First-principles investigation of native point defects in two-dimensional Ti3C2

Two-dimensional transition metal carbides or nitrides (MXenes) have attracted much research interest and have shown very promising applications. Recently, the Ti vacancies in Ti3C2 were observed in an experiment (Sang et al., 2016) and demonstrated an important influence on the electronic properties of Ti3C2. Based on the first-principles method, this study systemically investigated these issues and found that the formation energy of a single Ti or C vacancy comes from two competing factors. One is the dangling bonds introduced by the vacancies, leading to an increase in the total energy. The other is the splitting of the original degenerate states after the lattice deformation around the vacancies, leading to a decrease in the total energy. Different from a previous study, this report found that both the surface and inner Ti atoms see a C-3v crystal field. The bonding states among Ti 3d orbitals with the same symmetry form a network, which does not break down even around the Ti vacancy, making the system metallic. The diffusion of a single Ti vacancy from the surface Ti layer to the inner Ti layer is more difficult than the path return, whether or not surface functional groups exist. By deriving the relationship between the equilibrium concentration of the vacancy clusters and the temperature, this study found that the formation of vacancy clusters is possible, although their concentration is much lower than that of a single vacancy. The results of this report will enhance the understanding of the relationship between the electronic properties and the native point defects of Ti3C2.