Rare-earth ions doped transparent oxyfluoride glass-ceramics

In recent years, rare-earth ions doped transparent oxyfluoride glass-ceramics have attracted great attentions for their low phonon energy environments of fluoride nanocrystals and high chemical and mechanical stabilities of oxide glassy matrix. In this chapter, firstly, the crystallization behaviors of the transparent glass ceramics containing CaF2 nanocrystals are presented to demonstrate the controllable microstructure evolution of nano-composites. Secondly, the optical properties of the newly developed transparent glass-ceramics containing β-YF3 nanocrystals are systematically reviewed. The rare-earth ions are inclined to partition into the YF3 nanocrystals after crystallization. Through variation of the rare-earth doping and control of the microstructures, the glass-ceramics could exhibit high-stimulated emission cross-section, broadband near infrared emission, high efficient ultraviolet upconversion emission and bright white light emission, indicating their potential multifunctional applications in solid state laser, upconversion, optical amplifier, three-dimensional display, and so on

Manipulation of Ga2O3 Nanocrystals for the Design of Functional Phosphors

Transparent conducting oxides are of great interest in semiconductor research and industry. Their ability to carry electricity while remaining transparent allows them to be used for different applications including photovoltaics, lighting, and photocatalysis. Among transparent conducting oxides, Ga2O3 has the widest band gap and is characterized by a strong broad-band blue-green afterglow emission, making it attractive for various lighting applications. The main motivation of this thesis is to use these unique properties of Ga2O3 to design new phosphors with targeted optical properties. The research described in this thesis specifically focuses on generating white light by non-radiative energy transfer between Ga2O3 nanocrystals, as energy donors, and orange-red emitting semiconductor quantum dots, as energy acceptors, and using dopant-induced trap states to extend afterglow emission. Solution phase conjugation of colloidal nanocrystals allows for short-range energy transfer processes to occur with high probability, which is valuable for sensing and lighting. The first part of this thesis demonstrates the conjugation and electronic coupling of Ga2O3 nanocrystals with CdSe/CdS core/shell quantum dots. The introduction of a bifunctional organic ligand to the suspension mixture of these nanocrystals allows for their conjugation. The resulting Förster resonance energy transfer leads to quenching of Ga2O3 emission and an increase in the emission of CdSe/CdS quantum dots. As the ratio of CdSe/CdS quantum dots to Ga2O3 nanocrystals increases, so does the quenching of Ga2O3 emission and the CdSe/CdS photoluminescence intensity. The increase in quenching of Ga2O3 emission was successfully modelled assuming a Poisson distribution of CdSe/CdS quantum dots bound to Ga2O3 nanocrystals. Owing to the good emission colour complementarity of these materials, white light was observed for optimal nanoconjugate composition. Next, we demonstrated that the same phenomenon can be achieved without the organic linker by a careful deposition of the colloidal mixture on the glass substrate to remove the empty space between nanocrystals. This approach enables a surface-mediated Förster resonance energy transfer that allows for a design of all-inorganic white light emitting phosphors. The resulting films were highly luminescent and were tuned to give CIE coordinates of 0.31, 0.28. Gallium oxide also has a long luminescence lifetime, compared to other nanoparticles with similar emission strengths, enabled by its donor-acceptor pair recombination. This phenomenon makes Ga2O3 nanocrystals a prime candidate for attempting to design persistent afterglow nanophosphors through doping nanocrystals with selected trivalent rare earth metals. These rare earth elements provide a mechanism for trapping excited free carriers because relaxation within these dopant ions is Laporte forbidden. As a result of doping Ga2O3 with Dy3+, the emission of Ga2O3 nanocrystals becomes significantly longer at room temperature. More interestingly, the temperature-dependent emission of Ga2O3 nanocrystals doped with Dy3+ for doping levels between 3 and 13 % increased between 50 K and 200 K, where typical semiconducting nanocrystals show strong quenching. We attribute this anomalous behavior to the carriers trapped in Dy3+ excited states, that are thermally reactivated and subsequently relax to Ga2O3 native traps states. This assignment was validated by a kinetic Monte Carlo simulation, which was in good agreement with experimental results. As the importance of luminescence materials, particularly persistent phosphors, increases it is imperative that undergraduate students in chemistry and materials science become familiar with their properties and fabrication. A new laboratory exercise encompassing the combustion synthesis, processing, and characterization of SrAl2O4:(Eu2+, Dy3+) has been adopted by NE 320L course in the Nanotechnology Engineering program at the University of Waterloo. In this laboratory, students produced crude strontium aluminate containing Eu3+ which was subsequently annealed under hydrogen gas, resulting in the red europium emission. This emission becomes green upon reduction of Eu3+ to Eu2+. Students performed XRD showing a dramatic increase in crystallinity after annealing, while their SEM measurements did not show a significant change in morphology. Mechanoluminescence was observed using a ballistic setup and found to show a linear dependence on the projectile velocity. In this thesis I demonstrated the extrinsic (external functionalization) and intrinsic (doping) manipulation of the electronic structure of gallium oxide nanocrystals. The obtained results allow for technological application of the resulting materials (e.g., to generate white light and extend afterglow emission), and provide a framework to enrich upper-year undergraduate curriculum in materials science and nanotechnology

Luminescence in sulfides : a rich history and a bright future

Sulfide-based luminescent materials have attracted a lot of attention for a wide range of photo-, cathodo- and electroluminescent applications. Upon doping with Ce3+ and Eu2+, the luminescence can be varied over the entire visible region by appropriately choosing the composition of the sulfide host. Main application areas are flat panel displays based on thin film electroluminescence, field emission displays and ZnS-based powder electroluminescence for backlights. For these applications, special attention is given to BaAl2S4:Eu, ZnS:Mn and ZnS:Cu. Recently, sulfide materials have regained interest due to their ability (in contrast to oxide materials) to provide a broad band, Eu2+-based red emission for use as a color conversion material in white-light emitting diodes (LEDs). The potential application of rare-earth doped binary alkaline-earth sulfides, like CaS and SrS, thiogallates, thioaluminates and thiosilicates as conversion phosphors is discussed. Finally, this review concludes with the size-dependent luminescence in intrinsic colloidal quantum dots like PbS and CdS, and with the luminescence in doped nanoparticles

Highly Stable Luminous "snakes" from CsPbX3 Perovskite Nanocrystals Anchored on Amine-Coated Silica Nanowires

CsPbX3 (X = Cl, Br, and I) perovskite nanocrystals (NCs) are known for their exceptional optoelectronic properties, yet the material's instability toward polar solvents, heat, or UV irradiation greatly limits its further applications. Herein, an efficient in situ growing strategy has been developed to give highly stable perovskite NC composites (abbreviated CsPbX3@CA-SiO2) by anchoring CsPbX3 NCs onto silica nanowires (NWs), which effectively depresses the optical degradation of their photoluminescence (PL) and enhances stability. The preparation of surface-functionalized serpentine silica NWs is realized by a sol-gel process involving hydrolysis of a mixture of tetraethyl orthosilicate (TEOS), 3-aminopropyltriethoxysilane (APTES), and trimethoxy(octadecyl)silane (TMODS) in a water/oil emulsion. The serpentine NWs are formed via an anisotropic growth with lengths up to 8 μm. The free amino groups are employed as surface ligands for growing perovskite NCs, yielding distributed monodisperse NCs (∼8 nm) around the NW matrix. The emission wavelength is tunable by simple variation of the halide compositions (CsPbX3, X = Cl, Br, or I), and the composites demonstrate a high photoluminescence quantum yield (PLQY 32-69%). Additionally, we have demonstrated the composites CsPbX3@CA-SiO2 can be self-woven to form a porous 3D hierarchical NWs membrane, giving rise to a superhydrophobic surface with hierarchical micro/nano structural features. The resulting composites exhibit high stability toward water, heat, and UV irradiation. This work elucidates an effective strategy to incorporate perovskite nanocrystals onto functional matrices as multifunctional stable light sources

Luminescent and Scintillating Properties of Lanthanum Fluoride Nanocrystals in Response to Gamma/Neutron Irradiation: Codoping with Ce Activator, Yb Wavelength Shifter, and Gd Neutron Captor

A novel concept for detection and spectroscopy of gamma rays, and detection of thermal neutrons based on codoped lanthanum fluoride nanocrystals containing gadolinium is presented.The trends of colloidal synthesis of the mentioned material, LaF3 co-doped with Ce as the activator, Yb as the wavelength-shifter and Gd as the neutron captor, is reported. Nanocrystals of the mentioned material were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), optical absorption, and photoluminescence spectroscopy. Gamma detection and its potential spectroscopy feature have been confirmed. The neutron detection capability has been confirmed by experiments performed using a 252Cf neutron source.Comment: 5 figures, 16 page

Characterization of white light emitting CdSe quantum dots

A novel type of white light emitting semiconductor quantum dot was characterized at the ensemble and single-molecule level. This kind of semiconductor nanocrystal can be made into white light emitting diodes, which have the potential to replace conventional lighting sources. The quantum dots used in this thesis consisted of a cadmium selenide (CdSe) core, capped with ZnS, and have a surface polymer coating of poly(acrylic acid) (PAA). We have characterized the quantum dot size distribution by using dynamic light scattering (DLS), transmission electron microscopy (TEM), atomic force microscopy (AFM) and UV-Vis spectroscopy. Based on these measurements, it is clear that the white quantum dots are polydisperse, with a core size of 2.4 ± 0.5 nm, though the polymer coating swells considerably in aqueous solution. In order to explore the optical properties, the absorption and emission spectra of the ensemble quantum dots solution were measured and compared to “standard” commercial quantum dots. The emission spectrum of the white quantum dots showed two peaks, a strong blue emission peak and a weaker red emission peak. The fluorescence quantum yield of the white quantum dots was found to be less than that of commercial quantum dots. To explore the behavior of individual quantum dots, spatially-resolved single-molecule images were obtained by a dual-view single molecule fluorescence microscopy with a beam splitter which can separate the emission into red and blue components. It was found that individual white CdSe nanocrystals have a broad emission spectrum and the samples did not consist of a mixed population of red emitters and blue emitters. These results suggest that these white light emitting quantum dots can be used for pure white light LEDs and are a good candidate for the replacement for conventional lighting sources

All-Inorganic Metal Halide Perovskite Nanocrystals: Opportunities and Challenges.

The past decade has witnessed the growing interest in metal halide perovskites as driven by their promising applications in diverse fields. The low intrinsic stability of the early developed organic versions has however hampered their widespread applications. Very recently, all-inorganic perovskite nanocrystals have emerged as a new class of materials that hold great promise for the practical applications in solar cells, photodetectors, light-emitting diodes, and lasers, among others. In this Outlook, we first discuss the recent developments in the preparation, properties, and applications of all-inorganic metal halide perovskite nanocrystals, with a particular focus on CsPbX3, and then provide our view of current challenges and future directions in this emerging area. Our goal is to introduce the current status of this type of new materials to researchers from different areas and motivate them to explore all the potentials

Rare earth based nanostructured materials: Synthesis, functionalization, properties and bioimaging and biosensing applications

Rare earth based nanostructures constitute a type of functional materials widely used and studied in the recent literature. The purpose of this review is to provide a general and comprehensive overview of the current state of the art, with special focus on the commonly employed synthesis methods and functionalization strategies of rare earth based nanoparticles and on their different bioimaging and biosensing applications. The luminescent (including downconversion, upconversion and permanent luminescence) and magnetic properties of rare earth based nanoparticles, as well as their ability to absorb X-rays, will also be explained and connected with their luminescent, magnetic resonance and X-ray computed tomography bioimaging applications, respectively. This review is not only restricted to nanoparticles, and recent advances reported for in other nanostructures containing rare earths, such as metal organic frameworks and lanthanide complexes conjugated with biological structures, will also be commented on.European Union 267226Ministerio de Economía y Competitividad MAT2014-54852-

Lead halide perovskite nanowires stabilized by block copolymers for Langmuir-Blodgett assembly

The rapid development of solar cells based on lead halide perovskites (LHPs) has prompted very active research activities in other closely-related fields. Colloidal nanostructures of such materials display superior optoelectronic properties. Especially, one-dimensional (1D) LHPs nanowires show anisotropic optical properties when they are highly oriented. However, the ionic nature makes them very sensitive to external environment, limiting their large scale practical applications. Here, we introduce an amphiphilic block copolymer, polystyrene-block-poly(4-vinylpyridine) (PS-P4VP), to chemically modify the surface of colloidal CsPbBr3 nanowires. The resulting core-shell nanowires show enhanced photoluminescent emission and good colloidal stability against water. Taking advantage of the stability enhancement, we further applied a modified Langmuir-Blodgett technique to assemble monolayers of highly aligned nanowires, and studied their anisotropic optical properties. [Figure not available: see fulltext.]

Engineering Silicon Nanocrystals: Theoretical study of the effect of Codoping with Boron and Phosphorus

We show that the optical and electronic properties of nanocrystalline silicon can be efficiently tuned using impurity doping. In particular, we give evidence, by means of ab-initio calculations, that by properly controlling the doping with either one or two atomic species, a significant modification of both the absorption and the emission of light can be achieved. We have considered impurities, either boron or phosphorous (doping) or both (codoping), located at different substitutional sites of silicon nanocrystals with size ranging from 1.1 nm to 1.8 nm in diameter. We have found that the codoped nanocrystals have the lowest impurity formation energies when the two impurities occupy nearest neighbor sites near the surface. In addition, such systems present band-edge states localized on the impurities giving rise to a red-shift of the absorption thresholds with respect to that of undoped nanocrystals. Our detailed theoretical analysis shows that the creation of an electron-hole pair due to light absorption determines a geometry distortion that in turn results in a Stokes shift between adsorption and emission spectra. In order to give a deeper insight in this effect, in one case we have calculated the absorption and emission spectra going beyond the single-particle approach showing the important role played by many-body effects. The entire set of results we have collected in this work give a strong indication that with the doping it is possible to tune the optical properties of silicon nanocrystals.Comment: 14 pages 19 figure