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Christof WöllLehrstuhl Physikalische Chemie I

Ruhr-Universität Bochum

The Physics and Chemistry of Organic Surfaces: Fabrication of Model Systems

using the Selfassembly of Organothiols on Gold

Gold-Substrat

Bochum University

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Molecules and Surfaces

LCD Display

Sensor DevicesCell Membranes

Friction and LubricationP < 10mBar-9 P

(P < 10mBar)-9

HeterogenousCatalysis

Molecular Beam Epitaxy

Chemical Vapour Deposition

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Why Organic surfaces ?• Inorganic Surfaces

– Metals (Heterogeneous Catalysis, Electronics, Corrosion, Friction, Adhesion)

– Semiconductors (Electronics, Interfaces)– Insulators (Heterogeneous Catalysis, Corrosion inhibitions, Friction,

Adhesion)

• Organic Surfaces– Large Variety of Molecules– Relevance for Biology and Biochemistry– Friction, Corrosion, Adhesion, Sensor devices……– But…. application of standard Surface Science

techniques not straightforward (insulating, soft, sensitive to radiation,…)

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Organic molecule with liquid crystalline phase

Polymer substrate (Polyimide)

Liquid crystal displays

LCD

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5

Fabrication of organic surfaces

• Surfaces of polymers (plastics) prepared by cutting or casting– Disadvantage: Very difficult to obtain surfaces

exposing predefined chemical functionality– Quality of surfaces (cleanliness, structural

order) often not satisfying– Standard methods in Surface Science not

applicable

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• Adsorption of a simple (monofunctional) molecule not very efficient

• Either bonding is so strong that molecule is modified• Or bonding is so weak that system is unstable at

room-temperature

Grafting of interesting molecules to a metal substrate

~30°~30°

Better: Anchor – Chain - Function

Substrate

Example: benzene

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Required: Strong binding to substrate

Unspecific ionic coupling (LB films) not enough

Ideal: Formation of a covalent chemical bond

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Different anchor groups for different substrates

• Si, SiO2, Al2O3,…. (-OH groups available)– Trichlorsilane, Trimethoxysilane

e.g. (Cl3)-Si-(CH2) n-CH3 Anchoring by covalent bonds, formation of HCl(Trichlorsilane) or methanol CH3OH (Trimethoxysilane)

• Au, Cu, Ag– Organothiols

e.g. Alkanthiols HS-(CH2)n-CH3Anchoring by (weak) covalent bonds, Au-thiolates,

dihydrogen formation

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Most important system today:SAMs made from organothiols

Anchor

chain,backbone

Head group

Octanethiol, alkanethiols

H Gold substrate

Anchoring through Au-thiolate bond

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Most organic molecules are suited for incorporation into organothiols ….

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11

Characterisation of alkanethiolate adlayers

Pfennig (copper)coated uncoated

coated uncoated

Contact angleH2O

hydrophilichydrophobic

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Quantitative characterization of SAMsusing photoelectron spectroscopy

hν e--

X-ray photoelectronspectroscopy (XPS)

• Laboratory technique, no single crystals required, fast

binding energy of core electrons

EB=hν -Ekin

Clean Au (111) substrate

ca. 1 nm HeptanethiolateMonolayer

1000 800 600 400 200 00

500

1000

1500

2000

2500

3000

3500

Zähl

rate

Bindungsenergie [eV]

Au 4fAu 4f

Au 4dAu 4d

1000 800 600 400 200 00

500

1000

1500

2000

2500

3000

3500

Zähl

rate

Bindungsenergie [eV]

300 290 280 270400

600

800

1000

Zählr

a te

Bindungsenergie[eV]

C 1s

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Two principal ways to make SAMs …

Deposition in Ultrahigh Vacuum (UHV) Deposition from solution

10-10 mBar 1 Bar

Very difficult to determine preparation method for SAM after

formation!

All UHV-methods are applicable !

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Determination of molecular orientation using IR -spectroscopy

1000 950 900 850 800 750

0,00

0,05

0,10

0,15

0,20

Terphenylthiol Pellet

Wavenumbers [cm-1]

-0,0002

0,0000

0,0002

0,0004

0,0006

0,0008

prepared in EtOH solution

1000 950 900 850 800 750

Terphenylthiol/Au(111)

Θ

Thiolat-Adlayer on Au

E

Surface selection rule

Metal

Terphenylthiol

KBr pellet

IRRAS

IR bulk

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Near Edge X-ray absorption fine structure NEXAFS

Spectroscopy of unoccupied valence states using synchrotron radiation

IP

Unoccupiedmolecular orbitals

Synchrotron Synchrotron requiredrequired

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NEXAFS: Hexadecanethiol/Au(111)

280 290 300 310 3200

1

2

3

4 90° 55° 20°

HS(CH2)15CH3/Au

Photonenenergie [eV]

Inte

nsitä

t [Ei

nhei

ten

des

Kant

ensp

rung

s]

Goldkristall

35°

Determination of orientationby varying angle of incidence andanalyzing linear dichroism

• requires synchrotron

• fast (< 5 min/spectrum)

• straightforward analysis

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SAMs as model system for a systematicinvestigation of organic surfaces

• High degree of molecular orientation

• What about lateral order? What is the structural quality?

Goldkristall

35°

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Formation of highly orderedmolecular adlayers

LEEDDiffraction of low energy electrons(27 eV)

Decane thiolate

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Formation of highly orderedmolecular adlayers

LEEDDiffraction of low energy electrons(27 eV)

Decane thiolate

Substrate spots

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Formation of highly orderedmolecular adlayers

LEEDDiffraction of low energy electrons(27 eV)

Decane thiolate

Substrate spots

Superstructure spots

(2√3*√3)R30°

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Adsorption of Decanethiols on Au(111)Adsorption of Decanethiols on Au(111)1mM 24h@60 C°

depression

Domain boundary

5 nm Domain boundary

A

A

B

A-A translatuional domain boundaryA-B orientational domain boundary

0 10 20 30 40-0,20-0,15-0,10-0,050,000,050,100,150,20

Hei

ght(

nm)

Length (nm)

Au(111) Depression

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(2√3×3)

C10 adsorp on different sites such as : top, bridge, and hcphollow sites.

Au(111) Depression

NN brighter spot= 8.68A°

NN

bri

ghte

rsp

ot=

10A

°

(2√3

×3)

c(4×2)

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Self-assembled monolayers fabricated by immersion of Au-substrates into solutions of organothiols

• SAMs are ultrathin organic films with extremely high structural quality (2D single crystal)

• exhibits organic surfaces mainly defined by ω-function of thiol

• Basically all traditional techniques form traditional surface science can be applied (including XPS,UPS and STM)

• Ideal model system (?)

Goldkristall

35°

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Modification of SAM-surfacesubstrate

solutionof

thiols

adsorption fromsolution

termination-CH3

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Modification of SAM-surfacesubstrate

solutionof

thiols

adsorption fromsolution

termination-OH

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Modification of SAM-surfacesubstrate

solutionof

thiols

adsorption fromsolution

termination-COOH

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Characterization of organic surfaces using IR-Spectroscopy

3000 2900 2800 1600 1400 1200

0

2

4

6

8

10

12

14

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RO

O

IRRAS: MHDA/Au

Wavenumbers [cm-1]

Ester-bands

Sample 5: 0.02 mM MHDA/EtOH, 1% HCl

Sample 3: 0.5 mM MHDA/EtOH

Sample 4: 0.5 mM MHDA/EtOH, 5% AA

Sample 2: 0.02 mM MHDA/EtOH, 0.5 % TFA

Sample 1: 0.02 mM MHDA/EtOH

Abs

orba

nce

[10-3

AU

]

RO

O1800 1700 1600 15000

1

MDHAPellets

Wavenumbers [cm-1]

D H20

C=Omono-mer

C=O azyclic dimerC=O zyclic dimer

Abso

rban

ce [n

orm

.] R. Arnold, W. Azzam, A. Terfort, Ch. WöllLangmuir 18, 3980, (2002)

Mercapto-hexadecanoicAcid

A B

concentratedconcentrated

diluteddiluted

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Importance of organothiol backbone

High High degreedegree of order of order observedobserved forfor moremore rigidrigid backbonebackbone

Flexible backbone Rigid backbone

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SH

SH

BP3

BP4

SAMs from Organothiols with oligopheny-backbone

Systematic studies by

varying alkyl chain length

Circular depressions arenot defects in film, corrosion of Au-substrate

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2.5nm

A

BP3

BP4

STM LEED

(2√3×√3)-StructureMolecular area 21.6 Å2

Left: LEED patterns recorded for a BP3 monolayer at 345 K. Right: Schematic diffraction pattern for the (2√3×√3)- structure.

No ordered diffraction pattern could be observed for BP4 monolayers

(5√3×3)-StructureMolecular area 27.05 Å2

Pronounced differencebetween odd and evennumber of methylene units !

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Structural Models

BP3

BP4

Au(111)

S S S S S S

(2√3×√3)-Structure

(5√3×3)-Structure

Au(111)

S S S S S

W. Azzam, P. Cyganik, G. Witte, M. Buck, Ch. WöllLangmuir 19, 8262, (2003)

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Organic (or molecular) electronics: Conductivity of single molecules

SiO2

Au

Au

SSH H

S

S

S

S

S

S

S

S

Biphenyldithiol

H

Evaporation of metal onto SAM

HHH Goal: fabrication of a SH-terminated surface

Binding to metal-electrode

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Orientierungsbestimmung mittels IR-Spektroskopie

S

S

S

S

S

S

S

SSSH H

SSHH

W.Azzam, B.I.Wehner, R.A.Fischer, A.Terfort, Ch. WöllLangmuir 18, 7766, (2002)

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Orientierungsbestimmung mittels IR-Spektroskopie

S

S

S

S

S

S

S

SSSH H

SSHH

W.Azzam, B.I.Wehner, R.A.Fischer, A.Terfort, Ch. WöllLangmuir 18, 7766, (2002)

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Orientierungsbestimmung mittels IR-Spektroskopie

S

S

S

S

S

S

S

S

S

S

SS

S

S

S

S

SSH H

SSHH

W.Azzam, B.I.Wehner, R.A.Fischer, A.Terfort, Ch. WöllLangmuir 18, 7766, (2002)

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C=O

CH3C=O

CH3

C=O

CH3 C=O

CH3

C=O

CH3 C=O

CH3

C=O

CH3 C=O

CH3

C=O

CH3

=

S

S

R CS R’

O

R’SRC O

O -

O HR CO H

O SR-

+

R CS R’

O

R’SRC O

O -

O HR CO H

O SR-

+

RC

S S

H

S

R’ R’ R’

OR C

O

O

-OH -

R C OH

O -

Using protection group chemistry to fabricatedithiol SAMs of high structural quality

ThioesterThioester, , deprotectiondeprotection usingusingbasicbasic agentsagents

Post-synthesis modification

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b) The SAM spectrum of BPDMAc-1+EtOH+H2O+NaOH (2days).

c) BPDMAc-1 SAM (afterone day of immersion into dichloromethanesolution of BPDMAc-1 [100 µM] at RT )

Theory

Monitoring the de-protection of BPDMAC by IR spectroscopy

Bulk pellet

BPDMAC SAM

C=O

CH3C=O

CH3

C=O

CH3 C=O

CH3

C=O

CH3 C=O

CH3

C=O

CH3 C=O

CH3

C=O

CH3

0

2x10-3

0

2x10-30

2x10-3

1800 1600 1400 1200 1000 8000

1x102

2x1020

5x100

b)

e)

d)

c)

a)

Ab

sorb

ance

[AU

]

Wavenumber (cm-1)

=

S

S

Deprotected film

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XPSNEXAFS

292 290 288 286 284 282 280

284.4

287.6*

BPDMAc-1Deprotected BPDMAc-1

Inte

nsity

(Cps

)

Binding Energy (eV)

285,

2

287,

428

9,1

293,

8

285 290 295 300 305

Dep. BPDMAc-1 30° Dep. BPDMAc-1 90°

*

Norm

aliz

ed P

artia

l Ele

ctro

n Yi

eld

Photon Energy (eV)

BPDMAc-1 30° BPDMAc-1 90°

1800 1650 1500 1350 1200 1050 900

Dep. BPDMAc-1

Wavenumber (cm-1)

Abso

rban

ce

* BPDMAc-1

IR

Disappearance of C=O stretching vibration of the acetate group of BPDMAc-1 at 1695 cm-1 in IR, π* resonance of C=O of BPDMAc-1 at 287.4 eV in NEXAFS and the peak at 287.6 eV corresponding to the C=O of BPDMAc-1 in XPS indicates the deacylation

process.

The NEXAFS spectra of both BPDMAc1 and the deprotected BPDMAc1 show a pronounced dichroism, which suggests good orientational order in these SAMs.

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Deprotection: Problems with reproducibility

• Some samples: Deprotection done in a few hours, complete conversion after a day

• Other samples are not converted even after 2 days

R CS R’

O

R’S HR C

O

O

-

OH -

R C

OH

O

SR’

-

+

RC

S S

H

S

R’ R’ R’

OR C

O

O

-OH -

R C OH

O -

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How fast is the deprotection ?

1800 1700 1600 1500 1400 1300 1200 1100 1000

1604

.7

958.

610

05.0

1142

.8

1357

.0

1496

.7

1695

.4

-45x

10-3

10

DeprotectedBPDMAc-1

Wavenumber /cm−1

0 12 24 36 48 60 72 84

Sample -1Sample -2Sample -3

BPDMAc-1

Abs

orba

nce

900

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Deprotection speeds depends on sample conditions

• Reaction very fast in solution (minutes at most)

• At organic surfaces significantly delayed

• Influence of defects ?

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OH-

Thioacetate Thiol

Base

S

S

Au (111) Au (111)

S

S

HS

S

Au (111)

COO

+ R

OHR

-ORO

OH-

DeprotectedBPDMAc-1

BPDMAc-1

S

S SS S S S S

S S S S S SS S

S S

S S

S S S SS S

S S S SS S S S

In solution the deprotection reaction is fairly fast (<10 minutes).The attack of the basic agent requires nucleation centers

This makes the process very slow (3.5 days) for samples withlow defect density

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Before deprotection 3 h 5 h 25 h

SEM micrographs of deprotection process (0.01 M NaOH)

OH-

DeprotectedBPDMAc-1

BPDMAc-1

S

S SS S S S S

S S S S S SS S

S S

S S

S S S SS S

S S S SS S S S

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Self-assembled organic layers (monolayers) on metal substrates

• Why organic surfaces?• Fabricaton of organic surfaces• Why self-assembly?• Particular advantages of organothioles

• Characterisation of ultrathin organic adlayers

• Adressing topics in biology and biochemistry usingSAMs as model biomolecular surfacesAppying in-situ methods

• Generate lateral structures using μ-contact printing

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SPR - Surface Plasmon Resonance

Total reflection of light at the base plane of a prism

Excitation of surface plasmonson top of Au surface

Totalreflexion

Prisma

θ

θ

Gold-Schicht

Totalreflexion

Prisma

θ

ReflectedIntensity

θ

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SPR: Monitoring adsorption of octadecanethiol

15 20 25 30 35 40 45 50 55 60 65 70 750,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

42°

44.5°

SPR : Octadecanthiol-Layer on 50 nm gold film

clean gold film octadecanthiol adsorption (0.5 h)

46.1°

43.3°

Inte

nsity

[AU

]

θprism [deg]

Gold-Substrat

Gold-Substrat

Strong shift of surface plasmon frequency

Change in optical densityclose to Au-surface

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Adhesion of proteins to surfaces

Ophiopteris papillosa (Seestern)Zucht von Jakobsmuscheln Patella (Seeschnecken) auf Fels

Scallops Brittlestar

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Adsorption of RNaseon a CH3-terminated organic surface

M. Mrksich, G.B. Sigal, G. M. Whitesides

Langmuir 11, 4383, (1995)

Surface Plasmon Resonance (SPR)

Unspecific adsorptionof proteins

Reference

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Making surfaces protein resistantusing polyethyleneglycol

M. Mrksich, G.B. Sigal, G. M. Whitesides

Langmuir 11, 4383, (1995)

Au

HS-(CH2)11 (OCH2CH2) 6OH)

Organic surfaceexhibiting protein resistance

SPR

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Specific binding: Biotin/Streptavidin

StreptavidinBiotin

Au

Thiol with biotin attached

Protein-resistant thiol

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Biotin/Streptavidin: SPR-Measurements

No specificAdsorption

SpecificAdsorption

J.Spinke, M. Liley, F.-J. Schmitt, H.-J. Guder, L. Angermaier and W.Knoll,J. Chem. Phys. 99, 7012, (1993)

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Self-assembled organic layers (monolayers) on metal substrates

• Why organic surfaces?• Fabricaton of organic surfaces• Why self-assembly?• Particular advantages of organothioles

• Characterisation of ultrathin organic adlayers

• Adressing topics in biology and biochemistry usingSAMs as model biomolecular surfaces

• Generate lateral structures using μ-contact printing

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μ-contact printing

Poly-Dimethylsiloxane(PDMS) stamp Ink with Organothiol 1

Stamp, imprint

Immersion in Thiol 2

Thiol 2

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RememberRuhr University Bochum Department of Physical Chemistry I

adsorption of streptavidin on biotynilated surfaces

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Crystal structure data for streptavidin

results:

height of protein

40-43 Å

56

OEG(6)-Thiol

OH-Thiol/Biotinthiol+

Streptavidin

result:

height of streptavidin

(19,8±2)Å

height-measurement of streptavidin by AFM (air)

Not consistent !

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height-determination of streptavidin by AFM (liquid)

result:

height of streptavidin

(42,2±3)Å

Measurements in liquid

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Conclusions on specific binding of streptavidin

- high specific adsorption of

the streptavidin on the

patterned surface

- no unspecific adsorption on

the proteinresistant parts

of the surface

- good consistence in height of literature and experiments only when imaged in liquid

no proteinprotein

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5959

BiomolecularBiomolecular MultilayersMultilayers

10%-Biotinthiol 200nM Streptavdin 150nM bHRP

6060

AFM in Wasser:

Höhe des Proteinlayers

(76,5±4)Å

folglich für die Höhe der bHRP:

(34,3±3,5)Å

gute Übereinstimmung mit Literatur

Ruhr Universität Bochum Lehrstuhl für Physikalische Chemie I Proteinchemie an OberflächenR. Chelmowski

AFM in Wasser:

Höhe der SA-Lage:

(42,2±3)Å

„Phasen“-bild der strukturierten Thiolbereiche

Biomolekulare MehrfachschichtenBiomolekulare Mehrfachschichten

3 µm 3 µm 3 µm

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Fabrication of highly ordered molecular adlayers using organothiols

Some applicationsSPR

Topics

Characterization of organic surfacs

Tailoring properties of Organic Surfaces

Gold-Substrat

The Physics and Chemistry of Organic Surfaces: Fabrication of Model Systems

using the Selfassembly of Organothiols on Gold

Au80 µm 80 µm

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