Tunneling Photocurrent Assisted by Interlayer Excitons in Staggered van der Waals Hetero‐Bilayers

Vertically stacked van der Waals (vdW) heterostructures have been suggested as a robust platform for studying interfacial phenomena and related electric/optoelectronic devices. While the interlayer Coulomb interaction mediated by the vdW coupling has been extensively studied for carrier recombination processes in a diode transport, its correlation with the interlayer tunneling transport has not been elucidated. Here, a contrast is reported between tunneling and drift photocurrents tailored by the interlayer coupling strength in MoSe₂/MoS₂ hetero‐bilayers (HBs). The interfacial coupling modulated by thermal annealing is identified by the interlayer phonon coupling in Raman spectra and the emerging interlayer exciton peak in photoluminescence spectra. In strongly coupled HBs, positive photocurrents are observed owing to the inelastic band‐to‐band tunneling assisted by interlayer excitons that prevail over exciton recombinations. By contrast, weakly coupled HBs exhibit a negative photovoltaic diode behavior, manifested as a drift current without interlayer excitonic emissions. This study sheds light on tailoring the tunneling transport for numerous optoelectronic HB devices.     

Light‐Emitting Transition Metal Dichalcogenide Monolayers under Cellular Digestion

2D materials cover a wide spectrum of electronic properties. Their applications are extended from electronic, optical, and chemical to biological. In terms of biomedical uses of 2D materials, the interactions between living cells and 2D materials are of paramount importance. However, biointerfacial studies are still in their infancy. This work studies how living organisms interact with transition metal dichalcogenide monolayers. For the first time, cellular digestion of tungsten disulfide (WS₂) monolayers is observed. After digestion, cells intake WS₂ and become fluorescent. In addition, these light‐emitting cells are not only viable, but also able to pass fluorescent signals to their progeny cells after cell division. By combining synthesis of 2D materials and a cell culturing technique, a procedure for monitoring the interactions between WS₂ monolayers and cells is developed. These observations open up new avenues for developing novel cellular labeling and imaging approaches, thus triggering further studies on interactions between 2D materials and living organisms.     

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.     

Ultrahigh‐Sensitive Broadband Photodetectors Based on Dielectric Shielded MoTe2/Graphene/SnS2 p–g–n Junctions

2D atomic sheets of transition metal dichalcogenides (TMDs) have a tremendous potential for next‐generation optoelectronics since they can be stacked layer‐by‐layer to form van der Waals (vdW) heterostructures. This allows not only bypassing difficulties in heteroepitaxy of lattice‐mismatched semiconductors of desired functionalities but also providing a scheme to design new optoelectronics that can surpass the fundamental limitations on their conventional semiconductor counterparts. Herein, a novel 2D h‐BN/p‐MoTe₂/graphene/n‐SnS₂/h‐BN p–g–n junction, fabricated by a layer‐by‐layer dry transfer, demonstrates high‐sensitivity, broadband photodetection at room temperature. The combination of the MoTe₂ and SnS₂ of complementary bandgaps, and the graphene interlayer provides a unique vdW heterostructure with a vertical built‐in electric field for high‐efficiency broadband light absorption, exciton dissociation, and carrier transfer. The graphene interlayer plays a critical role in enhancing sensitivity and broadening the spectral range. An optimized device containing 5?7‐layer graphene has been achieved and shows an extraordinary responsivity exceeding 2600 A W^?1 with fast photoresponse and specific detectivity up to ≈10¹³ Jones in the ultraviolet–visible–near‐infrared spectrum. This result suggests that the vdW p–g–n junctions containing multiple photoactive TMDs can provide a viable approach toward future ultrahigh‐sensitivity and broadband photonic detectors.     

Van der Waals Epitaxial Growth of 2D Metallic Vanadium Diselenide Single Crystals and their Extra‐High Electrical Conductivity

2D metallic transition‐metal dichalcogenides (MTMDs) have recently emerged as a new class of materials for the engineering of novel electronic phases, 2D superconductors, magnets, as well as novel electronic applications. However, the mechanical exfoliation route is predominantly used to obtain such metallic 2D flakes, but the batch production remains challenging. Herein, the van der Waals epitaxial growth of monocrystalline, 1T‐phase, few‐layer metallic VSe₂ nanosheets on an atomically flat mica substrate via a “one‐step” chemical vapor deposition method is reported. The thickness of the VSe₂ nanosheets is precisely tuned from several nanometers to several tenths of nanometers. More significantly, the 2D VSe₂ single crystals are found to present an excellent metallic feature, as evidenced by the extra‐high electrical conductivity of up to 10⁶ S m^?1, 1–4 orders of magnitude higher than that of various conductive 2D materials. The thickness‐dependent charge‐density‐wave phase transitions are also examined through low‐temperature transport measurements, which reveal that the synthesized 2D metallic 1T‐VSe₂ nanosheets should serve as good research platforms for the detecting novel many‐body states. These results open a new path for the synthesis and property investigations of nanoscale‐thickness 2D MTMDs crystals.     

Cross‐Plane Carrier Transport in Van der Waals Layered Materials

The mechanisms of carrier transport in the cross‐plane crystal orientation of transition metal dichalcogenides are examined. The study of in‐plane electronic properties of these van der Waals compounds has been the main research focus in recent years. However, the distinctive physical anisotropies, short‐channel physics, and tunability of cross layer interactions can make the study of their electronic properties along the out‐of‐plane crystal orientation valuable. Here, the out‐of‐plane carrier transport mechanisms in niobium diselenide and hafnium disulfide are explored as two broadly different representative materials. Temperature‐dependent current–voltage measurements are preformed to examine the mechanisms involved. First principles simulations and a tunneling model are used to understand these results and quantify the barrier height and hopping distance properties. Using Raman spectroscopy, the thermal response of the chemical bonds is directly explored and the insight into the van der Waals gap properties is acquired. These results indicate that the distinct cross‐plane carrier transport characteristics of the two materials are a result of material thermal properties and thermally mediated transport of carriers through the van der Waals gaps. Exploring the cross‐plane electron transport, the exciting physics involved is unraveled and potential new avenues for the electronic applications of van der Waals layers are inspired.     

In‐Plane Anisotropic Thermal Conductivity of Few‐Layered Transition Metal Dichalcogenide Td‐WTe2

2D Td‐WTe₂ has attracted increasing attention due to its promising applications in spintronic, field‐effect chiral, and high‐efficiency thermoelectric devices. It is known that thermal conductivity plays a crucial role in condensed matter devices, especially in 2D systems where phonons, electrons, and magnons are highly confined and coupled. This work reports the first experimental evidence of in‐plane anisotropic thermal conductivities in suspended Td‐WTe₂ samples of different thicknesses, and is also the first demonstration of such anisotropy in 2D transition metal dichalcogenides. The results reveal an obvious anisotropy in the thermal conductivities between the zigzag and armchair axes. The theoretical calculation implies that the in‐plane anisotropy is attributed to the different mean free paths along the two orientations. As thickness decreases, the phonon‐boundary scattering increases faster along the armchair direction, resulting in stronger anisotropy. The findings here are crucial for developing efficient thermal management schemes when engineering thermal‐related applications of a 2D system.     

PtTe2‐Based Type‐II Dirac Semimetal and Its van der Waals Heterostructure for Sensitive Room Temperature Terahertz Photodetection

Recent years have witnessed rapid progresses made in the photoelectric performance of two‐dimensional materials represented by graphene, black phosphorus, and transition metal dichalcogenides. Despite significant efforts, a photodetection technique capable for longer wavelength, higher working temperature as well as fast responsivity, is still facing huge challenges due to a lack of best among bandgap, dark current, and absorption ability. Exploring topological materials with nontrivial band transport leads to peculiar properties of quantized phenomena such as chiral anomaly, and magnetic‐optical effect, which enables a novel feasibility for an advanced optoelectronic device working at longer wavelength. In this work, the direct generation of photocurrent at low energy terahertz (THz) band at room temperature is implemented in a planar metal–PtTe₂–metal structure. The results show that the THz photodetector based on PtTe₂ with bow‐tie‐type planar contacts possesses a high photoresponsivity (1.6 A W^?1 without bias voltage) with a response time less than 20 µs, while the PtTe₂–graphene heterostructure‐based detector can reach responsivity above 1.4 kV W^?1 and a response time shorter than 9 µs. Remarkably, it is already exploitable for large area imaging applications. These results suggest that topological semimetals such as PtTe₂ can be ideal materials for implementation in a high‐performing photodetection system at THz band.     

Atomic‐Level Customization of 4 in. Transition Metal Dichalcogenide Multilayer Alloys for Industrial Applications

Despite many encouraging properties of transition metal dichalcogenides (TMDs), a central challenge in the realm of industrial applications based on TMD materials is to connect the large‐scale synthesis and reproducible production of highly crystalline TMD materials. Here, the primary aim is to resolve simultaneously the two inversely related issues through the synthesis of MoS_{2(1?x )}Se_2x ternary alloys with customizable bichalcogen atomic (S and Se) ratio via atomic‐level substitution combined with a solution‐based large‐area compatible approach. The relative concentration of bichalcogen atoms in the 2D alloy can be effectively modulated by altering the selenization temperature, resulting in 4 in. scale production of MoS_1.62Se_0.38, MoS_1.37Se_0.63, MoS_1.15Se_0.85, and MoS_0.46Se_1.54 alloys, as well as MoS₂ and MoSe₂. Comprehensive spectroscopic evaluations for vertical and lateral homogeneity in terms of heteroatom distribution in the large‐scale 2D TMD alloys are implemented. Se‐stimulated strain effects and a detailed mechanism for the Se substitution in the MoS₂ crystal are further explored. Finally, the capability of the 2D alloy for industrial application in nanophotonic devices and hydrogen evolution reaction (HER) catalysts is validated. Substantial enhancements in the optoelectronic and HER performances of the 2D ternary alloy compared with those of its binary counterparts, including pure‐phase MoS₂ and MoSe₂, are unambiguously achieved.     

Van der Waals Epitaxial Growth of Atomic Layered HfS2 Crystals for Ultrasensitive Near‐Infrared Phototransistors

As a member of the group IVB transition metal dichalcogenides (TMDs) family, hafnium disulfide (HfS₂) is recently predicted to exhibit higher carrier mobility and higher tunneling current density than group VIB (Mo and W) TMDs. However, the synthesis of high‐quality HfS₂ crystals, sparsely reported, has greatly hindered the development of this new field. Here, a facile strategy for controlled synthesis of high‐quality atomic layered HfS₂ crystals by van der Waals epitaxy is reported. Density functional theory calculations are applied to elucidate the systematic epitaxial growth process of the S‐edge and Hf‐edge. Impressively, the HfS₂ back‐gate field‐effect transistors display a competitive mobility of 7.6 cm² V^?1 s^?1 and an ultrahigh on/off ratio exceeding 10⁸. Meanwhile, ultrasensitive near‐infrared phototransistors based on the HfS₂ crystals (indirect bandgap ≈1.45 eV) exhibit an ultrahigh responsivity exceeding 3.08 × 10⁵ A W^?1, which is 10⁹‐fold higher than 9 × 10^?5 A W^?1 obtained from the multilayer MoS₂ in near‐infrared photodetection. Moreover, an ultrahigh photogain exceeding 4.72 × 10⁵ and an ultrahigh detectivity exceeding 4.01 × 10¹² Jones, superior to the vast majority of the reported 2D‐materials‐based phototransistors, imply a great promise in TMD‐based 2D electronic and optoelectronic applications.     

Homoepitaxial Growth of Large‐Scale Highly Organized Transition Metal Dichalcogenide Patterns

Controllable growth of highly crystalline transition metal dichalcogenide (TMD) patterns with regular morphology and unique edge structure is highly desired and important for fundamental research and potential applications. Here, single‐crystalline MoS₂ flakes are reported with regular trigonal symmetric patterns that can be homoepitaxially grown on MoS₂ monolayer via chemical vapor deposition. The highly organized MoS₂ patterns are rhombohedral (3R)‐stacked with the underlying MoS₂ monolayer, and their boundaries are predominantly terminated by zigzag Mo edge structure. The epitaxial MoS₂ crystals can be tailored from compact triangles to fractal flakes, and the pattern formation can be explained by the anisotropic growth rates of the S and Mo edges under low sulfur chemical potential. The 3R‐stacked MoS₂ pattern demonstrates strong second and third‐harmonic‐generation signals, which exceed those reported for monolayer MoS₂ by a factor of 6 and 4, correspondingly. This homoepitaxial growth approach for making highly organized TMD patterns is also demonstrated for WS₂.     

Large‐Area Atomic Layers of the Charge‐Density‐Wave Conductor TiSe2

Layered transition metal (Ti, Ta, Nb, etc.) dichalcogenides are important prototypes for the study of the collective charge density wave (CDW). Reducing the system dimensionality is expected to lead to novel properties, as exemplified by the discovery of enhanced CDW order in ultrathin TiSe₂. However, the syntheses of monolayer and large‐area 2D CDW conductors can currently only be achieved by molecular beam epitaxy under ultrahigh vacuum. This study reports the growth of monolayer crystals and up to 5 × 10⁵ µm² large films of the typical 2D CDW conductor—TiSe₂—by ambient‐pressure chemical vapor deposition. Atomic resolution scanning transmission electron microscopy indicates the as‐grown samples are highly crystalline 1T‐phase TiSe₂. Variable‐temperature Raman spectroscopy shows a CDW phase transition temperature of 212.5 K in few layer TiSe₂, indicative of high crystal quality. This work not only allows the exploration of many‐body state of TiSe₂ in 2D limit but also offers the possibility of utilizing large‐area TiSe₂ in ultrathin electronic devices.     

Preparation of High‐Percentage 1T‐Phase Transition Metal Dichalcogenide Nanodots for Electrochemical Hydrogen Evolution

Nanostructured transition metal dichalcogenides (TMDs) are proven to be efficient and robust earth‐abundant electrocatalysts to potentially replace precious platinum‐based catalysts for the hydrogen evolution reaction (HER). However, the catalytic efficiency of reported TMD catalysts is still limited by their low‐density active sites, low conductivity, and/or uncleaned surface. Herein, a general and facile method is reported for high‐yield, large‐scale production of water‐dispersed, ultrasmall‐sized, high‐percentage 1T‐phase, single‐layer TMD nanodots with high‐density active edge sites and clean surface, including MoS₂, WS₂, MoSe₂, Mo_0.5W_0.5S₂, and MoSSe, which exhibit much enhanced electrochemical HER performances as compared to their corresponding nanosheets. Impressively, the obtained MoSSe nanodots achieve a low overpotential of ?140 mV at current density of 10 mA cm^?2, a Tafel slope of 40 mV dec^?1, and excellent long‐term durability. The experimental and theoretical results suggest that the excellent catalytic activity of MoSSe nanodots is attributed to the high‐density active edge sites, high‐percentage metallic 1T phase, alloying effect and basal‐plane Se‐vacancy. This work provides a universal and effective way toward the synthesis of TMD nanostructures with abundant active sites for electrocatalysis, which can also be used for other applications such as batteries, sensors, and bioimaging.     

A General Method for the Chemical Synthesis of Large‐Scale,Seamless Transition Metal Dichalcogenide Electronics

The capability to directly build atomically thin transition metal dichalcogenide (TMD) devices by chemical synthesis offers important opportunities to achieve large‐scale electronics and optoelectronics with seamless interfaces. Here, a general approach for the chemical synthesis of a variety of TMD (e.g., MoS₂, WS₂, and MoSe₂) device arrays over large areas is reported. During chemical vapor deposition, semiconducting TMD channels and metallic TMD/carbon nanotube (CNT) hybrid electrodes are simultaneously formed on CNT‐patterned substrate, and then coalesce into seamless devices. Chemically synthesized TMD devices exhibit attractive electrical and mechanical properties. It is demonstrated that chemically synthesized MoS₂–MoS₂/CNT devices have Ohmic contacts between MoS₂/CNT hybrid electrodes and MoS₂ channels. In addition, MoS₂–MoS₂/CNT devices show greatly enhanced mechanical stability and photoresponsivity compared with conventional gold‐contacted devices, which makes them suitable for flexible optoelectronics. Accordingly, a highly flexible pixel array based on chemically synthesized MoS₂–MoS₂/CNT photodetectors is applied for image sensing.