Nanophase Composites Produced by Ion Implantation: Properties, Problems, and Potential
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Nanophase Composites Produced by Ion Implantation: Properties, Problems, and Potential A. Meldrum,1 L. A. Boatner,2 C. W. White,2 and R. F. Haglund, Jr.3 1 Department of Physics, University of Alberta, Edmonton, AB T6G 2J1 Canada 2 Solid State Division, Oak Ridge National Laboratory, Oak Ridge TN 37831 U.S.A. 3 Department of Physics and Astronomy, Vanderbilt University, Nashville TN 37235 U.S.A. ABSTRACT Ion implantation has become a versatile and powerful technique for synthesizing nanometer-scale clusters and crystals embedded in the near-surface region of a variety of hosts. The resulting nanocomposite materials often show unique optical, magnetic, and electronic properties. Here we review some of the principal features of this nanophase materials synthesis technique and discuss the outstanding experimental difficulties that currently hamper the development of devices based on the many unique properties of these nanocomposite materials. Possible solutions to these problems and future research directions are discussed. OVERVIEW Nanophase materials are frequently characterized by novel properties that can be significantly different from those of the corresponding bulk phase. As precipitated nanocrystals (NCs) are formed on ever decreasing length scales, the differences between the bulk and smallparticle properties become increasingly pronounced. These differences have stimulated a growing worldwide effort that cuts across many disciplines and research areas that emphasize the synthesis and characterization of an increasingly wide variety of nanocomposite materials. The practical motivation for this intense research effort derives both from the fundamental characteristics of small particles as well as the numerous potential applications of these materials, particularly in the areas of optical devices, micromechanical devices, and information storage [1]. The novel properties of nanophase particles are dominated by two major effects. These are, first, the increasing relative significance of the surface-energy contributions associated with the larger surface-to-volume ratio of small particles and, second, the unique characteristics of electrons in confined systems. The first effect largely determines the thermodynamic properties of the particles or the nanocomposite (e.g., melting points, solid phase transitions, bulk modulus). Both the surface properties and electron confinement combine to produce novel electronic properties that can be manifested in a wide range of effects, such as a large nonlinear optical susceptibility, intense photoluminescence, altered band structures, and superparamagnetism, to name but a few. Many experimental techniques have been developed for synthesizing various types of nanocomposite materials. Ion implantation was first used for this purpose in the 1970s to form Ag and Au nanocrystals embedded in silica glass [2]. At that time, however, there were no obvious applications for such nanocomposites, so it was not until the 1990s that ion implantation became an important and widely used resea
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