Colloidal Germanium Nanoparticles
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The interest in semiconductor nanoparticles is primarily because of disparities in electronic properties from the bulk material they exhibit because of their extremely small size. However, the preparation method used and the ability to manipulate them make an enormous impact on the applications of semiconductor nanoparticles. In the case of Ge nanoparticles, interest in their preparation and properties traces back to the proposal by Canham in 1990 that the luminescence of porous Si was caused by quantum confinement (QC) of charge carriers. Because both bulk Si and Ge are diamond-lattice semiconductors with indirect band gaps in the near infrared, it seemed natural that the great interest in Si nanoparticles triggered by Canham's paper would cause a similar interest in Ge nanoparticles. This level of interest has not materialized because of the desire to link with the microelectronics community, which focuses so intensely on Si. Physically, the larger exciton Bohr radius of Ge of 11.5 nm, its higher dielectric constant, and its higher refractive index make Ge the material of choice for some applications. Unlike Si, very little work has been performed on nanoparticles derived from porous Ge or spark processed Ge. The purpose of this article is to discuss methods for preparation of Ge nanoparticles by colloidal chemistry; we will first discuss the initial methods and point out their strengths and weaknesses. A discussion of more recent preparation methods will follow. Finally, a comparison of the recent methods will point to some possible improvements in the colloidal preparation of Ge nanoparticles.