Few-layer ReS2 Research Articles

Few-layered ReS2 as saturable absorber for 2.8 μm solid state laser.

A novel two-dimensional (2D) material member in the transition metal dichalcogenides family, few-layered rhenium disulfide (ReS2) was prepared by liquid phase method successfully. By using the open-aperture Z-scan method, the saturable absorption properties at 2.8μm were characterized with a saturable fluence of 22.6 μJ/cm2 and a modulation depth of 9.7%. A passively Q-switched solid-state laser at 2.8μm was demonstrated by using the as-prepared ReS2 saturable absorber successfully. Under an absorbed pump power of 920mW, a maximum output power of 104mW was obtained with a pulse width of 324ns and a repetition rate of 126kHz. To the best of our knowledge, this is the first demonstration of applying ReS2 in an all-solid-state laser. Moreover, this represents the shortest pulses in Q-switched MIR lasers based on a 2D material as the saturable absorber, which demonstrated the superiority of ReS2 acting as an optical modulator for generating short-pulsed lasers. The results well prove that 2D ReS2 is a reliable optical modulator for MIR solid-state lasers.

Phase-Engineering-Induced Generation and Control of Highly Anisotropic and Robust Excitons in Few-Layer ReS2.

The anisotropic exciton behavior in two-dimensional materials induced by spin-orbit coupling or anisotropic spatial confinement has been exploited in imaging applications. Herein, we propose a new strategy to generate high-energy and robust anisotropic excitons in few-layer ReS2 nanosheets by phase engineering. This approach overcomes the limitation imposed by the layer thickness, enabling production of visible polarized photoluminescence at room temperature. Ultrasonic chemical exfoliation is implemented to introduce the metallic T phase of ReS2 into the few-layer semiconducting Td nanosheets. In this configuration, light excitation can readily produce "hot" electrons to tunnel to the Td phase via the metal-semiconductor interface to enhance the overlap between the wave functions and screened Coulomb interactions. Owing to the strong electron-hole interaction, significant increase in the optical band gap is observed. Highly anisotropic and tightly bound excitons with visible light emission (1.5-2.25 eV) are produced and can be controlled by tailoring the T phase concentration. This novel strategy allows manipulation of polarized optical information and has great potential in optoelectronic devices.

Few-layered ReS2 Nanosheets Grown on Carbon Nanotubes for Improved Hydrogen Evolution Reaction

For the first time, few-layered ReS2 nanosheets directly nucleated and grown on the tube walls of carbon nanotubes (CNTs) have been synthesized through a facile hydrothermal method. The ReS2/CNTs composite delivers remarkably enhanced electrocatalytic performance. It exhibits a smaller onset overpotential of 80 mV and lower Tafel slope of 93.5 mV dec-1 than that of bare ReS2 (100 mV, 152.7 mV dec-1). The unique architecture with a high conductive and porous internetwork guarantees easy electrolyte infiltration, efficient electron transfer and sufficient active edge sites, resulting in enhanced HER performance of ReS2/CNTs. The presented synthesis approach can be extended to synthesize other 2D-semiconductor-based composite for energy storage and catalytic devices.

Few-layered ReS2 nanosheets grown on carbon nanotubes: A highly efficient anode for high-performance lithium-ion batteries

Rhenium disulfide (ReS2) two-dimensional (2D) semiconductor, has attracted more and more attention due to its unique anisotropic electronic, optical, mechanical properties. However, the facile synthesis and electrochemical performance of ReS2 and corresponding composite are still necessary to be explored. In this study, for the first time, few-layered ReS2 nanosheets directly nucleated and grown on the tube walls of carbon nanotubes (CNTs) have been synthesized through a facile and one-pot hydrothermal method. Compared with bare ReS2, the ReS2/CNTs composite anode delivers remarkably enhanced electrochemical performance. It exhibits a very high capacity with 1048mAhg−1 at 0.2C and high capacity retention of 93.6% after 100 cycles at 0.5C, which are much larger than that of bare ReS2. The significant enhancement in electrochemical performance is mainly attributed to its unique architecture: the CNTs not only guarantee to construct a high conductive and porous internetwork, but also ensure a compact contact with ReS2, which allow for easy electrolyte infiltration, efficient electron transfer and ionic diffusion. The presented synthesis approach can be extended to synthesize other 2D-semiconductor-based composite for energy storage and catalytic devices.

Self-assembled chrysanthemum-like microspheres constructed by few-layer ReSe2 nanosheets as a highly efficient and stable electrocatalyst for hydrogen evolution reaction

For the first time, a facile one-pot synthesis of chrysanthemum-like ReSe2 microspheres (CRM), which are self-assembled by wrinkled few-layer ReSe2 nanosheets with highly crystalline quality, is presented. Compared to ReS2, the ReSe2 microspheres exhibit superior electrocatalytic activity for hydrogen evolution reaction (HER): the CRM catalyst delivers a small onset overpotential of 80mV and the lowest Tafel slope of 67.5mV/decade for all reported Re-based dichalcogenides; it also exhibits excellent stability. The unique chrysanthemum-like structure with rich wrinkled edges and abundant nanopores guarantees sufficient electrocatalytic active edge sites, resulting in excellent HER performance of ReSe2. Chrysanthemum-like ReSe2 microsphere is a promising candidate for high-performance HER.

Selectively tunable optical Stark effect of anisotropic excitons in atomically thin ReS2.

The optical Stark effect is a coherent light–matter interaction describing the modification of quantum states by non-resonant light illumination in atoms, solids and nanostructures. Researchers have strived to utilize this effect to control exciton states, aiming to realize ultra-high-speed optical switches and modulators. However, most studies have focused on the optical Stark effect of only the lowest exciton state due to lack of energy selectivity, resulting in low degree-of-freedom devices. Here, by applying a linearly polarized laser pulse to few-layer ReS2, where reduced symmetry leads to strong in-plane anisotropy of excitons, we control the optical Stark shift of two energetically separated exciton states. Especially, we selectively tune the Stark effect of an individual state with varying light polarization. This is possible because each state has a completely distinct dependence on light polarization due to different excitonic transition dipole moments. Our finding provides a methodology for energy-selective control of exciton states.

3D chrysanthemum-like ReS2 microspheres composed of curly few-layered nanosheets with enhanced electrochemical properties for lithium-ion batteries

As a new member of the transition metal dichalcogenides (TMDs) family, rhenium disulfide (ReS2) is attracting more and more attention because of its many distinctive characteristics, such as extremely weak interlayer coupling and anisotropic electronic, optical, and mechanical properties. The studies on synthesis method and electrochemical properties of ReS2 are still rare. For the first time, three-dimensional (3D) chrysanthemum-like microspheres composed of curly ReS2 nanosheets have been synthesized through a facile hydrothermal method. The high-resolution TEM image indicates that the ReS2 nanosheet is highly crystalline with a thickness of few monolayers. As anode for lithium-ion battery, the as-synthesized 3D chrysanthemum-like ReS2 (C-ReS2) microspheres deliver a large initial discharge capacity of 843.0 mAh g−1 and remain 421.1 mAh g−1 after 30 cycles. These values are much higher than that of commercial ReS2. The significant enhancement in electrochemical performance can be attributed to its porous and chrysanthemum-like microsphere structure constructed by few-layered curly ReS2 nanosheets. This unique architecture can allow for easy electrolyte infiltration, efficient electron transfer, and ionic diffusion. The facile synthesis approach can be extended to synthesize other two-dimensional TMDs semiconductors. The study renders ReS2 a promising future in lithium-ion batteries.

Facile growth of large-area and high-quality few-layer ReS2 by physical vapour deposition

As a new two-dimensional semiconductor, rhenium disulfide (ReS2), has lots of distinctive features and exhibits great potential for future novel device applications due to its unusual structure and unique anisotropic properties. In this study, for the first time, large-area few-layer ReS2 has been grown on the SiO2/Si substrate by physical vapour deposition (PVD) using ReS2 powder as source material. XPS and Raman data confirm the composition and bonding configurations of ReS2. Clear lattice fringes of the high-resolution TEM images reveal that the ReS2 is few-layer with highly crystalline quality. It suggests that PVD is promising to synthesize wafer-scale ReS2 film to realize its applications in electronics, optoelectronics, valleytronics and spintronics. The PVD method can be extended to grow other two-dimensional semiconductors with few-layer thickness and high quality.

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.

Coupling and Stacking Order of ReS2 Atomic Layers Revealed by Ultralow-Frequency Raman Spectroscopy.

We investigate the ultralow-frequency Raman response of atomically thin ReS2, a special type of two-dimensional (2D) semiconductors with unique distorted 1T structure. Bilayer and few-layer ReS2 exhibit rich Raman spectra at frequencies below 50 cm(-1), where a panoply of interlayer shear and breathing modes are observed. The emergence of these interlayer phonon modes indicate that the ReS2 layers are coupled and orderly stacked. Whereas the interlayer breathing modes behave similarly to those in other 2D layered crystals, the shear modes exhibit distinctive behavior due to the in-plane lattice distortion. In particular, the two shear modes in bilayer ReS2 are nondegenerate and clearly resolved in the Raman spectrum, in contrast to the doubly degenerate shear modes in other 2D materials. By carrying out comprehensive first-principles calculations, we can account for the frequency and Raman intensity of the interlayer modes and determine the stacking order in bilayer ReS2.

Enhanced Raman Scattering on In-Plane Anisotropic Layered Materials

Surface-enhanced Raman scattering (SERS) on two-dimensional (2D) layered materials has provided a unique platform to study the chemical mechanism (CM) of the enhancement due to its natural separation from electromagnetic enhancement. The CM stems from the charge interactions between the substrate and molecules. Despite the extensive studies of the energy alignment between 2D materials and molecules, an understanding of how the electronic properties of the substrate are explicitly involved in the charge interaction is still unclear. Lately, a new group of 2D layered materials with anisotropic structures, including orthorhombic black phosphorus (BP) and triclinic rhenium disulfide (ReS2), has attracted great interest due to their unique anisotropic electrical and optical properties. Herein, we report a unique anisotropic Raman enhancement on few-layered BP and ReS2 using copper phthalocyanine (CuPc) molecules as a Raman probe, which is absent on isotropic graphene and h-BN. According to detailed Raman tensor analysis and density functional theory calculations, anisotropic charge interactions between the 2D materials and molecules are responsible for the angular dependence of the Raman enhancement. Our findings not only provide new insights into the CM process in SERS, but also open up new avenues for the exploration and application of the electronic properties of anisotropic 2D layered materials.

Metal to Insulator Quantum-Phase Transition in Few-Layered ReS₂.

In ReS2, a layer-independent direct band gap of 1.5 eV implies a potential for its use in optoelectronic applications. ReS2 crystallizes in the 1T'-structure, which leads to anisotropic physical properties and whose concomitant electronic structure might host a nontrivial topology. Here, we report an overall evaluation of the anisotropic Raman response and the transport properties of few-layered ReS2 field-effect transistors. We find that ReS2 exfoliated on SiO2 behaves as an n-type semiconductor with an intrinsic carrier mobility surpassing μ(i) ∼ 30 cm(2)/(V s) at T = 300 K, which increases up to ∼350 cm(2)/(V s) at 2 K. Semiconducting behavior is observed at low electron densities n, but at high values of n the resistivity decreases by a factor of >7 upon cooling to 2 K and displays a metallic T(2)-dependence. This suggests that the band structure of 1T'-ReS2 is quite susceptible to an electric field applied perpendicularly to the layers. The electric-field induced metallic state observed in transition metal dichalcogenides was recently claimed to result from a percolation type of transition. Instead, through a scaling analysis of the conductivity as a function of T and n, we find that the metallic state of ReS2 results from a second-order metal-to-insulator transition driven by electronic correlations. This gate-induced metallic state offers an alternative to phase engineering for producing ohmic contacts and metallic interconnects in devices based on transition metal dichalcogenides.

Integrated digital inverters based on two-dimensional anisotropic ReS2 field-effect transistors.

Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behaviour, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here we present atomically thin rhenium disulfide (ReS2) flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated monolayer and few-layer ReS2 field-effect transistors, which exhibit competitive performance with large current on/off ratios (∼107) and low subthreshold swings (100 mV per decade). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known two-dimensional semiconducting materials. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic field-effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic applications.

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.