Formation and characterization of germanium nanoparticles
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Elemental germanium was mechanically milled with magnesium oxide with the intention of forming disperse nanoparticulate germanium in a soluble matrix. The crystallite size was determined by x-ray diffraction (XRD) and Raman spectroscopy using a phonon confinement model. The crystallite size was found to decrease exponentially with milling time; however, the size determined by XRD was typically five to ten times greater than that by Raman. This was attributed to the presence of two separate crystallite sizes, which were averaged when using the Scherrer equation for the XRD data. Sonication of the powder resulted in the breakup of >20 m aggregates into individual particles of approximately 40 nm. These particles are thought to compose a single crystal core with a crystallite size of approximately 28 nm surrounded by a layer of smaller crystallites (approximately 5 nm), which showed quantization during Raman spectroscopy. Separation of the germanium from the magnesium oxide was readily achieved using a simple acid leach, although some oxidation of germanium was evident when using an aqueous leach.
I. INTRODUCTION
In recent years there has been growing interest in nanoparticles because of their unusual properties. Semiconductors have been widely studied in particular because their electronic properties become controlled by quantum mechanics when the particles are below the Bohr exciton radius. This phenomenon has been particularly well studied in quantum dots and wires, which can be controllably grown by techniques such as molecular beam epitaxy or metalorganic chemical vapor deposition. In most instances the materials studied are compounds (e.g., AlGaAs, GaN, CdS) rather than the elemental semiconductors (e.g., Si, Ge) because the technology is more mature and the compound semiconductors are typically more easily controlled by variation of the chemical conditions for reaction. Nanoparticulate elements can be formed by a variety of methods such as laser ablation,1 ion implantation,2 epitaxial growth,3 plasma deposition,4 and sol-gel growth.5 However, these techniques typically produce small quantities of free particles or larger amounts of particles immobilized in another medium, and scale-up to produce grams (or kilograms) of powder is economically unfeasible, thus restricting the use of the powders to high-value applications. Although silicon,6 –8 germanium,6 and selenium9 have all previously been milled, it has been with the intention of amorphizing the powder and not with the formation of a)
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J. Mater. Res., Vol. 15, No. 11, Nov 2000 Downloaded: 16 Mar 2015
nanoparticles. It has been shown that a range of nanocrystalline compound semiconductors can be formed during high-energy milling either by direct reaction between elements, e.g., GeS,10 BiSb,11 ZnS,12 or between elements and gases e.g., GaN,13 Si3N4.14 In many cases, the final product is undoubtedly nanocrystalline, but the particle size is typically somewhat larger.15,16 Annealing of
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