Time-resolved Luminescence Spectroscopy Research Articles

Formation of mixed Eu(III)-CO3-fulvic acid complex: Spectroscopic evidence and NICA-Donnan modeling

Humic substances impact the fate of radionuclides in the environment. The formation of mixed complexes between lanthanides(III), carbonates, and humic substances is still a matter of debate and further studies are needed. Time-resolved luminescence spectroscopy (TRLS) is used to evidence the presence of Eu(III)-CO3-SRFA ternary complexes in media representative of environmental conditions. The evolutions of the Eu(III) luminescence spectra from the 5D0 → 7F0 to the 5D0 → 7F2 transitions are showing modifications of the asymmetry ratio — area ratio of the 5D0 → 7F2 over the 5D0 → 7F1 transition — that indicates the presence of several species in addition to the EuCO3+ and Eu-SRFA complexes. The occurrence of bi-exponential decay times, with one decay time faster than the one from Eu3+, is typical of the occurrence of fulvic complex, even in the presence of carbonate. The analyses of the indirect excitation of the Eu-CO3-SRFA system are used to build complexation isotherms varying either SRFA or total carbonate concentrations in the framework of the NICA-Donnan modeling of the Eu(III)-SRFA system. In view of the restricted pH range, no strict attribution to the different possible types of sites can be done. Different complex(es) formations are considered and discussed.

U(VI) sorption on Ca-bentonite at (hyper)alkaline conditions – Spectroscopic investigations of retention mechanisms

Environmental conditions in deep geological repositories for radioactive waste may involve high pH values due to the degradation of concrete. However, the U(VI) sorption at such (hyper)alkaline conditions is still poorly understood. In this study, batch sorption experiments with Ca-bentonite in the pH range 8–13 at different carbonate concentrations were combined with spectroscopic investigations in order to gain insight into the underlying retention mechanisms. It was found that U(VI) sorption strongly correlates with the aqueous U(VI) speciation determined by time-resolved laser-induced luminescence spectroscopy (TRLFS). Increasing retention with increasing pH was accompanied by a change in aqueous speciation from uranyl carbonates to uranyl hydroxides. The occurrence of luminescence line-narrowing and a decreased frequency of the symmetric stretch vibration, deduced from site-selective TRLFS, indicate the presence of adsorbed U(VI) surface complexes. X-ray absorption fine structure (EXAFS) spectroscopy confirms that surface precipitation does not contribute significantly to the removal of U(VI) from solution but that retention occurs through the formation of two non-equivalent U(VI)-complexes on the bentonite surface. The present study demonstrates that in alkaline environments, where often only precipitation processes are considered, adsorption can provide effective retention of U(VI), despite the anionic character of prevailing aqueous species.

Sorption of Eu(III) on Eibenstock granite studied by \xb5TRLFS: A novel spatially-resolved luminescence-spectroscopic technique

In this study a novel technique, micro-focus time-resolved laser-induced luminescence spectroscopy (µTRLFS) is presented to investigate heterogeneous systems like granite (mainly consisting of quartz, feldspar, and mica), regarding their sorption behavior. µTRLFS is a spatially-resolved upgrade of conventional TRLFS, which allows point-by-point analysis of single minerals by reducing the beam size of the analytic laser beam to below the size of mineral grains. This provides visualization of sorption capacity as well as speciation of a luminescent probe, here Eu3+. A thin-section of granitic rock from Eibenstock, Saxony, Germany was analyzed regarding its mineralogy with microprobe X-ray fluorescence (µXRF) and electron probe microanalysis (EPMA). Afterwards, it was reacted with 5.0 × 10−5 mol/L Eu3+ at pH 8.0 and uptake was quantified by autoradiography. Finally, the µTRLFS studies were conducted. The results clearly show that the materials interact differently with Eu3+, and often even on one mineral grain different speciations can be found. Alkali-feldspar shows very high uptake, with an inhomogeneous distribution, and intermediate sorption strength. On quartz uptake is almost 10-fold lower, while the complexation strength is higher than on feldspar. This may be indicative of adsorption only at surface defect sites, in accordance with low hydration of the observed species.

A Spectroscopic Investigation of Eu3+ Incorporation in LnPO4 (Ln = Tb, Gd1-xLux, X = 0.3, 0.5, 0.7, 1) Ceramics.

We have investigated the incorporation of the luminescent Eu3+ cation in different LnPO4 (Ln = Tb, Gd1−xLux, x = 0.3, 0.5, 0.7, 1) host phases. All samples were analyzed with powder X-ray diffraction (PXRD), Raman spectroscopy, and site-selective time-resolved laser-induced luminescence spectroscopy (TRLFS) directly after synthesis and after an aging time of one year at ambient conditions. The PXRD investigations demonstrate the formation of a TbPO4 phase in an uncommon anhydrite-like crystal structure evoked by a pressure-induced preparation step (grinding). In the Gd1−xLuxPO4 solid solution series, several different crystal structures are observed depending on the composition. The TRLFS emission spectra of LuPO4, Gd0.3Lu0.7PO4, and Gd0.5Lu0.5PO4 indicate Eu3+–incorporation within a xenotime-type crystal structure. TRLFS and PXRD investigations of the Gd0.7Lu0.3PO4 composition show the presence of anhydrite, xenotime, and monazite phases, implying that xenotime no longer is the favored crystal structure due to the predominance of the substantially larger Gd3+–cation in this solid phase. Eu3+–incorporation occurs predominantly in the anhydrite-like structure with smaller contributions of Eu3+ incorporated in monazite and xenotime. The electronic levels of the Eu3+–dopant in Gd0.3Lu0.7PO4 and Gd0.5Lu0.5PO4 xenotime hosts are strongly coupled to external lattice vibrations, giving rise to high-energy peaks in the obtained excitation spectra. The coupling becomes stronger after aging to such an extent that direct excitation of Eu3+ in the xenotime structure is strongly suppressed. This phenomenon, however, is only visible for materials where Eu3+ was predominantly incorporated within the xenotime structure. Single crystals of Eu3+–doped LuPO4 show no changes upon aging despite the presence of vibronically coupled excitation peaks in the excitation spectra measured directly after synthesis. Based on this observation, we propose a lattice relaxation process occurring in the powder samples during aging, resulting in Eu3+ migration within the crystal structure and Eu3+ accumulation at grain boundaries or xenotime surface sites.

Кинетика импульсной катодолюминесценции кристаллов ортобората лития-гадолиния, легированного примесью церия

The pulse cathodoluminescence kinetics of Li6GdB3O9 crystals in the form of both the single crystal and crystal-fiber have been studied using time-resolved absorption and luminescence spectroscopies upon excitation with a nanosecond electron beam at 293 K. Mechanism for pulse cathodoluminescence excitation has been developed on the basis of the numerical modeling of recombination processes.

Evolution of Size and Optical Properties of Upconverting Nanoparticles during High-Temperature Synthesis

We investigated the growth of β-phase NaYF4:Yb3+,Er3+ upconversion nanoparticles synthesized by the thermal decomposition method using a combination of in situ and offline analytical methods for determining the application-relevant optical properties, size, crystal phase, and chemical composition. This included in situ steady state luminescence in combination with offline time-resolved luminescence spectroscopy as well as small-angle X-ray scattering (SAXS) transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), and inductively coupled plasma optical emission spectrometry (ICP-OES). For assessing the suitability of our optical monitoring approach, the in situ-collected spectroscopic data, which reveal the luminescence evolution during nanocrystal synthesis, were compared to measurements done after cooling of the reaction mixture of the as-synthesized particles. The excellent correlation of the in situ and time-resolved upconversion luminescence with the nanoparticle sizes determined duri...

Stability of U(VI) doped calcium silicate hydrate gel in repository-relevant brines studied by leaching experiments and spectroscopy

The stability of calcium silicate hydrate (C-S-H) gel doped with uranium to form calcium uranium silicate hydrate (C-U-S-H) gel was investigated in 2.5 M NaCl, 2.5 M NaCl/0.02 M Na2SO4, 2.5 M NaCl/0.02 M NaHCO3 or 0.02 M NaHCO3 solutions relevant to the geological disposal of radioactive waste. The C-U-S-H gel samples were synthesized by direct U(VI) incorporation and characterized with time-resolved laser-induced luminescence spectroscopy (TRLFS), infrared (IR) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Time-dependent pH changes as well as the Ca, Si and U release from C-U-S-H gels into the brines, determined by inductively coupled plasma mass spectrometry (ICP-MS), were monitored for three calcium-to-silicon (C/S) ratios (0.99, 1.55 and 2.02) over 32 d. Subsequently, changes of the U(VI) speciation and C-S-H mineralogy caused by leaching were investigated with TRLFS, IR spectroscopy and XRD. Results indicated that composition and pH value of the leaching solution, the presence of portlandite as well as formation and solubility of calcite as secondary phase determine the U(VI) retention by C-S-H gel under high saline and alkaline conditions. At high ionic strengths, the Ca release from C-S-H and secondary phases like calcite is increased. Under hyperalkaline conditions only small amounts of U(VI) were released during leaching. A decrease of the pH due to the additional presence of carbonate was linked with an increased U(VI) release from C-S-H gel leading to the formation of aqueous calcium uranyl carbonate in the supernatant solution.

Electrostatic interactions and covalent binding effects on the energy transfer between quantum dots and reaction centers of purple bacteria

The effect of electrostatic interaction and covalent binding on the energy transfer from negatively charged CdSe/CdS/ZnS-COО− and positively charged CdSe/CdS/ZnS-N H3+ quantum dots to reaction centers of Rb. sphaeroides bacteria (RC) in aqueous solutions is reported. The study was performed using optical absorption and steady-state and time resolved luminescence spectroscopies. The experiments were accompanied by theoretic modelling of charge distribution on the RC surface. More effective energy transfer from positively charged QD to RC, as compared with negatively charged QD, was observed. This effect was associated with the different localization of positively and negatively charged QD on the RC surface. The theoretic analysis has demonstrated that the periplasmic RC side is characterized by larger negative charge density as compared with the cytoplasmic side. Therefore, positively charged QD are localized mainly on periplasmic RC side while negatively charged QD are localized on the cytoplasmic RC side. In RC energy acceptors (porphyrins) are localized closer to the periplasmic side. Therefore, the energy transfer efficiency from positively charged QD is higher. The increase in ionic strength equalizes the charge distribution on both RC sides, thus levelling affinity of differently charged QD with RC. Equal energy transfer efficiency for both covalently bound and unbound QD is due to the fact of QD localization on the same RC side. This study demonstrates that for binding of charged QD with RC the electrostatic interaction is principal.

Non-Singlet Oxygen Kinetic Solvent Isotope Effects in Aquatic Photochemistry.

The kinetic solvent isotope effect (KSIE) is typically utilized in environmental photochemistry to elucidate whether a compound is susceptible to photooxidation by singlet oxygen (1O2), due to its known difference in lifetime in water (H2O) versus heavy water (D2O). Here, the overall indirect photodegradation rates of diarylamines in the presence of dissolved organic matter (DOM) were enhanced in D2O to a greater extent than expected based on their reactivity with 1O2. For each diarylamine, the relative contribution of reaction with 1O2 to the observed KSIE was determined from high resolution data of 1O2 lifetimes by time-resolved infrared luminescence spectroscopy. The additional enhancement in D2O beyond reaction with 1O2 contributed significantly to the observed KSIE for diarylamines (8-65%) and diclofenac (100%). The enhancement was ascribed to slower reduction of transient radical species of the diarylamines due to H/D exchange at DOM's phenolic antioxidant moieties. A slower second-order reaction rate constant with a model antioxidant was verified for mefenamic acid radicals using transient absorption spectroscopy. Changes in lifetime and reactivity with triplet sensitizers were not responsible for the additional KSIE. Other pollutants with quenchable radical intermediates may also be susceptible to such an additional KSIE, which has to be considered when using the KSIE as a diagnostic tool.

Doping-Enhanced Short-Range Order of Perovskite Nanocrystals for Near-Unity Violet Luminescence Quantum Yield.

All-inorganic perovskite nanocrystals (NCs) have emerged as a new generation of low-cost semiconducting luminescent system for optoelectronic applications. The room-temperature photoluminescence quantum yields (PLQYs) of these NCs in the green and red spectral range approach unity. However, their PLQYs in the violet are much lower, and an insightful understanding of such poor performance remains missing. We report a general strategy for the synthesis of all-inorganic violet-emitting perovskite NCs with near-unity PLQYs through engineering local order of the lattice by nickel ion doping. A broad range of experimental characterizations, including steady-state and time-resolved luminescence spectroscopy, X-ray absorption spectra, and magic angle spinning nuclear magnetic resonance spectra, reveal that the low PLQY in undoped NCs is associated with short-range disorder of the lattice induced by intrinsic defects such as halide vacancies and that Ni doping can substantially eliminate these defects and result in increased short-range order of the lattice. Density functional theory calculations reveal that Ni doping of perovskites causes an increase of defect formation energy and does not introduce deep trap states in the band gap, which is suggested to be the main reason for the improved local structural order and near-unity PLQY. Our ability to obtain violet-emitting perovskite NCs with near-perfect properties opens the door for a range of applications in violet-emitting perovskite-based devices such as light-emitting diodes, single-photon sources, lasers, and beyond.

Förster Resonance Energy Transfer Distance Dependence from Upconverting Nanoparticles to Quantum Dots

Forster resonant energy transfer (FRET) with upconverting nanoparticles (UCNPs) as donors and quantum dots (QDs) as acceptors has been regarded as a promising tool for biosensing applications. In this work, we use time-resolved luminescence spectroscopy to analyze the UCNP-to-QD FRET and we focus on the most relevant parameter of the FRET phenomenon, UCNP–QD distance. This distance is controlled by a nanometric silica shell around the UCNP surface. We theoretically reproduce the experimental results applying FRET theory to the distribution of emitting erbium ions in the UCNP. This simple model allows us to estimate the contribution of every erbium ion to the final FRET response and to explore different strategies to improve FRET efficiency.

Improving the efficiency of solar cells by upconverting sunlight using field enhancement from optimized nano structures

Spectral conversion of the sunlight has been proposed as a method for enhancing the efficiency of photovoltaic devices, which are limited in current production by the mismatch between the solar spectrum and the wavelength range for efficient carrier generation. For example, the photo current can be increased by conversion of two low-energy photons (below the band gap of the absorber) to one higher-energy photon (i.e. upconversion). In this paper, we will review our ongoing activities aimed at enhancing such spectral-conversion processes by employing appropriately designed plasmonic nanoparticles. The nanoparticles serve as light-concentrating elements in order to enhance the non-linear upconversion process. From the theoretical side, we approach the optimization of nanoparticles by finite-element modelling of the plasmonic near fields in combination with topological optimization of the particle geometries. Experimentally, the nanostructures are formed by electron-beam lithography on thin films of Er3+-containing transparent materials, foremost TiO2 made by radio-frequency magnetron sputtering, and layers of chemically synthesized NaYF4 nanoparticles. The properties of the upconverter are measured using a variety of optical methods, including time-resolved luminescence spectroscopy on erbium transitions and spectrally resolved upconversion-yield measurements at ∼1500-nm-light excitation. The calculated near-field enhancements are validated using a technique of near-field-enhanced ablation by tunable, ultrashort laser pulses.

Influence of adipic acid on the speciation of Eu(III): Review of thermodynamic data in NaCl and NaClO4 media, and a new determination of Eu-adipate complexation constant in 0.5 mol·kgw−1 NaClO4 medium by time-resolved luminescence spectroscopy

The complexation of Eu(III) by adipic acid — AdipH2, hexane-1,6-dioic acid —, one of the major hydro-soluble degradation products (HDPs) from radio-oxidized polyesterurethane, is investigated by time-resolved luminescence spectroscopy (TRLS) in NaClO4 ionic medium (0.5 mol·kgw−1). The solubility and deprotonation constants of adipic acid, as well as specific interaction theory (SIT) coefficients of aqueous AdipH2(aq) and adipate ions (AdipH− and Adip2−) were estimated using available literature data in NaCl and NaClO4 ionic media. A 1:1 (EuAdip+) complex is evidenced in TRLS with log10β(0.5 m) = 2.09 ± 0.16. Using reaction constant at another NaClO4 ionic strength from literature, the logarithm of reaction constant log10β° = 3.88 ± 0.40 at 0 ionic strength and specific ion parameter ε(EuAdip+, ClO4-) = (0.10 ± 0.40) kgw·mol−1 is estimated. This implies that adipic acid can control the Eu(III) speciation from mildly acidic to mildly basic conditions, but not in highly basic solutions characteristic of cementitious interstitial waters.

Ligand-Tuneable, Red-Emitting Iridium(III) Complexes for Efficient Triplet-Triplet Annihilation Upconversion Performance.

A series of substituted 2-phenylquinoxaline ligands have been explored to finely tune the visible emission properties of a corresponding set of cationic, cyclometallated iridium(III) complexes. The electronic and redox properties of the complexes were investigated through experimental (including time-resolved luminescence and transient absorption spectroscopy) and theoretical methods. The complexes display absorption and phosphorescent emissions in the visible region that are attributed to metal to ligand charge-transfer transitions. The different substitution patterns of the ligands induce variations in these parameters. Time-dependent DFT studies support these assignments and show that there is likely to be a strong spin-forbidden contribution to the visible absorption bands at λ=500-600 nm. Calculations also reliably predict the magnitude and trends in triplet emitting wavelengths for the series of complexes. The complexes were assessed as potential sensitisers in triplet-triplet annihilation upconversion experiments by using 9,10-diphenylanthracene as the acceptor; the methylated variants performed especially well with impressive upconversion quantum yields of up to 39.3 %.

Exchange interaction of O2 and nitroxide radical encounter pair as studied by time-resolved near IR luminescence spectroscopy of O2(1Δg)

Phosphorescence spectral shift of singlet oxygen, O2(1Δg) was studied in the presence of a nitroxide radical quencher. The O2-nitroxide complex causes a blue-shifts in both molecular solvents and in ionic liquids. The amount of shift is found to be nearly the same for both solvents, about 50 cm−1. This shift was attributed to quartet-doublet energy separation of O2(3Σg−)-nitroxide pair by exchange interaction of O2 and nitroxide radical.

Upconversion/back-transfer losses and emission dynamics in Nd3+-Yb3+ co-doped phosphate glasses for multiple pump channel laser

An active phosphate glass system co-doped with Nd3+ and Yb3+ rare earth, optically active ions was studied using steady-state and time-resolved luminescence spectroscopies. Because this system exhibits great potential for the development of multiple pump channel Yb3+ lasers, this work attempts to identify losses in order to establish the limits of its radiative efficiency. To achieve this, multiple upconverted visible emissions related to Nd3+ ions were observed in experiments carried out under near-infrared excitation at around 800 nm. An analysis was carried out to clarify if these upconverted emissions were governed by excited-state absorption of pump radiation or energy transfer between the rare earth ions. Another potential source of losses is related to energy back-transfer from Yb3+ to Nd3+ ions. However, no evidence of this phenomenon was observed, which is a good result for laser applications. Finally, the luminescence dynamics of the Yb3+ ions were investigated in the framework of fast-diffusion processes.

Scintillating ceramics based on non-stoichiometric strontium hafnate

A set of non-stoichiometric strontium-hafnate ceramic samples was synthesized by the spark plasma sintering technique from powder precursors of optimized composition. The scintillation and optical properties of ceramic samples were investigated by means of time-resolved luminescence spectroscopy. We measured radioluminescence, photoluminescence excitation and emission spectra as well as luminescence decays excited by UV and soft X-rays, respectively. Multiple annealing was employed to optimize ceramic performance. Furthermore, optical properties of powder and ceramic sample of the same composition were compared. Application potential of studied ceramics in superfast imaging applications is discussed as well.

Unraveling the Ultrafast Exciton Relaxation and Hidden Energy State in CH3NH3PbBr3 Nanoparticles

Recently organic–inorganic lead-halide perovskite nanoparticles (NPs) have been attractive as low-cost and high-conversion-efficient solar cells and light-emitting diode. The generation of the exciton and dissociation into free carriers are quite important for primary photoelectric conversion processes. In this study, we have examined the initial exciton dynamics of CH3NH3PbBr3 (MAPbBr3) NPs by femtosecond transient absorption spectroscopy and picosecond time-resolved luminescence spectroscopy. The ultrafast exciton quenching with a time constant of 200 fs was observed, which may be related with longitudinal optical phonon and/or lurching motion of MA cation in the excited state. In addition, higher electronic state with a short lifetime was clearly detected by the excitation intensity dependence of time-resolved luminescence and transient absorption spectra. These findings of MAPbBr3 NPs are very important not only for understanding the generation of charge carrier but also for constructing the high-effi...

Facile preparation of multifunctionalisable 'stealth' upconverting nanoparticles for biomedical applications.

Pure hexagonal (β-phase) NaYF4-based hydrophobic upconverting nanoparticles (UCNPs) were surface-modified with O-phospho-l-threonine (OPLT), alendronic acid, and PEG-phosphate ligands to generate water-dispersible UCNPs. Fourier-transform infrared (FTIR) spectroscopy was used to establish the presence of the ligands on the UCNP surface. These UCNPs exhibit great colloidal stability and a near-neutral surface at physiological pH, as confirmed by dynamic light scattering (DLS) and zeta potential (ζ) measurements, respectively. The particles also display excellent long-term stability, with no major adverse effect on the size of UCNPs when kept at pH 7.4. Upon exposure to human serum, PEG-phosphate- and alendronate-coated UCNPs showed no formation of biomolecular corona, as confirmed by SDS-PAGE analysis. The photophysical properties of water-dispersible UCNPs were investigated using steady-state as well as time-resolved luminescence spectroscopy, under excitation at ca. 800 nm. The results clearly show that the UCNPs demonstrate bright upconversion (UC) luminescence. Furthermore, the presence of reactive groups on the NPs, such as free amine groups in alendronate-coated UCNPs, enables further functionalisation of UCNPs with, for example, small molecules, peptides, proteins, and antibodies. Overall these protein corona resistant UCNPs show great biocompatibility and are worthy of further investigation as potential new biomaging probes.

Photoluminescence and Lifetime Measurement for the Excitation and Temperature Dependence of Carrier Relaxation in III-V Semiconductors

Researches in the field of III-V semiconductor photonic devices have initiate applications in a number of disciplines including lighting, optical communications and biomedical engineering. One of the limiting factors for getting better the photonic devices is the carrier relaxation time. This is the time obligatory for energetic carriers to cool to the edge of their particular bands in a bulk semiconductor material, or to the bottom of a well throughout inter- and intra-sub-band spreading in a heterojunction structure. From these lower energy states, they can afterwards recombine radiatively in photonic devices. This study exploited the nonlinear optical practice of frequency up conversion to complete time-resolved luminescence spectroscopy on epitaxial bulk GaAs samples to analyse carrier relaxation times in each as a function of excitation irradiance and temperature of the sample. There is no electrons and defect energy level in the energy curve for p-type samples. In this study, we focus on the recombination process of yellow-luminescence, which causes the decrease in emission efficiency. The variation of yellow-photoluminescence spectrum shape and intensity, which is caused by occupation YL centers by electrons and thermal activation processes of energy level transitions of electrons by phonon collision in GaAs. The measurement model explains the dependence of the PL intensity on excitation intensity, as well as the PL lifetime and its temperature dependence. We demonstrate that time-resolved PL measurements can be used to find the concentrations of free electrons and acceptors contributing to PL in p-type semiconductors.