WSe2纳米结构的合成及减摩性能研究

将W粉和Se粉按一定比例混合,直接密封在石英管中加热或高能球磨、压片,在Ar气氛中加热,得到了不同形貌的WSe2纳米结构.利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)和透射电子显微镜(TEM、HRTEM)分析了WSe2纳米结构的组成、微观形貌和组织形态;利用UMT-2摩擦试验机考察了WSe2作为HVI500液体基础油添加剂的摩擦磨损性能.研究结果表明:直接密封加热得到的产物为棒状WSe2纳米材料,最小棒直径为6 nm;球磨、压片后加热得到WSe2纳米颗粒,颗粒的平均尺寸在50 nm以下,二者都具有层状结构和良好的结晶性.添加质量分数5%的WSe2纳米材料作为基础油添加剂能够显著降低摩擦系数,减少磨损,增强了材料抗疲劳磨损能力.

单层WSe2、MoSe2激子发光的压力诱导K-Λ交互转变

采用机械剥离法在金刚石对顶砧中制备了单层WSe_2和MoSe_2样品,利用高压微区荧光光谱测量技术,在氩传压介质环境下对其激子发光行为进行了高压调控研究。其中单层WSe_2的中性和负电激子演化趋势在2. 43 GPa处出现拐点,单层MoSe_2中性激子发光在3. 7 GPa处发生了劈裂。结合第一性原理计算分析,确认该不连续现象的产生机制为压力诱导的导带底K-Λ交互转变。该结果可以扩大至整个二维层状材料体系,为发展激子器件垫定基础。

单层与双层WSe2纳米片层的光致发光

采用气相沉积法制备了WSe2二维纳米材料,对其低温光致发光谱进行了研究。结果表明:随着WSe2层数的增加,其光致发光强度单调下降;当WSe2层数从单层增加为双层时,其发光强度急剧下降,表明其能带结构已从直接带隙转变为间接带隙。进一步研究了双层WSe2的变温光致发光谱,发现随着温度的升高,双层WSe2发光峰中A峰峰位的变化基本符合半导体带隙的温度变化规律,而I峰峰位红移与温度基本成线性关系,表明双层WSe2同时存在间接和直接跃迁,且直接跃迁和间接跃迁特性不同。

稀土元素Er掺杂提高WSe2纳米薄膜的光电特性

采用热蒸发法在Si片上沉积稀土元素Er掺杂的WSe2薄膜,研究了Er^3+掺杂对WSe2薄膜表面形貌、晶体结构﹑光致发光光谱、光吸收特性和电学特性的影响。研究发现薄膜在(004)晶面呈柱状择优生长,Er^3+掺杂不仅没有改变WSe2的晶体结构,还使薄膜结晶性显著增强,提高了光吸收特性,明显增加了光致发光强度。同时发现Er^3+掺杂WSe2薄膜的电子迁移率为未掺杂的4倍多。当光照强度由0增加到25 mW/cm^2时,Er^3+掺杂的WSe2/Si异质结的Ⅰ-Ⅴ曲线由整流特性逐步变成线性,相同偏压下光电流增加了约3倍,并且光电导灵敏度也明显增加。随着加热温度的升高,电导显著增大,电阻表现负温度系数。表明WSe2/Si异质结对温度和光强具有非常高的灵敏度,在高效率的温度传感器和光电探测器等方面有良好的应用前景。

固相烧结合成WSe2纳米棒的微观形貌及摩擦学性能

将钨粉和硒粉按物质的量比为1∶2.2混合并球磨后,再在600~800℃进行固相烧结合成了纳米WSe2,对其微观形貌进行表征;然后将在600℃合成的WSe2纳米棒作为润滑油添加剂加入到HVI500基础油中,研究了它的摩擦学性能。结果表明：在700,800℃合成的纳米WSe2为片状,在600℃合成的纳米WSe2为棒状,该纳米棒的直径约为500nm,长约为800nm;与HVI500基础油相比,2%WSe2纳米棒＋HVI500基础油的摩擦因数约降低了40%,具有较好的减摩作用。

Room-temperature sputtered electrocatalyst WSe2 nanomaterials for hydrogen evolution reaction

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.

尺寸效应对MoS2/WSe2范德华异质结构层间与俄歇复合的界面调控

为探索界面工程对二维材料范德华异质结构中载流子复合率的影响,本工作基于界面键弛豫理论和费米黄金定则,建立了范德华异质结俄歇和层间复合率与各结构组元尺寸之间的理论模型。结果表明,MoS2/WSe2异质结的俄歇复合寿命随着组元尺寸的增大而增加,且异质结的俄歇复合率远小于相应的单组元体系。在MoS2/WSe2双层异质结中引入薄h-BN插层后,体系的层间复合率和俄歇复合率随h-BN厚度的增加而分别呈现减小和增大的趋势;在组元处于单层MoS2和WSe2情况下,当界面插层h-BN厚度达到9.1 nm时,俄歇复合率将趋于5.3 ns^-1。该研究结果为二维过渡金属硫族化合物基异质结光电器件的优化设计提供了一种理论依据。

Single photon-chiral phonon entanglement in monolayer WSe2

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.

First-principles analysis of phonon thermal transport properties of two-dimensional WS2/WSe2 heterostructures

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.

富缺陷晶态WSe2纳米片:一种潜在的高效低成本析氢反应电催化剂（英文）

氢是高效的清洁能源,在应对全球能源危机和环境污染方面具有重要作用。电解水制氢是通过消耗可再生的电能(水电、光电、风电等)和储量丰富的水资源以获得氢气,该方法制氢颇具应用前景。然而,电解水过程中的析氢反应(HER)动力学迟缓、过电位高,导致制氢能耗较大。为提升析氢反应速率,需在电解水设备中引入贵金属作为催化剂,这进一步增加了制氢成本。开发高效低成本的析氢催化剂对电解水制氢的规模化应用至关重要。过渡金属硫属化合物(TMDs)因具有独特的层状结构和较低的氢原子吸附自由能,表现出良好的析氢催化活性,有望成为贵金属催化剂的替代品。近年来,MoS2、WS2、TiS2、TaS2、MoSe2、WSe2等TMDs材料被广泛用于催化析氢反应。TMDs的边缘位点被认为是其催化活性中心,且材料的催化性能与边缘位点数成正比。研究表明,通过缺陷调控增加边缘位点数是提升TMDs催化活性的不二法门。液相加工及其他低温合成法是目前制备富缺陷TMDs析氢催化剂的有效手段,然而该条件下得到的材料结晶性差、易发生电化学腐蚀、析氢稳定性低。高温处理可合成高结晶性的TMDs,具有较好的电化学稳定性。但高温结晶会使材料比表面积减小、缺陷和边缘位点数减少,造成催化活性不佳。采用化学/电化学剥离晶态TMDs样品,可在室温条件下制备富缺陷晶态TMDs析氢催化剂。然而,此方法受限于易燃溶剂的使用,且制备过程繁琐,难以实现规模化生产。因此,富缺陷晶态TMDs的制备,是高效析氢催化剂领域的研究重点和难点。在已报道的TMDs中,WSe2因带隙小(1.6 eV)、导电性好而备受关注,引发了微纳WSe2催化剂的研究热潮。其中,片状WSe2有利于材料活性位点与电解液直接接触,通常表现出更优异的析氢催化活性。类似其他TMDs材料,富缺陷晶态WSe2纳米片的制备,目前仍难以实现。本工作以WO3和Se粉为原料,先在高温条件下合成高结晶度WSe2,再经超声剥离得到晶态的WSe2纳米片。在随后的长时间超声作用下,晶态WSe2纳米片表面会进一步产生许多纳米级的岛状颗粒,得到富缺陷晶态WSe2纳米片材料。选区衍射分析表明,岛状颗粒的引入使WSe2纳米片新增了多晶衍射环。同时,材料的BET比表面积高达105.2 m2·g-1,且具有更宽的孔径分布和更大的孔体积。在三电极条件下,以0.5 mol/L H2SO4为电解液,富缺陷晶态WSe2纳米片在10 mA·cm-2时的过电位仅为277 mV,远低于未剥离的WSe2材料(385 mV)。此外,富缺陷晶态WSe2纳米片的Tafel斜率(58 mV·dec-1)也明显低于剥离前的WSe2(81 mV·dec-1)催化剂。虽然富缺陷晶态WSe2纳米片的析氢催化活性与商用Pt/C (20%)贵金属材料相比仍有一定差距,但其成本较低,在大规模电解水制氢产业中仍有重要应用价值。交流阻抗测试进一步表明,富缺陷晶态WSe2纳米片具有更低的电荷转移电阻,可有效提升析氢反应的电极动力学过程。长时间电解水析氢测试表明,富缺陷晶态WSe2纳米片不仅具有高的析氢催化活性,还具有良好的电化学稳定性。富缺陷晶态WSe2纳米片卓越的电化学性能主要得益于以下两点:一方面,超声剥离减小了催化剂的尺寸、增加了比表面积、拓宽了孔径分布,形成了富缺陷的WSe2结构;另一方面,较高的结晶性使材料能够抵御电化学腐蚀,在析氢反应中表现出良好的稳定性。

Heterostructured graphene quantum dot/WSe2/Si photo-detector with suppressed dark current and improved detectivity

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.

Electronic structure of exfoliated millimeter-sized monolayer WSe2 on silicon wafer

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.

Strong room-temperature emission from defect states in CVD-grown WSe2 nanosheets

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.

Robust n-type doping of WSe2 enabled by controllable proton irradiation

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.

Effects of dielectric stoichiometry on the photoluminescence properties of encapsulated WSe2 monolayers

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.

Gold-vapor-assisted chemical vapor deposition of aligned monolayer WSe2 with large domain size and fast growth rate

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.

Light-driven soft actuator based on graphene and WSe2 nanosheets composite for multimodal motion and remote manipulation

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.

Few-layer WSe2 lateral homo-and hetero-junctions with superior optoelectronic performance by laser manufacturing

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.

Temperature-dependent Raman spectroscopy studies of 1-5-layer WSe2

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.

Layer-dependent signatures for exciton dynamics in monolayer and multilayer WSe2 revealed by fluorescence lifetime imaging measurement

Two-dimensional(2D)transition-metal dichalcogenide(TMD)materials have aroused noticeable interest due to their distinguished electronic and optical properties.However,little is known about their complex exciton properties together with the exciton dynamics process which have been expected to influence the performance of optoelectronic devices.The process of fluorescence can well reveal the process of exciton transition after excitation.In this work,the room-temperature layer-dependent exciton dynamics properties in layered WSe2 are investigated by the fluorescence lifetime imaging microscopy(FLIM)for the first time.This paper focuses on two mainly kinds of excitons including the direct transition neutral excitons and trions.Compared with the lifetime of neutral excitons(<0.3 ns within four-layer),trions possess a longer lifetime(~6.6 ns within four-layer)which increases with the number of layers.We attribute the longer-lived lifetime to the increasing number of trions as well as the varieties of trion configurations in thicker WSe2.Besides,the whole average lifetime increases over 10%when WSe2 flakes added up from monolayer to four-layer.This paper provides a novel tuneable layer-dependent method to control the exciton dynamics process and finds a relatively longer transition lifetime of trions at room temperature,enabling to investigate in the charge transport in TMD-based optoelectronics devices in the future.