Lewis acid-doped transition metal dichalcogenides for ultraviolet–visible photodetectors

Ultraviolet photodetectors(UV PDs)are widely used in civilian,scientific,and military fields due to their high sensitivity and low false alarm rates.We present a temperature-dependent Lewis acid p-type doping method for transition metal dichalcogenides(TMDs),which can effectively be used to extend the optical response range.The p-type doping based on surface charge transfer involves the chemical adsorption of the Lewis acid SnCl_(4)as a light absorption layer on the surface of WS_(2),significantly enhancing its UV photodetection performance.Under 365 nm laser irradiation,WS_(2)PDs exhibit response speed of 24 ms/20 ms,responsivity of 660 mA/W,detectivity of 3.3×10^(11)Jones,and external quantum efficiency of 226%.Moreover,we successfully apply this doping method to other TMDs materials(such as MoS_(2),MoSe_(2),and WSe_(2))and fabricate WS_(2) lateral p–n heterojunction PDs.

Optical two-dimensional coherent spectroscopy of excitons in transition-metal dichalcogenides

Exciton physics in atomically thin transition-metal dichalcogenides(TMDCs)holds paramount importance for fundamental physics research and prospective applications.However,the experimental exploration of exciton physics,including excitonic coherence dynamics,exciton many-body interactions,and their optical properties,faces challenges stemming from factors such as spatial heterogeneity and intricate many-body effects.In this perspective,we elaborate upon how optical two-dimensional coherent spectroscopy(2DCS)emerges as an effective tool to tackle the challenges,and outline potential directions for gaining deeper insights into exciton physics in forthcoming experiments with the advancements in 2DCS techniques and new materials.

Recent Progress of Two-Dimensional Thermoelectric Materials

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power.Moreover,the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades.Among these compounds,layered two-dimensional(2D)materials,such as graphene,black phosphorus,transition metal dichalcogenides,IVA–VIA compounds,and MXenes,have generated a large research attention as a group of potentially high-performance thermoelectric materials.Due to their unique electronic,mechanical,thermal,and optoelectronic properties,thermoelectric devices based on such materials can be applied in a variety of applications.Herein,a comprehensive review on the development of 2D materials for thermoelectric applications,as well as theoretical simulations and experimental preparation,is presented.In addition,nanodevice and new applications of 2D thermoelectric materials are also introduced.At last,current challenges are discussed and several prospects in this field are proposed.

Environmental Analysis with 2D Transition-Metal Dichalcogenide-Based Field-Effect Transistors

Field-effect transistors(FETs)present highly sensitive,rapid,and in situ detection capability in chemical and biological analysis.Recently,two-dimensional(2D)transition-metal dichalcogenides(TMDCs)attract significant attention as FET channel due to their unique structures and outstanding properties.With the booming of studies on TMDC FETs,we aim to give a timely review on TMDCbased FET sensors for environmental analysis in different media.First,theoretical basics on TMDC and FET sensor are introduced.Then,recent advances of TMDC FET sensor for pollutant detection in gaseous and aqueous media are,respectively,discussed.At last,future perspectives and challenges in practical application and commercialization are given for TMDC FET sensors.This article provides an overview on TMDC sensors for a wide variety of analytes with an emphasize on the increasing demand of advanced sensing technologies in environmental analysis.

Defect repairing in two-dimensional transition metal dichalcogenides

Atomically thin two-dimensional(2D)transition metal dichalcogenides(TMDCs)have stimulated enormous research interest due to rich phase structure,high theoretical carrier mobility and layer-dependent bandgap.In view of the close correlation between defects and properties in 2D TMDCs,more attentions have been paid on the defect engineering in recent years,however the mechanism is still unclear.Herein,we review the critical progress of defect engineering and provide an extensive way to modulate the properties depressed by defects.To insight into the defect engineering,we firstly introduce two common kinds of defects during the growth progress of TMDCs and the possible distribution of energy levels those defects could induce.Then,various methods to improve point defects and grain boundaries during the period of growth are discussed intensively,with the assistance of which more large-area TMDCs films can be obtained.Considering the defects in TMDCs are inevitable regardless of concentration,we also highlight strategies to heal the defects after growth.Through dry methods or wet methods,the chalcogen vacancies can be repaired and thus,the performance of electronic device would be significantly enhanced.Finally,we propose the challenges and prospective for defect engineering in 2D TMDCs materials to support the optimization of device and lead them to wide applied fields.

Flexible electronics based on one-dimensional inorganic semiconductor nanowires and two-dimensional transition metal dichalcogenides

Flexible electronics technology is considered as a revolutionary technology to unlock the bottleneck of traditional rigid electronics that prevalent for decades,thereby fueling the next-generation electronics.In the past few decades,the research on flexible electronic devices based on organic materials has witnessed rapid development and substantial achievements,and inorganic semiconductors are also now beginning to shine in the field of flexible electronics.As validated by the latest research,some of the inorganic semiconductors,particularly those at low dimension,unexpectedly exhibited excellent mechanical flexibility on top of superior electrical properties.Herein,we bring together a comprehensive analysis on the recently burgeoning low-dimension inorganic semiconductor materials in flexible electronics,including one-dimensional(1D)inorganic semiconductor nanowires(NWs)and two-dimensional(2D)transition metal dichalcogenides(TMDs).The fundamental electrical properties,optical properties,mechanical properties and strain engineering of materials,and their performance in flexible device applications are discussed in detail.We also propose current challenges and predict future development directions including material synthesis and device fabrication and integration.

Two-dimensional transition metal dichalcogenides for post-silicon electronics

Rapid advancements in information technology push the explosive growth in data volume,requiring greater computing-capability logic circuits.However,conventional computing-capability improving technology,which mainly relies on increasing transistor number,encounters a significant challenge due to the weak field-effect characteristics of bulk siliconbased semiconductors.Still,the ultra-thin layered bodies of two-dimensional transition metal dichalcogenides(2D-TMDCs)materials enable excellent field-effect characteristics and multiple gate control ports,facilitating the integration of the functions of multiple transistors into one.Generally,the computing-capability improvement of the transistor cell in logic circuits will greatly alleviate the challenge in transistor numbers.In other words,one can only use a small number,or even just one,2DTMDCs-based transistors to conduct the sophisticated logic operations that have to be realized by using many traditional transistors.In this review,from material selection,device structure optimization,and circuit architecture design,we discuss the developments,challenges,and prospects for 2D-TMDCs-based logic circuits.

Strong correlations in two-dimensional transition metal dichalcogenides

Since the discovery of graphene,the development of two-dimensional material research has enabled the exploration of a rich variety of exotic quantum phenomena that are not accessible in bulk materials.These two-dimensional materials offer a unique platform to build novel quantum devices.Layered transition metal dichalcogenides,when thinned down to atomic thicknesses,exhibit intriguing physical properties such as strong electron correlations.The study of strongly-correlated phenomena in twodimensional transition metal dichalcogenides has been a major research frontier in condensed matter physics.In this article,we review recent progress on strongly-correlated phenomena in two-dimensional transition metal dichalcogenides,including Mott insulators,quantum spin liquids,and Wigner crystals.These topics represent a rapidly developing research area,where tremendous opportunities exist in discovering exotic quantum phenomena,and in exploring their applications for future electronic devices.

Growth of Noncentrosymmetric Two-Dimensional Single Crystals

Among the various two-dimensional(2D)materials,more than 99%of them are noncentrosymmetric.However,since the commonly used substrates are generally centrosymmetric,antiparallel islands are usually inevitable in the growth of noncentrosymmetric 2D materials because of the energetic equivalency of these two kinds of antiparallel islands on centrosymmetric substrates.Therefore,achieving the growth of noncentrosymmetric 2D single crystals has long been a great challenge compared with the centrosymmetric ones like graphene.In this review,we presented the remarkable efforts and progress in the past decade,through precise chemical processes.We first discussed the great challenge and possible strategies in the growth of noncentrosymmetric 2D single crystals.Then,we focused on the advancements made in producing representative noncentrosymmetric 2D single crystals,including hexagonal boron nitride(hBN),transition metal dichalcogenides(TMDs),and other noncentrosymmetric 2D materials.At last,we summarized and looked forward to future research on the growth of layer-,stacking-,and twist-controlled noncentrosymmetric 2D single crystals and their heterostructures.

Two-Dimensional van der Waals Superconductor Heterostructures:Josephson Junctions and Beyond

The past few years have witnessed prominent progress in two-dimensional(2D)van der Waals heterostructures.Vertically assembled in an artificial manner,these atomically thin layers possess distinctive electronic,magnetic,and other properties,which have provided a versatile platform for both fundamental exploration and practical applications in condensed matter physics and materials science.Within various potential combinations,a particular set of van der Waals superconductor(SC)heterostructures,which is realized by stacking fabrication based on two-dimensional SCs,is currently attracting intense attention.For example,the Josephson junction,a specific structure in which a nonsuperconducting barrier is inserted between two proximity-coupled SCs,shows phenomena and outstanding properties with atomic-scale thickness.In this Perspective,we first review this emerging research area of van der Waals SC heterostructures,especially progress on the 2D van der Waals Josephson junctions,from the aspects of preparation,performance,and application,and also propose our vision for the future direction and potential innovation opportunities.

Recent advances on liquid intercalation and exfoliation of transition metal dichalcogenides: From fundamentals to applications

The weak van der Waals gap endows two dimensional transition metal dichalcogenides(2D TMDs)with the potential to realize guest intercalation and host exfoliation.Intriguingly,the liquid intercalation and exfoliation is a facile,low-cost,versatile and scalable strategy to modulate the structure and physiochemical property of TMDs via introducing foreign species into interlayer.In this review,firstly,we briefly introduce the resultant hybrid superlattice and disperse nanosheets with tailored properties fabricated via liquid intercalation and exfoliation.Subsequently,we systematically analyze the intercalation phenomenon and limitations of various intercalants in chemical or electrochemical methods.Afterwards,we intensely discuss diverse functionalities of resultant materials,focusing on their potential applications in energy conversion,energy storage,water purification,electronics,thermoelectrics and superconductor.Finally,we highlight the challenges and outlooks for precise and mass production of 2D TMDs-based materials via liquid intercalation and exfoliation.This review enriches the overview of liquid intercalation and exfoliation strategy,and paves the path for relevant high-performance devices.

Two-dimensional alloyed transition metal dichalcogenide nanosheets:Synthesis and applications

Two-dimensional(2D)transition metal dichalcogenide(TMD)nanosheets have attracted considerable attention owing to their diverse properties and great potential in a wide range of applications.In order to further tune their properties and then broaden their application domain,large efforts have been devoted into engineering the structures of 2D TMD nanosheets at atomic scale,especially the alloying technology.Alloying different 2D TMD nanosheets into 2D alloys not only offers the opportunities to fine-tune their physical/chemical properties,but also opens up some unique properties,which are highly desirable for wide applications including electronics,optoelectronics and catalysis.This review summarizes the recent progress in the preparation,characterization and applications of 2D alloyed TMD nanosheets.

First-principle high-throughput calculations of carrier effective masses of two-dimensional transition metal dichalcogenides

Two-dimensional group-VIB transition metal dichalcogenides（with the formula of MX2） emerge as a family of intensely investigated semiconductors that are promising for both electronic（because of their reasonable carrier mobility） and optoelectronic（because of their direct band gap at monolayer thickness） applications. Effective mass is a crucial physical quantity determining carriers transport, and thus the performance of these applications. Here we present based on first-principles high-throughput calculations a computational study of carrier effective masses of the two-dimensional MX2 materials. Both electron and hole effective masses of different MX2（M = Mo, W and X = S, Se, Te）, including in-layer/out-of-layer components, thickness dependence, and magnitude variation in heterostructures, are systemically calculated. The numerical results, chemical trends, and the insights gained provide useful guidance for understanding the key factors controlling carrier effective masses in the MX2 system and further engineering the mass values to improve device performance.

Two-dimensional noble transition-metal dichalcogenides for nanophotonics and optoelectronics:Status and prospects

An emerging subclass of transition-metal dichalcogenides(TMDs),noble-transition-metal dichalcogenides(NMDs),has led to an increase in nanoscientific research in two-dimensional(2D)materials.NMDs feature a unique structure and several useful properties.2D NMDs are promising candidates for a broad range of applications in areas such as photodetectors,phototransistors,saturable absorbers,and meta optics.In this review,the state of the art of 2D NMDs research,their structures,properties,synthesis,and potential applications are discussed,and a perspective of expected future developments is provided.

Synthesis of two-dimensional transition metal dichalcogenides for electronics and optoelectronics

Tremendous efforts have been devoted to preparing the ultrathin two-dimensional(2D)transition-metal dichalcogenides(TMDCs)and TMDCS-based heterojunctions owing to their unique properties and great potential applications in next generation electronics and optoelectronics over the past decade.However,to fulfill the demands for practical applications,the batch production of 2D TMDCs with high quality and large area at the mild condi-tions is still a challenge.This feature article reviews the state-of-the art research progresses that focus on the preparation and the applications in elec-tronics and optoelectronics of 2D TMDCs and their van der Waals hetero-junctions.First,the preparation methods including chemical and physical vapor deposition growth are comprehensively outlined.Then,recent progress on the application of fabricated 2D TMDCs based materials is revealed with particular attention to electronic(eg,field effect transistors and logic circuits)and optoelectronic(eg,photodetectors,photovoltaics,and light emitting diodes)devices.Finally,the challenges and future prospects are considered based on the current advance of 2D TMDCs and related heterojunctions.

Two-dimensional square transition metal dichalcogenides with lateral heterostructures

Fabrication of lateral heterostructures （LHS） is promising for a wide range of next-generation devices and could sufficiently unlock the potential of two-dimensional materials.Herein,we demonstrate the design of lateral heterostructures based on new building materials,namely 1S-MX2 LHS,using first-principles calculations.1S-MX2 LHS exhibits excellent stability,demonstrating high feasibility in the experiment.The desired bandgap opening can endure application at room temperature and was confirmed in 1S-MX2 LHS with spin-orbit coupling （SOC）.A strain strategy further resulted in efficient bandgap engineering and an intriguing phase transition.We also found that black phosphorus can serve as a competent substrate to support 1S-MX2 LHS with a coveted type-Ⅱ band alignment,allowing versatile functionalized bidirectional heterostructures with built-in device functions.Furthermore,the robust electronic features could be maintained in the 1S-MX2 LHS with larger components.Our findings will not only renew interest in LHS studies by enriching their categories and properties,but also highlight the promise of these lateral heterostructures as appealing materials for future integrated devices.

The influence of two-dimensional organic adlayer thickness on the ultralow frequency Raman spectra of transition metal dichalcogenide nanosheets

Recently, it has been reported that physisorbed adsorbates can be trapped between the bottom surface of twodimensional(2D) materials and supported substrate to form2 D confined films. However, the influence of such 2D confined adsorbates on the properties of 2D materials is rarely explored. Herein, we combined atomic force microscopy(AFM), Kelvin probe force microscopy(KPFM) and Raman spectroscopy especially the ultralow frequency(ULF) Raman spectroscopy to explore the influence of 2D confined organic adlayer thickness on the ULF breathing modes of few-layer MoS2 and WSe2nanosheets. As the thickness of organic adlayers increased, red shift, coexistence of blue and red shifts as well as blue shift of ULF breathing mode was observed. KPFM measurement confirmed the enhanced n-doping and p-doping behaviors of organic adlayers as their thickness increased,respectively. Our results will provide new insights into the interaction between 2D confined adsorbates and bottom surface of 2D nanosheets, which could be useful for modulating properties of 2D materials.

Band structure engineering through van der Waals heterostructing superlattices of two-dimensional transition metal dichalcogenides

The indirect-to-direct band-gap transition in transition metal dichalcogenides(TMDCs)from bulk to monolayer,accompanying with other unique properties of two-dimensional materials,has endowed them great potential in optoelectronic devices.The easy transferability and feasible epitaxial growth pave a promising way to further tune the optical properties by constructing van der Waals heterostructures.Here,we performed a systematic high-throughput first-principles study of electronic structure and optical properties of the layerby-layer stacking TMDCs heterostructing superlattices,with the configuration space of[(MX2)n(M0X02)10−n](M/M0=Cr,Mo,W;X/X0=S,Se,Te;n=0-10).Our calculations involving long-range dispersive interaction show that the indirect-to-direct band-gap transition or even semiconductor-to-metal transition can be realized by changing component compositions of superlattices.Further analysis indicates that the indirect-to-direct band-gap transition can be ascribed to the in-plane strain induced by lattice mismatch.The semiconductor-to-metal transition may be attributed to the band offset among different components that is modified by the in-plane strain.The superlattices with direct band-gap show quite weak band-gap optical transition because of the spacial separation of the electronic states involved.In general,the layers stacking-order of superlattices results in a small up to 0.2 eV band gap fluctuation because of the built-in potential.Our results provide useful guidance for engineering band structure and optical properties in TMDCs heterostructing superlattices.

Recent developments in CVD growth and applications of 2D transition metal dichalcogenides

Two-dimensional(2D)transition metal dichalcogenides(TMDs)with fascinating electronic energy band structures,rich valley physical properties and strong spin–orbit coupling have attracted tremendous interest,and show great potential in electronic,optoelectronic,spintronic and valleytronic fields.Stacking 2D TMDs have provided unprecedented opportunities for constructing artificial functional structures.Due to the low cost,high yield and industrial compatibility,chemical vapor deposition(CVD)is regarded as one of the most promising growth strategies to obtain high-quality and large-area 2D TMDs and heterostructures.Here,state-of-the-art strategies for preparing TMDs details of growth control and related heterostructures construction via CVD method are reviewed and discussed,including wafer-scale synthesis,phase transition,doping,alloy and stacking engineering.Meanwhile,recent progress on the application of multi-functional devices is highlighted based on 2D TMDs.Finally,challenges and prospects are proposed for the practical device applications of 2D TMDs.

Phase engineering two-dimensional nanostructures for electrocatalytic hydrogen evolution reaction

Hydrogen(H2)is considered to be a promising substitute for fossil fuels.Two-dimensional(2D)nanomaterials have exhibited an efficient electrocatalytic capacity to catalyze hydrogen evolution reaction(HER).Particularly,phase engineering of 2D nanomaterials is opening a novel research direction to endow 2D nanostructures with fascinating properties for deep applications in catalyzing HER.In this review,we briefly summarize the research progress and present the current challenges on phase engineering of 2D nanomaterials for their applications in electrocatalytic HER.Our summary will be of significance to provide fundamental understanding for designing novel 2D nanomaterials with unconventional phases to electrochemically catalyze HER.