Simulation of STM Images

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. 

An explanation of why a gold/nickel surface alloy is formed, whereas no bulk three-dimensional gold/nickel alloy exists (reflecting the fact that the heat of 

The idea that the gold-nickel surface could have interesting catalytic properties for steam reforming originated from three fundamental findings  

The only difference between the two samples is the gold modification of the nickel nanoclusters. In these investigations, n-butane was used to test the activity, because it is known to cause the most severe graphite formation problems. Whereas the pure nickel catalyst was deactivated rapidly as a result of the formation of graphite, as confirmed by electron microscopy, for example, it was found that the conversion catalyzed by the gold/nickel sample was maintained almost constant. This comparison is consistent with the inference that the novel gold/nickel catalyst did not 

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Cerda J, van Hove MA (1997) Efficient method for the simulation of STM images. I. Generalized green-function formalism. Phys Rev B 56 15885... 

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. 

Figure 4.10 Simulated STM images (sample bias 1V) at constant density (2.5 x 10-6e/B3) of the (a) hydroxylated and (b) reduced (1 1 0) rutile Ti02 surface (one OH group and one oxygen vacancy, respectively) obtained with B3LYP localized basis set calculation. (Reprinted with permission from Ref. [20].)...

The combination of state-of-the-art first-principles calculations of the electronic structure with the Tersoff-Hamann method [38] to simulate STM images provides a successful approach to interpret the STM images from oxide surfaces at the atomic scale. Typically, the local energy-resolved density of states (DOS) is evaluated and isosurfaces of constant charge density are determined. The comparison between simulated and measured high-resolution STM images at different tunneling... 

Figure6.7 (a) STM image of (5 x /SJ-rectvanadium oxide islands on Rh(l 1 1) (1000A x 1000A, + 1.5 V, 0.1 nA). Inset enlarged section of an (5 x 03)-rect island (70A x 70 A, +0.5 V, 0.1 nA) (b) DFT-derived model of the (5 x. y3)-rect structure, unit cell and structural units are indicated (V green, O red, Rh gray). Inset simulated STM image. (Reproduced with permission from Refs [18, 101].)...

Figure 6.18. (Top) STM image of the flfe-plane of TTF-TCNQ taken at 63 K (Ft = 50 mV, /t = 1 nA). The image area is 5.3 nm x 5.3 nm. Reprinted with permission from Z. Z. Wang, J. C. Girard, C. Pasquier, D. Jerome and K. Bechgaard, Physical Review B, 67,121401 (2003). Copyright (2003) by the American Physical Society. (Bottom) Simulation of the STM image of the afe-plane of TTF-TCNQ, obtained with DFT calculations in the GGA performed with the Siesta code (Soler et al, 2002) using the Tersoff-Hamann approximation (see Section 4.2). The value of the charge density is 2 x 10 electrons/a.u., which is about 0.2 nm above the surface. Courtesy of Drs P. Ordejon and E. Canadell.

Figure 2. Simulated STM images of the non-rebonded Sb step edge, (a) the filled-state image and (b) the empty-state image, both under high-current conditions.

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