3 research outputs found
Interlayer Coupling and Gate-Tunable Excitons in Transition Metal Dichalcogenide Heterostructures
Bilayer van der Waals
(vdW) heterostructures such as MoS<sub>2</sub>/WS<sub>2</sub> and
MoSe<sub>2</sub>/WSe<sub>2</sub> have attracted
much attention recently, particularly because of their type II band
alignments and the formation of interlayer exciton as the lowest-energy
excitonic state. In this work, we calculate the electronic and optical
properties of such heterostructures with the first-principles GW+Bethe–Salpeter
Equation (BSE) method and reveal the important role of interlayer
coupling in deciding the excited-state properties, including the band
alignment and excitonic properties. Our calculation shows that due
to the interlayer coupling, the low energy excitons can be widely
tuned by a vertical gate field. In particular, the dipole oscillator
strength and radiative lifetime of the lowest energy exciton in these
bilayer heterostructures is varied by over an order of magnitude within
a practical external gate field. We also build a simple model that
captures the essential physics behind this tunability and allows the
extension of the <i>ab initio</i> results to a large range
of electric fields. Our work clarifies the physical picture of interlayer
excitons in bilayer vdW heterostructures and predicts a wide range
of gate-tunable excited-state properties of 2D optoelectronic devices
Dynamical Excitonic Effects in Doped Two-Dimensional Semiconductors
It is well-known
that excitonic effects can dominate the optical properties of two-dimensional
materials. These effects, however, can be substantially modified by
doping free carriers. We investigate these doping effects by solving
the first-principles Bethe–Salpeter equation. Dynamical screening
effects, included via the sum-rule preserving generalized plasmon-pole
model, are found to be important in the doped system. Using monolayer
MoS<sub>2</sub> as an example, we find that upon moderate doping,
the exciton binding energy can be tuned by a few hundred millielectronvolts,
while the exciton peak position stays nearly constant due to a cancellation
with the quasiparticle band gap renormalization. At higher doping
densities, the exciton peak position increases linearly in energy
and gradually merges into a Fermi-edge singularity. Our results are
crucial for the quantitative interpretation of optical properties
of two-dimensional materials and the further development of ab initio
theories of studying charged excitations such as trions
Heterointerface Screening Effects between Organic Monolayers and Monolayer Transition Metal Dichalcogenides
The
nature and extent of electronic screening at heterointerfaces
and their consequences on energy level alignment are of profound importance
in numerous applications, such as solar cells, electronics <i>etc.</i> The increasing availability of two-dimensional (2D)
transition metal dichalcogenides (TMDs) brings additional opportunities
for them to be used as interlayers in “van der Waals (vdW)
heterostructures” and organic/inorganic flexible devices. These
innovations raise the question of the extent to which the 2D TMDs
participate actively in dielectric screening at the interface. Here
we study perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) monolayers
adsorbed on single-layer tungsten diselenide (WSe<sub>2</sub>), bare
graphite, and Au(111) surfaces, revealing a strong dependence of the
PTCDA HOMO–LUMO gap on the electronic screening effects from
the substrate. The monolayer WSe<sub>2</sub> interlayer provides substantial,
but not complete, screening at the organic/inorganic interface. Our
results lay a foundation for the exploitation of the complex interfacial
properties of hybrid systems based on TMD materials