Atomic surface characterisation and modification of the layered compounds Bi2Se3, Bi1.9Sb0.1Se3 and Bi1.6Sb0.4Se3

In this work, we show atomic STM images of the layered compound Bi2Se3. We study the effect in the surface of the substitution of 5% and 20% of the Bi atoms for Sb in Bi1.9Sb0.1Se3 and Bi1.6Sb0.4Se3. The images of the three samples show similar trigonal structures corresponding probably to the van der Waals Se atoms. The distance measured between surface atoms in Bi2Se3 is 4.04 A, in Bi1.9Sb0.1Se3 is 4.16 A and in Bi1.6Sb0.4Se3 is 4.26 A. In Bi1.6Sb0.4Se3 some atomic sites appear brighter than others. The effect is accentuated at higher tunnelling currents and is not observed in the other compounds. Nanoscopic range depressions on the sample might be related to the skeletal crystal structure since the images show atomic corrugations that align slightly in one direction. We explain the results as the effects of the interactions between tip and sample, and discuss two interpretations: on the one hand, localised depression of the individual atomic sites, and on the other the possible elevation of the atoms of the surface due to a phase transition of the compounds induced by STM.