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) Ti2C 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 Ti2C exhibits metallic behaviour, which is retained even after surface functionalization with halogen atoms. The cohesive energy (EC) calculations confirm the thermodynamic stability of all studied systems, as evidenced by their negative EC. Among them, I-functionalized Ti2C shows the highest stability with an EC of approximately −6.05 eV. Halogenation of Ti2C significantly modifies its optical response, lowering the static refractive index from about 6–7 in pristine Ti2C 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 Ti2C (∼20) becoming broadened and termination-dependent, indicating altered interband transitions and optical loss. Surface functionalization of Ti2C 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 × 1012 Wm−1K−2s−1 due to an optimized Seebeck coefficient (-1.29 ×10−4 V/K) and a significantly suppressed thermal conductivity of 0.19 × 1015 Wm−1K−1s−1. Conversely, iodine termination induces a metallic state with an exceptional electrical conductivity of 5.12 × 1020 ohm−1m−1s−1 at 300 K, demonstrating the power of surface chemistry in tailoring transport channels for targeted TE applications.