Effect of Strain on the Band Alignment of TiO2/ZnS Core-Shell Nanostructures

Core-shell quantum dots (CSQDs), are a special class of nanostructures where a lower band gap core is encased in a higher band gap shell, leading to enhanced carrier confinement, stability, and reduced recombination. In CSQDs, the point of contact of the two dissimilar materials leads to the formation of a heterojunction. At this heterojunction, lattice mismatch arises inducing strain, which directly affects the CSQDs’ optical and electrical properties by altering band lineups. In our study, through theoretical analysis we focused on varying the shell width while keeping the core size constant and vice versa influencing the strain at the junction, thus impacting band alignment and the material’s optical and electronic properties. We considered a model using a TiO2 core enclosed within a ZnS shell layer. It has been observed that for a fixed core width, the strain imposed on TiO2 increases with increasing core size, while that imposed on ZnS is seen to diminish for the same. This strain has a direct impact on the band lineup of the CSQD, thereby impacting the bandgap of the material. For increasing shell width, the bandgap decreases for both TiO2 and ZnS. For the case of constant shell and varying core width, the opposite trend is noted across all outcomes. An interesting observation that has been derived from our work was that, as the core:shell ratio remains constant, the strain, band alignment and related properties remain consistent, irrespective of the absolute thickness of the core and shell.