214 research outputs found
Defect-induced modification of low-lying excitons and valley selectivity in monolayer transition metal dichalcogenides
We study the effect of point-defect chalcogen vacancies on the optical
properties of monolayer transition metal dichalcogenides using ab initio GW and
Bethe-Salpeter equation calculations. We find that chalcogen vacancies
introduce unoccupied in-gap states and occupied resonant defect states within
the quasiparticle continuum of the valence band. These defect states give rise
to a number of strongly-bound defect excitons and hybridize with excitons of
the pristine system, reducing the valley-selective circular dichroism. Our
results suggest a pathway to tune spin-valley polarization and other optical
properties through defect engineering
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
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
Energy Level Alignment at Molecule-Metal Interfaces from an Optimally-Tuned Range-Separated Hybrid Functional
The alignment of the frontier orbital energies of an adsorbed molecule with
the substrate Fermi level at metal-organic interfaces is a fundamental
observable of significant practical importance in nanoscience and beyond.
Typical density functional theory calculations, especially those using local
and semi-local functionals, often underestimate level alignment leading to
inaccurate electronic structure and charge transport properties. In this work,
we develop a new fully self-consistent predictive scheme to accurately compute
level alignment at certain classes of complex heterogeneous molecule-metal
interfaces based on optimally-tuned range-separated hybrid functionals.
Starting from a highly accurate description of the gas-phase electronic
structure, our method by construction captures important nonlocal surface
polarization effects via tuning of the long-range screened exchange in a
range-separated hybrid in a non-empirical and system-specific manner. We
implement this functional in a plane-wave code and apply it to several
physisorbed and chemisorbed molecule-metal interface systems. Our results are
in quantitative agreement with experiments, both the level alignment and work
function changes. Our approach constitutes a new practical scheme for accurate
and efficient calculations of the electronic structure of molecule-metal
interfaces.Comment: 15 pages, 8 figure
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
Quasiparticle spectra from a non-empirical optimally-tuned range-separated hybrid density functional
We present a method for obtaining outer valence quasiparticle excitation
energies from a DFT-based calculation, with accuracy that is comparable to that
of many-body perturbation theory within the GW approximation. The approach uses
a range-separated hybrid density functional, with asymptotically exact and
short-range fractional Fock exchange. The functional contains two parameters -
the range separation and the short-range Fock fraction. Both are determined
non-empirically, per system, based on satisfaction of exact physical
constraints for the ionization potential and many-electron self-interaction,
respectively. The accuracy of the method is demonstrated on four important
benchmark organic molecules: perylene, pentacene,
3,4,9,10-perylene-tetracarboxylic-dianydride (PTCDA) and
1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA). We envision that for
finite systems the approach could provide an inexpensive alternative to GW,
opening the door to the study of presently out of reach large-scale systems
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