Structural disorder has profound impacts on the extraordinary properties of graphene and atomically thin MoS2 material. Theoretically, some special point defects and adsorbates such as vacancies, chemical groups or metal adatoms can modify the band structures of these materials. In this talk, I present our resent research on detecting the resonant impurities induced by Ag adatoms deposited on SLG through quantum capacitance measurement. Different from long-range charged impurities and other types of conventional resonant impurities, Ag adatoms form very weak covalent bonds with carbon atoms and thus modify the band structure of graphene near the charge neutrality point (CNP) point. The midgap states induced by Ag adatoms are visible at room temperature and become even more evident at cryogenic temperatures. We found that the appearance of a robust resonant peak near the CNP and the splitting of the zero Landau level for Ag-adsorbed graphene were manifestations of the hybridization of electrons from graphene bands and the impurity bands. SLG decorated with a high density of Ag resonant impurities displays the unconventional phenomenon of negative quantum capacitance. We show that resonant impurities quench the kinetic energy and drive the electrons to the Coulomb energy dominated regime with negative compressibility [1, 2].
The metal-insulator transition (MIT) is one of the remarkable electrical properties of atomically thin MoS2. The underlying mechanism and detailed transition process in MoS2, however, still remain largely unexplored. We built a new type of capacitor structure based on the vertical metal-insulator-semiconductor heterostructures using atomically thin MoS2 for probing electron states. The vertical configuration offers the added advantage of eliminating the influence of large impedance at the band tails and allows the observation of fully excited electron states near the surface of MoS2 over a wide excitation frequency and temperature range. Different from the theory of electron–electron interactions which has been used in modelling the metal-insulator transition in MoS2, we suggested a percolation-type MIT in MoS2, driven by density inhomogeneity of electron states that describes the systems in which charge carriers are transported through percolating conductive channels in the disorder landscapes due to the poor screening effect at low carrier densities .
. Lin Wang, et. al., “Negative Quantum Capacitance Induced by Midgap States in Single-layer Graphene”, Scientific Reports 3 (2013)2041.
. Lin Wang, et. al., “Detection of resonant impurities in graphene by quantum capacitance measurement”, Phys. Rev. B 89 (2014) 075410.
. Xiaolong Chen, et. al., "Probing the electron states and metal-insulator transition mechanisms in molybdenum disulphide vertical heterostructures", Nature Comm. 6 (2015) 6088.