Impurity-trapped Excitons Research Articles

Luminescence of Eu2+ Ions in CsMgPO4 Phosphor: Anomalous Emission and its Origin

Eu2+-doped CsMgPO4 phosphors were synthesized by the conventional high-temperature solid-state reaction. The phase formations of the phosphors were confirmed by X-ray powder diffraction measurements. The photoluminescence excitation and emission spectra, the temperature dependent (10–300 K) luminescence intensities and the decay curves of the phosphor were measured. The Eu2+-doped CsMgPO4 shows a red emission band peaking at around 650 nm from 4f65d1→4f7 transitions of Eu2+ ions. The luminescence nature of CsMgPO4:Eu2+ is greatly different from the other Eu2+ doped ABPO4 orthophosphates (A and B are mono- and divalent cations, respectively) and is characterized by the red-shifted emission, the larger Stokes shift and a lower luminescence quenching temperature. The anomalous luminescence properties of CsMgPO4:Eu2+ are ascribed to impurity trapped exciton emission.

Luminescence Properties of Yb2+ Doped NaBaPO4 Phosphate Crystals

Yb2+-doped NaBaPO4 phosphors were synthesized by the conventional high-temperature solid-state reaction. The x-ray powder diffraction, photoluminescence excitation and emission spectra, and the decay curves, were measured. The crystallographic site-occupations of Yb2+ ions in NaBaPO4 host were assigned on the base of the luminescence spectra and the crystal structure. The luminescence natures of NaBaPO4:,Yb2+, e.g., the Stokes shift, the luminescence quenching temperature, and the activation energy of thermal quenching were reported. The thermal stability of luminescence was investigated on the base of the temperature-dependent emission intensities and the decay curves. Some anomalous luminescence properties, e.g., the larger Stokes shift, the short decay lifetimes, the lower quenching temperature and low activation energies for thermal quenching, were discussed. The luminescence was ascribed to impurity trapped exciton emission.

Excited states dynamics under high pressure in lanthanide-doped solids

Emission related to rare earth ions in solids takes place usually due to 4fn→4fn and 4fn−15d1→4fn internal transitions. In the case of band to band excitation the effective energy transfer from the host to optically active impurity is required. Among other processes one of the possibilities is capturing of the electron at the excited state and the hole at the ground state of impurity. The latest results on high pressure investigations of luminescence related to Pr3+ and Eu2+ in different lattices are briefly reviewed. The influence of pressure on anomalous luminescence and 4fn−15d1→4fn luminescence in BaSrF2:Eu2+ and LiBaF3:Eu2+ systems and Pr3+ 4fn→4fn emission quenching is presented and discussed. A theoretical model describing the impurity-trapped exciton as a system where a hole is localized at the impurity and an electron is captured by Coulomb potential at Rydberg-like states is developed. The results show the importance of local lattice relaxation for the creation of stable impurity-trapped exciton states. The ligands shifts create a potential barrier that controls the effect of mixing between the Rydberg-like electron and localized electron wave functions.

Yb 2 + -doped SrCl2: Electronic structure of impurity states and impurity-trapped excitons

First-principles electronic structure calculations of the excited states of Yb(2+)-doped SrCl(2) crystals up to 65,000 cm(-1) reveal the existence of unexpected excited states with double-well potential energy surfaces and dual electronic structure lying above and very close in energy to the 4f(13)5d manifold, with which they interact strongly through spin-orbit coupling. The double-well energy curves result from avoided crossings between Yb-trapped exciton states (more stable at short Yb-Cl distances) and 4f(13)6s impurity states (more stable at long Yb-Cl distances); the former are found to be preionization states in which the impurity holds the excited electron in close lying empty interstitials located outside the YbCl(8) moiety. Spin-orbit coupling between the double-well states and the lower lying 4f(13)5d impurity states spreads the dual electronic structure character to lower energies and, hence, the instability of the divalent oxidation state is also spread. To some extent, the dual electronic structure (impurity-trapped exciton-impurity state) of some excited states expresses and gives support to hypotheses of interaction between Yb(2+) and Yb(3+) pairs proposed to understand the complex spectroscopy of the material and conciliates these hypotheses with interpretations in terms of the existence of only one type of Yb(2+) defect. The results presented confirm the presence of impurity states of the 4f(13)6s configuration among the 4f(13)5d manifolds, as proposed in literature, but their energies are very different from those assumed. The Yb-trapped excitons found in this chloride host can be seen as precursors of the luminescent Yb-trapped excitons characterized experimentally in the isomorphous SrF(2) crystals.

High Pressure Study of Localized States Related to Lanthanide Ions in Solids

The latest results on high pressure investigations of luminescence related to and in different lattices are presented in this paper. In the latter case, the influence of pressure on anomalous luminescence in for , 0.3, 0.5, and 1, and the system as a function of pressure are presented and discussed. A theoretical model describing an impurity-trapped exciton (ITE) as a system where the first carrier is localized at the impurity and the second one is captured by the coulomb potential at Rydberg-like states is developed. The model is used to analyze the anomalous luminescence in -doped fluorides and the pressure quenching of luminescence in oxides. The model explains the differences in the formation of an ITE depending on whether the first excited impurity state belongs to or electronic configurations.

Impurity-trapped excitons: Experimental evidence and theoretical concept

The paper summarizes experimental evidence of anomalous luminescence in RE-doped dielectric materials. Special attention is paid to Pr3+-doped LiNbO3 and LiTaO3, and Eu2+-doped XF2, where X=Ba, Sr. Luminescence, luminescence excitation spectra and luminescence kinetics results are discussed obtained at ambient and high hydrostatic pressure at various temperatures. Hydrostatic pressure has been shown to cause a red shift of anomalous luminescence. The experimental data show the existence of temperature- and pressure-induced spectral transformation where anomalous luminescence is replaced with normal d–f emission in Eu2+ centers. The model predicts strong electron-lattice coupling of bound excitons and relaxation in the system’s excited states, as well as the pressure effect of spectral transformation from anomalous to normal emission.

New Observations on the Pressure Dependence of Luminescence from Eu2+-Doped MF2 (M = Ca, Sr, Ba) Fluorides

The luminescence from Eu(2+) ions in MF2 (M = Ca, Sr, Ba) fluorides has been investigated under the pressure range of 0-8 GPa. The emission band originating from the 4f(6)5d(1) --> 4f(7) transition of Eu(2+) ions in CaF2 and SrF2 shows the red-shift as increasing pressure with pressure coefficients of -17 meV/GPa for CaF2 and -18 meV/GPa for SrF2. At atmospheric pressure, the emission spectrum of BaF2:Eu(2+) comprises two peaks at 2.20 and 2.75 eV from the impurity trapped exciton (ITE) and the self-trapped exciton (STE), respectively. As the pressure is increased, both emission peaks shift to higher energies, and the shifting rate is slowed by the phase transition from the cubic to orthorhombic phase at 4 GPa. Due to the phase transition at 4-5 GPa pressure, the ITE emission disappears gradually, and the STE emission is gradually replaced by the 4f(6)5d(1) --> 4f(7) transition of Eu(2+). Above 5 GPa, the pressure behavior of the 4f(6)5d(1) --> 4f(7) transition of Eu(2+) in BaF2:Eu(2+) is the same as the normal emission of Eu(2+) in CaF2 and SrF2 phosphors.

Geometry and electronic structure of impurity-trapped excitons in Cs2GeF6:U4+ crystals. The 5f17s1 manifold

Excitons trapped at impurity centers in highly ionic crystals were first described by McClure and Pedrini [Phys. Rev. B 32, 8465 (1985)] as excited states consisting of a bound electron-hole pair with the hole localized on the impurity and the electron on nearby lattice sites, and a very short impurity-ligand bond length. In this work the authors present a detailed microscopic characterization of impurity-trapped excitons in U(4+)-doped Cs(2)GeF(6). Their electronic structure has been studied by means of relativistic ab initio model potential embedded cluster calculations on (UF(6))(2-) and (UF(6)Cs(8))(6+) clusters embedded in Cs(2)GeF(6), in combination with correlation methods based on multireference wave functions. The local geometry of the impurity-trapped excitons, their potential energy curves, and their multielectronic wave functions have been obtained as direct, nonempirical results of the methods. The calculated excited states appear to be significantly delocalized outside the UF(6) volume and their U-F bond length turns out to be very short, closer to that of a pentavalent uranium defect than to that of a tetravalent uranium defect. The wave functions of these excited states show a dominant U 5f(1)7s(1) configuration character. This result has never been anticipated by simpler models and reveals the unprecedented ability of diffuse orbitals of f-element impurities to act as electron traps in ionic crystals.

Photoionization processes of rare-earth dopant ions in ionic crystals

Photoionization of impurity ions in ionic host crystals has been studied through photocoductivity measurements. Besides the conventional method using blocking electrodes and a vibrating reed electrometer to measure directly the photocurrent, the microwave resonant cavity technique was developed and applied for the first time to dielectric crystals. This technique, based on the detection of the transient responses of the dielectric permittivity under pulsed laser excitation, is very promising since it allows to work on polycrystals including nanocrystals, to study details of the photoconductivity dynamics and to know about the nature of the released electrons (free or bound). A systematic study of the photoionization thresholds was able to provide pertinent information on the location of impurity ion states in comparison with the edge of the conduction band. In some divalent rare-earth doped alkaline earth fluorides, it was demonstrated that the delocalization of the electron in the conduction band may result in the formation of impurity-trapped excitons leading to unexpected luminescence. Models have been proposed to evaluate the photoionization energies and to describe exciton-like states.

Luminescence properties of SrSi 2O 2N 2 doped with divalent rare earth ions

The optical properties of SrSi 2O 2N 2 doped with divalent Eu 2+ and Yb 2+ are investigated. The Eu 2+ doped material shows efficient green emission peaking at around 540 nm that is consistent with 4f 7→4f 65d transitions of Eu 2+. Due to the high quantum yield (90%) and high quenching temperature (>500 K) of luminescence, SrSi 2O 2N 2:Eu 2+ is a promising material for application in phosphor conversion LEDs. The Yb 2+ luminescence is markedly different from Eu 2+ and is characterized by a larger Stokes shift and a lower quenching temperature. The anomalous luminescence properties are ascribed to impurity trapped exciton emission. Based on temperature and time dependent luminescence measurements, a schematic energy level diagram is derived for both Eu 2+ and Yb 2+ relative to the valence and conduction bands of the oxonitridosilicate host material.

Luminescent properties of Ce 3+ in MSO 4 (M: Ca, Sr and Ba) and the effect of Na + co-doping

The optical properties of Ce 3+ in CaSO 4, SrSO 4 and BaSO 4 are reported. The Ce 3+ ion shows 4 f 05 d 1→ 2 F 5/2, 2 F 7/2 luminescence in all three sulphates. Co-doping with Na + does not change the local surrounding of the Ce 3+ ion, but enhances the amount of Ce 3+ ions built in. Under optical excitation, besides the typical Ce 3+ doublet emission in the ultraviolet spectral region, band emission around 445 nm was observed. This band emission was not assigned to emission from a Ce 3+ centre, but to emission from an impurity-trapped exciton. Under X-ray excitation, both Ce 3+ emission and an emission band around 380 nm was observed. This band was assigned to emission from a self-trapped exciton.

UV induced afterglow of KCl:Eu,KBr:Eu and NaCl:Eu at low temperature

We propose a model for the production of defects by non-ionising radiation in impurity-doped alkali halides based on the formation of impurity-trapped excitons by excitation of the impurity, instead of ionisation of the impurity as has been proposed before. The dissociation of the trapped excitons into F Z and H centres in our model is similar to the case of the formation of F and H centres induced by ionising radiation in pure alkali halides. To gather information about the mechanism of defect creation by non-ionising radiation, we investigate the afterglow (AG) emission induced by UV light at low temperature (20 K) in three different alkali halides: KCl, KBr and NaCl doped with Eu impurity. We conclude that the AG emission can be split into two components; one of them has a fast decay and the other one remains for several minutes, decaying according to t −1/2. This second component has a close relation with the thermoluminescence emission. We show that the decay of the second component can be explained within the framework of our model if we assume that the H centres migrate along lines. We suggest that these lines are dislocation lines.

The influence of the host lattice on the luminescence of divalent europium

The luminescence of Eu 2+ in several silicate host lattices has been studied. In these structures there are chains of alkaline earth ions. In some cases the Eu 2+ ion in these chains shows emission at long wavelengths. This effect is due to preferential orientation of a d-orbital. In (Ca, Sr) 2MgSi 2O 7:Eu 2+ it is shown that if the cell parameters are increased by replacing (part of) the calcium by strontium ions, the effect of preferential orientation of a d-orbital in the chain direction decreases and the Eu 2+ emission shifts to shorter wavelengths. However, in SrSiO 3:Eu 2+ and CaSiO 3:Eu 2+ the chains of alkaline earth ions are close to each other and a blue Eu 2+ emission is observed. This indicates that preferential orientation of only one d-orbital is required to obtain a Eu 2+ emission at low energy. Finally, in BaSiO 3:Eu 2+, BaSi 2O 5:Eu 2+ and Sr 2LiSiO 4F:Eu 2+, where there is a network of alkaline earth ions through the lattice, the conduction band is lowered and an impurity-trapped exciton emission is observed. This situation is similar to that in BaF 2:Eu 2+.

Luminescence properties of GaBO 3: Bi 3+

The luminescence properties of GaBO 3: Bi 3+ have been studied. In contrast to other borates with calcite structure, GaBO 3: Bi 3+ shows a very efficient UV emission and a blue Stokes-shifted emission caused by an annihilation of the impurity-trapped excitons. The former is attributed to single Bi 3+ ions in S 6 sites, whereas the latter is related to bismuth defect centers which arise due to difference in the size of the Ga 3+ and Bi 3+ ions (0.645 Å versus 1.02 Å).

Photoionization and luminescence properties of Bi 3+ in In1 − xLuxBO3 solid solutions

The luminescence properties of the In 1 − x Lu x BO 3: Bi 3+ (0⩽ x⩽1) solid solutions were studied. In the concentration range 0 < x⩽0.30 both the emission of Bi 3+ ions and the emission due to an annihilation of the impurity-trapped excitons are observed. The impurity-trapped exciton band shifts towards higher energies with increasing x. At x ⪢ 0.30 only emission of Bi 3+ centers is observed. This behavior is correlated with increasing band gap energy of the solid solutions and can be described in terms of configuration coordinate curve diagrams.

An emission and kinetic study of the impurity-trapped exciton in CdCl2:Pb2+

We have studied the spectroscopic properties of Pb2+ in a CdCl2 host crystal. By excitation in the absorption band due to the 1A1g to 3T1u(3P1) transition, two emission bands are observed. At very low temperature a strong UV luminescence with a very long lifetime (2.5 ms) is detected. As the temperature increases, the lifetime of the UV emission drastically decreases to a few nanoseconds at 100 K, and the UV intensity declines. Meanwhile a strong green fluorescence appears. It is assumed that this emission is of the impurity-trapped exciton type, as in the case of CdCl2:Cu+ and BaF2:Eu2+. A detailed study of the kinetics of both fluorescences is presented.

One- and two-photon spectroscopy of f → d and f → f transitions of Eu 2+ ions in M 1- xN xF 2 mixed fluoride crystals (M, N = Ba, Sr, Ca; 0 ⩽ x ⩽ 1)

Divalent europium ions exhibit either normal blue or anomalous yellow emission in fluoride crystals, the latter being due to an impurity-trapped exciton resulting from a photoionization process. In (Ca-Ba) and (Sr-Ba) mixed crystals both luminescences are observed. Absorption and emission spectra have been analyzed for several crystals, all doped with 0.02 at% Eu 2+. As the Ba concentration increases, the broadening of the zero-phonon line is due to inhomogeneity and the smoothing of the first absorption band is interpreted as a Fano resonance due to the coupling of the excited “d” electron with the conduction band (i.e. the autoionization of Eu 2+ ion). For the first time, the two-photon transition 8S 7/2 → 6P 7/2 in BaF 2 as well as in M 1- x N x F 2, (M, N = Ba, Sr, Ca; 0 ⩽ x ⩽ 1) mixed crystals is presented. We analyze the evolution of the splitting of the 6P 7/2 energy level in the mixed crystals and show that the lowering of the site symmetry from O h to C 2v induces a fourth component to grow up. Besides, this level is essentially broadened by the inhomogeneity of the crystal but no Fano resonance process with the conduction band occurs. This is in good agreement with the expected fact that the coupling of well shielded “f” electrons with the conduction band is much weaker than this of the outer “d” electrons.

Fluorescence properties of Eu2+ ions in mixed fluoride crystals M1−xNxF2 (M,N = Ca,Sr,Ba)

Divalent europium ions exhibit either normal blue or anomalous yellow emission in fluoride crystals, the latter being due to an impurity-trapped exciton resulting from a photoionization process. In (Ca-Ba) and (Sr-Ba) mixed crystals both luminescences are observed. Absorption, excitation and emission spectra have been analyzed for several crystals, all doped with 0.02 at % Eu2+. The broadening of the zero phonon line as well as this of the two-photon transition 8S72→6P72 in (Sr-Ba) mixed crystals is much more important than the one observed in (Ca-Sr) mixed crystals and is not only due to the inhomogeneity of the crystal. A Fano resonance-type analysis is presented.

Luminescence excitation of pure and impure BeO single crystals using synchrotron radiation

Absorption, reflection, UV and VUV luminescence excitation and thermoluminescence excitation spectra have been measured for BeO and BeOZn crystals in the energy range from 8 to 35 eV, using synchrotron radiation. The nature of electronic excitations in the fundamental absorption edge region is discussed; the value of the forbidden-gap energy, E g, is estimated as 10.6 eV for BeO. Intrinsic luminescence with a 6.7 eV maximum for BeO and impurity luminescence with a 6.0 eV maximum for BeOZn are due to the relaxation of both optically created self-trapped or impurity-trapped excitons. The multiplication effect of electronic excitations with E>2 E g (or E≥2 E ex for 6.7 eV luminescence) for beryllium oxide is due to the inelastic scattering of hot photoelectrons or hot photoholes.

Biexciton decay and multimagnon sideband in emission spectra of CsMnF 3

Emission spectra of intrinsic and impurity-trapped excitons and their side- bands have been measured in CsMnF 3 by a polarized time-resolved spectroscopy. From a biexcitonic decay of an emission an exciton- exciton interaction was found to be very small in contrast with those of MnF 2 and KMnF 3. Multimagnon sidebands have been assigned with the aid of calculation of magnon density of states.