Monolayer ReS2 Research Articles

Strain-induced changes of electronic and optical properties of O adsorbed ReS2 monolayer

The electronic structure and optical properties of O adsorbed ReS2 monolayer under different biaxial strains were studied through Density Functional Theory. Strain reduces the band gap of O adsorbed ReS2 monolayer, the band gap varies greatly reaching 1.509 eV under strain of −10%. Both the absorption intensity and reflectivity change significantly under strains. The adsorption coefficient reaches maximum value of 27.08 × 104 cm−1 under strain of −10% within the ultraviolet range. Our research shows that strain can effectively modulate the optoelectronic properties of O adsorbed ReS2 monolayer, which provides helpful guidance for the application of ReS2 in nanoscale optoelectronic devices.

Doping-Mediated Lattice Engineering of Monolayer ReS2 for Modulating In-Plane Anisotropy of Optical and Transport Properties.

ReS2 exhibits strong anisotropic optical and electrical responses originating from the asymmetric lattice. Here, we show that the anisotropy of monolayer (1L) ReS2 in optical scattering and electrical transport can be practically erased by lattice engineering via lithium (Li) treatment. Scanning transmission electron microscopy revealed that significant strain is induced in the lattice of Li-treated 1L-ReS2, due to high-density electron doping and the resultant formation of continuous tiling of nanodomains with randomly rotating orientations of 60°, which produced a nearly isotropic response of polarized Raman scattering and absorption of Li-treated 1L-ReS2. With Li treatment, the in-plane conductance of 1L-ReS2 increased by an order of magnitude, and its angle dependence became negligible. Our result that the asymmetric phase was converted into the isotropic phase by electron injection could significantly expand the optoelectronic applications of polymorphic two-dimensional transition metal dichalcogenides.

Ultrafast hole transfer from monolayer ReS2 to thin-film F8ZnPc

We report a combined experimental and computational study on the charge transfer properties of heterostructures formed by monolayer 1T′-ReS2 and fluorinated zinc phthalocyanine, F8ZnPc. Two-dimensional (2D) ReS2 monolayer flakes were exfoliated from bulk crystals, and an F8ZnPc film with a thickness of 4 nm was thermally deposited on ReS2. Density functional theory shows that the two materials form a type-II band alignment. In transient absorption measurements, photocarriers are selectively excited in ReS2 and monitored in F8ZnPc. We found that holes in ReS2 can transfer to F8ZnPc on a timescale shorter than 0.35 ps. The transferred holes have a long lifetime in F8ZnPc on the order of 1 ns, confirming the lack of electron transfer. The efficient charge transfer from a 1T′ transition metal dichalcogenide monolayer to an organic semiconductor illustrates the feasibility of developing 2D/organic heterostructures with in-plane anisotropic electronic and optoelectronic properties.

Optical and electrical properties of monolayer ReS2 developed via chemical vapor deposition on SiO2/Si substrate

Optical and electrical properties of monolayer ReS2 developed via chemical vapor deposition on SiO2/Si substrate

Valence band offset of ReS2/BN heterojunction measured by X-ray photoelectron spectroscopy

We report the band alignment parameters of ReS2/BN van der Waals heterojunction where the ReS2 and BN monolayer are grown by chemical vapor deposition (CVD). The determination of the band alignment has been carried out by X-ray photoelectron spectroscopy (XPS). With a type-I band alignment, the valence band offset (VBO) and conduction band offset (CBO) values are measured to be 2.79±0.24 eV and 1.76±0.24 eV, respectively. The results can offer a reference for the electric and photoelectric devices based on the ReS2/BN heterojunction. In addition, the large VBO and CBO of this heterojunction make it a good choice for substrates and gate dielectrics of complementary metal oxide semiconductor (CMOS) transistors.

Large-area ReS2 monolayer films on flexible substrate for SERS based molecular sensing with strong fluorescence quenching

Two-dimensional (2D) transition metal dichalcogenides materials have recently been explored as possible substrates for surface-enhanced Raman spectroscopy (SERS). However, in comparison to the conventional noble-metal-based counterpart, most of the 2D materials have very small area for SERS measurement, which is a big limitation toward practical applications. Herein, we presented our study of SERS using substrate-scale continuous ReS2 monolayer films on flexible mica substrate. We demonstrated that the charge transfer between the target molecules and the atomically thin ReS2 can be modulated by the underlying substrate of ReS2, resulting in greatly different suppressing level of FL background. Kelvin probe force microscopy revealed a large difference of surface potential between ReS2/SiO2 and ReS2/mica, suggesting that the strong FL quenching of molecular on ReS2/mica might be attributed to the inert surface of layered mica. The large-area ReS2 monolayer films on mica show a detection limit of R6G molecule around 10−7 M and a robust SERS performance after an exposure in air for one month and repeated bending for 1000 times. Our work explores the practical application of 2D ReS2 for molecular detection via SERS technique.

Beyond Graphene: Low-Symmetry and Anisotropic 2D Materials

Low-symmetry 2D materials—such as ReS2 and ReSe2 monolayers, black phosphorus monolayers, group-IV monochalcogenide monolayers, borophene, among others—have more complex atomistic structures than the honeycomb lattices of graphene, hexagonal boron nitride, and transition metal dichalcogenides. The reduced symmetries of these emerging materials give rise to inhomogeneous electron, optical, valley, and spin responses, as well as entirely new properties such as ferroelasticity, ferroelectricity, magnetism, spin-wave phenomena, large nonlinear optical properties, photogalvanic effects, and superconductivity. Novel electronic topological properties, nonlinear elastic properties, and structural phase transformations can also take place due to low symmetry. The “Beyond Graphene: Low-Symmetry and Anisotropic 2D Materials” Special Topic was assembled to highlight recent experimental and theoretical research on these emerging materials.

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.

Layer-dependent band to band tunneling in WSe2/ReS2 van der Waals heterojunction

Van der Waals (vdW) heterostructures are promising for building tunneling field-effect transistors (TFETs), owing to an inherent narrow vdW gap between two stacked materials induced by the dangling bond free surface. However, the band to band tunneling (BTBT) of such a vdW heterostructure TFET strongly depends on the layer-dependent band structure variation at the interface. Here, we report a first principle simulation on the BTBT transition of the monolayer ReS2 based heterostructures with monolayer and bilayer WSe2. An obvious decrease of the turn-on gate voltage from 36 V to 12 V was achieved by adding a layer of WSe2 due to the band gap narrowing and momentum conservative Γ-Γ tunneling. Under the gate voltage of 20 V with bias of 0.271 V, the upper limit of the BTBT saturate current density in bilayer WSe2 vdW heterojunction can reach 934 μA μm−1. These results show the bilayer WSe2 heterojunction could be an ideal candidate for lower power and high operating speed TFETs.

Salt-assisted growth and ultrafast photocarrier dynamics of large-sized monolayer ReSe2

Owing to its anisotropic optical and electrical properties, rhenium diselenide (ReSe2) has garnered considerable attention recently as a candidate material for polarization-sensitive photodetectors. However, the direct and controllable synthesis of large-sized ReSe2 with a uniform thickness is still a great challenge. Herein, we have refined the synthesis method to obtain uniform monolayer ReSe2 flakes with a size of up to ~ 106 μm on sapphire via an ambient-pressure chemical vapor deposition technique using Na promoter from sodium chloride. Interestingly, optical pump-probe spectroscopy revealed a fast switching from saturable absorption (SA) to absorption enhancement (AE) in subpicosecond time scale, followed by a slower decay induced by exciton recombination. Furthermore, both AE and SA signals exhibited clear angular dependence with a periodicity of 180°, which reflected the dichroism in nonlinear absorption dynamics. In addition, the photocarrier dynamics including free-carrier transport and subpicosecond relaxation due to exciton formation or surface trapping was probed using time resolved terahertz spectroscopy. We believe that our study serves as a reference for atomically controlled synthesis of large-sized ReSe2 and provides useful insights on its optoelectronic properties for novel device applications.

Deeply Exploring Anisotropic Evolution toward Large-Scale Growth of Monolayer ReS2.

Among large numbers of transition metal dichalcogenides (TMDCs), monolayer rhenium disulfide (ReS2) is of particular interest due to its unique structural anisotropy, which opens up unprecedented opportunities in dichroic atomical electronics. Understanding the domain structure and controlling the anisotropic evolution of ReS2 during the growth is considered critical for increasing the domain size toward a large-scale growth of monolayer ReS2. Herein, by employing angle-resolved Raman spectroscopy, we reveal that the hexagonal ReS2 domain is constructed by six well-defined subdomains with each b-axis parallel to the diagonal of the hexagon. By further combining the first-principles calculations and the transmission electron microscopy (TEM) characterization, a dislocation-involved anisotropic evolution is proposed to explain the formation of the domain structures and understand the limitation of the domain size. Based on these findings, growth rates of different crystal planes are well controlled to enlarge the domain size, and moreover, single-crystal domains with a triangle shape are obtained. With the improved domain size, large-scale uniform, strictly monolayer ReS2 films are grown further. Scalable field-effect transistor (FET) arrays are constructed, which show good electrical performances comparable or even superior to that of the single domains reported at room temperature. This work not only sheds light on comprehending the novel growth mechanism of ReS2 but also offers a robust and controllable strategy for the synthesis of large-area and high-quality two-dimensional materials with low structural symmetry.

Strain-Mediated Stability of Structures and Electronic Properties of ReS2, Janus ReSSe, and ReSe2 Monolayers

Monolayers of transition metal ReX2 and ReSX (X=S, Se) have been proposed as new electronic materials for nanoscale devices. In this paper, there are three structures: ReS2, Janus ReSSe, and ReSe2. Based on the first-principles theory, we analyzed the structures, electronic properties, and Fermi speed. Remarkably, we studied the stability of structures of ReS2, Janus ReSSe, and ReSe2 monolayers under biaxial tensile and compressive strain by density functional approach. It is worth noting that when the strain changes, not only the band gap changes but also the band gap properties (direct and indirect) also change. The bond gaps decrease with the increase of tensile strain and compressive strain; Moreover, when the strain is greater than 0, the bond angle decreases as the strain increases, and when the strain is less than 0, the bond angle increases as the strain increases.

DFT + U studies of the electronic and optical properties of ReS2 mono-layer doped with lanthanide atoms

The structural, electronic and optical properties of various elements in the lanthanide series as dopant atoms in the triclinic mono-layered rhenium disulphide (ReS2) structure was evaluated using density functional theory with the Hubbard U correction. The Hubbard U correction term was introduced to account for the presence of the electron-electron correlation in the partial filled 4f orbitals of the lanthanide dopant atoms. The calculated lanthanide ion substitutional dopant energies in the ReS2 mono-layer under both the Re-rich and S-rich conditions show that the synthesis of the lanthanide metal doped ReS2 mono-layer is feasible. The pristine ReS2 mono-layer has an electronic band gap of 1.43 eV with a non-magnetic ground state. The introduction of lanthanide atoms (Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm) with partial filled 4f orbital modifies the ReS2 mono-layer from non-magnetic to magnetic ground state. The La atom without 4f as well as the Yb and Lu atoms with completely filled 4f orbitals retains its non-magnetic ground state. The presence of the lanthanide dopant atoms in the ReS2 mono-layer modifies the electronic ground structure leading to the introduction of defect states within the energy gap and minute changes in their overall density of states profiles. The calculated optical reflectivity spectra and absorption spectra was modified with significant red-shift observed in the absorption spectra as a result of the dopant atoms. These lanthanide doped ReS2 mono-layer could find potential applications in spintronic and optoelectronic devices.

Controllable growth of large-area monolayer ReS2 flakes by chemical vapor deposition

Two-dimensional (2D) ReS2 flakes are synthesized by the chemical vapor deposition under atmospheric pressure using Re–Te binary eutectic and sulfur as precursors. The morphologies, crystal structure, phonon vibration modes, chemical states and optical property of the 2D ReS2 flakes are characterized using optical microscopy, scanning electron microscope, transmission electron microscopy, the Raman spectroscopy, X-ray photoelectron spectroscopy and transmittance spectra. The lateral size of 2D ReS2 flakes can be tuned via changing the synthesis parameters. The monolayer 2D ReS2 flakes is single crystal structure. Its maximum lateral size is 30 μm, and optical band gap is 1.42 eV. The absorption coefficient is as large as 105 cm−1, transmittance of 92–98% and refractive index of 1.5–1.3 in the wavelength range from 400 to 800 nm. The effect of synthesis parameters on the morphologies and growth mechanism of 2D ReS2 flakes are investigated in detail. This is helpful for further application of ReS2 in electronic and optoelectronic devices.

Nanosized ReS2 Monolayers Embedded in Nitrogen‐Doped Carbon Nanotubes for High‐Rate Capacitive Lithium Storage

AbstractThe expected approach to improve the electrochemical performance of layered transition metal dichalcogenides is to thin them down to monolayer for minimizing the barrier of lithium‐ion intercalation and diffusion. In this work, a distinct nanocomposite constructed from nanosized ReS2 monolayers uniformly embedded in nitrogen‐doped carbon nanotubes is synthesized via a facile hydrothermal method followed by calcination. When evaluated as anode materials for lithium ion batteries, the robust nanocomposites exhibit a reversible capacity of 825 mAh g−1 at 100 mA g−1 after 100 cycles with an initial Coulombic efficiency as high as 83%, and more importantly, retain an outstanding capacity of 612 mAh g−1 at 1 A g−1 after 500 cycles with an excellent capacity retention of 80%. Kinetics analysis of the redox reactions reveals that 86% of the charge storage is controlled by a capacitive behavior, leading to both high‐rate and long‐life lithium storage performances. The design of the nanocomposites offers a promising strategy to achieve extra power over batteries by capacitive energy storage.

Distorted Monolayer ReS2 with Low-Magnetic-Field Controlled Magnetoelectricity.

Two dimensional (2D) materials possessing ferroelectric/ferromagnetic orders and especially low-magnetic-field controlled magnetoelectricity have great promise in spintronics and multistate data storage. However, ferroelectric and magnetoelectric (ME) dipoles in the atom-thick 2D materials are difficult to be realized due to structural inversion symmetry, thermal actuation, and depolarized field. To overcome these difficulties, the monolayer structure must possess an in-plane inversion asymmetry in order to provide out-of-plane ferroelectric polarization. Herein, crystal chemistry is adopted to engineer specific atomic displacement in monolayer ReS2 to change the crystal symmetry to induce out-of-plane ferroelectric polarization at room temperature. The cationic Re vacancy in the atom-displaced ReS2 monolayer causes spin polarization of two immediate neighbor sulfur atoms to generate magnetic ordering, and the ferroelectric distortion near the Re vacancy locally tunes the ferromagnetic order thereby triggering low-magnetic-field controlled ME polarization at about 28 K. As a result, 2D ME coupling multiferroics is achieved. Our results not only reveal a design methodology to attain coexistence of ferroelectric and ferromagnetic orders in 2D materials but also provide insights into magnetoelectricity in 2D materials.

Monolayer-ReS2 field effect transistor using monolayer-graphene as electrodes

Atomic thinness, excellent transport property and gate-tunable band structure of graphene render it a potential electrode material for assembling ultra thin electronic devices. Here, we present back gate field effect transistor based on exfoliated monolayer (ML) ReS2 as channel semiconductor and exfoliated ML-graphene as drain-source electrodes. The G and 2D peaks of ML-graphene stiffen significantly and their intensity ratio increases as well when the graphene is encapsulated between the SiO2/Si substrate and ReS2. Owing to the excellent electron transport properties of the ML-ReS2 and gate-tunable Fermi-level of the underlying ML-graphene, the transistor exhibits on/off current ratio exceeding 106, mobility of ∼1.1 cm2 V−1s−1 and subthreshold swing ∼740 mV per decade. This work indicates that ML-graphene is an excellent electrode material for fabricating atomically thin ReS2 electronic devices.

SINGLE LAYER ReS2H2 : STABILITY, RAMAN ACTIVITY AND ELECTRONIC PROPERTIES

In this study, the structural, vibrational and electronic properties of the hydrogenated single layer of ReS2 are investigated by performing the first principle calculations based on density functional theory. We found that the characteristic properties of the monolayer ReS2 can be manipulated upon the hydrogen functionalization. As the monolayer ReS2, the ReS2H2 has distorted 1Tphase; however, the bonding in Re slab significantly varies with the hydrogenation. Our results demonstrate that the full-surface hydrogenation leads to an expansion in lattice and the Re4 tetramer-chains in the monolayer ReS2 are separated into two dimers in the hydrogenated monolayer. It is calculated that the dynamically stable monolayer of ReS2H2 has 26 Raman-active vibrational modes. Constant volume specific heat calculations are also performed and the results indicate that at high temperature, the monolayer ReS2 approaches to limit of 3R before the monolayer ReS2H2. By performing the electronic band structure calculations, it is shown that when the ReS2 surface is fully hydrogenated, there occurs a direct to indirect band gap transition and the semiconducting hydrogen-induced monolayer has a band gap of 0.74 eV.

Epitaxial Growth of 1D Atomic Chain Based Se Nanoplates on Monolayer ReS2 for High‐Performance Photodetectors

AbstractMixed‐dimensional (0D, 1D, and 3D) heterostructures based on 2D layered materials have been proven as a promising candidate for future nanoelectronics and optoelectronics applications. In this work, it is demonstrated that 1D atomic chain based Se nanoplates (NPs) can be epitaxially grown on monolayer ReS2 by a chemical transport reaction, thereby creating an interesting mixed‐dimensional Se/ReS2 heterostructure. A unique epitaxial relationship is observed with the (110) planes of the Se NPs parallel to the corresponding ReS2 (010) planes. Experimental and theoretical studies reveal that the Se NPs could conjugate with underlying monolayer ReS2 via strong chemical hybridization at heterointerface, which is expected to originate from the intrinsic defects of ReS2. Remarkably, photodetectors based on Se/ReS2 heterostructures exhibit ultrahigh detectivity of up to 8 × 1012 Jones, and also show a fast response time of less than 10 ms. These results illustrate the great advantage of directly integrated 1D Se based nanostructure on planar semiconducting ReS2 films for optoelectronic applications. It opens up a feasible way to obtain mixed‐dimensional heterostructures with atomic interfacial contact by epitaxial growth.

Space-confined growth of monolayer ReSe2 under a graphene layer on Au foils

Vertical heterostructures based on two-dimensional (2D) materials have attracted widespread interest for their numerous applications in electronic and optoelectronic devices. Herein, we report the direct construction of an abnormal graphene/ReSe2 stack on Au foils by a two-step chemical vapor deposition (CVD) strategy. During the second growth stage, monolayer ReSe2 is found to preferentially evolve at the interface between the first-grown graphene layer and the Au substrate. The unusual stacking behavior is unraveled by in-situ “cutting open” the upper graphene from the defects to expose the lower ReSe2 using scanning tunneling microscopy (STM). From combination of these results with density functional theory calculations, the domain boundaries and edge sites of graphene are proposed to be adsorption sites for Re and Se precursors, further facilitating the growth of ReSe2 at the van der Waals gap of graphene/Au. This work hereby offers an intriguing strategy for obtaining vertical 2D heterostructures featured with an ultra-clean interface and a designed stacking geometry.