Less is more: A holey grail of materials science
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Less is more: A holey grail of materials science
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e have known for a long time that it is possible to make something out of the right kind of nothing. I should note right off the bat that I am not discussing nothingness in the sense of the recent Isaac Asimov Memorial Debate on “The Existence of Nothing”1 or the street-fight surrounding that debate.2 I will leave that for another day along with discussions of the Casimer effect,3 other vacuum engineering effects, and effects that might lead to discussions of less than nothingness.4 Instead, I am discussing the nothingness (from the atomic scale to the macroscale) that vacancies,5 clusters of vacancies, voids, porosity, structured holes, aerogels,6 and nanofoams7 are made of that are more familiar to materials researchers. We know that gases, liquids, and solids are mostly empty space—atoms floating in nothing. Additionally, solid crystals contain defects. Statistical mechanics informs us that, at temperatures above absolute zero, crystalline lattices will deviate from perfection and contain vacancies and interstitial atoms.8 Other space-emptying (holey) defects include di-vacancies, vacancies next to impurity atoms, vacancy clusters, voids, and so on. Existence of these defects in materials and devices can alter their performance, reduce their reliability, and shorten their lifetime. However, defects including vacancies and other forms of nothingness can have beneficial properties. Some vacancies are known to produce color centers in materials, including the alkali halides.9 The simplest color centers are electrons trapped at a vacancy site, and their properties can be modeled reasonably well using a particle-in-a-box approach. The changing color of glass (glass bottles, panes in stained-glass windows) over many years of exposure to UV light from solar exposure is attributed to the formation of color centers. Color centers have been used for broadly tunable laser media, radiation dosimeters, and have been proposed for high-density memory devices. More recently, the nitrogen-vacancy center in diamond is being explored as a useful medium for quantum information applications.10 Given the almost metaphysical sense in which quantum information is viewed by many these days, I am tempted to relate this to the deus ex machina, but I do not want to offend anyone’s beliefs. At a much more macroscopic scale, porosity11 plays an important role in the properties of a wide variety of materials, including concrete, brick, wood, and other structural materials, sieves and membranes, filters, paper, fabrics, and various other materials. The integration of controlled quantities of nothing at random into these materials can optimize their utility, while too much of nothing can cause them to fail (concrete, brick, wood) and too little of nothing can limit their performance (sieves, membranes, filters). Photonic bandgap structures (PBGs)12 are based upon geometric volumes of optical materials into which the controlled integration of nothing alters the optical frequency response of th
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