Sheryl Ehrman

Sometimes called the building blocks of nanotechnology, nanoparticles (particles with diameters less than 100nm) constitute a commercially important sector of the nanotechnology market. Unlike many speculative applications of nanotechnology, nanoparticles have been with us for a while. Nanoscale gold clusters have been used to color ancient glass as far back as Roman civilization.!!! More recently, in the twentieth century, carbon blacka material composed of nanoparticles of high-grade carbon "soot"was incorporated into tires, resulting in greatly improved durability. By the year 2000, carbon black for tires was a 6-million-tons-per-year global market.!!! Nanoparticles are useful when their properties at the nanoscale (mechanical, optical, magnetic, and so on) are different from those at the bulk in some beneficial way and also when their size enables interactions with biological systems.

An example nanoparticle is shown in Figure 13-1.

Figure 13-1. TiO2 particles (micron scale formed from nanoscale particles, used for clarity). (Courtesy of Altair Nanotechnologies, Inc.)

Figure 13-1. TiO2 particles (micron scale formed from nanoscale particles, used for clarity). (Courtesy of Altair Nanotechnologies, Inc.)

Applications of Nanoparticles

Remember white, pasty sunscreen? It is now transparent because the key ingredient, zinc oxide, which absorbs ultraviolet light, is transparent if it is made of nanoparticles but white when made of larger, micron-sized particles. The same holds for titanium dioxidewhich is a white paint pigment and an additive to improve opacity in colored paintsif the particles are hundreds of nanometers in diameter or greater. Now, these transparent nanoparticles, which still absorb in the UV range, are finding their way into modern, transparent, and highly effective sunscreens.!3!

Nanoparticles of titania are also a key ingredient in dye-sensitized solar cells. The efficiency of these cells is boosted by incorporating nanoparticles that increase the total surface area for harvesting light by a factor of a thousand as compared with a single crystal of titania.™ Compared with silicon-based photovoltaic materials, dye-sensitized solar cells are much less expensive to produce, and in 2001 a manufacturing plant opened in Australia.^

Another significant market for nanoparticles is in the semiconductor industry, in a process known as chemical mechanical planarization (CMP). In the manufacturing of computer chips, the component materials must be coated onto wafers, and at several points in the process, the coatings must be rendered nearly atomically smooth across the entire wafer. For 300-mm

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