162 research outputs found
Adsorption of Alkali, Alkaline Earth and Transition Metal Atoms on Silicene
The adsorption characteristics of alkali, alkaline earth and transition metal
adatoms on silicene, a graphene-like monolayer structure of silicon, are
analyzed by means of first-principles calculations. In contrast to graphene,
interaction between the metal atoms and the silicene surface is quite strong
due to its highly reactive buckled hexagonal structure. In addition to
structural properties, we also calculate the electronic band dispersion, net
magnetic moment, charge transfer, workfunction and dipole moment of the metal
adsorbed silicene sheets. Alkali metals, Li, Na and K, adsorb to hollow site
without any lattice distortion. As a consequence of the significant charge
transfer from alkalis to silicene metalization of silicene takes place. Trends
directly related to atomic size, adsorption height, workfunction and dipole
moment of the silicene/alkali adatom system are also revealed. We found that
the adsorption of alkaline earth metals on silicene are entirely different from
their adsorption on graphene. The adsorption of Be, Mg and Ca turns silicene
into a narrow gap semiconductor. Adsorption characteristics of eight transition
metals Ti, V, Cr, Mn, Fe, Co, Mo and W are also investigated. As a result of
their partially occupied d orbital, transition metals show diverse structural,
electronic and magnetic properties. Upon the adsorption of transition metals,
depending on the adatom type and atomic radius, the system can exhibit metal,
half-metal and semiconducting behavior. For all metal adsorbates the direction
of the charge transfer is from adsorbate to silicene, because of its high
surface reactivity. Our results indicate that the reactive crystal structure of
silicene provides a rich playground for functionalization at nanoscale.Comment: 8 Figures, 1 Table. under publication Physical Review B (2013
Characterization of the size and position of electron-hole puddles at a graphene p-n junction
The effect of an electron-hole puddle on the electrical transport when
governed by snake states in a bipolar graphene structure is investigated. Using
numerical simulations we show that information on the size and position of the
electron-hole puddle can be obtained using the dependence of the conductance on
magnetic field and electron density of the gated region. The presence of the
scatterer disrupts snake state transport which alters the conduction pattern.
We obtain a simple analytical formula that connects the position of the
electron-hole puddle with features observed in the conductance. Size of the
electron-hole puddle is estimated from the magnetic field and gate potential
that maximizes the effect of the puddle on the electrical transport.Comment: This is an author-created, un-copyedited version of an article
published in Nanotechnology. IOP Publishing Ltd is not responsible for any
errors or omissions in this version of the manuscript or any version derived
from it. The Version of Record is available online at
doi:10.1088/0957-4484/27/10/10520
Electronic and Vibrational Properties of PbI 2 : From Bulk to Monolayer
Using first-principles calculations, we study the dependence of the
electronic and vibrational properties of multi-layered PbI 2 crystals on the
number of layers and focus on the electronic-band structure and the Raman
spectrum. Electronic-band structure calculations reveal that the direct or
indirect semiconducting behavior of PbI 2 is strongly influenced by the number
of layers. We find that at 3L-thickness there is a direct-to-indirect band gap
transition (from bulk-to-monolayer). It is shown that in the Raman spectrum two
prominent peaks, A 1g and E g , exhibit phonon hardening with increasing number
of layers due to the inter-layer van der Waals interaction. Moreover, the Raman
activity of the A 1g mode significantly increases with increasing number of
layers due to the enhanced out-of-plane dielectric constant in the few-layer
case. We further characterize rigid-layer vibrations of low-frequency
inter-layer shear (C) and breathing (LB) modes in few-layer PbI 2 . A reduced
mono-atomic (linear) chain model (LCM) provides a fairly accurate picture of
the number of layers dependence of the low-frequency modes and it is shown also
to be a powerful tool to study the inter-layer coupling strength in layered PbI
2 .Comment: To appear in Phys. Rev.
Magnetic field dependence of the atomic collapse state in graphene
Quantum electrodynamics predicts that heavy atoms ()
will undergo the process of atomic collapse where electrons sink into the
positron continuum and a new family of so-called collapsing states emerges. The
relativistic electrons in graphene exhibit the same physics but at a much lower
critical charge () which has made it possible to confirm this
phenomenon experimentally. However, there exist conflicting predictions on the
effect of a magnetic field on atomic collapse. These theoretical predictions
are based on the continuum Dirac-Weyl equation, which does not have an exact
analytical solution for the interplay of a supercritical Coulomb potential and
the magnetic field. Approximative solutions have been proposed, but because the
two effects compete on similar energy scales, the theoretical treatment varies
depending on the regime which is being considered. These limitations are
overcome here by starting from a tight-binding approach and computing exact
numerical results. By avoiding special limit cases, we found a smooth evolution
between the different regimes. We predict that the atomic collapse effect
persists even after the magnetic field is activated and that the critical
charge remains unchanged. We show that the atomic collapse regime is
characterized: 1) by a series of Landau level anticrossings and 2) by the
absence of scaling of the Landau levels with regard to magnetic
field strength
Helical edge states in silicene and germanene nanorings in perpendicular magnetic field
Due to nonzero intrinsic spin-orbit interaction in buckled honeycomb crystal
structures, silicene and germanene exhibit interesting topological properties,
and are therefore candidates for the realization of the quantum spin Hall
effect. We employ the Kane-Mele model to investigate the electron states in
hexagonal silicene and germanene nanorings having either zigzag or armchair
edges in the presence of a perpendicular magnetic field. We present results for
the energy spectra as function of magnetic field, the electron density of the
spin-up and spin-down states in the ring plane, and the calculation of the
probability current density. The quantum spin Hall phase is found at the edges
between the nontrivial topological phase in silicene and germanene and vacuum.
We demonstrate that the helical edge states in zigzag silicene and germanene
nanorings can be qualitatively well understood by means of classical magnetic
moments. However, this is not the case for comparable-sized armchair nanorings,
where the eigenfunctions spread throughout the ring. Finally, we note that the
energy spectra of silicene and germanene nanorings are similar and that the
differences between the two are mainly related to the difference in magnitude
of the spin-orbit coupling.Comment: 17 pages, 10 figure
Gate induced monolayer behavior in twisted bilayer black phosphorus
Optical and electronic properties of black phosphorus strongly depend on the
number of layers and type of stacking. Using first-principles calculations
within the framework of density functional theory, we investigate the
electronic properties of bilayer black phosphorus with an interlayer twist
angle of 90. These calculations are complemented with a simple
model which is able to capture most of the low energy
features and is valid for arbitrary twist angles. The electronic spectrum of
90 twisted bilayer black phosphorus is found to be x-y isotropic in
contrast to the monolayer. However x-y anisotropy, and a partial return to
monolayer-like behavior, particularly in the valence band, can be induced by an
external out-of-plane electric field. Moreover, the preferred hole effective
mass can be rotated by 90 simply by changing the direction of the
applied electric field. In particular, a +0.4 (-0.4) V/{\AA} out-of-plane
electric field results in a 60\% increase in the hole effective mass
along the y (x) axis and enhances the ()
ratio as much as by a factor of 40. Our DFT and
simulations clearly indicate that the twist angle in combination with an
appropriate gate voltage is a novel way to tune the electronic and optical
properties of bilayer phosphorus and it gives us a new degree of freedom to
engineer the properties of black phosphorus based devices.Comment: 8 pages, 8 figure
Structure and energetics of hydrogen chemisorbed on a single graphene layer to produce graphane
Chemisorption of hydrogen on graphene is studied using atomistic simulations
with the second generation of reactive empirical bond order Brenner
inter-atomic potential. The lowest energy adsorption sites and the most
important metastable sites are determined. The H concentration is varied from a
single H atom, to clusters of H atoms up to full coverage. We found that when
two or more H atoms are present, the most stable configurations of H
chemisorption on a single graphene layer are ortho hydrogen pairs adsorbed on
one side or on both sides of the graphene sheet. The latter has the highest
hydrogen binding energy. The next stable configuration is the ortho-para pair
combination, and then para hydrogen pairs. The structural changes of graphene
caused by chemisorbed hydrogen are discussed and are compared with existing
experimental data and other theoretical calculations. The obtained results will
be useful for nanoengineering of graphene by hydrogenation and for hydrogen
storage.Comment: 22 pages, 8 figures, 2 tables; accepted, to appear in Carbo
The magnetic, electronic, and light-induced topological properties in two-dimensional hexagonal FeX2 (X = Cl, Br, I) monolayers
Topological materials are fertile ground for investigating topological phases
of matter and topological phase transitions. In particular, the quest for novel
topological phases in 2D materials is attracting fast growing attention. Here,
using Floquet-Bloch theory, we propose to realize chiral topological phases in
2D hexagonal FeX2 (X=Cl, Br, I) monolayers under irradiation of circularly
polarized light. Such 2D FeX2 monolayers are predicted to be dynamical stable,
and exhibit both ferromagnetic and semiconducting properties. To capture the
full topological physics of the magnetic semiconductor under periodic driving,
we adopt ab initio Wannier-based tight-binding methods for the Floquet-Bloch
bands, with the light-induced band gap closings and openings being obtained as
the light field strength increases. The calculations of slab with open
boundaries show the existence of chiral edge states. Interestingly, the
topological transitions with branches of chiral edge states changing from zero
to one and from one to two by tuning the light amplitude are obtained, showing
that the topological floquet phase of high Chern number can be induced in the
present Floquet-Bloch systems
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