Quantum capacitance measurement of high-quality graphene with resonant impurities and artificial modulations

Abstract

Based on graphene and the technique of quantum capacitance measurement, two types of high-quality graphene capacitances with Y₂O₃ and BN dielectric layers are fabricated respectively, and their electronic properties in the presence of resonant impurities and artificial modulations are investigated systematically. The properties of pristine single-layer graphene covered by Y₂O₃ layer as the dielectric material are firstly evaluated using structural characterization, DC transport and AC capacitance measurements, demonstrating the Y₂O₃ material to be a qualified insulating dielectric. Then a special type of resonant impurities is introduced by the Ag adatoms deposition, which creates midgap states in the vicinity of the charge neutrality point (CNP). The quantum capacitance measurement presents pronounced resonant peaks around the Dirac point, with significant dependences on temperature, impurity concentration and magnetic field. Moreover, under high concentration of Ag adatoms, an unconventional phenomenon of negative quantum capacitance is observed. The Ag adatoms with high concentration create a series of midgap states around the Dirac point, quenching the kinetic energy and enhancing the Coulomb interaction energy simultaneously, which results in the unconventional negative quantum capacitance. Furthermore, a new type of high-quality BN-Graphene-BN capacitances is developed by replacing the SiO₂ substrates and Y₂O₃ dielectric layers with the h-BN sheets, which presents extremely high electronic performance. Under high magnetic field, the Landau level (LL) oscillations are quite obvious, even with the LL splitting and slight negative quantum capacitance observed at the 0^th LL position, mainly due to the magnetic-field-enhanced electron-electron interactions. Afterwards the artificial modulations are introduced by the Ga⁺ ion-beam cutting and side-gate modulations. At high temperatures, the graphene nano-ribbon capacitance presents outstanding electronic performance and the side-gate modulations are consistent with the numerical simulations very well. While at cryogenic temperatures, an unconventional phenomenon of capacitance vanishing is observed, which is sensitive to the width of nano-ribbon and the type of edge-disorders. It is demonstrated that the Ga⁺ ions attached to the edges of graphene nano-ribbon will not influence graphene’s electronic structure and DOS, but introduce strong localizations and open the Coulomb quasigap, degrading the conductive abilities of graphene, leading to the extraordinary capacitance vanishing.

Date of Award	2014
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology