Vibrational properties of Ti3C2 and Ti3C2T2 (T = O, F, OH) monosheets by first-principles calculations: a comparative study

Abstract

We present a comparative study on the static and dynamical properties of bare Ti₃C₂ and T-terminated Ti₃C₂T₂ (T = O, F, OH) monosheets using density functional theory calculations. First, the crystal structures are optimized to be of trigonal configurations (Pm1), which are thermodynamically and dynamically stable. It is demonstrated that the terminations modulate the crystal structures through valence electron density redistribution of the atoms, particularly surface Ti (Ti2) in the monosheets. Second, lattice dynamical properties including phonon dispersion and partial density of states (PDOS) are investigated. Phonon PDOS analysis shows a clear collaborative feature in the vibrations, reflecting the covalent nature of corresponding bonds in the monosheets. In the bare Ti₃C₂ monosheet, there is a phonon band gap between 400 and 500 cm⁻¹, while it disappears in Ti₃C₂O₂ and Ti₃C₂(OH)₂ as the vibrations associated with the terminal atoms (O and OH) bridge the gap. Third, both Raman (E_g and A_1g) and infrared-active (E_u and A_2u) vibrational modes are predicted and conclusively assigned. A comparative study indicates that the terminal atoms remarkably influence the vibrational frequencies. Generally, the terminal atoms weaken the vibrations in which surface Ti atoms are involved while strengthening the out-of-plane vibration of C atoms. Temperature-dependent micro Raman measurements agree with the theoretical prediction if the complexity in the experimentally obtained lamellae for the Raman study is taken into account.