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Materials and Nanotechnology www.mrs.org/publications/bulletin MATERIAL MATTERS for designer chemicals. This is nanoscale science because the energetics at the inter- Materials and face of these zeolitic materials and the molecules within them, which either guides their synthesis or guides the reac- tivity of the molecules, enables a new set Nanotechnology of technologies. The ability to cleverly design and constitute materials is the Alexandra Navrotsky modern equivalent of a Maxwell demon— a creature that can select molecules at will. We tell molecules where to go and how to self-assemble. This is very critical to the The following is an edited transcript of a talk end product is essentially an equilibrium- nano revolution. presented during the Franklin Institute phase assemblage; then, in the nonclassi- Nanomaterials beg an exact definition. Awards program in the symposium on cal way, the preparative pathway still has A nanomaterial is something that has a “Materials Science and the Future of to be energetically possible, but the prod- nanoscale dimension. Scientists who have Nanotechnology,” co-hosted by Drexel uct we obtain is a kinetic result of the worked with colloid chemistry and sur- University on April 25, 2002. pathway and not an equilibrium material. face science for 60 years say, “What’s new This liberation from the tyranny of equi- about that? This is all just hype.” I think I would like to address three issues in librium has led, in part, to the nanotech- what is new, from a fundamental point of order of increasing generality. I first want nology revolution. The fact that we can view, is the realization that the basic ques- to comment on structural chemistry, now make inorganic materials, as well as tions of structure, of energetics, of proper- energetics, and materials in a world that inorganic–organic composite materials, by ties, are common whether we are talking now includes a much greater awareness processes such as layer-by-layer deposi- about a semiconductor, a carbon nano- of phenomena at the nanoscale. This also tion, chemical vapor deposition, and laser tube, a superconductor, a mineral, or an includes a more general definition, or a ablation, means that the material we make air-pollution particle. We might define a more general applicability, of nanomate- is tailored by the process, just as a bio- nanomaterial most broadly as one that has rials, not just to conventional nanotech- chemical reaction is tailored by the avail- dimensions larger than that of a molecular nology, but to other areas where the able enzymes and reagents. One does not cluster but smaller than that of a bulk same sort of understanding is essential. generally have to worry that we as organ- material, but more importantly has an The second is to talk about the impact of isms are metastable with respect to carbon interesting property that is different from this work and this sort of thinking on dioxide and water. So, the richness of either. So, in a sense, if it quacks like a education, particularly at the graduate materials we can make depends on having nanomaterial, it is a nanomaterial. If we and postdoctoral level. How do we best control at the molecular level of the have quantum confinement, a change in take advantage of changing needs to real- process that makes those materials. band structure, and a change in absorp- ly get people thinking properly? The A number of accidents, or perhaps fun- tion frequency, we are dealing with phe- third is a more general potpourri of soci- damentals, of nature make this possible. nomena on the nanoscale. If we have a etal issues that are with us whether or not The first is that in the solid state, many suppressed melting point, we are dealing we have nanomaterials. These issues are, different structures are in fact very close with nanoscale phenomena, and we seek perhaps, brought to the fore by this latest to each other in energy, sometimes with- to harness those phenomena for the appli- industrial revolution. in the available thermal energy. A good cation in question. It is no different from example is zeolites with their framework harnessing phenomena at the macroscop- Nanoscale Science structures. They are nanomaterials in the ic scale, except that we now have an Let us begin with the science. I am a sense that they have immensely high sur- added richness. solid-state chemist and thermodynamicist face areas of hundreds to thousands of From the point of view of earth science, by training and practice. We are interested square meters per gram. Those surfaces which is one of the hats I wear, nature has in the structure of the solid state and the are primarily internal surfaces, which are been practicing nanotechnology probably energies that hold molecules or atoms structurally controlled. That topology, since the beginning of the universe. together in solids. I sometimes say I count which is created by controlled synthesis, Within condensation from the solar nebu- calories for a living, for the energetics of often using organic structure-directing la, the initial particles that were formed materials offers tremendous insight, partic- agents, provides materials that, for exam- were probably not well-crystallized, huge, ularly in a systematic way, into the bond- ple, are used for the cracking of petrole- single crystals. They were likely little, ing of atoms and molecules. The energies um or the synthesis of fine chemicals. So, poorly crystalline, dust-grain nanoparti- of phase transitions and chemical reactions we have the challenge on the one hand of cles that eventually grew, nucleated, con- whisper of the interatomic forces and bond making zeolites cheaply for these mass densed, came under pressure, and started energies that put atoms and molecules applications and on the other hand of the evolution of planets. together. The entropies of materials sing of making small amounts of very special- The surface chemistry of a planet such lattice vibrations, of magnetics, of electron- ized materials, that is, designer zeolites as Earth involves nanoscale processes. The ic transitions, of order-disorder. These weathering of rocks is corrosion at the items, put together, determine the sorts of nanoscale. The geochemical cycle then turns materials that we can have and do have, Material Matters is a forum for these weathered rocks into soil, into dust both in classical and nonclassical ways. An expressing personal points of particles, and into sediments. Eventually, example in the classical equilibrium way is view on issues of interest to the the sediments coalesce again and become represented by “heat it and beat it” metal- materials community. rocks. That cycle involves primarily the lurgy and ceramic science, in which the reaction of particles at the nanoscale. If one 92 MRS BULLETIN/FEBRUARY 2003 MATERIAL MATTERS thinks of soil science and agriculture, a soil slightly metastable—silica zeolites. It is is a most complex nanomaterial: mixed By emphasizing the that richness we are exploiting. organic, inorganic, and biological materials of different sizes. Certainly, much of the technology, we have neither Education transportation of nutrients, pollutants, identified nor resolved the The second part of my talk deals with organics, and heavy metals occurs at the underlying social issues. education, particularly graduate and nanoscale, that is, takes place on or around postdoctoral education in the United small particles. So, as earth environmental States. What does one need to succeed in scientists, we are very interested in coated the structure of the ceramic may be this field? What cultural patterns deter- nanoparticles such as iron oxide nuclei with heterogeneous on the scale of a nanome- mine our careers? Nanoscience and nano- a bunch of harmful organics around them. ter, are continuum models. What do these technology are just part of our changing The question is, will this go downriver until bulk descriptions mean and, in particular, world, and I do not think “nano” can be it gets into my water supply? what do they mean when the particle or blamed or praised for all of these As atmospheric scientists, we are inter- the material deviates from the behavior of changes. What we certainly still need in ested in nanoparticles. The nucleation of its macroscopic counterpart—that is, graduate education is the fundamentals. I clouds is a nanoscale phenomenon: ice when it shows true nanoscale behavior? decry people who call themselves droplets, sulfuric acid droplets, cloud seed- To what extent are the bulk macroscopic chemists and physicists, and who sort of ing. Thus, nucleation science is really mean-field theories applicable when the skate on the surface of science, without nanoscale science. Any initial solid-state or order we are observing is that of a few really having grasped the fundamentals. condensed-phase reaction starts some- unit cells of the material? If such models While there are more papers being writ- where, and a few atoms do something. In are not applicable, what is? Certainly ab ten and published, there are more bad that sense, the start of an earthquake, initio quantum calculations are applicable, papers. I think we are having great diffi- which may affect hundreds of kilometers, but even with our most powerful comput- culty sorting the wheat from the chaff occurs somewhere with the breaking of a ers doing these on a scale accurate enough simply because there is so much of it. So few chemical bonds. The origin of life and for complex nanomaterials containing we need to teach our students the funda- biochemical reactions have probably been many different kinds of atoms, including mentals and encourage them in critical mediated by mineral surfaces and, very heavy atoms, such computations are still a thinking. We certainly need to train them likely, by mineral surfaces of small grains.
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