Abnormal n-type doping effect in nitrogen-doped tungsten diselenide prepared by moderate ammonia plasma treatment

To facilitate potential applications of tungsten diselenide （WSe2） in electronics, controllable doping is of great importance. As an industrially compatible technology, plasma treatment has been used to dope two-dimensional （2D） materials. However, owing to the strong etching effect in transition metal dichalcogenides （TMDCs）, it is difficult to controllably dope 2D WSe2 crystals by plasma. Herein, we develop a moderate ammonia plasma treatment method to prepare nitrogen-doped WSe2 with controlled nitrogen content. Interestingly, Raman, photoluminescence, X-ray photoelectron spectroscopy, and electrical Lts reveal abnormal n-doping behavior of nitrogen-doped WSe2, which is attributed to selenium anion vacancy introduced by hydrogen species in ammonia plasma. Nitrogen-doped WSe2 with abnormal n-doping behavior has potential applications in future TMDCs-based electronics.

High-performance electronics and optoelectronics of monolayer tungsten diselenide full film from pre-seeding strategy

Tungsten diselenide (WSe2) possesses extraordinary electronic properties forapplications in electronics, optoelectronics, and emerging exciton physics. Thesynthesis of monolayer WSe2 film is of topmost for device arrays and integratedcircuits. The monolayer WSe2 film has yet been reported by thermal chemicalvapor deposition (CVD) approach, and the nucleation mechanism remainsunclear. Here, we report a pre-seeding strategy for finely regulating the nucleidensity at an early stage and achieving a fully covered film after chemicalvapor deposition growth. The underlying mechanism is heterogeneous nucle-ation from the pre-seeding tungsten oxide nanoparticles. At first, we optimized the precursor concentration for pre-seeding. Besides, we confirmed the superi-ority of the pre-seeding method, compared with three types of substrate pre-treatments, including nontreatment, sonication in an organic solvent, andoxygen plasma. Eventually, the high-quality synthetic WSe2 monolayer filmexhibits excellent device performance in field-effect transistors and photodetec-tors. We extracted thermodynamic activation energy from the nucleation andgrowth data. Our results may shed light on the wafer-scale production ofhomogeneous monolayer films of WSe2, other 2D materials, and their van derWaals heterostructures.

Effect of electrical contact on performance of WSe_(2) field effect transistors

Two-dimensional(2 D) transition metal dichalcogenides(TMDCs) such as tungsten diselenide(WSe_(2)) have spead many interesting physical properties, which may become ideal candidates to develop new generation electronic and optoelectronic devices. In order to reveal essential features of 2 D TMDCs, it is necessary to fabricate high-quality devices with reliable electrical contact. We systematically analyze the effect of graphene and metal contacts on performance of multilayered WSe_(2) field effect transistors(FETs). The temperature-dependent transport characteristics of both devices are tested.Only graphene-contacted WSe_(2) FETs are observed with the metal-insulator transition phenomenon which mainly attributes to the ultra-clean contact interface and lowered contact barrier. Further characterization on contact barrier demonstrates that graphene contact enables lower contact barrier with WSe_(2) than metal contact, since the Fermi level of graphene can be modulated by the gate bias to match the Fermi level of the channel material. We also analyze the carrier mobility of both devices under different temperatures, revealing that graphene contact can reduce the charge scattering of the device caused by ionized impurities and phonon vibrations in low and room temperature regions, respectively. This work is expected to provide reference for fabricating 2 D material devices with decent performances.

Nanocrystalline WSe_(2)excels at high-performance anode for Na storage via a facile one-pot hydrothermal method

For the next large-scale energy storage systems,sodium-ion batteries(SIBs)with excellent electrochemical performance are promising.However,the exploration of anode materials with high specific capacity,fascinating cycling stability and rate capability is still restricted.Among transition metal dichalcogenides(TMDs),tungsten diselenide(WSe_(2))has been regarded as an effective anode material for SIBs and has been extensively studied,due to high theoretical capacity and unique twodimensional layered structure.Herein,nanocrystalline WSe_(2)is prepared by a facile one-pot hydrothermal method.Compared with the micro-scale WSe_(2),benefiting from the high specific surface area of highly ordered nano-flake structures and short ion/electron transport paths,nanocrystalline WSe_(2)shows excellent electrochemical performance in Na storage.After 1000 cycles at a current density of 2 A·g^(-1),a high specific capacity of 264.4 mA·h·g^(-1)is still maintained.The full cell matched with the Na_(3)V_(2)(PO_(4))_(3) cathode can deliver an excellent reversible specific capacity of 196.5 mA·h·g^(-1)after 100 cycles at 0.5 A·g^(-1).

Defects as a factor limiting carrier mobility in WSe2： A spectroscopic investigation

The electrical performance of two-dimensional transition metal dichalcogenides （TMDs） is strongly affected by the number of structural defects. In this work, we provide an optical spectroscopic characterization approach to correlate the number of structural defects and the electrical performance of WSe2 devices. Low-temperature photoluminescence （PL） spectra of electron-beam-lithography- processed WSe2 exhibit a clear defect-induced PL emission due to excitons bound to defects, which would strongly degrade the electrical performance. By adopting an electron-beam-free transfer-electrode technique, we successfully prepared a backgated WSe2 device containing a limited amount of defects. A maximum hole mobility of approximately 200 cm2.V -1.s-1 was achieved because of the reduced scattering sources, which is the highest reported value for this type of device. This work provides not only a versatile and nondestructive method to monitor the defects in TMDs but also a new route to approach the room-temperature phonon-limited mobility in high-performance TMD devices.

High performance IEICO-4F/WSe_(2)heterojunction photodetector based on photoluminescence quenching behavior

Heterostructure is the basic building block for functional optoelectronic devices.Heterostructures consisting of two-dimensional(2D)transition metal dichalcogenides(TMDs)and organic semiconductors are currently attracting great interest for highperformance optoelectronics.However,how to design heterostructure for highly efficient optoelectronic devices remains a big challenge.Here we design high-performance organic semiconductor/WSe_(2)heterostructure photodetectors by tailoring the charge transfer effect between 2,2ʹ-((2Z,2ʹZ)-(((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydros-indaceno[1,2-b:5,6-bʹ]dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile(IEICO-4F)organic semiconductors with various thicknesses and monolayer WSe_(2).With the increase of IEICO-4F layer thickness,the photoluminescence(PL)characteristics of WSe_(2)could be completely quenched due to the charge transfer from the lowest unoccupied molecular orbital(LUMO)level of IEICO-4F to the conduction band minimum(CBM)of WSe_(2).Benefiting from the exquisite charge transfer behavior,the IEICO-4F/WSe_(2)heterojunction photodetector with optimized 6.0-nm thick IEICO-4F shows high performance including the responsivity of 8.32 A/W and specific detectivity of 4.65×10^(11)Jones at incident light of 808 nm.This work demonstrates a simple approach based on PL characteristics to design high-performance IEICO-4F/WSe_(2)heterojunction,thus paving the way for the development of excellent optoelectronic devices based on organic/TMD heterostructures.

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.

Niobium doping induced mirror twin boundaries in MBE grown WSe2 monolayers

Mirror twin boundary(MTB)brings unique one-dimensional(1D)physics and properties into two-dimensional(2D)transition metal dichalcogenides(TMDCs),but they were rarely observed in non-Mo-based TMDCs.Herein,by post growth Nb doping,high density 4|4E-W and 4|4P-Se mirror twin boundaries(MTBs)were introduced into molecular beam epitaxy(MBE)grown WSe2 monolayers.Of them,4|4E-W MTB with a novel structure was discovered experimentally for the first time,while 4|4P-Se MTBs present a random permutations of W and Nb,forming a 1D alloy system.Comparison between the doped and non-doped WSe2 confirmed that Nb dopants are essential for MTB formation.Furthermore,quantitative statistics reveal the areal density of MTBs is directly proportional to the concentration of Nb dopants.To unravel the injection pathway of Nb dopants,first-principles calculations about a set of formation energies for excess Nb atoms with different configurations were conducted,based on which a model explaining the origin of MTBs introduced by excess metal was built.We conclude that the formation of MTBs is mainly driven by the collective evolution of excess Nb atoms introduced into the lttice of host WSe2 crystal and subsequent displacement of metal atoms(W or Nb).This study provides a novel way to tailor the MTBs in 2D TMDC materials via proper metal doping and presents new opportunities for exploring the intriguing properties.