Stable Monolayer Transition Metal Dichalcogenide Ordered Alloys with Tunable Electronic Properties

From first-principles calculations, we discover highly stable monolayer transition metal dichalcogenides (TMDs) ternary alloys consisting of group 5 and 7 transition metal elements. We show for Nb_1–<i>x</i>Re_<i>x</i>S₂, Ta_1–<i>x</i>Re_<i>x</i>S₂ and their selenide counterparts that the 1H ordered alloy structures for <i>x</i> ≤ 0.5 are thermodynamically stable, with formation energies an order of magnitude lower than currently known TMD alloys such as Mo_<i>x</i>W_1–<i>x</i>S₂, so that they could potentially be synthesizable using chemical vapor deposition or exfoliation techniques. This class of TMD alloys offer a wide tunable bandgap range of ∼1 eV, displaying metallic to semiconducting behavior versus alloy composition. Importantly, at <i>x</i> = 0.5, the alloys are valence isoelectronic with MoS₂. These stoichiometric compounds, Nb_0.5Re_0.5S₂, Ta_0.5Re_0.5S₂, and their selenide counterparts, exhibit band features similar to MoS₂, but possess significantly smaller bandgaps (∼1 to 1.2 eV). As a result, compared to MoS₂ and WS₂, this class of alloy TMDs display enhanced absorbance in the visible range of the solar spectrum where the solar spectral intensity is the strongest. These ordered monolayer TMD alloys could open doors for designing ultrathin solar absorbers with improved performance

Large Thermoelectricity via Variable Range Hopping in Chemical Vapor Deposition Grown Single-Layer MoS₂

Ultrathin layers of semiconducting molybdenum disulfide (MoS₂) offer significant prospects in future electronic and optoelectronic applications. Although an increasing number of experiments bring light into the electronic transport properties of these crystals, their thermoelectric properties are much less known. In particular, thermoelectricity in chemical vapor deposition grown MoS₂, which is more practical for wafer-scale applications, still remains unexplored. Here, for the first time, we investigate these properties in grown single layer MoS₂. Microfabricated heaters and thermometers are used to measure both electrical conductivity and thermopower. Large values of up to ∼30 mV/K at room temperature are observed, which are much larger than those observed in other two-dimensional crystals and bulk MoS₂. The thermopower is strongly dependent on temperature and applied gate voltage with a large enhancement at the vicinity of the conduction band edge. We also show that the Seebeck coefficient follows <i>S</i> ∼ <i>T</i>^1/3, suggesting a two-dimensional variable range hopping mechanism in the system, which is consistent with electrical transport measurements. Our results help to understand the physics behind the electrical and thermal transports in MoS₂ and the high thermopower value is of interest to future thermoelectronic research and application

Evolution of Electronic Structure in Atomically Thin Sheets of WS₂ and WSe₂

Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion in mono- to few-layer thickness regime. Molybdenum disulfide (MoS₂) was recently found to exhibit indirect-to-direct gap transition when the thickness is reduced to a single monolayer. Emerging photoluminescence (PL) from monolayer MoS₂ opens up opportunities for a range of novel optoelectronic applications of the material. Here we report differential reflectance and PL spectra of mono- to few-layer WS₂ and WSe₂ that indicate that the band structure of these materials undergoes similar indirect-to-direct gap transition when thinned to a single monolayer. The transition is evidenced by distinctly enhanced PL peak centered at 630 and 750 nm in monolayer WS₂ and WSe₂, respectively. Few-layer flakes are found to exhibit comparatively strong indirect gap emission along with direct gap hot electron emission, suggesting high quality of synthetic crystals prepared by a chemical vapor transport method. Fine absorption and emission features and their thickness dependence suggest a strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe₂

Origin of Indirect Optical Transitions in Few-Layer MoS₂, WS₂, and WSe₂

It has been well-established that single layer MX₂ (M = Mo, W and X = S, Se) are direct gap semiconductors with band edges coinciding at the K point in contrast to their indirect gap multilayer counterparts. In few-layer MX₂, there are two valleys along the Γ–K line with similar energy. There is little understanding on which of the two valleys forms the conduction band minimum (CBM) in this thickness regime. We investigate the conduction band valley structure in few-layer MX₂ by examining the temperature-dependent shift of indirect exciton photoluminescence peak. Highly anisotropic thermal expansion of the lattice and the corresponding evolution of the band structure result in a distinct peak shift for indirect transitions involving the K and Λ (midpoint along Γ-K) valleys. We identify the origin of the indirect emission and concurrently determine the relative energy of these valleys

Efficient Carrier-to-Exciton Conversion in Field Emission Tunnel Diodes Based on MIS-Type van der Waals Heterostack

We report on efficient carrier-to-exciton conversion and planar electroluminescence from tunnel diodes based on a metal–insulator–semiconductor (MIS) van der Waals heterostack consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS₂). These devices exhibit excitonic electroluminescence with extremely low threshold current density of a few pA·μm^–2, which is several orders of magnitude lower compared to the previously reported values for the best planar EL devices. Using a reference dye, we estimate the EL quantum efficiency to be ∼1% at low current density limit, which is of the same order of magnitude as photoluminescence quantum yield at the equivalent excitation rate. Our observations reveal that the efficiency of our devices is not limited by carrier-to-exciton conversion efficiency but by the inherent exciton-to-photon yield of the material. The device characteristics indicate that the light emission is triggered by injection of hot minority carriers (holes) to n-doped WS₂ by Fowler–Nordheim tunneling and that hBN serves as an efficient hole-transport and electron-blocking layer. Our findings offer insight into the intelligent design of van der Waals heterostructures and avenues for realizing efficient excitonic devices

Dynamic Structural Evolution of Metal–Metal Bonding Network in Monolayer WS₂

Layered transition metal dichalcogenides (TMDs) exist in a range of crystal phases with distinct electronic character. Some crystal phases are known to exhibit unique in-plane anisotropy characterized by a periodic distortion of the lattice and a formation of metal–metal bonding network. Here, we report in situ observation of dynamic structural evolution in the one-dimensional zigzag chains of single layer WS₂ induced by electron beam irradiation. Metastable zigzag chains of tungsten atoms are found to undergo reorganization of metal–metal bonds, resulting in emergence of tetramer clusters and zigzag chains with a new orientation. Our first-principles calculations reveal a small (∼0.1 eV per formula unit) activation energy barrier for monolayer WS₂ zigzag chain reorientation and a metastable transition state in the form of tetramer clusters. We further show that local tetramer clusters can be induced and stabilized by local electronic charging effects. The formation of local tetramer clusters indicate that this dynamic structural evolution is not a charge density wave phenomenon; we find instead that these lattice changes are a response to electronic instabilities that weaken the W–S bonds in the zigzag phase. Our findings shed light on the origin of structural instabilities and phases in two-dimensional materials, and constitute a step further toward their potential uses in phase change applications

Conducting MoS₂ Nanosheets as Catalysts for Hydrogen Evolution Reaction

We report chemically exfoliated MoS₂ nanosheets with a very high concentration of metallic 1T phase using a solvent free intercalation method. After removing the excess of negative charges from the surface of the nanosheets, highly conducting 1T phase MoS₂ nanosheets exhibit excellent catalytic activity toward the evolution of hydrogen with a notably low Tafel slope of 40 mV/dec. By partially oxidizing MoS₂, we found that the activity of 2H MoS₂ is significantly reduced after oxidation, consistent with edge oxidation. On the other hand, 1T MoS₂ remains unaffected after oxidation, suggesting that edges of the nanosheets are not the main active sites. The importance of electrical conductivity of the two phases on the hydrogen evolution reaction activity has been further confirmed by using carbon nanotubes to increase the conductivity of 2H MoS₂

Evidence for Fast Interlayer Energy Transfer in MoSe₂/WS₂ Heterostructures

Strongly bound excitons confined in two-dimensional (2D) semiconductors are dipoles with a perfect in-plane orientation. In a vertical stack of semiconducting 2D crystals, such in-plane excitonic dipoles are expected to efficiently couple across van der Waals gap due to strong interlayer Coulomb interaction and exchange their energy. However, previous studies on heterobilayers of group 6 transition metal dichalcogenides (TMDs) found that the exciton decay dynamics is dominated by interlayer charge transfer (CT) processes. Here, we report an experimental observation of fast interlayer energy transfer (ET) in MoSe₂/WS₂ heterostructures using photoluminescence excitation (PLE) spectroscopy. The temperature dependence of the transfer rates suggests that the ET is Förster-type involving excitons in the WS₂ layer resonantly exciting higher-order excitons in the MoSe₂ layer. The estimated ET time of the order of 1 ps is among the fastest compared to those reported for other nanostructure hybrid systems such as carbon nanotube bundles. Efficient ET in these systems offers prospects for optical amplification and energy harvesting through intelligent layer engineering

Transport Properties of Monolayer MoS₂ Grown by Chemical Vapor Deposition

Recent success in the growth of monolayer MoS₂ via chemical vapor deposition (CVD) has opened up prospects for the implementation of these materials into thin film electronic and optoelectronic devices. Here, we investigate the electronic transport properties of individual crystallites of high quality CVD-grown monolayer MoS₂. The devices show low temperature mobilities up to 500 cm² V^–1 s^–1 and a clear signature of metallic conduction at high doping densities. These characteristics are comparable to the electronic properties of the best mechanically exfoliated monolayers in literature, verifying the high electronic quality of the CVD-grown materials. We analyze the different scattering mechanisms and show that the short-range scattering plays a dominant role in the highly conducting regime at low temperatures. Additionally, the influence of optical phonons as a limiting factor is discussed

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 (WSe₂), 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 WSe₂ 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