1

Manipulation and Imaging of Aromatic Organic Molecules

with Atomically Sharp Tips:

The Last Atom Matters

Nicolae Atodiresei1, Vasile Caciuc2, Hendrik Hölscher2 and Stefan Blügel1

1Institut für Festkörperforschung (IFF), Research Center Jülich 2Center for NanoTechnology (CeNTech), University of Münster

Physikalisches Institut, WWU Münster, Germany

images taken from: Joachim, Gimzewski, Aviram, Nature 408, 541 (2007)

C60 amplifier carbon nanotube transistor

Molecular Electronics Based On Aromatic Type Sytems

Manipulation of Single Atoms and Molecules …

by STM by NC-AFM

conductor

lateralmovement

insulator

vert

ical

mo

vem

ent

AG Berndt, CAU Kiel

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In non-contact mode, the system vibrates a stiff cantilever near its resonant frequency (typically from 100 to 400 kHz) with an amplitude of a few tens of angstroms. Then it detects changes in the resonant frequency or vibration amplitude as the tip comes near the sample surface.

Dynamic Atomic Force Microscopy

Ab Initio Simulations of Tip-Sample InteractionsA Tool to Garantee The Succes of the Nanotechnology

• van der Waals force (long-range)

• chemical/bonding forces

(short-range)– small cluster at tip apex– ab initio calculations

Atomically Sharp Tips

Two chemically different tips:

An important issue is the chemical and structural nature of the tip used for the manipulation of the molecule.

During approach towards the sample surface the tip might crash into the sample and some material might be picked up from the surface.

As a consequence the exact geometrical and chemical nature is unknown in nearly all cases.

a clean Silicon tip (Si4H3)with [111]-orientation.

a Copper terminated tip(Si3H3Cu), where the foremost

tip atom was exchanged.

Two conditions have to be considered choosing a substrate for the controlled manipulation of specific molecules with a sharp tip:

(i) Isolated molecules have to adsorb on the substrate.

(ii) The bonding to the substrate has to be soft in order to allow a lateral manipulation of the molecule by the tip.

Manipulation and Imaging of Aromatic Organic Molecules with Atomically Sharp Tips

Benzene (C6H6) as a prototype for aromatic organic molecules

Imaging of Benzene on Cu(110) by STMDoering et al., Surf. Sci. 410, L736 (1998)

„ … the molecule appears to adsorb in the long-bridge site … .“

Benzene adsorbed on a Cu(110) surface

- the ”long-bridge”-site -between two rows of first layer Cu atoms- has the lowest adsorption energy.

- the ”hollow”-site position (gray shaded) has a slightly higher adsorption energy.

secondlayerfirst

layer

In the dynamic AFM simulations we approached the tip at three different sites on the C6H6 molecule marked by:

A - on top of a carbon atom.

B - carbon atom in ”ortho” position to A.

C - center of the carbon ring.

The clean Silicon tip at the B-site

d) Finally, as the tip is pushed into the surface an atomic defect is generated in the Cu-surface.

c) As the Benzene molecule is moved away the tip comes in direct contact with the Cu-surface.

b) During the approach the Benzene is pushed along the [110]-direction.

The clean Silicon tip at the B-site

During the approach the Benzene is pushed along the [110]-direction.

The clean Silicon tip at the B-siteAntibonding (repulsive) interaction

The Copper terminated tip at the B-site

The Copper terminated tip at the B-site

c) As the approach goes on the Benzene is squeezed between the tip and the surface.

d) The bonding between the benzene molecule and the Cu-terminated tip is more stable than the molecule-surface interaction.

b) During the approach the Benzene jumps towards the tip at a distance of 5.0°A. The molecule is attached to the tip by a covalent bond.

During the approach the Benzene jumps towards the tip at a distance of 5.0°A. The molecule is attached to the tip by a covalent bond.

The Copper terminated tip at the B-siteBonding (atractive) interaction

The Copper terminated tip at the B-siteThe clean Silicon tip at the B-site

Manipulation and Imaging of Aromatic Organic Molecules with Atomically Sharp Tips

Summary:

Manipulation and Imaging of Aromatic Organic Molecules with Atomically Sharp Tips: The Last Atom Matters!!!

Based on ab initio calculations we simulated how an aromatic organic

molecule like Benzene adsorbed on a Cu(110) surface can be imaged and

mechanically manipulated in non-contact atomic force microscopy using two

types of atomically sharp tips.

- A clean Silicon tip pushes the Benzene molecule from one adsorption site to

another and can therefore be used for lateral manipulation processes.

- A Copper terminated tip binds to the benzene molecule lifting it from the Cu

surface, thus leading to a vertical manipulation of this molecule.

Benzene orbitals

D

ts

z

dzzD

zF

Ac

fADf

)(

2

1),(

2/30

Giessibl, PRB 56, 16010 (1997); Dürig, APL 75, 433 (1999)

Theory of Dynamic Force Microscopy

),( DAf• a AFM image is a map of constant frequency shift

• topographic image:

maxima: larger tip-sample distance D

minima: smaller tip-sample distance D

• frequency shift is a weighted average of tip-sample force:

parameters:f0 = 160 kHz, f = -63 HzA = 12.7 nm, T = 14 Kunit cell: 4.27 Å x 6.04 Å

10 nm

[001]

[110]

noise (2 pm) comparable to low temperature STM

Schwarz et al., PRB 61, 2837 (2000).

True Atomic Resolution on InAs(110)by Noncontact Atomic Force Microscopy (NC-AFM)

lateral position [nm]

heig

ht

[pm

]

0 1 2 3 4

-10

0

10

OUTLOOK

π-system

π-system

AFM on magnetic moleculesπ-

syst

em

π-sy

stem

Interaction is NON-LOCAL therefore it is not included in local/semilocal DFT functionals commonly used today (LDA,GGA)

Van der Waals interaction - origin

Fluctuating dipoles

M. Dion, H. Rydberg, E. Schröder, D.C. Langreth and B.I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)

HOWEVER – THERE IS A REMEDY

The so called “SEAMLESS APPROACH”The only required input is the DFT density

Basically one needs to calculate:

Even though the integral looks simple it requires lots of CPU timeto be calculated.

Luckily the procedure can be practically perfectly parallelized.

Typical size of our calculations – charge density in real space is about 3003 points.

On 1 GHz CPU this calculation runs for ~ 1000 CPU DAYS

BENZENE MOLECULE ON Cu(100) SURFACE

Binding energies:

DFT with semilocal GGA (PBE) 300 meV

Including vdW in seamless fashion gives additional 400 meV.