RELATIONSHIP OF STM AND AFM IMAGES TO THE LOCAL DENSITY OF STATES IN THE VALENCE AND CONDUCTION BANDS OF RESE2

Atomic resolution scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images of the semiconducting layered transition-metal dichalcogenide, ReSe2, were obtained in air. STM images were obtained at bias voltages where tunneling was from the surface to the tip and from the tip to the surface. These STM images were analyzed on the basis of local densities of states (LDOS) calculated for the 4p orbitals of the surface layer Se atoms near the top of the valence band and the bottom of the conduction band. The calculations predict that Se atoms closer to the tip do not necessarily dominate the tunneling images because they may have lower charge densities. An assignment of the atomic resolution STM images to the four crystallographically distinct surface selenium atoms is proposed where the subsurface Re atoms appear as regions of smaller, but nonzero, tunneling current. The atomic force images resolve the higher and lower rows of Se atoms on the surface.