Takehisa Yoshinari

Reflectivity and Electronic Structures of Layered Ionic Crystals, (CnH2n+1NH3)2CdCl4: n=1, 2, 3

The reflection spectra of (C n H 2 n +1 NH 3 ) 2 CdCl 4 were measured in the photon energy region of 0.5–25 eV at room temperature (RT) and liquid nitrogen temperature (LNT) by means of synchrotron radiation. It was found that absorption spectra of the present crystals resemble that of CdCl 2 crystals except for an additional peak at 5.10 eV (RT). The energy of the additional peak shifts with temperature by as much as 0.38 eV from 5.10 eV (RT) to 5.48 eV (LNT). This absorption peak has doublet structure and the splitting energy is 0.1 eV, which is the same magnitude as the spinorbit splitting of a chlorine atom. The band is assigned to be an electronic transition from the chlorine 3 p valence band to a new-type conduction band which is composed of alkylammonium bases and is below the ordinary Cd 5 s conduction band in energy.

Weak ferromagnetism of the layered perovskite (CnH2n+1NH3)2MnCl4 (n=1, 2 and 3)

Abstract Weak ferromagnetism in the layered perovskite (C n H2n+1NH3)2MnCl4((n = 1, 2 and 3) has been studied by means of neutron diffraction and SQUID magnetometry. The antiferromagnetic spin arrangement in all the compounds is consistent with the chemical unit cell of the tilt system of MnCl6 octahedra, alternately rotating about the b axis. The weak ferromagnetic moment due to spin canting in the direction of the a axis of the compound with n = 3 is 0.42 × 10−3 μB at 4.2 K; the spin canting can be ascribed to an antisymmetric exchange interaction through a coupling of the spin directions and the tilting directions of the MnCl6 octahedra. The critical index β of the antiferromagnetic moment and that of the weak ferromagnetic moment for this compound are 0.25 + 0.02 and 0.229 + 0.005, respectively; the results reflect the character of the two-dimensional magnetic system. For the compounds with n = 1 and 2 the weak ferromagnetic moments are found in the c-plane.

Optical Properties of Electron Trapped Center in Layered Ionic Crystals (CH3NH3)2CdCl4 and (C2H5NH3)2 CdCl4 Irradiated with X-Rays at 15 K

New optical absorption bands (IR bands) were found at around 10∼20 kcm -1 besides absorption band of Cl 2 - molecular center in two-dimensional crystals of (CH 3 NH 3 ) 2 CdCl 4 and (C 2 H 5 NH 3 ) 2 CdCl 4 irradiated with X-rays at 15 K. The absorption intensities increase with the X-ray dosage in proportion to that of the Cl 2 - center and decrease at temperatures higher than 25 K for (CH 3 NH 3 ) 2 CdCl 4 and 35 K for (C 2 H 5 NH 3 ) 2 CdCl 4 accompanying the decrease of Cl 2 - . The both bands are also bleached by the light irradiation on the IR bands at 15 K. The IR bands are assigned to an electron center where an electron is trapped by an alkylammonium head in the neighborhood of a Cl - vacancy.

ESR and optical studies on Cl2− in single crystals of (CnH2n+1NH3)2CdCl4 with n=1, 2 and 3

The crystals of (C n H 2 n +1 NH 3 ) 2 CdCl 4 ( n =1, 2, 3) have been irradiated with X - and γ-rays at 77 K. From ESR and optical absorption spectra, it is found that Cl 2 - is created in the layer of CdCl 6 . With angular dependence of hyperfine coupling constant of Cl 2 - , the direction of its molecular axis is determined. The presence of any absorption bands corresponding to vacancy centers, such as F center in alkali halides, has not been detected in the irradiated crystals. Metallic luster as a silver mirror has been observed in the crystals which have been warmed up to room temperature after irradiation at 77 K.

Metal Precipitation in Transparent Layer Ionic Crystals (CnH2n+1NH3)2MCl4 with n=1, 2 and M=Mn, Cd

Layer ionic crystals (CnH2n+1NH3)2MCl4 with n=1, 2 and M=Mn, Cd have been irradiated with X- or γ-rays at 77 K. After warming the crystals up to room temperature, metallic luster as a silver mirror has been observed in them. Taking into account optical absorption spectra and elimination of chlorine from the crystals, it is concluded that the metallic luster is caused by metal precipitation in the crystals. The mechanism of the precipitation is considered with the concept of color center formation and destruction in low dimensional crystals.

Quantum dots of SnI4 and its clusters in cyclodextrins

Abstract From the reflection spectra of tin tetraiodide (SnI 4 ) single crystals, the exciton absorption band has been observed at peak energies of 3.14 eV (77 K) and 3.07 eV (room temperature). The exciton binding energy is estimated at 380 meV. The large binding energy suggests that each SnI 4 molecule has a quantum dot-like character. The reflection spectra of the SnI 4 clusters in γ-cyclodextrins were also measured. The first exciton peak of the clusters shift to a higher energy by 0.35 eV (77 K) and 0.40 eV (room temperature) compared with those of the single crystal due to the quantum size effect.

Emission spectra and decay characteristics in photo-stimulated (CnH2n+1NH3)2CdCl4 : n=1, 2, 3

Recombination luminescence from (CnH2n+1NH3)2CdCl4 : n=1, 2, 3 has been studied using synchrotron radiation pulses. A largely stokes-shifted emission band was found to appear at 2.50eV for the n=2 crystal when excited into the excitonic absorption range, while at 2.25eV when excited into the band-to-band transition range. Decay curves of both bands are composed of fast and slow components. Non-exponential decay of slow components suggests that the 2.25eV band is induced by tunneling recombination between self-trapped electrons and self-trapped holes. As for the 2.50eV band, recombination of correlated electron-hole pairs with strong lattice relaxation is supposed.

Polarization Dependence of Absorption and Luminescence Spectra of a 1,5-Dihydroxyanthraquinone Single Crystal

A study has been made on the polarization dependence of absorption and luminescence spectra of a 1,5-dihydroxyanthraquinone single crystal with the thickness of 0.5 mm, which was prepared by gas ph...

Coupling Between Electrons and Molecular Vibrations of an Anthracene Single Molecule in a Cyclodextrin

Optical studies of a single anthracene molecule succeeded by using a β-cyclodextrin (CyD) as an isolating cage. The absorption and the fluorescence spectra consist of several vibronic lines with 0.174–0.178 eV intervals and are mirror image with each other. In addition, their line shapes vary scarcely between RT and 2 K. These facts indicate that the anthracene molecule is almost free in the β-CyD cage. These spectra are theoretically analyzed by a free-molecule-model in which the electronic excited state couples with the vibrations \(\hbar \omega\) of the molecule with coupling strength s (Haung Rhys factor). Observed spectra are in good agreement with theoretical ones with s =1.56 and \(\hbar \omega=0.178\) eV. For the single crystal of anthracene, absorption and fluorescence were also measured. At 2 K, the zero-phonon peak becomes enhanced and the phonon lines becomes sharpened compared with the single molecule case. But at RT, the fluorescence spectrum becomes quite diffused shape. These facts suggest...

Optical Spectra of Inorganic-Organic Compounds (C2H5NH3)2CdCl4 in 3–30 eV Range

Reflection spectra of (C 2 H 5 NH 3 ) 2 CdCl 4 single crystals have been investigated in the range of 3–30 eV using synchrotron radiation. Two kinds of exciton bands with the splitting of the order of 0.1 eV, which is almost equal to the spin-orbit splitting of a Cl atom, are observed at 6.19 and 8.20 eV at 7 K. These bands are located at the energy positions close to the lowest exciton bands of CdCl 2 and C 2 H 5 NH 3 Cl crystals, respectively. On rising the sample temperatures, the 8.20 eV band shows a discontinuous peak-shift of 0.1 eV around the temperature where a structural phase transition occurs, while such a result is not observed for the 6.19 eV band. The 6.19 eV band is assigned to the electronic transition from the Cl - 3 p valence band to the lower Cd 2+ 5 s conduction band, and the 8.20 eV band to that from the Cl - 3 p valence band to the upper NH 3 - s -like conduction band.