Narrowing sputtered nanoparticle size distributions
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A. S. Edelstein Materials Science and Technology Division, Naval Research Laboratory, Washington, DC 20375-5000 (Received 28 October 1992; accepted 30 December 1992)
By adjusting the sputtering rate and gas pressure, it is possible to form nanoparticles of different sizes, phases, and materials. We have investigated the spatial distribution of sputtered particle formation using a vertical, linear arrangement of substrates. Collecting the particles soon after they are formed, before they have time to grow and agglomerate, allows one to obtain a narrow size distribution. In the case of molybdenum, a narrow distribution of cubic particles is formed at relatively large distances (8 cm) from the source. These cubic particles collide and self-assemble in the vapor into arrays of larger cubic particles. The particle size histograms are fitted to lognormal distribution functions. How supersaturation occurs is discussed qualitatively as a function of the distance from the substrate, sputtering rate, and the mean free path in the vapor. This method of nanocrystalline particle formation has potential use in magnetic and opto-electronic (quantum dot) applications, where a narrow size distribution is required.
I. INTRODUCTION Recently, there has been a growing interest1 in producing clusters and nanocrystalline material for potential catalytic,2 magnetic,3 and opto-electronic4 applications. For some applications, a narrow size distribution is required to obtain a uniform material response. For example, such a narrow distribution is necessary in order that the energy levels in the particles are shifted by approximately the same amount through quantum confinement. The usual synthesis techniques5 for producing nanoparticles include chemical techniques such as chemical precipitation6 and sol-gel processing,7 and physical techniques such as condensation from the vapor and high-energy milling8 processes. Vapor deposition of particles has been achieved by gas evaporation,9 laser ablation,10 and sputtering.11 Condensation using gas evaporation has been studied for over thirty years, but has been largely limited to resistive evaporation of metals. Sputtering is a more versatile procedure that can be applied to the deposition of metals, alloys, semiconductors, and ceramics. We examined the formation mechanisms of sputtered material and its effect on the growth of particles in the vapor. In this article, we describe a method, using sputtering, for producing nanoscale Mo particles with a narrow size distribution. J. Mater. Res., Vol. 8, No. 5, May 1993
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There have been several previous studies11 14 of particle formation in the vapor of normally bcc elements. In the case of high-pressure sputtering of Mo, evidence was found11 for the self-arrangement of small (4.8 nm) Mo cubes with a bcc structure into larger (17.5 nm) cubes. The formation of these self-arranged cubes is strongly dependent on the sputtering gas pressure. At pressures of 100 mTorr and below, only film formation occurs
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