First-principles investigation of elastic and electronic properties of double transition metal carbide MXenes

We use first-principles-based density functional theory (DFT) calculations to investigate the structural, elastic, and electronic properties of various pristine and oxygen (O)-functionalized double transition metal (DTM) MXenes with general formulas of M-2 ' M '' C-2 and M-2 ' M '' C2O2, where M ' = Mo, Cr and M '' = Ti, V, Nb, Ta. The dynamic stability of the DTM MXenes are assessed and elastic stiffness constants (C-ij) are used to investigate the mechanical stability and properties of the compositions. The calculated elastic properties of the pristine Mo-based MXenes are found to be superior compared to Cr-based compounds. Furthermore, the O-functionalized MXenes exhibit improved in-plane elastic constants, Young's moduli, and shear moduli compared to their pristine counterpart. We observe that the hybridization of the energy states results in stronger covalent interactions as such increased elastic properties for the M-2 ' M '' C2O2 MXenes. Ashby plot clearly demonstrates superior materials properties of O-functionalized Mo-based DTM MXenes compared to other commonly known two-dimensional materials. All the MXenes exhibit metallic character evident from the density of states (DOS) calculations. Additionally, the work functions are studied and the calculated values are higher in the case of O-functionalized MXenes. Overall, this work will be a guide for future investigations on the mechanical properties of DTM MXenes for their targeted applications in structural nanocomposites.