Tailoring Janus In₂SeTe monolayers with Al doping: a computational study for NO _x sensing applications

Anthropogenic emissions of harmful gases from fossil fuel combustion and industrial processes require advanced sensing platforms with exceptional sensitivity and selectivity. Herein, we present a comprehensive first-principles study of a Janus In₂SeTe monolayer and its aluminum-doped analogue as multifunctional gas sensors. Using Density Functional Theory (DFT), we examine the adsorption of six common pollutants, including CO, NH₃, CO₂, CH₄, NO, and NO₂ on geometrically optimized surfaces, analyzing adsorption energy, distance, charge transfer, Density of States (DOS), Electron Density Difference (EDD), and recovery time. We further evaluate the resultant changes in electronic band structure, conductivity, optical absorption, refractive index, and spin-resolved DOS. Our results reveal that only NO and NO₂ chemisorb strongly, inducing the highest adsorption energies and shortest distances, while the other gases physiosorbed weakly. The adsorption of NO _x narrows the bandgap by 0.46/0.408 eV (NO/NO₂) and 1.068/0.612 eV (NO/NO₂) for pristine and Al-doped In₂SeTe, respectively, translating into dramatic chemiresistive responses. At the optimized Al-doping level (2.33% Al for NO, 5.5% Al for NO₂), the sensitivity was boosted by a factor of 1.26 × 10⁵ (NO) and 51.6 (NO₂) compared to the pristine In₂SeTe. The optical absorption and refractive index spectra exhibit gas-specific shifts in pristine In₂SeTe for NO and Al-doped In₂SeTe for NO₂, while only NO _x adsorption generates a nonzero magnetic moment, enabling concurrent optical, chemiresistive, and magnetic transduction. The obtained faster recovery time supports real-time and reusable detection of NO _x . Thus, these findings establish Al-doped In₂SeTe as an excellent sensing material for next-generation environmental NO _x sensing.