Time: April 15, 2014 (Tuesday) 14:30pm
Location: Conference Room of Basic Energy Science Building
Lecturer: Prof. Ning Wang, Hong Kong University of Science and Technology
Abstract:
Structural disorder has profound impacts on graphene’s extraordinary properties. Theoretically, some special point defects and adsorbates such as vacancies, chemical groups or metal adatoms can modify the band structure of single-layer graphene (SLG). 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 effect of electrons from graphene bands and the impurity bands.
Single-layer graphene 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. In the absence of a magnetic field, negative quantum capacitance is first observed near the CNP. In the quantum Hall regime, the negative quantum capacitance behavior is also displayed. We believe that this novel behavior is associated with the quenching effect of kinetic energy by the formation of Landau levels. Moreover, the negative quantum capacitance effect is further enhanced in the presence of Landau levels due to the magnetic-field-enhanced Coulomb interactions. The negative compressibility induced in graphene may have potential applications in graphene-based field-effect transistors and resonant tunneling transistors.
Contract: Group 502 Ying Shi （9128）