Three-dimensional hierarchical MoS_2/CoS_2 heterostructure arrays for highly efficient electrocatalytic hydrogen evolution

Developing non-expensive, highly active and highly stable electrocatalysts for hydrogen evolution has aroused extensive attention, owing to the necessity of novel clean and sustainable energy carriers. In this paper, we report a synthesis of free-standing three-dimensional hierarchical MoS_2/CoS_2 heterostructure arrays through a convenient process. The investigation of electrocatalytic HER performance suggests that the MoS_2/CoS_2 hybrid catalyst exhibits significant enhancement in HER(onsetpotential and potential at a current density of 100 mA cm^(-2) are 20 mV and125 mV, respectively) and superior durability(no shift of current density is observed after a continuous scanning of 3000 times) compared with individual CoS_2 and MoS_2. The superior HER performance was attributed to the formation of the interface between CoS_2 and MoS_2 through the electrochemical characterization, Raman, XPS analysis, and the control experiment. The lower onsetpotential, higher current density, excellent durability, and the free-standing structure of the three-dimensional hierarchical MoS_2/CoS_2 heterostructure array make it a promising cathode catalyst suitable for widespread application.

采用PECVD方法制作SiO2绝缘层的AlGaN/GaNMOS-HFET器件

为了进一步减小栅漏电,提高击穿电压,将MOS结构的优点引入ALGaN/GaNHEMT器件中,研制并分析了新型的基于AlGaN/GaN的MOS-HFET结构。采用等离子增强气相化学沉积(PECVD)的方法生长了50nm的SiO2作为栅绝缘层,新型的AlGaN/GaNMOS-HFET器件栅长1μm,栅宽80μm,测得最大饱和输出电流为784mA/mm,最大跨导为44.25ms/mm,最高栅偏压+6V。

2D hierarchical yolk-shell heterostructures as advanced host-interlayer integrated electrode for enhanced Li-S batteries

Lithium sulfur(Li-S)batteries hold great promising for high-energy-density batteries,but appear rapid capacity fading due to the lack of overall and elaborated design of both sulfur host and interlayer.Herein,we developed a novel two-dimensional(2D)hierarchical yolk-shell heterostructure,constructed by a graphene yolk,2D void and outer shell of vertically aligned carbon-mediated MoS2 nanosheets(G@void@MoS2/C),as advanced host-interlayer integrated electrode for Li-S batteries.Notably,the 2D void,with a typical thickness of^80 nm,provided suitable space for loading and confining nano sulfur,and vertically aligned ultrathin MoS2 nanosheets guaranteed enriched catalytically active sites to effectively promote the transition of soluble polysulfides.The conductive graphene yolk and carbon mediated shell sufficiently accelerated electron transport.Therefore,the integrated electrode of G@void@MoS2/C not only exceptionally confined the sulfur/polysulfides in 2D yolk-shell heterostructures,but also achieved catalytic transition of the residual polysulfides dissolved in electrolyte to solid Li2S2/Li2S,both of which synergistically achieved an extremely low capacity fading rate of 0.05%per cycle over 1000 times at 2C,outperforming most reported Mo based cathodes and interlayers for Li-S batteries.2D hierarchical yolkshell heterostructures developed here may shed new insight on elaborated design of integrated electrodes for Li-S batteries.

Abnormal phenomenon of source-drain current of AlGaN/GaN heterostructure device under UV/visible light irradiation

We report an abnormal phenomenon that the source-drain current(I_(D))of AlGaN/GaN heterostructure devices decreases under visible light irradiation.When the incident light wavelength is 390 nm,the photon energy is less than the band gaps of GaN and AlGaN whereas it can causes an increase of ID.Based on the UV light irradiation,a decrease of I_(D) can still be observed when turning on the visible light.We speculate that this abnormal phenomenon is related to the surface barrier height,the unionized donor-like surface states below the surface Fermi level and the ionized donor-like surface states above the surface Fermi level.For visible light,its photon energy is less than the surface barrier height of the AlGaN layer.The electrons bound in the donor-like surface states below the Fermi level are excited and trapped by the ionized donor-like surface states between the Fermi level and the conduction band of AlGaN.The electrons trapped in ionized donor-like surface states show a long relaxation time,and the newly ionized donor-like surface states below the surface Fermi level are filled with electrons from the two-dimensional electron gas(2DEG)channel at AlGaN/GaN interface,which causes the decrease of ID.For the UV light,when its photon energy is larger than the surface barrier height of the AlGaN layer,electrons in the donor-like surface states below the Fermi level are excited to the conduction band and then drift into the 2DEG channel quickly,which cause the increase of ID.

不同衬底上层状MoS2薄膜制备及应用的研究进展

二维层状MoS2薄膜具有超高的光响应度、高导电特性、良好的光学透明度以及优异的机械性能,是制造多功能和高性能光电探测器、传感器等最理想的半导体材料之一。在不同衬底上制备的MoS2薄膜性质有所差异,其构成的异质结性能也各具特色。首先,介绍了常用于制备层状MoS2薄膜的化学气相沉积(CVD)法和高温热分解法;然后,综述了在Si、塑料、GaN、GaAs、Si纳米线、蓝宝石、SiO2/Si和SiC等不同衬底上制备层状MoS2薄膜的方法,利用原子力显微镜、X射线衍射、拉曼光谱、X射线光电子能谱等测试方法对各衬底上制备的MoS2薄膜结构和性能进行了表征;同时讨论了相应的异质结器件的特性及应用,并对高质量MoS2薄膜在光电探测器、气体传感器、压电器件等光电子和纳电子器件中的应用进行了展望。

Tunable Syngas Synthesis from Photocatalytic CO2 Reduction Under Visible-Light Irradiation by Interfacial Engineering

Visible-light-driven CO2 photoreduction to achieve renewable materials,such as syngas,hydrocarbons,and alcohols,is a key process that could relieve environmental problems and the energy crisis simultaneously.Reduction of syngas products with diff erent H2:CO proportions is highly expected to produce high value-added chemicals in the industry.However,the development of technologies employing long-wavelength irradiation to achieve CO2 photoreduction and simultaneous tuning of the resultant H2:CO proportion remains a challenging endeavor.In this work,we carried out interfacial engineering by designing a series of heterostructured layered double-hydroxide/MoS2 nanocomposites via electrostatic self-assembly.The syngas proportion(H 2:CO)obtained from CO2 photoreduction could be modulated from 1:1 to 9:1 by visible-light irradiation(λ>400 nm)under the control of the interface-rich heterostructures.This work provides a cost-eff ective strategy for solar-tofuel conversion in an artificial photosynthetic system and describes a novel route to produce syngas with targeted proportions.

Optical properties and applications for MoS_2-Sb_2Te_3-MoS_2 heterostructure materials

Two-dimensional(2D) materials with potential applications in photonic and optoelectronic devices have attracted increasing attention due to their unique structures and captivating properties. However, generation of stable high-energy ultrashort pulses requires further boosting of these materials' optical properties, such as higher damage threshold and larger modulation depth. Here we investigate a new type of heterostructure material with uniformity by employing the magnetron sputtering technique. Heterostructure materials are synthesized with van der Waals heterostructures consisting of MoS_2 and Sb_2Te_3. The bandgap, carrier mobility, and carrier concentration of the MoS_2-Sb_2Te_3-MoS_2 heterostructure materials are calculated theoretically. By using these materials as saturable absorbers(SAs), applications in fiber lasers with Q-switching and mode-locking states are demonstrated experimentally. The modulation depth and damage threshold of SAs are measured to be 64.17%and 14.13 J∕cm^2, respectively. Both theoretical and experimental results indicate that MoS_2-Sb_2Te_3-MoS_2 heterostructure materials have large modulation depth, and can resist high power during the generation of ultrashort pulses. The MoS_2-Sb_2Te_3-MoS_2 heterostructure materials have the advantages of low cost, high reliability, and suitability for mass production, and provide a promising solution for the development of 2D-material-based devices with desirable electronic and optoelectronic properties.

Effect of defects on the electronic structure of a PbI2/MoS2 van der Waals heterostructure: A first-principles study

PbI2/MoS2,as a typical van der Waals(vdW)heterostructure,has attracted intensive attention owing to its remarkable electronic and optoelectronic properties.In this work,the effect of defects on the electronic structures of a PbI2/MoS2 heterointerface has been systematically investigated.The manner in which the defects modulate the band structure of PbI2/MoS2,including the band gap,band edge,band alignment,and defect energy-level density within the band gap is discussed herein.It is shown that sulfur defects tune the band gaps,iodine defects shift the positions of the band edge and Fermi level,and lead defects realize the conversions between the straddling-gap band alignment and valence-band-aligned gap,thus enhancing the light-absorption ability of the material.

Edge reconstruction of layer-dependentβ-In2Se3/MoS2 vertical heterostructures for accelerated hydrogen evolution

The layer-dependent properties are still unclarified in two-dimensional(2D)vertical heterostructures.In this study,we layer-bylayer deposited semimetalβ-In2Se3 on monolayer MoS2 to form verticalβ-In2Se3/MoS2 heterostructures by chemical vapor deposition.The defect-mediated nucleation mechanism inducesβ-In2Se3 nanosheets to grow on monolayer MoS2,and the layer number of stackedβ-In2Se3 can be precisely regulated from 1 layer(L)to 13 L by prolonging the growth time.Theβ-In2Se3/MoS2 heterostructures reveal tunable type-Ⅱband alignment arrangement by altering the layer number ofβ-In2Se3,which optimizes the internal electron transfer.Meanwhile,the edge atomic structure ofβ-In2Se3 stacking on monolayer MoS2 shows the reconstruction derived from large lattice mismatch(~29%),and the presence ofβ-In2Se3 also further increases the electrical conductivity ofβ-In2Se3/MoS2 heterostructures.Attributed to abundant layer-dependent edge active sites,edge reconstruction,improved hydrophilicity,and high electrical conductivity ofβ-In2Se3/MoS2 heterostructures,the edge ofβ-In2Se3/MoS2 heterostructures exhibits excellent electrocatalytic hydrogen evolution performance.Lower onset potential and smaller Tafel slope can be observed at the edge of monolayer MoS2 coupled with 13-Lβ-In2Se3.Hence,the outstanding conductive layers coupled with edge reconstruction in 2D vertical heterostructures play decisive roles in the optimization of electron energy levels and improvement of layer-dependent catalytic performance.

Ultrahigh-temperature ferromagnetism in MoS_(2) Moirésuperlattice/graphene hybrid heterostructures

Realizing high-temperature ferromagnetism in two-dimensional(2D)semiconductor nanosheets is significant for their applications in next-generation magnetic and electronic nanodevices.Herein,this goal could be achieved on a MoS_(2) Moirésuperlattice grown on the reduced graphene oxide(RGO)substrate by a hydrothermal approach.The as-synthesized bilayer MoS_(2) superlattice structure with rotating angle(ϕ=13°±1°)of two hexagonal MoS_(2) lattices,possesses outstanding ferromagnetic property and an ultra-high Curie temperature of 990 K.The X-ray absorption near-edge structure and ultraviolet photoelectron spectroscopies combined with density functional theory calculation indicate that the covalent interactions between MoS_(2) Moirésuperlattice and RGO substrate lead to the formation of interfacial Mo-S-C bonds and complete spin polarization of Mo 4d electrons near the Fermi level.This design could be generalized and may open up a possibility for tailoring the magnetism of other 2D materials.

Insights into the photocatalytic mechanism of the C4N/MoS2 heterostructure: A first-principle study

Constructing heterostructures by combining COFs and TMD is a new strategy to design efficient photocatalysts for CO2 reduction reaction(CO2RR) due to their good stability,tunable band gaps and efficient charge separation.Based on the synthesis of completely novel C4N-COF in our previous re ported work,a new C4N/MoS2 heterostructure was constructed and then the related structural,electronic and optical properties were also studied using first principle calculations.The interlayer coupling effect and charge transfer between the C4N and MoS2 layer are systematically illuminated.The reduced band gap of the C4N/MoS2 heterostructure is beneficial to absorb more visible light.For the formation of type-Ⅱ band alignment,a built-in electric field appears which separates the photogene rated electrons and holes into different layers efficiently and produces redox active sites.The band alignment of the heterostructure ensures its photocatalytic activities of the whole CO2 reduction reaction.Furthermore,the charge density difference and charge carrier mobility confirm the existence of the built-in electric field at the interface of the C4N/MoS2 heterostructure directly.Finally,the high optical absorption indicates it is an efficient visible light harvesting photocatalyst.Therefore,this wo rk could provide strong insights into the internal mechanism and high photocatalytic activity of the C4N/MoS2 heterostructure and offer guiding of designing and synthesizing COF/TMD heterostructure photocatalysts.

Electrical characteristics of AlInN/GaN HEMTs under cryogenic operation

Electrical properties of an AIlnN/GaN high-electron mobility transistor （HEMT） on a sapphire substrate are investigated in a cryogenic temperature range from 295 K down to 50 K. It is shown that drain saturation current and conductance increase as transistor operation temperature decreases. A self-heating effect is observed over the entire range of temperature under high power consumption. The dependence of channel electron mobility on electron density is investigated in detail. It is found that aside from Coulomb scattering, electrons that have been pushed away from the AIInN/GaN interface into the bulk GaN substrate at a large reverse gate voltage are also responsible for the electron mobility drop with the decrease of electron density.

Enabling novel device functions with black phosphorus/MoS2 van der Waals heterostructures

Van der Waals heterostructure, which consists of various two- dimensional （2D） layered materials stacked along the direction perpendicular to their 2D plane, has emerged as a promising material system for device applications in recent years .

Influence of annealed ohmic contact metals on electron mobility of strained AlGaN/GaN heterostructures

The influence of annealed ohmic contact metals on the electron mobility of a two dimensional electron gas （2DEG） is investigated on ungated AlGaN/GaN heterostructures and AlGaN/GaN heterostructure field effect transistors （AlGaN/GaN HFETs）. Current-voltage （I-V） characteristics for ungated AlGaN/GaN heterostructures and capacitance-voltage （C-V） characteristics for AlGaN/GaN HFETs are obtained, and the electron mobility for the ungated AlGaN/GaN heterostructure is calculated. It is found that the electron mobility of the 2DEG for the ungated AlGaN/GaN heterostructure is decreased by more than 50% compared with the electron mobility of Hall measurements. We propose that defects are introduced into the AlGaN barrier layer and the strain of the AlGaN barrier layer is changed during the annealing process of the source and drain, causing the decrease in the electron mobility.

The mobility of two-dimensional electron gas in AlGaN/GaN heterostructures with varied Al content

The mobility of the two-dimensional electron gas （2DEG） in AIGaN/GaN hetero-structures changes significantly with AI content in the AIGaN barrier layer, while few mechanism analyses focus on it. Theoretical calculation and analysis of the 2DEG mobility in AIGaN/GaN heterostructures with varied AI content are carried out based on the recently reported experimental data. The 2DEG mobility is modeled analytically as the total effects of the scattering mechanisms including acoustic deformation-potential, piezoelectric, polar optic phonon, alloy disorder, interface roughness, dislocation and remote modulation doping scattering. We show that the increase of the 2DEG density, caused by the ascension of the AI content in the barrier layer, is a dominant factor that leads to the changes of the individual scat- tering processes. The change of the 2DEG mobility with AI content are mainly determined by the interface roughness scattering and the alloy disorder scattering at 77 K, and the polar optic phonon scattering and the interface roughness scattering at the room temperature. The calculated function of the interface roughness pa- rameters on the AI content shows that the stress caused AIGaN/GaN interface degradation at higher AI content is an important factor in the limitation of the in- terface roughness scattering on the 2DEG mobility in AIGaN/GaN heterostructures with high AI content.

MoS2-wrapped silicon nanowires for photoelectro- chemical water reduction

Integration of molybdenum disulfide （MoS2） onto high surface area photocathod is highly desired to minimize the overpotential for the solar-powered hydrogen evolution reaction （HER）. Semiconductor nanowires （NWs） are beneficial use in photoelectrochemistry because of their large electrochemically availab surface area and inherent ability to decouple light absorption and the transpo of minority carriers. Here, silicon （Si） NW arrays were employed as a mod photocathode system for MoS2 wrapping, and their solar-driven HER activil was evaluated. The photocathode is made up of a well-defined MoSJTiO2/Si coaxial NW heterostructure, which yielded photocurrent density up to 15 mA/cm2 （at 0 V vs. the reversible hydrogen electrode （RHE）） with goo stability under the operating conditions employed. This work reveals the earth-abundant electrocatalysts coupled with high surface area NW electrod~ can provide performance comparable to noble metal catalysts for photocathod hydrogen evolution.

Fast response speed of mechanically exfoliated MoS2 modified by PbS in detecting NO2

MoS2,acting as a promising gas sensing material,has shown huge potential in monitoring of toxic and harmful gases at room temperature.However,MoS2-based gas sensors still suffer from poor gas sensing performance such as poor sensitivity,long response time.Constructing the hete ro structure is an effective approach to improve gas-sensing performance of MoS2.Herein,PbS@MoS2 composites synthesized by mechanical exfoliation combining with wet-chemical precipitation are used to investigate its performance in detecting NO2 at room temperature.The response value of PbS@MoS2 gas sensor against NO2 is significantly improved compared with the pure MoS2 gas sensor.At the same time,the modification with PbS also accelerates the response speed of MoS2,and the response time is almost reduced by two orders of magnitude,from hundreds of seconds to less than ten seconds.The enhanced response value and fast response time are mainly benefited from the modulation effect of NO2 to PbS@MoS2 heterostructure and the mechanically exfoliated MoS2 surface with few defects.This work can be expected to provide useful guidance for designing composite materials with excellent gas sensing properties.

Synthesis, properties, and applications of large-scale two-dimensional materials by polymer-assisted deposition

Two-dimensional(2D) materials have attracted considerable attention because of their novel and tunable electronic,optical, ferromagnetic, and chemical properties. Compared to mechanical exfoliation and chemical vapor deposition, polymer-assisted deposition(PAD) is more suitable for mass production of 2D materials owing to its good reproducibility and reliability. In this review, we summarize the recent development of PAD on syntheses of 2D materials. First, we introduce principles and processing steps of PAD. Second, 2D materials, including graphene, MoS2, and MoS2/glassy-graphene heterostructures, are presented to illustrate the power of PAD and provide readers with the opportunity to assess the method. Last, we discuss the future prospects and challenges in this research field. This review provides a novel technique for preparing 2D layered materials and may inspire new applications of 2D layered materials.

Vapor transport growth of MoS2 nucleated on SiO2 patterns and graphene flakes

Vapor transport growth of atomically thin MoS2 layers on patterned substrates is investigated, as it is a step towards the self-aligned growth and formation of heterojunctions, which could be useful in future applications. Enhanced formation of MoS2 flakes at the pattern edges is observed on both the substrates examined, namely, patterned thermal SiO2 on Si（100） and graphene flakes on SiO2. The diffusion driven growth leads to the formation of MoS2 monolayers （MLs） with sizes of tens of micrometers around the edges of SiO2 patterns. The growth mode and the optical quality of the MoS2 flakes can be controlled by varying the substrate temperature. Besides the lateral growth, 3R-type pyramids are obtained on prolonging the growth. Lateral MoS2-graphene heterostructures are obtained by using graphene flakes on SiO2 as a substrate.

Boosting reaction kinetics and reversibility in Mott-Schottky VS/MoS heterojunctions for enhanced lithium storage

Heterostructures have lately been recognized as a viable implement to achieve high-energy Li-ion batteries(LIBs) because the as-formed built-in electric field can greatly accelerate the charge transfer kinetics. Herein, we have constructed the Mott-Schottky heterostructured VS2/MoS2 hybrids with tailorable 1T/2H phase based on their matchable formation energy, which are made of metallic and few-layered VS2 vertically grown on MoS2 surface. The density functional theory(DFT) calculations unveil that such heterojunctions drive the rearrangement of energy band with a facilitated reaction kinetics and enhance the Li adsorption energy more than twice compared to the MoS2 surface. Furthermore, the VS2 catalytically expedites the Li–S bond fracture and meantime the enriched Mo6+ enables the sulfur anchoring toward the oriented reaction with Li+to form Li2S, synergistically enhancing the reversibility of electrochemical redox. Consequently, the as-obtained VS2/MoS2 hybrids deliver a very large specific capacity of 1273 m Ah g^-1 at 0.1 A g^-1 with 61% retention even at 5 A g^-1. It can also stabilize 100 cycles at 0.5 A g^-1 and 500 cycles at 1 A g^-1. The findings provide in-depth insights into engineering heterojunctions towards the enhancement of reaction kinetics and reversibility for LIBs.