NATURE OF THE CHARGE-DENSITY-WAVE IMAGES OF LAYERED DICHALCOGENIDES 1T-TAX2 (X = S, SE) IN SCANNING TUNNELING AND ATOMIC FORCE MICROSCOPY

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images of the charge density wave (CDW) materials 1T-TaX2 (X = S, Se) were examined by calculating the partial electron density p(r0, e(f)) and total electron density rho(r0) of a single 1T-TaSe2 layer. Our calculations show that the bright spots of the STM and AFM images are associated with the surface chalcogen atoms, and that both images should possess a pattern of six-chalcogen-atom triangles at CDW maxima. The present study explains why STM images of the CDW's have apparent height amplitudes an order of magnitude greater than the true atomic displacements of the surface chalcogen atoms, why the bright spots of the STM images have a circular shape with a radius smaller than about 2 angstrom, and why these bright spots appear as three-atom clusters in some images. The patterns of the STM and AFM images predicted from our calculations agree well with those of the observed images.