Lanthanide ions form an important class of optical dopants in a wide variety of materials used in modern photonic devices. Everyday examples are the Ce and Eu based conversion phosphors used to build white LEDs and Er based infrared lasers for telecommunication [1]. Energy level schemes offer a natural connection between the electronic structure of materials and their spectroscopic properties. During the last decades, empirical methods and relationships were devised for constructing energy level schemes of lanthanide defects in wide band gap solids [2]. These simple techniques allow to locate the charge-state transition levels as well as the excited 5d levels of all 14 lanthanide ions with respect to the host’s valence and conduction bands with only limited experimental input. These energy level schemes offer a lot of information on optical and electronic properties. The features in the absorption and excitation spectra of the luminescent materials can be determined from location of absorption bands, both originating from interconfigurational 4f-4f5d transitions as well as charge transfer transitions [2]. In this talk, the empirical energy level models for lanthanides will be introduced and applied to the luminescent material SrAl2O4:Eu . Co-doped with Dy, this material features a strong green afterglow, well-known from emergency signalization and a lot of different gadgets [3]. Although this phenomenon was discovered in the 1990’s, the exact mechanism of the energy storage is not yet completely resolved. The delayed light emission of the persistent luminescence is thought to be the consequence of the temporary trapping of charge carriers after photoionization of the Eu ion. The exact nature of the trapping centers as well as the role of the Dy ion is not clear [3]. To resolve these issues, energy level modeling of the lanthanide defects can be an interesting tool to get a deeper understanding on the incorporation of the lanthanide ions as well as their possibilities to capture charge carriers. On the other hand, the SrAl2O4 host poses a specific challenge for the empirical energy level modeling as two nonequivalent defects are expected due to incorporation on two different Sr sites in the crystal structure [4]. To enable the construction of a realistic energy level scheme for this material, the conventional energy level model is extended to accommodate materials in which two – or more – nonequivalent lanthanide defects exist. This obviously requires site-selective spectroscopy. In the case of SrAl2O4:Eu , we show that a blue emission band which only appears at sufficiently low temperature can be related to the second type of europium defect [4].

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.

Correction for ‘Plasma enhanced atomic layer deposition of Ga2O3 thin films’ by Ranjith K. Ramachandran et al., J. Mater. Chem. A, 2014, 2, 19232–19238.

The electro-optical properties of lead zirconate titanate (PZT) thin films depend strongly on the quality and crystallographic orientation of the thin films. We demonstrate a novel method to grow highly textured PZT thin films on silicon using the chemical solution deposition (CSD) process. We report the use of ultrathin (5-15 nm) lanthanide (La, Pr, Nd, Sm) based intermediate layers for obtaining preferentially (100) oriented PZT thin films. X-ray diffraction measurements indicate preferentially oriented intermediate Ln2O2CO3 layers providing an excellent lattice match with the PZT thin films grown on top. The XRD and scanning electron microscopy measurements reveal that the annealed layers are dense, uniform, crack-free and highly oriented (>99.8%) without apparent defects or secondary phases. The EDX and HRTEM characterization confirm that the template layers act as an efficient diffusion barrier and form a sharp interface between the substrate and the PZT. The electrical measurements indicate a dielectric constant of ∼650, low dielectric loss of ∼0.02, coercive field of 70 kV/cm, remnant polarization of 25 μC/cm(2), and large breakdown electric field of 1000 kV/cm. Finally, the effective electro-optic coefficients of the films are estimated with a spectroscopic ellipsometer measurement, considering the electric field induced variations in the phase reflectance ratio. The electro-optic measurements reveal excellent linear effective pockels coefficients of 110 to 240 pm/V, which makes the CSD deposited PZT thin film an ideal candidate for Si-based active integrated nanophotonic devices.

The luminescence properties of the blue emitting phosphor Sr 0.25 Ba 0.75 Si 2 O 2 N 2 :Eu 2+ are extensively investigated. This oxonitridosilicate phosphor features strong 4f 6 5d 1 - 4f 7 luminescence originating from the Eu 2+ 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 Properties PACS 42.70.-a Optical materials

The research field of persistent luminescence has experienced a strong growth in the past two decades, with a steady development of new materials and applications. Here we give an overview of the recent progress in a specific class of host materials, namely oxynitride and nitride persistent phosphors. These are interesting hosts to explore because of their unique characteristics, such as chemical stability and tunability of the emission over the entire visible range upon doping with divalent europium. To yield persistent luminescence however, co-dopants have to be added or the synthesis conditions have to be adjusted. Specific materials, such as Ca2Si5N8:Eu,Tm and BaSi2O2N2:Eu, are highlighted and their properties are put into the context of emerging applications such as in vivo imaging and pressure sensing via mechanoluminescence. Finally, directions for future research are given. (C) 2014 Elsevier B.V. All rights reserved.

The research work performed in this PhD initially focussed on the technical development of an advanced TL-spectroscopy setup, suitable for the investigation of persistent luminescent phosphors. It soon became clear that this setup, called \textit{$\lambda$T-TL}, is also perfectly suited for investigating the temperature dependency of the luminescence properties of regular photoluminescent materials. Although not yet entirely completed, the \textit{$\lambda$T-TL} setup has already proven its usefulness in thermal quenching (TQ) studies of regular phosphors and temperature and wavelength dependent charging, afterglow and thermoluminescence (TL) measurements on persistent luminescent phosphors. Based on the unique capabilities of this setup, a feasibility study for the use of persistent luminescent materials in traffic markings was performed. The main conclusions of these investigations are summarized in this dissertation.

We demonstrate an ALD process for Ga2O3that relies upon sequential pulsing of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)gallium(iii), [Ga(TMHD)3] and O2plasma and enables the deposition from temperatures as low as 100 °C.

The luminescence properties of the blue emitting phosphor Sr0.25Ba0.75Si2O2N2:Eu 2+ are extensively investigated. This oxonitridosilicate phosphor features strong 4f5d 4f luminescence originating from the Eu 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 Properties PACS 42.70.-a Optical materials

SrAl2O4:Eu,Dy is presumably the best known persistent luminescent phosphor. At room temperature, its green emission remains visible for hours after switching off the excitation. It is known that upon lowering the temperature of the phosphor a second photoluminescence emission band arises in the blue part of the visible spectrum, although its origin is still the subject of discussion. In this paper we thoroughly study the origin of both emission bands in SrAl2O4:Eu,Dy and we attribute this to europium ions substituting for the two different Sr sites in the phosphor's monoclinic host lattice. The photoluminescence properties, the thermal quenching behavior, and photoluminescence lifetime of both emission bands are investigated. A lanthanide energy level scheme is constructed for both sites. Using an integrated approach, i.e., combining charging, afterglow, and thermoluminescence measurements in the same run, we study the charging or trap filling processes in SrAl2O4:Eu,Dy upon excitation with site selective excitation wavelengths and at different temperatures. We show that trap filling is a thermally activated process when the green emitting center is excited at 435 nm. Furthermore, we also demonstrate that the distribution of filled traps after charging depends strongly on the excitation wavelength and thus on which Eu2+ center has been excited. This suggests trapping of the electron close to the ionized Eu2+ ion, without full delocalization to the conduction band during the trapping process. Finally, the quantum efficiency of the persistent luminescence is estimated at 65 (+/-10)%.

Europium (Eu2+) doped calcium thiosilicate (Ca2SiS4) is interesting as conversion phosphor for light-emitting diodes (LEDs), because of its yellow-to-red emission, depending on the europium concentration. However, for elevated europium concentrations two phases appear which is reflected in the temperature dependency of the emission spectrum. In this work we study the emission spectrum and thermal quenching behaviour at the microscopic level, by performing temperature dependent cathodoluminescence spectroscopy in a scanning electron microscope. A clear relation between the local europium concentration and the thermal quenching is obtained. This proves useful to explain the luminescence properties of the bulk phosphor.

Solid state lighting (SSL) has proven to be the sole viable alternative to replace energy consuming incandescent or mercury based lighting technology. Light-emitting diodes or LEDs are gaining market share every day although improvements in color rendering and luminous efficacy are still desirable. In white LEDs, the intrinsically narrow emission band of the pumping diode is combined with one or multiple phosphor materials which convert a part of the pumping light into light of longer wavelengths. The different colors mix together, yielding white light. Blue phosphors are required for high color rendering white LEDs, in which they are combined with a near-ultraviolet pumping diode. The phosphors have to fulfill several requirements in order to be suitable for the application [1]. Oxonitridosilicates are well known host materials for conversion phosphors. Doped with divalent europium (Eu2+), they are able to convert light with very high conversion efficiencies, up to 90% [2]. Furthermore, this material class is chemically stable. The luminescence is driven by the parity allowed 4f65d1 → 4f7 transition in the optical dopant. In this work, a blue emitting phosphor, Sr0.25Ba0.75Si2O2N2:1%Eu2+ is considered. The crystallographic structure of this specific composition deviates from the structures of the well-known green and bluish green phosphors SrSi2O2N2:Eu2+ and BaSi2O2N2:Eu2+ [3]. First, emphasis is put on the synthesis process. Secondly, important physical properties like photoluminescence, thermal quenching, luminescent decay and quantum efficiency of this new material are evaluated. Thirdly, variations in chemical composition and its influence on the luminescence could be studied by means of cathodoluminescence spectroscopy inside a scanning electron microscope (SEM-CL), giving a deeper insight into the luminescent properties of the powders.

Persistent phosphors or glow-in-the-dark materials are luminescent materials with the ability to emit light for minutes up to hours after the excitation has ended. Although they are currently used in various established and emerging applications, the origin of the energy storage mechanism is still largely unknown [1,2]. Up to now, there are still several competing models on the energy storage mechanism in these phosphors, with respect to the energetic position and the nature of the trap levels. We now report on both facets of the energy storage. First of all, the depth and shape of trap level distribution can be estimated by performing a series of thermoluminescence experiments with varying excitation duration and at varying excitation temperature, in combination with the initial rise analysis method. It is shown that the traps responsible for the energy storage in the blueemitting CaAl2O4:Eu,Nd are not discrete, but rather have a Gaussian-like broadened distribution (with trap depth centered around 0.9eV) [3]. Secondly, a possible valence state change of the rare earth co-dopants in SrAl2O4:Eu,RE is evaluated by combined optical excitation and x-ray absorption studies [4]. Based on both types of observations, the presently available models are critically evaluated.

Persistent luminescent materials are ubiquitous in our daily life, from watch dials, toys and gadgets to pictograms for emergency exit signs [1]. However the mechanism behind this persistent luminescence is not yet fully understood. Strontium aluminates doped with europium and dysprosium (SrAl2O4:Eu,Dy) is one of the most widely used persistent luminescent phosphor. It features a broad green emission band and bright long-lasting afterglow at room temperature [2]. Besides the green emission band a second, blue emission band becomes visible at low temperatures (< 220 K).