石墨烯与TMDCs缺陷工程的研究进展

晶体缺陷是晶体中的常见现象,它的存在会改变晶体局部的周期性排列,从而对晶体的力学性质、电学性质等产生影响。在石墨烯和二维过渡金属硫族化合物这两种典型的二维材料中,缺陷的作用尤为显著,如何利用缺陷工程实现对石墨烯和TMDCs性质的调节并设计可控的功能化应用已经成为该领域中的核心问题。本文基于成熟的研究结果,同时也结合了近些年最新的一些研究进展,全面地介绍了几种在石墨烯和TMDCs中常见的缺陷。文章的介绍阐明了缺陷对石墨烯和TMDCs性质的重要影响,同时也体现出了缺陷工程对于实现石墨烯与TMDCs功能化应用的关键意义。

Electronic structure engineering of transition metal dichalcogenides for boosting hydrogen energy conversion electrocatalysts

Electrocatalytic water splitting for hydrogen production is an appealing strategy to reduce carbon emissions and generate renewable fuels.This promising process,however,is limited by its sluggish reaction kinetics and high-cost catalysts.The two-dimensional(2D)transition metal dichalcogenides(TMDCs)have presented great potential as electrocatalytic materials due to their tunable bandgaps,abundant defective active sites,and good chemical stability.Consequently,phase engineering,defect engineering and interface engineering have been adopted to manipulate the electronic structure of TMDCs for boosting their exceptional catalytic performance.Particularly,it is essential to clarify the local structure of catalytically active sites of TMDCs and their structural evolution in catalytic reactions using atomic resolution electron microscopy and the booming in situ technologies,which is beneficial for exploring the underlying reaction mechanism.In this review,the growth regulation,characterization,particularly atomic configurations of active sites in TMDCs are summarized.The significant role of electron microscopy in the understanding of the growth mechanism,the controlled synthesis and functional optimization of 2D TMDCs are discussed.This review will shed light on the design and synthesis of novel electrocatalysts with high performance,as well as prompt the application of advanced electron microscopy in the research of materials science.

How defects influence the photoluminescence of TMDCs

Two-dimensional(2D)transition metal dichalcogenide(TMDC)monolayers,a class of ultrathin materials with a direct bandgap and high exciton binding energies,provide an ideal platform to study the photoluminescence(PL)of light-emitting devices.Atomically thin TMDCs usually contain various defects,which enrich the lattice structure and give rise to many intriguing properties.As the influences of defects can be either detrimental or beneficial,a comprehensive understanding of the internal mechanisms underlying defect behaviour is required for PL tailoring.Herein,recent advances in the defect influences on PL emission are summarized and discussed.Fundamental mechanisms are the focus of this review,such as radiative/nonradiative recombination kinetics and band structure modification.Both challenges and opportunities are present in the field of defect manipulation,and the exploration of mechanisms is expected tofacilitate the applications of 2D TMDCs in the future.

Transition metal dichalcogenides (TMDCs) heterostructures: Optoelectric properties

Transition metal dichalcogenides(TMDCs)have suitable and adjustable band gaps,high carrier mobility and yield.Layered TMDCs have attracted great attention due to the structure diversity,stable existence in normal temperature environment and the band gap corresponding to wavelength between infrared and visible region.The ultra-thin,flat,almost defect-free surface,excellent mechanical flexibility and chemical stability provide convenient conditions for the construction of different types of TMDCs heterojunctions.The optoelectric properties of heterojunctions based on TMDCs materials are summarized in this review.Special electronic band structures of TMDCs heterojunctions lead to excellent optoelectric properties.The emitter,p-n diodes,photodetectors and photosensitive devices based on TMDCs heterojunction materials show excellent performance.These devices provide a prototype for the design and development of future high-performance optoelectric devices.

二维层状材料过渡金属硫化物

过渡金属硫化物(TMDCs)纳米材料优异的性能有利于它在纳米电子学、光电子学和自旋电子学等领域广泛地应用。主要介绍了TMDCs材料最新的进展,概述了TMDCs的制备方法、性能以及纳米结构形态。纳米TMDCs层状材料边缘态对材料性能会产生重要的影响,着重介绍了低维纳米材料边缘态对性能(化学性能、电子性能和磁性能)的影响以及边缘态最新的研究进展,特别是TMDCs材料边缘态的磁性能为其在磁性和自旋领域潜在性的应用提供了可能,并且引起了广大研究者们的兴趣,从实验和理论2个方面总结了该领域的研究成果,为二维层状TMDCs材料的研究提供了支持。

超级电容器中的二维材料

以石墨烯为代表的具有层状结构的二维材料因具有大比表面积等特性成为超级电容器电极材料的热门候选。文章着眼于针对诸如石墨烯、过渡金属二硫族化合物、过渡金属碳/氮化物、层状过渡金属氧化物/氢氧化物等二维材料在超级电容器领域应用的研究,尝试总结了其制备方法、产物形貌特征以及作为电极的性能等,并对这一领域的未来发展和面临的挑战提出了看法与预测。

Spintronics in Two-Dimensional Materials

Spintronics,exploiting the spin degree of electrons as the information vector,is an attractive field for implementing the beyond Complemetary metal-oxide-semiconductor(CMOS)devices.Recently,two-dimensional(2D)materials have been drawing tremendous attention in spintronics owing to their distinctive spin-dependent properties,such as the ultralong spin relaxation time of graphene and the spin-valley locking of transition metal dichalcogenides.Moreover,the related heterostructures provide an unprecedented probability of combining the di erent characteristics via proximity e ect,which could remedy the limitation of individual 2D materials.Hence,the proximity engineering has been growing extremely fast and has made significant achievements in the spin injection and manipulation.Nevertheless,there are still challenges toward practical application;for example,the mechanism of spin relaxation in 2D materials is unclear,and the high-effciency spin gating is not yet achieved.In this review,we focus on 2D materials and related heterostructures to systematically summarize the progress of the spin injection,transport,manipulation,and application for information storage and processing.We also highlight the current challenges and future perspectives on the studies of spintronic devices based on 2D materials.

2D metamaterials coherently steer nonlinear valley photons of 2D semiconductor

Monolayer transition-metal dichacolgenides (TMDCs) present a direct optical bandgap at the Brillouin zones, socalled valleys. Those energetically degenerate valleys (K and K’) present different valley pseudospins, emitting the valley photons with opposite spin angular momentums due to nonlinear optical selection rules. Furthermore, although atomically thin, two-dimensional (2D) TMDCs have giant nonlinearity, which can be enhanced by the valley-excitons.

基于二硫化钼纳米片的高性能湿度传感器

采用研磨辅助液相剥离法结合高功率超声处理,制备了分散性良好、尺寸均匀的MoS_2纳米片。通过XRD、SEM以及拉曼光谱对材料进行了表征,并通过滴注法构筑基于MoS_2纳米片的电阻式湿度传感器。湿敏测试结果表明:该器件在11%RH~98%RH的检测范围内具有良好的线性、快速响应特性;在11%RH时的灵敏度为0.64,响应时间为17 s,恢复时间为2 s。除此之外,器件具有良好的重复性和长期稳定性。这些研究结果进一步拓展了MoS_2纳米片在高性能湿度传感器中的应用前景。

Electronic structure of molecular beam epitaxy grown 1T’-MoTe2 film and strain effect

Atomically thin transition metal dichalcogenide films with distorted trigonal(1T') phase have been predicted to be candidates for realizing quantum spin Hall effect. Growth of 1T' film and experimental investigation of its electronic structure are critical. Here we report the electronic structure of 1T'-MoTe2 films grown by molecular beam epitaxy(MBE).Growth of the 1T'-MoTe2 film depends critically on the substrate temperature, and successful growth of the film is indicated by streaky stripes in the reflection high energy electron diffraction(RHEED) and sharp diffraction spots in the low energy electron diffraction(LEED). Angle-resolved photoemission spectroscopy(ARPES) measurements reveal a metallic behavior in the as-grown film with an overlap between the conduction and valence bands. First principles calculation suggests that a suitable tensile strain along the a-axis direction is needed to induce a gap to make it an insulator. Our work not only reports the electronic structure of MBE grown 1T'-MoTe2 films, but also provides insights for strain engineering to make it possible for quantum spin Hall effect.

Valley dynamics of different excitonic states in monolayer WSe_(2)grown by molecular beam epitaxy

Monolayer transition-metal dichalcogenides possess rich excitonic physics and unique valley-contrasting optical selection rule,and offer a great platform for long spin/valley lifetime engineering and the associated spin/valleytronics exploration.Using two-color time-resolved Kerr rotation and time-resolved reflectivity spectroscopy,we investigate the spin/valley dynamics of different excitonic states in monolayer WSe_(2)grown by molecular beam epitaxy.With fine tuning of the photon energy of both pump and probe beams,the valley relaxation process for the neutral excitons and trions is found to be remarkably different-their characteristic spin/valley lifetimes vary from picoseconds to nanoseconds,respectively.The observed long trion spin lifetime of>2.0 ns is discussed to be associated with the dark trion states,which is evidenced by the photon-energy dependent valley polarization relaxation.Our results also reveal that valley depolarization for these different excitonic states is intimately connected with the strong Coulomb interaction when the optical excitation energy is above the exciton resonance.

过渡金属硫族化物纳米片的制备与生物应用

过渡金属硫族化物(TMDCs)是一类类石墨烯材料,具有高近红外吸收率,因此常被用作肿瘤光热治疗(PTT)过程中的光热剂以及光声成像(PAI)中的造影剂.TMDCs大多数情况下以二维纳米片的形式存在,具有更大的比表面积和更多的活性位点,可以通过负载不同功能的纳米小颗粒、药物小分子或生物大分子来赋予材料更多的可能性.文章主要介绍了TMDCs的合成与修饰,总结了常用的几种合成方法及其优缺点,以及该纳米材料在肿瘤诊疗中的一些应用.

MOCVD法生长二维范德华材料的研究进展

二维范德华材料凭借其优异的光学和电学特性,自被发现以来一直作为延续集成电路摩尔定律的重要基础电子材料而备受关注。通过机械剥离的方法得到高质量的二维材料进行实验室层面的研究工作已经不能满足现阶段的需要。采用金属有机化学气相沉积(MOCVD)技术可以得到高质量的大面积二维范德华材料,并具有生长层数和成核密度可控的优势。以过渡金属硫化物(TMDC)为例,分别从生长条件、金属有机源材料、衬底、催化剂等方面综述了采用MOCVD技术生长二维范德华材料的研究进展,同时讨论了二维材料的范德华异质结构的特性及应用。利用MOCVD技术优势可以推动二维范德华材料的大规模应用。最后总结了MOCVD法生长二维范德华材料现阶段的优势与不足,并对其未来的发展进行了展望。

Two-Dimensional Transition Metal Dichalcogenides and Their Charge Carrier Mobilities in Field-Effect Transistors

Two-dimensional(2D) materials have attracted extensive interest due to their excellent electrical, thermal,mechanical, and optical properties. Graphene has been one of the most explored 2D materials. However, its zero band gap has limited its applications in electronic devices. Transition metal dichalcogenide(TMDC), another kind of 2D material,has a nonzero direct band gap(same charge carrier momentum in valence and conduction band) at monolayer state,promising for the efficient switching devices(e.g., field-effect transistors). This review mainly focuses on the recent advances in charge carrier mobility and the challenges to achieve high mobility in the electronic devices based on 2DTMDC materials and also includes an introduction of 2D materials along with the synthesis techniques. Finally, this review describes the possible methodology and future prospective to enhance the charge carrier mobility for electronic devices.

碲协助法制备二维过渡金属硫属化合物合金及异质结

二维过渡金属硫属化合物(TMDC)合金和异质结由于具有独特的光电性能,因而在下一代光电器件中有着广泛的应用前景.然而,如何可控地制备出二维TMDC合金和异质结是一个重大的挑战.以WS2与MoS2的异质结和合金为例,通过在钨粉中引入不同含量的低熔点碲粉,有效降低了WS2的生长温度,继而调节了WS2的成核和生长速率,利用一步化学气相沉积方法,可控地制备出WS2/MoS2垂直异质结和Mo1-xWxS2合金.拉曼光谱、光致发光谱、拉曼成像和光致发光成像技术表明,制备出的WS2/MoS2垂直异质结是由单层的WS2和MoS2上下叠加而成,而在Mo1-xWxS2合金中,W的含量(x)为0.83.此研究为人们提供了一个简单、有效的可控制备二维TMDC异质结和合金的化学气相沉积方法.

Field distribution of the Z_(2)topological edge state revealed by cathodoluminescence nanoscopy

Photonic topological insulators with robust boundary states can enable great applications for optical communication and quantum emission,such as unidirectional waveguide and single-mode laser.However,because of the diffraction limit of light,the physical insight of topological resonance remains unexplored in detail,like the dark line that exists with the crys-talline symmetry-protected topological edge state.Here,we experimentally observe the dark line of the Z_(2)photonic topo-logical insulator in the visible range by photoluminescence and specify its location by cathodoluminescence characteriza-tion,and elucidate its mechanism with the p-d orbital electromagnetic field distribution which calculated by numerical sim-ulation.Our investigation provides a deeper understanding of Z_(2)topological edge states and may have great signific-ance to the design of future on-chip topological devices.

Controlled growth of vertically stacked In_(2)Se_(3)/WSe_(2) heterostructures for ultrahigh responsivity photodetector

Transition metal dichalcogenides(TMDCs)are promising candidates for future optoelectronic devices accounting for their high carrier mobility and excellent quantum efficiency.However,the limited light absorption efficiency in atomically thin layers significantly hinders photocarrier generation,thereby impairing the optoelectronic performance and hindering practical applications.Herein,we successfully synthesized In_(2)Se_(3)/WSe_(2) heterostructures through a typical two-step chemical vapor deposition(CVD)method.The In_(2)Se_(3) nanosheet with strong light absorption capability,serving as the light absorption layer,was integrated with the monolayer WSe_(2),enhancing the photosensitivity of WSe_(2) significantly.Upon laser irradiation with a wavelength of 520 nm,the In_(2)Se_(3)/WSe_(2) heterostructure device shows an ultrahigh photoresponsivity with a value as high as 2333.5 A/W and a remarkable detectivity reaching up to 6.7×10^(12) Jones,which is the highest among almost the reported TMDCs-based heterostructures grown via CVD even some fabricated by mechanical exfoliation(ME).Combing the advantages of CVD method such as large scale,high yield,and clean interface,the In_(2)Se_(3)/WSe_(2) heterostructures would provide a novel path for future high-performance optoelectronic device.

Interlayer exciton dynamics of transition metal dichalcogenide heterostructures under electric fields

Stacking single layers of atoms on top of each other provides a fundamental way to achieve novel material systems and engineer their physical properties,which offers opportunities for exploring fundamental physics and realizing next-generation optoelectronic devices.Among the two-dimensional(2D)-stacked systems,transition metal dichalcogenide(TMDC)heterostructures are particularly attractive because they host tightly-bonded interlayer excitons which possess various novel and appealing properties.These interlayer excitons have drawn significant research attention and hold high potential for the application in unique optoelectronic devices,such as polarization-and wavelength-tunable single photon emitters,valley Hall transistors,and possible high-temperature superconductors.The development of these devices requires a comprehensive understanding of the fundamental properties of these interlayer excitons and the impact of electric fields on their behaviors.In this review,we summarize the recent advances on the understanding of interlayer exciton dynamics under electric fields in TMDC heterostructures.We put emphasis on the electrical modulation of interlayer excitons’emission,the valley Hall transport of charge carriers after the separation of interlayer excitons by an electric field,and the correlation physics of interlayer excitons and charges under electrical doping and tuning.Challenges and perspectives are finally discussed for the application of TMDC heterostructures in future optoelectronics.

Photoluminescence manipulation in two-dimensional transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides(TMDCs)have been regarded as an intriguing platform for exploring novel physical phenomena and optoelectronic devices due to their excitonic emission characteristics derived from the atomic thin thickness and reduced dielectric screening effect.Notably,monolayer TMDCs with a direct bandgap exhibiting strong photoluminescence(PL)are promising candidates for the light-emitting devices,while the interlayer excitons in heterostructures hold great potential for the photonic chips and optical communication applications.However,the non-ideal photoluminescent intensity and quality due to the ultrathin thickness and high defect density of experimentally obtained monolayer TMDCs limit the further development for the light-emission applications.Here,we summarize the research progress on the PL manipulation of the excitonic emission in TMDCs,where the PL intensity enhancement and emission wavelength regulation are included.The concept and characteristics of excitons are overviewed firstly,followed by the discussion on the evaluation and characterization of excitonic emission.The state-of-the-art progress on the manipulation of the neutral excitons and interlayer excitons PL are then summarized.Finally,the challenges and prospects are proposed.

Optoelectronic devices based on two-dimensional transition metal dichalcogenides

In the past few years, two-dimensional （2D） transition metal dichalcogenide （TMDC） materials have attracted increasing attention of the research community, owing to their unique electronic and optical properties, ranging from the valley-spin coupling to the indirect-to-direct bandgap transition when scaling the materials from multi-layer to monolayer. These properties are appealing for the development of novel electronic and optoelectronic devices with important applications in the broad fields of communication, computation, and healthcare. One of the key features of the TMDC family is the indirect-to-direct bandgap transition that occurs when the material thickness decreases from multilayer to monolayer, which is favorable for many photonic applications. TMDCs have also demonstrated unprecedented flexibility and versatility for constructing a wide range of heterostructures with atomic-level control over their layer thickness that is also free of lattice mismatch issues. As a result, layered TMDCs in combination with other 2D materials have the potential for realizing novel high-performance optoelectronic devices over a broad operating spectral range. In this article, we review the recent progress in the synthesis of 2D TMDCs and optoelectronic devices research. We also discuss the challenges facing the scalable applications of the family of 2D materials and provide our perspective on the opportunities offered by these materials for future generations of nanophotonics technology.