Compressive behavior of nanoscale silicon: amorphization vs crystalline phase transitions

Silicon (Si) nanostructures, especially Si nanowire arrays, have been suggested as promising, high-performance, and scalable thermoelectric materials. Knowledge of the mechanical properties of various silicon nanostructures is essential for their use in thermoelectric and other applications. Silicon nanowires and nanocrystalline powders have been investigated for their mechanical behaviors and structural transitions under stress by various groups, yielding contradictory results. Stress-induced conversion of diamond-Si (DS) nanocrystals embedded in porous Si (π-Si) to a high-density amorphous (HDA) form that transforms to a low-density amorphous (LDA) form upon decompression has been reported. By contrast, crystal-crystal transition of nanoscale DS under stress has also been reported. Here, we review the available data on the compressive behaviors of Si nanostructures and present in situ high-pressure Raman and angle-dispersive synchrotron powder x-ray diffraction (XRD) data on nanocrystalline silicon. The data show crystal-crystal transitions in silicon nanocrystals with sizes down to ∼4 nm.