Recovery of Si-IV nanowires from extreme GPa pressure

TL;DR

This study demonstrates that extreme GPa pressure can transform silicon nanowires from cubic to hexagonal phase while maintaining their shape, using Raman spectroscopy, electron microscopy, and simulations to analyze the phase transitions and recovery process.

Contribution

It provides the first detailed experimental and computational evidence of Si-IV phase recovery in silicon nanowires subjected to high pressure and decompression.

Findings

Silicon nanowires undergo phase transitions at specific GPa pressures.

Si-IV phase can be recovered after decompression from 17 GPa.

High-pressure conditions favor formation of hexagonal silicon nanowires.

Abstract

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.