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

Simultaneous measurement of force and tunneling current
Tu-1200
Daisuke Sawada, Yoshiaki Sugimoto, Ken-ichi Morita, Masayuki Abe, and Seizo Morita
Graduate school of Engineering, Osaka University, Osaka, Japan
We have performed simultaneous STM and AFM measurements in the dynamic mode
using Pt-Ir coated Si cantilevers at room temperature. Frequency shift (Δf) and timeaverage
tunneling current () images were obtained by tip scanning on the Si(111)-
(7×7) surface at constant height mode to prevent a crosstalk between these two channels
[Fig.1 (a)]. To compensate the thermal drift of the tip-surface distance, feed forward
technique was applied [1]. Analysis of 25 sets of AFM/STM images using different tips
shows that when atomic resolution is obtained simultaneously by both AFM and STM,
the tunneling current is much larger than the typical values in conventional STM. We
have also performed simultaneous measurements of site-specific force/tunneling
spectroscopy. The Δf and versus tip-surface distance curves were converted into the
short-range force (FSR) and the tunneling current (It) at closest separation between the
sample surface and the oscillating tip [Fig.1 (b)]. We observed the drop in the tunneling
current due to the chemical interaction between the tip apex atom and the surface adatom,
which was found recently [2], and estimated the value of the chemical bonding force.
Furthermore, scanning tunneling spectroscopy (STS) was performed on the same site
using the same tip [Fig.1 (c)]. The spectrum is in good agreement with previous STM
results. Our results demonstrate that one can quantitatively measure the local density of
state and the chemical bonding force above the same atom using the same tip [3].
Figure 1: (a) Δf and images of a Si(111)-(7×7) surface simultaneously obtained in the
constant height mode at room temperature. The acquisition parameters are f0=284092.5 Hz,
A=300 Å, k=22.9 N/m, and Vs=-400 mV respectively. (b) It-z and FSR-z curves derived from z
and Δf-z curves obtained above a center adatom in a faulted half unit cell, respectively. (c) STS
spectrum and Δf - Vs curves obtained at z=4.4 Å.
[1] M. Abe, et al., Appl. Phys. Lett. 90, 203103 (2007).
[2] P. Jelinek, M. Svec, P. Pou, R. Perez, and V. Chab, Phys. Rev. Lett. 101, 176101 (2008).
[3] D. Sawada, Y. Sugimoto, K. Morita, M. Abe, and S. Morita, Appl. Phys. Lett. in press.
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