Shengxue Yang

Photoresponsive and Gas Sensing Field-Effect Transistors based on Multilayer WS2 Nanoflakes

The photoelectrical properties of multilayer WS2 nanoflakes including field-effect, photosensitive and gas sensing are comprehensively and systematically studied. The transistors perform an n-type behavior with electron mobility of 12 cm2/Vs and exhibit high photosensitive characteristics with response time (τ) of <20 ms, photo-responsivity (Rλ) of 5.7 A/W and external quantum efficiency (EQE) of 1118%. In addition, charge transfer can appear between the multilayer WS2 nanoflakes and the physical-adsorbed gas molecules, greatly influencing the photoelectrical properties of our devices. The ethanol and NH3 molecules can serve as electron donors to enhance the Rλ and EQE significantly. Under the NH3 atmosphere, the maximum Rλ and EQE can even reach 884 A/W and 1.7 × 105%, respectively. This work demonstrates that multilayer WS2 nanoflakes possess important potential for applications in field-effect transistors, highly sensitive photodetectors, and gas sensors, and it will open new way to develop two-dimensional (2D) WS2-based optoelectronics.

Self-Driven Photodetector and Ambipolar Transistor in Atomically Thin GaTe-MoS2 p–n vdW Heterostructure

Heterostructure engineering of atomically thin two-dimensional materials offers an exciting opportunity to fabricate atomically sharp interfaces for highly tunable electronic and optoelectronic devices. Here, we demonstrate abrupt interface between two completely dissimilar material systems, i.e, GaTe-MoS2 p-n heterojunction transistors, where the resulting device possesses unique electronic properties and self-driven photoelectric characteristics. Fabricated heterostructure transistors exhibit forward biased rectifying behavior where the transport is ambipolar with both electron and hole carriers contributing to the overall transport. Under illumination, photoexcited electron-hole pairs are readily separated by large built-in potential formed at the GaTe-MoS2 interface efficiently generating self-driven photocurrent within <10 ms. Overall results suggest that abrupt interfaces between vastly different material systems with different crystal symmetries still allow efficient charge transfer mechanisms at the interface and are attractive for photoswitch, photodetector, and photovoltaic applications because of large built-in potential at the interface.

High-Performance Few-layer Mo-doped ReSe2 Nanosheet Photodetectors

Transition metal dichalcogenides (TMDCs) have recently been the focus of extensive research activity owing to their fascinating physical properties. As a new member of TMDCs, Mo doped ReSe2 (Mo:ReSe2) is an octahedral structure semiconductor being optically biaxial and highly anisotropic, different from most of hexagonal layered TMDCs with optically uniaxial and relatively high crystal symmetry. We investigated the effects of physisorption of gas molecule on the few-layer Mo:ReSe2 nanosheet based photodetectors. We compared the photoresponse of the as-exfoliated device with annealed device both in air or ammonia (NH3) environment. After annealing at sub-decomposition temperatures, the Mo:ReSe2 photodetectors show a better photoresponsivity (~55.5 A/W) and higher EQE (10893%) in NH3 than in air. By theoretical investigation, we conclude that the physisorption of NH3 molecule on Mo:ReSe2 monolayer can cause the charge transfer between NH3 molecule and Mo:ReSe2 monolayer, increasing the n-type carrier density of Mo:ReSe2 monolayer. The prompt photoswitching, high photoresponsivity and different sensitivity to surrounding environment from the few-layer anisotropic Mo:ReSe2 can be used to design multifunctional optoelectronic and sensing devices.

Synthesis of MoSe2 flower-like nanostructures and their photo-responsive properties

Novel MoSe2 flower-like nanostructures were produced by a facile hydrothermal method. Devices containing such nanostructures were also fabricated and reveal obvious photo-responsive characteristics. SEM and HRTEM images show that the as-prepared products have a flower-like structure, and the diameter of a single particle is about 500 nm. The average atomic ratio between Se and Mo is 2.68 according to the results of the EDS measurements. The photo-responsive characteristics responding to red illumination of a device with MoSe2 flower-like nanostructures were investigated for the first time and a mechanism for the photoresponse was proposed. Our findings reveal that such nanostructures have excellent electric conductivity and new photo-responsive properties.

Crystal orientation dependence of the dielectric properties for epitaxial BaZr0.15Ti0.85O3 thin films

Epitaxial Ba0.15Zr0.85TiO3 (BZT) ferroelectric thin films with (001), (011), and (111) orientations were, respectively, grown on La0.67Sr0.33MnO3 (LSMO) buffered LaAlO3 substrates by pulsed laser deposition method. The dc electric-field dependence of permittivity and dielectric loss of (001)-, (011)-, and (111)-oriented BZT/LSMO heterostructures obeys the Johnson formula, and the ac electric-field dependence of that obeys the Rayleigh law under the subswitching field region. The anisotropic dielectric properties are attributed to the higher mobility of the charge carriers, the concentration of mobile interfacial domain walls, and boundaries in the (111)-oriental films than in the (110)- and (100)-oriented films.

Synthesis and Transport Properties of Large-Scale Alloy Co0.16Mo0.84S2 Bilayer Nanosheets

Synthesis of large-scale highly crystalline two-dimensional alloys is significant for revealing properties. Here, we have investigated the vapor growth process of high-quality bilayer CoxMo1-xS2 (x = 0.16) hexagonal nanosheets systematically. As the initial loading of the sulfur increases, the morphology of the CoxMo1-xS2 (0 < x ≤ 1) nanosheets becomes hexagons from David stars step by step at 680 °C. We find that Co atoms mainly distribute at the edge of nanosheets. When the temperature increases from 680 to 750 °C, high-quality cubic pyrite-type crystal structure CoS2 grows on the surface of CoxMo1-xS2 nanosheet gradually and forms hexagonal film induced by the nanosheet. Electrical transport measurements reveal that the CoxMo1-xS2 nanosheets and CoS2 films exhibit n-type semiconducting transport behavior and half-metallic behavior, respectively. Theoretical calculations of their band structures agree well with the experimental results.

Synthesis of WO3 nanostructures and their ultraviolet photoresponse properties

Tungsten oxide (WO3) nanostructures such as nanowires, nanorod bundles and nanotube bundles are synthesized by a facile hydrothermal method. The ultraviolet (UV) photoresponse characteristics of devices containing these WO3 nanostructures are investigated for the first time and new photosensitive mechanisms involving both photo-generated electron–hole pairs and reversible electrochemical reactions are proposed. We find that h-WO3 nanowires with large specific surface areas and fewer defects exhibit excellent UV photoresponse properties with switch ratios (defined as Iphoto/Idark) as high as 60, which is due to the existing large tunnels serving as channels and intercalation sites for mobile ions and active electrochemical reactions, and our findings provide a new family and more selectivity for UV photosensitive nanomaterials in the future.

Highly efficient gas molecule-tunable few-layer GaSe phototransistors

Herein we present a systematic study on the effects of different gas molecules on the photoelectric response of few-layer GaSe phototransistors before and after introducing defects. After introducing defects by thermal annealing, the phototransistors become largely photo-responsive (18.75 A W−1) with high external quantum efficiency (EQE) (∼91.53%), high photocurrent on–off ratio, fast photo-response, and good stability in an O2 rich environment when illuminated by 254 nm ultraviolet light.

Abnormal photocurrent response and enhanced photocatalytic activity induced by charge transfer between WS(2) nanosheets and WO(3) nanoparticles.

Sun trap: Pure WS2 nanosheets are prepared that exhibit excellent photosensitive properties. After functionalization with WO3 nanoparticles, abnormal photocurrent responses, enhanced photocatalytic activity, and induced photoluminescence is observed.

Gate-tunable diode-like current rectification and ambipolar transport in multilayer van der Waals ReSe2/WS2 p–n heterojunctions

Vertically stacked van der Waals (vdW) heterojunctions of two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a great deal of attention due to their fascinating properties. In this work, we report two important gate-tunable phenomena in new artificial vdW p-n heterojunctions created by vertically stacking p-type multilayer ReSe2 and n-type multilayer WS2: (1) well-defined strong gate-tunable diode-like current rectification across the p-n interface is observed, and the tunability of the electronic processes is attributed to the tunneling-assisted interlayer recombination induced by majority carriers across the vdW interface; (2) the distinct ambipolar behavior under gate voltage modulation both at forward and reverse bias voltages is found in the vdW ReSe2/WS2 heterojunction transistors and a corresponding transport model is proposed for the tunable polarity behaviors. The findings may provide some new opportunities for building nanoscale electronic and optoelectronic devices.