Stable Nanotubular Crystal of Silicon: A Predicted Allotrope With Direct   Band Gap
  On basis of the first principle calculation we show that a crystalline structure of silicon, as a novel allotrope with nanotubular holes along two perpendicular directions, is stable. The calculations on geometrical and electronic properties reveal that this allotrope possesses a direct band gap wider by 0.5eV than the indirect one of silicon with diamond structure. The crystal belongs to I41/AMD space group, showing anisotropic optical properties and Young modulus. The bulk modulus is 64.4GPa and the density is 1.9g/cm$^{3}$, lower than that of the diamond silicon due to the presence of nanotubular holes. It is hopeful that the allotrope may widely expand applications of silicon in many fields due to its direct band gap and specific nanotubular structure. 
Chi-Pui Tang, Jie Cao, and Shi-Jie Xiong
cond-mat.mtrl-sci