1 Manipulation and Imaging of Aromatic Organic Molecules with Atomically Sharp Tips: The Last Atom Matters Nicolae Atodiresei 1, Vasile Caciuc 2, Hendrik Hölscher 2 and Stefan Blügel 1 1 Institut für Festkörperforschung (IFF), Research Center Jülich 2 Center for NanoTechnology (CeNTech), University of Münster Physikalisches Institut, WWU Münster, Germany

images taken from: Joachim, Gimzewski, Aviram, Nature 408, 541 (2007) C 60 amplifier carbon nanotube transistor Molecular Electronics Based On Aromatic Type Sytems

Manipulation of Single Atoms and Molecules … by STM by NC-AFM conductor lateral movement insulator vertical movement AG Berndt, CAU Kiel ?

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 Interactions A 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 (Si 4 H 3 ) with [111]-orientation. a Copper terminated tip (Si 3 H 3 Cu), 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 (C 6 H 6 ) as a prototype for aromatic organic molecules

Imaging of Benzene on Cu(110) by STM Doering 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. second layer first layer In the dynamic AFM simulations we approached the tip at three different sites on the C 6 H 6 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-site Antibonding (repulsive) interaction

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-site Bonding (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

Giessibl, PRB 56, (1997); Dürig, APL 75, 433 (1999) Theory of Dynamic Force Microscopy 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: f 0 = 160 kHz,  f = -63 Hz A = 12.7 nm,T= 14 K unit 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] height [pm]

OUTLOOK π -system AFM on magnetic molecules π -system

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, (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 time to be calculated. Luckily the procedure can be practically perfectly parallelized. Typical size of our calculations – charge density in real space is about 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.