Origin of atomic resolution in STM

As we have discussed in Section 1.3, experimentally, atomic resolution has been observed on literally every clean surfaces of metals and semiconductors. Today, atomic resolution on rigid surfaces has become a must in STM operation (Rohrer, 1992). In order to resolve single atoms, a lateral resolution of 2 A is required. The importance of the STM — the feature that sets it apart from other instruments — is that it can resolve details in the vicinity of a single atom, otherwise it would not have created the excitement that now surrounds it (Quate, 1986). Here, we briefly discuss the origin of its atomic resolution. 

An early systematic experimental study on the imaging mechanism was conducted on Al(lll) (Wintterlin et al., 1989). The observed corrugation amplitude was more than one order of magnitude larger than the Fermi-level LDOS corrugation. Aluminum is a textbook example of simple metals. The electronic states on the AI(lll) surface have been studied thoroughly. 

The surface charge density of Al(lll) has been well characterized by first-principles calculations as well as helium scattering experiments. The asymptote of the corrugation amplitude Az of equal-LDOS surface contours follows an exponential law, as obtained from a first-principles calculation of the electronic structure of the Al(l 11) surface (Mednick and Kleinman, 1980)  

An exponential dependence of corrugation amplitude with distance is clearly observed. 

The atomic resolution in STM can be understood in terms of tip electronic states and tip-sample interactions. We will discuss the effect of tip electronic states in this section, and the tip-sample interactions in the next section. 

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