Bromine and Iodine Stabilise Materials Showing Promising Superconductivity Properties

Researchers are actively pursuing novel two-dimensional materials with enhanced superconducting properties, and a new study details significant progress in this area. Jakkapat Seeyangnok and Udomsilp Pinsook, both from the Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok, Thailand, present a first-principles investigation into halogen-functionalized molybdenum carbide (Mo2C) MXenes, revealing substantially improved superconductivity compared to the pristine material. Their work demonstrates that bromine and iodine functionalisation creates dynamically stable monolayers exhibiting strong electron-phonon coupling, leading to predicted superconducting transition temperatures of up to 18.1 K. This research is significant because it not only identifies mechanically robust, phonon-mediated two-dimensional superconductors but also highlights the potential for tuning their performance through carrier doping and strain, offering a promising pathway towards advanced superconducting technologies.

This work details a comprehensive theoretical investigation into halogen-functionalized molybdenum dicarbide (Mo2C) MXene monolayers, revealing that bromine and iodine functionalization yields dynamically stable structures with significantly improved superconducting properties. Superconductivity, the lossless transmission of electrical current, typically requires extremely low temperatures or high pressures, limiting practical applications. This research demonstrates a route to achieving superconductivity at more accessible temperatures through careful materials design and manipulation at the nanoscale. The study establishes that pristine Mo2C exhibits a superconducting transition temperature of 7.2 K, but strategically adding bromine or iodine atoms dramatically alters its electronic behaviour. Calculations predict that Mo2C functionalized with bromine achieves a transition temperature of 13.1 K, while the iodine-functionalized material reaches 18.1 K, both within the strong-coupling regime for superconductivity. By introducing electron doping, the transition temperature can be further elevated to 21.7 K for Mo2CBr2 and 21.3 K for Mo2CI2. Similarly, applying biaxial tensile strain also influences the superconducting properties, offering another degree of control over the material’s behaviour. These findings position halogen-functionalized Mo2C MXenes as mechanically robust, two-dimensional superconductors with tunable properties, opening new avenues for designing advanced superconducting materials for future technologies. Density functional theory and density functional perturbation theory underpinned the investigation into the properties of halogen-functionalized Mo2YX2 monolayers, where Y represents carbon or nitrogen and X denotes fluorine, chlorine, bromine, or iodine. Calculations were performed using the Quantum ESPRESSO package, employing a plane-wave basis set with an energy cutoff of 80 Ry for electronic wave functions and 240 Ry for the charge density. A 24 × 24 × 1 Monkhorst, Pack k-point mesh facilitated Brillouin-zone integrations, and van der Waals interactions were incorporated to accurately model interlayer forces. Structural relaxations continued until residual forces on each atom diminished to less than 10−5 Ry/Bohr, ensuring geometric optimisation. The generalised gradient approximation, specifically the Perdew, Burke, Ernzerhof functional, alongside norm-conserving optimised Vanderbilt pseudopotentials, described electron-ion interactions. Methfessel, Paxton smearing, with a width of 0.02 Ry, addressed metallic occupations. Electron-phonon coupling calculations, conducted using density functional perturbation theory, sampled phonon wave vectors on a 12 × 12 × 1 q-point mesh. This methodological approach provides a comprehensive and accurate description of the electronic and vibrational properties crucial for understanding superconductivity. The Allen-Dynes formalism was then used to estimate the superconducting transition temperature, Tc, relying on the isotropic Eliashberg spectral function α2F(ω). The electron-phonon coupling constant, λ, was determined from this spectral function via integration, and the logarithmic average phonon frequency, ωln, and the second moment of the phonon spectrum, ω2, were calculated as intermediate values. This detailed analysis allowed for the identification of systems exhibiting strong electron-phonon coupling and, consequently, potential superconductivity. Calculations reveal superconducting transition temperatures of 13.1 K for Mo2CBr2 and 18.1 K for Mo2CI2, determined within the Allen-Dynes formalism. These values represent a significant increase compared to pristine Mo2C, which exhibits a Tc of 7.2 K, demonstrating that halogen functionalization strengthens electron-phonon coupling and enhances superconductivity. Detailed analysis of phonon spectra confirms that only bromine and iodine functionalized Mo2C monolayers possess dynamic stability throughout the Brillouin zone, indicating structural robustness. Electronic structure calculations demonstrate metallic behaviour in these systems, with states near the Fermi level dominated by molybdenum d orbitals and a pronounced density of states, creating favourable conditions for strong electron-phonon coupling. The resulting electron-phonon coupling constants place both Mo2CBr2 and Mo2CI2 firmly within the strong coupling regime, a prerequisite for achieving relatively high superconducting transition temperatures. Further investigation shows that carrier doping substantially modifies the superconducting properties of these materials, elevating the Tc for Mo2CBr2 to 21.7 K and for Mo2CI2 to 21.3 K, highlighting the tunability of superconductivity through external control. Scientists have long sought materials exhibiting superconductivity at temperatures achievable without exotic and costly cooling systems. This work on halogen-functionalized molybdenum carbide MXenes represents a subtle but potentially significant step towards that goal. The challenge has always been balancing the necessary conditions for strong electron-phonon coupling, the mechanism driving superconductivity in many materials, with structural stability and practical synthesis. MXenes, two-dimensional materials with a unique layered structure, have emerged as promising candidates, but achieving sufficiently high transition temperatures has remained elusive. What distinguishes this research is the focused application of halogen functionalization to tailor the electronic properties of Mo2C. By strategically attaching bromine and iodine atoms, the researchers have not only created dynamically stable structures but also demonstrably enhanced the superconducting potential, pushing the critical temperature to nearly 20 Kelvin. However, the reliance on computational modelling introduces inherent limitations. While density functional theory provides valuable insights, it’s not a perfect representation of reality, and experimental verification is crucial. Furthermore, the achieved temperatures still require liquid nitrogen cooling, limiting immediate practical applications. The next logical step involves synthesizing these materials and rigorously testing their properties, alongside exploring the effects of different halogen combinations and concentrations. Beyond this specific MXene system, functionalization offers a viable route to engineer superconductivity in two-dimensional materials, potentially unlocking a new generation of high-performance electronic devices and energy-efficient technologies.