Wide band gap tunability in complex transition metal oxides by site-specific substitution

Abstract: Fabricating complex transition metal oxides with a tuneable band gap without compromising their intriguing physical properties is a longstanding challenge. Here we examine the layered ferroelectric bismuth titanate and demonstrate that, by site-specific substitution with the Mott insulator lanthanum cobaltite, its band gap can be narrowed as much as one electron volt, while remaining strongly ferroelectric. We find that when a specific site in the host material is preferentially substituted, a split-off state responsible for the band gap reduction is created just below the conduction band of bismuth titanate. This provides a route for controlling the band gap in complex oxides for use in emerging oxide opto-electronic and energy applications.

• The paper shows that site-specific substitution in BiTiO3 achieves nearly a 1 eV band gap reduction without impairing ferroelectric properties.
• The researchers used pulsed laser epitaxy and confirmed the structure with XRD and STEM, ensuring high-quality epitaxial thin films.
• DFT analysis and enhanced photoresponse reveal that LaCoO3 substitution modifies the electronic structure, paving the way for advanced optoelectronic devices.

Band Gap Tunability Via Site-Specific Substitution in Transition Metal Oxides

The paper presents a comprehensive study on the controllability of band gaps in complex transition metal oxides, specifically in the context of layered ferroelectric BiTiO$_3$ (BiT) through site-specific substitution with lanthanum cobaltite (LaCoO$_3$, or LCO), a Mott insulator. The research addresses a significant challenge in materials science, which is to manipulate the band gap in such materials without impairing their fundamental physical properties. This advancement paves the way for potential applications in optoelectronic and energy devices, including more efficient transparent conductors and photovoltaic materials.

Key Findings and Methodology

BiT, with its Aurivillius phase structure, presents an opportunity for substitution without disrupting its ferroelectric properties, partly due to its tolerance to oxygen vacancies. The researchers employed pulsed laser epitaxy to fabricate high-quality epitaxial thin films of BiT-LCO on SrTiO$_3$ substrates. These thin films demonstrated successful site-specific substitutional alloying, achieving a significant band gap reduction of nearly 1 eV.

Crystallographic and Ferroelectric Properties: The team utilized x-ray diffraction (XRD) and Z-contrast scanning transmission electron microscopy (STEM) to confirm the retention of the Aurivillius structure post-substitution. The ferroelectric properties remained largely unaffected, with polarization measurements showing consistent remnant polarization values, indicating sustained ferroelectric behavior despite substitution.

Optical and Photoelectric Characteristics: The optical band gap was notably reduced from 3.55 eV in pure BiT to 2.65 eV in the LCO-substituted BiT. This was corroborated by spectroscopic ellipsometry. Furthermore, the photocurrent investigations revealed enhanced photoelectrical response, with a marked increase in photocurrent generation under light exposure, demonstrating the improved photoresponse capabilities due to band gap tuning.

Theoretical Insights and Implications

Density functional theory (DFT) calculations bolstered the experimental findings, indicating that the insertion of Co along with oxygen vacancies leads to split-off states just below the conduction band of BiT. This effect primarily arises from the interaction between Bi atoms and Bi 6p states, which modifies the electronic structure substantially. The reduction in the band gap opens up pathways for charge transfer below the original gap, an insight that matches the experimental absorptive characteristics observed.

The efficacy of Co substitution further underlines its significant role, where Co plays a pivotal part in band gap modulation, unlike other elements such as Ti or Al, which do not effectuate similar changes. This highlights the importance of specific chemical substitution in altering electronic properties while maintaining desirable physical attributes.

Future Prospects and Applications

This study presents a promising approach for band gap engineering in ferroelectric and potentially other complex oxide systems. The work underscores the potential for integrating these findings into wider oxide systems such as the Ruddlesden-Popper series and other artificially structured materials, possibly leading to advancements in electronic and energy applications. The ability to tailor band gaps judiciously while preserving inherent material properties could facilitate novel device functionalities and improve existing technologies, particularly in the areas of photovoltaics and transparent conductive materials.

By providing a deeper understanding of the interaction between electronic structure and chemical substitution within complex oxides, this research may spur the development of new materials with customizable electronic and optical properties for various scientific and industrial applications. As AI research and applications continue to expand, breakthroughs such as these in material science could provide foundational technologies critical for energy-efficient and high-performance AI systems.