This work demonstrates that it is possible to synthesize crystallized Ge nanostructures directly in an aqueous medium under ambient conditions by using widely available GeO2 (in the form of germanate ions) as a precursor. The reaction of germanate ions with NaBH4 in an aqueous medium resulted in highly hydrogenated Ge that could be transformed into crystallized Ge after an air-drying treatment. The NaBH4/GeO2 molar ratio, reaction time and drying temperature were optimized for the synthesis of crystallized Ge products. Furthermore, the reaction time has an influence on the size and shape of the final crystallized Ge products. A reaction time of 12 h could result in crystallized Ge powder samples that contain ultra-small (5-20 nm) particles and larger (50-100 nm) particles. By controlling the reaction time to 24 h, a Ge powder product consisting of worm-like crystallized Ge nanostructures with diameters of 10-80 nm and lengths up to 1000 nm was obtained. The possible reaction and growth mechanisms involved in this method were investigated. This new synthetic route may be a good candidate for synthesizing a wide variety of crystallized Ge nanomaterials and devices due to its low cost, low safety risk, facileness, high yield (above 70% and in gram scale) and convenience for adding other chemicals (i.e. dopants or morphology modifying agents) into the reaction system.

Porous germanium films with good adhesion to the substrate were produced by annealing GeO 2 ceramic films in H 2 atmosphere. The reduction of GeO 2 started at the top of a film and resulted in a Ge layer with a highly porous surface. TEM and Raman measurements reveal small Ge crystallites at the top layer and a higher degree of crystallinity at the bottom part of the Ge film; visible photoluminescence was detected from the small crystallites. Porous Ge films exhibit high density of holes (10 20 cm −3 ) and a maximum of Hall mobility at ∼225 K. Their p-type conductivity is dominated by the defect scattering mechanism.

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