Group 14 elements in the Cmcm phase with a direct band structure for photoelectric application

This work presents two novel group 14 element allotropes, oC24 Si-24 and oC24 Ge-24, and the physical properties of oC24 C-24, oC24 Si-24, and oC24 Ge-24 are studied in detail by means of first-principles calculations. The mechanical stability, dynamic stability, and thermodynamic stability of oC24 Si-24 and oC24 Ge-24 are proven by the elastic parameters, phonon spectrum, and relative enthalpy, respectively. According to the B/G value, all oC24 C-24, oC24 Si-24, and oC24 Ge-24 are brittle materials. By analysing the directional dependence of Young's modulus, it is clear that oC24 Si-24 possesses the largest mechanical anisotropy. From the electronic properties, it can be concluded that oC24 Si-24 is a semiconductor material with a direct band gap of 1.047 eV, which suggests that oC24 Si-24 may be suitable to make solar cells. More importantly, the light absorption ability of oC24 Si-24 is stronger than that of diamond Si in the visible light region. In addition, the kappa ( min ) of oC24 Si-24 and oC24 Ge-24 have better thermal conductivity, indicating that oC24 Si-24 and oC24 Ge-24 have great application potential in solar cells, optoelectronic devices and other microelectronic devices.