Table 9)

92

(80°) and after irradiation (65°) with

ted in Figure 67 and the measurements

e disordered film structure as well, where

e photoresponse on the SAM unclear and

nd Azo 2 by annealing at 60 °C and then

form trans- and after irradiation 1 h with

.

1h irradiated UV light

(Cis)

56

54

1h irradiated UV light

(Cis)

59

57

o 2 at 60 °C, before and after

rved a decreasing of the values for both

to 57°(see Table 10). The photoresponse

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

93

from trans- to cis form by the irradiation with the UV light does not exist, which reveal that

the comportation of the SAMs by annealing at 60°C is different from the comportation at

room temperature presented above.

7.3.1.3. STM

Figure 68 shows STM images of the Azo 2 molecules in the trans position after different

times of immersion in ethanolic solution, at room temperature and at 60° under ambient

conditions.

We find a densely packed monolayer consisting of small domains ( Azo 2- 1 day and Azo 2- 4

days) without so-called “etch pits”, which are known from SAMs of simple alkanethiols and

are caused by the relaxation of the Au(111) surface reconstruction upon adsorption of the

thiol group [68]. After 1 day and 4 days of immersion in ethanolic solution the monolayer

shows the typical atomically flat terraces of the Au substrate, which has a height of 2.4 Å [9]

but Azo 2 is unable to form a highly ordered monolayer at room temperature, or it is because

of the impossibility to image these films at the molecular resolution. At elevated temperature

of 60 °C, one can observe a big difference between the monolayers, the domains are bigger

and full of the so-called “etch pits”. In general it was problematic to image these SAMs,

which can be interpreted by the disordered film structure of the Azo 2 conjugated molecules

and one can speculate that the azobenzene group plays a critical role in the SAM formation

and in the structure of the SAM [161].

In the following we tried to image with the STM in situ the photoisomerisation of these Azo 2

molecules prepared at 60 °C by the irradiation with UV light. By the irradiation of these Azo

2 SAMs prepared at elevated temperature we expected a different arrangement of the

molecules within the unit cell. As presented in the image above one can observe that the

morphologie of the surface changed and present a plenty of so-called “edge pits”, which are

showing a normal comportation of adsorption of these molecules on the Au substrate.

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

94

Figure 69: STM images of Azo 2 SAM prepared at elevated temperature of 60 °C: A)

Before irradiation with UV light. B) After irradiation with UV light during 1h.

The STM measurements shown in Figure 69, exhibit the same morpfologie of the surface

Figure 68: STM images showing the a SAM of Azo 2 on Au(111): the first row after 1 day

immersion in the ethanolic solution, after 4 days immersion and after 1 day at elevated

temperature of 60 °C.

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

95

before and after irradiation with UV light. We expected to see some differences because of

the photoisomerization of the molecule from the stable form trans to the cis form, more at the

limits of the domains or at the defects of the substrate.

7.4. Discussion

The present work demonstrates the use of light on a photoresponsive solid surface and in

solution. The detailed preparation and structural characterization of the solutions and SAMs

made from two azobenzene molecules (Azo 1 and Azo 2) are presented in this chapter.

The molecules change their conformation between trans and cis forms under visible (Vis,

440nm) and ultraviolet (UV, 360 nm) light. Therefore, when functional molecules are

modified using this moiety, new functions associated with the switching mechanisms can be

added to the original functions such as molecular recognition, sensing and memory as shown

in Figure 70 (b).

Figure 70: a) Schematic of photoisomerization of azobenzene molecule. b) Schematic of

functional control using an azobenzene molecule [162].

In the case of solutions containing azobenzene molecules Azo 1 or Azo 2, we saw with the

help of UV/VIS spectroscopy that the photoirradiation process is reversible and the molecules

are very sensitive to day light (blue light). After measuring two cis-spectra with alternating

irradiation of UV light and blue light, one can conclude that the reproducibility of the

switching process is very high.

SAMs were prepared by immersing the gold substrates in ethanolic solutions of the azo-

molecules for 24 h. In the cis configuration the footprint area is twice as large as for trans

configuration, that means for a efficient photoisomerization in the film one needs larger free

volume [153].

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

96

From the IR measurements one can deduce that the azobenzene molecules Azo 1 and Azo 2

are adsorbed on the surface. The bands in the KBr pellet spectra and the bands in the SAM

spectra of the Azo 1 molecules measured on a gold substrate show a good agreement. In IR

measurements obtained after irradiating the samples with UV light one can not observe a

difference between the stable trans form and cis-form that should have been formed after

irradiation (the measurements are not shown in this work) [157, 163].

The IR spectra indicate that the azobenzene units are tilted from the surface normal and form

ordered monolayers on the gold substrates. These SAMs show a photoresponse that is

reflected in a change of the static contact angle measurements indicating that there is a free

space for the isomerization of the azobenzene moieties. To increase the stability of the SAMs,

ordered and densely packed structures are needed, whereas on the other hand free space is

necessary to achieve a reproducible molecular transformation due to photoisomerization, as is

observed in the case of the contact angle measurements [154, 160]. Towards to this

circumstance, by annealing the samples at 60 °C they show by the contact angle

measurements a decreasing in the values and the effect of photoisomerization is missing

completely.

The STM measurements are indicating the formation of densely packed monolayer at room

temperature, but at increased temperature the surface shows a difference. The domain size is

increased and the number of the so-called “etch-pits” is also increased. We can say that this

phase is dependent on the temperature, but in both cases it was impossible to get the

molecular resolution. This observation indicates that the substituent on the azobenzene

moieties is also an important factor in regulating the photoreactivity (as described in section

7.3.1.2 part) as well as the SAM structures shown in section 7.3.1.3. Regarding the STM

measurements of Azo 2 using in situ irradiation with UV light for 1h, the sample looks the

same before and after using the UV light. The STM measurements are in good agreement

with the contact angle measured for samples prepared at 60 °C, where the effect of

photoisomerization is missing. Concerning the STM measurements at elevated temperature of

60 °C and NEXAFS measurements (are not presented in this work) of Azo 1 and Azo 2

molecules, where the calculated tilt angle from the surface normal of the azobenzene moiety

and it has been found to be 275° [157] a model of the trans- and cis form is shown below.

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Chapter Seven Determination of Trans/Cis Isomerization of Azobenzene Molecules

Figure 71: Model possibility of Azo 2 molecules on the gold (111) in trans- isomerization

(tilting of ~ 275° from the surface normal).

97

Figure 72: Model possibility of Azo 2 molecules on the gold (111) in cis- isomerization

(disorder of the molecules).

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Chapter Eight Summary and Conclusions

98

Chapter Eight

Summary and Conclusions

The purpose of this work was to investigate and characterize the structure and the ordering of

self-assembled monolayers made from alkane thioacetates, aromatic and rigid thiols as

triptycenethiol, and triarylaminethiols based on a tertiary amine bonded to three phenyl rings

and the investigation of the photoswitching in the case of azobenzene disulfides by using UV

light and blue light.

For the preparation of the gold substrates two methods have been used: the evaporation of

gold onto silicon wafers and evaporation of gold onto mica (see sec.3.2). All the

measurements have been done on gold surfaces, because gold is an inert material which is not

oxidized in comparison to silver and copper under atmospheric conditions [164]. The gold on

silicon wafer is suitable for IR measurements, ellipsometry, contact angle, x-ray photoelectron

spectroscopy and near-edge x-rax absorption fine structure but is not suitable for the STM

measurements which need atomically flat gold surface with terraces separated by one atomic

layer of Au (111). High quality gold on mica substrates are prepared and imaged at an atomic

level using STM measurements.

Therefore, SAMs from the bulky triptycenethiol molecules have been prepared and the results

reveal that triptycenethiols adsorb on the surface. The reproducibility of C0T is showing

difficulties because of the high rigidity of the molecule, which plays an important role. About

C0T and their orientation one can conclude that, because in the NEXAFS measurements the

C0T spectra are not showing any dichroism this molecule did not present a dependence of the

angle, and C1T and C3T are presenting a tilting of the triple symmetry axis of the triptycene

frame against to the surface normal [107] due to the methylene spacer inserted between

sulphur and the triptycene frame and in NEXAFS spectra they are showing an angular

dependence, which is not observed for the C0T films. About the structure of the SAMs made

by triptycenethiol, one can conclude from the STM measurements that these thiols are

forming densely packed monolayers on the Au (111). The arrangements of these molecules

present differences between the surface morphology of C0T, C1T and C3T and based on

LEED data [23] the C1T and C3T form (m3 x n3) R30° structure, where m and n can be

possibly 2. For future work, the formation of highly ordered monolayers should be attemped.

The experiments on substituted triarylamines show that these protected molecules with an

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Chapter Eight Summary and Conclusions

99

acetyl group are able to deprotect themselves during a longer and slower reaction [115, 116].

The objective of this work was to understand better why there is almost no difference between

protected triarylaminethioacetate and the corresponding thiol after deprotection and to find the

explanation of a kinetic stabilization which occurs upon the leaving of the thioacetate group

during the deprotection process.

The IR measurements are showing that the quality of the SAMs formed after deprotection by

themselves is really similar with the quality of the SAMs deprotected by using the 10mM

(C2H5)2NH, to generate the SAM directly. The IR and XPS measurements possess

information about the chemisorption of substituted- triarylaminethiols on the gold surface.

The determination of the orientation of these molecules on the surface was impossible to

understand through NEXAFS, where these films are not showing even a small dichroism and

by the performance of the STM measurements one can conclude that these molecules are not

forming highly ordered monolayers, one can suppose that these molecules are still in the lying

down phase or disordered on the surface.

To understand better the effect of the deprotection process without using any acid or base, it

has been done the investigation of a simple alkanethioacetate likewise dodecyl thioacetate

C12SAc onto gold (111). These measurements were done in comparison with the

measurements of the corresponding alkanethiol (C12SH). While ellipsometry suggests the

formation of incomplete SAMs from C12SAc with a thickness of 30% relative to the C12SH

reference system, (6 Å vs 18 Å), the water contact angle of 65° for the C12SAc SAMs implies

the presence of an organic surface exposing methylene groups (as opposed to high density

dodecanethiolate SAMs with a contact-angle of ~110° characteristic for the presence of CH3-

groups [84]. This could either be the effect of strong disorder or of flat-lying molecules

(shown in Figure 73).

More definitive clues can be found in the XPS and NEXAFS data, where a shift of the C1s

core-levels and unoccupied orbital resonances, respectively, indicates the presence of n-alkyl

chains adsorbed in a flat geometry on a metal surface [134, 139]. Also the IR spectra reveal

significant differences to those obtained for dense thiolate SAMs, which also indicate the

presence of a saturated hydrocarbon chain with its C-C-C backbone orientated parallel to the

substrate. The STM data reproduced in sec. 6.2.2.6 clearly demonstrate the presence of a

striped phase, a typical signature of low-density, intermediate SAM phases, which appears in

conflict with previous reports [140, 141, 145]. This behavior can be rationalized by the notion

that for the transformation of C12SAc into the binding thiolate some reagent must be present,

since these molecules are chemically stable in pure ethanol. In our case this reagent is the gold

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Chapter Eight Summary and Conclusions

100

surface, facilitating the cleavage by formation of the stable Au-S bond (the leaving acetyl

group probably reacts with the ethanol). As soon as the gold surface is covered by the low

density monolayer, the contact between the gold surface and the C12SAc molecules is

hindered, suppressing the quick formation of the denser phase. Only on domain boundaries or

structural defects the gold substrate is sufficiently exposed to permit a further generation of

thiolate molecules, eventually leading to the formation of the upright phase in these places.

Figure 73: The time dependence of the apparent thickness (as determined by ellipsometry)

as well as the contact angle (advancing, for water) show that the monolayer obtained from

the thioacetates must mostly consist of flat lying molecules and remains stable even after

prolonged immersion times [11].

Instead, the thioacetates form a highly-ordered striped phase with flat-lying molecules anti-

parallel to each other, a structure which has not yet been reported for SAMs formed from the

corresponding alkanethiols and the presence of leaving-group effects on the structure and

quality of organothiolate SAMs opens a new possibility to control the properties of this

versatile class of materials.

In the category of the photoswitching exhibiting behavior we investigated the azobenzene

molecules (Azo 1 and Azo 2) by alternating the irradiation with UV light (366 nm) and blue

light (440 nm) in the solutions and also by the SAMs prepared on gold substrate. The

azobenzene molecules are very representative photoresponsive molecules, therefore one can

find several reports about azobenzene on hybrid material with mesoporous silica [165].

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Chapter Eight Summary and Conclusions

101

By investigation the photoswitching exhibiting in the solutions of Azo 1 and Azo 2 it has been

used a UV/VIS spectrometer to show if the irradiation process by alternating UV light and

blue light is reversible. The UV/VIS spectra shown in the sec.7.2.1.1. are indicating that the

irradiation process is reproducible and one can reach very easy the cis isomerisation of the

azobenzene molecules.

In case of the SAMs prepared from azobenzene molecules, is not easy to reach the expected

photoresponse, because on the surface the azobenzene molecules need more space and a

larger free volume. From the IR measurements one can observe that these molecules adsorbed

rapidly in the trans form on the gold surface. After irradiation with the UV light it is

impossible to observe the isomerization to the cis form. One can also conclude from the IR

spectra that the azobenzene moiety is almost tilted from the surface normal.

The photoswitching of the SAMs is suitable for the contact angle measurements, because after

several cycles of alternating the irradiation with UV and blue light the photoresponse of Azo 1

and Azo 2 is reversible and reproducible.

By STM measurements in the trans stable form, these molecules form up densely packed

monolayers at room temperature, but with the increasing of the temperature the domains

become larger and the number of so-called “etch-pits” is incrementing. Regarding the STM

measurements proceeded on Azo 2 prepared at 60 °C and continued to irradiate the sample

with UV light for 1h, the sample looks the same before and after using the UV light.

The impossibility to obtain the high resolution of these azobenzene monolayers indicate that

Azo 1 and Azo 2 do not form highly ordered monolayers.

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Chapter Nine Appendix

102

Chapter Nine

9.1. List of figures

Figure 1: Structure of thiols used in this study. ......................................................................... 2

Figure 2: The IR regions of the electromagnetic spectrum. ..................................................... 6

Figure 3: The IR spectrum of octadecanethiol, plotted as transmission (left) and absorbance

(right). ........................................................................................................................................ 7

Figure 6: Stretching and bending vibrational modes for a –CH2 group. .................................. 9

Figure 7: Harmonic approximation via potential of the oscillator V(r) [23].......................... 10

Figure 8: Schematic setup of FTIR- Spectrometer [23]. ........................................................ 13

Figure 9: Set up of rinsing gas supply. ................................................................................... 14

Figure 10: Spectrum of air in the sample area. ....................................................................... 15

Figure 11: Deduction of the surface selection rules at metallic surfaces [23]....................... 16

Figure 12: Measuring accessories of the “Uniflex” of the Bio-Rad FTS-3000 [23]. ............ 16

Figure 13: Energy pattern of x-ray photoelectron spectroscopy. ............................................ 18

Figure 14: Schematic diagram of the photoelectronic spectroscopy....................................... 19

Figure 15: XPS overview spectrum of octadecanethiol (C18) on Au(111)............................. 20

Figure 16: Energy pattern of NEXAFS spectroscopy. ............................................................ 21

Figure 17: Different methods of recording x-ray absorption spectra. ................................... 21

Figure 18: Definition and orientation of the angles in the surface coordinate system of the

NEXAFS experiment. E║ and E┴ are the p and the s-polarized portions of the incident light,

and TDM is the situation of the dipole transition moment of the excited transition................ 22

Figure 19: Principle of the imaging process by the STM. The lower part shows a band

structure diagram for a tunnel contact between the tip and the sample. ................................. 24

Figure 20: Schematic of the geometry of an ellipsometry experiment..................................... 25

Figure 21: Schematic of a sessile-drop contact angle system. ................................................ 27

Figure 22: Electronic excitations for an organic molecule. .................................................... 28

Figure 23: Compariosn between LB films and SAMs films. .................................................... 30

Figure 24: Schematic mechanism diagram for the self-assembly of thiols on Au(111): a)

Initial adsorption. b) Striped phase or lying-down phase. c) 2D phase with a transition from

lying-down to standing-up phase. d) Formation of a complete SAM [74]. ............................. 32

Figure 25: Schematic diagram of a SAM. Shaded circle indicates adsorbed or chemisorbed

headgroup and open circle endgroup, which can be chosen from variety of chemical

functionalities. .......................................................................................................................... 32

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Chapter Nine Appendix

103

Figure 26: Schematic diagram of different energies in the adsorbed SAM [71]..................... 33

Figure 27: Constant-current STM topograph of an octanethiol monolayer on Au(111) which

shows a c(4 x 2) superlattice of a R3033 )( overlay structure [68]. ................................. 34

Figure 28: The tilt angle Θt of the alkanethiol chain relative to the surface normal [71]. .... 34

Figure 29: AFM (atomic force microscopy) image of gold evaporate on Si(100)-Wafer (1μm

× 1μm). ..................................................................................................................................... 35

Figure 30: STM images of Au/Mica (111) and Au/Si (111). .................................................... 36

Figure 32: XP spectra of C 1s and S 2p regions of the C0T, C1T and C3T molecules in

comparison with an alkanethiol ODT. ..................................................................................... 42

Figure 36: NEXAFS spectra of C0T, C1T and C3T thiolates for different angles of incidence

of the synchrotron light. The solvent used for the solutions was EtOH. .................................. 47

Figure 37: STM image of C0T molecule on Au(111) at room temperature............................. 48

Figure 38: STM image of C1T molecule on gold (111) at room temperature ......................... 48

Figure 39: STM image of C1T molecule on gold (111) at high temperature .......................... 48

Figure 40: The crystal structure of the triptycene [12, 23]. .................................................... 50

Figure 41: Model for the arrangement of the C1T on gold surface. ....................................... 51

Figure 42: Molecular structures of substituted triarylamine- AH-10, VK-55, TS-10 and AH-4.

.................................................................................................................................................. 53

Figure 43: Infrared spectra of substituted triarylaminethiols AH-10, TS-10, AH-4 and VK-55.

The red trace displays are IRRAS spectra of the monolayer and the black trace measured for

a KBr pellet. ............................................................................................................................. 56

Figure 44: XP spectra recorded for polycristalline gold substrates showing the carbon 1s,

nitrogen 1s, sulfur 2p and oxygen 1s region for the triarylaminethiols AH-10, TS-10, VK-55

and AH-4. ................................................................................................................................. 57

Figure 45: XPS S 2p spectrum of AH-10 adsorbed onto gold/Si. Two S 2p doublets with 2:1

area ratios and splittings of 1,2 eV were used to peak fit experimental spectrum................... 58

Figure 47: IRRAS spectra of AH-10, TS-10 and AH-4. The red lines are showing the spectra

without deprotection process and the black lines the spectra with deproctection (used 10mM

(C2H5)2NH) [13]....................................................................................................................... 61

Figure 48: STM measurements on the AH-4 molecule. Conditions: with deprotection, 10-20

µM DCM solution; Ut= 600 mV, It= 70 pA............................................................................. 62

Figure 49: STM measurements on the AH-4 molecule. Conditions: with deprotection at 60°,

.................................................................................................................................................. 63

Figure 50: STM measurements on AH-4 molecule. Conditions: without deprotection, 10-20

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µM DCM solution; Ut= 600 mV, It= 70 pA............................................................................. 63

Figure 51: Model of an AH-4 molecule on the Au (111) surface flat lying or tilted away from

the surface. ............................................................................................................................... 65

Figure 52: Infrared spectra of C12SH and C12SAc a) in ethanolic solution, b) in ethanolic ... 69

Figure 53: RAIRS spectra of C12SH on the gold substrate (111) showing the region of C-H

stretch mode vibrations and the approximation of these peaks. .............................................. 70

Figure 54: XP spectra recorded for polycristalline gold substrates showing the carbon ....... 72

Figure 55: NEXAFS spectra of a) C12SAc and b) C12SH thiol for different angles of incidence

of the synchrotron light. The solvent used for the solutions was EtOH. .................................. 74

Figure 56: (a) Constant-current STM micrographs showing the gold substrate after............ 76

Figure 57: Constant-current STM micrographs showing the gold surface after immersion into

a 10-20µM ethanolic solution of C12SAc at 273 K for 48 h. In (b), the unit cell of the

(2√3×4)R30° structure is marked by the rectangular box. Tunneling parameters: (a) Ut=800

mV, It= 95 pA; (b) Ut= 1000 mV, It= 75 pA. ........................................................................... 77

Figure 58: C-H vibrational area of the IRRA spectra of the monolayers formed from C12SAc,

.................................................................................................................................................. 78

Figure 59: XP spectra recorded for polycristalline gold substrates showing the carbon ....... 79

Figure 60: Model of the surface layer formed upon adsorption of C12SAc (top view). ........... 81

Figure 61: Conformational change of azobenzene [151]........................................................ 84

Figure 62: Azo 1 and Azo 2 molecules..................................................................................... 84

Figure 63: UV-VIS spectra of Azo 1 and Azo 2 solutions in the ground state of trans isomers

and after irradiation with the UV light. ................................................................................... 86

Figure 64: Trans- and cis-Azo 2 on the gold substrate............................................................ 87

Figure 65: KBr, calculated and SAM IR spectra in low and high frequency regions [157]. .. 88

Figure 66: Comparison between Azo 1 and Azo 2 SAMs in low and the high frequency........ 90

Figure 67: Droplet of water on Azo 2 SAM before (80°) and after irradiation (65°) with UV

light........................................................................................................................................... 92

Figure 68: STM images showing the a SAM of Azo 2 on Au(111): the first row after 1 day

immersion in the ethanolic solution, after 4 days immersion and after 1 day at elevated

temperature of 60 °C. ............................................................................................................... 94

Figure 69: STM images of Azo 2 SAM prepared at elevated temperature of 60 °C: A) Before

irradiation with UV light. B) After irradiation with UV light during 1h. ................................ 94

Figure 70: a) Schematic of photoisomerization of azobenzene molecule. b) Schematic of

functional control using an azobenzene molecule [162]. ........................................................ 95

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Figure 71: Model possibility of Azo 2 molecules on the gold (111) in trans- isomerization

(tilting of ~ 275° from the surface normal). ........................................................................... 97

Figure 72: Model possibility of Azo 2 molecules on the gold (111) in cis- isomerization

(disorder of the molecules)....................................................................................................... 97

Figure 73: The time dependence of the apparent thickness (as determined by ellipsometry) as

well as the contact angle (advancing, for water) show that the monolayer obtained from the

thioacetates must mostly consist of flat lying molecules and remains stable even after

prolonged immersion times [11]. ........................................................................................... 100

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9.2. List of tables

Table 1: Number of vibrational degrees of freedom of nonlinear and linear molecules........... 7

Table 2: Absorption by single, double and triple bonds observed in an IR spectrum. ............ 11

Table 3: The layer thicknesses (in Å) as determined by XPS and ellipsometry as well as the

expected values for molecules standing upright on the substrate. ........................................... 41

Table 5:Tilt angles of the triptycene units with respect to the surface normal in the triptycene

thiol films on gold..................................................................................................................... 50

Table 6: Positions and assignment of the IR-modes In case of Au surface and KBr pellet. .... 55

Table 7: The assigment of the peaks of C-H stretch mode vibrations from Figure 53. .......... 70

Table 8: The assignment of the more intense peaks in the SAM of Azo 1. ............................... 89

Table 9: Contact angles of Azo 1 and Azo 2 before and after irradiation (liquid: water). ..... 91

Table 10: Contact angle of annealed Azo 1 and Azo 2 at 60 °C, before and after irradiation

with the UV light. ..................................................................................................................... 92

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ACKNOWLEDGEMENTS

I would like to express my gratitude to Prof. Dr. Christof Wöll for his great support and

encouragement during this study. His suggestions and critical advices in the course of

numerous fruitful discussions we had - were and are highly appreciated and were also a

significant contribution to the success of this work.

Prof. Dr. Roland Fischer I would like to thank for taking over the part of the co-referee and its

with-stated interest in the present work.

I am also indebted to Dr. Andreas Terfort for his assistance, guidance and motivation during

my entire doctoral study. His enthusiasm, exploratory mind and his friendship have been truly

inspiring.

Special thanks I would like to express to Dr. Waleed Azzam for his help and support in all

what self-assembled monolayers means and for the good atmosphere which we had during his

visit in Germany.

I would like to give special thanks to Dr. Thomas Strunskus for the comprehensive scientific

collaboration, for reviewing important parts of this manuscript, as well as for the numerous

suggestions he gave me related to the spectroscopy methods.

I would like to thank Dr. Alexander Birkner for introducing me to the field of scanning

tunnelling microscopy at the very beginning of my doctoral studies and Dr. Gregor Witte for

his fruitful and critical discussions about the scientific work during the course of my studies.

A critical review of the manuscript was carried out also by Dr. Franziska Träger and by Dipl.

Phys. Dorothee Meier, - their comments are highly appreciated.

Dorothee Meier I would like to thank for her guidance and patience to introduce me to the

field of atomic force microscopy and for her friendship, which means a lot to me.

I would further like to thank all my collegues in the Physical Chemistry group, especially to

Asif Bashir, Deler Langenberg, Dr. Kathrin Hänel, Ketheeswari Rajalingam, Jennifer Haag,

Milusche Krzikalla, Jan Götzen and Dr. Carsten Busse to whom I owe countless valuable

discussions and advice, and - last but not least - great fun in the course of my work.

Special thanks are reserved to all technicians from the Physical Chemistry department for

their very good support with the equipments and for their technical assistance.

Very, very special thanks I would like to express to the secretaries’ chair, Mrs. Uhde, Mrs.

van Eerd, Mrs. Knoedleseder Mutschler and Mrs. Kruse Fernkorn (second Mom) for the

assistance of the administrative questions, for their friendship, motivation and for their

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“warmness”. These four ladies are very special to me and one of the brightest things which

happened to me in Germany.

I would like to express special thanks to my partner Marius Pesavento for supporting and

motivating me in the course of my doctoral study and for his faith in me.

Finally, I would like to express thanks and gratitude to my both parents, Aurica and Stelian

Badin and to my lovely sister Simona Mocanu. Without their help and support I would never

arrived here, where I am now.

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List of publication

1. Kinetically Stable, Flat Lying Thiolate Monolayers

M. G. Badin, A. Bashir, S. Krakert, Th. Strunskus, A. Terfort and Ch. Wöll, Angewandte.

Chemie, Int. Ed. 46 (2007) 3762-3764

2. Chemistry in confined geometries: Reactions at an organic surface

K. Rajalingam, A. Bashir, M. Badin, F. Schröder, N. Hardman, Th. Strunskus, R. A. Fischer,

Ch.Wöll, ChemPhysChem 8 (2007) 657-660

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Curriculum Vitae

Family name: BADIN

First name: MIHAELA GEORGETA

Adress: Seippelstr. 2, 44803 Bochum

Email: [email protected]

Date of birth: 20.09.1978

Nationality: Romanian

Academic degrees:

1997-2002: Studies at the University „Politehnica” Temesvar, Romania, Faculty of Industrial

Chemistry and Environmental Engineering, department of „Knowledge and engineering

science of the oxide materials- Silicates”. Degree: Diploma engineer

Diploma Thesis: „Thermally steady pigments in the system CoO-ZnO-Al2O3-TiO2” and

„Development of the methods for the evaluation of the thin coatings”.

Since April 2007: Process Engineer in Air Quality Control Systems, Hitachi Power

Europe, Duisburg.

Dec. 2003- March 2007: doctorand by Prof. Dr. Christof Wöll, Lehrstuhl für

Physikalische Chemie I, Ruhr-Universität Bochum.

March 2002-July 2002: Scholarship "Socrates Erasmus" at the University of Applied

Sciences Gelsenkirchen, department of Recklinghausen, Germany.

Oct. 1997-Sept. 2002: Studies at the University „Politehnica” Temesvar, Romania,

Faculty of Industrial Chemistry and Environmental Engineering, department of

„Knowledge and engineering science of the oxide materials- Silicates”

Sept. 1993-June 1997: High school: Theoretical lyceum „C.D.Nenitescu”, Brasov,

Romania.

Sept. 1985-June 1993: Primary school Brasov, Romania.