Role of Vacancy Condensation in the Formation of Voids in Rutile TiO₂ Nanowires

Titanium dioxide nanowire (NW) arrays are incorporated in many devices for energy conversion, energy storage, and catalysis. A common approach to fabricate these NWs is based on hydrothermal synthesis strategies. A drawback of this low-temperature method is that the NWs have a high density of defects, such as stacking faults, dislocations, and oxygen vacancies. These defects compromise the performance of devices. Here, we report a postgrowth thermal annealing procedure to remove these lattice defects and propose a mechanism to explain the underlying changes in the structure of the NWs. A detailed transmission electron microscopy study including in situ observation at elevated temperatures reveals a two-stage process. Additional spectroscopic analyses and X-ray diffraction experiments clarify the underlying mechanisms. In an early, low-temperature stage, the as-grown mesocrystalline NW converts to a single crystal by the dehydration of surface-bound OH groups. At temperatures above 500 °C, condensation of oxygen vacancies takes place, which leads to the fabrication of NWs with internal voids. These voids are faceted and covered with Ti³⁺-rich amorphous TiO_<i>x</i>

Heat-Induced Phase Transformation of Three-Dimensional Nb₃O₇(OH) Superstructures: Effect of Atmosphere and Electron Beam

Nanostructured niobium oxides and hydroxides are potential candidates for photochemical applications due to their excellent optical and electronic properties. In the present work the thermal stability of Nb₃O₇(OH) superstructures prepared by a simple hydrothermal approach is investigated at the atomic scale. Transmission electron microscopy and electron energy-loss spectroscopy provide insights into the phase transformation occurring at elevated temperatures and probe the effect of the atmospheric conditions. In the presence of oxygen, H₂O is released from the crystal at temperatures above 500 °C, and the crystallographic structure changes to H-Nb₂O₅. In addition to the high thermal stability of Nb₃O₇(OH), the morphology was found to be stable, and first changes in the form of a merging of nanowires are not observed until 850 °C. Under reducing conditions in a transmission electron microscope and during electron beam bombardment, an oxygen-deficient phase is formed at temperatures above 750 °C. This transformation starts with the formation of defects in the crystal lattice at 450 °C and goes along with the formation of pores in the nanowires which accommodate the volume differences of the two crystal phases

Synchronous Emission from Nanometric Silver Particles through Plasmonic Coupling on Silver Nanowires

We investigated silver nanowires using correlative wide-field fluorescence and transmission electron microscopy. In the wide-field fluorescence images, synchronous emission from different distinct positions along the silver nanowires was observed. The sites of emission were separated spatially by up to several micrometers. Nanowires emitting in such cooperative manner were then also investigated with a combination of transmission electron microscopy based techniques, such as high-resolution, bright-field imaging, electron diffraction, high-angle annular dark-field imaging, and energy-dispersive X-ray spectroscopy. In particular, analyzing the chemical composition of the emissive areas using energy-dispersive X-ray spectroscopy led to the model that the active emissive centers are small silver clusters generated photochemically and that individual clusters are coupled <i>via</i> surface plasmons of the nanowire

Synchronous Emission from Nanometric Silver Particles through Plasmonic Coupling on Silver Nanowires

We investigated silver nanowires using correlative wide-field fluorescence and transmission electron microscopy. In the wide-field fluorescence images, synchronous emission from different distinct positions along the silver nanowires was observed. The sites of emission were separated spatially by up to several micrometers. Nanowires emitting in such cooperative manner were then also investigated with a combination of transmission electron microscopy based techniques, such as high-resolution, bright-field imaging, electron diffraction, high-angle annular dark-field imaging, and energy-dispersive X-ray spectroscopy. In particular, analyzing the chemical composition of the emissive areas using energy-dispersive X-ray spectroscopy led to the model that the active emissive centers are small silver clusters generated photochemically and that individual clusters are coupled <i>via</i> surface plasmons of the nanowire

Theoretical and Experimental Study on the Optoelectronic Properties of Nb₃O₇(OH) and Nb₂O₅ Photoelectrodes

Nb₃O₇(OH) and Nb₂O₅ nanostructures are promising alternative materials to conventionally used oxides, e.g. TiO₂, in the field of photoelectrodes in dye-sensitized solar cells and photoelectrochemical cells. Despite this important future application, some of their central electronic properties such as the density of states, band gap, and dielectric function are not well understood. In this work, we present combined theoretical and experimental studies on Nb₃O₇(OH) and H–Nb₂O₅ to elucidate their spectroscopic, electronic, and transport properties. The theoretical results were obtained within the framework of density functional theory based on the full potential linearized augmented plane wave method. In particular, we show that the position of the H atom in Nb₃O₇(OH) has an important effect on its electronic properties. To verify theoretical predictions, we measured electron energy-loss spectra (EELS) in the low loss region, as well as, the O–K and Nb–M₃ element-specific edges. These results are compared with corresponding theoretical EELS calculations and are discussed in detail. In addition, our calculations of thermoelectric conductivity show that Nb₃O₇(OH) has more suitable optoelectronic and transport properties for photochemical application than the calcined H–Nb₂O₅ phase

Synchronous Emission from Nanometric Silver Particles through Plasmonic Coupling on Silver Nanowires

We investigated silver nanowires using correlative wide-field fluorescence and transmission electron microscopy. In the wide-field fluorescence images, synchronous emission from different distinct positions along the silver nanowires was observed. The sites of emission were separated spatially by up to several micrometers. Nanowires emitting in such cooperative manner were then also investigated with a combination of transmission electron microscopy based techniques, such as high-resolution, bright-field imaging, electron diffraction, high-angle annular dark-field imaging, and energy-dispersive X-ray spectroscopy. In particular, analyzing the chemical composition of the emissive areas using energy-dispersive X-ray spectroscopy led to the model that the active emissive centers are small silver clusters generated photochemically and that individual clusters are coupled <i>via</i> surface plasmons of the nanowire

Ca_18.75Li_10.5[Al₃₉N₅₅]:Eu²⁺Supertetrahedron Phosphor for Solid-State Lighting

Highly efficient red-emitting luminescent materials deliver the foundation for next-generation illumination-grade white light-emitting diodes (LEDs). Recent studies demonstrate that the hardly explored class of nitridoaluminates comprises intriguing phosphor materials, e.g., Sr­[LiAl₃N₄]:Eu²⁺ or Ca­[LiAl₃N₄]:Eu²⁺. Here, we describe the novel material Ca_18.75Li_10.5[Al₃₉N₅₅]:Eu²⁺ with highly efficient narrow-band red emission (λ_em ≈ 647 nm, full width at half-maximum, fwhm ≈ 1280 cm^–1). This compound features a rather uncommon crystal structure, comprising sphalerite-like T₅ supertetrahedra that are composed of tetrahedral AlN₄ units that are interconnected by additional AlN₄ moieties. The network charge is compensated by Ca²⁺ and Li⁺ ions located between the supertetrahedra. The crystal structure was solved and refined from single-crystal and powder X-ray diffraction data in the cubic space group <i>Fd</i>3̅<i>m</i> (No. 227) with <i>a</i> = 22.415(3) Å and <i>Z</i> = 8. To verify the presence of Li, transmission electron microscopy (TEM) investigations including electron energy-loss spectroscopy (EELS) were performed. Based on the intriguing luminescence properties, we proclaim high potential for application in high-power phosphor-converted white LEDs

Dorothea Lutherinna Helena Stränigstie to Carl Linnaeus

Dorothea Lutherinna Helena Stränigstie to Carl Linnaeu

Synchronous Emission from Nanometric Silver Particles through Plasmonic Coupling on Silver Nanowires

We investigated silver nanowires using correlative wide-field fluorescence and transmission electron microscopy. In the wide-field fluorescence images, synchronous emission from different distinct positions along the silver nanowires was observed. The sites of emission were separated spatially by up to several micrometers. Nanowires emitting in such cooperative manner were then also investigated with a combination of transmission electron microscopy based techniques, such as high-resolution, bright-field imaging, electron diffraction, high-angle annular dark-field imaging, and energy-dispersive X-ray spectroscopy. In particular, analyzing the chemical composition of the emissive areas using energy-dispersive X-ray spectroscopy led to the model that the active emissive centers are small silver clusters generated photochemically and that individual clusters are coupled <i>via</i> surface plasmons of the nanowire

Synchronous Emission from Nanometric Silver Particles through Plasmonic Coupling on Silver Nanowires

We investigated silver nanowires using correlative wide-field fluorescence and transmission electron microscopy. In the wide-field fluorescence images, synchronous emission from different distinct positions along the silver nanowires was observed. The sites of emission were separated spatially by up to several micrometers. Nanowires emitting in such cooperative manner were then also investigated with a combination of transmission electron microscopy based techniques, such as high-resolution, bright-field imaging, electron diffraction, high-angle annular dark-field imaging, and energy-dispersive X-ray spectroscopy. In particular, analyzing the chemical composition of the emissive areas using energy-dispersive X-ray spectroscopy led to the model that the active emissive centers are small silver clusters generated photochemically and that individual clusters are coupled <i>via</i> surface plasmons of the nanowire