Lithium Niobate Tantalate Solid Solutions Probed by Luminescence Spectroscopy

Using continuous and time-resolved spectroscopy, we investigate Raman and luminescence signals from synthetic hydroxylapatites doped with trivalent REE including Dy3+, Eu3+, Nd3+, and Sm3+, as well as REE in natural apatites, with laser excitations at 532 and 785 nm. We demonstrate that time-resolved spectroscopy is an efficient method to reduce luminescence from Raman spectra or, alternatively, to investigate the luminescence signals without interference from the Raman contribution. Time-resolved luminescence spectroscopy is a powerful technique for generating specific high-quality luminescence spectra for the REE emission activators in apatites by using appropriate combinations of delay and gate width for time synchronization of the laser pulse and ICCD detector. This allows the unambiguous detection and identification of the activators by avoiding overlapping of various emission signals in the luminescence spectra. This is particularly useful in the case of natural samples, which often have several activators for luminescence. In the case of synthetic REE-doped apatites, a quenching process for luminescence due to activator concentration is seen for Eu3+ and Sm3+, i.e., the higher the concentration, the shorter the luminescence decay time. The interpretation of luminescence decay time in natural apatites is promising but more complex because of energy transfers between the various luminescence activators present in the crystal lattice. Luminescence is a powerful technique for detecting the presence of REE in apatites down to parts per million levels, though quantifying the concentration is still a challenge.

The spectroscopy of solid anthracene is examined both experimentally and theoretically. To avoid experimental complications such as self-absorption and polariton effects, ultrathin polycrystalline films deposited on transparent substrates are studied. To separate the contributions from different emitting species, the emission is resolved in both time and wavelength. The spectroscopic data are interpreted in terms of a three-state kinetic model, where two excited states, a high energy state 1 and a low energy state 2, both contribute to the luminescence and are kinetically coupled. Using this model, we analyze the spectral lineshape, relative quantum yield, and relaxation rates as a function of temperature. For state 1, we find that the ratio of the 0-0 vibronic peak to the 0-1 peak is enhanced by roughly a factor of 3.5 at low temperature, while the quantum yield and decay rates also increase by a similar factor. These observations are explained using a theoretical model previously developed for herringbone polyacene crystals. The early-time emission lineshape is consistent with that expected for a linear aggregate corresponding to an edge-dislocation defect. The results of experiment and theory are quantitatively compared at different temperatures in order to estimate that the singlet exciton in our polycrystalline films is delocalized over about ten molecules. Within these domains, the exciton's coherence length steadily increases as the temperature drops, until it reaches the limits of the domain, whereupon it saturates and remains constant as the temperature is lowered further. While the theoretical modeling correctly reproduces the temperature dependence of the fluorescence spectral lineshape, the decay of the singlet exciton appears to be determined by a trapping process that becomes more rapid as the temperature is lowered. This more rapid decay is consistent with accelerated trapping due to increased delocalization of the exciton at lower temperatures. These observations suggest that exciton coherence can play an important role in both radiative and nonradiative decay channels in these materials. Our results show that the spectroscopy of polyacene solids can be analyzed in a self-consistent fashion to obtain information about electronic delocalization and domain sizes.

We show that there exists a sum rule for the local mode density that is associated with radiative spontaneous decay of an atom in an absorbing dielectric material. The existence of such a sum rule in particular means that dielectric surroundings merely redistribute the local density of states along the real frequency axis. This result supercedes a previously obtained sum rule [S. M. Barnett and R. Loudon, Phys. Rev. Lett. 77, 2444 (1996)] that is valid only for strictly nonabsorbing dielectric materials. The new sum rule also provides a way of computing the integrated density of states associated with nonradiative decay channels.

Excitation spectra of luminescence quantum yield in alkali and silver halides are measured at low temperatures using synchrotron radiation for the energy range up to 30 eV. The yields at the valence exciton excitation are low (0.1 at most) in alkali halides (KCl, KBr and NaBr), but high (0.4) in AgCl. Enhancement of the yield indicative of multiple exciton production is observed at higher energies, and the yields in NaBr and AgCl exceed unity in the energy range studied. Discussion is given for the competition between radiative and non-radiative decay channels during relaxation of excited states.

Following excited states in molecular systems using density-based indexes: A dual emissive system as a test case

The optical properties of as-grown and electron-irradiated α-Al₂0₃ (corundum) single crystals were investigated at low temperatures, using vacuum-W (VUV) time-resolved luminescence spectroscopy under the synchrotron radiation excitation. It has been shown that in the electron-irradiated crystals the excitation of self-shrunk exciton emission extends to the region of interband transitions. The decay time of the 7.6-eV emission of the self-shrunk excitons in as-grown crystals was found to be about 7 and 23.8 ns respectively for the excitations in the excitonic band and at energies at which several electron-hole pairs are generated. In the irradiated crystals, these lifetimes were by about 2 ns longer than in the as-grown ones. For both crystal states, the nature and dynamics of the electronic excitations are discussed.

It is shown on the basis of an analysis of the J-band shape, luminescence spectra, and luminescence decay that self-trapping of excitons takes place in J-aggregates with strong topological disorder. A self-trapping barrier must be overcome for this to occur. A microscopic model of the self-trapped state is presented.

Time evolution of tunnelling recombination luminescence in CsI:Na

The regulation of hollow morphology, band structure modulation of solid solution, and introduction of cocatalysts greatly promote the separation of electron-hole pairs in photocatalytic processes, which is of great significance for the process of photocatalytic hydrogen evolution (PHE). In this study, we constructed Zn1-xCdxS hollow solid solution photocatalysts using template and ion exchange methods, and successfully loaded PdS quantum dots (PdS QDs) onto the solid solution through in situ sulfidation. Significantly, the 0.5 wt% PdS QDs/Zn0.6Cd0.4S composite material achieved a H2 production rate of 27.63 mmol g-1 h-1 in the PHE process. The hollow structure of the composite material enhances processes such as light reflection and scattering, the band structure modulation of the solid solution enables the electron-hole pairs to reach an optimal exciton recombination balance, and the modification of PdS QDs provides abundant sites for oxidation, thereby promoting the proton reduction and hydrogen evolution rate. This work provides valuable guidance for the rational design of efficient composite PHE catalysts with strong internal electric field.

Decay dynamics of Eu 3+/Tb 3+ in solution-processed Y 2O 3

The synthesis and characterization of wide bandgap II–VI (~3.6 eV) semiconductor materials has been the major area of research in the field of nanotechnology since last 10 years. The nano-sized wide bandgap semiconductor crystallites have unique chemical, physical, and electronic properties, which support their potential application in opto-electronics, data storage devices, quantum dot lasers, nanosensors, nanophosphors, biological markers, and efficient photocatalytic applications [1–6]. When the size of semiconductor nanoparticles is reduced below a critical diameter (<100 nm), the spatial confinement of the charge carrier causes them to behave quantum mechanically. As a result, the bands split into discrete electronic states in the valence and conduction bands, and nanoparticles behave more like a giant atom. Due to the quantum confinement effect [7,8], semiconductor nanomaterials also exhibit large surface area to volume ratio. The high surface-to-volume ratio of nanoclusters leads to high surface reactivity and enhance the chemical reaction dynamics and photon absorption efficiency. In case of photocatalytic processes, the electron–hole pairs generated on the surface of semiconductor materials by visible or ultraviolet (UV) photon undergo redox reactions with molecules adsorbed onto the surface, thereby breaking them into smaller fragments. The process finds its applications for the mineralization of toxic organic compounds, hazardous inorganic constituents, and bacteria disinfection. An ideal photocatalyst should be stable, inexpensive, nontoxic, and highly photoactive. Doping of pure semiconductor nanomaterials with rare earths and/or transition metals creates quasistable energy states within the bandgap, thereby affecting the electronic structure, optical, and transition probabilities. In case of doped semiconductor nanocrystals, impurity ion occupies the host atom lattice site and behaves as trap site for electron and holes. The electrons are excited from the conduction band by absorbing the energy equal or greater than their bandgap energy. Subsequent relaxation of these photoexcited electrons to some surface states or levels is followed by radiative and nonradiative decay processes. In case of photocatalysis, increased electron trapping due to higher defect sites leads to enhancement in the photocatalytic efficiency. This increase in photocatalytic efficiency is possible provided the electron– hole pair recombination rate is lower than the rate of electron transfer to adsorbed molecules. Therefore, the photocatalytic and luminescent properties of wide bandgap semiconductor nanomaterials are strongly dependent upon the type of dopant ions. Moreover, photophysical and photochemical behavior correlation and their dependence on doping concentration and nanostructure morphology will be very beneficial for making the phosphor and photocatalytic calibration curves, which will serve as the backbone for the future applications of these smart materials. CONTENTS

Plasmon enhancement of optical properties is both fundamentally important and appealing for many biological and photonic applications. Although metal-enhanced two-photon excitation fluorescence has been demonstrated in the solid substrates, there is no report on metal enhanced overall two-photon excitation fluorescence in the colloid system. Here we systematically investigated gold nanorod enhanced one- and two-photon excitation fluorescence of a porphyrin molecule, T790. The separation distance between the metal core and T790 was varied by adjusting the silica shell thickness from 13 to 42 nm. One- and two-photon excitation fluorescence intensities of T790 were found to strongly depend on the thickness of silica shell that separates gold nanorod and T790. The optimum one- and two-photon excitation fluorescence enhancement was found to occur at shell thicknesses of 34 and 20 nm, with enhancement factors of 2.1 and 11.8, respectively. Fluorescence lifetime of T790 steadily decreased as the shell thickness decreased. The observed two-photon excitation fluorescence enhancement is ascribed to a combination effect of local electric field amplification and competition between increased radiative and non-radiative decay rates. Core-shell nanoparticles that displayed enhanced two-photon excitation fluorescence were also found to exhibit significantly improved singlet oxygen generation capability under two-photon excitation. The applications of these nanoparticles as effective agents for two-photon cell imaging and nano-photosensitizers for two-photon photodynamic therapy with improved efficiency have also been demonstrated in HepG2 cancer cells. The combined advantages of enhanced two-photon excitation fluorescence and two-photon induced singlet oxygen generation make these core-shell nanoparticles as attractive agents for two-photon imaging guided two-photon photodynamic therapy.

Spectroscopic properties of CaMoO4 and MgMoO4 crystals were studied in view of their application to cryogenic scintillation detectors.Luminescence spectra and the luminescence decay kinetics were measured over a wide range oftemperatures (8–300 K). For the first time we measured time-resolved luminescence spectra ofCaMoO4. In addition to the green emission arising from the triplet state of self-trappedexcitons (STEs), a new band at around 430 nm with a decay time constant10 ± 3 ns wasobserved at T = 8 K. This emission is assigned to the radiative decay of a singlet STE.The relaxation of electronic excitations in the crystals under study is discussed on the basisof our current understanding of their electronic structures and a configurationcoordinate model for the radiative decay of STEs. The model includes adiabaticpotential energy surfaces (APESs) associated with singlet and triplet states andexplains the variation of the luminescence kinetics with temperature as a result of are-distribution in the population of these states. Thus, judging from the change of thesinglet STE emission due to temperature variation, we infer the existence of anenergy barrier between the singlet and triplet APESs. The multi-exponentialcharacter of the decay of the triplet emission can be understood assuming that therelevant radiative transitions originate from different minima of the triplet APES.Non-radiative energy transfer processes control the population of these states,resulting in thermal variation of the intensities of the different emission components.

The decay time of an exciton depends on the coupling between the dipole oscillator and the optical environment in which it is placed. For an organic light-emitting device this environment is determined by the thin-film layer structure. The radiative decay competes with nonradiative decay channels and in this way influences the luminescent efficiency and the external quantum efficiency of the device. We describe a method to estimate the dependency of the exciton decay time and the luminescent efficiency on the thin-film stack and validate the results experimentally.

The electronic structure and the excited state relaxation phenomena in condensed rare gases have been studied by absorption spectroscopy, photoelectron spectroscopy, and time resolved luminescence spectroscopy, all using selective excitation by monochromatized synchrotron radiation. Radiationless and radiative electronic transitions in excited states and the rearrangement of surrounding matrix atoms will be discussed for excited rare gas atoms in rare gas matrices. Furthermore, experimental results concerning the scattering of excitons within the free exciton branches, localization of excitons, relaxation within the self-trapped exciton states, and energy transfer to guest atoms and boundaries will be presented.