Sometimes it can be hard to really conceptualize the breadth of research work that’s coming out of our contamination-controlled environments. In Class 100/1000 cleanrooms from coast to coast, unless you work directly in the field, it is a challenge to keep up with the multitude of advances in technology and process. With that said, if we take a moment to focus and really look closely, we can normally distinguish a pattern to the work being done in different industry silos. Take aeronautics, optics, or automotive, for instance: these silos are ones in which advances in materials technology continue to play an enormous role and unless we’re talking about some top secret, ‘eyes only’ military project, usually the results of the research are pretty easy to see. Unless, of course, we’re talking about new work that’s coming out of Brookhaven Lab’s Center for Functional Nanomaterials where cleanroom-based researchers are ahead of the field in the formulation of invisible glass.
Invisible glass? If you are as confused as we initially were, read on!
Composed of a ‘former’ (most commonly silicon dioxide from sand), soda ash which acts as a flux to lower the melting temperature, and a stabilizer (commonly calcium oxide in the form of limestone) that makes it water insoluble, glass is an ancient material forged in flame. According to Roman historian Plinny, Phoenician merchants were the first to make glass in Syria around 5000BC, although archeological evidence in Eastern Mesopotamia and Egypt suggest the date to be closer to 3500BC. By the time of the Crusades, glass making had developed as an industry in Italy with centers in Venice and later Murano – two areas still famed for their glass production – and 1674 saw the first lead glass being created by English glass-smith George Ravencroft. With the advent of the pressing machine came cylinder glass, float glass, and rolled plate, and by the middle of the twentieth century glass science had become an academic pursuit worthy of a research center, established by the Ford Motor Co. And, although we take it for granted most of the time, glass is actually a remarkably interesting material. Forged in fire from dry materials, it initially becomes viscous before being cooled sufficiently rapidly that an irregular crystalline structure is formed. In fact, according to the Corning Museum of Glass the cooling atoms of what will become a piece of glass ‘become locked in a disordered state like a liquid before they can form into the perfect crystal arrangement of a solid. Being neither a liquid nor a solid, but sharing the qualities of both, glass is its own state of matter.’(1)
And when it comes to this material, one-type-fits-all is no longer the norm. All forms of glass share a unique chemistry that allows it to be non-toxic, transparent but still impermeable, and almost inert, but some formulations are also anti-microbial, soluble, or even flexible. And in part due to these new formulations, even with its long history glass is very much a material of the modern era. From bio-medical implants and dentistry to water filtration to a source medium for laser technologies, and even for use in the disposal of radioactive waste materials, glass lends itself to myriad applications.
But there are problems that, stemming from its inherent physical properties, have seemed intractable. Until now. Glass has, for instance, an ability to reflect and to refract light, which can be a major problem. Why? Let’s take a look.
As we know from high school physics classes, visible light travels in waves of a continuous range of frequencies. When a wave comes into contact with an object, one of two things can happen depending on how closely the natural frequencies of the light wave and the object match: it can be absorbed; or it can be reflected. In the first instance, the wave has a matching frequency to the electrons of the object it strikes and so they absorb its energy and translate it into vibrational motion. In other words, it becomes heat energy. In the second scenario, the frequency of the light wave is incompatible with that of the object it connects with and, instead of absorbing the energy, the electrons re-emit the energy as a light wave. In other words, the light bounces off and continues its journey.
So what are the practical implications of this? Light that bounces off a surface can be a nuisance as anyone squinting at a screen while trying to take a smartphone selfie on a sunny day can attest. With that glare it’s impossible to really capture the exquisitely mesmerizing beauty of your subject – am I right? But joking aside, the impacts of reflected light are more serious in applications such as aerospace technology, solar cells, or medical device manufacturing. As an article in Physics.org suggests: reducing the amount of power lost to reflection could ‘enhance the energy-conversion efficiency of solar cells by minimizing the amount of sunlight lost to refection. It could also be a promising alternative to the damage-prone antireflective coatings conventionally used in lasers that emit powerful pulses of light, such as those applied to the manufacture of medical devices and aerospace components.’(2)
But don’t we already have the technology to reduce reflection and refraction? To a certain extent, yes we do. But there’s plenty of room for improvement. Current solutions to the glare problem of modern glass screens, for instance, center on the use of anti-reflective coatings. Coatings such as the ones developed by premier glass manufacturer Corning Inc., based in Corning, NY, work by dramatically reducing the ‘intensity and magnitude of light reflecting off a device.’(3) Conventional coatings, however, tend to reduce the brightness of the screen making images dull and text harder to read. Corning’s proprietary formula, however, allows a coating of thin inorganic film applied to the screen surface to manipulate incoming light waves by causing them to cancel (‘interfere with’) each other out, allowing the screen colors to remain true and the text as legible as before.
But could there be an even more effective way to minimize reflection? If we return to the work being done in Brookhaven, perhaps so. The Center for Functional Nanomaterials (CFN) is a project of the U.S. Department of Energy Office of Science and researchers there are working on changing the refractive index of light as it crosses from one material to another. The refractive index is the amount by which a light wave must bend as it moves between – say – air and glass and depending upon how great that index is, a certain amount of the light is lost to reflection. But by etching features that look like a forest of cones onto the glass on the nano scale, the refractive index is lowered from that of air to that of glass.
Does this interfere with the transparency of the material? Not at all. The etchings, which allow the material to be anti-reflective over the full visible spectrum and in the near-infrared, are visible only through an electron microscope, with the cones standing a full 170-nanometers high and 52 nanometers apart. For an idea of scale, 300 nanometers is equivalent to 0.3 microns (μm), where 75μm is the average diameter of a human hair in cross section. At this size and with this configuration, around 900 billion cones will co-exist on a 2-inch piece of glass.(4)
And, tiny as they might be, the effect of these etchings is startling. In essence, in changing the geometry of the surface from a smooth plane to a ‘forest’ of cones, the reflectivity of the material’s surface is cut down to such an extent as to render a piece of glass ‘invisible.’ In addition to its efficacy, this technology also has the great advantage of being a much more durable solution than a simple coating would be. Geometric etchings cannot abrade or be corroded, they don’t shrink or peel, and they are a part of the object rather than an addition. And when used, for instance, in solar cells, on aircraft, on satellites or spacecraft this enhanced durability could be a major selling point. Indeed it is the flexibility of this application that has won it the Grand Prize for the 2018 Create the Future Design Contest, a program of the magazine NASA Tech Briefs with a mandate to ‘stimulate and reward engineering innovation.’(5) From over 800 applications spanning 60 different countries, Brookhaven’s ‘Invisible Glass’ swooped in to claim the $20,000 purse for being a technology with ‘the potential to impact several areas, including electronics, energy, medicine, and aerospace [and to] improve users’ experience using smartphones, computers, and other electronic displays, […] enhance the energy-conversion efficiency of solar cells by minimizing the amount of sunlight lost to reflection […and] replace the antireflective coatings that are conventionally put into high-intensity lasers, such as those used to manufacture medical devices and aerospace components [which…] often get damaged when exposed to powerful laser light.’(6)
It almost goes without saying that this prize, and indeed this level of innovation, would not have been possible without the use of a sophisticated cleanroom environment in which to develop the components and capabilities to create these nanofabricated etchings.
The CFN is a user-focused center that offers researchers a 5,000 sq. ft. Class 100/1000 (ISO Class 5 and Class 6) cleanroom with equipment dedicated to leading edge patterning and processing of devices, thin films, and nanomaterials. According to the Center’s website, the cleanroom instrumentation offers the ability to pattern materials across a wide spectrum of sizes – from 10 nanometers to 10 millimeters and is used to ‘fabricate devise for nanoelectronics, nanophotonics, biomedical engineering, photovoltaics, x-ray optics, nanomagnetics, and beyond.’(7) Moreover, on-site laboratories allow for the synthesis of both organic and inorganic nanomaterials, and the facility incorporates a cluster of electron microscopes to offer not only scanning transmission (STEM), but also transmission (TEM), and scanning tunneling electron microscopy (STM). Optical spectroscopy is also offered for studying materials in single-molecule and confocal methods using ultrafast and nonlinear spectroscopy.
That’s quite the lineup for an academic facility so perhaps it should come as no surprise that Brookhaven is also associated with the U. S. Department of Energy in the context of national security. Where Corning may be developing consumer-focused applications, it is unsurprising that Brookhaven is approaching the question from a different angle. But another interest that both organizations must share is in the cleaning of their products. Whether it’s the surface of a space-age anti-reflective coating or that of a forest of nano-textured cones, we do have to wonder whether the tried and trusted methods of cleaning will be applicable? Will either surface be chemically compatible with IPA or other solvents? And given the coniferous, etched surface of the Brookhaven material, will new techniques for ensuring complete contamination control need to be crafted? For a non-smooth surface, it may well be that new protocols for accessing those 170-nanometer high crevices between the cones will need to be devised, and indeed perhaps new materials formulated to address a different materials sensitivity.
At this stage, we simply do not know and it would be pure speculation to guess, but we do know that the issue is very much in the pipeline and we are excited to see the lines of future development for these intriguing products. Furthermore, whichever direction the research ultimately takes, we can be absolutely certain of one thing: when Brookhaven, Corning, and other institutions and corporations innovate in this area, we too will be ready with our own contamination-control innovations to continue in the vital role of serving their needs.
Intrigued at the research coming out of our nation’s cleanrooms? Would you like to know more about how different industries are using contamination-controlled environments? Let us know your thoughts!