Characterization of the charge transfer luminescence of the [WO6]6- octahedron in Ca3WO6 and the [WO5]4- square pyramid in Ca3WO5Cl2.

Charge transfer (CT) luminescence of different types of polyhedra, [WO5]4- in Ca3WO5Cl2 and [WO6]6- in Ca3WO6, is characterized by spectroscopic experiments and ab initio calculations. According to the geometry optimization, W6+ ions form five-fold [WO5]4- square pyramids in Ca3WO5Cl2 because of a large interatomic distance between W6+ and Cl- of 3.266 Å. The analysis of the density of electronic states reveals the ionic character of Cl- ions to the W6+ ions in the Ca3WO5Cl2 lattice, resulting in the observed broad luminescence band peak at 488 nm of the single-crystal Ca3WO5Cl2 sample being assigned to the CT transition in the [WO5]4- square pyramid. Compared with the [WO6]6- octahedron in Ca3WO6, the [WO5]4- square pyramid shows an inconsistent CT energy shift: higher CT absorption and lower luminescence energies. The larger bandgap brings about higher absorption energy due to the structural and compositional features of the orthorhombic Ca3WO5Cl2. The redshifted CT luminescence band and small activation energy for the thermal quenching of the Ca3WO5Cl2 sample are explained, assuming that the CT states of the anisotropic [WO5]4- square pyramid take a larger offset in the configurational coordinate diagram than the [WO6]6- octahedron.

Deep-red to near-infrared luminescence from Eu2+-trapped exciton states in YSiO2N.

The valence state of Eu ions doped in inorganic compounds is easily influenced by the synthesizing conditions. In this study, X-ray absorption spectroscopy revealed that almost half of Eu ions incorporated in the YSiO2N host were reduced into the divalent state through the sintering process at 1600 °C under a N2 gas atmosphere without any annealing processes. The prepared Eu2+/3+-doped YSiO2N sample showed anomalous deep-red to near-infrared luminescence below 300 K under violet light illumination, whose luminescent properties are discussed through detailed spectroscopic analyses. In the photoluminescence spectra at 4 K, the broad luminescence band ranging from 550 to 1100 nm with a large Stokes shift of 5677 cm-1 was observed, assigned to the recombination emission related to the Eu2+-trapped exciton state. The temperature dependence of luminescence lifetime suggests that the thermal quenching of Eu2+-trapped exciton luminescence takes place through complicated processes in addition to thermal ionization. The energy diagrams based on the spectroscopic results indicate that Eu2+-trapped exciton luminescence in the YSiO2N:Eu2+/3+ sample was observed because all the Eu2+: 5d excited levels are degenerated with the host conduction band, and the relatively stable Eu2+-trapped exciton state in the Y3+ sites is formed just below the conduction band bottom. A comprehensive discussion on the deep-red to near-infrared luminescence in the YSiO2N host could give new insights into the mechanism of Eu2+-trapped exciton luminescence in Y3+ sites, which has potential in near-infrared emitting devices.

High-Pressure Photoluminescence Properties of Cr3+-Doped LaGaO3 Perovskites Modulated by Pressure-Induced Phase Transition.

The photoluminescence properties of Cr3+-doped LaGaO3 perovskites are investigated by high-pressure spectroscopy. The pressure-induced phase transition from orthorhombic (Pbnm) to rhombohedral (R3̅c) at around 2 GPa is confirmed by Raman spectroscopy. Cr3+-doped LaGaO3 shows deep-red emission peaks around 730 nm due to the zero-phonon line (R-line) and the phonon sidebands, which correspond to Cr3+: 2Eg → 4A2g transitions in the ideal octahedral site and the Cr-Cr pair luminescence (N-line) under ambient condition. Under a high pressure, the R-line shifts to a lower energy at a rate of -13 cm-1/GPa. From the pressure dependence of photoluminescence excitation (PLE) spectra, it is suggested that the redshift of the R-line is caused by the decrease of Racah parameters B and C. Moreover, the N-line luminescence becomes stronger relative to the R-line with increasing pressure and the N-line/R-line can be used to monitor the phase transition pressure. Under a high pressure, the tilt angle of the GaO6 octahedral unit becomes smaller. It implies that the enhanced N-line luminescence is caused by the stronger superexchange interaction between Cr3+ ions due to the increased Cr-O-Cr bond angle closer to 180°.

Microsized Red Luminescent MgAl2O4:Mn4+ Single-Crystal Phosphor Grown in Molten Salt for White LEDs.

A single-crystalline defect-less phosphor is desired for efficient luminescence of the therein doped optical activators. In this paper, microsized MgAl2O4:Mn4+ single-crystal phosphors with bright red luminescence were grown in molten LiCl salt at 950 °C, for application in blue LED pumped white lighting. By comparing the phosphor formation from various Mg2+- and Al3+-bearing sources, both the template-formation and the dissolution-diffusion processes were evidenced to account for the formation of the microsized MgAl2O4:Mn4+ crystallites. Using nano γ-Al2O3 as the Al3+-bearing precursor, the uniform MgAl2O4:Mn4+ microcrystallites with a {111} planes-exposed tetragonal bipyramid morphology were obtained. The photoluminescence property was studied at various temperatures, and Mg ↔ Al anti-site disorder induced luminescence broadening was discussed. The Mn4+ 2Eg → 4A2g transition in MgAl2O4 shows a quite short luminescence wavelength peaking at 651 nm and ultrabroadband emission extending to 850 nm. The luminescence is relatively robust against thermal effect with relatively high thermal quenching temperature of 400 K and activation energy of 0.23 eV. Employing the red-emitting MgAl2O4:Mn4+ crystallites, blue LED pumped white lighting prototypes were fabricated which simulate the solar-like spectrum and show neutral to warm white.

Pressure-induced variation of persistent luminescence characteristics in Y3Al5-xGaxO12:Ce3+-M3+ (M = Yb, and Cr) phosphors: opposite trend of trap depth for 4f and 3d metal ions.

Changing the electronic structure of materials by pressure and the accompanying changes in optical properties have attracted scientific interest. We have reported that the energy position of the conduction band (CB) bottom and the crystal field splitting of the Ce3+:5d excited level in Y3Al5-xGaxO12:Ce3+ are changed by applying pressure, which results in the red shifting of the Ce3+:5d → 4f luminescence and the increase of the quenching temperature. We also reported dramatic improvement of the persistent luminescence performance by either Cr3+ or Yb3+ codoping into the Y3Al5-xGaxO12:Ce3+ phosphors. The different trap depths formed by Cr3+ and Yb3+ affect the initial persistent luminescence intensity and the persistent luminescence duration. In this study, the effect of pressure on the persistent luminescence performance was investigated. For the Y3AlGa4O12:Ce3+-Yb3+ phosphor, the slope of persistent luminescence decay curve becomes more gentle with increasing pressure, while for the Y3AlGa4O12:Ce3+-Cr3+ phosphor the slope becomes steeper. These results indicate that the trap depth of Yb3+ becomes deeper and that of Cr3+ becomes shallower with increasing pressure. Based on the pressure-dependence of the luminescence quenching and the trap depth change estimated from the decay slopes, the relative electronic energies of the CB bottom and the Yb2+ (4f14) or Cr2+ (3d4) levels are discussed. The CB bottom energy is increased relative to the ground 1S0 state of Yb2+ with increasing pressure, which results in deepening of the electron trap depth of the Yb2+ state. The opposite tendency of the Cr3+ codoped sample was described by a decreasing tendency of the energy gap between the CB bottom and the Cr2+:eg level, the relative energy level of which is increased by the increase of the crystal field with increasing pressure in the garnet host material, where the electron-trapping Cr2+ ions take the high spin state (t32ge1g) rather than the low-spin state (t42g).

Reply to the 'Comment on "Spectroscopic properties and location of the Ce3+ energy levels in Y3Al2Ga3O12 and Y3Ga5O12 at ambient and high hydrostatic pressure"' by Y. Wang, M. Glowacki, M. Berkowski, A. Kaminska and A. Suchocki, Phys. Chem. Chem. Phys., 2019, 21, DOI.

New results presented by Wang et al. showing the temperature dependence of the Y3Ga5O12:Ce3+ energy bandgap have been taken into account in the calculations of the changes of the energy distance between the lowest 5d state of Ce3+ and the edge of the conduction band. Our calculations show that the diminishing of the band gap energy with temperature has a negligible effect on the difference between the energy of the conduction band and the localized states of the 5d configuration of Ce3+, which means that the new experimental results do not undermine the validity of the conclusions of our previous paper.

Reply to the 'Comment on "Spectroscopic properties and location of the Ce3+ energy levels in Y3Al2Ga3O12 and Y3Ga5O12 at ambient and high hydrostatic pressure"' by Y. Wang, M. Glowacki, M. Berkowski, A. Kaminska and A. Suchocki, Phys. Chem. Chem. Phys., 2019, 21, DOI: 10.1039/C8CP06154H.

New results presented by Wang et al. showing the temperature dependence of the Y3Ga5O12:Ce3+ energy bandgap have been taken into account in the calculations of the changes of the energy distance between the lowest 5d state of Ce3+ and the edge of the conduction band. Our calculations show that the diminishing of the band gap energy with temperature has a negligible effect on the difference between the energy of the conduction band and the localized states of the 5d configuration of Ce3+, which means that the new experimental results do not undermine the validity of the conclusions of our previous paper.

Intense deep-red zero phonon line emission of Mn4+ in double perovskite La4Ti3O12.

Phosphors that emit in the deep-red spectral region are critical for plant cultivation light-emitting diodes. Herein, ultrabroadband deep-red luminescence of Mn4+ in La4Ti3O12 was studied, which showed intense zero phonon line emission. The double-perovskite structural La4Ti3O12 simultaneously contains two Ti4+ sites forming slightly- and highly-distorted TiO6 octahedra, respectively. The influence of octahedral distortion on the Mn4+ emission energy in the two distinct Ti4+ sites was studied both experimentally and theoretically. The spectral measurements indicated that Mn4+ in La4Ti3O12 showed intense zero phonon line emission (ZPL) at deep-red 710-740 nm under excitation of 400 nm charging the O2-→ Mn4+ charge transfer transition. The splitting of the ZPL of the Mn4+ 2Eg→4A2g transition as well as the intensity of ZPL relative to the vibronic phonon sideband emissions were found to be greatly influenced by the degree of octahedral distortion. The crystal-field strength and Racah parameters of Mn4+ in each Ti4+ site were also estimated. The Mn4+ 2Eg→4A2g luminescence exhibited severe thermal quenching, which was explained by the low-lying 4T2g level and charge-transfer state.

Persistent energy transfer in ZGO:Cr3+,Yb3+: a new strategy to design nano glass-ceramics featuring deep red and near infrared persistent luminescence.

ZnGa2O4:Cr3+, owing to its persistent luminescence properties in the deep red range, is an exceptional material in view of foreseen in vivo imaging applications. In the present work, we report the elaboration process and detailed investigations of the optical properties of nano glass-ceramics composed of spinel ZnGa2O4:Cr3+,Yb3+ nanocrystals embedded in a transparent, silica rich, glass matrix. The as-prepared materials show good incorporation of the dopants in the crystallites leading in both Cr3+ and Yb3+ emissions. These emissions occur while exciting in the Cr3+ bands, indicating an energy transfer process from Cr3+ to Yb3+. Furthermore, excitation in the Yb3+ band in the near-infrared (NIR) range suggests an interesting up-conversion process, which promotes the Cr3+ emission. Persistent luminescence of both Cr3+ and Yb3+ doping ions can be activated by charging the Cr3+ excitation bands, leading to persistent luminescence of zinc gallate nanocrystals in both first and second biological windows. The influence of Yb3+ co-doping on persistent luminescence properties has been investigated by persistent luminescence decay profiles and thermoluminescence studies. Indeed, thermoluminescence glow curves of Yb3+ exhibit similar shape to those of Cr3+ but appear broader and shifted towards higher temperatures. This temperature shift may be explained by the temperature dependence of the involved energy transfer process.

Toward Rechargeable Persistent Luminescence for the First and Third Biological Windows via Persistent Energy Transfer and Electron Trap Redistribution.

Persistent luminescence (PersL) imaging without real-time external excitation has been regarded as the next generation of autofluorescence-free optical imaging technology. However, to achieve improved imaging resolution and deep tissue penetration, developing new near-infrared (NIR) persistent phosphors with intense and long duration PersL over 1000 nm is still a challenging but urgent task in this field. Herein, making use of the persistent energy transfer process from Cr3+ to Er3+, we report a novel garnet persistent phosphor of Y3Al2Ga3O12 codoped with Er3+ and Cr3+ (YAG G:Er-Cr), which shows intense Cr3+ PersL (∼690 nm) in the deep red region matching well with the first biological window (NIR-I, 650-950 nm) and Er3+ PersL (∼1532 nm) in the NIR region matching well with the third biological window (NIR-III, 1500-1800 nm). The optical imaging through raw-pork tissues (thickness of 1 cm) suggests that the emission band of Er3+ can achieve higher spatial resolution and more accurate signal location than that of Cr3+ due to the reduced light scattering at longer wavelengths. Furthermore, by utilizing two independent electron traps with two different trap depths in YAG G:Er-Cr, the Cr3+/Er3+ PersL can even be recharged in situ by photostimulation with 660 nm LED thanks to the redistribution of trapped electrons from the deep trap to the shallow one. Our results serve as a guide in developing promising NIR (>1000 nm) persistent phosphors for long-term optical imaging.

Comparison of quenching mechanisms in Gd3Al5-xGaxO12:Ce3+ (x = 3 and 5) garnet phosphors by photocurrent excitation spectroscopy.

In this work we present the results of photocurrent excitation spectroscopy (PCE) of Gd3Al2Ga3O12:Ce3+ (GAGG:Ce3+) and Gd3Ga5O12:Ce3+ (GGG:Ce3+) performed at temperatures ranging from 100 to 500 K supplemented by spectroscopic measurements (steady state and time resolved photoluminescence spectroscopy) performed at temperatures ranging from 10 to 500 K and at high pressure up to 300 kbar. The PCE spectra contain bands related to transitions from the ground state 2F5/2 of the 4f1 electronic configuration to the crystal field split states related to the 5d1 electronic configuration of Ce3+. This implicates the presence of the autoionization process - transfer of electrons from the localized, excited states of Ce3+ to the conduction band (CB), directly linked to luminescence quenching of Ce3+. The mechanism of autoionization of GAGG:Ce3+ and GGG:Ce3+ was determined to be different on the grounds of differences in temperature dependence of photocurrent intensity. The latter system exhibits autoionization, which occurs when all of the 5d excited states are degenerated with the CB, whereas in the former system, the autoionization process is thermally assisted with an activation energy barrier (distance to the edge of the CB) of approximately 1600 cm-1. In GGG:Ce3+ the degeneracy of 5d1 states of Ce3+ was lifted by application of high pressure, shifting the edge of the CB up and exposing Ce3+ luminescence at 20 kbar. Further spectroscopic analyses of the pressure-temperature dependence of the luminescence decay time as well as the temperature dependence of photocurrent intensity of GGG:Ce3+ have independently shown existence of a luminescence quenching state located approximately 600 cm-1 below the CB, attributed to the impurity trapped exciton.

Vacuum Referred Binding Energy (VRBE)-Guided Design of Orange Persistent Ca3Si2O7:Eu2+ Phosphors.

Orange persistent phosphors of Ca3Si2O7 (CSO) doped with Eu2+ were strategically developed by codoping Sm3+ or Tm3+. First, a vacuum referred binding energy, VRBE, diagram of Ca3Si2O7 (CSO) was constructed from the measured spectroscopic data. By the zigzag curve of the divalent lanthanide ions in the VRBE diagram, Sm3+ and Tm3+ ions were predicted to be a suitable electron trap for the persistent luminescence. The initial persistent luminance of CSO:Eu2+-Sm3+ and CSO:Eu2+-Tm3+ was found to be 290 times and 9300 times stronger, respectively, compared with CSO:Eu2+. By optimizing Eu2+ and Tm3+ concentrations, the persistent luminescence duration on 0.32 mcd/m2 reached approximately 50 min in CSO:Eu2+-Tm3+. From the VRBE diagram and the persistent luminescence properties, we discuss the persistent mechanism including the charging process, detrapping process, and electron trapping centers.

Study on Trap Levels in SrSi2AlO2N3:Eu2+,Ln3+ Persistent Phosphors Based on Host-Referred Binding Energy Scheme and Thermoluminescence Analysis.

We investigated the effect of trivalent lanthanide substitution on a novel oxynitride persistent phosphor SrSi2AlO2N3:Eu2+,Ln3+, which shows green persistent luminescence for more than 2 h. First, an energy level diagram by using the host-referred binding energy (HRBE) scheme was constructed. The location of the energy levels of all divalent and trivalent lanthanides referred to the energy band of the host SrSi2AlO2N3 was estimated. Then, thermoluminescence (TL) measurements in the target persistent phosphors were performed to obtain direct experimental results on the trap depth. We found that the trap levels based on the TL measurements coincided well with the 4f ground states of divalent lanthanide codopants in SrSi2AlO2N3:Eu2+,Ln3+. The result strongly suggests the effective traps for persistent luminescence in SrSi2AlO2N3:Eu2+,Ln3+ could be due to the aliovalent substitution of Ln3+ for Sr2+, which can be controlled by selecting suitable codopant Ln3+. The work shows the HRBE scheme may offer a way to understand the nature of defects in the persistent phosphor as well as a possible guideline to design new persistent phosphors with required trap depths.

A brief review of characteristic luminescence properties of Eu3+ in mixed-anion compounds.

Trivalent europium (Eu3+) ions show red luminescence with sharp spectral lines owing to the intraconfigurational 4f-4f transitions. Because of their characteristic luminescence properties, various Eu3+-doped inorganic compounds have been developed to meet the demands of optoelectronic devices. Regardless of shielding by the outer 5s and 5p orbitals, the properties of the Eu3+:4f-4f transition depend on the local environment, such as the shapes of the coordination polyhedra, site symmetry, nephelauxetic effects, crystal field effects, and bonding character. Mixed-anion coordination, where multiple types of anions surround a single Eu3+ ion, can directly affect the optical properties of Eu3+. We review the luminescence properties of Eu3+ ions in mixed-anion compounds of the oxynitride YSiO2N and oxyhalides YOX (X = Cl or Br). Oxynitride and oxyhalide coordination results in characteristic transition probabilities and branching ratios of the 5D0 → 7F0-6 transitions due to distorted structural environments and red-shifted charge transfer excitation bands due to an upward shift of the valence band. The expected and experimentally observed features of Eu3+ luminescence in mixed-anion compounds are outlined based on band and Judd-Ofelt theories. Future applications of the intense red luminescence at ∼620 nm under near-ultraviolet light illumination in Eu3+-doped mixed-anion compounds are introduced, and material design guidelines for new functional Eu3+-doped phosphors are presented.

Addressing Current Challenges in OSL Dosimetry Using MgB4O7:Ce,Li: State of the Art, Limitations and Avenues of Research.

The objective of this work is to review and assess the potential of MgB4O7:Ce,Li to fill in the gaps where the need for a new material for optically stimulated luminescence (OSL) dosimetry has been identified. We offer a critical assessment of the operational properties of MgB4O7:Ce,Li for OSL dosimetry, as reviewed in the literature and complemented by measurements of thermoluminescence spectroscopy, sensitivity, thermal stability, lifetime of the luminescence emission, dose response at high doses (>1000 Gy), fading and bleachability. Overall, compared with Al2O3:C, for example, MgB4O7:Ce,Li shows a comparable OSL signal intensity following exposure to ionizing radiation, a higher saturation limit (ca 7000 Gy) and a shorter luminescence lifetime (31.5 ns). MgB4O7:Ce,Li is, however, not yet an optimum material for OSL dosimetry, as it exhibits anomalous fading and shallow traps. Further optimization is therefore needed, and possible avenues of investigation encompass gaining a better understanding of the roles of the synthesis route and dopants and of the nature of defects.

Reversible Phase Segregation and Amorphization of Mixed-Halide Perovskite Nanocrystals in Glass Matrices.

Mixed-halide perovskites have attracted great attention in applications of lighting and photovoltaic devices due to their excellent properties. Understanding the phase segregation mechanism of mixed-halide perovskite has significance for suppressing the performance degradation of optoelectronic devices. Herein, we investigate the mixed-halide perovskite nanocrystals (NCs) in isolation from the external factors (oxygen, moisture, and pressure) using glass encapsulation, which shows excellent photostability against phase segregation. By monitoring the structural evolution of the NCs in glass matrices, the coexisting phase segregation and amorphization of mixed-halide perovskites are observed in real-time. The results show that thermal-induced local temperature increase plays a dominant role in the phase segregation of mixed-halide perovskite NCs. The recovery process is driven by the spontaneous crystallization of the amorphous mixed-halide phase. The clarified dynamic equilibrium process between the compositional segregation (mixing) and structural disorder (order) gives us a better insight into the reversible phase segregation mechanism of mixed-halide perovskite.

Effect of Glass Composition on Luminescence and Structure of CsPbBr3 Quantum Dots in an Amorphous Matrix.

Glass matrix embedding is an efficient way to improve the chemical and thermal stability of the halide perovskite QDs. However, CsPbX3 QDs exhibit distinct optical properties in different glass matrixes, including photoluminescence (PL) peak position, PL peak width, and optical band gap. In this work, the temperature-dependent PL spectra, absorption spectra, high-energy X-ray structure factor S(Q), and pair distribution function (PDF) were integrated to analyze the structural evolution of CsPbBr3 QDs in different glass matrixes. The results show that the lattice parameters and atomic spacing of CsPbBr3 QDs are affected by the glass composition in which they are embedded. The most possibility can be attributed to the thermal expansion mismatch between CsPbBr3 QDs and the glass matrix. The results may provide a new way to understand the effect of the glass composition on the optical properties of CsPbBr3 QDs in a glass matrix.

Photoluminescent coordination polymer bulk glasses and laser-induced crystallization.

We synthesized luminescent coordination polymer glasses composed of d10 metal cyanides and triphenylphosphine through melt-quenching and mechanical milling protocols. Synchrotron X-ray total scattering measurements and solid-state NMR revealed their one-dimensional chain structures and high structural dynamics. Thermodynamic and photoluminescence properties were tunable by the combination of heterometallic ions (Ag+, Au+, and Cu+) in the structures. The glasses are moldable and thermally stable, and over centimeter-sized glass monoliths were fabricated by the hot-press technique. They showed high transparency over 80% from the visible to near-infrared region and strong green emission at room temperature. Furthermore, the glass-to-crystal transformation was demonstrated by laser irradiation through the photothermal effect of the glasses.

Formation of PbCl2-type AHF (A = Ca, Sr, Ba) with partial anion order at high pressure.

The high-pressure structures of alkaline earth metal hydride-fluorides (AHFs) (A = Ca, Sr, Ba) were investigated up to 8 GPa. While AHF adopts the fluorite-type structure (Fm3[combining macron]m) at ambient pressure without anion ordering, the PbCl2-type (cotunnite-type) structure (Pnma) is formed by pressurization, with a declining trend of critical pressure as the ionic radius of the A2+ cation increases. In contrast to PbCl2-type LaHO and LaOF whose anions are fully ordered, the H-/F- anions in the high-pressure polymorph of SrHF and BaHF are partially ordered, with a preferential occupation of H- at the square-pyramidal site (vs. tetrahedral site). First-principles calculations partially support the preferential anion occupation and suggest occupation switching at higher pressure. These results provide a strategy for controlling the anion ordering and local structure in mixed-anion compounds.

Broadband mid-infrared amplified spontaneous emission from Er/Dy co-doped fluoride fiber with a simple diode-pumped configuration.

Er3+/Dy3+ co-doped double-clad ZBLAN optical fiber has been used to obtain amplified spontaneous emission (ASE) broadband light sources cladding-pumped by 980-nm multimode laser diode (LD) sources. It has been demonstrated that mid-infrared broadband emission extending from 2515 to 3735 nm was obtained by energy transfer between Er3+ and Dy3+. We experimentally investigated the optimum design of Er3+/Dy3+ co-doped ZBLAN fiber in terms of ion concentration, fiber length, pumping configuration, and pumping power. The ASE output power was more than 2.5 mW when the LD pump power was set at 5 W. To assess its potential for gas sensing applications, the fabricated ASE light source was used to successfully detect methane gas with concentrations at 1% and 5%. The simple and stable construction of our ASE light source is suitable for practical purposes.