Emerging NO₂ gas sensing on substitutionally doped Fe on NiWO₄ SCES insulators

In this study, we demonstrate the emergence of NO₂ gas sensing capabilities in the typically non-active NiWO₄, a strongly correlated electron system (SCES), by introducing substitutional Fe at the Ni site. NiWO₄ typically exhibits strong Coulombic repulsion between Ni atoms, resulting in a large band gap of over 3.0 eV and insulating behavior. This correlated behavior is clearly reflected in the significant increase of band gap when considering the Hubbard U correction for the cations, bringing the theoretical value closer to the observed value. The single-phase Fe_0.5Ni_0.5WO₄ displays a notable shift in the [NiO₆] symmetric vibration mode and an increase in magnetization. Additionally, theoretical calculations confirm the preservation of the wide band gap, with the Fe and O levels generated within the band gap. These findings indicate that Fe located in the Ni sites modulate Coulombic repulsion in NiWO₄ SCES insulators. Unlike the poor gas-sensing performance of intrinsic NiWO₄, Fe_0.5Ni_0.5WO₄ exhibits a significant NO₂ response (R_g/R_a) of 11 at 200°C than other gases and a limit of detection (LOD) of 46.4 ppb. This study provides a pathway for realizing gas-sensing performance in strongly correlated electron insulators with large band gaps through the introduction of dopant levels at the cation sites.