Polarization-Sensitive Self-Powered Schottky Photodetector with High Photovoltaic Performance Induced by Geometry-Asymmetric Contacts

Polarization-sensitive photodetectors have attracted considerable attention owing to their potential application prospects in navigation, optical switching, and communication. However, it remains challenging to develop a facile and effective strategy to simultaneously meet the demands of low power consumption, high performance, and excellent polarization sensitivity. Herein, a series of low-symmetry two-dimensional (2D) ReSe2 Schottky photodetectors with geometry-asymmetric contacts are constructed. These devices exhibit excellent photoelectrical performance and impressive polarization sensitivity in the self-powered mode owing to the difference in the Schottky barrier height induced by the asymmetric contact areas, interfacial states, and thickness difference. Particularly, an outstanding responsivity of 379 mA/W, a decent specific detectivity of 6.8 × 1011 Jones, and a high light on/off ratio (Ilight/Idark) of over 105 under 635 nm light illumination are achieved. Scanning photocurrent mapping (SPCM) measurements further confirm that the ReSe2/drain overlapped region (corresponding to the smaller contact area side) with a higher Schottky barrier height plays a dominant role in the generation of photocurrent. Furthermore, the proposed device displays impressive polarization ratios (PRs) of 3.1 and 3.6 at zero bias under 635 and 808 nm irradiation, respectively. The high-resolution single-pixel imaging capability is also demonstrated. This work reveals the great potential of the ReSe2 Schottky photodetector with geometry-asymmetric contacts for high-performance, self-powered, and polarization-sensitive photodetection.

High-Performance Photoinduced Tunneling Self-Driven Photodetector for Polarized Imaging and Polarization-Coded Optical Communication based on Broken-Gap ReSe2 /SnSe2 van der Waals Heterojunction

Novel 2D materials with low-symmetry structures exhibit great potential applications in developing monolithic polarization-sensitive photodetectors with small volume. However, owing to the fact that at least half of them presented a small anisotropic factor of ≈2, comprehensive performance of present polarization-sensitive photodetectors based on 2D materials is still lower than the practical application requirements. Herein, a self-driven photodetector with high polarization sensitivity using a broken-gap ReSe2 /SnSe2 van der Waals heterojunction (vdWH) is demonstrated. Anisotropic ratio of the photocurrent (Imax /Imin ) could reach 12.26 (635 nm, 179 mW cm-2 ). Furthermore, after a facile combination of the ReSe2 /SnSe2 device with multilayer graphene (MLG), Imax /Imin of the MLG/ReSe2 /SnSe2 can be further increased up to13.27, which is 4 times more than that of pristine ReSe2 photodetector (3.1) and other 2D material photodetectors even at a bias voltage. Additionally, benefitting from the synergistic effect of unilateral depletion and photoinduced tunneling mechanism, the MLG/ReSe2 /SnSe2 device exhibits a fast response speed (752/928 µs) and an ultrahigh light on/off ratio (105 ). More importantly, MLG/ReSe2 /SnSe2 device exhibits excellent potential applications in polarized imaging and polarization-coded optical communication with quaternary logic state without any power supply. This work provides a novel feasible avenue for constructing next-generation smart polarization-sensitive photodetector with low energy consumption.

Machine-learning-assisted analysis of transition metal dichalcogenide thin-film growth

In situ reflective high-energy electron diffraction (RHEED) is widely used to monitor the surface crystalline state during thin-film growth by molecular beam epitaxy (MBE) and pulsed laser deposition. With the recent development of machine learning (ML), ML-assisted analysis of RHEED videos aids in interpreting the complete RHEED data of oxide thin films. The quantitative analysis of RHEED data allows us to characterize and categorize the growth modes step by step, and extract hidden knowledge of the epitaxial film growth process. In this study, we employed the ML-assisted RHEED analysis method to investigate the growth of 2D thin films of transition metal dichalcogenides (ReSe₂) on graphene substrates by MBE. Principal component analysis (PCA) and K-means clustering were used to separate statistically important patterns and visualize the trend of pattern evolution without any notable loss of information. Using the modified PCA, we could monitor the diffraction intensity of solely the ReSe₂ layers by filtering out the substrate contribution. These findings demonstrate that ML analysis can be successfully employed to examine and understand the film-growth dynamics of 2D materials. Further, the ML-based method can pave the way for the development of advanced real-time monitoring and autonomous material synthesis techniques.

Bias-Modified Schottky Barrier Height-Dependent Graphene/ReSe2 van der Waals Heterostructures for Excellent Photodetector and NO2 Gas Sensing Applications

Herein, we reported a unique photo device consisting of monolayer graphene and a few-layer rhenium diselenide (ReSe₂) heterojunction. The prepared Gr/ReSe₂-HS demonstrated an excellent mobility of 380 cm²/Vs, current on/off ratio ~ 10⁴, photoresponsivity (R ~ 74 AW^-1 @ 82 mW cm^-2), detectivity (D^* ~ 1.25 × 10¹¹ Jones), external quantum efficiency (EQE ~ 173%) and rapid photoresponse (rise/fall time ~ 75/3 µs) significantly higher to an individual ReSe₂ device (mobility = 36 cm² V^-1s^-1, Ion/Ioff ratio = 1.4 × 10⁵-1.8 × 10⁵, R = 11.2 AW^-1, D* = 1.02 × 10¹⁰, EQE ~ 26.1%, rise/fall time = 2.37/5.03 s). Additionally, gate-bias dependent Schottky barrier height (SBH) estimation for individual ReSe₂ (45 meV at V_bg = 40 V) and Gr/ReSe₂-HS (9.02 meV at V_bg = 40 V) revealed a low value for the heterostructure, confirming dry transfer technique to be successful in fabricating an interfacial defects-free junction. In addition, HS is fully capable to demonstrate an excellent gas sensing response with rapid response/recovery time (39/126 s for NO₂ at 200 ppb) and is operational at room temperature (26.85 °C). The proposed Gr/ReSe₂-HS is capable of demonstrating excellent electro-optical, as well as gas sensing, performance simultaneously and, therefore, can be used as a building block to fabricate next-generation photodetectors and gas sensors.

Electric Transport in Few-Layer ReSe2 Transistors Modulated by Air Pressure and Light

We report the fabrication and optoelectronic characterization of field-effect transistors (FETs) based on few-layer ReSe₂. The devices show n-type conduction due to the Cr contacts that form low Schottky barriers with the ReSe₂ nanosheet. We show that the optoelectronic performance of these FETs is strongly affected by air pressure, and it undergoes a dramatic increase in conductivity when the pressure is lowered below the atmospheric one. Surface-adsorbed oxygen and water molecules are very effective in doping ReSe₂; hence, FETs based on this two-dimensional (2D) semiconductor can be used as an effective air pressure gauge. Finally, we report negative photoconductivity in the ReSe₂ channel that we attribute to a back-gate-dependent trapping of the photo-excited charges.

Efficient Suppression of Charge Recombination in Self-Powered Photodetectors with Band-Aligned Transferred van der Waals Metal Electrodes

Recombination of photogenerated electron-hole pairs dominates the photocarrier lifetime and then influences the performance of photodetectors and solar cells. In this work, we report the design and fabrication of band-aligned van der Waals-contacted photodetectors with atomically sharp and flat metal-semiconductor interfaces through transferred metal integration. A unity factor α is achieved, which is essentially independent of the wavelength of the light, from ultraviolet to near-infrared, indicating effective suppression of charge recombination by the device. The short-circuit current (0.16 μA) and open-circuit voltage (0.72 V) of the band-aligned van der Waals-contacted devices are at least 1 order of magnitude greater than those of band-aligned deposited devices and 2 orders of magnitude greater than those of non-band-aligned deposited devices. High responsivity, detectivity, and polarization sensitivity ratio of 283 mA/W, 6.89 × 10¹² cm Hz^1/2 W^-1, and 3.05, respectively, are also obtained for the device at zero bias. Moreover, the efficient suppression of charge recombination in our air-stable self-powered photodetectors also results in a fast response speed and leads to polarization-sensitive performance.

Phase Evolution of Re1-xMoxSe2 Alloy Nanosheets and Their Enhanced Catalytic Activity toward Hydrogen Evolution Reaction

Two-dimensional ReSe₂ has emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER), but its catalytic activity needs to be further improved. Herein, we synthesized Re_1-xMo_xSe₂ alloy nanosheets with the whole range of x (0-100%) using a hydrothermal reaction. The phase evolved in the order of 1T″ (triclinic) → 1T' (monoclinic) → 2H (hexagonal) upon increasing x. In the nanosheets with x = 10%, the substitutional Mo atoms tended to aggregate in the 1T″ ReSe₂ phase with Se vacancies. The incorporation of the 1T' phase makes the alloy nanosheets more metallic than the end compositions. The 10% Mo substitution significantly enhanced the electrocatalytic performance toward HER (in 0.5 M H₂SO₄), with a current of 10 mA cm^-2 at an overpotential of 77 mV (vs RHE) and a Tafel slope of 42 mV dec^-1. First-principles calculations of the three phases (1T″, 2H, and 1T') predicted a phase transition of 1T″-2H at x ≈ 65% as well as the production of a 1T' phase along the composition tuning, which are consistent with the experiments. At x = 12.5%, two Mo atoms prefer to form a pair along the Re₄ chains. Gibbs free energy along the reaction path indicates that the best HER performance of nanosheets with 10% Mo originates from the Mo atoms that form Mo-H when there are adjacent Se vacancies.

Adatom Doping of Transition Metals in ReSe2 Nanosheets for Enhanced Electrocatalytic Hydrogen Evolution Reaction

Two-dimensional Re dichalcogenide nanostructures are promising electrocatalysts for the hydrogen evolution reaction (HER). Herein, we report the adatom doping of various transition metals (TM = Mn, Fe, Co, Ni, and Cu) in ReSe₂ nanosheets synthesized using a solvothermal reaction. As the atomic number of TM increases from Mn to Cu, the adatoms on Re sites become more favored over the substitution. In the case of Ni, the fraction of adatoms reaches 90%. Ni doping resulted in the most effective enhancement in the HER catalytic performance, which was characterized by overpotentials of 82 and 109 mV at 10 mA cm^-2 in 0.5 M H₂SO₄ and 1 M KOH, respectively, and the Tafel slopes of 54 and 81 mV dec^-1. First-principles calculations predicted that the adatom doping structures (TMs on Re sites) have higher catalytic activity compared with the substitution ones. The adsorbed H atoms formed a midgap hybridized state via direct bonding with the orbitals of TM adatom. The present work provides a deeper understanding into how TM doping can provide the catalytically active sites in these ReSe₂ nanosheets.

Rhenium Diselenide (ReSe2) Near-Infrared Photodetector: Performance Enhancement by Selective p-Doping Technique

In this study, a near-infrared photodetector featuring a high photoresponsivity and a short photoresponse time is demonstrated, which is fabricated on rhenium diselenide (ReSe2) with a relatively narrow bandgap (0.9-1.0 eV) compared to conventional transition-metal dichalcogenides (TMDs). The excellent photo and temporal responses, which generally show a trade-off relation, are achieved simultaneously by applying a p-doping technique based on hydrochloric acid (HCl) to a selected ReSe2 region. Because the p-doping of ReSe2 originates from the charge transfer from un-ionized Cl molecules in the HCl to the ReSe2 surface, by adjusting the concentration of the HCl solution from 0.1 to 10 m, the doping concentration of the ReSe2 is controlled between 3.64 × 1010 and 3.61 × 1011 cm-2. Especially, the application of the selective HCl doping technique to the ReSe2 photodetector increases the photoresponsivity from 79.99 to 1.93 × 103 A W-1, and it also enhances the rise and decay times from 10.5 to 1.4 ms and from 291 to 3.1 ms, respectively, compared with the undoped ReSe2 device. The proposed selective p-doping technique and its fundamental analysis will provide a scientific foundation for implementing high-performance TMD-based electronic and optoelectronic devices.

Current Rectification in a Structure: ReSe2/Au Contacts on Both Sides of ReSe2

Schottky effect of two-dimensional materials is important for nanoscale electrics. A ReSe₂ flake is transferred to be suspended between an Au sink and an Au nanofilm. This device is initially designed to measure the transport properties of the ReSe₂ flake. However, a rectification behavior is observed in the experiment from 273 to 340 K. The rectification coefficient is about 10. The microstructure and elements composition are systematically analyzed. The ReSe₂ flake and the Au film are found to be in contact with the Si substrate from the scanning electron microscope image in slant view of 45°. The ReSe₂/Si and Si/Au contacts are p-n heterojunction and Schottky contacts. Asymmetry of both contacts results in the rectification behavior. The prediction based on the thermionic emission theory agrees well with experimental data.

Identifying the Non-Identical Outermost Selenium Atoms and Invariable Band Gaps across the Grain Boundary of Anisotropic Rhenium Diselenide

Rhenium diselenide (ReSe₂) is a unique transition-metal dichalcogenide (TMDC) possessing distorted 1T structure with a triclinic symmetry, strong in-plane anisotropy, and promising applications in optoelectronics and energy-related fields. So far, the structural and physical properties of ReSe₂ are mainly uncovered by transmission electron microscopy and spectroscopy characterizations. Herein, by combining scanning tunneling microscopy and spectroscopy (STM and STS) with first-principles calculations, we accomplish the on-site atomic-scale identification of the top four non-identical Se atoms in a unit cell of the anisotropic monolayer ReSe₂ on the Au substrate. According to STS and photoluminescence results, we also determine the quasiparticle and optical band gaps as well as the exciton binding energy of monolayer ReSe₂. In particular, we detect a perfect lattice coherence and an invariable band gap across the mirror-symmetric grain boundaries in monolayer and bilayer ReSe₂, which considerably differ from the traditional isotropic TMDCs featured with defect structures and additional states inside the band gap. Such essential findings should deepen our understanding of the intrinsic properties of two-dimensional anisotropic materials and provide fundamental references for their applications in related fields.

Highly Anisotropic in-Plane Excitons in Atomically Thin and Bulklike 1T'-ReSe2

Atomically thin materials such as graphene or MoS₂ are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel optoelectronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe₂. On reducing the crystal thickness from bulk to a monolayer, we observe a strong blue shift of the optical band gap from 1.37 to 1.50 eV. The excitons are strongly polarized with dipole vectors along different crystal directions, which persist from bulk down to monolayer thickness. The experimental results are well reproduced by ab initio calculations based on the GW-BSE approach within LDA+GdW approximation. The excitons have high binding energies of 860 meV for the monolayer and 120 meV for bulk. They are strongly confined within a single layer even for the bulk crystal. In addition, we find in our calculations a direct band gap in 1T'-ReSe₂ regardless of crystal thickness, indicating weak interlayer coupling effects on the band gap characteristics. Our results pave the way for polarization-sensitive applications, such as optical logic circuits operating in the infrared spectral region.

Chemical Vapor Deposition Synthesis of Ultrathin Hexagonal ReSe2 Flakes for Anisotropic Raman Property and Optoelectronic Application

Hexagonal crystalline ultrathin ReSe₂ flakes are synthesized for the first time by a chemical vapor deposition (CVD) method. The as-synthesized ReSe₂ flake is revealed as a novel structure, which has mirror-symmetric single-crystal domains inside, by polarization incident Raman and HRTEM. The successful development of the CVD method will facilitate research on the novel anisotropic electronic/optoelectronic properties of ReSe₂ in the future.

Tunable Ambipolar Polarization-Sensitive Photodetectors Based on High-Anisotropy ReSe2 Nanosheets

Atomically thin 2D-layered transition-metal dichalcogenides have been studied extensively in recent years because of their intriguing physical properties and promising applications in nanoelectronic devices. Among them, ReSe2 is an emerging material that exhibits a stable distorted 1T phase and strong in-plane anisotropy due to its reduced crystal symmetry. Here, the anisotropic nature of ReSe2 is revealed by Raman spectroscopy under linearly polarized excitations in which different vibration modes exhibit pronounced periodic variations in intensity. Utilizing high-quality ReSe2 nanosheets, top-gate ReSe2 field-effect transistors were built that show an excellent on/off current ratio exceeding 10(7) and a well-developed current saturation in the current-voltage characteristics at room temperature. Importantly, the successful synthesis of ReSe2 directly onto hexagonal boron nitride substrates has effectively improved the electron motility over 500 times and the hole mobility over 100 times at low temperatures. Strikingly, corroborating with our density-functional calculations, the ReSe2-based photodetectors exhibit a polarization-sensitive photoresponsivity due to the intrinsic linear dichroism originated from high in-plane optical anisotropy. With a back-gate voltage, the linear dichroism photodetection can be unambiguously tuned both in the electron and hole regime. The appealing physical properties demonstrated in this study clearly identify ReSe2 as a highly anisotropic 2D material for exotic electronic and optoelectronic applications.

Splitting of Interlayer Shear Modes and Photon Energy Dependent Anisotropic Raman Response in N-Layer ReSe₂ and ReS₂

We investigate the interlayer phonon modes in N-layer rhenium diselenide (ReSe2) and rhenium disulfide (ReS2) by means of ultralow-frequency micro-Raman spectroscopy. These transition metal dichalcogenides exhibit a stable distorted octahedral (1T') phase with significant in-plane anisotropy, leading to sizable splitting of the (in-plane) layer shear modes. The fan-diagrams associated with the measured frequencies of the interlayer shear modes and the (out-of-plane) interlayer breathing modes are perfectly described by a finite linear chain model and allow the determination of the interlayer force constants. Nearly identical values are found for ReSe2 and ReS2. The latter are appreciably smaller than but on the same order of magnitude as the interlayer force constants reported in graphite and in trigonal prismatic (2Hc) transition metal dichalcogenides (such as MoS2, MoSe2, MoTe2, WS2, WSe2), demonstrating the importance of van der Waals interactions in N-layer ReSe2 and ReS2. In-plane anisotropy results in a complex angular dependence of the intensity of all Raman modes, which can be empirically utilized to determine the crystal orientation. However, we also demonstrate that the angular dependence of the Raman response drastically depends on the incoming photon energy, shedding light on the importance of resonant exciton-phonon coupling in ReSe2 and ReS2.

Rhenium Dichalcogenides: Layered Semiconductors with Two Vertical Orientations

The rhenium and technetium diselenides and disulfides are van der Waals layered semiconductors in some respects similar to more well-known transition metal dichalcogenides (TMD) such as molybdenum sulfide. However, their symmetry is lower, consisting only of an inversion center, so that turning a layer upside-down (that is, applying a C2 rotation about an in-plane axis) is not a symmetry operation, but reverses the sign of the angle between the two nonequivalent in-plane crystallographic axes. A given layer thus can be placed on a substrate in two symmetrically nonequivalent (but energetically similar) ways. This has consequences for the exploitation of the anisotropic properties of these materials in TMD heterostructures and is expected to lead to a new source of domain structure in large-area layer growth. We produced few-layer ReS2 and ReSe2 samples with controlled "up" or "down" orientations by micromechanical cleavage and we show how polarized Raman microscopy can be used to distinguish these two orientations, thus establishing Raman as an essential tool for the characterization of large-area layers.

Single-Layer ReS₂: Two-Dimensional Semiconductor with Tunable In-Plane Anisotropy

Rhenium disulfide (ReS2) and diselenide (ReSe2), the group 7 transition metal dichalcogenides (TMDs), are known to have a layered atomic structure showing an in-plane motif of diamond-shaped-chains (DS-chains) arranged in parallel. Using a combination of transmission electron microscopy and transport measurements, we demonstrate here the direct correlation of electron transport anisotropy in single-layered ReS2 with the atomic orientation of the DS-chains, as also supported by our density functional theory calculations. We further show that the direction of conducting channels in ReS2 and ReSe2 can be controlled by electron beam irradiation at elevated temperatures and follows the strain induced to the sample. Furthermore, high chalcogen deficiency can induce a structural transformation to a nonstoichiometric phase, which is again strongly direction-dependent. This tunable in-plane transport behavior opens up great avenues for creating nanoelectronic circuits in 2D materials.

Raman spectra of monolayer, few-layer, and bulk ReSe₂: an anisotropic layered semiconductor

Rhenium diselenide (ReSe2) is a layered indirect gap semiconductor for which micromechanical cleavage can produce monolayers consisting of a plane of rhenium atoms with selenium atoms above and below. ReSe2 is unusual among the transition-metal dichalcogenides in having a low symmetry; it is triclinic, with four formula units per unit cell, and has the bulk space group P1̅. Experimental studies of Raman scattering in monolayer, few-layer, and bulk ReSe2 show a rich spectrum consisting of up to 16 of the 18 expected lines with good signal strength, pronounced in-plane anisotropy of the intensities, and no evidence of degradation of the sample during typical measurements. No changes in the frequencies of the Raman bands with layer thickness down to one monolayer are observed, but significant changes in relative intensity of the bands allow the determination of crystal orientation and of monolayer regions. Supporting theory includes calculations of the electronic band structure and Brillouin zone center phonon modes of bulk and monolayer ReSe2 as well as the Raman tensors determining the scattering intensity of each mode. It is found that, as for other transition-metal dichalcogenides, Raman scattering provides a powerful diagnostic tool for studying layer thickness and also layer orientation in few-layer ReSe2.