TRANSPARENT ELECTRICALLY - CONDUCTIVE SOLID MATERIAL, AND METHOD AND COMPOSITION FOR FORMING TRANSPARENT ELECTRICALLY - CONDUCTIVE SOLID MATERIAL

Abstract

A method of preparing a transparent, electrically-conductive solid material is provided, typically a coating. The method comprises: (i) Providing a liquid composition comprising a matrix-forming material and a plurality of metal nanowires, and optionally a carrier liquid in which the matrix-forming material and the plurality of metal nanowires may be dispersed; and (ii) Forming the solid material from the liquid composition. A solid material, typically in the form of a coating, provided by such a method is also described.

Claims

1. A method of forming an electrically-conductive, transparent solid material, said method comprising: (i) Providing a liquid composition comprising a matrix-forming material and a plurality of metal nanowires; and (ii) Forming said transparent solid material from said liquid composition.

2. A method according to claim 1, wherein said transparent solid material is an electrically-conductive, transparent coating on a surface of a first substrate, said method comprising contacting said surface with said liquid composition, or comprising forming a free-standing film of said liquid composition and forming a solid free-standing film from the free-standing film of said liquid composition.

3-67. (canceled)

68. A method according to claim 1, wherein the liquid composition comprises a carrier liquid in which the matrix-forming material and plurality of metal nanowires are dispersed, and wherein forming a solid coating from said composition comprises removing the carrier liquid to form said solid material.

69. A method according to claim 68, wherein the carrier liquid is a polar liquid or comprises a mixture of polar liquids

70. A method according to claim 1, wherein the metal nanowires comprise one or more of silver, gold, copper and nickel.

71. A method according to claim 1, wherein the matrix-forming material comprises one or more polymers.

72. A method according to claim 71, wherein the one or more polymers is water-soluble.

73. A method according to claim 71, wherein at least one of said polymers exhibits surfactant-like behaviour.

74. A method according to claim 71, wherein the one or more polymers is selected from one or more of the group consisting of a poly(alkenol); a polyether; and a block co-polymer comprising one or more blocks of poly(alkenol) or polyether, with one or more blocks of a polymer comprising repeat groups of a tertiary amine.

75. A method according to claim 1, wherein the liquid composition comprises no more than 1 wt % metal nanowires.

76. A method according to claim 1, wherein the liquid composition comprises no more than 1 wt % matrix-forming material.

77. A method according to claim 1, wherein the liquid composition comprises from 0.1 wt % to 1 wt % metal nanowires and from 0.1 wt % to 1 wt % matrix-forming material, and wherein the matrix-forming material comprises a polymer.

78. A method according to claim 77, wherein the liquid composition comprises 0.1 to 0.6 wt % metal nanowires and 0.1 to 0.6 wt % matrix-forming material, and the matrix-forming material comprises a polymer.

79. A method according to claim 1, wherein the ratio of the weight of the matrix-forming material relative to the sum of the weight of the matrix-forming material and the metal nanowires is no more than 0.5:1 and at least 0.05:1.

80. A transparent, electrically-conductive solid material producible in accordance with the method of claim 1.

81. A substrate provided with an electrically-conductive transparent solid coating comprising a plurality of metal nanowires dispersed within a matrix.

82. A substrate according to claim 81, wherein the solid material has a percentage light transmission of at least 50%.

83. A substrate according to claim 81, wherein the haze generated by the solid material is no more than 20%.

84. A substrate according to claim 81, in which the metal nanowires are distributed approximately uniformly throughout the thickness of the solid material.

85. A liquid composition for forming a transparent electrically-conductive solid material, optionally in the form of a coating, the liquid composition comprising a matrix-forming material and a plurality of metal nanowires.

Description

[0060] The prevent invention will now be described by way of example only with reference to the following figures of which:

[0061] FIG. 1 is a photograph of the belted crest device of the University of Oxford, part of which (the top left) is covered with a substrate which has been provided with a coating made using an example of an embodiment of a method of the present invention;

[0062] FIG. 2 is a cross-sectional scanning electron microscope image of a coating made using an example of an embodiment of a method of the present invention;

[0063] FIG. 3 is a plan view scanning electron microscope image of a coating made using an example of an embodiment of a method of the present invention;

[0064] FIG. 4 is a graph showing the sheet resistance of a coating made using an example of an embodiment of a method of the present invention as a function of the wt % of polyvinyl alcohol relative to the sum of the weights of polyvinyl alcohol and silver nanowires;

[0065] FIG. 5 is a graph showing the optical transmission of a coating made using an example of an embodiment of a method of the present invention as a function of the wt % of polyvinyl alcohol relative to the sum of the weights of polyvinyl alcohol and silver nanowires; and

[0066] FIG. 6 is a graph showing the optical transmission of a coating made using an example of an embodiment of a method of the present invention as a function of sheet resistance of the coating.

[0067] A general example of a method in accordance with the present invention will now be described. Silver nanowires were made essentially as described in Rapid synthesis of silver nanowires through a CuCl- or CuCl.sub.2-mediated polyol process, K. E. Korte et al., J. Mat. Chem., 2008, vol. 18, pages 437-441. Instead of using an oil bath to heat the reaction (as described by Korte et al.), the applicant used a hot plate and glass wool. Optical transmission spectroscopy of aliquots of the reaction mixture in the UV-visible spectrum was used to monitor the progress of the reaction, a peak in absorption at about 400 nm indicating the presence of nanowires. The silver nanowires were characterised in several ways. The nanowires were deposited from suspension onto a substrate for examination using scanning electron microscopy (SEM). The SEM images allowed the size of the nanowires to be examined. Transmission electron microscopy was used to examine individual nanowires.

[0068] A suspension of nanowires in a carrier liquid was then prepared. A separate solution of polymer in the carrier liquid was prepared. An appropriate amount of the polymer solution was then combined with an appropriate amount of the suspension of nanowires to produce a liquid composition for depositing onto a substrate. The liquid composition was deposited onto a substrate (typically glass) using an off-the-shelf, low cost spray system to form a liquid layer on the substrate. The carrier liquid was then removed by heating the substrate and liquid layer to about 50 C.

[0069] The thickness of the coating was investigated using scanning electron microscopy (using a JEOL 840F).

[0070] The optical characteristics of the coating could be observed by eye and by measuring the optical transmission of the coating. This was performed using a UV-vis spectrometer in transmission mode in the visible region (390-750 nm wavelength light) against a reference substrate. The coating could also be examined using scanning electron microscopy.

[0071] The sheet resistance of the coating was measured using a 4-point probe, as is well known to those skilled in the art.

EXAMPLE 1

[0072] A 1 wt % suspension of silver nanowires in water was prepared. A 1 wt % solution of polyvinyl alcohol (PVA) in water was prepared. In the present example, the PVA was 99+% hydrolysed, having an average M.sub.W of 85000 to 124000 (Aldrich, UK). The PVA solution and the silver nanowire suspension were mixed in appropriate amounts to form the liquid composition for forming the coating. The coating was formed as described above in the general method.

[0073] FIG. 1 shows a photograph of the University of Oxford logo, with a substrate and coating formed in accordance with the method of Example 1 placed over the top left part of the logo. The coating was formed from a liquid comprising the same weights of PVA and silver nanowires. As can be seen from FIG. 1, whilst the coating exhibits some unwanted scattering (giving rise to a slight misting effect), the coating is substantially transparent.

[0074] FIGS. 2 and 3 show scanning electron micrograph images of coatings formed in accordance with Example 1. The weight ratio of PVA:silver nanowires was 60:40. FIG. 2 shows that the coating is relatively thick (about 3 microns thick). This may be beneficial when applying coatings to relatively rough underlying substrates. FIG. 3 indicates that the silver nanowires have a high aspect ratio, and are relatively evenly dispersed in a lateral direction throughout the coating. There appears to be significant contact between adjacent nanowires (thereby enhancing conductivity), whilst there appears to be little aggregation of nanowires over distances comparable to the wavelength of light (such aggregation causing unwanted scattering of light).

[0075] FIG. 4 shows the sheet resistance of a coating made using Example 1. Different amounts of silver nanowire suspension and polymer solution were mixed in order to vary the weight percentage of polymer. As can be seen from FIG. 4, the sheet resistance values are low, certainly compared to commercially available indium tin oxide. As the percentage weight of the polymer decreases, the sheet resistance decreases. Referring now to FIG. 5, it was observed that the % optical transmission was at a consistent level of from 60-70% for a percentage weight of polymer of from about 25% to 60%, but as the percentage weight of the polymer decreased below about 25%, the % optical transmission dropped considerably.

[0076] FIG. 6 shows the % optical transmission as a function of sheet resistance. The data shown in FIG. 6 were derived from the data of FIGS. 4 and 5. This suggests that there is a range of optimum values of resistivity which produce good optical transmission.

EXAMPLE 2

[0077] The method of Example 1 was reproduced using methanol as the carrier liquid. The coatings produced were optically and electrically acceptable.

EXAMPLE 3

[0078] The method of Example 1 was reproduced using tetrahydrofuran as the carrier liquid. The coatings produced were optically and electrically acceptable.

EXAMPLE 4

[0079] The method of Example 1 was reproduced using iso-propyl alcohol as the carrier liquid. The coatings produced were optically and electrically acceptable, if the liquid composition comprising the carrier liquid, metal nanowires and the polymer was used quickly after preparation.

EXAMPLE 5

[0080] The method of Example 1 was reproduced using polyvinylpyrrolidone (Aldrich catalogue #85,656-8, M.sub.W of about 55000, Aldrich, UK) instead of PVA, but was unsuccessful. Whilst not wishing to be constrained by theory, it is thought that the polymer inhibits contact between metal nanowires, thereby inhibiting electrical conductivity. Furthermore, poor quality films were formed by spray coating.

EXAMPLE 6

[0081] The method of Example 1 was reproduced using PEDOT.ESS (Clevios P polymer, H.C.Starck GmbH, Germany) instead of PVA, but was relatively unsuccessful. Without wishing to be bound by theory, the applicant believes that the liquid used was too viscous.

[0082] The examples above illustrate the use of spray dispensing to form a liquid layer on a substrate. Those skilled in the art will realise that alternative methods for forming a layer may be used, for example, dipping or screen printing. Screen printing may be of particular benefit because screen printing permits effective deposition of the liquid composition through a screen onto substrates to generate patterning, for example, by providing strips or regions of coating separated by electrically-isolating gaps.

[0083] The examples above illustrate the use of polyvinyl alcohol as a polymer from which the layer is formed. Those skilled in the art will realise that other materials may be used. For example, alternative polymers could be used, such as other polymers which nave surfactant properties, such as polyethers, poly(methyl methacrylate) and substituted polystyrenes. Alternatively or additionally, other materials could be used, such as monomer or low molecular mass moieties which form polymers in the layer, for example, on exposure to a stimulus, such as illumination with UV or other suitable electromagnetic radiation.

[0084] The examples above describe the production of relatively thick films. Whilst this may be desirable in certain circumstances, it is possible to make thinner films, for example from 10 to 100 nm thick.

[0085] The examples above describe the deposition of coatings onto a substrate. Whilst this may be desirable in certain circumstances, it may be desirable to make a solid material which is not in the form of a coating but rather, for example, in the form of a free-standing film.

[0086] The examples above describe the use of a liquid composition comprising a carrier liquid which is subsequently removed, for example, by heating. Whilst this may be desirable in certain circumstances, it may be desirable not to have a carrier liquid, for instance, if the matrix-forming material itself is a liquid.

[0087] The examples above describe the use of silver nanowires. Those skilled in the art will realise that, additionally or alternatively, different metal nanowires may be used, for example, gold nanowires. Furthermore, additional conductive species, such as carbon nanostructures, could be added.

[0088] The examples above illustrate the use of a liquid composition comprising a polymer and metal nanowires to produce a conductive film. Those skilled in the art will realise that it would be possible to add further components to the liquid composition, such as one or more of a low molecular weight surfactant, an adhesion promoter, dye, a corrosion inhibitor, a refractive index modifier and a viscosity modifier.

[0089] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.