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
Entanglement is a defining feature of quantum physics with no classical analog. On the one hand, it is a powerful concept used in the fundamental study of quantum systems, many-body physics and even black hole physics...Entanglement is a defining feature of quantum physics with no classical analog. On the one hand, it is a powerful concept used in the fundamental study of quantum systems, many-body physics and even black hole physics. On the other hand, it is a key resource in quantum communication and information processing. Entanglement has been realized between photons[1], ions[2], spins[3], quantum dots[4] and even larger objects such as macroscopic diamonds[5]. Entanglement involving macroscopic objects is particularly intriguing and holds promise for novel quantum technologies. In a recent paper[6] published in Nature Physics, Srivastava and coworkers have unveiled an intriguing entanglement between collective and macroscopic vibration involving billions of atoms of the crystal (phonon) and a single optical excitation of a quantum dot (QD) in monolayer WSe2 (Fig. 1(a)). This is the first report on single photon entangled with phonon.展开更多
In this work, the spin dynamics of a centrosymmetric WSe2 bilayer has been investigated by the two-color timeresolved Kerr rotation together with helicity-resolved transient reflectance techniques. Two depolarization ...In this work, the spin dynamics of a centrosymmetric WSe2 bilayer has been investigated by the two-color timeresolved Kerr rotation together with helicity-resolved transient reflectance techniques. Two depolarization processes associated with the direct transition are discovered at a low temperature of 10 K, with the characteristic decaying time of～3.8 ps and ～20 ps, respectively. The short decay time of ～3.8 ps is suggested to be the exciton spin lifetime of the WSe2 bilayer, which is limited by the short exciton lifetime of the WSe2 bilayer and the rapid intervalley electron–hole exchange interaction between K^＋ and K^- valley in the same layer as that of monolayer. The long decay time of ～20 ps is suggested to be the spin lifetime of photo-excited electrons, whose spin relaxation is governed by the rapid intervalley scattering from the K valley to the global minimum Σ valley and the subsequent interlayer charge transfer in WSe2 bilayer.Our experimental results prove the existence of the spin-polarized excitons and carriers even in centrosymmetric transition metal dichalcogenides（TMDCs） bilayers, suggesting their potential valleytronic and spintronic device applications.展开更多
The van der Waals(vdW)heterostructures of bilayer transition metal dichalcogenide obtained by vertically stacking have drawn increasing attention for their enormous potential applications in semiconductors and insulat...The van der Waals(vdW)heterostructures of bilayer transition metal dichalcogenide obtained by vertically stacking have drawn increasing attention for their enormous potential applications in semiconductors and insulators.Here,by using the first-principles calculations and the phonon Boltzmann transport equation(BTE),we studied the phonon transport properties of WS2/WSe2 bilayer heterostructures(WS2/WSe2-BHs).The lattice thermal conductivity of the ideal WS2/WSe2-BHs crystals at room temperature(RT)was 62.98 W/mK,which was clearly lower than the average lattice thermal conductivity of WS2 and WSe2 single layers.Another interesting finding is that the optical branches below 4.73 THz and acoustic branches have powerful coupling,mainly dominating the lattice thermal conductivity.Further,we also noticed that the phonon mean free path(MFP)of the WS2/WSe2-BHs(233 nm)was remarkably attenuated by the free-standing monolayer WS2(526 nm)and WSe2(1720 nm),leading to a small significant size effect of the WS2/WSe2-BHs.Our results systematically demonstrate the low optical and acoustic phonon modes-dominated phonon thermal transport in heterostructures and give a few important guidelines for the synthesis of van der Waals heterostructures with excellent phonon transport properties.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
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
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
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
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
Orientation-controlled growth of two-dimensional(2D)transition metal dichalcogenides(TMDCs)may enable many new electronic and optical applications.However,previous studies reporting aligned growth of WSe2 usually yiel...Orientation-controlled growth of two-dimensional(2D)transition metal dichalcogenides(TMDCs)may enable many new electronic and optical applications.However,previous studies reporting aligned growth of WSe2 usually yielded very small domain sizes.Herein,we introduced gold vapor into the chemical vapor deposition(CVD)process as a catalyst to assist the growth of WSe2 and successfully achieved highly aligned monolayer WSe2 triangular flakes grown on c-plane sapphire with large domain sizes(130μm)and fast growth rate(4.3μm·s^−1).When the aligned WSe2 domains merged together,a continuous monolayer WSe2 was formed with good uniformity.After transferring to Si/SiO2 substrates,field effect transistors were fabricated on the continuous monolayer WSe2,and an average mobility of 12 cm^2·V^−1·s−1 was achieved,demonstrating the good quality of the material.This report paves the way to study the effect of catalytic metal vapor in the CVD process of TMDCs and contributes a novel approach to realize the growth of aligned TMDC flakes.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
Lateral hetero-junctions are considered as potential candidate for building blocks in modern electronics and optoelectronics,however,the construction of which remains a challenge.In this work,by using a laser-assisted...Lateral hetero-junctions are considered as potential candidate for building blocks in modern electronics and optoelectronics,however,the construction of which remains a challenge.In this work,by using a laser-assisted manufacture technique,WSe2/WO3-x hetero-junction and monolayer/trilayer WSe2 homo-junction with Schottky diode like behavior are fabricated,both of which present competitive performance for photodetection and power generation in a wide range of wavelengths from ultraviolet to infrared,with maximum photoresponsivity of 10 A/W,external quantum efficiency of 14%,and power conversion efficiency of 1.3%.Combined with Kelvin probe microscopy and electrical transport measurements,it is demonstrated that the barrier-induced built-in electric field at WSe2/WO3-x interface,and the energy band discontinuities at the monolayer/trilayer WSe2 interface facilitate the separation of photo-generated electron-hole pairs.Our work provides a solid step towards the controllable construction of lateral junctions by laser-assisted manufacture for exploiting van der Waals materials-based novel electronic and optoelectronic applications.展开更多
In this work,Raman measurements of 1-5-layer WSe2 supported on SiO2/Si in the temperature range of 133 to 533 K are reported.A physical model including both volume effect and temperature effect is used to quantitative...In this work,Raman measurements of 1-5-layer WSe2 supported on SiO2/Si in the temperature range of 133 to 533 K are reported.A physical model including both volume effect and temperature effect is used to quantitatively understand the nonlinear temperature dependence of E2g^1Raman mode.It is found this nonlinear dependence of Raman mode mainly originates from thermal expansion effect and three-phonon scattering.The former effect increases with an increase in number of layers,which is inverse for the latter effect.The temperature-dependent thermal expansion coefficients of 1-5-layer WSe2 are also obtained from Raman spectra.The full width at half maximum(FWHM)of E;g mode is also systematically studied both experimentally and theoretically in the temperature range of 133 to 413 K.It is found that the increase in FWHM of E2g^1 mode originates from decaying of E2g^1 phonon.This work will promote the understanding of anharmonic behaviors of phonons in WSe2 flakes with different thicknesses.展开更多
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.
文摘Entanglement is a defining feature of quantum physics with no classical analog. On the one hand, it is a powerful concept used in the fundamental study of quantum systems, many-body physics and even black hole physics. On the other hand, it is a key resource in quantum communication and information processing. Entanglement has been realized between photons[1], ions[2], spins[3], quantum dots[4] and even larger objects such as macroscopic diamonds[5]. Entanglement involving macroscopic objects is particularly intriguing and holds promise for novel quantum technologies. In a recent paper[6] published in Nature Physics, Srivastava and coworkers have unveiled an intriguing entanglement between collective and macroscopic vibration involving billions of atoms of the crystal (phonon) and a single optical excitation of a quantum dot (QD) in monolayer WSe2 (Fig. 1(a)). This is the first report on single photon entangled with phonon.
基金Project supported by the National Natural Science Foundation of China(Grant No.11474276)the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDPB0603)
文摘In this work, the spin dynamics of a centrosymmetric WSe2 bilayer has been investigated by the two-color timeresolved Kerr rotation together with helicity-resolved transient reflectance techniques. Two depolarization processes associated with the direct transition are discovered at a low temperature of 10 K, with the characteristic decaying time of～3.8 ps and ～20 ps, respectively. The short decay time of ～3.8 ps is suggested to be the exciton spin lifetime of the WSe2 bilayer, which is limited by the short exciton lifetime of the WSe2 bilayer and the rapid intervalley electron–hole exchange interaction between K^＋ and K^- valley in the same layer as that of monolayer. The long decay time of ～20 ps is suggested to be the spin lifetime of photo-excited electrons, whose spin relaxation is governed by the rapid intervalley scattering from the K valley to the global minimum Σ valley and the subsequent interlayer charge transfer in WSe2 bilayer.Our experimental results prove the existence of the spin-polarized excitons and carriers even in centrosymmetric transition metal dichalcogenides（TMDCs） bilayers, suggesting their potential valleytronic and spintronic device applications.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51720105007,51806031,11602149,and GZ1257)the Fundamental Research Funds for the Central Universities,China(Grant Nos.DUT16RC(3)116 and DUT19RC(3)006)The computing resources from Supercomputer Center of Dalian University of Technology and ScGrid are greatly acknowledged。
文摘The van der Waals(vdW)heterostructures of bilayer transition metal dichalcogenide obtained by vertically stacking have drawn increasing attention for their enormous potential applications in semiconductors and insulators.Here,by using the first-principles calculations and the phonon Boltzmann transport equation(BTE),we studied the phonon transport properties of WS2/WSe2 bilayer heterostructures(WS2/WSe2-BHs).The lattice thermal conductivity of the ideal WS2/WSe2-BHs crystals at room temperature(RT)was 62.98 W/mK,which was clearly lower than the average lattice thermal conductivity of WS2 and WSe2 single layers.Another interesting finding is that the optical branches below 4.73 THz and acoustic branches have powerful coupling,mainly dominating the lattice thermal conductivity.Further,we also noticed that the phonon mean free path(MFP)of the WS2/WSe2-BHs(233 nm)was remarkably attenuated by the free-standing monolayer WS2(526 nm)and WSe2(1720 nm),leading to a small significant size effect of the WS2/WSe2-BHs.Our results systematically demonstrate the low optical and acoustic phonon modes-dominated phonon thermal transport in heterostructures and give a few important guidelines for the synthesis of van der Waals heterostructures with excellent phonon transport properties.
基金supported by the National Natural Science Foundation of China(51902261)the Natural Science Basic Research Program of Shaanxi(2019JQ-025)Fundamental Research Funds for the Central Universities(31020180QD094,31020180QD116)
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.
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.
文摘Orientation-controlled growth of two-dimensional(2D)transition metal dichalcogenides(TMDCs)may enable many new electronic and optical applications.However,previous studies reporting aligned growth of WSe2 usually yielded very small domain sizes.Herein,we introduced gold vapor into the chemical vapor deposition(CVD)process as a catalyst to assist the growth of WSe2 and successfully achieved highly aligned monolayer WSe2 triangular flakes grown on c-plane sapphire with large domain sizes(130μm)and fast growth rate(4.3μm·s^−1).When the aligned WSe2 domains merged together,a continuous monolayer WSe2 was formed with good uniformity.After transferring to Si/SiO2 substrates,field effect transistors were fabricated on the continuous monolayer WSe2,and an average mobility of 12 cm^2·V^−1·s−1 was achieved,demonstrating the good quality of the material.This report paves the way to study the effect of catalytic metal vapor in the CVD process of TMDCs and contributes a novel approach to realize the growth of aligned TMDC flakes.
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supported by the National Natural Science Foundation of China(Grant Nos.51572057,51902069,GZ213054,21571101)the Natural Science Foundation of Jiangsu Province(Grant No.BK20161543)the support from the Start-up Grant(Grant Nos.7200656,9610482)from City University of Hong Kong。
文摘Lateral hetero-junctions are considered as potential candidate for building blocks in modern electronics and optoelectronics,however,the construction of which remains a challenge.In this work,by using a laser-assisted manufacture technique,WSe2/WO3-x hetero-junction and monolayer/trilayer WSe2 homo-junction with Schottky diode like behavior are fabricated,both of which present competitive performance for photodetection and power generation in a wide range of wavelengths from ultraviolet to infrared,with maximum photoresponsivity of 10 A/W,external quantum efficiency of 14%,and power conversion efficiency of 1.3%.Combined with Kelvin probe microscopy and electrical transport measurements,it is demonstrated that the barrier-induced built-in electric field at WSe2/WO3-x interface,and the energy band discontinuities at the monolayer/trilayer WSe2 interface facilitate the separation of photo-generated electron-hole pairs.Our work provides a solid step towards the controllable construction of lateral junctions by laser-assisted manufacture for exploiting van der Waals materials-based novel electronic and optoelectronic applications.
基金This work was supported the National Key R&D Program of China(Nos.2017YFF0108900,2016YFF0203802,and 2017YFD080120203)the Dalian Youth Science and Technology Star Project(No.2017RQ138)+2 种基金the project from Liaoning Education Department(No.JDL2019015)the National Natural Science Foundation of China(Nos.51872058 and 21904029)the Fundamental Research Funds for Central Universities(No.HIT.NSRIF 2019071).
文摘In this work,Raman measurements of 1-5-layer WSe2 supported on SiO2/Si in the temperature range of 133 to 533 K are reported.A physical model including both volume effect and temperature effect is used to quantitatively understand the nonlinear temperature dependence of E2g^1Raman mode.It is found this nonlinear dependence of Raman mode mainly originates from thermal expansion effect and three-phonon scattering.The former effect increases with an increase in number of layers,which is inverse for the latter effect.The temperature-dependent thermal expansion coefficients of 1-5-layer WSe2 are also obtained from Raman spectra.The full width at half maximum(FWHM)of E;g mode is also systematically studied both experimentally and theoretically in the temperature range of 133 to 413 K.It is found that the increase in FWHM of E2g^1 mode originates from decaying of E2g^1 phonon.This work will promote the understanding of anharmonic behaviors of phonons in WSe2 flakes with different thicknesses.
