WO2024263908A2

WO2024263908A2 - Tns nanowire and fabrication method

Info

Publication number: WO2024263908A2
Application number: PCT/US2024/035008
Authority: WO
Inventors: Jiang Wei; Abin JOSHY; Fei Wang
Original assignee: The Administrators Of The Tulane Educational Fund
Priority date: 2023-06-23
Filing date: 2024-06-21
Publication date: 2024-12-26
Also published as: WO2024263908A3; WO2024263908A9

Abstract

A nanowire fabrication method includes heating, to a first temperature, an ampoule that includes a mixture of elemental powders; and maintaining the ampoule at the first temperature for a first duration. The method also includes heating the ampoule to a second temperature that exceeds the first temperature; maintaining the ampoule at the second temperature for a second duration; and cooling the ampoule to an ambient temperature. A TNS nanowire has a material composition that includes Ta₂Ni₃Se₈, and a length-to-thickness ratio greater than one thousand. A semiconductor device includes the TNS nanowire, which has a first position and a second position along its length. The semiconductor device also includes a first metal contact on the TNS nanowire at the first position and a second metal contact on the TNS nanowire at the second position.

Description

TNS NANOWIRE AND FABRICATION METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63/522,879, filed on 23 June 2023, the disclosure ofwhich is incorporated herein by reference in its entirety.

GOVERNMENT RIGHTS

[0002] This invention was made with U.S. Government support under Grant No. 1752997 awarded by the U.S. National Science Foundation. The United States Government has certain rights in the invention.

BACKGROUND

[0003] One-dimensional (ID) van der Waals (vdW) nanowires are crystals of molecular chains bundled with weak interactions. Unlike traditional silicon nanowires, they show promise of downsizing functional devices to the molecular level without losing crystallinity due to their exfoliable nature and inert surface. However, the investigation of their fundamental properties and applications is limited by the lack of efficient synthesis methods.

SUMMARY OF THE EMBODIMENTS

[0004] Direct synthesis of vdW nanowires has yet to be realized. Herein, we disclose a solid state growth with high yield production of single-crystal TazNisSes (TNS) nanowires with lengths reaching several millimeters and aspect ratios up to approximately 100,000. Wafer-scalable alignment of as-grown nanowires maybe achieved using the soft-lock drawing method. In embodiments, further mechanical exfoliation from the as-grown nanowires can produce a-few-nanometer-thick air-stable nanowires. In addition, arrays of Schottky devices are fabricated on a single TNS nanowire. Electrical transport measurements reveal that these nanowires have uniform electrical properties throughout their length. The synthesis of structurally and electronically uniform ultralong TNS nanowires paves the way for developing integrated molecular electronics and sensors based on ID vdW materials. [0005] In a first aspect, a nanowire fabrication method is disclosed. The method includes heating, to a first temperature, an ampoule that includes a mixture of elemental powders; and maintaining the ampoule at the first temperature for a first duration. The method also includes heating the ampoule to a second temperature that exceeds the first temperature; maintaining the ampoule at the second temperature for a second duration; and cooling the ampoule to an ambient temperature.

[0006] In a second aspect, a TNS nanowire is disclosed. The TNS nanowire has a material composition that includes Ta₂Ni₃Se₈, and a length-to-thickness ratio greater than one thousand.

[0007] In a third aspect, a semiconductor device is disclosed. The semiconductor device includes TNS nanowire of the second aspect that has a first position and a second position along its length. The semiconductor device also includes a first metal contact on the TNS nanowire at the first position and a second metal contact on the TNS nanowire at the second position.

BRIEF DESCRIPTION OF THE FIGURES

[0008] FIGs. 1 and 2 illustrate synthesis of crystalline TNS nanowires using solid state growth, in an embodiment.

[0009] FIG. 3 shows alignment of millimeter-long TNS nanowires, in an embodiment.

[0010] FIG. 4. shows transmission electron microscopy images of ultra-long TNS nanowires, in an embodiment.

[0011] FIG. 5 shows crystal structure and exfoliation of nanowires, in an embodiment.

[0012] FIG. 6 illustrates a one-dimensional array of Schottky devices on a single ultralong TNS nanowire, in an embodiment.

[0013] FIGs. 7 and 8 illustrate electrical characterization of Schottky devices made on an embodiment of a TNS nanowire.

[0014] FIG. 9 shows SEM images of TNS nanowire embodiments.

[0015] FIG. 10 shows an example of turf roll growth by controlling the distribution of source powder before growth.

[0016] FIG. 11 shows an energy-dispersive X-ray spectrum of embodiments of TNS nanowires and a stoichiometric ratios determined therefrom. [0017] FIG. 12 is a powder x-ray diffraction (XRD) pattern of TNS crystal, in an embodiment.

[0018] FIG. 13 includes XRD spectra ofthe highlighted regions marked in FIG. 12.

[0019] FIG. 14 is an XRD peak-fitting to the quantify the ratio of constituent phases of the TNS crystal of FIG. 12 and includes the weight ratio of constituent phases in the as-grown crystals.

[0020] FIG. 15 shows SEM image of randomly oriented TNS nanowires from the turf roll growth and a histogram ofthe thickness distribution ofthe TNS nanowires, in an embodiment.

[0021] FIG. 16 shows orbital diagrams for Ta⁵⁺ and Ni²⁺ for different coordination numbers.

[0022] FIG. 17 is a schematic showing the locations of cleavage bonds, cleave bond energy, and surface cleavage schemes in an a TNS crystal structure.

[0023] FIG. 18 is a graph of the total density of states for TNS bulk, in an embodiment.

[0024] FIGs. 19, 20, and 21 are graphs used for extraction of Schottky barrier for TNS device using a metal-semiconductor-metal (MSM) model and different channel lengths.

[0025] FIG. 22 is a schematic of a Ta₂Ni₃Se₈ nanowire, in an embodiment. Nanowires of FIGs. 1-7 and 9 are examples ofthis Ta₂Ni₃Se₈ nanowire.

[0026] FIG. 23 is a schematic plan view of a semiconductor device that includes a TNS-nanowire bundle, which includes at least on TNS nanowires of FIG. 22, in an embodiment.

[0027] FIG. 24 is a schematic of a strain sensor that includes a TNS-nanowire bundle of FIG. 23, in an embodiment.

[0028] FIG. 25 is a schematic of a chemical sensor that includes a TNS-nanowire bundle of FIG. 23, in an embodiment.

[0029] FIGs. 26 and 27 are schematics of an optical sensor that includes a TNS- nanowire bundle of FIG. 23, in embodiments.

[0030] FIG. 28 is a flowchart illustrating a method for fabricating a TNS nanowire of FIG. 22, in an embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

1. Introduction

[0031] Two dimensional [2D] van der Waals [vdW] materials uncovered novel electrical properties due to quantum confinement effect and reduced symmetry, allowing investigation of thickness dependent bandgaps, topological phase transition, excitons, and valley polarization, enabling their application in electronics and sensors. One-dimensional (ID ] van der Waals materials, made of ID molecular chains, can extend dimensional confinement further. Within each chain, atoms are connected through covalent bonds. In between chains, weaker bonding, as weak as vdW interactions, holds chains together.

[0032] Such ID vdW materials with strict ID confinement, large surface-to-volume ratio, and strong electron-electron interaction bring new properties for fundamental research and technological innovation. Examples include the highly anisotropic conductivity of TiSs and the significant breakdown current density found in TaSes and ZrTes nanoribbons. Ballistic heat transport has been discovered in TazPdsSes. High anisotropy in optical absorption/emission has been demonstrated in ZrSs. In addition, the applications of interconnects, field-effect transistors(FET), and photodetectors have been demonstrated.

[0033] However, unlike the blooming number of materials for 2D vdW materials research, a limited number of ID vdW materials have been studied. The limited number of reported methods to produce vdW nanowires is hindering research progress in this field. Two main synthesis routes are currently utilized: exfoliation and chemical vapor deposition (CVD). Exfoliation involves splitting bulk crystals through the use of tapes or sonication in a liquid medium. Although the tape method yields high-quality nanowires, it produces a low yield and often results in the formation of short nanowires. An alternative method, liquid exfoliation, can improve yield, but it causes crystal breakages, leading to the formation of short nanowires with a high number of defects. The use of CVD growth to synthesize vdW nanowires has not yet been systematically established due to the complexity of the chemical reaction and growth dynamics, resulting in a lack of established CVD reaction recipes.

[0034] Therefore, developing new synthesis techniques is necessary to broaden the scope of ID vdW nanowires. In particular, it is highly desirable to acquire ultralong and uniform vdW nanowires with consistent structural and electrical properties, which promise the easy integration of many devices. Furthermore, cost-effective methods for commercially scalable production would also be critical at the stage of large-scale integration of nanodevices towards straightforward applications.

[0035] Embodiments disclosed herein include a method for synthesizing semiconducting Ta2Ni3Ses (TNS) van der Waals nanowires through a single-step solid- state reaction. Method 2800 of FIG. 28 is an example ofthis method. The approach used in this work is cost-effective, resulting in the production of large-scale, millimeter-long TNS nanowires with uniform morphology and aspect ratios exceeding 10,000. Following growth, mechanical exfoliation was demonstrated to produce nanowires with thicknesses in the tens of nanometers range. The use of transmission electron microscopy (TEM) revealed that the TNS nanowires are homogeneous single crystals that remain resistant to oxidation at room temperature. Additionally, a TNS nanowirebased nanodevice was fabricated with nickel contacts, which exhibit Schottky diode properties with a barrier height of approximately 0.41 eV.

2. Solid-state growth of TNS nanowires

[0036] The schematics of solid state growth of TNS nanowires are illustrated in FIG. 1(a) (see methods for detail], FIG. 1(b) shows the ampoule after growth. The product of growth includes tightly packed nanowires resembling a turf of rolling grass. After removing it from the ampoule, the turf roll remains a single piece, as shown in FIG. 1(c). The nanowire turf roll is cut open along the longitudinal direction and unfolded for examining the morphology.

[0037] FIGs. 1 and 2 illustrate synthesis of crystalline TNS nanowires using method 2800 described in FIG. 28. FIG. 1(a) is a schematic of TNS nanowire growth via the solid state reaction. FIG. 1(b) is photograph of the ampoule containing TNS nanowires after synthesis. FIG. 1(c) SEM image ofthe TNS turf roll after removing from the ampoule, showing a cross section relative to the ampoule's longitudinal direction (Scotch tape is attached to the perimeter to maintain the shape of the turf roll). FIG. 2 (a) is an SEM image ofthe unfolded TNS turf roll (obtained by stitching 64 SEM images) showing the overall size of grown TNS nanowire turf. FIG. 2(e) is a side view ofthe turf roll section shown in (a). [0038] The TNS turf roll is first examined using a scanning electron microscope (SEM) by taking high-resolution micrographs of the nanowire at various locations. FIG. 2 [a] shows the unfolded turf roll lying flat on Scotch tape. The whole image is formed by stitching 64 SEM images, demonstrating the abundance of the growth. FIG. 2 b) shows a side view of a TNS turf, which exhibits an overall 2 mm thickness measured from the baseline. Most of the nanowires can be seen to go from the bottom to the top, which suggests that each nanowire may be a few millimeters long on average. More SEM examinations of the nanowires on the bottom side surface of the turf roll are shown in FIGs. 9(a) and 9(b). We notice brighter regions or spots dispersed throughout the bottom surface. Unlike the bottom side, the top side of the nanowire turf, as shown in FIGs. 9(c) and (d), is free of seed regions and fully covered with clean and long nanowires. Therefore, it is reasonable to speculate that brighter regions serve as nucleation sites, mostly located on the sidewall of the ampoule (i.e., the bottom of the turf). Nanowires grow from the seed regions toward the center of the quartz tube to eventually form a turf structure.

[0039] We suspect that the source particles, initially attached to the inner wall, form the seeding area that leads to abundant nanowire growth. To verify this hypothesis, we prepared two ampoules, one with source powders covering only two ends of the ampoule with an uncovered region in the center (Ampoule A) and the other with powder covering the entire inner wall (Ampoule B). This is realized by rubbing an O-ring from outside to induce electrostatic charges, which attract the source powder onto the quartz inner wall. Then, both ampoules were grown with the same conditions (see FIG. 10(a) & 10(d)) simultaneously. The corresponding ampoules after growth are shown in FIG. 10(b) & 10(e), showing distinctive growth yield. The ampoule with seeds covering the entire wall results in a dense growth of nanowires (FIG. 10(c) & 10(f)). Complementary to the earlier understanding, this result provides strong evidence of the direct correlation between seed particles and nanowire growth.

[0040] Energy-dispersive X-ray spectroscopy (EDS) and x-ray diffraction (XRD) characterizations were performed on the as-grown nanowires to assess their chemical composition and crystal structure. FIG. 11(a) shows EDS data obtained from a collection of free-standing nanowires. All the peaks belong to Ta, Ni, and Se except C and 0, originating from carbon tape as the sample mount. A quantitative EDS analysis shows thatthe atomic ratio of Ta/Ni/Se is about 2:3:8, consistent with the expected stoichiometric ratio of the TNS crystal. It is also confirmed that all nanowires at the seed region exhibited a consistent stoichiometric ratio, as shown in FIG. 11 (b- d]. The crystal structure of as-grown samples was determined by comparing our XRD powder diffraction peaks to those from the database (PDF#86-0186] (FIG. 12]. Except for a few extra minor peaks, which correspond to monoclinic TazNiSe?, all the XRD peaks match orthorhombic Ta2NisSe8, as shown in FIG. 13(a]-(c], The two phases can be further estimated by comparing the weights of major peak profiles (see supplementary FIG. 14 for more details]. The as-grown nanowires contain 84.3 wt % of Ta2NisSe8 crystals and 15.7 wt% Ta2NiSe? crystals.

[0041] The as-grown TNS nanowires are long but entangled. The soft-lock drawing method is adopted to achieve alignment for the whole turf roll. We cut a piece of as- grown turf roll and lay it flat onto a silicon wafer. A razor blade is pressed to fix one side of the roll. Then another razor blade, wrapped with an ethanol-wetted nylon filter membrane, is used to press and sweep from the fixed razor until an aligned nanowire region is observed. The ethanol acts as a liquid medium to reduce the resistance between the nanowire and the substrate. The razor blade is kept static for a few seconds after finishing the sweeping to keep aligned nanowires from retraction until the solvent evaporates.

[0042] FIG. 3 shows alignment of millimeter-long TNS nanowires. FIG. 3(a] is a low-magnification SEM image of TNS nanowires aligned using the soft-lock drawing method. FIG. 3 (a] is an SEM image of an isolated nanowire obtained through alignment. FIGs. 3(c]-(g] are high magnification SEM images of nanowire segment marked by white crosses in (b]. The scale bar in FIGs. 3(c]-(g] is 200 nm.

[0043] We use SEM to study the morphology of the aligned nanowires. FIG. 3 (a] shows the low magnification SEM image of the nanowires after the alignment, with the blade moving from left to right. In the beginning, nanowires are randomly oriented. With the sweeping force from the blade, the nanowires become aligned, forming a distinct region ofclean and oriented nanowires in the middle. In FIG. 3(b], a 2.1 mm long nanowire with a 260 nm thickness is clearly visible in the high magnification SEM image taken from a location chosen from FIG. 3 (a]. It is worth mentioning that the nanowires could be longer if more precise control of the force can be realized to avoid breaking the nanowire during the aligning process. FIG. 3(c]-(g] represents the high- resolution images taken from multiple locations along the aforementioned 2.1 mm long nanowire.

[0044] Notably, the nanowire exhibits a high uniformity in thickness, with a thickness fluctuation of 5nm over the entire length. In addition, we have sampled about 200 TNS nanowires. The overall dimension analysis is shown in FIG. 15. The longest nanowire can reach 4 mm. Most nanowires have a thickness between 100 to 400nm. The average thickness is 265 nm, while the most frequently observed value is 170 nm. Only a few samples with a thickness above 400 nm were found.

[0045] We use TEM to examine the crystallinity and structure integrity of the nanowires. The TEM samples were prepared by directly rubbing bundles of nanowires with TEM grids. FIG. 3(a) shows a TEM image of a 30 nm TNS nanowire. It exhibits well- resolved lattice fringes with a 0.62 nm separation, corresponding to the interplanar distance of (2 1 0) lattice planes. The lower left inset highlights that the atomic-level clarity of molecular chains is most pronounced on the edge of the nanowire. Additionally, neither the edge nor the surface reveals any amorphous layer associated with lattice degradation or defects. By comparing diffraction patterns taken from different incident angles, we found that the nanowire was grown along the [001] direction, parallel to the molecular chains. In order to check the crystallinity of the nanowire, a 23um-long nanowire (shown in FIG. 3(b)) was selected for the selective area electron diffraction (SAED). FIG. 3(d)-(h) show the diffraction patterns from five different locations along the nanowire. All SAEDs exhibit an identical pattern of diffraction spots but with varying intensities, indicating a slight twist occurred when the nanowire was placed on the TEM grid.

[0046] FIG. 4 shows TEM images of ultra-long TNS nanowires. FIG. 4(a) is a high resolution TEM image of a TNS nanowire. The inset shows the SAED with the scale bar corresponding to 2 nm^-1. FIG. 4(b) is a low magnification TEM image of the nanowire shown in FIG. 4(c). FIG. 4(d)-(h) SAED data for five locations along the TNS nanowire shown in (c). The scale bar in FIGs. 4(d)-(h) is 2 nm^-1.

[0047] TNS is a member of the isostructural group M2X3Ses (M = Ta or Nb; X = Ni, Pd, or Pt), with the orthorhombic space group D2h⁹-Pbam. As depicted in FIGs. 4(a) and 4(b) its crystal structure resembles a framework of molecular ribbons extending down the c-axis. Each ribbon (as shown in FIG. 4(c)) consists of two chains of edge-sharing Tacentered trigonal Se prisms. They are connected at the ribbon's center and capped on both sides with Ni atoms. In order to form a bulk, TNS molecular ribbons are joined by weak interaction between the edge-terminating Ni atoms and trans-Se atoms, which in return, slightly distorts the Se prisms. Overall, the bulk crystal can be viewed as composed of "windmill" units. Each unit includes four molecular ribbons rotated 90 degrees with a sizable channel in the center.

[0048] FIG. 5 shows crystal structure and exfoliation of nanowires. FIG. 5(a) is a stereo-view of TNS bulk crystal structure projected along the c-axis. FIG. 5(b) shows a 4- blade "windmill" unit highlighted from the bulk TNS structure. FIG. 5(c) is a top view of a single ribbon extended along the c direction. FIG. 5(d) s an optical image of exfoliated TNS nanowires under darkfield illumination. FIG. 5(e) is an AFM image of the region marked by a dashed rectangle 511 in FIG. 5(d) showing nanowires with 19 nm and 46 nm thickness. FIG. 5(f) is an AFM image of a thin nanowire with a thickness of approximately 7 nm.

[0049] We demonstrate that thinner TNS nanowires can be further obtained using micro-mechanical exfoliation of as-grown nanowires. The as-grown TNS nanowire was cleaved several times using tape and then press-transferred onto a thermally oxidized (300 nm SiOz) silicon wafer. Optical microscopy was adopted to identify the TNS nanowires after exfoliation. In particular, darkfield illumination was used to enhance the visibility of nanowires, as shown in FIG. 4(d). The TNS nanowires have different colors corresponding to different thicknesses. The accurate thicknesses of nanowires can be further identified by atomic force microscopy (AFM), as shown in FIG. 4(e). TNS nanowires as thin as 7 nm can be readily achieved (FIG. 4(f)). Most identifiable exfoliated nanowires on silicon substrates have length/thickness ratios larger than 1000.

[0050] Here we discuss exfoliation mechanism of TNS nanowires. Conventional van der Waals materials, represented by graphene, are known to be mechanically exfoliable because they contain significantly weaker van der Waals bonds. However, TNS has all chemical bonds with comparable bond strength. Especially, interchain bonds in TNS are between nickel and selenium atoms (see analysis in supplementary FIG. 17), also a conventional chemical bond. However, TNS still exhibits remarkable exfoliability.

[0051] We performed density functional theory (DFT) calculations to understand this dilemma. We first located four possible cleavage bonds (labeled Ei to E4), as shown in FIG. 17(a), and calculated the cleavage energies of each bond. All four bonds (E1-E4) possess comparable cleavage energy varying from 1.5 to 1.9 eV. However, because the bond densities (i.e., bonds per unit cell) vary drastically, it is necessary to count net cleavage energy per unit cell that directly determines the exfoliability of the nanowire. As shown in FIG. 17(b), net cleavage energy per unit cell along Ei's direction is the lowest (3.12 eV) in comparison with that of other directions E2 (7.76 eV), E3 (13.36 eV), and Er (6.16 eV). Hence, if the external force is applied to the nanowire, we expect the nanowire incline to split along the c direction (Ei). This calculated result agrees with the exfoliation finding from the TEM study.

[0052] FIG. 6 illustrates a one-dimensional array of Schottky devices on a single ultralong TNS nanowire. FIG. 6(a) is an optical image of 100 contacts on a single TNS nanowire. The bottom inset shows the SEM image of the region marked by dotted rectangle 611 in FIG. 6(a). FIG. 6(b) is an SEM image of the ultralong nanowire used for device fabrication (marked by a dotted rectangle 612 in FIG. 6(a). Note: only one-third of the nanowire is used for placing electrical contacts. FIG. 6(c) is a zoomed-in SEM image of the region marked by a dotted rectangle 621 in FIG. 6(b). FIG. 6(d) is a zoomed-in SEM image of the region marked by a dotted rectangle 622 in FIG. 6(b).

[0053] We demonstrated that TNS nanowires can grow to lengths of a few millimeters while maintaining uniform crystallinity. Further, our DFT calculation of the TNS band structure reveals a semiconducting gap of 0.24eV (FIG. 18). All these results indicate that TNS can be easily fabricated into electronic devices for various applications. To illustrate this advantage of TNS nanowires, we patterned metal contacts up to hundreds on a single nanowire using standard e-beam lithography. FIG. 6(a) shows an optical image of a representative device highlighting a hundred contacts defined on a TNS nanowire. The SEM image of the 1.5mm wire used for this device fabrication is shown in FIG. 6(b). The magnified image (FIG. 6(c)) of the region marked by the dashed rectangle shows the contacts passing vertically, corresponding to source/drain electrodes with 1.5 pm width and 3 -pm separation. This candidate device covers a third of the nanowire length, indicating the potential to fabricate a large array of devices. All the contacts are conductive, as discussed in the following sections.

[0054] Since research about TNS is in its infancy, understanding TNS/metal interface is crucial for its potential applications. For example, an ohmic interface allows carrier transport across the interface easily, making it ideal for field effect transistor applications. On the other hand, a Schottky barrier interface rectifies the charge transport. It can be engineered as the building blocks of various applications, including rectifiers, logic gates, solar cells, photodetectors, biosensors, gas sensors, and strain sensors. A broad range of metals can be used to investigate the interface's electrical functionalities and chemical nature. In embodiments, this metal is nickel because it forms a strong and stable adhesion with TNS due to the expected strong orbital overlap between the d orbitals of the Ni atoms in the metal and TNS.

[0055] FIGs. 7 and 8 illustrate electrical characterization of Schottky devices made on TNS nanowire. FIG. 7(a) is an optical image of TNS 2T devices fabricated from a single TNS nanowire, such as any of the TNS nanowires of FIGs 1-6. FIG. 7(b) is an SEM image of the metal-semiconductor-metal (MSM) device marked in a dashed box 711 in FIG. 7(a) with a channel length of 1.5 pm. Inset shows the schematic of an MSM device structure. FIG. 8(a) Experimental I-V characteristics of TNS nanowire devices with different channel lengths (empty squares) and the fitting with the MSM model (solid lines). FIG. 8(a) Extracted Schottky barrier heights for devices with different TNS nanowires.

[0056] A two-terminal (2T) device structure, which includes two symmetrical TNS/Ni interfaces connected in series with a TNS nano wire, is used for the investigation. Multiple 2T devices were fabricated on a single nanowire using standard electron beam lithography followed by e-beam metal deposition (70 nm Ni /25 nm Au). The optical micrograph in FIG. 7(a) shows a representative as-fabricated device. The individual nanowire was contacted with 2 pm wide metal contacts with variable spacing ranging from 1 to 4 um. From the SEM imaging, as shown in FIG. 7(b), the single crystal nanowire and metal contacts remained uniform and clean after fabrication.

[0057] The TNS device is then measured by sweeping source-drain voltage (V) while recording the source-drain current (I). A typical I-V characteristic from a single TNS nanowire with multiple 2T devices is shown in FIG. 8(a) (labeled with empty squares). All the I-V curves exhibit superlinear characteristics, indicating a Schottky barrier formation at the TNS/Ni interface. Based on the device structure of metalsemiconductor-metal (MSM), an equivalent circuit can be described as a serial connection of two back-to-back aligned Schottky diodes and one semiconductor, as illustrated in the inset of FIG. 7 (b). To extract Schottky barrier height, we fit our I-V data using the well-established MSM model, (see Methods for more details) The solid lines in FIG. 8(a) show the fitting of the model, which agrees well with the experimental I-V data. FIG. 8(b) shows that forty-four devices made on three nanowires were measured. The extracted Schottky barrier height average is about 0.38eV with a fluctuation of +7%.

[0058] The details of data fitting to the devices from each nanowire are shown in FIGs. 19, 20 and 21. A few hundred devices were fabricated and measured for this investigation, and the extracted Schottky barriers are in good agreement and consistent. The uniformity of the Schottky barrier height may be attributed to the homogeneity of the barrier interface between the nanowire and the nickel metal. While the intricate mechanism remains a subject for future investigation, the present findings provide insight into the potential utility of incorporating TNS Schottky devices in potential applications, such as photodetectors, solar cells, and chemical sensors.

3. Conclusion

[0059] We have achieved high yield growth of millimeter-long TNS nanowires via a straightforward solid-state reaction. The TNS nanowires were confirmed to be single crystals with uniform crystallinity throughout their entire length. Additionally, we demonstrated that scalable alignment of as-grown nanowires can be implemented, and mechanical exfoliation of individual as-grown nanowires can further produce nanowires with a few nanometers of thickness. DFT investigation reveals that the significant exfoliating capability of TNS nanowires originates from the considerable anisotropy in cleavage energy. Moreover, the charge transport study of TNS nanodevices concludes that the electrical contact between Ni and TNS is of Schottky type, with a consistent barrier height of 0.41 eV. These results open up new avenues for the exploration of one-dimensional van der Waals materials and their potential utilization in advanced technologies.

4. Methods

[0060] Synthesis of TNS nanowires-. Any of the following steps for synthesis of TNS nanowires may be part of, or incorporated into, embodiments of method 2800, FIG. 28. TNS nanowires were synthesized using the solid state reaction, as illustrated in FIG. 1(a). Stoichiometric elemental powders of tantalum (157 mg, Alfa Aesar, 99.97% purity), nickel (76.4 mg, Alfa Aesar, 99.99% purity), and selenium (274 mg, Alfa Aesar, 99.99% purity) were mixed and placed in a quartz ampoule (14 x 1.6 cm). The ampoule was evacuated and sealed at a pressure of around 5 x IO ⁵ torr. Then, the ampoule was placed in a tube furnace (Lindberg Mini Blue) with the powder source end positioned in the middle of the furnace. The furnace was then heated to 400 °C at a rate of 20 °C hr^-1 and kept atthis temperature for 10 hours. Then, the furnace was ramped to 725 °C with a rate of 10 °C hr^-1 and maintained at 725 °C for one week. Finally, the furnace was cooled to room temperature at ~12 °C hr^-1. FIG. 1(b) shows the ampoule after growth, in which a wool-like dark product can be seen.

[0061] Material characterization-. The crystallinity of the sample was examined using a Rigaku DMAX 2200 powder diffractometer, configured with Cu Ka radiation operating at 40 kV and 40 mA with a scan rate of 1° per minute. The chemical composition was confirmed with EDS (Oxford Instruments) attached to SEM (Hitachi S- 3400) operating at an accelerating voltage of 30 kV. The morphology and nanostructure of the samples were characterized by SEM (Hitachi S4800) working at an acceleration voltage of 10 kV and TEM (Tecnai G2-F30). All TEM images and SAED patterns were taken at an accelerating voltage of 300 kV. The thickness of nanowires was determined using AFM (Bruker Dimension ICON).

[0062] First-principles calculations: DFT calculations are performed within the projector augmented wave method as implemented in the Vienna Ab-initio Simulation Package. The generalized gradient approximation (GGA) functional with the Perdew- Burke-Ernzerhof parametrization is used. A plane-wave energy cut-off is set to 400. Structural relaxation is done until the forces acting on each atom become less than 10'³ eV/A. An accurate computational description of the M2X3Y8 nanoribbons system is conjugated with a proper choice ofvdW dispersion functional. Here we test 4 possible cases, including (i) vdW is switched off, (ii) DFT-D3 semi-empirical method, (iii) nonlocal VDW-DF1 method, and (iv) fractional ionic atoms (FIA) method. The latter is currently proposed to be one of the best nowadays, especially in the presence of pronounced atomic polarizability that should take place in nanoribbon systems. Interchain £_bind is intrinsically found as the normalized energy difference between the bulk system and two single ribbons that form a bulk unit cell:

[0063] Results from different approaches are described in Table SI, and we can see that absence of vdW functional significantly underestimates binding energy. At the same time, all three compared vdW methods provide a consistent range of Ebind [I- 1.4 eV/bond) and bond length (2.4-2.7 A). Also, our data reveals relatively close Ebind and di_nter values for FIA and DFT-D3 methods. Since vdW corrections do not explicitly influence electronic properties, we used a less time-consuming DFT-D3 approach.

[0064] Finally, DFT-D3 functional is applied to adequately describe weak interactions in the nanoribbons system. The intra-bond E2-E4 (as well as Ei for comparison) is evaluated through surface cleavage with necessary bonds following the equation:

where E_slab is the total energy of the cleaved TNS slab, E_buik is bulk TNS energy, and I bonds is the number of broken bonds relevant to exfoliation. The Brillouin zone is sampled according to the Monkhorst-Pack scheme with a k-point spacing of 0.15 A^-1 and 0.07 A^-1 in the periodic directions for structural relaxation and DOS calculations, respectively. To avoid spurious interactions between the neighboring unit cells, we set the translation vector along the non-periodic direction more than 20 A while calculating single ribbons and TNS slabs. Found results are checked to be not influenced by spin- polarized calculations. More details on the theoretical approaches used are presented in the supporting Information.

[0065] Device fabrication and electrical characterization-. To fabricate the TNS nanodevice, we first create a multi-terminal contact pattern over an exfoliated thin nanowire by standard electron beam lithography (Raith Voyager 100). Then a metal deposition of 75 nm nickel and 25 nm gold as the contact metal was conducted with an electron beam evaporator (Angstrom Engineering Nexdep) with a deposition rate of 0.5 A s’¹. Before metal deposition, samples were ion-milled (AJA ion miller) using argon gas for 7s to remove any organic residual layers and surface contaminants. We loaded samples immediately (within 1-2 min) into an e-beam evaporation system for metal deposition, followed by a lift-off process. Transport measurements were conducted in the ambient environment using a semiconductor parameter analyzer (HP 4155A). Current-voltage characteristics were fitted using MATLAB-based software PKUMSM.m, to determine the Schottky barrier heights. The PKUMSM software inputs are the TNS nanowire's physical dimensions and Eo (obtained by fitting the linear region in In(Ids) vs. V). 5. Turf Roll Growth of Semiconducting van der Waals Ta₂Ni₃Se₈ Nanowire

[0066] FIG. 9 shows SEM images of TNS nanowires. FIGs. 9(a) and 9(c) are respected images of the outer surface and the inner part of the turf roll. FIGs. 9(b) and 9(d) show a magnified view of the regions 911 and 913 marked in FIGs. 9(a) and 9(c), respectively.

[0067] FIG. 10 shows turf roll growth by controlling the distribution of source powder before growth. FIGs. 10(a)-(c) are optical images showing ampoules: pregrowth with source powder covering the left half inner wall, post-growth, and as-grown nanowire exposed with the top glass removed. FIGs. 10 (d)-(f) are optical images showing ampoules: pre-growth with source powder covering most of the inner wall, post-growth, and as-grown nanowire exposed with the top glass removed.

[0068] FIG. 11(a) is an EDS spectrum from the TNS nanowires shown in the bottom inset. The composition obtained from spectrum data is shown in the table of the top inset. FIG. 11(b) is an SEM image of TNS nanowires from the outer surface ofthe turf roll, color-marked areas are used for EDS analysis. FIG. 11(c) and (d) show EDS data ofthe TNS nanowires at locations marked by rectangles 1111 and 1112 in image FIG. 11(b).

[0069] FIG. 12 is a powder XRD pattern of TNS crystal. Lines 1210, 1220, and 1230 denote the measured data, references for Ta2NisSe8 (JCPDS 86-0186) and Ta2NiSe? (JCPDS 78-0457), respectively. FIG. 13(a)-(c) are XRD spectra ofthe highlighted regions marked in FIG. 12. FIG. 14(a) is an XRD peak fitting to quantify the ratio of constituent phases (Ta NisSes and Ta2NiSe?) based on data from the dotted rectangle shown in FIG. 12(a). FIG. 14(b) shows the weight ratio of constituent phases in the as-grown crystals.

[0070] FIG. 15(a) is an SEM image of randomly oriented TNS nanowires from the turf roll growth. FIG. 15(b) shows the thickness distribution of TNS nanowires.

5.1 TazNisSes crystal structure

[0071] The crystalline TazNisSes is a coordination compound that includes (i) tantalum with a trigonal prismatic Dsh coordination, (ii) one nickel with a flat square D4h coordination, and (hi) two nickels with a square pyramid C4v coordination. The difference in nickel geometry arises from the different occupancy of its d-orbital. For detail, see FIG. 16 and the explanation below.

[0072] FIG. 16 shows Orbital diagrams for Ta⁵⁺ and Ni²⁺ for different coordination numbers. Ta⁵⁺ atomic orbitals are empty. While forming a bond with Se^2-, vacant orbitals of tantalum hybridize into a sp³d² state, forming a trigonal prismatic configuration with a coordination number equal to 6.

[0073] Nickel atoms are not all equivalent in TNS structure. Indeed, one Ni atom in the ribbon's center has a coordination number 4. In contrast to edging Ni atoms, it is equal to 5. The eight d-electrons are pairing, which leads to empty d_xz__yz orbital. This empty d_xz__yz orbital hybridizes, as well as empty s and p-orbitals, and forms an inner- orbital complex with a coordination number 4 d_xz__yzsp_xp_y — dsp²) - square planar.

[0074] However, the p_z orbital can also interact with unshared electron pairs of Se from neighboring ribbons, forming the fifth bond and square pyramidal complex with a coordination number 5.

5.2 DFT calculations

[0075] An accurate computational description of the M2X3Y8 nanoribbon system is conjugated with a proper choice ofvdW dispersion functional. Here we test 4 possible cases, including (ij vdW is switched off, (if) DFT-D3 semi-empirical method, (iiij nonlocal VDW-DF1 method, and (ivj fractional ionic atoms ( FIA] method. The latter is proposed to be one of the best nowadays in the presence of pronounced atomic polarizability that should take place in nanoribbon systems. Three bulk structures are considered, namely Ta2NisSe8, Ta2PdsSe8, and ^PdsSes.

[0076] As benchmarks, we evaluate inter-chain binding energy and inter-chain bond length. Inter-chain E_bind ^s intrinsically found as the normalized energy difference between the bulk system and two single ribbons that form a bulk unit cell according to equation (1).

[0077] The complete results with reference data from other DFT calculations are presented in Table SI.

Table SI. Comparison of inter-chain E_bind and bond length calculated via different vdW approaches for different M₂/₃T₈ bulk nanoribbons systems.

[0078] FIG. 17(a) is a schematic showingthe locations of cleavage bonds (E1-E4) in the TNS crystal structure. Planes are displayed perpendicular to the bonds for visual guidance. FIG. 17(b) is a table showing cleavage energy of various bonds in TNS. FIGs. 17(c)— (f) show schemes of surface cleavage along marked bonds (E1-E4) in TNS bulk. The dashed box represents the unit cell used to calculate the net energy of bonds in FIG. 17(b).

[0079] FIG. 18 is a graph of the total density of states for TNS bulk, in an embodiment. The Fermi level is set to zero. The atomic structure of the TNS bulk is shown in the inset. FIGs. 19- 21 are graphs used for extraction of Schottky barrier for TNS device using MSM model and different channel lengths.

6. Embodiments of TNS nanowires

[0080] FIG. 22 is a schematic of a TNS nanowire 2200. Examples of TNS nanowire 2200 include nanowires of FIGs. 1, 2, 3(a), 3(b), 4, 5(d), 5(e), 6, 7(a), 9, 10, and 15. TNS nanowire 2200 has a material composition that includes Ta₂Ni₃Se₈. TNS nanowire 2200 may be a single Ta₂Ni₃Se₈ crystal. In embodiments, the material composition of TNS nanowire 2200 is at least eighty percent Ta₂Ni₃Se₈ by weight. The remaining material composition may include other compounds of Ta, Ni, and Se, such as Ta₂NiSe₇.

[0081] TNS nanowire 2200 has a length 2210 and a thickness 2220. A ratio of length 2210 to thickness 2220 may exceed one thousand. In embodiments, TNS nanowire 2200 has at least one of the following properties: thickness 2220 is between 0.1 micrometers and 0.4 micrometers, length 2210 exceeds 500 micrometers, and its length-to-thickness ratio exceeds ten thousand. [0082] FIG. 23 is a schematic plan view of a device 2300. Examples of device 2300 include the Schottky devices of FIGs. 6, 7(a], and 7(b ] . Device 2300 includes a TNS- nanowire bundle 2370 and electrical contacts 2310(1] and 2310(2] thereon. Electrical contacts 2310 are electrically connected to TNS -nanowire bundle 2370 and each may be attached to TNS-nanowire bundle 2370. Device 2300 may also include a substrate 2350 on which TNS nanowire 2200 is disposed. Substrate 2350 maybe a semiconductor substrate. TNS-nanowire bundle 2370 includes one or more TNS nanowires 2200.

[0083] Along the length of TNS nanowire 2200, metal contacts 2310(1] and 2310(2] are on TNS nanowire 2200 at a longitudinal position 2231 and a longitudinal position 2232, respectively along the length dimension of TNS nanowire 2200. The material composition of each of electrical contact 2310(1] and 2310(2] may include at least one of nickel and gold. A distance 2315 along TNS nanowire 2200 between the first and second longitudinal positions may be between one micrometer and four micrometers. A distance 2314 between metal contacts 2310(1] and 2310(2] along TNS nanowire 2200 may also be between one micrometer and four micrometers.

[0084] TNS nanowire 2200 has a thickness 2221 in a transverse direction 2391 at longitudinal position 2231. Electrical contact 2310(1] has a width 2311 in transverse direction 2391 that exceeds the thickness 2221. TNS nanowire 2200 has a thickness 2222 in a transverse direction 2392 at longitudinal position 2232. Electrical contact 2310(2] has a width 2321 in transverse direction 2392 that exceeds the thickness 2222. Transverse direction 2392 maybe parallel to transverse direction 2391, e.g., when the segment of TNS nanowire 2200 between longitudinal positions 2231 and 2232 is straight. At least one of distances 2314 and 2315 may be in a direction perpendicular to one or both of directions 2391 and 2392.

[0085] FIG. 24 is a schematic of a strain sensor 2400 in a use scenario where it measures mechanical strain of a subject 2490. Strain sensor 2400 includes electrical contacts 2410(1,2,3], a substrate 2450, and TNS-nanowire bundles 2470(1,2], which are respective examples of electrical contacts 2310, substrate 2350, and TNS-nanowire bundle 2370, respectively. Strain sensor 2400 may also include a flexible packaging layer 2460. Substrate 2450 is flexible, and may be attached to subject 2490, e.g., with adhesives. TNS-nanowire bundle 2470(1], electrical contacts 2410(1,2], and substrate 2450 together constitute a first example of device 2300. TNS-nanowire bundle 2470(2] and electrical contacts 2410(2,3), and substrate 2450 together constitute a second example of device 2300.

[0086] FIG. 24 denotes orthogonal axis Al and A2, which may define a plane of a surface of subject 2490 when the surface is planar, and hence not experiencing any stress. TNS-nanowire bundles 2370(1) and 2370(2) detect bending about axes Al and A2, respectively, via their stress-dependent conductivity. Electrical contacts 2310 connect to external or integrated electric circuits for strain measurement. These circuits may monitor the resistance, through current or voltage change of one or both of TNS- nanowire bundles 2370, upon bending of subject 2490.

[0087] FIG. 25 is a schematic of a chemical sensor 2500 in a use scenario where it is characterizing a liquid 2590 contained by a vessel 2592. Chemical sensor 2500 includes electrical contacts 2510(1,2,3), a substrate 2550, and a TNS-nanowire bundle 2570, which together constitute an example of device 2300 and are respective examples of electrical contacts 2310, substrate 2350, and TNS-nanowire bundle 2370. The material composition of electrical contacts 2510 may include silver chloride or platinum depending on the chemical being detected in liquid 2590.

[0088] FIG. 26 is a schematic of an optical sensor 2600 in a use scenario where it is detecting light 2690. Optical sensor 2600 includes electrical contacts 2610(1,2), a substrate 2650, and a TNS-nanowire bundle 2670, which together constitute an example of device 2300 and are respective examples of electrical contacts 2310, substrate 2350, and TNS-nanowire bundle 2370. As a semiconductor, the conductivity and resistivity of TNS-nanowire bundle 2670 change when it is illuminated by light 2690, which may be focused on TNS-nanowire bundle 2670. Electrical contacts 2610 may be electrically connected to external or integrated electrical circuits for photoconductance measurement, and hence detect light 2690 incident on TNS- nanowire bundle 2670.

[0089] FIG. 27 is a schematic of optical sensor 2600 in a use scenario where it is detecting light 2790 incident on an interface between TNS-nanowire bundle 2670 and electrical contact 2610(2). The interface between TNS-nanowire bundle 2670 and electrical contact 2610(2) is a Schottky barrier that functions as a photodiode that generates a voltage when illuminated by incident light 2790.

[0090] FIG. 28 is a flowchart illustrating a method 2800 for fabricating a nanowire, such as TNS nanowire 2200 and examples of TNS nanowire 2200 described herein. Method 2800 includes at least one of steps 2810, 2820 2830, 2840, 2850, 2860, and 2870.

[0091] Step 2810 includes evacuating gas an ampoule that includes a mixture of elemental powders. The 2810 may include evacuating the ampoule to a pressure between 4 x 10^-4 Torr and 6 x 10^-6 Torr. The mixture of elemental powders may include one or more of tantalum, nickel, and selenium. In embodiments, mixture of elemental powders includes each of tantalum, nickel, and selenium, and has a mole ratio of tantalum to nickel to selenium equaling two to three to eight. Method 2800 may include sealing the ampoule after step 2810.

[0092] Step 2820 includes distributing the mixture on an interior surface of the ampoule in part by inducing electrostatic charges on an exterior surface of the ampoule. The exterior surface may be a region of the exterior surface of the ampoule. Step 2820 may be implemented after step 2810 and before any of steps 2830, 2840, 2850, 2860, and 2870.

[0093] Step 2830 includes heating the ampoule to a first temperature. In embodiments, the first temperature is between 3250 °C and 600 °C. Step 2830 may include heating the ampoule at a rate between 5 °C per hour and 30 °C per hour. When method 2800 includes step 2810, step 2810 may precede step 2830.

[0094] Step 2840 includes maintaining the ampoule at the first temperature for a first duration, which may exceed one hour. Step 2840 may follow step 2830. Step 2850 includes heating the ampoule to a second temperature that exceeds the first temperature. The second temperature may be between 650 °C and 850 °C. Step 2850 may include at least one of (i) heating the ampoule from the first temperature to the second temperature and (if) heating the ampoule at a rate between 5 °C per hour and 30 °C per hour. Step 2850 may follow at least one of steps 2830 and 2840.

[0095] Step 2860 includes maintaining the ampoule at the second temperature for a second duration, which may exceed one hour. Step 2860 may follow step 2850.

[0096] Step 2870 includes cooling the ampoule to an ambient temperature. Step 2870 may follow steps 2850 and 2860. The ambient temperature is less than the first temperature, and may be between 24 °C and 30 °C. Step 2870 may include cooling the ampoule at a rate that exceeds 3 °C per hour.

[0097] In embodiments, step 2870 yields a mass of tightly packed nanowires, such as the nanowires shown in FIG. 1(b). In such embodiments, method 2800 may include a step 2880. Step 2880 includes mechanically exfoliating the mass to yield a plurality of individuated nanowires.

Combinations of Features

[0098] Features described above, as well as those claimed below, maybe combined in various ways without departing from the scope hereof. The following enumerated examples illustrate some possible, non-limiting combinations.

[0099] Embodiment 1. A nanowire fabrication method includes: heating, to a first temperature, an ampoule that includes a mixture of elemental powders; maintaining the ampoule at the first temperature for a first duration; heating the ampoule to a second temperature that exceeds the first temperature; maintaining the ampoule at the second temperature for a second duration; and cooling the ampoule to an ambient temperature.

[0100] Embodiment 2. The method of embodiment 1, said cooling yielding a mass of tightly packed nanowires, the method further including mechanically exfoliating the mass to yield a plurality of individuated nanowires.

[0101] Embodiment 3. The method of either one of embodiments 1 and 2, heating the ampoule to the first temperature including: heating the ampoule at a rate between 5 °C per hour and 30 °C per hour.

[0102] Embodiment 4. The method of any one of embodiments 1-3, in said heating the ampoule to the first temperature, the first temperature being between 3250 °C and 600 °C.

[0103] Embodiment 5. The method of any one of embodiments 1-4, in said maintaining the ampoule at the first temperature, the first duration exceeds one hour.

[0104] Embodiment 6. The method of any one of embodiments 1-5, heating the ampoule to the second temperature including heating the ampoule from the first temperature to the second temperature.

[0105] Embodiment 7. The method of any one of embodiments 1-6, heating the ampoule to the second temperature including: heating the ampoule at a rate between 5 °C per hour and 30 °C per hour.

[0106] Embodiment 8. The method of any one of embodiments 1-7, the second temperature being between 650 °C and 850 °C.

[0107] Embodiment 9. The method of any one of embodiments 1-8, the second duration exceeds one hour. [0108] Embodiment 10. The method of any one of embodiments 1-9, the cooling including cooling the ampoule at a rate that exceeds 3 °C per hour.

[0109] Embodiment 11. The method of any one of embodiments 1-10, further including: distributing the mixture on an interior surface of the ampoule in part by inducing electrostatic charges on an exterior surface of the ampoule.

[0110] Embodiment 12. The method of any one of embodiments 1-11, the elemental powders including tantalum, nickel, and selenium.

[0111] Embodiment 13. The method of embodiments 12, the mixture having a mole ratio of tantalum to nickel to selenium equaling two to three to eight.

[0112] Embodiment 14. The method of any one of embodiments 1-13, further including, before heating the ampoule to the first temperature, evacuating gas from an ampoule.

[0113] Embodiment 15. The method of embodiment 14, evacuating including evacuating the ampoule to a pressure between 4 x Torr and 6 x Torr.

[0114] Embodiment 16. A TNS nanowire having a material composition that includes , and a length-to-thickness ratio greater than one thousand.

[0115] Embodiment 17. The TNS nanowire of embodiment 16 being a single crystal.

[0116] Embodiment 18. The TNS nanowire either one of embodiment 16 and 17 having a thickness between 0.1 micrometers and 0.4 micrometers.

[0117] Embodiment 19. The TNS nanowire of any one of embodiments 16-18 having a length exceeding 500 micrometers.

[0118] Embodiment 20. The TNS nanowire of any one of embodiments 16-19, the length-to-thickness ratio exceeding ten thousand.

[0119] Embodiment 21. The TNS nanowire of any one of embodiments 16-20, the material composition being at least eighty percent .

[0120] Embodiment 22. A semiconductor device includes: the TNS nanowire of embodiment 16 having a first position and a second position along a length of the TNS nanowire; a first metal contact on the TNS nanowire at the first position; and a second metal contact on the TNS nanowire at the second position.

[0121] Embodiment 23. The semiconductor device of embodiment 22, a distance between the first position and the second position being between one micrometer and four micrometers. [0122] Embodiment 24. The semiconductor device of either one of embodiment 22 and 23, a material composition of each of the first metal contact and the second metal contact including nickel.

[0123] Embodiment 25. The semiconductor device of embodiment 24, the material composition also including gold.

[0124] Embodiment 26. The semiconductor device of any one of embodiments 22- 25, the TNS nanowire having a first thickness in a first transverse direction at the first position, the first metal contact having a first width in the first transverse direction that exceeds the first thickness; and the TNS nanowire having a second thickness in a second transverse direction at the second position, the first metal contact having a second width in the second transverse direction that exceeds the second thickness.

[0125] Changes may be made in the above methods and systems without departing from the scope of the present embodiments. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. Herein, and unless otherwise indicated the phrase "in embodiments” is equivalent to the phrase "in certain embodiments," and does not refer to all embodiments.

[0126] Regarding instances of the terms "and/or" and "at least one of,” for example, in the cases of "A and/or B" and "at least one of A and B," such phrasing encompasses the selection of (i) A only, or (if) B only, or (hi) both A and B. In the cases of "A, B, and/or C” and "at least one of A, B, and C,” such phrasing encompasses the selection of (i) A only, or (ii) B only, or (iii) C only, or (iv) A and B only, or (v) A and C only, or (vi) B and C only, or (vii) each of A and B and C. This may be extended for as many items as are listed.

[0127] The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.

Claims

We claim:

1. A nanowire fabrication method comprising: heating, to a first temperature, an ampoule that includes a mixture of elemental powders; maintaining the ampoule at the first temperature for a first duration; heating the ampoule to a second temperature that exceeds the first temperature; maintaining the ampoule at the second temperature for a second duration; and cooling the ampoule to an ambient temperature.

2. The method of claim 1, said cooling yielding a mass of tightly packed nanowires, the method further comprising mechanically exfoliating the mass to yield a plurality of individuated nanowires.

3. The method of claim 1, heating the ampoule to the first temperature comprising: heating the ampoule at a rate between 5 °C per hour and 30 °C per hour.

4. The method of claim 1, in said heating the ampoule to the first temperature, the first temperature being between 3250 °C and 600 °C.

5. The method of claim 1, in said maintaining the ampoule at the first temperature, the first duration exceeds one hour.

6. The method of claim 1, heating the ampoule to the second temperature comprising heating the ampoule from the first temperature to the second temperature.

7. The method of claim 1, heating the ampoule to the second temperature comprising: heating the ampoule at a rate between 5 °C per hour and 30 °C per hour.

8. The method of claim 1, the second temperature being between 650 °C and 850 °C.

9. The method of claim 1, the second duration exceeds one hour.

10. The method of claim 1, the cooling comprising cooling the ampoule at a rate that aour.

11. The method of claim 1, further corr distributing the mixture on an interior surface of the ampoule in part by inducing electrostatic charges on an exterior surface of the ampoule.

12. The method of claim 1, the elemental powders including tantalum, nickel, and selenium.

13. The method of claim 12, the mixture having a mole ratio of tantalum to nickel to selenium equaling two to three to eight.

14. The method of claim 1, further comprising, before heating the ampoule to the first temperature, evacuating gas from an ampoule.

15. The method of claim 14, evacuating comprising evacuating the ampoule to a pressure between 4 x 10^-4 Torr and 6 x 10^-6 Torr.

16. A TNS nanowire having a material composition that includes Ta₂Ni₃Se₈, and a length-to-thickness ratio greater than one thousand.

17. The TNS nanowire of claim 16 being a single Ta₂Ni₃Se₈ crystal.

18. The TNS nanowire of claim 16 having a thickness between 0.1 micrometers and 0.4 micrometers.

19. The TNS nanowire of claim 16 having a length exceeding 500 micrometers.

20. The TNS nanowire of claim 16, the length-to-thickness ratio exceeding ten thousand.

21. The TNS nanowire of claim 16, the material composition being at least eighty percentTa₂Ni₃Se₈.

22. A semiconductor device comprising: the TNS nanowire of claim 16 having a first position and a second position along a length of the TNS nanowire; a first metal contact on the TNS nanowire at the first position; and a second metal contact on the TNS nanowire at the second position.

23. The semiconductor device of claim second position being between one micrometer and four micrometers.

24. The semiconductor device of claim 22, a material composition of each of the first metal contact and the second metal contact including nickel.

25. The semiconductor device of claim 24, the material composition also including gold.

26. The semiconductor device of claim 22, the TN S nanowire having a first thickness in a first transverse direction at the first position, the first metal contact having a first width in the first transverse direction that exceeds the first thickness; and the TNS nanowire having a second thickness in a second transverse direction at the second position, the first metal contact having a second width in the second transverse direction that exceeds the second thickness.

We claim:

9. The method of claim 1, the second duration exceeds one hour.

10. The method of claim 1, the cooling comprising cooling the ampoule at a rate that exceeds 3 °C per hour.

11. The method of claim 1, further comprising: distributing the mixture on an interior surface of the ampoule in part by inducing electrostatic charges on an exterior surface of the ampoule.

17. The TNS nanowire of claim 16 being a single Ta₂Ni₃Se₈ crystal.

19. The TNS nanowire of claim 16 having a length exceeding 500 micrometers.

21. The TNS nanowire of claim 16, the material composition being at least eighty percent Ta₂Ni₃Se₈.

23. The semiconductor device of claim 22, a distance between the first position and the second position being between one micrometer and four micrometers.

26. The semiconductor device of claim 22, the TNS nanowire having a first thickness in a first transverse direction at the first position, the first metal contact having a first width in the first transverse direction that exceeds the first thickness; and the TNS nanowire having a second thickness in a second transverse direction at the second position, the first metal contact having a second width in the second transverse direction that exceeds the second thickness.