Theoretical and Experimental Insight into the Mechanism for Spontaneous Vertical Growth of ReS2 Nanosheets

Rhenium disulfide (ReS₂) differs fundamentally from other group‐VI transition metal dichalcogenides (TMDs) due to its low structural symmetry, which results in its optical and electrical anisotropy. Although vertical growth is observed in some TMDs under special growth conditions, vertical growth in ReS₂ is very different in that it is highly spontaneous and substrate‐independent. In this study, the mechanism that underpins the thermodynamically favorable vertical growth mode of ReS₂ is uncovered. It is found that the governing mechanism for ReS₂ growth involves two distinct stages. In the first stage, ReS₂ grows parallel to the growth substrate, consistent with conventional TMD growth. However, subsequent vertical growth is nucleated at points on the lattice where Re atoms are “pinched” together. At such sites, an additional Re atom binds with the cluster of pinched Re atoms, leaving an under‐coordinated S atom protruding out of the ReS₂ plane. This under‐coordinated S is “reactive” and binds to free Re and S atoms, initiating growth in a direction perpendicular to the ReS₂ surface. The utility of such vertical ReS₂ arrays in applications where high surface‐to‐volume ratio and electric‐field enhancement are essential, such as surface enhanced Raman spectroscopy, field emission, and solar‐based disinfection of bacteria, is demonstrated.     

ReS2‐Based Field‐Effect Transistors and Photodetectors

Atomically thin 2D layered transition metal dichalcogenides (TMDs) have been extensively studied in recent years because of their appealing electrical and optical properties. Here, the fabrication of ReS₂ field‐effect transistors is reported via the encapsulation of ReS₂ nanosheets in a high‐κ Al₂O₃ dielectric environment. Low‐temperature transport measurements allow to observe a direct metal‐to‐insulator transition originating from strong electron–electron interactions. Remarkably, the photodetectors based on ReS₂ exhibit gate‐tunable photoresponsivity up to 16.14 A W^?1 and external quantum efficiency reaching 3168%, showing a competitive device performance to those reported in graphene, MoSe₂, GaS, and GaSe‐based photodetectors. This study unambiguously distinguishes ReS₂ as a new candidate for future applications in electronics and optoelectronics.     

Experimental Evidence of Anisotropic and Stable Charged Excitons (Trions) in Atomically Thin 2D ReS2

Experimentally observed, stable trions with large binding energy (≈25 meV) in atomically thin monolayer 2D transition metal dichalcogenides MX₂ (M = Mo, W, X = S, Se, and Te) with an isotropic crystal structure have been extensively studied. In contrast, the characteristics of trions in atomically thin 2D materials with an anisotropic crystal structure are not completely understood. Low‐temperature photoluminescence (PL) spectroscopy in few‐layer ReS₂ with an anisotropic crystal structure by applying a gate voltage is described. A new PL peak that emerges below the lower‐energy side of neutral excitons obtained by tuning the gate voltages is attributed to emission from negative trions. Furthermore, the trion binding energy that is strongly dependent on the layer thickness reaches a large value of ≈60 meV in 1L–ReS₂, which is ≈2 times larger than that in other isotropic 2D materials (MX₂). The enhancement of the binding energy reflects the quasi‐1D nature of the trions in anisotropic atomically thin ReS₂. These experimental observations will promote a better understanding of the optical response and applications in new categories of the anisotropic atomically thin 2D materials with a quasi‐1D nature.     

Synthesis of 2H‐1T′ WS2‐ReS2 Heterophase Structures with Atomically Sharp Interface via Hydrogen‐Triggered One‐Pot Growth

2D transition metal chalcogenides (TMDs) with different compositions, phase structures, and properties offer giant opportunities for building novel 2D lateral heterostructures. However, the studies to date have been largely limited to homophase TMD heterostructures, while the construction of heterophase TMD heterostructures remains a challenge. Herein, the synthesis of 2H‐1T′ WS₂‐ReS₂ heterophase junctions with high‐quality interface structure via a hydrogen‐triggered one‐pot growth approach is reported. Sequential introduction of hydrogen during growth system, which acts as a “switch” to selectively turn off the growth of ReS₂ while turning on the growth of WS₂, allows WS₂ to seamlessly grow around ReS₂ to form the WS₂‐ReS₂ heterojunction. Moreover, WS₂ prefers to nucleate at the vertices of ReS₂ grain with fixed lattice orientation, which makes the surrounding WS₂ grains merge into single crystal. Scanning transmission electron microscopy reveals high crystal quality of the heterojunction with an atomically sharp 2H‐1T′ heterophase interface. Transient absorption spectroscopy indicates that the photocarriers can effectively separate at the heterophase interface. Based on the high quality heterophase junction, prominent rectification characteristics and polarization‐dependent photodiode properties are achieved. This study provides a robust way for the controlled synthesis of 2D heterophase structures, which is essential for their fundamental studies and device applications.     

Tunable Room‐Temperature Synthesis of ReS2 Bicatalyst on 3D‐ and 2D‐Printed Electrodes for Photo‐ and Electrochemical Energy Applications

The advancement in 3D‐printing technologies conveniently offers boundless opportunities for the customization of a practical substrate or electrode for diverse functionalities. ReS₂ is an attractive transition metal dichalcogenide (TMD), showing strong photoelectrochemical activities. Two advanced systems are merged for the next step in electrochemistry—the limits of the prevailing synthesis techniques of TMDs operating at high temperature or low pressure, which are not compatible with 3D‐printed polymer electrodes that can withstand only comparatively low temperatures, are overcome. A unique NH₄ReS₄ precursor is separately prepared to conduct subsequent ReS₂ electrodeposition at room temperature on 3D‐printed carbon and 2D‐printed carbon electrodes. The deposited ReS₂ is investigated as a dual‐functional electro‐ and photocatalyst in hydrogen evolution reaction and photoelectrochemical oxidation of water. Moreover, the electrodeposition conditions can be adjusted to optimize the catalytic activities. These encouraging outcomes demonstrate the simplicity yet versatility of TMDs based on electrodeposition technique on a rationally designed conductive platform, which creates numerous possibilities for other TMDs and on other low‐temperature substrates for electrochemical energy devices.     

Controlled Vapor Growth and Nonlinear Optical Applications of Large‐Area 3R Phase WS2 and WSe2 Atomic Layers

2D layered 3‐rhombohedral (3R) phase transition metal dichalcogenides (TMDs) have received significantly increased research interest in nonlinear optical applications due to their unique crystal structures and broken inversion symmetry. However, controlled growth of 2D 3R phase TMDs still remains a great challenge. In this work, a direct growth of large‐area WS₂ and WSe₂ atomic layers with controllable crystal phases via a developed temperature selective physical vapor deposition route is reported. Large‐area triangular 3R phase layers are synthesized at a lower deposition temperature. Steady state and time‐resolved photoluminescence spectroscopy and Raman spectroscopy are used to study the unique properties of 3R phase layers due to different layer stacking and interlayer coupling. More importantly, with broken inversion symmetry, 3R phase layers show a quadratically increased second harmonic generation (SHG) intensity with respect to layer numbers. Furthermore, by polarization‐resolved SHG, a uniform polarization preference is observed in bilayer and trilayer 3R phase WS₂, which could be a benefit for practical applications. The results not only contribute to the controlled growth of 2D TMDs layers with different phases but also pave the way to promising nonlinear optical devices.     

Highly Sensitive Detection of Polarized Light Using Anisotropic 2D ReS2

Due to the novel optical and optoelectronic properties, 2D materials have received increasing interests for optoelectronics applications. Discovering new properties and functionalities of 2D materials is challenging yet promising. Here broadband polarization sensitive photodetectors based on few layer ReS₂ are demonstrated. The transistor based on few layer ReS₂ shows an n‐type behavior with the mobility of about 40 cm² V^?1 s^?1 and on/off ratio of 10⁵. The polarization dependence of photoresponse is ascribed to the unique anisotropic in‐plane crystal structure, consistent with the optical absorption anisotropy. The linear dichroic photodetection with a high photoresponsivity reported here demonstrates a route to exploit the intrinsic anisotropy of 2D materials and the possibility to open up new ways for the applications of 2D materials for light polarization detection.     

Vertical Heterophase for Electrical,Electrochemical, and Mechanical Manipulations of Layered MoTe2

Phase engineering is a breakthrough for various electronic and energy device applications with transition metal dichalcogenides (TMDs). Chemical methods, such as lithium intercalation, are mostly used for phase engineering, which achieves atomically thin flakes and high catalytic performances in several group 6 TMDs including MoS₂. However, chemical methods cannot be applied to MoTe₂, a widely investigated group 6 TMD with intriguing semiconducting, topological, and catalytic properties. The lack of modifying MoTe₂ by chemical methods remains a puzzling issue considering the small energy difference between the polymorphs of MoTe₂. Here, a convection‐assisted lithium ion intercalation and phase transition is reported to achieve a vertical heterophase in a MoTe₂ crystal. The vertical heterophase in MoTe₂ reduces the Schottky barrier with metal electrodes down to 66 meV, enhancing the overall ion conductance for electrochemical hydrogen production. Moreover, the weakened adhesion of the 1T' phase layers on the top and bottom surfaces in the vertical heterophase, formed by the intercalation, enables a unique surface tension‐driven exfoliation of MoTe₂ flakes. The heterophase chemical engineering suggests a new platform for hybrid catalysts and next‐generation electronic devices based on 2D materials.     

Low‐Temperature Eutectic Synthesis of PtTe2 with Weak Antilocalization and Controlled Layer Thinning

Metallic transition metal dichalcogenides (TMDs) have exhibited various exotic physical properties and hold the promise of novel optoelectronic and topological devices applications. However, the synthesis of metallic TMDs is based on gas‐phase methods and requires high‐temperature condition. As an alternative to the gas‐phase synthetic approach, lower temperature eutectic liquid‐phase synthesis presents a very promising approach with the potential for larger‐scale and controllable growth of high‐quality thin metallic TMD single crystals. Here, the first realization of low‐temperature eutectic liquid‐phase synthesis of type‐II Dirac semimetal PtTe₂ single crystals with thickness ranging from 2 to 200 nm is presented. The electrical measurement of synthesized PtTe₂ reveals a record‐high conductivity of as high as 3.3 × 10⁶ S m⁻¹ at room temperature. Besides, the weak antilocalization behavior is identified experimentally in the type‐II Dirac semimetal PtTe₂ for the first time. Furthermore, a simple and general strategy is developed to obtain atomically thin PtTe₂ crystal by thinning as‐synthesized bulk samples, which can still retain highly crystalline and exhibits excellent electrical conductivity. The results of controllable and scalable low‐temperature eutectic liquid‐phase synthesis and layer‐by‐layer thinning of high‐quality thin PtTe₂ single crystals offer a simple and general approach for obtaining different thickness metallic TMDs with high melting‐point transition metal.     

Domain Engineering in ReS2 by Coupling Strain during Electrochemical Exfoliation

Chemical exfoliation has been used for the fast and large‐scale production of 2D nanosheets from graphene and transition metal dichalcogenides; however, it is rarely used for domain engineering of exfoliated nanosheets. Herein, it is found that the use of large sized molecular intercalants during electrochemical intercalation induce atomic row dislocation and parallel mirror twin boundaries (MTBs) on an otherwise pristine rhenium disulfide (ReS₂) crystal, such that the exfoliated flakes possess a parallel, multi‐domain structure. These domains can be distinguished under a polarized microscope owing to the intrinsic in‐plane optical dichroic properties of ReS₂, thereby affording a way to track the number of domains introduced versus the size of the molecular intercalant during electrochemical exfoliation. Ferromagnetism is detected on the intercalated sample using large sized molecular intercalants. Density function theory suggests that these may be due to the coupled effects of lattice strain and S vacancies in the MTBs.     

Understanding Dopant–Host Interactions on Electronic Structures and Optical Properties in Ce-Doped WS2 Monolayers

Substitutional lanthanide doping of 2D transition metal dichalcogenides (TMDs) is expected to be a promising strategy to engineer optical, electronic, and optoelectronic properties of TMDs. Understanding the interactions between lanthanide dopants and 2D TMDs host is one of the key problems to be resolved for their profound research studies. Herein, the interactions between Ce dopants and monolayer WS₂ in a physical vapor deposition grown Ce-doped WS₂ monolayer are studied by combining scanning tunneling microscopy with optical characterizations with high spatial and temporal resolution. It is found that the highly anisotropic crystal field can effectively split the energy levels of the Ce dopants’ f orbital. The electrons in the split energy levels can bind the holes in the valence band maximum of the Ce-doped WS₂, forming optical bright excitons. These excitons collide with the free A excitons when increasing the pump fluences, reducing the A exciton's lifetime. This study may be beneficial for the design and fabrication of optical, electronic, and optoelectronic devices based on lanthanide-doped TMDs.     

Space‐Confined Chemical Vapor Deposition Synthesis of Ultrathin HfS2 Flakes for Optoelectronic Application

Due to the predicted excellent electronic properties superior to group VIB (Mo and W) transition metal dichalcogenides (TMDs), group IVB TMDs have enormous potential in nanoelectronics. Here, the synthesis of ultrathin HfS₂ flakes via space‐confined chemical vapor deposition, realized by an inner quartz tube, is demonstrated. Moreover, the effect of key growth parameters including the dimensions of confined space and deposition temperature on the growth behavior of products is systematically studied. Typical as‐synthesized HfS₂ is a hexagonal‐like flake with a smallest thickness of ≈1.2 nm (bilayer) and an edge size of ≈5 µm. The photodetector based on as‐synthesized HfS₂ flakes demonstrates excellent optoelectronic performance with a fast photoresponse time (55 ms), which is attributed to the high‐quality crystal structure obtained at a high deposition temperature and the ultraclean interface between HfS₂ and the mica substrate. With such properties HfS₂ holds great potential for optoelectronics applications.     

Nanoassembly Growth Model for Subdomain and Grain Boundary Formation in 1T′ Layered ReS2

Grain boundaries (GBs) significantly affect the electrical, optical, magnetic, and mechanical properties of 2D materials. An anisotropic 2D material like ReS₂ provides unprecedented opportunities to explore novel GB properties, since the reduced lattice symmetry offers greater degrees of freedom to build new GB structures. Here the atomic structure and formation mechanism of unusual multidomain and diverse GB structures in the vapor phase synthesized ReS₂ atomic layers are reported. Using high‐resolution electron microscopy, two major categories of GBs are observed in each ReS₂ domain, namely, the joint GB including three structures, and the GBs formed from a reconstruction of Re4‐chains including seven different structures. Based on the experimental observations, a novel “nanoassembly growth model” is proposed to elucidate the growth process of ReS₂, where three types of Re4‐chain reconstruction give rise to a multidomain structure. Moreover, it is shown that by controlling the thermodynamics of the growth process, the structure and density of GB in the ReS₂ domain can be tailored. First‐principles calculations point to interesting new properties resulting from such GBs, such as a new electron state or ferromagnetism, which are highly sought after in the construction of novel 2D devices.     

Piezo‐Phototronic Effect for Enhanced Flexible MoS2/WSe2 van der Waals Photodiodes

The recent discoveries of transition‐metal dichalcogenides (TMDs) as novel 2D electronic materials hold great promise to a rich variety of artificial van der Waals (vdWs) heterojunctions and superlattices. Moreover, most of the monolayer TMDs become intrinsically piezoelectric due to the lack of structural centrosymmetry, which offers them a new degree of freedom to interact with external mechanical stimuli. Here, fabrication of flexible vdWs p–n diode by vertically stacking monolayer n‐MoS₂ and a few‐layer p‐WSe₂ is achieved. Electrical measurement of the junction reveals excellent current rectification behavior with an ideality factor of 1.68 and photovoltaic response is realized. Performance modulation of the photodiode via piezo‐phototronic effect is also demonstrated. The optimized photoresponsivity increases by 86% when introducing a −0.62% compressive strain along MoS₂ armchair direction, which originates from realigned energy‐band profile at MoS₂/WSe₂ interface under strain‐induced piezoelectric polarization charges. This new coupling mode among piezoelectricity, semiconducting, and optical properties in 2D materials provides a new route to strain‐tunable vdWs heterojunctions and may enable the development of novel ultrathin optoelectronics.     

High Responsivity Phototransistors Based on Few‐Layer ReS2 for Weak Signal Detection

2D transition metal dichalcogenides are emerging with tremendous potential in many optoelectronic applications due to their strong light–matter interactions. To fully explore their potential in photoconductive detectors, high responsivity is required. Here, high responsivity phototransistors based on few‐layer rhenium disulfide (ReS₂) are presented. Depending on the back gate voltage, source drain bias and incident optical light intensity, the maximum attainable photoresponsivity can reach as high as 88 600 A W^?1, which is a record value compared to other individual 2D materials with similar device structures and two orders of magnitude higher than that of monolayer MoS₂. Such high photoresponsivity is attributed to the increased light absorption as well as the gain enhancement due to the existence of trap states in the few‐layer ReS₂ flakes. It further enables the detection of weak signals, as successfully demonstrated with weak light sources including a lighter and limited fluorescent lighting. Our studies underscore ReS₂ as a promising material for future sensitive optoelectronic applications.     

2D Rhenium Dichalcogenides: From Fundamental Properties to Recent Advances in Photodetector Technology

Van der Waals (vdW) materials of transition metal dichalcogenides (TMD) family with semiconducting properties are currently at the forefront of research in the field of optoelectronics. The ability to couple them with one another at atomic interface precision in a synergistic way opens up unprecedented opportunities to design photodetectors of broad spectral range with excellent figures of merits not accessible to discrete materials. Recent years have seen a surge of interest in group VII TMD materials (ReS₂ and ReSe₂) due to their strong optical response from bulk to monolayer and good ambient stability. Their band gap energies spanning over visible and near-infrared ranges and the strong linear polarization sensitivity stemming from the distorted octahedral symmetry, are ideally suited for polarization-sensitive photodetectors. This review aims at providing a comprehensive understanding of the fundamental properties, optical identification of various structural features, long-debated question of band gap nature and interlayer coupling, and recent advances in the development of photodetectors based on ReS₂, ReSe₂, and their vdW heterostructures with other layered materials of practical importance. We critically review various conceptual device designs implemented based on band engineering, emphasize on the merits of these photodetectors and their potential applications, and provide an outlook for future prospects.     

Large‐Area Bilayer ReS2 Film/Multilayer ReS2 Flakes Synthesized by Chemical Vapor Deposition for High Performance Photodetectors

Rhenium disulfide (ReS₂) is attracting more and more attention for its thickness‐depended direct band gap. As a new appearing 2D transition metal dichalcogenide, the studies on synthesis method via chemical vapor deposition (CVD) is still rare. Here a systematically study on the CVD growth of continuous bilayer ReS₂ film and single crystalline hexagonal ReS₂ flake, as well as their corresponding optoelectronic properties is reported. Moreover, the growth mechanism has been proposed, accompanied with simulation study. High‐performance photodetector based on ReS₂ flake shows a high responsivity of 604 A·W^?1, high external quantum efficiency of 1.50 × 10⁵ %, and fast response time of 2 ms. ReS₂ film‐based photodetector exhibits weaker performance than the flake one; however, it still demonstrates a much faster response time (≈10³ ms) than other reported CVD‐grown ReS₂‐based photodetector (≈10⁴–10⁵ ms). Such good properties of ReS₂ render it a promising future in 2D optoelectronics.     

Dual Functionalization of Liquid‐Exfoliated Semiconducting 2H‐MoS2 with Lanthanide Complexes Bearing Magnetic and Luminescence Properties

Liquid exfoliated, atomically thin semiconducting transition metal dichalcogenides (TMDs), as inorganic equivalents of graphene, have attracted great interest due to their distinctive physical, optoelectronic, and chemical properties. Functionalization of 2D TMDs brings new prospects for applications in optoelectronics, quantum technologies, catalysis, and medicine. In this report, dual functionalization of 2D semiconducting 2H‐MoS₂ nanosheets through simultaneous incorporation of magnetic and luminescent properties is demonstrated. A facile method is proposed for tuning the properties of the TDM semiconductors and accessing multimodal platforms, consisting in covalent grafting of lanthanide complexes onto the surface of 2D TMDs. Dual functionalization of liquid‐exfoliated MoS₂ nanosheets is demonstrated simultaneously with both europium (III) and gadolinium (III) complexes to form a colloidally stable luminescent (with millisecond lifetimes) and paramagnetic MoS₂‐based nanohybrid material. This work is the first example of transition metal dichalcogenide nanosheets functionalized with preformed lanthanide complexes. These findings open new prospects for covalent functionalization of TMDs with molecular species bearing specific functionalities as a means to tune the optoelectronic properties of the semiconductors, in order to create advanced materials and devices with a wide range of functionalities.     

Direct and Indirect Exciton Dynamics in Few‐Layered ReS2 Revealed by Photoluminescence and Pump‐Probe Spectroscopy

Atomically thin‐layered ReS₂ with a distorted 1T structure has attracted attention because of its intriguing optical and electronic properties. Here, the direct and indirect exciton dynamics of a three‐layered ReS₂ is investigated by polarization‐resolved transient photoluminescence (PL) and ultrafast pump‐probe spectroscopy. The various time scales of the decay signals of the time‐resolved PL (<10 ps), with monitoring of the populations of electron–hole pairs (exciton), and the transient differential reflectance (≈1 and 100 ps), with monitoring of the populations of electrons and/or holes in the excited states, are observed. These results reveal the characteristic exciton dynamics: rapid relaxation of direct excitons (electron–hole pairs) and slow relaxation of the momentum‐mismatched indirect excitons accompanied by a one‐phonon emission process. These findings provide important information regarding the indirect bandgap nature of few‐layered ReS₂ and its characteristic exciton dynamics, boosting the understanding of the novel electronic and optical properties of atomically thin‐layered ReS₂.     

Surface‐Shielding Nanostructures Derived from Self‐Assembled Block Copolymers Enable Reliable Plasma Doping for Few‐Layer Transition Metal Dichalcogenides

Precise modulation of electrical and optical properties of 2D transition metal dichalcogenides (TMDs) is required for their application to high‐performance devices. Although conventional plasma‐based doping methods have provided excellent controllability and reproducibility for bulk or relatively thick TMDs, the application of plasma doping for ultrathin few‐layer TMDs has been hindered by serious degradation of their properties. Herein, a reliable and universal doping route is reported for few‐layer TMDs by employing surface‐shielding nanostructures during a plasma‐doping process. It is shown that the surface‐protection oxidized polydimethylsiloxane nanostructures obtained from the sub‐20 nm self‐assembly of Si‐containing block copolymers can preserve the integrity of 2D TMDs and maintain high mobility while affording extensive control over the doping level. For example, the self‐assembled nanostructures form periodically arranged plasma‐blocking and plasma‐accepting nanoscale regions for realizing modulated plasma doping on few‐layer MoS₂, controlling the n‐doping level of few‐layer MoS₂ from 1.9 × 10¹¹ cm^?2 to 8.1 × 10¹¹ cm^?2 via the local generation of extra sulfur vacancies without compromising the carrier mobility.