Fermiology of two-dimensional titanium carbide and nitride MXenes

TL;DR

This study investigates the Fermi-surface characteristics of Ti-based MXenes with various surface functional groups, revealing how surface chemistry influences electronic properties and conductivity, validated by theoretical calculations and experimental data.

Contribution

It introduces a method to approximate complex mixed-surface MXene band structures with pseudohydrogenated models, enabling detailed Fermi-surface analysis.

Findings

Fermi-surface properties vary with surface functionalization.

Pseudohydrogenated models closely match experimental ARPES results.

Electrical conductivity can be estimated from Fermi-surface data.

Abstract

MXenes are a family two-dimensional transition metal carbide and nitride materials, which often exhibit very good metallic conductivity and are thus of great interest for applications in, e.g., flexible electronics, electrocatalysis, and electromagnetic interference shielding. However, surprisingly little is known about the fermiology of MXenes, i.e, the shape and size of their Fermi-surfaces, and its effect on the material properties. One reason for this may be that MXene surfaces are almost always covered by a mixture of functional groups, and studying Fermi-surfaces of disordered systems is cumbersome. Here, we study fermiology of four common Ti-based MXenes as a function of the surface functional group composition. We first calculate the effective band structures of systems with explicit mixed surfaces and observe gradual evolution in the filling of the Ti-d band and resulting shift of Fermi-level. We then demonstrate that these band structures can be closely approximated by using pseudohydrogenated surfaces, and also compare favorably to the experimental angle-resolved photoemission spectroscopy results. By modifying the pseudohydrogen charge we then proceed to plot Fermi-surfaces for all systems and extract their properties, such as the Fermi-surface area and average Fermi-velocity. These are in turn used to evaluate the electrical conductivity with the relaxation time fitted to experimentally measured conductivities.