Single-crystal synthesis and properties of the open-framework allotrope Si₂₄

Si-24 is a new, open-framework silicon allotrope that is metastable at ambient conditions. Unlike diamond cubic silicon, which is an indirect-gap semiconductor, Si-24 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 Na4Si24 precursor to create Si-24 powders at ambient conditions. Remarkably, we demonstrate here that Na remains highly mobile within large (similar to 100 mu m) Na4Si24 single crystals. Na readily diffuses out of Na4Si24 crystals under vacuum with gentle heating (10(-4) mbar at 125 degrees C) and can be further reacted with iodine to produce large Si-24 crystals that are 99.9985 at% silicon, as measured by wavelength-dispersive x-ray spectroscopy. Si-24 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(4)Si(24)to render single-crystalline semiconducting samples of Si-24. 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-24 indicate promise for use of this material in bulk and thin film forms with potential applications in optoelectronic technologies.