Noncontact Atomic Force Microscopy - Yale School of Engineering ...

We-1140
Theoretical DFT simulations of the STM/AFM atomic scale imaging on
graphite.
Vít Rozsíval 1 , Martin Ondráček 1 , Pavel Jelínek 1
1 Department of Thin Films, Institute of Physics of the ASCR, Prague, Czech Republic
Graphite is used as a standard benchmark surface to achieve atomic resolution by
Scanning Probe Methods (SPM). True atomic scale contrast of graphite provided by
simultaneous measurement of both forces and tunneling current is still source of
controversy [1-3]. The hexagonal surface unit cell contains to non-equivalent atoms: the
α-site atom located directly above adjacent planes and β-site atom above the hollow site.
While there is a consensus on STM measurements seeing every second, β-site, atom in
the hexagonal unit cell [4], the situation turns more conflicting in the case of nc-AFM
experiments. Main doubt arises where the maximum of the contrast is located: (i) over
hollow site [2] or one C atom [1]; and (ii) if the α and β atoms can be distinguished by
nc-AFM.
The direct quantitative interpretation of SPM measurements in the near-to-contact
mode is a not straightforward task due to several factors playing an important role in
imaging processes. Realistic conductance calculations have to involve properly several
effects: e.g. the tip/surface structural relaxations and multiple electron scattering to
understand properly the relation between forces and currents measured in experiments
[5]. Here we performed state-of-the-art theoretical studies based on a combination of total
energy DFT calculations with a Green's function approach for the electron STM transport
[5] between silicon or tungsten tip and graphite. We analyzed the electronic structure
evolution along tip-sample distance. In addition, we analyzed the relation between the
chemical short-range force and the conductance from the tunnelling to the contact
regime.
[1] S. Hembacher, F. J. Giessibl, J. Mannhart, and C. F. Quate, PNAS 100, 12539 (2003).
[2] H. Holscher, W. Allers, U.D. Schwarz, A. Schwarz, and R. Wisendanger Phys. Rev. B 62, 6967 (2000)
[3] S, Kawai and H. Kawakatsu, Phys. Rev. B 79, 115440 (2009).
[4] G. Binning et al, Europhys. Lett. 1, 31 (1986).
[5] P. Jelínek, M. Švec, P. Pou, R. Pérez and V. Cháb, Phys.Rev. Lett. 101 (2008).
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