Stacking-Order-Driven Optical Properties and Carrier Dynamics in ReS2

Two distinct stacking orders in ReS₂ are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principles calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm⁻¹ for AA stacking, and 20 cm⁻¹ for AB stacking, making a simple tool for determining the stacking orders in ReS₂. Polarized photoluminescence (PL) reveals that AB stacking possesses blueshifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump–probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. The findings underscore the stacking-order driven optical properties and carrier dynamics of ReS₂, mediate many seemingly contradictory results in the literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.     

Unraveling the Raman Enhancement Mechanism on 1T′‐Phase ReS2 Nanosheets

2D transition metal dichalcogenides materials are explored as potential surface‐enhanced Raman spectroscopy substrates. Herein, a systematic study of the Raman enhancement mechanism on distorted 1T (1T′) rhenium disulfide (ReS₂) nanosheets is demonstrated. Combined Raman and photoluminescence studies with the introduction of an Al₂O₃ dielectric layer unambiguously reveal that Raman enhancement on ReS₂ materials is from a charge transfer process rather than from an energy transfer process, and Raman enhancement is inversely proportional while the photoluminescence quenching effect is proportional to the layer number (thickness) of ReS₂ nanosheets. On monolayer ReS₂ film, a strong resonance‐enhanced Raman scattering effect dependent on the laser excitation energy is detected, and a detection limit as low as 10^?9m can be reached from the studied dye molecules such as rhodamine 6G and methylene blue. Such a high enhancement factor achieved through enhanced charge interaction between target molecule and substrate suggests that with careful consideration of the layer‐number‐dependent feature and excitation‐energy‐related resonance effect, ReS₂ is a promising Raman enhancement platform for sensing applications.     

Highly Efficient Photocatalytic Hydrogen Evolution by ReS2 via a Two‐Electron Catalytic Reaction

Highly efficient photocatalytic hydrogen evolution (PHE) is highly desirable for addressing the global energy crisis and environmental problems. Although much attention has been given to electron–hole separation, ridding photocatalysts of poor efficiency remains challenging. Here, a two‐electron catalytic reaction is developed by utilizing the distinct trion behavior of ReS₂ and the efficient reduction of two H⁺ (2H⁺ + 2e^? → H₂) is realized. Due to the monolayer‐like structure of the catalyst, the free electrons in ReS₂ can be captured by the tightly bound excitons to form trions consisting of two electrons and one hole. These trions can migrate to the surface and participate in the two‐electron reaction at the abundant active sites. As expected, such a two‐electron catalytic reaction endows ReS₂ with a PHE rate of 13 mmol g^?1 h^?1 under visible light irradiation. Meanwhile, this reaction allows the typically poor PHE efficiency of pure transition metal dichalcogenides to be overcome. The proposed two‐electron catalytic reaction provides a new approach to the design of photocatalysts for PHE.     

Transient Absorption Measurements on Anisotropic Monolayer ReS2

Anisotropic optical and transport properties of monolayer ReS₂ fabricated by mechanical exfoliation are reported. Transient absorption measurements with different polarization configurations and sample orientations reveal that the absorption coefficient and transient absorption are both anisotropic, with maximal and minimal values occurring when the light polarization is parallel and perpendicular to the Re atomic chains, respectively. The maximal values are about a factor of 2.5 of the minimal values. By resolving the spatiotemporal dynamics of excitons, it is found that the diffusion coefficient of excitons moving along Re atomic chains is about 16 cm² s⁻¹ at room temperature, which is about a factor of three larger than those moving perpendicular to that direction. An exciton lifetime of 40 ps is also extracted. These findings establish monolayer ReS₂ as an anisotropic 2D transition metal dichalcogenide.     

垂直排列ReS2(1-x)Se2x合金纳米片的控制合成及带隙调控

二维过渡金属硫属化合物具有优异的电学和光学特性, 形貌控制及带隙调控对于其在光电子学、光子学、纳米电子学领域中的应用至关重要。研究采用CVD技术在SiO₂/Si衬底上生长了垂直排列ReS₂纳米片材料, 硒化处理后得到ReS_2(1-x)Se_2x合金纳米片, 并研究了硒化温度(700、850 和 920℃)及硒化时间(0.5、1和1.5 h)对ReS_2(1-x)Se_2x合金纳米片形貌及组分的影响。XPS元素定量分析及紫外-可见-近红外吸收光谱研究表明ReS_2(1-x)Se_2x样品中Se含量可以在x=0(纯ReS₂)到x=0.86之间调变, 相应材料的带隙可从1.55 eV (800 nm)调变到1.28 eV (969 nm)。SEM结果显示ReS_2(1-x)Se_2x纳米片的结构受到硒化温度和硒化时间的影响, 硒化温度升高和硒化时间延长会破坏纳米片的垂直结构。上述结果表明本研究成功合成了垂直排列ReS_2(1-x)Se_2x合金纳米片, 该材料在电化学催化、功能电子器件和光电子器件方面具有潜在应用价值。     

Identifying,Resolving, and Quantifying Anisotropy in ReS2 Nanomechanical Resonators

As an emerging two-dimensional semiconductor, rhenium disulfide (ReS₂) is renowned for its strong in-plane anisotropy in electrical, optical, and thermal properties. In contrast to the electrical, optical, optoelectrical, and thermal anisotropies that are extensively studied in ReS₂, experimental characterization of mechanical properties has largely remained elusive. Here, it is demonstrated that the dynamic response in ReS₂ nanomechanical resonators can be leveraged to unambiguously resolve such disputes. Using anisotropic modal analysis, the parameter space for ReS₂ resonators in which mechanical anisotropy is best manifested in resonant responses is determined. By measuring their dynamic response in both spectral and spatial domains using resonant nanomechanical spectromicroscopy, it is clearly shown that ReS₂ crystal is mechanically anisotropic. Through fitting numerical models to experimental results, it is quantitatively determined that the in-plane Young's moduli are 127 and 201 GPa along the two orthogonal mechanical axes. In combination with polarized reflectance measurements, it is shown that the mechanical soft axis aligns with the Re-Re chain in the ReS₂ crystal. These results demonstrate that dynamic responses in nanomechanical devices can offer important insights into intrinsic properties in 2D crystals and provide design guidelines for future nanodevices with anisotropic resonant responses.     

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.     

Synthesis of Large‐Size 1T′ ReS2xSe2(1−x) Alloy Monolayer with Tunable Bandgap and Carrier Type

Chemical vapor deposition growth of 1T′ ReS_2x Se_{2(1?x )} alloy monolayers is reported for the first time. The composition and the corresponding bandgap of the alloy can be continuously tuned from ReSe₂ (1.32 eV) to ReS₂ (1.62 eV) by precisely controlling the growth conditions. Atomic‐resolution scanning transmission electron microscopy reveals an interesting local atomic distribution in ReS_2x Se_{2(1?x )} alloy, where S and Se atoms are selectively occupied at different X sites in each Re‐X₆ octahedral unit cell with perfect matching between their atomic radius and space size of each X site. This structure is much attractive as it can induce the generation of highly desired localized electronic states in the 2D surface. The carrier type, threshold voltage, and carrier mobility of the alloy‐based field effect transistors can be systematically modulated by tuning the alloy composition. Especially, for the first time the fully tunable conductivity of ReS_2x Se_{2(1?x )} alloys from n‐type to bipolar and p‐type is realized. Owing to the 1T′ structure of ReS_2x Se_{2(1?x )} alloys, they exhibit strong anisotropic optical, electrical, and photoelectric properties. The controllable growth of monolayer ReS_2x Se_{2(1?x )} alloy with tunable bandgaps and electrical properties as well as superior anisotropic feature provides the feasibility for designing multifunctional 2D optoelectronic devices.     

Modulating the Function of GeAs/ReS2 van der Waals Heterojunction with its Potential Application for Short-Wave Infrared and Polarization-Sensitive Photodetection

Van der Waals heterojunction (vdWs) of 2D materials with integrated or extended superior characteristics, opening up new opportunities in functional electronic and optoelectric device applications. Exploring methods to achieve multifunctional vdWs heterojunction devices is one of the most promising prospects in this area. Herein, a diverse function of forward rectifying diode, Zener tunneling diode, and backward rectifying diodes are realized in GeAs/ReS₂ heterojunction by modulating the doping level of GeAs. The tunneling diode presents an interesting trend forward negative differential resistance (NDR) behavior which may facilitate the application of multi-value logic. More importantly, the GeAs/ReS₂ forward rectifying diode exhibits highly sensitive photodetection in the wide-spectrum range up to 1550 nm corresponding to a short-wave infrared (SWIR) region. In addition, as two strong anisotropic 2D materials of GeAs and ReS₂, the heterojunction exhibits strong polarization-sensitive photodetection behavior with a dichroic photocurrent ratio of 1.7. This work provides an effective strategy to achieve multifunctional 2D vdW heterojunction devices and develops more possibilities to broaden their functionalities and applications.     

Chemical Vapor Deposition of High‐Quality and Atomically Layered ReS2

Recently, anisotropic 2D materials, such as black phosphorus and rhenium disulfides (ReS₂), have attracted a lot attention because of their unique applications on electronics and optoelectronics. In this work, the direct growth of high‐quality ReS₂ atomic layers and nanoribbons has been demonstrated by using chemical vapor deposition (CVD) method. A possible growth mechanism is proposed according to the controlled experiments. The CVD ReS₂‐based filed‐effect transistors (FETs) show n‐type semiconducting behavior with a current on/off ratio of ≈10⁶ and a charge carrier mobility of ≈9.3 cm² Vs⁻¹. These results suggested that the quality of CVD grown ReS₂ is comparable to mechanically exfoliated ReS₂, which is also further supported by atomic force microscopy imaging, high‐resolution transmission electron microscopy imaging and thickness‐dependent Raman spectra. The study here indicates that CVD grown ReS₂ may pave the way for the large‐scale fabrication of ReS₂‐based high‐performance optoelectronic devices, such as anisotropic FETs and polarization detection.