The surface superstructures in niobium disulfide and diselenide intercalated by Cu, Co and Fe

A series of intercalated transition-metal dichalcogenides with nominal comp ositions A(x)NbCh(2), with A = Cu, Co, Fe, Ch = S, Se and x = 0.25, 0.33, w ere studied by scanning tunneling microscopy (STM). Evidence is given that intercalation influences the STM images. Intercalated Cu atoms occupy tetra hedral interstices in the van der Waals gaps, forming pairs with Nb atoms a nd adopting the 2H(b)-MoS2 stacking of the host structure. Effects associat ed with nonstoichiometry, such as formation of agglomerates and incomplete ordering of the intercalated Cu are frequently observed. Fe and Co atoms oc cupy octahedral interstices in the gaps, form chains with alternating Nb an d stabilize the 2H(a)-NbS2 polytype. In case of (Fe,Co)(x)NbS2 2a(0) x 2a(0) and root 3a(0) x root 3a(0) hexagon al superstructures are formed for x = 1/4 and x = 1/3, respectively, while in case of CuxNb(S,Se)(2) only the x = 1/4 fraction orders into a 2a(0) x 2 a(0) hexagonal superstructure at room temperature. In all cases the superst ructures are visible at the surfaces and are attributed to changes in the l ocal densities of states caused by a charge transfer between the intercalat ed and the surface atoms. Typically these superstructures were observed to change during prolonged scanning, probably as a result of tip-induced fluct uations in the charge density. The experimental STM images are compared to simulated images calculated with the extended Huckel tight binding approxim ation. (C) 2001 Elsevier Science B.V. All rights reserved.