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Low-Symmetry Two-Dimensional ReS2 and its Heterostructures:Chemical Vapor Deposition Synthesis and Properties

Hui Zhang , Shanshan Wang , Jinshan Yu

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (6) : 1440 -1452.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (6) : 1440-1452 DOI: 10.7536/PC210846
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Low-Symmetry Two-Dimensional ReS2 and its Heterostructures:Chemical Vapor Deposition Synthesis and Properties
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Abstract

Two-dimensional (2D) rhenium disulfide (ReS2) is a layer-structured functional nanomaterial with atomic thickness and few lattice symmetry elements. The low symmetry of the crystal structure endows 2D ReS2 with rich anisotropic physical and chemical properties, giving it great potential in the fieldsof nanophotonics, tactile sensors, and anisotropic electronic devices. The applications of 2D ReS2 rely on high-quality synthesis and a deep understanding of its properties. This review firstly categorizes the chemical vapor deposition (CVD) methods of ReS2 into three groups based on the types of the metallic and non-metallic precursors applied in the growth, as well as the substrates. Different CVD strategies and the corresponding growth mechanisms are systematically summarized. Subsequently, the recent progress in the preparation of ReS2 in-plane and vertical 2D heterostructures is introduced. Approaches are divided into “one-step method” and “two-step method” based on the number of steps used in the CVD process. Finally, the anisotropic optical and electronic properties of 2D ReS2 are discussed. This review also puts forward an outlook on the challenges and opportunities of the synthesis and property investigation of 2D ReS2.

Contents

1 Introduction

2 CVD growth of 2D ReS2and its heterostructures

2.1 CVD growth of 2D ReS2

2.2 CVD growth of ReS2-based heterostructures

3 Properties of 2D ReS2

3.1 Vibrational and optical properties

3.2 Electrical properties

4 Summary and Prospect

Graphical abstract

Keywords

ReS2 / chemical vapor deposition / heterostructures / low symmetry / anisotropy

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Hui Zhang, Shanshan Wang, Jinshan Yu. Low-Symmetry Two-Dimensional ReS2 and its Heterostructures:Chemical Vapor Deposition Synthesis and Properties. Progress in Chemistry, 2022, 34(6): 1440-1452 DOI:10.7536/PC210846

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1 Introduction

The discovery of graphene has opened a whole new era in the study of two-dimensional materials[1⇓~3]. Researchers have realized that dimensional control can significantly change the physical and chemical properties of materials and stimulate new properties of materials, and a large number of new two-dimensional materials have emerged[4⇓~6]. Among them, two-dimensional Transition metal dichalcogenides (TMDs) have become an important branch of two-dimensional materials research because of their abundant elements, diverse crystal structures, excellent optical, electrical, sensing, catalytic and other properties[7⇓~9].
In recent years, rhenium disulfide (ReS2), a member of the family of two-dimensional TMDs, has attracted much attention because of its unique low-symmetry crystal structure[10⇓~12]. As shown in Figure 1A, the monolayer ReS2 is composed of three atomic layers of S-Re-S, and the upper and lower layers of S atoms sandwich the Re atomic layer to form a "sandwich" structure, with strong chemical bonds in the layers and weak van der Waals forces between the layers[13]. Different from a large number of two-dimensional TMDs with hexagonal high symmetry, ReS2 belongs to the triclinic system with the lowest symmetry among the seven major crystal systems, and has only one symmetry element, the center of symmetry, showing a distorted 1T 'phase structure. As shown in Figure 1b, the Re atom is in octahedral coordination, and the redundant valence electrons in the 5d orbital cause Peierls distortion, and the four adjacent Re atoms in the layer are connected by metallic bonds, so the Re atoms are displaced to form a Re4 atom cluster, which causes the original hexagon in the ReS2 crystal structure to be distorted into a parallelogram[14][15].
Due to the low symmetry of lattice structure and the weak coupling between layers, two-dimensional Chemical vapor deposition crystals prepared by Chemical vapor deposition (CVD) exhibit many unique growth behaviors, including asymmetric growth, out-of-plane growth and nano-assembly growth[13,16,17]. In addition, the reduction of structural symmetry also brings unique optical and electrical properties to ReS2, which shows great potential in the fields of photoelectric detection devices and new logic devices[15,18⇓~20]. In this paper, the CVD fabrication methods of two-dimensional ReS2 and their heterojunctions are systematically summarized, and the special optical and electrical properties caused by the low symmetry of the structure are discussed.Finally, the CVD fabrication methods of two-dimensional ReS2 and their heterojunctions are prospected.

2 CVD process for preparing 2D ReS2 and its heterojunction

The common preparation methods of two-dimensional materials include mechanical exfoliation, liquid phase exfoliation and vapor deposition[21,22][23][7]. The CVD method shows significant overall advantages in terms of preparation efficiency, energy consumption, and product quality[8,24,25]. As shown in fig. 2, the CVD process can be generally divided into the following steps: first, a gaseous precursor containing the elements required for the two-dimensional product is transferred to the reaction zone (step a)[26]; Then, the gaseous precursor reacts homogeneously in the gas phase to produce intermediate products and gaseous by-products (step d), or is adsorbed on the boundary layer of the substrate surface, and surface diffusion and migration occur at a suitable temperature (steps B, C); Ubsequently, the adsorbed precursor or intermediate product is subjected to further chemical reaction on the surface of the substrate to realize heterogeneous nucleation and growth to obtain a two-dimensional product (step e); Finally, the by-products desorb from the substrate surface and exit the system with the carrier gas (step f). At present, the CVD method has basically realized the high-efficiency, low-cost and controllable synthesis of two-dimensional materials with high symmetry, such as graphene and molybdenum sulfide, but due to the particularity of the crystal structure of two-dimensional ReS2 and the fact that the research is still in its early stage, there are still many challenges in the synthesis of products with large grain size, high crystallinity, uniform thickness and high coverage by CVD method[27⇓⇓~30]. In order to optimize the preparation results and realize the large-scale application of ReS2, we can start from the basic steps of CVD process: on the one hand, changing the type of precursor can improve the efficiency and uniformity of gas phase mass transfer process; On the other hand, the surface characteristics of the substrate can be controlled by selecting the appropriate substrate or modifying the substrate, so as to affect the adsorption, migration and nucleation of the precursor or intermediate on the substrate surface. In this paper, the preparation strategies of two-dimensional ReS2 are systematically summarized from two main lines: the optimization of precursor types and the substrate engineering, and the preparation of two-dimensional ReS2 heterojunction by CVD method is introduced.Finally, the unique optical and electrical properties of two-dimensional ReS2 are discussed.

2.1 CVD method for preparing two-dimensional ReS2 film

The preparation of two-dimensional ReS2 by chemical vapor deposition (CVD) is usually based on the principle of gas phase reaction, and the nucleation density, grain size and growth thickness of the product can be precisely controlled by fine adjustment of the parameters such as precursor gas concentration, growth temperature, pressure and growth time. In addition, selecting a substrate with appropriate surface structure can regulate the interfacial interaction, change the adsorption energy and migration barrier, and affect the nucleation and growth process of the product. By studying the mechanism of CVD process, the large area, high quality and controllable preparation of two-dimensional ReS2 can be realized.

2.1.1 Control of metal precursors containing rhenium

Ammonium perrhenate (NH4ReO4), rhenium (Re) powder, and rhenium trioxide (ReO3) are currently the three most common solid rhenium-containing precursors for the preparation of two-dimensional ReS2. The researchers volatilize the solid precursor into a gaseous state by high temperature preheating, and transfer the mass to the vicinity of the substrate with the assistance of the carrier gas, so as to realize the subsequent growth of two-dimensional materials. Therefore, how to control the concentration, stability and spatial uniformity of precursor volatilization and diffusion has become the focus of researchers. On the one hand, the melting point of the solid precursor determines the degree of its volatilization and affects the vapor pressure of the precursor in the system at a certain heating temperature, so the sufficient supply of the gaseous precursor in the system can be ensured by reducing the melting point of the precursor; On the other hand, too high precursor concentration in the system will lead to too high nucleation density and easy to obtain thick layer products. Therefore, obtaining the gaseous precursor with appropriate concentration and stable supply is one of the key factors to control the quality of ReS2 preparation.
Ajayan et al. Selected the reaction of NH4ReO4 with sulfur (S) vapor to realize the large area controlled synthesis of monolayer ReS2 under atmospheric pressure for the first time[17]. The melting point of NH4ReO4 is about 365 ℃, as shown in Fig. 3A. When NH4ReO4 is heated at 450 ℃, argon (Ar) is used as the carrier gas to transport S vapor to the reaction area. The precursor is adsorbed and migrated on the surface of the substrate. When the concentration exceeds the critical nucleation concentration, it will nucleate and grow at the active sites, resulting in monolayer ReS2 grains with sizes ranging from 2 to 60 μm (Fig. 3B). The NH4ReO4 with low melting point is selected as the precursor to increase the gaseous concentration of the precursor and reduce the reaction temperature, and a nearly hexagonal or circular monolayer product can be obtained at a temperature of 400 to 450 deg C. Scanning transmission electron microscopy (STEM) can clearly characterize the Re4 atomic chain of Scanning transmission electron microscopy, and it can be seen that there are a few vacancy defects in the product, which reduces the local atomic arrangement periodicity of Scanning transmission electron microscopy (Figure 3C). The average mobility of the two-dimensional ReS2 field effect transistor fabricated based on this method is 7.2×10-2cm2·V-1·s-1.
Liu et al. Chose metal Re to react with gaseous S to form two-dimensional ReS2 or nanoribbons on the surface of SiO2/Si at 750 ° C (Fig. 3D, e)[31]. From the High resolution transmission electron microscope (HRTEM) of Fig. 3F ~ G, it can be seen that the Re chain atoms are arranged clearly and completely, indicating that the High resolution transmission electron microscope directly grown under this condition has good crystallinity. Therefore, the obtained ReS2 has a current on-off ratio of about 106 and a carrier mobility of 9.3 cm2·V-1·s-1, which is equivalent to the product prepared by the mechanical exfoliation method. In addition, multilayer films, monolayer films and multilayer nanoribbons with ReS2 can also be obtained by adjusting the growth time. It is worth noting that the reaction temperature of CVD system is mostly not more than 1100 ℃, while the melting point of Re is as high as 3180 ℃, so the direct use of Re as a precursor has the problems of difficult volatilization and low concentration of gaseous precursor in the system, which makes it difficult to achieve efficient synthesis of large-area ReS2.
For this reason, researchers have developed three strategies to solve the problems of high melting point and low volatility of Re: low melting point eutectic method, alkali metal salt auxiliary method and Re and oxide blending method. Xu et al. Innovatively added tellurium (Te) powder to Re powder to form Re-Te binary low melting point eutectic[32]. By adjusting the proportion of Te added, the melting point of the Re source can be reduced to 850 deg C or even 430 deg C, and the gaseous concentration of the precursor is increased. As shown in Fig. 3 H, the large-area and high-crystallinity ReS2 can be synthesized on mica substrate by mixing Re powder with Te in a certain proportion under atmospheric pressure with Ar as the carrier gas (Fig. 3 I). It is worth noting that the weak oxidizing Te introduced in the S-rich environment will be carried out of the reaction system with the carrier gas, which will not affect the purity or crystal structure of the ReS2 products such as doping. This conclusion was verified by Raman spectroscopy and X-ray photo electron spectroscopy (XPS). No characteristic signal peak of Te was observed in the Raman and XPS spectra, indicating that there was no Te element in the product. The Selected area electron diffraction (SAED) image shows only one set of diffraction spots corresponding to the Selected area electron diffraction crystal form, which proves that the product is a single crystal. In addition, well-structured Re chain quadrilaterals can be observed in the HRTEM image, indicating that the crystallinity of the synthesized ReS2 is good (Fig. 3j, K). In addition to using Re-Te binary blend to reduce the melting point, Li et al. Also controlled the precursor concentration by substrate interlayer confinement, and obtained a stable flow rate environment[33]. ReS2 films from monolayer to few-layer were synthesized on mica with chemically inert surface by varying the distance between the substrate and the precursor. In addition, the addition of alkali metal compounds to the Re powder can also improve the volatility of the Re source. Liu et al. Heated the mixture of Re and KI powder to form a low-melting point intermediate product containing Re, and synthesized monolayer ReS2 grains at 650 – 750 ° C, the grain size could reach 50 μm, and the high mobility of 6.5 cm2·V-1·s-1 was realized[34]. Liu et al. Used a similar method to obtain a single-layer ReS2 film with an area of 100 microns on a SiO2/Si[35]. Re vacancies can be introduced into ReS2 by adjusting the precursor concentration ratio and mixing different proportions of H2 into Ar, and the hydrogen adsorption free energy (Δ G H *) of ReS2 in catalytic hydrogen evolution can be close to the optimal value (0 eV) after the introduction of vacancies. In acidic environment, the overpotential of 10 mA·cm-2 is − 147 mV at current density, the Turnover frequency (TOF) per sulfur atom can reach 1~10 s-1, and it has excellent cycling stability. The boundary angles of monolayer ReS2 grains obtained by Zhou et al. With the same preparation method are mostly 60 ° and 120 °[36]. Further study shows that the boundary formation energy of ReS2 along [100] (a axis), [010] (B axis) and [110] directions is the lowest, so the grains will preferentially grow along the main lattice direction[37]. Zhen et al. Used a mixture of high melting point Re and volatile Re2O7 as a rhenium-containing precursor. At the initial stage of the reaction, the Re2O7 was adsorbed on the surface of the freshly exfoliated fluorophlogopite substrate and became the nucleation site for growth. With the increase of the heating temperature of the precursor, the concentration of Re gas increased, which provided abundant Re sources for the growth of crystal nuclei. By prolonging the reaction time, continuous multi-layer ReS2 films could be obtained[38].
In addition to Re element, Re oxide is also a common metal precursor for the synthesis of ReS2. Zhai et al. Selected ReO3 with a melting point of 400 ° C as the Re precursor, introduced reducing hydrogen (H2), and synthesized a large-area uniform double-layer continuous film on the sapphire substrate, and obtained hexagonal ReS2 grains on the SiO2/Si substrate with an average grain size of 2 μm, which were uniformly distributed on the substrate[39]. With the increase of reaction temperature, the dismutation reaction of ReO3 occurs, which rapidly decomposes into volatile Re2O7 and less volatile ReO2, resulting in the rapid increase of Re precursor concentration in the growth system and the uncontrollable growth behavior. In order to alleviate this problem, Xu et al. Used two mica substrates to construct a micro-reaction cavity on the premise of still using ReO3 as the Re source precursor, which limited the concentration of precursor entering the substrate gap (Fig. 3l), thereby reducing the nucleation density and growth rate of ReS2, and synthesized a large area of high-quality uniform monolayer ReS2 on the surface of fluorophlogopite mica (Fig. 3m, n)[16]. In addition, Xu et al. Dispersed the crushed molecular sieve uniformly into the ReO3, controlled the volatilization of rhenium source by the adsorption of the inorganic porous material, realized the uniform supply of rhenium-containing precursor at a suitable growth temperature, prepared a monolayer ReS2 on the surface of freshly cleaved mica by sulfur powder sulfurization, and studied its structural characteristics and growth mechanism from the atomic scale[14].

2.1.2 Control of nonmetallic sulfur-containing precursor

Solid S powder and H2S gas are commonly used sulfur-containing precursors for the preparation of TMDCs materials by CVD, both of which have their own advantages and disadvantages. Using S powder as sulfur-containing precursor has low cost and high experimental safety, but the solid sulfur-containing precursor is easy to form a spatial concentration gradient in the tube after heating and gasification, resulting in uneven two-dimensional product quality at different positions of the substrate. In addition, the partial pressure of S vapor is also easily affected by many factors, such as heating temperature, sulfur powder dosage, powder surface area and so on, which makes it difficult to achieve stable control of different batch conditions. In contrast, the use of gaseous sulfur-containing precursors (such as H2S) has more advantages in the above two points, but the gas has chemical toxicity and has limitations from the safety and environmental protection point of view.
Lee et al. Directly selected the gaseous H2S precursor as the sulfur source to realize the preparation of highly uniform continuous ReS2 films on transparent flexible glass substrates[40]. By controlling the gas flow rate, the precursor concentration can be accurately controlled, and the growth uniformity of the TMDC material can be improved[41]. As shown in Fig. 4A, ReO3 was selected as the precursor, and gaseous H2S was introduced into the low-pressure (10-3Torr) system, and the ReS2 films with different layers were obtained by sulfurization for 30 min (Fig. 4B). The Raman test was carried out at multiple positions randomly selected on the obtained ReS2 film, and the positions and intensities of the spectral peaks were consistent, which proved that the film synthesized by this method was uniform everywhere (Fig. 4C). The ReS2 thin film FET fabricated by this method has typical n-type characteristics, with a threshold voltage of 0.75 V, an on-off ratio of about 105, and a carrier mobility of 0.13 cm2·V-1·s-1. In addition, members of this group grew highly uniform and continuous bilayer ReS2 films on SiO2/Si substrates using the same experimental conditions. Cho et al. Also selected H2S as a sulfur-containing precursor to sulfurize ReO3 at a growth temperature of 450 ° C to synthesize multilayer ReS2 on a SiO2/Si substrate[42]. It is easier to obtain uniform and continuous ReS2 films by selecting H2S as the sulfur-containing precursor, which lays a solid foundation for the application of various devices based on ReS2 materials.

2.1.3 Foundation base engineering

The choice of substrate is very important for the growth of materials, and its type, roughness, surface energy, and interaction force with materials greatly affect the growth conditions and mechanisms of materials. Substrate engineering is to improve the horizontal growth process of two-dimensional materials by selecting appropriate substrates or surface pretreatment of substrates, including optimizing grain size, accelerating growth rate and inducing epitaxial orientation of grains on the surface of crystalline substrates. The adsorption, migration and nucleation of precursor molecules on the substrate surface are closely related to the properties of the substrate surface. For inert substrates such as SiO2/Si, mica and sapphire, surface modification by oxygen plasma treatment or seeding with planar fused ring aromatic molecules (PTAS) can improve the growth process of two-dimensional materials. For example, PTAS is often used to synthesize large-area MoS2 continuous films by CVD, which can change the wettability of the substrate, reduce the free energy of material nucleation and effectively enhance the interaction force between the material and the substrate, thus promoting the layered growth of two-dimensional materials[28,43]. Lee et al. Precoated PTAS uniformly on the surface of SiO2/Si substrate as the nucleation site of ReS2[44]. Under low pressure, the gasified Re and S molecules synthesized a large-area multilayer ReS2 film on the surface of Atomic force microscope containing PTAS (Figure 5A, B). Atomic force microscope (AFM) shows that the thickness of the product is 3.28 nm (Figure 5C). PTAS has a high volatilization temperature and excellent chemical stability, and does not decompose during growth. In addition, because PTAS are macromolecules, it is difficult to substitute or dope atoms in the lattice of ReS2. Before growing ReS2, Tongay et al. Used oxygen plasma to chemically treat the c-plane sapphire substrate to completely remove the contaminants on the substrate surface, and obtained monolayer ReS2 grains at the growth temperature of 490 – 520 ° C[13]. The morphology of the grains can be controlled by adjusting the growth temperature, gas flow rate, etc., resulting in triangular and hexagonal ReS2 (Fig. 5d, e). In the CVD growth process of ReS2, the Re chain orientation is easily reconstructed, so the product is easy to produce a large number of sub-crystal domains and grain boundaries, and the shape of triangle and hexagon is not matched with the low lattice symmetry of ReS2 macroscopically. Further characterization by AFM shows that the synthesized ReS2 grains with different morphologies are all monolayer, and the edges are rough and dendritic (Fig. 5F, G).
The low-symmetry structure of intrinsic ReS2 brings unique in-plane anisotropic optical and electrical properties, which are promising for applications in the field of optical and electrical devices. The prerequisite for obtaining anisotropic physical and chemical properties is that the product is single crystal. However, most of the existing CVD methods can only obtain polycrystalline two-dimensional products which are assembled by grains with different orientations. The randomness of grain orientation averages the overall in-plane anisotropy of ReS2 films. In addition, the dense grain boundaries also seriously weaken the carrier migration performance of the material. Therefore, it is of practical significance to explore the preparation of single crystal two-dimensional ReS2 with controllable orientation. By selecting the substrate with strong interfacial interaction with ReS2, the reconstruction of Re chain orientation can be effectively suppressed to obtain single crystal products. Xu et al. Chose Au foil as the substrate to achieve this goal[45]. They first annealed Au foil at about 1050 ℃ to prepare Au substrate with multiple crystal planes, and then, using ReO3 and S powder as precursors, ReS2 grains were obtained on Au substrates with different symmetries. It is found that the lattice orientation and growth behavior of ReS2 materials are closely related to the lattice symmetry of Au crystal planes, and ReS2 with two or even a single orientation is obtained on the four-fold symmetric Au (001) crystal plane and the two-fold symmetric Au (101) crystal plane, respectively (Fig. 5 H, I). Density functional theory (DFT) calculations and experiments show that the interfacial coupling between Density functional theory and Au substrate is much stronger than that between Au and other substrates. As shown in Fig. 5j, the broadening of the ReS2 vibration peak can be observed in the Raman spectrum, indicating that there is heavy electron doping in the ReS2 prepared by CVD on the Au substrate. In addition, a new Raman peak appears at the 195 cm-1, proving that there is a strong coupling effect between the electron orbitals of the ReS2 and Au. It is this strong ReS2-Au interaction that enables orientation-controlled growth of ReS2 on Au.
In conclusion, the CVD method for the preparation of ReS2 is a convenient and effective synthesis method, which is most likely to achieve high quality and mass production in the future. Compared with other methods, the products prepared by CVD method have a series of advantages, such as uniform morphology, controllable thickness, low defect rate, clean interface and high synthesis efficiency. The orientation, shape, thickness, nucleation density, grain size and the like of the product can be precisely controlled by adjusting the type of the precursor, the surface property of the substrate, the growth temperature, the gas flow rate, the reaction time and the like in the CVD process.

2.2 CVD method for preparing two-dimensional ReS2 heterojunction

The weak van der Waals interaction between layers of two-dimensional materials enables them to stack vertically freely like Lego building blocks, forming vertical heterojunctions with rich varieties and diverse properties.It can also realize the in-plane splicing of different two-dimensional materials by forming chemical bonds at the interface to form horizontal heterojunctions with special interface properties, which greatly expands the application of such materials in the fields of electronics, optoelectronics, sensing, energy and so on[46⇓⇓~49][50⇓⇓⇓⇓~55]. Vertical heterojunctions can be fabricated by mechanical exfoliation transfer, CVD, and liquid phase exfoliation, among which vertical heterojunctions grown by CVD have high yield and excellent interfacial contact properties. Horizontal heterojunctions can only be fabricated by CVD. Since Song et al. Prepared graphene/h-BN vertical heterojunction by one-step method, researchers have found that this structure can bring electronic structure different from intrinsic single two-dimensional material, and the research on two-dimensional TMDCs heterojunction including ReS2 has been gradually carried out. The CVD preparation of heterojunction can be divided into "one-step method" and "two-step method" according to the number of synthesis steps[49][56⇓⇓~59]. The one-step method puts the precursor source containing the synthesized heterojunction element into the reaction system at one time, and uses the difference of substrate adsorption energy or controls the growth conditions, such as growth temperature or carrier gas type, to realize the sequential growth of two materials. The two-step method is to carry out two CVD processes, and the growth of the heterojunction is carried out in two steps in turn. In the two-step method, attention should be paid to the selection of preparation conditions in the second step to avoid the destruction of the two-dimensional material obtained in the first step during the second step.

2.2.1 One-step process

Fu et al. Realized the sequential growth of WS2 and ReS2 by using the adsorption energy difference of different TMDCs materials on the substrate surface, and synthesized ReS2/WS2 vertical heterojunction on Au/W-Re alloy foil in one step (Fig. 6a – C)[60]. The calculated results of DFT for the adsorption energy of different atoms show that the adsorption energy of Re atom and W atom on Au substrate is 1.52 and 2.77 eV, respectively. Therefore, at high temperature, Re atoms are more easily desorbed from the substrate surface and difficult to nucleate. On the contrary, W atoms with larger adsorption energy can adsorb on the substrate surface and subsequently react with S to obtain WS2. On the other hand, the adsorption energy of Re atom on the surface of WS2 is relatively high (3. 04 eV), which ensures that Re atom can adsorb, nucleate and grow on the surface of WS2 to form ReS2/WS2 vertical heterojunction. As shown in Fig. 6 d, e, integrating the characteristic peak positions of ReS2 and WS2 respectively, a complete and uniform characteristic peak integrated Raman spectrum image response can be observed, proving that the product is a heterojunction rather than an alloy. In addition, Xu et al. Used hydrogen to control the volatilization characteristics of the two precursors, thus controlling the heterojunction growth like a switch, and synthesized 2H-WS2/1T’-ReS2 horizontal heterojunction with high interface quality by "one-pot method"[61]. As shown in Fig. 6f, under the temperature condition of 850 ° C, WO3 and ReO3 powders were placed on both sides of the ceramic boat respectively, and when Ar was the carrier gas,The WO3 with high melting point is difficult to volatilize, while the ReO3 is decomposed into Re2O7 with high volatility at high temperature, so that only a monolayer of ReS2 can be obtained on the surface of the substrate. However, when H2 is introduced into the system, the high-melting WO3 is mainly reduced to the volatile W18O49, while the highly volatile Re2O7 is reduced to the non-volatile Re. At this time, only monolayer WS2 product can be obtained on the substrate surface. Therefore, by changing the composition of the carrier gas, metal precursors with different volatilities can be obtained, WS2-ReS2 horizontal heterojunction is obtained on the c-plane sapphire, and the seamless splicing of the WS2 around the ReS2 is realized. As shown in fig. 6G, a clear boundary between the inner Scanning electron microscopy (bright) and the outer WS2 (dark) can be observed in the Scanning electron microscopy (SEM) image, and the SEM image shows a clear heterojunction interface at the atomic scale. WS2 tends to nucleate at the apex of ReS2 grains, and the zigzag edge of WS2 is parallel to the b-axis direction of ReS2 grains (Fig. 6 H). The heterojunction has polarization-dependent photodiode characteristics and exhibits excellent rectification characteristics. According to the transient absorption spectrum, the photogenerated carriers can be effectively separated at the interface of the heterojunction. The photovoltaic effect can be effectively modulated by changing the polarization direction of the light relative to the heterogeneous crystal lattice. When the polarization direction of the light is perpendicular (parallel) to the b-axis direction of the ReS2 grains, both the open-circuit voltage and short-circuit current of the device reach their minimum (maximum) values.

2.2.2 Two-step method

Xu et al. Epitaxially grew 1T’ReS2-ReSe2 horizontal heterojunction on mica substrate by two-step method through reducing ReO3 of S powder and Se powder. In the first step, ReS2 was prepared at relatively high temperature (≥ 850 ℃).Then lowering the reaction temperature to below 700 deg C, introducing a small amount of H2 to ensure that the structure of the ReS2 is complete, and epitaxially growing a ReSe2 on the edge of the structure[62]. A two-step growth process can avoid obtaining a more thermodynamically stable alloy. The ReS2 and the ReSe2 are both distorted 1T 'structures, which are easy to grow asymmetrically and form complex dendritic interface structures. In response to the above problems, the researchers introduced molecular sieves to effectively reduce the volatilization rate of the Re source precursor, thereby reducing the growth rate of ReX2(X=S,Se) and obtaining products with relatively smooth edges (Figure 7A). The results of OM, SEM and AFM show that the synthesized heterojunction is a monolayer (Fig. 7B – d), with ReS2 in the central region and ReSe2 at the edge. In addition, the Raman characteristic peaks of ReS2 and ReSe2 in the heterojunction interface region appear simultaneously, that is, ReS2 is not alloyed with external ReSe2. In-depth study of the 1T’ReS2-ReSe2 interface shows that there are three kinds of angles in the b-axis direction when the ReSe2 epitaxially grows along the edge of the ReS2: 0 °, 120 ° and 180 °. DFT calculation of the angle formation energy shows that the structures with angles of 0 ° and 180 ° have higher thermodynamic stability and are easier to form. The heterojunction exhibits horizontal intrinsic p-n junction rectification characteristics and polarization-dependent photodiode characteristics, providing a material basis for the development of multifunctional optoelectronic devices.
Tongay et al. Prepared ReS2-WS2 horizontal heterojunction by a two-step method[63]. In the first step, monolayer WS2 was synthesized on sapphire substrate by using WO3 and S as precursors and introducing Ar/H2 gas mixture at low pressure. Subsequently, NH4ReO4 and S were selected as precursors to synthesize monolayer ReS2 at the WS2 edge (Fig. 7e, f). It should be noted that the growth temperature of ReS2 (490 ℃) should be controlled to be much lower than the decomposition temperature of WS2 (600 ℃) when preparing ReS2-WS2 heterojunction, so as to ensure the structural integrity of heterojunction. The Raman surface scanning image consistent with the complete grain morphology can be obtained by integrating the characteristic peak positions of WS2 and ReS2 (Fig. 7G), which proves that the heterojunction structure is generated.
Researchers combined TMDCs materials to form different kinds of heterojunctions, and also selected two-dimensional materials such as graphene and ReS2 to construct heterojunction structures by CVD method. Park et al. Grew graphene /ReS2 vertical heterojunction by CVD two-step method (Fig. 7h)[64]. Graphene on the surface of copper foil prepared by CVD was transferred to SiO2/Si substrate, and then ReS2 was synthesized by CVD with ReO3 and S as precursors. The surface of two-dimensional graphene is smooth and chemically inert, which has a low adsorption and migration barrier, and the surface of graphene has a strong binding force with S atoms, which promotes the in-plane growth of ReS2 on the surface of graphene. It can be seen in the OM versus AFM images that a monolayer of ReS2 grains was grown on the graphene surface (Fig. 7 I, J).

3 Properties of two-dimensional ReS2.

Neumann's theorem shows that the symmetry exhibited by an arbitrary macroscopic physical property of a crystal must have all the symmetry elements of the corresponding point group of the crystal. Therefore, two-dimensional ReS2, which belong to the triclinic system with the lowest symmetry among the seven major crystal systems and contain only one symmetry element, will show significant anisotropy in physics[12]. In addition, the formation of Re4 chains in ReS2 distorts the lattice structure, changes the original electronic distribution, reduces the order of interlayer stacking of ReS2, minimizes the interlayer overlap of wave function, and has a very weak interlayer coupling effect, so its vibrational, optical and electrical properties are not related to the number of layers of materials. In this section, the unique properties of this class of materials will be introduced from three aspects: vibration, optics and electricity.

3.1 Vibrational and optical properties

Raman spectrum is a kind of scattering spectrum to characterize the rotation and vibration of materials. The low lattice symmetry of ReS2 brings about complex and diverse Raman vibration modes, which can be divided into low-frequency interlayer coupling vibration and high-frequency intralayer vibration according to the Raman peak position. The interlayer coupling vibration is usually observed at low frequency (<50 cm-1) and subdivided into shear mode and breathing mode; The intralayer vibration appears in the high frequency (100~450 cm-1) range. The 18 intralayer vibration modes of the ReS2 are shown in Figure 8 a. Among them, the vibration modes below 250 cm-1 are mainly Re atomic vibration, and about 213 cm-1 corresponds to Re-Re chain vibration; Above 250 cm-1, it is mainly S atom vibration. In addition, due to the weak interlayer coupling of ReS2, its Raman vibration does not change with the number of layers[65]. Hone et al. Used angle-resolved Raman spectroscopy to find that the peak intensity of monolayer ReS2 has polarization angle dependence (Fig. 8 B), and the peak intensity changes with the angle from 0 ° to 180 °[19].
Fig. 8 C shows the polar coordinates of the peak intensity and angle function of the few-layer ReS2 at mode Ⅲ(152 cm-1) and mode Ⅳ(213 cm-1), where the two modes correspond to the Re atomic vibration and the Re-Re chain vibration, respectively. The intensity of the two mode peaks shows a 180 ° periodic change with the change of angle, which proves that the ReS2 has anisotropic Raman vibration. High-symmetry monolayer TMDC materials (such as MoS2, WSe2, etc.) Have isotropic optical absorption and emission properties, while ReS2 are triclinic and exhibit in-plane anisotropic optical properties due to the Peierls distortion in the low-symmetry crystal structure[66,67]. It is known from the study of electron-electron and hole-hole interactions in the excited state that strongly bound excitons can induce optical anisotropic absorption in ReS2, and their optical absorption properties depend on the polarization direction of the incident laser. When the polarization direction of the light is perpendicular to the Re chains, the anisotropic absorption coefficient and transient absorption are minimum, while they are maximum when the light is parallel to the Re chains[68][69][70]. Since the Raman spectra of ReS2 materials depend significantly on their lattice orientation, they can be used to fabricate polarization-dependent photodetectors[5,71].

3.2 Electrical property

DFT calculations show that the band structure of ReS2 is independent of the number of layers, and the size of the band gap does not change from bulk thinning to monolayer (Fig. 8 d). In addition, as shown in Figure 8 e, the interlayer coupling energy of ReS2 is only 18 meV, which is much lower than 460 meV of MoS2[72]. The band structure of two-dimensional ReS2 is sensitive to electric field and strain, which can be tuned by applying electric field and force field. As shown in fig. 8f, strain applied along different directions can also change the resistance of the ReS2 along different lattice directions[73]. Strain can also raise the valence band maximum and lower the conduction band minimum of the ReS2, resulting in a reduction of the band gap. In Order to study the dependence of the electrical properties of Field effect transistor on its lattice direction, and to design and construct Field effect transistor devices with different lattice directions, Liu et al.FET) to study the anisotropic conductivity and carrier mobility of the material. In Fig. 8g, there is a significant difference between the conductivity along the a-axis and b-axis of the ReS2, and both the theoretical and experimental values show that the conductivity changes along different crystal directions[74]. Liu et al. Also found that the carrier mobility of single-layer ReS2 is significantly related to the lattice direction, with the maximum carrier mobility and conductivity along the B axis and the minimum along the a axis, and the carrier mobility along the B axis is about 3 times of that along the a axis (Fig. 8h). This electrical anisotropy changing with the lattice direction can be applied to logic circuits[73].

4 Conclusion and prospect

Low symmetry crystal structure brings structural complexity and property anisotropy to two-dimensional ReS2, which makes it unique in growth behavior and physical and chemical properties, and has broad application prospects in micro-nano electronic devices, photoelectric detection, energy and other fields[12,75]. In this paper, the crystal structure of ReS2 and the general process and basic steps of preparing two-dimensional materials by CVD are summarized. Then, the existing preparation methods of two-dimensional ReS2 are systematically classified and reviewed in detail from the aspects of precursor types and substrate engineering. Then, combined with the current frontier hot spots of low-dimensional heterostructure fabrication, according to the number of synthesis steps, the frontier progress of CVD fabrication of horizontal and vertical heterojunctions containing ReS2 is introduced, which is divided into "one-step method" and "two-step method". Finally, the vibrational, optical, and electrical anisotropic physical properties of two-dimensional ReS2 are briefly summarized.
Although important progress has been made in the study of ReS2, the understanding of such materials is still in its infancy, and challenges and opportunities coexist. Firstly, the application of anisotropic physical and chemical properties of ReS2 depends on the research and development of large area, high-quality, single-crystal two-dimensional materials. However, the orientation of Re quadrilateral chains is very easy to change during the preparation process, which leads to the fact that only polycrystalline products can be obtained by a large number of synthesis methods. The randomly oriented grains average the anisotropic physical properties of ReS2, and the existence of grain boundaries deteriorates the electrical transport properties. The use of Au with a specific crystal face as a substrate can partially solve the problem of ReS2 single crystal preparation, but the cost of the Au substrate is high, and the interface interaction between Au and ReS2 is too strong, which causes difficulties in subsequent transfer, and the size of the existing single crystal product is only in the micron level. How to develop large area single crystal ReS2 synthesis on the surface of cheap and universal substrates is the premise of the application of this kind of materials in the field of anisotropic devices. Secondly, the mapping relationship between the low lattice symmetry in real space and the property space of this kind of two-dimensional materials needs to be further understood. At present, two-dimensional materials corner electronics, represented by "magic angle" graphene, has brought an upsurge of research on electronic strong coupling systems.Current research mainly focuses on graphene, boron nitride and hexagonal TMDCs systems, and it is still unknown whether coupling two-dimensional materials with low lattice symmetry, such as ReS2, can bring new exotic properties. Of course, this will also lead to the development of "bottom-up" vertical heterojunction growth methodology in the field of ReS2 heterojunction. In a word, two-dimensional ReS2, as a typical class of low-symmetry two-dimensional crystals, provide a good platform for studying the intrinsic relationship between structural symmetry and properties of materials.

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Funding

National Natural Science Foundation of China(52172032)

National Natural Science Foundation of China(21805305)

State Administration of Science, Technology and Industry for National Defense(WDZC20195500503)

China Postdoctoral Science Foundation(2020M680231)

National University of Defense Technology(ZK18-01-03)

National University of Defense Technology(ZZKY-YX-09-01)

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