In-gap states and local structures around substitutional defects in La- or Nb-doped n-type SrTiO₃

First-principles calculations with DFT+U method were conducted on the local structures and the in-gap electronic states of n-type SrTiO₃, whose electron doping was realized by substitution of Sr by La or Ti by Nb. For both La-doped and Nb-doped SrTiO₃ cubic structures were proved to be stable e3ven when tetragonal structures were assumed, as suggested also in a reported experimental study. In La-doped SrTiO₃, O atoms neighboring the substitutional La were symmetrically attracted toward the dopants. Sharp electron band observed at the top of the bandgap, which gave n-type conductivity and resulted from the intersection of Fermi energy level at the conduction band bottom with La-doping, was composed of 3d orbitals in Ti. The La-doping transformed some of the p electrons in O into the d electrons in Ti in the sharp band, and these electrons at Fermi level were expected to act as n-type carriers. In Nb-doped SrTiO₃, on the other hand, Ti atoms at the first, second and third nearest neighbors of the Nb, which were in the same Ti-O plane surrounding the dopants, were repelled away from the Nb dopants to be isotropic local expansion, while O atoms neighboring Nb were attracted toward the dopants in the expanded local structure. Sharp band of the in-gap states just below the Fermi energy of the Nb-doped SrTiO₃ was not only of the 3d orbitals in the first nearest-neighbor Ti atoms, but also of those in the second and third nearest-neighbor Ti atoms of the dopants. The degree of Fermi level intrusion into conduction band and atomic rearrangements near Nb substitutions were milder than those of La substitutions. As replacement of Sr atoms by La not contained in Ti-O network does not disturb the electron transport within Ti-O conduction planes, Nb-doping in Ti site does not interrupt the n-type carrier transportation even though Nb atoms are in Ti-O network, because of the formation of conduction path which bypasses the unit cell containing the dopants

Generation of time-multiplexed chiroptical information from multilayer-type luminescence-based circular polarization conversion films

時間変化する円偏光スペクトルの生成と読み出しに成功 --光記録や偽造防止技術への円偏光利用に期待--. 京都大学プレスリリース. 2024-02-09.Circularly polarized (CP) light generated from photoluminescence (PL) has great potential for the transmission of diverse forms of optical information including light intensity (brightness), spectral profile (color), and polarization (left-handed (LH)/right-handed (RH)), as well as temporal information corresponding to the PL lifetime of the CP light source. However, a systematic approach to the design of CP light-generating materials for the conveyance of time-multiplexed chiroptical information has not yet been reported. Herein, we demonstrate a novel approach to time-multiplexing chiroptical information using multilayered luminescence-based CP convertors comprising two linearly polarized luminescence (LPL) films with different PL lifetimes and a quarter-wave retardation film. We prepared LPL films with short and long PL lifetimes by stretching films comprising poly[2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) and CdSe/CdS core/shell quantum rod (QR) luminogens, respectively. We then fabricated four types of multilayered luminescence-based CP convertors by laminating the LPLMEH-PPV and LPLQR films with quarter-wave retardation films, so that the azimuthal angles between the polarization axes of the LPL films and the fast axes of the quarter-wave films differed in each case. The resulting CP light comprised short- and long-lifetime components. Subsequently, we used a time-resolved spectroscopic technique to extract time-multiplexed chiroptical information from changes in the time-course of the spectral profile of the LH- and RH-CP light. The time-varying of CP light profiles were thereby read-out as time-multiplexed chiroptical information. Our findings will pave the way for the design of CP light-generating materials for conveying time-multiplexed chiroptical information