The adsorption and desorption of electrolyte ions strongly modulates the carrier density or carrier type on the surface of monolayer-MoS2 catalyst during the hydrogen evolution reaction (HER). The ...buildup of electrolyte ions onto the surface of monolayer MoS2 during the HER may also result in the formation of excitons and trions, similar to those observed in gate-controlled field-effect transistor devices. Using the distinct carrier relaxation dynamics of excitons and trions of monolayer MoS2 as sensitive descriptors, an in situ microcell-based scanning time-resolved liquid cell microscope is set up to simultaneously measure the bias-dependent exciton/trion dynamics and spatially map the catalytic activity of monolayer MoS2 during the HER. This operando probing technique used to monitor the interplay between exciton/trion dynamics and electrocatalytic activity for two-dimensional transition metal dichalcogenides provides an excellent platform to investigate the local carrier behaviors at the atomic layer/liquid electrolyte interfaces during electrocatalytic reaction.
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.
Palladium diselenide (PdSe
), a peculiar noble metal dichalcogenide, has emerged as a new two-dimensional material with high predicted carrier mobility and a widely tunable band gap for device ...applications. The inherent in-plane anisotropy endowed by the pentagonal structure further renders PdSe
promising for novel electronic, photonic, and thermoelectric applications. However, the direct synthesis of few-layer PdSe
is still challenging and rarely reported. Here, we demonstrate that few-layer, single-crystal PdSe
flakes can be synthesized at a relatively low growth temperature (300 °C) on sapphire substrates using low-pressure chemical vapor deposition (CVD). The well-defined rectangular domain shape and precisely determined layer number of the CVD-grown PdSe
enable us to investigate their layer-dependent and in-plane anisotropic properties. The experimentally determined layer-dependent band gap shrinkage combined with first-principle calculations suggest that the interlayer interaction is weaker in few-layer PdSe
in comparison with that in bulk crystals. Field-effect transistors based on the CVD-grown PdSe
also show performances comparable to those based on exfoliated samples. The low-temperature synthesis method reported here provides a feasible approach to fabricate high-quality few-layer PdSe
for device applications.
Abstract
2D ferroelectric material has emerged as an attractive building block for high‐density data storage nanodevices. Although monolayer van der Waals ferroelectrics have been theoretically ...predicted, a key experimental breakthrough for such calculations is still not realized. Here, hexagonally stacking α‐In
2
Se
3
nanoflake, a rarely studied van der Waals polymorph, is reported to exhibit out‐of‐plane (OOP) and in‐plane (IP) ferroelectricity at room temperature. Ferroelectric multidomain states in a hexagonal α‐In
2
Se
3
nanoflake with uniform thickness can survive to 6 nm. Most strikingly, the electric‐field‐induced polarization switching and hysteresis loop are, respectively, observed down to the bilayer and monolayer (≈1.2 nm) thicknesses, which designates it as the thinnest layered ferroelectric and verifies the corresponding theoretical calculation. In addition, two types of ferroelectric nanodevices employing the OOP and IP polarizations in 2H α‐In
2
Se
3
are developed, which are applicable for nonvolatile memories and heterostructure‐based nanoelectronics/optoelectronics.
The dominant defect types in chemical-vapor-deposited (CVD) tungsten disulfide (WS2) monolayers (ML) were regulated through mask-assisted scattered-oxygen-ion (O+) implantation. A shadow mask allowed ...for two distinctive implantation regions: directly bombarded and mask-shaded. Upon direct implantation, photoluminescence (PL) was universally suppressed, whereas in the mask-shaded region, PL was enhanced by up to 500% at low doses before suppression at doses of >3 × 1013 ions/cm2. We verified that the introduction of scattered O+ ions and low-density structural atomic defects are the two prerequisites for PL enhancement by replacing O+ ions with C+ ions and eliminating the involvement of physisorbed gases or laser treatment. Density functional theory calculations were carried out, suggesting a possible mechanism for the vacancy-induced dangling bonds in WS2. Sulfur- or tungsten-related vacancies create in-gap deep trap states, hindering electron–hole recombination. Scattered-oxygen treatment passivates these sulfur vacancies by effectively eliminating these in-gap nonradiative pathways. In addition, it further increases the transition probability by creating more tungsten-dominated states near the conduction band edge through charge transfer. This work demonstrates a facile and successful single-step method to passivate sulfur vacancies with scattered oxygen ions. It has the potential to heal the PL quenching that originated from the intrinsic high defect density in a CVD-grown WS2 ML.
Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ...ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.
Electronic junctions on edge
Two-dimensional materials such as graphene are attractive materials for making smaller transistors because they are inherently nanoscale and can carry high currents. ...However, graphene has no band gap and the transistors are “leaky”; that is, they are hard to turn off. Related transition metal dichalcogenides (TMDCs) such as molybdenum sulfide have band gaps. Transistors based on these materials can have high ratios of “on” to “off” currents. However, it is often difficult to make a good voltage-biased (p-n) junction between different TMDC materials. Li
et al.
succeeded in making p-n heterojunctions between two of these materials, molybdenum sulfide and tungsten selenide. They did this not by stacking the layers, which make a weak junction, but by growing molybdenum sulfide on the edge of a triangle of tungsten selenide with an atomically sharp boundary
Science
, this issue p.
524
The regrowth of the second transition metal dichalcogenide monolayer by edge epitaxy creates a lateral p-n heterojunction.
Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS
2
and tungsten sulfide WSe
2
have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe
2
-MoS
2
heterojunction, where the edge of WSe
2
induces the epitaxial MoS
2
growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.
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