Katleen Korthout

Microencapsulation of Moisture-Sensitive CaS : Eu2 + Particles with Aluminum Oxide

Single-crystal, submicrometer-sized CaS:Eu luminescent particles were synthesized via a solvothermal route, and these moisture-sensitive particles were coated with aluminum oxide using atomic layer deposition (ALD). Photoluminescence (PL) spectra of coated and uncoated particles were compared. They both showed a broad-band PL emission with a maximum of 650 nm. Microencapsulation by aluminum oxide layers did not have a pronounced effect on the intensity of the emission. In situ luminescence measurements during the accelerated aging (80 degrees C, 80% relative humidity) of coated and uncoated CaS: Eu particles were performed. While the uncoated phosphor was largely degraded within 30 h of aging, it was observed that a 20 nm thick aluminum oxide coating dramatically increased the resistance of the luminescent material against moisture, showing the conformity of the Al2O3 coating by the ALD process. Upon degradation, CaCO3 was formed, leading to Eu3+ emission as observed in cathodoluminescence. Finally, the use of these coated particles as a wavelength conversion material in light-emitting diodes was evaluated.

Rare earth doped core-shell particles as phosphor for warm-white light-emitting diodes

Light-emitting diodes (LEDs) are efficient, energy-saving light sources. Unfortunately, designing phosphors for LEDs that emit warm white light is not straightforward. We solvothermally prepared rare earth doped alkaline earth sulfides with a core-shell structure in order to obtain a physical separation between different dopants (europium and cerium). Cathodoluminescence of a single phosphor particle in an electron microscope proves simultaneous Eu2+ and Ce3+ broad band emission. The emission color can be tuned by variation of the composition, core size, and shell thickness. Upon excitation of SrS:Eu2+-SrS:Ce3+ core-shell structures at 430 nm, white light emission with good color rendering and a color temperature around 3000 K is obtained.

Valence states of europium in CaAl 2 O 4 :Eu phosphors

Persistent luminescent CaAl2O4:Eu2+,Nd3+ powders were prepared by a non-aqueous sol-gel technique. The crystallization of calcium aluminate by heat-treatment of the sols is described in detail. After heat treatment in air, the europium dopant ions are mainly in a trivalent state. For the reduction to the divalent state post-annealing in a reducing nitrogen-hydrogen atmosphere is used. The reduction of europium ions is monitored by photoluminescence and x-ray absorption (XANES) spectroscopy. The degree of reduction is strongly dependent on the annealing temperature. Although for high temperature a strong enhancement of the Eu2+ emission is observed, this also leads to powders with a gray body color.

Whispering gallery modes in micron-sized SrS:Eu octahedrons

Optical whispering gallery modes were observed in micron-sized SrS:Eu2+ octahedrons. Cathodoluminescence in a scanning electron microscope was used to evaluate the emission characteristics of individual octahedrons with various sizes. The resonance modes superposed on the orange Eu2+ broadband emission were explained by a plane wave model, taking the wavelength dispersion of the refractive index into account. The resonance modes occur in the three equatorial planes in the octahedrons, which were approximated by corner-cut square disks.

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.

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.

LaAlO3:Mn4+ as Near-Infrared Emitting Persistent Luminescence Phosphor for Medical Imaging: A Charge Compensation Study

Mn4+-activated phosphors are emerging as a novel class of deep red/near-infrared emitting persistent luminescence materials for medical imaging as a promising alternative to Cr3+-doped nanomaterials. Currently, it remains a challenge to improve the afterglow and photoluminescence properties of these phosphors through a traditional high-temperature solid-state reaction method in air. Herein we propose a charge compensation strategy for enhancing the photoluminescence and afterglow performance of Mn4+-activated LaAlO3 phosphors. LaAlO3:Mn4+ (LAO:Mn4+) was synthesized by high-temperature solid-state reaction in air. The charge compensation strategies for LaAlO3:Mn4+ phosphors were systematically discussed. Interestingly, Cl−/Na+/Ca2+/Sr2+/Ba2+/Ge4+ co-dopants were all found to be beneficial for enhancing LaAlO3:Mn4+ luminescence and afterglow intensity. This strategy shows great promise and opens up new avenues for the exploration of more promising near-infrared emitting long persistent phosphors for medical imaging.

Probing the local structure of the near-infrared emitting persistent phosphor LiGa5O8:Cr3+

Near-infrared emitting persistent phosphors have promising applications in the field of in vivo medical imaging. In this paper, we prepared the persistent phosphor LiGa5O8:Cr3+ (LGO:Cr) which exhibits emission in the tissue transparency window and shows afterglow for multiple hours after excitation. Addition of Si or Ge improves the persistent luminescence. X-ray diffraction and electron microscopy, coupled with elemental analysis, revealed that there is a maximum amount of Si or Ge that can be incorporated in the host. Via X-ray absorption spectroscopy and electron paramagnetic resonance experiments, we studied the local environment of chromium in the LGO spinel host. The presence of both Cr3+ and Cr4+ on octahedral sites in LGO was confirmed. Electron paramagnetic resonance showed that Cr3+ resides in a rhombically distorted octahedral lattice site and that the Cr3+ local environment is sensitive to variation in point defects in the surrounding.

K2MnF6 as a precursor for saturated red fluoride phosphors: the struggle for structural stability

Phosphor-converted white light-emitting diodes (LEDs) are currently taking over the lighting market because of their high luminous efficiency, environmentally friendly nature and long lifetime. A new generation of saturated red fluoride phosphors, using Mn4+ as the activator, has gained interest in further enhancing the color rendering properties and efficiency of white LEDs for lighting and display applications. They can be described as A2MF6:Mn4+ (A = K, Na, Sc, NH4 and M = Si, Ge, Ti, Sn), KNaMF6:Mn4+, BaMF6:Mn4+ (M = Si, Ti) or ZnMF6·H2O (M = Si, Ge) compounds, in which Mn is a substitute for the M(IV) element of the fluoride host. A two-step co-precipitation synthesis method has recently been developed because of the increased control of the Mn valence state and the relatively low cost. In this method, K2MnF6 is first synthesized as a precursor which then serves as a source for the preparation of [MnF6]2− complexes in further phosphor synthesis. In-house production of K2MnF6 is required as it quickly degrades. Here, we investigate the structural properties after synthesis, as well as the main degradation routes of K2MnF6 when the material is subjected to heat and humidity or used in further synthesis reactions. It is found that impurities, such as KHF2, K2MnF5·H2O and Mn ions in an oxygen coordination, can be formed as a result of parasitic reactions during synthesis. Even in pure K2MnF6, degradation occurs due to heat and hydrolysis both of which induce reduction of the Mn4+ ion. Heating in air causes the material to form Mn2+ as KMnF3/KF·MnF2 starts to form at high temperatures due to hydrolysis. In dilute HF solutions the Mn4+ ion is partially reduced to Mn3+, often incorporated in hydrated structures as KMnF4·H2O and K2MnF5·H2O. The Mn3+ ion is found to affect the optical absorption properties.

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%.