Christina Scheu

Narrow-band red-emitting Sr[LiAl3N4]:Eu2+ as a next-generation LED-phosphor material

To facilitate the next generation of high-power white-light-emitting diodes (white LEDs), the discovery of more efficient red-emitting phosphor materials is essential. In this regard, the hardly explored compound class of nitridoaluminates affords a new material with superior luminescence properties. Doped with Eu(2+), Sr[LiAl3N4] emerged as a new high-performance narrow-band red-emitting phosphor material, which can efficiently be excited by GaN-based blue LEDs. Owing to the highly efficient red emission at λ(max) ~ 650 nm with a full-width at half-maximum of ~1,180 cm(-1) (~50 nm) that shows only very low thermal quenching (>95% relative to the quantum efficiency at 200 °C), a prototype phosphor-converted LED (pc-LED), employing Sr[LiAl3N4]:Eu(2+) as the red-emitting component, already shows an increase of 14% in luminous efficacy compared with a commercially available high colour rendering index (CRI) LED, together with an excellent colour rendition (R(a)8 = 91, R9 = 57). Therefore, we predict great potential for industrial applications in high-power white pc-LEDs.

Strong Efficiency Improvements in Ultra-low-Cost Inorganic Nanowire Solar Cells

The need for sustainable power generation has encouraged research into a variety of photovoltaic materials and structures, with a greater emphasis being placed on a balance between performance and cost. The stability of many semiconducting oxides relative to other inexpensive solar cell technologies, such as organic [ 1 ] and dye-sensitized [ 2 ] cells, makes them an attractive alternative. Yet low-cost, non-toxic, inorganic solar cell technologies have received comparatively little attention. In a recent report, nine inorganic semiconductors were identifi ed as having both the potential for annual electricity production in excess of worldwide demand and material extraction costs less than that of crystalline silicon. [ 3 ] Further to materials costs, a recent study examined the high cost of modern vacuum deposition methods and highlighted the need for low-temperature, atmospheric, solution-based synthesis. [ 4 ] Solution-based synthesis of several of the nine, promising inorganic materials has been demonstrated previously. [ 5–7 ] Copper (I) oxide (Cu 2 O), in particular, has been synthesized extensively in polycrystalline form by electrodeposition from solutions near room temperature. [ 5 , 8 , 9 ]

Oscillatory Mass Transport in Vapor-Liquid-Solid Growth of Sapphire Nanowires

Growing Nanowires In vapor-liquid-solid (VLS) growth of nanowires, the liquid phase acts as a transporter to bring material from the gas phase to the growing solid. By heating a single crystal of sapphire in a high-resolution transmission microscope, Oh et al. (p. 489) monitored the growth of sapphire (α-Al2O3) nanowires out of an aluminum droplet. The liquid aluminum brings oxygen to the growing wire surface, in alternating growth and dissolution reactions at the edge of the wire. The oscillation created an optimum face at the self-catalytic site for atomic stacking and regenerated the junction between the VLS phases, allowing growth of the nanowire. High-resolution transmission electron microscopy is used to identify oscillatory growth of a sapphire nanowire. In vapor-liquid-solid (VLS) growth, the liquid phase plays a pivotal role in mediating mass transport from the vapor source to the growth front of a nanowire. Such transport often takes place through the liquid phase. However, we observed by in situ transmission electron microscopy a different behavior for self-catalytic VLS growth of sapphire nanowires. The growth occurs in a layer-by-layer fashion and is accomplished by interfacial diffusion of oxygen through the ordered liquid aluminum atoms. Oscillatory growth and dissolution reactions at the top rim of the nanowires occur and supply the oxygen required to grow a new (0006) sapphire layer. A periodic modulation of the VLS triple-junction configuration accompanies these oscillatory reactions.

Tin doping speeds up hole transfer during light-driven water oxidation at hematite photoanodes

Numerous studies have shown that the performance of hematite photoanodes for light-driven water splitting is improved substantially by doping with various metals, including tin. Although the enhanced performance has commonly been attributed to bulk effects such as increased conductivity, recent studies have noted an impact of doping on the efficiency of the interfacial transfer of holes involved in the oxygen evolution reaction. However, the methods used were not able to elucidate the origin of this improved efficiency, which could originate from passivation of surface electron-hole recombination or catalysis of the oxygen evolution reaction. The present study used intensity-modulated photocurrent spectroscopy (IMPS), which is a powerful small amplitude perturbation technique that can de-convolute the rate constants for charge transfer and recombination at illuminated semiconductor electrodes. The method was applied to examine the kinetics of water oxidation on thin solution-processed hematite model photoanodes, which can be Sn-doped without morphological change. We observed a significant increase in photocurrent upon Sn-doping, which is attributed to a higher transfer efficiency. The kinetic data obtained using IMPS show that Sn-doping brings about a more than tenfold increase in the rate constant for water oxidation by photogenerated holes. This result provides the first demonstration that Sn-doping speeds up water oxidation on hematite by increasing the rate constant for hole transfer.

Perylene-Labeled Silica Nanoparticles: Synthesis and Characterization of Three Novel Silica Nanoparticle Species for Live-Cell Imaging

The increasing exposure of humans to nanoscaled particles requires well-defined systems that enable the investigation of the toxicity of nanoparticles on the cellular level. To facilitate this, surface-labeled silica nanoparticles, nanoparticles with a labeled core and a silica shell, and a labeled nanoparticle network-all designed for live-cell imaging-are synthesized. The nanoparticles are functionalized with perylene derivatives. For this purpose, two different perylene species containing one or two reactive silica functionalities are prepared. The nanoparticles are studied by transmission electron microscopy, widefield and confocal fluorescence microscopy, as well as by fluorescence spectroscopy in combination with fluorescence anisotropy, in order to characterize the size and morphology of the nanoparticles and to prove the success and homogeneity of the labeling. Using spinning-disc confocal measurements, silica nanoparticles are demonstrated to be taken up by HeLa cells, and they are clearly detectable inside the cytoplasm of the cells.

Towards Mesostructured Zinc Imidazolate Frameworks

The transfer of supramolecular templating to the realm of metal-organic frameworks opens up new avenues to the design of novel hierarchically structured materials. We demonstrate the first synthesis of mesostructured zinc imidazolates in the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB), which acts as a template giving rise to ordered lamellar hybrid materials. A high degree of order spanning the atomic and mesoscale was ascertained by powder X-ray diffraction, electron diffraction, as well as solid-state NMR and IR spectroscopy. The metrics of the unit cells obtained for the zinc methylimidazolate and imidazolate species are a=(11.43±0.45), b=(9.55±0.35), c=(27.19±0.34) Å, and a=(10.98±0.90), b=(8.95±0.95), c=(26.33±0.34) Å, respectively, assuming orthorhombic symmetry. The derived structure model is consistent with a mesolamellar structure composed of bromine-terminated zinc (methyl)imidazolate chains interleaved with motionally rigid cationic surfactant molecules in an all-trans conformation. The hybrid materials exhibit unusually high thermal stability up to 300 °C, at which point CTAB is lost and evidence for a thermally induced transformation into poorly crystalline mesostructures with larger feature sizes is obtained. Treatment with ethanol effects the extraction of CTAB from the material, followed by facile transformation into pure microporous ZIF-8 nanoparticles within minutes, thus demonstrating a unique transition from a mesostructure into a microporous zinc imidazolate.

Quantitative Understanding of the Optical Properties of a Single, Complex-Shaped Gold Nanoparticle from Experiment and Theory

We report on a combined study of Rayleigh and Raman scattering spectroscopy, 3D electron tomography, and discrete dipole approximation (DDA) calculations of a single, complex-shaped gold nanoparticle (NP). Using the exact reconstructed 3D morphology of the NP as input for the DDA calculations, the experimental results can be reproduced with unprecedented precision and detail. We find that not only the exact NP morphology but also the surroundings including the points of contact with the substrate are of crucial importance for a correct prediction of the NP optical properties. The achieved accuracy of the calculations allows determining how many of the adsorbed molecules have a major contribution to the Raman signal, a fact that has important implications for analyzing experiments and designing sensing applications.

A 3D insight on the catalytic nanostructuration of few-layer graphene

The catalytic cutting of few-layer graphene is nowadays a hot topic in materials research due to its potential applications in the catalysis field and the graphene nanoribbons fabrication. We show here a 3D analysis of the nanostructuration of few-layer graphene by iron-based nanoparticles under hydrogen flow. The nanoparticles located at the edges or attached to the steps on the FLG sheets create trenches and tunnels with orientations, lengths and morphologies defined by the crystallography and the topography of the carbon substrate. The cross-sectional analysis of the 3D volumes highlights the role of the active nanoparticle identity on the trench size and shape, with emphasis on the topographical stability of the basal planes within the resulting trenches and channels, no matter the obstacle encountered. The actual study gives a deep insight on the impact of nanoparticles morphology and support topography on the 3D character of nanostructures built up by catalytic cutting.

Template-free synthesis of novel, highly-ordered 3D hierarchical Nb3O7(OH) superstructures with semiconductive and photoactive properties

3D hierarchical Nb3O7(OH) mesocrystals can be formed by self-organization from nanometer sized building blocks. The present study focuses on the synthesis and detailed investigation of mesocrystals, which can be achieved from a one-step, template-free hydrothermal synthesis approach. The obtained cubic superstructures consist of a periodic nanowire-network and combine a large surface area, high crystallinity, with a band gap of 3.2 eV and photocatalytic activity. Their easy processability in combination with the named excellent properties makes them promising candidates for a large number of applications. These include photochemical and photophysical devices where the Nb3O7(OH) mesocrystals can be used as electrode material since they are semiconducting and possess a large surface area. Generally the forces involved in the self-organized formation of mesocrystals are not fully understood. In this regard, the assembly of the Nb3O7(OH) mesocrystals was investigated in-depth applying transmission electron microscopy, scanning electron microscopy, UV/Vis measurements and electron energy-loss spectroscopy. Based on the achieved results a formation mechanisms is proposed, which expands the number of mechanisms for mesocrystal formation reported in literature. In addition, our study reveals different types of nanowire junctions and investigates their role at the stabilization of the networks.

Nanoprobe crystallographic orientation studies of isolated shield elements of the coccolithophore species Emiliania huxleyi

Coccolithophore algae produce elaborately structured skeletons composed of sub-micrometer-scale calcite crystals. In order to understand calcite crystallization and assembly in a coccosphere with nanoscale resolution, the crystal orientation and interdigitation of the structural units were investigated by transmission electron microscopy imaging, selected-area and nano-probe electron diffraction. Focused ion beam sectioning of coccoliths of the coccolithophore species Emiliania huxleyi is used to obtain target-prepared specimens in suitable orientation. We were able to detect and analyze the V-unit, which is overgrown by the R-unit. For the V-unit the 001 direction points perpendicular to the coccolith plane while the 110 axis is tangential to the coccolith ring. The R-unit c-axis is parallel and the b-axis is perpendicular to the coccolith plane, thus confirming the R- and V-model which was based on scanning electron microscopy and optical microscopy. Furthermore we show that the distal- and the proximal shield element of an individual R-unit of a single segment are tilted by 4 degrees +/- 1 degrees with respect to each other. This orientation change is required to obtain the flat domed character of the coccoliths, which is necessary to form the coccosphere. The orientation change between the distal- and the proximal shield element appears continuous.