Probing Anisotropic Thermal Conductivity of Transition Metal Dichalcogenides MX2 (M = Mo,W and X = S,Se) using Time‐Domain Thermoreflectance

Transition metal dichalcogenides (TMDs) are a group of layered 2D semiconductors that have shown many intriguing electrical and optical properties. However, the thermal transport properties in TMDs are not well understood due to the challenges in characterizing anisotropic thermal conductivity. Here, a variable‐spot‐size time‐domain thermoreflectance approach is developed to simultaneously measure both the in‐plane and the through‐plane thermal conductivity of four kinds of layered TMDs (MoS₂, WS₂, MoSe₂, and WSe₂) over a wide temperature range, 80–300 K. Interestingly, it is found that both the through‐plane thermal conductivity and the Al/TMD interface conductance depend on the modulation frequency of the pump beam for all these four compounds. The frequency‐dependent thermal properties are attributed to the nonequilibrium thermal resistance between the different groups of phonons in the substrate. A two‐channel thermal model is used to analyze the nonequilibrium phonon transport and to derive the intrinsic thermal conductivity at the thermal equilibrium limit. The measurements of the thermal conductivities of bulk TMDs serve as an important benchmark for understanding the thermal conductivity of single‐ and few‐layer TMDs.     

Quaternary 2D Transition Metal Dichalcogenides (TMDs) with Tunable Bandgap

Alloying/doping in 2D material is important due to wide range bandgap tunability. Increasing the number of components would increase the degree of freedom which can provide more flexibility in tuning the bandgap and also reduces the growth temperature. Here, synthesis of quaternary alloys Mo_x W_1?x S_2y Se_{2(1?y )} is reported using chemical vapor deposition. The composition of alloys is tuned by changing the growth temperatures. As a result, the bandgap can be tuned which varies from 1.61 to 1.85 eV. The detailed theoretical calculation supports the experimental observation and shows a possibility of wide tunability of bandgap.     

Anisotropic MoS2 Nanosheets Grown on Self‐Organized Nanopatterned Substrates

Manipulating the anisotropy in 2D nanosheets is a promising way to tune or trigger functional properties at the nanoscale. Here, a novel approach is presented to introduce a one‐directional anisotropy in MoS₂ nanosheets via chemical vapor deposition (CVD) onto rippled patterns prepared on ion‐sputtered SiO₂/Si substrates. The optoelectronic properties of MoS₂ are dramatically affected by the rippled MoS₂ morphology both at the macro‐ and the nanoscale. In particular, strongly anisotropic phonon modes are observed depending on the polarization orientation with respect to the ripple axis. Moreover, the rippled morphology induces localization of strain and charge doping at the nanoscale, thus causing substantial redshifts of the phonon mode frequencies and a topography‐dependent modulation of the MoS₂ workfunction, respectively. This study paves the way to a controllable tuning of the anisotropy via substrate pattern engineering in CVD‐grown 2D nanosheets.     

Flexible Nonvolatile Transistor Memory with Solution‐Processed Transition Metal Dichalcogenides

Nonvolatile field‐effect transistor (FET) memories containing transition metal dichalcogenide (TMD) nanosheets have been recently developed with great interest by utilizing some of the intriguing photoelectronic properties of TMDs. The TMD nanosheets are, however, employed as semiconducting channels in most of the memories, and only a few works address their function as floating gates. Here, a floating‐gate organic‐FET memory with an all‐in‐one floating‐gate/tunneling layer of the solution‐processed TMD nanosheets is demonstrated. Molybdenum disulfide (MoS₂) is efficiently liquid‐exfoliated by amine‐terminated polystyrene with a controlled amount of MoS₂ nanosheets in an all‐in‐one floating‐gate/tunneling layer, allowing for systematic investigation of concentration‐dependent charge‐trapping and detrapping properties of MoS₂ nanosheets. At an optimized condition, the nonvolatile memory exhibits memory performances with an ON/OFF ratio greater than 10⁴, a program/erase endurance cycle over 400 times, and data retention longer than 7 × 10³ s. All‐in‐one floating‐gate/tunneling layers containing molybdenum diselenide and tungsten disulfide are also developed. Furthermore, a mechanically‐flexible TMD memory on a plastic substrate shows a performance comparable with that on a hard substrate, and the memory properties are rarely altered after outer‐bending events over 500 times at the bending radius of 4.0 mm.     

Composition and Interface Engineering of Alloyed MoS2xSe2(1–x) Nanotubes for Enhanced Hydrogen Evolution Reaction Activity

Hierarchical MoS_2xSe_2(1?x) nanotubes assembled from several‐layered nanosheets featuring tunable chalcogen compositions, expanded interlayer spacing and carbon modification, are synthesized for enhanced electrocatalytic hydrogen evolution reaction (HER). The chalcogen compositions of the MoS_2xSe_2(1?x) nanotubes are controllable by adjusting the selenization temperature and duration while the expanded (002) interlayer spacing varies from 0.98 to 0.68 nm. It is found that the MoS_2xSe_2(1?x) (x = 0.54) nanotubes with expanded interlayer spacing of 0.98 nm exhibit the highest electrocatalytic HER activity with a low onset potential of 101 mV and a Tafel slope of 55 mV dec^?1. The improved electrocatalytic performance is attributed to the chalcogen composition tuning and the interlayer distance expansion to achieve benefitting hydrogen adsorption energy. The present work suggests a potential way to design advanced HER electrocatalysts through modulating their compositions and interlayer distances.     

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High‐Mobility MoS2 Field‐Effect Transistors

2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS₂ devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS₂ film and a Ag electrode as an interfacial layer. The MoS₂ field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm² V^?1 s^?1, an on/off current ratio of 4 × 10⁸, and a photoresponsivity of 2160 A W^?1, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS₂ and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS₂ and graphene is a key step toward the practical application of atomically thin TMDC‐based devices with low‐resistance contacts for high‐performance large‐area electronics and optoelectronics.     

Hybridizing Plasmonic Materials with 2D‐Transition Metal Dichalcogenides toward Functional Applications

Recently, 2D transition metal dichalcogenides (TMDs) have become intriguing materials in the versatile field of photonics and optoelectronics because of their strong light–matter interaction that stems from the atomic layer thickness, broadband optical response, controllable optoelectronic properties, and high nonlinearity, as well as compatibility. Nevertheless, the low optical cross‐section of 2D‐TMDs inhibits the light–matter interaction, resulting in lower quantum yield. Therefore, hybridizing the 2D‐TMDs with plasmonic nanomaterials has become one of the promising strategies to boost the optical absorption of thin 2D‐TMDs. The appeal of plasmonics is based on their capability to localize and enhance the electromagnetic field and increase the optical path length of light by scattering and injecting hot electrons to TMDs. In this regard, recent achievements with respect to hybridization of the plasmonic effect in 2D‐TMDs systems and its augmented optical and optoelectronic properties are reviewed. The phenomenon of plasmon‐enhanced interaction in 2D‐TMDs is briefly described and state‐of‐the‐art hybrid device applications are comprehensively discussed. Finally, an outlook on future applications of these hybrid devices is provided.     

One‐Step Solution Phase Growth of Transition Metal Dichalcogenide Thin Films Directly on Solid Substrates

Ultrathin transition metal dichalcogenides (TMDs) have exotic electronic properties. With success in easy synthesis of high quality TMD thin films, the potential applications will become more viable in electronics, optics, energy storage, and catalysis. Synthesis of TMD thin films has been mostly performed in vacuum or by thermolysis. So far, there is no solution phase synthesis to produce large‐area thin films directly on target substrates. Here, this paper reports a one‐step quick synthesis (within 45–90 s) of TMD thin films (MoS₂, WS₂, MoSe₂, WSe₂, etc.) on solid substrates by using microwave irradiation on a precursor‐containing electrolyte solution. The numbers of the quintuple layers of the TMD thin films are precisely controllable by varying the precursor's concentration in the electrolyte solution. A photodetector made of MoS₂ thin film comprising of small size grains shows near‐IR absorption, supported by the first principle calculation, exhibits a high photoresponsivity (>300 mA W^?1) and a fast response (124 µs). This study paves a robust way for the synthesis of various TMD thin films in solution phases.     

Photoluminescence Architectures for Disease Diagnosis: From Graphene to Thin‐Layer Transition Metal Dichalcogenides and Oxides

Ever since the discovery of graphene, increasing efforts have been devoted to the use of this stellar material as well as the development of other graphene‐like materials such as thin‐layer transition metal dichalcogenides and oxides (TMD/Os) for a variety of applications. Because of their large surface area and unique optical properties, these two‐dimensional materials with a size ranging from the micro‐ to the nanoscale have been employed as the substrate to construct photoluminescence architectures for disease diagnosis as well as theranostics. These architectures are built through the simple self‐assembly of labeled biomolecular probes with the substrate material, leading to signal quenching. Upon the specific interaction of the architecture with a target biomarker, the signal can be spontaneously restored in a reversible manner. Meanwhile, by co‐loading therapeutic agents and employing the inherent photo‐thermal properties of the material substrates, a combined disease imaging and therapy (theranostics) can be achieved. This review highlights the latest advances in the construction and application of graphene and TMD/O based thin‐layer material composites for single‐target and multiplexed detection of a variety of biomarkers and theranostics. These versatile material architectures, owing to their ease in preparation, low cost and flexibility in functionalization, provide promising tools for both basic biochemical research and clinical applications.     

Type‐I Transition Metal Dichalcogenides Lateral Homojunctions: Layer Thickness and External Electric Field Effects

Transition metal dichalcogenide (TMD) heterostructures have been widely explored due to the formation of type‐II band alignment and interlayer exciton. However, the studies of type‐I TMD heterostructures are still lacking, which limit their applications in luminescence devices. Here, the 1L/nL MX₂ (n = 2, 3, 4; M = Mo, W; X = S, Se) lateral homojunction based on the layer‐dependent band gaps of TMD nanosheets is theoretically simulated. The studies show that the TMD homojunction presents with high thermal stability and type‐I band alignment. The band offset and quantum confinement of carriers can be easily tuned by controlling the thickness of the multilayer region. Moreover, the electric field can decrease the band gaps of 1L/3L and 1L/4L homojunctions linearly. Interestingly, for the 1L/2L MX₂ homojunction, the gap value is robust to the weak electric field, while it drops sharply under a strong electric field. This study sheds light on the physical pictures in the TMD lateral homojunction, and provides a practicable and general approach to engineer a type‐I homojunction based 2D semiconductor materials.     

Direct Bandgap Transition in Many‐Layer MoS2 by Plasma‐Induced Layer Decoupling

We report a robust method for engineering the optoelectronic properties of many‐layer MoS₂ using low‐energy oxygen plasma treatment. Gas phase treatment of MoS₂ with oxygen radicals generated in an upstream N₂–O₂ plasma is shown to enhance the photoluminescence (PL) of many‐layer, mechanically exfoliated MoS₂ flakes by up to 20 times, without reducing the layer thickness of the material. A blueshift in the PL spectra and narrowing of linewidth are consistent with a transition of MoS₂ from indirect to direct bandgap material. Atomic force microscopy and Raman spectra reveal that the flake thickness actually increases as a result of the plasma treatment, indicating an increase in the interlayer separation in MoS₂. Ab initio calculations reveal that the increased interlayer separation is sufficient to decouple the electronic states in individual layers, leading to a transition from an indirect to direct gap semiconductor. With optimized plasma treatment parameters, we observed enhanced PL signals for 32 out of 35 many‐layer MoS₂ flakes (2–15 layers) tested, indicating that this method is robust and scalable. Monolayer MoS₂, while direct bandgap, has a small optical density, which limits its potential use in practical devices. The results presented here provide a material with the direct bandgap of monolayer MoS₂, without reducing sample thickness, and hence optical density.