WSe2 Films Research Articles

Opto-electronic responses of large area WSe2 layers synthesized by chemical conversion of WO3 thin films

Atomically thin tungsten diselenide (WSe2) has drawn much interest due to its remarkable optical properties and potential applications in next-generation optoelectronics and energy harvesting devices. High-quality, wafer-scale synthesis of such two-dimensional (2D) semiconductors is necessary for their practical applications. Chemical vapor deposition (CVD) is a promising method for synthesizing large-area thin films of 2D materials where the constituent elements are of widely different vapor pressure. Herein, we demonstrate large-area synthesis of the 2H phase of WSe2 with a precise control over the 2D film thickness. The first step of this process is the deposition of tungsten trioxide (WO3) films of a well-defined thickness by thermal evaporation over large areas. The subsequent selenization of these films in an atmospheric pressure CVD reactor, yields high-quality uniform films of WSe2. The quality of these films is ascertained by measuring their characteristic Raman active vibrational modes and layer thickness dependent optical absorption and photoluminescence. The photoconductivity (PC) of these films has been measured with 405, 532 and 915 nm laser excitation. We note an increase in the PC of the films with their thickness. The best photoresponsivity and detectivity obtained are 11.3 mA/W and 1.42 × 108 Jones respectively. These results are promising to create photo-transistor arrays over large areas using such WO3 processed films.

Toward Mass Production of Transition Metal Dichalcogenide Solar Cells: Scalable Growth of Photovoltaic-Grade Multilayer WSe2 by Tungsten Selenization.

Semiconducting transition metal dichalcogenides (TMDs) are promising for high-specific-power photovoltaics due to their desirable band gaps, high absorption coefficients, and ideally dangling-bond-free surfaces. Despite their potential, the majority of TMD solar cells to date are fabricated in a nonscalable fashion, with exfoliated materials, due to the lack of high-quality, large-area, multilayer TMDs. Here, we present the scalable, thickness-tunable synthesis of multilayer WSe2 films by selenizing prepatterned tungsten with either solid-source selenium at 900 °C or H2Se precursors at 650 °C. Both methods yield photovoltaic-grade, wafer-scale WSe2 films with a layered van der Waals structure and superior characteristics, including charge carrier lifetimes up to 144 ns, over 14× higher than those of any other large-area TMD films previously demonstrated. Simulations show that such carrier lifetimes correspond to ∼22% power conversion efficiency and ∼64 W g-1 specific power in a packaged solar cell, or ∼3 W g-1 in a fully packaged solar module. The results of this study could facilitate the mass production of high-efficiency multilayer WSe2 solar cells at low cost.

Highly sensitive near-field leakage-enhanced optical fiber SPR sensor based on gold nanoarrays modulation

A novel near-field leakage-enhanced optical fiber surface plasmon resonance (NLF-SPR) sensor based on the synergistic sensitization of cuboid gold nanoarrays and WSe2 film is proposed for the first time to our knowledge. The sensor is composed of a side-polished multimode fiber, a continuous gold film, a continuous tungsten selenide (WSe2) film, and well-arranged cuboid gold nanoarrays. The sensor leverages the high carrier mobility characteristic of the WSe2 film, the near-field leakage enhancement arising from the nanoarrays structure, and the localized electric field enhancement property introduced by nanotips to enhance the refractive index sensitivity. In order to optimize the spectral characteristic, the modulation of electric field distribution and spectral characteristic by the structural parameters of gold nanoarrays is investigated through three-dimensional (3D) finite element simulation. Results show that in the refractive index range of 1.3332–1.3432 refractive index unit (RIU), the average sensitivity of the NLF-SPR sensor can reach up to 10108 nm/RIU, and the highest sensitivity can reach up to 14692.46 nm/RIU. The average sensitivity and the highest sensitivity are 1.87 times and 2.56 times higher, respectively, than those of the side-polished fiber SPR sensor with a monolayer gold film. Furthermore, the experimental results demonstrate that the near-field leakage enhancement introduced by gold nanoarrays improves the performance of the fiber SPR sensor, resulting in a 49.3 % increment in sensitivity. The experimental results are in agreement with the trend of the simulation results. Even better performance improvements have been achieved compared to existing methods and a more flexible approach to performance tuning is provided. With the maturity and development of various nanoarray processing techniques such as focused ion beam, electron beam lithography, and nanoimprint lithography, the proposed NLF-SPR sensor has the potential to be fabricated on a large scale and is expected to be widely employed in biomedical, clinical applications, and other fields.

High performance Al/WSe2/CuO/ITO structure based broadband photodetector

This letter reports a Al/WSe2/CuO/ITO structure based broadband photodetector. The low cost spin coating technique is utilized to deposit, p-type WSe2 film and n-type CuO film. The Al contacts were deposited over WSe2 film using thermal evaporation unit. The suggested photodetector gives the broad photo response with maximum Responsivity RS (A/W) and External Quantum Efficiency (EQE) value of 3099 A/W, 3680 A/W, and 493 A/W; 1.20 × 106%, 1.52× 106% and 2.04 × 105% at 350 nm (UV), 500 nm (visible) and 950 nm (IR) under application of −2 V bias. The EQE value beyond 100% are attributed to trap assisted photomultiplication mechanism.

Interface engineering and heterometal doping Cu3Se2/WSe2 for efficient charge transfer and nonlinear optical performance

Integrating interface engineering and precious heterometal doping with semiconductors is a feasible pathway to construct efficient charge transfer and improve the nonlinear optical (NLO) performance of materials. Here, we report a bimetallic selenide type-II heterostructure of Cu3Se2/WSe2 (CWS) with a simple one-step magnetron co-sputtering of Cu and WSe2 targets at low argon flow rates. Se first nucleates and precipitates to form nanospheres at low flow rates. Due to the electronegativity of Cu being greater than that of W and Cu prioritizes attachment to the outside of the Se nanospheres and promotes the formation of the nanorods-grown Cu3Se2. A combined analysis of X-ray photoelectron spectroscopy and X-ray diffraction data was used to investigate the minor shift in the W 4 f/Se 2p ratio revealing that electrons shifted from Cu3Se2 to WSe2. The diffraction peaks of CWS and the valence electronic structure of Cu1+/Cu2+ confirmed the implantation of Cu3Se2 nanorods on WSe2. The NLO properties of CWS composite films were modulated by varying the power of Cu. More importantly, the nonlinear absorption coefficient of the CWS composite films formed after Cu3Se2 promotion is much better than that of other unmodified pure WSe2 films. The photogenerated carrier dynamics of CWS composite films were investigated by femtosecond transient absorption spectroscopy, and density functional theory calculations show that CWS composite films inhibit efficient electron transfer photogenerated carrier complexation. This work proposes a few strategies for designing heterogeneous structures with interface engineering and precious metal doping to promote adjustable electron transfer dynamics and achieve high NLO conversion efficiency.

Autonomous Synthesis of Thin Film Materials with Pulsed Laser Deposition Enabled by In Situ Spectroscopy and Automation.

Autonomous systems that combine synthesis, characterization, and artificial intelligence can greatly accelerate the discovery and optimization of materials, however platforms for growth of macroscale thin films by physical vapor deposition techniques have lagged far behind others. Here this study demonstrates autonomous synthesis by pulsed laser deposition (PLD), a highly versatile synthesis technique, in the growth of ultrathin WSe2 films. By combing the automation of PLD synthesis and in situ diagnostic feedback with a high-throughput methodology, this study demonstrates a workflow and platform which uses Gaussian process regression and Bayesian optimization to autonomously identify growth regimes for WSe2 films based on Raman spectral criteria by efficiently sampling 0.25% of the chosen 4D parameter space. With throughputs at least 10x faster than traditional PLD workflows, this platform and workflow enables the accelerated discovery and autonomous optimization of the vast number of materials that can be synthesized by PLD.

Low-Temperature Synthesis of WSe2 by the Selenization Process under Ultrahigh Vacuum for BEOL Compatible Reconfigurable Neurons.

Low-temperature large-area growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) is critical for their integration with silicon chips. Especially, if the growth temperatures can be lowered below the back-end-of-line (BEOL) processing temperatures, the Si transistors can interface with 2D devices (in the back end) to enable high-density heterogeneous circuits. Such configurations are particularly useful for neuromorphic computing applications where a dense network of neurons interacts to compute the output. In this work, we present low-temperature synthesis (400 °C) of 2D tungsten diselenide (WSe2) via the selenization of the W film under ultrahigh vacuum (UHV) conditions. This simple yet effective process yields large-area, homogeneous films of 2D TMDs, as confirmed by several characterization techniques, including reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and different spectroscopy methods. Memristors fabricated using the grown WSe2 film are leveraged to realize a novel compact neuron circuit that can be reconfigured to enable homeostasis.

Preparation of 2D Tungsten Diselenide (WSe2) thin films by selenization of DC-sputtered W precursors

Two-dimensional layered transition metal dichalcogenides based on WSe2 films demonstrated promising properties for nano-electronics and photovoltaic applications. In this work, WSe2 films were prepared by selenization of DC-sputtered W precursors. The influence of the selenization temperature on the structural, morphology, and optical properties of the WSe2 films was investigated. The selenization temperature was varied from 350 °C to 450 °C at the interval of 50 °C. Structural, morphology and optical properties of the WSe2 were investigated using X-ray diffractometry (XRD), Atomic Force Microscope (AFM), and UV-VIS-NIR spectrophotometer, respectively. XRD analysis revealed that all WSe2 were polycrystalline and exhibited the co-existence of c-axis perpendicular and parallel substrate texture. Samples selenized at 400 °C demonstrated strong - types of crystal orientations – perpendicular c-axis substrate texture – dominated crystal growth. The AFM images further revealed the co-existence of parallel and perpendicular crystal orientations for samples selenized at 350 °C and 450 °C. Optical measurement showed that all WSe2 samples were transparent and consisted of an excitonic peak at the wavelength of around 620 nm. The estimated bandgap values were in the range of 1.22 eV to 1.37 eV which is somchat lower than expected – the presence of W5O14 phases is suggested to be the main cause.

Directly seeding epitaxial growth of tungsten oxides/tungsten diselenide mixed-dimensional heterostructures with excellent optical properties

Mixed-dimensional heterostructures have drawn significant attention due to their intriguing physical properties and potential applications in electronic and optoelectronic nanodevices. However, limited by the lattice matching, the preparation of heterostructures is experimentally difficult and the underlying growth mechanism has not been well established. Here, we report a three-step seeding epitaxial growth strategy for synthesizing mixed-dimensional heterostructures of one-dimensional microwire (MW) and two-dimensional atomic thin film. Our growth strategy has successfully realized direct epitaxial growth of WSe2 film on WOx MW and significantly improves the quality of the epitaxial WSe2 monolayer, which is evidenced by the remarkably enhanced photoluminescence (PL). More intriguingly, the as-synthesized WOx MWs exhibit a strong nonlinear optical response due to the enhancement effect of the core (WOx)-shell (WSe2) nanocavity. Our work provides a feasible route for direct growth of WOx-based mixed-dimensional heterostructures, which possess potential applications in high-performance optoelectronic devices.

Simultaneous quantification of Young’s modulus and dispersion forces with nanoscale spatial resolution

Many advances in polymers and layered materials rely on a precise understanding of the local interactions between adjacent molecular or atomic layers. Quantifying dispersion forces at the nanoscale is particularly challenging with existing methods often time consuming, destructive, relying on surface averaging or requiring bespoke equipment. Here, we present a non-invasive method able to quantify the local mechanical and dispersion properties of a given sample with nanometer lateral precision. The method, based on atomic force microscopy (AFM), uses the frequency shift of a vibrating AFM cantilever in combination with established contact mechanics models to simultaneously derive the Hamaker constant and the effective Young’s modulus at a given sample location. The derived Hamaker constant and Young’s modulus represent an average over a small (typically <100) number of molecules or atoms. The oscillation amplitude of the vibrating AFM probe is used to select the length-scale of the features to analyse, with small vibrations able to resolve the contribution of sub-nanometric defects and large ones exploring effectively homogeneous areas. The accuracy of the method is validated on a range of 2D materials in air and water as well as on polymer thin films. We also provide the first experimental measurements of the Hamaker constant of HBN, MoT2, WSe2 and polymer films, verifying theoretical predictions and computer simulations. The simplicity and robustness of the method, implemented with a commercial AFM, may support a broad range of technological applications in the growing field of polymers and nanostructured materials where a fine control of the van der Waals interactions is crucial to tune their properties.

Large-Scale Complementary Logic Circuit Enabled by Al2O3 Passivation-Induced Carrier Polarity Modulation in Tungsten Diselenide.

Achieving effective polarity control of n- and p-type transistors based on two-dimensional (2D) materials is a critical challenge in the process of integrating transition metal dichalcogenides (TMDC) into complementary metal-oxide semiconductor (CMOS) logic circuits. Herein, we utilized a proficient and nondestructive method of electron-charge transfer to achieve a complete carrier polarity conversion from p-to n-type by depositing a thin layer of aluminum oxide (Al2O3) onto tungsten diselenide (WSe2). By utilizing the Al2O3 passivation layer, we observed precisely tuned n-type behavior in contrast to transistors fabricated on the as-grown WSe2 film without any passivation layer, which display prominent p-type behavior. The polarity-transformed n-type WSe2 transistor from the pristine p-type shows the maximum ON current of ∼0.1 μA accompanied by a high electron mobility of 7 cm2 V-1 s-1 at a drain voltage (VDS) of 1 V. We successfully showcased a homogeneous CMOS inverter utilizing 2D-TMDC which exhibits an impressive voltage gain of 7 at VDD = 5 V. Moreover, this effective polarity control approach was further expanded upon to successfully demonstrate a range of logic circuits such as AND, OR, NAND, NOR logic gates, and SRAM. The proposed methodology possesses significant promise for facilitating the advancement of high-density circuitry components utilizing 2D-TMDC.

Demonstration of an AI-driven workflow for autonomous high-resolution scanning microscopy

Modern scanning microscopes can image materials with up to sub-atomic spatial and sub-picosecond time resolutions, but these capabilities come with large volumes of data, which can be difficult to store and analyze. We report the Fast Autonomous Scanning Toolkit (FAST) that addresses this challenge by combining a neural network, route optimization, and efficient hardware controls to enable a self-driving experiment that actively identifies and measures a sparse but representative data subset in lieu of the full dataset. FAST requires no prior information about the sample, is computationally efficient, and uses generic hardware controls with minimal experiment-specific wrapping. We test FAST in simulations and a dark-field X-ray microscopy experiment of a WSe2 film. Our studies show that a FAST scan of <25% is sufficient to accurately image and analyze the sample. FAST is easy to adapt for any scanning microscope; its broad adoption will empower general multi-level studies of materials evolution with respect to time, temperature, or other parameters.

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.

Effect of gamma-ray irradiation on structural and optical property of WSe2 film

Effect of gamma-ray irradiation on structural and optical property of WSe2 film

Phase/Interfacial-Engineered Two-Dimensional-Layered WSe2 Films by a Plasma-Assisted Selenization Process: Modulation of Oxygen Vacancies in Resistive Random-Access Memory.

Here, we propose phase and interfacial engineering by inserting a functional WO3 layer and selenized it to achieve a 2D-layered WSe2/WO3 heterolayer structure by a plasma-assisted selenization process. The 2D-layered WSe2/WO3 heterolayer was coupled with an Al2O3 film as a resistive switching (RS) layer to form a hybrid structure, with which Pt and W films were used as the top and bottom electrodes, respectively. The device with good uniformity in SET/RESET voltage and high low-/high-resistance window can be obtained by controlling a conversion ratio from a WO3 film to a 2D-layered WSe2 thin film. The Pt/Al2O3/(2D-layered WSe2/WO3)/W structure shows remarkable improvement to the pristine Pt/Al2O3/W and Pt/Al2O3/2D-layered WO3/W in terms of low SET/RESET voltage variability (-20/20)%, multilevel characteristics (uniform LRS/HRS distribution), high on/off ratio (104-105), and retention (∼105 s). The thickness of the obtained WSe2 was tuned at different gas ratios to optimize different 2D-layered WSe2/WO3 (%) ratios, showing a distinctive trend of reduced and uniform SET/RESET voltage variability as 2D-layered WSe2/WO3 (%) changes from 90/10 (%) to 45/55 (%), respectively. The electrical measurements confirm the superior ability of the metallic 1T phase of the 2D-layered WSe2 over the semiconducting 2H phase. Through systemic studies of RS behaviors on the effect of 1T/2H phases and 2D-layered WSe2/WO3 ratios, the low-temperature plasma-assisted selenization offers compatibility with the temperature-limited 3D integration process and also provides much better thickness control over a large area.

Self-driven Vis-NIR broadband photodetector based on nano-hedge-like MoS2/WSe2 heterostructure

TMDC heterostructures provide alternative bases for optoelectronic devices besides conventional Schottky devices. With the continued expansion of the modern semiconductor industry, self-powered devices have emerged as an integral part of electronic and optoelectronic equipments. This work reports a unique seamless nano-hedge (nh) like morphology of MoS2 deposited on WSe2 film to form nh-MoS2/WSe2 heterostructure as a self-powered photodetector with a broad spectral range from visible to NIR. The developed heterostructure exhibits a very high responsivity of 900 mAW-1 when illuminated by 860 nm under zero bias (self-powered mode). In addition, the device also shows a high responsivity of 177 mAW-1, 271 mAW-1, and 506 mAW-1 for illumination of 532 nm, 625 nm light, and 950 nm, respectively, at zero applied bias. With low noise equivalent power of 2.6 × 10-14 WHz1/2 and ultra-high responsivity of 2.6 × 104 mAW-1 in photoconductive mode at 860 nm light irradiation, the device exhibits extremely high external quantum efficiency of 6.29 × 103%. Further, the device also displays detectivity of 5 × 1011 Jones, an LDR value of 26 dB, with a fast response time of 15 ms. Developing a broadband photodetector based on nh-MoS2/WSe2 heterostructure opens the route for highly responsive, self-powered future optoelectronic devices.

Multilayer WSe2/ZnO heterojunctions for self-powered, broadband, and high-speed photodetectors

Self-powered broadband photodetectors have attracted great interest due to their applications in biomedical imaging, integrated circuits, wireless communication systems, and optical switches. Recently, significant research is being carried out to develop high-performance self-powered photodetectors based on thin 2D materials and their heterostructures due to their unique optoelectronic properties. Herein, a vertical heterostructure based on p-type 2D WSe2 and n-type thin film ZnO is realized for photodetectors with a broadband response in the wavelength range of 300–850 nm. Due to the formation of a built-in electric field at the WSe2/ZnO interface and the photovoltaic effect, this structure exhibits a rectifying behavior with a maximum photoresponsivity and detectivity of ∼131 mA W−1 and ∼3.92 × 1010 Jones, respectively, under an incident light wavelength of λ = 300 nm at zero voltage bias. It also shows a 3-dB cut-off frequency of ∼300 Hz along with a fast response time of ∼496 μs, making it suitable for high-speed self-powered optoelectronic applications. Furthermore, the facilitation of charge collection under reverse voltage bias results in a photoresponsivity as high as ∼7160 mA W−1 and a large detectivity of ∼1.18 × 1011 Jones at a bias voltage of −5 V. Hence, the p-WSe2/n-ZnO heterojunction is proposed as an excellent candidate for high-performance, self-powered, and broadband photodetectors.

Electronic and Optoelectronic Monolayer WSe2 Devices via Transfer-Free Fabrication Method.

Monolayer transition metal dichalcogenides (TMDs) have drawn significant attention for their potential applications in electronics and optoelectronics. To achieve consistent electronic properties and high device yield, uniform large monolayer crystals are crucial. In this report, we describe the growth of high-quality and uniform monolayer WSe2 film using chemical vapor deposition on polycrystalline Au substrates. This method allows for the fabrication of continuous large-area WSe2 film with large-size domains. Additionally, a novel transfer-free method is used to fabricate field-effect transistors (FETs) based on the as-grown WSe2. The exceptional metal/semiconductor interfaces achieved through this fabrication method result in monolayer WSe2 FETs with extraordinary electrical performance comparable to those with thermal deposition electrodes, with a high mobility of up to ≈62.95 cm2 V-1 s-1 at room temperature. In addition, the as-fabricated transfer-free devices can maintain their original performance after weeks without obvious device decay. The transfer-free WSe2-based photodetectors exhibit prominent photoresponse with a high photoresponsivity of ~1.7 × 104 A W-1 at Vds = 1 V and Vg = -60 V and a maximum detectivity value of ~1.2 × 1013 Jones. Our study presents a robust pathway for the growth of high-quality monolayer TMDs thin films and large-scale device fabrication.

Carrier and Phonon Dynamics in Multilayer WSe2 Captured by Extreme Ultraviolet Transient Absorption Spectroscopy

Carrier and phonon dynamics in a multilayer WSe2 film are captured by extreme ultraviolet (XUV) transient absorption (TA) spectroscopy at the W N6,7, W O2,3, and Se M4,5 edges (30–60 eV). After the broadband optical pump pulse, the XUV probe directly reports on occupations of optically excited holes and phonon-induced band renormalizations. When compared with density functional theory calculations, XUV transient absorption due to holes is identified below the W O3 edge whereas signals at the Se M4,5 edges are dominated by phonon dynamics. Therein, 0.4 ps hole relaxation time, 1.5 ps carrier recombination time, and 1.7 ps phonon heating time are extracted. The acquisition of hole and phonon-induced signals in a single experiment can facilitate the investigation of the correlations between electron and phonon dynamics. Furthermore, the simultaneous observation of signals from different elements can be further extended to explore photochemical processes in multilayers and alloys, thereby providing key information for their applications in electronics, photocatalysts, and spintronics.

Тонкие наноструктурированные пленки n-WSe2 и их применение в полупроводниковых фотокатодах p-Si для получения водорода расщеплением воды

The possibilities of modification of the structure and type of conductivity of WSe2 films formed on p-type silicon by thermal treatment of a thin-film precursor, which was preliminarily created by pulsed laser deposition, are studied. Pulsed laser ablation of WSe2 and rhenium targets made it possible to obtain amorphous films WSex (x &gt; 2) containing rhenium atoms and inclusions of β-W nanoparticles. Heat treatment at 450 °C caused the crystallization of the amorphous matrix and the formation of a layered 2H-WSe2 shell surrounded the metal nanoparticles. Doping with rhenium led to the production of n-type WSe2 semiconductor films, which, in terms of their properties (band gap ~ 1.2 eV, high catalytic activity, low resistance to current transport), represent a promising material for creating p-Si photocathodes for efficient light-activated hydrogen evolution in acid solution. Theoretical calculations are carried out, which make it possible to identify local areas on the surface of the formed WSe2 films with enhanced catalytic activity in hydrogen evolution reaction.