Mechanisms of Controlled Growth Of Metallic Nanocrystals
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Mechanisms of Controlled Growth Of Metallic Nanocrystals
Victor F Puntes, Zoltan Konya, Can Erdonmez*, J. Zhu, Gabor A. Somorjai and A. Paul Alivisatos College of Chemistry & *Dept. of Matl. Sci., U.C. Berkeley, 94720 USA
ABSTRACT Spherical and cubic Pt nanoparticles and spherical, cubic and disk-shaped Co nanocrystals are produced by reduction of Pt2+-ion and decomposition of organometallic reagents in a hot solvent in presence of the surfactant molecules respectively. This displays the ability to control crystal growth and produce a range of nanoparticle shapes in the case of two different metallic elements. INTRODUCTION Continuous interest in size and shape control at the nanometer scale in the synthesis of inorganic nanocrystals has yielded significant advances [1]. A key point driving the research is that at the nanometer scale, physical, chemical and structural properties of the solid state may be modified and controlled [2]. For example, the stable phase of Co at room temperature for small particles (< 10-20 nm) is fcc while for larger ones, it is hcp, due to the slightly larger surface energy of the hexagonal structure [3]. Similarly, as the size of a semiconductor nanocrystal approaches the molecular scale, the energy level spacing and the density of the states become discrete while the melting temperature follows a 1/r relationship [4]. Two promising approaches to size and shape control are the SLS method in Chemical Vapor Deposition and the growth of colloidal particles in solution. In this paper, we focus on wet chemistry routes, which possess the natural advantage of being simple, inexpensive and versatile. These methods consist of the production of a supersaturated solution of atoms, which are highly insoluble and rapidly collapse into nuclei and grow into particles. By attaching selected molecules during the process to the surface of the nanocrystals, stable nanoparticles are formed, which can be highly monodisperse and with shapes far from equilibrium shapes. The atoms are produced in solution by reducing salts or by thermally decomposing organometallic precursors of the desired metal. In both cases, similar results have been achieved. For example, similar Co nanoparticles may be produced by reduction of CoCl 2 [5] or Co 2 (CO) 8 decomposition [6]. Wet chemistry approaches can be subdivided into two categories, depending on if the particles are produced in polar solvents (like water) or non-polar solvents (like toluene). Due to their low reactivity, more noble metals like Au can be easily prepared in reactive media. Meanwhile more reactive metals like Cd and Se, have to be synthesized in low reactive (low polar) solvents, and highly reactive metals like Co, Fe and Ni have to be synthesized in non-reactive, non-polar solvents as toluene, dichlorobenzene or dioctylether.
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