Few-layer ReS2 Research Articles

Precise Crystal Orientation Identification and Twist-Induced Giant Modulation of Optical Anisotropy in 1T'-ReS2.

The ability to precisely identify crystal orientation as well as to nondestructively modulate optical anisotropy in atomically thin rhenium dichalcogenides is critical for the future development of polarization programmable optoelectronic devices, which remains challenging. Here, we report a modified polarized optical imaging (POI) method capable of simultaneously identifying in-plane (Re chain) and out-of-plane (c-axis) crystal orientations of the monolayer to few-layer ReS2, meanwhile, propose a nondestructive approach to modulate the optical anisotropy in ReS2 via twist stacking. The results show that parallel and near-cross POI are effective to independently identify the in-plane and out-of-plane crystal orientations, respectively, while regulating the twist angle allows for giant modulation of in-plane optical anisotropy from highly intrinsic anisotropy to complete optical isotropy in the stacked ReS2 bilayer (with either the same or opposite c-axes), as well modeled by linear electromagnetic theory. Overall, this study not only develops a simple optical method for precise crystal orientation identification but also offers an efficient light polarization control strategy, which is a big step toward the practical application of anisotropic van der Waals materials in the design of nanophotonic and optoelectronic devices.

Sandwiched ReS2 nanocables with dual carbon coating for efficient K+/Na+ storage performance

In this work, the nanocables of few-layered ReS2 nanosheets sandwiched between carbon nanotubes (CNTs) and nitrogen-doped amorphous carbon (NC) coating (i.e., CNT@ReS2@NC) are synthesized as high-performance anodes of both potassium-ion batteries (PIBs) and sodium-ion batteries (SIBs). The CNT@ReS2@NC nanocables with dual carbon modifications have the several advantages for efficient K+/Na+ storage. The few-layered ReS2 nanosheets with a wide interlayer spacing of 0.64 nm contribute to accelerated reaction kinetics for fast K+/Na+ intercalation/extraction. The carbon nanotube skeleton with a hollow interior can effectively relieve the volume change and serve as a robust conductive network to boost structural stability. The NC layer provides rich defects as active sites and suppresses the shuttle effect of polysulfides produced in discharge/charge processes. Consequently, the CNT@ReS2@NC nanocables possess outstanding electrochemical performance in both PIBs and SIBs owing to the synergistic effect from the different components. A long cycling lifespan of 3500 cycles with a maintained discharge capacity of 125 mAh/g is achieved for CNT@ReS2@NC at 1 A/g in PIBs. In SIBs, it can keep a high capacity of 202 mAh/g over 3000 cycles at 5 A/g. Moreover, the CNT@ReS2@NC||Na3V2(PO4)3 full cell can exhibit remarkable cycling performance, yielding a low capacity decay rate of 0.019 % per cycle over 1000 cycles at 2C.

Fabrication of Rhenium Disulfide/Mesoporous Silica Core-Shell Nanoparticles for a pH-Responsive Drug Release and Combined Chemo-Photothermal Therapy.

A stimuli-responsive drug delivery nanocarrier with a core-shell structure combining photothermal therapy and chemotherapy for killing cancer cells was constructed in this study. The multifunctional nanocarrier ReS2@mSiO2-RhB entails an ReS2 hierarchical nanosphere coated with a fluorescent mesoporous silica shell. The three-dimensional hierarchical ReS2 nanostructure is capable of effectively absorbing near-infrared (NIR) light and converting it into heat. These ReS2 nanospheres were generated by a hydrothermal synthesis process leading to the self-assembly of few-layered ReS2 nanosheets. The mesoporous silica shell was further coated on the surface of the ReS2 nanospheres through a surfactant-templating sol-gel approach to provide accessible mesopores for drug uploading. A fluorescent dye (Rhodamine B) was covalently attached to silica precursors and incorporated during synthesis in the mesoporous silica walls toward conferring imaging capability to the nanocarrier. Doxorubicin (DOX), a known cancer drug, was used in a proof-of-concept study to assess the material's ability to function as a drug delivery carrier. While the silica pores are not capped, the drug molecule loading and release take advantage of the pH-governed electrostatic interactions between the drug and silica wall. The ReS2@mSiO2-RhB enabled a drug loading content as high as 19.83 mg/g doxorubicin. The ReS2@mSiO2-RhB-DOX nanocarrier's cumulative drug release rate at pH values that simulate physiological conditions showed significant pH responsiveness, reaching 59.8% at pH 6.8 and 98.5% and pH 5.5. The in vitro testing using HeLa cervical cancer cells proved that ReS2@mSiO2-RhB-DOX has a strong cancer eradication ability upon irradiation with an NIR laser owing to the combined drug delivery and photothermal effect. The results highlight the potential of ReS2@mSiO2-RhB nanoparticles for combined cancer therapy in the future.

Dual carbon modification of few-layered ReS2 nanosheets to retard polysulfide shuttling for long-cycling sodium-ion batteries

Rhenium disulfide (ReS2) is promising to work as a robust host for sodium storage, owing to the extremely weak interlayer van der Waals interactions and large interlayer spacing. However, the low electronic conductivity and huge volume expansion seriously restrict high-rate capability and cycling lifespan. In this work, few-layered ReS2 nanosheets with carbon coating (ReS2/C) grown on the hollow mesoporous carbon spheres (HMCS) are designed to form the HMCS@ReS2/C architectures that have several merits for the sodium storage, attributed to the dual carbon modification and the hollow structure. (i) HMCS provides a large interior void to relieve the volume change during discharge/charge processes and superior structural stability for long-term cycling. (ii) The large hetero-interfacial contact can enhance electron/Na+ transport in ReS2, enabling accelerated reaction kinetics. (iii) The outer carbon layer formed on ReS2 surface is capable of inhibiting dissolution of polysulfide ions and their corrosion to the copper current collector, preventing the “under-voltage failure” phenomenon. Consequently, HMCS@ReS2/C delivers superior sodium storage performance, yielding excellent high-rate capability (211 mAh g−1 at 10 A g−1), long lifespan (4000 cycles at 10 A g−1) and stable cycling stability with an extremely low capacity decay rate of 0.0096% per cycle. The high-rate cyclability surpasses the record of ReS2-based anodes reported previously. Furthermore, the fabricated Na3V2(PO4)3║HMCS@ReS2/C full cells can also exhibit outstanding cycling performance.

Polarization-tuneable excitonic spectral features in the optoelectronic response of atomically thin ReS2

The low crystal symmetry of rhenium disulphide (ReS2) leads to the emergence of dichroic optical and optoelectronic response, absent in other layered transition metal dichalcogenides, which could be exploited for device applications requiring polarization resolution. To date, spectroscopy studies on the optical response of ReS2 have relied almost exclusively in characterization techniques involving optical detection, such as photoluminescence, absorbance, or reflectance spectroscopy. However, to realize the full potential of this material, it is necessary to develop knowledge on its optoelectronic response with spectral resolution. In this work, we study the polarization-dependent photocurrent spectra of few-layer ReS2 photodetectors, both in room conditions and at cryogenic temperature. Our spectral measurements reveal two main exciton lines at energies matching those reported for optical spectroscopy measurements, as well as their excited states. Moreover, we also observe an additional exciton-like spectral feature with a photoresponse intensity comparable to the two main exciton lines. We attribute this feature, not observed in earlier photoluminescence measurements, to a non-radiative exciton transition. The intensities of the three main exciton features, as well as their excited states, modulate with linear polarization of light, each one acquiring maximal strength at a different polarization angle. We have performed first-principles exciton calculations employing the Bethe-Salpeter formalism, which corroborate our experimental findings. Our results bring new perspectives for the development of ReS2-based nanodevices.

Highly in-plane anisotropic 2D ReSe2 for polarization-sensitive photodetectors

Here we report a new member of in-plane anisotropic 2D materials Rhenium Diselenide (ReSe2) which holds a stable distorted 1 T’ phase with its relatively narrow bandgap to explore the in-plane anisotropic structural, electrical, and optical properties. The proposed polarization-sensitive photodetectors based on few-layer ReSe2 provides highly anisotropic photodetection behavior with linearly dichroic ratio up to ≈2.62 under 808 nm illumination. We measured carrier transport in space and time by mapping carrier density in a ReSe2 plate using transient absorption microscopy. These results directly determined the carrier diffusion coefficients of about 4100 cm2 s-1 and 2900 cm2 s-1 along the armchair and zigzag directions, respectively, providing fundamental parameters to better understand the anisotropy of ReSe2 and promote future 2D optoelectronic applications in polarization-sensitive detection.

Thickness-dependent exciton relaxation dynamics of few-layer rhenium diselenide

Rhenium diselenide (ReSe2) has gathered much attention due to its low symmetry of lattice structure, which makes it possess in-plane anisotropic optical, electrical as well as excitonic properties and further enables ReSe2 have an important application in optoelectronic devices. Here, we report the thickness-dependent exciton relaxation dynamics of mechanically exfoliated few-layer ReSe2 flakes by using time-resolved pump–probe transient transmission spectroscopies. The results reveal two thickness-dependent relaxation processes of the excitons. The fast one correlates with the exciton formation (i.e., the conversion of hot carriers to excitons), while the slow one is attributed to the exciton recombination dominated by defect-assisted exciton trapping besides photon emission channel. The decrease of scattering probability caused by defects leads to the increase of fast lifetime with thickness, and the increase of slow lifetime with thickness is related to the trap-mediated exciton depopulation induced by surface defects. Polarization-dependent transient spectroscopy indicates the isotropic exciton dynamics in the two-dimensional (2D) plane. These results are insightful for better understanding of excitonic dynamics of ReSe2 materials and its application in future optoelectronic and electronic devices.

Local Electric-Field-Induced Spin Photocurrent in ReS2.

A spin-related photocurrent excited by circularly polarized light is observed near the electrodes on a few-layer ReS2 sample at room temperature. For both electrodes, the spatial distribution of the spin photocurrent shows a feature of two wings, with one positive and the other negative. In this work, it is suggested that this phenomenon arises from the inverse spin Hall effect due to the local electric field near the electrode. Bias voltage that modulates this field further controls the sign and magnitude of the spin photocurrent. Our research shows that the electric field near the electrodes has a significant impact on the spin transmission operation, and hence it could be taken into account for manufacturing spintronic devices in the future.

Expanding the ReS2 Interlayer Promises High-Performance Potassium-Ion Storage

Improving the electrochemical kinetics and the intrinsic poor conductivity of transition metal dichalcogenide (TMD) electrodes is meaningful for developing next-generation energy storage systems. As one of the most promising TMD anode materials, ReS2 shows attractive performance in potassium-ion batteries (PIBs). To overcome the poor kinetic ion diffusion and limited cycling stability of the ReS2-based electrode, herein, the interlayer distance expanding strategy was employed, and reduced graphene oxide (rGO) was introduced into ReS2. Few-layered ReS2 nanosheets were grown on the surface of the rGO with expanded interlayer distance. The prepared ReS2 nanosheets show an expanded distance (∼0.77 nm). The synthesized EI-ReS2@rGO composites were used in PIBs as anode materials. The K-ion storage mechanism of the ReS2-based anode was investigated by in situ X-ray diffraction (XRD) technology, which shows the intercalation and conversion types. The EI-ReS2@rGO nanocomposites show high specific capacities of 432.5, 316.5, and 241 mAh g-1 under 0.05, 0.2, and 1.0 A g-1 current densities and exhibit excellent reversibility at 1.0 A g-1. Overall, this strategy, which finely tunes the local chemistry and orbital hybridization for high-performance PIBs, will open up a new field for other materials.

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.

Boosting Second-Harmonic Generation in Monolayer Rhenium Disulfide by Reversible Laser Patterning

Active modification and control of transition metal dichalcogenides (TMDs) properties are highly desirable for next-generation optoelectronic applications. In particular, controlling one of the most important characteristics of TMDs─their crystal structure and symmetry─may open means for manipulating their optical nonlinearities and electrical transport properties. Here, we show that a monolayer ReS2, which does not have a broken inversion symmetry due to its stable 1T′-distorted phase and correspondingly shows only weak second-harmonic generation (SHG), can produce a significantly enhanced (∼2 orders of magnitude) SHG upon reversible laser patterning. This enhancement can be explained by the laser-induced transition from centrosymmetric 1T′ to noncentrosymmetric 2H-phase. This hypothesis is confirmed by polarization-resolved SHG measurements, which reveal a gradual change from the 2-fold to 6-fold symmetry profiles upon laser patterning. Additionally, we found that laser patterning of the bilayer ReS2 samples, contrary to the monolayers, leads to a substantially reduced SHG signal. This result corroborates the 1T′-to-2H laser-induced phase transition. Finally, we show that the laser-induced patterning is reversible by heat. These results open a possibility to actively and reversibly control the crystal structure of mono- and few-layer ReS2 and thus its optical and electronic properties.

Anomalous polarization pattern evolution of Raman modes in few-layer ReS2 by angle-resolved polarized Raman spectroscopy.

Low-symmetry of ReS2 has not only in-plane but also out-of-plane anisotropic light scattering, which is complicated, yet interesting with intrinsic strong electron-phonon coupling. In such a case, the Raman tensor also gets sophisticated with nine non-zero elements, which is layer-dependent for different Raman modes. Herein, we systematically investigated the polarization pattern evolution of both in-plane and out-of-plane Raman modes of few-layer ReS2 by angle-resolved polarized Raman spectroscopy. We found that in-plane Raman modes with less layer-dependence could be used to determine the crystal orientation (Re-chain direction) due to the weak electron-phonon interaction between layers. However, the out-of-plane and mixed vibration Raman modes demonstrate much evident layer-dependence due to the obvious electron-phonon interaction between layers. As such, the polarization patterns for the out-of-plane vibration Raman modes are distorted with layers in not only petal types but also maximum Raman intensity directions. This distortion is mainly due to the phase difference between Raman elements, which are complex values due to the near bandgap excitation laser. The results reveal that deep insights into anisotropy in low-symmetry two-dimensional materials could afford not only rich physics but also potential polarized optoelectronic devices.

Photocurrent response in few-layered ReS2 devices with short and open circuits

Photocurrent response in few-layered ReS2 devices with short and open circuits

Radiative lifetime and dynamics of trions in few-layered ReS2

Few-layered ReS2 with anisotropic electrical and optical properties has attracted much attention in the research field of two-dimensional (2D) materials. Recently, a stable charged exciton (trion) composed of bound states of two electrons and a hole with large binding energy has been experimentally observed in a few-layered ReS2. However, the dynamics of stable trions remain unclear. We studied the excited-state dynamics, including the radiative lifetime of the negative trion in three layer (3L)-ReS2 via temperature-dependent photoluminescence spectroscopy and phenomenological rate-equation analysis. The derived radiative lifetime of the trion increases linearly with increasing temperature, reaching a few nanoseconds at a high temperature of 150 K; this observation implies that the experimentally observed negative trion in 3L-ReS2 lies in the optically allowed state. Our results provide significant insights into the physics of excitonic states in 2D materials.

Hierarchical ReS 2 / nitrogen‐doped graphene hybrid nanoarchitectures for efficient oxygen reduction

The highly efficient oxygen reduction reaction (ORR) electrocatalysts are desirable for the future energy storage scenario. Herein, we report three-dimensional (3D) nanostructured electrocatalysts constructed by in situ vertical assembly of few-layer ReS2 nanosheets onto the surface of nitrogen-doped reduced graphene oxide (ReS2/NRGO). The ReS2/NRGO hybrids are featured with the fully exposed active sites, interconnected conducting channels, and facilitated charge transfer. Accordingly, the as-synthesized ReS2/NRGO hybrids achieve a relatively small Tafel slope of 71 mV dec−1 and high diffusion-limited current density of 5.58 mA cm−2, which holds potential promise for practical applications in fuel cells and metal-air batteries.

Fabrication of van der Waals heterostructures through direct growth of rhenium disulfide on van der Waals surfaces

The growth of van der Waals (vdW) heterostructures has been desired to achieve two-dimensional (2D) layered heterostructures with various types of junctions for new functional device applications. ReS2 is one of the most promising semiconducting 2D materials for providing new electronic and optoelectronic properties when integrated into a vdW heterostructure. However, owing to its low-symmetry distorted 1T crystalline structure, there are few reports on continuous high-quality vdW heterostructures unlike those on 2D materials with a symmetric 2H crystalline structure. Herein, we report on the first demonstrated ReS2/HfS2 and ReS2/h-BN vdW heterostructures through the direct growth of ReS2 films on exfoliated HfS2 and h-BN with symmetric 1T (HfS2) and 2H (h-BN) crystalline structures. Few-layered continuous ReS2 films were grown on vdW surfaces of exfoliated 2D material flakes with highly crystalline structures, and the growth behavior is explained through a surface-dependent direct growth process with vdW epitaxy. This work verifies that symmetry-free direct growth of 2D vdW heterostructures can be achieved using a 2D material as the growth template with a high growth rate, which provides a path for developing various types of multi-functional vdW heterostructures.

Liquid-Phase Exfoliation of Rhenium Disulfide by Solubility Parameter Matching.

In this work, we provide a detailed account of the liquid-phase exfoliation (LPE) of rhenium disulfide (ReS2), a promising new-generation two-dimensional material. By screening LPE in a wide range of solvents, we show that the most optimal solvents are characterized by similar Hildebrand or dispersive Hansen solubility parameters of 25 and 18 MPa1/2, respectively. Such values are attained by solvents such as N-methyl-2-pyrrolidone, N,N-dimethylformamide, and 1-butanol. In line with solution thermodynamics, we interpret the conditions for high-yield exfoliation as a matching of the solvent and ReS2 solubility parameters. Using N-methyl-2-pyrrolidone as an exemplary exfoliation solvent, we undertook a detailed analysis of the exfoliated ReS2. In-depth morphological, structural, and elemental characterization outlined that the LPE procedure presented here produces few-layer, anisotropically stacked, and chemically pure ReS2 platelets with long-term stability against oxidation. These results underscore the suitability of LPE to batch-produce few-layer and pristine ReS2 in solvents that have a solubility parameter close to 25 MPa1/2.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Summary Grain boundaries (GBs) are significant microstructures that dominate properties of polycrystalline two-dimensional (2D) materials. Low-symmetry rhenium disulfide monolayers provide an ideal platform to investigate diverse configurations of GBs and their orientation-dependent characteristics. Here, we utilize the preferential deposition of platinum-based nanoparticles on grain edges to rapidly locate nanoscale-wide overlapping GBs during microscopic-scale observation. Atomic-resolution investigations by aberration-corrected annular dark-field scanning transmission electron microscopy reveal diverse overlapping GBs constructed by parallel grains with transversal displacement. They prefer to align along several primary lattice directions with different interlayer atomic registries. Calculations show that the electronic band structures of overlapping GBs are strongly direction dependent, suggesting their potential in tunable electronic and photonic devices. Incommensurate overlapping GBs formed by quasi-parallel grains with ultrasmall twist angles are also observed. These results unveil the rich polymorphism of GBs and new opportunities to tailor properties of anisotropic 2D materials via GB engineering.

Synthesis and characterization of GaN/ReS2, ZnS/ReS2 and ZnO/ReS2 core/shell nanowire heterostructures

Layered 2D van der Waals (vdW) materials such as graphene and transition metal dichalcogenides have recently gained a great deal of scientific attention due to their unique properties and prospective applications in various fields such as electronics and optoelectronics, sensors and energy. As a direct bandgap semiconductor in both bulk and monolayer forms, ReS2 stands out for its unique distorted octahedral structure that results in distinctive anisotropic physical properties; however, only a few scalable synthesis methods for few-layer ReS2 have been proposed thus far. Here, the growth of high-quality few-layer ReS2 is demonstrated via sulfurization of a pre-deposited rhenium oxide coating on different semiconductor material nanowires (GaN, ZnS, ZnO). As-produced core-shell heterostructures were characterized by X-ray diffraction, scanning and transmission electron microscopy, micro-Raman spectroscopy and X-ray absorption spectroscopy. Experimental characterizations were supported by total energy calculations of the electronic structure of ReS2 nanosheets and GaN, ZnS, and ZnO substrates. Our results demonstrate the potential of using nanowires as a template for the growth of layered vdW materials to create novel core-shell heterostructures for energy applications involving photocatalytic and electrocatalytic hydrogen evolution.

Polarized Light-Emitting Diodes Based on Anisotropic Excitons in Few-Layer ReS2.

An on-chip polarized light source is desirable in signal processing, optical communication, and display applications. Layered semiconductors with reduced in-plane symmetry have inherent anisotropic excitons that are attractive candidates as polarized dipole emitters. Herein, the demonstration of polarized light-emitting diode based on anisotropic excitons in few-layer ReS2 , a 2D semiconductor with excitonic transition energy of 1.5-1.6 eV, is reported. The light-emitting device is based on minority carrier (hole) injection into n-type ReS2 through a hexagonal boron nitride (hBN) tunnel barrier in a metal-insulator-semiconductor (MIS) van der Waals heterostack. Two distinct emission peaks from excitons are observed at near-infrared wavelength regime from few-layer ReS2 . The emissions exhibit a degree of polarization of 80% reflecting the nearly 1D nature of excitons in ReS2 .