Few-layer ReS2 Research Articles

Enhanced thermally aided memory performance using few-layer ReS2 transistors

Thermally varying hysteretic gate operation in few-layer ReS2 and MoS2 back gate field effect transistors (FETs) is studied and compared for memory applications. Clockwise hysteresis at room temperature and anti-clockwise hysteresis at higher temperature (373 K for ReS2 and 400 K for MoS2) are accompanied by step-like jumps in transfer curves for both forward and reverse voltage sweeps. Hence, a step-like conductance (STC) crossover hysteresis between the transfer curves for the two sweeps is observed at high temperature. Furthermore, memory parameters such as the RESET-to-WRITE window and READ window are defined and compared for clockwise hysteresis at low temperature and STC-type hysteresis at high temperature, showing better memory performance for ReS2 FETs as compared to MoS2 FETs. Smaller operating temperature and voltage along with larger READ and RESET-to-WRITE windows make ReS2 FETs a better choice for thermally aided memory applications. Finally, temperature dependent Kelvin probe force microscopy measurements show decreasing (constant) surface potential with increasing temperature for ReS2 (MoS2). This indicates less effective intrinsic trapping at high temperature in ReS2, leading to earlier occurrence of STC-type hysteresis in ReS2 FETs as compared to MoS2 FETs with increasing temperature.

Boosting the stable sodium-ion storage performance by tailoring the 1D TiO2@ReS2 core-shell heterostructures

ReS2 has been considered as an emerging transition metal dichalcogenides (TMDs) material for sodium-ion batteries (SIBs). However, its electrochemical performance is severely limited by the structural aggregation and damage during deep charge-discharge. Here, a new 1D TiO2@ReS2 core-shell structure is reported for boosting the stable performance as the TiO2 has durable structural stability. The 1D TiO2 nanotubes with rough surface and large surface area are helpful to grow few-layer (≤4 layers) ReS2 nanosheets onto their surface. In the obtained 1D TiO2 nanotube@ReS2 nanosheet (1D TiO2-NT@ReS2-NS) core-shell heterostructures, the exposed ultrathin ReS2 nanosheets offer high contact area for rapid Na+ diffusion, whilst the TiO2 nanotubes work as robust backbone for accommodating volume change and strain. Moreover, the chemical interfacial interaction between TiO2 and ReS2 gives rise to favorable synergistic effect, leading to enhanced electrical conductivity, Na+ diffusion kinetics, and structural stability at both electrode and materials levels. These findings can be supported by various characterization technologies such as X-ray photoelectron spectrum and high-resolution transmission electron microscopy. As a result, the 1D TiO2-NT@ReS2-NS electrode displays a desirable long-life span cycling performance of 118 mAh g−1 at 1 A g−1 after 1000 cycles in sodium-ion batteries. This work not only reports a stable SIBs anode material, but also provides fundamental understanding for designing and fabricating electrode materials for alkali metal ion batteries.

Complete determination of the crystallographic orientation of ReX2 (X = S, Se) by polarized Raman spectroscopy

Polarized Raman spectroscopy on few-layer ReS2 and ReSe2 was carried out to determine the crystallographic orientations.

High mobility ReSe2 field effect transistors: Schottky-barrier-height-dependent photoresponsivity and broadband light detection with Co decoration

2D transition metal dichalcogenides are promising in various electronics and optoelectronics applications and have gained popularity owing to their carrier transport and strong light–matter interactions. To fully realize their potential in field-effect transistors (FETs) and photodetectors, high mobility and high responsivity are imperative. Here, we demonstrate the highest mobility of ~166 cm2 V−1 s−1 at 200 K for single-layer rhenium diselenide (ReSe2) FETs encapsulated between h-BN flakes at Vg = 47 V. The high mobility is attributed to low-resistance contacts of scandium/gold (Sc/Au), with a low Schottky barrier height and reduced charge scattering platform of h-BN. Further, we elucidated the Schottky-barrier-height dependent high photoresponsivity (~3.2 × 106 A W−1) of few-layer ReSe2 (FL-ReSe2) at 532 nm-wavelength laser light on an h-BN substrate with Sc/Au contacts. Moreover, broadband light detection of undoped and Co-doped few-layer (FL) ReSe2 was performed under different laser wavelengths (400–1100 nm). After the deposition of Co nanoparticles, the photocurrent of FL-ReSe2 increased due to n-doping, as confirmed by the transfer curves of the FL-ReSe2-based undoped and co-doped FETs. Further, the work function decreased from 4.856 to 4.791 eV in FL-ReSe2, as measured by Kelvin probe force microscopy. No light signal was observed at 1100 nm for the undoped ReSe2 (1050 nm < λcut-off < 1100 nm); however, after doping with Co nanoparticles, the cut-off wavelength exceeded to (λcut-off > 1100 nm), due to the additional trap states generated in the energy band gap of ReSe2 after Co doping. Further, the transient response of ReSe2 and Co + ReSe2 FETs was estimated so that the rise and decay times are decreased from 1.9 s & 2.7 s to 1.1 s & 1.8 s, respectively. ReSe2 is therefore a promising semiconducting material for electrical and optoelectrical applications.

Ultrafast nonlinear absorption and carrier relaxation in ReS2 and ReSe2 films

As two important members of two-dimensional (2D) transition metal dichalcogenides, ReS2 and ReSe2 have gained interest for optoelectronic and photonic applications. The key to the application of the 2D materials to the optoelectronic devices is to understand the interaction between light and matter. Here, we report that the chemical vapor deposition-grown few-layer ReS2 and ReSe2 display saturable absorption under 400 nm pulse excitation, measured through intensity dependent transmission and confirmed by ΔT/T spectroscopy. ΔT/T spectroscopy substantiates the coexistence of saturable absorption and excited state absorption at 800 nm for both ReS2 and ReSe2. Two time constants are extracted from time-resolved spectroscopy; the short time constant of 10–20 ps is associated with the relaxation of hot carriers and exciton formation, and the long time constant of 70–100 ps is assigned to exciton lifetime. The polarization dependence of ΔT/T reveals that the initial distribution of photoexcited carriers centered at excitation state is anisotropic, and this initial anisotropy loses rapidly with carrier relaxation. The nonequilibrium carriers scattered far away from excitation state are fully isotropic in the entire relaxation process. These findings provide fundamental information for using the two materials in ultrafast optoelectronic and photonic devices.

Complete-series excitonic dipole emissions in few layer ReS2 and ReSe2 observed by polarized photoluminescence spectroscopy

Excitonic emission is a key factor for a two-dimensional (2D) semiconductor to be fabricated in light-emitting device for optoelectronics application. Especially when the 2D material has a specific in-plane optical anisotropy, its excitons would become polarized dipole emitters. In this study, the whole series excitonic emissions of few-layer ReS2 and ReSe2 have been detailed characterized using polarized micro-photoluminescence (μPL) measurement. There are totally five excitonic emissions denoted as E1ex, E2ex, E3ex, ES1ex and ES2ex can be detected by the μPL experiment for both the few-layer ReS2 and ReSe2. The angular dependencies of the polarized excitonic emissions with respect to b axis (Re4 diamond-chain direction) are determined. The emission feature E1ex is the band-edge exciton while it shows orthogonal polarization with respect to both E2ex and E3ex in layered ReX2 (X = S, Se). The formation of Re4 diamond chains (DCs) in the 1 T′-phase ReS2 and ReSe2 will also assist the development of in-plane built-in electric field and forming dipoles around the DC regions. The built-in electric field effect greatly enhances the higher ES1ex and ES2ex excitonic features as evident in the photo-modulated reflectance spectra of the layered ReX2 with in-plane electrical field modulation. The binding energy and transition origin of all the excitons are also analyzed and discussed.

Photogating and high gain in ReS2 field-effect transistors

Two-dimensional layered transition metal dichalcogenides have shown much promise due to their remarkable electro-optical properties and potential use as photodetectors. We observed photogating in our few-layered (3–4 layers) ReS2 field-effect transistors (FETs) in which varying the incident optical power shifted the FETs’ threshold voltage. The photogating effect produced a significant gain in the electrical response of the FETs to incident light as measured by the responsivity (R) and external quantum efficiency (EQE). We obtained a maximum R of 45 A/W corresponding to an EQE of ∼10 500% in a four-terminal measurement of the photoconductivity in the ON-state. We attribute both the photogating and the observed gain to the influence of charge traps. An estimate of the device gain based on our observations is calculated to be 5×104.

High-efficiency hydrogen evolution from seawater using hetero-structured T/Td phase ReS2 nanosheets with cationic vacancies

Hydrogen production by water splitting with electrochemical/photoelectrochemical techniques relies mainly on fresh water which only represents 7% of the total water resource in the world. The difficulty with performing seawater splitting arises from the complex seawater composition and environment, but from the viewpoint of better utilization of natural resources, it is highly desirable to conduct the hydrogen evolution reaction (HER) in seawater. Herein, we design and synthesize few-layer heterostructured ReS2 nanosheets with the lateral metallic T and semiconducting Td phase interface, in which cationic vacancies are intentionally introduced as active sites. The ReS2 nanosheets respond to the whole spectrum of visible light and produce hydrogen from seawater efficiently. Theoretical calculation and experiments indicate that the cationic vacancies are favorable to H+ adsorption because H+ has the lowest adsorption energy compared to other cations. The nanosheets deliver stable HER performance for over 12 h in a wide range of salinity. This effective strategy to produce hydrogen from seawater may be extended to other 2D materials to accomplish highly efficient hydrogen evolution from seawater.

Diode-Pumped Solid-State Q-Switched Laser with Rhenium Diselenide as Saturable Absorber

We report a solid-state passively Q-switched Nd:YVO4 laser adopting rhenium diselenide (ReSe2) as saturable absorber (SA) materials. ReSe2 belongs to a type of transition metal dichalcogenides (TMDs) materials and it has the weak-layered dependent feature beneficial for the preparation of few-layer materials. The few-layer ReSe2 was prepared by ultrasonic exfoliation method. Using a power-dependent transmission experiment, its modulation depth and saturation intensity were measured to be 1.89% and 6.37 MW/cm2. Pumped by diode laser and based on few-layer ReSe2 SA, the Q-switched Nd:YVO4 laser obtained the shortest Q-switched pulse width of 682 ns with the highest repetition rate of 84.16 kHz. The maximum average output power was 125 mW with the slope efficiency of 17.27%. Our experiment, to the best of our knowledge, is the first demonstration that used ReSe2 as SA materials in an all-solid-state laser. The results show that the few-layer ReSe2 owns the nonlinear saturable absorption properties and it has the capacity to act as SA in an all-solid-state laser.

Uniform ReS2 porous nanospheres assembled from curly ReS2 few-layers with an expanded interlayer spacing for high-performance lithium-ion batteries

A facile hydrothermal method has been exploited to synthesize the uniform ReS2 three-dimensional hierarchical nanospheres assembled from few-layered ReS2 nanosheets with expanded interlayers (designated as porous NSs), resulting in a lot of large pores formed from the curly ReS2 nanosheets. The expanded interlayers of the ReS2 porous NSs could accommodate more lithium ions and facilitate lithium ion transports. Especially notable is that the large void spaces in the three-dimensional hierarchical nanospheres is very favorable for buffering the volume expansion, very much favoring the preservation of the electrode integrity during the lithiation/delithiation process. When evaluated as anode materials for lithium-ion batteries at a current density of 100 mA g−1 after 100 cycles, the unique ReS2 porous NSs deliver a remarkably enhanced reversible capacity of 1011 mAh g−1 with a capacity retention of over 100%, which is significantly superior to the ReS2 solid microparticles (solid MPs) with a capacity of 458 mAh g−1 and a capacity retention of merely 75%. The very high reversible capacity is indicative of the robustness of the present ReS2 porous NSs as high-performance anodes for lithium-ion batteries, which may pave the way to further study the three-dimensional self-assembly of two-dimensional layered materials towards the superior electrochemical energy storage.

Rational design of few-layered ReS2 nanosheets/N-doped mesoporous carbon nanocomposites for high-performance pseudocapacitive lithium storage

A chemical vapor deposition-based two-step nanocasting process is employed to fabricate two different types of few-Layered ReS2 nanosheets/N-doped ordered mesoporous carbon CMK-3 nanocomposites: the ReS2 nanosheets are either confined within the channels of the N-doped CMK-3 or supported on the outside of the N-doped CMK-3. Siginificantly, the ReS2 nanosheets within the channels of N-doped CMK-3 delivers a much higher reversible capacity of 1175 mA h g−1 than does the ReS2 nanosheets on the external surface of N-doped CMK-3 (719 mA h g−1), and the superiority of the ReS2 nanosheets within the channels of N-doped CMK-3 is well demonstrated over the ReS2 nanosheets on the external surface of N-doped CMK-3, i.e., in terms of specific surface area, structural stability, lithium-ion and electron transport kinetics, capacitive charge storage, and the resulting high-performance lithium storage. This work clearly demonstrates the advantage of the few layered ReS2 within the channels of N-doped CMK-3 than on the external surface of N-doped CMK-3 as an anode material in lithium-ion batteries, and meanwhile provides new insight into the rational design of the transition metal dichalcogenides/porous carbon nanocomposites for electrochemical energy storage.

Few-layered ReS2 nanosheets grown on graphene as electrocatalyst for hydrogen evolution reaction

Few-layered ReS2 two-dimensional (2D) semiconductor nanosheets directly nucleated and grown on reduced graphene oxide (RGO) were synthesized through a facile hydrothermal method. Compared with bare ReS2, the ReS2/RGO hybrid delivers much better electrocatalytic activity for hydrogen evolution reaction (HER) in acidic media. It exhibits a lower Tafel slope of 107.4 mV·dec−1 and a larger current density of − 5.2 mA·cm−2 at − 250 mV (vs. RHE), compared with ReS2 (152.7 mV·dec−1, − 3.1 mA·cm−2). The ReS2/RGO hybrid has a unique architecture constructed by highly conductive and porous RGO internetworks, which guarantees easy electrolyte infiltration and efficient charge transfer and provides sufficient active edge sites, resulting in enhanced HER performance. The present synthesis approach can be extended to synthesize other 2D-semiconductor-based composites for energy storage and catalytic devices.

Comparison of Electrical and Photoelectrical Properties of ReS2 Field-Effect Transistors on Different Dielectric Substrates.

As one of the newly discovered transition-metal dichalcogenides (TMDs), rhenium disulfide (ReS2) has been investigated mostly because of its unique characteristics such as the direct band gap nature even in bulk form, which is not prominent in other TMDs (e.g., MoS2, WSe2, etc.). However, this material possesses a low mobility and an on/off ratio, which restrict its usage in high-speed and fast switching applications. Low mobilities or on/off ratios can also be caused by substrate scattering as well as environmental effects. In this study, we used few-layer ReS2 (FL-ReS2) as a channel material to investigate the substrate-dependent mobility, current on/off ratio, Schottky barrier height (SBH), and trap density of states of different dielectric substrates. The hexagonal boron nitride (h-BN)/FL-ReS2/h-BN structure was observed to exhibit a high mobility of 45 cm2 V-1 s-1, current on/off ratio of about 107, the lowest SBH of about 12 mV at a zero back-gate voltage ( Vbg), and a low trap density of states of about 5 × 1013 cm-3. These quantities are reasonably superior compared to the FL-ReS2 devices on SiO2 substrates. We also observed a nearly 5-fold improvement in the photoresponsivity and external quantum efficiency values for the FL-ReS2 devices on h-BN substrates. We believe that the photonic characteristics of TMDs can be improved by using h-BN as the substrate and capping layer.

1.3 μm Q-switched solid-state laser based on few-layer ReS2 saturable absorber

A novel two-dimensional saturable absorber, few-layered rhenium disulfide (ReS2), was prepared by liquid phase exfoliation method successfully. Moreover, for the first time, it was successfully employed in passively Q-switched bulk Nd:YAG lasers at 1.3 μm in this paper. The maximum average output power of 78 mW was obtained with the shortest pulse width of 403 ns and the repetition rate of 214 kHz, corresponding to single pulse energy and pulse peak power of 0.42 µJ and 0.9 W, respectively. The results indicate that ReS2 has the potential to be used for passive Q-switching laser generation at 1.3 μm wavelength.

Rhenium disulfide-based passively Q-switched dual-wavelength laser at 0.95 μm and 1.06 μm in Nd:YAG

We demonstrate a passively Q-switched dual-wavelength (0.95 μm, 1.06 μm) neodymium-doped laser based on a few-layer rhenium disulfide (ReS2) saturable absorber (SA). By a liquid exfoliation method, few-layer ReS2 was prepared, and then its nonlinear optical response was examined by an open aperture Z-scan method. By using a ReS2 SA, a 0.95, 1.06 μm dual-wavelength pulsed solid state laser with a mode waist controllable linear multi-mirror cavity has been reported for the first time. The ReS2 SA Q-switched dual-wavelength oscillation threshold was recorded to be 1.35 W. At the absorbed pump power of 3.07 W, a maximum average output power of 81 mW was obtained, corresponding to the shortest pulse width of 834 ns and a repetition frequency of 165 kHz. This is, to our knowledge, the first assessment of saturable absorption and dual-wavelength Q-switching ability adjacent to the bandgap of ReS2. The result indicates that few-layered ReS2 SAs are good candidates for 0.95, 1.06 μm simultaneous pulsed laser oscillating systems.

Broadband rhenium disulfide optical modulator for solid-state lasers

Rhenium disulfide (ReS2), a member of group VII transition metal dichalcogenides (TMDs), has attracted increasing attention because of its unique distorted 1T structure and electronic and optical properties, which are much different from those of group VI TMDs (MoS2, WS2, MoSe2, WSe2, etc.). It has been proved that bulk ReS2 behaves as a stack of electronically and vibrationally decoupled monolayers, which offers remarkable possibilities to prepare a monolayer ReS2 facilely and offers a novel platform to study photonic properties of TMDs. However, due to the large and layer-independent bandgap, the nonlinear optical properties of ReS2 from the visible to mid-infrared spectral range have not yet been investigated. Here, the band structure of ReS2 with the introduction of defects is simulated by the ab initio method, and the results indicate that the bandgap can be reduced from 1.38 to 0.54 eV with the introduction of defects in a suitable range. In the experiment, using a bulk ReS2 with suitable defects as the raw material, a few-layered broadband ReS2 saturable absorber (SA) is prepared by the liquid phase exfoliation method. Using the as-prepared ReS2 SA, passively Q-switched solid-state lasers at wavelengths of 0.64, 1.064, and 1.991 μm are investigated systematically. Moreover, with cavity design, a femtosecond passively mode-locked laser at 1.06 μm is successfully realized based on the as-prepared ReS2 SA for the first time. The results present a promising alternative for a rare broadband optical modulator and indicate the potential of ReS2 in generating Q-switched and mode-locked pulsed lasers. It is further anticipated that this work may be helpful for the design of 2D optoelectronic devices with variable bandgaps.

Identification of few-layer ReS2 as photo-electro integrated catalyst for hydrogen evolution

Efficient water splitting via photo-electrocatalytic (PEC) systems holds great promise for producing hydrogen fuel in future energy devices. The artificial integration of photosensitive semiconductors and co-catalysts into effective PEC electrodes is becoming significantly challenging. Here, few-layer ReS2 nanosheets (NSs) were found to have both favorable solar light harvesting and proton reducing kinetics based on the assessments for electronic band structure and Gibbs free energy (ΔGH*) of hydrogen adsorption. Furthermore, the naturally coupling between optical and catalytic functions was demonstrated using ReS2 NSs grown on conductive carbon fiber clothes (CFC) as a hydrogen evolution reaction (HER) cathode, where the simulated 1 sun irradiation (100 mW cm−2) could significantly improve the catalytic activity along with anodic shifts of ca. 37 mV in onset potential and ca. 39 mV in over potential, and near 3-fold increase in exchange current density. The comparative analyses on Tafel, turnover and active sites revealed that the light-improved HER performance was due to the photo-generated hot electron injection into edge active sites. This work identifies ReS2 as a naturally PEC function-coupling material, opening a prospect of utilizing light-activated catalysts or co-catalysts to integrate catalysis with solar energy in PEC systems.

Few-layer rhenium diselenide: an ambient-stable nonlinear optical modulator

Transition metal dichalcogenides (TMDs), a family of two-dimensional layered materials exhibiting unusual electronic, optical, mechanical properties, have aroused much attention in recent years. Here, we focus on the nonlinear saturable absorption behavior of a new family member of TMDs, rhenium diselenide (ReSe2), and its application in pulsed laser generation. We have prepared the few-layer ReSe2 by the mechanical exfoliation method, and validated its ambient-stable nonlinear optical responses and all-optical tunable potential at 1.55 µm. In addition, the ambient-stable and parameter-tunable Q-switched fiber laser modulated by the ReSe2 has been realized.

Probing in-plane anisotropy in few-layer ReS2 using low frequency noise measurement

ReS2, a layered two-dimensional material popular for its in-plane anisotropic properties, is emerging as one of the potential candidates for flexible electronics and ultrafast optical applications. It is an n-type semiconducting material having a layer independent bandgap of 1.55 eV. In this paper we have characterized the intrinsic electronic noise level of few-layer ReS2 for the first time. Few-layer ReS2 field effect transistor devices show a 1/f nature of noise for frequency ranging over three orders of magnitude. We have also observed that not only the electrical response of the material is anisotropic; the noise level is also dependent on direction. In fact the noise is found to be more sensitive towards the anisotropy. This fact has been explained by evoking the theory where the Hooge parameter is not a constant quantity, but has a distinct power law dependence on mobility along the two-axes direction. The anisotropy in 1/f noise measurement will pave the way to quantify the anisotropic nature of two-dimensional (2D) materials, which will be helpful for the design of low-noise transistors in future.

Few-layered ReS2 nanosheets vertically aligned on reduced graphene oxide for superior lithium and sodium storage

Hybrid 2D–2D heterostructures, consisting of few-layered ReS2 nanosheets vertically anchored on both sides of reduced graphene oxide, have been prepared and evaluated for energy storage application.