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

Facile and Scalable Colloidal Synthesis of Transition Metal Dichalcogenide Nanoparticles with High-Performance Hydrogen Production

Transition metal dichalcogenides (TMDs) have garnered significant attention as efficient electrocatalysts for the hydrogen evolution reaction (HER) due to their high activity, stability, and cost-effectiveness. However, the development of a convenient and economical approach for large-scale HER applications remains a persistent challenge. In this study, we present the successful synthesis of TMD nanoparticles (including MoS2, RuS2, ReS2, MoSe2, RuSe2, and ReSe2) using a general colloidal method at room temperature. Notably, the ReSe2 nanoparticles synthesized in this study exhibit superior HER performance compared with previously reported nanostructured TMDs. Importantly, the synthesis of these TMD nanoparticles can readily be scaled up to gram quantities while preserving their exceptional HER performance. These findings highlight the potential of colloidal synthesis as a versatile and scalable approach for producing TMD nanomaterials with outstanding electrocatalytic properties for water splitting.

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.

Universal Platform for Robust Dual-Atom Doped 2D Catalysts with Superior Hydrogen Evolution in Wide pH Media

Layered 2D transition metal dichalcogenides (TMDs) have been suggested as efficient substitutes for Pt-group metal electrocatalysts in the hydrogen evolution reaction (HER). However, poor catalytic activities in neutral and alkaline electrolytes considerably hinder their practical applications. Furthermore, the weak adhesion between TMDs and electrodes often impedes long-term durability and thus requires a binder. Here, a universal platform is reported for robust dual-atom doped 2D electrocatalysts with superior HER performance over a wide pH range media. V:Co-ReS2 on a wafer scale is directly grown on oxidized Ti foil by a liquid-phase precursor-assisted approach and subsequently used as highly efficient electrocatalysts. The catalytic performance surpasses that of Pt group metals in a high current regime (≥ 100 mA cm-2) at pH ≥ 7, with a high durability of more than 70 h in all media at 200 mA cm-2. First-principles calculations reveal that V:Co dual doping in ReS2 significantly reduces the water dissociation barrier and simultaneously enables the material to achieve the thermoneutral Gibbs free energy for hydrogen adsorption.

Gate tunable linear dichroism in monolayer 1T'-MoS2

The linear dichroism demonstrates promising applications in the fields of polarization-resolved photodetectors and polarization optical imaging. Herein, we study the optical properties of monolayer 1T'-MoS2 based on a four-band effective k · p Hamiltonian within the framework of linear response theory. Owing to the anisotropic band structure, the k-resolved optical transition matrix elements associated with armchair(x) and zigzag(y) direction polarized light exhibit a staggered pattern. The anisotropy of the optical absorption spectrum is shown to sensitively depend on the photon energy, the light polarization and the gate voltage. A gate voltage can continuously modulate the anisotropy of the optical absorption spectra, rendering it isotropic or even reversing the initial anisotropy. This modulation leads to linear dichroism conversions across multiple wavelengths. Our findings are useful to design polarized photodetectors and sensors based on monolayer 1T'-MoS2. Our results are also applicable to other monolayer transition metal dichalcogenides with 1T' structure.

Triggering the Channel-Sulfur Sites in 1T'-ReS2 Cocatalyst toward Splendid Photocatalytic Hydrogen Generation

Electron density manipulation of active sites in cocatalysts is of great essential to realize the optimal hydrogen adsorption/desorption behavior for constructing high-efficient H2 -evolution photocatalyst. Herein, a strategy about weakening metal-metal bond strength to directionally optimize the electron density of channel-sulfur(S) sites in 1T' Re1- x Mox S2 cocatalyst is clarified to improve their hydrogen adsorption strength (S─H bond) for rapid H2 -production reaction. In this case, the ultrathin Re1- x Mox S2 nanosheet is in situ anchored on the TiO2 surface to form Re1- x Mox S2 /TiO2 photocatalyst by a facial molten salt method. Remarkably, numerous visual H2 bubbles are constantly generated on the optimal Re0.92 Mo0.08 S2 /TiO2 sample with a 10.56 mmol g-1  h-1 rate (apparent quantum efficiency is about 50.6%), which is 2.6 times higher than that of traditional ReS2 /TiO2 sample. Density functional theory and in situ/ex situ X-ray photoelectron spectroscopy results collectively demonstrate that the weakened Re─Re bond strength via Mo introduction can induce the formation of unique electron-deficient channel-S sites with suitable electron density, which yield thermoneutral S─H bonds to realize superior interfacial H2 -generation performance. This work provides fundamental guidance on purposely optimizing the electronic state of active sites by manipulating the intrinsic bonding structure, which opens an avenue for designing efficacious photocatalytic materials.

Self-Oxidation Resistance of the Curved Surface of Achromatic Copper

Copper surfaces that exhibit a wide range of achromatic colors while still metallic have not been studied, despite advancements in antireflection coatings. A series of achromatic copper films grown with [111] preferred orientation by depositing 3D porous nanostructures is introduced via coherent/incoherent atomic sputtering epitaxy. The porous copper nanostructures self-regulate the giant oxidation resistance by constructing a curved surface that generates a series of monoatomic steps, followed by shrinkage of the lattice spacing of one or two surface layers. First-principles calculations confirm that these structural components cooperatively increase the energy barrier against oxygen penetration. The achromaticity of the single-crystalline porous copper films is systematically tuned by geometrical parameters such as pore size distribution and 3D linkage. The optimized achromatic copper films with high oxidation resistance show an unusual switching effect between superhydrophilicity and superhydrophobicity. The tailored 3D porous nanostructures can be a candidate material for numerous applications, such as antireflection coatings, microfluidic devices, droplet tweezers, and reversible wettability switches.

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.

In-Plane Anisotropic Rectification and Photovoltaic Effects on ZnO Nonpolar (101̅0) Crystal Plane and the Physical Mechanism

The concept of spontaneous electric field (Es) in polar structures is crucial for understanding the physical and chemical properties of compound semiconductors and improving their performanes. However, this concept has not been widely accepted so far. Here, we show the first observation of rectification and photovoltaic effects in the polar [0001] direction on the nonpolar ZnO (1010) crystal plane. However, no rectification and photovoltaic effects are observed in the nonpolar [1210] direction perpendicular to the [0001]. When a stress was applied in the [0001] direction of the ZnO single crystal, the rectification and photovoltaic effects are abated and disappeared. The disappearance of the two effects results from the pressure-induced disappearance of the polar structure. The results fully demonstrated that the rectification and photovoltaic effects arise from the existence of Es in the polar [0001] direction. The Es motivates the directional transfer of the electrons and photocreated charges along the [0001] direction, and the rectification and photovoltaic effects are thus observed. These results provide direct evidence for the polar structure theory and suggest that the polar structures can be employed to develop new types of photovoltaic and other electronic and photoelectronic devices.

Scalable-doped Nanoporous 1T″ ReSe2 via a General Surface Co-Alloy Strategy

Reliable and controllable doping of 2D transition metal dichalcogenides is an efficient approach to tailor their physicochemical properties and expand their functional applications. However, precise control over dopant distribution and scalability of the process remains a challenge. Here, we report a general method to achieve scalable in situ doping of centimeter-sized bicontinuous nanoporous ReSe₂ films with transition metal atoms via surface coalloy growth. The distinct strains induced by the bending curvature of nanoporous structures and uniform dopants result in a local 1T' to 1T″ structure phase transition over nanoporous ReSe₂ films. The as-prepared nanoporous Ru-ReSe₂ with high 1T″ phase exhibits preferable electrochemical activity in hydrogen evolution reaction. The work demonstrates a unique and general approach to synthesize uniformly-doped transition metal dichalcogenides with 3D bicontinuous nanoporous structure, which can be scaled up to batch production for various applications.

Analyzing Anisotropy in 2D Rhenium Disulfide Using Dichromatic Polarized Reflectance

In-plane anisotropy in 2D rhenium disulfide (ReS₂ ) offers intriguing opportunities for designing future electronic and optical devices, and toward such applications, it is crucial to identify the crystal orientation in such 2D anisotropic materials. Existing spectroscopy or electron microscopy methods for determining the crystalline orientation often require complicated sample preparing procedures and specialized equipment, which could sometimes limit their application. In this work, a dichromatic polarized reflectance method is demonstrated, which can quickly and accurately resolve the crystal orientation (Re-Re chain) in 2D ReS₂ crystals with different thicknesses. Furthermore, it can be readily extended to multi-chromatic schemes to achieve greater measurement capability and can be easily tailored to work for different 2D materials. The method offers a simple and effective approach for studying anisotropy in 2D materials.