We report the modification of the band gap in the Sc2CF2 MXene monolayer through C atom substitution by Si, Ge, Sn, F, S, N, B, and B + N.

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The electronic structures of Si and Ge substitutionally doped ScC(OH) MXene monolayers are investigated in density functional theory. The doped systems exhibit band inversion, and are found to be topological invariants in Z theory. The inclusion of spin orbit coupling results in band gap openings. Our results point out that the Si and Ge doped ScC(OH) MXene monolayers are topological insulators. The band inversion is observed to have a new mechanism that involves s and pd states.

We suggest an MXene-based magnetic tunnel junction (MTJ) design. The device characteristics of the MTJ were investigated by nonequilibrium Green’s function formalism within the density functional theory. Inspired by the first synthesized magnetic MAX crystal of Mn2GaC, its two-dimensional (2D) counterpart of the half-metallic Mn2CF2 MXene layer was selected as the magnetic electrode. The tunneling barrier was chosen as Ti2CO2 MXene, which is one of the most studied MXenes in experimental and theoretical works. It is beneficial that both the electrodes and the tunneling barrier are 2D materials from the same material family and have similar structures. The common device problem of lattice mismatch does not occur in our MTJ design because the lattice parameters are compatible. In addition, the band gap of Ti2CO2 tunneling barrier is almost the same as the half-metallic gap of Mn2CF2 electrodes. Both the barrier and the electrodes have a common C layer that contributes the most to the transmission. Our MTJ design consists of structurally and electronically well-matched components. We find that the tunneling magnetoresistance ratio has a peak value of ≈106 and stays higher than ≈103 under the bias voltages up to 1 V. Since the applied bias voltages are within the energy gap of the tunneling barrier, the half-metallic character of the conduction is preserved up to 1 V. The tunneling-based transmission was observed in all of the three devices with varying tunneling barrier widths, and the current decreases with increasing width. The MXene-based MTJ has promising device characteristics.

The electric field- and charge transfer-induced topological properties of Sc2C(OH)2 MXene layers are reported. The system transforms from a trivial insulator to a topological insulator and to a metal. The s-pd orbitals are exchanged during the topological phase transition. The modification of the bandgap along with the transition to the topological phase reveals that Sc2C(OH)2 MXene is a multifunctional material. The nearly free states, which form ideal conduction channels, are responsible for the topological electronic band structures.