2D Re‐Based Transition Metal Chalcogenides: Progress, Challenges, and Opportunities

Abstract The rise of 2D transition‐metal dichalcogenides (TMDs) materials has enormous implications for the scientific community and beyond. Among TMDs, ReX2 (X = S, Se) has attracted significant interest regarding its unusual 1T′ structure and extraordinary properties in various fields during the past 7 years. For instance, ReX2 possesses large bandgaps (ReSe2: 1.3 eV, ReS2: 1.6 eV), distinctive interlayer decoupling, and strong anisotropic properties, which endow more degree of freedom for constructing novel optoelectronic, logic circuit, and sensor devices. Moreover, facile ion intercalation, abundant active sites, together with stable 1T′ structure enable them great perspective to fabricate high‐performance catalysts and advanced energy storage devices. In this review, the structural features, fundamental physicochemical properties, as well as all existing applications of Re‐based TMDs materials are comprehensively introduced. Especially, the emerging synthesis strategies are critically analyzed and pay particular attention is paid to its growth mechanism with probing the assembly process of domain architectures. Finally, current challenges and future opportunities regarding the controlled preparation methods, property, and application exploration of Re‐based TMDs are discussed.


Introduction
Over the past decade, the superlative properties of graphene have inspired explosive interest in exploration of other 2D materials, initially focusing on transition metal dichalcogenides (TMDs) and recently broadening to a larger family of layered and nonlayered materials. [1][2][3] 2D TMDs are MX 2 -type compounds where M is a transition element from the periodic table and X represents the chalcogen species S, Se, and Te. The abundant  [26] Copyright 2018, Wiley-VCH; Reproduced with permission. [19] Copyright 2015, Springer Nature; Reproduced with permission. [28] Copyright 2018, Wiley-VCH; Reproduced with permission. [14] Copyright 2016, Springer Nature; Reproduced with permission. [15] Copyright 2015, American Chemical Society; Reproduced with permission. [132] Copyright 2016, Wiley-VCH; Reproduced with permission. [152] Copyright 2018, Wiley-VCH; Reproduced with permission. [38] Copyright 2019, Wiley-VCH. possess broader bandgap variability and high-performance electrical properties, ReX 2 exhibit several advantages compared with these materials. For example, ReX 2 has excellent environmental stability in ambient air, especially, it can be controlled prepared in large scale. Given the unusual structural features, better structural stability, outstanding properties, promising applications, and controllable preparation, ReX 2 has now become one of the most attractive research topics in the 2D world.
Although there are several early reviews have introduced the research progress of ReS 2 , [34][35][36][37] the complete and systematic studies of novel properties and applications about ReX 2 have been explored just in recent years. Moreover, the requirements for different functions of ReX 2 stimulate the booming development of various synthetic techniques, and an entirely new cognition to the crystal growth behavior has been studying. Especially, in the wake of the deepening of ReX 2 research, many new challenges and opportunities ahead for researchers, which will further motivate us to devote persistent efforts. Hence, a timely comprehensive review is of special significance to afford the latest advances and perspectives about this attractive material.
Here, this review provides a systematic introduction to the research progress of 2D Re-based TMDs materials as schematically shown in Figure 1. Initially, we outline their unusual structure features, booming properties and various preparation approaches especially focus on their chemical vapor deposition (CVD) growth progresses. Additionally, based on the structural and functional features of the Re-based TMDs, we categorize all reported applications into five main areas, including i) electronics, ii) optoelectronic, iii) energy conversion and storage, iv) surface-enhanced Raman spectra, and v) sensors. Each subsection is organized from the brief introduction of basic concepts following by some typical applications. Finally, current challenges and future perspectives regarding the controlled preparation, property, and application exploration of ReX 2 are discussed. Table 1. Unit-cell dimensions of ReS 2 and ReSe 2 . Z is the number of formula units per cell. a and b are the in-plane axes. The space group is P-1. [42] a [Å] b  91 4 split to break the degeneracy, leading ReS 2 as a distorted octahedral (1T′) structure, in which the threefold rotational symmetry of the parent 1T structure is broken. [39][40][41] As a result, the adjacent Re atoms are bonded in the form of zigzag Re4 clusters and aligned along the direction of the lattice vector b to form a Re4chain which is defined as the b-axis of ReX 2 crystal (Figure 2b). This distinguishing crystal structure endows ReX 2 crystal with many novel properties for diverse applications. The lattice parameters and space groups of ReX 2 (ReS 2 and ReSe 2 ) are summarized in Table 1. [42] In theory, the stability of layered structure materials is thermodynamically preferred with the d-electron count of the transition metal. [39] For d 0 , the octahedral structure is more stable, then filling the lowest d bands starts to favor the trigonalprismatic structure. For still larger d-electron counts again the octahedral geometry is more stable and the crossover of the two effects is at the d-electron equal to 2. When electron counts over d 3 , the stability becomes worse for the Fermi surface of the 1T phase is partially or weakly nested. Thus, for the ReX 2 with d 3 coordination, the undistorted 1T phase is not stable. In this regard, a periodic distortion of crystal lattice is adopted. The splitting of partially filled degenerate orbital causes a reduction of the free energy in 1T′ phase ReX 2 , resulting in it to form a stable 1T′ phase. Therefore, the most stable phase structure of ReX 2 should be the distorted 1T phase (1T′ phase), and nearly all presently reported experimental works on ReX 2 prepared by either CVT or CVD are 1T′ phase structure as discussed below. Certainly, recent theory works have proposed new phase (Tri phase) of ReS 2 , and they predicted that the transition from 1T′ to Tri phase can be induced with either electron doping or lithium atom intercalating. [43] Moreover, the full-surface hydrogenation can induce the structural transition of single layer ReS 2 from 1T′ to 1T-ReS 2 H 2 which is predicted as an indirect-gap semiconductor. [44] However, the new phase structure of ReS 2 has not been realized in experiment, which deserve to be further explored in the future. Moreover, the 1T′ ReS 2 can stable in ambient air for about 2-3 years or even longer, showing much high environmental stability, which is different from the frail environment stability of phosphorus and 1T′ MoTe 2 /WTe 2 . [45]

Electronic Band Structure
The valence band and conduction band edges of ReX 2 are contributed by the d-orbitals of Re atoms and p-orbitals of X atoms. Density functional theory (DFT) calculations show that ReS 2 nanosheets with thickness from bulk to monolayer have almost the same band structures except a minor bandgap widening. [13] Especially, all ReS 2 with different thickness belong to the direct bandgap semiconductors with bandgap of 1.35 eV for bulk, 1.40 eV for trilayer, and 1.44 eV for monolayer ( Figure 2c). [19] This is a significant difference compared with other 2D TMDs where the electronic band structures show strongly layer number dependence and undergo a transition from indirect to direct bandgap as their thickness reduced from bulk to monolayer. [8] This stark difference is attributed to the weaker interlayer coupling of ReS 2 induced by the distorted 1T structure, which leads to the absence of band renormalization, and eventually allows bulk ReS 2 behave as electronically and vibrationally decoupled monolayers. [13] Notably, the electronic band structure of ReSe 2 is obviously different from that of ReS 2 . Wolverson et al. found that the bulk ReSe 2 behaves as a direct bandgap (1.09-1.31 eV) semiconductor via projector augmented wave calculation, while single layer ReSe 2 has an indirect bandgap (1.34 eV). [46] Angle-resolved photoemission spectroscopy (ARPES) was utilized to provide an accurate description of the electronic band structure of ReS 2 , as shown in Figure 2d. [47] The k z dispersion of the band-edge states explains the observed tiny widening of bandgap when ReS 2 is thinned from bulk to monolayer, as quantum confinement in the z direction will increase the energy of the direct bandgap in ReS 2 . Moreover, though the surface Brillouin zone of ReS 2 remains almost hexagonal, its electronic structure presents obvious anisotropic feature along different in-plane directions, which brings many novel anisotropic optical and electrical properties as discussed in the following section. Energy band engineering of ReX 2 , which is critical to fulfilling the requirements of diverse optoelectronics, has been widely explored via strain, doping, molecular modification or high pressure. [48][49][50][51] For instance, theory calculations and experiments have shown that both the compressive and tensile strain applied in different lattice directions of ReX 2 can modulate the electronic band structures, and the bandgap is monotonically reduced with increasing strain in each direction. In addition, the high pressure can induce metallization and superconducting phase in ReS 2 and ReSe 2 with a band overlap mechanism.

Optical and Vibration Properties
DFT calculations demonstrate that the interlayer coupling energy of ReS 2 is about 18 meV per unit cell, while that of MoS 2 is about 460 meV for the 2 × 2 conventional cell (Figure 2e), [13] indicating a much weak interlayer coupling of ReS 2 . Such weak interlayer coupling of ReS 2 compared with other 2D materials (like MoS 2 , GeSe 2 , BP, SnS and PtSe 2 , etc.) are also confirmed via the thickness-dependent Raman and fluorescence spectra, the temperature-dependent Raman spectra, and the force constants per unit of low frequency modes. [52][53][54][55] For example, the PL spectra of ReX 2 shows negligible layer number dependence. With the thickness of ReS 2 and ReSe 2 thining from bulk to monolayer, their PL intensity decreases along with negligible blueshift of peak position (Figure 2f,g), [13,56] which is consistent well with theoretical bandgap. This result indicates that thinning down the flake does not enhance the quantum confinement of electron, and the neighbouring monolayers in the flake are largely electronically decoupled. In the optical absorption spectra, three optical transitions were identified for both bulk and monolayer ReS 2 , indicating decoupled excitonic and emission properties. [57] Moreover, the exciton peaks of ReS 2 (1.51 eV) and ReSe 2 (1.32 eV) are corresponding well with the bandgap achieved from their PL spectra, and that of ReSSe alloy located at 1.39 eV demonstrate the effective energy band engineering of their alloys ( Figure 2h). [58] It is worth noting that the low symmetry structure of ReX 2 leads to the emission of excitons exhibit interesting linear dichroism, [14,17,[59][60][61] as illustrated by the blue and red electronhole pairs in Figure 3a. [14] The nondegenerate absorption peak of X 1 (1.58 eV) and X 2 (1.63 eV) originate from the recombination of neutral exciton show strong polarized dependence (Figure 3b). Especially, the optical Stark shift of two energetically nondegenerate exciton states can be selectively tuned by manipulating light polarization. Moreover, the emission of the negative trions X 1 − (1.56 eV) and X 2 − (1.62 eV) also exhibit a distinct polarizationdependent characteristic (Figure 3c). [18] An interesting ultrafast exciton quantum beats was observed in the atomically thin ReS 2 , which can be further modulated by tuning the laser polarization. In this regard, the highly anisotropic excitons and trions in ReX 2 provide great feasibility for polarization-sensitive applications, such as anisotropic ultrafast optics and optical logic circuits.
As a nondestructive and powerful technique for characterizing the Brillouin-zone center (Γ-point) phonon properties of materials, Raman spectroscopy provides information about the structure, lattice symmetry, crystal quality, thickness, defects, and dopants. [62][63][64][65] Figure 3d shows the high frequency Raman spec-tra of bulk ReS 2 crystal, 18 first-order intralayer modes can be clearly observed within 100-450 cm −1 , which belong to A g symmetry and named as A g 1 −A g 18 modes. [66] For convenience, Roman numerals I, II, III, IV… are utilized to represent the 18 Raman modes of ReS 2 , in which three primary intralayer Raman modes III, IV and V are located at 152, 162, and 213 cm −1 , respectively. Notably, the frequency shift of each Raman mode shows tiny variation from monolayer to bulk ReS 2 , which is different from the strong thickness-dependent Raman spectra variation of most of 2D layered materials and is attributed to the weak interlayer coupling. [67] Moreover, ReSe 2 also has abundant Raman vibration modes with negligible layer number dependence because of its structural similarity with ReS 2 . [46,67] In this respect, the high frequency Raman spectra of ReX 2 cannot be utilized to identify their layer numbers as that used in other 2D materials.
The weak van der Waals (vdW) coupling in 2D layered materials usually leads to the emergence of interlayer phonon modes which are typically located in ultralow frequency regime. [67][68][69] Hence, the ultralow frequency Raman spectra can offer valuable information about the interlayer charge exchanges, screenings, scatterings and their stacking order, and thus renders that a favorable sign for determining the thickness of the 2D material. Figure 3e displays the ultralow frequency Raman modes (0-35 cm −1 ) of 2-layer ReS 2 , which belong to the layer shear modes (S) and www.advancedsciencenews.com www.advancedscience.com layer breathing modes (B). [68] For the S modes, the layers oscillate rigidly against each other in the crystal plane, while for the B mode (28 cm −1 ), the oscillation amplitude is perpendicular to the layer plane. Unlike the high symmetry TMDs such as MoS 2 , where S modes are doubly degenerate and have the same frequency, [70][71] the S modes of ReS 2 are nondegenerate due to the low crystal symmetry, and thus two types of S modes (S ∥ located at 13.0 cm −1 and S ⊥ located at 16.5 cm −1 ) are clearly resolved in the Raman spectra. Furthermore, the frequency of all the observed S and B modes of ReS 2 and ReSe 2 decrease with the layer number increases, and ultimately merge into the Rayleigh background when the thickness is more than ten layers. [67] The peak position of the low frequency Raman modes have a linear relationship with the number of layers, providing an effective method to identify the thickness of ReX 2 . In particular, the low frequency Raman spectra can also be used to judge the stacking order in ReX 2 crystal, as the interlayer phonon modes are sensitive to lattice coupling between the ReX 2 layers. [68][69]72] Additionally, from the aspect of force constants per unit (k S ) of low frequency modes, the k S of ReS 2 and ReSe 2 are 17.1 and 17.1 × 10 18 N m −3 , much smaller than the force constant of other materials (MoS 2 : 25 × 10 18 N m −3 , MoTe 2 : 34 × 10 18 N m −3 , and PtSe 2 : 46 × 10 18 N m −3 , etc.). [55,67,70,73] Above results indicate that ReX 2 has a rather weak interlayer coupling.
As low symmetry 2D material, the phonon coupling of ReX 2 shows strong anisotropic properties, which has been confirmed via angle-resolved polarized Raman spectra. [16,46,[68][69]74] Figure 3f shows the polarized Raman spectra of monolayer ReS 2 as a function of sample rotation angle . [16] The intensity of each Raman peak varies strongly with the change of , while the peak positions remain unchanged, indicating strong 2D in-plane anisotropic properties. Figure 3g presents the polar plots of the peak intensity of mode III (152 cm −1 ) and mode V (213 cm −1 ) as a function of . The direction of the maximum of the Raman tensor for the mode V is coincident with the direction of the b axis (Figure 3h), which can be utilized as a convenient optical method to determine the crystallographic orientation of ReS 2 . More than that, the intensity of both S and B modes in the ultralow frequency Raman of ReS 2 show strong polarization dependence as well, which can also be used to identify the in-plane crystal orientation of ReX 2 . [68][69] In view of 1T′ ReX 2 belongs to the Ci space group, its structure has only one inversion center. Hart et al. found turning a layer upside-down is not a symmetry operation, but reverses the sign of the angle between the two nonequivalent in-plane crystallographic axes. [75] They produced few-layer ReS 2 (and ReSe 2 ) samples with controlled "up" or "down" orientations by micromechanical cleavage and utilized the angle-resolved polarized Raman microscopy to distinguish these two orientations. The angles between the maximum of mode III and mode IV and mode V in the polar plots are opposite for the two ReS 2 flakes, indicating the distinct vertical orientations, "upward" and "downward." In reality, this is a very important discovery because different vertical orientation may lead to a new source of domain structures, especially for the large area growth of ReX 2 by CVD or other similar methods. All of the above research works establish angle-resolved polarized Raman spectra as an effective approach for the identification of both in-plane and vertical orientation of ReX 2 .
Besides, Zhao and coworkers reported an unexpected layerdependent, strong, and anisotropic second harmonic generations (SHGs) in atomically thin ReS 2 . [76] Appreciable (negligible) SHGs are obtained from even (odd) numbers of ReS 2 layers, which is opposite to the layer dependence of SHGs obtained in group-VI TMDs. The SHG signals of ReS 2 show strong anisotropy, indicating its distorted lattice structure with more unequal and nonzero second-order susceptibility elements.

Preparation and Structure Modulation of ReX 2
The controlled preparation of 2D ReX 2 with desirable size, morphology, thickness, and crystal quality is of significant importance for the investigation of their optical, electronic and thermal properties, as well as the exploration of their potential applications. In this section, we first summarize the synthetic methods of ReX 2 , which can be categorized into two parts: i) top-down production method and ii) bottom-up production method. The topdown production methods include mechanical exfoliation and liquid-phase exfoliation. The bottom-up production methods incorporate chemical vapor transport (CVT), physical vapor deposition (PVD) and CVD. On account of the high productivity and better controllability of the CVD method, we will give more emphasis on the CVD synthesis strategies of ReX 2 . Not only that, we also discuss the advantages and limitations of each approach and propose some future potential research directions for the improvement of ReX 2 synthesis methods. Finally, exploration of the unusual structural features and growth mechanism of ReX 2 as well as their structure modulation (such as constructing Re-based 2D alloys and heterostructures) are introduced.

Chemical Vapor Transport
Chemical vapor transport (CVT) growth is a classic and mature technique for the synthesis of bulk crystal materials. [77] At present, the fundamental property investigation and functional device fabrication of the most 2D materials are highly relied on the 2D nanosheets mechanically exfoliated from the bulk crystals synthesized via CVT method, [78][79] and ReX 2 is no exception. In general, the CVT method is equipped with a sealed ampoule tube containing the precursor and transport agent, and then the growth is performed in low pressure and high temperature for about several to ten days. [77] Bhattacharya and coworkers developed a modified Bridgman method without transport agent to synthesize high-quality ReS 2 and ReSe 2 bulk single crystals (Figure 4a-d) by using the pure Re powder and S/Se powder. [80] Just recently, Jiao and coworkers developed an approach for directly synthesizing thin ReX 2 flakes by CVT through carefully tuning the growth kinetics together with a specially designed quartz ampoule with a neck to separate the powders from the growth substrate. [81] With this approach, few-layer and even monolayer ReSe 2 nanosheets with high quality were grown on mica or sapphire substrate. Hence, CVT is an ideal approach for preparing the high crystallinity single crystal ReX 2 bulk and thin flakes. Nevertheless, the time-consuming, strict and harsh reaction conditions, as well as uncontrollable morphology, size and thickness limit its widespread exploitation, especially regarding the preparation of large-area monolayer material for device applications.  [80] Copyright 2016, American Chemical Society. Optical images of mechanical exfoliated monolayer and few-layer e) ReS 2 and f) ReS 2 nanosheets. Reproduced with permission. [67] Copyright 2016, American Chemical Society. g) Photograph and schematic diagram of ReS 2 dispersions prepared by the density gradient ultracentrifugation. h) AFM image of solution-processed ReS 2 nanosheets following deposition on a Si wafer. Reproduced with permission. [87] Copyright 2016, American Chemical Society.

Mechanical Exfoliation
Since the discovery of graphene, mechanical exfoliation of 2D layered materials from bulk parent materials has been the optimal preparation method for investigating their intrinsic physical and chemical properties. [78,[82][83] The main step of this top-down method is illustrated as follows: first the single crystal bulk material is attached to the adhesive on the Scotch tape and then another piece of tape is placed on the other side of the material, following by peeling off the two pieces of tape for several times; the freshly cleaved thin flake on the Scotch tape is then attached to a clean, flat substrate (generally 300 nm SiO 2 /Si substrate); finally few layers or monolayer material can be obtained on the target substrate. Actually, mechanical cleavage has been widely used as a facile technique to obtain few-layer ReX 2 with high quality, clean surface, and relatively large area for studying their fundamental properties and potential applications. [13,19,84] Figure 4e,f shows the typical OM images of ReSe 2 and ReS 2 on SiO 2 /Si substrate with different layers exfoliated from their bulk materials. [67] However, this method still has many shortcomings such as lack of control over thickness, randomness in size and shape, as well as low productivity. An improved exfoliated method needs to be explored toward high yield, large area and easy control with good repeatability for preparing desirable ReX 2 film in the future.

Liquid-Phase Exfoliation
Liquid-phase exfoliation is a powerful method for the scaledup fabrication of ultrathin nanosheets form bulk layered ma-terials. Dispersing them in the solvent is a direct and effective way to reduce the interlayer van der Waals force, which allows the liquid-phase exfoliation method possible to achieve industrialization and commercialization due to its high-yield mass production. [85][86] At present, the liquid-phase exfoliation method of ReX 2 materials can be divided into two primary types: sonication-assisted exfoliation and ion intercalation exfoliation. [87] Hersam and coworkers reported the exfoliation and layer-by-layer isopycnic density gradient ultracentrifugation sorting of high-density ReS 2 nanosheets in aqueous surfactant solutions ( Figure 4g). [87] First, ReS 2 powder was ultrasonically exfoliated in deionized water with the amphiphilic small-molecule surfactant sodium cholate. Then, the ReS 2 dispersion was purified by using centrifugal separation at 7500 rmp aims to remove the unexfoliated ReS 2 flakes. The supernatant was subsequently collected and further ultracentrifuged at 20 000 rpm to precipitate large-size ReS 2 nanosheets. Finally, the relatively uniform ReS 2 nanosheets with average thickness of ≈3 nm and lateral size of 50-100 nm were achieved as confirmed by AFM (Figure 4h). Additionally, based on the Hansen solubility theory, a mixedsolvent strategy was developed for the facile and green preparation of few-layered ReS 2 nanosheets by exfoliating bulk ReS 2 in an ethanol-water mixture. [88] It was found that 28% ethanol in 72% deionized water is the best mixture solvent for the efficient exfoliation of ReS 2 . Combining the mixture solvent and sonication assistance, large-scale ReS 2 nanosheets with an average lateral size of 2.3 nm and a thickness of 50-80 nm were obtained. Furthermore, ion intercalation exfoliation is another effective method for preparing ReS 2 nanosheets, as the cation (e.g., Li + , Na + , K + ) with small ionic radius can easily insert into the www.advancedsciencenews.com www.advancedscience.com interspace of layered bulk crystals, which can significantly expand the interspace and weaken the van der Waals interaction between adjacent layers. A solvent-free approach involving the reaction of ReS 2 powder with lithium borohydride (LiBH 4 ) was developed to effectively exfoliate ReS 2 nanosheets, which could replace the conventional protocol involving a butyl lithium solution. [89] The aforementioned methods demonstrate that liquidphase exfoliation is an effective approach for exfoliating ReX 2 nanosheets in large quantities with crystal quality comparable to mechanically exfoliated ReX 2 . Nonetheless, the resulting ReX 2 nanosheets are polydisperse with respect to lateral size and thickness, and it is challenging to achieve a uniform monolayer ReX 2 film with large size. Additionally, some reagents and solvents may contaminate the product. In this regard, the ReX 2 nanosheets prepared by liquid-phase exfoliation is more suitable for energy conversion and biological applications.

Physical Vapor Deposition
Physical vapor deposition (PVD) is a typical bottom-up method for the preparation of 2D nanosheets. PVD growth is an atomistic deposition process in which the solid or liquid precursors evaporate in the form of atoms or molecules, and then transports through a high vacuum or low pressure gas (or plasma) environment to the target substrate where it condenses or deposits. [90][91] Take the PVD growth of ReS 2 as an example, pure ReS 2 powder was first placed at the center of quartz tube, and then the target SiO 2 /Si substrate with facing up was placed at downstream far from the ReS 2 powder. [92] The growth was conducted at temperature of 900°C with 50 sccm Ar as carrier gas for 1 h. Then, the as-grown ReS 2 sample was annealed in a sulfur-rich environment to further improve its crystalline quality. Using this approach, centimeter-size ReS 2 film can be grown on SiO 2 /Si substrate, and the AFM results indicate that the thickness is about 2.3 nm corresponding to three layers ReS 2 film. Hence, PVD growth is an effective method for preparing large-area high-quality ReX 2 film. However, this method may not be versatile due to the higher melting point of the precursor material and higher vacuum requirements.

Chemical Vapor Deposition
Chemical vapor deposition (CVD) is one of the representative bottom-up approaches for fabricating traditional semiconductor films, and has also been widely utilized to prepare 2D materials. [3,[93][94][95][96] The CVD process involves depositing solid material from gaseous phase, which is achieved by means of a chemical reaction between volatile precursors. As the gas precursors pass over the surface of the heated substrate, the resulting chemical reaction forms a solid phase and deposits onto the substrate via nucleation and following epitaxy growth. In this process, several parameters must be controlled well, like growth temperature, gas flow rate, substrate type, precursor selection, etc. Hence, as a conventional and versatile approach, CVD growth plays a critical role in the preparation of scalable and high-quality 2D ReX 2 film for device applications. [97][98][99][100][101][102][103] Meanwhile, the easy command of CVD process facilitates the construction of ReX 2 -based alloys and heterostructures for tuning their optical, electrical and magnetic properties.
Owing to the 1T′ structure and weak interlayer coupling, ReX 2 has been found to show many unusual growth behaviors, such as off-symmetry growth, out-of-plane growth and nano-assembly growth. [38,97,102] As a result, the morphology of CVD-grown ReX 2 are mostly thick flakes, dendritic shape or 3D flower-like structures, especially large amounts of subdomains and grain boundaries exist in the ReX 2 nanosheets. These diverse morphologies and structures of ReX 2 together with the strong anisotropic properties and weak interlayer coupling endow them great application potential in wide fields. For example, the planar ReX 2 nanosheets have been utilized to construct diverse electronic and optoelectronic devices. The 3D ReX 2 nanoarrays display excellent application prospects in catalysis, lithium ion battery and sensors where abundant exposed active edges are needed. In this section, we summarized the progress of CVD growth of ReX 2 in three aspects: the growth of planar ReX 2 nanosheets, the exploration of the formation of subdomain and grain boundary in ReX 2 , and the growth of vertically oriented 3D ReX 2 nanoarrays.

CVD Growth of Planar ReX 2 Nanosheets
Ajayan and coworkers reported the first synthesis of monolayer ReS 2 nanosheets on SiO 2 /Si substrate using CVD growth (Figure 5a) with ammonium perrhenate (NH 4 ReO 4 ) and sulfur as Re and S precursors, respectively. [97] The size of as-grown ReS 2 nanosheets varies from 2 to 60 µm at different growth conditions. When the growth temperature is 400-450°C, the morphology of ReS 2 grains is mainly hexagon with an average thickness of 13 nm. With the growth temperature rises to 450-500°C, rounded ReS 2 grains with the thickness of 110 nm are obtained. Notably, all the ReS 2 grains grown at different temperatures show dendritic edges, which are attributed to the offsymmetry anisotropic growth of ReS 2 . In addition, the Z-contrast scanning transmission electron microscopy (STEM) image distinctly resolves the 1T′ structure of ReS 2 with Re4-chains are clearly visible (Figure 5a). However, a few defects can also be identified in the HRTEM image, which may be due to the excessive by-products like SO 2 especially NH 3 produced during the growth process, as shown in the reaction equation below For improving the crystal quality of CVD-grown ReS 2 , another early research work reported the CVD synthesis of ReS 2 using pure element Re powder as precursor, as shown in Figure 5b. [98] The growth was performed at the temperature of 750°C under atmospheric pressure condition. Few-layer ReS 2 flakes and 1D nanoribbons were obtained on Si/SiO 2 substrate as shown in the OM images. Furthermore, the HRTEM image shows that the crystal quality of ReS 2 is higher than that prepared with NH 4 ReO 4 as Re precursor, indicating the role of pure element Re in producing high crystallinity ReS 2 , as shown in the reaction equation below Re + 2S → ReS 2 (2) Figure 5. Development process of the preparation of ReX 2 material by CVD growth. a) Schematic diagram of CVD growth of ReS 2 using NH 4 ReO 4 as Re precursor, and the corresponding HRTEM image. Reproduced with permission. [97] Copyright 2015, Wiley-VCH. b) Optical and AFM images showing the morphologies and thickness of ReS 2 grown using pure Re powder as rhenium precursor. Reproduced with permission. [98] Copyright 2015, Wiley-VCH. c) Schematic diagram of the tellurium-assisted epitaxy growth of ReS 2 , and the corresponding OM images of monolayer on mica and SiO 2 /Si substrates. Reproduced with permission. Copyright 2016, Wiley-VCH. d) OM image of the ReSe 2 hexagonal flakes grown on SiO 2 /Si substrates with ReO 3 as Re precursor, inset shows the magnified OM image for the clarity. Reproduced with permission. [101] Copyright 2016, Wiley-VCH. e) Schematic diagram of the space-confined CVD growth of monolayer ReS 2 on mica substrate. Reproduced with permission. [102] Copyright 2016, Royal Society of Chemistry. f) Optical image of CVD-grown monolayer ReS 2 on c-cut sapphire substrate. Reproduced with permission. [103] Copyright 2016, American Chemical Society. g) Schematic diagram of the CVD growth of monolayer ReSe 2 on Au foil substrate, and the corresponding low-energy electron microscopy images. Reproduced with permission. [109] Copyright 2018, Springer Nature. h) Optical image showing the subdomains and grain boundaries in CVDgrown monolayer ReS 2 domain. Reproduced with permission. [38] Copyright 2019, Wiley-VCH.
However, there are only a few ReS 2 grains scattered on the substrate, indicating quite low growth efficiency. This is reasonable, given that the much high melting point of Re (3180°C) makes its vapor pressure in the growth system quite low at the general CVD growth temperature (<1000°C), which greatly decreases the efficiency of nuclear and epitaxial growth of ReS 2 . To solve the above problems, our group developed a tellurium (Te)-assisted epitaxial growth method to synthesize large-area, highly crystalline ReS 2 with uniform monolayer thickness on mica substrate (Figure 5c). [99] The inspired strategy derived from the Re-Te binary eutectic, whose eutectic point can be reduced to 850°C, and even to 430°C when the weight ratio of Te and Re reach to 90%. With this approach, large-scale, highly crystalline monolayer ReS 2 grains with a lateral size of ≈20 µm were synthesized at growth temperature range from 500 to 900°C. These results show that the growth efficiency was largely increased by introducing Te powder into the growth system, because it lowers the originally high melting point of Re and thus increases the vapor pressure of the Re precursor in the growth system. Furthermore, the morphology of ReS 2 changes from round at 500°C to hexagon at 600°C, and then to serration at 700°C, fi-nally to dendritic structure at 850°C. The dendritic morphology of ReS 2 indicates an off-symmetry growth which is attributed to the anisotropic interfacial energy induced by 1T′ structure. This work opens up new avenues for the synthesis of other 2D materials where the precursor has a high-melting point.
At the same time, Zhai and coworkers reported the CVD synthesis of ReS 2 on sapphire substrate using ReO 3 as Re precursor. [100] The chemical reaction is illustrated as follows Using this improved growth approach, large-scale hexagonal ReS 2 flakes and continuous bilayers ReS 2 film were successfully synthesized. HRTEM and AFM reveal that the hexagonal flakes are layered stacking crystals, while the continuous film is bilayer polycrystal. Combine with theory calculation, they found that the formation of hexagon ReS 2 grain takes place only when the growth rate on the (100) plane is comparable to the growth rate on the (020) plane. Moreover, the partial pressure of ReS 2 species plays a key role in the growth rate of the ReS 2 , as larger partial pressure may increase the growth rate and enhance the nucleation density at the same time. As a result, under higher partial pressure, small-sized ReS 2 grains nucleate simultaneously and connect to each other forming a continuous polycrystalline bilayer ReS 2 film. Besides the synthesis of ReS 2 , Zhai and coworkers also reported the first CVD growth of ReSe 2 by using ReO 3 as Re precursor. [101] Large-scale hexagonal-shape ReSe 2 flakes with thickness of ≈4.2 nm and lateral size of ≈5 µm were grown on SiO 2/ Si substrate (Figure 5d). The perfect lattice structure shown in HRTEM image evidence the high-quality of CVDgrown ReSe 2 flakes. Above results demonstrate that ReO 3 is an ideal Re precursor for the CVD synthesis of large-scale, high crystallinity ReS 2 and ReSe 2 materials.
Though tremendous progresses have been achieved on the synthesis of ReX 2 , the high volatility and versatile valence states of Re oxides make the controllable synthesis of high-quality ReX 2 film with large grain size, planar and uniform monolayer thickness quite difficult. In view of these problems, our group conducted an in-depth study on the decomposition process of ReO 3 powder at different temperatures through thermogravimetric analysis. [102] It was found that the unstable ReO 3 can quickly decompose into Re 2 O 7 (which sublimes easily) and ReO 2 (which is less volatile) via a disproportionation reaction (reaction 4) when the temperature rises to 400°C. Thus, the detailed reaction process for the growth of ReS 2 via ReO 3 is shown below The high volatility of Re 2 O 7 (melting point: 220°C and boiling point: 360°C) brings abundant Re precursor vapor and nucleation sites during the growth system, which leads to the growth of ReS 2 uncontrollable, and thus large amount of thick flakes, polycrystal films, and even 3D flower-like structures are grown on substrate.
Based on the above understanding, a space-confined CVD growth approach was developed to synthesize large-area and consecutive monolayer ReX 2 film on mica substrate. [102,104] Figure 5e illustrated the schematic diagram of the space-confined CVD growth approach. In brief, two pieces of freshly exfoliated mica were stacked together and then placed on the top of ceramic boat containing ReO 3 powder. The electrostatic attraction between two stacked micas forms a microreaction space which greatly decreases the precursors vapor and thus reduces the nuclear density and growth rate of ReS 2 . As a result, large-scale monolayer ReS 2 hexagonal domains with size up to 60 µm were grown on the inside of the stacked mica, and even continuous uniform monolayer ReS 2 film can be obtained by extending the growth time. Owing to the simplicity and effectivity, the space-confined CVD growth approach has been widely utilized to controllably synthesize large-area monolayer ReSe 2 film and other 2D TMDs materials. [104][105][106][107] Besides the above works, Zhang's group reported the synthesis of ReSe 2 with high crystal quality on gold foil substrate, which provides the convenience for in-situ studying their electrical properties by using scanning tunneling microscope (STM). [108][109][110][111] Lee and coworkers prepared high uniformity and continuity ReS 2 film on the transparent flexible glasses at a relatively low temperature. [112] Combining the tellurium (Te)-assisted epitaxial growth with another type of space-confined CVD configuration where an inner tube was placed in the reactor, Li and coworkers realized the growth of substrate-scale continuous atomically thin ReS 2 film with uniform monolayer thickness on mica substrate. [113] Furthermore, Zhen and coworkers prepared large-area continuous ReS 2 films by CVD growth on mica substrate with the mixture of metal Re and Re 2 O 7 as Re precursor. [114] After achieving the controlled synthesis of large-area monolayer ReX 2 film, researchers begin to explore its unusual structure, growth behavior and mechanism, as well as to clarify the relationship between them (Figure 5f-h). In view of the importance of this part, we will give it a special and systematic discussion in the following section.
All in all, CVD growth is the most convenient and effective method for the controlled preparation of 2D Re-based materials. An obvious advantage of this method, in addition to its relative simplicity, is the possibility of growing large-scale ReX 2 films with their lateral sizes only limited by the size of the substrate used. However, it is still challenging to precisely control over the thickness and uniformity of the ReX 2 film due to the point diffusion volatilization of precursor powder. In this regard, an improved CVD method like metal-organic chemical vapor deposition (MOCVD) which uses gaseous precursors may be an effective method to solve the above problems, because the growth process can be well controlled by regulating the flow rate of gas precursors just as the CVD growth of graphene. Therefore, the research in the synthesis of 2D ReX 2 materials is still in the infant stage. There are still greater opportunities for research innovations in developing new synthesis approaches with the goal of achieving high quality and large-scale ReX 2 materials at relatively low cost and better controllability.

Subdomains and Grain Boundaries in ReX 2
As is well known, the CVD growth of 2D TMDs experiences the nucleation of grain and the subsequent grain merger until they grow into large-area films. [3,115] During this process, grain boundaries (GBs) are inevitably formed between adjacent grains. ReX 2 naturally cannot avoid the arise of GBs during present CVD growth processes and technical means. However, different from the high-symmetry 2H TMDs (such as MoS 2 and WS 2 ), the lowsymmetry structure of ReX 2 endows them more degree of freedom during the growth process and thus bring more complex GBs. In the past several years, along with the discovery of a series of new GB structures, many abnormal growth behaviors, morphology and structure features of ReX 2 have been gradually uncovered. In this section, we mainly introduce the discovery of subdomains and GBs in CVD-grown ReX 2 and the exploration of growth mechanism of unusual multi-domain structures, which are critical to further control its preparation toward large-scale device applications.
In the earliest work about CVD growth of ultrathin hexagonal ReSe 2 flakes carried out by Zhai's group, [101] they performed the polarized Raman mapping of ReSe 2 to confirm the dependence of Raman vibration modes on the b-axis orientation. Surprisingly, the Raman intensity of E g -like (Figure 6a) and A g -like ( Figure 6b) modes vary at different areas and show distinct polarization dependence. Accordingly, they deduced that this might be due to Figure 6. Raman intensity mapping of a) in-plane E g -like and b) out-of-plane A g -like modes of CVD-grown ReSe 2 flakes, the variation in the integrated intensities of these two modes with the incident light polarization are overlapped with the respective Raman mapping image. Reproduced with permission. [101] Copyright 2016, Wiley-VCH. c) OM image of CVD-grown monolayer ReS 2 domain, and the corresponding ARPOM images taken at the angle of d) +60°and e) −60°between polarizer and analyzer. f,g) Polarized Raman mapping images of the ReS 2 domain for the vibrational modes at 160 cm −1 under parallel (P//) and perpendicular (P ┴ ) polarization configurations. h) Schematic structure of the ReS 2 composed of several subdomains and grain boundaries. Reproduced with permission. [38] Copyright 2019, Wiley-VCH. i) The ADF-STEM image of ReSe 2 . Two domains with different orientations are separated by a twin boundary, which are marked in blue, green, and yellow line, respectively. j) The ADF-STEM image of ReS 2 with divergent diamond chain orientations. Reproduced with permission. [15] Copyright 2015, American Chemical Society. k) Schematic diagram of the "nanoassembly growth model" of ReS 2 showing the formation process of subdomains and GBs. Reproduced with permission. [38] Copyright 2019, Wiley-VCH.
the grain orientation alteration in the same single ReSe 2 grain. This is the first time researchers have realized that the hexagonal ReSe 2 grain should not be single crystal but polycrystalline. To visualize the possible existed GBs, they heated the sample at about 300°C in the air just as the visualization method of graphene grown on copper substrate. After heat treatment, the GBs can be observed clearly and a complete ReSe 2 flake is divided into six parts, confirming the existence of GBs in the ReSe 2 grain. Tongay and coworkers reported the CVD synthesis of ReS 2 monolayer on c-cut sapphire substrate and got insight into the multidomain architectures in the CVD-grown ReS 2 . [103] Angle-resolved polarized Raman spectroscopy (ARPRM) measurements demonstrate the presence of structurally anisotropic ReS 2 subdomains in the CVD-grown ReS 2 triangle or hexagon domain. Furthermore, our group developed an angle-resolved polarized optical microscopy (ARPOM) imaging technique to directly visualize the subdomains and GBs in CVD-grown monolayer ReS 2 (Figure 6ce). [38] The OM image of CVD-grown monolayer ReS 2 domain shows it seems to be a single crystal, while six subdomains and the corresponding GBs were clearly distinguished in the same ReS 2 domain from their ARPOM images. Similar multi-domain structures were also observed from polarized Raman mapping of the two primary vibration modes at 160 and 211 cm −1 measured under parallel-polarized (P // ) and cross-polarized (P ┴ ) configurations of incident and collection lights (Figure 6f,g). These results demonstrate that the CVD-grown ReS 2 domain is composed of several subdomains with different lattice orientations, and GBs are formed between each adjacent subdomain, as schematically shown in Figure 6h. Notably, the GBs and subdomains in the ReS 2 domain are grown from one nucleation center, which is different from that in high-symmetry 2D materials where the GBs only originate from the splice of different domains.
Above research works convincingly confirmed the existence of GBs and subdomains in the CVD-grown ReX 2 domain, and preliminarily established its domain architectures, where each subdomain has different Re-chain directions and splicing together forms GBs. In view of the novel domain architectures, researchers are interested in the atomic-scale structure of CVDgrown ReX 2 , especially with respect to how the subdomains and GBs are formed in space and in energy scale, which are critical to revealing the growth mechanism of ReX 2 and tailored design of them in a controllable way. Toward that end, researchers www.advancedsciencenews.com www.advancedscience.com have given a detailed study to the atomic structure features of ReS 2 under the assistance of aberration-corrected STEM. Suenaga and coworkers discovered two distinct Re-chain orientations with an angle of ≈121°in mechanical exfoliated monolayer ReSe 2 domain ( Figure 6i) and ReS 2 domain (Figure 6j). [15] A mirrorsymmetric GB where the two different Re-chains sharing the four Re atoms separates the ReX 2 domain into two subdomains. Especially, the Re-chain direction of monolayer ReX 2 can be altered through in-situ electron beam irradiation at 500°C, indicating a Re-chain reconstruction/flip occurs through slight displacement of Re atoms from its perfect structure. It was deduced that the Re-chain flip can reduce the energy of the system and keep the structure become energetically favored. Importantly, the similar Re-chain flip was commonly observed in the CVD-grown ReS 2 and ReSe 2 domains, [103,109] indicating the possibility of Rechain flip inducing the formation of ReX 2 multidomain structures. Zhang and coworkers reported the on-site investigations of the structural and electronic properties of CVD-grown ReSe 2 on a conductive Au foil substrate. [108][109] Using scanning tunneling microscopy and spectroscopy (STM and STS), they identified the top four non-identical Se atoms in a unit cell of the anisotropic ReSe 2 . In particular, a perfect lattice coherence and an invariable bandgap were found across the commonly observed mirror-symmetric GBs in ReSe 2 , which considerably differ from the conventional isotropic TMDs featured with defect structures and additional states inside the bandgap. This work provides a key reference to the fundamental properties of 2D anisotropic ReX 2 and the abnormal structural and electronic properties at the GBs, which could promote their practical applications in electronic and photonic devices as well as the energy-related fields.
To further get insight into the nature behind the CVD growth of multidomain ReX 2 , our group reported a systematic exploration of the atomic structure, formation mechanism, and modulation strategy of GBs in the ReS 2 . [38] Using aberration-corrected STEM, we identified two major categories of GBs in one ReS 2 domain: the joint GBs, and the Re4-chain reconstruction-induced GBs, which are significantly different from those observed in the high-symmetry 2D TMDs materials. These diverse GBs imply the occurrence of different Re-chain reconstructions during the ReS 2 growth process, which induces subdomains with altered in-plane or/and vertical orientations. Based on the structural features of these GBs, we proposed a novel "nanoassembly growth model" to describe the formation mechanism of ReS 2 multi-domain structures. First, the usually observed hexagonal ReS 2 domain is started from a single crystal nucleus. Then, the growth moves to the Re4-chain reconstruction stage due to the deviation of atom bonding from its equilibrium state and the strain field effect, which changes the in-plane and vertical orientations of as-grown ReS 2 by 120°and 180°, respectively. During further growth process, each pair of adjacent subdomains merging together bring a particular in-plane (60°angle) and vertical (0°or 180°) orientational relationship. Finally, a hexagonal ReS 2 domain composed of six subdomains with different in-plane and vertical orientations is obtained, where both the twin boundary (mainly located in the nucleation region) and the joint GB (between adjacent subdomains) are present. This growth model accounts for the abnormal growth behavior, morphology, and a variety of structural features observed in CVD-grown ReS 2 and is sufficiently versatile to be extended to ReSe 2 and other low-symmetry 2D materials.
First-principle calculations show that these GBs have several interesting properties, including new electron state and ferromagnetism, which are highly desirable for the realization of novel 2D electronics. This work highlighted here presents a clear picture about the complicated assemble process of ReX 2 multi-domain structure from its nucleary to growth, and provides useful guidance for further studying the low-symmetry 2D materials. Afterward, Kang and coworkers also investigated the structure evolution of CVD-grown hexangular ReS 2 domains and proposed a dislocation-involved anisotropic growth mechanism which is quite similar to the above demonstrated nano-assembly growth mechanism. [116] So far, nearly all the abnormal growth behaviors and structure features of CVD-grown ReX 2 have been clearly revealed. Importantly, the growth mechanism of ReX 2 could be extended to understand and control the growth of other lowsymmetry 2D materials (such as GaTe, 1T′ MoTe 2 /WTe 2 , etc.) where similar multidomain structures were also observed. [117][118][119] Besides, there are also many defects in the CVD-grown ReX 2 , which affect its properties and applications. However, compared to the widely and deeply studies about defects of group-VI TMDs (such as MoS 2 ), the study about defects of Re-based TMDs is really lack. Presently, several research works have explored the defects in ReX 2 and their effect on its phase structure, anisotropy electrical transport and the catalysis properties. [15,103,[120][121][122] For instance, chalcogen-deficiency mediated structure transformation of ReS 2 shows that the sulfur atom loss reduces the spacing between two Re diamond chains by 20.6% from the original value (≈0.34 nm) in the b direction to form more constricted structure (≈0.27 nm). [15] Such sulfur deficiency can also give rise to a distinct phase transformation in the a direction. This "diamond flip" transformation leads to a new phase consisting of wider double Re zigzag chain structure found in the a direction, which have never been observed in TMDs of group-VI. Moreover, the commonly observed S-Re-S pair vacancies in CVD-grown ReS 2 change the atomic level interaction sufficiently that can lead to a 180°in-plane chain rotation. [103] Zhai and coworkers found the electronic anisotropy of CVD-grown bilayer ReS 2 can be largely adjustable, which is attributed to the angle-dependent scattering from defects or vacancies. [120] Furthermore, the abundant Re atoms vacancies with lower Gibbs free energy for H adsorption activate the inert basal plane of ReS 2 as highly active sites for hydrogen evolution reaction. [122] Notably, owing to the distorted 1T structure of ReX 2 , the six X atoms bonded to one Re atoms are inequivalent in space, and the four Re atoms in one Re4 cluster are also inequivalent. In this regard, there should be several different types of X-defect and Re-defect in ReX 2 , which would bring different effects to its properties and applications. Therefore, deeply study to these various types of defects from structure to property and controllably create (or repair) these diverse defects to modify (or improve) the properties of ReS 2 are deserve to be further explored in the future.

CVD Growth of Vertically Oriented ReX 2 Arrays
Taking advantage of the fact that ReX 2 is prone to out-ofplane growth, several research groups have prepared a variety of vertically oriented 3D ReX 2 arrays. Koratkar and coworkers demonstrated that with the proper precursors and appropriate  [123] Copyright 2016, American Chemical Society. b) SEM images of ReS 2 flowers grown on the SiO 2 substrate and the arrays of ReS 2 flowers fabricated on the Pt metals. Reproduced with permission. [124] c) Cross-sectional TEM image of a deposited ReS 2 film and SEM images of the film deposited on 3D structure. Reproduced with permission. [125] Copyright 2018, Wiley-VCH. d) Schematic diagrams of the two different growth behaviors of ReS 2 on SiO 2 /Si and mica substrates. Reproduced with permission. [102] Copyright 2016, Royal Society of Chemistry. e) Atomic resolution imaging to probe the mechanism for vertical growth and a cross section of adjacent vertical sheets and a single vertical sheet attached to the horizontal sheet along a row of adjacent B-sites. Reproduced with permission. [127] Copyright 2018, Wiley-VCH.
tuning of the CVD growth conditions, vertically orientated ReS 2 nanosheets can be grown on a series of substrates such as SiO 2 /Si, mica, and carbon as well as gold foil (Figure 7a). [123] Meanwhile, the packing density, sheet size, and crystal quality of the ReS 2 nanosheets can be controlled by adjusting growth temperature and sulfur concentration. Wang and coworkers demonstrated a Pt metal induced nucleation to successfully realize the CVD growth of ReS 2 flowers at controlled locations (Figure 7b). [124] To achieve the large-scale fabrication, a highly controllable and precise atomic layer deposition (ALD) technique, another bottom-up synthesis approach, is applied to deposit 3D ReS 2 nanoarrays. [125] By controlling the growth temperature (from 120 to 500°C), 3D ReS 2 nanosheets with vertical orientation and petal-like morphology (Figure 7c) were obtained on large-area (5 cm × 5 cm) Al 2 O 3 coated Si (110) substrate using easily sublimated ReCl 5 and H 2 S as precursors. The developed ALD technique offers a route to upgrade the preparation of 3D ReS 2 nanoarrays to an industrial scale. Similarly, several other research works have also prepared the 3D ReX 2 materials with various morphologies and shapes via the CVD growth for diverse function applications. [25,126] Above research works demonstrate a fact that ReX 2 exhibits much greater propensity to grow into ver-tical 3D nanoarrays compared to other TMDs (such as MoS 2 or WSe 2 ), which highlights the potential of the material for applications beyond planar structure architectures.
To reveal the underlying growth mechanism of vertically oriented ReX 2 , our group compared its different growth behaviors on SiO 2 /Si and mica substrates as shown in Figure 7d. [102] Obviously, thick flakes and flower-like shapes of ReS 2 tended to grow on the SiO 2 /Si substrate, whereas uniform monolayer ReS 2 film with large domain size and hexagonal morphology were obtained on the mica substrate. The distinct growth behaviors can be understood from the 1T′ structure of ReS 2 itself and the surface properties of the two substrates. The rough SiO 2 surface with abundant dangling bonds and defects brings a high energy barrier for surface migration of adatoms, which hinders the free migration of adatoms on the substrate during ReS 2 growth. In contrast, the flat and inert surface of the mica allows the surface migration energy barrier of the adatoms much lower than that on the SiO 2 /Si substrate. Hence, the atomically flat surface of mica facilitates the migration of adatoms on it during CVD growth. In addition, the weak van der Waals interaction between the mica and atom clusters in favor of the surface-confined growth while avoiding out-of-plane growth, which is critical to growing ReS 2 film with uniform monolayer thickness. Koratkar and coworkers further investigated the growth mechanism of spontaneous vertical growth of ReS 2 nanosheets from theoretical and experimental insight (Figure 7e). [127] It was found that the governing mechanism for the vertical growth of ReS 2 involves two distinct stages. In the first stage, ReS 2 grows parallel to the growth substrate. However, the 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 ReS 2 plane. This under-coordinated S is "reactive" and binds to free Re and S atoms, initiating growth in a direction perpendicular to the ReS 2 surface. Using DFT and binding energy calculations, the author explained the anisotropy in the out-of-plane growth wherein maximal vertical growth occurs at the center of the predeposited flat ReS 2 flake and is almost nonexistent along the flake edges. The growth model developed in this work could apply to a broad class of material systems with distorted 1T phase, wherein an out-of-plane metal-metal bond causes an under-coordinated chalcogen atom to be displaced, triggering vertical growth from the surface of the flat flake. Above research works presented a clear understanding about the out-of-plane growth behavior of ReX 2 , which is of significant importance to tailored design of vertical ReX 2 nanoarrays for diverse functional device applications.
The vertical 3D ReX 2 arrays with high surface-to-volume ratio and edge exposure make them ideal candidates in electrocatalysis and battery applications as discussed in section 4.2. Moreover, the vertical ReS 2 arrays also show great application potential in sensor, field emission, photothermal radiotherapy and solarbased disinfection of bacteria, etc. [28,126,128] Together with the advantages of spontaneous out-of-plane growth of ReX 2 and the developments of large-scale fabrication techniques, these materials will exhibit more abundant applications.

ReX 2 -Based 2D Alloys
Realizing the controllable modulation of energy band structure and electrical conduction is of crucial importance to tailored design of 2D materials for diverse device applications. Alloying of multi-component metals or chalcogen elements with different stoichiometries in 2D materials provides a versatile and efficient approach for continuously tuning the bandgaps in a wide energy range. As such, researchers have successfully synthesized a series of 2D TMDs alloys (such as MoS 2x Se 2(1-x) , WS 2x Se 2(1-x) , Mo x W 1-x S 2 , and Mo 1−x W x Se 2(1−y )S 2y , etc.) which exhibit great feasibility in the tunability of energy band structure, carrier concentration and conduction type. [129][130][131] Notably, current research works are mainly focused on the 2H phase TMDs alloys. In this regard, 1T′ phase ReX 2 which shows distinctive phase structures and properties compared with 2H TMDs provides new opportunities to enrich the structures and properties of 2D TMDs alloys.
In view of ReS 2 and ReSe 2 have disparate bandgap and opposite carrier type (ReS 2 : 1.6 eV, n-type; ReSe 2 : 1.3 eV, p-type), that offer a reasonable route to modulate the bandgap and conduction type by alloying of them. Our group successfully synthesized the spatially composition-modulated monolayer ReS 2x Se 2(1−x) alloys by systematically adjusting the mass ratio of S and Se pow-ders during the CVD growth process (Figure 8a). [132] The size of alloy domain can up to a few hundred microns (≈206 µm) as shown in Figure 8b. The STEM image of ReS 1. 52 Se 0.48 alloy shows that the 1T′ structure is well maintained and the diamond shaped Re4 chains can be visualized clearly (Figure 8c). The bandgaps of ReS 2x Se 2(1−x) alloys were continuously modulated from 1.32 to 1.62 eV along with the composition x varying from 0 to 1. Especially, the carrier type, threshold voltage and carrier mobility of ReS 2x Se 2(1-x) alloy FETs can be systematically modulated by tuning the alloy composition. Furthermore, using the synergy of salt assistance with the confined space CVD strategy, high-quality and large-size ReS 2x Se 2(1−x) monolayer crystals can be directly grown on SiO 2 /Si substrates. [133] Owing to the lowsymmetry 1T′ structure and tunable bandgap of ReS 2(1−x) Se 2x alloys, they exhibit strong anisotropic optical, electrical, and photoelectric properties, [132,134] as well as broadband photoresponse to visible and near-infrared light. [133] Beside 1T′ ReS 2x Se 2(1-x) alloys, constructing 2D alloys from two parent TMDs materials with different phase structures have attracted great attention, as the phase structure can provide more degree of freedom for engineering their structures and properties. [135] Ajayan and coworkers demonstrated the successful synthesis of 2D Mo 1−x Re x Se 2 and Re x Mo 1-x S 2 alloys with different compositions by alloying 2H phase MoSe 2 /MoS 2 and 1T′ phase ReSe 2 /ReS 2 via the CVD technique. [29,136] Unlike the widely studied 2H phase TMDs alloys, the Mo 1−x Re x Se 2 and Re x Mo 1-x S 2 alloys showed a structural phase transformation from 2H to 1T′ phase as a function of Re doping. Notably, the Re content in the 1T′ structures was found to be controllable between 55% and 100%, while only a small amount of Re (2-4%) was incorporated in the 2H phase. According to the energy calculations and the crystal field theory, a transition from 2H to 1T′ phase is predicted to be more energetically favorable when the Re/(Re+Mo) concentration is greater than ≈50%, which is consistent with the above experiment observations. [29,136] Figure 8d,e shows the STEM images of two representative alloys of 2H Re 0.02 Mo 0.98 S 2 and 1T′ Re 0.55 Mo 0.45 S 2 , respectively. As emphasized in the above work, the stable 1T′ phase MoS 2 which is metastable in nature can be obtained by alloying with Re when Re-content reached the ≈50% level. As a result, 1T′ Re 0.55 Mo 0.45 S 2 alloy shows enhanced HER activity on the basal plane with a low onset potential and a small Tafel slope as well as excellent stability, which is even superior to the benchmark lithium intercalated 1T phase MoS 2 . This study provides a new strategy to improve the overall HER performance of MoS 2 -based materials via alloying/doping. Furthermore, through precisely tuning the growth thermodynamics of Mo x Re 1-x S 2 alloys, two serious problems, the phase separation and structure reconstruction were successfully suppressed, which are usually neglected but critical to exploring their intrinsic properties. [137] The tunable phase structures and the distinct valence electron structures of Mo/W and Re endow 2D ReX 2 -based alloys many attracting electronic band structure features, photonic and electronic properties. [137][138][139][140][141][142] Figure 8g presents the electronic band structures calculated from experimental 3D atomic coordinates including dopant-free MoS 2 structure, single Re dopant, two Re dopants and single Re dopant with a S vacancy. [138] The band structures show highly distorted indirect bandgaps or metallike behavior with a large number of shadow bands. Moreover, Figure 8. a) Schematic illustration of the atomic structure of ReS 2x Se 2(1−x) alloy and its CVD synthesis approach. b) Typical optical image of a ReS 2x Se 2(1−x) grain. Reproduced with permission. [132] Copyright 2016, Wiley-VCH. STEM images of c) 1T′ ReS 2x Se 2(1−x) alloy, d) 2H Re 0.02 Mo 0.98 S 2 alloy, e) 1T′ Re 0.55 Mo 0.45 S 2 alloy, and f) 2H Re 0.5 Nb 0.5 S 2 alloy. Reproduced with permission. [132,136,140] Copyright 2017, Wiley-VCH. Copyright 2018, Wiley-VCH. Copyright 2019, American Physical Society. g) Electronic band structures of MoS 2 containing two Re dopants calculated from experimental 3D atomic coordinates of 6 × 6 × 1 unit cells. Reproduced with permission. [138] Copyright 2020, Springer Nature. h) Transfer curves (I ds -V g ) of Mo 0.98 Re 0.02 S 2 alloy FET. The V ds in changes from 0 to 2 V with an interval of 0.25 V. Reproduced with permission. [137] Copyright 2020, Wiley-VCH.
based on the high-quality Mo x Re 1-x S 2 alloys, several novel energy band structure features were revealed, such as subgap formation, Burstein-Moss effect, especially strong band bowing effect from 1T′ to 2H phase transition. [137] What's more, the Mo x Re 1-x S 2 alloys FET devices revealed tunable conduction transition from ntype to bipolar and p-type in 1T′ phase, as well as novel "bipolarlike" electron conduction behavior in 2H phase (Figure 8h). Furthermore, using salt-assisted CVD growth, Zhou and coworkers synthesized Re x W (1−x) S 2 alloys with different compositions which show greatly enhanced responsivity than that of CVDgrown WS 2 . [139] Zettl and coworkers reported the compositiondependent bandgap of monolayer Re x Nb 1−x S 2 in the 2H and 1T′ phases. It shows a broad range of bandgaps, from metallic (x = 0, NbS 2 ) to semiconducting (x = 1, ReS 2 ), in contrast to a limited bandgap range offered by isovalent TMDs alloys. [140] The above research works highlight the unique alloying effects which do not exist in the single-phase 2D alloys, and provide the feasibility for potential applications in building novel electronic and optoelectronic devices.
For 2D alloy materials, quantify the spatial and local distribution of atoms is much critical to investigate their structure and properties at the atomic scale. Since the low-symmetry of 1T′ ReX 2 , the local coordination and elemental distribution in ReX 2 -based alloys would be significant difference to that of highsymmetry 2H TMDs alloys. As expected, our group discovered an interesting "sub-nanometer-scale local atomic distribution" in the CVD-grown ReS 2x Se 2(1-x) alloys. It was found that the Se atoms are preferentially located at the outside of Re4-chain, while the S atoms are preferentially located at the inside of Re4-chain. The underlying reason for the preferential S/Se occupation in 1T′ ReS 2(1−x) Se 2x alloys is attributed to the stability of different Se/S coordination configurations, where the energy for Se atoms substitution of S atoms is much lower at the relatively larger volume sites. [143] Furthermore, an interesting frustration and atomic ordering feature were discovered in mechanically exfoliated monolayer Re x Nb (1−x) S 2 alloys. [140] Interestingly, the induced atomic ordering can be used as another degree of freedom to considerably modify the bandgap of monolayer semiconductor alloys. Therefore, the local coordination and elemental distribution features of above ReX 2 -based alloys would offer many novel properties and applications, which are worth to further explore in the future.

ReX 2 -Based Heterostructures
Various 2D materials can be flexibly combined to build diverse heterostructures with tunable band alignment, opening up giant opportunities for exploring novel properties as well as potential applications in optoelectronics. [144][145] In the past several years, vast permutations of heterostructures based on ReX 2 have been created with novel electronic and optoelectronic properties and applied in photovoltaic cells, rectifiers, photodetectors, light-emitting diodes, and even catalysis, etc. [146][147][148][149][150][151] According to the structure geometries, ReX 2 -based heterostructures can be divided into two categories: vertical heterostructure and lateral heterostructure. The vertical heterostructure which is bonded by weak Van der Waals interaction can be easily fabricated by assembling ReX 2 with other 2D materials via mechanical exfoliation and restacking procedure. This flexible approach is suitable for studying the fundamental properties of heterostructures, but difficult to realize the large-scale device applications, and it is unable to create lateral heterostructures. Besides, CVD growth which is considered as an effective way to realize large-scale preparation of 2D materials, has been widely used to fabricate both vertical and lateral ReX 2 -based heterostructures with clean and atomically sharp interface. In this section, we will mainly introduce the research progress of ReX 2 -based heterojunctions prepared by CVD growth and briefly discuss their electronic and optoelectronic properties.
The unusual 1T′ phase structure of ReX 2 offers new possibilities to construct both the homophase (1T′-1T′) and heterophase (1T′-2H) heterostructures for exploring novel interface structures and electrical properties. As shown in Figure 9a, our group reported the synthesis of 1T′ ReS 2 -ReSe 2 lateral heterostructure by two-step CVD growth on mica substrate. [152] The obtained heterojunction shows high crystal quality with an atomically sharp interface. Interestingly, three types of epitaxy growth modes accompanying formation of three different interfaces were revealed in the growth of 1T′ lateral heterostructures, where the angles between the b-axis of ReS 2 and ReSe 2 are 0°, 120°, and 180°, respectively. Electrical transport demonstrates that the 1T′ ReS 2 -ReSe 2 heterostructure forms lateral p-n junction with intrinsic rectification characteristics and exhibits polarization-dependent photodiode properties.
The stable 1T′ phase structure of ReX 2 provides great opportunities for building 2D heterophase structures with other 2H phase TMDs, which could bring abundant interface structures and novel properties. Tongay and coworkers reported the first synthesis of ReS 2 -WS 2 pseudo-1D-2D lateral heterophase structures via a two-step CVD process on sapphire substrate (Figure 9b). [30] Structure characterization of the interface between these dissimilar 2D materials reveals that ReS 2 lateral heterojunctions to WS 2 produce well-oriented (highly anisotropic) Rechains perpendicular to WS 2 edges. Notably, the lateral heterostructure conforms around the entire WS 2 edges followed by the formation a few of vertical heterostructures. Moreover, serious surface-deposition exists at the defect-rich 1D interface of the heterostructure where exists high-density nucleation centers. As is well known, the surface-deposition is an inevitable problem in the synthesis of 2D heterojunction via two-step CVD growth, and such problem is further aggravated in the synthesis of heterophase junction because of the large lattice mismatch. Ajayan and coworkers addressed the synthesis of 1T′-2H ReSe 2 -MoSe 2 lateral heterophase structure on sapphire substrate by one-step CVD growth with NH 4 ReO 4 and (NH 4 ) 2 MoO 4 as Re and Mo precursors ( Figure 9c). [153] The authors utilized the phase diagrams of the binary Mo-Re system to differentiate it from commonly reported Mo-W chalcogenide systems toward its unique thermodynamically dictated preference for alloy versus heterostructure formation at the monolayer limit. Though atomic sharp heterophase interface can be obtained, interdiffusion of Re and Mo atoms in the two phases still occur, and plenty of defects exist in the entire heterojunctions. Actually, the defect and alloying problems are intrinsic drawbacks of heterostructures prepared via one-step growth. Recently, our group developed a hydrogentriggered one-pot growth approach to synthesize 2H-1T′ WS 2 -ReS 2 heterophase junctions with high-quality interface structure (Figure 9d). [154] The sequential introduction of hydrogen during growth system, which acts as a "switch" to selectively turn off the growth of ReS 2 while turning on the growth of WS 2 , allows WS 2 to seamlessly grow around ReS 2 to form the lateral WS 2 -ReS 2 heterojunction. As a result, an atomically sharp 2H-1T′ heterophase interface was obtained and no cross-doping existed in the two phase regions as confirmed by the Raman, PL and STEM characterization. Moreover, the outer WS 2 grains are preferred to nucleate at the vertices of inner ReS 2 grain with fixed lattice orientation, promoting the merger of surrounding WS 2 grains to form a single crystal. Based on the high-quality WS 2 -ReS 2 heterophase junction, effective photocarriers separation, prominent rectification characteristics, and polarization-sensitive photodiode properties were achieved.
Beside above lateral heterojunctions, several research works also reported the synthesis of 2D ReX 2 -based vertical heterojunctions. Fu and coworkers reported the synthesis of ReS 2 -WS 2 vertical heterostructures using an innovative twinned growth relationship between ReS 2 and WS 2 . [155] For the twinned growth of ReS 2 -WS 2 vertical heterostructures, Au was chose as the growth substrate and W-Re alloy foil was used as the Re and W sources, which lower the barrier energies for this special twinned growth process. With this approach, 100% overlap for each of the stacked ReS 2 -WS 2 heterostructures with crystal size up to 600 µm 2 were achieved. Interestingly, owing to the weak interlayer van der Waals interactions, the ReS 2 -WS 2 vertical heterostructures can be disassembled into individual building blocks under the assistance of disassembly promoters (Te). [156] Moreover, Ye and Figure 9. a) Schematic illustration for the synthesis of 1T′ ReS 2 -ReSe 2 heterostructure through two-step epitaxial growth, and the corresponding optoelectronic properties. Reproduced with permission. [152] Copyright 2018, Wiley-VCH. b) Schematic diagram of the two-step CVD growth of 1T′-2H ReS 2 -WS 2 heterostructures, and the corresponding STEM image. Reproduced with permission. [30] Copyright 2017, Wiley-VCH. c) Schematic diagram for synthesizing MoSe 2 -ReSe 2 heterojunction via a one-pot growth process, and the corresponding colored STEM image. Reproduced with permission. [153] Copyright 2019, American Chemical Society. d) Schematic illustration for synthesizing 2H-1T′ WS 2 -ReS 2 heterostructures via a hydrogen-triggered onepot growth process, and the corresponding STEM image. Reproduced with permission. [154] Copyright 2020, Wiley-VCH.
coworkers reported the growth of Se nanoplates on monolayer ReS 2 through a two-step CVD process to build the Se-ReS 2 vertical heterostructures. [157] The Se nanoplates could conjugate with underlying ReS 2 via strong chemical hybridization at heterointerface due to the existed defects in ReS 2 , which allow it grown with the [001] axis vertically aligned with underlying ReS 2 . In addition, utilizing the advantage of atomically smooth and chemically inert surface of graphene, graphene-ReS 2 vertical heterostructures were achieved via CVD growth. [158] Compared with rough surface of SiO 2 /Si substrate with dangling bonds which hinders the uniform growth of ReS 2 , the inert and smooth surface of graphene provides lower energy barrier for migration of the adatoms, thereby promoting the in-plane growth of ReS 2 on it. In-situ patterning of the graphene-ReS 2 heterostructure was achieved by the selective growth of ReS 2 on the patterned graphene, which is attributed to the strong binding energy between sulfur atoms and graphene surface.
Overall, the above research works provide useful insights for understanding the growth dynamics and stability of ReX 2 -based heterostructures from chemically, structurally, and electronically different phases. Present studies are mainly focused on developing different synthesis strategies to build novel 2D heterostructures with diverse interface structures, while exploring their novel electronic and optoelectronic properties are still quite scarce. Moreover, the diverse phase structures and abundant properties of group-VI TMDs and ReX 2 offer significant opportunities to build a new class of 2D heterophase structures for engineering their interface structures and energy band alignments. In this regard, further exploring novel interface structures, properties and applications of 2D ReX 2 -based heterostructures should be the main emphasis of future research works. Furthermore, the synthesis approach for obtaining large-scale and highly uniform 2D ReX 2 -based heterostructures with desired interface structures (lateral or vertical heterojunction) still needs to be optimized. It could be optimistically anticipated that various epitaxial growth methods as well as superior properties of abundant ReX 2 -based heterostructures would spring out in the near future.

Application of ReX 2
This section is devoted to specifically discuss the various applications of ReX 2 . For this purpose, we categorized all reported applications of ReX 2 into three main areas based on the structural and functional features of each application. The pertinent subsections including electronic and optoelectronic devices, energy conversion and storage, as well as other emerging applications.

Electronic and Optoelectronic Devices
2D ReX 2 materials profit from their unusual 1T′ structure, finite bandgap, novel optical and electrical properties have attracted considerable research interest in constructing next-generation solid-state electronic devices. [13,19] In this subsection, we focus on three primary applications of ReX 2 in the field of electronic and optoelectronic devices: field effect transistors (FETs), logic circuits, and photodetectors.

Field Effect Transistors
FETs play an important role in our modern information technology as one of the most effective technologies of all time and have facilitated the development of computers, the Internet, thin mobile displays, and much more. [159] Atomically thin 2D TMDs as channel materials in FETs can eliminate drain-induced shortchannel effects to achieve high performance. In the past several years, FETs based on mechanically exfoliated ReS 2 have been reported with a current on/off ratio up to 10 7 and electron mobility varying from 0.1 to 18 cm 2 V −1 s −1 , the device is schematically shown in Figure 10a. The I DS varies linearly with V DS in the output curves measured at different temperatures (Figure 10b), indicating well-developed contact between the electrodes and the ReS 2 channel. The dependence of the channel conductance G on V BG at different temperatures in Figure 10c shows an interesting metallic state associated with the metal-to-insulator transition due to the increase of Fermi level when V BG is larger than 15 V. Moreover, FET devices based on the CVD-grown ReS 2 have also been extensively investigated, the mobility varies from 0.072 to 40 cm 2 V −1 s −1 depending on the crystal quality of the samples prepared in different works. Moreover, FETs based on ReSe 2 exhibit a p-type conduction behavior with mobility in the range of 1.36 × 10 −3 -14.1 cm 2 V −1 s −1 which is relatively lower than that of ReS 2 and most 2D TMDs. The relatively lower carrier mobility of ReX 2 attributed to its large electron effective mass may restrict its highperformance electronic device applications. Certainly, the carrier mobility of ReX 2 can be further improved by the chemical modification of materials or the optimization of the electrode-material contact. Anyway, the different bandgap together with the opposite conductive features of ReS 2 and ReSe 2 provides great perspectives for optoelectronic applications, although their mobility still has great room for improvement.
Several strategies have been adopted to optimize the performance of ReX 2 -based FET devices. Using graphene as contact electrodes, ambipolar behavior of ReSe 2 transistors with electron and hole mobilities of 1.1 and 3.1 cm 2 V −1 s −1 , respectively, can be achieved due to the tunable Fermi level of graphene by electrical field. [160] Xiu and coworkers constructed a top-gated ReSe 2 FET device with high-Al 2 O 3 as dielectric layer, which achieved a significant improvement of the electron mobility over 500 times and the hole mobility over 100 times at low temperature. [23] Moreover, by fabricating ReS 2 FET device onto h-BN substrate, the on/off switching ratio can reach to 10 8 and the mobility up to 30 cm 2 V −1 s −1 . [22] It is the reduced charged impurities that decrease the charge scattering of ReS 2 on h-BN substrate and thus accounting for the enhancement of mobility. The h-BN encapsulated ReSe 2 FETs, which isolate the ReSe 2 channel from both SiO 2 /Si substrate and air, can ulteriorly improve the device performance (Figure 10d,e). [160] Constructing suspended ReS 2 FET device is also an effective method to eliminate the interface traps between ReS 2 and SiO 2 /Si substrate for obtaining more intrinsic electrical properties of ReS 2 (Figure 10f). [161] Liu and coworkers reported the monolayer ReS 2 FET mobility up to 40 cm 2 V −1 S −1 via measuring at low temperature condition which can reduce the electron-phonon scattering. [20] Furthermore, Park and coworkers achieved a better-performance ReS 2 FET by simple oxygen plasma treatment. [84] The current on/off ratio enhancing from 10 3 to 10 4 and mobility enhancing from 4 to 7.6 cm 2 V −1 s −1 via increasing the O 2 plasma treatment time from 0 to 60 s. The improvements are attributed to the surface oxidation induced electron-accepting sites, and consequently cause the ptype doping of ReS 2 , which reduced the electron injection from metal to ReS 2 .
Different from conventional isotropic 2D materials like MoS 2 , the effective mass of electron and hole of ReX 2 varies along different crystal directions owing to the unusual low-symmetry crystal structure. [15,120,162] It affords ReX 2 inspiring 2D in-plane anisotropic electronic properties which have been revealed by fabricating ReX 2 FETs with multipair probes configuration and performing angle-resolved transport measurements. For example, Suenaga and coworkers investigated the relationship between crystalline orientation of ReS 2 and the electron transport by combination of transport measurement and transmission electron microscopy (Figure 10g,h). [15] It is found that the conductance and electron mobility were strongly dependent on the crystalline orientation. The conductance along the b-axis direction (0.82 S) was an order of magnitude higher than that parallel to the a-axis (0.075 S). The electron mobility along the baxis (23.1 cm 2 V −1 s −1 ) was nearly two times higher than that crossing the b-axis (14.8 cm 2 V −1 s −1 ). In the following study, the anisotropic ratio of polarization mobility at different crystal orientations reached up to ≈3 which is the highest value compared with other anisotropic 2D materials (Figure 10i). [19] Besides ReS 2 material, the ReSe 2 and even the Re-based 2D TMDs alloys and heterostructures with 1T′ structure also show anisotropic electrical transport properties. [132,152] It is worth noting that the strong anisotropic electronic characteristics of ReX 2 open up new opportunities for constructing novel FETs and logical devices.

Logical Circuits
These preliminary works mentioned in the previous subsection have aroused great interest in the applications of 2D ReX 2 in smart integrated circuits toward practical microprocessors. The key step to this application is the development of basic logic gates, especially the inverter (NOT gate). For the anisotropic ReX 2 , the  [21] Copyright 2015, Wiley-VCH. d) Schematic diagram of a ReSe 2 FET encapsulated with hBN. e) Transfer characteristics for ReSe 2 FET at different V bg . Reproduced with permission. [160] Copyright 2019, Wiley-VCH. f) Schematic of suspended ReS 2 device. Reproduced with permission. [161] Copyright 2018, American Chemical Society. The direction-dependent g) output curves and h) transfer curves of ReS 2 device. Reproduced with permission. [15] Copyright 2015, American Chemical Society. i) Normalized field-effect mobility of a six-layer device along 12 directions evenly spaced at 30°apart plotted in polar coordinates (red dots with left axis). The direction with the lowest mobility was set to be the 0°(or 180°) reference. The optical image of the device is shown in the inset. Reproduced with permission. [19] Copyright 2015, Springer Nature.
lattice orientation could be used as an alternate design variable to turn transport properties and optimize performance of 2D integrated circuits. Xing and coworkers fabricated the first ReS 2based digital inverter which is composed of two anisotropic ReS 2 FETs along the a-axis and b-axis of a quadrilateral-shaped ReS 2 flake with a 60°inner angle as shown in Figure 11a. [19] Figure 11b presents the OM image of the inverter device, where HfO 2 and Au were used as insulating dielectric layer and top gate electrode, respectively. The voltage transfer curves (Figure 11c) show excellent logic-level conservation between logic 0 and 1 at different input voltages (V in ). The signal gain (Figure 11d) of the inverter reaches 4.4 when the supply voltage (V DD ) is 3V, indicating its outstanding sensitivity to the changes of V in . This work provided a way to regulate the performance of logic circuits based on different crystal orientations of low-symmetry ReS 2 material, and injected fresh blood into the research field of 2D ReX 2 materials. In general, a Schottky barrier exists at the interface between ReS 2 material and metal, which degrades the efficiency of carriers injection from channel to electrode. Quantum transport simulation shows that selecting appropriate electrode materials which have strong interface interaction and relatively smaller work function for ReS 2 with an Ohmic contact could improve the FET performance for logic circuits application. [163] In experiment, Cho and coworkers adopted CVD-grown graphene as the electrodes to fabricate large-scale low-voltage-operated ReS 2 transistors and logic gates based on CVD-grown large-area multilayer ReS 2 film (Figure 11e,f). [164] The contact resistance of the ReS 2 transistors with graphene electrodes decreased dramatically  [19] Copyright 2015, Springer Nature. e) Schematic diagram of the CVD-grown ReS 2 transistors with graphene electrodes and ion gel gate dielectrics. f) Optical top-view image of the ReS 2 transistors and logic gates, such as NOR, NAND, and NOT gates. The inset presents an optical image of a single ReS 2 transistor with graphene drain, source, and gate electrodes and an ion gel gate dielectric. g-i) Optical top-view image and schematic band diagram of the g) NOT, h) NAND, and i) NOR gates gate. j) Voltage transfer characteristics of the NOT gate and signal gain at V D = 1 V. k) The input and output voltages of the NAND and NOR gates as a function of time. Reproduced with permission. [164] Copyright 2017, American Chemical Society. l) Schematic illustration of the phosphorene/ReS 2 ternary inverter. m) V IN versus V OUT characteristic of the ternary inverter. The inset shows an input-output table of the ternary inverter. Reproduced with permission. [146] Copyright 2016, Springer Nature. compared with that prepared with metal Cr electrodes. Then, the multiple ReS 2 transistors were successfully assembled to build the logic devices, including NOT, NAND, and NOR gates (Figure 11g-i). Notably, the signal inverter gain of NOT logic gate exceeded 3.5 (Figure 11j), which is sufficient to drive a next-stage component in a logic circuit. For NAND and NOR logic gates, the input and output voltages as a function of time exhibit excellent switching capacities between logic state "0" and logic state "1" (Figure 11k), indicating the outstanding computing capacities. Park et al. demonstrated a ternary inverter as multi-valued logic application by constructing a negative differential resistance device based on phosphorene/ReS 2 heterojunction (Figure 11l). [164] Three distinct logic values (states "2," "1" and "0") were achieved in the V IN versus V OUT characteristic of the device (Figure 11m). The above research progresses demonstrate that it is possible to construct a wide variety of complex functional digital logical circuits using ReX 2 materials in the future.

Photodetectors
Photodetector, a key component in many devices, has a wide range of applications in various fields including military and national economy. The working principle of most photodetectors is based on photoelectric effect, which can convert the information stored in light into electrical signals that can be processed by standard electronic devices. [165] For photodetector based on 2D materials, the incident photons with energy larger than its bandgap can excite bound excitons which are separated by built-in field or applied electrical field and ultimately generate photocurrent. [166] The large bandgap (ReS 2 : ≈ 1.3 eV and ReSe 2 : ≈ 1.6 eV) and the superior conductivity of ReX 2 make them ideal candidates for constructing high-performance photodetectors. Furthermore, the anisotropic structure of ReX 2 can afford the polarization state into its photodetection, and thus increase the detection dimension to get more accurate detection information. In this regard, Figure 12. a) Schematic structure of ReS 2 back-gate photodetector. Reproduced with permission. [21] Copyright 2015, Wiley-VCH. b) Photocurrent response of the ReS 2 photodetector under various V ds , fixing P light at 20 nW and V bg at 50 V. c) Weak signal detection in the ReS 2 photodetector. d) The dependence of R P on P light . For comparison, the results from monolayer MoS 2 , multilayer GaTe, and graphene are also presented. Reproduced with permission. [22] Copyright 2015, Wiley-VCH. e) Schematic diagram and optical image of back-gated ReSe 2 devices doped by PPh 3 and a descriptive illustration for the n-doping mechanism by PPh 3 at the PPh 3 /ReSe 2 interface. Reproduced with permission. [168] Copyright 2016, Wiley-VCH. f) Schematic illustration of O 2 -plasma-treated ReS 2 FET. g) Photoresponsivity of the ReS 2 photodetectors as a function of incident laser power according to various O 2 plasma treatment times. Reproduced with permission. [84] Copyright 2015, Wiley-VCH. h) 3D schematic view of the photodetection device. i) The photocurrent with drain bias of 1 V (red squire) and absorption (green circle) measured under different polarization angle of green light and plotted in polar coordinates. The blue lines are the fitting results using sinusoidal function. Reproduced with permission. [20] Copyright 2015, Wiley-VCH. j) SEM image of the ReSe 2 photodetector followed by polarization-dependent photocurrent mapping of the device, showing prominent linear dichroic photodetection. Reproduced with permission. [112] Copyright 2016, American Chemical Society.
ReX 2 material would have broad application prospects in the field of photodetection.
In the past several years, plenty of ReX 2 -based photodetectors with diverse structures have been fabricated to explore their practical applications. Figure 12a shows the schematic structure of ReX 2 photodetector which is a standard photoconductor-type detector. [21] Here, we focus on the three parameters: responsivity (R), external quantum efficiency (EQE) and response time ( : rising ( r ) and decaying ( d )), which are the primary indicators for assessing the performance of photodetectors. [165] Liu and coworkers reported the first ReS 2 -based photodetector fabricated by the mechanically exfoliated few-layer ReS 2 , and the device achieved a responsivity of 1000 A W −1 . [20] Xing and coworkers further presented a high responsivity photodetector based on few-layer ReS 2 . [22] As shown in Figure 12b, the time-resolved response curve of the ReS 2 device exhibited a stable and repeatable response to the 532 nm laser illumination but its response time was on the order of seconds. Importantly, the optimized photoresponsivity reached as high as 88 600 A W −1 and EQE up to 2 × 10 7 %, which are the record values compared to other individual 2D materials with similar two-terminal device structures. Taking advantage of the high photoresponsivity of the ReS 2 photodetectors, it can even be used to detect the weak signals from a lighter and limited fluorescent lighting in a dark room (Figure 12c). 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 ReS 2 flakes. With the fast development of CVD synthesis of ReX 2 , CVD-grown ReS 2 and ReSe 2 were widely utilized to construct the high-performance photodetectors. Xiu and coworkers reported the photodetector based on CVD-grown few-layer ReS 2 with the photo responsivity of 16.14 A W −1 and the EQE reached 3168% under the irradiation of 633 nm laser at power of 25 nW. [21] Notably, the response rate is quite slow, especially the d is as long as ≈1000 s. Zhai and coworkers explored the light response properties of CVD-grown single crystal ReS 2 flake and poly-crystalline ReS 2 bilayer film. [100] They achieved the responsivity of 604 A W −1 and EQE of 1.50 × 10 5 % in the single crystal ReS 2 flake device, and found that the response rate of the former ( r and d ≈ 2 ms) is shorter than the latter ( r : 5.5 s and d : 11.7 s). In general, photodectors based on mechanical exfoliated and CVD-grown ReS 2 routinely achieve high responsivity, indicating high photogain, originating from the long lifetime of photogenerated carriers which are boosted by trap states. In contrast, the photodetectors based on ReSe 2 have relatively low responsivity (R: 2.98-5.1 A W −1 and EQE: 4.58 × 10 2 %) due to the low carrier mobility, but exhibit quite fast response rate (both r and d at ms level). [23,104] Especially, owing to the relatively smaller bandgap (≈1.3 eV) of ReSe 2 , its photodetectors show a broadband photoresponse from visible to NIR (940 nm). [101,104] From the above research works, we can see a distinct trade-off existed between responsivity and response speed of ReS 2 photodetectors, which is known as the typical feature of photoconductive-type detector. [165] Moreover, the performance of photodetectors based on mechanically exfoliated ReX 2 is superior to that based on CVD-grown ReX 2 , [22][23] which is reasonable because of the higher crystal quality of exfoliated ReX 2 . The performance of photodetector fabricated by the relatively thicker ReX 2 is superior to that of monolayer, and the single crystal is better than the poly-crystalline. [100] Despite considerable research works have performed on the optoelectric properties of ReX 2 material, the relatively low responsivity and slow response time still restrict its usage in high-sensitive and fast switching applications.
For promoting the applicability of ReX 2 photodetectors, several recent studies have strived to elevate their performance through modifying the materials or device configuration, such as doping, chemical modification, suspended channel, heterostructure construction, and so on. [157,161,[167][168][169][170] For instance, a very high performance ReSe 2 photodetector with a broad detection range ( >1064 nm), high responsivity (1.18 × 10 6 A W −1 ), and fast temporal response ( r : 30 ms and d : 64 ms) was obtained by the modification of (3-Aminopropyl) Triethoxysilane (APTES) and Triphenylphosphine (PPh 3 ), as shown in Figure 12e. [168] This high performance is achieved by suppressing the interfacial carrier scattering with APTES treatment and improving the sourceside contact resistance through a PPh 3 -based n-doping technique. Similarly, using HCl-based selective p-doping technique can also increase the photoresponsivity of ReSe 2 photodetecor from 79.99 to 1.93 × 10 3 A W −1 , and the the r decreases from 10.5 to 1.4 ms with d from 291 to 3.1 ms compared with the intrinsic ReSe 2 device. [167] Moreover, Park and coworkers demonstrated a high-performance ReS 2 -based photodetector with high photoresponsivity (2.5 × 10 7 A W −1 ) and fast temporal response ( r : 670 ms and d : 5.6 s) through a simple oxygen (O 2 ) plasma treatment (Figure 12f,g). [84] This improvement originates from the controlled thickness of ReS 2 and the intentionally created defects (traps) in the ReS 2 during the O 2 plasma treatment, which affected the drain current at the off-state and the recombination of photocarriers. In another work, the molecular O 2 physisorption can be used as "gating" to modulate the carrier density of single-layer ReSe 2 transistors, resulting in a high R of 95 A W −1 and EQE of 18 645% in O 2 environment. [171] Furthermore, Lodha and coworkers investigated the responsivity and response rate in supported (SiO 2 /Si substrate) and suspended ReS 2 photodetectors to distinguish the roles of intrinsic bulk traps in ReS 2 and extrinsic traps at the ReS 2 /SiO 2 interface. [161] It was found that the dominant operating mechanism of these photodetectors can be switched between photogating and photoconductive regimes under gate voltage modulation. As a result, high responsibility of 4 A W −1 and much low response time of 20 µs were achieved on the suspended ReS 2 photodetector.
Beside engineering ReX 2 itself, the performance of ReX 2 photodetectors can also be enhanced by combining them with other 0D/1D/2D semiconductors to build heterostructure devices. The responsivity of the ReS 2 photodetector decorated with 0D CdSe-CdS-ZnS quantum dots could be enhanced by more than 25 times (up to ≈654 A W −1 ) and the r and d can be also reduced from 12s and 13s to 3.2 s and 2.8 s, respectively. [172] The excellent optoelectronic performance is originated from the coupling effect of quantum dots light absorber and cross-linker ligands 1,2-ethanedithiol, where the photoexcited electron-hole pairs in quantum dots can separate and transfer efficiently due to the type-II band alignment. Moreover, the photodetectors based on Se/ReS 2 heterostructures exhibit ultrahigh detectivity of up to 8 × 10 12 Jones and fast response time of less than 10 ms, illustrating the great advantage of directly integrated 1D Se based nanostructure on planar ReS 2 films for optoelectronic applications. [157] Moreover, by building a type II WSe 2 -ReS 2 p-n junction, a neardirect heterointerface bandgap (0.7 eV) was achieved to enhance photogeneration. [150] The device exhibits ultrafast response time (5 µs) and high responsivity (3 A W −1 ) at the entire hetero-overlap region, and broadband photoresponse to IR (1250 nm). Table 2 summarizes the main figures-of-merit of ReX 2 -based photodetectors.
It has been mentioned that, unlike other isotropic 2D materials, the structure anisotropy induced linear dichroism in ReX 2 enables researchers to build the polarization-sensitive photodetectors. Liu et al. demonstrated the first polarization-dependent photodetector with high photoresponsivity based on atomically thin ReS 2 . [20] Figure 12h shows the schematic view of the photodetection device, where the polarization angle of light is controlled by a polarizer and a half-wave plate. By rotating the polarization of the light while keeping the incident power constant, the photocurrent changes dramatically. The photocurrent with the incident light polarized along the b-axis of ReS 2 is much stronger than that perpendicular to the b-axis. Notably, the photocurrent and absorption have a quite similar dependence on the incident light polarization, which strongly suggests that the intrinsic polarization-dependent photoresponse originates from the anisotropic structure of ReS 2 itself (Figure 12i). Similarly, the polarization-sensitive photoresponse was also achieved in the few-layer ReSe 2 photodetector. [23] Figure 12j shows the large-area photocurrent mapping images for the whole channel under different polarization directions of the incident laser of 633 nm. The photocurrent reached to the maximum (minimum) value when the incident light is polarized parallel (perpendicular) to FL ReS 2 0.5 10 532 nm (10 nW) 10 4 20.2 45 10 500 - [170] CVD-grown FL ReS 2 0.5 50 500 nm (3.11 mW cm −2 ) 1 0 2 -10 3 30 604 1.5 × 10 5 2 ms [100] CVD-grown FL ReS 2 0.1 50 633 nm (25 nW) 10 6 1 16.14 3168 - [21] CVD-grown FL the b-axis. The ReSe 2 device also exhibits ambipolar (electron and hole regimes) gate-tunable linear dichroism photodetection. Moreover, the polarization-dependent photoresponse was also achieved from ReX 2 -based alloys and heterostructures. [132,152,154] Hence, the robust linear dichroism photodetection with high photoresponsivity reported on the above ReX 2 -based materials demonstrates a route to exploit the intrinsic anisotropy of 2D materials and opens up new ways for the applications of 2D materials for light polarization detection. At this stage, a deeper understanding of the photocurrent and carrier recombination mechanisms is needed for the optimization of photodetector performance. From the application perspective, the sizable variation in the figure-of-merit reported by different works on the ReX 2 photodetectors indicates that device fabrication, contact metals and measurement environment play important roles in the photodetection performance. Hence, more sophisticated and fundamental advances for the improvement of device performance of ReX 2 -based photodetectors are urgently needed.

Energy Conversion and Storage
Developing new energy conversion and storage techniques is highly desirable for addressing the global energy crisis and environmental problems. In order to lessen the reliance on fossil fuels, the production of hydrogen from water splitting has become an important subject as H 2 has been viewed as the most promising sustainable clean energy source. [173] Currently, highly efficient photocatalytic and electrocatalytic hydrogen evolution are considered as the two most important routes to produce H 2 . [173][174] Moreover, the new generation batteries have tremendous application prospects in future intelligence power system owing to its high power density and high energy density. [175] The rational design of electrode materials is the key to all above devices. Among various electrode materials, 2D layered materials have shown significant advantages in energy conversion and storage applications, i) high specific surface area, ii) abundant active sites, iii) weak interface force between layer and layer, iv) short ion path length, and so on. [176][177][178] Compared with most 2H TMDs materials, ReX 2 materials with stable 1T′ structure and much weaker interlayer coupling make them highly attractive in the applications of energy conversion and storage devices. In this section, we primarily introduce two major application fields of ReX 2 : catalysis and battery.

Photocatalytic and Electrocatalytic
The suitable bandgap with neglect layer dependence of ReX 2 enables it to be ideal photocatalyst for high-efficiency solar-energy conversion. Based on theory calculations, researchers have found that for monolayer and multilayer ReS 2 , the bandgap and band edge positions are excellently matched with the water-splitting energy levels. Moreover, the effective masses of the carriers are relatively light, and the optical absorption coefficients are high under visible illumination. Above superior properties suggest that ReS 2 should be an efficient photocatalyst for water splitting. In experiment, Fu and coworkers demonstrated a twoelectron catalytic reaction to achieve the efficient reduction of two H + (2H + + 2e − → H 2 ) utilizing the distinct trion behavior of ReS 2 (Figure 13,b). [26] The free electrons in vertical ReS 2 can be captured by the tightly bound excitons to form trions. Notably, the extremely weak interlayer interactions of ReS 2 cause the bulk to behave as electronically decoupled monolayers, which allows ReS 2 to generate numerous trions, even in a multilayer nanosheet. These trions can migrate to the surface and participate in the two-electron reaction at the abundant active sites. . Reproduced with permission. [26] Copyright 2018, Wiley-VCH. c) Polarization curves as well as d) tafel slope and onset overpotentials of different Re-doped MoS 2 catalysts and reference samples. Reproduced with permission. [136] Copyright 2018, Wiley-VCH. e) Schematic illustration of lithium intercalation in V-ReS 2 /3DGF-based lithium-ion battery. f) Schematic illustration of the weak interlayer interaction of ReS 2 , compared with MoS 2 and graphite. Reproduced with permission. [25] Copyright 2018, Wiley-VCH.
With this approach, a superior photocatalytic hydrogen evolution rate of 13 mmol g −1 h −1 was achieved under visible light irradiation, which surpassed the rates of most reported TMDs composite photocatalysts.
Interestingly, the anisotropic structure of ReX 2 endows them polarization-dependent photocatalytic degradation behavior. [179] The enhanced photocatalytic behavior of the layered ReS 2 with optical polarization along the b-axis demonstrated that the formation of a nano-diamond-chain and a one-layer trigonal crystalline phase is beneficial for the versatile energy applications of ReX 2 . Moreover, strain engineering of ReX 2 can effectively turn the band structure and density of states, [48] which could be considered as a factor to improve the photocatalytic performance. A theoretical study has predicted that 1-5% y-axial tensile strain can greatly enhance the performance of single-layer ReS 2 for overall photocatalytic water splitting. [180] Moreover, a dual-enhancement in ReSe 2 nanosheets with high concentration of active sites and efficient use of hot electrons is simultaneously achieved with moderate Mo doping. [181] Constructing nanocomposites of ReX 2 with other semiconductor catalytics to realize the co-catalytic is another route to elevate its photocatalysts performance. [182][183][184] The TiO 2 @ReS 2 nanocomposites present significantly enhanced degradation activity of organic pigments under sunlight illumination in comparison with pure TiO 2 nanoparticles. [184] The underlying mechanism of enhanced photocatalytic activity was attributed to the improved separation efficiency of photogenerated electron-hole pairs.
Besides the superior photocatalytic performance, ReX 2 has been considered as an ideal catalyst for the electrocatalytic hydrogen evolution reaction (HER). [123,185] On the one hand, the atomic sites on edges of ReS 2 nanosheets have dangling and unsaturated coordination bonds, which are the primary active sites of HER. On the other hand, the reduced dimension resulting in the extensive exposure of ReX 2 edges, and thus the specific surface area is enhanced effectively. In particular, as discussed in Section 3.5.3, the unusual out-of-plane growth and off-symmetry growth of ReX 2 provide more feasibilities to tailored design the materials with diverse structures for high performance HER. For example, Zhang and coworkers synthesized monolayer ReSe 2 with a sunflower-like shape on SiO 2 /Si substrate via CVD growth, and then transferred it onto Au foil to construct the ReSe 2 /Au electrode for HER. [185] The edge-abundant sunflower-shaped ReSe 2 nanosheets originating from the off-symmetry growth exhibited superior electrocatalytic HER activity, featuring a relatively low overpotential of 270 mV and Tafel slope of ≈76 mV dec −1 and an exchange current density of 10.5 µA cm −2 , which were superior to those of compact truncated-triangle-shaped ReS 2 nanosheets. Certainly, the catalysis performance of CVD-grown planar ReX 2 nanosheet is limited by the tiny amount of electrode materials. Koratkar and coworkers prepared vertically oriented arrays of 3D ReS 2 nanosheets via CVD growth to increase the amount of electrode materials and the exposed surface area, sharp edges and corners. [123] As HER catalysts, the vertically oriented arrays of ReS 2 provided very small onset overpotential (<100 mV) and an exceptional exchange-current density (≈67.6 µA cm −2 ), which is vastly superior to the planar ReS 2 electrode. Besides engineering the edges of ReX 2 to improve its catalytic activity, how to increase the active site at the basal plane of ReX 2 is currently a subject of great interest and intensive investigation. DFT calculation has shown that the catalytic activity of the ReS 2 basal plane can be efficiently activated by doping with transition metal atoms (such as Mo, Cr, Mn, Fe, Co, Pt, Au, and Ag). [186] The predicated performance improvement is attributed to the decreased hydrogen adsorption free energy (ΔG H* ) and exposed active sites by introducing more unsaturated electrons. Experimentally, Zhou and coworkers indeed found that the normally inert basal planes of 2D layers of ReS 2 (MoS 2 ) can generate highly catalytic activity for the HER when alloyed with Mo (Re). [136] The CVD-grown 1T′ Re 0.55 Mo 0.45 S 2 alloy as HER catalyst has a much lower onset potential of 90 mV and a small Tafel slope of 56 mV dec −1 , which are compared to most of the 1T′ MoS 2 -based catalysts (Figure 13c,d). Moreover, introducing defect is another effective way to enhance the HER performance of ReS 2 basal plane. [121][122] Abundant Re atoms vacancies were produced by visible-light assisted pre-electrolysis treatment, which can effectively decrease the overpotential from 170 to 88 mV compared with defect-free ReS 2 . [122] Theoretical calculations confirmed that the abundant Re atoms vacancies with lower Gibbs free energy for H adsorption activate the inert basal plane as highly active sites for hydrogen evolution.
As is well known, the conductivity of electrode materials is an important parameter for HER performance. From the perspective of improving contact between catalyst and electrode, several works have explored the direct CVD growth of vertically aligned ReX 2 nanosheets on conductive substrates like carbon cloth and glass carbon to improve the HER performance. [123,187] Recently, Pumera and coworkers developed a simple electrodeposition approach using NH 4 ReS 2 precursor to synthesize the electrocatalyst ReS 2 on 3D and 2D-printed carbon electrodes. [188] Additionally, the HER performance of ReX 2 can also be improved by the preparation of composites containing conductive additives, such as ReS 2 @ReO 2 , ReS 2 @TiO 2 , and ReS 2 @RGO (reduced oxidation graphene) composite electrode materials. [183,[189][190] Overall, these positive results demonstrate that ReX 2 is a superior dual functional, photo-and electrocatalytic material. In the future, further exploring various methods such as heteroatom doping, defect engineering, phase engineering and interface structure optimization is encouraged to advance the catalysis activity of ReX 2 . Moreover, band structure or Fermi energy level tuning should be an effective route to improve the photocatalysis performance. In addition to the preceding applications in photocatalytic and HER, we have faith that ReX 2 will emerge to extend the scope of applications in other catalytic systems.

Lithium-Ion Battery
Lithium-ion battery (LIB), an important energy storage technology, has attracted tremendous attention in recent years owing to its wide applications in portable electronics, power tools and environmentally friendly transportation, and so on. [175] Developing alternative anode materials with higher energy density, better rate capability and longer cycling life to replace the traditionally and commercially used graphite is of increasing interest to fulfill the growing demands for LIBs. Among various candidates, 2D layered materials including graphene and TMDs have shown great promise as anode materials of LIBs due to their high conductivity and natural layered structures for ion intercalation. [177] In particular, ReX 2 appears to have great potential in high-current-density LIBs because of its extremely weak interlayer coupling compared with graphene and other 2D TMDs materials. [37] For instance, the interlayer coupling of ReS 2 is about 18 meV per unit cell, while that of MoS 2 is about 460 meV for comparison. [13] The weak interlayer coupling and the larger interlayer spacing afford new opportunities for massive alkali-metal ions to efficiently diffuse without significant volume expansion. This is a highly significant advantage over other 2D layered materials. Theory calculation shows the capacity value for Li + storage of pure ReS 2 can up to ≈430 mAh g −1 . As thus, ReX 2 has been widely utilized as anode material to construct high-performance LIBs. In this section, we provide an overview of the recent progresses of LIBs based on ReX 2 electrode materials, including the structure design, composite ingredients, reaction mechanism, and electrochemical performance.
As mentioned in section 3.5.3, vertically oriented 3D ReS 2 nanosheets can increase catalytic active sites, here, this structural feature can also significantly improve the performance of ReS 2based Li-S battery. [123] The specific capacity of the battery is retained above 750 mAh g −1 after 300 cycles with only ≈0.063% capacity decay per cycle, which is much better than the baseline battery (without ReS 2 ) with ≈0.184% capacity decay per cycle under the same test conditions. Utilizing the extremely weak van der Waals coupling of ReX 2 , Fu and coworkers demonstrated a highcurrent-density LIBs by synthesizing uniformly distributed vertical ReS 2 nanowalls (V-ReS 2 ) on 3D graphene foam (3DGF) via CVD growth (Figure 13e,f). [25] The 3DGF was used as a template to enhance the conductivity and enlarge the specific surface area of the whole electrode material. This favorable vertical structure shortens the pathways and facilitates the fast diffusion of both Li + and electrolyte ions in the battery system as schematically shown in Figure 13e. As a result, the V-ReS 2 /3DGF composite exhibited good cycling stability at high-current-densities (1000 mA g −1 ) when serving as anode material for LIBs. Especially, at the high current density of 1000 mA g −1 , its capacity still maintained over 200 mAh g −1 even after 500 cycles. To further enhance the conductivity and ion diffusion rate for improving both high-rate capability and long life, a series of ReS 2 /carbon composite electrode materials were constructed. [27,[191][192][193] For example, Qiao and coworkers fabricated a rGO@ReS 2 @N-C composite where ReS 2 is confined in 2D-honeycombed carbon nanosheets composed of rGO and N-doped carbon coating layer. [27] The rGO@ReS 2 @N-C composites achieved rate performance of 231 mAh g −1 at 10 A g −1 , together with ultra-stable long-term cycling of 192 mAh g −1 at 2 A g −1 after 4000 cycles.
Additionally, enlarging the interlayer spacing of ReS 2 is conducive to the diffusion and migration of alkali-metal ions through electrode materials. Park and coworkers demonstrated oxygen incorporation and interlayer expansion of ReS 2 nanosheets deposited on hollow mesoporous carbon spheres, which facilitates the redox kinetics by means of the improved electronic conductivity and rapid ion diffusion. [194] With this approach, the composite electrode displayed a stable operation up to 10 A g −1 and a resilient recovery of capacity near 318 mAh g −1 . Besides, the role of crystallographic orientation and anisotropy in the Na + and Li + battery electrochemistry was investigated at the atomic scale via in-situ TEM. [195] It was found that the reaction speed of lithiation/sodiation of ReS 2 is highly anisotropic, occurring faster along the in-plane ReS 2 layer than along the out-of-plane Figure 14. a) Schematic illustration of the experimental setup for understanding the Raman enhancement mechanism on ReS 2 nanosheets. b) Raman spectra of 1 nm thick CuPc on ReS 2 films with different layer numbers and SiO 2 /Si substrate. c) Concentration-dependent Raman spectra of different fluorescent dyes adsorbed on monolayer ReS 2 . Reproduced with permission. [200] Copyright 2018, Wiley-VCH. d) Energy diagram of charge transfer process between R6G molecule and single-layer (1L) and few-layer (FL) ReS 2 nanosheets, reproduced with permission. e) Raman spectra of CuPc molecules on ReS 2 /gold/PMMA/300 nm SiO 2 /Si substrate. f) Polar plots of the normalized intensities of 1450 cm −1 (CuPc, B 2g ) modes as a function of sample rotation angle on ReS 2 . Reproduced with permission. [202] Copyright 2019, Tsinghua University Press and Springer Nature.
direction. This work suggests that crystallographic orientation in the highly anisotropic ReS 2 electrode materials can be exploited as a design parameter to further improve battery performance.
On the whole, great progresses have been achieved in the applications of ReX 2 in LIBs, including the synthesis of vertical 3D nanoarrays, the expansion of interlayer spacing, and the enhancement of conductance via constructing various composites, etc. Certainly, there is still considerable room for further elevating the specific capacity and current-density of ReX 2 -based LIBs. For instance, developing effective synthesis approach to improve the electrical contact between ReS 2 and electrode is highly desirable. An in-depth understanding of ions diffusion and migration along different crystallographic orientations of the anisotropic ReS 2 is significantly critical to effectively design and synthesize the material with preferential structure architectures. Moreover, the study of battery application of ReSe 2 is still blank, which is worth exploring in the future.

Raman Enhancement
In the past ten years, 2D materials including graphene and TMDs have been widely used as SERS substrates, which show great application prospect due to their good electrical and thermal conductivity, atomic level flatness, and chemical stability. [196][197][198][199] Dif-ferent from 2H TMDs materials, the 1T′ phase ReX 2 with low lattice symmetry as a SERS substrate endow more degree of freedom for exploring novel enhancement effects.
Xu and coworkers adopted a series of probe molecular including phthalocyanine (CuPc), rhodamine 6G, rhodamine B (RhB), crystal violet (CV) and methylene blue (MB) to investigate the Raman enhancement effect and possible enhancement mechanism of ReS 2 substrate (Figure 14a). [200] The Raman signal of CuPc molecular on the ReS 2 film decreases with the increasing of layer numbers (Figure 14b), indicating strong layer-numberdependence of the Raman enhancement effect. The Raman enhancement factor of CuPc on monolayer ReS 2 is 2.94. Moreover, Raman signals of probe molecule with different concentrations on monolayer ReS 2 show that the detection limit of R6G and MB molecular as low as 10 −9 m (Figure 14c), demonstrating the feasibility of utilizing monolayer ReS 2 as highly sensitive Raman enhancement substrate for detection applications. Furthermore, combining Raman and photoluminescence studies with the assistance of an Al 2 O 3 dielectric layer strongly suggests that the Raman enhancement effect of molecular on ReS 2 originates from a charge transfer process rather than from an energy transfer process. Figure 14d presents the energy diagram of charge transfer process between R6G molecule and monolayer as well as fewlayer ReS 2 , in which the degree of charge transfer from R6G to ReS 2 is heavily impacted by the band structure of ReS 2 . Furthermore, the authors also performed the angle-resolved polarized Raman spectra of CuPc on monolayer ReS 2 , but they haven't observed obvious polarization-dependence Raman enhancement effect. This should be attributed to the existence of abundant nanosized subdomains with different lattice orientations in the CVD-grown ReS 2 film as demonstrated in the preparation section.
Using mechanically exfoliated high-quality ReS 2 as substrate, Zhang and coworkers reported the polarization dependence of the enhanced Raman scattering of CuPc molecules, which is attributed to the different charge transfer probabilities between ReS 2 and molecules oriented along different crystalline directions. [201] To further understand the anisotropic Raman enhancement mechanism of the ReS 2 substrate, they investigated the doping modulated in-plane anisotropic Raman enhancement by stacking monolayer ReS 2 on a flat gold film or graphene, where the electrons doping in ReS 2 can decrease the anisotropic Raman enhancement. [202] The degeneration of anisotropy Raman enhancement was attributed to the electrons transferring from the Fermi level of gold/graphene to the conduction band of ReS 2 , which increased the electrons density in the conduction band and thus decreased the probability of charge transfer from CuPc molecules with major axes parallel to the b-axis of ReS 2 . These unique anisotropic charge interactions between molecules and ReX 2 not only provide new insights into the chemical enhancement mechanism in SERS, but also suggest new application perspectives in optoelectronics, such as polarizationcontrolled molecular electronic switches.

Sensor
2D ReX 2 materials with high surface-to-volume ratio and semiconductor characteristic make them an ideal building block and platform to construct diverse sensors, because changing their surrounding environment can modulate their electronic, optoelectronic and chemical properties. [203] Hence, the design and fabrication of ReX 2 -based materials for sensor applications have intrigued considerable attention, and several novel ReX 2 -based sensors have been developed recently. [28,[204][205] Fu's group presented a smart self-adapting wettability (SAW) of ReS 2 under sustaining light irradiation, where the ReS 2 film was grown on the Si/SiO 2 substrate via CVD growth. [204] Figure 15a shows the schematic diagram of the SAW process, and the corresponding contact angle variation reflects the wetting behavior during the whole process (Figure 15b). The surface of ReS 2 realized an automatic transition of hydrophobichydrophilic-hydrophobic state, indicating a self-adapting wettability behavior. The SAW process could be divided into three considered procedures: hydroxyl substitution, formation of hydrogen bonds, and water desorption (Figure 15c). Notably, the SAW behavior achieved in ReS 2 breaks the stereotype that a single stimulus leads to a monotonic change in structures or properties. Using the CVD-grown ReS 2 nanowalls, Fu and coworkers further presented an interesting temperature-responsive behavior that was triggered by stable and reversible thermally induced bending (TIB) (Figure 15d). [28] During the heating process, deformation and interlayer sliding occur successively in the ReS 2 nanowalls, which were reflected in the contact angle tests, and its wetting ability increases with increasing temperature (Figure 15e). Notably, the flexibility of ReS 2 enables its nanosheet to recover to the initial configuration when the temperature drops, and show good cycling stability during multiple TIB process (Figure 15f). Above all, the unique SAW and TIB behaviors provide new insight to broaden the applications of ReX 2 materials, and open up new perspectives for multifunctional intelligent devices. By combining the solution-based synthesis with surface functionalization strategies, the feasibility of colloidal ReS 2 nanosheet films for sensing different gases (such as NH 3 , H 2 O, CO 2 in Figure 15g, alcohol, acetone and dry air in Figure 15h) is demonstrated with highly competitive performance in comparison with devices built with CVD-grown ReS 2 and MoS 2 . [205] Moreover, a pH sensor was achieved via a bilayer ReS 2 FET with HfO 2 as a sensing oxide (Figure 15i). [206] It is observed that I ds increases stepwise with discrete changes in pH value from 8.11 to 3.21 and quickly goes to a stable value at each pH value (Figure 15j), demonstrating the fast response characteristic. The excellent gas and pH sensing performance suggest the great potential of ReX 2 in hazardous and toxic sensor and biosensor applications. Therefore, ReS 2 has been demonstrated as a promising material for constructing diverse sensors to respond with temperature, wetness and a few chemicals, which still have considerable room for improvement by exploiting device designs, finely tuned structures and surface modifications.
Besides the above applications, 2D ReX 2 materials have been demonstrated to show remarkable elastic modulus and large strain before fracture. [48] Applying mechanical strain to ReX 2 can hugely modulate their electronic conduction and optical behavior. Especially, the different crystalline directions of anisotropic ReX 2 respond differently to the uniaxial strain, which indicates distinct strain-electronic structure interaction along anisotropic directions. A recent study shows that ReS 2 has completely opposite (positive and negative) piezoresistance along two principal axes, which differed from any previously reported anisotropic piezoresistive effect in other 2D materials. [207] The novel property is ideally useful in accurately sense/recognize multidimensional strain signals for the development of strain sensors, conductance-switch FETs, electronic skin, human-machine interfaces, etc. Furthermore, theory calculations show that doping, strain or high pressure of ReX 2 could induce robust magnetic properties and even superconduction, [51,[208][209] which provide tremendous potential for novel applications such as ultrasensitive sensors, nanoscale electro-mechanical systems and spintronic devices for information storage.

Summary and Perspectives
Within just a few years, there has been a rapid surge of interest and tremendous progress for the attractive ReX 2 material. Besides the communal properties similar to other 2D TMDs, the unusual 1T′ structure of ReX 2 endows them abundant exciting physicochemical properties, such as weak interlayer coupling and strong anisotropic properties. As a result, it has been shown to have potential for a wide range of important applications including various solid state devices, SERS, sensors, catalytic hydrogen production and lithium ion batteries. Despite it has achieved considerable progress and been revealed extraordinary application potential, the research about ReX 2 is still in its infancy stage, and many challenges and opportunities deserve to be further explored. It mainly includes the following aspects:  [204] Copyright 2018, Wiley-VCH. d) Schematic representing the TIB process of the ReS 2 nanowalls. e) Wetting characteristics of the ReS 2 nanowalls with increasing temperature. f) The wetting switch is reversible for 10 cycles. Reproduced with permission. [28] Copyright 2018, Wiley-VCH. g,h) Representative gas-induced time response of the device built with ReS 2 exposed to different gases. Reproduced with permission. [205] Copyright 2019, Wiley-VCH. i) Schematic diagram of a ReS 2 FET for pH sensing. j) Response curve of ReS 2 for pH sensing. Reproduced with permission. [206] Copyright 2018, American Chemical Society.

Properties
The low crystal symmetry of ReX 2 makes it an anisotropic 2D material which exhibits strong anisotropic electrical, optical, vibrational, thermal, and magnetic properties. These unique properties of ReX 2 open up excellent potential for designing conceptually new devices where the strong anisotropic properties are required. Certainly, the anisotropy ratio of ReX 2 needs to be further enhanced to fulfill the real applications, which could be realized by applying strain or coupling with the anisotropic plasma nanostructures. Furthermore, it has been shown that doping metal elements into ReS 2 is an effective knob for tuning the magnetic properties, which broadens the way to realize new functional devices.
Tuning optical, electrical and magnetic properties of ReX 2 by strain engineering is still at the theory stage. Strain engineering is a unique technique for tuning physical and chemical properties at nanoscale for 2D materials. For example, wrinkle-generated local strain in ReS 2 modulates optical gap, enhances light emission, induces magnetism, and alters the electrical properties. Whilst precise control the intensity, direction and manner (tensile or compression) of applied strain is critical but challenging to the controlled modulate the properties of anisotropic ReX 2 . Obviously, carrying out deep experimental research for ReX 2 is needed. Furthermore, the weak interlayer coupling of ReX 2 facilitates the intercalation of functional ions provides a great opportunity to explore the intercalation chemistry, a new research direction to modify the properties of 2D materials.

Preparation
Despite tremendous progress have been achieved in the preparation of ReX 2 , the controlled synthesis of single crystal ReX 2 still faces a great challenge. As demonstrated in Section 3.5, the Re4chain reconstruction easily occurs during the synthesis of ReX 2 , www.advancedsciencenews.com www.advancedscience.com which brings plenty of subdomains and GBs in the CVD-grown ReX 2 . These GBs seriously degrade the properties of ReX 2 and thus hinders their application in large-scale optoelectronic devices. Moreover, the attractive anisotropy feature of ReX 2 makes their device applications have higher requirements for the lattice orientation than that of other 2D TMDs, as the random orientations of different ReX 2 domains inevitably bring heterogeneous properties. Hence, developing effective approaches to synthesize large-scale ReX 2 single crystal domains with uniform orientation is highly desired, and that should be the primary task in future research work. We believe, once the above goal is achieved, the optical and electrical properties of CVD-grown ReX 2 would have huge room to be further improved.
From another point of view, effectively utilizing these GBs would burst out new potential of ReX 2 . As is well known, GBs can provide a platform to explore emerging properties that are not manifest in a single crystal, since atomic displacement at GB can modify its electronic state to a large extent. Many novel properties such as free-electron creation, intrinsic magnetism, tunable bandgap, excitonic relaxation, memristive effect, and enhanced catalysis have been revealed in the GB of 2H TMDs. [210][211][212][213][214][215] As for 1T′ ReX 2 , the abundant GB structures provide rare opportunities to explore more novel properties and tailored design of them with desired properties. In this regard, it's worth further exploring the properties and applications of GBs in ReX 2 from both theory and experiment aspects.
The unusual and stable 1T′ structure and the distinctive properties of ReX 2 offer both opportunities and challenges for the integration of them with other TMDs materials to build many novel 2D structures for engineering their phase structures, energy band structures, properties and device applications. Moreover, the naturally out-of-plane growth tendency of ReX 2 provides a great chance to construct many novel 3D nanostructures which are highly desired in catalysis and sensor applications. Though initially progresses have been achieved in all above aspects, they are far away from the expectations, and thus further in-depth studies are required. All in all, controlled preparation is the key that determines the future of ReX 2 .

Application
Photodetectors based on ReX 2 show competitive performance compared with other TMDs in terms of the responsivity and EQE, especially in the ability to detect polarized light. Surface modification and plasma treatment were demonstrated to improve device performance. However, this improvement is not sufficient. An in-depth understanding of the photocurrent and carrier recombination mechanisms in ReX 2 is needed for further improving the performance of their optoelectronic devices to satisfy the practical applications. Moreover, line dichroism absorption, polarized excitons and selectively tunable optical Stark effects have been experimentally achieved in ReX 2 , which could provide more degree of freedom for designing novel multi-functional optoelectronic devices. For the energy conversion and storage, there are several possible strategies to further improve their performance: i) increase the active sites via introducing dopant or vacancy, ii) improve the conductivity via constructing heterostructures with other high conductivity materials, iii) optimize the ma-terials structures through effectively utilizing the off-symmetry growth and out-of-plane growth features of ReX 2 to build 3D vertical arrays with high surface area and large interlayer space. Moreover, ReX 2 has large exposed surface areas available for the material-analyte interaction, suitable active sites for effective and selective analyte binding, the ability to transduce the binding events into a detectable signal, as well as good mechanical and processing properties. These features enable ReX 2 huge potential for diverse sensor applications. As described above, ReS 2 is currently responsive to moisture and temperature, while other stimuli-responsive fields such as electricity, gas, as well as living systems-related taste, sight, touch and hearing urgently need to be explored. Some real-world smart sensor applications like intelligent wearable electronic devices, biosensors, and printing sensors may need ReX 2 material to fill this gap. Furthermore, owing to the intrinsic anisotropy in electronic properties, ReX 2 may realize a dissimilarly synaptic response of multiterminal threeterminal electrolyte-gated transistor devices, which exhibits great potential for developing an artificial system to mimic the behaviors of the human brain. Among the above potential applications, the intelligent sensors may take an early lead for practical application, because diverse intelligent sensors are highly required to satisfy the demand of modern life, and they have relatively lower requirements to the key materials compared with that for electronic device like a chip. Certainly, element Re is not earthabundant and relatively expensive. In this regard, the technoeconomic feasibility should be considered during developing the applications of Re-based TMDs. Developing the killer application of ReX 2 should be encouraged.
In a word, the above features that enable ReX 2 to be unique and deserve to be studied in depth. Here, ReX 2 is just a more general representation of other TMDs or those similarly low symmetry 2D materials. We believe that this review article will promote the exploration of similar 2D materials and accelerate the development of nanomaterials science.