Computational modelling of anion-substituted quantum dots reveals that controlled oxygen incorporation into CsInTiS₄ nanostructures induces profound modifications in electronic structure and optical response. Therefore, in our study, we examined 20% anion-site oxygen incorporation into CsInTiS4 (i.e., CsInTiS3.2O0.8 QDs) using methods based on density functional theory and dielectric function optical modelling. Our results regarding structural relaxation and simulated X-ray diffraction indicate that sulfur’s substitution with oxygen induces lattice contraction and structural reorganization. This is accompanied by a change in the lattice symmetry from polar \(\:{P}_{21}\) to centrosymmetric P2/m in the optimized geometry. CsInTiS₃.₂O₀.₈ exhibits a pronounced bandgap contraction from \(\:2.22\) \(\:\text{e}\text{V}\) in CsInTiS₄ to \(\:0.69\:\text{e}\text{V}\), whilst the optical carrier concentration-to-effective mass ratio increases by \(\:74\%\) to \(\:1.08\: \times \:\:{10^{52}}\:k{g^{ - 1}}\:{m^{ - 3}}\). Furthermore, it reveals a Wemple–DiDomenico dispersion energy of \(\:45.27\:\text{e}\text{V}\), an oscillator strength of \(\:455\:{\text{e}\text{V}}^{2}\), a static refractive index of \(\:2.35\), and a high-frequency dielectric constant of \(\:5.51\). Moreover, carrier dynamics characterised by an ultrafast relaxation time of \(\:1.22\: \times \:\:{10^{ - 16}}s\) point to dominant scattering pathways intrinsic to the mixed-anion framework. These findings suggest that oxygen anion substitution plays a viable role in band structure engineering and light–matter interactions in a CsInTiS₄ parent lattice. Thus, the predicted narrow band gap, enhanced light–matter interaction, and oscillator strength make CsInTiS₃.₂O₀.₈ a strong candidate for infrared photodetection, tunable plasmonics, and third-order nonlinear photonics.
