30 research outputs found
Designable exciton mixing through layer alignment in WS-graphene heterostructures
Optical properties of heterostructures composed of layered 2D materials, such
as transition metal dichalcogenides (TMDs) and graphene, are broadly explored.
Of particular interest are light-induced energy transfer mechanisms in these
materials and their structural roots. Here, we use state-of-the-art
first-principles calculations to study the excitonic composition and the
absorption properties of WS-graphene heterostructures as a function of
interlayer alignment and the local strain resulting from it. We find that
Brillouin zone mismatch and the associated energy level alignment between the
graphene Dirac cone and the TMD bands dictate an interplay between interlayer
and intralayer excitons, mixing together in the many-body representation upon
the strain-induced symmetry breaking in the interacting layers. Examining the
representative cases of the 0 and 30 interlayer twist angles,
we find that this exciton mixing strongly varies as a function of the relative
alignment. We quantify the effect of these structural modifications on exciton
charge separation between the layers and the associated graphene-induced
homogeneous broadening of the absorption resonances. Our findings provide
guidelines for controllable optical excitations upon interface design and shed
light on the importance of many-body effects in the understanding of optical
phenomena in complex heterostructures.Comment: 8 pages, 4 figure
Spin-orbit torque in single-molecule junctions from ab initio
The use of electric fields applied across magnetic heterojunctions that lack
spatial inversion symmetry has been previously proposed as a non-magnetic mean
of controlling localized magnetic moments through spin-orbit torques (SOT). The
implementation of this concept at the single-molecule level has remained a
challenge, however. Here, we present first-principle calculations of SOT in a
single-molecule junction under bias and beyond linear response. Employing a
self-consistency scheme invoking density functional theory and non-equilibrium
Green's function theory, we compute the current-induced SOT. Responding to this
torque, a localized magnetic moment can tilt. Within the linear regime our
quantitative estimates for the SOT in single-molecule junctions yield values
similar to those known for magnetic interfaces. Our findings contribute to an
improved microscopic understanding of SOT in single molecules.Comment: 7 pages + 5 figures; Supporting Information (16 pages + 3 figures
Charge quenching at defect states in transition metal dichalcogenide-graphene van der Waals heterobilayers
We study the dynamical properties of point-like defects, represented by
monoatomic chalcogen vacancies, in WS-graphene and MoS-graphene
heterobilayers. Employing a multidisciplinary approach based on the combination
of ab initio, model Hamiltonian and density matrix techniques, we propose a
minimal interacting model that allows for the calculation of electronic
transition times associated to population and depopulation of the vacancy by an
additional electron. We obtain the "coarse-grained" semiclassical dynamics by
means of a master equation approach and discuss the potential role of virtual
charge fluctuations in the internal dynamics of impurity quasi-degenerate
states. The interplay between the symmetry of the lattice and the spin degree
of freedom through the spin-orbit interaction and its impact on charge
quenching is studied in detail.Comment: 17 pages + 9 figures; Supplemental Material (10 pages + 4 figures
Reduced absorption due to defect-localized interlayer excitons in transition metal dichalcogenide-graphene heterostructures
Associating the presence of atomic vacancies to excited-state transport
phenomena in two dimensional semiconductors is of emerging interest, and
demands detailed understanding of the involved exciton transitions. Here we
study the effect of such defects on the electronic and optical properties of
WS-graphene and MoS-graphene van der Waals heterobilayers by employing
many-body perturbation theory. We find that the combination of chalcogen
defects and graphene adsorption onto the transition metal dichalcogenide layer
can radically alter the optical properties of the heterobilayer, due to a
combination of dielectric screening, the impact of the missing chalcogen atoms
in the intralayer and interlayer optical transitions, and the different nature
of each layer. By analyzing the intrinsic radiative rates of the most stable
subgap excitonic features, we find that while the presence of defects
introduces low-lying optical transitions, resulting in excitons with larger
oscillator strength, it also decreases the optical response associated to the
pristine-like transition-metal dichalcogenide intralayer excitons. Our findings
relate excitonic features with interface design for defect engineering in
photovoltaic and transport applications.Comment: 7 pages + 3 figures; Supporting Information (20 pages + 13 figures
Low-scaling GW algorithm applied to twisted transition-metal dichalcogenide heterobilayers
The method is widely used for calculating the electronic band structure
of materials. The high computational cost of algorithms prohibits their
application to many systems of interest. We present a periodic, low-scaling and
highly efficient algorithm that benefits from the locality of the Gaussian
basis and the polarizability. The algorithm enables calculations on a
MoSe/WS bilayer with 984 atoms per unit cell, in 42 hours using 1536
cores. This is four orders of magnitude faster than a plane-wave
algorithm, allowing for unprecedented computational studies of electronic
excitations at the nanoscale
Solitonics with Polyacetylenes
Polyacetylene molecular wires have attracted a long-standing interest for the past 40 years. From a fundamental perspective, there are two main reasons for the interest. First, polyacetylenes are a prime realization of a one-dimensional topological insulator. Second, long molecules support freely propagating topological domain-wall states, so-called "solitons," which provide an early paradigm for spin-charge separation. Because of recent experimental developments, individual poly- acetylene chains can now be synthesized on substrates. Motivated by this breakthrough, we here propose a novel way for chemically supported soliton design in these systems. We demonstrate how to control the soliton position and how to read it out via external means. Also, we show how extra soliton-antisoliton pairs arise when applying a moderate static electric field. We thus make a step toward functionality of electronic devices based on soliton manipulation, that is, "solitonics"
Mixed excitonic nature in water-oxidized BiVO surfaces with defects
BiVO is a promising photocatalyst for efficient water oxidation, with
surface reactivity determined by the structure of active catalytic sites.
Surface oxidation in the presence of oxygen vacancies induces electron
localization, suggesting an atomistic route to improve the charge transfer
efficiency within the catalytic cycle. In this work, we study the effect of
oxygen vacancies on the electronic and optical properties at BiVO surfaces
upon water oxidation. We use density functional theory and many-body
perturbation theory to explore the change in the electronic and quasiparticle
energy levels and to evaluate the electron-hole coupling as a function of the
underlying structure. We show that while the presence of defects alters the
atomic structure and largely modifies the wavefunction nature, leading to
defect-localized states at the quasipatricle gap region, the optical
excitations remain largely unchanged due to substantial hybridization of defect
and non-defect electron-hole transitions. Our findings suggest that
defect-induced surface oxidation supports improved electron transport, both
through bound and tunable electronic states and via a mixed nature of the
optical transitions, expected to reduce electron-hole defect trapping
Low-Scaling GW Algorithm Applied to Twisted Transition-Metal Dichalcogenide Heterobilayers
The GW method is widely used for calculating the electronic band structure of
materials. The high computational cost of GW algorithms prohibits their application to many systems of interest. We present a periodic, low-scaling, and highly efficient GW algorithm that benefits from the locality of the Gaussian basis and the polarizability. The algorithm enables G0W0 calculations on a MoSe2/WS2 bilayer with 984 atoms per unit cell, in 42 h using 1536 cores. This is 4 orders of magnitude faster than a plane-wave G0W0 algorithm, allowing for unprecedented computational studies of electronic excitations at the nanoscale
Low-Scaling GW Algorithm Applied to Twisted Transition-Metal Dichalcogenide Heterobilayers
The GW method is widely used for calculating the electronic band structure of materials. The high computational cost of GW algorithms prohibits their application to many systems of interest. We present a periodic, low-scaling, and highly efficient GW algorithm that benefits from the locality of the Gaussian basis and the polarizability. The algorithm enables G0W0 calculations on a MoSe2/WS2 bilayer with 984 atoms per unit cell, in 42 h using 1536 cores. This is 4 orders of magnitude faster than a plane-wave G0W0 algorithm, allowing for unprecedented computational studies of electronic excitations at the nanoscale