Computational prediction of stable semiconducting Zn-C binary compounds

Elemental carbon has been successfully used to tune the light emission properties of zinc oxide (ZnO) through artificially doping but the underlying mechanism remains controversial. At present, carbon-related defect complexes are the main explanation. Nevertheless, the possibility of forming semiconducting Zn-C compounds has not been discussed. In this study, we reveal the existence of various stable semiconducting Zn-C compounds. Based on particle swarm optimization and first-principles calculations, we perform a structural search of Zn-C binary compounds and report four stable semiconducting structures, in which the covalent Zn-C bonding characteristics are stronger compared with that in the metal rocksalt zinc carbide (ZnC). Crucially, three of the four Zn-C compounds have direct or quasi-direct band gaps in the range of 1.09-2.94 eV which are energies highly desirable for optoelectronic applications. Electronic transitions across the band gaps of these Zn-C structures could contribute to blue and near-infrared light emissions of C-doped ZnO. Our results have not only unraveled a new perspective to explain and tailor the light emission properties of ZnO but also provide a deeper understanding of possible Zn-C compounds.