IONIC VACANCY DIFFUSION DRIVEN GROWTH OF ALIGNED NANOSTRUCTURES

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/263,143, filed on November 20, 2009, the entirety of this application is hereby incorporated herein by reference for the teachings therein.

FIELD

The embodiments disclosed herein relate to ionic vacancy diffusion driven growth of aligned nanostructures, and more particularly to the fabrication of highly uniform and well aligned nanowires that are capable of orthogonalization of the directions of light absorption and charge separation in photovoltaic cells.

BACKGROUND

Renewable energy harvesting with marketable efficiency based on materials that are cheap and environmentally benign is a promising solution to the energy and environment crises we are facing today. One-dimensional nanoscale semiconductor materials, such as nanowire arrays, have generated recent interest because of their favorable properties for use in photovoltaic devices. Namely, nanowire devices have shown improvements over thin-films in both light absorption and charge separation and transport. However, common methods of nanowire formation require the use of high temperatures, hazardous precursors, expensive catalyst materials, and/or sophisticated instrumentation, all of which make the processes costly and complicated.

Cu ₂ S-based solar cells were intensively studied in the 1980s. The main advantage of Cu ₂ S as a candidate for photovoltaics lies in its bandgap, -1.2 eV, a desirable value to match the solar spectrum, and high absorption coefficient. Power conversion efficiency of 9.15% has been achieved on solar cells made of Cu ₂ S and CdS thin films. A practical efficiency of 17.8% was predicted on devices consisting of Cu ₂ S and ZnO. The structural complexity and undesired Cu ion diffusion at the p/n junction led to abandonment of research on Cu ₂ S film based solar cells. Recently, the prospect that Cu ₂ S nano structures may help solve the challenges has renewed the interest in Cu ₂ S. Cu ₂ S nanoparticles and randomly oriented Cu ₂ S nanowires (NWs) have been recently reported. Among these morphologies, NWs are particularly appealing for solar energy conversion because the anisotropic nature promises an optimum combination of light absorption and charge collection. Successful synthesis of Cu ₂ S nanostructures, especially one dimensional nanowires that orthogonalize light absorption and charge separation, may overcome the challenges seen in conventional planar devices, and provide information on ion diffusion, which is important to the development of Cu ₂ S solar cells with long lifetime.

SUMMARY

Ionic vacancy diffusion driven growth of aligned nanostructures is disclosed herein.

According to aspects illustrated herein, there is provided a method of fabricating a cuprous sulfide nanowire array that includes providing a copper substrate; subjecting the copper substrate to an atmosphere of oxygen gas, hydrogen sulfide gas and nitrogen gas at ambient conditions, wherein the nitrogen gas is passed through a bubbler for mixing the nitrogen gas with water vapor so that a relative humidity is between about 25% and about 100%; creating a cuprous sulfide film on the copper substrate; and growing a plurality of vertically aligned cuprous sulfide nanowires from the film so as to fabricate the cuprous sulfide nanowire array. In an embodiment, the cuprous sulfide nanowires in the array are highly uniform and densely packed. In an embodiment, vertical growth occurs via cation vacancy diffusion, where the incorporation of sulfur atoms generate vacant copper sites (Vc _u ') which diffuse down toward the copper substrate, and wherein copper oxidation at an interface between the cuprous sulfide film and the copper substrate produces Cu ⁺ cations which diffuse up to complete the cuprous sulfide propagation. In an embodiment, the copper substrate is pre-treated using electropolishing.

According to aspects illustrated herein, there is provided a sulfide nanostructure array that includes a plurality of copper (I) sulfide nanostructures, wherein a proximal end of the nanostructures form a copper (I) sulfide film layer along an upper surface of a copper substrate, wherein a distal end of each of the nanostructures forms a tip, and wherein vertical growth of the plurality of copper (I) sulfide nanostructures is capable of occurring via cation vacancy diffusion, where incorporation of sulfur atoms generate vacant copper sites (Vc _u ') which diffuse down towards the copper substrate, and wherein copper oxidation at an interface between the copper (I) sulfide film layer and the copper substrate produces Cu ⁺ cations which diffuse up towards the tip to complete copper (I) sulfide propagation.

According to aspects illustrated herein, there is provided a solar cell that includes a cuprous sulfide nanowire array of the present disclosure.

According to aspects illustrated herein, there is provided a lithium-ion battery that includes as anode a cuprous sulfide nanowire array of the present disclosure.

According to aspects illustrated herein, a cuprous sulfide nanowire array of the present disclosure can be used as an atomic switch for memory.

According to aspects illustrated herein, a cuprous sulfide nanowire array of the present disclosure can be used to create a nanosensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The presently disclosed embodiments will be further explained with reference to the attached drawings, wherein like structures are referred to by like numerals throughout the several views. The drawings shown are not necessarily to scale, with emphasis instead generally being placed upon illustrating the principles of the presently disclosed embodiments.

FIGS. 1A, IB and 1C show electron micrographs of an embodiment of a cuprous sulfide (Cu ₂ S) nanowire (NW) array of the present disclosure. FIG. 1A is a scanning electron micrograph (SEM) showing the uniformity of the Cu ₂ S NWs manifested in the perspective view (main frame) and top view (inset; scale bar 1 μιη) pictures. FIG. IB is a cross-sectional transmission electron micrograph (TEM) revealing the existence of a Cu ₂ S buffer layer. FIG. 1C is a high resolution transmission electron micrograph (HRTEM) showing a tip and a side of a Cu ₂ S NW. The inset electron diffraction pattern verifies that the Cu ₂ S NWs are low chalcocite.

FIG. 2 shows X-ray diffraction (XRD) patterns of a cuprous sulfide (Cu ₂ S) NW array of the present disclosure grown on polished Cu foil. The pattern is indexed as the low chalcocite structure. The horizontal indices mark the peaks from the Cu substrate. The preferential growth direction of [001] is evidenced by the high ( 1 04), ( 204) and ( 304 ) peaks as well as the suppressed (630) and (060) peaks. FIG. 3 shows corrected optical absorption spectrum of a Cu ₂ S NW array of the present disclosure dispersed in ethanol. The linear correlation of the square root of the absorbance with the photon energy reveals the indirect nature of the band structure. Extrapolation of the linear fit was used to calculate the indirect band gap, which was 1.20 eV.

FIG. 4 is a schematic of ionic vacancy diffusion growth model for a Cu ₂ S NW array of the present disclosure. H ₂ S reacts with 0 ₂ to produce H ₂ 0, S ^" lattice and Vc _u ', the latter of which diffuses to the Cu substrate (the actual atomic arrangements of Cu ₂ S is not represented in the schematic). A schematic diffusion channel is shown by the arrow.

FIG. 5A and FIG. 5B show length and chemical composition of NWs of a Cu ₂ S nanowire array of the present disclosure versus different relative humidity (RH). FIG. 5A shows NW growth occurs for relative humidity (RH) >25%, and the average NW length increases with RH. FIG. 5B shows the sulfur atomic ratio increases with increasing RH, and when RH is larger than 60% the sulfur atomic ratio saturates at -30%, close to the ideal value of 33% in Cu ₂ S.

FIGS. 6A-6E show SEM images illustrating the morphology of various embodiments of

Cu ₂ S NW arrays of the present disclosure grown at different RHs at a fixed growth duration of 5 hours. NW did not form at 0% RH or 25% RH (FIG. 6A). The quality of the NWs is higher with higher RH, though there is no remarkable difference in chemical composition for the samples grown at RH > 60% (FIG. 5B). FIG. 6B shows NW formation at 50% RH. FIG. 6C shows NW formation at 62.5% RH. FIG. 6D shows NW formation at 75% RH. FIG. 6E shows NW formation at 100% RH.

FIGS. 7A, 7B, 7C and 7D show Cu ₂ S NW size dependence on gas flow rates. NW length (FIG. 7A) and diameter (FIG. 7B) for growths of varying durations under typical (10 seem) and double (20 seem) H ₂ S flow rates are shown. FIG. 7C shows the dependence of Cu ₂ S NW diameters on the H ₂ S flow rates. FIG. 7D shows NW diameters for a series of 3 hour growths under varying 0 ₂ flow rates. The growths were performed on Cu foils after typical electrochemical polishing.

FIG. 8A and FIG. 8B show SEM images showing the influence of the Cu substrate on the growth of a Cu ₂ S NW array of the present disclosure. FIG. 8A shows binary growth occurring when a high density of defects are present in Cu. FIG. 8B shows binary growth of long and short NWs close to the edge of the Cu foil. Long NWs are observed on the edges but are rarely seen on a flat substrate without deformation.

FIG. 9A and FIG. 9B show SEM images of some embodiments of Cu ₂ S nanostructures of the present disclosure. FIG. 9A shows a SEM image of double-comb Cu ₂ S nanostructures and FIG. 9B shows a SEM image of a Cu ₂ S nano-helix structure.

FIG. 10A and FIG. 10B show various morphologies of some embodiments of Cu ₂ S nanostructures of the present disclosure. FIG. 10A shows Cu ₂ S nanostructures grown on Cu micro crystals, with the inset showing the crystallography correlations. FIG. 10B shows aligned Cu ₂ S NWs grown on an indium-tin-oxide (ITO) substrate.

While the above-identified drawings set forth presently disclosed embodiments, other embodiments are also contemplated, as noted in the discussion. This disclosure presents illustrative embodiments by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of the presently disclosed embodiments.

DETAILED DESCRIPTION

One-dimensional nanoscale semiconductor materials, such as nanowire arrays, have generated recent interest because of their favorable properties for use in photovoltaic devices. Namely, nanowire devices have shown improvements over thin-film in both light absorption and charge separation and transport. However, common methods of nanowire formation require the use of high temperatures, hazardous precursors, expensive catalyst materials, and/or sophisticated instrumentation, all of which make the processes costly and complicated.

The present disclosure relates to a growth model that produces uniform and aligned nanostructures at room temperature. The nanostructure growth occurs spontaneously at room temperature and pressure (ambient conditions) and requires no template or catalyst, producing vertical nanostructure arrays of controllable length and diameter. The growth result is significant - vertically aligned nanostructure arrays without heating or pressurizing or vacuum pumping. In an embodiment, the methods disclosed herein provide a growth mechanism for binary compounds of copper and sulfur vertical nanowire arrays by a well-controlled gas-solid reaction on a copper metal foil or film. In an embodiment, the binary compound is copper (I) sulfide Cu ₂ S. In an embodiment, the unique growth of vertically aligned Cu ₂ S nanowires of the present disclosure takes place via Cu ion vacancy diffusion. The driving force for the Cu ion diffusion is generated by chemical addition of sulfur on the top from the gas phase. The following detailed description will focus on the fabrication of Cu ₂ S vertical nanowire arrays. However, it should be noted that other sulfide nanowire arrays can be fabricated using the methods of the presently disclosed embodiments, including, but not limited to, villamaninite nanowire arrays, covellite nanowire arrays, yarrowite nanowire arrays, spionkopite nanowire arrays, geerite nanowire arrays, anilite nanowire arrays, digenite nanowire arrays, djurleite nanowire arrays, bornite nanowire arrays, chalcopyrite nanowire arrays, zinc sulfide nanowire arrays, cadmium sulfide nanowire arrays, iron sulfide nanowire arrays, germanium sulfide nanowire arrays and bismuth sulfide nanowire arrays.

Although the embodiments disclosed herein demonstrate the production of vertically aligned nanowire arrays, it should be understood that other types of nanostructure arrays can be produced using the disclosed methods. Examples of nanostructure arrays that can be produced according to the disclosed methods include, but are not limited to, nanotube arrays, nanorod arrays and nanobeam arrays.

The geometry of the nanowires in a Cu ₂ S nanowire array of the present disclosure can be controlled by varying precursor concentrations and growth time. The nanowires of a Cu ₂ S nanowire array of the present disclosure can nominally be grown on any substrate, which can be transparent, conductive or flexible. The fabrication of a Cu ₂ S nanowire array of the present disclosure is compatible with modern microelectronic process and can be easily scaled up. The nanostructure topology appears to help overcome the degradation problem that hindered development of Cu ₂ S-based thin film solar cells.

Ion diffusion is involved in growth of many functional materials including oxides, sulfides, silicides from various metals, including, but not limited to, copper, silver, nickel, iron, and cobalt. The entity of the diffusion can be cation, anion, and/or their vacancies. By manipulating the ion flux through varying synthesis parameters, high quality nanostructures can be produced for potential applications. However, nanowires or other desired structures only grow when the ion diffusion is controlled properly. One step for the synthesis of vertically aligned nanowire arrays is the generation of the driving force for ion diffusion. Another step for the synthesis of vertically aligned nanowire arrays is an ion diffusion barrier to control the growth kinetics.

A Cu ₂ S nanowire array of the present disclosure was fabricated by co-flowing approximately 10 standard cubic centimeter per minute (seem) of hydrogen sulfide gas (H ₂ S), approximately 80 seem of oxygen gas (0 ₂ ) and approximately 160 seem of nitrogen gas (N ₂ ) (saturated with water) at ambient conditions, over a polished Cu substrate that was placed in a reaction chamber with a gas inlet and outlet. By "ambient conditions" is meant at room temperature and atmospheric pressure. The hydrogen sulfide gas was present at a concentration of about 1.7 ^x 10 - ^" 3 mole/L, the oxygen gas was present at a concentration of about 13.3 ^x 10 - ^" 3 mole/L, and the nitrogen gas was present at a concentration of about 26.5 ^x 10 ^" mole/L. The gas mixture was introduced into the chamber through an inlet and vented out of the chamber through an outlet. The reaction took place at room temperature and under atmospheric pressure (ambient conditions). Prior to the reaction, a high-purity Cu foil (Sigma Aldrich, 99.99%) was first cleaned in acetone, methanol and isopropanol by ultrasonication to remove organic contaminations. The foil was then immersed in concentrated orthophosphoric acid (85 wt%, Alfa Aesar) together with a Pt wire as the counter electrode. First, 4.7 V (DC) was applied for 5 min; then a lower voltage of 1.7 V (DC) was used, and the reaction continued for 10 min. The surface of Cu foil became mirror smooth after this electrochemical planarization (electropolishing) treatment. The electropolishing treatment results in the anodic leveling of the Cu surface. Typical scanning electron micrographs (SEM, JOEL 6340F or 700 IF) of an as- grown Cu ₂ S NW array are shown in FIG. 1A. The Cu ₂ S NW array includes a plurality of vertically aligned nanowires. Uniform Cu ₂ S NWs with diameters of about 100 nm and lengths of about 500 nm were produced by a 3 hour growth. FIG. 1A shows the vertical orientation and uniformity of the array. Cross-sectional transmission electron microscopy (TEM, JOEL 201 OF) examinations revealed that the NW tips and sides are free of impurities and are atomically flat (FIG. IB and FIG. 1C), ruling out the possibility of seeded growth. The initial Cu ₂ S formation is similar to that of the metal scale growth, that is, the formation and thickening process of the oxide or sulfide or other compound layer formed on the surface of a metal as a result of oxidation/corrosion when exposed to the air or other environments. Once the Cu ₂ S film exceeds a critical thickness of approximately 250 nm, Cu ⁺ vacancies (Vcu') can be annihilated both in the Cu substrate and in the Cu ₂ S film, causing the cracking, hence the nanowire growth. The Cu ₂ S NW array comprises a plurality of vertically aligned copper (I) sulfide nanostructures, wherein a proximal end of the nanostructures form a cuprous sulfide film layer along an upper surface of a copper substrate, wherein a distal end of each of the nanostructures forms a tip, and wherein vertical growth of the plurality of copper (I) sulfide nanostructures is capable of occuring via cation vacancy diffusion, where incorporation of sulfur atoms generate vacant copper sites (Vcu') which diffuse down towards the copper substrate, and wherein copper oxidation at an interface between the cuprous film layer and the copper substrate produces Cu ⁺ cations which diffuse up to complete copper (I) sulfide propagation. The top view of the initially formed Cu ₂ S nanowires show that the tip of the nanowires are free of impurities, and the cross-section represents the symmetry of the S packing in Cu ₂ S, (see inset of FIG. 1A).

A method of fabricating a Cu ₂ S nanowire array using a gas-solid reaction includes providing a Cu substrate; subjecting the Cu substrate to an atmosphere of 0 ₂ , H ₂ S and N ₂ at ambient conditions; creating a Cu ₂ S film; and growing a plurality of vertically aligned Cu2S nanowires on the substrate. The Cu and S feeding are spatially separated. The synthesis needs no energy input, expensive elements or vacuum facilities, and can be readily scaled up. In an embodiment, the method further includes passing the N ₂ through an H ₂ 0 bubbler for the inclusion of humidity.

The cross-sectional sample for TEM characterization was prepared with an Ar ion miller (Gatan, PIPS-691). The NW array is separated from the Cu substrate by a Cu ₂ S layer of approximately 250 nm. Both electron diffraction (ED, FIG. 1C inset) and X-ray diffraction studies (FIG. 2) confirmed that the as-grown Cu ₂ S nanowires are of the low chalcocite structure. The X-ray diffraction pattern (FIG. 2) was taken with a Bruker diffractometer using Cu K _a irriadiation. FIG. 2 shows X-ray diffraction (XRD) patterns of cuprous sulfide (Cu ₂ S) NWs grown on polished Cu foil. The pattern is indexed as the low chalcocite structure. The horizontal indices mark the peaks from the Cu substrate. The preferential growth direction of

[001] is evidenced by the high ( 1 04), ( 204) and ( 304 ) peaks as well as the suppressed (630) and (060) peaks.

In order to obtain the absorption spectrum of Cu ₂ S nanowires, the Cu ₂ S nanowires were suspended in ethanol (anhydrous, >99.5%, Sigma- Aldrich) and the absorbance was measured using a UV-vis-NIR spectrometer (Ocean Optics HR4000CG-UV-NIR, with Mikropack DH-2000-BAL UV-VIS-NIR light source) in the wavelength range of 200 -1100 nm. The method reported by Partain et al. was utilized to exclude the influence of the absorbance by the absorptive loss mechanism. Plots of the corrected absorbance as a function of the photon energy reveal the indirect nature of the band structure, the gap of which was calculated as 1.20 eV, in good agreement with the literature. The λ ^η power law dependence of the absorptive loss was estimated by fitting the data beyond 920 nm to y = m λ ^η . Optical characterizations showed that the product has an indirect bandgap of 1.20 eV (FIG. 3), in good agreement with the literature. FIG. 3 shows corrected optical absorption spectrum of Cu ₂ S NWs dispersed in ethanol. The linear correlation of the square root of the absorbance with the photon energy reveals the indirect nature of the band structure. Extrapolation of the linear fit was used to calculate the indirect band gap, which was 1.20 eV. The

1/2 2 indirect and direct band gap energies were then estimated by extrapolating A and A plots, to find values of 1.19 eV and 1.85 eV, respectively.

Existing reports of NW growth can be generalized into two broad categories, base- feeding and tip-feeding. When fed from the base, the new addition of atoms (or molecules) "pushes" up the NWs and leads to axial elongation. Conversely, the elongation of NWs is a natural consequence of tip addition, as in a vapor-liquid-solid or solution-liquid-solid growth. The present synthesis method, however, requires the addition of one component, S, from the tip and the other component, Cu, from the base. The mechanism by which Cu and S are fed is unique. The feeding of Cu and S is from opposite directions along the nanowire axis through ionic vacancy diffusion, as evidenced by the presence of a Cu ₂ S film between the Cu substrate and H ₂ S gas. The existence of the Cu ₂ S buffer layer between the Cu substrate and the Cu ₂ S NWs rules out the possibility of surface diffusion and instead supports an internal diffusion model - ionic vacancy diffusion. In an embodiment, a buffer layer for ion diffusion can be intentionally introduced into a reaction system to control the kinetics of a nanostructure synthesis. For example, for compounds such as oxide, sulfide, carbide, and silicide requiring ion diffusion (cation, anion or their vacancies), a buffer layer can be introduced to control the ion flux thus to control the yield of desired morphology, for instance, converting film growth to nanowire growth. Ionic vacancies can be intentionally generated at specified sites to yield more complex structures. By varying the vacancy generation and annihilation process, atomic addition can be controlled more precisely, for instance, by switching the gas reactant, to produce other morphologies like hyperbranched structures, helical structures, or heterostructures. In an embodiment, any measure that makes a chemical potential gradient of the ions, for instance, ion concentration, temperature gradient, or other charges (like electron) concentration, can be applied to manipulate the ion flux and the growth. An example lies in the electron beam introduced growth that fabrication of a single nanostructure can be triggered by a precisely controlled electron beam.

The addition of water into the growth reaction helps gas absorption and charge transfer. This is applicable to many oxidation processes facilitated by humidity, such as in the Fe-0 and Ag-S systems. FIG. 4 illustrates the proposed growth mechanism of Cu ₂ S nanowires of the present disclosure. H ₂ 0, 0 ₂ and H ₂ S molecules are adsorbed on the surface of the Cu substrate. Water has been found to play a critical role in the growth of the disclosed nanowire arrays. The presence of H ₂ 0 facilitates electron transfer to 0 ₂ , leading to the disassociation of H ₂ S and

2- 2- 2- producing H ₂ 0, S ^" and holes. S ^" and the holes are incorporated into the Cu ₂ S lattice, with S ^" packing to extend the existing lattice and the holes occupying Cu ⁺ sites. In effect, the holes can be regarded as Cu ⁺ vacancies (VQ _I ')- VQ _I ' is highly mobile in the lattice of Cu ₂ S. The concentration difference of Vc _u ' near the interface of Cu ₂ S/Cu (low) and that of Cu ₂ S/gas (high) drives Vc _u ' to diffuse toward the Cu ₂ S/Cu interface. Once reaching the Cu substrate, Vc _u ' is annihilated by structural defects in the crystal of Cu, in a way similar to the metal scale growth. Initially, this process yields a layer of Cu ₂ S that is continuous. As the reaction continues, the volumetric expansion, as well as the annihilation of Vc _u ' by the defects in the Cu ₂ S film, leads to cracks in the Cu ₂ S film, creating NWs (FIG. 1A, inset). The total reaction can be expressed as:

0 ₂ + 2H ₂ S→ 2S (lattice) + 4V _Cu ' + 4h ⁺ + 2H ₂ 0 (1)

Cu + V _Cu '→Cu _C u ^X + e' (2)

As discussed above, H ₂ 0 facilitates electron transfer hence the reduction of 0 ₂ . Adding H ₂ 0 in the beginning of the reaction significantly speeds up the reaction, permitting the growth of uniform Cu ₂ S nanowires. Because H ₂ 0 is also a product of reaction 1, the overall growth of Cu ₂ S nanowires can occur even without the initial introduction of H ₂ 0, however the reaction would be slower. Humidity facilitates electron transfer and ion formation in the growth of Cu ₂ S thin films of vertically aligned nanostructures. The indispensable role of H ₂ 0 suggests that the growth depends on ionic behaviors. In an embodiment, a method of fabricating a Cu ₂ S nanowire array of the present disclosure included providing a Cu substrate; subjecting the Cu substrate to an atmosphere of 0 ₂ , H ₂ S and N ₂ at ambient conditions, wherein the N ₂ was passed through an H ₂ 0 bubbler to increase the relative humidity of the growth reaction; creating a Cu ₂ S film; and growing a plurality of vertically aligned Cu2S nanowires on the substrate. The degree of sulfidation is increased with increasing relative humidity, as characterized by increasing sulfur atomic ratio and decreasing copper ratio. Oxygen ratio was almost constantly low at all relative humidities, consistent with the ion-diffusion model disclosed herein. FIG. 5A and FIG. 5B show the length and chemical composition of Cu ₂ S NWs of the present disclosure versus different relative humidity (RH). FIG. 5A shows NW growth occurs for relative humidity (RH) >25%, and the average NW length increases with RH. FIG. 5B shows the sulfur atomic ratio increases with increasing RH, and when RH is larger than 60% the sulfur atomic ratio saturates at -30%, close to the ideal value of 33% in Cu ₂ S. The discrepancy from the ideal value may arise from the pure copper substrate beneath the Cu ₂ S. All growths were conducted with the following identical parameters except the specified RH: room temperature, atmosphere pressure, co-flow of approximately 10 seem of H ₂ S, approximately 80 seem of O ₂ , and approximately 160 seem N ₂ . The growth duration was fixed to 5 hours for all samples. FIGS. 6A-6E show SEM images illustrating the morphology of Cu ₂ S NW arrays of the present disclosure grown at different RHs. When H ₂ O was not intentionally added (0% RH), no appreciable formation of Cu ₂ S or Cu ₂ O was observed in the first five hours of reactions. Prolonged reactions, e.g., 24 hours, did yield detectable Cu ₂ S nanostructures, however, the quality of the product was significantly lower than when H ₂ O was added. (FIG. 6A). The quality of the NWs is higher with higher RH, though there is no remarkable difference in chemical composition for the samples grown at RH > 60% (FIG. 5B). FIG. 6B shows NW formation at 50% RH. FIG. 6C shows NW formation at 62.5% RH. FIG. 6D shows NW formation at 75% RH. FIG. 6E shows NW formation at 100% RH. The balance of the reaction kinetics of reactions 1 and 2 may be important to this observation. Other polar solvents such as ethanol were also used to replace H ₂ O. These solvents also facilitate charge transfer and hence O ₂ reduction. While the formation of Cu ₂ S was observed after 5 hours reactions, no distinguishable nanoscale features were seen. This phenomenon highlights the critical role of H ₂ 0 to the growth of aligned Cu ₂ S nanowire arrays of the present disclosure.

Vc _u ' can be generated on the tip or on the sidewalls of a growing Cu ₂ S nanowire. Vc _u ', however, diffuses significantly faster along the [001] direction of Cu ₂ S, leading to the anisotropic growth and the production of the nanowire morphology. By increasing the precursor concentrations, Vc _u ' generation will be promoted, both on the tip and on the sidewalls. When the rate of Vc _u ' generation exceeds the rate of Vc _u ' diffusion, the growth cannot be sustained. As a result, thinner and shorter nanowires will be produced. Nanowire growth is controlled by the transport of Cu from the substrate to the tip, resulting from Cu vacancy diffusion through the crystal. The diffusion is primarily driven by the adsorption of reactant species onto the crystal surfaces, and thus the concentration of said species influences the property of tip growth versus growth from the nanowire wall. As shown in FIG. 7A, FIG. 7B and FIG. 7C, higher H ₂ S flow rate means higher H ₂ S concentrations in the reactor, hence faster Vc _u ' generation. The influence of the flow rate of H ₂ S on the diameter is more pronounced than that on the length because Vc _u ' diffuses more easily in the nanowire axial direction. Varying the 0 ₂ flow rate (see FIG. 7D) for a series of 5 hour growths also creates trends in length and diameter. Under extreme conditions, e.g., when only trace amounts of 0 ₂ or H ₂ S or both is available, the nanostructure will cease to existence. Instead, only thin films can be produced.

The growth of aligned Cu ₂ S nanowires is highly sensitive to the quality of the Cu substrate. Cu ₂ S nanostructures with random morphologies were obtained on Cu substrates with high density of defects. The high density of defects appears to facilitate rapid Vc _u ' annihilation, hence fast nanostructure growth occurs and produces random morphologies. Experiments performed using high-crystalline Cu particle substrates yield epitaxial growth of Cu ₂ S nanorods. Micron sized Cu crystals were fabricated by electrochemical deposition. When the crystal was small (<10 μηι), uniform epitaxial growth was obtained as shown in FIG. 10A. The triple growth directions are seen on a flat (1 11) facet, while aligned nanorods on the edges emerge from (1 10) planes. On the other hand, a mixture of long and short NWs grown on large crystals showed more defects, FIG. 8 A and FIG. 8B. Such a phenomenon was more pronounced when more defects were present in the Cu substrate, such as the edge of the Cu substrate where a large number of defects are introduced by the mechanical treatments. Similar phenomenon was also observed on Cu crystals prepared by electrochemical deposition. FIG. 8A represents a typical example of this observation, where ultra-long nanostructures are seen. Reaction (2) above relies on the inadvertent existence of structural defects in Cu. The model predicts that high density of defects will lead to fast Cu ₂ S nanowire growth.

When the density of structural defects was exceedingly high, the production of Cu ₂ S nanostructures of various interesting morphologies were observed, such as the double-comb structure (branched) and helical nanowires, as shown in FIG. 9 A and FIG. 9B. The high density of defects were achieved by using Cu flakes obtained from machining Cu objects, Cu micro crystals by fast electrochemical deposition or Cu foils treated by repeated mechanical bending. This phenomenon and that described above confirm that the structural defects in the Cu substrate plays an important role in the nanowire growth. The occurrence of formation is likely due to characteristics of the underlying Cu substrate, which can be explained by the annihilation of Vc _u ' by the defects.

A Cu ₂ S nanowire array of the present disclosure was fabricated by co-flowing 10 standard cubic centimeter per minute (seem) of H ₂ S, 80 seem of 0 ₂ and 160 seem of N ₂ (saturated with water) over a Cu film deposited on another substrate, for instance, transparent conductive oxide (TCO) such as ITO on glass. In an embodiment, a seed layer of Cu (80 nm) was thermally evaporated onto glass. The electrolyte for plating was 0.16 M CuS0 ₄ aqueous solution. A Cu electrode served as the anode. The plating process was conducted at room temperature with the current density between 1 and 10 mA/cm . The Cu crystal sizes varied from approximately 1 μηι to approximately 20 μηι, the general trend being that larger current density produced larger Cu grain sizes. The growth resulted in uniform and aligned NW arrays on transparent ITO glass, see FIG. 10B. The length of the resulting NWs was approximately 2.5 times the starting film thickness.

In an embodiment, the disclosed nanowire array can be transferred to another substrate for device fabrication. In an embodiment, the nanowire array can be peeled off the growth substrate without damaging the nanowires. The lack of damage to the nanowires appears to be due to annihilation of ion vacancies at the metal/scale interface that weakens the bonding between the nanostructure and substrate. This enables transfer of the structure from the growth substrate to another substrate, like glass, plastic, or silicon wafer, for device making. In an embodiment, the methods disclosed herein are used to produce substantially uniform (e.g., having substantially the same properties) and substantially aligned (e.g., substantially parallel) Cu ₂ S NWs at room temperature. In an embodiment, Cu ⁺ vacancy diffusion in Cu ₂ S is the driving force of this growth. H ₂ 0 acts as an indispensable reagent to facilitate charge transfer and ionic vacancy formation. In an embodiment, the NW formation is governed by the difference of Cu ⁺ vacancy diffusion in various crystal directions.

In an embodiment, a highly uniform and dense array of Cu ₂ S nanowires of the present disclosure can be used to construct solar energy harvesting devices. In an embodiment, a Cu ₂ S nanowire array of the present disclosure can be used in a solar cell. Compared to conventional solar cells based on single crystal Si, the cost of a Cu ₂ S nanowire array of the present disclosure can be significantly reduced because both Cu and S are abundant on the Earth and the growth reaction takes place at room temperature. The nanostructure nature may help solve the degradation problem existing in conventional thin film cells. These all add to an increased probability of producing better solar cells with higher efficiency and longer lifetime, even with the identical materials such as CdS. A solar cell can convert solar light energy into electricity or energy-carrying chemicals (like H ₂ ). To achieve this, at least one p-n junction should be formed either by contacting a p-type semiconductor and an n-type one (for a "photovoltaic" cell), or immersing a semiconductor into a proper electrolyte (for a "photoelectrochemical" cell). Cu ₂ S is a p-type semiconductor with a proper bandgap that fits the solar spectrum well. Thin film solar cells based on Cu ₂ S have demonstrated appreciable efficiency. A Cu ₂ S nanowire array disclosed herein further optimizes the design of a solar cell by orthogonalizing the directions of light absorption and charge separation. There are many n-type semiconductors such as Ti0 ₂ , ZnO, CdS, W0 ₃ , Fe ₂ 0 ₃ , Sn0 ₂ , SiC, CdSe, each having its own band structures and band gaps. Using various methods, different n-type semiconductors can be coated onto the p-type Cu ₂ S nanowires disclosed herein to form an array of p-n junctions. Ti0 ₂ has been used in photoelectrochemical cells due to the high efficiency in the ultra-violet range of the solar spectrum. Cu ₂ S nanowires of the present disclosure can be incorporated as the p-type component to improve light absorption and to complete the photolysis of water.

In an embodiment, Cu ₂ S/Ti0 ₂ cells can be fabricated for water splitting using solar energy. Cu ₂ S/ZnO cells were predicted to have high efficiency. These solar cells are all made of cheap elements thus are very promising for practical usage. Among many possible combinations, Cu ₂ S/CdS solar cell has been demonstrated to function in the thin film form, i.e. both Cu ₂ S and CdS are thin films in the device. In an embodiment, Cu ₂ S/CdS solar cells can be fabricated based on the disclosed nanostructures resulting in improved performance characterized by higher efficiency and longer lifetime. Cu ₂ S/ZnO solar cell is also very promising because a very high efficiency has been predicted. All the materials involved are cheap and nontoxic. Besides Ti0 ₂ , other n-type semiconductors, such as WO3 and Fe ₂ 0 ₃ , can also be used in combination with Cu ₂ S nanowires of the present disclosure for device fabrication. Other semiconductor nanowire arrays can be synthesized using Cu ₂ S nanowires of the present disclosure as a template. For instance, Cu ₂ 0, another p-type semiconductor, can be obtained by oxidizing the existing Cu ₂ S nanostructures of the present disclosure.

Lithium-ion batteries are a type of rechargeable battery in which lithium ions move from the negative electrode (anode) to the positive electrode (cathode) during discharge, and from the cathode to the anode during charge. Lithium-ion batteries are common in portable consumer electronics because of their high energy-to-weight ratios, lack of memory effect, and slow self- discharge when not in use. In addition to consumer electronics, lithium-ion batteries are increasingly used in defense, automotive, and aerospace applications due to their high energy density. In an embodiment, a highly uniform and dense array of Cu ₂ S nanowires of the present disclosure can be attached to a conductive substrate and readily used as an anode in lithium ion batteries. A Cu ₂ S nanowire array may be used as a high efficiency anode for at least three reasons. First, there is room for Li ions in the crystal because Cu ₂ S has a unique layer-structure and Cu ion vacancies in the lattice. Second, the nanowire morphology facilitates fast ion insertion and extraction. Third, the Cu ₂ S nanowires are conductive, especially when a highly doped Cu ₂ S material is needed for this application. These factors enable Cu ₂ S nanowire array to be a good anode that permits fast charge and discharge.

In an embodiment, the high ion mobility of Cu ₂ S results in a highly uniform and dense array of Cu ₂ S nanowires of the present disclosure suitable for switching devices. In an embodiment, a highly uniform and dense array of the present disclosure can be used in memory devices in which a single nanowire could store a single bit of information when fully charged, a "0", a "1", or a "2" depending on the position of the electrons. In an embodiment, a highly uniform and dense array of the present disclosure can be used in memory devices in which a single nanowire could pack a hundred bits of data. In an embodiment, the memory could store 10 to 100 times more data than flash—the type of memory used in digital cameras and other small portable devices—while operating at much faster speeds. The study on mobility of Cu ions in a Cu ₂ S nanowire array of the present disclosure may shed light on functioning and failure analyses of devices made of Cu-based materials. The array disclosed herein of high density and uniform Cu ₂ S nanowires provides a possibility to make real memory devices consisting of many units. By designing the ion diffusion path and barriers, memory devices can be operated at low voltage while having a long lifetime. The units in the memory device based on spatially separated nanowires can be operated with little crosstalk despite being packed in a dense array.

Devices based on nanowires are emerging as a powerful tool and general platform for ultrasensitive, direct electrical detection of biological and chemical species. In an embodiment, the uniform NWs of the present disclosure can be used as a sensor. In an embodiment, the sensor can be used for detection of a wide -range of biological and chemical species, including, but not limited to, ions, single molecules, proteins, DNA, drug molecules and viruses. In an embodiment, the sensor can be used for the detection and quantification of biological and chemical species, ranging from diagnosing disease to the discovery and screening of new drug molecules. In an embodiment, the sensor is configured as field-effect transistors (FET), which will exhibit a conductivity change in response to variation in the electric field or potential at the surface of the NW FET. Compared to the widely used Cu ₂ S film, Cu ₂ S nanowire arrays are competitive because the increased surface area may contact the solution better and improve the sensitivity. The fabrication of the electrode is also feasible. The Cu ₂ S nanowire arrays disclosed herein can be directly grown on a conductive electrode, or can be transferred to the electrode easily. As a base material for ion exchange, sensors can be configured as ion exchange under external electric field or chemical environments, leading to a change in resistance or color as sensing indicator. As a strong light absorbing material, a Cu ₂ S nanowire array of the present disclosure also can provide additional flexibility in sensing chemical species by incorporating light manipulation in sensors. The spectrum of detection can be expanded by introducing light on the sensor to change the sensitivity to different species.

A method of fabricating a Cu ₂ S nanowire array using a gas-solid reaction includes providing a Cu substrate; subjecting the Cu substrate to an atmosphere of 0 ₂ , H ₂ S and N ₂ at ambient conditions; creating a Cu ₂ S film; and growing a plurality of vertically aligned Cu ₂ S nanowires on the substrate. In an embodiment, the Cu ₂ S nanowires in the array are highly uniform and densely packed. In an embodiment, vertical growth occurs via cation vacancy diffusion, wherein the incorporation of S atoms generate vacant Cu sites (Vcu') which diffuse down toward the Cu substrate, and wherein Cu oxidation at the film/substrate interface produce Cu ⁺ cations which diffuse up to complete the Cu ₂ S propagation. In an embodiment, the Cu substrate is pre-treated using electropolishing. In an embodiment, the method further includes passing the N ₂ through an H ₂ 0 bubbler for the inclusion of humidity. In an embodiment, the relative humidity in the reaction ranges from about 25% to about 60%. In an embodiment, the relative humidity in the reaction ranges from about 30% to about 40%. In an embodiment, the relative humidity in the reaction is about 33%. In an embodiment, the flow rate of 0 ₂ ranges from about 25 standard cubic centimeters per minute (seem) to about 300 seem. In an embodiment, the flow rate of 0 ₂ ranges from about 50 seem to about 200 seem. In an embodiment, the flow rate of 0 ₂ ranges from about 60 seem to about 100 seem. In an embodiment, the flow rate of 0 ₂ is about 80 seem. In an embodiment, the flow rate of H ₂ S ranges from about 2.5 seem to about 100 seem. In an embodiment, the flow rate of H ₂ S ranges from about 5 seem to about 50 seem. In an embodiment, the flow rate of H ₂ S is about 10 seem. In an embodiment, the flow rate of N ₂ ranges from about 100 seem to about 200 seem. In an embodiment, the flow rate of N ₂ is about 160 seem.

A method of fabricating a cuprous sulfide nanowire array includes providing a copper substrate; subjecting the copper substrate to an atmosphere of oxygen gas, hydrogen sulfide gas and nitrogen gas at ambient conditions, wherein the nitrogen gas is passed through a bubbler for mixing the nitrogen gas with water vapor so that a relative humidity is between about 25% and about 100%; creating a cuprous sulfide film on the copper substrate; and growing a plurality of vertically aligned cuprous sulfide nanowires from the film so as to fabricate the cuprous sulfide nanowire array. In an embodiment, the cuprous sulfide nanowires in the array are highly uniform and densely packed. In an embodiment, vertical growth occurs via cation vacancy diffusion, where the incorporation of sulfur atoms generate vacant copper sites (Vc _u ') which diffuse down toward the copper substrate, and wherein copper oxidation at an interface between the cuprous sulfide film and the copper substrate produces Cu ⁺ cations which diffuse up to complete the cuprous sulfide propagation. In an embodiment, the copper substrate is pre-treated using electropolishing. In an embodiment, the relative humidity in the reaction ranges from about 25% to about 60%. In an embodiment, the relative humidity in the reaction ranges from about 30% to about 40%. In an embodiment, the relative humidity in the reaction is about 33%. In an embodiment, the flow rate of oxygen gas ranges from about 25 standard cubic centimeters per minute (seem) to about 300 seem. In an embodiment, the flow rate of oxygen gas ranges from about 50 seem to about 200 seem. In an embodiment, the flow rate of oxygen gas ranges from about 60 seem to about 100 seem. In an embodiment, the flow rate of oxygen gas is about 80 seem. In an embodiment, the flow rate of hydrogen sulfide gas ranges from about 2.5 seem to about 100 seem. In an embodiment, the flow rate of hydrogen sulfide gas ranges from about 5 seem to about 50 seem. In an embodiment, the flow rate of hydrogen sulfide gas is about 10 seem. In an embodiment, the flow rate of nitrogen gas ranges from about 100 seem to about 200 seem. In an embodiment, the flow rate of nitrogen gas is about 160 seem.

A sulfide nanostructure array includes a plurality of copper (I) sulfide nanostructures, wherein a proximal end of the nanostructures form a copper (I) sulfide film layer along an upper surface of a copper substrate, wherein a distal end of each of the nanostructures forms a tip, and wherein vertical growth of the plurality of copper (I) sulfide nanostructures is capable of occurring via cation vacancy diffusion, where incorporation of sulfur atoms generate vacant copper sites (Vc _u ') which diffuse down towards the copper substrate, and wherein copper oxidation at an interface between the copper (I) sulfide film layer and the copper substrate produces Cu ⁺ cations which diffuse up towards the tip to complete copper (I) sulfide propagation.

All patents, patent applications, and published references cited herein are hereby incorporated by reference in their entirety. It will be appreciated that several of the above- disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or application. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art.