Assessment of crystalline materials for solid state lighting applications: Beyond the rare earth elements

In everyday life, we are continually exposed to different lighting systems, from the home interior to car lights and from public lighting to displays. The basic emission principles on which they are based range from the old incandescent lamps to the well-established compact fluorescent lamps (CFL) and to the more modern Light Emitting Diode (LEDs) that are dominating the actual market and also promise greater development in the coming years. In the LED technology, the key point is the electroluminescence material, but the fundamental role of proper phosphors is sometimes underestimated even when it is essential for an ideal color rendering. In this review, we analyze the main solid-state techniques for lighting applications, paying attention to the fundamental properties of phosphors to be successfully applied. Currently, the most widely used materials are based on rare-earth elements (REEs) whereas Ce:YAG represents the benchmark for white LEDs. However, there are several drawbacks to the REEs’ supply chain and several concerns from an environmental point of view. We analyze these critical issues and review alternative materials that can overcome their use. New compounds with reduced or totally REE free, quantum dots, metal–organic framework, and organic phosphors will be examined with reference to the current state-of-the-art

High Resolution Spectroscopy of Erbium Doped Solids

This thesis investigates the potential of Er:YSO and Er:Si for quantum communication and computation applications. Erbium uniquely possess optical transitions in the 1.5 um region, making it suitable for both fibre telecommunication and silicon photonics. The properties of the ${I}_{15/2}\leftrightarrow{I}_{13/2}$ optical transition in Er:YSO have already been extensively studied. Over two decades ago, improvements in $Er^{3+}$ dephasing time at 1.5 um were achieved by applying a 5T field along the D1 axis. More recently, a record 4.4 ms coherence time on the same optical transition was achieved using a 7T field. These investigations, among others, illustrate that large Er electron spins become thermally polarised with sufficient magnetic field. However, no long lived and coherent spin transitions associated with the Er ions had previously been identified, and such transitions are necessary for on-demand quantum state storage. To address this requirement, the optical and hyperfine transition properties of 167-Er:YSO were investigated in large magnetic fields. In a field of 7T, spectral hole lifetimes of 1 minute and hyperfine population lifetimes of 12 minutes were observed. These measurements illustrate the effect of spin-lattice relaxation in this system, and how it can be mitigated. Efficient spin-polarisation of the entire 167-Er hyperfine ensemble is also demonstrated. This is the first such demonstration in rare earth systems, and a key requirement for broadband optical storage. Moreover, a 1.3 second coherence time was recorded for an 167-Er:YSO hyperfine transition at 7T and 1.4 K. This is an improvement of several orders-of-magnitude over previous coherence measurements on spin-transitions in Er doped solids. This is also sufficient for maximal entanglement rates in quantum repeater networks that span distances of 1000 km or greater. With an optical transition at 1.5 um, Er is also an ideal candidate to connect silicon based quantum computers to the future quantum Internet. In particular, single Er:Si ions could be used to develop an optical-spin bus between P:Si qubits and fibre based quantum networks. Presented here is the first spectroscopic investigation of single Er:Si ions. This required a novel opto-electronic approach to single ion detection, where the Er ions are implanted into a nanometre scale fin-shaped Field Effect Transistor. With this approach it was possible to develop high resolution optical spectra, where both the electronic and hyperfine levels of individual Er ions were resolved. Long optical and spin coherence times are also important requirements for an optical-spin bus. To address the first requirement, an investigation of the optical lineshape was undertaken. Here it was determined that sources of Stark noise external to the transistor channel contribute a significant amount to optical homogeneous linewidth. However, the dominant noise contribution was determined to be short-range (from within the 30 nm wide channel) and the total homogeneous linewidth was measured to be 50 MHz. The site structure of an individual Er:Si ion was then analysed, using magnetic field rotation patterns and optical transitions between multiple crystal field levels. This site was determined to have approximately axial ($C_{3}$) symmetry. The purpose of this study was to determine a magnetic field regime in which the Er electrons spin can be polarised, which is necessary for realising of long hyperfine lifetimes and coherence times

Photoionisation dynamics and ion-ion interaction of individual erbium ions in silicon nanotransistors

Erbium has been widely studied in a variety of host materials for its 1.5 m optical transition, which is compatible with fibre communication systems. Recent studies demonstrated 4.4 ms optical coherence time and 1.3 s nuclear spin coherence time with Er-doped Y2SiO5. This has stimulated the interest in building Er-based quantum memory and spin qubits with optical interface for large-scale quantum systems. This thesis focuses on the investigation of erbium ions in silicon nanotransistors for developing erbium quantum devices on the silicon platform. First, an efficient detection method for individual erbium ions is introduced. This method relies on the short laser pulse excitation and latched current signal readout. The latched signal can be reset by an off-resonance laser pulse and the latching period can be tuned by a gate voltage. This allows for adjustment of the detection speed for higher readout fidelity or faster readout speed. Based on the pulsed method, the dependence of the linewidth and signal intensity on excitation pulse length have been investigated. This allows us to understand the line shape and broadening of the spectrum under laser excitation. Finally, the higher bound of the optical lifetime was estimated based on a Markov model, and a Rabi oscillation process is simulated base on the optical Bloch equation. Then, the Zeeman effect of two coupled erbium ions was studied at high spectral resolution. The spectrum is distinctly different from that of a single Er ion as there are zero field splitting and anticrossings at multiple places. A model based on magnetic dipole-dipole interaction can match not only the Zeeman splitting slopes, but also the anti-crossings in the observed spectrum. Additionally, the potential of using single Er ions to map the electric field and strain in silicon nanotransistors has been explored. This provides a new method to characterise the electric field and strain in silicon sub-10-nanometer-node devices for optimising modern microelectronic devices

Samarium-Doped Fluorophosphate and Fluoroaluminate Glasses for High-Dose High-Resolution Dosimetry for Microbeam Radiation Therapy

Microbeam Radiation Therapy (MRT) is an important and developing radiotherapy technique that uses spatially fractionated doses, several orders of magnitude larger than that of the doses used in conventional radiation therapy. Healthy tissue displays remarkable resistance to damage caused from microscopically narrow, fractionated, planar beams of x-rays, while showing preferential damage towards cancerous growths, allowing for a high potential towards the treatment of often inoperable tumours. These synchrotron generated, spatially fractionated, planar beams are referred to as microbeams, and have a thickness of 20 – 50 µm and are separated by distances of 100 – 400 µm. The dose delivered at the center of the microbeam can be on the order of thousands of Grays (Gy), whereas the dose between each microbeam should be kept below tens of Gy. An important aspect of MRT is the spatial distribution of the dose delivered to the patient, which must be accurately measured. Ultimately, both high resolution and large dynamic range dosimetric measurements must be done simultaneously. The objective of this Ph.D. research involves the development and characterization of a dosimetric technique that fulfills the requirements of measuring dose distributions of microbeams. The proposed technique uses the indirect detection of x-rays, where the dose is recorded in a glass plate which can then be readout using a confocal microscopy system. The dose delivered is recorded by using trivalent samarium (Sm3+) doped fluoroaluminate and fluoro-phosphate glasses, where conversion from the trivalent form of samarium to the divalent form (Sm2+) occurs after exposure to x-rays. The extent of this conversion can be readout and digitized with high resolution using a confocal microscopy system that utilizes the easily distinguishable photoluminescent spectra of Sm3+ to Sm2+. The work carried out in this research involves the high resolution recording of microbeam profiles performed at the Canadian synchrotron, using samarium doped glass plates under a variety of irradiation parameters in order to determine their suitability for dosimetric applications. In particular, the dose rate and x-ray energy dependence of these materials is investigated, as well as the determination of the optimum Sm3+ dopant concentration. Further, the confocal measurement technique is investigated, as well as the suitability of ion implantation of samarium ions in order to improve the signal readout. Lastly, the change in dose distributions of microbeams is investigated by performing irradiations over a wide range of monochromatic x-rays, so that the potential effect of the selected energy on MRT treatment planning can be examined

Nanomateriales basados en lantánidos: caracterización y visualización

Las nanopartículas upconversion (UCNP) comenzaron a desarrollarse a inicios del presente siglo, generando una gran expectación e interés por sus propiedades fotofísicas particulares. Las UCNP son nanomateriales capaces de producir el fenómeno upconversion (UC) mediante el cual fotones de baja energía (de la región del infrarrojo cercano, NIR) se transforman en fotones de alta energía (en la región del UV-vis). Los principales actores del fenómeno son los iones lantánidos que se encuentran como dopantes de una matriz nanométrica transparente. Por tanto, en las UCNP, a diferencia de las nanopartículas (NP) semiconductoras, el confinamiento nanométrico no es el responsable de las propiedades ópticas sino las interacciones entre los iones lantánidos de la matriz. Aunque, sin duda, la escala nanométrica afectará a sus propiedades emisivas. Esta tesis estructurada en cuatro capítulos comienza con una introducción a las propiedades ópticas de las UCNP, con unas breves reseñas históricas y una descripción detallada de los protagonistas de este fenómeno tan inusual como interesante: los iones lantánidos. En el primer capítulo se describe una técnica de microscopía desarrollada por el grupo y que permite caracterizar con resolución temporal la luminiscencia de las emisiones upconversion (UC). En el segundo se demuestra cómo la técnica anterior permite medir el alargamiento del tiempo de vida de un colorante orgánico funcionalizado en la superficie de la UCNP con una estrategia basada en ligandos macrocíclicos de cucurbiturilo. El nanohíbrido upconversion (UCNH) resultante incorpora sinérgicamente las propiedades únicas de la UCNP con las de detección de la sonda orgánica, dando lugar a un sensor prueba de concepto. En el tercer capítulo se evalúa la citotoxicidad de la plataforma utilizada para desarrollar el sensor en comparación con la UCNP de partida. Por último, el cuarto capítulo profundiza en la caracterización fotofísica de un UCNH prueba de concepto compuesto por UCNP y un colorante orgánico. De tal manera que el hilo conductor de la tesis es la caracterización fotofísica de las UCNP y de los UCNH, pues es precisamente la limitación en el desarrollo de estos materiales. Cada capítulo está, a su vez, apoyado en una introducción inicial con resultados bibliográficos relevantes que describen la motivación que llevó a empezar la investigación. Asimismo, cuenta con una sección de objetivos, de resultados y discusión y conclusiones. Finalmente se completa la tesis con las conclusiones más relevantes obtenidas en cada capítulo.Upconversion nanoparticles (UCNP) started to be developed at the beginning of this century, generating great expectations and interest due to their particular photophysical properties. UCNPs are nanomaterials capable of producing the upconversion (UC) phenomenon by which low-energy photons (in the near-infrared region, NIR) are transformed into high-energy photons (in the UV-vis region). The responsible species of this phenomenon are the lanthanide ions found as dopants of a transparent nanometric matrix. Therefore, in UCNPs, unlike semiconductor nanoparticles (NPs), nanometric confinement is not responsible for the optical properties but rather the interactions between the lanthanide ions in the matrix. Although, undoubtedly, the nanometer scale will affect its emissive properties. This thesis structured in four chapters. It begins with an introduction to the optical properties of UCNPs, with brief historical reviews and a detailed description of the protagonists of this unusual and interesting phenomenon: the lanthanide ions. The first chapter describes a microscopy technique developed by the group that allows characterizing the luminescence of upconversion (UC) emissions with temporal resolution. The second chapter shows how the previous technique allows measuring the lengthening of the lifetime of a functionalized organic dye on the surface of the UCNP with a strategy based on macrocyclic cucurbituril ligands. The upconversion nanohybrid (UCNH) synergistically incorporates the unique UCNP detection properties with those of the organic probe, resulting in a proof-of-concept sensor. In the third chapter, the cytotoxicity of the platform used to develop the sensor is evaluated in comparison with the starting UCNP. Finally, the fourth chapter delves into the photophysical characterization of a proof-of-concept UCNH composed of UCNP and an organic dye. In such a way that the guiding thread of the thesis is the photophysical characterization of the UCNP and the UCNH, since it is precisely the limitation in the development of these materials. Each chapter is, in turn, supported by an initial introduction with relevant bibliographic results that described the motivation that led to start the research. It also has a section on objectives, results and discussion and conclusions. Finally, the thesis is completed with the most relevant conclusions obtained in each chapter

Synthesis and Physicochemical Characterization of Noncentrosymmetric Quaternary Sulfides with Potential Nonlinear Optical Applications

In this work, several new quaternary sulfides were synthesized and investigated for their potential applications in optics. Chapter 1 provides an overview of nonlinear optical (NLO) materials in general, including their applications, ideal characteristics, current deficiencies, and strategies for the discovery of new candidate NLO materials, particularly those for use in the infrared (IR). Two families of compounds are investigated in this work, including Ln3LiTS7 (3-1-1-7) compounds and I4-II-IV2-VI7 (4-1-2-7) diamond-like semiconductors (DLSs). All products were synthesized via traditional high-temperature, solid-state synthesis and their structures were solved and refined through single crystal X-ray diffraction. Chapter 2 focuses on the rare-earth-containing 3-1-1-7 compounds, including nine previously unreported and nine further characterized compounds. Following the general formula of Ln3LiTS7 where Ln = La, Ce, Pr, Nd, Sm, Gd, Dy and T = Si, Ge, Sn, a systematic study of homovalently substituted compounds was carried out. Chapter 3 reports the synthesis and crystal structures of two new Cu-containing 4-1-2-7 DLSs, Cu4FeGe2S7 and Cu4CoGe2S7. Chapter 4 focuses on a novel Li-containing 4-1-2-7 DLS, Li4CdGe2S7. This compound, derived from the hexagonal form of diamond, crystallizes in the Cc space group with the Cu5Si2S7 structure type

Lanthanide Doped Alkaline-Earth Metal Nanocrystalline as Ionising Radiation Storage Phosphors

This work involved the preparation methods, structural and spectroscopic characterisations of lanthanide ions (Ln = Sm, Eu) doped nanocrystalline alkaline-earth metal fluorides MF2 (M = Ca, Sr) as ionising radiation storage phosphors for potential applications in the field of computed radiography, dosimetry, and optical data storage. The structural characterisation was conducted using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and SEM and TEM energy dispersive spectroscopy (EDS). The storage mechanism of the phosphors was studied by photoluminescence spectroscopy (PL) and spectral hole- burning. The X-ray storage phosphor properties i.e. X-irradiation induced reduction of Sm3+ to Sm2+ and reverse photoionisation of Sm2+ to Sm3+, in CaF2:Sm3+ nanocrystals prepared by co-precipitation method were investigated by monitoring the PL intensities of both Sm3+ and Sm2+ ions. Both processes can be modelled by first-order dispersive kinetics. Besides, the Sm3+ to Sm2+ conversion upon X-irradiation in SrF2:Sm3+ were compared with CaF2:Sm3+. In addition, the X-ray storage phosphor properties in mechanochemically ball milled nanocrystalline CaF2:Sm3+ were explored in detail. The photoionisation of Sm2+ ions was also demonstrated by spectral hole-burning experiments of the X-irradiated nanocrystalline CaF2:Sm3+. The hole-burning rate decreased with the X-irradiation dose, while an increase was observed with an increase of the Sm concentration, manifesting the significance of the Sm3+ electron trap in the photoionisation of Sm2+ in CaF2. Furthermore, the Zeeman effects in magnetic fields up to 9 Tesla on the Sm2+ luminescence were investigated, in particular the splitting of the 7F1 ground state level and the quadratic dependence of the intensity of a forbidden transition. The photoinduced electron transfer between Eu2+ and Sm3+ in CaF2 nanocrystals prepared by a facile co-precipitation method was explored. The doping of divalent Eu was realised by reducing Eu3+ to Eu2+ in solution under nitrogen employing granular zinc. The forward and backward photoinduced electron transfer in CaF2:Eu2+,Sm3+ was investigated by monitoring the Eu2+ and Sm2+ luminescence signals, under ultraviolet (UV) light exposure at 310 and 340 nm. Importantly, this is the first report on the X-ray storage properties, spectral hole-burning, and photoinduced electron transfer phenomena of nanocrystalline CaF2:Sm3+, SrF2:Sm3+, and CaF2:Eu2+,Sm3+ powders

Spectroscopy and crystal field analysis of rare-earth doped yttrium orthosilicate for quantum information processing.

The long coherence times of trivalent rare-earth ions in insulating crystals make them good candidates for implementing on-demand, long-storage time quantum devices. In addition, the optical transitions of these ions provide near-telecommunication wavelengths useful for low-loss transmission of light in the existing optical fibre infrastructure coupled with quantum repeaters to facilitate the implementation of long-distance quantum communications, i.e., quantum net- works. Further, research on the optical-to-microwave down-conversion capability of rare earth crystals is underway that may lead to the realisation of quantum networks. Ground state hyperfine level coherence times of seconds have been observed for these ions which can be extended to hours when techniques such as zero first order Zeeman (ZEFOZ) and dynamic decoupling are employed. Of great significance is the ability to predict these ZEFOZ points using the results of crystal field calculations. Y₂SiO₅ (YSO) crystal doped with rare-earth ions has been recently proposed as a candidate material for quantum storage implementation due to the low nuclear spins of its constituents. However, the C1 site symmetry of this crystal makes such calculations challenging. To address these difficulties, crystal field analyses have been performed for Y₂SiO₅:Nd³⁺ through fits to experimentally observed crystal field energy levels, directional Zeeman g-values as well as the ground state Zeeman g tensor from electron paramagnetic resonance measurements. Sufficiently good agreement between the experimental and calculated crystal field energy levels as well as the experimental and calculated directional g-values was obtained through these fits. The experimental energy levels and directional Zeeman g-values have been observed for the two Nd³⁺ sites in YSO using high-resolution Zeeman absorption spectroscopy. Laser site-selective fluorescence and excitation spectroscopy have been employed to assign the observed absorption lines to their respective Nd³⁺ site in YSO. In addition, temperature-dependent absorption spectroscopy has been utilised in order to make more accurate site assignments for transitions not within the range of the laser spectroscopy equipment’s spectral response. A total of 64 crystal field energy levels have been observed for site 2 while 61 levels were recorded for site 1 up to 19000 cm⁻¹. With the crystal field energy levels and Zeeman g-value data acquired for Y₂SiO₅:Nd³⁺, crystal field fits were performed obtaining a total of 35 fitted parameters including 27 real-valued one-electron and 3 correlation crystal field parameters. The directional Zeeman splitting data incorporated the orientation information to the fits in order to obtain a unique set of crystal field parameters. Further, the addition of the correlation crystal field parameters successfully accounted for the discrepancies that arose between the theoretical and experimental values relevant to the ²H11/2(2) multiplet in a one-electron crystal field model attributed to correlation effects. Absorption spectra of the unreported 4f -4f intra-configurational transitions of Y2SiO5:Ce³⁺ have been recorded using Fourier transform infrared absorption spectroscopy. Of significant note was the existence of the absorption lines of only one Ce³⁺ site in the absorption spectra. The crystal field calculation performed for both 4f and 4f 5d configurations of Y2SiO5:Ce³⁺resulted in great convergence between the experimental and calculated values. Moreover, the Zeeman absorption spectra of Y₂SiO₅:Yb³⁺ in both substitutional sites have been observed and the crystal field energy levels and directional g-values were inferred from the experimental data. The corresponding crystal field fits also gave good account of the experimental data. With the obtained crystal field parameters for the studied ions and Er³⁺ the existence of a trend in the size of the crystal field strength parameters across the lanthanide series was investigated which partially followed the patterns shown by the crystal field parameters of rare-earth doped LaF3. Further, the temperature dependence of the inhomogeneous linewidths and line positions of the absorption lines of these ions using the existing theoretical models based on a Debye phonon density of states was investigated. The corresponding electron-phonon coupling coefficients were calculated through fits to the temperature-dependent linewidth and position data. These coefficients assisted with identifying the contributions of different non-radiative processes to the inhomogeneous linewidth and peak positions of the spectral lines. Temperature-dependent fluorescence lifetime measurements for the two Nd³⁺sites in Y₂SiO₅ were also performed and the number of phonons bridging the energy gap in a multiphonon emission and their effective energies were obtained through fits to the lifetime data