Taketoshi Kawai

Optical Absorption in Band-Edge Region of ( CH3NH3)3Bi2I9 Single Crystals

Optical absorption spectra were measured around the band edge in ( CH 3 NH 3 ) 3 Bi 2 I 9 single crystals. In the very thin crystals, an intrinsic exciton band appears clearly at 2.51 eV at low temperatures, and the absorption peak remains even at room temperature reflecting a large binding energy of more than 300 meV. The exciton band does not show a discontinuous shift near the temperature where the methylammonium ions induce a structural phase transition. This fact indicates that the exciton states are independent of the methylammonium ions. The origin of the exciton band is examined on the basis of the annealing effect on the absorption bands in vacuum-evaporated films. The lower energy tail of the exciton band obeys the Urbach rule between 78 and 350 K. The detailed analysis by the Urbach rule reveals that the exciton-phonon interaction in the present system is relatively strong.

Phase Transition in (CH3NH3)3Bi2Br9 Single Crystal

The successive phase transitions in (CH 3 NH 3 ) 3 Bi 2 Br 9 (MABB) single crystal have been investigated by measuring of dielectric and elastic dispersion and Brillouin and Raman scattering. Softening of the elastic constant c 66 and critical slowing down of the order parameter, which belongs to the E g representation, have been found in the phase I. No optical soft mode has been observed. The phase transition from the phase I to the phase II has been discussed by the Landau theory.

Effect of Phase Transition on the Optical Absorption Edge and Electronic States in Ferroelectric Materials of (CH3NH3)3M2Br9 (M: Sb,Bi).

We have measured the temperature dependence of the absorption spectra in the band-edge region and the reflection spectra of the (CH 3 NH 3 ) 3 M 2 Br 9 (M:Sb,Bi) single crystals exhibiting serveral successive phase transitions. The dispersion-like reflection structure due to the band-edge exciton transition is observed at 2.92 eV and 2.95 eV for (CH 3 NH 3 ) 3 Bi 2 Br 9 and (CH 3 NH 3 ) 3 Sb 2 Br 9 , respectively. The reflection structure remains even at room temperature reflecting a large binding energy. The fundamental absorption edge in both crystals possesses an exponential shape following the empirical Urbach rule. The steepness coefficients characterizing a broadening of the absorption edge exhibit a distinct jump at the phase transition temperature. The results indicate that the ordering of orientational motion of methylammonium cations changes not only the crystal structure and dielectric properties but also the Urbach tail in both crystals.

Origin of the Absorption Bands of Cesium Halides Doped with Tl+ Ions

We have systematically investigated the absorption spectra of CsX:Tl + (X = Cl, Br, I) and their mixed crystals with CsCl-type crystal structure. The lowest energy absorption bands, which are peaked at 5.18, 4.85, and 4.27 eV for CsCl:Tl + , CsBr:Tl + , and CsI:Tl + , respectively, exhibit the similar temperature dependence as the A band of Tl + centres in NaCI-type alkali halides. In the higher energy region, several absorption bands are observed with no extremely different absorption intensity. The temperature evolution of the higher energy absorption bands is not characteristic of the Tl + centres in NaCI-type alkali halides. Absorption bands in CsBr 1-x CsI x :Tl + mixed crystals can be classified into two groups: the rapidly disappearing and gradually fading absorption bands with increasing composition of the hetero-halogen ions. The origin of the higher absorption bands is discussed on the basis of the spectral changes in the CsBr 1-x CsI x :Tl (0.0≤ x ≤1.0) mixed crystals.

Investigation of photoluminescence of Si−O−C(−H) ceramics at an early stage of decarbonization by using high energy excitation

Si−O−C(−H) ceramics with reduced carbon contents were prepared by pyrolyzing polysiloxane particles in hydrogen at temperatures of 750, 800 and 850 °C. Under HeCd laser irradiation (325 nm), the obtained ceramics show broad spectra peaking at 400–415 nm. On the other hand, the excitation on the higher energy region by an ArF excimer laser (193 nm) induces new PL bands located at short wavelength region of 300 and 355 nm. Such high energy PL bands appear prominently in the ceramics synthesized at 750 °C, and are minor in ceramics synthesized at 800 and 850 °C.

Absorption Bands of Tl+ Centers Doped in Ionic Crystals with the CsCl Structure

We have investigated the absorption spectra of Tl + centers doped in ammonium and cesium halides crystals with the CsCl structure up to the vacuum ultraviolet energy range. In the NH 4 X:Tl + (X = Cl, Br) crystals, the A, B and C absorption bands due to the intraionic transitions in the Tl + ion are observed in the energy region below the fundamental absorption band edge of the host crystals. In the CsX:Tl + (X = Cl, Br, I) crystals, on the other hands, several absorption bands having no extremely different absorption intensity appear below the absorption band edge. The lowest absorption bands are assigned to the A absorption band, and other absorption bands have no feature of the B and C bands. The appearance of the absorption bands is remarkably different between the NH 4 X:Tl + and CsX:Tl + crystals, in spite of the same ion arrangements around the Tl + ion which is surrounded by eight nearest-neighbor halogen ions. The difference of the absorption bands between the NH 4 X:Tl + and CsX:Tl + crystals ca...

Nonlinear optical responses due to the lateral transport of excitons in a quasi two-dimensional system in BiI3 crystals

Abstract We have studied high density effects and their spatial distribution on the absorption spectra of excitons localized at a specific planar-defect structure in layered BiI3 crystals in which a quasi two-dimensional space with the translational symmetry is realized and the center-of-mass motion of excitons is confined. The exciton transition energy was examined by a space-resolved pump-probe method. The blue-shift of the transition energy has been detected not only at the exciting laser spot but also at the different site away from the exciting point. This is the first observation of high density effects on the exciton spectra at the isolated area from the source due to the spatial propagation of excitons.

Comparison of Luminescence Properties of CsI Crystals Activated with Ag-, Au-, and Tl+ Ions at Room Temperature

We have investigated the luminescence properties of CsI crystals activated with Ag-, Au-, and Tl+ ions at room temperature. When the activated CsI crystals are irradiated by light with a wavelength shorter than the absorption edge of CsI bulk crystals, luminescence bands related to the activated centers are observed in the visible wavelength region, in addition to intrinsic luminescence bands due to self-trapped excitons. In CsI:Au- and CsI:Tl+, the visible luminescence bands exhibit decay profiles with a decay time of about 1 µs. On the other hand, the visible luminescence band in CsI:Ag- consists of two luminescence components with decay times of 25 and 250 µs. Their luminescence properties are discussed from the viewpoint of their application as a scintillator material.

Coherent and incoherent transport properties of excitons at high density in a quasi-two-dimensional space of a stacking disorder interface in a layered crystal BiI3

Abstract We present transport properties of excitons at high density in a quasi-two-dimensional space of a specific stacking disorder interface in layered crystal BiI 3 , observing temporal behavior and peak energy shifts in space- and time-resolved luminescence spectra. Coherent transport component was resolved as a ballistic wave front of exciton-polariton with a very large group velocity. On the other hand, incoherent exciton transport is responsible for the relatively slower component in the space-resolved regime.

Energy Transfer and Relaxation Mechanism among Exciton States in Layered Crystal BiI3. I. Energy Transfer by Triplet Excitons

Energy transfer from the triplet state of the bulk exciton to the exciton states localized at a stacking fault interface (stacking fault exciton; SFE) in BiI 3 crystals is investigated by observing the temporal behavior and space-resolved spectra of their resonant luminescence. The recombination luminescence with decay time constant of 1.5 nsec from the triplet exciton appears in the sample having the SFE transition, by taking the SFE states as intermediate states. For the excitation at the indirect exciton region, one of the SFE luminescence shows a slow decay with the same time constant as that of the triplet state in addition to an intrinsic fast decay component with the time constant of 40 psec. The space-resolved luminescence spectra indicate that the triplet excitons transport their energy far from the exciting light spot and transfer it to the SFE states efficiently.