Tikalon Blog by Dev Gualtieri

Crystals without Facets

November 7, 2014

I was born and raised in Upstate New York, about fifteen miles from Herkimer, New York, a village with a population of less than 10,000 people. One summer, I tended to the transmitter of a small AM radio station there while its chief engineer was on vacation.

The Herkimer area is famous for Herkimer diamonds. These are large, clear crystals found in dolomite rock, and they aren't really diamonds. They're quartz, the crystalline form of silicon dioxide (silica, SiO₂), and they're impressive since they are large and double-terminated; that is, they exhibit facets all around.

Crystal facets of quartz.

Quartz grows as left- and right-handed crystals, as shown.

(Left image and right image by "strickja," via Via Wikimedia Commons, modified.)

Herkimer crystals were formed in nature by hydrothermal crystallization in which silica, dissolved in hot water, is transported to a seed crystal to allow the crystal to grow to a larger size. The principal use of quartz crystals for the last century was as the frequency-determining element in radio circuits, followed by its use in such things as digital watches and computers. Natural quartz was mined for this purpose until World War II made the usual supply sources inaccessible. industrial synthesis of quartz was developed at Bell Labs by A. C. Walker;[1] and by Ernie Buehler,[2] with whom I worked for several years.

Crystals form in regular shapes, since the attractive forces of their constituent atoms or molecules, as constrained by the lowest energy packing arrangement, allow just certain combinations. These shapes can be as simple as a cube, as found in table salt and iron crystals; or, much more complex, as shown in quartz crystals. Crystals demonstrate facets, since the rate of growth for the crystal is different for different crystal planes because of surface energy effects.

Synthetic Berlinite (AlPO₄), crystals grown by hydrothermal synthesis.[3] The small divisions on the ruler are millimeters.

This material is isostructural with quartz.

(Photo by the author, as uploaded to Wikimedia Commons.)

Facet-free crystals are occasionally found in nature, but not in laboratory synthesis. The usual product is either an amorphous film, or faceted crystallites. That's why it's significant that engineers and physicists from the University of Michigan (Ann Arbor, Michigan) have found a way to routinely produce rounded, facet-free, nanoscale crystals of boron subphthalocyanine chloride, an organic solar cell material.[4-7]

The process produces a crystalline layer with a lobe texture like that of starfish shells (see figure). Says Olga Shalev, a doctoral student in materials science and engineering at the University of Michigan and lead member of the research team, "We call them nanolobes. They look like little hot air balloons that are rising from the surface."[6]

Micrograph of the non-faceted texture of a boron subphthalocyanine chloride layer.

(Image: University of Michigan/Max Shtein.)

Echinoderms, such as starfish, display similar rounded structures on their bodies that act as lenses for their primitive eyes. As Max Shtein, a University of Michigan associate professor in several departments, including materials science and engineering, comments,

"In my years of working with these kinds of materials, I've never seen shapes that looked like these. They're reminiscent of what you get from biological processes... Nature can sometimes produce crystals that are smooth, but engineers haven't been able to do it reliably."[6]

Boron subphthalocyanine chloride, from which these layers are made, is a small molecule that makes either flat films or faceted crystals with sharp edges. Once again, serendipity advanced science when Shaurjo Biswas, a doctoral student at the University of Michigan in 2010, was using organic vapor jet printing to make solar cells from this material. He found that layers of thickness greater than about 600 nm exhibited a texture, so he decided to make thicker films. The nanolobe pattern emerged at a thickness of 800 nm.[6]

After a while, Shalev continued production and analysis of these layers, systematically varying the growth conditions in an improved apparatus. Eventually, others joined the research effort that culminated in a paper in a recent issue ofNature Communications.[6] Shtein had helped to develop the organic vapor jet printing process as a graduate student. It's essentially like spray painting, but with a gas instead of a liquid, and it has the advantage that the process doesn't require a vacuum.[4,6]

Portion of the organic vapor jet printing apparatus.

This photo conveys no useful information, but I always enjoy looking at laboratory apparatus.

(Still image from a YouTube video.)[7]

One possible application is textured surfaces with controlled wettability, technologies for which surface area films are desired, and nonreflective coatings.[4,6] This research was funded by the U.S. Department of Energy, Office of Basic Energy Sciences, the National Science Foundation, and the Air Force Office of Scientific Research.[6]

Crystals without Facets

References: