Rhenium dichalcogenides (ReX₂, X = S or Se): an emerging class of TMDs family

Abstract

The two-dimensional transition metal dichalcogenides (TMDs) have been attracting increasing interest due to their unique structures and remarkable properties. As a new member of the TMDs family, rhenium dichalcogenides (ReX₂, X = S or Se) possess many distinctive features because of an unusually distorted octahedral (1T) crystal structure with triclinic symmetry. In this unique crystal structure, each monolayer contains diamond-shaped chains (DS-chains) comprising interlinked Re₄ clusters, which makes the structure anisotropic. This novel structure renders ReX₂ with wide applications in (opto-)electronics, such as photodetectors and field effect transistors (FETs). ReX₂ are new materials, so this review presents mainly basic and research work done in recent years. In the first part, the unique crystal and electronic structures are introduced. The second part summarizes the various growth methods for ReX₂. Finally, the third part focuses on the applications in high-performance photodetectors and FETs based on 2D ReX₂.

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Muhammad Hafeez

Dr Muhammad Hafeez is an Assistant Professor in the Department of Physics, COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan. In 2005, he received his BS degree from the University of Punjab (Lahore, Pakistan). He completed his MS (physics) in 2009 and received his doctorate (physics) in 2013 from the Department of Physics of CIIT under the supervision of Professor Arshad Bhatti. In 2014, he joined the School of Materials Science and Engineering of Huazhong University of Science and Technology (HUST), Wuhan, China, as a postdoctoral fellow. His research interest focus on the controllable scalable synthesis of 2D TMDc via CVD methods for electronic and optoelectronic applications.

Lin Gan

Lin Gan received his BS degree in chemistry from Wuhan University in 2005, and then received his doctorate in physical chemistry from Peking University under the supervision of Professor Xuefeng Guo in 2012. Currently, he is working at the School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research interest focuses mainly on the chemical-vapor deposition synthesis of two-dimensional (2D) nanomaterials such as graphene and TMDs, and on the electronic/optoelectronic properties and applications of these 2D materials.

Arshad Saleem Bhatti

Arshad Bhatti is a tenured professor of Physics at the COMSATS Institute of Information Technology. Bhatti received his first degree in physics from the University of the Punjab (Lahore, Pakistan) in 1987. He did an MPhil and doctorate in microelectronics and optoelectronics under the supervision of Dr Howard P. Hughes from the University of Cambridge (Cambridge, UK) in 1992 and 1995, respectively. He also did postdoctoral work at the University of Rome (Rome, Italy) from 1998 to 2000 with Professor A. Frova, at the University of Nottingham (Nottingham, UK) from 2000 to 2001, and with Professor L. Eaves at the University of Illinois (Urbana Champaign, Urbana, USA). His research interests include the synthesis and characterization of low-dimensional semiconductor structures for applications in biophysics and the environment. He has published more than 75 papers. He is also a team leader for CIIT in A Large Ion Collider Experiment (ALICE) at CERN. For his services, the Government of Pakistan bestowed him with Tamgha-e-Imtiaz in 2011.

Tianyou Zhai

Tianyou Zhai received his BS degree in chemistry from Zhengzhou University in 2003, and then received his doctorate in physical chemistry from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS), under the supervision of Professor Jiannian Yao in 2008. He joined the National Institute for Materials Science (NIMS) as a JSPS postdoctoral fellow of Professor Yoshio Bando's group and then as an ICYS-MANA researcher within NIMS. Currently, he is a Chief Professor of the School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research interests include the controlled synthesis and exploration of fundamental physical properties of inorganic functional nanomaterials, as well as their promising applications in energy science, electronics and optoelectronics. He has authored and co-authored ≈140 peer-reviewed journal articles, five book chapters, co-edited one book on nanotechnology, and held seven patents. His publications have been cited more than 6500 times (H-index is 45).

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Introduction

Graphene has triggered a wave of research interest in ultrathin two-dimensional (2D) transition metal dichalcogenides (TMDs).^1–10 TMDs are compounds of the general form MX₂, where M denotes a transition metal (Mo, W, Ti, Hf, Re) and X denotes a chalcogen (S, Se, Te).^11,12 The variety of TMDs covers a wide range from semiconductors to semi-metals/metals and even super-conductors due to the presence of unsaturated d-orbitals of transition metal.¹³

In particular, atomically thin 2D TMDs are potentially ideal for eliminating short-channel effects, such as drain-induced barrier lowering (DIBL) in metal oxide semiconductor field-effect transistors (MOSFETs).¹⁴ Therefore, TMDs could have applications of charge density waves,¹⁵ logic transistors,^16,17 optoelectronics^18–20 and interband tunnel FETs.^21–23

Rhenium dichalcogenides (ReX₂) are new members among the TMDs family and possess the same typical “sandwich” structure like other TMDs,^24–29e.g. MoS₂,^13,22,30 WS₂,^31,32 MoSe₂^33,34 and WSe₂.^35,36 However, ReX₂ are unique in that they have much weak interlayer coupling resulting from a distorted octahedral (1T) crystal structure with triclinic symmetry, which renders ReX₂ many novel properties, such as anisotropic electronic, optical and mechanical properties.^{26,28,29,37–39} In this context, we present a comprehensive review about the ReX₂ which mainly focus on their various interesting physical properties (crystal structures, electronic structures), followed by their synthetic methods, including bulk crystal growth, chemical exfoliation and vapor-deposition methods. Furthermore, recent reports about the application of these 2D ReX₂ into devices (photodetectors, FETs) are presented. Finally, this review is summarized with prospects for future research in this area.

Crystal structure

ReX₂ are known to have a layered atomic structure like graphene and other TMDs. Fig. 1a shows the side and top view of single layer of the distorted 1T structure of a ReX₂,^37,40–42 which is different from the conventional 1H structure of TMDs such as MoS₂^23,43,44 and WS₂⁴⁵etc. The crystal structure of ReX₂ is very special due to their in-plane motif of diamond-shaped chains (DS-chains) arranged in parallel, as can be seen in the side and top view (Fig. 1a) of the crystal structure.^37,46–48Fig. 1b shows an annular dark field (ADF) image of exfoliated ReS₂ flakes with mono and trilayer regions where the single-layer region exhibits clearly a DS phase. It is clear from the ADF image of the single layer region that four Re atoms are arranged in a diamond-like shape, and the diamonds form atomic chains in the b direction, as mentioned in Fig. 1b. These structures are clearly visible in the typical ADF images of single-layer ReS₂ and trilayer ReSe₂ (Fig. 1c and d). The lattice parameters determined using density functional theory (DFT) calculations are presented in Table 1.

Fig. 1 Crystal structure. (a) Schematic depiction of monolayer ReX₂ identifying the b-axis [010] Re-chain direction and a-axis [100] across Re-chains. (b) High-magnification ADF image of ReS₂. The upper part is single-layer with a diamond-shaped (DS) phase structure, whereas the lower part is three-layer stacking. Four Re atoms form a diamond cluster (green line). The DS clusters form a chain-like structure along the b[010] direction. (c and d) ADF images of ReS₂ and ReSe₂. The spacing between Re diamond chains is 0.34 and 0.39 nm for ReS₂ and ReSe₂, respectively. The spacing between Re DS clusters in the chain is 0.31 and 0.35 nm for ReS₂ and ReSe₂, respectively. Scale bar is 0.5 nm. Reproduced with permission.³⁷ Copyright 2015, American Chemical Society. (e) DFT calculated electronic band structure of bulk (orange solid curves) and monolayer (purple dashed curves) ReS₂. Reproduced with permission.²⁶ Copyright 2014, Nature Publishing Group. Both are predicted to be a direct bandgap semiconductor with nearly identical bandgap value at the Gamma point. (f and g) Band structure of monolayer and bulk ReSe₂, reproduced with permission.²⁸ Copyright 2014, American Chemical Society. PL spectra of (h) ReS₂, reproduced with permission,²⁶ (i) ReSe₂, reproduced with permission,²⁵ flakes with different number of layers. (j) Absorption spectrum of ReS₂, ReSSe, and ReSe₂, reproduced with permission.⁵³ Copyright 2014, Royal Society of Chemistry.

Table 1 Dimensions of unit cells of ReS₂ and ReSe₂ (ref. 49)

	a (Å)	b (Å)	c (Å)	α (deg)	β (deg)	γ (deg)
ReS2	6.352	6.446	12.779	91.51	105.17	118.97
ReSe2	6.597	6.710	6.721	91.84	104.90	118.91

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The direction of the DS Re₄ chain is along the b[010] axis (which is also known as the in-plane axis) whereas the a[100] axis (which is known as an out-of-plane axis) and the angle between b[010] and a[100] axes is ∼119.8°, as mentioned in Fig. 1c.²⁶ The spacing between two vicinal diamond-shaped clusters in the direction of b and a is 0.34 and 0.31 nm for ReS₂ whereas the spacing is 0.39 and 0.35 nm for ReSe₂, respectively, as calculated from ADF images in Fig. 1c and d.³⁷ In the three-layer region in Fig. 1b, the stacking sequence is not 1T, 2H, or 3R, which are well-known for group-VI TMDs.⁵⁰

The most significant feature of ReS₂ is that the bulk and monolayer ReS₂ have nearly identical band structures (Fig. 1e). DFT calculations show that the monolayer and bulk form have direct-bandgap and predicted generalized gradient approximation (GGA) bandgaps at 1.35 eV (bulk) and 1.43 eV (monolayer), respectively.²⁶ This nature of band structure is in stark contrast to conventional TMDs, where the band structure is strongly dependent on the number of layers.⁵¹ The behaviour of the ReSe₂ band structure is quite different and the bandgap is direct in the bulk, and it changes to an indirect bandgap when the layer number decreases to monolayer, as shown in Fig. 1f and g.⁵² Most importantly, unlike many group-VI TMDs, the indirect gap remains near the K point and the difference between the indirect gap minima and the direct gap is ∼20 meV.⁴⁶

DFT within GGA and local density approximation (LDA) usually underestimates the bandgap. Photoluminescence (PL) and absorption spectroscopy provide the exact value of bandgap energy and also the effect of thickness on the bandgap of ReS₂ and ReSe₂. PL measurements were carried out on monolayer, few-layer, and bulk ReS₂ (ReSe₂) samples and plotted in Fig. 1h (Fig. 1i), respectively. The bandgap for ReS₂ was determined to be 1.58 eV (1.5 eV) for monolayer (bulk) samples.²² In the case of ReSe₂, owing to the indirect bandgap nature in the monolayer form, the peak intensity is relatively weak for monolayer ReSe₂ and increases monotonically when adding layer numbers.

The bandgap increases with deceasing layer numbers, ranging from 1.26 eV for the bulk to 1.32 eV for the monolayer ReSe₂ crystals.²⁵ The absorption spectroscopy results of ReS₂, ReSSe and ReSe₂, are in accordance with PL results.⁵³

Raman spectroscopy is a powerful tool for evaluation of crystal orientations, and identification of the vibration symmetry of chemical bonds.^52,54 Recently, it has been used for the determination of layer number in conventional group-VI TMDs such as MoS₂ and WS₂, where the in-plane and out-of-plane modes show variation in the Raman shift with a variation in the layer number.^55,56 However, the situation is completely different for ReX₂, where Raman spectra display a similar pattern with the change in thickness of ReX₂.^42,57–59Fig. 2a and b shows the Raman spectra of N = 1 to N = 7 layers and bulk ReS₂ and ReSe₂, respectively, recorded at 532 nm laser for randomly oriented samples. Raman measurements from exfoliated ReS₂ and ReSe₂ flakes display a similar Raman signal regardless of its thickness, as indicated by the data shown in Fig. 2a and b.⁶⁰ More than 10 modes in the 100–400 cm⁻¹ range can be observed for ReS₂ and ReSe₂, which is much more than that in TMDs. A large number of Raman peaks is attributed to the low crystal symmetry of ReX₂ and associated with fundamental Raman modes (E_1g, E_2g, and A_1g) coupled to each other and to acoustic phonons.^26,42 In the case of ReS₂, five peaks (151, 161, 213, 238, and 312 cm⁻¹) were assigned to in-plane E_g-like modes, and the peak at 139 cm⁻¹ corresponds to the out-of-plane A_g-like mode.^61,62 In the case of ReSe₂, the peak at 124 cm⁻¹ was assigned to an in-plane E_g-like mode and the peaks at 162 cm⁻¹ and 174 cm⁻¹ correspond to an out-of-plane A_g-like mode.⁶⁰

Fig. 2 Raman spectra recorded on (a) N-layer ReSe₂ and (b) N-layer ReS₂ at E_L = 2.33 eV in the parallel polarization (XX) configuration; the fifth intralayer mode (V) is highlighted with a gray rectangle. (c) Optical image followed by HRTEM image of the ReSe₂ bilayer showing the direction of the Re chains. At θ = 0°, the laser polarization is parallel to the Re chains. (d) Angular dependence of the Raman mode V of ReSe₂ recorded as a function of the angle between the linearly polarized laser field and the Re chains in the parallel (XX) polarization configuration. The raw spectra (spheres) are fitted to Voigt profiles (red lines) and are vertically offset for clarity. (e and f) At two photon energies, E_L = 1.96 eV (red squares) and E_L = 2.33 eV (open green triangles), in the parallel (XX) configuration on the ReSe₂ and ReS₂ bilayer. The angular patterns are normalized for a clearer comparison. The solid lines fit to the experimental data considering a Raman tensor with complex elements. Reproduced with permission.⁶⁰ Copyright 2016, American Chemical Society.

Determination of crystal orientation

Raman spectra can also be used to identify the direction of the DS Re₄ chain.^{24,47,52,57,63} For identification of the Re₄ direction, only the fifth-lowest frequency mode (denoted as mode V) at ∼162 cm⁻¹ (respectively, 213 cm⁻¹) in ReSe₂ (respectively, ReS₂) was considered and mentioned in Fig. 2a and b.⁶⁰ Mainly mode ‘V’ describes the atomic displacement of the Re–Re bond. Three steps are involved in the determination of crystal orientation and direction of the DS Re₄ chain. First is an observation: Re chains are found to coincide with one of the cleaved edges of the ReX₂ crystals, providing the first hint of the crystal orientation, as can be seen in optical microscope and corresponding high-resolution transmission electron microscopy (HRTEM) images of exfoliated ReSe₂ flakes (Fig. 2c). Secondly, the empirical approach that “the integrated intensity of the mode V is maximum when the laser polarization is parallel to the Re chains” was used to confirm the orientation of the ReSe₂ chains.

Hone and co-workers stated that, at θ = 0°, the laser polarization is parallel to the Re chains, as marked in Fig. 2c.⁶⁴ Polarization angle-dependent Raman spectroscopy was utilized to study the Raman mode V of ReS₂ and ReSe₂. Angular dependence of the Raman mode of ReSe₂ mode V recorded as a function of the angle between the linearly polarized laser light and the Re chains in the parallel (XX) polarization configuration are shown in Fig. 2d. The third and most important factor is the incoming laser photon energy. These materials efficiently absorb visible light, so the Raman tensor elements are complex numbers. Yang and co-workers predicted a complex manifold of strongly anisotropic excitons in monolayer ReSe₂ and ReS₂, with several associated optical resonances in the spectral window (1.96–2.33 eV).⁶⁵ Due to the photon-energy dependence of the anisotropic Raman response in N-layer ReX₂, great care must be taken when correlating the angular dependence of the Raman response of an N-layer ReX₂ sample to its crystal orientation. Thus, the applicability of this method is valid for ReSe₂ while using a 1.96 eV laser, as shown in Fig. 2e. Conversely, it is clear from Fig. 2f that 1.96 eV and 2.33 eV laser lights are acceptable for identification of the DS Re₄ chains in ReS₂.

Preparation methods and characterizations

Reliable production of large-scale ultrathin 2D ReX₂ is essential for further research and applications. In general, two types of approaches (top-down and bottom-up) have been used for preparation of 2D TMDs. Mechanical exfoliation from the bulk is a top-down method and vapor-phase deposition is a bottom-up method. In the following section, we focus on these preparation methods.

Conventional Bridgman method

Bhattacharya and co-workers⁶⁶ employed modified-Bridgman method to grow single crystals of ReS₂ and ReSe₂ directly from the constituent elements (i.e. pure Re, and S/Se). A 2 cm diameter and 20 cm-long quartz tube was used for the growth, which was cleaned using HF/H₂O (1 : 10) solution and rinsed thoroughly with distilled water prior to the growth process. This step is crucial because it helps to remove impurities and the etchant also causes a slight roughening to promote nucleation during growth. The constituent elements Re, and S/Se (high purity, ∼5 N) were weighed in stoichiometric proportions individually, introduced into the tube and sealed under vacuum (2 × 10⁻⁶ mbar). Growth was initiated by placing the ampoule into a vertical Bridgman furnace with a specific temperature profile used to obtain single crystals of ReX₂ materials. Initially, the temperature was increased to 1100 °C over 72 h. After a stabilization time of 24 h, the temperature was very slowly decreased to 900 °C at 1 °C h⁻¹ over 10 days, after which the tube was cooled down to room temperature at 60 °C h⁻¹. This resulted in the formation of shiny crystals in the form of very thin, plate-like flakes (thickness = hundreds of micrometers; lateral size, approximately 4 × 5 mm²). The optical micrographs and scanning electron microscope (SEM) images of the grown crystals are shown in Fig. 3a–d. High-resolution X-ray diffraction measurements were done to study the structural properties and phase identification of the ReS₂ and ReSe₂ single crystals, as shown in Fig. 3e and f, respectively. An enlarged view of the most prominent (0 0 l) peak is shown in the inset of Fig. 3e and f. For both samples, only the (0 0 l) reflections were observed, which clearly represent the highly oriented single crystal in the c-axis direction. The sharpness of diffraction peaks, all with full width at half maximum (fwhm) values of <0.1°, clearly indicates the high crystalline quality of the material. Hence, the conventional Bridgman method without utilizing a halogen transport agent can provide high-quality ReX₂ crystals. For measurement of layer-dependent properties and device fabrication, thin sheets were used to mechanically exfoliate these flakes and transfer them to SiO₂-coated Si substrates for measurements. Another very famous chemical-vapor transport (CVT) method is being widely used to synthesize the bulk of single crystals of ReS₂ and ReSe₂. The procedure is similar to the method described above using the Bridgman method except that a halogen (I₂ or Br₂) is used as a transport agent.^37,67

Fig. 3 Optical image of the as-grown crystals of (a) ReS₂ (inset shows the ingot as removed from the quartz tube) and (b) ReSe₂. SEM images of (c) ReS₂ and (d) ReSe₂, showing the surface morphology of the flakes. HRXRD 2θ scan for (e) ReS₂ and (f) ReSe₂ single crystals. The patterns were indexed with International Centre for Diffraction Data Powder Diffraction Files 00-052-0818 and 04-007-1113, [2014], for ReS₂ and ReSe₂, respectively. The y axis is plotted on a log scale. Insets show expanded views of the most prominent reflection. The absence of other (hkl) reflections and the small fwhm values of 0.1 and 0.06° for (0 0 1) reflection of ReS₂ and ReSe₂, respectively, point to high crystal quality. Reproduced with permission.⁶⁶ Copyright 2016, American chemical Society.

Physical vapor deposition (PVD)

Zhang and co-workers⁶⁸ employed a simple thermal-evaporation method to produce a large-area, continuous ReS₂ film using ReS₂ powder (Alfa-Aesar, 99% purity). ReS₂ powder (30 mg) was placed in the middle of quartz tube and SiO₂/Si substrate with a 280 nm oxide layer was placed in the downstream, as shown in Fig. 4a. After evacuation of the quartz tube, argon at a flow rate of 50 SCCM was used as the carrier gas. The temperature of the furnace was increased to 900 °C in 60 min and kept at 900 °C for 1 h to grow the ReS₂ film and, after the growth, the samples were furnace-cooled to room temperature. Then, the as-grown samples were annealed at 800 °C for 60 min in the argon atmosphere, and then 400 mg of sulfur powder was placed in the upstream at 165 °C. The optical image of as-grown ReS₂ film shows a pink (Fig. 4b) continuous ReS₂ film of the size of centimeters (Fig. 4c). The pink color indicates that the as-grown film comprises a few layers, and this was confirmed further by atomic force microscopy (AFM) images (Fig. 4d). The height profile shows that the film is homogenous with a thickness of 2.30 nm, which indicates that about three monolayers are present. Furthermore, TEM shows that the thin film is continuous at a nanometer scale (Fig. 4e). Dark-field TEM was employed to observe the grain boundaries, and the average grain size of the as-grown ReS₂ film was 250 nm (inset in Fig. 4e). Furthermore, the sharpness of the Raman peaks indicates the good quality of the as-grown ReS₂ film, as shown in Fig. 4f.

Fig. 4 (a) Schematic diagram of synthesized ReS₂ film by PVD. Optical (b) photograph and (c) microscope image of grown ReS₂ film on the SiO₂/Si substrate. (d) is the AFM and (e) TEM image of the ReS₂ film. The inset shown in Fig. 3e is the DF-TEM image of ReS₂ film. (f) Raman spectrum of ReS₂ from 100 cm⁻¹ to 400 cm⁻¹. Reproduced with permission.⁶⁸ Copyright 2016, Elsevier.

Chemical exfoliation method

Chemical exfoliation from bulk materials is an effective method of TMD production, and lithium intercalation of bulk materials is a well-known exfoliation technique for the scalable production of thin films and composites.^69–71 Chen and co workers⁷² used a solvent-free chemical exfoliated method for the production of ReS₂ nanosheets from bulk powders. They used the reaction of ReS₂ powder with lithium borohydride (LiBH₄) for intercalating Li. This new method could replace the conventional method, which involves a solution of butyl lithium. Lithium intercalation was carried out in a standard oxygen-free, Ar-filled glove box. ReS₂ powder (0.1 g) with lithium borohydride (35 mg) were mixed to ensure homogeneity, and then heated to 350 °C for 3 days. The resulting product was immersed in Ar-bubbled water at a ratio of 1 mg per mL of water, and then sonicated for 1 h. Then, lithium cations and non-exfoliated ReS₂ were removed by centrifugation. Comparison of the size and shape of the as-received ReS₂ powders and as-exfoliated ReS₂ nanosheets was carried out by SEM. The grain size of as-received ReS₂ powder varied from 100 to 500 nm, whereas the size of as-exfoliated ReS₂ nanosheets was reduced to 50–100 nm, as shown in Fig. 5a and b. Optical photographs of a dark-brown homogeneous suspension of chemically exfoliated ReS₂ in water is shown in the inset of Fig. 5b. It is clear from the TEM image that the as-exfoliated ReS₂ nanosheets were very thin and some were exfoliated down to monolayers, aas confirmed by high-angle annular dark-field STEM (HAADF STEM) image, where the formation of super-lattice DS Re₄ chains can be seen easily (Fig. 5c). Raman spectra were taken from as-received powders and as-exfoliated ReS₂ nanosheets (Fig. 5d), and significant peak shift or change in phase were not observed. PL spectra (Fig. 5e) were obtained from as-received powders and as-exfoliated ReS₂ nanosheets. They revealed that the ReS₂ continued to have semiconducting features after exfoliation, which is in the stark contrast to the case of MoS₂⁷³ and WS₂,⁷⁴ which undergo phase transformation and become metallic.

Fig. 5 (a) SEM image of as-received ReS₂ powders. (b) TEM image of as-exfoliated ReS₂ nanosheets; the inset is a photograph of a typical dark-brown exfoliated ReS₂ suspension in water. (c) High-resolution STEM image of as-exfoliated ReS₂ nanosheets. (d) Raman spectra of as-exfoliated ReS₂ nanosheets and as-received powders. (e) PL spectra of as-exfoliated ReS₂ nanosheets on an oxidized Si substrate. The spectrum from the bulk (as-received powders) is shown for comparison. Reproduced with permission.⁷² Copyright 2014, Royal Society of Chemistry.

Chemical vapor deposition (CVD)

The CVD method has been adopted intensively due to the large-area, high-quality and uniform products obtained.^17,75,76 The success of achieving large-area CVD-grown graphene^77,78 has led the CVD method to be intensively applied to produce large-area, high-quality 2D-layer materials, especially MoS₂.^5,79 Ajayan and co-workers⁸⁰ demonstrated the CVD method for the scalable synthesis of large-area monolayer ReS₂. They reported a CVD method in which the lowest growth temperature (450 °C) occurs, which is less than all previously CVD-synthesized monolayers (Fig. 6a). Ammonium perrhenate (NH₄ReO₄) and sulfur powder were used as precursor materials and 280 nm SiO₂/Si was used as receiving substrates. NH₄ReO₄ (0.1 g) was loaded in an alumina boat and SiO₂/Si substrates were placed on top of the alumina boat with the polished side facing downwards. The boat was placed in the center of the furnace, as shown in Fig. 6b. Sulfur (2 g) was placed at the opening of the quartz tube furnace. Ar was used as the carrier gas with a flow rate of 50 sccm. The furnace was ramped to 450 °C in 15 min (sulfur temperature was ∼300 °C) and then held at 450 °C for 20 min with a constant Ar flow rate under atmospheric pressure. Then, the samples were furnace-cooled to 35 °C. SEM was employed for the surface analysis. As-grown ReS₂ samples and SEM images revealed the growth of thin and clean ReS₂ crystals with different sizes and hexagonal shapes, as shown in Fig. 6c. The size of the crystals varied from 2 to 60 μm, with a mean size of 40 μm.

Fig. 6 (a) Growth temperatures for various CVD grown monolayers and for bulk crystal growth of various TMDs along with temperatures for ReS₂. (b) Schematic for the CVD growth of monolayer ReS₂. (c) SEM image showing various ReS₂ monolayer crystals. Reproduced with permission.⁸⁰ Copyright 2015, Wiley-VCH. (d) Schematic for the hydrogen-assisted large-area CVD growth approach. (e) A picture of bare and as-grown ReS₂ bilayer (2L) film on sapphire wafer. (f) Optical microscope image of the ReS₂ bilayer film grown on sapphire substrates; a scratch was made intentionally for clarity, and the inset shows the height profile. Reproduced with permission.⁶² Copyright 2016, Wiley-VCH. (g) Schematic for the tellurium-assisted CVD growth approach. Temperature-dependent growth behavior of ReS₂. (h) OM images of ReS₂ grown at 850, 700, 600, and 500 °C after transfer to a SiO₂/Si substrate. Followed by enlarged images. (i) Raman spectra of ReS₂ grown at different temperatures. Reproduced with permission.⁸⁷ Copyright 2016, Wiley-VCH.

After the first report of the CVD growth of ReS₂, many attempts were made with mixed success to increase the quality and maintenance of the shape and large-area growth.^{47,59,79,81,82} Zhai and co-worker⁶² employed a hydrogen-assisted CVD method for the synthesis of large-area uniform polycrystalline ReS₂ film and hexagonal single-crystal ReS₂ flakes. Similar to the conventional method of TMDs growth in which MoO₃⁸³ was used to synthesize MoS₂,^79,84 MoSe₂⁸⁵ and WO₃ for the growth of WS₂⁸⁶ and WSe₂,³⁵ ReO₃ (10 mg) and S (3 g) were used as precursor materials for the growth of ReS₂. The synthesis of bilayers ReS₂ was carried out in a quartz tube using a three-zone horizontal tube furnace. In brief, sapphires pieces (2 cm × 2 cm) were placed in the tube furnace onto an alumina boat containing precursor material (ReO₃), as shown in Fig. 6d. Growth of large-area ReS₂ bilayers was carried out at low pressure (∼1 Torr) and a carrier gas of high purity (N₂ and H₂) was used at 800 °C for 10 min. After the growth, the samples were furnace-cooled to room temperature. After the growth, an obvious dark membrane could be observed compared with the bare substrate, as shown in Fig. 6e. An AFM height profile was employed to measure the thickness, and found to be ∼1.45 nm from the edge of a scratch (Fig. 6f). A highly uniform large-growth area was observed only for the sapphire substrate, which might be attributed to the weak van der Waals interaction between ReS₂ and sapphire.

Zhang and co-workers⁸⁷ introduced a new (tellurium-assisted) CVD method to reduce the melting point of the precursor material, and achieved high-quality monolayer ReS₂ at a large scale on mica substrate. The actual melting temperature of Re metal is 3180 °C and the eutectic point of Re–Te binary alloy is very low (down to 850 °C) and can be 430 °C if the Te–Re weight ratio is ≤90%. The whole synthetic process is illustrated in Fig. 6g. Briefly, Re and Te powders were mixed together and the weight ratio kept at 1 : 6. These mixed powders were loaded into a ceramic boat, and freshly cleaved mica substrates were put on the ceramic boat. Then, the boat was placed in the center of quartz tube. The boat (which contained sulfur powder) was placed near the outside edge of the hot zone. Ar was used as a carrier gas at a constant flow rate of 80 sccm, growth temperatures of 460–900 °C and growth time of 10 min, as described in Fig. 6g. Furthermore, the sharpness of Raman spectra indicated that high-quality ReS₂ could be obtained above 600 °C (Fig. 6i). This shows that the growth temperature has an important role in the crystal quality and morphological control of ReS₂ (Fig. 6h and i).

Zhai and co-workers⁸⁸ used a CVD method for the first time to synthesize ReSe₂ flakes and continuous film. The existence of pure H₂ gas in the carrier gas and the ratio of N₂ to H₂ was the key to synthesize high-quality ReSe₂. High-purity ReO₃ and Se powders were used as precursor materials and SiO₂/Si wafers with a 280 nm SiO₂ layer as substrates. Several SiO₂/Si (2 cm × 2 cm) and sapphire pieces were placed on an alumina boat containing precursor material (ReO₃) and the boat was loaded in the middle of a tube furnace, as shown in Fig. 7a. Se (1 g) was placed in the low-temperature zone at 200 °C and high purity N₂ and H₂ was used as a carrier gas, as shown in Fig. 7a. Then, the furnace was heated to 625 °C in 15 min and stayed at a maximum temperature for 10 min. After the growth, the samples were furnace-cooled to room temperature. As-synthesized samples were characterized by optical microscopy and the final product was hexagonal-shaped ReSe₂ flakes with lateral size up to 5 μm (Fig. 7b). A slight variation was observed between different ReSe₂ flakes, as can be seen from the color variation in the magnified image in the inset of Fig. 7b. The thickness of ReSe₂ flakes was further investigated by height profile using AFM. The measured height was ∼4.2 nm, which corresponded to six layers of ReSe₂ (Fig. 7c). Interestingly, a wrinkle feature was observed in thin ReSe₂ flakes, which is common in metal selenides such as WSe₂⁸⁹ and Bi₂Se₃.⁹⁰ Raman signals of ReSe₂ hexagonal flakes displayed a similar Raman signal, which is in accordance with reported work.⁶⁰ The sharpness of Raman modes clearly indicates the high crystalline quality of as-synthesized ReSe₂ flakes. Sample composition was evaluated by energy dispersive X-ray spectroscopy (EDX) and an atomic ratio of 33.03 Re to 66.97 Se was deduced from the EDX spectrum, which matched well with the stoichiometric ratio of ReSe₂. All these characterizations indicated that as-grown ReSe₂ flakes were of high-quality crystal.

Fig. 7 CVD synthesis of ReSe₂, (a) schematic the CVD growth approach, (b) optical microscope (OM) image of the ReSe₂ hexagonal flakes grown on SiO₂/Si substrates; inset shows the magnified OM image for the clarity. (c) AFM image of the corresponding ReSe₂ hexagons and the inset shows the AFM height profile for the ReSe₂ hexagon. (d) Raman spectra collected from different thicknesses of the as-synthesized ReSe₂ hexagons. (e) EDX spectrum of the ReSe₂ flake. Reproduced with permission.⁸⁸ Copyright 2016, Wiley-VCH.

Device applications

Due to weak interlayer coupling, ReS₂ remains a direct band-gap material upon reduction of the thickness from bulk to monolayer. This implies that thicker samples of ReS₂ could have a higher density of states and enhanced light absorption. Therefore, few-layer ReS₂ becomes an ideal candidate for highly responsive photodetectors.

Photodetectors based on ReX₂

Xiu and co-workers⁸² fabricated two-terminal back-gate photodetectors based on CVD-grown few-layers of ReS₂, as shown in Fig. 8a. The device was probed using a focused laser beam (633 nm) with laser power ranging from 12.5 to 1000 nW, with a continuous 50 V back gate bias (Fig. 8b). A significant increase of the photocurrent was observed and showed a strong dependence on gate bias and laser power. Maximum photoresponsivity was measured up to 16.14 A W⁻¹ and the external quantum efficiency reached to 3168%. Time-resolved photo-detection exhibited a stable and repeatable response, but the response was very slow (∼10⁴ s) with a small on/off ratio of ≈2.8 (Fig. 8c), as compared with photodetectors based on conventional TMDs. This slow photo-response may be due to poor morphologies and ambiguous, trailing electron diffraction spots of synthesized ReS₂.

Fig. 8 (a) Schematic structure of a ReS₂ photodetector device. (b) IV characteristics of the device under different incident laser power P_in. (c) Time-dependent I_DS of the device with and without laser illumination. Inset, plot of the photoresponse peak over a small range, which shows a tendency toward saturation. Reproduced with permission.⁸² Copyright 2015, Wiley-VCH. (d) The characteristic current–voltage (I–V) curve of the single ReS₂ hexagon device fabricated on a freshly cleaned SiO₂/Si substrate; inset shows the schematic image of the device. (e) The stable and repetitive switching characteristic curves with the light (500 nm) turned on/off periodically at 10 s intervals with a bias of 0.5 V. (f) Time-dependent photo-response curves in different environments (air and vacuum). Reproduced with permission.⁸³ Copyright 2016, Wiley-VCH. (g) Schematic and measurement circuit of few-layer ReS₂ phototransistors. (h) Optical microscopy image of a few-layer ReS₂ phototransistor. The scale bar is 10 μm. (i) The dependence of RP on light. Under weak illumination (≈6 pW), RP reached ≤88 600 A W⁻¹. For comparison, the results from monolayer MoS₂, multilayer GaTe, and graphene are also presented. The error bar is smaller than the size of the dots. Weak signal detection in a 5-layer ReS₂ phototransistor using a lighter and limited fluorescent lighting as the weak light sources. V_bg was fixed at −50 V and V_ds was fixed at 2.0 V. The areas with light-green background represent the states under illumination. Reproduced with permission.⁹² Copyright 2016, Wiley-VCH.

Zhai and co-workers⁶² fabricated photodetector devices based on high-quality CVD-grown single-crystal hexagonal ReS₂ flakes. A 500 nm incident light of power intensity 3.11 mW cm⁻² was illuminated perpendicularly to the devices. The measured photocurrent was 1.85 μA with an on–off ratio ≈32 and an ultrafast response time of 2 ms, as shown in Fig. 8e. The calculated responsivity of ReS₂ hexagons was ≈604 A W⁻¹ with an EQE ≈ 1.50 × 10⁵% and a specific detectivity D* ≈ 4.44 × 10¹⁰ Jones. Moreover, the photocurrent in a vacuum was larger than that in air, which revealed the n-type nature of as-grown ReS₂ flakes (Fig. 8f). The high performance of the photodetector was attributed to the Schottky contact, high quality of samples, and the weak interlayer coupling in ReS₂.

Xing and co-worker⁹¹ introduced a new method for the fabrication of photodetector devices. They used a buffer layer between the few-layer ReS₂ and SiO₂/Si substrate, as shown in Fig. 8g. The h-BN layer was helpful to reduce the scattering from impurities in the substrate, as has been shown to improve the device performance in graphene⁹² and MoS₂.⁹³ The devices were illuminated under 532 nm laser light with power ranging from 6 pW to 29 μW, under the fixed bias of 4.0 V and a back gate voltage fixed at −50 V. Under the minimum power 6 pW, the measured photoresponsivity of the ReS₂ phototransistor reached ≤88 600 A W⁻¹ and the external quantum efficiency reached 2 × 10⁷%. These values are the highest values among the reported photodetectors based on 2D materials, monolayer MoS₂,⁹⁴ multilayer GaTe⁹⁵ and graphene⁹⁶ (Fig. 8h). Due to the ultra-high sensitivity of few-layer ReS₂ phototransistors, they are suitable for the detection of very weak signals such as lighter (photocurrent ∼1 nA) and fluorescent lighting (photocurrent ∼7.6 nA), as shown in Fig. 8i.

In the case of ReSe₂, the presence of Re₄ chains makes it an optically biaxial material that shows exceptionally anisotropic electrical and optical behavior for linearly polarized light. Wei and co-workers⁴⁵ reported the first monolayer ReSe₂-based photodetector. The monolayers were exfoliated from a bulk sample and vacuum-annealed to remove remaining residue. Devices were fabricated by standard e-beam lithography with 5 nm-thick Cr and 50 nm-thick gold contacts, whereas the width (W) and length (L) of the channel in the device were both ∼2 μm, as shown in the schematic and AFM image (inset) in Fig. 9a. A 633 nm laser with optical power intensity of 100 mW cm⁻² was focused onto the devices, and time-dependent photo-responses were measured in various gas environments. The photo-responsivity (in air) was calculated to be 17.8 A W⁻¹ and external quantum efficiency (EQE) was 3484%. These values are quite good and reasonable compared with other conventional TMDs.

Fig. 9 (a) A schematic of a single-layer ReSe₂ FET together with two gold electrodes. Inset: Atomic force microscopy (AFM) image of the device based on the ReSe₂ single layer. (b) Time-dependent photoresponse with bias voltage V_ds = 0.5 V in O₂, air or NH₃ environment with light (633 nm, 100 mW cm⁻²). Reproduced with permission.⁴⁵ Copyright 2014, Royal Society of Chemistry. (c) Characteristic current–voltage (I–V) curve of the single ReSe₂ hexagon device fabricated on a SiO₂/Si substrate; inset shows a schematic image of the device. (d) The stable and repetitive switching characteristic curves with the light (808 nm) turned on/off periodically at 60 s intervals with a bias of 5 V. Reproduced with permission.⁸⁸ Copyright 2016, Wiley-VCH. (e) Schematic diagram and optical image of back-gated (90 nm thick SiO₂ gate dielectric) ReSe₂ devices doped by PPh₃ and a descriptive illustration for the n-doping mechanism by PPh₃ at the PPh₃/ReSe₂ interface. Reproduced with permission.⁹⁷ Copyright 2016, Wiley-VCH. (f) Photoresponse as a function of time obtained before and after PPh₃ doping. (g) Photoresponsivity of ReSe₂ photodetectors as a function of the wavelength. (h) Schematic structure of polarization-sensitive photodetectors based on ReSe₂ nanosheets. (i) SEM image of the ReSe₂ photodetector followed by polarization-dependent photocurrent mapping of the device, showing prominent linear dichroic photodetection. The thickness of the ReSe₂ channel is 12 nm. Reproduced with permission.⁹⁸ Copyright 2016, American Chemical Society.

Zhai and coworkers⁸⁸ fabricated photodetector devices by using same device structure employed by Wei et al. but with CVD-grown ReSe₂ hexagonal flakes, as shown schematically in Fig. 9c. A significant change in the current (∼100 nA) was observed under a 808 nm incident light of power intensity 566 mW cm⁻², as shown in Fig. 9c. The photodetector devices were very stable and completely reversible, as shown in a cyclability test (Fig. 9d). The photo-responsivity was calculated to be ∼2.98 A W⁻¹ and maximum EQE was 4.58 × 10²%. These values are comparable with an earlier report of exfoliated samples.

Park and co-workers⁹⁷ introduced a new method for the enhancement of photoresponses by improving the source-side contact resistance through a triphenyl phosphine (PPh₃)-based n-doping technique, as shown schematically in Fig. 9e. The temporal photoresponse was measured in the undoped (control) and PPh₃-doped ReSe₂ photodetectors by using 520 nm laser light with 1 nW laser power, as presented in Fig. 9f. By performing the PPh₃-based n-doping process, a significant enhancement from 3.97 × 10³ A W⁻¹ to 3.68 × 10⁴ A W⁻¹ was observed. Fig. 9g reveals that a broad range of photo-detection (possible above 1064 nm) can be achieved with ultrafast photo-switching (τ_r = 30 ms and τ_d = 64 ms).

Xiu and co-workers⁹⁸ reported polarization-sensitive photodetection behavior due to the intrinsic linear dichroism originating from high in-plane optical anisotropy. Usually, linear dichroism requires an anisotropic crystal and Raman spectroscopy reveals high in-plane crystal anisotropy in ReSe₂. A light polarizer and a half-wave plate were used to change the angle θ between the polarization direction of incident laser (633 nm) and the b-axis (DS chains direction) of the sample, as shown in Fig. 9h. Fig. 9i shows that the photocurrent has a maximum value when the incident light is polarized along the b crystal axis (0° polarization) and reaches a minimum at the perpendicular setup (90° polarization).

FET based on ReX₂

FET is one of the most elementary components in electronics, and consists of three parts: (i) drain–source metal contacts, (ii) dielectric layer (gate electrode), and (iii) semiconducting channel. The drain–source current is controlled by the gate voltage on the dielectric layer. Xing and co-workers⁶¹ reported the performance of back-gated FET devices based on the different thicknesses of ReS₂ channels. Fig. 10a shows the FET transfer curve (for monolayer and trilayers) obtained by measuring the source–drain current I_ds while sweeping the back-gate voltage V_bg from −50 to +50 V with a fixed 100 mV source–drain bias voltage V_sd. The inset of Fig. 10a presents the optical image of a typical monolayer device. Both devices behaved as excellent n-type semiconductors with an excellent current on/off ratio of ∼10⁷, which indicates that ReS₂ is a suitable material for many applications in electronics. Subthreshold swing can also be measured from the transfer curves, and was measured as 310 mV per decade for a monolayer device and 100 mV per decade for the trilayer device, as shown in Fig. 10a. These values are comparable with top-gated MoS₂ transistors⁹⁹ and outperform black-phosphorus¹⁰⁰ devices. The layer number (monolayer to 7 layers)-dependent properties of FET devices were studied and plotted in Fig. 10c. The mobility of monolayer device varied between 0.1 and 2.6 cm² V⁻¹ s⁻¹, and the highest mobility was obtained as 15.4 cm² V⁻¹ s⁻¹ for a six-layer device. This difference in mobility values with the same number of layers could be induced by the anisotropic property. Suenaga and coworkers³⁷ investigated the relationship between the atomic structure and anisotropic transport properties. ReS₂ flakes were exfoliated onto SiO₂/Si and the number of layers was identified using color contrast in the optical microscope, as shown in Fig. 10d. Electrodes were deposited opposite to each other and marked as 1, 2, 3, and 4 (Fig. 10d). Electrical measurements were carried out and plotted in Fig. 10e. The high-magnification ADF image of the ReS₂ device shows Re diamond chains to be aligned parallel to the 1–4 and 2–3 direction. The two-terminal conductance parallel to the b-axis was 0.82 μS for V_ds = 1 V and V_g = 20 V, whereas conductance perpendicular to the b axis was one order of magnitude smaller (0.075 μS). The mobility of the device along the b-axis was calculated to be μ_1–4 = 23.1 cm² (V s)⁻¹, whereas electrons exhibited lower mobility (14.8 cm² (V s)⁻¹) perpendicular to the b-axis (Fig. 10f).

Fig. 10 ReS₂ field-effect transistor devices. (a) Transfer curves of monolayer (red) and trilayer (blue) ReS₂ FET devices. V_ds is fixed at 100 mV. The on/off ratio is ∼10⁷ for the monolayer device and 10⁷ for the seven-layer device. The subthreshold swings are 310 mV per decade (monolayer) and 100 mV per decade (trilayer), respectively. Inset: Optical image of a typical monolayer ReS₂ FET device. Scale bar, 5 mm. (b) I_ds–V_ds curves of a monolayer ReS₂ FET at different V_bg, with linear dependence indicating ohmic contact. (c) The dependence of device mobility on the number of layers. In general, the mobility increases monotonically with the number of layers with some scattering. Reproduced with permission.⁶¹ Copyright 2015, Nature Publishing Group. (d) Optical microscope image of a ReS₂ four-probe transistor on a SiO₂/Si substrate. Channel length and width of the device between the opposite contacts, e.g., 1–3 and 2–4, are 1 and 0.3 μm. The approximate channel length and width for adjacent contacts, e.g., 1–2, 2–3, 3–4, and 1–4, are 0.7 and 0.3 μm. (b) Back-lighting optical microscopic image of the identical ReS₂ device transferred onto a TEM quantifoil. The magnified ADF images taken are at a scale bar of 1 nm. (e) The direction-dependent I–V characteristics with applied back gate voltage V_g = 20 V. (f) The direction-dependent transfer characteristics with drain–source bias of −3 V. Reproduced with permission.³⁶ Copyright 2015, American Chemical Society.

The intrinsic transport properties of ReSe₂ were studied and showed ambipolar behaviour:⁹⁸ two types of a four-terminal back-gate ReSe₂ FETs devices were fabricated on, SiO₂ (285 nm)/Si or hBN/SiO₂ (7/285 nm). Fig. 11(a) and (d) display a schematic device structure of two kinds of devices; correspondingly, the optical images are provided in the insets of Fig. 11b and e. A four-probe configuration was used for the conductance measurement to avoid contact resistance, and hBN and SiO₂ served as the gate dielectric. Parts b and e of Fig. 11 display the temperature-dependent conductance as a function of VBG of the devices built on SiO₂ and hBN/SiO₂ substrates, respectively. Both devices exhibited ambipolar behavior and the temperature-dependent electron and hole mobilities are plotted in Fig. 11c and f. The ReSe₂ devices on hBN/SiO₂ exhibited a significant increase in electron motility over 500 (3) times and hole mobility over 100 (60) times at low (room) temperature. This improvement in mobility from the devices on hBN/SiO₂ can be attributed to less scattering from charged impurities.

Fig. 11 Temperature-dependent transport properties of four-terminal few-layer ReSe₂ FETs on SiO₂ and hBN substrates. (a) Schematic structure of a ReSe₂ back-gate FET on a SiO₂ substrate. (b) Temperature-dependent sheet conductance versus back-gate voltage showing evident ambipolar behavior of the device on a SiO₂ (285 nm) substrate. The channel is 6 nm thick. Inset: Optical image of the FET device. Scale bar, 5 μm. (c) Extracted four-terminal field-effect electron and hole mobility versus temperature for the device on a SiO₂ substrate. (d) Schematic structure of the device on a hBN/SiO₂ substrate. (e) Temperature-dependent sheet conductance versus back-gate voltage of the device on a hBN/SiO₂ (7 nm/285 nm) substrate with a channel thickness of 7 nm. Inset: Optical image of the sample before and after the electrode fabrication. Scale bar, 4 μm. (f) Extracted four-terminal field-effect electron and hole mobility versus temperature for the device on a hBN/SiO₂ substrate. Reproduced with permission.⁹⁸ Copyright 2016, American Chemical Society.

Prospects for thermoelectric applications

The 2D structure and low symmetry-induced anisotropy have affected the thermoelectric performance significantly, as reported recently for the case of black phosphorus,¹⁰¹ SnSe¹⁰² and TaX₂ (X = S, Se).¹⁰³ Hence, these unique anisotropic ReX₂ materials may also be useful for enhancing the thermoelectric performance. Unlike other 2D materials, these ReX₂ have weak interlayer coupling, and the electronic free carriers may have to transport across the van der Waals interface through a tunneling effect whereas the phonon transport will go through van der Waals interfaces. Thus, the ratio between these two types of transport may be different from the usually strong interlayer coupling interfaces, which may result in an enhancement of the thermoelectric figure of merit (ZT).

Conclusions

In this review, we have made a concise and comprehensive summary of the typical fundamental properties and potential applications of a new member, ReX₂ (X = S, Se), in the 2D TMDs family. Compared with other methods, vapor deposition is considered to be the most successful method for high-quality ReX₂ fabrication. Crystal orientation-dependent photodetector and FETs measurements revealed that the performance has a strong positive relationship with the b-axis direction. Raman spectroscopy can be a powerful tool for the determination of crystal orientation, which paves the way for future applications in the anisotropic properties of ReX₂. With regard to synthesis, future research is expected to focus on large-area high-quality ReX₂ film or pattern growth of ReX₂ single crystals, which are the basic requirements for integrated circuits. For study of devices, (opto-)electronic properties combined with the anisotropic features of ReX₂ could be a hot topic. Finally, the recent exciting achievements on the study of 2D ReX₂ have opened up new opportunities for further research on 2D TMDs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (91622117, 51472097, and 21501060), National Key Research and Development Program of “Strategic Advanced Electronic Materials” (2016YFB0401100), National Basic Research Program of China (2015CB932600), Program or HUST Interdisciplinary Innovation Team (2015ZDTD038) and the Fundamental Research Funds for the Central University. The authors are indebted for the kind permission from the corresponding publishers/authors to reproduce their materials (especially figures) in this review article.

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