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possess different properties. Semiconductor nanoclusters, which emit light whose wavelength depends on cluster size, offer the possibility of tailoring material properties to suit a particular need. Even mature techniques, such as those for bulk crystal growth, demand continuous improvements in process control to produce the size or quality of material required for either technological applications or fundamental studies.

Better understanding of the mechanisms at play in materials that have been known for decades can lead to new approaches that alleviate detrimental properties. An excellent example is the introduction of metallic oxide electrodes in ferroelectric devices, which reduces aging effects dramatically. Better understanding of the details of materials preparation can give rise to improvements in processing. Improved insight into the kinetics of epitaxial growth can dramati-

BOX 2.1 Additions to the Zoo: New Materials and Structures of the Past Fifteen Years

There have been far too many new developments in the past 15 years or so to document them all in detail, but all these developments have been made possible by advances in two intertwined areas: complexity and processing. Many of the new materials and structures are dramatically more complex, compositionally or structurally, than have been studied previously. In general, this trend has required advances in processing to allow control of the increased complexity. In other cases, the final product may not be much more complex than other well-known materials or structures, but the processing itself may need to be altered to achieve more control over the growth process in order to obtain the new material.

Advances giving rise to new materials and structures fall into three categories. Some involve the synthesis of an entirely new compound or material. The advance may have been revolutionary, meaning that the properties of the new material (or in some cases its existence) could not have been predicted. In other cases, advances in processing have allowed fabrication of new or modified materials or structures whose properties were suspected before the material was actually made. This may allow a well-known compound to be remade in a new form with different properties. Third, well-known materials are sometimes found to exhibit new (in some cases unexpected) properties that appear when the ability to process them is improved. The new property may be found in a known material simply by looking at it in a new light, which shines on it as a result of insight gained from another materials system.

The materials advances listed in Table 2.1.1 were driven by different motivations. Many addressed a technological need, such as the need to transfer or store information. Others were driven by scientific curiosity. Although the driver can be clearly identified in each case, the two sets are not mutually exclusive. Many discoveries that result from pure scientific curiosity ultimately find their way into products. For example, low-temperature superconductors are now used in magnets for magnetic resonance imaging. Other discoveries, though originally motivated by a technological need, give rise to very beautiful and fundamental insights.

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