paper-Nanomeeting-NMartsinovich

Full text

PHYSICS, CHEMISTRY AND APPLICATION OF NANOSTRUCTURES, 2009
THEORETICAL MODELLING OF TIP-INDUCED
MANIPULATION OF C60 IN THE SI(001) SURFACE
N. MARTSINOVICH
Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK
Department of Physics, King’s College London, London, WC2R 2LS, UK
L. KANTOROVICH
Department of Physics, King’s College London, London, WC2R 2LS, UK
Constant height repulsive (pushing mode) manipulation of a C60 molecule covalently
bound to the Si(001) surface is modelled using ab initio density functional theory, with
the scanning tunneling microscope (STM) tip included explicitly in the calculations. The
formation of chemical bonds between the tip and the molecule is demonstrated. The
bonds between the molecule, tip and surface are constantly rearranging, so that a
continuous manipulation process is possible. Tip-induced manipulation considered here is
compared with the tip-free model, and the effects due to the tip are discussed.
1. Introduction
Creation of nanostructures with desired geometries and physical properties
requires the ability to assemble molecules or atoms in a controllable way. Two
approaches to nanostructure formation are self-assembly, where molecules
assemble by themselves into large-scale structures due to chemical or Van der
Waals interactions between them, and manipulation, where individual atoms or
molecules are placed in required positions by an external force. Scanning probe
microscopy is often used as a tool for manipulating atoms and molecules [1,2].
In particular, the C60 molecule is often a system of choice for manipulation
experiments [3-8] because of its compact nearly spherical shape, which makes it
easy to image and ensures that its manipulation trajectories are simple enough.
Besides its ability to roll, C60 is a prototype molecule for endohedral metal-
doped fullerenes, which are a promising material for quantum computers [9].
Manipulation of C60 has been performed using scanning tunneling microscopy
(STM) on copper [3], gold [4] and silicon [5-8] surfaces.
In this paper we address the manipulation of the C60 on the Si(001) surface.
Manipulation of C60 on this surface has been achieved at room temperature with
STM [5-7]. Different regimes of manipulation (pushing and pulling) have been
identified [6] depending on the tip height.
1

The movement of the C60 molecule on the Si(001) surface has been
modelled in our group [7,10,11]. A pivoting mechanism of the C60 movement
based on the molecule pivoting over its front two C-Si bonds between two stable
adsorption configurations, followed by consecutive adsorbate-surface bond
breaking and bond formation events, has been proposed in Ref. [7] and
confirmed by extensive density-functional theory (DFT) calculations [7,11].
The model proposed in Refs. [7,11] did not include the effect of the STM
tip, and the molecule was forced to move by displacing a single C atom of the
C60 in small steps. However, it is known from previous theoretical work [12] that
the tip structure and position are important in determining which regime of
manipulation will operate. The tip should also be able to maintain contact with
the molecule over the whole manipulation path.
In this paper we include the STM tip explicitly in the ab initio modelling of
the C60 manipulation on Si(001). The presence of the tip allows us to simulate
the manipulation process more realistically. We discuss the atomic-scale details
of the manipulation and the effects due to the presence of the tip.
2. Method
We use the DFT SIESTA code [13], which implements the generalized gradient
approximation (GGA), Perdew-Burke-Ernzerhof (PBE) density functional [14],
norm-conserving pseudopotentials and periodic boundary conditions. A
localised double-ζ polarised (DZP) basis set was used for the valence electrons.
The computational cell included the C60 molecule, the silicon slab
containing 96 Si atoms (6 layers) and 32 terminating H atoms at the bottom of
the slab, and a (111)-oriented atomically sharp silicon tip terminated with a
single dangling bond [15]. The coordinates of the two lowest Si layers of the
slab, the top layer of Si atoms of the tip, and all H atoms were fixed during the
simulations. The remaining atoms were allowed to relax until forces on atoms
were 0.02 eV/Ǻ. The BSSE (basis set superposition error) correction to the
binding energy was handled using the Boys-Bernardi counterpoise method [16].
3. Results and Discussion
The initial structure involved the C60 adsorbed in the trough of the Si(001)
c(4x2)-reconstructed surface in the lowest-energy adsorption configuration t4c
[17]. The tip was placed with its apex at the same height as the top of the C60 and
displaced towards the molecule in the direction along the trough in steps of 0.05
Ǻ, at a constant height. The system was relaxed after each tip displacement.
2

The binding energies of the tip-C60-surface system along the manipulation
path are plotted in Fig. 1. Several stages (labelled A-E in Fig. 1) can be
identified in the manipulation path. Each stage consists of a graduate reduction
of the binding energy (the curve goes up), which terminates with an abrupt
increase (vertical drop of the curves). The chemical processes during each stage
correspond to different bond rearrangement events: (A) C60-tip bond formation;
(B) formation of the first new front C60-surface bond; (C) rear C60-surface bonds
breaking; (D) C60-tip bond rearrangement; (E) formation of the second front C60-
surface bond. The tip-C60 bonds are maintained throughout the manipulation.
Figure 1. (a) Binding energies during the C60 manipulation by one surface lattice constant (a0 = 3.84
Ǻ) using the dangling bond-terminated tip; (b) deformation and pairwise binding energies of the C60,
tip and surface. Vertical dotted lines indicate the bond rearrangements events labelled A, B, C, D, E.
When the results in Fig. 1 (a) are compared with our earlier simulations of
diffusion/manipulation without the tip [7,11] where only a single energy barrier
was found, the difference is immediately obvious: the energetics of the tip-
induced manipulation is more complex. Also, notably, the binding energies in
Fig. 1 (a) at and near the energy barriers are positive. This result may seem
unreasonable at first sight; however, it has a physical explanation lying in the
very nature of the C60-surface-tip system. At these points, the molecule is
3

significantly deformed and its bonds to the surface strained, but the tip-C60
bonds hold the molecule in place and prevent it from moving into a more
favourable configuration. Only near the pivoting point the molecule is able to
break the tip-C60 bonds and slip into the next adsorption configuration. Thus, the
movement of the C60 is controlled by the tip, unlike in self-diffusion.
The main contributions to the binding energy can be identified by analysing
the deformation energies of each of the three components (C60, tip and surface),
as well as their pairwise binding energies, which are all shown in Fig. 1 (b). The
Figure shows that the tip deformation energy increases abruptly at the point of
C60-tip bond formation (point A); however, there is also an energy gain due to
the C60-tip binding. The C60 and the surface are both significantly deformed
(large deformation energies), especially in the pivoting region (B-C). The effect
of this deformation is partially counterbalanced by the increase in the C60-
surface binding energy in the pivoting region. Overall, a significant energy gain
is observed when the molecule arrives in the new configuration (point E). Thus,
during each bond rearrangement event there is interplay of binding energies
becoming more negative as a result of bonding, and deformation energies
increasing, as the C60, tip and surface are distorted from their ideal geometries.
In conclusion, our theoretical study of the constant-height pushing
manipulation of the C60 molecule with a Si tip shows that the manipulation is a
complex process, where the strong C60-surface interaction controls the
adsorption configurations. We show that the tip forms tip-C60 chemical bonds
and drives the manipulation process. The tip-C60 bonds rearrangement provides a
possibility of the continuous manipulation of the C60 by STM.
References
1. D. Eigher and E. Schweizer, Nature 344, 524 (1990).
2. L. Bartels, G. Meyer, and K. H. Rieder, Phys. Rev. Lett. 79, 697 (1997).
3. M. T. Cuberes, R. R. Schlittler et al., Appl. Phys. Lett. 69, 3016 (1996).
4. N. Katsonis et al., J. Photochem. Photobiol. A 158, 101 (2003).
5. P. Moriarty, Y. R. Ma, et al., Surf. Sci. 407, 27 (1998).
6. D. L. Keeling, M. J. Humphry et al., Chem. Phys. Lett. 366, 300 (2002).
7. D. L. Keeling et al., Phys. Rev. Lett. 94, 146104 (2005).
8. M. Kageshima, H. Ogiso et al., Surf. Sci. Lett. 517, L557 (2002).
9. W. Harneit, Phys. Rev. A 65, 032322 (2002).
10. N. Martsinovich, L.Kantorovich, R.H.J.Fawcett et al., Small, 4, 765
(2008).
11. N. Martsinovich et al., Phys. Rev. B 76, 085304 (2006).
12. L. Pizzagalli and A. Baratoff, Phys. Rev. B 68, 115427 (2003).
13. J. M. Soler et al., J. Phys.: Condens. Matter 14, 2745 (2002).
4

14. J. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865
(1996).
15. L. Kantorovich and C. Hobbs, Phys. Rev. B 73 (2006).
16. S. Boys and F. Bernardi, Molecular Physics 19, 553 (1970).
17. C. Hobbs, L. Kantorovich, and J. Gale, Surf. Sci. 591, 45 (2005).
5