Strain tuning of electronic structure in Bi4Ti3O12-LaCoO3 epitaxial thin films

TL;DR

This study demonstrates that epitaxial strain and LaCoO3 substitution can systematically tune the electronic structure and band gap of Bi4Ti3O12 thin films, revealing strain as a powerful tool for electronic property control.

Contribution

It provides new insights into how strain and compositional modifications influence the electronic structure of ferroelectric oxides, enabling targeted property tuning.

Findings

Band gap decreases with LaCoO3 substitution.

Tensile strain further narrows the band gap.

Overlap of Co 3d and Ti t2g states reduces the optical transition energy.

Abstract

We investigated the crystal and electronic structures of ferroelectric Bi4Ti3O12 (BiT) single crystalline thin films site-specifically substituted with LaCoO3 (LCO). The epitaxial films were grown by pulsed laser epitaxy on NdGaO3 and SrTiO3 substrates to vary the degree of strain. With increasing the LCO substitution, we observed a systematic increase in the c-axis lattice constant of the Aurivillius phase related with the modification of pseudo-orthorhombic unit cells. These compositional and structural changes resulted in a systematic decrease in the band gap, i.e., the optical transition energy between the oxygen 2p and transition metal 3d states, based on a spectroscopic ellipsometry study. In particular, the Co 3d state seems to largely overlap with the Ti t2g state, decreasing the band gap. Interestingly, the applied tensile strain facilitates the band gap narrowing, demonstrating that epitaxial strain is a useful tool to tune the electronic structure of ferroelectric transition metal oxides.