Bond Confinement-Dependent Peierls Distortion in Epitaxially Grown Bismuth Films

A systematic study of the impact of film thickness on the properties of thin Bi films is presented. To this end, epitaxial films of high quality have been grown on a Si (111) substrate with thicknesses ranging from 1.9 to 29.9 nm. Broadband optical spectroscopy reveals a notable decline in the optical dielectric constant and the absorption peak height as the film thickness decreases, alongside a shift of the absorption maximum to higher photon energies. Raman and pump-probe spectroscopy show that the phonon mode frequencies increase upon decreasing film thickness, with the in-plane mode frequency rising by 10% from the thickest to the thinnest sample. The X-ray diffraction analysis reveals an increasing Peierls distortion for thinner films, explaining the observed property changes. Quantum chemical bonding analysis and density functional theory calculations show that the properties of thin bismuth are influenced by the interplay between electron localization and delocalization, characteristic of metavalently bonded solids. This study shows that for solids that utilize metavalent bonding, a thickness reduction leads to significant property changes. The effect can even be employed to tailor material properties without the need to change material stoichiometry.