2 research outputs found
Gate-Controlled Nonlinear Conductivity of Dirac Fermion in Graphene Field-Effect Transistors Measured by Terahertz Time-Domain Spectroscopy
We present terahertz spectroscopic measurements of Dirac
fermion
dynamics from a large-scale graphene that was grown by chemical vapor
deposition and on which carrier density was modulated by electrostatic
and chemical doping. The measured frequency-dependent optical sheet
conductivity of graphene shows electron-density-dependence characteristics,
which can be understood by a simple Drude model. In a low carrier
density regime, the optical sheet conductivity of graphene is constant
regardless of the applied gate voltage, but in a high carrier density
regime, it has nonlinear behavior with respect to the applied gate
voltage. Chemical doping using viologen was found to be efficient
in controlling the equilibrium Fermi level without sacrificing the
unique carrier dynamics of graphene
Diffusion Mechanism of Lithium Ion through Basal Plane of Layered Graphene
Coexistence of both edge plane and basal plane in graphite
often
hinders the understanding of lithium ion diffusion mechanism. In this
report, two types of graphene samples were prepared by chemical vapor
deposition (CVD): (i) well-defined basal plane graphene grown on Cu
foil and (ii) edge plane-enriched graphene layers grown on Ni film.
Electrochemical performance of the graphene electrode can be split
into two regimes depending on the number of graphene layers: (i) the
corrosion-dominant regime and (ii) the lithiation-dominant regime.
Li ion diffusion perpendicular to the basal plane of graphene is facilitated
by defects, whereas diffusion parallel to the plane is limited by
the steric hindrance that originates from aggregated Li ions adsorbed
on the abundant defect sites. The critical layer thickness (<i>l</i><sub>c</sub>) to effectively prohibit substrate reaction
using CVD-grown graphene layers was predicted to be ∼6 layers,
independent of defect population. Our density functional theory calculations
demonstrate that divacancies and higher order defects have reasonable
diffusion barrier heights allowing lithium diffusion through the basal
plane but neither monovacancies nor Stone-Wales defect