Theoretical and experimental STM images of fresh and oxidized pyrrhotite

STM images taken at negative bias voltages exhibit triangular terraces separated by steps. Measured step heights are integer multiples of 2.9 Å, which is one quarter of the 4C $\rm Fe_7S_8$ unit cell size in the c direction, and corresponds to the distance between two consecutive Fe or S layers. Although the STM images of single terraces appear to have an atomic arrangement corresponding to the ordering within those Fe layers that contain vacancies, bright spots in the images are most likely to represent S atoms, with a vacancy ordering that is induced by the Fe vacancies. This conclusion is supported by experimental STM images, which show a reversed orientation of the surface geometry on successive terraces when separated by steps of 2.9 Å, and by quantum mechanical calculations of STM images that show S 3p-like states at the top of the valence band.

Pyrrhotite (001) surfaces contain triangular etch pits with dimensions ranging from the atomic scale (Fig. 16.2) to more than 100 Å across. These are formed by the successive removal of $\rm Fe_3S_3$ units from the surface.

  

Figure: Experimental STM image (bias voltage of -3.5 V) showing pits at the atomic scale. Pits with only one bright spot (one FeS unit, labeled X) removed can combine to form ``hook-like'' pits with a missing Fe₂S₂ unit ( Y) or a triangular pit with a missing Fe₃S₃ unit( Z). The image also shows a dislocation ( D), going through a triangular pit ( Z_D)

Images taken following exposure of the surface to 6000 L O₂ did not alter the flat terraces but formation of adsorptive structures near steps, especially at corners, was observed (Fig. 16.3).

  

Figure: Experimental STM image of a pyrrhotite (001) surface after 6000 L of O₂ exposure. The terraces are unaltered by oxygen exposure, whereas adsorption features can be observed at the corners of terraces. The step heights shown are 5.8 Å. The image was taken at a bias voltage of -0.4 V.