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Silver Nanoparticle Ink for Transparent Conductive Coatings

By Will Soutter

Cima NanoTech has developed a unique method for coating a transparent mesh of conductive silver nanoparticles, which could revolutionize touchscreen displays, solar cells, and a whole host of other technologies.

Cima's "Self-Assembling Nanoparticle Technology" (SANTE) is a nanoparticle ink, which self-assembles into a fine, random mesh when it dries. This material is ideal for transparent, highly conductive coatings for a huge variety of applications.

The key to the SANTE Coating is Cima's proprietary nanoparticle emulsion formulation, which is protected by around 111 patents. It ensures that the lines of silver nanoparticles which make up the mesh are narrow enough to make the coating transparent, but connect with each other enough to maintain a good conductive path from edge to edge.

SANTE® films are currently being commercialized for use as a replacement for indium tin oxide (ITO) in touch-enabled displays, as well as in EMI shielding and transparent heating, and is being developed for other hi-tech applications like Bluetooth antennae.

Several other nanomaterials have been hailed as replacements for the ubiquitous ITO - including graphene, carbon nanotubes, and silver or copper nanowires. Cima's silver nanoparticle inks have a distinct advantage over these material - because of the nature of the nanoparticle mesh, they have much lower sheet resistances than other materials can attain, even over large areas.

CEO of Cima NanoTech, Jon Brodd, explains the benefit of this unique property in everyday use:

"Large-format touch screens are really suffering from a lack of good conductive materials right now. Small-screened smartphones have spoiled us, as we are used to basically instant response from our touch-screen devices.

"Larger touch monitors and 'all-in-one' devices then appear to be really slow to the user, because the lower conductivity across the larger area drags response times down. That's where SANTE technology really shines - we have really low sheet resistances, and it's very scalable to large areas. The high-transparency films we launched in May have resistances of 25 Ohms/m2, compared to 150 for ITO."

These attractive electrical properties are minimally affected when the coating is formed - making flexible devices and unusually shaped touch-enabled objects feasible.

"Partly because of the mesh structure, and partly because the nanoparticle is lightly sintered together, the five micron-wide lines in the mesh have a sponge-like structure, which you can see with a SEM.

"That allows it to be stretched and flexed, and even thermoformed. We were able to take, say, half of a golfball, and thermoform the coating to coat that half-dome shape - and not lose the electrical property."

SANTE Coatings also have excellent optical properties - "transparent" ceramics like ITO and AZO (aluminium zinc oxide) are not perfectly transparent, and tend to lend a little of their own colour to the display.

"Because it's a mesh-like structure, there's virtually no color. We're not tampering with the light waves - we're just blocking 5% of them. Also, we eliminate the Moiré effect that you can get from lithographic meshes, because our mesh is random - it has no regular geometry at all."

Touchscreens aren't the only application to benefit from lower sheet resistances and better transparency. These impressive numbers will also help convince manufacturers of EMI shielding, flat panel OLED-based lighting and of course solar cells.

"We have been working with some partners in China and other places, to integrate our technology with conventional silicon solar cells - we have a small focus team working on scaling that up at the moment. There's going to be some really exciting developments in the next year or two in that area when those products start coming out."

Another major benefit of the SANTE Nanoparticle Emulsion over ceramics like ITO and AZO is the relatively simple, low-temperature coating process that uses conventional optical film coating lines.

The process works almost like printing a newspaper, and Cima have done significant development work to make the unusual nanoparticle inks compatible with conventional roll-to-roll coating.

This makes the manufacturing process very affordable, with very low capital investment required.

This is a major bottleneck for development of graphene for applications like this - manufacturing graphene layers at a high enough quality, and transferring them between substrates, is a notoriously tricky process which is hard to scale.

The process isn't limited to silver nanoparticles either - the same principle works to make a whole range of nanomaterials coatable. Cima have proven the process with several of their own materials, as well as those provided by partners.

"It's just a very, very nice way of getting very small amounts of materials down into a usable three-dimensional structure.

"Transparent conductors are a very nice place to start because of the market demand, but we're not limited to silver, and we're not limited to transparent applications.

"At the lab scale, we've done the same thing with carbon nanotubes, graphene, carbon black, gold - lots of different materials."

Cima NanoTech is well-placed to drive their innovations to market - many of the management team were previously at legendary technology company 3M - even Dr Geoff Nicholson, inventor of the Post-It note, is on the board of directors, along with the former CTO of 3M Japan, and senior IP attorneys.

Jon Brodd himself led a 3M team developing multilayer nano-scale optical films, at a time when nanotechnology wasn't quite such a commercial buzz-word.

"We just never really thought about the fact that we were working at the nanoscale - we called it multi-layered optical film. We were extruding polymers into the 15 or 20 nanometer size range, and then controlling it. It was way ahead of its time - in fact, 3M has recently rebranded that product line as 'nano'!

"I think there are probably some really good engineering firms out there that have been working on that scale for quite some time. I think in the early 2000's, there was just too much buzz around the word - I've never considered it an industry, I just see it as a measurement.

"But as you well know, it's a measurement that can create a lot of really exciting features and materials. We really feel lucky at this stage to be able to hold our heads up and say that our products are truly nano-enabled."

Cima NanoTech seems well-placed to take advantage of the coming boom in ITO-replacement technologies - with a solid product and plentiful business expertise, it will be very interesting to see where the company can take their technology in the next few years.

"I think the coatings industry is getting really exciting, the more people learn about it. The more experts that really are, fundamentally, coating experts, the more interesting it gets. We're really just scratching the surface in terms of where the future can lead for emulsion technology."

 

Date Added: Jul 30, 2013 | Updated: Aug 2, 2013
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