Thermally stable one-dimensional hexagonal phase vanadium sulfide nanowire and preparation method thereof

Abstract

The invention provides a general salt-assisted chemical vapor deposition (SA-CVD) synthesis method for preparing 1D hexagonal phase MxV6S8 (M = K, Rb, Cs) and KxV6SySe8-y nanowires with high yield. The obtained nanowire shows typical metal characteristics and can be used as good Van der Waals contact so as to realize a high-performance Fermi-level-free pinned transistor. The synthesis method allows more systematic research on the inherent characteristics of the hexagonal phase V6S8 structure.

Description

Cross - Reference to Related Applications

[0001] This application claims priority to U.S. Patent Application No. 18 / 523,934, filed Nov. 30, 2023, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0002] The present invention generally relates to at least the following fields: materials science, nanomaterial synthesis, chemical synthesis techniques, crystal structure research, and potential applications in catalysis and electronic devices. More specifically, the present invention relates to salt - assisted synthesis of one - dimensional hexagonal phase vanadium sulfide nanowires. Background Art

[0003] In the past decade, transition metal - chalcogenides (TMCs) have received significant attention due to their potential in applications such as gas sensing, catalysis, optoelectronics, energy storage, and as an ideal platform for fundamental research on layered quantum systems. One - dimensional (1D) TMC nanowires, such as M 6 X 6 (M: Mo or W and X: S, Se or Te) nanowires have been widely studied due to their unique crystal structures and resulting unique physical and chemical properties. 1-5 . These properties lead to various possible applications, including catalysis (e.g., hydrogen evolution reaction) 6 and electronics (e.g., van der Waals contacts for two - dimensional (2D) semiconductors). 7-8 .

[0004] As typical transition metal chalcogenides, vanadium sulfides have multiple compositions, including VS, VS 2 , V 2 S 3 , V 3 S 4 , V 5 S 8 and V 6 S 8 , some of which can crystallize into different phases with different physical and chemical properties. For example, V 6 S 8 exhibits two polymorphs: monoclinic and hexagonal. However, most studies have mainly focused on the synthesis of VS 2 , and few reports have been made on the synthesis of other vanadium sulfides, especially V 6 S 8 .

[0005] Non - stoichiometric M x V 6 S 8(M = K, Rb, Cs) crystals crystallize in a hexagonal structure consisting of VS connected together by shared edges and faces. 6 Octahedral structure, thus forming a large hexagonal channel extending parallel to the c-axis 9-11 These channels can accommodate a variety of atoms, including K, Rb, Cs, etc., allowing fine-tuning of the electronic configuration of the structure. For example, Zheng Chong et al. reported the synthesis of Ba x V 6 S 8 The molten salt growth method of the crystal. Specifically, BaS, V, S and KCl powders were mixed in a vacuum quartz ampoule and the ampoule was heated for 6 days. 9 However, the reaction time throughout the entire synthesis process is too long, which is not conducive to the industry that requires large-scale production; and GA Wiegers et al. reported a method for synthesizing K 0.2 V 6 S 8 Closed system gas-solid method for crystal formation. Specifically, K 2 S, V and S powders were prepared and the ampoule was then heated at 1200 K for about a week. In addition, the wall of the quartz ampoule was severely damaged during the reaction process, which made the synthesis very challenging. 10 Although the reported method for preparing hexagonal M x V 6 S 8 Several synthetic strategies for bulk crystals have been proposed, including closed-system gas-solid methods and ion exchange methods, but these methods generally require complex procedures, multiple steps, and long reaction times of up to one week, and can only produce limited types of M in low yields. x V 6 S 8 Crystal 9-11 .

[0006] Salt-assisted chemical vapor deposition (SA-CVD) techniques using salt or molten salt as precursors offer a robust approach with significant potential for synthesizing 1D materials. These methods effectively lower the melting point of the precursor, increase the vapor pressure, and accelerate the growth rate. 12-15 Compared with conventional chemical vapor deposition (CVD) methods, the SA-CVD method shows clear advantages in producing 1D TMC nanowires, thereby increasing yield and improving reproducibility. 4,16 Nevertheless, the application of SA-CVD as a catalyst for the synthesis of 1D hexagonal M phases has not yet been explored. x V 6 S 8 Therefore, it is highly desirable to develop a general strategy for synthesizing hexagonal M nanowires in high yield. x V 6S 8 General method for nanowires. Summary of the Invention

[0007] To address the foregoing drawbacks, the present invention has developed a novel CVD method for synthesizing V with a specific morphology and crystal phase 6 S 8 crystals.

[0008] The present invention provides a direct CVD method for synthesizing one-dimensional non-stoichiometric hexagonal phase vanadium sulfide nanowires with high yield, high electrical conductivity, and high catalytic activity. In this structure, VS 6 octahedral units are interconnected by sharing faces and edges to form a stable three-dimensional (3D) V with hexagonal channels parallel to the c-axis 6 S 8 network. The diameter of the channels is large enough to accommodate different atoms, such as K, Rb, Cs, etc.

[0009] In a first aspect, the present invention provides a thermally stable 1D hexagonal phase M x V y S z nanowires, where M is potassium (K), rubidium (Rb), or cesium (Cs); x is about 0.2 or 1.12; y is 6; z is 8. The thermally stable 1D hexagonal phase M x V y S z nanowires have c-axis aligned hexagonal channels. The thermally stable 1D hexagonal phase M x V y S z nanowires exhibit an invariant Raman spectrum in the temperature range of 30 °C to 300 °C. Preferably, y is 6 and z is 8.

[0010] According to one embodiment, the c-axis aligned hexagonal channels have a diameter in the range of to

[0011] According to one embodiment, the thermally stable 1D hexagonal phase M x V y S z nanowires serve as an effective van der Waals contact for MoS 2 -based field effect transistors, which have good ohmic contact, large charge mobility, and reduced Fermi-level pinning effect.

[0012] According to one embodiment, the M x V y Sz The nanowire has a thickness in the range of 1 to 15 nanometers.

[0013] According to another embodiment, the M x V y S z The nanowire is K 0.2 V 6 S 8 , exhibiting five distinct Raman peaks recorded at 532 nanometers, including 169.4, 221.5, 326.7, 340.0, and 374.1 cm -1 .

[0014] In a second aspect, the present invention provides a thermally stable one-dimensional hexagonal phase K x V 6 S y Se 8-y nanowire, where x is approximately 0.68 or 1.34; y is 7.02 or 6.79. The thermally stable one-dimensional hexagonal phase K x V 6 S y Se 8-y nanowire has c-axis aligned hexagonal channels. The thermally stable one-dimensional hexagonal phase K x V 6 S y Se 8-y nanowire exhibits an invariant Raman spectrum in the temperature range of 30 °C to 300 °C.

[0015] According to one embodiment, the c-axis aligned hexagonal channels have a diameter in the range of to .

[0016] According to one embodiment, the thermally stable one-dimensional hexagonal phase K x V 6 S y Se 8-y nanowire serves as an effective van der Waals contact for a field-effect transistor based on MoS 2 , the field-effect transistor having good ohmic contact, large charge mobility, and reduced Fermi level pinning effect.

[0017] According to one embodiment, the K x V 6 S y Se 8-y nanowire is K 0.68 V 6 S 7.02 Se 0.98, showing five unique Raman peaks recorded at 532 nm, including 165.2, 221.3, 328.4, 337.9, and 375.7 cm -1 .

[0018] According to another embodiment, the K x V 6 S y Se 8-y nanowire is K 1.34 V 6 S 6.79 Se 1.21 , showing five unique Raman peaks recorded at 532 nm, including 157.5, 213.6, 328.4, 337.9, and 370.1 cm -1 .

[0019] In a third aspect, the present invention provides a salt-assisted method for synthesizing one-dimensional hexagonal vanadium sulfide nanowires on a substrate, the salt-assisted method comprising the following steps: Preparing a substrate; Providing a precursor by mixing a vanadium(V) compound and a sulfur(S) compound; Introducing a metal salt into the precursor via the SA-CVD method to obtain a mixture; Transferring the mixture to a quartz boat and covering the top of the loaded mixture with two freshly cleaved fluorophlogopite mica substrates; Transferring the quartz boat to the center of a quartz tube, and then introducing Ar and H 2 to provide an optimal synthesis atmosphere; Heating the quartz tube, and then cooling the quartz boat to room temperature; and Growing the one-dimensional hexagonal M x V 6 S 8 nanowires and K x V 6 S y Se 8-y nanowires.

[0020] According to one embodiment, the ratio between the precursor and the metal salt is in the range of 1:1 to 1:1.5.

[0021] According to one embodiment, the step of providing the precursor includes mixing V 2 S 3 powder and S powder.

[0022] According to one embodiment, the substrate is a mica substrate.

[0023] According to one embodiment, the metal salt is selected from the group consisting of: KCl, K 2 S, K 2 CO 3 、KHCO 3 、K 2 C 2 O 4 ·H 2 O, Rb 2 CO 3 and Cs 2 CO 3 .

[0024] According to one embodiment, the quartz tube is purged with 500 sccm for 20 minutes in advance to remove internal air and moisture.

[0025] According to one embodiment, the flow rate ratio between Ar and H 2 is 4:1.

[0026] According to one embodiment, the precursor further comprises a selenium (Se) compound.

[0027] Compared with the prior art, the present invention provides the following main advantages: (1) The developed synthetic strategy employs a highly robust and direct salt-assisted CVD method, enabling the large-scale preparation of 1D hexagonal phase M x V 6 S 8 nanowires with excellent yields and reproducibility. This method can be easily scaled up by using a larger CVD furnace, thus demonstrating bright potential for the future electronics industry. (2) The present invention provides a general method for directly synthesizing a series of 1D MxV6S8 nanowires, which have the same 3D V6S8 network structure but different atomic compositions within the hexagonal channels. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Embodiments of the present invention will be described in more detail below with reference to the drawings, in which:

[0029] Figure 1A A schematic diagram of the hexagonal phase M x V 6 S 8 structure along the c-axis is depicted. Figure 1B A schematic diagram of the unit cell of the hexagonal phase M x V 6 S 8 structure is depicted. Figure 1C A schematic diagram of the hexagonal phase M x V 6 S 8Three-dimensional (3D) schematic diagram of the structure along the c-axis;

[0030] Figure 2 Depicts the experimental setup for synthesizing 1D hexagonal phase M x V 6 S 8 nanowires;

[0031] Figure 3A and Figure 3B Shows the optical and SEM images of the prepared K x V 6 S 8 nanowires on a mica substrate. The arrows indicate three different preferred growth orientations of the nanowires;

[0032] Figure 4 Shows the atomic force microscope (AFM) height image of the prepared K x V 6 S 8 nanowires;

[0033] Figure 5 Depicts the X-ray diffraction (XRD) pattern of the prepared hexagonal phase K x V 6 S 8 nanowires;

[0034] Figure 6A and Figure 6B Shows the low magnification and high resolution transmission electron microscope (TEM) images of a typical hexagonal phase K x V 6 S 8 nanowire along the zone axis. Figure 6C Shows the corresponding fast Fourier transform (FFT) pattern cropped from the high resolution transmission electron microscope (HRTEM) image in Figure 6B ; Figure 6D Shows the selected area electron diffraction (SAED) pattern of a typical K x V 6 S 8 nanowire;

[0035] Figure 7A and Figure 7B Shows the experimental and simulated SAED patterns of the hexagonal phase KxV6S8 structure along the zone axis, showing good agreement;

[0036] Figure 8A Shows the prepared K x V 6 S 8 nanowire along Spherical aberration corrected aberration corrected high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) (HAADF-STEM) images of the zone axis. Figure 8B Shows the prepared K x V 6 S 8 nanowires along the large area HAADF-STEM image of the zone axis. Figure 8C Shows the prepared K x V 6 S 8 nanowires along the simulated STEM image of the zone axis;

[0037] Figure 9A and Figure 9B Shows K x V 6 S 8 the cross-sectional HAADF-STEM image of the nanowires along the c-axis. Figure 9B The white arrows in show the presence of potassium ions in some hexagonal channels. Figure 9C Shows the hexagonal phase K x V 6 S 8 the simulated cross-sectional STEM image of the structure along the c-axis. Figure 9D Depicts the hexagonal phase K x V 6 S 8 the atomic structure model of the structure along the c-axis;

[0038] Figure 10A Shows the corresponding fast Fourier transform (FFT) pattern cropped from the cross-sectional HAADF-STEM image in Figure 9A . Figure 10B Shows the hexagonal phase K x V 6 S 8 the simulated SAED pattern of the structure along the c-axis;

[0039] Figure 11 Shows the prepared K x V 6 S 8 the dark field STEM image of the nanowires and the corresponding elemental mapping image of the K x V 6 S 8 nanowires. Also shows the hexagonal phase K x V 6 S 8Element mapping and energy-dispersive X-ray spectroscopy (EDS) characterization of the nanowires in STEM mode. Additional elemental signals are from the Cu TEM grid. Inset: corresponding weight and atomic ratios of V, S, and K elements obtained from the EDS spectra;

[0040] Figure 12A Depicts the prepared hexagonal K x V 6 S 8 Raman spectra of the nanowires (gray curve) and the bare Si / SiO 2 substrate (black curve). Figure 12B Shows the optical image (left) and Raman mapping image (right) of a single K x V 6 S 8 nanowire. Figure 12C Depicts the prepared hexagonal K x V 6 S 8 PL spectra of the nanowires (gray curve) and the bare Si / SiO 2 substrate (black curve);

[0041] Figure 13 Shows 0° and 90° between the incident laser polarization (arrow) and the c-axis of the synthesized K x V 6 S 8 nanowires;

[0042] Figure 14A Depicts the angle-resolved polarization Raman spectra of the synthesized K x V 6 S 8 nanowires measured at a 532-nm laser and polarization angles varying from 0° to 180°. Figure 14B Depicts the corresponding hot-spot mapping of the Raman spectra at different polarization angles in the range between 0° and 360°;

[0043] Figure 15 Depicts the polar plot of the Raman peak intensity as a function of the polarization angle;

[0044] Figure 16 Depicts the Raman spectra of the prepared K x V 6 S 8 nanowires at different heating temperatures in the range between 30 °C and 300 °C;

[0045] Figure 17A Depicts the X-ray photoelectron spectroscopy (XPS) spectra of V 2p in the grown K x V 6 S 8 nanowires.Figure 17B Depicts K x V 6 S 8 XPS spectrum of S2p in the nanowires. Figure 17C Depicts K x V 6 S 8 XPS spectrum of K 2p in the nanowires;

[0046] Figure 18A Shows the prepared K on a mica substrate by adding 1 mmol of selenium powder to the reaction mixture x V 6 S y Se 8-y Optical image of the nanowires. Figure 18B Shows the prepared K on a mica substrate by adding 2 mmol of selenium powder to the reaction mixture x V 6 S y Se 8-y Optical image of the nanowires;

[0047] Figure 19A and Figure 19B Depicts the synthesized hexagonal K x V 6 S y Se 8-y nanowires (gray curve) and the Raman spectrum of the bare Si / SiO 2 substrate (black curve);

[0048] Figure 20A and Figure 20B Depicts the synthesized hexagonal K x V 6 S y Se 8-y nanowires (black curve) and the PL spectrum of the bare Si / SiO 2 substrate (gray curve);

[0049] Figure 21A and Figure 21B Shows the typical hexagonal K synthesized by adding 2 mmol of selenium powder to the reaction mixture x V 6 S y Se 8-y Low magnification and high resolution TEM images of the nanowires. Figure 21C Shows the corresponding FFT pattern cropped from the HRTEM image in Figure 21B . Figure 21D Shows the typical K x V 6 S y Se 8-ySAED pattern of the nanowires;

[0050] Figure 22 Depicts the prepared K x V 6 S y Se 8-y Dark-field STEM image of the nanowires and K x V 6 S y Se 8-y Corresponding elemental mapping images of the nanowires. Also shown is the hexagonal phase K x V 6 S y Se 8-y EDS characterization of the nanowires in STEM mode. The additional elemental signals originate from the Cu TEM grid. Inset: Corresponding weight and atomic ratios of V, S, Se, and K elements obtained from the EDS spectra;

[0051] Figures 23A to 23C Depicts the hexagonal phase K x V 6 S 8 nanowires synthesized with the assistance of different metal salts, Rb x V 6 S 8 nanowires, and Cs x V 6 S 8 optical images of the nanowires; and

[0052] Figure 24A Shows the optical image of the three-terminal device based on a single K x V 6 S 8 nanowire. Figure 24B Depicts the typical V DS -I DS electrical curve without gate bias. Figure 24C Depicts the typical transfer curve of the device at different source-drain biases. Figure 24D Shows the optical image of the field-effect transistor based on MoS 2 with asymmetric electrode contacts consisting of a nanowire contact on one side and a Cr / Au contact on the other side. Figure 24E Depicts the typical transfer of the field-effect transistor based on MoS 2 Figure 24F Depicts the output curve of the field-effect transistor based on MoS 2 Detailed Description

[0053] ​​The present invention will be described in detail through the following embodiments and accompanying drawings. It should be understood that the specific embodiments are provided for illustrative purposes only and should not be construed in a limiting manner. Those skilled in the art will understand that changes and modifications can be made to the present invention described herein other than those specifically described.

[0054] The present invention encompasses all such changes and modifications. The present invention also encompasses all the steps and features individually or jointly referred to or indicated in the specification, as well as any and all combinations of any two or more of said steps or features. Other aspects and advantages of the present invention will be apparent to those skilled in the art upon reading the following description.

[0055] 1D TMCs have attracted increasing scientific and technological interest, especially for applications in ultra-small electronic interconnections and highly active catalysts. Nevertheless, the range of available TMC nanowires remains rather limited, and the synthesis of new 1D TMC nanowires remains a highly challenging endeavor.

[0056] To date, due to complex growth conditions, most research efforts have mainly focused on the synthesis of VS 2 , while there has been limited attention to other vanadium sulfides, especially V 6 S 8 . Only a few studies have reported the closed-system gas-solid synthesis of the V 6 S 8 structure. Nevertheless, previous gas-solid methods typically involve complex procedures, multiple steps, and extended reaction times of up to a week, making the synthesis process complex and time-consuming. In addition, this method cannot be used as a general method for producing various M x V 6 S 8 structures.

[0057] In view of this, the present invention introduces a thermally stable 1D hexagonal phase M x V y S z nanowire, where M is potassium (K), rubidium (Rb), or cesium (Cs); x is about 0.2 or 1.12; y is 6; and z is 8. The thermally stable 1D hexagonal phase M x V y S z nanowire has c-axis aligned hexagonal channels. The thermally stable 1D hexagonal phase M x V y S z nanowire exhibits an invariant Raman spectrum in the temperature range of 30 °C to 300 °C. Preferably, y is 6 and z is 8.

[0058] These nanowires serve as a basis for MoS2 Effective van der Waals contact of the field-effect transistor, which has good ohmic contact, large charge mobility, and reduced Fermi level pinning effect.

[0059] Hexagonal phase M synthesized by CVD x V 6 S 8 The structure is superconducting due to its spacious c-axis aligned channels that can accommodate various ions, which enables precise adjustment of the electronic configuration and improvement of the superconducting properties.

[0060] The present invention also provides a thermally stable 1D hexagonal phase K x V 6 S y Se 8-y nanowire, where x is about 0.68 or 1.34; y is 7.02 or 6.79. The thermally stable 1D hexagonal phase K x V 6 S y Se 8-y nanowire has c-axis aligned hexagonal channels. The thermally stable 1D hexagonal phase K x V 6 S y Se 8-y nanowire exhibits an invariant Raman spectrum in the temperature range of 30 °C to 300 °C.

[0061] In addition, the present invention further provides a general SA-CVD synthesis method for high-yield preparation of 1D hexagonal phase M x V 6 S 8 and K x V 6 S y Se 8-y nanowires on a mica substrate. The method includes: preparing a substrate; providing a precursor by mixing vanadium (V) and sulfur (S) compounds; introducing a metal salt into the precursor via the SA-CVD method to obtain a mixture; transferring the mixture to a quartz boat and covering the loaded mixture with two freshly cleaved fluorophlogopite substrates on top; transferring the quartz boat to the center of a quartz tube, and then introducing Ar and H 2 to provide an optimal synthesis atmosphere; heating the quartz tube and then cooling the quartz boat to room temperature; and growing one-dimensional hexagonal phase MxV6S8 nanowires and KxV6SySe8-y nanowires on the substrate. Chemical vapor deposition (CVD) method provides a controllable method for synthesizing vanadium chalcogenide crystals with different compositions, sizes, morphologies, and phases by tuning growth parameters. The present invention first introduces V 6 S 8Direct CVD growth of crystals overcomes the complex and challenging control of growth conditions.

[0062] Some readily available metal salts include but are not limited to KHCO 3 、KCl、K 2 S、K 2 CO 3 、K 2 C 2 O 4 ·H 2 O (for synthesizing K x V 6 S 8 nanowires), Rb 2 CO 3 (for synthesizing Rb x V 6 S 8 nanowires) and Cs 2 CO 3 (for synthesizing Cs x V 6 S 8 nanowires), and can be used to synthesize 1D hexagonal phase nanowires.

[0063] The prepared nanowires are single-crystalline and are characterized by XRD, HRTEM, HAADF-STEM, SAED, and Raman spectroscopy.

[0064] Non-stoichiometric M x V 6 S 8 (M = K, Rb, Cs) crystals are in the shape of a hexagonal structure with the space group P6 3 / m. Referring to Figure 1A , the region marked by the black line corresponds to the unit cell of the hexagonal phase M x V 6 S 8 structure. Six sulfur atoms coordinate with each vanadium atom, and the unit cell consists of two octahedral trimers connected by sharing octahedral edges ( Figure 1B ). Turning to Figure 1C , VS 6 octahedral units are joined together by sharing faces and edges to produce a stable three-dimensional V 6 S 8 network 9-11 with hexagonal channels parallel to the c-axis. The diameter of the channels is approximately which is large enough to accommodate various atoms, including K, Rb, Cs, etc. Different alkali metal atoms can form columns within the hexagonal channels.

[0065] Although there is a large amount of available space in the channels, the voids are not completely occupied. Figure 2Depicts a schematic of a salt-assisted CVD method for synthesizing 1D hexagonal phase M x V 6 S 8 nanowires. More specifically, commercially available V 2 S 3 powder, S powder, and one of the following metal salts: containing KCl, K 2 S, K 2 CO 3 , KHCO 3 , K 2 C 2 O 4 ·H 2 O (for synthesizing K x V 6 S 8 nanowires), Rb 2 CO 3 (for synthesizing Rb x V 6 S 8 nanowires) or Cs 2 CO 3 (for synthesizing Cs x V 6 S 8 nanowires) are mixed and then ground using an agate mortar. After that, the obtained homogeneous mixture is covered with two freshly cleaved fluorophlogopite substrates and then heated at 820 to 850 °C under a gas flow of 80% Ar and 20% H 2 for 1 to 10 minutes.

[0066] Overall, a direct, practical, robust, and general SA-CVD strategy has been developed to enable the high-yield synthesis of 1D hexagonal phase M x V 6 S 8 and K x V 6 S y Se 8-y nanowires. In addition to KHCO 3 , several other metal salts have been observed to be useful for synthesizing 1D hexagonal phase M x V 6 S 8 nanowires, including KCl, K 2 S, K 2 CO 3 , K 2 C 2 O 4 ·H 2 O (for synthesizing K x V 6 S8 nanowires), Rb 2 CO 3 (for generating Rb x V 6 S 8 nanowires) and Cs 2 CO 3 (for forming Cs x V 6 S 8 nanowires). In addition, this method has also been used to prepare hexagonal alloys, such as K x V 6 S y Se 8-y nanowires.

[0067] The following examples illustrate the invention and are not intended to limit the invention.

[0068] Example

[0069] Example 1 - Materials and Methods

[0070] Chemicals

[0071] Vanadium sulfide (V 2 S 3 , 99.9%), anhydrous potassium sulfide (K 2 S, analytical reagent) and potassium chloride (KCl, 99.5%) were purchased from Macklin (China). Potassium carbonate (K 2 CO 3 , 99%), cesium carbonate (Cs 2 CO 3 , 99.9%), rubidium carbonate (Rb 2 CO 3 , 99.8%) and sulfur powder (S, 99.98%) were purchased from Sigma-Aldrich (Germany). Potassium bicarbonate (KHCO 3 , 99.9%), potassium oxalate monohydrate (K 2 C 2 O 4 ·H 2 O, 99.8%) and selenium powder (Se, ≥99.99%) were purchased from Aladdin (China). Ultra-high purity (99.999%) H 2 and Ar were purchased from Linde HKO Limited (Hong Kong, China). All chemicals were used as received without purification.

[0072] Characterization

[0073] Optical images were obtained on an optical microscope (Nikon, LV100ND, Japan). Scanning electron microscope (SEM) images were acquired on a scanning electron microscope (Thermo Fisher Scientific, QUATTRO S). AFM measurements were performed in ambient air using a Bruker Icon atomic force microscope (AFM) system, and AFM height profiles were obtained in tapping mode using an AFM tip (OTESPA-R3, Bruker Nano Inc., USA). Using a Cu Kα radiation source X-ray diffraction (XRD) patterns were recorded using a Rigaku SmartLab X-ray diffractometer. Samples for XRD characterization were prepared by transferring the as-prepared hexagonal nanowires from a mica substrate to a freshly cleaned glass substrate. Transmission electron microscope (TEM) images, high-resolution TEM (HRTEM) images, and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2100F (JEOL, Tokyo, Japan) transmission electron microscope. High-angle annular dark-field scanning TEM (HAADF-STEM) images were captured on a JEOL ARM200F (JEOL) spherical aberration-corrected transmission electron microscope equipped with a cold field emission gun and an advanced aberration corrector. The acceleration voltage used during all TEM operations was 200 kV. Samples for cross-sectional analysis were produced using a focused ion beam (FIB) system (JEOL JIB-4500) and subsequently transferred onto a porous carbon-coated Cu grid. EDS data were collected using a JEOL JEM-2100F microscope. Raman spectra, Raman mapping images, and photoluminescence (PL) spectra were obtained on a confocal Raman microscope (Renishaw inVia TM ) with an excitation wavelength of 532 nm and a laser spot diameter of approximately 1 μm. Angular-resolved Raman spectroscopy measurements were performed by keeping the polarized laser constant and incrementally rotating the sample by 15°. XPS spectra were acquired on an ESCALAB 250Xi (Thermo Fisher Scientific) instrument, and the XPS results were calibrated by using the C1s peak at a binding energy of 284.8 eV as a reference. Samples for XPS characterization were prepared by transferring the as-prepared hexagonal nanowires from a mica substrate to a freshly cleaned Si substrate.

[0074] Example 2 - Synthesis of Nanowire M x V 6 S 8

[0075] Synthesis of 1DK x V 6 S 8 Nanowire

[0076] 1 mmol of V 2 S 3 powder, 1.5 mmol of KHCO3 1 mmol of V 2 S powder, 1.6 mmol of Rb x V 6 S 8 nanowires. It is worth noting that 1D hexagonal phase K 2 S, KCl, K 2 CO 3 and K 2 C 2 O 4 ·H 2 O can also be synthesized by replacing KHCO 3 with other metal salts such as K x V 6 S 8 while keeping other growth parameters constant.

[0077] Synthesis of 1D Rb x V 6 S 8 nanowire

[0078] 1 mmol of V 2 S 3 powder, 1.6 mmol of Rb 2 CO 3 powder and 5 mmol of S powder are uniformly blended and then ground in an agate mortar for at least 20 minutes. 20 mg of the obtained mixed powder is transferred to a quartz boat, and then two freshly cleaved fluorophlogopite substrates are precisely covered on top of the loaded mixture. Then, the quartz boat is transferred to the center of a quartz tube (1 inch in diameter). Before heating, the tube is purged with 500 s.c.c.m (standard cubic centimeters per minute) for 20 minutes to remove the air and moisture inside it, and then with 80 s.c.c.m Ar and 20 s.c.c.m H 2Purge for 20 minutes to provide an optimal synthesis atmosphere. Subsequently, insert the tube into a tube furnace heated to 820 °C and maintain for 6 minutes. Then, remove the tube furnace from the reaction area and close it, and let the boat cool naturally to room temperature. Finally, synthesize 1D Rb nanowires on the mica substrate. x V 6 S 8 nanowires.

[0079] Synthesis of 1DCs x V 6 S 8 Nanowire

[0080] Mix 1 mmol of V 2 S 3 powder, 1.1 mmol of Cs 2 CO 3 powder and 5 mmol of S powder uniformly, and then grind with an agate mortar for at least 20 minutes. Transfer 20 mg of the obtained mixed powder to a quartz boat, and then precisely cover the top of the loaded mixture with two freshly cleaved fluorophlogopite substrates. Then, transfer the quartz boat to the center of a quartz tube (1-inch diameter). Before heating, purge the tube at 500 s.c.c.m (standard cubic centimeters per minute) for 20 minutes to remove the air and moisture inside it, and then purge with 80 s.c.c.m Ar and 20 s.c.c.m H 2 Purge for 20 minutes to provide an optimal synthesis atmosphere. Subsequently, insert the tube into a tube furnace heated to 850 °C and maintain for 6 minutes. Then, remove the tube furnace from the reaction area and close it, and let the boat cool naturally to room temperature. Finally, synthesize 1D Cs x V 6 S 8 nanowires.

[0081] Example 3-1D M x V 6 S 8 Characterization of Nanowires

[0082] As an example, consider K x V 6 S 8 . Figures 3A to 3B Shows the optical image and SEM image of the K 3 synthesized on the mica substrate by means of KHCO x V 6 S 8 crystals, demonstrating the high yield of the crystals and showing a clear 1D nanowire morphology with lengths extending to dozens of micrometers. The growth of K x V 6 S 8 nanowires seems to be restricted to three main directions, rather than growing uniformly in all directions on the surface of the mica substrate. Additionally, Figure 4 the AFM image in x V6 S 8 The nanowires have a thickness in the range of 5.6 nanometers to 12.7 nanometers, determined by the height profile of the nanowires. For example, the obtained K x V 6 S 8 The nanowires have a thickness of 5.6 nanometers, 6.0 nanometers, 6.1 nanometers, 6.6 nanometers, 7.1 nanometers, 9.5 nanometers, 9.7 nanometers, 10.0 nanometers, or 12.7 nanometers.

[0083] To display the crystal structure and crystallinity of the prepared K x V 6 S 8 XRD was performed on the nanowires synthesized with KHCO 3 . Referring to Figure 5 , the XRD pattern of the hexagonal phase K 0.2 V 6 S 8 crystals is based on the data of JCPDS No. 78 - 2418. The obtained XRD pattern exhibits six different diffraction peaks, which match well with the 0.2 V 6 S 8 planes of the hexagonal phase K and crystals (JCPDS No. 78 - 2418). However, only the group planes were found. This indicates that the K x V 6 S 8 nanowires grow anisotropically perpendicular to the a - axis.

[0084] TEM was also performed to study the crystal structure and elemental composition of the prepared hexagonal K x V 6 S 8 nanowires. Figure 6A Shows a low - magnification TEM image of a typical 1D K x V 6 S 8 nanowire, which is consistent with the morphology depicted in the optical ( Figure 3A ) and SEM ( Figure 3B ) images. Figure 6B The HRTEM image in x V 6 S 8 indicates the excellent crystallinity of the synthesized K x V 6 S 8 nanowires. Correspondingly, the zone - axis obtained from different regions of the prepared K Figure 6C in the FFT pattern inFigure 6D The SAED pattern in Figure 7A only shows a set of clear diffraction spots, thus confirming its single-crystalline nature and hexagonal phase structure. Turning to x V 6 S 8 structure, the SAED result is in good agreement with the simulated SAED pattern along the zone axis of the hexagonal phase K Figure 7B ).

[0085] In addition, aberration-corrected HAADF-STEM is used to image the atomic structure of the as-prepared K x V 6 S 8 nanowires. In Figures 8A to 8B , the STEM image taken along the zone axis shows the typical atomic arrangement of the hexagonal phase structure, which is consistent with the simulated STEM image shown in Figure 8C .

[0086] To further clarify the crystal structure of the 1D K x V 6 S 8 nanowires, FIB technology is carried out to cut the K x V 6 S 8 nanowires along the crystal plane perpendicular to the

[0001] zone axis. In Figures 9A to 9B , the cross-sectional HAADF-STEM image clearly shows that the nanowires including many regularly arranged hexagonal channels grow along the c-axis. Figure 9B shows the magnified cross-sectional HAADF-STEM image. The result shows that potassium ions (indicated by the white arrow in Figure 9B ) are accommodated in the centers of some channels to form columns surrounded by six sulfur atoms of octahedrons with triangular deformation. These results are consistent with the simulated cross-sectional STEM image along the

[0001] zone axis ( Figure 9C ) and the atomic structure model ( Figure 9D ). In addition, the corresponding FFT pattern of the cross-sectional STEM image ( Figure 9A ) is also consistent with the simulated SAED pattern ( Figure 10A ), thus further proving the successful synthesis of the hexagonal phase K Figure 10B nanowires. x V 6 S 8 .

[0087] In addition, the EDS spectrum obtained in the STEM mode confirms the uniform dispersion of strong V and S signals and very weak K signals, where throughout the K x V 6 S 8The atomic ratio in the nanowires is approximately 0.2% ( Figure 11 ).

[0088] Reference Figure 12A , K x V 6 S 8 The Raman spectrum of the nanowires (recorded at 532 nm) shows five distinct Raman peaks located at 169.4, 221.5, 326.7, 340.0, and 374.1 cm -1 respectively. These Raman peaks are different from the Raman peaks in previously reported vanadium sulfides 17-19 . A single K x V 6 S 8 The Raman mapping image of the nanowires ( Figure 12B ) exhibits a uniform Raman signal, indicating the uniform chemical composition and crystal structure of the nanowires, as well as the high quality of the as-prepared 1D K x V 6 S 8 nanowires.

[0089] In addition, Figure 12C depicts the photoluminescence (PL) spectra of the synthesized K x V 6 S 8 nanowires and the bare Si / SiO 2 substrate. The photoluminescence PL spectra exhibit a PL quenching phenomenon without distinct absorption peaks, indicating the metallic properties of the 1D K x V 6 S 8 nanowires.

[0090] In addition, angle-resolved polarized Raman spectroscopy (ARPRS) was performed to study the anisotropic phonon vibrations and crystal orientation of the as-prepared K x V 6 S 8 nanowires 20-21 . During the testing process, 0° and 90° indicate the polarization of the laser parallel and perpendicular to the c-axis alignment of the synthesized K x V 6 S 8 nanowires ( Figure 13 ). Figures 14A to 14B shows the polarized Raman spectra and corresponding hotspot maps from 0° to 180°, showing that when the parallel polarization configuration is applied, the peak intensities of all five Raman modes exhibit periodic variations. This observation indicates the structural anisotropy of the as-prepared K x V 6 S 8 nanowires.

[0091] To further study the correlation between Raman peaks and polarization angles, Figure 15 also depicts the polar coordinate plots of all five Raman modes (Raman peaks at 169.4, 221.5, 326.7, 340.0, and 374.1 cm -1 ). The intensities of the Raman peaks at 221.5, 340.0, and 374.1 cm -1 show four maximum intensity angles. In contrast, the intensities of the Raman modes at 169.4 and 326.7 cm -1 exhibit a 180° period, with the minimum Raman intensity at polarization angles of 0° and 180°. This observation is in good alignment with the c-axis of the synthesized K x V 6 S 8 nanowires, thus allowing this observation to be used to determine the crystal orientation of the nanowires.

[0092] In addition, temperature-dependent Raman spectroscopy is performed to show the thermal stability of the synthesized hexagonal K x V 6 S 8 nanowires. As Figure 16 shown, the Raman spectrum remains unchanged in the temperature range of 30 °C to 300 °C, thus indicating the remarkable stability of the nanowires at high temperatures. This extraordinary thermal stability provides significant advantages in various applications, especially in the fabrication of devices for exploring the electrical properties of the grown K x V 6 S 8 nanowires.

[0093] Furthermore, XPS measurements are performed on the prepared K x V 6 S 8 nanowires to study the electronic states of the elements ( Figures 17A to 17C ). In Figure 17A , as shown in the V 2p spectrum, both V 2+ and V 3+ coexist in the K x V 6 S 8 nanowires, thus demonstrating the formation of the V–S bond. The V 2p 1 / 2 and V 2p 3 / 2 located at 520.2 and 512.6 eV correspond to the unreacted V element, while the V 2p 1 / 2 and V 2p 3 / 2 located at 524.6 and 517.0 eV are attributed to the oxidation of the grown K x V 6 S 8 nanowires. In Figure 17BAmong them, as shown in the S2p spectrum, the peaks at 161.4 and 162.5 eV are S2p 3 / 2 and S2p 1 / 2 , while the peaks at high binding energy are induced by the oxidized nanowires, which is consistent with the XPS spectrum of V. In Figure 17C , as shown in the K 2p spectrum, the K 2p at 295.7 and 292.9 eV 1 / 2 and K 2p 3 / 2 confirm the presence of potassium ions in the prepared K x V 6 S 8 nanowires. The above XPS results are in good agreement with the previous reports on vanadium sulfides 11,17,22 .

[0094] Example 4 - Hexagonal Phase K x V 6 S y Se 8-y Synthesis of Nanowires

[0095] Mix 1 mmol of V 2 S 3 powder, 1.5 mmol of KHCO 3 powder, 10 mmol of S powder and 1 or 2 mmol of Se powder uniformly, and then grind them using an agate mortar for at least 20 minutes. Transfer 20 mg of the obtained mixed powder to a quartz boat, and then precisely cover the top of the loaded mixture with two freshly cleaved fluorophlogopite substrates. Then, transfer the quartz boat to the center of a quartz tube (1-inch diameter). Before heating, purge the tube with 500 s.c.c.m for 20 minutes to remove the air and moisture inside it, and then purge it with 80 s.c.c.m of Ar and 20 s.c.c.m of H 2 for 20 minutes to provide an optimal synthesis atmosphere. Subsequently, insert the tube into a tube furnace heated to 850 °C and maintain it for 6 minutes. After that, remove the tube furnace from the reaction area and turn it off, and let the boat cool naturally to room temperature. Finally, synthesize 1D K x V 6 S y Se 8-y nanowires on the mica substrate. It should be noted that 1D hexagonal phase K 2 S, KCl, K 2 CO 3 and K 2 C 2 O 4 ·H 2 O can also be synthesized by replacing KHCO 3 with other metal salts of K while keeping other growth parameters constant x V 6 S y Se 8-y nanowires.

[0096] Example 5 - Hexagonal Phase K x V 6 S y Se 8-y Characterization of Nanowires

[0097] 1D hexagonal phase K with different chemical compositions (sulfur to selenium ratio) can be easily synthesized by adding 1 or 2 mmol Se powder to the reaction mixture x V 6 S y Se 8-y nanowires.

[0098] As Figures 18A to 18B shown, the two grown hexagonal phase K x V 6 S y Se 8-y alloys present a 1D nanowire morphology with lengths of tens of micrometers, similar to that of K x V 6 S 8 nanowires( Figures 3A to 3B ). Figures 19A to 19B The synthesized K x V 6 S y Se 8-y nanowires' Raman spectra and Figures 20A to 20B the synthesized K x V 6 S y Se 8-y nanowires' PL spectra also exhibit similar Raman peaks and PL quenching phenomena, indicating the metallic nature of the nanowires. K x V 6 S y Se 8-y nanowires' crystal structures are further characterized by HRTEM and SAED( Figures 21A to 21D ), demonstrating good crystallinity and a hexagonal phase structure. In addition, K x V 6 S y Se 8-y nanowires' elemental mapping images and EDS characterization show a uniform distribution of V, S, Se, and K signals throughout the nanowires( Figure 22 ).

[0099] More impressively, in addition to KHCO 3 , compounds such as KCl, K 2 S, K 2 CO 3 、K 2 C 2 O 4 ·H2O (used for synthesizing K x V 6 S 8nanowires), Rb 2 CO 3 (for synthesizing Rb x V 6 S 8 nanowires) and Cs 2 CO 3 (for synthesizing Cs x V 6 S 8 Several other metal salts (for synthesizing 1D hexagonal phase M x V 6 S 8 nanowires) can also be used to synthesize 1D hexagonal phase M

[0100] Reference Figure 23A , by means of different metal salts, the prepared M x V 6 S 8 crystals all exhibit a 1D nanowire morphology, a unique Raman spectrum ([[]] Figure 23B ) and complete PL quenching ([[]] Figure 23C ), which is very consistent with the K 3 V x S 6 nanowires synthesized by means of KHCO 8 nanowires. In addition, the grown Rb x V 6 S 8 and Cs x V 6 S 8 nanowires have also been characterized by HRTEM, SAED, elemental mapping and EDS, clearly showing the hexagonal phase structure of the nanowires and the uniformly distributed elemental signals in the nanowires.

[0101] Example 6 - Transfer of hexagonal phase M x V 6 S 8 and K x V 6 S y Se 8-y nanowires

[0102] The grown nanowires were transferred based on a previously reported poly(methyl methacrylate) (PMMA)-assisted method with minor modifications. First, a fluorophlogopite substrate with the prepared hexagonal phase nanowires was spin-coated with PMMA (PMMA 495, A8, Microchem) at 3000 revolutions per minute (r.p.m.) for 50 s, and then baked at 75 °C for 18 min. After removing the edges of the mica substrate, the PMMA-coated nanowires were easily peeled off from the substrate using deionized water. Subsequently, the floating PMMA film with the nanowires was transferred to a clean Si / SiO 2On the substrate, it was then heated at 90 °C for 30 min to increase the adhesion between the PMMA film and the Si / SiO 2 substrate. After that, the PMMA was removed by immersing the Si / SiO 2 substrate in acetone for 10 h. Finally, the Si / SiO 2 substrate was dried using nitrogen, and the synthesized hexagonal nanowires were left on the Si / SiO 2 substrate.

[0103] Example 7 - Device Fabrication and Measurement

[0104] To fabricate a three-terminal device based on a single K x V 6 S 8 nanowire, first, the transfer of 1D K x V 6 S 8 nanowire was performed according to the PMMA-assisted wet transfer method. After transferring the 1D K x V 6 S 8 nanowire from mica to a Si / 50-nm SiO 2 substrate, drain and source electrodes were fabricated on the nanowire by standard electron beam lithography. Then 8 nm of Cr and 70 nm of Au were thermally evaporated as the electrodes, and a lift-off process was subsequently carried out.

[0105] To fabricate a MoS 2 -based field-effect transistor with an asymmetric electrode contact consisting of a nanowire contact on one side and a Cr / Au contact on the other side, first, a monolayer of MoS 2 was grown on a Si / 300-nm SiO 2 substrate at 850 °C for 30 min using a 500 / 1 sccm Ar / O 2 mixed flow by the oxygen-assisted CVD method. The transfer of 1D K x V 6 S 8 nanowire was performed according to the PMMA-assisted wet transfer method. After transferring the 1D K x V 6 S 8 nanowire from mica to a sacrificial Si / 50-nm SiO 2After the substrate, a poly(propylene carbonate) (PPC) layer was spin-coated on the silicon substrate at 3000 rpm and baked at 60 °C for 2 minutes. These nanowires were mechanically exfoliated (picked up) in DI water and released (pulled down) onto the target substrate via a mechanical aligner under an optical microscope through a PPC-coated poly(dimethylsiloxane) (PDMS) block mounted on a glass slide. Before the PPC removal, the transferred samples covered with the PPC film were baked at 150 °C for >30 min to enhance the vdWs coupling strength of the metal-semiconductor contact. Then, drain and source electrodes were fabricated on the device by standard electron beam lithography. And 8 nm of Cr and 70 nm of Au were thermally evaporated as the electrodes, and then a lift-off process was performed.

[0106] The electrical characteristics of these devices were measured in a probe station using a Keysight B1500A semiconductor device parameter analyzer.

[0107] Example 8-1D K x V 6 S 8 Electrical characteristics of nanowires

[0108] As Figure 24A shown, three-terminal devices based on single K x V 6 S 8 nanowires were fabricated. In the devices, Si served as the substrate, a 50-nm SiO 2 layer was used as the dielectric, KxV6S8 nanowires were used as the semiconductor channel, and Cr / Au were used as the source-drain electrode and the gate electrode, respectively.

[0109] AFM images of the devices showed that the thickness of the 1D K x V 6 S 8 nanowires was approximately 12 nm. The electrical curves ( Figure 24B ) without gate bias showed a linear V DS -I DS relationship and almost ideal Ohmic contacts. Moreover, the transfer curves ( Figure 24C ) of the devices exhibited weak gate-tunable conduction without electro-switching behavior 23 . These results demonstrated the metallic nature of the 1D K x V 6 S 8 nanowires, which was consistent with the results of photoluminescence characterization.

[0110] Moreover, field-effect transistors based on MoS 2 were fabricated using asymmetric electrode contacts, where the nanowire contact was on one side and the Cr / Au contact was on the other side, as Figure 24D shown. Unexpectedly, the devices exhibited good n-type semiconducting behavior with an on / off ratio of 10 6 (Figure 24E ), and the output curve ( Figure 24F ) shows that the Schottky barriers of the contacts on both sides are substantially the same, indicating that the synthesized nanowires can be good van der Waals contacts for achieving high-performance transistors without Fermi level pinning. 24-26 .

[0111] Define

[0112] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising" shall be understood to imply the inclusion of the stated integer or group of integers but not the exclusion of any other integer or group of integers. It should also be noted that in this disclosure and particularly in the claims and / or paragraphs, terms such as "comprises", "comprised of", "comprising", etc. may have the meaning ascribed to them in US patent law; for example, such terms permit elements not expressly recited, but exclude elements found in the prior art or that affect the basic or novel characteristics of the invention.

[0113] Furthermore, throughout this specification and the claims, unless the context requires otherwise, the word "include" or variations such as "includes" or "including" shall be understood to imply the inclusion of the stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0114] References in this specification to "one embodiment", "an embodiment", "example embodiment", etc., indicate that the described embodiment may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases do not necessarily refer to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that, whether or not explicitly described, the feature, structure, or characteristic is within the knowledge of those skilled in the art in connection with other embodiments.

[0115] Other definitions for selected terms used herein may be considered within the detailed description of the invention and apply throughout the detailed description of the invention. Unless otherwise defined, all other technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0116] Those skilled in the art will appreciate that, given these teachings, alternative embodiments can be implemented without undue experimentation or departing from the spirit or scope of the invention as set forth in the appended claims. The invention should be limited only by the appended claims, which encompass all such embodiments and modifications when viewed in conjunction with the above specification and drawings.

[0117] Industrial Applicability:

[0118] The present invention provides a robust and direct CVD method for synthesizing highly conductive 1D metal hexagonal phase M x V 6 S 8 nanowires. These nanowires serve as excellent van der Waals contacts in MoS 2 -based field-effect transistors, resulting in good ohmic contacts, significant charge mobility, and reduced Fermi level pinning. This synthesis strategy shows bright potential in electronic applications. Additionally, the CVD-synthesized hexagonal phase MxV 6 S 8 structure is a promising candidate for superconductivity due to its large channels aligned with the c-axis. These channels can accommodate various ions, allowing precise tuning of the electronic structure and enhancement of its superconducting properties.

[0119] Due to the high conductivity of the M x V 6 S 8 nanowires, the prepared nanowires can also act as electrode materials for supercapacitors and batteries.

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Claims

1. A thermally stable one-dimensional hexagonal phase M x V y S z Nanowires, characterized in that M=K, Rb, Cs; x = 0.2 or 1.12; y＝6； z＝8， The thermally stable one-dimensional hexagonal phase M x V y S z The nanowires have c-axis aligned hexagonal channels, and wherein the thermally stable one-dimensional hexagonal phase M x V y S z The nanowires exhibit an unchanged Raman spectrum in the temperature range of 30°C to 300°C.

2. The thermally stable one-dimensional hexagonal phase M according to claim 1 x V y S z Nanowires where y=6 and z=8.

3. The thermally stable one-dimensional hexagonal phase M according to claim 1 x V y S z Nanowires in which the c-axis aligned hexagonal channels have to within the range of diameter.

4. The thermally stable one-dimensional hexagonal phase M according to claim 1 x V y S z Nanowires, wherein the thermally stable one-dimensional hexagonal phase M x V y S z The nanowires act as effective van der Waals contacts for MoS2-based field-effect transistors with good ohmic contact, large charge mobility, and reduced Fermi-level pinning.

5. The thermally stable one-dimensional hexagonal phase M according to claim 1 x V y S z Nanowires, wherein the M x V y S z The nanowires have a thickness in the range of 1 to 15 nanometers.

6. The thermally stable one-dimensional hexagonal phase M according to claim 1 x V y S z Nanowires, wherein the M x V y S z Nanowires are K 0.2 V6S8 exhibits five unique Raman peaks recorded at 532 nm, including 169.4, 221.5, 326.7, 340.0, and 374.1 cm -1 .

7. A thermally stable one-dimensional hexagonal phase K x V6S y Se 8-y Nanowires, characterized in that x = 0.68 or 1.34; y = 7.02 or 6.79, The thermally stable one-dimensional hexagonal phase K x V6S y Se 8-y The nanowires have c-axis aligned hexagonal channels, and wherein the thermally stable one-dimensional hexagonal phase K x V6S y Se 8-y The nanowires exhibited an unchanged Raman spectrum over the temperature range of 30°C to 300°C.

8. The thermally stable one-dimensional hexagonal phase K according to claim 7 x V6S y Se 8-y Nanowires in which the c-axis aligned hexagonal channels have to within the range of diameter.

9. The thermally stable one-dimensional hexagonal phase K according to claim 7 x V6S y Se 8-y Nanowires wherein the thermally stable one-dimensional hexagonal phase K x V6S y Se 8-y The nanowires serve as effective van der Waals contacts for MoS2-based field-effect transistors with good ohmic contact, large charge mobility, and reduced Fermi level pinning.

10. The thermally stable one-dimensional hexagonal phase K according to claim 7 x V6S y Se 8-y Nanowires, wherein the K x V6S y Se 8-y Nanowires are K 0.68 V6S 7.02 Se 0.98 , showing five unique Raman peaks recorded at 532 nm, including 165.2, 221.3, 328.4, 337.9, and 375.7 cm -1 .

11. The thermally stable one-dimensional hexagonal phase K according to claim 7 x V6S y Se 8-y Nanowires, wherein the K x V6S y Se 8-y Nanowires are K 1.34 V6S 6.79 Se 1.21 , showing five unique Raman peaks recorded at 532 nm, including 157.5, 213.6, 328.4, 337.9, and 370.1 cm -1 .

12. A salt-assisted method for synthesizing one-dimensional hexagonal vanadium sulfide nanowires on a substrate, characterized in that: The following steps are involved: preparing a substrate; providing a precursor by mixing a vanadium (V) compound and a sulfur (S) compound; introducing a metal salt into the precursor via a salt-assisted chemical vapor deposition (SA-CVD) method to obtain a mixture; The mixture was transferred to a quartz boat and two freshly cleaved fluorophlogopite mica substrates were covered on top of the loaded mixture; The quartz boat is transferred to the center of the quartz tube, and then Ar and H2 are introduced to provide an optimal synthesis atmosphere; heating the quartz tube, and then cooling the quartz boat to room temperature; and The one-dimensional hexagonal phase M is grown on the substrate x V6S8 nanowires and K x V6S y Se 8-y Nanowires.

13. The method according to claim 12, wherein the ratio between the precursor and the metal salt is in the range of 1:1 to 1:1.

5.

14. The method of claim 12, wherein the step of providing a precursor comprises mixing V2S3 powder and S powder.

15. The method of claim 12, wherein the substrate comprises a mica substrate.

16. The method of claim 12, wherein the metal salt is selected from the group consisting of KCl, K2S, K2CO3, KHCO3, K2C2O4.H2O, Rb2CO3 and Cs2CO3.

17. The method according to claim 12, wherein the quartz tube is purged in advance at 500 s.ccm for 20 minutes to remove internal air and moisture.

18. The method of claim 12, wherein the flow rate ratio between Ar and H2 is 4:

1. The method of claim 12 , wherein the precursor further comprises a selenium (Se) compound.

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