Atsushi Ishizumi

Structural and luminescence properties of Eu-doped ZnO nanorods fabricated by a microemulsion method

We have studied photoluminescence (PL) properties of Eu-doped ZnO (ZnO:Eu) nanorods fabricated by a microemulsion method. The ZnO:Eu nanorods have a hexagonal crystal structure and exhibit a sharp luminescence due to the intra-4f transitions of Eu3+ ions. The excitation energy and temperature dependence of the PL intensity show that the Eu3+-related PL efficiency is determined by the energy relaxation process of excited Eu3+ ions, rather than by the energy transfer process from ZnO nanorods to Eu3+ ions. The energy transfer and luminescence processes in ZnO:Eu nanorods are discussed.

Blue luminescence from electron-doped SrTiO3

Blue-light emissions from electron-doped SrTiO3 single crystals at room temperature were observed. Substituting La3+ for Sr2+ and Nb5+ for Ti4+ in SrTiO3 provides electron carriers in Ti 3d conduction bands; these carriers are responsible for the room-temperature blue-light emission. This blue-light emission is essentially the same as that observed in Ar+-irradiated oxygen-deficient SrTiO3. This blue luminescence is independent of the defect type. The chemical substitution of La3+ for Sr2+ changes the temperature of the structural phase transition from cubic to tetragonal symmetry. The relation between the photoluminescence properties and the structural phase transition is also discussed.

Space-resolved photoluminescence of ZnS:Cu,Al nanocrystals fabricated by sequential ion implantation

We report on photoluminescence (PL) properties of Cu- and Al-doped ZnS nanocrystals fabricated by sequential implantation of Zn+, S+, Cu+, and Al+ ions into Al2O3 matrices. The spatially resolved PL spectrum has been studied by a scanning near-field optical microscope (SNOM). In the SNOM image, bright spots are observed on the sample surface. The PL spectrum at each bright spot is broad and is not sensitive to the monitored positions. The broad SNOM-PL spectrum at each spot is very similar to the macroscopic PL spectrum measured by conventional optics. The donor-acceptor pair luminescence process in nanocrystals is discussed.

Mn―Mn couplings in Mn-doped CdS nanocrystals studied by magnetic circular dichroism spectroscopy

We report on the optical and magnetic properties of Mn-doped CdS nanocrystals coated with a ZnS shell layer (CdS:Mn/ZnS core-shell nanocrystals) by magnetic circular dichroism (MCD) spectroscopy. The magnetic field and temperature dependences of the MCD spectrum show paramagnetic behavior of the CdS:Mn/ZnS core-shell nanocrystals. The MCD intensity increases with the Mn concentration up to a few mol %, and then starts to decrease rapidly. This Mn-concentration dependence of the MCD intensity can be explained by the formation of Mn–Mn pairs in heavily doped nanocrystals.

Photoluminescence properties of single Mn-doped CdS nanocrystals studied by scanning near-field optical microscopy

We have fabricated Mn-doped CdS (CdS:Mn) nanocrystals embedded in Al2O3 matrices by sequential ion implantation and studied their photoluminescence (PL) properties by a scanning near-field optical microscope (SNOM). In the PL spectra of CdS:Mn nanocrystals measured by the SNOM, several sharp PL lines and a broad PL band were observed. The sharp PL lines are related to bound excitons at shallow impurities in CdS nanocrystals. The Mn-related PL spectrum is very broad even in single nanocrystals at low temperatures, and both the peak energy and the spectral width of the PL band depend on the excitation laser intensity. The PL properties of single CdS:Mn nanocrystals are discussed.

Fe-catalytic growth of ZnSe nanowires on a ZnSe(001) surface at low temperatures by molecular-beam epitaxy

We grew ZnSe needle-like nanowires on a ZnSe∕GaAs epilayer using Fe catalysts by means of molecular-beam epitaxy operated at low temperatures of 250–350 °C, which are comparable to the usual temperatures for fabrication of ZnSe-based optoelectronic devices. The diameters at the tops of the nanowires ranged from 8 to 20 nm, and the typical length was about 200 nm. The number density of the nanowires was the order of 109cm−2. A nanowire was the zinc blende structure and the longitudinal direction was ⟨001⟩,⟨111⟩,⟨110⟩, or ⟨112⟩. Photoluminescence spectroscopy implied that the optical property of the nanowires differs from that of the bulk crystals.

Photoluminescence Dynamics of Mn2+-Doped CdS/ZnS Core/Shell Nanocrystals: Mn2+ Concentration Dependence

The controlled doping of impurities into semiconductor nanocrystals is essential for producing and enhancing optical and magnetic functionalities. We fabricated Mn 2+ -doped CdS nanocrystals overcoated with a ZnS shell layer (CdS:Mn/ZnS core/shell nanocrystals) and studied photoluminescence (PL) properties of CdS:Mn/ZnS core/shell nanocrystals with different Mn 2+ concentrations. The intensity and decay profile of the Mn 2+ intra-3d transition PL are determined by the Mn 2+ concentration. Our findings suggest that the Mn 2+ concentration dependence of the PL dynamics is caused by exchange-coupled Mn pairs in nanocrystals.

Dye-doped polymer microring laser coupled with stimulated resonant Raman scattering

Polymer ring laser was fabricated with 1,4-bis[2-[4-[N,N-di(p-totyl)amino]phenyl]vinyl]benzene (DADSB)-doped polyvinyl-pyrrolidone (PVP) thin films coated on an inner surface of a quartz microcapillary. Ring-waveguided multimode lasing was observed under pulsed optical pumping. With increasing excitation wavelengths, an additional emission peak based on stimulated resonant Raman scattering (SRRS) appeared with a Raman shift of the stretching vibration of DADSB. When the SRRS peak just overlapped with each ring laser modes, the emission intensity was periodically enhanced. Such a wavelength-selective Raman coupling can demonstrate a tunable single-mode microring laser.

Origin of efficient luminescence from GaN:Eu3+ epitaxial films revealed by microscopic photoluminescence imaging spectroscopy

We have studied photoluminescence (PL) properties of Eu3+-doped GaN (GaN:Eu3+) epitaxial films by microscopic PL imaging spectroscopy. The GaN:Eu3+ epitaxial films exhibit efficient red luminescence related to intra-4f transitions of Eu3+ ions. The intensity and the spectral shape of the Eu3+-related PL are sensitive to the Eu3+ concentration, the excitation wavelength, and the monitored position. Microscopic PL imaging spectroscopy revealed that efficient red luminescence of GaN:Eu3+ epitaxial films is due to Eu3+ ions around point defects and dislocations.

Multicarrier Recombination and Energy Transfer in Mn-Doped CdS Nanocrystals Studied by Femtosecond Pump-Probe Spectroscopy

We studied the photocarrier decay dynamics of Mn-doped CdS nanocrystals (NCs) coated with a ZnS shell layer by femtosecond pump–probe transient absorption spectroscopy. At low excitation intensities, the decay dynamics of photocarriers is determined by energy transfer from electron–hole pairs in CdS NCs to Mn ions. At high excitation intensities, the photocarrier decay curves of Mn-doped CdS NCs are very similar to those of undoped CdS NCs, and they are determined by the fast Auger recombination of photocarriers. These density-dependent photocarrier behaviors in CdS NCs are confirmed by Mn 2+ luminescence spectroscopy.