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Laser-induced particle size tuning and structural transformations in germanium nanoparticles prepared by stain etching and colloidal synthesis route
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See supplementary material http://dx.doi.org/10.1063/1.4939066
for a TEM measurement of free-standing Ge nanoparticles, table of the values of the size of Ge nanoparticles found by the RFC model upon the exposure of a He-Ne laser light (laser power = 2mW), comparison of Raman shift from a reference bulk Ge powder and that from 40 min exposed Ge nanoparticles by the laser light (laser power = 2mW), Raman spectra of Ge nanoparticles exposed to a He-Ne laser light with an excitation wavelength of 633 nm (laser power = 2mW) from 40 min to 100 min, schematic of the growth of free-standing Ge nanoparticles upon exposure of a 2 mW He-Ne laser, schematic representation of energy states referring to crystalline and amorphous state of germanium and two videos (a) and (b) on the explosive crystallisation effect.[Supplementary Material]
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In this study, with the aid of Raman measurements, we have observed transformations in small (∼3 nm and ∼10 nm) free-standing Genanoparticles under laser light exposure. The nanoparticles were obtained by the chemical stain etching of a monocrystalline Ge wafer and of Gepowder and by colloidal synthesis route. We found that the transformation path depends on laser power and exposure time. At relatively low values of the laser power (2 mW) over a period of 100 min, the Raman signal indicates transformation of the sample from a nanocrystaline to bulk-like state, followed by partial oxidation and finally a conversion of the entire sample into alpha-quartz type GeO2. However, when the laser power is set at 60 mW, we observed a heat release during an explosive crystallization of the nanocrystalline material into bulk Ge without noticeable signs of oxidation. Together with the transmission electron microscopy measurements, these results suggest that the chemical stain etching method for the preparation of porous Ge may not be a top-down process as has been widely considered, but a bottom up one. Systematic studies of the laser exposure on Genanoparticles prepared by colloidal synthesis results in the fact that the explosive crystallisation is common for H-terminated and partially disordered Genanoparticles regardless of its particle size. We suggest possible bio-medical applications for the observed phenomena.
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