Green on the Industrial Scale

Molecular Layer Deposition of Polymers – George, Yoon & Dameron [4]

Many exotic materials or materials with special properties are processed using extreme temperature and pressure often with toxic starting materials. In semiconductors, molecular beam epitaxy (MBE) to build single crystal structures and sputtering are common methods of physical deposition to deposit thin films. Both are done using a very high vacuum. MBE heats the atomic materials until they sublimate and land on the desired surface. Sputtering uses a gas plasma to knock a few atoms of material off a “target” and onto the desired surface. There are also different chemical deposition processes including electroplating which uses metal salts dissolved in a solution bath, chemical vapor deposition (CVD) which uses high vacuum, and atomic layer deposition (ALD) which is similar to CVD but uses two half-reactions of gas phase precursors

Limitations imposed by extreme temperature, extreme pressure, and toxic materials combined with a typically slow deposition rate make it is difficult to economically run these processes on an industrial scale for high volume manufacturing. But what if there was a process that produced extremely uniform materials economically at room temperature and standard (normal) pressure? And used aqueous solutions with low toxicity while running at industrial scale?

According to Dr. Ben Wang, one of the founders, Savya Nanotechnologies has created exactly that. It was my pleasure to host Ben last week to present “Nanostructured Thin Films via Layer-by-Layer Assembly at the Industrial Scale” at the IEEE San Francisco Bay Area Nanotechnology Council monthly event. Svaya has moved layer-by-layer (LBL) assembly from “molecular beaker epitaxy” to an unprecedented scale. As he describes it, molecular beaker epitaxy is simply a graduate student with a steady hand dipping substrates into beakers in an attempt to uniformly coat them. A slightly more advanced process is to use an automated system which dips the substrates into glass beakers. What Svaya has achieved is building high volume equipment including a roll-to-roll system that can process forty-eight inch wide films at fifteen meters per minute (approximately ten inches per second).

Instead of dip coating the substrates, Svaya has implemented spray coating to reduce the volume of liquid required and to decrease the drying time between layers. In layer-by-layer assembly multiple layers, typically with a washing step between each layer, are deposited on the substrate to “build up” the finished coating. Each layer has a uniform thickness due to the molecular layer deposition [1,2,3] (MLD) process which is based upon self-limiting surface reactions. In order to bond (cross-link) to the surface, each molecule of reactant needs a site. If there is no site to bond, the extra fluid is washed away leaving an extremely uniform layer one molecule thick. The illustration above shows a simplified version of MLD using two homobifunctional reactants. Multiple reactants, assuming they will cross-link, may be used and as many layers as required can be applied.

Ben listed many application examples of their MLD technology including several which were “green applications” in themselves: water filtration films, heat reflecting optical nano films, and biocompatible coatings. A car windshield film to reduce the solar heating of a car’s interior, reducing the energy required to cool the car, is a specific application he discussed in detail. This type of film is a distributed Bragg reflector where the thickness and refractive index of multiple layers are designed to determine which frequencies are reflected and which frequencies are passed. As such, they have designed this film to block (reflect) the infrared spectrum but allow visible light to pass through the film. Their film is designed to replace the usual film that is laminated in the center of the windshield to make safety glass. This is certainly a high volume and cost sensitive application which has a green footprint.

In addition to films on flat surfaces, Ben discussed the coating of irregular surfaces. Examples include the coating of nylon fibers, textured silicon solar cells, and concave or convex surfaces. Since the MLD process is self-limiting the layer follows the contour of the surface and the thickness is uniform everywhere. An additional unique application of the technology is the creation of pigments by crushing the nano film. The desired color is selected by the design of the Bragg reflector layers of the film. Then when the film is removed from the substrate and crushed, it leaves the structure of the layers intact – just in much smaller particles than a continuous film – which can then be used as pigments creating unique colors.

Although Ben said that Svaya is building a “printer and ink” business model typical of product companies, it appears as though Svaya is more of a technology company.* As a technology company, the key to growth is to find as many applications as possible to use that technology. They have already started seeding those via their Technology Access Program by providing spray based automation equipment to researchers in the MLD field. (Probably earning the gratitude of many graduate students relieved of at least this drudgery.) The applications already identified, the beneficial economics of high volume, and the attractive environmental profile of their process should position Svaya for significant growth and profitability. 

 

  

* A good topic for a future blog post. See this blog post by Binnur Al-Kazily for an introduction of the difference between technology and product management.

References:

1. Molecular Layer Deposition Links on Svaya website.

2. S.M. George Research Group, University of Colorado at Boulder, “Molecular Layer Deposition of Polymers“.

3. ibid. “Introduction to Atomic Layer Deposition & Molecular Layer Deposition“.

4.  “Surface Chemistry for Molecular Layer Deposition of Organic and Hybrid Organic−Inorganic Polymers“, Steven M. George, Byunghoon Yoon, and Arrelaine A. Dameron Accounts of Chemical Research 2009 42 (4), 498-508. Image above reprinted with permission. Copyright 2009 American Chemical Society.

 

 

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