  We use Raman spectroscopy in tandem with transmission electron microscopy and
DFT simulations to show that extreme (GPa) pressure converts the phase of
silicon nanowires from cubic (Si-I) to hexagonal (Si-IV) while preserving the
nanowire's cylindrical morphology. In situ Raman scattering of the TO mode
demonstrates the high-pressure Si-I to Si-II phase transition near 9 GPa. Raman
signal of the TO phonon shows a decrease in intensity in the range 9 to 14 GPa.
Then, at 17 GPa, it is no longer detectable, indicating a second phase change
(Si-II to Si-V) in the 14 to 17 GPa range. Recovery of exotic phases in
individual silicon nanowires from diamond anvil cell experiments reaching 17
GPa is also shown. Raman measurements indicate Si-IV as the dominant phase in
pressurized nanowires after decompression. Transmission electron microscopy and
electron diffraction confirm crystalline Si-IV domains in individual nanowires.
Computational electromagnetic simulations suggest that heating from the Raman
laser probe is negligible and that near-hydrostatic pressure is the primary
driving force for the formation of hexagonal silicon nanowires.
