Impurity-trapped Excitons Research Articles

Intrinsic electronic excitations and impurity luminescent centres in NaMgF3 and MgF2 doped with Yb2+

The relaxation pathways of states in the energy region of the fundamental absorption of NaMgF3:Yb2+ and MgF2:Yb2+ are investigated using time-resolved vacuum ultraviolet spectroscopy. For NaMgF3:Yb2+, excitation into intrinsic free exciton states or above the band gap results in emission features associated with self-trapped excitons and impurity-trapped excitons. Excitation into host-related states of MgF2:Yb2+ similarly exhibits self-trapped exciton emission as well as emission from Yb2+ 4f135d states. The excitation features related to Yb2+ 4f14→4f135d transitions are interpreted using semi-empirical crystal field models. For both materials, the interplay between intrinsic distortions and Yb impurity centres, as the excitation wavelength and sample temperature are varied, is presented and analysed.

Synthesis and optical properties of the Eu2+-doped alkaline-earth metal hydride chlorides AE7H12Cl2 (AE = Ca and Sr)

In our study of the optical properties of Eu2+-doped alkaline-earth metal hydride chlorides AE7H12Cl2 (AE = Ca and Sr) we observe yellow (AE = Sr) and orange-red (AE = Ca) emission. Compared to the emission energies of Eu2+ -doped fluorides, chlorides and mixed fluoride chlorides, this corresponds to a wide redshift of the Eu2+ 4f65d1–4f7 emission. We explain this observation with the strong polarizability of the hydride anion and therefore its strong nephelauxetic effect, which shifts the 5d barycenter to lower energy. Furthermore, the redshift is even significant compared to other known hydride chlorides and hydride oxide chlorides, which is probably caused by the relatively short AE2+/Eu2+−H− distances, or it might also be caused by impurity-trapped exciton states. We have also investigated the temperature dependence of the photoluminescence lifetimes and the thermal stability. Even though the compounds are probably not ideal for phosphor application due to their low stability against air and moisture, our study underlines the large influence of the highly polarizable hydride anion on the optical properties of divalent europium and shows that Eu2+ could be used as a probe in mixed anionic systems to determine the hydride content.

Unusual temperature and excitation energy dependences of impurity-trapped excitons in LiBaF3:Eu2+ crystals

Luminescence properties of impurity-trapped excitons (ITEs) in LiBaF3:Eu2+ crystals are investigated by optical and time-resolved laser-excitation spectroscopy. The three excited states of 4f65d, 4f7(6P7/2) and ITE are identified in the excitation and emission spectra. The intensity of ITEs’ emission increases unexpectedly with increasing temperature and excitation energy. The initial intensity of the ITE emission just after pulsed-laser excitation increases continuously with temperature in the temperature range 7–600K. The results are discussed in relation to radiative and nonradiative rates and excitonic processes in this system. The theoretical model that includes the ITE, 4f7(6P7/2) and 4f6(7FJ)5d(eg) states is developed to describe and support the experimental observations.

Temporal dynamics of the frequency non-degenerate transient photoluminescence enhancement observed following excitation of inter-configurational [formula omitted] transitions in CaF2:Yb2+

We present a detailed investigation of the dynamics of impurity-trapped excitons in CaF2:Yb2+ using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing a UV optical parametric amplifier (OPA) synchronized to an infrared free electron laser (FEL). The temporal behaviour is well approximated by a multi-level rate equation model as relaxation between excited states of the exciton as well as a small contribution from local lattice heating by the FEL which becomes evident due to the 40cm−1 splitting of the exciton excited states giving rise to the transient photoluminescence enhancement itself.

Evidence That the Anomalous Emission from CaF2:Yb2+ Is Not Described by the Impurity Trapped Exciton Model.

Yb-substituted CaF2 exhibits an anomalous red-shifted luminescence after UV excitation, attributed to the relaxation of impurity trapped excitons (ITE). CaF2:Yb is the archetype system for this model, in which the Yb2+ ions can be excited into a long-lived (ms) exciton state. Upon de-excitation, the emission intensity should be proportional to the Yb2+ concentration, but that could not be checked when this model was first proposed. Using the X-ray absorption near edge structure (XANES) technique, we determine the fractions of Yb2+ and Yb3+ for low Yb concentrations, 0.01% to 0.1%, and thus determine the net concentration of Yb2+. A comparison with luminescence data shows that the intensity is not proportional to the Yb2+ concentration, and only a fraction of Yb2+ ions contributes to the anomalous luminescence. This is inconsistent with the ITE model and illustrates the importance of checking the dependence of the emission intensity on the Yb2+ concentration.

Vacuum referred binding energy scheme for rare earth ions in RE2BaZnO5 [RE = Y, Gd, La

The chief purpose of this paper is to examine the nature of optical transitions that are responsible for the broad bands in the room temperature excitation spectrum of Ce3+, Pr3+ and Tb3+ impurities in RE2BaZnO5. This goal is accomplished by constructing the vacuum referred binding energy (VRBE) scheme for the rare-earth ions from the optical data that is available in the archival literature. The resulting VRBE scheme provides convincing evidence for associating the broad band in the excitation spectra of Pr3+ (and Tb3+) to intervalence charge transfer (IVCT) transition. This refutes the usual assignment of this band to the 4fn →4fn-15d1 optical transition. The excitation band for Ce3+ is associated with the 4f1→5d1 transition and the luminescence with impurity-trapped exciton emission. The dominance of Pr3+ 3PJ emission in Gd2BaZnO5 is consistent with the predictive placement of the Pr3+3PJ levels much below the host lattice exciton state. We reanalyze the reported energy transfer within the Ce-Yb couple in Gd2BaZnO5 and conclude that the tetravalent cerium (Ce4+) ion is responsible for the energy transfer energy to Yb3+. For Y2BaZnO5, band structure calculations indicate that the host lattice excitation band is due to electronic transition from the O 2p state (top of the valence band) to the Y 4d and Zn 4s states (bottom of the conduction band).

Charge transfer bands in optical materials and related defect level location

Charge transfer (CT)-bands, electron trapping, hole trapping, electron release, hole release, metal-to-metal-charge transfer, CT-luminescence, anomalous emission, impurity trapped exciton emission, inter-valence charge transfer, pair-emission, tunneling, photo-electron spectroscopy, redox potentials, photo-ionization, thermal-ionization. All these phenomena deal with the transfer of an electron from one atom in a compound to either another atom in the compound or to the ambient, i.e., outside the compound. The energy needed for, or released in, such transfer carries information on the electron binding energy in the defect levels with respect to the host band levels or the levels in the ambient. First the different types of charge transfer between a lanthanide and the host bands, and how they can be used to construct a host referred binding energy (HRBE) diagram, are reviewed. Then briefly the chemical shift model is introduced in order to convert the HRBE diagram into a vacuum referred binding energy diagram (VRBE). Next charge transfer between transition metal elements and host bands and between Bi3+ and host bands are treated, and finally electron transfer from one defect to another and to the ambient. Illustrating examples are provided.

Pressure dependence of the emission in CaF2 : Yb2+

We present a detailed spectroscopic investigation of CaF2 doped with Yb2+ performed at high hydrostatic pressure which is applied in a diamond anvil cell. At ambient pressure and at temperatures lower than 175 K, the luminescence consists of a single broad band peaked at 18 500 cm−1, attributed to the recombination of impurity-trapped excitons. Increasing pressure causes the luminescence to be observable at higher temperature. At a pressure of 72 kbar luminescence can be observed up to 275 K. The emission lineshape does not strongly depend on pressure below 85 kbar. However, at 85 kbar it is blue shifted to 21 630 cm−1. This is attributed to the known phase transition of the CaF2 crystal from cubic to the orthorhombic phase. The absolute energy of the ground and 4f135d states of Yb2+ as well as the energy of the impurity-trapped exciton with respect to valence and conduction bands have been estimated. The results, are discussed in comparison with the pressure dependences observed for the luminescence of BaF2 : Eu2+ and CaF2 : Eu2+. The difference between the spectral properties of Eu2+ and Yb2+ is attributable to the fact that the ground and 4f65d states of Eu2+ are placed deeper in the CaF2 bandgap than the ground and excited 4f135d states of Yb2+, whereas the energies of the impurity-trapped exciton states for Yb2+ and Eu2+ with respect to the conduction band are approximately the same.

Frequency non-degenerate sequential excitation of the impurity trapped exciton in strontium fluoride crystals doped with ytterbium

We model the dynamic behaviour observed for impurity-trapped excitons in SrF2:Yb2+ using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing an UV optical parametric amplifier synchronized to an IR free electron laser (FEL). We observe sharp transitions interpreted as a change of state of the localized hole and broad bands interpreted as a change of state of the delocalized electron. Our modeling indicates that the 4f crystal-field interaction is 25% smaller than in CaF2. The photoluminescence enhancement transients are analyzed across a range of excitation frequencies using a system of rate equations. The temporal behavior is explained in terms of intra-excitonic relaxation, local lattice heating by the FEL, and liberation of electrons from trap states.

Vacuum ultraviolet synchrotron measurements of excitons in NaMgF3:Yb2+

Results of a vacuum ultraviolet spectroscopic characterization of NaMgF3:Yb2+ are presented. The material demonstrates emission features associated with self-trapped excitons and impurity-trapped excitons. The emission features noticeably overlap giving rise to a broad emission band from 17000 to 35000cm−1 at a sample temperature of 8K. To identify the true profiles of the emission features we have used a deconvolution procedure. The deconvolution was possible due to the thermal quenching of self-trapped excitons at room temperature that allowed for direct observations of the impurity trapped exciton emission band. Energy transfer between host electronic excitations (excitons and e–h pairs) and Yb2+ ions leading to the formation of impurity-trapped excitons is evident from excitation spectra.

Intervalence charge transfer luminescence: interplay between anomalous and 5d - 4f emissions in Yb-doped fluorite-type crystals.

In this paper, we report the existence of intervalence charge transfer (IVCT) luminescence in Yb-doped fluorite-type crystals associated with Yb(2+)-Yb(3+) mixed valence pairs. By means of embedded cluster, wave function theory ab initio calculations, we show that the widely studied, very broad band, anomalous emission of Yb(2+)-doped CaF2 and SrF2, usually associated with impurity-trapped excitons, is, rather, an IVCT luminescence associated with Yb(2+)-Yb(3+) mixed valence pairs. The IVCT luminescence is very efficiently excited by a two-photon upconversion mechanism where each photon provokes the same strong 4f(14)-1A1g→ 4f(13)((2)F7/2)5deg-1T1u absorption in the Yb(2+) part of the pair: the first one, from the pair ground state; the second one, from an excited state of the pair whose Yb(3+) moiety is in the higher 4f(13)((2)F5/2) multiplet. The Yb(2+)-Yb(3+) → Yb(3+)-Yb(2+) IVCT emission consists of an Yb(2+) 5deg → Yb(3+) 4f7/2 charge transfer accompanied by a 4f7/2 → 4f5/2 deexcitation within the Yb(2+) 4f(13) subshell: [(2)F5/25deg,(2)F7/2] → [(2)F7/2,4f(14)]. The IVCT vertical transition leaves the oxidized and reduced moieties of the pair after electron transfer very far from their equilibrium structures; this explains the unexpectedly large band width of the emission band and its low peak energy, because the large reorganization energies are subtracted from the normal emission. The IVCT energy diagrams resulting from the quantum mechanical calculations explain the different luminescent properties of Yb-doped CaF2, SrF2, BaF2, and SrCl2: the presence of IVCT luminescence in Yb-doped CaF2 and SrF2; its coexistence with regular 5d-4f emission in SrF2; its absence in BaF2 and SrCl2; the quenching of all emissions in BaF2; and the presence of additional 5d-4f emissions in SrCl2 which are absent in SrF2. They also allow to interpret and reproduce recent experiments on transient photoluminescence enhancement in Yb(2+)-doped CaF2 and SrF2, the appearance of Yb(2+) 4f-5d absorption bands in the excitation spectra of the IR Yb(3+) emission in partly reduced CaF2:Yb(3+) samples, and to identify the broadband observed in the excitation spectrum of the so far called anomalous emission of SrF2:Yb(2+) as an IVCT absorption, which corresponds to an Yb(2+) 4f5/2 → Yb(3+) 4f7/2 electron transfer.

Excitons and interconfigurational transitions in CaF2:Yb2+ crystals

A time-resolved VUV spectroscopic study of emission and excitation spectra of CaF2:Yb2+ has been performed to investigate excitation and relaxation mechanisms of both impurity-trapped excitons and intrinsic excitons in CaF2. Host-to-impurity energy transfer mechanisms leading to formation of impurity-trapped excitons have been discussed. The change in free exciton excitation peak position with increasing lattice temperature has been measured and is well approximated by Viña׳s expression for the temperature shift of a semiconductor band gap. The 4f14→4f135d CaF2:Yb2+ absorption bands are successfully modeled with a semi-empirical effective Hamiltonian calculation.

Effective Hamiltonian parameters for ab initio energy-level calculations of SrCl2:Yb2+ and CsCaBr3:Yb2+

Calculated energy levels from recent ab initio studies of the electronic structure of SrCl2:Yb2+ and CsCaBr3:Yb2+ are fitted with a semi-empirical ‘crystal-field’ Hamiltonian, which acts within the model space 4f14 + 4f135d + 4f136s. Parameters are obtained for the minima of the potential energy curves for each energy level and also for a range of anion–cation separations. The parameters are compared with published parameters fitted to experimental data and to atomic calculations. The states with significant 4f136s character give a good approximation to the impurity-trapped exciton states that appear in the ab initio calculations.

Site-selective transient photoluminescence enhancement of impurity-trapped excitons in NaMgF3:Yb2+

The excited-state structure of impurity-trapped excitons are measured in a multisite system. We use a two-color (UV-IR) pulsed photoluminescence enhancement technique, which probes the interlevel transitions and dynamics of impurity-trapped excitons in doped insulating phosphor materials. The technique is applied to NaMgF${}_{3}$:Yb${}^{2+}$, which exhibits emission from two charge-compensation centers with peaks at 22300 cm${}^{\ensuremath{-}1}$ (448 nm) and 24000 cm${}^{\ensuremath{-}1}$ (417 nm). The observed photoluminescence enhancement is caused by a combination of intraexcitonic excitation and electron trap liberation. The electron traps are inferred to have a depth of approximately 800 cm${}^{\ensuremath{-}1}$.

Red, Green, and Blue Photoluminescence of Ba₂SiO₄:M (M = Eu3+, Eu2+, Sr2+) Nanophosphors.

Divalent europium doped barium orthosilicate is a very important phosphor for the production of light emitting diodes (LEDs), generally associated to the green emission color of micron-sized crystals synthesized by means of solid-state reactions. This work presents the combustion synthesis as an energy and time-saving preparation method for very small nano-sized Ba2SiO4 particles, flexibly doped to acquire different emission energies. The size of the resulting spherical nanoparticles (NPs) of the green emitting Ba2SiO4:Eu2+ was estimated to about 35 nm applying the Scherrer equation and further characterized with aid of atomic force microscopy (AFM) as well as scanning electron microscopy (SEM). This phosphor is able to build homogeneous luminescent suspensions and was successfully down-sized without changing the optical properties in comparison to the bulk phosphors. Besides the X-ray diffraction (XRD) analysis and the different types of microscopy, the samples were characterized by luminescence spectroscopy. Undoped Ba2SiO4 NPs are not luminescent, but show characteristic red emission of the 5D0 → 7FJ (J = 0–4) electronic transitions when doped with Eu3+ ions. Moreover, these orthosilicate nanoparticles generate blue light at low temperatures due to impurity-trapped excitons, introduced by the partial substitution of the Ba2+ with Sr2+ ions in the Ba2SiO4 lattice causing a substantial distortion. A model for the temperature behavior of the defect luminescence as well as for their nature is provided, based on temperature-dependent luminescence spectra and lifetime measurements.

Luminescence of CaWO4:Pr3+ and CaWO4:Tb3+ at ambient and high hydrostatic pressures

Photoluminescence spectra of CaWO4 doped with Pr3+ and Tb3+ obtained at high hydrostatic pressures up to 315 kbar applied in a diamond anvil cell (DAC) are presented. The intensities of the luminescence from the 3P0 state of Pr3+ and from the 5D3 state of Tb3+ decreased with increasing pressure. At pressures greater than 50 kbar, the 1D2 → 3HJ transitions in Pr3+ and the 5D4 → 7FJ transitions in Tb3+ dominated the spectra. At pressures greater than 100 kbar, only emissions from the lower excited states were observed. At pressures greater than 150 kbar, luminescence from the 1D2 and 5D4 states also decreased with increasing pressure, and at a pressure of 315 kbar for CaWO4:Pr3+ and 190 kbar for CaWO4:Tb3+, the emissions related to the Pr3+ and Tb3+ were quenched. These effects were related to the influence of impurity trapped excitons (ITEs) on the efficiency of the f–f emission in the Pr3+ and Tb3+ ions. Analysis of the emission spectra collected at different pressures allowed the energies of the ground states of the Pr3+ and Tb3+ ions with respect to the band edges of the CaWO4 host to be estimated.

Impurity trapped exciton states related to rare earth ions in crystals under high hydrostatic pressure

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 excited state and hole at the ground state of impurity. Localization of electron or hole at the dopand site creates a long range Coulomb potential that attracts the second carrier which then occupies the localized Rydberg-like states. Such a system can be considered as impurity trapped exciton. Usually impurity trapped exciton is a short living phenomenon which decays non-radiatively leaving the impurity ion in the excited state. However, in several compounds doped with Eu2+ the impurity trapped exciton states become stable and contribute to the radiative processes though anomalous luminescence that appears apart of the 4f7 → 4f7 and 4f75d1 → 5f7 emission. In this contribution pressure effect on energies of the 4fn−15d1→5fn transitions in Ln doped oxides and fluorides as well as influence of pressure on the energy of impurity trapped exciton states is discussed. The latest results on high pressure investigations of luminescence related to Pr3+, and Eu2+ in different lattices are reviewed.

Impurity trapped excitons under high hydrostatic pressure

Paper summarizes the results on pressure effect on energies of the 4fn→4fn and 4fn−15d1→4fn transitions as well as influence of pressure on anomalous luminescence in Lnα+ doped oxides and fluorides. A model of impurity trapped exciton (ITE) was developed. Two types of ITE were considered. The first where a hole is localized at the Lnα+ ion (creation of Ln(α+1)+) and an electron is attracted by Coulomb potential at Rydberg-like states and the second where an electron captured at the Lnα+ ion (creation of Ln(α−1)+) and a hole is attracted by Coulomb potential at Rydberg-like states. Paper presents detailed analysis of nonlinear changes of energy of anomalous luminescence of BaxSr1−xF2:Eu2+ (x>0.3) and LiBaF3:Eu2+, and relate them to ITE-4f65d1 states mixing.

Impurity-trapped excitons and electron traps in CaF2:Yb2+ and SrF2:Yb2+ probed by transient photoluminescence enhancement

CaF2:Yb2+ and SrF2:Yb2+ crystals have been investigated by a two-color UV + IR transient photoluminescence enhancement technique. The enhancement gives information about both changes in internal energy levels of the excitons and liberation of electrons from traps in the crystals.

Transient photoluminescence enhancement as a probe of the structure of impurity-trapped excitons in CaF2:Yb2+

We demonstrate a direct measurement of the energy levels of impurity-trapped excitons in CaF$_2$:Yb$^{2+}$. The radically different radiative decay rates of the lowest exciton state and higher excited states enable the generation of a transient photoluminescence enhancement measured via a two-step excitation process. We observe sharp transitions arising from changes of state of localized electrons, broad bands associated with changes of state of delocalized electrons, and broad bands arising from trap liberation.