28 research outputs found
Edge Saturation effects on the magnetism and band gaps in multilayer graphene ribbons and flakes
Using a density functional theory based electronic structure method and
semi-local density approximation, we study the interplay of geometric
confinement, magnetism and external electric fields on the electronic structure
and the resulting band gaps of multilayer graphene ribbons whose edges are
saturated with molecular hydrogen (H) or hydroxyl (OH) groups. We discuss
the similarities and differences of computed features in comparison with the
atomic hydrogen (or H-) saturated ribbons and flakes. For H
edge-saturation, we find \emph{shifted} labeling of three armchair ribbon
classes and magnetic to non-magnetic transition in narrow zigzag ribbons whose
critical width changes with the number of layers. Other computed
characteristics, such as the existence of a critical gap and external electric
field behavior, layer dependent electronic structure, stacking-dependent band
gap induction and the length confinement effects remain qualitatively same with
those of H-saturated ribbons.Comment: 9 pages, 10 figures, submitte
Effects of edge magnetism and external electric field on energy gaps in multilayer graphene nanoribbons
Using first-principles density-functional theory, we study the electronic
structure of multilayer graphene nanoribbons as a function of the ribbon width
and the external electric field, applied perpendicular to the ribbon layers. We
consider two types of edges (armchair and zigzag), each with two edge
alignments (referred to as alpha- and beta-alignments). We show that, as in
monolayer and bilayer armchair nanoribbons, multilayer armchair nanoribbons
exhibit three classes of energy gaps which decrease with increasing width.
Nonmagnetic multilayer zigzag nanoribbons have band structures that are
sensitive to the edge alignments and the number of layers, indicating different
magnetic properties and resulting energy gaps. We find that energy gaps can be
induced in ABC-stacked ribbons with a perpendicular external electric field
while in other stacking sequences, the gaps decrease or remain closed as the
external electric field increases.Comment: 7 pages, 9 figures, text revised, last version before publicatio
Ab Initio Theory of Gate Induced Gaps in Graphene Bilayers
We study the gate voltage induced gap that occurs in graphene bilayers using
\textit{ab initio} density functional theory. Our calculations confirm the
qualitative picture suggested by phenomenological tight-binding and continuum
models. We discuss enhanced screening of the external interlayer potential at
small gate voltages, which is more pronounced in the \textit{ab initio}
calculations, and quantify the role of crystalline inhomogeneity using a
tight-binding model self-consistent Hartree calculation.Comment: 7 pages, 7 figures; the effect of r3 coupling included; typo
correcte
Density Functional Study of Ternary Topological Insulator Thin Films
Using an ab-initio density functional theory based electronic structure
method with a semi-local density approximation, we study thin-film electronic
properties of two topological insulators based on ternary compounds of Tl
(Thallium) and Bi (Bismuth). We consider TlBiX (X=Se, Te) and Bi_2_2_3$ (X=Se, Te). With this property in combination with
a structurally perfect bulk crystal, the latter ternary compound has been found
to have improved surface electronic transport in recent experiments. In this
article, we discuss the nature of surface states, their locations in the
Brillouin zone and their interactions within the bulk region. Our calculations
suggest a critical thin film thickness to maintain the Dirac cone which is
significantly smaller than that in binary Bi-based compounds. Atomic
relaxations or rearrangements are found to affect the Dirac cone in some of
these compounds. And with the help of layer-projected surface charge densities,
we discuss the penetration depth of the surface states into the bulk region.
The electronic spectrum of these ternary compounds agrees very well with the
available experimental results.Comment: 9 pages, 11 figures, 1 table, Accepted for publication in Physical
Review
Dielectric capping effects on binary and ternary topological insulator surface states
Using a density functional based electronic structure method, we study the
effect of crystalline dielectrics on the metallic surface states of Bismuth-
and chalcogen-based binary and ternary three dimensional topological insulator
(TI) thin films. Crystalline quartz (SiO2) and boron nitride (BN) dielectrics
were considered. Crystalline approximation to the amorphous quartz allows to
study the effect of oxygen coverage or environmental effects on the surface
states degradation which has gained attention recently in the experimental
community. We considered both symmetric and asymmetric dielectric cappings to
the sufaces of TI thin films. Our studies suggest that BN and quartz cappings
have negligible effects on the Dirac cone surface states of both binary and
ternary TIs, except in the case of an oxygen-terminated quartz surface.
Dangling bond states of oxygens in oxygen-terminated quartz dominate the region
close to Fermi level, thereby distorting the TI Dirac cone feature and burying
the Dirac point in the quartz valence band region. Passivating the
oxygen-terminated surface with atomic hydrogen removes these dangling bond
states from the Fermi surface region, and consequently the clear Dirac cone is
recovered. Our results are consistent with recent experimental studies of TI
surface degradation in the presence of oxygen coverage.Comment: 11 pages, 15 figures, Accepted for publication in Physical Review
Density functional theory based study of graphene and dielectric oxide interfaces
We study the effects of insulating oxides in their crystalline forms on the
energy band structure of monolayer and bilayer graphene using a \textit{first
principles} density functional theory based electronic structure method and a
local density approximation. We consider the dielectric oxides, SiO
(-quartz) and AlO (alumina or -sapphire) each with
two surface terminations. Our study suggests that atomic relaxations and
resulting equilibrium separations play a critical role in perturbing the linear
band structure of graphene in contrast to the less critical role played by
dangling bonds that result from cleaving the crystal in a particular direction.
We also see that with the addition of a second graphene layer, the Dirac cone
is restored for the quartz surface terminations. Alumina needs more than two
graphene layers to preserve the Dirac cone. Our results are at best
semi-quantitative for the common amorphous forms of the oxides considered.
However, crystalline oxides for which our results are quantitative provide an
interesting option for graphene based electronics, particularly in light of
recent experiments on graphene with crystalline dielectrics (hexagonal BN) that
find considerable improvements in transport properties as compared to the those
with amorphous dielectrics