Molybdenum and tungsten chalcogenides have attracted tremendous attention in energy storage and conversion due to their outstanding physicochemical and electrochemical properties.There are intensive studies on molybde...Molybdenum and tungsten chalcogenides have attracted tremendous attention in energy storage and conversion due to their outstanding physicochemical and electrochemical properties.There are intensive studies on molybdenum and tungsten chalcogenides for energy storage and conversion,however,there is no systematic review on the applications of WS2,Mo Se2and WSe2as anode materials for lithium-ion batteries(LIBs)and sodium-ion batteries(SIBs),except Mo S2.Considering the importance of these contents,it is extremely necessary to overview the recent development of novel layered WS2,Mo Se2and WSe2beyond Mo S2in energy storage.Here,we will systematically overview the recent progress of WS2,Mo Se2and WSe2as anode materials in LIBs and SIBs.This review will also discuss the opportunities,and perspectives of these materials in the energy storage fields.展开更多
A high-performance heterojunction photodetector is formed by combining an n-type Si substrate with p-type monolayer WSe2 obtained using physical vapor deposition. The high quality of the WSe2/Si heterojunction is demo...A high-performance heterojunction photodetector is formed by combining an n-type Si substrate with p-type monolayer WSe2 obtained using physical vapor deposition. The high quality of the WSe2/Si heterojunction is demonstrated by the suppressed dark current of I nA and the extremely high rectification ratio of 107. Under illumination, the heterojunction exhibits a wide photoresponse range from ultraviolet to near-infrared radiation. The introduction of graphene quantum dots （GQDs） greatly elevates the photodetective capabilities of the heterojunction with strong light absorption and long carrier lifetimes. The GQDs/WSe2/Si heterojunction exhibits a high responsivity of -707 mA·W^-1, short response time of 0.2 ms, and good specific detectivity of -4.51×10^9 Jones. These properties suggest that the GQDs/WSe2/Si heterojunction holds great potential for application in future high- performance photodetectors.展开更多
Two-dimensional （2D） layered transition metal dichalcogenides （TMDs） have attracted enormous research interests and efforts towards the development of versatile electronic and optical devices, owing to their extra...Two-dimensional （2D） layered transition metal dichalcogenides （TMDs） have attracted enormous research interests and efforts towards the development of versatile electronic and optical devices, owing to their extraordinary and unique fundamental properties and remarkable prospects in nanoelectronic applications. Among the TMDs, tungsten diselenide （WSe2） exhibits tunable ambipolar transport characteristics and superior optical properties such as high quantum efficiency. Herein, we demonstrate significant enhancement in the device performance of WSe2 phototransistor by in situ surface functionalization with cesium carbonate （Cs2CO3）. WSe2 was found to be strongly doped with electrons after Cs2CO3 modification. The electron mobility of WSe2 increased by almost one order of magnitude after surface functionalization with 1.6-nm-thick Cs2CO3 decoration. Furthermore, the photocurrent of the WSe2-based phototransistor increased by nearly three orders of magnitude with the deposition of 1.6-nm-thick Cs2CO3. Characterizations by in situ photoelectron spectroscopy techniques confirmed the significant surface charge transfer occurring at the Cs2COB/WSe2 interface. Our findings coupled with the tunable nature of the surface transfer doping method establish WSe2 as a promising candidate for future 2D materials- based optoelectronic devices.展开更多
High-performance field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors have demonstrated great promise in post-Moore integrated circuits. However, unipolar p-type 2D semiconducto...High-performance field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors have demonstrated great promise in post-Moore integrated circuits. However, unipolar p-type 2D semiconductor transistors yet remain challenging and suffer from low saturation current density (less than 10 µA·µm^(−1)) and high contact resistance (larger than 100 kΩ·µm), mainly limited by the Schottky barrier induced by the mismatch of the work-functions and the Fermi level pinning at the metal contact interfaces. Here, we overcome these two obstacles through van der Waals (vdW) integration of high work-function metal palladium (Pd) as the contacts onto monolayer WSe2 grown by chemical vapor deposition (CVD) method. We demonstrate unipolar p-type monolayer WSe2 FETs with superior device performance: room temperature on-state current density exceeding 100 µA·µm^(−1), contact resistance of 12 kΩ·µm, on/off ratio over 107, and field-effect hole mobility of ~ 103 cm2·V^(−1)·s^(−1). Electrical transport measurements reveal that the Fermi level pinning effect is completely effectively eliminated in monolayer WSe2 with vdW Pd contacts, leading to a Schottky barrier-free Ohmic contact at the metal-semiconductor junctions. Combining the advantages of large-scale vdW contact strategy and CVD growth, our results pave the way for wafer-scale fabrication of complementary-metal-oxide-semiconductor (CMOS) logic circuits based on atomically thin 2D semiconductors.展开更多
Two-dimensional(2D)transition metal dichalcogenides(TMDs)are considered to be promising building blocks for the next generation electronic and optoelectronic devices.Various doping schemes and work function engineerin...Two-dimensional(2D)transition metal dichalcogenides(TMDs)are considered to be promising building blocks for the next generation electronic and optoelectronic devices.Various doping schemes and work function engineering techniques have been explored to overcome the intrinsic performance limits of 2D TMDs.However,a reliable and long-time air stable doping scheme is still lacking in this field.In this work,we utilize keV ion beams of H2+to irradiate layered WSe2 crystals and obtain efficient n-type doping effect for all irradiated crystals within a fluence of 1×1014 protons·cm−2(1e14).Moreover,the irradiated WSe2 remains an n-type semiconductor even after it is exposed to ambient conditions for a year.Localized ion irradiation with a focused beam can directly pattern on the sample to make high performance homogenous p-n junction diodes.Raman and photoluminescence(PL)spectra demonstrate that the WSe2 crystal lattice stays intact after irradiation within 1e14.We attribute the reliable electrondoping to the significant increase in Se vacancies after the proton irradiation,which is confirmed by our scanning transmission electron microscope(STEM)results.Our work demonstrates a reliable and long-term air stable n-type doping scheme to realize high-performance electronic TMD devices,which is also suitable for further integration with other 2D devices.展开更多
The low-temperature physical vapor deposition process of atomically thin two-dimensional transition metal dichalcogenide(2D TMD) has been gaining attention owing to the cost-effective production of diverse electrochem...The low-temperature physical vapor deposition process of atomically thin two-dimensional transition metal dichalcogenide(2D TMD) has been gaining attention owing to the cost-effective production of diverse electrochemical catalysts for hydrogen evolution reaction(HER) applications. We, herein, propose a simple route toward the cost-effective physical vapor deposition process of 2D WSe2 layered nanofilms as HER electrochemical catalysts using RF magnetron sputtering at room temperature(<27℃). By controlling the variable sputtering parameters, such as RF power and deposition time, the loading amount and electrochemical surface area(ECSA) of WSe2 films deposited on carbon paper can be carefully determined. The surface of the sputtered WSe2 films are partially oxidized, which may cause spherical-shaped particles. Regardless of the loading amount of WSe2, Tafel slopes of WSe2 electrodes in the HER test are narrowly distributed to be ~120–138 mV dec-1, which indicates the excellent reproducibility of intrinsic catalytic activity. By considering the trade-off between the loading amount and ECSA, the best HER performance is clearly observed in the 200 W-15 min sample with an overpotential of 220 mV at a current density of 10 mA cm-2. Such a simple sputtering method at low temperature can be easily expanded to other 2D TMD electrochemical catalysts, promising potentially practical electrocatalysts.展开更多
Monolayer transition metal dichalcogenides （TMDCs）, as direct bandgap semiconductors, show promise for applications in ultra-thin flexible optoelec- tronic devices. However, the optical properties and device perform...Monolayer transition metal dichalcogenides （TMDCs）, as direct bandgap semiconductors, show promise for applications in ultra-thin flexible optoelec- tronic devices. However, the optical properties and device performance are greatly affected by defects, such as vacancies, present in these materials. Vacancies exist unavoidably in mechanically exfoliated or grown by chemical vapor deposition （CVD） monolayer TMDCs; therefore, their influence on the electric and optical properties of host materials has been widely studied. Here, we report a new defect state located at 1.54 eV, which is 70 meV lower than the neutral exciton energy in as-prepared WSe2 monolayers grown by CVD. This defect state is clearly observed in photoluminescence （PL） and Raman spectra at ambient conditions. PL mapping, Rarnan mapping, and atomic force microscopy analysis indicate a solid-vapor reaction growth mechanism of the defect state formation. During a certain growth stage, nuclei with the composition of WOxSey do not fully react with the Se vapor, leading to the defect formation. This type of defects permits radiative recombination of bound neutral excitons, which can make the PL intensity as strong as the intrinsic excitation. Our findings reveal a new way to tailor the optical properties of two-dimensional TMDCs without any additional processes performed after growth.展开更多
Transition-metal dichalcogenides （TMDs） exhibit immense potential as lithium/ sodium-ion electrode materials owing to their sandwich-like layered structures. To optimize their lithium/sodium-storage performance, two...Transition-metal dichalcogenides （TMDs） exhibit immense potential as lithium/ sodium-ion electrode materials owing to their sandwich-like layered structures. To optimize their lithium/sodium-storage performance, two issues should be addressed： fundamentally understanding the chemical reaction occurring in TMD electrodes and developing novel TMDs. In this study, WSe2 hexagonal nanoplates were synthesized as lithium/sodium-ion battery （LIB/SIB） electrode materials. For LIBs, the WSe2-nanoplate electrodes achieved a stable reversible capacity and a high rate capability, as well as an ultralong cycle life of up to 1,500 cycles at 1,000 mA·g^-1. Most importantly, in situ Raman spectroscopy, ex situ X-ray diffraction （XRD）, transmission electron microscopy, and electrochemical impedance spectroscopy measurements performed during the discharge-charge process clearly verified the reversible conversion mechanism, which can be summarized as follows： WSe2 ＋ 4Li^＋ ＋ 4e^- ←→ W ＋ 2Li2Se. The WSe2 nanoplates also exhibited excellent cycling performance and a high rate capability as SIB electrodes. Ex situ XRD and Raman spectroscopy results demonstrate that WSe2 reacted with Na^＋ more easily and thoroughly than with Li^＋ and converted to Na2Se and tungsten in the Ist sodiated state. The subsequent charging reaction can be expressed as Na2Se → Se ＋ 2Na^＋＋ 2e^-, which differs from the traditional conversion mechanism for LIBs. To our knowledge, this is the first systematic exploration of the lithium/sodium-storage performance of WSe2 and the mechanism involved.展开更多
The mono layer WSe2 is in teresting and important for future application in nanoelectronics,spintronics and valleytronics devices,because it has the largest spin splitting and Ion gest valley coherence time among all ...The mono layer WSe2 is in teresting and important for future application in nanoelectronics,spintronics and valleytronics devices,because it has the largest spin splitting and Ion gest valley coherence time among all the known monolayer transition-metal dichalcogenides(TMDs).Toobtain the large-area monolayer TMDs'crystal is the first step to manu facture scalable and high-performance electronic devices.In this letter,we have successfully fabricated millimeter-sized mono layer WSe2 single crystals with very high quality,based on our improved mecha nicalexfoliation method.With such superior samples,using standard high resolution angle-resolved photoemission spectroscopy,we didcomprehe nsive electronic band structure measurements on our mono layer WSe2.The overall band features point it to be a 1.2 eV direct bandgap semico nductor.Its spin splitting of the valence band at K point is found as 460 meV,which is 30 meV less than the corresponding band splitting in its bulk counterpart.The effective hole masses of valence bands are determined as 2.344 me atГ,and 0.529 me as well as 0.532 meat K for the upper and lower branch of splitting ban ds,respectively.And screening effect from substrate is shown to substa ntially impact onthe electronic properties.Our results provide importa nt insights into band structure engineering in mono layer TMDs.Our mono layer WSe2 crystals may constitute a valuable device platform.展开更多
Two-dimensional transition metal dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact footprints. However, the high sensitivity ...Two-dimensional transition metal dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact footprints. However, the high sensitivity of atomically thin materials to the surrounding dielectric media imposes severe limitations on their practical applicability. Hence, to enable the effective integration of these materials in devices, the development of reliable encapsulation procedures that preserve their physical properties is required. Here, the excitonic photoluminescence （at room temperature and 10 K） is assessed on mechanically exfoliated WSe2 monolayer flakes encapsulated with SiOx and AlxOy layers by means of chemical and physical deposition techniques. Conformal coating on untreated and non- functionalized flakes is successfully achieved by all the techniques examined, with the exception of atomic layer deposition, for which a cluster-like oxide coating is formed. No significant compositional or strain state changes in the flakes are detected upon encapsulation, independently of the technique adopted. Remarkably, our results show that the optical emission of the flakes is strongly influenced by the stoichiometry quality of the encapsulating oxide. When the encapsulation is carried out with slightly sub-stoichiometric oxides, two remarkable phenomena are observed. First, dominant trion （charged exciton） photoluminescence is detected at room temperature, revealing a clear electrical doping of the monolayers. Second, a strong decrease in the optical emission of the monolayers is observed, and attributed to non-radiative recombination processes and/or carrier transfer from the flake to the oxide. Power- and temperature-dependent photoluminescence measurements further confirm that stoichiometric oxides obtained by physical deposition lead to a successful encapsulation, opening a promising route for the development of integrated two-dimensional devices.展开更多
The effects of biaxial strain on the electronic structure and thermoelectric properties of monolayer WSe2 have been investigated by using first-principles calculations and the semi-classical Boltzmann transport theory...The effects of biaxial strain on the electronic structure and thermoelectric properties of monolayer WSe2 have been investigated by using first-principles calculations and the semi-classical Boltzmann transport theory. The electronic band gap decreases under strain, and the band structure near the Fermi level of monolayer WSe2 is modified by the applied biaxial strain. Furthermore, the doping dependence of the thermoelectric properties of n-and p-doped monolayer WSe2 under biaxial strain is estimated. The obtained results show that the power factor of n-doped monolayer WSe2 can be increased by compressive strain while that of p-doping can be increased with tensile strain. Strain engineering thus provides a direct method to control the electronic and thermoelectric properties in these two-dimensional transition metal dichalcogenides materials.展开更多
Monolayer transition-metal dichalcogenides （TMDs） are considered to be fantastic building blocks for a wide variety of optical and optoelectronic devices such as sensors, photodetectors, and quantum emitters, owing ...Monolayer transition-metal dichalcogenides （TMDs） are considered to be fantastic building blocks for a wide variety of optical and optoelectronic devices such as sensors, photodetectors, and quantum emitters, owing to their direct band gap, transparency, and mechanical flexibility. The core element of many conventional electronic and optoelectronic devices is the p-n junction, in which the p- and n-types of the semiconductor are formed by chemical doping in different regions. Here, we report a series of optoelectronic studies on a monolayer WSe2 in-plane p-n photodetector, demonstrating a low- power dissipation by showing an ambipolar behavior with a reduced threshold voltage by a factor of two compared with the previous results on a lateral electrostatically doped WSe2 p-n junction. The fabrication of the device is based on a polycarbonates （PC） transfer technique and hence no electron-beam exposure induced damage to the monolayer WSe2 is expected. Upon optical excitation, the photodetector demonstrates a photoresponsivity of 0.12 mA.W-1 and a maximum external quantum efficiency of 0.03%. Our study provides an alternative platform for a flexible and transparent two- dimensional photodetector, from which we expect to further promote the development of next-generation optoelectronic devices.展开更多
Remote controlled soft actuators have attracted ever-increasing interest in industrial,medical,robotics,and engineering fields.Soft actuators are charming than normal tools in executing dedicate tasks due to small vol...Remote controlled soft actuators have attracted ever-increasing interest in industrial,medical,robotics,and engineering fields.Soft actuators are charming than normal tools in executing dedicate tasks due to small volume and flexible body they have.However,it remains a challenge to design soft actuator that can adapt to multi-environments under remote stimuli with promising nano materials.Herein,we have developed a kind of near-infrared laser driven soft actuators with multi locomotive modes based on WSe2 and graphene nanosheets heterojunction.Different locomotion modes are driven by photothermal effect induced deformation to adapt to different working conditions.Moreover,the specially designed gripper driven by pulsed laser can lift a heavy load which is four times of its weight.This work broadens the choice of advanced nanomaterials for photothermal conversion of soft actuators.It is promising to realize applications including photothermal therapy and complex environment detection through the combination of the intelligent robot design and optical fiber system.展开更多
基金supported by the National Natural Science Foundation of China (Grant No. 51302079)the Natural Science Foundation of Hunan Province (Grant No. 2017JJ1008)
文摘Molybdenum and tungsten chalcogenides have attracted tremendous attention in energy storage and conversion due to their outstanding physicochemical and electrochemical properties.There are intensive studies on molybdenum and tungsten chalcogenides for energy storage and conversion,however,there is no systematic review on the applications of WS2,Mo Se2and WSe2as anode materials for lithium-ion batteries(LIBs)and sodium-ion batteries(SIBs),except Mo S2.Considering the importance of these contents,it is extremely necessary to overview the recent development of novel layered WS2,Mo Se2and WSe2beyond Mo S2in energy storage.Here,we will systematically overview the recent progress of WS2,Mo Se2and WSe2as anode materials in LIBs and SIBs.This review will also discuss the opportunities,and perspectives of these materials in the energy storage fields.
文摘A high-performance heterojunction photodetector is formed by combining an n-type Si substrate with p-type monolayer WSe2 obtained using physical vapor deposition. The high quality of the WSe2/Si heterojunction is demonstrated by the suppressed dark current of I nA and the extremely high rectification ratio of 107. Under illumination, the heterojunction exhibits a wide photoresponse range from ultraviolet to near-infrared radiation. The introduction of graphene quantum dots （GQDs） greatly elevates the photodetective capabilities of the heterojunction with strong light absorption and long carrier lifetimes. The GQDs/WSe2/Si heterojunction exhibits a high responsivity of -707 mA·W^-1, short response time of 0.2 ms, and good specific detectivity of -4.51×10^9 Jones. These properties suggest that the GQDs/WSe2/Si heterojunction holds great potential for application in future high- performance photodetectors.
基金Acknowledgements W. C. acknowledges the financial support from Singapore MOE Grant R143-000-652-112, National Natural Science Foundation of China （No. 21573156） and the technical support from Centre for Advanced 2D Materials and Graphene Research Centre for the device fabrication. G. E. acknowledges Singapore National Research Foundation, Prime Minister＇s Office, Singapore, for funding the research under its Medium-sized Centre program as well as NRF Research Fellowship （No. NRF-NRFF2011-02）. G. E. also acknowledges financial support from Singapore MOE （No. MOE2015-T2-2-123）.
文摘Two-dimensional （2D） layered transition metal dichalcogenides （TMDs） have attracted enormous research interests and efforts towards the development of versatile electronic and optical devices, owing to their extraordinary and unique fundamental properties and remarkable prospects in nanoelectronic applications. Among the TMDs, tungsten diselenide （WSe2） exhibits tunable ambipolar transport characteristics and superior optical properties such as high quantum efficiency. Herein, we demonstrate significant enhancement in the device performance of WSe2 phototransistor by in situ surface functionalization with cesium carbonate （Cs2CO3）. WSe2 was found to be strongly doped with electrons after Cs2CO3 modification. The electron mobility of WSe2 increased by almost one order of magnitude after surface functionalization with 1.6-nm-thick Cs2CO3 decoration. Furthermore, the photocurrent of the WSe2-based phototransistor increased by nearly three orders of magnitude with the deposition of 1.6-nm-thick Cs2CO3. Characterizations by in situ photoelectron spectroscopy techniques confirmed the significant surface charge transfer occurring at the Cs2COB/WSe2 interface. Our findings coupled with the tunable nature of the surface transfer doping method establish WSe2 as a promising candidate for future 2D materials- based optoelectronic devices.
基金financially supported by the National Natural Science Foundation of China(No.12174444)M.Zhu acknowledges the fruitful discussion with Dr.Jinbao Jiang at National University of Defense Technology.
文摘High-performance field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors have demonstrated great promise in post-Moore integrated circuits. However, unipolar p-type 2D semiconductor transistors yet remain challenging and suffer from low saturation current density (less than 10 µA·µm^(−1)) and high contact resistance (larger than 100 kΩ·µm), mainly limited by the Schottky barrier induced by the mismatch of the work-functions and the Fermi level pinning at the metal contact interfaces. Here, we overcome these two obstacles through van der Waals (vdW) integration of high work-function metal palladium (Pd) as the contacts onto monolayer WSe2 grown by chemical vapor deposition (CVD) method. We demonstrate unipolar p-type monolayer WSe2 FETs with superior device performance: room temperature on-state current density exceeding 100 µA·µm^(−1), contact resistance of 12 kΩ·µm, on/off ratio over 107, and field-effect hole mobility of ~ 103 cm2·V^(−1)·s^(−1). Electrical transport measurements reveal that the Fermi level pinning effect is completely effectively eliminated in monolayer WSe2 with vdW Pd contacts, leading to a Schottky barrier-free Ohmic contact at the metal-semiconductor junctions. Combining the advantages of large-scale vdW contact strategy and CVD growth, our results pave the way for wafer-scale fabrication of complementary-metal-oxide-semiconductor (CMOS) logic circuits based on atomically thin 2D semiconductors.
基金The authors acknowledge financial support from NRF CRP on Oxide Electronics on Silicon Beyond Moore(NRF-CRP15-2015-01)the National Natural Science Foundation of China(Nos.U2032147,21872100,and 62004128)+2 种基金Singapore MOE Grant T2EP50220-0001,MOE AcRF Tier 1 Startup grant R-284-000-179-133the Science and Engineering Research Council of A*STAR(Agency for Science,Technology and Research)Singapore,under Grant No.A20G9b0135the Fundamental Research Foundation of Shenzhen(No.JCYJ20190808152607389).
文摘Two-dimensional(2D)transition metal dichalcogenides(TMDs)are considered to be promising building blocks for the next generation electronic and optoelectronic devices.Various doping schemes and work function engineering techniques have been explored to overcome the intrinsic performance limits of 2D TMDs.However,a reliable and long-time air stable doping scheme is still lacking in this field.In this work,we utilize keV ion beams of H2+to irradiate layered WSe2 crystals and obtain efficient n-type doping effect for all irradiated crystals within a fluence of 1×1014 protons·cm−2(1e14).Moreover,the irradiated WSe2 remains an n-type semiconductor even after it is exposed to ambient conditions for a year.Localized ion irradiation with a focused beam can directly pattern on the sample to make high performance homogenous p-n junction diodes.Raman and photoluminescence(PL)spectra demonstrate that the WSe2 crystal lattice stays intact after irradiation within 1e14.We attribute the reliable electrondoping to the significant increase in Se vacancies after the proton irradiation,which is confirmed by our scanning transmission electron microscope(STEM)results.Our work demonstrates a reliable and long-term air stable n-type doping scheme to realize high-performance electronic TMD devices,which is also suitable for further integration with other 2D devices.
基金supported by the Fundamental Research Program of the Korean Institute of Materials Science(Grant PNK6130)the National Research Foundation of Korea(NRF)grant funded by the Korea government(MSIT,Ministry of Science and ICT)(No.2017R1C1B1005076)+1 种基金financially supported by the Ministry of Trade,Industry and Energy(MOTIE)Korea Institute for Advancement of Technology(KIAT)through the National Innovation Cluster R&D program(P0006704_Development of energy saving advanced parts)。
文摘The low-temperature physical vapor deposition process of atomically thin two-dimensional transition metal dichalcogenide(2D TMD) has been gaining attention owing to the cost-effective production of diverse electrochemical catalysts for hydrogen evolution reaction(HER) applications. We, herein, propose a simple route toward the cost-effective physical vapor deposition process of 2D WSe2 layered nanofilms as HER electrochemical catalysts using RF magnetron sputtering at room temperature(<27℃). By controlling the variable sputtering parameters, such as RF power and deposition time, the loading amount and electrochemical surface area(ECSA) of WSe2 films deposited on carbon paper can be carefully determined. The surface of the sputtered WSe2 films are partially oxidized, which may cause spherical-shaped particles. Regardless of the loading amount of WSe2, Tafel slopes of WSe2 electrodes in the HER test are narrowly distributed to be ~120–138 mV dec-1, which indicates the excellent reproducibility of intrinsic catalytic activity. By considering the trade-off between the loading amount and ECSA, the best HER performance is clearly observed in the 200 W-15 min sample with an overpotential of 220 mV at a current density of 10 mA cm-2. Such a simple sputtering method at low temperature can be easily expanded to other 2D TMD electrochemical catalysts, promising potentially practical electrocatalysts.
基金This research was financiaUy supported by the National Natural Science Foundation of China （No. 11304060） and the Foundation of Harbin Institute of Technology for the Incubation Program of the Development of Basic Research Outstanding Talents （No. 01509321）.
文摘Monolayer transition metal dichalcogenides （TMDCs）, as direct bandgap semiconductors, show promise for applications in ultra-thin flexible optoelec- tronic devices. However, the optical properties and device performance are greatly affected by defects, such as vacancies, present in these materials. Vacancies exist unavoidably in mechanically exfoliated or grown by chemical vapor deposition （CVD） monolayer TMDCs; therefore, their influence on the electric and optical properties of host materials has been widely studied. Here, we report a new defect state located at 1.54 eV, which is 70 meV lower than the neutral exciton energy in as-prepared WSe2 monolayers grown by CVD. This defect state is clearly observed in photoluminescence （PL） and Raman spectra at ambient conditions. PL mapping, Rarnan mapping, and atomic force microscopy analysis indicate a solid-vapor reaction growth mechanism of the defect state formation. During a certain growth stage, nuclei with the composition of WOxSey do not fully react with the Se vapor, leading to the defect formation. This type of defects permits radiative recombination of bound neutral excitons, which can make the PL intensity as strong as the intrinsic excitation. Our findings reveal a new way to tailor the optical properties of two-dimensional TMDCs without any additional processes performed after growth.
基金The authors gratefully acknowledge financial support by National Natural Science Foundation of China （Nos. 51371106 and 51671115）, and Young Tip-top Talent Support Project （the Organization Department of the Central Committee of the CPC）.
文摘Transition-metal dichalcogenides （TMDs） exhibit immense potential as lithium/ sodium-ion electrode materials owing to their sandwich-like layered structures. To optimize their lithium/sodium-storage performance, two issues should be addressed： fundamentally understanding the chemical reaction occurring in TMD electrodes and developing novel TMDs. In this study, WSe2 hexagonal nanoplates were synthesized as lithium/sodium-ion battery （LIB/SIB） electrode materials. For LIBs, the WSe2-nanoplate electrodes achieved a stable reversible capacity and a high rate capability, as well as an ultralong cycle life of up to 1,500 cycles at 1,000 mA·g^-1. Most importantly, in situ Raman spectroscopy, ex situ X-ray diffraction （XRD）, transmission electron microscopy, and electrochemical impedance spectroscopy measurements performed during the discharge-charge process clearly verified the reversible conversion mechanism, which can be summarized as follows： WSe2 ＋ 4Li^＋ ＋ 4e^- ←→ W ＋ 2Li2Se. The WSe2 nanoplates also exhibited excellent cycling performance and a high rate capability as SIB electrodes. Ex situ XRD and Raman spectroscopy results demonstrate that WSe2 reacted with Na^＋ more easily and thoroughly than with Li^＋ and converted to Na2Se and tungsten in the Ist sodiated state. The subsequent charging reaction can be expressed as Na2Se → Se ＋ 2Na^＋＋ 2e^-, which differs from the traditional conversion mechanism for LIBs. To our knowledge, this is the first systematic exploration of the lithium/sodium-storage performance of WSe2 and the mechanism involved.
基金This work is supported by the National Science Foundation of China(Nos.11574367 and 11874405)the National Key Research and Development Program of China(Nos.2016YFA0300600,2018YFA0704200,and 2019YFA0308000)the Youth Innovation Promotion Association of CAS(Nos.2017013 and 2019007).
文摘The mono layer WSe2 is in teresting and important for future application in nanoelectronics,spintronics and valleytronics devices,because it has the largest spin splitting and Ion gest valley coherence time among all the known monolayer transition-metal dichalcogenides(TMDs).Toobtain the large-area monolayer TMDs'crystal is the first step to manu facture scalable and high-performance electronic devices.In this letter,we have successfully fabricated millimeter-sized mono layer WSe2 single crystals with very high quality,based on our improved mecha nicalexfoliation method.With such superior samples,using standard high resolution angle-resolved photoemission spectroscopy,we didcomprehe nsive electronic band structure measurements on our mono layer WSe2.The overall band features point it to be a 1.2 eV direct bandgap semico nductor.Its spin splitting of the valence band at K point is found as 460 meV,which is 30 meV less than the corresponding band splitting in its bulk counterpart.The effective hole masses of valence bands are determined as 2.344 me atГ,and 0.529 me as well as 0.532 meat K for the upper and lower branch of splitting ban ds,respectively.And screening effect from substrate is shown to substa ntially impact onthe electronic properties.Our results provide importa nt insights into band structure engineering in mono layer TMDs.Our mono layer WSe2 crystals may constitute a valuable device platform.
基金The authors would like to thank Georgios Katsaros and Tim Wehling for valuable discussions. Stephan Br~iuer, Albin Schwarz, and Ursula Kainz are ackno- wledged for technical support. A. M. acknowledges the financial support through BES-2013-062593. G. G. acknowledges support from the Austrian Science Fund through project P 28018-B27. I. Z. acknowledges financial support from the Swiss National Science Foundation research grant （No. 200021_165784）. This work was partially funded by the Austrian Science Fund through the projects P24471 and P26830, and by the Spanish Ministry for Economy and Competitiveness trough the project MINECO/FEDER TEC2015-69916- C2-1-R.
文摘Two-dimensional transition metal dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact footprints. However, the high sensitivity of atomically thin materials to the surrounding dielectric media imposes severe limitations on their practical applicability. Hence, to enable the effective integration of these materials in devices, the development of reliable encapsulation procedures that preserve their physical properties is required. Here, the excitonic photoluminescence （at room temperature and 10 K） is assessed on mechanically exfoliated WSe2 monolayer flakes encapsulated with SiOx and AlxOy layers by means of chemical and physical deposition techniques. Conformal coating on untreated and non- functionalized flakes is successfully achieved by all the techniques examined, with the exception of atomic layer deposition, for which a cluster-like oxide coating is formed. No significant compositional or strain state changes in the flakes are detected upon encapsulation, independently of the technique adopted. Remarkably, our results show that the optical emission of the flakes is strongly influenced by the stoichiometry quality of the encapsulating oxide. When the encapsulation is carried out with slightly sub-stoichiometric oxides, two remarkable phenomena are observed. First, dominant trion （charged exciton） photoluminescence is detected at room temperature, revealing a clear electrical doping of the monolayers. Second, a strong decrease in the optical emission of the monolayers is observed, and attributed to non-radiative recombination processes and/or carrier transfer from the flake to the oxide. Power- and temperature-dependent photoluminescence measurements further confirm that stoichiometric oxides obtained by physical deposition lead to a successful encapsulation, opening a promising route for the development of integrated two-dimensional devices.
基金Project supported by the National Natural Science Foundation of China(Grant No.11627801)the Research Foundation of Education Bureau of Hunan Province of China(Grant Nos.15B083 and 17B090)
文摘The effects of biaxial strain on the electronic structure and thermoelectric properties of monolayer WSe2 have been investigated by using first-principles calculations and the semi-classical Boltzmann transport theory. The electronic band gap decreases under strain, and the band structure near the Fermi level of monolayer WSe2 is modified by the applied biaxial strain. Furthermore, the doping dependence of the thermoelectric properties of n-and p-doped monolayer WSe2 under biaxial strain is estimated. The obtained results show that the power factor of n-doped monolayer WSe2 can be increased by compressive strain while that of p-doping can be increased with tensile strain. Strain engineering thus provides a direct method to control the electronic and thermoelectric properties in these two-dimensional transition metal dichalcogenides materials.
基金Project supported by the National Key Research and Development Program of China(Grant No.2016YFA0301700)the National Natural Science Foundation of China(Grant Nos.61590932,11774333,61674132,11674300,11575172,and 11625419)+2 种基金the Anhui Provincial Initiative in Quantum Information Technologies,China(Grant Nos.AHY080000 and AHY130300)the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB24030601)the Fundamental Research Funds for the Central Universities,China
文摘Monolayer transition-metal dichalcogenides （TMDs） are considered to be fantastic building blocks for a wide variety of optical and optoelectronic devices such as sensors, photodetectors, and quantum emitters, owing to their direct band gap, transparency, and mechanical flexibility. The core element of many conventional electronic and optoelectronic devices is the p-n junction, in which the p- and n-types of the semiconductor are formed by chemical doping in different regions. Here, we report a series of optoelectronic studies on a monolayer WSe2 in-plane p-n photodetector, demonstrating a low- power dissipation by showing an ambipolar behavior with a reduced threshold voltage by a factor of two compared with the previous results on a lateral electrostatically doped WSe2 p-n junction. The fabrication of the device is based on a polycarbonates （PC） transfer technique and hence no electron-beam exposure induced damage to the monolayer WSe2 is expected. Upon optical excitation, the photodetector demonstrates a photoresponsivity of 0.12 mA.W-1 and a maximum external quantum efficiency of 0.03%. Our study provides an alternative platform for a flexible and transparent two- dimensional photodetector, from which we expect to further promote the development of next-generation optoelectronic devices.
基金This work was financed by the National Natural Science Foundation of China(No.62175225)Zhejiang Provincial Natural Science Foundation of China(No.LZ21E020004)Fundamental Research Funds for the Provincial Universities of Zhejiang,Young Top Talent Plan of Zhejiang(No.ZJWR0308004).
文摘Remote controlled soft actuators have attracted ever-increasing interest in industrial,medical,robotics,and engineering fields.Soft actuators are charming than normal tools in executing dedicate tasks due to small volume and flexible body they have.However,it remains a challenge to design soft actuator that can adapt to multi-environments under remote stimuli with promising nano materials.Herein,we have developed a kind of near-infrared laser driven soft actuators with multi locomotive modes based on WSe2 and graphene nanosheets heterojunction.Different locomotion modes are driven by photothermal effect induced deformation to adapt to different working conditions.Moreover,the specially designed gripper driven by pulsed laser can lift a heavy load which is four times of its weight.This work broadens the choice of advanced nanomaterials for photothermal conversion of soft actuators.It is promising to realize applications including photothermal therapy and complex environment detection through the combination of the intelligent robot design and optical fiber system.
