Silicon has several technologically promising allotropes that are formed via high-pressure synthesis. One of these phases (hd) has been predicted to have a direct band gap under tensile strain, whereas other (r8 and bc8) phases are predicted to have narrow band gaps and good absorption across the solar spectrum. Pure volumes of these phases cannot be made using conventional nanowire growth techniques. In this work, Si nanowires were compressed up to ∼20 GPa and then decompressed using a diamond anvil cell in the temperature range of 25–165 °C. It was found that at intermediate temperatures, near-phase-pure bc8-Si nanowires were produced, whereas amorphous Si (a-Si) dominated at lower temperatures, and a direct transformation to the diamond cubic phase (dc-Si) occurred at higher temperatures under compression. Thus this study has opened up a new pressure–temperature pathway for the synthesis of novel Si nanowires consisting of designed phase components with transformative properties.

Larissa Q. Huston, Alois Lugstein, Guoyin Shen, David A. Cullen, Bianca Haberl, Jim S. Williams, and Jodie E. Bradby, Nano Letters Article ASAP, Synthesis of Novel Phases in Si Nanowires Using Diamond Anvil Cells at High Pressures and Temperatures, DOI: 10.1021/acs.nanolett.0c04354 abstract

TEM micrographs and inset SAEDs of SiNWs decompressed at different temperatures. Dark-field images of nanowires compressed at (a) 25 °C that contain dc-Si, (b) 70 °C that contain dc-Si, and (c) 105 °C that contain bc8-Si. (d) Bright-field image of nanowires compressed at 165 °C containing dc-Si. Note that all SAEDs have a width of 1.5 Å–1.