1,721,134 research outputs found
Electronic and optical properties of graphane and related 2-D systems
No full textWith the aid of ab-initio calculations, we have studied the effect of
hydrogen functionalization on
graphene. This new material, theoretically predicted in 2007 [1] and
synthesized in 2008 [2],
is known with the name 'graphane'.
We have investigated the effect of hydrogen also on silicon and germanium based
counterparts of graphene,
polysilane and polygermyne monolayers. Our main objective of this study is to
understand the effect
of hydrogen in the optical absorption spectra and in the electronic
affinity of such
systems, in view of
their possible application in photovoltaic and optoelectronic devices. Being
the optical absorption spectrum and the electron affinity excited
state properties
of materials, we go beyond standard density-functional theory (DFT)
calculations by
introducing quasi-particle and excitonic effects within GW and Bethe Salpeter
approaches. We show that upon H functionalization, graphene undergoes
a metal-insulator transition,
with an opening of an electronic gap at Gamma of about 6 eV. The
electron affinity stays positive, at odd
with hydrogen covered diamond surfaces.
A smaller opening of a gap is observed
in silicon and germanium 2-D counterparts, making these materials eligible for
optoelectronic applications.
[1] J.O. Sofo, A.S. Chaudhari, and G.D. Barber, Phys. Rev. B 75, 153401 (2007)
[2] D.C. Elias, R.R. Nair, T.M.G. Mohiuddin, S.V. Morozov, P.Blake,
M.P., A.C. Ferrari, D.W.
Boukhvalov, M.I. Katsnelson, A.K. Geim, and K.S. Novoselov,
Science 323, 610 (2009); S.Ryu, M.Y. Han, J.Maultzsch, T.F. Heinz,
P.Kim, M.L. Steigerwald, and L.E. Brus
Nanoletters 8, 4597 (2008)
The fascinating physics of carbon surfaces: first-principles study of hydrogen on C(001)
Full text linkWith the aid of ab initio, parameter free calculations based on density-functional and many-body perturbation theory, we investigate the electronic band structure and electron affinity of diamond surfaces. We focus on clean, ideal (0 0 1) and (1 1 1) surfaces and on the effect of hydrogen adsorption. Also single sheets of graphane, that is graphene functionalized upon hydrogen, are investigated. At full H-coverage nearly free electron states (NFESs) appear near the conduction band minimum in all the systems under study. At the same time, the electron affinity is strongly reduced becoming negative for the hydrogenated diamond surfaces, and almost zero in graphane. The effects of quasi-particle corrections on the electron affinity and on the NFESs are discussed.</jats:p
Beyond graphene: Clean, hydrogenated and halogenated silicene, germanene, stanene, and plumbene
No full textThe fascinating electronic and optoelectronic properties of freestanding graphene and the possible inclusion of novel two-dimensional (2D) systems in silicon-based electronics have driven the search for atomic layers consisting of other group-IV elements Si, Ge, Sn, and Pb, which form similar hexagonal lattices and are isoelectronic to graphene. The resulting 2D crystals silicene, germanene, stanene and plumbene, referred as Xenes, but also their functionalized counterparts, e.g. the hydrogenated sheet crystals, named as Xanes, silicane, germanane, and stanane, are in the focus of this review article. In addition, halogenated Xenes are investigated. The consequences of the larger atomic radii on the atomic geometry, the energetic stability, and possible epitaxial preparations are discussed. In the case of honeycomb atomic arrangements, the low-energy electronic excitations are ruled by almost linear bands. Spin–orbit coupling opens small gaps leading to Dirac fermions with finite effective masses. The linear bands give rise to an absorbance of the Xenes determined by the finestructure constant in the long-wavelength regime. While for vanishing photon energies the excitonic influence is still an open question, saddle-point excitons and excitons at M0 van Hove singularities appear at higher frequencies. After opening substantial fundamental gaps by hydrogenation, the absorption edges of the Xanes, silicane, germanane, and stanane, are dominated by bound excitons with extremely large binding energies. Other chemical functionalizations, but also vertical electric fields, yield electronic structures ranging from topological to trivial insulators. Even a quantum spin Hall phase is predicted at room temperature. The topological character and the possible quantization of the spin Hall conductivity are studied versus gap inversion, chemical functionalization, and Rashba spin–orbit interaction. The drastic changes of the electronic properties of Xenes with chemical functionalization, interaction with the substrate, and external perturbations, open future opportunities for tailoring fundamental properties and, therefore, interesting applications in novel electronic and optoelectronic nanodevices
Electronic and optical properties of metal decorated nitrogen-doped vacancy defects in graphene
No full textWe present a first principles study of the stability, and of the electronic and optical properties of graphene with nitrogen doped vacancies. Moreover, we use the vacancies as anchoring sites for Mg, Zn, Pd al Pt atoms and vary the concentration of defects. Decoration of the defects with metal atoms produces semi-metallic systems for any studied size of the cell, with linear bands crossing at the Fermi level. The peculiar electronic properties of massless Dirac fermions in graphene are hence kept, although with anisotropic Fermi velocities. New sharp peaks appear in the optical conductivity in the visible range, thus strongly enhancing the optical response of graphene
Electronic and optical properties of topological semimetal Cd3As2
Full text linkUsing ab initio density functional theory the band structure and the dielectric function of a bct Cd3As2 crystal are calculated. We find a Dirac semimetal with two Dirac nodes k± near the Γ point on the tetragonal axis. The bands near the Fermi level exhibit a linear behavior. The resulting Dirac cones are anisotropic and the electron-hole symmetry is destroyed along the tetragonal axis. Along this axis the symmetry-protected band linearity only exists in a small energy interval. The Dirac cones seemingly found by ARPES in a wider energy range are interpreted in terms of pseudo-linear bands. The behavior as 3D graphene-like material is traced back to As p orbital pointing to Cd vacancies, in directions which vary throughout the unit cell. Because of the Dirac nodes the dielectric functions (imaginary part) show a plateau for vanishing frequencies whose finite value is proportional to the Sommerfeld fine structure constant but varies with the light polarization. The consequences of the anisotropy of the Dirac cones are highlighted for the polarization dependence of the infrared optical conductivity
Thermal properties of Dirac fermions in Xenes: Model studies
No full textThe thermal properties and the electrical conductance are studied for 2D electron gases in doped Xenes - graphene, silicene, germanene, stanene, and plumbene - applying a four-band model to describe the low-energy Dirac-fermion-like electronic excitations. Spin-orbit interactions to discriminate the five Xenes and the influence of an electric field in the normal direction, are taken into account. The density of states and the spectral behavior of the current-current correlation function allow the calculation of the Onsager coefficients. They give analytical formulas for the electronic contributions to the heat capacity and thermal conductance. Also, the electrical conductance can be described within the same framework, if the scattering properties of the electron gases are simulated by constant broadening parameters. For all these thermal and transport properties, only a weak variation with the Xene is found, because of their small spin-orbit-induced gaps, with the exception of plumbene. The heat capacity of Xenes does not show a Schottky anomaly. The thermal conductance increases linearly or quadratically with temperature depending on the temperature range. A similar behavior characterizes the electrical conductance. The dominance of Dirac fermions, i.e., linear bands, determines the ratio of electron thermal conductance and electric conductance, which depends on doping level and temperature. It violates the Wiedemann-Franz law known for 3D electron gases with parabolic energy-momentum dispersion. The Lorenz number is generally much larger for 2D electron gases with almost Dirac character of the dispersion relation
Surface Structure and Energy Bands of 1/3 ML Sn/Ge(111)
No full textThe geometrical and electronic structure of 1/3
monolayer of Sn on the Ge(111) surface is investigated within
density functional theory. The well known and reconstructions are studied in details, performing
also band structure calculations and simulating STM images. Our
results confirm that the 1U-2D structure is the more
favorable one, not only from an energetic point of view but also
from a comparison with STM experiments. On the other hand, the
static room temperature electronic band structure of the hardly compares with the available photoemission
data, thus supporting the idea of a dynamical flipping of the Sn
ad-atoms
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