LETTER pubs.acs.org/Langmuir
Formation of Large-Scale Flexible Transparent Conductive Films Using Evaporative Migration Characteristics of Au Nanoparticles Ko Higashitani,*,†,|| Cathy E. McNamee,‡ and Masaki Nakayama§ †
Department of Chemical Engineering, Kyoto University - Katsura, Nishikyo-ku, Kyoto, Kyoto 615-8510, Japan Shinshu University, Tokida 3-15-1, Ueda-shi, Nagano-ken 386-8567, Japan § Mitsubishi Rayon Co., 10-1, Daikoku-cho, Tsurumi-ku, Yokohama, 230-0053, Japan ‡
ABSTRACT: To sustain the growing demand of transparent conductive films for wide applications, such as flat panel displays, a much more cost-effective film is required over the widely used indium tin oxide film. Here we developed a promising method to manufacture a cost-effective flexible transparent conductive film of high performance by first making grid-iron patterns of thin lines on a large scale using evaporative migration characteristics of gold nanoparticles, and then by burying the grid-iron pattern into a poly(ethylene terephthalate) film.
1. INTRODUCTION Demand for transparent conductive films is rapidly growing for use in the broad applications of flat panel displays, such as the liquid crystal display, the touch panel, electronic paper, the light emitting diode, electroluminescence, and the solar cell. Indium tin oxide (ITO) film has been widely used up to now for such applications.1,2 In order to sustain future demands, various trials have been actively carried out to develop substitutive films,3-9 but many of them are neither performance- nor cost-effective for real applications on a large scale. Recently, evaporative lithography using colloidal solutions of nanoparticles was shown to be a promising method to make such films on a large scale, requiring only a proper template.10,11 It is known that when a drop of a colloidal solution of nanoparticles dries on a substrate, the particles move with the flow of medium during the evaporation and a circular ring of accumulated particles, the so-called “coffee ring”, will be formed along the perimeter of the dried droplet.12 These evaporative self-assembly characteristics of colloidal solutions may be employed to make an invisible or hardly visible conductive network; this process is called evaporative lithography. Layani et al. used the principle to make a film of high transparency and conductivity by coating a surface of poly(ethylene terphthalate) (PET) film with overlapping arrays of self-assembled coffee rings of silver nanoparticles.13 Harris and co-workers reported that there is a possibility to make a hexagonal network of thin lines using evaporative lithography.14,15 When the small gap between an upper mask plate containing hexagonally arrayed holes and a lower substrate is filled with the colloidal solution and the solution medium is evaporated uniformly, a hexagonal network is formed on the substrate by the accumulation of nanoparticles due to the Marangoni effect of the colloidal solution. We previously proposed a similar but different method to make networks of thin r 2011 American Chemical Society
lines on the substrate using a layer of micrometer-size closepacked latex particles as the template.10 By dropping a small amount of colloidal solution of gold nanoparticles over the template, the gap between the latex particles and a substrate of glass plate was instantaneously filled with the solution. The solution was then slowly evaporated in a refrigerator of 4 °C. Beautiful straight lines were formed between the neighboring contact points of the latex particles on the substrate, in addition to circular lines around the contact points. A further study on this network was carried out by Vakarelski et al.,11 but the detailed mechanism for the reason why such straight lines are formed between the neighboring contact points has not yet been clarified. The above experimental results indicate that the following factors are necessary to make uniform networks on the substrate by evaporative lithography. (1) The colloidal particles must be highly hydrophilic and small enough, say less than 20 nm, because all the particles must migrate smoothly with the flow of medium throughout the evaporation process. (2) The colloidal solution must spread instantaneously into the gap between the template and the substrate, and the gap must be evenly filled with the solution in order to make a uniform network. (3) The medium of the colloidal solution must evaporate uniformly to the direction perpendicular to the substrate. If the medium evaporates parallel to the substrate, the network line will be broken at that point. The networks are certainly able to be made by the method of evaporative lithography, if the conditions described above are satisfied. Received: September 29, 2010 Revised: January 2, 2011 Published: January 25, 2011 2080
dx.doi.org/10.1021/la103902z | Langmuir 2011, 27, 2080–2083
Langmuir
Figure 1. Series of macroscopic observations from above, showing the drying process at room temperature. A drop of 35 μL aqueous solution of gold nanoparticles with a nominal diameter 10 nm, supplied by Nippon Paint Co., was dropped at the center of a 5 5 cm2 plain-fabric screen (wire diameter = 20 μm, 250 mesh) placed on a 5 5 cm2 float glass plate. The solution spread instantaneously as shown in (a).
However, the methods described above are not suitable to make the networks on a large scale, which is a requirement for real industrial applications. For example, in the case of the method proposed by Harris co-workers, it is quite difficult to make a nanoscale gap uniformly between the template and substrate on a large scale. In our case in which closed-packed latex particles were used as the template, a single layer of monodisperse latex particles is difficult to make on a large scale. These examples seem to indicate that evaporative lithography is inapplicable for the formation of industrial-scale networks. We have, however, succeeded in this present study to use the principle of evaporative lithography to make a flexible transparent conductive film on a large scale. This was achieved by using any type of nanoparticle and screens for screen printing as the template, which have a large variety of availability in the mesh, wire size, and thread.
2. EXPERIMENTAL SECTION In this study, we used a 5 5 cm2 plain-fabric screen of 420 mesh made of stainless steel wire of 20 μm in diameter as the template (Kansai Wire Netting) and a 5 5 cm2 float glass plate (Matsunami Glass) as the substrate. Both the surfaces of the wire and glass were cleaned and highly hydrophilized by the usual cleaning procedure of acetone (Wako, high purity), ethanol (Wako, high purity), and pure water, followed by plasma cleaning (PDC-32G, Harrick) in air for 2 min. The water used in this study was distilled and deionized to give a resistivity of 18.2 MΩ cm and a total organic content of
