Self-trapped Exciton Luminescence Research Articles

Fluorescence properties of <italic>A</italic><sub>2</sub>InCl<sub>5</sub>·H<sub>2</sub>O ∶Sb<sup>3+</sup>(<italic>A</italic>=Cs, Rb, K)

The photoluminescence of self-trapped excitons is a common luminescence behavior of metal halide perovskite fluorescent materials. The ability of Lattice deformation has a significant impact on its self-trapped exciton luminescence. The 0-dimensional inorganic perovskite <italic>A</italic>₂InCl₅·H₂O (<italic>A</italic>=Cs, Rb, K) doped with Sb³⁺ is characterized by X-ray diffraction, fluorescence spectroscopy and Raman spectroscopy. The effect of octahedral deformation of [InCl₅H₂O]^2-/[SbCl₅H₂O]^2- due to cation radius change on the fluorescence properties is investigated. The decrease of the cation radius causes the decrease of the interplanar spacing of material, the increase of band gapand the blue shift of the excitation spectrum. At the same time, the electro-acoustic coupling effect of material is enhanced, then the non-radiative recombination process is enhanced, the Stokes shift increases, the spectral line broadens, the spectrum red shift happens, and the photoluminescence quantum yield decreases. This paper provides an effective way to regulate the self-trapped excitons luminescence of perovskites.

Zero-dimensional Sb&lt;sup&gt;3+&lt;/sup&gt; doped Rb&lt;sub&gt;7&lt;/sub&gt;Bi&lt;sub&gt;3&lt;/sub&gt;Cl&lt;sub&gt;16&lt;/sub&gt; metal halides with triplet self-trapped exciton emission

Low-dimensional metal halides have attracted extensive attention due to their excellent optical properties, especially zero-dimensional metal halides, which can improve the radiation recombination probability due to the characteristics of their isolated octahedral structures. In this paper, we report a zero-dimensional metal halide Sb&lt;sup&gt;3+&lt;/sup&gt; doped Rb&lt;sub&gt;7&lt;/sub&gt;Bi&lt;sub&gt;3&lt;/sub&gt;Cl&lt;sub&gt;16&lt;/sub&gt; with a broadband orange-yellow emission at 613 nm. When the Sb&lt;sup&gt;3+&lt;/sup&gt; doping concentration is 30%, the highest photoluminescence quantum yield of the system reaches 30.7%. This high-efficiency luminescence is derived from the self-trapped excitons generated by the strong interaction between electrons and the crystal lattice. The specific physical mechanism and energy transfer process of self-trapped exciton luminescence are further studied through characterizing the optical performances. The electronic states in the singlet &lt;sup&gt;1&lt;/sup&gt;P&lt;sub&gt;1 &lt;/sub&gt;level are relaxed to the triplet &lt;sup&gt;3&lt;/sup&gt;P&lt;sub&gt;1 &lt;/sub&gt;via an intersystem crossing process, and the strong orange-yellow emission comes from the triplet state &lt;sup&gt;3&lt;/sup&gt;P&lt;sub&gt;1&lt;/sub&gt;→&lt;sup&gt;1&lt;/sup&gt;S&lt;sub&gt;0&lt;/sub&gt; radiation recombination process. In addition, Sb&lt;sup&gt;3+&lt;/sup&gt; doped Rb&lt;sub&gt;7&lt;/sub&gt;Bi&lt;sub&gt;3&lt;/sub&gt;Cl&lt;sub&gt;16&lt;/sub&gt; has satisfactory environmental stability, the Sb&lt;sup&gt;3+&lt;/sup&gt;:Rb&lt;sub&gt;7&lt;/sub&gt;Bi&lt;sub&gt;3&lt;/sub&gt;Cl&lt;sub&gt;16&lt;/sub&gt;-based light-emitting diodes (LED) are fabricated here in this work, and the color coordinates and correlated color temperature of the LED are (0.4886, 0.4534) and 2641 K, respectively. The highly efficient and stable Sb&lt;sup&gt;3+&lt;/sup&gt; doped Rb&lt;sub&gt;7&lt;/sub&gt;Bi&lt;sub&gt;3&lt;/sub&gt;Cl&lt;sub&gt;16&lt;/sub&gt; is expected to be used in solid-state lighting and display fields.

Low-temperature luminescence of ScF3 single crystals under excitation by VUV synchrotron radiation

Photoluminescence and excitation spectra of ScF3 single crystals have been measured under vacuum ultraviolet excitations utilizing undulator synchrotron radiation from 1.5 GeV storage ring of MAX IV synchrotron. The emission peak at 280 nm is explained as emission band of self-trapped excitons in ScF3. This emission is quenched at 50 K and activation energy of thermal quenching was obtained. The excitation spectrum in vacuum ultraviolet spectral range exhibits that the luminescence of self-trapped excitons effectively occurs under direct excitation in the excitonic absorption band, whereas under higher energies this excitation is strongly suppressed, however, multiplication of electronic excitation processes have been successfully identified.

In-situ investigation of point defects kinetics in LiF using ion luminescence technique

The present work reports the in-situ ion beam induced luminescence measurements using 2 MeV proton excitation of LiF single crystal with the aim of obtaining a further insight on F-type centers kinetics and its correlation with the self-trapped exciton (STE). It is found that the STE luminescence intensity decreases from the beginning of irradiation. While the dose-dependent evolution of F3+ and F2 centers reveals three kinetic stages: (i) linear increase from the beginning of irradiation up to 10.1 MGy, followed by (ii) monotonic decrease to 18.1 MGy, and (iii) steady state from 18.1 to 26.2 MGy where the luminescence intensity remains constant. The STE luminescence completely extinguished at high dose; 18.1 MGy in our case. Whereas, the F3+and F2luminescence persist and remain constant above 18.1 MGy. The latter behavior revealed in the present study can be useful for LiF application in radiation dosimetry and an imaging detector.

Luminescence of self-trapped excitons in alkali halide crystals at low temperature uniaxial deformation

The processes of temperature quenching of the luminescence of the self-trapped excitons (STEs) from 93 to 200 K in the conditions of applied elastic low-temperature deformation for several alkali-halide crystals (AHCs) are presented in the paper. The theoretical explanation of the influence of low-temperature uniaxial deformation on the formation of the luminescence of the STEs was done. The increase in activation energy of the temperature quenching of the luminescence of the STEs, narrowing of the emission bands, increase in the frequency of the active oscillations of the STEs and the decrease in the Huang-Rhys parameter are shown as the results. These changes indicate a weakening of the exciton-phonon interaction in elastically deformed AHCs, which, in turn, leads to an increase in the probability of radiative annihilation of excitons.

CsI(Na) micron-scale particles-based composite material for fast pulsed X-ray detection

We report a method for the preparation of fast scintillation composite material based on CsI(Na) micron-scale particles. The composite material was prepared by solidifying CsI(Na) micron-scale particles in an epoxy resin adhesive. The scintillation properties under X-ray excitation were investigated. The luminescence time characteristic of the CsI(Na) composite material is significantly improved, with the fall time range of 15.6–19.6 ns, over 20 times faster than that of bulk single crystal CsI(Na). Furthermore, the deliquescence property of CsI(Na) is overcome. The luminescence spectra show that the Na-related luminescence and self-trapped excitons luminescence of the composite material are strongly concentration-dependent. This kind of composite material could be a promising scintillator used in fast pulse X-ray detection.

X-ray induced radiation damage in CLYC(Ce)

The radiation hardness of the 6Li loaded scintillator CLYC(Ce) to X-rays was investigated. Two crystals were studied; one crystal was irradiated with X-rays and one was kept as a control and only exposed to a moderated 241Am–Be source. The control crystal was used as a reference sample for photoluminescence excitation and emission measurements. The exposed crystal was given two doses of X-rays, the first was 2.3 Gy and the second was 118.2 Gy after a 22 week annealing period. The total dose was 120.5 Gy and was found to significantly reduce the light yield of the crystal. Pulse height spectra from the moderated 241Am–Be taken with the irradiated crystal showed a 54% decrease in pulse height and a broadening of the FHWM of the peak from 14.6 % to 44.2%. Analysis of the pulse shapes showed no change to the median decay time to 37% of the scintillator, with values of ∼350 ns for the irradiated and control crystal. Photoluminescence excitation and emission measurements revealed a reduction in the relative intensity of the main Ce3+ emission with respect to the self-trapped exciton luminescence. This indicated a modification to the Ce3+ luminescence center. There was little recovery at room temperature over the course of 22 weeks after the first irradiation. The irradiated crystal was also found to display significant room temperature luminescence after X-ray irradiation with it being measurable up to 120 min after the X-ray irradiation.

Bluish-white-light-emitting diodes based on two-dimensional lead halide perovskite (C6H5C2H4NH3)2PbCl2Br2

Bluish-white-light-emitting diodes (BWLEDs) are designed based on the two-dimensional mixed halide perovskite (C6H5C2H4NH3)2PbCl2Br2 at room temperature. Bluish-white electroluminescence devices were fabricated by a spin-coating method. The BWLEDs can be turned on at 4.9 V and depict a maximum luminance of ∼70 cd/m2 at 7 V. Low and room temperature photoluminescence spectra show the coexistence of free exciton and self-trapped exciton luminescence in a deformable lattice. The strategy of achieving white electroluminescence (EL) from mixed halide perovskite reported here can be applied to other two-dimensional perovskites to increase the optoelectronic efficiency of the device in the future.

Quenching of exciton luminescence in SrF2 nanoparticles within a diffusion model

Quenching processes of the self-trapped exciton luminescence were studied analyzing the shape of X-ray luminescence decay kinetics curves of SrF2 nanoparticles of different sizes. To describe the curves of luminescence decay kinetics, an equation was obtained which is based on the model which takes into account the diffusion of self-trapped excitons and which considers the case of strong surface quenching. The obtained relation was shown to describe the shape of the experimental curves of X-ray luminescence decay kinetics of SrF2 nanoparticles if their size distribution is bi-modal, i.e., a log-normal distribution about a mean size in the tens of nanometers range plus a distribution of particles larger than 130 nm. The presence of large particles is implied in this model by the single-exponential decay with time constant of 1.2 μs. From the fitting of kinetics curves using the proposed relation, the average diffusion length of self-trapped excitons in SrF2 was estimated to be (15 ± 2) nm.

Intrinsic luminescence of SrF2 nanoparticles

The influence of SrF2 nanoparticle size on their luminescence and kinetic properties under photo- and X-ray excitation was studied. The self-trapped exciton luminescence intensity was shown to reveal dependence on both the nanoparticle size and the range of excitation energies. The lowest sensitivity to the nanoparticle size is observed in the case of excitation in the region of optical exciton formation. The recombination of electrons with surface defects and non-radiative decay of excitons due to diffusion to the surface is found to be the determining factors of the luminescence quenching in the case of X-ray excitation.

Luminescence of α-quartz crystal and silica glass under excitation of excimer lasers ArF (193 nm), KrF (248 nm)

Luminescence of crystalline α-quartz and silica glass was studied under focused laser excitation. It was found that in crystalline α-quartz the luminescence of self-trapped exciton (STE) is excited in two-photon regime with ArF (193nm) excimer laser. In the case of KrF (248nm) laser excitation mainly luminescence related to near surface area is seen even without laser beam focusing. The near surface luminescence has an emission band similar to that of STE in bulk. Temperature quenching is also similar, therefore this luminescence is attributed to STE created in the area of surface. Luminescence decay kinetic of surface STE is longer than bulk STE decay (tens of ms compared to 1ms at 80K). Electron or/and hole self-trapping near the surface is assumed. Their recombination could provide longer duration of surface STE luminescence. Similar surface luminescence was not excited with ArF (193nm) excimer laser. The nature of absorption for laser 248nm photons at surface is not yet clear.Luminescence of silica glass of III type excited with focused beam of KrF (248nm) laser resembles that of previously studied luminescence of STE in silica.

The deformation stimulated luminescence in KCl, KBr and KI crystals

Currently, strengthening of the intensity of luminescence in alkali halide crystals (AHC) at lattice symmetry lowering is discussed as a promising direction for the development of scintillation detectors [1-3]. In this regard, for the study of anion excitons and radiation defects in the AHC anion sublattice at deformation, the crystals with the same sizes of cations and different sizes of anions were chosen. In the X-ray spectra of KCl at 10 K, the luminescence at 3.88 eV; 3.05 eV and 2.3 eV is clearly visible. The luminescence at 3.05 eV corresponds to the tunneling recharge [F*, H]. Luminescence at 3.88 eV is quenched in the region of thermal destruction of F′-centers and characterizes tunneling recharge of F′, VK-centers. In KCl at 90 K, the luminescence of self-trapped excitons (STE) is completely absent. In KBr at deformation not only STE luminescence, but also deformation stimulated luminescence at 3.58 eV were recorded, the last one corresponds to tunneling recharge of F′, VK-centers. In KI crystal at 10 K and 90 K at deformation, only STE luminescence is enhanced. There are no deformation luminescence bands in KI compares with KBr and KCl crystals.

Host and defect-related photoluminescence of structurally disordered K3WO3F3 oxyfluoride crystals

Spectra of photoluminescence (PL) in the region of 1.5–5.5 eV, PL excitation spectra (3–22 eV), PL decay kinetics and PL temperature dependence were measured for single crystals and ceramics K3WO3F3 as well as for ceramics K3WO3F3 irradiated by fast electrons. Synchrotron radiation was used for low temperature PL experiments with time resolution. Single crystals are transparent in microwave, visible and near UV range, inter-band transition energy is Eg = 4.3 eV. In K3WO3F3, the wide band luminescence in the region of 2.5 eV with the Stokes shift of 1.5 eV with the microsecond decay kinetics is connected with luminescence of triplet self-trapped excitons (STE). This luminescence is formed by electronic transitions in [WO3F3] octahedron. Different distortion of KWOF crystal lattice is manifested in the change of the Stokes shift of the STE luminescence band. The 3.2 eV emission band in low-temperature PL spectrum with decay times of 1.8 ns and 11 ns corresponds to singlet STE luminescence. A new 2.9 eV emission band is discovered in low-temperature PL spectrum in the samples irradiated by fast electrons (E = 10 MeV, D = 160 kGy). This emission band is excited not through the intracenter mechanism but through the creation of excitons bound on the defects. It is suggested that it is F-like centers of anionic sublattice induced by the mechanism of elastic collision.

X-ray excited luminescence of polystyrene composites loaded with SrF2 nanoparticles

The polystyrene film nanocomposites of 0.3mm thickness with embedded SrF2 nanoparticles up to 40wt% have been synthesized. The luminescent and kinetic properties of the polystyrene composites with embedded SrF2 nanoparticles upon the pulse X-ray excitation have been investigated. The luminescence intensity of the pure polystyrene scintillator film significantly increases when it is loaded with the inorganic SrF2 nanoparticles. The film nanocomposites show fast (∼2.8ns) and slow (∼700ns) luminescence decay components typical for a luminescence of polystyrene activators (p-Terphenyl and POPOP) and SrF2 nanoparticles, respectively. It is revealed that the fast decay luminescence component of the polystyrene composites is caused by the excitation of polystyrene by the photoelectrons escaped from the nanoparticles due to photoeffect, and the slow component is caused by reabsorption of the self-trapped exciton luminescence of SrF2 nanoparticles by polystyrene.

The specifics of radiative annihilation of self-trapped excitons in a KI–Tl crystal under low-temperature deformation

The effect of low-temperature uniaxial deformation on the self-trapping-limited mean free path of excitons in a KI–Tl crystal was revealed from x-ray luminescence spectra. The analysis of the dependence of the intensity ratio of the Tl-center emission (2.85 eV) and the luminescence of self-trapped excitons (π-component; 3.3 eV) on the extent of low-temperature deformation showed that in the KI–Tl crystal (3 × 10−3 mol. %) the self-trapping-limited mean free path of excitons is comparable with the distance between Tl atoms (20–27)a under a deformation ε = 2%. As the compression increases to ε ≥ 2%–5%, the mean free path drops to (27-5.35)a. The results of modeling based on the continuum approximation showed that with increasing temperature and the degree of low-temperature deformation the height of the potential barrier for the exciton self-trapping drops, which is consistent with the reduction of the mean free path of excitons in the KI–Tl crystal.

Luminescence of SiO2 and GeO2 crystals with rutile structure. Comparison with α-quartz crystals and relevant glasses (Review Article)

Luminescence properties of SiO2 in different structural states are compared. Similar comparison is made for GeO2. Rutile and α-quartz structures as well as glassy state of these materials are considered. Main results are that for α-quartz crystals the luminescence of self-trapped exciton is the general phenomenon that is absent in the crystal with rutile structure. In rutile structured SiO2 (stishovite) and GeO2 (argutite) the main luminescence is due to a host material defect existing in as-received (as-grown) samples. The defect luminescence possesses specific two bands, one of which has a slow decay (for SiO2 in the blue and for GeO2, in green range) and another, a fast ultraviolet (UV) band (4.75 eV in SiO2 and at 3 eV in GeO2). In silica and germania glasses, the luminescence of self-trapped exciton coexists with defect luminescence. The latter also contains two bands: one in the visible range and another in the UV range. The defect luminescence of glasses was studied in details during last 60–70 years and is ascribed to oxygen deficient defects. Analogous defect luminescence in the corresponding pure nonirradiated crystals with α-quartz structure is absent. Only irradiation of a α-quartz crystal by energetic electron beam, γ-rays and neutrons provides defect luminescence analogous to glasses and crystals with rutile structure. Therefore, in glassy state the structure containing tetrahedron motifs is responsible for existence of self-trapped excitons and defects in octahedral motifs are responsible for oxygen deficient defects.

Mechanism for Broadband White-Light Emission from Two-Dimensional (110) Hybrid Perovskites.

The recently discovered phenomenon of broadband white-light emission at room temperature in the (110) two-dimensional organic-inorganic perovskite (N-MEDA)[PbBr4] (N-MEDA = N(1)-methylethane-1,2-diammonium) is promising for applications in solid-state lighting. However, the spectral broadening mechanism and, in particular, the processes and dynamics associated with the emissive species are still unclear. Herein, we apply a suite of ultrafast spectroscopic probes to measure the primary events directly following photoexcitation, which allows us to resolve the evolution of light-induced emissive states associated with white-light emission at femtosecond resolution. Terahertz spectra show fast free carrier trapping and transient absorption spectra show the formation of self-trapped excitons on femtosecond time-scales. Emission-wavelength-dependent dynamics of the self-trapped exciton luminescence are observed, indicative of an energy distribution of photogenerated emissive states in the perovskite. Our results are consistent with photogenerated carriers self-trapped in a deformable lattice due to strong electron-phonon coupling, where permanent lattice defects and correlated self-trapped states lend further inhomogeneity to the excited-state potential energy surface.

Cross-shaped photoluminescence of excimers in perylene crystals

Cross-shaped excimer (self-trapped exciton) luminescence from α- and β-perylene single crystals of 50–100 μm was found when they were excited at the center of the crystals with a continuous-wave (cw) laser resonant with the exciton absorption. The cross shape is formed by the two lines which intersect at the excited position and are perpendicular to the sides of the crystals of parallelogram shape. Luminescence is emitted from the excited spot and 4 side edges in the cross shape. The most striking feature is that the luminescence intensity at the edges was as high as or higher than at the excited spot. The possibility of the exciton propagation or the waveguide effect is rejected both experimentally and theoretically. This phenomenon can be reasonably explained only when the radiative transition probability of excimers is significantly enhanced at the crystals side edges than at the center due to the lower symmetry.

Luminescent and structural properties of ZnxMg1-xWO4 mixed crystals

The structural and luminescent properties of perspective scintillating ZnxMg1-xWO4 mixed crystals were studied. The following characteristics were found to depend linearly on x value: the energy of several vibrational modes detected by Raman spectroscopy, the bandgap width deduced from the shift of the excitation spectrum onset of a self-trapped exciton (STE) emission, the position of thermally stimulated luminescence peaks. It is also shown that the thermal stability of the STE luminescence decreases gradually when x decreases. These data indicate that each ZnxMg1-xWO4 mixed crystal is not a mixture of two constituents, but possesses its original crystalline structure, as well as optical and luminescent properties.

Luminescence of polymorphous SiO2

The luminescence of self-trapped exciton (STE) was found and systematically studied in tetrahedron structured silica crystals (α-quartz, coesite, cristobalite) and glass. In octahedron structured stishovite only host material defect luminescence was observed. It strongly resembles luminescence of oxygen deficient silica glass and γ or neutron irradiated α-quartz. The energetic yield of STE luminescence for α-quartz and coesite is about 20% of absorbed energy and about 5(7)% for cristobalite. Two types of STE were found in α-quartz. Two overlapping bands of STEs are located at 2.5–2.7 eV. The model of STE is proposed as Si–O bond rupture, relaxation of created non-bridging oxygen (NBO) with foundation of a bond with bridging oxygen (BO) on opposite side of c or x,y channel. The strength of this bond is responsible for thermal stability of STE. Similar model of STE was ascribed for coesite and cristobalite with difference related to different structure. STE of Silica glass is strongly affected by disordered structure.