Stefan Lis

Energy transfer in solution of lanthanide complexes

The lanthanides with their well-defined energy levels provide an excellent basis to study different Ln(III)-specific energy transfer processes in a variety of chemical environments. The studies concerning intramolecular and intermolecular energy transfer processes with participation of Ln(III) ions and a variety of ligand groups in solution are reviewed. Phenomena of energy transfer from ligands to Ln(III) ions, resulting consequently in a great enhancement of the Ln(III) ion luminescence (ligand sensitized luminescence), as well as from Ln(III) to other species and between Ln(III) ions are presented.

Luminescence spectroscopy of lanthanide(III) ions in solution

Luminescence spectroscopy of Ln(III) ions, characterised by very narrow emission bands and a long decay time, is an important technique for the study of coordination, analytical and photophysical aspects of lanthanide chemistry. Factors affecting the Ln(III) luminescence in solution that can both quench or increase the intensity and lifetime of luminescence, are described. The quenching processes of the Ln(III) excited states, strongly dependent on energy of vibrators resulting from ligands (and/or solvents) and energy gaps, AE, between the emissive state and the highest sublevel of the ground state of Ln(III), are presented. Effectiveness of quenching of the luminescent excited state of the Ln(III) ions by O-H, N-H and a strong deactivating power of the azide ion, N - 3 , are described. The factors which markedly increase luminescence, efficiently reducing nonradiative energy degradation of Ln(III) ions are presented. Highly luminescent Ln(III) systems based on complex and ternary complex formation with several groups of ligands (e.g. crytptands, β-diketones, macrocyclic ligands, heterobiaryl ligands, etc.) as well as energy transfer processes are discussed. The use of europium luminescence excitation spectroscopy of the 7 F 0 → 5 D 0 transition as a unique and sensitive way to characterize the number of Eu(III) species present in solution, binding sites of various ligands and complex stoichiometries, is briefly reviewed. Recent developments in the use of the excitation spectroscopy of Eu(III) and a wealth of information which can be obtained from this method are presented.

Structural and spectroscopic properties of LaOF:Eu3+ nanocrystals prepared by the sol-gel Pechini method.

A new method was used to obtain Eu(3+)-doped LaOF nanocrystals. The obtained nanocrystals were synthesized for the first time using a modified Pechini sol-gel method. The products were analyzed by X-ray powder diffraction and the Rietveld method. Optimal conditions for the synthesis were found. Luminescent properties of the tetragonal and rhombohedral LaOF:Eu(3+) nanocrystals were investigated by collecting excitation and luminescence spectra. The most effective dopant concentrations in both hosts were found. Luminescent lifetimes were also measured. The time-resolved luminescent traces showed both a growth and a decay, which pointed to energy transfer processes between Eu(3+) ions in the LaOF host. In order to explain these phenomena, an adequate mechanism has been proposed. Intensity parameters Ω(2), Ω(4) and quantum efficiencies were calculated using the Judd-Ofelt theory, allowing for an extensive study of the luminescent properties of Eu(3+) ion in the LaOF matrix.

Multifunctionality of GdPO4:Yb3+,Tb3+ nanocrystals – luminescence and magnetic behaviour

Multifunctional, luminescent and magnetic systems were synthesised by a co-precipitation method and broadly investigated. In our studies down- and visible up-conversion of Yb3+ and Tb3+ doped GdPO4 nanocrystals were observed. The synthesised nanomaterials were analysed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Spectroscopic properties were determined from excitation and emission spectra together with a subsequent analysis. The build-up characteristics of luminescence decays showed the energy transfer processes that occurred. Magnetic measurements were performed by means of vibrating sample magnetometry (VSM). These nanomaterials showed intense green emission under ultraviolet (UV), 272 nm, and near infrared (NIR), 980 nm, laser excitation. The mechanism lying behind the observed spectroscopic properties was proposed. Energy transfer between Gd3+ and Tb3+ ions, as well as cooperative energy transfer from Yb3+ to Tb3+, was found to give a dual-mode luminescence. Also, quenching processes by cross-relaxation and phonon-assisted energy transfer were observed. The presence of different lanthanide ions and the influence of the finite-size effect underlie the complex behaviour observed.

Revision of structural properties of GdBO3 nanopowders doped with Eu3+ ions through spectroscopic studies.

Monoclinic Gd(1-x)Eu(x)BO(3) nanopowders were successfully synthesized using a modified Pechini method. The crystal structure of the prepared materials was revised and confirmed using several techniques such as: IR, XRD, TEM, Raman spectroscopy and EDX analysis. The obtained material was comprised of particles, consisting of parts with the average size 350 nm. The luminescence properties of the prepared phosphors with different concentrations of Eu(3+) ions were characterized by excitation and emission spectra and its kinetic decay. The Judd-Ofelt parameters (Ω(2), Ω(4)), quantum efficiency, η, and chromaticity coordinates were also calculated.

Photoluminescent properties of LaF3:Eu3+ and GdF3:Eu3+ nanoparticles prepared by co-precipitation method

Abstract LaF 3 :Eu 3+ and GdF 3 :Eu 3+ nanoparticles were prepared by a co-precipitation method in the presence of the chelating agent, citric acid. The structural properties of the products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average crystallite size was estimated from the full-width at half-maximum (FWHM) of the diffraction peaks by the Scherrer equation. The sizes of the nanoparticles were 12 nm for LaF 3 :Eu 3+ and 17 nm for GdF 3 :Eu 3+ . The luminescent properties of the nanoparticles were investigated by excitation and emission spectra. Energy transfer from Gd 3+ to Eu 3+ was observed.

Aqueous Solutions of Uranium(VI) as Studied by Time-Resolved Emission Spectroscopy: A Round-Robin Test

Results of an inter-laboratory round-robin study of the application of time-resolved emission spectroscopy (TRES) to the speciation of uranium(VI) in aqueous media are presented. The round-robin study involved 13 independent laboratories, using various instrumentation and data analysis methods. Samples were prepared based on appropriate speciation diagrams and, in general, were found to be chemically stable for at least six months. Four different types of aqueous uranyl solutions were studied: (1) acidic medium where UO22+aq is the single emitting species, (2) uranyl in the presence of fluoride ions, (3) uranyl in the presence of sulfate ions, and (4) uranyl in aqueous solutions at different pH, promoting the formation of hydrolyzed species. Results between the laboratories are compared in terms of the number of decay components, luminescence lifetimes, and spectral band positions. The successes and limitations of TRES in uranyl analysis and speciation in aqueous solutions are discussed.

The effects of down- and up-conversion on dual-mode green luminescence from Yb3+- and Tb3+-doped LaPO4 nanocrystals

Monoclinic LaPO4 nanocrystals doped with Yb3+ and Tb3+ ions were synthesised by co-precipitation followed by annealing at 900 °C. These materials exhibited intense luminescence under ultraviolet (UV) and near infrared (NIR) irradiation. The structural properties of the products were analysed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The obtained nanomaterials had a single phase, a spherical shape of nanocrystals and an average size of 42 ± 5 nm. The spectroscopic properties of LaPO4:Yb3+,Tb3+ nanocrystals were investigated in detail using excitation and emission spectra. Additionally, the luminescence lifetimes for both Tb3+ and Yb3+ ions were calculated from the measured decays. The optimal dopant concentrations were estimated with respect to the intensity of luminescence. A cooperative energy transfer (CET) mechanism, which causes the observed up-conversion, was proposed because of the observed dependence of integral intensity on the power of the pumping laser. The calculated efficiencies of the CET were much higher than the efficiencies of the opposing quenching processes; these parameters were measured in samples doped with high amounts of Tb3+ ions. These materials demonstrated intense green up-conversion in addition to UV-excited emission.

Luminescence studies of Eu(III) mixed ligand complexes

Abstract A series of europium(III) mixed ligand complexes, Ln/L/X based on pyridine carboxylic acid N-oxides, L, plus 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen), X, have been studied. The complexes were characterized using Eu luminescence (conventional and laser-induced) spectroscopy. A number of N-oxide ligand designs, including mono- and dicarboxylic acids and their derivatives (aryl-substituted), having various steric groups, have been tested with the main goal of optimizing the luminescence properties under various experimental conditions. Potentiometric titration of acid N-oxides have been carried out in aqueous solution in order to determine their p K a and the pH values, important for complexation conditions. Based on the Eu(III) luminescence lifetime measurements, hydration numbers of the Eu(III) complexes have been obtained, which evidenced the formation of binary EuL and EuL 2 complexes and mixed ligand EuL 2 X complexes. The results suggest that in ternary complex formation the steric requirement of the secondary ligand play an important role. The results of these studies lead us to achieve a better understanding of the factors, as aryl substituted groups (e.g. Br, NO 2 , NH 2 , CH 3 , OCH 3 ) and a kind of solvent, influencing the luminescence intensity and lifetime of the Eu(III) complexes studied.

Hydrothermal Synthesis and Structural and Spectroscopic Properties of the New Triclinic Form of GdBO3:Eu3+ Nanocrystals

Triclinic Gd1-xEuxBO3 nanophosphors have been prepared by a hydrothermal method without using additional coreagents and prior precipitation of precursor (in situ). The formation of the borate nanorods and their crystal structure was refined on the basis of X-ray diffraction patterns (XRD) and well confirmed using various techniques such as infrared spectroscopy (IR), Raman spectroscopy, transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX). The new triclinic crystal structure (space group P1) for the GdBO3 nanocrystals and detailed structure parameters were determined with the help of the Rietveld analysis. The spectroscopic characteristics of the synthesized nanomaterials with different concentrations of Eu(3+) ions were defined with the use of luminescence excitation spectra as well as emission spectra and decay kinetics. The Judd-Ofelt parameters (Ω2, Ω4) and quantum efficiency, η, were also calculated for the more detailed analysis of Eu(3+) spectra in the GdBO3 host.