Site-selective Excitation Research Articles

Luminescence and crystallographic sites for Eu3+ ions in Sr5(PO4)3F phosphor

Abstract This paper reports on the luminescence studies of Eu 3+ -doped strontium fluoroapatite. The goal of this paper is to establish and characterize the possible sites for Eu 3+ substitution in Sr 5 (PO 4 ) 3 F lattice. Eu 3+ -doped Sr 5 (PO 4 ) 3 F phosphor was prepared by the solid state reaction method. The X-ray powder diffraction result of as-synthesized powder phosphor reveals the single phase Sr 5 (PO 4 ) 3 F and it also indicates that the incorporation of Eu 3+ ions does not affect the crystal structure. Photoluminescence (PL) studies of Eu 3+ -doped Sr 5 (PO 4 ) 3 F phosphor are performed to study spectral properties of the sample. Site-selective excitation and emission spectra together with the decay curves are investigated by site-selective laser-excitation spectroscopy. The three crystallographic sites for Eu 3+ ions are identified in the 7 F 0 → 5 D 0 excitation spectra by using a pulsed, tunable, and narrowband dye laser. The luminescence due to the 5 D 0 → 7 F J ( J = 1, 2) transitions under excitation at each crystallographic site exhibits its own spectral features. Three crystallographic sites in Sr 5 (PO 4 ) 3 F give rise to different crystal-field splits of the 7 F 1 and 7 F 2 multiplets. Heterovalent substitution by rare-earth ions (Eu 3+ ) for Ca 2+ positions in the hexagonal crystal lattice requires charge compensation. The charge-compensation mechanism, site symmetry, and the crystal-field strength on Eu 3+ in Sr 5 (PO 4 ) 3 F are discussed for better understanding of preferential substitution of Eu 3+ in the Sr 5 (PO 4 ) 3 F lattice.

Probing of local alloy disorder in InGaN using Er3+ ions

Abstract We report on studies to probe the local lattice disorder in InGaN:Er epilayers using the 1.54 μm emission of Er3+ ions. The InGaN layers were doped during MOCVD growth with Er to a concentration of about 2.3%. Site selective optical spectroscopy and excitation photoluminescence spectroscopy were used to show that in InGaN:Er two classes of the emitting Er centers can be distinguished: (i) one leading to emission of Er3+ which is nearly the same as in GaN:Er, except nonuniform broadening and (ii) another that yields new emission features not having its counterpart in GaN:Er. The latter emission is interpreted as originating from Er-complexes involving one or more In atoms in the second coordination sphere. The observed fluorescence line broadening and wavelength shifts in the emission wavelength indicate the extent of the disorder in the metallic sublattice.

Broadband, site selective and time resolved photoluminescence spectroscopic studies of finely size-modulated Y2O3:Eu3+ phosphors synthesized by a complex based precursor solution method

Undoped and Eu 3þ -doped cubic yttria (Y2O3) nanophosphors of good crystallinity, with selective particle sizes ranging between 6 and 37 nm and showing narrow size distributions, have been synthesized by a complex-based precursor solution method. The systematic size tuning has been evidenced by transmission electron microscopy, X-ray diffraction, and Raman scattering measurements. Furthermore, sizemodulated properties of Eu 3þ ions have been correlated with the local structure of Eu 3þ ion in different sized Y2O3:Eu 3þ nanophosphors by means of steady-state and time-resolved site-selective laser spectroscopies. Time-resolved site-selective excitation measurements performed in the 7 F0 / 5 D0 peaks of the Eu 3þ ions at C2 sites have allowed us to conclude that Eu 3þ ions close to the nanocrystal surface experience a larger crystal field than those in the nanocrystal core. Under the site-selective excitation in the 7 F0 / 5 D0 peaks, energy transfer between the sites has also been observed.

Luminescence properties of Eu-doped GaN under resonant excitation and quantitative evaluation of luminescent sites

We report a study on the Eu luminescence properties of Eu-doped GaN grown on a GaN substrate by organometallic vapor phase epitaxy. The site-selective excitation of Eu ions revealed the concentration of each luminescent site using the luminescence properties under resonant excitation. The quantitative evaluation of the Eu luminescent sites showed that more than 80% of Eu ions are incorporated into a high-symmetry site. However, the photoluminescence spectrum under indirect excitation is markedly different from that under resonant excitation, which indicates that the luminescent site with high symmetry exhibits low-efficiency energy transfer from the GaN host to the luminescent site.

Site-selective excitation and emission of Eu^3+-doped transparent glass ceramic containing Ca_5(PO_4)_3F nanocrystals

An Eu3+-doped transparent oxyfluoride glass ceramic containing Ca5(PO4)3F nanocrystals was prepared by melt quenching and subsequent thermal treatment. The transmittance of the glass ceramic with a thickness of 1.92 mm is up to 80.5% in the visible range at room temperature. Site-selective excitation and emission spectra indicate that Eu3+ ions in the Ca5(PO4)3F nanocrystals occupy two types of sites, A and B, with a same point symmetry Cs. As for Eu3+ ions at site A, the crystal field is more deformed and stronger. The crystal field may appear in the form of a ligand ion F- being replaced by O2- in accordance with the charge compensation scheme: Ca2+ + F-→Eu3+ + O2-. As for Eu3+ ions at site B, the possible charge compensation scheme is 3Ca2+→2Eu3+ + Vacancy. Furthermore, an energy transfer process from Eu3+ ions at site A to that at site B in the glass ceramic at low temperature was also discussed.

Synthesis and investigation of luminescence properties of Eu3+-doped cubic perovskite Ba3Y2WO9

Abstract Eu 3+ -doped cubic double perovskite Ba 3 Y 2 WO 9 phosphors have been firstly successfully synthesized by sol–gel method. The prepared samples were characterized by X-ray diffraction (XRD) and field emission-scanning electron microscopy (FE-SEM). The luminescence excitation and emission spectra in the ultraviolet–visible (UV) region were used to investigate the luminescence properties of these phosphors. Then the site-selective excitation and emission spectroscopy along with luminescence decay have been investigated in the 5 D 0 → 7 F 0 region under a pulsed, tunable, narrowband dye laser at room temperature. In our study, the crystallographic occupations of Eu 3+ ions were explored based on both spectroscopy and crystal structure of cubic Ba 3 Y 2 WO 9 :Eu 3+ . The multiple site structure of Eu 3+ ions with highly disordered distributions in cubic Ba 3 Y 2 WO 9 lattices was suggested. The results help us to understand the site assignments of rare earth ions doped in cubic Ba 3 Y 2 WO 9 .

Spectral study of the stimulated emission of Nd^3+ in fluorotellurite bulk glass

In this work we present, for the first time to our knowledge, laser emission under wavelength selective laser-pulsed pumping in Nd(3+)-doped TeO(2)-ZnO-ZnF(2) bulk glass for two different Nd(3+) concentrations. The fluorescence properties of Nd(3+) ions in this matrix which include, Judd-Ofelt calculation, stimulated emission cross-section of the laser transition and lifetimes are also presented. The site-selective emission and excitation spectra along the (4)I(9/2)→(4)F(3/2) absorption band show the inhomogeneous behaviour of the crystal field felt by Nd(3+) ions in this fluorotellurite glass which allows for spectral tuning of the laser output pulse as a function of the pumping wavelength. The emission cross-section obtained from the Judd-Ofelt analysis and spectral data (4.9x10(-20) cm(2)) is in fairly good agreement with the value obtained from the analysis of the laser threshold data (4x10(-20) cm(2)).

The site-selective excitation and the dynamical electron–lattice interaction on the luminescence of YBO3: Sb3+

The phosphor of YBO3: Sb3+ was synthesized by solid state reaction. Its crystal structure was examined by using X-ray diffraction analysis. The morphology and particle size were characterized with a scanning electron microscopy. The properties of YBO3: Sb3+ luminescence was systematically studied by exciting borate host, the P-1(1) and P-3(1) levels of Sb3+ at the C-i and C-1 sites with 168, 203, 218, 252 and 262 nm, respectively, measured on the synchrotron radiation instruments. The different configurations of emission spectra excited with variant wavelengths show the character of wavelength-selective excitation, which was attributed to different occupations of the Sb3+ site in the YBO3 crystal lattice. Moreover, strong electron-lattice interaction (i.e., the Jahn-Teller effect) between Sb3+ and YBO3 host was discriminated by comparing low temperature and room temperature spectra. The site-selective luminescence was confirmed by YBO3 activated with multiple Sb3+ concentrations. The spectral assignment was verified with fluorescence lifetime achieved by using the Time-Correlated Single-Photon Counting method. (C) 2013 Elsevier Inc. All rights reserved.

Correction to “Site-Selective Excitation of “Exciplex Tuning” for Luminescent Nanoclusters of Dicyanoargentate(I) Ions Doped in Different Alkali Halide Crystals”

Correction to “Site-Selective Excitation of “Exciplex Tuning” for Luminescent Nanoclusters of Dicyanoargentate(I) Ions Doped in Different Alkali Halide Crystals”

Site-Selective Chelation and Excitation Energy Transfer between the Higher Plant Major Light-Harvesting Complexes and Quantum Dots

The major light-harvesting complexes of photosystem II(LHCII) have been linked to many organic or inorganic materials to construct bio-inspired new hybrid materials. In this study, the authors reported a hybrid complex of recombinant LHCII and water-soluble quantum dots(QD) linked by nickel-nitriloacetic acid(NTA·Ni) with a series of molar ratio between LHCII and QD(1∶64, 1∶32, 1∶16, 1∶12, 1∶8, 1∶4,1∶2.7, 1∶2, 1∶1.6) on the QD and histidine tag on the LHCII. The spectroscopic results showed that the fluorescence at 680 nm from LHCII increased when more QD molecules were put into the hybrid system.The enhancement reached the maximum and saturated when the molar ratio between LHCII and QD was2.7∶1. The fluorescence spectra results could be explained by resonance energy transfer mechanism from QD to LHCII. Moreover, no obvious energy transfer was observed between native LHCII and QD mixture and this highlighted the necessity of linkage for such energy transfer.

The thermal stability and structural site-distribution of Eu3+ ions in the red-emitting phosphors Ca9Eu2W4O24 and Sr9Eu2W4O24

► The photoluminescence spectra of Ca 9 Eu 2 W 4 O 24 and Sr 9 Eu 2 W 4 O 24 were investigated. ► The site-occupancy of Eu 3+ ions were determined by the laser site-selective spectra. ► Eu 3+ ions have only one site in Ca 9 Eu 2 W 4 O 24 and have two disordered sites in Sr 9 Eu 2 W 4 O 24 . ► The luminescence thermal stability was discussed on the base of the crystal structure. ► The microstructure of Eu 3+ ions has great influence on the luminescence. The red-emitting phosphors Ca 9 Eu 2 W 4 O 24 and Sr 9 Eu 2 W 4 O 24 were synthesized by the solid-state reaction method. The crystal phases were characterized by X-ray powder diffraction. The photoluminescence excitation and emission spectra were investigated. The luminescence excitation and emission spectra confirm that the phosphors are efficiently excited by near UV light. The dependence of luminescence intensities on the heating temperatures was investigated. The Ca 9 Eu 2 W 4 O 24 phosphor exhibits higher thermal stability than that of Sr 9 Eu 2 W 4 O 24 . The crystallographic sites for Eu 3+ ions in Ca 9 Eu 2 W 4 O 24 and Sr 9 Eu 2 W 4 O 24 are investigated by the site-selective excitation spectra in the 5 D 0 → 7 F 0 wavelength region. It is identified that the Eu 3+ ions occupy only M sites (statistically occupied by 0.5Eu and 0.5Ca) in Ca 9 Eu 2 W 4 O 24 and, however, the Eu 3+ ions can substitute both M sites (Eu 3+ + Sr 2+ ) and Sr 2+ sites in Sr 9 Eu 2 W 4 O 24 . The luminescence spectra and the thermal stability are discussed on the basis of the crystal structure, Eu 3+ site-distributions and the energy transfer.

Site-selective photoluminescence in thiol-capped gold nanoclusters

Photoluminescence (PL) from the thiol-capped Au nonoclusters (NCs) has been investigated under site-selective excitation. Upon scanning the excitation light with energy below 2.1 eV down to 1.6 eV, the PL narrows and begins shifting linearly with excitation energy. The time-resolved PL was studied and the PL decay traces of Au NCs were found to depend on the excitation and emission energies. The slow carrier relaxation in the localized states is suggested to be responsible for the line narrowing and peak-shift in the site-selective PL.

Investigation of photoluminescence properties of Eu3+-doped GdAlO3 and LaAlO3 by site-selective laser spectroscopy

Photoluminescence properties of Eu3+-doped GdAlO3 and LaAlO3 phosphors prepared by sol–gel method have been investigated by the site-selective laser spectroscopy along with the decay time measurements at room temperature. The site-selective excitation spectra showed one 7 F0 → 5D0 transition line in GdAlO3:Eu3+ and three 7 F0 → 5D0 transition lines in LaAlO3:Eu3+, indicating that there was only one crystallographic site for Eu3+ in GdAlO3 and three crystallographic sites for Eu3+ in LaAlO3. The result of LaAlO3 was not in agreement with its crystal structure. The stark energy levels for Eu3+ at different sites were characterized and discussed by site-selective emission spectra arose from 5D0 level of Eu3+.

Vibrational energy relaxation of benzene dimer and trimer in the CH stretching region studied by picosecond time-resolved IR-UV pump-probe spectroscopy

Vibrational energy relaxation (VER) of the Fermi polyads in the CH stretching vibration of the benzene dimer (Bz(2)) and trimer (Bz(3)) has been investigated by picosecond (ps) time-resolved IR-UV pump-probe spectroscopy in a supersonic beam. The vibrational bands in the 3000-3100 cm(-1) region were excited by a ps IR pulse and the time evolutions at the pumped and redistributed (bath) levels were probed by resonance enhanced multiphoton ionization with a ps UV pulse. For Bz(2), a site-selective excitation in the T-shaped structure was achieved by using the isotope-substituted heterodimer hd, where h = C(6)H(6) and d = C(6)D(6), and its result was compared with that of hh homodimer. In the hd heterodimer, the two isomers, h(stem)d(top) and h(top)d(stem), show remarkable site-dependence of the lifetime of intracluster vibrational energy redistribution (IVR); the lifetime of the Stem site [h(stem)d(top), 140-170 ps] is ~2.5 times shorter than that of the Top site [h(top)d(stem), 370-400 ps]. In the transient UV spectra, a broad electronic transition due to the bath modes emerges and gradually decays with a nanosecond time scale. The broad transition shows different time profile depending on UV frequency monitored. These time profiles are described by a three-step VER model involving IVR and vibrational predissociation: initial → bath1(intramolecular) → bath2(intermolecular) → fragments. This model also describes well the observed time profile of the Bz fragment. The hh homodimer shows the stepwise VER process with time constants similar to those of the hd dimer, suggesting that the excitation-exchange coupling of the vibrations between the two sites is very weak. Bz(3) also exhibited the stepwise VER process, though each step is faster than Bz(2).

Luminescence characteristics and site-occupancy of Eu2+- and Eu3+-doped MgZn2(PO4)2 phosphors

Eu2+- and Eu3+-doped MgZn2(PO4)2 were prepared using the high temperature solid-state reaction method. The excitation spectra in the UV and VUV region, the emission spectra, and the luminescence decay curves of the Eu2+ and Eu3+ ions were investigated. The crystallographic distributions of the Eu2+ and Eu3+ sites in MgZn2(PO4)2 lattices are different. MgZn2(PO4)2:Eu2+ presents a bluish green luminescence with two distinct emission centers at 450 and 525 nm. MgZn2(PO4)2:Eu3+ presents an reddish orange color with luminescence transitions from the 5D0 level of the Eu3+ ions. There exist two Eu2+-crystallographic sites in MgZn2(PO4)2:Eu2+, which occupy both Mg2+ and Zn2+ sites. However, there is only one Eu3+-site in the MgZn2(PO4)2:Eu3+ lattices, which was confirmed by site-selective excitation and emission spectroscopy, and the luminescence decay in the 5D0 → 7F0 region of the Eu3+ ions under a pulsed, tunable, narrowband dye laser. The Eu3+ ion was suggested to occupy the octahedral Mg2+-site in the MgZn2(PO4)2 lattices. The spectral characteristics, the temperature-dependent luminescence intensity and activation energies of thermal stabilities and the microstructures were discussed on the basis of the crystal structure and the luminescence results.

Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption

Despite the central role of light absorption and the subsequent generation of free charge carriers in organic and hybrid organic-inorganic photovoltaics, the precise process of this initial photoconversion is still debated. We employ a novel broadband (UV-Vis-NIR) transient absorption spectroscopy setup to probe charge generation and recombination in the thin films of the recently suggested hybrid material combination poly(3-hexylthiophene)/silicon (P3HT/Si) with 40 fs time resolution. Our approach allows for monitoring the time evolution of the relevant transient species under various excitation intensities and excitation wavelengths. Both in regioregular (RR) and regiorandom (RRa) P3HT, we observe an instant (<40 fs) creation of singlet excitons, which subsequently dissociate to form polarons in 140 fs. The quantum yield of polaron formation through dissociation of delocalized excitons is significantly enhanced by adding Si as an electron acceptor, revealing ultrafast electron transfer from P3HT to Si. P3HT/Si films with aggregated RR-P3HT are found to provide free charge carriers in planar as well as in bulk heterojunctions, and losses are due to nongeminate recombination. In contrast for RRa-P3HT/Si, geminate recombination of bound carriers is observed as the dominant loss mechanism. Site-selective excitation by variation of pump wavelength uncovers an energy transfer from P3HT coils to aggregates with a 1/e transfer time of 3 ps and reveals a factor of 2 more efficient polaron formation using aggregated RR-P3HT compared to disordered RRa-P3HT. Therefore, we find that polymer structural order rather than excess energy is the key criterion for free charge generation in hybrid P3HT/Si solar cells.

Site-selected luminescence of atomic europium in the solid rare gases

Site-selective excitation has been used to simplify complex emission recorded in the visible spectral region for atomic europium isolated in the solid rare gases. In addition to y(8)P resonance fluorescence, excitation of the y(8)P state produces emission from the z(6)P state and the metastable a(10)D state. Very weak emission at 690 nm is tentatively assigned to the J = 9/2 level of the z(10)P state. Eu atoms isolated in the red and blue sites exhibit very different temperature dependence both spectrally and temporally. For the y(8)P state emission the red site atoms exhibit small Stokes shifts and yield radiative lifetimes while the emission from the blue site loses intensity and the temporal profiles shorten dramatically between 10 and 16 K indicating very efficient non-radiative relaxation in this site. An analysis of the Stokes shifts exhibited for the y(8)P state in each site supports the attributions made in a previous publication [O. Byrne and J.G. McCaffrey, J. Chem. Phys. 134, 124501 (2011)] that the smaller blue tetravacancy site has a greater repulsive interaction with the guest. With the exception of the y(8)P state resonance fluorescence, the recorded decay profiles of all the other emissions exhibit multiple components. This behaviour has been attributed to the existence of multiple crystal field levels arising from the splitting of the distinct spin-orbit levels from which emission occurs.

The luminescence and structural characteristics of Eu 3+-doped NaBaPO 4 phosphor

Abstract Eu 3+ -doped NaBaPO 4 was prepared by a high-temperature solid-state reaction. The phase formation was confirmed by X-ray powder diffraction measurements. The laser site-selective excitation and emission spectra have been investigated in the 5 D 0 → 7 F 0 region by using a pulsed, tunable and narrowband dye laser. The excitation spectra corresponding to the 7 F 0 → 5 D 0 transition consist of two transitions at 579.6 nm Eu(I) and 578.9 nm Eu(II), indicating the Eu 3+ ions occupy two crystallographic sites of Ba 2+ ions. The decay lifetimes of the two Eu 3+ sites were measured. Two crystallographic sites for Eu 3+ ions doped in NaBaPO 4 lattice were assigned from the luminescence characteristic and structure features. Meanwhile, the charge compensation mechanism of Eu 3+ doping in NaBaPO 4 was discussed.

Investigations of Yb-doped optical fiber using selective laser excitation

The method of tunable site-selective laser excitation was used for a study of the spectral and fluorescence characteristics of Yb3+ optical centers in laser optical fiber. Decay curves and positions of fluorescence spectra maxima at different wavelengths of selective laser excitation and positions of excitation spectra maxima at different fluorescence selective registration provide new information on lifetimes and Stark energy distribution among inhomogeneously broadened Yb3+ lines. The obtained Stark splitting energy distribution diagram demonstrates the tuneability of Yb3+ laser oscillation wavelength under tunable excitation of a pump laser.

The luminescence spectroscopy and thermal stability of red-emitting phosphor Ca 9Eu(VO 4) 7

Eu-based vanadate Ca 9 Eu(VO 4 ) 7 phosphor was synthesized by the solid state reaction method and was characterized by X-ray powder diffraction (XRD). The photoluminescence excitation and emission spectra, fluorescence decay curves and the dependence of luminescence intensity on temperature were investigated. The phosphor can be efficiently excited by near UV light to realize an intense red luminescence (614 nm) corresponding to the electric dipole transition 5 D 0 → 7 F 2 of Eu 3+ ions. The crystallographic site-occupations of the Eu 3+ ions in Ca 9 Eu(VO 4 ) 7 were investigated by the site-selective excitation and emission spectra, and the fluorescence decay curves in the 5 D 0 → 7 F 0 region using a pulsed, tunable, narrowband dye laser. The red luminescence together with the thermal stability was discussed on the base of the Eu 3+ site-distribution in Ca 9 Eu(VO 4 ) 7 host.