This chapter discusses the imaging mechanism of STM at the nanometer scale, where the features of interest are of about one nanometer and up. Using an s-wave tip model, using the Bardeen tunneling theory, Tersoff and Hamann showed that the STM image in this case is tip-independent: it is determined by the local density of states of the bare sample surface at Fermi level, taken at the center of curvature of the tip. The Tersoff-Hamann model has found numerous applications in interpreting the STM images, from the superstructure of surface reconstruction to the confined or scattered waves of the surface states. However, as shown by Tersoff and Hamann in their original papers, for features much smaller than one nanometer, such as at the atomic features of 0.3 nm, the non-spherical electronic states of the tip could play a significant role and thus cannot be overlooked.
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