WSe2 Thin Films Research Articles

Probing the effect of selenization on RF sputtered WSe2 thin films toward enhanced photoelectrochemical and photodetection performance

Tungsten diselenide (WSe2) is one of the promising two-dimensional materials among transition metal dichalcogenide (TMDCs) for its application in solar hydrogen production and optoelectronics devices. In this paper, we report the synthesis of WSe2 thin films using RF-magnetron sputtering followed by post-selenizationin in purview of their use for photoelectrochemical (PEC) water splitting and photodetection applications. Characterization of selenized WSe2 film shows improved structural, optical, and electrical properties compared to as-deposited WSe2 film. The PEC evaluation shows that the selenized WSe2 photocathode has improved photocurrent density, stability, and charge transfer properties compared to the as-deposited film. Furthermore, the selenized WSe2 thin film photodetector demonstrated excellent performance, including better responsivity of 1.2 μA/W and detectivity of 9.5 × 105 Jones with enhanced response time under one sun illumination. These results highlight a simple fabrication approach for producing scalable and high-quality WSe2 thin films for next-generation optoelectronics devices. The post-selenization step can effectively heal defects and optimize optoelectronic properties.

Formula omitted] thin-films as efficient optical limiters: A study on impact of annealing

We present a detailed investigation on the impact of thermal annealing on the third-order nonlinear optical (NLO) properties of sputtered WSe2 thin films. The process of annealing results in an appreciable change in morphology and texture of WSe2 films that is governed by the annealing temperature and duration of annealing. These modifications are apparent from the electron micrographs as well as X-ray diffraction and Raman spectrum. The visible absorption spectroscopy point towards a gradual shrinkage of electronic bandgap which motivated us to explore the impact of NLO properties of the annealed WSe2 films. The NLO measurements were carried out using a single-beam Z-scan technique using ultrashort pulses centered at 1030 nm excitation wavelength. The investigations reveal a distinct two-photon absorption (TPA) signature for the annealed WSe2 films along with a self-focusing effect. The observations indicate the strengthening of optical limiting behavior exhibited by WSe2 films when they are annealed for a longer duration. The outcomes suggest that the process of annealing in WSe2 films allows controlling the NLO properties and renders them more suitable candidates as optical limiters for high-power applications.

Lateral WSe2 Homojunction through Metal Contact Doping: Excellent Self‐powered Photovoltaic Photodetector

AbstractHere an IR‐heating chemical vapor deposition (CVD) approach enabling fast 2D‐growth of WSe2 thin films is reported, and the great potential of metal contact doping in building CVD‐grown WSe2‐based lateral homojunction is demonstrated by contacting with TiN/Ni metals in favor of holes/electrons injection. Shortening nanosheet channel to ≈2 µm leads to pronounced enhancement in the performance of diode. The fabricated WSe2‐based diode exhibits high rectification ratios without the need of gate modulation and can work efficiently as photovoltaic cell, with maximum open circuit voltage reaching up to 620 mV and a high power conversion efficiency over 15%, empowering it as superb self‐powered photodetector for visible to near‐infrared lights, with photoresponsivity over 0.5 A W−1 and a fast photoresponse speed of 10 µs under 520 nm illumination. It is of practical significance to achieve well‐performed photovoltaic devices with CVD‐grown WSe2 using fab‐friendly metals and simple processing, which will help pave the way toward future mass production of optoelectronic chips.

Nonlinear Optical Absorption Properties of Sputtered WSe2 Thin-films in the Near-Infrared Spectral Band

Nonlinear Optical Absorption Properties of Sputtered WSe2 Thin-films in the Near-Infrared Spectral Band

Impact of defects on the χ(3) optical nonlinearity of sputtered WSe2 thin films in the optical communication band

We present an experimental investigation on third-order nonlinear optical properties of RF-sputtered WSe2 thin films using a single-beam Z-scan technique around the optical communication window in a near-infrared spectral band. The nonlinear absorption coefficient was obtained from the open-aperture Z-scan transmission using ultrashort pulses centered at 1520nm excitation wavelength. The results show a reverse saturable absorption signature for all the WSe2 films due to two-photon absorption (TPA) mediated electronic transitions. The dependency of the TPA coefficient on film thickness is analyzed through the modification in an electronic band structure essentially caused due to defects induced in the samples. In addition, we also explored the impact of high laser intensity on the TPA process, which essentially pointed toward a significant contribution of free-carrier absorption at small laser intensities in WSe2 thin films. This investigation provides a basis for optimally tailoring the nonlinear optical properties of transition-metal-dichalcogenides via bandgap engineering for improving photonic device functionality.

Comparative study of WSe2 thin films synthesized via pre-deposited WO3 and W precursor material selenization

Layered two-dimensional (2D) materials, such as p-type WSe2, are potential key materials in the manufacture of the next generation electronic devices. One of the remaining main challenges is the large area growth of high-quality films. A potential large-scale 2D WSe2 synthesis method is conversion (selenization) of a pre-deposited sacrificial precursor coating. However, its use is still limited, mainly due to a lack of understanding the growth mechanisms involved. Here, we have studied and compared properties of thin crystalline WSe2 films prepared via selenization of sputter-deposited sacrificial WO3 and W films. Surface morphology of the as-grown films was studied using a scanning electron microscope complemented with an atomic force microscope. The structure and chemical composition were confirmed by X-ray diffraction and micro-Raman spectroscopy, respectively. On-chip photoconductive devices were made using the standard photolithography process, and their photoresponse was investigated with 405 nm wavelength light. For the electrical characterization, field effect transistors (FETs) were made to measure output and transfer curves. The results obtained give insight into the growth of crystalline WSe2 via sacrificial film selenization.

Impact of pauli-blocking effect on optical limiting properties of WSe2 thin films

We present a detailed investigation on thickness-dependent third-order (χ(3)) nonlinear optical properties of RF-sputtered WSe2 thin-films using ultrashort pulses centered at different excitation wavelengths. The single-beam Z-scan based investigation in the visible spectrum reveals a predominant Pauli-blocking effect led saturable absorption or optical limiting in WSe2 thin-films. The study also explores the dispersion in third-order nonlinear susceptibility (χ(3)) in WSe2 thin films. Interestingly, WSe2 thin-films of any thickness exhibit a self-focusing effect depicting a positive nonlinear refractive index (n2 > 0). The frequency-dependent nonlinear absorption (β) bear distinct correlations with the bandgap of the films which is also investigated through density-functional-theory (DFT) based simulations. The alteration in bandstructure is primarily due to the Se-deficiency induced defect bands in WSe2 thin-films which has discernible impact on the optical limiting characteristics.

Transient Surface Photovoltage Spectroscopy of (NH4)2Mo3S13/WSe2 Thin-Film Photocathodes for Photoelectrochemical Hydrogen Evolution.

Hydrogen produced from solar energy has the potential to replace petroleum in the future. To this respect, there is a need in the abandoned and efficient materials that can continuously split water molecules using solar energy. In this report, an ammonium thiomolybdate (ATM: (NH4)2Mo3S13) is evaluated as a p-type semiconductor film photocathode for hydrogen evolution reaction. The ATM thin films are prepared by spin-coating on fluorine-doped tin oxide substrates, and their structural, morphological, optical, photoelectrical, and photoelectrochemical (PEC) properties are studied. Transient surface photovoltage (TSPV) spectroscopy and spectroscopic ellipsometry indicate the band gap Eg = 1.9 eV for the ATM thin films. Furthermore, the photovoltage of the ATM thin films measured by TSPV is correlated to the photocurrents measured by the PEC characterization that can be used to evaluate the material potential for hydrogen generation. The films exhibit a low photocurrent density of 46 μA cm-2 at 0 VRHE. However, its combination with WSe2 thin-film photocathodes results in a significant increase in photocurrent density up to 4.6 mA cm-2 at 0 VRHE (100 times). The reason for such a strong charge carrier transfer effect for ATM/WSe2 heterojunction photocathodes is studied by TSPV spectroscopy that allows a comprehensive evaluation of potential photovoltaic materials toward PEC hydrogen production. Furthermore, the photovoltage generated by a WSe2 thin film is 30 times lower than that of its single crystal, which indicates that the quality of WSe2 thin films should be improved for faster PEC hydrogen evolution.

Tungsten(VI) selenide tetrachloride, WSeCl4 - synthesis, properties, coordination complexes and application of [WSeCl4(SenBu2)] for CVD growth of WSe2 thin films.

WSeCl4 was obtained in good yield by heating WCl6 and Sb2Se3in vacuo. Green crystals grown by sublimation were shown by single crystal X-ray structure analysis to contain square pyramidal monomers with apical WSe, and powder X-ray diffraction (PXRD) analysis confirmed this to be the only form present in the bulk sample. Density functional theory (DFT) calculations using the B3LYP-D3 functional replicated the structure, identified the key bonding orbitals, and were used to aid assignment of the IR spectrum of WSeCl4. Reaction of WSeCl4 with ligands L gave [WSeCl4(L)] (L = MeCN, DMF, thf, py, OPPh3, 2,2'-bipy, SeMe2, SenBu2), whilst the dimers [(WSeCl4)2(μ-L-L)] were formed with L-L = Ph2P(O)CH2P(O)Ph2, 1,4-dioxane and 4,4'-bipyridyl. The complexes were characterised by microanalysis, IR and 1H NMR spectroscopy, and single crystal X-ray structures determined for [WSeCl4(L)] (L = OPPh3, MeCN, DMF) and [(WSeCl4)2(μ-L-L)] (L-L = 1,4-dioxane, 4,4'-bipyridyl). All except the 2,2'-bipy complex, which is probably seven-coordinate, contain six-coordinate tungsten with the neutral donor trans to WSe. Alkylphosphines, including PMe3 and PEt3, decompose WSeCl4 upon contact, forming phosphine selenides (SePR3). In contrast, the selenoether complexes [WSeCl4(SeMe2)] and [WSeCl4(SenBu2)] were isolated and characterised. The crystal structure of the minor W(VI) by-product, [(WSeCl4)2(μ-SeMe2)], was determined and using SMe2, a few crystals of the W(V) species, [{WCl3(SMe2)}2(μ-Se)(μ-Se2)], were obtained and structurally characterised. The isolated W(VI) complexes are compared with those of WOCl4 and WSCl4 and the combination of experimental and computational data are consistent with WSeCl4 being a weaker Lewis acid and its complexes significantly less stable than those of the lighter analogues, especially in solution. Low pressure chemical vapour deposition (LPCVD) using [WSeCl4(SenBu2)] in the range 660-700 °C (0.1 mmHg) produced highly reflective thin films, which were identified to be WSe2 by grazing incidence X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XRD analysis of the thinner films revealed them to be highly oriented in the <00l> direction.

Nucleation, growth, and stability of WSe2 thin films deposited on HOPG examined using in situ, real-time synchrotron x-ray radiation

We have examined the nucleation and growth of WSe2 thin films in ultrahigh vacuum on highly oriented pyrolytic graphite (HOPG) using in situ real-time x-ray fluorescence (XRF), and ex situ x-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy. We employed W(CO)6 as the W source delivered via a supersonic molecular beam, Sen delivered via an effusion cell, and we examined substrate temperatures from 400 to 540 °C. Crystalline, near stoichiometric thin films were formed at temperatures Ts ≤ 470 °C, whereas those formed at 540 °C were very W-rich. The thin films were not continuous but consisted of filamentlike features with spikelike edges. A focus of our work was to examine the initial stages of growth and the effects of extinguishing one of the species (W or Se) both before and during growth. First, in all cases examined, there was a delay in the onset of a measurable rate of growth on the clean HOPG surface following the introduction of both species, W(CO)6 and Sen. In cases where the incident flux of W(CO)6 was gated, once WSe2 growth had commenced, extinguishing the flux of W(CO)6 quenched growth immediately and did not result in the deposition of additional Se. Once the incident flux of W(CO)6 was re-started, growth began essentially immediately. The pattern with Sen gating was strikingly different. In this case, once WSe2 growth had commenced, extinguishing the flux of Sen resulted in a continuing uptake of W essentially unabated, while the amount of Se in the thin film decreased, which resulted in an oscillation in the Se-to-W content in the thin film. As the thin films were stable in UHV in the absence of both species, the incident W(CO)6 is responsible for the etching of Se, which we postulate is due to a ligand exchange reaction forming volatile SeCO.

Low temperature carrier transport mechanism and photo-conductivity of WSe2

This work reports the electrical-transport and temperature-dependent photoconductivity in tungsten diselenide (WSe2) thin films. Temperature-dependent electrical conductivity has been demonstrated using different models. At lower temperatures (< 190 K), carriers become localized to small regions in the film due to Mott’s hopping mechanism. The middle-temperature region (190–273 K) follows Seto’s parameters and obtained low barrier height (0.0873 eV) may be responsible for the improved carrier mobility. At higher temperature (> 273 K) region, thermally activated conduction is dominated with two activation energies of ~138 meV and 98 meV. The peaks obtained in photoluminescent analysis attributes to the presence of mid-bandgap states or defect states which play an important role in the photoconductivity of WSe2. The transient photoconductivity measurements show consistent temperature-dependent behaviour. The effect of light intensity and wavelength variation on the photoconductivity of WSe2 thin films is also discussed. The photo-current is 1.19 * 10−5 A at 125 K, while at 350 K was observed to be 3.12 * 10−4 A. The light-on/off current cycles show that the current can recover to its initial state after various cycles, which points to the WSe2 thin-film device's stable and reversible properties that can be used in optoelectronic applications.

The enhanced photoelectric properties of ZnO-doped WSe2 thin film

The photoelectric properties of ZnO-doped WSe2 thin films created on Si substrates by a thermal evaporation method were investigated. The effects of ZnO on the surface morphology, structure, photoluminescence, light absorption characteristics and electrical properties of WSe2 thin films were analyzed. It is found that the nucleation density and the crystallinity of the ZnO-doped WSe2 nanowires are higher than without doping, and the electron mobility of the doped sample is about 1.4 times that of the undoped sample. Also, doping improved the light absorption and photoluminescence efficiency. Additionally, the [Formula: see text]–[Formula: see text] curve of the ZnO-doped WSe2/Si heterojunction gradually changes from a rectification characteristic to a linear dependence, and the photocurrent increases by about four times when the light power increases from 0 to 25 mW/cm2. Moreover, the heterojunction has a very high sensitivity to operating temperature; the current significantly increases as the temperature increases to [Formula: see text]. With high absorptivity and photoluminescence efficiency, and sensitivity to light and temperature, ZnO-doped WSe2 film is promising for use in optoelectronic devices.

Hydrazine-free chemical bath deposition of WSe2 thin films and bi-layers for photovoltaic applications

We report the photovoltaic performance of a diode based on a p-type tungsten diselenide (WSe2) active region. The device is formed by WSe2 and cadmium sulfide (CdS) layers, both obtained via chemical bath deposition. A novel hydrazine-free chemical formulation allows deposition of WSe2 on glass and flexible Hastelloy substrates. Characterization of WSe2 via Raman spectroscopy shows four bands at 189, 244, 293, and 588 cm−1. The strongest band at 293 cm−1 is assigned to the interlayer interaction mode of the WSe2 layers. The photoluminescent characterization of Hastelloy/CdS/WSe2 samples exhibits two excitonic transitions at 1.7860 and 1.7899 eV. The current density-voltage (J-V) measurements of the CdS/WSe2 structure show a photovoltaic response with VOC = 180 mV, JSC = 0.07 mA cm−2, and FF = 0.22.

Two-dimensional electric-double-layer Esaki diode

Two-dimensional van der Waals materials offer unique advantages for the development of band-to-band tunneling devices given their lack of dangling bonds, atomically flat thickness and steep band edges. Here, we present the experimental demonstration of an electric double layer (EDL) Esaki junction in synthetic WSe2 thin films. A Si-compatible process is developed for the fabrication of nanoscale FETs utilizing molecular beam epitaxy of WSe2 performed directly on top of a high-κ dielectric at back-end-of-line-friendly temperatures (<550 °C). Degenerate and abrupt doping profiles are obtained by modulating the electron/cation and hole/anion EDLs formed at the interface between a tens-of-nanometer long WSe2 channel and a solid polymer electrolyte, polyethylene oxide:cesium perchlorate (PEO:CsClO4). Numerical simulations are used to determine the bias dependence of the equilibrium ion and carrier density profiles. The EDL-doped tunnel diode exhibits repeatable, gate-tunable band-to-band tunneling with negative differential resistance in the forward bias regime at temperatures up to 140 K, and strong conduction in reverse bias. A maximum peak-to-valley current ratio of 3.5 is measured at 110 K.

Ultrafast THz modulators with WSe2 thin films [Invited

Tungsten diselenide (WSe2) thin films exhibit ultrafast carrier recombination lifetimes, which makes them promising candidates for high speed modulators. With pulsed optical excitation, they could be used to realize all-optical, frequency agile, terahertz devices. Looking into the potential of this material for such applications, time-resolved terahertz spectroscopy can provide significant insight into its free carrier and exciton dynamics such as recombination lifetimes, photo-induced conductivity and decay pathways. In this study, we measure transient terahertz conductivity and photo-generated carrier lifetimes in custom-grown large-area WSe2 thin-films. We discuss its dependence on grain size and number of layers. By analyzing the tradeoffs between carrier-lifetimes, photo-generated conductivity, grain size, and the number of layers, we show that the response of these films can be tailored by controlling the growth parameters. Customizing the film terahertz response can enable large modulation without the need for integration with bulk semiconductors, as widely reported in the literature, thereby achieve high terahertz photoconductivity and high-speed operation. Across samples, our measurements show carrier decay timescale on the order ~10 to 100 ps and a transient conductivity that shows non-Drude behavior. This deviation from a Drude response is dominant within the first few picoseconds (<10 ps) before changing into a Drude like free-carrier response at longer delays. Based on our grown films, we experimentally demonstrate a metamaterial terahertz modulator with WSe2 as the only active element, attaining ~40% modulation depth.

Passivation of recombination active PdSex centers in (001)-textured photoactive WSe2 films

Highly (001)-textured tungsten diselenide WSe2 thin films are crystallized from X-ray amorphous WSex films with Pd-promotion in an H2Se atmosphere. During the crystallization process, the liquid promoter PdSex is driven to the grain boundaries due to the lateral growth of WSe2 platelets and their coalescence. The photoactivity of the WSe2 films is limited by the PdSex crystallites at the grain boundaries of WSe2 platelets. The presence of the PdSex crystallites leads to a metal like behavior of the conductivity versus temperature, whereas their etching leads to a semiconducting behavior of the WSe2 film. After selective etching of these crystallites, the more active WSe2 sites (edges, cavities) are exposed, enhancing the electron transfer to the electrolyte. A photocurrent density of 2 mA cm−2 at 0.35 V vs. reversible hydrogen electrode (RHE) is observed in sulfuric acid (0.5 M H2SO4) using Fe2+/Fe3+ as redox pair under 1 sun (Air Mass (AM) 1.5, 100 mW cm−2) illumination.

Growth of Centimeter‐Scale Monolayer and Few‐Layer WSe2 Thin Films on SiO2/Si Substrate via Pulsed Laser Deposition

AbstractTo date, the fabrication of highly uniform large‐scale monolayer and few‐layer p‐type WSe2 throughout the entire substrate, especially on SiO2/Si substrates is significantly challenging and one of the most important issues. Here, the fabrication of highly uniform monolayer and few‐layer WSe2 thin films on a centimeter‐scale SiO2/Si substrate via pulsed laser deposition is reported. The number of WSe2 layers and the thickness, controlled by varying the number of laser pulses, is determined by transmission electron microscopy analysis. The high uniformity of the as‐grown WSe2 thin film on the entire SiO2/Si substrate is verified by the uniform Raman peak position and intensity, estimated by Raman mapping. Using a conventional photolithography process, field effect transistors based on the as‐grown WSe2 thin films are fabricated and their electrical characteristic is analyzed.

Non-vacuum Preparation of wse2 Thin Films via the Selenization of Hydrated Tungsten Oxide Prepared using Chemical Solution Methods

ABSTRACTIt is known that tungsten oxide may be reacted with selenium sources to form WSe2 but literature reports include processing steps that involve high temperatures, reducing atmospheres, and/or oxidative pre-treatments of tungsten oxide. In this work, we report a non-vacuum process for the fabrication of compositionally high quality WSe2 thin films via the selenization of tungsten oxide under milder conditions. Tungsten source materials were various hydrated WO3 and WO2.9 compounds that were prepared using chemical solution techniques. Resulting films were selenized using a two-stage heating profile (250 °C for 15 minutes and 550 °C for 30 minutes) under a static argon atmosphere. Effects of the starting tungsten oxide phase on WSe2 formation after single and double selenization cycles were investigated using Raman spectroscopy and X-ray diffraction (XRD). After two selenization cycles, hydrated WO3 was converted to (002)-oriented WSe2 that exhibits well-resolved peaks for E12g and A1g phonon modes. Only a single selenization cycle was required to convert amorphous WO2.9 to WSe2. All selenizations in this work were achieved in non-reducing atmospheres and at lower temperatures and shorter times than any non-laser-assisted processes reported for WO3-to-WSe2 conversions.

Field‐Dependent Electrical and Thermal Transport in Polycrystalline WSe2

AbstractOwing to their desirable electrical and thermoelectric properties, transition metal dichalcogenides (TMDs) have attracted significant attention. It is important to develop an easy synthetic method and a simple device fabrication process for TMDs. In this study, WSe2 films were synthesized on a large scale by thermally assisted conversion (TAC) of W films on SiO2/Si substrates at 600 °C. The TAC process yields homogeneous polycrystalline films of controlled thickness over large areas which have the advantage that they can be adapted for mass production for applications in electronics and thermoelectrics. In this regard, pre‐patterning of the deposited metal films allows for devices to be easily fabricated without any etch process. UV‐lithography‐defined W structures have been deposited and after conversion to WSe2 their electrical and thermoelectric properties have been studied. Using e‐beam lithography, a field effect transistor (FET) with a WSe2 channel was fabricated. This showed p‐type behavior and reasonable field effect mobility value. The thermoelectric properties of WSe2 thin films were analyzed by additionally integrating micro heating elements to the WSe2 FET. The maximum Seebeck coefficient and power factor (S2·σ) values were calculated to be ≈61 mV·K−1 (Vg = 45 V) and ≈1.3 nW·K−2·cm−1, respectively.

Unraveling the Structural and Electronic Properties at the WSe2–Graphene Interface for a Rational Design of van der Waals Heterostructures

WSe2 thin films grown by chemical vapor deposition on graphene on SiC(0001) are investigated using photoelectron spectromicroscopy and electron diffraction. By tuning of the growth conditions, micrometer-sized single or multilayer WSe2 crystalline islands preferentially aligned with the main crystallographic directions of the substrate are obtained. Our experiments suggest that the WSe2 islands nucleate from defective WSex seeds embedded in the support. We explore the electronic properties of prototypical van der Waals heterostructures by performing μ-angle resolved photoemission spectroscopy on WSe2 islands of varying thickness (mono- and bilayer) supported on single layer, bilayer, and trilayer graphene. The experiments are substantiated by DFT calculations indicating that the interaction between WSe2 and graphene is weak and the electronic properties of the resulting heterostructures are unaffected by the thickness of the supporting graphene layer or by the crystallographic orientation. Yet the WSe2–gr...