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

Th-1120
Combined Qplus-AFM and STM imaging of the Si(100) surface:
Activating the c(4x2) to p(2x1) transition with subnanometre oscillation
amplitudes
A. Sweetman, S. Gangopadhyay, R. Danza, and P. Moriarty
School of Physics and Astronomy, University of Nottingham,
Nottingham NG7 2RD, UK
We report the first combined Qplus AFM and STM images of the Si(100) surface. The
study of the Si(100)-(2x1) and c(4x2) reconstructions using scanning probe methods
(both STM and NC-AFM) has led to a variety of intriguing and controversial issues
associated with identifying the ground state of the system [1]. Foremost among these
have been the determination of whether the silicon dimers, which give rise to the (2x)
periodicity at the Si(100) surface, are asymmetric or symmetric, and the extent to which
the measurement technique might influence the energy balance. Previous experimental
NC-AFM measurements (with ~ 10 nm amplitude) combined with theoretical
calculations [2] showed that the AFM probe could strongly influence the Si(100) surface,
stabilising a p(2x1) phase at low temperatures. Our Qplus measurements, taken with
probe oscillation amplitudes ranging from 250 pm to 10 nm, not only confirm that the
strong influence of the tip is retained at subnanometre oscillation amplitudes, but, as
shown in Fig. 1, suggest that tip symmetry plays a central role in stabilisation of a
particular surface phase. In addition, there are clear and striking differences between
STM and Qplus AFM images of the Si(100)-(2x1) phase arising from the residual dimer
buckling associated with probe-induced stabilisation of the p(2x1) structure.
[1] Lev Kantorovich and Chris Hobbs, Phys. Rev. B 73 245420 (2006) and references therein
[2] Yan Jun Li et al., Phys. Rev. Lett. 96 106104 (2006)
Fig. 1 Qplus AFM images of the Si(100) surface. (a) Regions of both (2x1) and c(4x2)
symmetry are visible. (df=-10.9 Hz, A vib=350 pm (700 pm pk-pk), V gap=+0.3V). (b) and (c)
Images of the same surface region taken in parallel with identical scan parameters showing the
influence of the scan direction on the surface phase. Image (b) was taken while the tip was
moving from left to right. For Image (c) the tip was moving from right to left.
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