Marion A. Stevens-Kalceff

Defects induced in fused silica by high fluence ultraviolet laser pulses at 355 nm

Point defects induced at the exit surface of optical-grade fused silica by high power (>30 J/cm2) 355 nm laser pulses have been investigated to elucidate the nature of laser damage in optics for use at high peak powers (>2 GW/cm2). Six defects have been identified. Eγ′ and E74′ defects were identified using electron spin resonance spectroscopy. The nonbridging oxygen hole center, a self-trapped exciton, an oxygen-deficient center, and interstitial O2 were identified and spatially resolved in the damage craters using cathodoluminescence microanalysis. The defects are associated with the laser generated shock waves and/or thermal explosion.

Correlation between morphology and cathodoluminescence in porous GaP

Porous layers fabricated by anodic etching of n-GaP substrates in a sulfuric acid solution were studied by electron microscopy and cathodoluminescence (CL) microanalysis. The morphology of porous layers was found to depend strongly upon the anodization conditions. When the etching process starts at the initial surface, “catacomb-like” pores and current-line oriented pores are introduced at low and high anodic current densities, respectively. After the initial development of either kind of pore, further anodization at the current density of about 1 mA/cm2 favors the propagation of pores along 〈111〉 crystallographic directions. The spatial and spectral distribution of CL in bulk and porous samples is presented. A comparative analysis of the secondary electron and panchromatic CL images evidenced a porosity induced increase in the emission efficiency.

Strong light scattering and broadband (UV to IR) photoabsorption in stretchable 3D hybrid architectures based on Aerographite decorated by ZnO nanocrystallites

In present work, the nano- and microscale tetrapods from zinc oxide were integrated on the surface of Aerographite material (as backbone) in carbon-metal oxide hybrid hierarchical network via a simple and single step magnetron sputtering process. The fabricated hybrid networks are characterized for morphology, microstructural and optical properties. The cathodoluminescence investigations revealed interesting luminescence features related to carbon impurities and inherent host defects in zinc oxide. Because of the wide bandgap of zinc oxide and its intrinsic defects, the hybrid network absorbs light in the UV and visible regions, however, this broadband photoabsorption behavior extends to the infrared (IR) region due to the dependence of the optical properties of ZnO architectures upon size and shape of constituent nanostructures and their doping by carbon impurities. Such a phenomenon of broadband photoabsorption ranging from UV to IR for zinc oxide based hybrid materials is novel. Additionally, the fabricated network exhibits strong visible light scattering behavior. The developed Aerographite/nanocrystalline ZnO hybrid network materials, equipped with broadband photoabsorption and strong light scattering, are very promising candidates for optoelectronic technologies.

Three-dimensional Aerographite-GaN hybrid networks: Single step fabrication of porous and mechanically flexible materials for multifunctional applications

Three dimensional (3D) elastic hybrid networks built from interconnected nano- and microstructure building units, in the form of semiconducting-carbonaceous materials, are potential candidates for advanced technological applications. However, fabrication of these 3D hybrid networks by simple and versatile methods is a challenging task due to the involvement of complex and multiple synthesis processes. In this paper, we demonstrate the growth of Aerographite-GaN 3D hybrid networks using ultralight and extremely porous carbon based Aerographite material as templates by a single step hydride vapor phase epitaxy process. The GaN nano- and microstructures grow on the surface of Aerographite tubes and follow the network architecture of the Aerographite template without agglomeration. The synthesized 3D networks are integrated with the properties from both, i.e., nanoscale GaN structures and Aerographite in the form of flexible and semiconducting composites which could be exploited as next generation materials for electronic, photonic, and sensors applications.

Mechanochemical preparation of nanocrystalline BaFCl doped with samarium in the 2+ oxidation state.

We report a facile mechanochemical preparation method for nanocrystalline BaFCl doped with samarium in the 2+ oxidation state by ball milling BaCl2, BaF2, and SmI2 under a nitrogen atmosphere. The resulting phosphors were characterized by powder X-ray diffraction; electron microscopy, X-ray photoelectron spectroscopy; and photoluminescence, photoexcitation, cathodoluminescence, and diffuse reflectance spectroscopy. This is the first report of a direct preparation method of Sm(2+) doped alkaline earth fluorohalides at room temperature and points to a significant potential for the preparation of a wide range of related X-ray storage phosphors containing rare earth ions in divalent and trivalent cationic states by mechanochemical methods.

Effects of postannealing on the photoluminescence properties of coprecipitated nanocrystalline BaFCl:Sm3+.

Nanocrystalline BaFCl:Sm(3+), as prepared by coprecipitation from aqueous solutions, is an efficient photoluminescent X-ray storage phosphor. In the present study, we report effects on its photoluminescence properties resulting from postannealing treatment in air in the temperature range between 100 to 900 °C. Interestingly, upon annealing at temperatures from 200 to 600 °C in air, a small fraction of the Sm(3+) ions in nanocrystalline BaFCl can be reduced to Sm(2+) ions. In addition to the creation of Sm(2+) ions, two different sites of Sm(3+) ions, denoted as sites A and B, are observed when the nanocrystalline BaFCl:Sm(3+) is annealed between 500 to 900 °C. The temperature dependence of photoluminescence properties of the two different sites in the 500 °C annealed sample reveals that the Sm(3+) ions at site A are possibly located at or near the crystallite surface, whereas site B is situated in a very ordered environment. To the best of our knowledge, this is the first report on the reduction of Sm(3+) ions doped in alkaline-earth fluorohalides to Sm(2+) ions by annealing in air.

Micromodification of silicon dioxide in a variable pressure/environmental scanning electron microscope

Electron irradiation in the ionized gaseous environment of a variable pressure/environmental scanning electron microscope induces modifications of poorly conducting specimens. In particular it is shown, using nondestructive depth-resolved cathodoluminescence microanalysis, that environmental ions can penetrate into the bulk of the irradiated specimen and modify the local microstructure of the irradiated specimen. The observed modifications are attributed to electric fields associated with trapped electrons and environmental ions. These effects can be controlled by varying the environmental gas and/or electron beam parameters.

Dislocation-induced changes in quantum dots: step alignment and radiative emission.

A transition between two types of step alignment was observed in a multilayered InGaAs/GaAs quantum-dot (QD) structure. A change to larger QD sizes in smaller concentrations occurred after formation of a dislocation array. Cathodoluminescence(CL) spectra show a bimodal peak with lower energy peak enhancement when probing at lower e-beam energies. The two peaks separate as a result of QDinterdiffusion.CLimaging and cross-sectional transmission electron microscopy showed contrast from a dislocation array formed at the interface between GaAs and the first InGaAs QD layer. Strong QD emission in the near infrared (800–1100 nm) was obtained despite the presence of dislocations.

Origin and significance of the yellow cathodoluminescence (CL) of quartz

Abstract The origin of yellow cathodoluminescence (CL) in quartz has been investigated by a combination of CL microscopy and spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and spatially resolved trace-element analysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The study shows that the appearance of a ~570 nm (2.17 eV) emission band can be attributed to high oxygen deficiency and local structural disorder in quartz. A proposed luminescence center model implies self-trapped exciton (STE) emission from localized amorphized regions in quartz. Although the high-intensity emission at 570 nm is in general consistent with high concentrations of E′1 defects detected by EPR spectroscopy, CL studies with different electron beam parameters and annealing experiments up to 600 °C show a temperature and irradiation dependence of the luminescence related defects excluding the role of E′1 centers as direct luminescence activators for the 570 nm emission. The evaluation of geochemical data shows that quartz with yellow CL occurs in low-temperature hydrothermal environment (mostly <250 °C) and is related to fast crystallization in an environment with oxygen deficiency.

Cathodoluminescence microanalysis of silica and amorphized quartz

Cathodoluminescence (CL) techniques are used to investigate the defect structures of pure synthetic silicon dioxide (SiO2) polymorphs. Pure, synthetic Types I, II, III and IV amorphous SiO2 polymorphs, pure, synthetic crystal α-SiO2 and pure, synthetic amorphized crystal α-SiO2 have been investigated and their characteristic defects have been determined and compared. The CL emission from pure SiO2 polymorphs is generally related to local point defects in the tetrahedrally coordinated SiO2 host lattice. A range of CL emissions associated with non bridging oxygen defects, oxygen deficient defects and the radiative recombination of self trapped excitons are observed from both the pure synthetic crystal and amorphous SiO2 polymorphs. In addition CL emissions associated with residual concentrations of Aluminium impurities are also observed from α-SiO2 (quartz) and Type I and II a-SiO2 (fused quartz). Localised amorphous micro-volumes may exist within natural α-SiO2 due to the presence of a high concentration of pre-existing or induced defects. Amorphization of α-SiO2 diminishes the difference between the defect structures and associated CL from α-SiO2 and a-SiO2. Thus CL investigation of the defect structure of a-SiO2 polymorphs provides useful insight into the microstructure of amorphized α-SiO2.