Printing of nanowire films

Abstract

Provided is a novel printing process for fabricating metallic, conductive and transparent ultra-thin nanowires and patterns including same on a substrate. The process includes two different controllable steps, each designed to achieving a useful and efficient pattern.

Claims

1. A process for forming a transparent and conductive metal nanowire film on a surface region of a substrate, the process comprising: (a) applying on at least one surface region of a substrate a solution comprising metal seeds; (b) applying a growth solution on the at least one surface region, wherein the growth solution comprises at least one metal reducing agent, and at least one surfactant, and wherein the growth solution is free of metal seeds; and (c) allowing formation of a metal nanowire film in a region where both the solution comprising metal seeds and the growth solution have been applied, wherein said solution comprising metal seeds or said growth solution further comprises at least one metal precursor, wherein the metal seeds and the at least one metal precursor are of the same metal element; and wherein the metal nanowire film comprises a plurality of metal nanowires formed in said the at least one surfactant.

2. The process according to claim 1, wherein the solution comprising metal seeds is deposited by printing to obtain a pattern of said seeds; and applying the growth solution onto said pattern of metal seeds.

3. The process according to claim 1, wherein the growth solution and/or the solution comprising metal seeds further comprises at least one metal reducing agent.

4. The process according to claim 1, wherein the solution comprising metal seeds is free of at least one metal reducing agent and/or at least one metal precursor.

5. The process according to claim 1, said process further comprising a step of applying a metal enhancement solution onto the metal nanowire film.

6. The process according to claim 1, wherein the metal seeds comprise a metal selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Nb, Tc, Ru, Mo, Rh, W, Au, Pt, Pd, Ag, Mn, Co, Cd, Hf, Ta, Re, Os, Ir and Hg.

7. The process according to claim 1, wherein the metal seeds consist of a metal selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Nb, Tc, Ru, Mo, Rh, W, Au, Pt, Pd, Ag, Mn, Co, Cd, Hf, Ta, Re, Os, Ir and Hg.

8. The process according to claim 6, wherein the at least one metal precursor is at least one precursor of gold or silver.

9. The process according to claim 1, wherein the solution comprising metal seeds comprises seeds of gold metal and/or silver metal and at least one metal reducing agent selected from the group consisting of metal hydrides; the growth solution comprises at least one metal precursor, at least one reducing agent and at least one surfactant.

10. The process according to claim 1, wherein the process for forming the metal nanowire film is a printing process.

11. The process according to claim 1, wherein the metal nanowire film comprises one or more ultra-thin nanowires having a diameter equal to or smaller than 3 nm.

12. The process according to claim 1, wherein the metal nanowire film having a sheet resistance of below 1,000 Ω/square.

13. The process according to claim 10, wherein the process for forming a metal nanowire film is ink-jet printing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to better understand the subject matter that is disclosed herein and to exemplify 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) FIG. 1 presents a general scheme of a nanowires printing process according to the present invention.

(3) FIGS. 2A-B present TEM images of FIG. 2A—gold nanoparticles and FIG. 2B-silver nanoparticles used as seed particles in a nanowire production process according to the invention.

(4) FIGS. 3A-B present TEM images of nanowires formed according to a process of the invention. The nanowires were prepared from FIG. 3A—gold seeds and FIG. 3B-silver seeds.

(5) FIGS. 4A-D present TEM images of nanowires prepared using gold seeds and silver seeds before (FIGS. 4A and 4B) and after (FIGS. 4C and 4D) silver enhancement.

(6) FIG. 5 presents a SEM image of a film of gold nanowires, grown with BDAC solution, after silver enhancement, on a 2.5×2.5 cm.sup.2 glass substrate. The measured sheet resistance was 30 Ω/sq. and seed areal density was 60 seeds/μm.sup.2. The inset shows a coated 10×10 cm.sup.2 PET substrate, demonstrating flexibility and high coating uniformity and optical quality.

(7) FIG. 6 provides a plot of sheet resistance measured for nanowire films (grown with BDAC, after silver enhancement) vs. number of seeds per 1 μm.sup.2 area. The insets are representative SEM images of the films produced at different seed densities.

(8) FIG. 7 presents optical transmission vs. sheet resistance for nanowire films prepared with different levels of silver plating and/or density.

(9) FIG. 8 presents haze vs. sheet resistance for nanowire films at varying amounts of silver plating and/or nanowire density.

(10) FIGS. 9A-D provide SEM images CTAB based nanowire stripes, patterned on a PET substrate in lines of ˜70 μm wide seed droplets and separated by ˜1 mm FIG. 9A) shows the printed lines of seed droplets imaged after silver enhancement, FIG. 9B-D) show, in increasing magnification, grown nanowire stripes, confined to the same width as the seed droplets. The blurred bright areas seen in FIG. 9C-D are due to charging of the insulating areas out of the nanowire stripes.

(11) FIGS. 10A-C presents SEM images of CTAB based nanowire film grown on a single seed droplet printed by an 80 μm orifice at the center of a PET substrate, taken after silver plating.

(12) FIGS. 11A-D presents SEM images of BDAC based nanowire films deposited on a PET substrate, with seeds patterned at ˜100 μm wide lines (single seed droplet width) and separated by ˜1 mm FIGS. 11A-B show at low magnification, SEM images of greater metal density along the patterned seed lines, due to some seed particles remaining attached to the surface and silver enhanced. FIGS. 11C-D show at higher magnification, SEM images of nanowire films that extend beyond the deposited seed droplet lines, demonstrating bulk solution nanowire growth.

DETAILED DESCRIPTION OF EMBODIMENTS

(13) This invention provides a novel printing process for forming conductive and transparent ultra-thin metal nanowire films (the process scheme can be seen in FIG. 1). This process, which may be achieved by printing, comprises two separate controllable steps: step 1—metal seed particle deposition, and step 2—deposition of a nanowires growth solution. The sequence of the steps shown in FIG. 1 is for purpose of illustration only and is not to be taken as limiting. The sequence, for example, may be reversed.

(14) A third deposition step further improves the film conductivity and stability by, e.g., electroless plating of the nanowires with silver. However, this third step is not generally necessary. In contrast to previous metal nanowire film deposition techniques, where the nanowires were first synthesized and then deposited from a colloidal dispersion, and the micrometer scale nanowires could easily clog ink jetting orifices, the deposition process of the invention involves printing of only 1.5-3 nm size seed particles and aqueous solutions of precursors. It therefore enables reliable deposition by inkjet devices and control over the patterning of the film with high resolution and on a variety of substrates (e.g. PET, glass, etc.).

(15) The separation of the process into two stages, in which a seed particle film is deposited separately from the deposition of a growth solution, allows better control of various film parameters. The fact that the two patterning steps are carried out from separate solutions also prolongs the shelf-life of the two solutions, as neither reacts with the other while in stock. Thus, while the present process permits patterning of the two solutions at different time points, thereby controlling the process conditions, where the two solutions were mixed before deposition, the deposition time would have been limited to several minutes.

(16) The two-stage printing process allows unique in-situ controllable deposition process on many different substrates (e.g. glass and many types of polymers) with a high resolution patterning of the film.

(17) For the formation of ultra-thin metal nanowires directly on a substrate of interest (glass or various polymers), very small metal seed particles (1-4 nm) may be employed. The nanowires formed would typically be ultra-thin (2-3 nm in diameter) and occur in bundles or clusters, where each bundle comprises between one to several hundred of nanowires. To increase the stability of the nanowires a third metal deposition step may be carried out. In some cases, the additional step involves selective electroless silver plating.

(18) The density of the nanowires on the surface can be controlled by the amount of the deposited seed particles and growth solution specifications. The parameters of the electroless metal deposition step would determine the final thickness of the metal nanowires; hence determine the final sheet resistance and transparency of the film.

(19) This process of the invention is highly advantageous as compared with similar processes of the art, at least in the following:

(20) 1. In processes of the art the nanorods/nanowires are coated by an insulating polymer (typically PVP) so that electrical contacts may not form between them.

(21) 2. The pre-formed, typically micron scale long nanowires, can easily clog inkjet printing nozzles, especially if small orifice nozzles are used for high resolution patterning. In the process of the invention, the largest objects may be the 1-3 nm seed particles utilized for the patterning.

(22) 3. Nanowires formed by the other processes have typically diameters greater than 50 nm, causing substantial light scattering (hence haze). However, nanowires of the invention, even after controlled thickening typically have thicknesses below 50 nm. Therefore less haze is expected.

(23) Inks made of dispersions of large nano-objects like the silver/copper nanowires are very difficult to prepare, and dispense by inkjet. With the technology of the invention simple solutions are utilized—no clogging problems, hence no need for special orifice cleaning procedures that one would use when working with large metal nanoparticle dispersions

(24) Experimental

(25) Chemicals.

(26) The reagents used for the metal nanowires synthesis, including BDAC, CTAB, HAuCl.sub.4, AgNO.sub.3, ascorbic acid, sodium ascorbate, bovine serum albumin (BSA), citric acid, trisodium citrate, hydroquinone and sodium borohydride (NaBH.sub.4) were purchased from Sigma-Aldrich and used without any further purification. The Au seeds (“Nanogold”) were purchased from Nanoprobes Inc. and used without further treatment. All water used was ultrapure (18 MΩ.Math.cm), obtained from a USF ELGA UHQ system.

(27) Method 1

(28) Step 1: A Metal Seed Particles Solution is Printed on a Specific Desired Area on the Substrate

(29) Small seed nanoparticles of different types were used:

(30) 1. Commercial gold nanoparticles (“Nanogold”): ˜1.4 nm (˜55 gold atoms). Produces by Nanoprobes Inc.

(31) 2. Homemade silver nanoparticles of ˜3.5 nm were made according to: Zhaoxia Qian and So-Jung Park [4].

(32) Ag seeds were synthesized by mixing 4.75 mL of 0.1 M BDAC and 200 μL of 0.01 M AgNO.sub.3 in a 15 mL aluminum-foil-wrapped plastic vial at 30° C., followed by a quick injection of 600 μL of 0.01 M ice-cold fresh NaBH.sub.4 solution. The solution was gently mixed and left undisturbed at 30° C. for 2.5 h and used immediately without further aging.

(33) 10.sup.−8M or 10.sup.−7M solution of seeds in a mixture of 1:1 water and methanol was used for printing a pattern where the growth of the nanowire film is desired, on various types of substrates (glass, poly(ethylene terphtalate) (PET) etc.).

(34) The gold nanoparticles are in average size of 1.4±0.3 nm as can be seen from the TEM image (FIG. 2A). The silver nanoparticles are in average size of 3.6±1.3 nm as can be seen from the TEM image (FIG. 2B). Typical seed particle coverage was about 100 particles per a square micrometer.

(35) Step 2: A Growth Solution Deposited on the Seeds Pattern is Used to Create the Nanowires at the Same Location

(36) Deposition of a nanowires growth solution may be carried out either only where the seed solution was deposited or uniformly over the whole substrate. In any case the nanowire film would grow only where the seed particles were deposited. Three types of growth solutions using two different surfactants were used:

(37) BDAC Growth Solution:

(38) The growth solution was prepared according to Zhaoxia Qian and So-Jung Park [4]. The growth solution was prepared by mixing aqueous solutions of BDAC (0.1 M, 10 mL), HAuCl.sub.4 (0.01 M, 421 μL), AgNO.sub.3 (0.01 M, 512 μL), and ascorbic acid (0.1 M, 268 μL) sequentially at 30° C. in a 20 mL glass vial. Usable time of the solution is at least 1 hour (different from the reference).

(39) 100-300 μl of the growth solution was deposited on the substrate for 5-40 minutes on various substrates that were held at room temperature up to 80° C. Then the substrate was washed by immersing for 1 minute in methanol and 1 minute in water.

(40) CTAB Growth Solution:

(41) The growth solution was prepared by mixing aqueous solutions of CTAB (0.25M, 10 mL), HAuCl.sub.4 (0.025M, 500 μL), AgNO.sub.3 (0.1M, 250 μL), and sodium ascorbate (1.8M, 425 μL) sequentially at 35° C. in a 20 mL glass vial.

(42) 300 mL of the growth solution was deposited on the substrate for 5 minutes at room temperature on a 2.5×2.5 cm.sup.2 area. Then the substrate was washed by dipping for 1 minute in 70% ethanol and 1 minute in 100% ethanol.

(43) Mixed CTAB and BDAC Growth Solution:

(44) This growth solution was prepared by mixing aqueous solutions of CTAB (250 mM, 7.5 ml) and BDAC (100 mM, 2.5 ml) at 35° C. in a 20 mL glass vial, and adding aqueous solutions of HAuCl.sub.4 (25 mM, 500 μL), AgNO.sub.3 (100 mM, 250 μL), and sodium ascorbate (1.8M, 425 μL) sequentially.

(45) 300 mL of the growth solution was deposited on the substrate for 5 minutes in room temperature on different substrates (2.5×2.5 cm.sup.2). Then the substrate was washed for 1 minute in methanol, 1 minute in 70% ethanol and 1 minute in 100% ethanol.

(46) The nanowires formed at this stage would typically be ultra-thin (2-3 nm diameter) and occur in bundles consisting of anywhere between single wires to hundreds of wires. Nanowires prepared from gold seeds and silver seeds with the same growth solution can be seen in FIGS. 3A and 3B, respectively. After this step the sample sheet resistance is in the order of 300-700 Ohm sq., and with visible transmission of nearly 100% on PET and glass (2.5×2.5 cm.sup.2).

(47) Step 3: A Possible Third Step is a Selective Metal Deposition in Order to Enhance the Film Conductivity and Stabilize the Grown Ultra-Thin Nanowires

(48) A selective electroless silver plating process (also called silver enhancement) was used in order to thicken and stabilize the ultra-thin nanowires.

(49) A silver enhancement solution was prepared according to either one of two methods:

(50) Method 1:

(51) A solution of 79 mL of 2.7 mM polyvinylpyrrolidone (PVP) (3500 MW) or BSA 0.2-0.8% or BDAC 0.1M or gum Arabic 1% in water was heated to 35-45° C. while stirring. The following solutions were added in the order of their appearance: AgNO.sub.3 (0.1 M, 500 μL), 5 mL of 1.2 M Citric acid and 1.6M Citrate buffer (pH-4.5), and Hydroquinone (0.3M, 12.5 mL), sequentially at 30° C. in a 200 mL glass vessel.

(52) The substrates with nanowire film were dipped for 1-10 minutes in the stirred silver enhancement solution. Then the samples were washed for 1 minute in methanol and 1 minute in water.

(53) Method 2:

(54) The silver plating solution was prepared by mixing aqueous solutions of BSA (0.5% w/v, 200 mL), AgNO.sub.3 (0.1M, 1110 μL), Citric acid and Citrate buffer (1.2M and 1.6M, 11.11 mL), and Hydroquinone (0.3M, 33 mL), sequentially at 26° C.

(55) The samples were dipped for 4-20 minutes in the silver plating solution. Then the samples were washed for 1 minute in methanol and 1 minute in water.

(56) Microscopy images were taken using Quanta 200 FEG Environmental Scanning Electron Microscope (ESEM) and Philips Tecnai F20 Transmission Electron Microscope (TEM). Sheet resistance was measured with a multimeter using silver paste paint lines at 2 opposite edges of the substrate as conductive contacts. The measurement was carried out by contacting the two silver paste contacts with the multimeter electrodes.

(57) Optical transmission for nanowire films deposited on a substrate was measured against a reference clean substrate in a spectrophotometer (Ocean Optics, S2000) at a wavelength range of 400-800 nm. Each sample was measured in several places and the averaged value is the sample's transmission.

(58) Seeds and growth solutions were printed using Jetlab4® system of MicroFab Technologies Inc. Drop volume and diameter were in the range of 150-300 pL and 60-80 μm, respectively.

(59) After the nanowires preparation (step 2), the nanowires would not be stable over time and a third metal deposition step would be required. The last step has to be selective to thicken the pre-formed nanowires without depositing metal elsewhere on the substrate.

(60) The inventors have used a selective electroless silver plating process (called silver enhancement) to thicken and stabilize the ultra-thin nanowires. The density of the nanowires on the surface can be controlled by the amount of the deposited seed particles and the parameters of the electroless metal deposition step would determine the final thickness of the metal nanowires, hence the final film's sheet resistance and transparency.

(61) TEM images of the nanowires (prepared from gold and silver seeds) before and after the silver enhancement process can be seen in FIGS. 4A-D. It can be clearly seen that the process was selective and only the nanowires were thickened without metal deposition directly on the substrate. SEM image of nanowires after silver enhancement on 2.5×2.5 cm.sup.2 glass substrate can be seen in FIG. 5. The film sheet resistance of this sample was 30 Ohm sq.

(62) After selective metal deposition, the sample sheet resistance was in the range of 40-70 Ohm sq., and with visible transmission of 90% on PET and glass (2.5×2.5 cm.sup.2).

(63) Transmission Electron Microscopy (TEM).

(64) All samples for TEM were deposited on carbon-coated copper grids (SPI). A XX mL seeds solution was manually deposited followed by XX mL of growth solution. For the silver coated nanowires, nickel grids were used. Images were recorded using an FEI Tecnai F20 TEM.

(65) Scanning Electron Microscopy (SEM).

(66) SEM measurements were carried out in Quanta200 field emission gun ESEM using the FEI wet-STEM detector. To examine non-conducting substrates (glass, PET), water vapor environment (low vacuum) was used.

(67) Sheet Resistance, Transparency and Haze Characterization.

(68) The optical transmission and haze of the films were measured against a reference blank substrate in a fiber-coupled array spectrophotometer (Ocean Optics, S2000) connected to an integrating sphere, at a wavelength range of 400-900 nm. The haze was calculated from four optical transmission measurements, made by mounting the specimen at the input of the integrating sphere: Haze=[(T.sub.4/T.sub.2)−(T.sub.3/T.sub.1)]×100%, where T.sub.1 is a measurement of the incident light with no specimen and closed sphere, T.sub.2 is a measurement of the total light transmitted through the specimen with a closed sphere, T.sub.3 is a measurement of the light scattered by the instrument with no specimen, with an open sphere and a light trap, and T.sub.4 is a measurement of the light scattered by the instrument and the specimen, with an open sphere and a light trap.

(69) Sheet resistance measurements were carried out employing a Fluke multimeter using silver paint at the edges of the substrates for defining the contacts.

(70) The density of seed particles deposited on the substrate could be controlled by the concentration of the seed solution, droplet size (via the diameter of the capillary used for dispensing) and the droplet spacing. Typical dispensing capillary size was 80 μm and seed concentration was in the range 4.2×10.sup.−8M-1.8×10.sup.−7M. The droplets were typically printed with their rims nearly touching or slightly overlapping to obtain an average surface density of ˜1-500 seeds/μm.sup.2.

(71) After drying the seed solution droplets on the substrate, a typical amount of ˜50 μL of the BDAC based growth solution per 1 cm.sup.2 of seed coated substrate was deposited. This resulted in bundles of ultra-thin (2-3 nm diameter) gold/silver nanowires, as shown in FIGS. 4A-B.

(72) The nanowires formation mechanism using BDAC as the templating surfactant, is initiated in the solution with the reduction of Au(III) ions to Au(I) state by the ascorbic acid molecules. The Au and Ag ions form complexes with the BDAC molecules, which allows for reduction to the final metallic state only in the presence of the catalytic metal seeds particles printed on the surface of the substrate. The nanowires formation mechanism is a self-assembly process occurring in the thin layer of growth solution deposited on the substrate. It should be emphasized that the BDAC based growth solution leads to bulk growth of metal nanowires in the solution volume. On the other hand, CTAB based growth of nanowires, only produces nanowires at the substrate-solution interface (“surface growth”). Hence, The nanowire network which grows in the whole volume of the BDAC based solution is believed to weakly attach to the substrates at random locations, and finally, after washing, flattens on the surface, forming strong van der Waals attraction to the substrate. This will be further illustrated in the discussion of patterned deposition later on.

(73) A selective electroless silver plating process (also called silver enhancement) has been use to thicken and stabilize the ultra-thin nanowires and to enhance the film conductivity. Silver deposition was preferred over the previously used gold deposition (2) since unlike gold, silver does not have interband transitions in the visible range and is therefore more transparent across this range.

(74) TEM images of the nanowires after the silver enhancement process can be seen in FIGS. 4C-D. Before the silver enhancement the sample sheet resistance is in the order of 1 kΩ/sq. (which degrades over time), and with visible transmission of nearly 100% on PET and glass.

(75) FIG. 6 presents the results of a study of the influence of the areal density of printed seeds on the nanowires final density and hence on the final sheet resistance. When the density of seeds is very large (˜>500 seeds/μm.sup.2) or very small <5 seeds/μm.sup.2) the sheet resistance rises to very high values (>1 kΩ/sq.) with good correlation to low nanowires density (shown in the inset SEM images). Between ˜8 seeds/μm.sup.2 to ˜60 seeds/μm.sup.2 has obtained relatively low sheet resistance values with high nanowires density.

(76) A significant improvement in conductance and nanowires density occurs when areal density is increased from ˜4 seeds/μm.sup.2 to ˜8 seeds/μm.sup.2. Only ˜8 seeds/μm.sup.2 are needed to grow dense nanowires mesh on 20×20 mm.sup.2 area with less than 50 Ω/sq.

(77) As the areal density of seeds grows larger than ˜60 seeds/μm.sup.2, more low aspect-ratio particles are seen in the SEM images. It is therefore clear that too high concentration of seed particles interrupts the nanowires growth, probably due to distribution of the deposited gold and silver atoms over too many seed particles.

(78) The seed density/concentration used in the current work is similar to that used by Azulai et al [3], in CTAB based nanowires formation. Their seeds concentration was in the range of 0.67-2.1×10.sup.−8M. Taking such a concentration in a growth solution comparable to the present work (50 μL per 1 μm.sup.2) one obtains the equivalent of ˜50-100 seeds/μm.sup.2. It is therefore concluded that the nanowire growth mechanism seems to be somewhat different between the surface growth mode of the CTAB based system which requires nearly an order of magnitude larger concentration of seed particles compared with the BDAC based system where bulk solution growth requires much less seed particles.

(79) After silver plating the samples (prepared with suitable seeds concentration) sheet resistance was in the range of 30-70 Ω/sq., and with visible transmission of ˜90% on PET and glass (2.5×2.5 cm.sup.2). SEM image of a nanowire film after silver enhancement on 2.5×2.5 cm.sup.2 glass substrate can be seen in FIG. 5. The sheet resistance of this sample was 30 Ω/sq. It can be clearly seen that the silver deposition process was selective and only the nanowires were thickened without metal deposition directly on the substrate, and with almost no “satellite” seeds of significant size contaminating the sample after the enhancement process. It is highly probable that some metal particles adsorbed to the substrate prior to the silver coating were washed away from the substrate during the silver deposition or washing steps.

(80) Moreover, using TEM-EDX and SEM-EDX, is has been found that the Au/Ag composition in the metal nanowires before the silver deposition was ˜75%/˜25%, and after the silver deposition it changed to ˜5%/˜95%, which together with the TEM images of the silver-coated nanowires proves that a substantial silver layer formed on top of the nanowires.

(81) FIG. 7 presents optical transmission versus sheet resistance for films deposited on glass and PET. Tunability of the two parameters is achieved through the amount of silver deposited and nanowire density. It can be seen that even at relatively low sheet resistance (˜60 Ω/sq.) the transparency is ˜90%. Moreover, the haze vs. sheet resistance graph in FIG. 8 shows that relatively low haze is achieved for films (˜0.6%) at ˜100 Ω/sq., and at lower sheet resistance (˜50 Ω/sq.) has reached ˜1% haze. There are different slopes to the graphs in FIGS. 7 and 8 above and below ˜70 Ω/sq. While sheet resistance drops with the increase in thickness as the diameter squared, light scattering increases as a higher power (˜2-6) of the dimension and causes the non-linear increase in light scattering.

(82) Patterning of the Nanowires Films

(83) In order to optimize the nanowire deposition process for narrow line patterns the two nanowire growth modes were compared, “surface growth” using CTAB based solution vs. bulk growth using BDAC based solution. In these experiments single seed droplet thick lines were printed on PET substrates followed by deposition of the nanowire growth solution over the whole substrate, followed by the silver enhancement bath.

(84) FIG. 9A displays deposited seed spots after silver enhancement of the seed particles to make them visible in high-resolution SEM. The 80:20 water:ethanol droplets, deposited from a 40 μm orifice, spread on the PET substrate to ˜70 μm diameter spots. The grown nanowire film stripes were confined to about the same dimension. To further learn about the confined nanowire surface growth mode, a single seed spot was deposited on a PET substrate and the grown nanowires were imaged. As seen in FIG. 10, the nanowires were tightly confined to the ˜100 μm seed spot with a fairly sharp edge, where the nanowires abruptly terminate, except for occasional small protrusions of the order of ˜10 μm.

(85) FIG. 11 displays the results of growing the nanowires over a single seed drop wide line using a BDAC growth solution. The BDAC based nanowires growth is clearly occurring in the bulk solution by seed particles which detached from the substrate and become mobile in the solution through convection and diffusion. BDAC is substantially more soluble in water compared with CTAB and thus accumulates more slowly on the surface when the growth solution touches the substrate. Hence, more seed particle may diffuse deeper into bulk solution film and lead to the nanowire formation away from their original position. With the CTAB based growth solution the seeds are confined to the proximity of the surface due to very fast accumulation of CTAB on the surface, and thus grow at the substrate-solution interface.

(86) Consequently, the CTAB based confined growth is highly suitable for patterning the nanowire films on substrates. As seen in FIGS. 9 and 10, the formed nanowires are constricted to the original spots where the seed particles were deposited.

(87) The difference between the obtained line patterns in the two nanowire growth modes is also demonstrated through conductance measurements. The lines grown with the CTAB growth showed sheet resistance of ˜50 Ω/sq. along the lines and no conductivity was measured in the perpendicular direction (across the printed lines). On the other hand, the lines grown with BDAC showed higher sheet resistance along the lines (>100 Ω/sq.) and finite conductance has been measured also perpendicular to the printed lines.

(88) It is clear that various combinations of the two surfactants within the growth solution may be used to control the nanowire density and their spread on the substrate, as well as the required thickness of growth solution needed for optimal nanowire growth.

(89) The separated seed deposition and nanowire growth steps allow a tightly controllable printing mode on a variety of substrates, in particular, using small diameter orifices for inkjet printing of the seed solution should enable patterning of the nanowire films with <100 μm features (possibly down to 20-30 μm). The density of the nanowires on the surface can be controlled by the amount of the deposited seed particles (2) and the time of growth and the parameters of the silver plating step would determine the final thickness of the metal nanowires, hence the final film sheet resistance and transparency.