Enhanced and Tunable Superconductivity Enabled by Mechanically Stable Halogen-Functionalized Mo2C MXenes

TL;DR

This study targets practical 2D superconductivity in MXenes by halogen functionalization of Mo2YX2 (Y=C,N; X=F,Cl,Br,I). Using first-principles DFT/DFPT, Br- and I-functionalized Mo2C MXenes are found to be dynamically stable with strong electron–phonon coupling, yielding $T_c$ values of $13.1$ K and $18.1$ K, respectively, significantly higher than pristine Mo2C ($T_c$ ≈ $7.2$ K). Superconductivity is highly tunable via carrier doping and biaxial strain, with electron doping pushing $T_c$ above 21 K for both Br- and I-functionalized systems. Overall, halogen-functionalized Mo2C MXenes emerge as mechanically robust, tunable, phonon-mediated 2D superconductors, offering a design principle for MXene-based superconducting materials.

Abstract

We present a comprehensive first-principles investigation of the structural, electronic, vibrational, and superconducting properties of halogen-functionalized Mo2YX2 (Y = C, N; X = F, Cl, Br, I) MXene monolayers. Density functional theory and density functional perturbation theory calculations reveal that, among the halogenated systems considered, only Br- and I-functionalized Mo2C monolayers are dynamically stable, as confirmed by positive definite phonon spectra throughout the Brillouin zone. Electronic structure calculations show metallic behavior with states near the Fermi level dominated by Mo d orbitals with pronounced electronic density of states, providing favorable conditions for strong electron-phonon coupling (EPC). The resulting EPC constants place both systems in the strong coupling regime, yielding superconducting transition temperatures of Tc = 13.1 K for Mo2CBr2 and Tc = 18.1 K for Mo2CI2 within the Allen-Dynes formalism. Notably, halogen functionalization itself plays a crucial role in enhancing superconductivity in Mo2C, which has Tc = 7.2 K, leading to a substantial increase in the superconducting transition temperature compared with pristine Mo2C through strengthened electron-phonon coupling. Furthermore, we demonstrate that superconductivity in these systems is highly tunable via carrier doping and biaxial tensile strain. Electron doping significantly enhances EPC and raises Tc up to 21.7 K for Mo2CBr2 and 21.3 K for Mo2CI2. Our results identify halogen-functionalized Mo2C MXenes as mechanically robust, phonon mediated two dimensional superconductors and highlight carrier doping as an effective strategy for optimizing their superconducting performance.

Figures (5)

• Figure 1: Crystal structure of halogen-functionalized Mo$_2$C and Mo$_2$N MXenes, Mo$_2$C$X_2$ ($X=$ F, Cl, Br, I). (a) Side view and (b) top view of the optimized monolayer structure. Purple, brown, and orange spheres represent Mo, C (or N), and halogen atoms, respectively.
• Figure 2: Electronic structure of halogen-functionalized Mo$_2$C$X_2$ monolayers. (a)–(c) Orbital-projected electronic band structures and the corresponding projected density of states (PDOS) of pristine Mo$2$C, Mo$2$Br$2$, and Mo$2$CI$2$, together with their respective Fermi surfaces. Colored circles denote the orbital contributions from Mo $d{z^2}$, Mo $d{zx}/d{zy}$, and Mo $d{x^2-y^2}/d{xy}$ states, while the PDOS highlights the contributions from Mo-$d$, halogen-$p$, and C-$p$ orbitals.
• Figure 3: Representative zone-center ($\Gamma$-point) phonon eigenmodes of the halogen-functionalized Mo$_2$C$X_2$ monolayer. The vibrational displacement patterns for selected Raman- and infrared-active modes are shown, including the $E_u$, $E_g$, $A_{1g}$, and $A_{2u}$ symmetries, as indicated beneath each panel together with their corresponding mode indices. Red arrows denote the atomic displacement vectors, illustrating both in-plane and out-of-plane vibrational character. Purple, brown, and orange spheres represent Mo, C, and halogen atoms, respectively.
• Figure 4: Phonon dispersion relations weighted by the electron--phonon coupling (EPC), phonon density of states (PhDOS), and Eliashberg spectral function of halogen-functionalized Mo$_2$C$X_2$ monolayers. (a-c) EPC-weighted phonon dispersion, atom-resolved PhDOS, Eliashberg spectral function $\alpha^2F(\omega)$, and cumulative EPC constant $\lambda(\omega)$ for Mo$_2$C, Mo$_2$CBr$_2$, and Mo$_2$CI$_2$. The phonon dispersions are plotted along the high-symmetry path $\Gamma$--$K$--$M$--$\Gamma$, with the color scale indicating the magnitude of EPC strength, from weak (blue) to strong (red). The PhDOS highlights the vibrational contributions from Mo, C, and halogen atoms, while $\alpha^2F(\omega)$ and $\lambda(\omega)$ quantify the phonon-mode-resolved EPC contributions.
• Figure 5: Tunability of the superconducting transition temperature $T_c$ of halogen-functionalized Mo$_2$C$X_2$ monolayers under biaxial tensile strain and electron doping. The calculated $T_c$ values for Mo$_2$CBr$_2$ (blue) and Mo$_2$CI$_2$ (orange) are shown for $+1\%$ and $+2\%$ biaxial strain, the pristine structures, and electron doping levels of $-0.1$ and $-0.2~e$/cell.