AVS 49th International Symposium
    Surface Science Wednesday Sessions
       Session SS-WeM

Paper SS-WeM6
First Principles Simulation for NC-AFM Images of Si(111) @sr@3x@sr@3-Ag Surface

Wednesday, November 6, 2002, 10:00 am, Room C-112C

Session: New Opportunities and Technique Innovations
Presenter: M. Tsukada, University of Tokyo, Japan
Authors: N. Sasaki, University of Tokyo and Japan Science and Technology Corp. (JST), Japan
S. Watanabe, University of Tokyo, Japan
M. Tsukada, University of Tokyo, Japan
Correspondent: Click to Email
Quantitative, or sometimes even qualitative interpretation of non-contact atomic force microscopy NC-AFM images is extremely difficult, which contracts with the case of STM (scanning tunneling microscopy). Therefore theoretical simulations of NC-AFM images based on the first-principles density functional theory play very important role for the analyses of the experimental date. Effects of the tip structure and atom kind dependence can be also clarified, by the theoretical simulation. There have been some extrordinary features have been experimentally reported for the Si(111) @sr@3x@sr@3 surface(refferd to @sr@3-Ag hereafter), which await theoretical explanation. Recently Scanning Tunneling Microscopy (STM) experiment of @sr@3-Ag surface have been reproduced well based on the assumption of fluctuated Ag atoms among different energetically stable phases of Inequivalent-Triangle (IET) structures@footnote 1@ without the tip effects using Monte Carlo simulation.@footnote 2@ However NC-AFM experiments of the @sr@3-Ag surface at room temperature@footnote 3@ have not been fully understood yet. Important point here is that, in the case of NC-AFM, the tip effects are much stronger than in the case of STM, which give remarkable influences on the surface dynamics.Therefore in this work, NC-AFM images of @sr@3-Ag surface at both room and lower temperatures have been reproduced by DFT calculations.@footnote 4,5@ First we report that calculated images successfully reproduce experimental ones for both room temperature@footnote 5@ and lower one. For the room temperature image, thermal fluctuation is described by the weighted average of the two IET phases with the Boltzmann factor counting the difference of the interaction energies. Energetically stable IET structure appears in the lower temperature image. Thus it is clarified that, as far as only the room temperature NC-AFM experiment is observed, we cannot obtain information of truly stable IET structure of @sr@3-Ag surface.Next we show a remarkable tip-height dependence of the lower temperature NC-AFM images. In this case the Hydrogen-terminated Si tip is used. As the tip approaches the surface the NC-AFM image pattern changes from that of IET phase to HCT phase, and another IET phase. We explain this transition from the standpoint of the atom relaxation of tip-surface system. Thus our calculated results mean that Scanning Probe Microscopy (SPM) has an ability of not only directly observing dynamic feature of the surface but also mechanically controling the surface structures. @FootnoteText@ @footnote 1@ H. Aizawa, M. Tsukada, N. Sato, and S. Hasegawa, Surf. Sci. 429, L5c09 (1999).@footnote 2@ Y. Nakamura, Y. Kondo, J. Nakamura, and S. Watanabe, Surf. Sci. 493 206 (2001).@footnote 3@ Y. Sugawara et al., Surf. Interface Anal, 27, 456 (1999).@footnote 4@ N. Sasaki, S. Watanabe, H. Aizawa, M. Tsukada, Surf. Sci. 493, 188 (2001). @footnote 5@ N. Sasaki, S. Watanabe, M. Tsukada, Phys. Rev. Lett. 88, 046106 (2002).