Tyunina Marina, Yao Lide, Chvostova Dagmar, Dejneka Alexandr, Kocourek Tomas, Jelinek Miroslav, Trepakov Vladimir, van Dijken Sebastiaan
NanoSpin, Department of Applied Physics, Aalto University School of Science, PO Box 15100, FI-00076 Aalto, Finland.
Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic.
Sci Technol Adv Mater. 2015 Apr 8;16(2):026002. doi: 10.1088/1468-6996/16/2/026002. eCollection 2015 Apr.
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 KTaO, KNbO (KNO), and NaNbO (NNO) with a thickness of 10-30 unit cells. Control of the bandgap is achieved using heteroepitaxial growth of new structural phases on SrTiO (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.
钙钛矿型铁电（FE）晶体是具有技术上有价值的光学和光电特性的宽带隙材料。在此，在厚度为10 - 30个晶胞的KTaO、KNbO（KNO）和NaNbO（NNO）的FE纳米薄膜中展示了电子跃迁的通用工程。通过在SrTiO（001）衬底上异质外延生长新的结构相来实现带隙的控制。与块状晶体相比，在KNO和NNO薄膜的FE状态下获得了异常的带隙变窄。这种效应与通常在FE材料中发现的极化诱导带隙变宽相反。透射电子显微镜和光谱椭偏测量表明，KNO和NNO的高对称结构相的形成产生了吸收光谱向可见光方向所需的红移，同时稳定了强大的FE有序性。FE薄膜中光学性质的调谐对于纳米级光子和光电器件具有重要意义。
