title: Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms.
abstract: Perovskite-type ferroelectric (FE) crystals are wide bandgap materials with technologically valuable optical and photoelectric properties. Here, versatile engineering of electronic transitions is demonstrated in FE nanofilms of KTaO3, KNbO3(KNO), and NaNbO3(NNO) with a thickness of 10-30 unit cells. Control of the bandgap is achieved using heteroepitaxial growth of new structural phases on SrTiO3(001) substrates. Compared to bulk crystals, anomalous bandgap narrowing is obtained in the FE state of KNO and NNO films. This effect opposes polarization-induced bandgap widening, which is typically found for FE materials. Transmission electron microscopy and spectroscopic ellipsometry measurements indicate that the formation of higher-symmetry structural phases of KNO and NNO produces the desirable red shift of the absorption spectrum towards visible light, while simultaneously stabilizing robust FE order. Tuning of optical properties in FE films is of interest for nanoscale photonic and optoelectronic devices.
authors.forename: Marina
authors.initials: M
authors.lastname: Tyunina
authors.affiliation: NanoSpin, Department of Applied Physics, Aalto University School of Science, PO Box 15100, FI-00076 Aalto, Finland.
authors.forename: Lide
authors.initials: L
authors.lastname: Yao
authors.affiliation: Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic.
authors.forename: Dagmar
authors.initials: D
authors.lastname: Chvostova
authors.affiliation: Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic.
authors.forename: Alexandr
authors.initials: A
authors.lastname: Dejneka
authors.affiliation: Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic.
authors.forename: Tomas
authors.initials: T
authors.lastname: Kocourek
authors.affiliation: Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic.
authors.forename: Miroslav
authors.initials: M
authors.lastname: Jelinek
authors.affiliation: Ioffe Physical-Technical Institute RAS, 194021 St. Petersburg, Russia.
authors.forename: Vladimir
authors.initials: V
authors.lastname: Trepakov
authors.affiliation: NanoSpin, Department of Applied Physics, Aalto University School of Science, PO Box 15100, FI-00076 Aalto, Finland.
authors.forename: Sebastiaan
authors.initials: S
authors.lastname: van Dijken
keywords: epitaxial growth; ferroelectric nanofilms; optical properties