Characterization of the structural, electronic and tribological properties of metal dichalcogenides by scanning probe microscopies

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used to characterize the structural and electronic properties of single-crystal MoS2, nickel-substituted MoS2 (Ni x Mo1−x S2), and chalcogenide-substituted MoS2 (MoS2−x Ch x , Ch ≡ Se, Te) at the atomic level. Images of Ni0.1Mo0.9S2 demonstrate that nickel substitution causes localized changes in the electronic states, although the structure of the surface sulfur layer is unchanged compared with MoS2. Investigations of MoS1.75Se0.25 also show that within the detection limits of AFM selenium substitution does not perturb the sulfur surface structure, and STM data further indicate that the substituted selenium is electronically delocalized. In contrast, AFM studies of MoS1.75Te0.25 show that tellurium substitution produces atomic-sized structural protrusions that may modify significantly the tribological properties of MoS2. In addition, we demonstrate that material wear can be characterized on an atomic scale by AFM. These studies indicate that the microscopic origin of material wear as well as the local structure and electronic properties should be considered to develop further models of friction and wear in metal dichalcogenide materials.