First Principles Study of a New Large-Gap Nonoporous Silicon Crystal: Hex-Si40

We present an ab initio calculation of the structural and electronic properties of Si in a novel hexagonal, fourfold coordinated structure with 40 atoms per unit cell, obtained from the coalescence of small fullerenic cages. Hex-S${\mathrm{i}}_{40}$ has a tubular structure, inducing confined electronic states near the gap, which is widened by $\sim 0.4\mathrm{eV}$ with respect to normal silicon. This system is predicted to be a very interesting, possibly photoluminescent material for optoelectronics.