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Negative Quantum Capacitance Induced by Midgap States
  • Negative Quantum Capacitance induced by Mid-gap States


One of our recent discoveries in graphene:


→ Single-layer graphene (SLG) decorated with a high density of Ag adatoms (figure (a)) displays the unconventional phenomenon of negative quantum capacitance.

We found that Ag adatoms act as resonant impurities and form nearly dispersionless resonant impurity bands (see figure (d) near the charge neutrality point (CNP).

 

→ We prepared Ag adatoms on high quality graphene surfaces and measured the DOS changes from graphene's quantum capacitance.

 

images/stories/QC1.jpg

Figure 1 | (a) Schematic diagram of the Ag-adsorbed single-layer graphene capacitor. (b) An optical image of the Ag-adsorbed single-layer graphene capacitor device; dashed line represents the graphene flake and the scale bar is 5 mm. (c) Circuit diagram of the capacitance measurements of Y2O3 top-gated graphene devices. (d) Density plot of spectral function A (E,k) in k 2 E plane for impurity concentration ni 5 1%, where a 5 0.142 nm is the nearest-neighbor distance.


images/stories/QC2.jpg

 

Figure 2 The relationship between quantum capacitance Cq and Fermi energy EF measured at different temperatures for graphene samples with increasing Ag concentration or deposition duration for (a) 0s, (b) 1s and (c) 2s , respectively. The insets enlarge the area near the charge neutrality point.

images/stories/QC3.jpg

Figure 3 (a) The measured capacitance Cm versus top gate voltage Vtg of the Ag-adsorbed SLG measured at T = 2K, where the gray dashed line denotes the value of Cg = 0.65 µF/cm2 for the dielectric layer. (b) Inverse compressibility of Ag-adsorbed graphene, where the orange dashed line denotes the zero value; (c) 2D mapping of Cm measured at T = 2 K as a function of top-gate Vtg and magnetic field B, where the regions in which the negative quantum capacitance (i.e. Cm > Cg ) emerges are colored in yellow (the LL positions are labeled).

 

  • Why negative quantum capacitance?

 

We believe 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 behavior at several Landau level positions is displayed, which is associated with the quenching of kinetic energy by 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. Details can be found in our recent publication (Scientific Reports 3 (2013)2041).

 

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Copyright © 2013  Dr. Ning WANG. Department of Physics, The Hong Kong University of Science and Technology. All rights reserved.