Metal nanowire thin-films

Abstract

A conductive nanowire film based on a high aspect-ratio metal is disclosed. The nanowire film is produced by inducing metal reduction in a concentrated surfactant solution containing metal precursor ions, a surfactant and a reducing agent. The metal nanostructures demonstrate utility in a great variety of applications.

Claims

1. A process for the preparation of a conductive nanowire film on a surface of a substrate, wherein the aspect ratio of the nanowires is greater than 1000 and the cross-sectional diameter of the nanowires is less than 10 nm, said process comprising: (a) obtaining an aqueous precursor solution comprising at least one metal precursor, at least one surfactant and at least one metal reducing agent wherein the concentration of the at least one surfactant in said solution is at least 5% (w/w), wherein the metal precursor is selected from metal precursor in a form of metal ions or in a form which under the reaction conditions dissociate into metal ions; (b) after obtaining the precursor solution, placing the precursor solution on the surface of a substrate, forming a film of the precursor solution on at least a portion of the surface of the substrate; and (c) allowing said nanowires to form in said film; thereby obtaining a conductive nanowire film on at least a portion of said surface.

2. The process according to claim 1, further comprising irradiating the film of the precursor solution on at least a portion of a surface of a substrate with ultraviolet radiation.

3. The process according to claim 1, wherein said aqueous precursor solution is prepared by first forming a solution of at least one first metal precursor and at least one surfactant at a temperature allowing dissolution followed by the addition of at least one metal reducing agent and/or at least one second metal precursor.

4. The process according to claim 3, wherein said aqueous precursor solution is prepared by first forming a solution of at least one first metal precursor, at least one surfactant and at least one metal reducing agent at a temperature allowing dissolution followed by the addition of at least one second metal precursor.

5. The process according to claim 3, wherein said aqueous precursor solution is prepared by first forming a solution of at least one first metal precursor, at least one surfactant and at least one second metal precursor and at least one reducing agent at a temperature allowing dissolution, followed by the addition of at least one second reducing agent.

6. The process according to claim 5, wherein said at least one second metal precursor is a silver metal precursor.

7. The process according to claim 1, wherein said at least one metal precursor is selected from the group consisting of gold metal precursors, silver gold precursors, palladium metal precursors, copper metal precursors, nickel metal precursors and platinum metal precursors.

8. The process according to claim 7, wherein said at least one metal precursor is selected from the group consisting of HAuCl.sub.4; AgNO.sub.3; (NH.sub.4).sub.2PdCl.sub.6; Cu(NO.sub.3).sub.2; NiCl.sub.2; and H.sub.2PtCl.sub.6.

9. The process according to claim 7, wherein said at least one metal precursor is at least one gold metal precursor.

10. The process according to claim 1, wherein said at least one surfactant is at least one cationic surfactant.

11. The process according to claim 10, wherein said at least one cationic surfactant comprises at least one quaternary ammonium group.

12. The process according to claim 11, wherein said at least one surfactant is selected from the group consisting of cetyltrimethylammonium bromide (CTAB), didodecyldimethylammonium bromide, tetradecyltrimethylammonium bromide, didecyldimethylammonium bromide, wherein the bromide counterion, alternatively, may be a chloride or an iodide.

13. The process according to claim 1, wherein said at least one metal reducing agent is selected from the group consisting of inorganic and organic reducing agents.

14. The process according to claim 13, wherein said at least one reducing agent is selected from the group consisting of metal borohydride, metal ascorbate, hydroquinone and hydroquinone derivatives, hydrazine and hydrazine derivatives and any combination thereof.

15. The process according to claim 14, wherein said metal borohydride and metal ascorbate are sodium borohydride and sodium ascorbate, respectively.

16. The process according to claim 1, wherein the thickness of the thin-film is between 10 and 100 m.

17. The process according to claim 1, wherein the aqueous precursor solution in step (a) being prepared by combining the at least one surfactant, the at least one metal precursor being at least one first metal precursor, the at least one metal reducing agent, and at least one second metal precursor in an aqueous medium.

18. The process according to claim 17, wherein the reduction of the at least one first metal precursor is induced by the addition of the at least one second metal precursor.

19. The process according to claim 18, wherein said at least one second metal precursor is a silver metal precursor.

20. The process according to claim 17, wherein said at least one first metal precursor is a gold metal precursor and the aqueous precursor solution is obtained by: (i) forming a solution of at least one surfactant, at least one gold metal precursor and at least one metal reducing agent in an aqueous medium; and (ii) adding into the aqueous solution the at least one second metal precursor, wherein the at least one second metal precursor is at least one silver metal precursor to thereby induce reduction of said at least one gold metal precursor.

21. The process according to claim 17, wherein said at least one metal reducing agent is sodium ascorbate and said at least one surfactant is CTAB.

22. The process according to claim 1, wherein in the obtaining the aqueous precursor solution, said solution being prepared by: (i) combining the at least one surfactant, the at least one first metal precursor, at least one second metal precursor and the at least one metal reducing agent in an aqueous medium, wherein the at least one metal precursor and the at least one second metal precursor are each, independently selected from metal precursors in a form of metal ions or in a form which under the reaction conditions dissociate into metal ions; and (ii) introducing at least one second reducing agent.

23. The process according to claim 22, wherein the reduction of the at least one first metal precursor is induced by the at least one second reducing agent after a solution has been formed of the first and second metal precursors, first reducing agent and the at least one surfactant.

24. The process according to claim 22, wherein the at least one reducing agent and the at least one second reducing agent employed are a hydride or a metal borohydride and sodium ascorbate.

25. The process according to claim 22, wherein the at least one first metal precursor is a gold metal precursor and the at least one second metal precursor is silver.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIGS. 1A-1B are transmission electron microscope (TEM) images of a dried growth solution containing 7.5% CTAB deposited on a carbon coated copper grid after washing most of the CTAB with water and ethanol.

(3) FIGS. 2A-2B are scanning electron microscope (SEM) images of thin-films prepared from the 7.5% growth solution deposited on (FIG. 2A) fused silica and (FIG. 2B) Si with native oxide 1010 mm substrates.

(4) FIG. 3 presents a visible light transmission curve of a film deposited from a 7.5% CTAB solution on a fused silica substrate. This film had a 500 sq resistance.

(5) FIG. 4 depicts a film prepared according to a process of the art.

(6) FIG. 5A-5D are TEM images of nanowire films deposited on carbon coated grids:

(7) FIGS. 5A-5B show TEM obtained from 0.25 M CTAB solution. The inset in FIG. 5A shows the uniformity of the nanowire film over a macroscopic area (>100 m.sup.2 area).

(8) FIGS. 5C-5D show TEM obtained from 0.6 M CTAB solution. The inset in FIG. 5C shows a case where silver ion concentration was too low relative to the CTAB 0.6 M concentration and the small nucleated segments could not connect.

(9) FIG. 6 shows a SEM image of a part of a nanowire film prepared on a silicon substrate and washed with 70% ethanol.

(10) FIGS. 7A-7D show SEM image of nanowire bundle, a typical measurement configuration and current-voltage curves:

(11) FIG. 7A shows the current-voltage measurement for the SEM image shown in FIG. 7B of nanowire bundle conductance measurement using clean tungsten nanoprobes in the Zyvex S100 system.

(12) FIG. 7C shows a typical measurement configuration with the nanowire film deposited on a pre-patterned silicon substrate with gold electrodes.

(13) FIG. 7D shows current-voltage curves measured with various inter-electrode spacings as indicated in the legend.

(14) FIGS. 8A-8C show visible light transmission curve of a nanowire film, bending of PET substrate coated with a nanwire film and the periodic table as observed through the PET film.

(15) FIG. 8A shows a visible light transmission curve of a nanowire film deposited on a fused silica substrate with a sheet resistance of 200 /sq.

(16) FIG. 8B demonstrates bending of a PET substrate coated with a nanowire film to a curvature radius of 1.5 cm maintains a 100 /sq sheet resistance.

(17) FIG. 8C displays the periodic table as observed through the same PET film which had 80-85% optical transmission in the visible range. The bright stripes are silver paint lines used to estimate the sheet resistance. The upper right corner is film-free.

DETAILED DESCRIPTION OF EMBODIMENTS

General Experimental Procedures

(18) The preparation of a high aspect-ratio metal nanowire mesh films with high conductivity, flexibility and transparency was based on an in-situ formation of the nanowires which occurred after the deposition of a thin film of precursor solution on top of a substrate of choice.

(19) Gold-silver nanowires were grown in a drying thin film containing a high cationic surfactant concentration which formed a liquid-crystalline template phase for the formation of a nanowire network. The nanowire network films were uniform over macroscopic (cm.sup.2 scale) areas and on a variety of substrates. These films, measuring only few nanometers in thickness were characterized by low sheet resistivities, in the range of 60-300 /sq, as formed, and a high transparency, comparable to indium tin oxide (ITO) films.

(20) One process for the preparation of the metal nanowire mesh films begins with the preparation of a relatively concentrated surfactant solution having at least 5%, or at least 7.5%, or from 5% to 30%, or from 5% to 21%, or from 7.5% to 21% (w/w) of a surfactant such as cetyltrimethylammonium bromide (CTAB) in ultrapure water. The formerly published process [9, 10] had only 1.6% CTAB. Such high concentrations require heating of the solution so as to produce a uniform micro-emulsion phase of the surfactant/water mixture.

(21) A solution of chloroauric acid was added to this emulsion to yield a final Au precursor concentration of between 1 and 4 mM and a higher concentration of sodium ascorbate was added at a concentration of 40 to 60 times higher than the gold concentration. The initiation of the metal deposition process occurred by adding a concentrated AgNO.sub.3 solution to the prepared solution at 30-40 C., while stirring, to a final silver concentration 2 times higher than that of the gold. The silver ions added were being reduced by the ascorbate ions and when small silver metal seeds formed, the reduction of gold ions by the ascorbate was catalyzed and the metal nanostructures began growing. Immediately after silver addition a thin-film of the solution was spread on the substrate of choice either by drop casting, dip-coating or spin-coating. The thickness of such a film depended on the viscosity (determined by surfactant concentration and temperature) and the spread conditions and was measured to be between 10 and 100 Next, the film was dried, in some cases by placing the substrate under mild heating by a lamp at 35-40 C. until the film fully dried, after about 10 minutes.

(22) For microscopy studies of the dried films, most of the surfactant was washed out with various solvents. For conductance measurements, a quick ethanol wash was sufficient to allow for good electrical contact, either to pre-fabricated electrodes patterned on the substrate or to electrodes patterned post-film-deposition, either by metal evaporation or by spreading silver paint on the film.

(23) In another process according to the invention, the aqueous solution was first formed by preparing a relatively concentrated surfactant solution having at least 5%, or at least 7.5%, or from 5% to 30%, or from 5% to 21%, or from 7.5% to 21% (w/w) of a surfactant such as cetyltrimethylammonium bromide (CTAB) in ultrapure water. A solution of chloroauric acid was added to this emulsion together with a concentrated AgNO.sub.3 solution, while stirring, to a final silver concentration 2 times higher than that of the gold. After a few minutes, a solution of sodium borohydride was added followed by a solution of sodium ascorbate. The silver and gold ions in the presence of the strongly reducing agent began undergoing reduction, forming silver/gold metal seeds, the reduction of gold and silver ions by the ascorbate was catalyzed and the metal nanostructures began growing.

Example 1

(24) A 8.3% (w/w) cetyltrimethylammonium bromide (CTAB) solution was prepared by heating and stirring the CTAB/water mixture at 50 C. for 5 minutes. To this solution, at 40 C., 500 L of 25 mM HAuCl.sub.4 solution and 425 L of 1.8 M freshly prepared sodium ascorbate solution were added and stirred together. Then, 250 L of 100 mM AgNO.sub.3 solution were added while stirring. The final CTAB concentration in the nanowire growth solution was 7.5%. 30 seconds after the addition of the Ag solution, the stirring was discontinued and the solution was deposited on a substrate and let dry for 15-45 minutes at 35-40 C.

(25) Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) imaging revealed metal nanowire networks of varying wire densities and entanglement, depending on the exact solution and deposition conditions, uniformly spread over the substrates (See, FIGS. 1A to 1B showing transmission electron microscope images, and FIGS. 2A to 2B showing scanning electron microscope images). The nanowires were typically 3-5 nm wide and many micrometers long and in this case composed of 85-90% gold and 15-10% silver. A varying amount of non-elongated metal nanostructures was also observed. The minimization of the concentration of such structures in the films was a key to improving their optical transmission.

(26) Nanowire films were obtained from solutions that had up to 21% CTAB concentrations, 5 mM HAuCl.sub.4, 0.2M sodium ascorbate and 10 mM AgNO.sub.3. These concentrated CTAB solutions were highly viscous and required longer mixing and heating times to prepare homogeneous solution thereof. With such growth solutions it was easy to coat the substrates by simple dip-coating.

(27) The composition of the substrate did not influence the final results since the high surfactant concentration ensured proper wetting of either hydrophobic or hydrophilic surfaces. So far, the process produced similar results on silicon, fused silica, polycarbonate and carbon substrates. Differences between various substrates were mostly due to edge effects of the drying film which were more substantial in cases of small substrates such as TEM grids. The high level of uniformity and thus nanowire percolation, as seen in FIGS. 1A to 1B and 2A to 2B could not be obtained using the procedure described in the art [e.g., in refs. 9 and 10].

(28) Electrical measurements, done on several length scales at various arbitrary positions on the substrates using various types of contacts have shown ohmic conductance of the order of 100-500 /sq and 75-85% transmittance in the visible range (FIG. 3), which is comparable to indium tin oxide (ITO) films. The estimated conductivity per Au/Ag wire was of the order of bulk gold conductivity. It should be noted that a significant part of the 20% extinction observed in these experiments came from light scattering that the simple spectrophotometer used for these measurements could not collect, while in a thin-film photovoltaic device most of the scattered light would be collected. Thus, the total transmitted light was probably significantly higher than the observed average 80%.

(29) Contrary to the metal nanowire films prepared by the processes of the invention, films prepared by the methods of the art, particularly those described in references [9 and 10] do not result in the formation of mesh film arrangements of the type observed in FIGS. 1A to 1B and disclosed herein. In fact, and as FIG. 4 demonstrates, the previously published procedure typically yield a film of spherical nanoparticles rather than a film of nanowires on scaling up surfactant and reagent concentrations. The process of the invention reproducibly yields metal nanowire films.

Example 2

(30) A solution comprising surfactant cetyltrimethylammonium bromide (CTAB), chloroauric acid, as a gold precursor at a molar ratio of 1:200 relative to the CTAB concentration, and sodium ascorbate, at a molar ratio of 60:1 relative to the gold concentration, was prepared. The nanowire growth solutions had CTAB concentrations of 0.25 M and 0.6 M, significantly higher than the 0.1 M used by Murphy [12]. In addition, the growth solution contained a relatively high concentration of silver nitrate, twice the concentration of the Au(III) ions. When the four components were mixed together at 35 C. the gold ions were reduced to the colorless Au(I) state, forming a [AuX.sub.2].sup.-CTA.sup.+ complex (=Cl, Br) but further reduction to the metallic state required the addition of catalytic metal seed particles. Similarly, the silver ions formed an AgBr-CTAB complex.

(31) As an alternative, a small amount of sodium borohydride dissolved in water (e.g., 0.001-0.0001%) was added to the precursor solution in order to initiate metal reduction in this solution. The borohydride amount was enough to reduce up to 0.02% of the metal ions to form small metallic seed particles which catalyzed the reduction of the rest of the metal ions by the ascorbate. Immediately after the borohydride addition, the solution was deposited as a thin film, 100 m thick, on the substrate of choice that was kept at 35 C. and a relative humidity of 50% for drying. The viscosity of the deposited solution at 35 C. was 2 cP for the 0.25 M CTAB solution and 100 cP for the 0.6 M CTAB solution.

(32) FIGS. 5A to 5D display the results of drying the thin growth solution films on transmission electron microscopy (TEM) carbon coated grids for samples prepared with two CTAB concentrations: 0.25M and 0.6M. FIGS. 5A to 5B show TEM obtained from 0.25M CTAB solution. It may be noted that a highly uniform nanowire coating appeared across the 3 mm diameter grid for the 0.25 M CTAB sample. Most of the nanowires appeared in wavy bundles with characteristic bundle size of 20 wires, in the case of the 0.25 M CTAB sample and thicker nanowire domains for the 0.6 M CTAB sample (FIG. 5C). The high magnification image (FIG. 5D) provides more quantitative information about the structure of the nanowire bundles; Average nanowire diameters are in the range 2-2.5 nm, and inter-wire spacing is 2.5 nm, which is significantly smaller than the 3.9 nm estimated for a CTAB bilayer covering thicker gold nanorods [13]. This difference may be due to a larger radius of curvature around the ultra-thin nanowires of the invention, which would lead to a different bilayer packing. Thus, it appears that the metal was deposited at locally ordered surfactant mesostructure domains that were previously found to have liquid-crystalline characteristics, probably close to a reverse hexagonal phase. The nanowire bundle density and morphology varied with deposited solution thickness, drying temperature and drying rate (by control of relative humidity). One of the important parameters was the initial surfactant concentration; when it was increased to about 0.6 M the liquid crystalline domains were thicker than those formed at lower concentrations (FIG. 5C), but also with a larger number of spherical particles that were apparently formed out of the tubular mesostructures. In the case of the higher CTAB concentration the formed metal mesostructures bear a closer resemblance to the oxide based mesoporous materials.

(33) A closer inspection of a sample with high surfactant concentration (0.6 M) and relatively low silver concentration (4 mM, relative to the usual 6 mM) revealed regions with discontinuous, segmented nanowires (inset of FIG. 5C) with typical segment size and separations of the order of few nm up to 30 nm. Accordingly and without wishing to be bound by theory the nanowire formation process began in a large number of small metal clusters triggered by the borohydride addition. These small metal particles were apparently caught within the surfactant template structure as the film became progressively more concentrated on drying. While drying, additional metal atoms deposited on the seeds through catalytic reduction of the metal ions by ascorbate ions. It has been previously shown for mesostructured silica that regions of the mesophase ordered parallel to the interface were induced by proximity to the interface, as also appears to be the case in the present invention.

(34) The processes of the invention may be performed using various different substrates such as silicon, quartz and polyethylene terphtalate (PET). FIG. 6 displays a scanning electron microscope (SEM) image of the film as disclosed herein above deposited on a silicon substrate after gentle washing with 70%/30% ethanol/water solution. In this case it was not possible to resolve individual nanowires and only whole bundles of the CTAB coated nanowires were observable.

(35) Conductance measurements of the nanowire films were performed on various length scales. For example, FIGS. 7A-7D show SEM images of nanowire bundle, a typical measurement configuration and current-voltage curves. FIG. 7A shows the current-voltage measurement for the SEM image shown in FIG. 7B of nanowire bundle conductance measurement using clean tungsten nanoprobes in the Zyvex S 100 system. FIG. 7C shows a typical measurement configuration with the nanowire film deposited on a pre-patterned silicon substrate with gold electrodes. FIG. 7D shows current-voltage curves measured with various inter-electrode spacings as indicated in the legend. On the smallest scale, sharp tungsten probes (500 nm in diameter) were used in a Zyvex 8100 nanomanipulator system to probe individual nanowire bundles in situ, while imaging with the SEM, as shown in FIG. 7 A. In order to avoid large contact resistance the tungsten probes were chemically cleaned in KOH solution followed by in-situ oxidation removal process in the SEM, which resulted in a probe-to-probe resistance of the order of 10. In addition, the substrate with the deposited nanowires was thoroughly washed with 70%/30% ethanol/water and shortly exposed to low-power oxygen plasma, which removed part of the nanowire film in addition to the surfactant coating. The current-voltage curves of the nanowire bundles were ohmic with typical resistance values of the order of 1 k/m. Several measurements on isolated nanowire bundles as the one shown in the inset of FIG. 7A were performed. Assuming an average bundle of 20 nanowires and a diameter of 2.5 nm, an estimated nanowire resistivity of the order of 10.sup.7 m was obtained, which is about 4 times the resistivity of bulk gold. Considering the roughness of the estimate and possible probe-wire contact resistance, this result is roughly consistent with bulk gold like nanowire resistivity.

(36) In addition, the films were deposited over Si wafers with a 100 nm thick oxide layer and gold electrodes patterned on top with inter-electrode 2-20 m gaps (FIG. 7B). The bundle resistances measured over these gaps, together with the bundle densities apparent in the SEM images, were used to estimate effective sheet resistances that were in the range of 100-300 /sq. They also exhibited an ohmic behavior down to 4 K. Rough estimates of nanowires' width and length, connecting the micro-electrodes provided wire resistivities which are of the same order as bulk gold (10.sup.8 m). This indicates that at least part of the nanowires grown within the CTAB meso-structures were formed at the bottom of the dried CTAB film, forming good electrical contact with the pre-formed gold electrodes. Optical dark field microscopy confirmed that the nanowire bundles were located at the bottom of the 5-10 m thick dried CTAB films.

(37) Furthermore, the nanowire films were deposited on a 1 cm.sup.2 fused silica substrates (also from 0.6 M CTAB), silver paint was applied in two parallel lines at the edges of the substrate and sheet resistances of the order of 100 /sq were measured after mild ethanol washing. In particular, the high flexibility of the film was demonstrated (FIG. 8B) where only up to 10% increase in the 100 /sq sheet resistance occurred for a film deposited on a PET substrate which was bended with a curvature radius of 1.5 cm. Upon relaxing the bend in the film the sheet resistance returned to its exact original value, demonstrating the high flexibility of the nanowire film. The films deposited on PET have shown the lowest resistivity results, down to 60 /sq.

(38) FIGS. 8A-8C show visible light transmission curve of a nanowire film, bending of PET substrate coated with a nano wire film and the periodic table as observed through the PET film. FIG. 8A shows a visible light transmission curve of a nanowire film deposited on a fused silica substrate with a sheet resistance of 200 /sq. FIG. 8B demonstrates bending of a PET substrate coated with a nanowire film to a curvature radius of 1.5 cm maintains a 100 /sq sheet resistance. FIG. 8C displays the periodic table as observed through the same PET film which had 80-85% optical transmission in the visible range. The bright stripes are silver paint lines used to estimate the sheet resistance. The upper right corner is film-free.

(39) The optical extinction of the films was measured using a standard spectrophotometer. A transmission curve for a film with relatively high transparency and sheet resistance of 200 /sq is presented in FIG. 8A. Typical far-field transmission of all samples was in the range of 80-90%. This extinction contained a large scattering component which, in the case of photovoltaic devices, may be collected within the device. Without wishing to be bound by theory, the varying amounts of residual spherical particles, which had relatively large diameters relative to the nanowires, may be responsible for a substantial part of the extinction.

Example 3

(40) As recited above, in some experiments, prior to the addition of the reducing agent (e.g., sodium ascorbate) to the Au precursor solution, the silver solution was added to the Au precursor solution and only then the mild reducing agent e.g., sodium ascorbate was added. Under such conditions no metal reduction was induced. Subsequently, low concentration (e.g., 1/100 of that of the sodium ascorbate or lower) of a stronger reducing agent with respect to ascorbate was added to the solution. Such stronger reducing agent should have a reduction potential)(E.sup.0) of 0.5 V or more negative. Non-limiting examples are sodium borohydride, sodium cyanoborohydride and hydrazine. The addition of the strong reducing agent initiated metal reduction in this solution and subsequent metal deposition on the substrate.