In this study, with the aid of Raman measurements we observe transformations in small (~ 3 nm and ~ 10 nm) free-standing Ge nanoparticles under laser light exposure. The nanoparticles were obtained by chemical stain etching of a monocrystalline Ge wafer and of Ge powder, and by colloidal synthesis route. We found that transformation path depends on laser power and exposure time. At relatively low values of the laser power (2 mW) over the period of 100 minutes, the Raman signal indicates transformation of the sample from a nanocrystaline to bulk-like state, followed by partial oxidation and finally conversion of entire sample into alpha-quartz type GeO2. However, when the laser power is set at 60 mW, we observed 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 chemical stain etching method for 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 Ge nanoparticles prepared by colloidal synthesis results in that the explosive crystallisation is common for H-terminated and partially disordered Ge nanoparticles regardless of its particle size. We suggest possible bio-medical applications for the observed phenomena.
Available in Journal of Applied Physics
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