Halogenated Ti2C MXene: A promising multifunctional 2D material for optoelectronic and thermoelectric applications — Predictions from DFT

Two-dimensional (2D) MXenes have attracted significant attention owing to their remarkable electronic, optical, and thermoelectric (TE) characteristics, making them promising candidates for advanced multifunctional devices. In this work, the structural stability and physical properties of pristine and halogen-functionalized (-F, -Cl, -Br, and -I) Ti 2 C MXenes were systematically investigated using first-principles based density functional theory (DFT) calculations. The electronic band structure (EBS) and projected density of states (PDOS) analyses reveal that pristine Ti 2 C exhibits metallic behaviour, which is retained even after surface functionalization with halogen atoms. The cohesive energy (E C ) calculations confirm the thermodynamic stability of all studied systems, as evidenced by their negative E C . Among them, I-functionalized Ti 2 C shows the highest stability with an E C of approximately −6.05 eV. Halogenation of Ti 2 C significantly modifies its optical response, lowering the static refractive index from about 6–7 in pristine Ti 2 C to roughly 2–4 and shifting the main optical features into the 1.5–3.5 eV range. The extinction coefficient also changes, with the strong low-energy peak of pristine Ti 2 C (∼20) becoming broadened and termination-dependent, indicating altered interband transitions and optical loss. Surface functionalization of Ti 2 C effectively modulates their TE properties, inducing a carrier type transition from n-type (pristine and F-terminated) to p-type (Cl and Br-terminated). Notably, fluorine functionalization yields the highest room-temperature power factor of 1.93 × 10 12 Wm −1 K −2 s −1 due to an optimized Seebeck coefficient (-1.29 ×10 −4 V/K) and a significantly suppressed thermal conductivity of 0.19 × 10 15 Wm −1 K −1 s −1 . Conversely, iodine termination induces a metallic state with an exceptional electrical conductivity of 5.12 × 10 20 ohm −1 m −1 s −1 at 300 K, demonstrating the power of surface chemistry in tailoring transport channels for targeted TE applications.