Nanoprobing electrical properties of topological insulators and graphene using scanning probe microscopy

Abstract

Scanning probe microscopy (SPM) is powerful tools to illustrate the local physical characterization in response to the growing needs for energy technologies. We investigated the electronic properties of topological insulator and graphene using scanning probe microscopy. Three-dimensional topological insulators are insulating in the bulk with unusual metallic surface states that consist of spin-polarized Dirac fermions. Graphene has outstanding electrical properties that stem from its massless Dirac fermions with ultrahigh mobility, as well as graphene having excellent mechanical, chemical and thermal stability. In section 2, correlation between electrical transport and mechanical stress were observed in a topological insulator, $Bi_2Te_3$, using conductive probe atomic force microscopy in an ultrahigh vacuum environment. After directly measuring charge transport on the cleaved $Bi_2Te_3$ surface, we found that the current density varied with applied load. This variation of current density was explained in light of the combined effect of changes in the in-plane conductance due to spin？orbit coupling and hexagonal warping. By comparing surface conduction on a fresh surface versus a surface exposed to air, we observed a minor change in resistance when surface oxide was present. In section 3, we report scanning tunneling microscopy (STM) images of graphene on hydrophilic surface, which is influenced by intercalated water layers. Graphene on three water layers exhibits various atomic patterns of graphene, while the atomic structure of graphene on the single water layer shows a typical honeycomb lattice structure, which is also visible on the graphene layer directly contacting the mica surface. Moreover, we report that the conductance of graphene is influenced by intercalated water layers using current sensing atomic force microscopy (C-AFM). The surface conductance is perturbed by the presence of a water layer between the graphene and mica, which is not found in the STM topographic image. We attribute the perturbation of conductance to structural defects from the water film and a weak interaction between the edge of the water and graphene.