Electronic inhomogeneities in graphene prevent us from precisely accessing its Dirac point and related physics. One rather gets electron and hole puddles due to this disorder, primarily arising from charge of the defects near the graphene-dielectric interface. The resultant potential inhomogeneity in graphene is also dependent on the screening of the defect potential by graphene carriers with the screening length strongly dependent on the carrier density. Thus the electron inhomogenieties are expected to evolve significantly even with fixed interface defects. In this talk I plan to discuss our recent studies on the electronic inhomogeneities in graphene using scanning tunneling microscopy and spectroscopy (STM/S). We measure local tunnel spectra and conductance maps as a function of back-gate voltage, which controls the carrier density. The local spectra, by way of tip-gating effect, give the variation in local Fermi energy, and the conductance maps explicitly probe the evolution of electronic inhomogeneity, with the back-gate voltage. Finally, both point to a role played by interface states in dictating the local graphene-potential in addition to the carrier density dependent screening length which, at least partially, dictates the length scale of the inhomogeneities. Understanding the physics and chemistry of the interface defects in graphene devices can help us engineer the interfaces for memory applications.
Prof. Gupta received an M.Sc. (Integ.) from IIT Kanpur in 1995 and a Ph.D. from Univ. of Kentucky in 2001 on tunnelling studies of high-Tc superconductors. In the later years of his Ph.D. he developed a scanning tunneling microscope (STM) with a novel design. A further improved version of this STM is commercially available in India from Quazar Tech. in Delhi. After Ph.D. He spent about two years in a CNRS lab at Grenoble (France) as a post-doc researcher working on superconducting proximity effect using a 60 mK STM. He joined IIT Kanpur in May 2003 where he has set-up two STM systems which are being used to study electronic inhomogenieties in correlated systems including various oxides and graphene. He is also interested in superconducting devices (SQUIDs) for probing magnetism at nano-scales. At IITK he have supervised five Ph.D. thesis while two are in progress. He was a young associate of Indian Academy of sciences (2004-07) and received P. K. Kelkar fellowship (2009-12). Currently he is an Associate Professor of physics in IIT Kanpur.