200 research outputs found
Energy-resolved Photoconductivity Mapping in a Monolayer-bilayer WSe2 Lateral Heterostructure
Vertical and lateral heterostructures of van der Waals materials provide
tremendous flexibility for band structure engineering. Since electronic bands
are sensitively affected by defects, strain, and interlayer coupling, the edge
and heterojunction of these two-dimensional (2D) systems may exhibit novel
physical properties, which can be fully revealed only by spatially resolved
probes. Here, we report the spatial mapping of photoconductivity in a
monolayer-bilayer WSe2 lateral heterostructure under multiple excitation
lasers. As the photon energy increases, the light-induced conductivity detected
by microwave impedance microscopy first appears along the hetero-interface and
bilayer edge, then along the monolayer edge, inside the bilayer area, and
finally in the interior of the monolayer region. The sequential emergence of
mobile carriers in different sections of the sample is consistent with the
theoretical calculation of local energy gaps. Quantitative analysis of the
microscopy and transport data also reveals the linear dependence of
photoconductivity on the laser intensity and the influence of interlayer
coupling on carrier recombination. Combining theoretical modeling, atomic scale
imaging, mesoscale impedance microscopy, and device-level characterization, our
work suggests an exciting perspective to control the intrinsic band-gap
variation in 2D heterostructures down to the few-nanometer regime.Comment: 18 pages, 5 figures; Nano Lett., Just Accepted Manuscrip
Tailoring excitonic states of van der Waals bilayers through stacking configuration, band alignment and valley-spin
Excitons in monolayer semiconductors have large optical transition dipole for
strong coupling with light field. Interlayer excitons in heterobilayers, with
layer separation of electron and hole components, feature large electric dipole
that enables strong coupling with electric field and exciton-exciton
interaction, at the cost that the optical dipole is substantially quenched (by
several orders of magnitude). In this letter, we demonstrate the ability to
create a new class of excitons in transition metal dichalcogenide (TMD) hetero-
and homo-bilayers that combines the advantages of monolayer- and
interlayer-excitons, i.e. featuring both large optical dipole and large
electric dipole. These excitons consist of an electron that is well confined in
an individual layer, and a hole that is well extended in both layers, realized
here through the carrier-species specific layer-hybridization controlled
through the interplay of rotational, translational, band offset, and
valley-spin degrees of freedom. We observe different species of such
layer-hybridized valley excitons in different heterobilayer and homobilayer
systems, which can be utilized for realizing strongly interacting
excitonic/polaritonic gases, as well as optical quantum coherent controls of
bidirectional interlayer carrier transfer either with upper conversion or down
conversion in energy
Point Defects and Localized Excitons in 2D WSe2
Identifying the point defects in 2D materials is important for many
applications. Recent studies have proposed that W vacancies are the predominant
point defect in 2D WSe2, in contrast to theoretical studies, which predict that
chalcogen vacancies are the most likely intrinsic point defects in transition
metal dichalcogenide semiconductors. We show using first principles
calculations, scanning tunneling microscopy (STM) and scanning transmission
electron microscopy experiments, that W vacancies are not present in our
CVD-grown 2D WSe2. We predict that O-passivated Se vacancies (O_Se) and O
interstitials (Oins) are present in 2D WSe2, because of facile O2 dissociation
at Se vacancies, or due to the presence of WO3 precursors in CVD growth. These
defects give STM images in good agreement with experiment. The optical
properties of point defects in 2D WSe2 are important because single photon
emission (SPE) from 2D WSe2 has been observed experimentally. While strain
gradients funnel the exciton in real space, point defects are necessary for the
localization of the exciton at length scales that enable photons to be emitted
one at a time. Using state-of-the-art GW-Bethe-Salpeter-equation calculations,
we predict that only Oins defects give localized excitons within the energy
range of SPE in previous experiments, making them a likely source of previously
observed SPE. No other point defects (O_Se, Se vacancies, W vacancies and Se_W
antisites) give localized excitons in the same energy range. Our predictions
suggest ways to realize SPE in related 2D materials and point experimentalists
toward other energy ranges for SPE in 2D WSe2
G band Raman double resonance in twisted bilayer graphene: an evidence of band splitting and folding
The stacking faults (deviates from Bernal) will break the translational
symmetry of multilayer graphenes and modify their electronic and optical
behaviors to the extent depending on the interlayer coupling strength. This
paper addresses the stacking-induced band splitting and folding effect on the
electronic band structure of twisted bilayer graphene. Based on the
first-principles density functional theory study, we predict that the band
folding effect of graphene layers may enable the G band Raman double resonance
in the visible excitation range. Such prediction is confirmed experimentally
with our Raman observation that the resonant energies of the resonant G mode
are strongly dependent on the stacking geometry of graphene layers.Comment: 16 pages, 4 figures, Accepted by Phys. Rev.
Atomically Thin Resonant Tunnel Diodes built from Synthetic van der Waals Heterostructures
Vertical integration of two-dimensional van der Waals materials is predicted
to lead to novel electronic and optical properties not found in the constituent
layers. Here, we present the direct synthesis of two unique, atomically thin,
multi-junction heterostructures by combining graphene with the monolayer
transition-metal dichalocogenides: MoS2, MoSe2, and WSe2.The realization of
MoS2-WSe2-Graphene and WSe2-MoSe2-Graphene heterostructures leads toresonant
tunneling in an atomically thin stack with spectrally narrow room temperature
negative differential resistance characteristics
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