Strain anisotropy in freestanding germanium nanoparticles synthesized by ball milling.

Although embedded Ge nanocrystals (NCs) have been grown by variety of techniques and its properties have been studied extensively, intrinsic properties of isolated Ge NCs have not been studied properly due to lack of proper synthesis technique. Here we report on the synthesis of freestanding Ge nanoparticles (NPs) down to approximately 7 nm using ball milling technique and study its structural evolution as a function of milling time. Morphology and microstructure of the freestanding Ge NPs are studied using atomic force microscopy, transmission electron microscopy and X-ray diffraction (XRD) analysis. A systematic study of the XRD line profile using Williamson-Hall method reveals presence of anisotropic strain in the milled NPs. Strain anisotropy factor is calculated using a modified Williamson-Hall method by taking into consideration a dislocation contrast factor, assuming that dislocations are main contributors to the strain in these NPs. Our calculations suggest that screw type dislocations are main contributors to the strain anisotropy in the Ge NPs. We find that for milling time up to 40 hrs, NPs size monotonically goes down to approximately 7.3 nm and then almost saturates, while the dislocation density first increases from 1.64 x 10(16) m(-2) to 11.62 x 10(17) m(-2) for milling time up to 20 hrs and then decreases drastically during further milling. We have monitored a low temperature heat release at approximately 310 degrees C from the milled NPs using differential scanning calorimetry, clearly indicating a kind of structural relaxation of the strained NPs.