Showing papers by "Margit Zacharias published in 2008"
TL;DR: It is shown, based on measurements in high magnetic fields, that defects are the dominant source of light from Si nanocrystals, and it is shown that it is possible to control the origin of the photoluminescence in a single sample.
Abstract: Silicon dominates the electronics industry, but its poor optical properties mean that III–V compound semiconductors are preferred for photonics applications. Photoluminescence at visible wavelengths was observed from porous Si at room temperature in 1990, but the origin of these photons (do they arise from highly localized defect states or quantum confinement effects?) has been the subject of intense debate ever since. Attention has subsequently shifted from porous Si to Si nanocrystals, but the same fundamental question about the origin of the photoluminescence has remained. Here we show, based on measurements in high magnetic fields, that defects are the dominant source of light from Si nanocrystals. Moreover, we show that it is possible to control the origin of the photoluminescence in a single sample: passivation with hydrogen removes the defects, resulting in photoluminescence from quantum-confined states, but subsequent ultraviolet illumination reintroduces the defects, making them the origin of the light again.
480 citations
TL;DR: The influence of an Al(2)O(3) shell on the photoluminescence emission of ZnO nanowires is reported and a model explaining the spectral changes is proposed.
Abstract: We report the influence of an Al2O3 shell on the photoluminescence emission of ZnO nanowires. At room temperature, the spectrum of the core?shell nanowires shows a strong reduction of the relative intensity of the green defect emission with respect to the near-band-edge emission. At 5?K an increase of the relative intensity of the surface exciton band with respect to the donor-bound exciton emission is observed. Annealing the core?shell nanowires at 500??C does not increase the green defect luminescence at 5?K. We propose a model explaining the spectral changes.
169 citations
TL;DR: In this paper, the influence of temperature on the Kirkendall effect-related evolution of coaxial ZnO/Al2O3 one-dimensional heterostructures based on a solid-state reaction is presented.
Abstract: In this paper, we present the influence of temperature on the Kirkendall effect-related evolution of coaxial ZnO/Al2O3 one-dimensional heterostructures based on a solid-state reaction. Controllable fabrication of spinel ZnAl2O4-based nanotubes and porous nanowires can step-by-step be achieved from ZnO/Al2O3 core−shell nanowires with a change in the reaction temperature. In particular, we demonstrate that the formation of completely hollow nanotubes in this system is not strictly limited to a fixed stoichiometry of ZnO/Al2O3 required for the spinel-forming reaction when an annealing temperature of 800 °C is employed. Combined with atomic layer deposition, the wall thickness of the formed nanotubes can be further precisely tailored and defined. Our finding provides an effective route to fabricate spinel nanotubes as well as nanotube arrays on a large scale.
73 citations
TL;DR: Uniform Si-CdSSe core/shell nanowires were controllably synthesized by a multisource thermal evaporation route and may have potential applications in multicolor nanoscaled light-emitting devices.
Abstract: Uniform Si-CdSSe core/shell nanowires were controllably synthesized by a multisource thermal evaporation route. Both the silicon core and the alloyed CdSSe shell are of high-quality and single crystalline. The silicon core is grown via the gold-catalyzed VLS route with a silicon wafer piece at the high temperature zone as the source. These preferentially grown Si nanowires further serve as templates for the afterward depositions of CdSSe shells using CdS/CdSe powders at the low temperature zone of the furnace as sources. The composition/band gap of the shells can be continuously modulated by the S/Se ratio of the evaporation sources, making these prepared heterostructures have strong and spectral position/color largely tunable light emission at the visible region. These kind of structures may have potential applications in multicolor nanoscaled light-emitting devices. This flexible growth route will also be applicable for controllable synthesis of other Si wire containing heterostructures.
61 citations
TL;DR: In this paper, the authors presented novel 3D hierarchical ZnO and ZnAl2O4 nanostructures produced by multitransformation reactions using Zn as the starting material.
Abstract: We present novel three-dimensional (3D) hierarchical ZnO and ZnAl2O4 nanostructures produced by multitransformation reactions using Zn as the starting material. In two representative instances selected for illustrating this strategy, various Zn precursors like Zn nanowires and polyhedral Zn microcrystals were first prepared by physical vapor deposition without any catalyst. Subsequently, we successfully synthesized branched ZnO nanowires and hollow ZnO polyhedrons with vertically aligned ZnO nanowires by oxidizing Zn nanowires and polyhedral Zn microcrystals, respectively, under optimal conditions. The growth of both ZnO nanobranches and vertically aligned nanowires was suggested as a self-supplying and self-catalytic process following the vapor transport solid condensation mechanism. Furthermore, the obtained ZnO nanostructures were homogeneously covered by a shell of amorphous Al2O3 using atomic layer deposition. Through the solid–solid interfacial reaction of ZnO and Al2O3, the 3D nanostructures were t...
52 citations
TL;DR: Semiconductingnanowires have many uniquefeature properties required for potential applications in future electronics andoptical devices, and nanowires with reliable functionality can be incorporated into advanced electronic devices.
Abstract: Semiconductingnanowireshavemanyuniquefeaturesthatarerequired for potential applications in future electronics andoptoelectronics. Nanowires with reliable functionality can beincorporated into advanced electronic devices. Much efforthas been devoted to engineering nanowire properties, forexample, to achieve segmented doping along a nanowire andprecise controlling over interfaces within a heterostructurenanowire.
47 citations
TL;DR: In this paper, an irregular structured shell composed of ZnO nanocrystals was deposited on pristine nanotubes, while a highly defined znO shell was placed on the tubes after its functionalization with Al2O3.
Abstract: Carbon nanotubes and ZnO based functional coaxial heterostructured nanotubes have been fabricated by using atomic layer deposition. An irregular structured shell composed of ZnO nanocrystals was deposited on pristine nanotubes, while a highly defined ZnO shell was deposited on the tubes after its functionalization with Al2O3. Photoluminescence measurements of the ZnO shell on Al2O3/nanotube show a broad green band emission, whereas the shell grown on the bare nanotube shows a band shifted to the orange spectral range.
38 citations
TL;DR: In this paper, the electrical properties of individual as-grown ZnO nanowires (NWs) on SiO2∕Si and GaN/sapphire substrates were reported.
Abstract: We report the electrical properties of individual as-grown ZnO nanowires (NWs) on SiO2∕Si and GaN/sapphire substrates. Carrier transport in metal-semiconductor-metal junction is interpreted in terms of thermionic emission and tunneling current which was assumed to dominate independently on reverse and forward contacts. Current flow in ZnO NWs grown on SiO2∕Si was dominated by thermionic emission. In contrast, both thermionic emission and tunneling current contributed to the transport in ZnO NWs epitaxially grown on GaN/sapphire. Carrier concentrations of ZnO NWs were estimated to be around 8.88×1017cm−3 if grown on SiO2∕Si and 1.18×1018cm−3 if grown on GaN/sapphire, respectively.
32 citations
TL;DR: Transmission electron microscopy (TEM) imaging is one of the primary tools for assessing core/shell 1D nanoscale heterostructures and clear evidence of this stability was shown from studies of the structural degradation of ZnS nanotubes during electron-beam irradiation in a TEM.
Abstract: One-dimensional (1D) nanoscale heterocomposites are currently receiving much attention due to their tunable chemical reactivity and physical properties. An attractive approach for producing coaxial core/shell 1D nanoscale heterostructures involves coating the nanowires in a conformal metal-oxide concentric layer via atomic-layer deposition (ALD). Encapsulation of ZnO nanowires by an ALD-deposited Al2O3 insulator layer has been widely used to prepare ZnO/Al2O3 core/shell nanowires. [5–8] Such nanostructures attract particular interest because of their possible applications in dye-sensitized solar cells and in the fabrication of other functional nanomaterials, such as wall-thickness-controllable Al2O3 nanotubes [6] and well-crystallized 1D ZnAl2O4 nanostructures. [7,8] Transmission electron microscopy (TEM) imaging is one of the primary tools for assessing core/shell 1D nanoscale heterostructures. Differences in electron density in the samples lead to contrast changes in the TEM images, from which the coaxial core/shell structure of ZnO/Al2O3 nanowires can be confirmed. ZnO is usually considered a fairly stable material under electron-beam irradiation on account of its high decomposition temperature (1975 8C) and relatively high Zn–O bond energy (2.94 eV/284.1 kJ mol ). Moreover, ZnO presents very little redox activation in vacuum or atmosphere at moderate temperatures. Clear evidence of this stability was shown from studies of the structural degradation of ZnS nanotubes during electron-beam irradiation in a TEM
31 citations
TL;DR: In this article, an extensive electron spin resonance (ESR) analysis has been carried out on structures comprised of Si nanoparticles embedded in a regular pattern in an amorphous SiO2 matrix, fabricated by the SiO/SiO2 superlattice approach, with the intent to reveal and quantify occurring paramagnetic defects.
Abstract: An extensive electron spin resonance (ESR) analysis has been carried out on structures comprised of Si nanoparticles (∼2 nm across) embedded in a regular pattern in an amorphous SiO2 matrix, fabricated by the SiO/SiO2 superlattice approach, with the intent to reveal and quantify occurring paramagnetic defects. The as-grown state is found to exhibit only a Si dangling bond (DB) signal, which through combination of first and second harmonic X-, K-, and Q-band observations in combination with computer spectra simulation, could be conclusively disentangled as solely comprised of overlapping powder pattern spectra of Pb(0) and Pb1 defects, the archetypal intrinsic defects of the Si/SiO2 interface, with no evidence for a D line (Si DBs in disordered Si). This indicates a full crystalline system of randomly oriented Si nanocrystals (NCs). The Pb(0)/Pb1 defect system, pertaining to the NC-Si/SiO2 interfaces, is found to be both qualitatively and quantitatively much alike that of standard (high-quality) thermal Si...
28 citations
TL;DR: In this article, a K-and Q-band electron spin resonance study was performed on structures composed of Si nanoparticles (∼2nm across) embedded in a SiO2 matrix obtained by annealing of SiO∕SiO2 superstructures.
Abstract: A K- and Q-band electron spin resonance study, combining two detection modes, on structures composed of Si nanoparticles (∼2nm across) embedded in a SiO2 matrix obtained by annealing of SiO∕SiO2 superstructures unveils the observed principal Si dangling bond signal as solely comprised of the intrinsic Pb(0) and Pb1 Si∕SiO2 interface defect spectra, of comparable densities. Based on independent particle density counting, about one Pb-type defect is found at every 1.4 Si nanoparticles. Relying on the known properties of Pb-type defects in standard microscopic Si∕SiO2, the data would comply with Si nanocrystallites with morphology, schematically, of [100] truncated (111) octahedrons. Both types and densities of observed point defects indicate a structure of quality much alike that of standard thermal Si∕SiO2.
TL;DR: In this paper, single-crystal ZnO nanowires are reacted at 800−900 °C in vacuum with alumina vapor generated by electron beam evaporation, and the morphology changes after the solid-vapor reactions are studied in detail using electron microscopy and compared to other similar spinel nanostructures.
Abstract: Single-crystal ZnO nanowires are reacted at 800−900 °C in vacuum with alumina vapor generated by electron beam evaporation. The morphology changes after the solid−vapor reactions are studied in detail using electron microscopy and compared to other similar spinel nanostructures. Unlike other solid−vapor reactions like MgO−Al2O3 and ZnO−Ga2O3 where a continuous spinel layer is formed, the reaction of ZnO nanowires with alumina vapor is unique. The initially smooth surfaces of ZnO nanowires become rugged due to surface decomposition without the growth of spinel layers. A formation mechanism is proposed that the surface reaction of ZnO with alumina vapor might constitute a process of a unilateral transport of ZnO and the associated surface diffusion.
TL;DR: A typing error unfortunately led to a wrong expression on page 1406 in the paragraph between equations (1) and (2) as discussed by the authors, which was later corrected by the author.
Abstract: A typing error unfortunately lead to a wrong expression on page 1406 in the paragraph between equations (1) and (2).
TL;DR: In this article, the authors studied the typical lasing dynamics after pulsed excitation, which is caused by the interplay of gain competition among several lasing modes, the bandgap renormalization, and the carrier-density-dependent refractive index.
Abstract: ZnO nanorods grown via vapor transport methods have been studied under high optical excitation. Via a focused ion beam (FIB), particular nanorods have been marked on the substrate for optical measurements on as-grown single rods. In the particular nanorods, stimulated emission occurs above a certain excitation threshold. A time-resolved measurement of the single nanorod emission is accomplished by the use of a streak-camera. We focus on the typical lasing dynamics, which is found in these measurements on several samples. We explain the complex dynamics after pulsed excitation, which is caused by the interplay of gain competition among several lasing modes, the bandgap renormalization, and the carrier-density-dependent refractive index.
TL;DR: In this paper, the authors verandert sich die Diffusion, wenn die diffundierenden Atome in Nanokugeln oder Nanodrahten eingeschlossen sind.
Abstract: Beim Ubergang vom Mikro- zum Nanobereich konnen altbekannte Effekte plotzlich ganz erstaunliche Auswirkungen haben. So verandert sich die Diffusion, wenn die diffundierenden Atome in Nanokugeln oder Nanodrahten eingeschlossen sind. Dieser Effekt hat es unserer Gruppe am Max-Planck-Institut fur Mikrostukturphysik in Halle und der Universitat Freiburg ermoglicht, mikrometerlange Hohlraume mit Durchmessern von 30 bis 100 nm in Nanodrahten definiert zu erzeugen.