Infrared Optical Absorption in Low-spin Fe$^{2+}$-doped SrTiO${}_{3}$

TL;DR

This study introduces a controlled doping method for SrTiO3 thin films that stabilizes Fe2+ ions in a low-spin state, enabling near-infrared optical transitions and advancing band gap engineering for energy applications.

Contribution

It presents a novel co-doping technique to stabilize Fe2+ in SrTiO3 without defects, revealing low-spin electronic states for improved optical properties.

Findings

Fe2+ ions exhibit low-spin configuration in doped SrTiO3.

Co-doping with La stabilizes Fe2+ without compensating defects.

Optical transitions occur in the near-infrared regime.

Abstract

Band gap engineering in SrTiO${}_{3}$ and related titanate perovskites has long been explored due to the intriguing properties of the materials for photocatalysis and photovoltaic applications. A popular approach in the materials chemistry community is to substitutionally dope aliovalent transition metal ions onto the B site in the lattice to alter the valence band. However, in such a scheme there is limited control over the dopant valence, and compensating defects often form. Here we demonstrate a novel technique to controllably synthesize Fe$^{2+}$- and Fe$^{3+}$-doped SrTiO${}_{3}$ thin films without formation of compensating defects by co-doping with La$^{3+}$ ions on the A site. We stabilize Fe$^{2+}$-doped films by doping with two La ions for every Fe dopant, and find that the Fe ions exhibit a low-spin electronic configuration, producing optical transitions in the near infrared regime and degenerate doping. The novel electronic states observed here offer a new avenue for band gap engineering in perovskites for photocatalytic and photovoltaic applications.