Single-crystal synthesis and properties of the open-framework allotrope Si 24

Si is a new, open-framework silicon allotrope that is metastable at ambient conditions. Unlike diamond cubic silicon, which is an indirect-gap semiconductor, Si has a quasidirect gap near 1.4 eV, presenting new opportunities for optoelectronic and solar energy conversion devices. Previous studies indicate that Na can diffuse from micron-sized grains of a high-pressure Na Si precursor to create Si powders at ambient conditions. Remarkably, we demonstrate here that Na remains highly mobile within large (~100 µm) Na Si single crystals. Na readily diffuses out of Na Si crystals under vacuum with gentle heating (10 mbar at 125 °C) and can be further reacted with iodine to produce large Si crystals that are 99.9985 at% silicon, as measured by wavelength-dispersive x-ray spectroscopy. Si crystals display a sharp, direct optical absorption edge at 1.51(1) eV with an absorption coefficient near the band edge that is demonstrably greater than diamond cubic silicon. Temperature-dependent electrical transport measurements confirm the removal of Na from metallic Na Si to render single-crystalline semiconducting samples of Si . These optical and electrical measurements provide insights into key parameters such as the electron donor impurity level from residual Na, reduced electron mass, and electron relaxation time. Effective Na removal on bulk length scales and the high absorption coefficient of single-crystal Si indicate promise for use of this material in bulk and thin film forms with potential applications in optoelectronic technologies.