Abstract

Silicon (Si) products have been extensively investigated during the last 60 years and are currently the most important and dominant semiconductors used in communication transmission, microelectronics, and solar cells. With the development of nanotechnology, numerous low-dimensional Si nanomaterials, such as nanowires, nanobelts, and nanoparticles, have been synthesized in the last 20 years. ^1–4 Si nanomaterials possess unique physical and chemical properties, such as size confinement and high surface area, and are of growing interest to researchers. On the other hand, along with the research of Si products, the complex phase diagram of Si has also attracted intense research interest whether in theoretical or in experimental studies for more than 50 years, because of its key position for technological application and fundamental condensed matter physics study. ^5–8 Moreover, the phase transformation research of Si nanomaterials and unique phase synthesis of Si nanocrystals (NCs) have also been of high concern from 20 years ago, ^9–12 because of the unique Si nanomaterials, especially their special nanophase which may be applied in field-emission devices,^106,107 biomedical imaging, biosensors, and energy storage. ^5–7 For example, by using high-pressure X-ray diffraction (XRD) and resistivity measurements in a diamond anvil cell (DAC), or micro-Raman spectroscopy associate indentation experiments with precisely controlled strain rates, researchers have found that Si undergoes a series of phase transitions during compression. Si can be transformed from the diamond cubic structure (Si-I) at ambient condition to the metallic ?-Sn phase (Si-II) at about 12 GPa, then to Imma (Si-XI) at about 13 GPa, and to a primitive hexagonal structure (Si-V) at about 16G Pa. ¹⁰ Moreover, these phase transitions can be reversible upon decompression. Instead of a reversible transition to the original Si-I phase, the Si-II phase would transform to a semimetallic R8 (Si-XII) phase at approximately 9.3 GPa and further to a metastable body-centered BC8 (Si-III) phase when pressure is completely released. ¹³ However, it should be pointed out that the abundant and unexpected mestable phase of Si has not been explored to the end, just utilizing DAC or indentation experiment (including nanoindentation research) or pressure-induced transformation methods. ^14–17 Some of the predicted metastable phase—zincblende structure of Si—has not been achieved by the traditional high-pressure compression method, however, it may be trapped utilizing a far-from-thermodynamic equilibrium process. ¹⁸ In addition, the knowledge of the phase and stability of these metastable structures are therefore critical for developing practical applications.