76 research outputs found
Tunable Hybridization Between Electronic States of Graphene and Physisorbed Hexacene
Non-covalent functionalization via physisorption of organic molecules
provides a scalable approach for modifying the electronic structure of graphene
while preserving its excellent carrier mobilities. Here we investigated the
physisorption of long-chain acenes, namely, hexacene and its fluorinated
derivative perfluorohexacene, on bilayer graphene for tunable graphene devices
using first principles methods. We find that the adsorption of these molecules
leads to the formation of localized states in the electronic structure of
graphene close to its Fermi level, which could be readily tuned by an external
electric field. The electric field not only creates a variable band gap as
large as 250 meV in bilayer graphene, but also strongly influences the charge
redistribution within the molecule-graphene system. This charge redistribution
is found to be weak enough not to induce strong surface doping, but strong
enough to help preserve the electronic states near the Dirac point of graphene.Comment: 17 pages, 7 figures, supporting informatio
From Half-metal to Semiconductor: Electron-correlation Effects in Zigzag SiC Nanoribbons From First Principles
We performed electronic structure calculations based on the first-principles
many-body theory approach in order to study quasiparticle band gaps, and
optical absorption spectra of hydrogen-passivated zigzag SiC nanoribbons.
Self-energy corrections are included using the GW approximation, and excitonic
effects are included using the Bethe-Salpeter equation. We have systematically
studied nanoribbons that have widths between 0.6 and 2.2
. Quasiparticle corrections widened the Kohn-Sham band gaps because
of enhanced interaction effects, caused by reduced dimensionality. Zigzag SiC
nanoribbons with widths larger than 1 nm, exhibit half-metallicity at the
mean-field level. The self-energy corrections increased band gaps
substantially, thereby transforming the half-metallic zigzag SiC nanoribbons,
to narrow gap spin-polarized semiconductors. Optical absorption spectra of
these nanoribbons get dramatically modified upon inclusion of electron-hole
interactions, and the narrowest ribbon exhibits strongly bound excitons, with
binding energy of 2.1 eV. Thus, the narrowest zigzag SiC nanoribbon has the
potential to be used in optoelectronic devices operating in the IR region of
the spectrum, while the broader ones, exhibiting spin polarization, can be
utilized in spintronic applications.Comment: 22 pages, 6 figures (included
Selective Control of Surface Spin Current in Topological Materials based on Pyrite-type OsX2 (X = Se, Te) Crystals
Topological materials host robust surface states, which could form the basis
for future electronic devices. As such states have spins that are locked to the
momentum, they are of particular interest for spintronic applications.
Understanding spin textures of the surface states of topologically nontrivial
materials, and being able to manipulate their polarization, is therefore
essential if they are to be utilized in future technologies. Here we use
first-principles calculations to show that pyrite-type crystals OsX2 (X= Se,
Te) are a class of topological material that can host surface states with spin
polarization that can be either in-plane or out-of-plane. We show that the
formation of low-energy states with symmetry-protected energy- and
direction-dependent spin textures on the (001) surface of these materials is a
consequence of a transformation from a topologically trivial to nontrivial
state, induced by spin orbit interactions. The unconventional spin textures of
these surface states feature an in-plane to out-of-plane spin polarization
transition in the momentum space protected by local symmetries. Moreover, the
surface spin direction and magnitude can be selectively filtered in specific
energy ranges. Our demonstration of a new class of topological material with
controllable spin textures provide a platform for experimentalists to detect
and exploit unconventional surface spin textures in future spin-based
nanoelectronic devices
Extracting unconventional spin texture in two dimensional topological crystalline insulators via tuning bulk-edge interactions
Tuning the interaction between the bulk and edge states of topological
materials is a powerful tool for manipulating edge transport behavior, opening
up exciting opportunities for novel electronic and spintronic applications.
This approach is particularly suited to topological crystalline insulators
(TCI), a class of topologically nontrivial compounds that are endowed with
multiple degrees of topological protection. In this study, we investigate how
bulk-edge interactions can influence the edge transport in planar bismuthene, a
TCI with metallic edge states protected by in-plane mirror symmetry, using
first principles calculations and symmetrized Wannier tight-binding models. By
exploring the impact of various perturbation effects, such as device size,
substrate potentials, and applied transverse electric field, we examine the
evolution of the electronic structure and edge transport in planar bismuthene.
Our findings demonstrate that the TCI states of planar bismuthene can be
engineered to exhibit either a gapped or conducting unconventional helical spin
texture via a combination of substrate and electric field effects. Furthermore,
under strong electric fields, the edge states can be stabilized through a
delicate control of the bulk-edge interactions. These results open up new
directions for discovering novel spin transport patterns in topological
materials and provide critical insights for the fabrication of topological
spintronic devices.Comment: 23 pages, 8 figure
Electrically Controlled Reversible Strain Modulation in MoS Field-effect Transistors via an Electro-mechanically Coupled Piezoelectric Thin Film
Strain can efficiently modulate the bandgap and carrier mobilities in
two-dimensional (2D) materials. Conventional mechanical strain-application
methodologies that rely on flexible, patterned or nano-indented substrates are
severely limited by low thermal tolerance, lack of tunability and/or poor
scalability. Here, we leverage the converse piezoelectric effect to
electrically generate and control strain transfer from a piezoelectric thin
film to electro-mechanically coupled ultra-thin 2D MoS. Electrical bias
polarity change across the piezoelectric film tunes the nature of strain
transferred to MoS from compressive 0.23% to tensile 0.14% as
verified through peak shifts in Raman and photoluminescence spectroscopies and
substantiated by density functional theory calculations. The device
architecture, built on a silicon substrate, uniquely integrates an MoS
field-effect transistor on top of a metal-piezoelectric-metal stack enabling
strain modulation of transistor drain current 130, on/off current ratio
150, and mobility 1.19 with high precision, reversibility and
resolution. Large, tunable tensile (1056) and compressive (-1498) strain gauge
factors, easy electrical strain modulation, high thermal tolerance and
substrate compatibility make this technique promising for integration with
silicon-based CMOS and micro-electro-mechanical systems.Comment: Manuscript and Supplementary Informatio
- …