Experimental and calculated STM images

STM images of the upper valence band of galena show a periodic array of spots which could be matched to either Pb or S. Our calculations show that these spots mainly stem from electronic states with sulfur 3p character near the Fermi level. In addition, images of the mid-band gap and lower conduction band also show states that we calculate as being due to orbitals with lead 6p character. If the galena surface is exposed to air, more and more of the formerly bright sulfur spots appear as dark spots. This agrees with the calculated depletion of the upper valence band electronic density near sulfur sites after adsorbing oxygen atoms to them. Therefore, tunneling microscopy on semiconducting sulfides can not only be understood as a tool of visualizing single atoms, but also as a probe for surface sites at an atomic level that can be most easily oxidized.

Modeling galena with infinitely wide slabs (Fig. 16.1) as opposed to clusters shows significant differences in the oxidation mechanisms. On slabs, suitable for modeling the electronic structure of terraces, the calculations predict that oxygen will be adsorbed on top of the surface, whereas on clusters, portions of which can be used to represent steps and kinks, oxygen was calculated to migrate underneath the surface.

  

Figure: (a) Experimental constant-height STM image at -405 mV, setpoint current 2.2 nA, after air exposure of 20 minutes, showing dark patches (labeled as A) of low tunneling current. These patches grow with oxidation time and represent a loss of S3p valence band density. Around the patches, areas of increased electronic density are observed (labeled as B). The height and width of the image is 40 Å). (b) Theoretical image as calculated by using a 3$\times$1 supercell on Crystal92 (periodic HF) with one oxygen atom adsorbed per supercell. The supercells are repeating units and thus, as an artifact of the calculation procedure, the oxidation appears to propagate in vertical rows with oxygen atoms 2.96 Å apart (height of the supercell) and rows 8.88 Å apart (width of the supercell). The height and width of the image is 13 Å).