SCANNING TUNNELING AND ATOMIC-FORCE MICROSCOPY STUDY OF THE TE-ATOM SURFACES OF COMMENSURATE LAYERED TELLURIDES NBAXTE2 (A=GE, SI)

The surfaces of commensurate layered tellurides NbGe2/5Te2, NbGe1/3Te2, and NbSi1/2Te2 were examined by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The observed atomic-resolution STM and AFM images were analyzed in terms of the partial and total electron density plots of their surfaces calculated by the extended Huckel tight-binding electronic band structure method. The STM and AFM images of these phases are characterized by a pseudohexagonal arrangement of spots representing the surface Te atoms, and the surface unit-cell parameters of these commensurate MAxTe2 phases are well reproduced by STM and AFM. The atomic size patterns of the AFM and STM images differ in their relative contrasts from those expected on the basis of the surface Te-atom corrugations. This discrepancy is explained if the surface Te-atom sheets undergo a slight height reconstruction, most likely because the short interlayer Te...Te contacts present in the bulk crystals are truncated at the surface.