Simulation of Noncontact Atomic Force Microscopy of Hydrogen- and Methyl-Terminated Si(001) Surfaces

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Simulations of noncontact atomic force microscopy (ncAFM) are presented for Si(111), Si(001), and hydrogen-terminated and methyl-terminated Si(001) surfaces. The force between tip and surface consists of the long-range van der Waals force and the short-range chemical force. The long-range van der Waals force is estimated by a continuum model using only the Si–Si Hamaker constant in a vacuum, whether or not the surface is terminated by H atoms or methyl radicals. The short-range chemical force is predicted by the density-functional based tight-binding method because its computational cost is much lower than ab initio calculations using density functional theory. The experimental range of the frequency shifts and the ncAFM images of Si(111), Si(001), and H-terminated and methyl-terminated Si(001) surfaces were reproduced excellently. In the images, the methyl radical adsorbed is observed as a cavity-like region despite of its protrusion.
2009-02-25