Jonas Joos

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.

White light-emitting diodes: Stabilizing colour and intensity

Thermally activated defects in a blue-emitting phosphor can enhance energy transfer to the activator, and compensate for thermal quenching.

Hybrid remote quantum dot|[sol]|powder phosphor designs for display backlights

Quantum dots are ideally suited for color conversion in light emitting diodes owing to their spectral tunability, high conversion efficiency and narrow emission bands. These properties are particularly important for display backlights; the highly saturated colors generated by quantum dots justify their higher production cost. Here, we demonstrate the benefits of a hybrid remote phosphor approach that combines a green-emitting europium-doped phosphor with red-emitting CdSe/CdS core/shell quantum dots. Different stacking geometries, including mixed and separate layers of both materials, are studied at the macroscopic and microscopic levels to identify the configuration that achieves maximum device efficiency while minimizing material usage. The influence of reabsorption, optical outcoupling and refractive index-matching between the layers is evaluated in detail with respect to device efficiency and cost. From the findings of this study, general guidelines are derived to optimize both the cost and efficiency of CdSe/CdS and other (potentially cadmium-free) quantum dot systems. When reabsorption of the green and/or red emission is significant compared to the absorption strength for the blue emission of the pumping light emitting diode, the hybrid remote phosphor approach becomes beneficial.

K 2 SiF 6 :Mn 4+ as a red phosphor for displays and warm-white LEDs: a review of properties and perspectives

Phosphor-converted white light-emitting diodes (pc-WLEDs) are energy efficient and environmentally friendly light sources with a long lifetime, applicable in both display backlights and general lighting. Adding red-emitting phosphors improves the color quality of white LEDs compared to the prototype combination of a blue LED and the yellow Y3Al5O12:Ce3+. Efficient narrow-band red-emitting phosphors like K2SiF6:Mn4+ can meet the market’s needs. This review recapitulates research since 2008 on K2SiF6:Mn4+ as the first and most discussed fluoride phosphor. The limited nephelauxetic effect, typical for fluorides, allows for the tuning of the Mn4+ emission in the red part of the spectrum below 650 nm. This is reflected in the spectroscopic parameters of the crystal field theory. Synthesis methods are described, showing the evolution from etching Si wafers to solution synthesis resulting in consistent luminescent and thermal properties. Though important for applications, long-term stability is often neglected, although (in)organic coatings improving stability emerge. This leads not only to warm-white LEDs with high efficacies and good color rendering, but also to efficient displays with a large color gamut.

Origin of saturated green emission from europium in zinc thiogallate

Europium doped zinc thiogallate, ZnGa2S4:Eu2+, has been reported as a saturated green emitting phosphor, suitable as conversion phosphor in white LEDs for lighting or displays. Up to now, no direct proof for the incorporation of Eu2+ in ZnGa2S4 has been given. We combined X-ray diffraction (XRD), cathodoluminescence in electron microscopy (SEM-CL) and X-ray absorption spectroscopy (XAS) to study the incorporation of the europium ions in the host material. The previously reported green luminescence was found to originate from small amounts of unintentionally formed EuGa2S4, and not from europium ions incorporated into ZnGa2S4. EuGa2S4 has a low quantum efficiency (< 20%) and shows strong thermal quenching, already below room temperature. The XAS data analysis suggests that a certain amount of europium might occupy octahedral voids inside the zinc thiogallate lattice in a divalent state. The zinc ion next to these interstitial dopants is then removed for charge compensation. Notwithstanding the possible, but limited, incorporation of Eu2+ in ZnGa2S4, these ions do not activate any luminescence as was shown with SEM-CL.

Evaluating the use of blue phosphors in white LEDs: the case of Sr0.25Ba0.75Si2O2N2:Eu2+

The luminescence properties of the blue emitting phosphor Sr0.25Ba0.75Si2O2N2:Eu2+ are extensively investigated. This oxonitridosilicate phosphor features strong 4f65d1 - 4f7 luminescence originating from the Eu2+ ion, with a narrow emission band peaking at 467 nm and a full width at half maximum of only 41 nm. Thermal quenching of the blue luminescence only sets in above 450 K, making this material an interesting candidate as LED conversion phosphor. The fast decay of the luminescence prevents the phosphor to be susceptible to saturation effects at high excitation fluxes. Furthermore it is proven to be chemically stable against moisture. The only drawback is the relatively low quantum efficiency of the synthesized powder, provisionally preventing this material to be used in applications. In addition, the phosphor features a weak yellow emission band, originating from small domains featuring a different crystal structure. It is shown that the majority of the powder grains only exhibit blue emission. Finally, the spectrum of a white LED, based on a UV pumping LED and three (oxy)nitride phosphors is simulated in order to assess the usefulness of blue phosphors in LEDs for lighting. Only a marginal improvement in terms of color quality can be achieved with a narrow banded phosphor, at the expense of a decrease in luminous efficacy and overall electrical to optical power efficiency.PACS 70 – Condensed Matter: Electronic structure, Electrical, Magnetic, and Optical PropertiesPACS 42.70.-a Optical materials

2D and 3D lanthanide metal–organic frameworks constructed from three benzenedicarboxylate ligands: synthesis, structure and luminescent properties

Nineteen lanthanide–benzenedicarboxylate complexes were obtained under similar hydrothermal conditions: [Ln(IP-Py)(HIP-Py)(H2O)2]n (Ln = Pr (1), Sm (2), Eu (3), Gd (4), Tb (5), Dy (6)), [Ln4(IP)6(H2O)4(DMF)·DMF·mH2O]n (Ln = Sm, m = 1 (7), Eu, m = 1.5 (8), Gd, m = 1.5 (9), Tb, m = 1.5 (10), Dy, m = 1 (11), Ho, m = 1.5 (12)), [Ln2(IP-Py)2(IP)(H2O)4·H2O]n (Ln = Sm (13), Eu (14), Tb (15), Dy (16)) and [Ln2(IP-Py)2(TP)(H2O)4·H2O]n (Ln = Sm (17), Eu (18), Gd (19)), where: H2IP-Py = 5-(4-pyridyl)-isophthalic acid), H2IP = isophthalic acid, H2TP = terephthalic acid. Complexes 1–6 exhibit a two-dimensional layered structure based on H2IP-Py. For 7–12, the Ln3+ ions are linked into one-dimensional chains with two triple-stranded helices of opposite handedness and further constructed into a two-dimensional layer by coordination of the IP2− ligands. For complexes 13–16 and 17–19, a similar two-dimensional layer was formed by coordination of Ln3+ and IP-Py2−, and three-dimensional pillar-layered frameworks were formed due to the layers bridged by the auxiliary IP2− or TP2− ligand, respectively. Most complexes show the characteristic narrow-banded lanthanide 4fN–4fN emission in combination with luminescence from the ligands, with the relative contribution depending on the energy transfer from the ligands to the lanthanides. The ligands play an important role in the sensitization of the lanthanide emission. The onset of the main excitation band for the Eu3+ complexes is found at the lowest energy for 14, enabling excitation by near-UV LEDs. The complexes containing the technologically relevant ions for (back) lighting applications, Tb3+ (green) and Eu3+ (red), are evaluated in detail in terms of luminescence lifetime and thermal stability. The emission intensity of selected Eu3+ and Tb3+ compounds is stable up to room temperature.

Counting the Photons: Determining the Absolute Storage Capacity of Persistent Phosphors

The performance of a persistent phosphor is often determined by comparing luminance decay curves, expressed in cd/m2. However, these photometric units do not enable a straightforward, objective comparison between different phosphors in terms of the total number of emitted photons, as these units are dependent on the emission spectrum of the phosphor. This may lead to incorrect conclusions regarding the storage capacity of the phosphor. An alternative and convenient technique of characterizing the performance of a phosphor was developed on the basis of the absolute storage capacity of phosphors. In this technique, the phosphor is incorporated in a transparent polymer and the measured afterglow is converted into an absolute number of emitted photons, effectively quantifying the amount of energy that can be stored in the material. This method was applied to the benchmark phosphor SrAl2O4:Eu,Dy and to the nano-sized phosphor CaS:Eu. The results indicated that only a fraction of the Eu ions (around 1.6% in the case of SrAl2O4:Eu,Dy) participated in the energy storage process, which is in line with earlier reports based on X-ray absorption spectroscopy. These findings imply that there is still a significant margin for improving the storage capacity of persistent phosphors.

Red Mn4+-Doped Fluoride Phosphors: Why Purity Matters

Traditional light sources, e.g., incandescent and fluorescent lamps, are currently being replaced by white light-emitting diodes (wLEDs) because of their improved efficiency, prolonged lifetime, and environmental friendliness. Much effort has recently been spent to the development of Mn4+-doped fluoride phosphors that can enhance the color gamut in displays and improve the color rendering index, luminous efficacy of the radiation, and correlated color temperature of wLEDs used for lighting. Purity, stability, and degradation of fluoride phosphors are, however, rarely discussed. Nevertheless, the typical wet chemical synthesis routes (involving hydrogen fluoride (HF)) and the large variety of possible Mn valence states often lead to impurities that drastically influence the performance and stability of these phosphors. In this article, the origins and consequences of impurities formed during synthesis and aging of K2SiF6:Mn4+ are revealed. Both crystalline impurities such as KHF2 and ionic impurities such as Mn3+ are found to affect the phosphor performance. While Mn3+ mainly influences the optical absorption behavior, KHF2 can affect both the optical performance and chemical stability of the phosphor. Moisture leads to decomposition of KHF2, forming HF and amorphous hydrated potassium fluoride. As a consequence of hydrate formation, significant amounts of water can be absorbed in impure phosphor powders containing KHF2, facilitating the hydrolysis of [MnF6]2- complexes and affecting the optical absorption of the phosphors. Strategies are discussed to identify impurities and to achieve pure and stable phosphors with internal quantum efficiencies of more than 90%.

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.