Fermiology of two-dimensional titanium carbide and nitride MXenes

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.

Tuning the Electronic and Magnetic Properties of Double Transition Metal (MCrCT2, M = Ti, Mo) Janus MXenes for Enhanced Spintronics and Nanoelectronics

“…Here, τ 0 is the relaxation time, and d is the interlayer spacing. For all calculations, we are assuming τ 0 to be 1 fs as it gives the value of conductivity close to experimental reports . The values of d taken from experiments are 7.7 Å for Ti 2 CT z and 9.16 Å for Mo 2 CT z .…”

“…Within the constant relaxation time approximation, the sheet conductivity of a 2D material can be calculated with the knowledge of the average Fermi velocity and Fermi surface area. The detailed methodology is presented by M. Bagheri et al in their Supporting Information. The final expression for sheet conductivity of MXene is given by ( σ 2 normalD τ 0 ) = e 2 2 π 2 ℏ false⟨ v F false⟩ 2 l F where ℏ is the reduced Planck’s constant, l F is the Fermi surface length ( l F = a normalF b z and b z = 2 π L z : a F , Fermi surface area and L z , length of unit cell in the z -direction), and ⟨ v F ⟩ is the average Fermi velocity.…”

“…In other words, Figure 4 confirms that the magnetic moment in the Cr atoms is mainly restricted. conductivity from first-principles calculations 41 by including Fermi velocity. Using this method, we calculate the film conductivity of the Janus MXenes investigated in this report.…”

Highly Selective H2S Gas Sensor Based on Ti3C2Tx MXene–Organic Composites

“…Mechanism 2 can dominate over mechanism 1 in the case of H 2 S and NH 3 for which the adsorption energy is comparable or higher than that of H 2 O. In order to estimate how the charge transfer would affect resistance, instead of ballistic conductance calculations, , we here rely on ref where it was computationally shown that when O content of Ti 3 C 2 T x surfaces is more than 50%, an increasing electron concentration (positive charge transfer from the analyte) leads to increasing conductivity and thereby a negative sensor response, as is the case with H 2 S.…”

“…Mechanism 2 can dominate over mechanism 1 in the case of H 2 S and NH 3 for which the adsorption energy is comparable or higher than that of H 2 O. In order to estimate how the charge transfer would affect resistance, instead of ballistic conductance calculations, 57,58 we here rely on ref 59 where it was computationally shown that when O content of Ti 3 C 2 T x surfaces is more than 50%, an increasing electron concentration (positive charge transfer from the analyte) leads to increasing conductivity and thereby a negative sensor response, 59 as is the case with H 2 S. The response of NH 3 is more complicated, which makes it difficult to draw firm conclusions based on the calculations. Because NH 3 captures H + and 0.29e from MXenes, and H atoms in the OH group contains about 0.8e, 0.51e remains in the MXene and the response should be in the same direction as with H 2 S, in contrast to the experiments.…”