Thickness-Dependent Band Gap Modification in BaBiO(3)

The material BaBiO [Formula: see text] is known for its insulating character. However, for thin films, in the ultra-thin limit, metallicity is expected because the oxygen octahedra breathing mode will be suppressed as reported recently. Here, we confirm the influence of the oxygen breathing mode on the size of the band gap. The electronic properties of a BaBiO [Formula: see text] thickness series are studied using in-situ scanning tunneling microscopy. We observe a wide-gap ([Formula: see text] > 1.2 V) to small-gap ([Formula: see text] ≈ 0.07 eV) semiconductor transition as a function of a decreasing BaBiO [Formula: see text] film thickness. However, even for an ultra-thin BaBiO [Formula: see text] film, no metallic state is present. The dependence of the band gap size is found to be coinciding with the intensity of the Raman response of the breathing phonon mode as a function of thickness.