335 research outputs found
Variability of structural and electronic properties of bulk and monolayer Si2Te3
Since the emergence of monolayer graphene as a promising two-dimensional
material, many other monolayer and few-layer materials have been investigated
extensively. An experimental study of few-layer Si2Te3 was recently reported,
showing that the material has diverse properties for potential applications in
Si-based devices ranging from fully integrated thermoelectrics to
optoelectronics to chemical sensors. This material has a unique layered
structure: it has a hexagonal closed-packed Te sublattice, with Si dimers
occupying octahedral intercalation sites. Here we report a theoretical study of
this material in both bulk and monolayer form, unveiling a fascinating array of
diverse properties arising from reorientations of the silicon dimers between
planes of Te atoms. The lattice constant varies up to 5% and the band gap
varies up to 40% depending on dimer orientations. The monolayer band gap is 0.4
eV larger than the bulk-phase value for the lowest-energy configuration of Si
dimers. These properties are, in principle, controllable by temperature and
strain, making Si2T3 a promising candidate material for nanoscale mechanical,
optical, and memristive devices.Comment: 9 pages, 4 figure
Mapping the wavefunction of transition metal acceptor states in the GaAs surface
We utilize a single atom substitution technique with spectroscopic imaging in
a scanning tunneling microscope (STM) to visualize the anisotropic spatial
structure of magnetic and non-magnetic transition metal acceptor states in the
GaAs (110) surface. The character of the defect states play a critical role in
the properties of the semiconductor, the localization of the states influencing
such things as the onset of the metal-insulator transition, and in dilute
magnetic semiconductors the mechanism and strength of magnetic interactions
that lead to the emergence of ferromagnetism. We study these states in the GaAs
surface finding remarkable similarities between the shape of the acceptor state
wavefunction for Mn, Fe, Co and Zn dopants, which is determined by the GaAs
host and is generally reproduced by tight binding calculations of Mn in bulk
GaAs [Tang, J.M. & Flatte, M.E., Phys. Rev. Lett. 92, 047201 (2004)]. The
similarities originate from the antibonding nature of the acceptor states that
arise from the hybridization of the impurity d-levels with the host. A second
deeper in-gap state is also observed for Fe and Co that can be explained by the
symmetry breaking of the surface.Comment: 19 pages, 6 figure
Atomic-Scale Dynamics of the Formation and Dissolution of Carbon Clusters in SiO2
Oxidation of SiC produces SiO2 while CO is released. A `reoxidation' step at
lower temperatures is, however, necessary to produce high-quality SiO2. This
step is believed to cleanse the oxide of residual C without further oxidation
of the SiC substrate. We report first-principles calculations that describe the
nucleation and growth of O-deficient C clusters in SiO2 under oxidation
conditions, fed by the production of CO at the advancing interface, and their
gradual dissolution by the supply of O under reoxidation conditions. We predict
that both CO and CO2 are released during both steps.Comment: RevTex, 4 pages, 2 ps figures, to appear in Phys. Rev. Lett. (June
25, 2001
Zero-bias molecular electronics: Exchange-correlation corrections to Landauer's formula
Standard first principles calculations of transport through single molecules
miss exchange-correlation corrections to the Landauer formula. From Kubo
response theory, both the Landauer formula and these corrections in the limit
of zero bias are derived and calculations are presented.Comment: 4 pages, 3 figures, final version to appear in Phys. Rev. B, Rapid
Communication
Theory-assisted determination of nano-rippling and impurities in atomic resolution images of angle-mismatched bilayer graphene
Ripples and impurity atoms are universally present in 2D materials, limiting carrier mobility, creating pseudo–magnetic fields, or affecting the electronic and magnetic properties. Scanning transmission electron microscopy (STEM) generally provides picometer-level precision in the determination of the location of atoms or atomic 'columns' in the in-image plane (xy plane). However, precise atomic positions in the z-direction as well as the presence of certain impurities are difficult to detect. Furthermore, images containing moiré patterns such as those in angle-mismatched bilayer graphene compound the problem by limiting the determination of atomic positions in the xy plane. Here, we introduce a reconstructive approach for the analysis of STEM images of twisted bilayers that combines the accessible xy coordinates of atomic positions in a STEM image with density-functional-theory calculations. The approach allows us to determine all three coordinates of all atomic positions in the bilayer and establishes the presence and identity of impurities. The deduced strain-induced rippling in a twisted bilayer graphene sample is consistent with the continuum model of elasticity. We also find that the moiré pattern induces undulations in the z direction that are approximately an order of magnitude smaller than the strain-induced rippling. A single substitutional impurity, identified as nitrogen, is detected. The present reconstructive approach can, therefore, distinguish between moiré and strain-induced effects and allows for the full reconstruction of 3D positions and atomic identities
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