Shosuke Mochizuki

Photoluminescence and reversible photo-induced spectral change of SrTiO3

When strontium titanate (SrTiO3) single crystal is irradiated at room temperature with a 325 nm laser light in an evacuated specimen chamber, the luminescence intensity increases, creating a broad visible luminescence centred at about 2.4 eV. Then, introducing oxygen gas into the specimen chamber, the photoluminescence spectrum returns reversibly to the original weak luminescence under the same laser light irradiation. After removing the laser light irradiation, each photoluminescent state is stored for a long time at room temperature under room light, regardless of any changes of atmosphere. Such photo-induced spectral change has been observed also at different temperatures from 13 K to room temperature. The observed phenomenon is explained by means of the photo-induced oxygen defect formation at the surfaces of SrTiO3 crystal. For the same SrTiO3 single crystal, we have studied the photoluminescence properties. Besides the 2.4 eV luminescence band, we have observed new two luminescence bands centred at about 3.2 eV and about 2.9 eV. The energy, 3.2 eV, is close to both the photoluminescence excitation edge energy and the reported band edge energy of SrTiO3 crystal. Both the 3.2 eV luminescence and the 2.9 eV luminescence decay rapidly after a pulsed photoexcitation, while the 2.4 eV luminescence lasts for several seconds at 13 K. The excitation light intensity dependence of these luminescence bands has been also measured at 13 K. The 2.4 eV luminescence increases in intensity with increasing excitation intensity up to 4 mJ cm−2, and then it becomes decreased with further increase in the excitation intensity. On the other hand, both the 3.2 eV luminescence and the 2.9 eV luminescence increase in intensity with increasing excitation intensity, without any saturation. Although the 2.4 eV luminescence had been assigned to the radiative decay of intrinsic self-trapped excitons in a superparaelectric state by several workers, the present studies have clarified that the luminescence originates mainly from crystal defects (oxygen defects and chemical heterogeneity in the surface region). Both the 3.2 eV luminescence and the 2.9 eV luminescence are discussed qualitatively.

Short-range order in ultrathin film titanium dioxide studied by Raman spectroscopy

It is demonstrated here that nonresonant Raman spectroscopy can be used for unequivocal determination of short-range order in ultrathin films, using different structures of titanium dioxide as the model system. Titania films as thin as 7 nm sputter deposited on 〈111〉 Si have been analyzed and their phase content determined.

Reversible photoinduced spectral change in Eu2O3 at room temperature

When Eu2O3 powder compact and film are irradiated with ultraviolet (UV) laser light in a vacuum, their photoluminescence (PL) spectra change from a red sharp-line structure to a white broad band, which can be clearly seen with the naked eye. After removing the UV laser light, the white PL continues for more than several months at room temperature under room light, in spite of any changes of atmosphere. By irradiating with the same UV laser light at room temperature under O2 gas atmosphere, the original red PL state reappears. Such a reversible phenomenon may well yield materials for white-light-emitting devices and erasable optical storage.

Validity of the Sugano-Tanabe diagram for band states in MnO and MnS under high pressure

Abstract Optical transition between 3d bands in cubic MnO and MnS is measured upon applying hydrostatic pressure. Pressure dependence of the Racah parameter B and the crystal field strength Dq is determined. The pressure coefficients of transition energies from 6 A 1 to 4 T 1 and 4 T 2 states calculated using the Sugano-Tanabe diagram agree well with experimental values. An emission band at 1.63 eV in MnS at 1.5 K. is identified to be 6 T 1 → 4 A 1 transition accompanied by a large lattice relaxation.

An optical study on vapour, microcrystal beam and film

During evaporation of powder in confined noble gas, flowing noble gas and vacuum, the optical absorption spectra of vapour zones, microcrystal beam and film on a substrate were measured as functions of the time elapsed since the beginning of the evaporation. The results show clearly the spectral evolution from molecule to solid. Also, to clarify the details of the optical excitation and relaxation in the condensed solid, the absorption, photoluminescence and photoconduction spectra were measured for films at various temperatures from 7 K to room temperature.

Optical spectra of free and quasi-free AgI microcrystals

Abstract The optical spectra of free and quasi-free AgI microcrystals have been measured. The evolution of the Z 1,2 and Z 3 exciton bands was observed clearly. The exciton bands were blue-shifted from their bulk positions, owing to quantum confinement effects. Also, the splitting of the Z 1,2 band was observed in the small crystallite region.

Structural, electrical and optical studies on AgI?anatase composites

AgI?anatase composites, (x)AgI?(1 ? x)anatase, were fabricated over a wide composition range of 0?100% AgI for the first time. The electrical conductivity at 300 K increases with increasing AgI content and reaches a maximum (about 3 ? 10?2?S?m?1) at about 40% AgI. The conductivity is enhanced by about three orders of magnitude in comparison with that of pristine AgI. These composite specimens have been characterized by both scanning electron microscopy and x-ray diffractometry. The observations with a scanning electron microscope show that anatase fine particles are densely packed in an AgI particle of several micrometres in size and such composite particles are three-dimensionally connected to each other to form a composite specimen. The photoluminescence (PL), PL excitation and time-resolved PL spectra of these composites have also been measured at different temperatures between 8 and 278 K. The exciton spectra obtained related to the anatase particles and the AgI domains in anatase-packed AgI particles are discussed in the light of the morphological, structural and conductivity data.

Optical Study of MnO under High Pressure

Raman and optical absorption studies of the antiferromagnetic insulator MnO were performed at several temperatures under high pressure. In Raman scattering at 2 K, it was observed that the two-magnon peak shifts to higher energy with increasing pressure and then disappears at around 20 GPa. This suggests that a change in spin arrangement occurs at that pressure. At 77 K the peak-vanishing pressure increases to 25 GPa. At room temperature, a jump of the Raman peak position was observed at 30 GPa and the peak vanishes at 89 GPa. These results are consistent with previous X-ray diffraction studies which showed phase transitions at 30 and 90 GPa. The vanishing of the Raman peak suggests a metallization transition. The pressure dependence of the Mn2+ d–d transition observed in optical absorptions was consistent with the crystal field theory up to around 25 GPa at room temperature.

Optical spectra of free silver clusters and microcrystal produced by the gas evaporation technique: transition from atom to microcrystal

By setting an appropriate condition for the gas evaporation of silver, two vertically well separated zones-a transparent emission zone and a bright smoky zone-have been produced above the evaporation source in a confined helium atmosphere. Optical data were obtained by measuring time-resolved and space-resolved transmissivity spectra. The structure of the spectra and their dependence on time on the one hand and on the distance from the source on the other hand support the occurrence of atoms, clusters and microcrystals. The mechanism of the growth of silver clusters and microcrystals by the gas evaporation technique is also studied.

Spin-wave-assisted photoluminescence in MnO crystals

The authors report the observation of photoluminescence in antiferromagnetic and paramagnetic MnO. Two strong emission bands centred at 1.66 and 1.25 eV are observed below the Neel temperature, 118 K, in MnO crystals. Above the Neel temperature, the emission band centred at 1.66 eV vanishes, while the band centred at 1.25 eV persists and remains in the paramagnetic phase at least to room temperature. The photoluminescence is interpreted in terms of a model in which the emissions originate from the impurity-perturbed Mn2+ excitons. In the model, the observed emissions arise from the transitions assisted by magnons and the short-wavelength magnetic excitations. The excitation spectra, the time-resolved emission spectra, and the heat-treatment effect on the emission spectra are also reported and discussed.