Anomalous photoluminescence enhancement and resonance charge transfer in type-II 2D lateral heterostructures

Type-Ⅱband alignment can realize the efficient charge transfer and separation at the semiconductor heterointerface,which results in photoluminescence(PL)quenching.Recently,several researches demonstrated great enhancement of localized PL at the interface of type-Ⅱtwo-dimensional(2D)heterostructure.However,the dominant physical mechanism of this enhanced PL emission has not been well understood.In this work,we symmetrically study the exciton dynamics of type-Ⅱlateral heterostructures of monolayer MoS_(2) and WS_(2) at room temperatures.The strong PL enhancement along the one-dimensional(1D)heterointerface is associated with the trion emission of the WS_(2) shell,while a dramatic PL quenching of neutral exciton is observed on the MoS_(2) core.The enhanced quantum yield of WS2trion emission can be explained by charge-transfer-enhanced photoexcited carrier dynamics,which is facilitated by resonance hole transfer from MoS_(2) side to WS_(2) side.This work sheds light on the 1D exciton photophysics in lateral heterostructures,which has the potential to lead to new concepts and applications of optoelectronic device.

Visualizing interface states in In_(2)Se_(3)–WSe_(2)monolayer lateral heterostructures

Recent findings of two-dimensional(2D)ferroelectric(FE)materials provide more possibilities for the development of 2D FE heterostructure electronic devices based on van der Waals materials and the application of FE devices under the limit of atomic layer thickness.In this paper,we report the in-situ fabrication and probing of electronic structures of In_(2)Se_(3)–WSe_(2) lateral heterostructures,compared with most vertical FE heterostructures at present.Through molecular beam epitaxy,we fabricated lateral heterostructures with monolayer WSe_2(three atomic layers)and monolayer In_(2)Se_(3)(five atomic layers).Type-Ⅱband alignment was found to exist in either the lateral heterostructure composed of anti-FEβ′-In_(2)Se_(3) and WSe_(2) or the lateral heterostructure composed of FEβ*-In_(2)Se_(3)and WSe_2,and the band offsets could be modulated by ferroelectric polarization.More interestingly,interface states in both lateral heterostructures acted as narrow gap quantum wires,and the band gap of the interface state in theβ*-In_(2)Se_(3)–WSe_(2)heterostructure was smaller than that in theβ′-In_(2)Se_(3)heterostructure.The fabrication of 2D FE heterostructure and the modulation of interface state provide a new platform for the development of FE devices.

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.

Facile synthesis of two-dimensional MoS_(2)/WS_(2) lateral heterostructures with controllable core/shell size ratio by a one-step chemical vapor deposition method

Heterostructures based on two-dimensional(2D) transition-metal dichalcogenides(TMDCs) possess unique electronic and optical properties, which open up unprecedented opportunities in nanoscale optoelectronic devices. Synthesizing high-quality 2D TMDC heterostructures with different core/shell size ratios is of great significance for practical applications. Here, we report a simple one-step chemical vapor deposition(CVD) method for fabricating MoS2/WS2 lateral heterostructures with controllable core/shell size ratio. An ultrathin MoO3/WO3 film prepared by thermal evaporation was used as the precursor, and a step-like heating process was adopted to separately grow MoS2 and WS2 monolayers by taking advantage of the different melting points of MoO3 and WO3 sources. High-quality MoS2/WS2 lateral heterostructures with sharp interfaces were fabricated by optimizing the key growth parameters. Furthermore, the core/shell size ratio of heterostructures could be easily controlled by changing the thickness ratio of MoO3/WO3 film, and an approximately linear dependence between them is revealed. Compared with MoS2 or WS2 monolayers, the MoS2/WS2 heterostructure exhibited a shortened exciton lifetime owing to the type-Ⅱ energy band alignment, which is conducive to the application of high-performance devices. This work provides a facile strategy for the synthesis of 2D lateral heterostructures with controllable size ratio.

In-plane epitaxial growth of 2D CoSe-WSe2 metalsemiconductor lateral heterostructures with improved WSe2 transistors performance

The two-dimensional(2D)in-plane(lateral)heterostructures have attracted increasing interest for potential applications in the atomically thin electronics and optoelectronics.While most studies focus on semiconductorsemiconductor lateral heterostructures with highly similar lattice structures between the constituent components,the synthesis of metal-semiconductor lateral heterostructures is much less explored and usually more challenging due to more distinct lattice structures or chemical properties.Herein,a vapor phase epitaxy growth method of high-quality metal-semiconductor lateral heterostructures between tetragonal CoSe and hexagonal WSe2 is reported.The 2D CoSe can selectively nucleate at the edge of pre-grown WSe2 nanosheets to form CoSe-WSe2 metal-semiconductor lateral heterostructures.Optical microscopy(OM),scanning electron microscopy(SEM),and atomic force microscopy(AFM)studies reveal distinct contrast across the heterostructure interface.High-resolution transmission electron microscopy(HRTEM)and selected area electron diffraction(SAED)studies further confirm the microstructure modulation across the heterostructure interface.The field-effect transistors(FETs)of CoSe-WSe2 lateral heterostructures show satisfactory Ohmic contacts and considerably better FET performance over those with deposited Cr/Au contacts,suggesting the in-plane metal-semiconductor junctions may function as improved contacts for the atomically thin electronics.

Edge-and strain-induced band bending in bilayer-monolayer Pb_(2)Se_(3) heterostructures

By using scanning tunneling microscope/microscopy(STM/STS), we reveal the detailed electronic structures around the sharp edges and strained terraces of lateral monolayer-bilayer Pd_(2)Se_(3) heterostructures. We find that the edges of such heterostructures are well-defined zigzag type. Band bending and alignment are observed across the zigzag edge, forming a monolayer-bilayer heterojunction. In addition, an n-type band bending is induced by strain on a confined bilayer Pd_(2)Se_(3) terrace. These results provide effective toolsets to tune the band structures in Pd_(2)Se_(3)-based heterostructures and devices.

Mechanical proper ties of lateral transition metal dichalcogenide heterostructures

Transition metal dichalcogenide(TMD)monolayers attract great attention due to their specific structural,electronic and mechanical properties.The formation of their lateral heterostructures allows a new degree of flexibility in engineering electronic and optoelectronic dervices.However,the mechanical properties of the lateral heterostructures are rarely investigated.In this study,a comparative investigation on the mechanical characteristics of 1H,IT'and 1H/1T'heterostructure phases of different TMD monolayers including molybdenum disulfide(M0S_(2))molybdenum diselenide(MoSe_(2)),Tungsten disulfide(WS_(2)),and Tungsten diselenide(WSe_(2))was conducted by means of density functional theory(DFT)calculations.Our results indicate that the impact of the lateral heterostructures has a relatively weak mechanical strength for all the TMD monolayers.The significant correlation bet ween the mechanical properties of the TMD monolayers and their structural phases can be used to tune their stiffness of the materials.Our findings,therefore,suggest a novel strategy to manipulate the mechanical characteristics of TMDs by engineering their structural phases for their practical applications.

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.

Recent progress about 2D metal dichalcogenides: Synthesis and application in photodetectors

In recent years, two-dimensional (2D) layered metal dichalcogenides (MDCs) have received enormous attention on account of their excellent optoelectronic properties. Especially, various MDCs can be constructed into vertical/lateral heterostructures with many novel optical and electrical properties, exhibiting great potential for the application in photodetectors. Therefore, the batch production of 2D MDCs and their heterostructures is crucial for the practical application. Recently, the vapour phase methods have been proved to be dependable for growing large-scale MDCs and related heterostructures with high quality. In this paper, we summarize the latest progress about the synthesis of 2D MDCs and their heterostructures by vapour phase methods. Particular focus is paid to the control of influence factors during the vapour phase growth process. Furthermore, the application of MDCs and their heterostructures in photodetectors with outstanding performance is also outlined. Finally, the challenges and prospects for the future application are presented.

Controlled growth of transition metal dichalcogenide via ther-mogravimetric prediction of precursors vapor concentration

Transition metal dichalcogenide(TMD)alloys and heterostructures are attracting increasing attention thanks to their unique electronic,optical,and interfacial properties.However,the growth fundamental of TMD alloys and heterostructures during one-step growth is still beyond understanding.Here,thermogravimetric(TG/DTG)technology is introduced to predict the evolution of the precursor(MoO_(3)and WO_(3))concentration in the vapor during growth.We establish the correlation between precursor concentration and the corresponding growth behavior.TG/DTG predication suggests that tuning precursor temperature and powder ratio can alter their concentration in the vapor,well explaining the formation of Mo_(x)W_(1-x)Se_(2) alloy or MoSe_(2)-WSe_(2) heterostructure at different growth conditions.Based on the TG/DTG analysis,we further design and grow a complex MoSe_(2)-Mo_(x)W_(1-x)Se_(2)-WSe_(2) heterostructure and Mo_(x)W_(1-x)Se_(2) monolayer alloys,confirming the validity of TG/DTG prediction in TMD crystal synthesis.Thus,employing TG/DTG to predict the synthesis of two-dimensional materials is of importance to understand the TMD growth behavior and provide guidance to the desired TMD heterostructure formation for future photoelectric devices.