Epitaxial Core/Shell Nanocrystals of (Europium-Doped) Zirconia and Hafnia.

A careful design of the nanocrystal architecture can strongly enhance the nanocrystal function. So far, this strategy has faced a synthetic bottleneck in the case of refractory oxides. Here we demonstrate the epitaxial growth of hafnia shells onto zirconia cores and pure zirconia shells onto europium-doped zirconia cores. The core/shell structures are fully crystalline. Upon shelling, the optical properties of the europium dopant are dramatically improved (featuring a more uniform coordination and a longer photoluminescence lifetime), indicating the suppression of nonradiative pathways. These results launch the stable zirconium and hafnium oxide hosts as alternatives for the established NaYF4 systems.

Identification of vanadium dopant sites in the metal-organic framework DUT-5(Al).

Studying the structural environment of the VIV ions doped in the metal-organic framework (MOF) DUT-5(Al) ((AlIIIOH)BPDC) with electron paramagnetic resonance (EPR) reveals four different vanadium-related spectral components. The spin-Hamiltonian parameters are derived by analysis of X-, Q- and W-band powder EPR spectra. Complementary Q-band Electron Nuclear DOuble Resonance (ENDOR) experiments, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX), X-Ray Diffraction (XRD) and Fourier Transform InfraRed (FTIR) measurements are performed to investigate the origin of these spectral components. Two spectral components with well resolved 51V hyperfine structure are visible, one corresponding to VIV[double bond, length as m-dash]O substitution in a large (or open) pore and one to a narrow (or closed) pore variant of this MOF. Furthermore, a broad structureless Lorentzian line assigned to interacting vanadyl centers in each other's close neighborhood grows with increasing V-concentration. The last spectral component is best visible at low V-concentrations. We tentatively attribute it to (VIV[double bond, length as m-dash]O)2+ linked with DMF or dimethylamine in the pores of the MOF. Simulations using these four spectral components convincingly reproduce the experimental spectra and allow to estimate the contribution of each vanadyl species as a function of V-concentration.

Identification of Dy

Laser excitation and x-ray spectroscopy are combined to settle a long-standing question in persistent luminescence. A reversible electron transfer is demonstrated, controlled by light and showing the same kinetics as the persistent luminescence. Exposure to violet light induces charging by oxidation of the excited Eu^{2+} while Dy^{3+} is simultaneously reduced. Oppositely, detrapping of Dy^{2+} occurs at ambient temperature or by infrared illumination, yielding afterglow or optically stimulated luminescence, respectively.

Doping homogeneity in co-doped materials investigated at different length scales.

Doping homogeneity is important for the properties of co-doped phosphors, as it can affect the energy transfer between sensitizer and activator ions. In a case study we apply different methods, that is scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX) mapping, SEM combined with cathodoluminescence (CL) and solid-state nuclear magnetic resonance (NMR), to study the doping homogeneity of the host system monazite LaPO4 doped with two different lanthanide ions on different length scales. A new criterion for doping heterogeneity in co-doped systems is developed, which is based on the NMR visibility function, which for this purpose is extended to doping with two or more paramagnetic dopants. A deviation from this function is indicative of doping heterogeneity on the length-scale of the blind-spheres of the paramagnetic dopants. A discussion of the advantages and disadvantages of the different methods is presented. The combined approach allows to study doping homogeneity from the nm to the µm scale.

Vibrational Quenching in Near-Infrared Emitting Lanthanide Complexes: A Quantitative Experimental Study and Novel Insights.

Two series of novel NIR-emissive complexes of Nd3+ , Sm3+ , Er3+ and Yb3+ with two different ß-diketonate ligands (L1 =4,4,4-trifluoro-1-phenyl-1,3-butadione and L2 =4,4,4-trifluoro-1-(4-chlorophenyl)-1,3-butadione) are reported. The neutral triphenylphosphine oxide (tppo) ligand was used to replace coordinated water molecules in the first coordination sphere of the as-obtained [Ln(L1(2) )3 (H2 O)2 ] complexes to afford water-free [Ln(L1(2) )3 (tppo)2 ] molecular species. Upon replacement of water molecules by tppo units, the NIR emission lifetimes of the Nd3+ , Er3+ and Sm3+ complexes increase by about one order of magnitude up to values of ≈9, 8 and 113 ms while Yb3+ complexes reach intrinsic quantum yields as high as to ΦYb =6.5 %., which are remarkably high for fully hydrogenated complexes. Vibrational quenching by CH and OH oscillators has been quantitatively assessed by implementing the Förster's model of resonance energy transfer on the basis of experimental data. This study demonstrates that highly efficient NIR-emitting lanthanide complexes can be obtained with facile, cheap and accessible syntheses through a rational design.

Blind spheres of paramagnetic dopants in solid state NMR.

Solid-state NMR on paramagnetically doped crystal structures gives information about the spatial distribution of dopants in the host. Paramagnetic dopants may render NMR active nuclei virtually invisible by relaxation, paramagnetic broadening or shielding. In this contribution blind sphere radii r0 have been reported, which could be extracted through fitting the NMR signal visibility function f(x) = exp(-ar03x) to experimental data obtained on several model compound series: La1-xLnxPO4 (Ln = Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb), Sr1-xEuxGa2S4 and (Zn1-xMnx)3(PO4)2·4H2O. Radii were extracted for 1H, 31P and 71Ga, and dopants like Nd3+, Gd3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+ and Mn2+. The observed radii determined differed in all cases and covered a range from 5.5 to 13.5 Å. While these radii were obtained from the amount of invisible NMR signal, we also show how to link the visibility function to lineshape parameters. We show under which conditions empirical correlations of linewidth and doping concentration can be used to extract blind sphere radii from second moment or linewidth parameter data. From the second moment analysis of La1-xSmxPO431P MAS NMR spectra for example, a blind sphere size of Sm3+ can be determined, even though the visibility function remains close to 100% over the entire doping range. Dependence of the blind sphere radius r0 on the NMR isotope and on the paramagnetic dopant could be suggested and verified: for different nuclei, r0 shows a -dependence, γ being the gyromagnetic ratio. The blind sphere radii r0 for different paramagnetic dopants in a lanthanide series could be predicted from the pseudo-contact term.

Switchable Piezoresistive SmS Thin Films on Large Area.

Samarium monosulfide (SmS) is a switchable material, showing a pressure-induced semiconductor to metal transition. As such, it can be used in different applications such as piezoresistive sensors and memory devices. In this work, we present how e-beam sublimation of samarium metal in a reactive atmosphere can be used for the deposition of semiconducting SmS thin films on 150 mm diameter silicon wafers. The deposition parameters influencing the composition and properties of the thin films are evaluated, such as the deposition rate of Sm metal, the substrate temperature and the H2S partial pressure. We then present the changes in the optical, structural and electrical properties of this compound after the pressure-induced switching to the metallic state. The back-switching and stability of SmS thin films are studied as a function of temperature and atmosphere via in-situ X-ray diffraction. The thermally induced back switching initiates at 250 °C, while above 500 °C, Sm2O2S is formed. Lastly, we explore the possibility to determine the valence state of the samarium ions by means of X-ray photoelectron spectroscopy.

Exploring Lanthanide Doping in UiO-66: A Combined Experimental and Computational Study of the Electronic Structure.

Lanthanide-based metal-organic frameworks show very limited stabilities, which impedes their use in applications exploiting their extraordinary electronic properties, such as luminescence and photocatalysis. This study demonstrates a fast and easy microwave procedure to dope UiO-66, an exceptionally stable and tunable Zr-based metal-organic framework. The generally applicable synthesis methodology is used to incorporate different transition metal and lanthanide ions. Selected experiments on these newly synthesized materials allow us to construct an energy scheme of lanthanide energy levels with respect to the UiO-66 host. The model is confirmed via absolute intensity measurements and provides an intuitive way to understand charge transfer mechanisms in these doped UiO-66 materials. Density functional theory calculations on a subset of materials moreover improve our understanding of the electronic changes in doped UiO-66 and corroborate our empirical model.

Predicting the afterglow duration in persistent phosphors: a validated approach to derive trap depth distributions.

Persistent phosphors are increasingly investigated due to their potential applications in various fields, such as safety signage, dosimetry and in vivo imaging. These materials act as optical batteries that store and gradually release energy supplied during optical charging. As the energy is stored, or 'trapped', at specific defect sites in the host lattice, a clear understanding of the defects and trapping mechanisms in these materials is important for systematic improvement of their properties. Here, the thermoluminescence and afterglow properties of the near-infrared (NIR) emitting persistent phosphor LiGa5O8:Cr3+ (LGO:Cr) are studied. This phosphor is used as a model system for illustrating a more general approach to reliably derive trap depth distributions in persistent luminescent and storage materials. The combination of the Tstop-Tmax method with initial rise analysis is used to experimentally determine the presence of a broad distribution of trapping states. Computerized glow curve fitting is subsequently used to extract the relevant trapping parameters of the system in a rigorous, consistent manner, by fitting all the experimentally recorded data simultaneously. The resulting, single set of model parameters is able to describe all measured thermoluminescence and afterglow data and hence can be used to predict afterglow and storage properties of the phosphor under various conditions. The methods to analyze and describe the trap structure of the persistent phosphor LGO:Cr are straightforwardly applicable for other persistent and storage phosphors and result in a reliable determination of the relevant trapping parameters of a given material.

Charge transfer induced energy storage in CaZnOS:Mn - insight from experimental and computational spectroscopy.

CaZnOS:Mn2+ is a rare-earth-free luminescent compound with an orange broadband emission at 612 nm, featuring pressure sensing capabilities, often explained by defect levels where energy can be stored. Despite recent efforts from experimental and theoretical points of view, the underlying luminescence mechanisms in this phosphor still lack a profound understanding. By the evaluation of thermoluminescence as a function of the charging wavelength, we probe the defect levels allowing energy storage. Multiple trap depths and trapping routes are found, suggesting predominantly local trapping close to Mn2+ impurities. We demonstrate that this phosphor shows mechanoluminescence which is unexpectedly stable at high temperature (up to 200 °C), allowing pressure sensing in a wide temperature range. Next, we correlate the spectroscopic results with a theoretical study of the electronic structure and stability of the Mn defects in CaZnOS. DFT calculations at the PBE+U level indicate that Mn impurities are incorporated on the Zn site in a divalent charge state, which is confirmed by X-ray absorption spectroscopy (XAS). Ligand-to-metal charge transfer (LMCT) is predicted from the location of the Mn impurity levels, obtained from the calculated defect formation energies. This LMCT proves to be a very efficient pathway for energy storage. The excited state landscape of the Mn2+ 3d5 electron configuration is assessed through the spin-correlated crystal field and a good correspondence with the emission and excitation spectra is found. In conclusion, studying phosphors at both a single-particle level (i.e. via calculation of defect formation energies) and a many-particle level (i.e. by accurately localizing the excited states) is necessary to obtain a complete picture of luminescent defects, as demonstrated in the case of CaZnOS:Mn2+.

First-Principles Study of Antisite Defect Configurations in ZnGa2O4:Cr Persistent Phosphors.

Zinc gallate doped with chromium is a recently developed near-infrared emitting persistent phosphor, which is now extensively studied for in vivo bioimaging and security applications. The precise mechanism of this persistent luminescence relies on defects, in particular, on antisite defects and antisite pairs. A theoretical model combining the solid host, the dopant, and/or antisite defects is constructed to elucidate the mutual interactions in these complex materials. Energies of formation as well as dopant, and defect energies are calculated through density-functional theory simulations of large periodic supercells. The calculations support the chromium substitution on the slightly distorted octahedrally coordinated gallium site, and additional energy levels are introduced in the band gap of the host. Antisite pairs are found to be energetically favored over isolated antisites due to significant charge compensation as shown by calculated Hirshfeld-I charges. Significant structural distortions are found around all antisite defects. The local Cr surrounding is mainly distorted due to a ZnGa antisite. The stability analysis reveals that the distance between both antisites dominates the overall stability picture of the material containing the Cr dopant and an antisite pair. The findings are further rationalized using calculated densities of states and Hirshfeld-I charges.

Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution.

Persistent phosphors are a specific type of luminescent materials having the unique ability to emit light long after the excitation has ended. They are commonly used as emergency signage in near ideal, isothermal indoor situations. Recently, their energy storage capacity was relied on for outdoor situations, e.g. for glow-in-the-dark road marks and in combination with solar cells and photo catalytic processes. In this work the influence of temperature, illumination intensity and the duration of the night is critically evaluated on the performance of afterglow phosphors. The persistent luminescence of SrAl2O4:Eu,Dy green emitting phosphors is studied under realistic and idealized conditions. It is found that the light output profile is hardly influenced by the ambient temperature in a wide range. This is due to the presence of a broad trap depth distribution, which is beneficial to cover the longer and colder winter nights. Temperature drops during the night are however detrimental. For traffic applications, the total light output of glow-in-the-dark road marks at the end of the night is not sufficient for the studied compound, although re-charging by the car's headlamps partially alleviates this. For energy storage applications, the trap density should be improved and tunneling recombination processes might be needed to overcome overnight temperature drops.

Energy level modeling of lanthanide materials: review and uncertainty analysis.

Energy level schemes are an essential tool for the description and interpretation of atomic spectra. During the last 40 years, several empirical methods and relationships were devised for constructing energy level schemes of lanthanide defects in wide band gap solids, culminating in the chemical shift model by Thiel and Dorenbos. This model allows us to calculate the electronic and optical properties of the considered materials. However, an unbiased assessment of the accuracy of the obtained values of the calculated parameters is still lacking to a large extent. In this paper, error margins for calculated electronic and optical properties are deduced. It is found that optical transitions can be predicted within an acceptable error margin, while the description of phenomena involving conduction band states is limited to qualitative interpretation due to the large error margins for physical observables such as thermal quenching temperature, corresponding to standard deviations in the range 0.3-0.5 eV for the relevant energy differences. As an example, the electronic structure of lanthanide doped calcium thiogallate (CaGa2S4) is determined, taking the experimental spectra of CaGa2S4:Ln(Q+) (Ln(Q+) = Ce(3+), Eu(2+), Tm(3+)) as input. Two different approaches to obtain the shape of the zig-zag curves connecting the 4f levels of the different lanthanides are explored and compared.

Energy transfer in Eu³âº doped scheelites: use as thermographic phosphor.

In this paper the luminescence of the scheelite-based CaGd2(1-x)Eu2x(WO4)4 solid solutions is investigated as a function of the Eu content and temperature. All phosphors show intense red luminescence due to the 5D0 - 7F2 transition in Eu³âº, along with other transitions from the 5D1 and 5D0 excited states. For high Eu³âº concentrations the intensity ratio of the emission originating from the 5D1 and 5D0 levels has a non-conventional temperature dependence, which could be explained by a phonon-assisted cross-relaxation process. It is demonstrated that this intensity ratio can be used as a measure of temperature with high spatial resolution, allowing the use of these scheelites as thermographic phosphor. The main disadvantage of many thermographic phosphors, a decreasing signal for increasing temperature, is absent.

Hydrophilic, bright CuInS2 quantum dots as Cd-free fluorescent labels in quantitative immunoassay.

We report on the synthesis of core-shell CuInS2/ZnS quantum dots (QDs) in organic solution, their encapsulation with a PEG-containing amphiphilic polymer, and the application of the resulting water-soluble QDs as fluorescent label in quantitative immunoassay. By optimizing the methods for core synthesis and shell growth, CuInS2/ZnS QDs were obtained with a quantum yield of 50% on average after hydrophilization. After conjugation with an aflatoxin B1-protein derivative, the obtained QDs were used as fluorescent labels in microplate immunoassay for the quantitative determination of the mycotoxin aflatoxin B1. QDs-based immunoassay showed higher sensitivity compared to enzyme-based immunoassay.

Cs7Nd11(SeO3)12Cl16: first noncentrosymmetric structure among alkaline-metal lanthanide selenite halides.

Cs7Nd11(SeO3)12Cl16, the complex selenite chloride of cesium and neodymium, was synthesized in the NdOCl-SeO2-CsCl system. The compound has been characterized using single-crystal X-ray diffraction, electron diffraction, transmission electron microscopy, luminescence spectroscopy, and second-harmonic-generation techniques. Cs7Nd11(SeO3)12Cl16 crystallizes in an orthorhombic unit cell with a = 15.911(1) Å, b = 15.951(1) Å, and c = 25.860(1) Å and a noncentrosymmetric space group Pna2(1) (No. 33). The crystal structure of Cs7Nd11(SeO3)12Cl16 can be represented as a stacking of Nd11(SeO3)12 lamellas and CsCl-like layers. Because of the layered nature of the Cs7Nd11(SeO3)12Cl16 structure, it features numerous planar defects originating from occasionally missing the CsCl-like layer and violating the perfect stacking of the Nd11(SeO3)12 lamellas. Cs7Nd11(SeO3)12Cl16 represents the first example of a noncentrosymmetric structure among alkaline-metal lanthanide selenite halides. Cs7Nd11(SeO3)12Cl16 demonstrates luminescence emission in the near-IR region with reduced efficiency due to a high concentration of Nd(3+) ions causing nonradiative cross-relaxation.

A XAS study of the luminescent Eu centers in thiosilicate phosphors.

Due to its bright yellow-to-red emission, europium doped Ca2SiS4 is a very interesting material for phosphor converted light emitting diodes. The emission spectrum is highly dependent on the Eu concentration and can consist of more than one emission band. We combined X-ray absorption fine structure and photoluminescence measurements to analyze the structure of europium centers in (Ca,Eu)2SiS4 luminescent powders. This paper provides an explanation for the concentration dependency of the emission spectra. We find that at low dopant concentrations a large fraction of trivalent europium ions is unexpectedly present in the powders. These trivalent europium ions tend to form defect clusters in the luminescent powders. Furthermore we observe a preferential substitution of the europium ions over the two different substitutional Ca sites, which changes upon increasing the dopant concentration. At high dopant concentration, the powder crystallizes in the monoclinic Eu2SiS4 structure. Once more a preferential substitution of the europium ions is observed. Summarizing, the influence of the concentration on the emission spectrum is explained by a difference in preferential occupation of the Eu ions in the lattice.

Strontium Aluminate Persistent Luminescent Single Crystals: Linear Scaling of Emission Intensity with Size Is Affected by Reabsorption.

The green-emitting SrAl2O4:Eu,Dy phosphor is the most widely used and well-studied persistent luminescent phosphor available today. Recent efforts to boost its performance in terms of luminescence intensity and duration are challenged by complex loss mechanisms, including the optically stimulated release of previously trapped charges by excitation light. Here, we present minimally scattering SrAl2O4:Eu,Dy single crystals, which, as opposed to powder phosphors, allow to profit from a so-called volume effect, resulting in a significantly increased emission intensity. Additionally, they allow for the identification of the reabsorption of the afterglow emission by trapped charges as an important loss mechanism, leading to a nonlinear scaling of the emission intensity with the crystal size. If circumvented, the emission intensity could be further increased, in persistent luminescent powders, ceramics, and single crystals.

Optimizing the Mechanoluminescent Properties of CaZnOS:Tb via Microwave-Assisted Synthesis: A Comparative Study with Conventional Thermal Methods.

Recent developments in lighting and display technologies have led to an increased focus on materials and phosphors with high efficiency, chemical stability, and eco-friendliness. Mechanoluminescence (ML) is a promising technology for new lighting devices, specifically in pressure sensors and displays. CaZnOS has been identified as an efficient ML material, with potential applications as a stress sensor. This study focuses on optimizing the mechanoluminescent properties of CaZnOS:Tb through microwave-assisted synthesis. We successfully synthesized CaZnOS doped with Tb3+ using this method and compared it with samples obtained through conventional solid-state methods. We analyzed the material's characteristics using various techniques to investigate their structural, morphological, and optical properties. We then studied the material's mechanoluminescent properties through single impacts with varying energies. Our results show that materials synthesized through microwave methods exhibit similar optical and, primarily, mechanoluminescent properties, making them suitable for use in photonics applications. The comparison of the microwave and conventional solid-state synthesis methods highlights the potential of microwave-assisted methods to optimize the properties of mechanoluminescent materials for practical applications.