Hybrid Density Functional Study of Structural and Electronic Properties of Functionalized \ce{Ti_{n+1}X_n} (X= C, N) monolayers

TL;DR

This study uses density functional theory to analyze the structural and electronic properties of functionalized Ti-based MXene monolayers, revealing their metallic or semiconducting nature and how properties vary with thickness and surface groups.

Contribution

It provides a comprehensive computational analysis of Ti_{n+1}X_n MXenes with various surface terminations, highlighting differences in electronic behavior and the impact of layer thickness.

Findings

Most MXenes are metallic, except Ti2CO2 which is semiconducting.

Thicker MXenes have higher density of states at the Fermi level.

Carbides and nitrides exhibit different behaviors with the same functional groups.

Abstract

Density functional theory simulations with conventional (PBE) and hybrid (HSE06) functionals were performed to investigate the structural and electronic properties of MXene monolayers, \ce{Ti_{n+1}C_n} and \ce{Ti_{n+1}N_n} ($n$ = 1--9) with surfaces terminated by O, F, H, and OH groups. We find that PBE and HSE06 give similar results. Without functional groups, MXenes have magnetically ordered ground states. All the studied materials are metallic except for \ce{Ti_{2}CO_{2}}, which we predict to be semiconducting. The calculated density of states at the Fermi level of the thicker MXenes ($n$ $\geqslant$ 5) is much higher than for thin MXenes, indicating that properties such as electronic conductivity and surface chemistry will be different. In general, the carbides and nitrides behave differently with the same functional groups.