Carbon vacancies in Ti₂CT₂ MXenes: defects or a new opportunity?

Carbon vacancies are commonly present in two-dimensional (2D) MXenes that hold promise in a variety of applications whereas their behavior remains unknown. Here we report on the influence of carbon vacancies on the structural stability, electronic properties and stiffness of MXenes by taking Ti₂CT₂ (T = O, F, and OH) as an example. According to the first-principles calculations, the formation energies of carbon vacancies in MXenes are lower than those in other typical 2D materials including graphene and MoS₂, in combination with high migration energies. These two features mean that carbon-vacant MXenes are thermodynamically and dynamically stable as further evidenced by the absence of structural reconstruction both in the ground state and at ambient temperature. Interestingly, carbon vacancies that are usually considered as defects substantially offer a new opportunity on at least two aspects: enhanced electronic conduction and reduced stiffness corresponding to improved flexibility. The localized states in the vicinity of the Fermi level introduced by carbon vacancies account for the prominent metallic characteristics in carbon-vacant Ti₂CT₂ MXenes.