Thickness-Dependent Band Gap Modification in BaBiO₃

Area of Science:

• Condensed matter physics
• Materials science
• Thin film physics

Background:

• Barium bismuthate (BaBiO3) is typically an insulator.
• Theoretical models predict metallicity in ultra-thin BaBiO3 films due to suppressed oxygen octahedra breathing modes.
• Recent studies suggest a link between oxygen breathing mode and electronic properties.

Purpose of the Study:

• To investigate the influence of the oxygen breathing mode on the band gap of BaBiO3 thin films.
• To study the electronic properties of BaBiO3 films across a range of thicknesses.
• To determine if metallicity is achieved in ultra-thin BaBiO3 films.

Main Methods:

• In-situ scanning tunneling microscopy (STM) was used to probe electronic properties.
• A series of BaBiO3 films with varying thicknesses were prepared.
• Raman spectroscopy was employed to analyze phonon modes.

Main Results:

• A transition from a wide band gap (> 1.2 eV) to a small band gap (≈ 0.07 eV) was observed with decreasing BaBiO3 film thickness.
• No metallic state was detected, even in the ultra-thin film limit.
• The band gap size correlated with the intensity of the oxygen breathing phonon mode, measured by Raman spectroscopy.

Conclusions:

• The oxygen breathing mode significantly influences the band gap of BaBiO3 thin films.
• The transition to a smaller band gap is driven by changes in the oxygen breathing mode, not metallicity.
• Ultra-thin BaBiO3 films exhibit semiconducting behavior, contrary to some theoretical predictions.