The Interpretation of STM Images

Theoretical models of STM images initially treated the STM tip as a point source of current, since, from the theoretician s point of view and despite all the efforts put in by experimentalists (see the discussion later), little is known 

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The electronic structure of hematite (001) surfaces Applications to the interpretation of STM image and heterogeneous surface reactions. Am. Min. 81 1301-1314... 

A review of First Principles simulation of oxide surhices is presented, focussing on the interplay between atomic-scale structure and reactivity. Practical aspects of the First Principles method are outlined choice of functional, role of pseudopotential, size of basis, estimation of bulk and surface energies and inclusion of the chemical potential of an ambient. The suitability of various surface models is discussed in terms of planarity, polarity, lateral reconstruction and vertical thickness. These density functional calculations can aid in the interpretation of STM images, as the simulated images for the rutile (110) surface illustrate. Non-stoichiometric reconstructions of this titanium oxide surface are discussed, as well as those of ruthenium oxide, vanadium oxide, silver oxide and alumina (corundum). This demonstrates the link between structure and reactivity in vacuum versus an oxygen-rich atmosphere. This link is also evident for interaction with water, where a survey of relevant ab initio computational work on the reactivity of oxide surfaces is presented. 

In view of the huge number of surface systems that have been successfully studied by STM we give in the following section examples for the different categories of systems. It must be mentioned that the interpretation of STM images can become difficult or even impossible unless other standard surface analytical tools, such as LEED/RHEED (Section 3.2), AES (Section 4.3), or LEIS (Section 2.2) are employed. 

The simplest interpretation of stm images is in terms of surface topography. However, care must be exercised in this interpretation, since in teahty, tunneling probabiUty is really measured. The many subdeties of stm data interpretation ate beyond the scope of this article. The interested reader is referred to references 14 and 15 for a more detailed discussion of these issues. 

As mentioned above, the information contained in STM images pertains principally to the electronic structure of the surface, and (as for most types of microscopy) STM provides no direct insight into the chemical identities of structures. This lack of chemical specificity often makes it difficult to relate the observed structures of complex clusters, molecular adsorbates, or reaction intermediates to their chemical nature and conformation on the surface. Theoretical electronic-structure calculations are therefore commonly employed to assist in the interpretation of STM results. The theoretical calculations provide complementary information about the possible ground-state configurations of samples and can be used to generate fairly accurate simulations of STM images. 

STM probes (e.g., from W or Pt-It wire) are fabricated by either mechanical cutting or electrochemical etching. Further treatments are sometimes used to sharpen them, such as annealing under high fields 133434 in techniques handed down from the original atomic resolution microscopies—field emission microscopy and field ion microscopy.135 136 Another method employed is to lift an atom or molecule onto the probe tip so as to define the tip precisely. One important issue that can prevent clear interpretation of STM images but can also be used to tremendous advantage is the fact that the atom at the very apex of the STM probe... 

The results for the apparent radius of STM images for individual states can be used to interpret experimental images directly. For surfaces with complex periodic structures, such as Si(lll)-7 X 7 and Si(lll)-5 X 5, the concept of imaging individual atomic states is a much better description than surface Bloch functions. For adatoms and defects, the individual state description is the only possible one. 

Equation (3) indicates that the STM tip does probe the density of electronic contours rather than the surface topography in terms of a hard-sphere model [47]. Electronic contours generally coincide with atoms on metals. With semiconductors the interpretation of images is not trivial because electrons are located in bound states (dangling bonds on clean surfaces in UHV, chemical bonds between surface atoms and ligands in liquids) whose density and occupation can vary from one atom to another. 

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