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Investigations of EuGa2S4 have been done on the photoluminescence (PL) related to the transition between 4f65d and 4f7 configuration of the Eu2+ ion and its excitation (PLE) spectra, Raman scattering and infrared absorption. The energies of phonons coupled to the ground and the excited states of the transition are analyzed to be 34 and 19 meV from the shapes of the PL and PLE bands, respectively. The former corresponds to the energy of the Raman line showing the highest intensity. The latter is close to the value obtained from analysis of the temperature dependence of the half width of the PL band. These correspondences indicate that the relevant emission of EuGa2S4 surely has phonon-terminated character.

Luminescence properties of highly -oriented Y2O3:Tb3+ whiskers obtained by chemical vapor deposition on Si substrate have been investigated over a wide temperature range for different dopant concentrations. A considerable thermally stimulated increase of the green Tb3+ emission intensity was found in the temperature range of 98–433 K under 325 nm laser excitation. At temperatures higher than 433 K, the emission exhibits temperature quenching with the activation energy of ~0.98 eV. This behavior is discussed on the basis of the results of the time-decay measurements of the emission in the temperature range of 300–773 K. The thermally stimulated intensity increase is attributed to the thermal broadening of the photoluminescence excitation spectrum, whereas the temperature quenching is tentatively ascribed to the thermal activation of electrons from the excited 4f75d configuration to the conduction band. The decay time of the green Tb3+ emission exhibits shortening with increasing dopant concentration, which is considered in terms of the energy transfer processes between the Tb3+ ions.

Luminescence properties of highly -oriented Y2O3:Eu3+ whiskers obtained by chemical vapor deposition on YSZ substrate have been investigated over a wide temperature range. A considerable thermally stimulated increase of the red Eu3+ emission intensity was found under the excitation at the wavelength corresponding to the intrinsic transition within the Eu3+ ion, in contrast to the charge-transfer excitation. This behavior is discussed on the basis of the results of the time-decay measurements of the emissions from each electronic level of the 5DJ (J=0,1,2) manifold in the temperature range 300–1155 K. A model is proposed in which this thermally stimulated increase is assigned to the multiphonon relaxation from the upper-lying 5DJ (J=1,2) levels to the terminal level 5D0, responsible for the intensive red emission.

Ce-doped CaGa2S4 and SrGa2S4 thin films were prepared for the first time by the flash evaporation method. The films were characterized before and after annealing in H2S(10%)+Ar gas stream by measuring photoluminescence and absorption spectra, X-ray diffraction and electron probe micro analyses. X-ray diffraction curves and absorption spectra before annealing show amorphous behaviour, whereas the annealing leads to a significant crystallization and improves the stoichiometry of the films. Based on the performance data obtained from dispersive type EL cells using CaGa2S4:Eu powder together with photoluminescence property comparison between CaGa2S4:Eu and CaGa2S4:Ce powders, the annealed films prepared by flash evaporation can be considered to become one of the candidates for TFEL flat panel devices.

The Ce emission appearing in the blue–green region was investigated for CaGa2S4 and SrGa2S4. The room temperature quantum efficiency as high as 73% was observed for a 0.4 wt% Ce-doped CaGa2S4 single crystal grown by the melt method. The quantum efficiency decreased for a 0.8 wt% Ce-doped CaGa2S4 crystal and clear photoacoustic signal was detected in this sample, indicating an increase of non-radiative transition. For each of these compounds, ultraviolet (UV) excitation produced a hitherto unreported new UV emission which is thought to originate from 5d(Eg) level of the Ce ion. Temperature dependent intensity changes observed in this UV emission and in the blue–green emission suggest temperature dependent population transfer from the upper level of Eg to the lower level of T2g in the excited state of 5d electrons.

Photoluminescence properties of the stoichiometric compound of EuGa 2 S 4 has been investigated for the first time. The emission appearing in the region of 510–620 nm has a high quantum efficiency value reaching 100% at low temperatures. The characteristics of the emission are similar to those of CaGa 2 S 4 :Eu, and can be considered to be due to phonon-terminated transition of the Eu ion. Based on the measured luminescence data, construction feasibility of unique high gain tunable lasers is discussed.

Luminescence properties of highly (0001) oriented ZnO whiskers obtained by atmospheric chemical vapor deposition have been investigated over wide temperature range of 12 K to 300 K. An ultraviolet emission associated with bound-exciton recombination has been observed at 12 K under the low power photoexcitation. The emission exhibited temperature quenching with an activation energy of ~9 meV, which is treated as the binding energy of the bound exciton. The shape of the emission was considerably modified at high-power UV laser excitation. The principal feature of this modification is an occurrence of a series of equally spaced, narrow emission components, decreasing in intensity at longer wavelength. The separation between these components is found to correspond to the energy of a longitudinal optical phonon (~70 meV) determined by Raman scattering measurements. The activation energy of the temperature quenching of the emission considerably enhanced up to ~58 meV under the laser excitation. This enhancement of the activation energy is discussed as being due to the domination of the free-exciton emission in the photoluminescense spectra under the high-power excitation. The balance between free- and bound-exciton states is discussed in terms of exciton generation and the annihilation scheme.

Thermoluminescence of Eu and Ce co-doped CaGa 2 S 4 has been investigated for the first time. All co-doped samples were found to exhibit a long afterglow of the Eu emission having a peak energy of ∼2.20 eV. Analyses of measured transient thermoluminescence curves showed two activation energies, ∼0.2 eV and ∼0.94 eV, for most co-doped samples. These two activation energies are shown to be explainable by the model which includes one trap level with two activation processes, and hole release from the terminal state of the Eu ion. Experimentally observed thermoluminescence curves are shown to be well reproduced by the theoretically formulated model which takes into account the above-mentioned hole release process.

The resonance-type energy transfer process has been investigated for the first time in Ce and Eu co-doped CaGa2S4 polycrystalline samples with different dopant concentrations in the range from 0.2 at.% to 4 at.%. The energy transfer from Ce to Eu ions increases with increasing Ce and Eu concentrations. The values of the rate determined from measured Ce emission decay time constants with different dopant concentrations showed a reasonable agreement with those obtained from intensity analyses of the Ce emission which takes into account the re-absorption effect by Eu ions. In addition, the observed change of the transfer rate with dopant concentration was discussed in terms of the distance between Ce and Eu ions.

Significant acceleration of the photoluminescence (PL) decay rate was observed in water solutions of two rare earth ions (REIs), Tb and Eu. We propose that the time-resolved PL spectroscopy data are explained by a fluorescence resonance energy transfer (FRET) between the REIs. FRET was directly confirmed by detecting the induced PL of the energy acceptor, Eu ion, under the PL excitation of the donor ion, Tb, with FRET efficiency reaching 7% in the most saturated solution, where the distance between the unlike REIs is the shortest. Using this as a calibration experiment, a comparable FRET was measured in the mixed solution of REIs with single-wall nanotubes (SWNTs) wrapped with DNA. From the FRET efficiency of 10% and 7% for Tb and Eu, respectively, the characteristic distance between the REI and SWNT/DNA was obtained as 15.9 ± 1.3 A, suggesting that the complexes are formed because of Coulomb attraction between the REI and the ionized phosphate groups of the DNA.

The rubrene molecular crystal has the unique property of showing a strong photoconductivity for light wavelengths that are close to the absorption edge. We studied the microsecond dynamics of the photoconductivity induced by short light pulses to characterize the way in which photoinduced excitons efficiently ionize to produce free charge carriers. We found that the photoconductivity is produced by carriers released after an average time of $100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ from a ``reservoir state'' that originates from the photoexcited molecular excitons. The conversion of photoexcited excitons into this reservoir state happens only at the surface of the crystal within a depth of the order of a few micrometers, but in this region close to the surface, a photoexcited molecular exciton has a probability of the order of unity to ultimately lead to a mobile charge carrier. This high carrier photoexcitation efficiency leads to a pronounced shortening of the photocurrent rise time for decreasing wavelength or increasing energy of the excitation pulses because of the effect of quadratic recombination of the photocarriers.

By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are only created through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime., Comment: 4 pages, 3 figures

This paper describes the development of a 3.5 inch diagonal Active Matrix Organic Light Emitting Diode Display on flexible metal foils. The active matrix array had the VGA format and was fabricated using the polysilicon TFT technology. The advantages that the metal foil substrates offer for flexible display applications will first be discussed, followed by a discussion on the multilayer coatings that were investigated in order to achieve a high quality insulating layer on the metal foil substrate prior to TFT fabrication. Then the polysilicon TFT device performance will be presented as a function of the polysilicon crystallization method. Both laser crystallized polysilicon and solid phased crystallized polysilicon films were investigated for the TFT device fabrication. Due to the opaque nature of the metal foil substrates the display had a top emission structure. Both small molecule and polymer based organic material were investigated for the display emissive part. The former were evaporated while the latter were applied by spin-cast. Various transparent multi-layer metal films were investigated as the top cathode. The approach used to package the finished AMOLED display in order to protect the organic layers from environmental degradation will be described. The display had integrated polysilicon TFT scan drivers consisting of shift registers and buffers but external data drivers. The driving approach of the display will be discussed in detail. The performance of the finished display will be discussed as a function of the various materials and fabrication processes that were investigated.

Excitons in rubrene single crystals dissociate into free charge carriers via two mechanisms whose relative importance depends on the illumination wavelength through the optical penetration depth into the crystal. The first mechanism is defect-induced dissociation in less than 10 ns after photoexcitation. For low photoexcitation densities, about 10% of the excitons that survive radiative recombination dissociate through this channel. The second mechanism, affecting the remaining 90% of the excitons, involves a previously reported state localized close to the surface of the crystal that leads to a delayed release of photocarriers a fraction of a millisecond after photoexcitation.

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Luminescence properties of anatase TiO2 films grown on sapphire substrates by atmospheric chemical vapor deposition have been investigated over a wide temperature range of 20 K to 300 K. An intense green emission band associated with localized exciton recombination has been observed at low temperatures under photoexcitation. The emission exhibited temperature quenching with an activation energy of ∼48 meV. The shape of the emission spectrum was considerably modified upon electron beam excitation. The principal feature of this modification is a reversible redistribution of the spectrum intensity, which causes the appearance of a new UV emission band at high acceleration voltages. Details of the mutual relationship between the green and UV emission processes are considered in terms of the linear coupling configuration coordinate model. We propose that the temperature quenching of green emission is attributed to a combination of various thermally stimulated processes within the configuration coordinate diagram and energy migration through the excited state of the UV emission.