The electronic behavior of single-layer graphene (SLG) containing resonant impurities was investigated, particularly by quantum capacitance measurements. Before introducing resonant impurities into SLG, the properties of pristine SLG devices top-gated using ultra-thin Y₂O₃ dielectric layers were systematically studied by structure characterization, DC transport measurements and AC quantum capacitance measurements. Y₂O₃ is an ideal candidate of dielectric materials for SLG top-gated devices by introducing very few short-range impurities. This facilitates us to probe the quantum capacitance and the density of states (D= C_q /e²) of pristine and disordered graphene due to its very large capacitance.

A new type of resonant impurities of Ag adatoms deposited on SLG was successfully d...[ Read more ]

The electronic behavior of single-layer graphene (SLG) containing resonant impurities was investigated, particularly by quantum capacitance measurements. Before introducing resonant impurities into SLG, the properties of pristine SLG devices top-gated using ultra-thin Y₂O₃ dielectric layers were systematically studied by structure characterization, DC transport measurements and AC quantum capacitance measurements. Y₂O₃ is an ideal candidate of dielectric materials for SLG top-gated devices by introducing very few short-range impurities. This facilitates us to probe the quantum capacitance and the density of states (D= C_q /e²) of pristine and disordered graphene due to its very large capacitance.

A new type of resonant impurities of Ag adatoms deposited on SLG was successfully detected through quantum capacitance measurements. The midgap states induced by Ag-adatoms are visible at room temperature and more evident at cryogenic temperatures. The intensity of Ag-adatom-induced resonances becomes stronger at higher impurity concentration and higher magnetic fields, which agrees fairly well with theoretical calculations based on the density functional theory (DFT) and tight-binding model (TB). We elucidated that the appearance of the robust resonant peak near the charge neutrality point (CNP) and the splitting of zero Landau level (LL) for Ag-adsorbed graphene are manifestations of the hybridization effect of electrons from graphene bands and the resonant impurity bands.

With a very high density of Ag adatoms, SLG capacitors show unconventional negative quantum capacitance behavior. The Ag adatoms act as resonant impurities and form nearly dispersionless resonant impurity bands near the CNP. 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 observed near the CNP. In the quantum Hall regime, negative quantum capacitance at several Landau level positions is observed, which is associated with the quenching effect of kinetic energy due to the formation of Landau levels. The negative quantum capacitance effect near the CNP is further enhanced in the presence of Landau levels due to the magnetic-field-enhanced Coulomb interactions.