Electronic Nature Transition and Magnetism Creation in Vacancy-Defected Ti₂CO₂ MXene under Biaxial Strain: A DFTB + U Study

Abstract

The structural, electronic, and magnetic properties of vacancy defect in Ti 2 CO 2 MXene and the effect of strain have been investigated using the density functional tight-binding (DFTB) approach including spin-polarization with Hubbard onsite correction (DFTB + U). The band gap of pure Ti 2 CO 2 is ∼1.3 eV, which decreases to ∼0.4 and ∼1.1 eV in the case of C-and O-vacancies, respectively, i.e., the semiconducting behavior is retained. In contrast, Ti 2 CO 2 undergoes semiconductor-to-metal transition by the introduction of a single Ti-vacancy. This transition is the result of introduced localized states in the vicinity of the Fermi level by the vacancy. Both Ti-and O-vacancies have zero net magnetic moments. Interestingly, the nonmagnetic (NM) ground state of semiconducting Ti 2 CO 2 turns into a magnetic semiconductor by introducing a C-vacancy with a magnetization of ∼2 μ B /cell. Furthermore, we studied the effect of strain on the electronic structure and magnetic properties of Ti-, C-, and O-vacant Ti 2 CO 2 . The nature of the band gap in the presence of single O-vacancy remains indirect in both compression and tensile strain, and the size of the band gap decreases. Compression strain on Ti-vacant Ti 2 CO 2 changes metal into a direct semiconductor, and the metallic character remains under tensile biaxial strain. In opposition, a semiconductor-to-metal transition occurs by applying a compressive biaxial strain on C-vacant Ti 2 CO 2 . We also find that the magnetism is preserved under tensile strain and suppressed under compression strain on V C -Ti 2 CO 2 . Moreover, we show that double C-vacancies maintain magnetism. Our findings provide important characteristics for the application of the most frequent MXene material and should motivate further investigations because experimentally achieved MXenes always contain point defects.

Modeling size and edge functionalization of MXene-based quantum dots and their effect on electronic and magnetic properties

“…2), degeneracy for both HOMO/HOMO+1 and LUMO/LUMO−1 pairs is found in the case of non-hybrid functionals, while in the case of hybrid functionals, degeneracy is observed in the case of LUMO/LUMO−1. Our results, in line with the conclusion we made for Ti 2 CO 2 MXene, 7 suggest that the GGA and meta-GGA density functionals underestimate the energy gap and incorrectly describe the magnetism in MXQD. For all hybrid functionals, the spin density distribution (Fig.…”

“…8, where for QDs with lateral sizes above 1.5 nm (QD3 and QD4), only a weak polarization at the edge of the QD is observed, which is induced by the oxygen atoms at the edge. The edge effect of the oxygen atoms is not spread inside the quantum dot, and the non-magnetic behavior rather mimics the 2D material (2D Ti 2 CO 2 is non-magnetic 5,7 ). In contrast, for QDs with lower lateral dimensions up to 1.4 nm (QD1 and QD2), the edge-to-area ratio is higher and the edge-atom effect dominates, producing more spin-polarized segments that interact and can generate magnetic behavior inside the quantum dot.…”

“…We, therefore, decided to perform additional calculations on the Ti 2 CO 2 monolayer (using the periodic plane-wave VASP program). We reoptimized the geometrical structure from our previous studies 5,7 and tested the energy spectrum of the monolayer system. Besides GGA (PBE), meta-GGA (RevTPSS), and hybrid density functionals (HSE06 and B3LYP), we also used the many-body perturbation GW method.…”

“…, biosensors, batteries, adsorption, catalysis, energy storage, and environmental research). 2–4 Despite the appearance of some interesting phases (semiconductors, 5 excitonic insulators, 6 or antiferromagnets 7 ), unfortunately, most of the MXenes have exhibited metallic conductivity without an intrinsic band gap, which limits their applications in several fields ( e.g. , applications in laser diodes, light emitting diodes, and field-effect transistors).…”

“…1 They have a typical planar morphology with excellent structural stability, good electrical conductivity, a tunable surface, and other unique chemical properties, thus having a wide range of applications in various elds (e.g., biosensors, batteries, adsorption, catalysis, energy storage, and environmental research). [2][3][4] Despite the appearance of some interesting phases (semiconductors, 5 excitonic insulators, 6 or antiferromagnets 7 ), unfortunately, most of the MXenes have exhibited metallic conductivity without an intrinsic band gap, which limits their applications in several elds (e.g., applications in laser diodes, light emitting diodes, and eld-effect transistors). 8 Furthermore, MXenes exhibit a low photoluminescence (PL) response in aqueous solution and their use in biological and optical applications is signicantly limited.…”