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

A series of intercalated transition-metal dichalcogenides with nominal compositions AxNbCh2, with A=Cu, Co, Fe, Ch=S, Se and x=0.25, 0.33, were studied by scanning tunneling microscopy (STM). Evidence is given that intercalation influences the STM images. Intercalated Cu atoms occupy tetrahedral interstices in the van der Waals gaps, forming pairs with Nb atoms and adopting the 2Hb–MoS2 stacking of the host structure. Effects associated with non-stoichiometry, such as formation of agglomerates and incomplete ordering of the intercalated Cu are frequently observed. Fe and Co atoms occupy octahedral interstices in the gaps, form chains with alternating Nb and stabilize the 2Ha–NbS2 polytype. In case of (Fe,Co)xNbS2 2a0×2a0 and √3a0×√3a0 hexagonal 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 2a0×2a0 hexagonal superstructure at room temperature. In all cases the superstructures are visible at the surfaces and are attributed to changes in the local densities of states caused by a charge transfer between the intercalated and the surface atoms. Typically these superstructures were observed to change during prolonged scanning, probably as a result of tip-induced fluctuations in the charge density. The experimental STM images are compared to simulated images calculated with the extended Huckel tight binding approximation.