Showing papers by "Jakob Birkedal Wagner published in 2009"

Giant, level-dependent g factors in InSb nanowire quantum dots.

Abstract: We report on magnetotransport measurements on InSb nanowire quantum dots. The measurements show that the quantum levels of the InSb quantum dots have giant g factors, with absolute values up to approximately 70, the largest value ever reported for semiconductor quantum dots. We also observe that the values of these g factors are quantum level dependent and can differ strongly between different quantum levels. The presence of giant g factors indicates that considerable contributions from the orbital motion of electrons are preserved in the measured InSb nanowire quantum dots, while the level-to-level fluctuations arise from spin-orbit interaction. We have deduced a value of Delta(SO) = 280 mueV for the strength of spin-orbit interaction from an avoided level crossing between the ground state and first excited state of an InSb nanowire quantum dot with a fixed number of electrons.

Junctions in axial III-V heterostructure nanowires obtained via an interchange of group III elements.

Abstract: We present an investigation of the morphology and composition of novel types of axial nanowire heterostructures where GaxIn1-xAs is used as barrier material in InAs nanowires. Using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray analysis we demonstrate that it is possible to grow junctions by changing the group III elements, and we find that a substantial fraction of Ga can be incorporated in axial InAs/GaxIn1-xAs/InAs, retaining straight nanowire configurations. We explain how the adatoms are transferred to the incorporation site at the growth interface via two different routes, (1) interface diffusion and (2) volume diffusion through the catalyst particle.

In situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Transmission Electron Microscope

Abstract: Environmental transmission electron microscopy was used to characterize in situ the reduction and oxidation of nickel from a Ni/YSZ solid oxide fuel cell anode support between 300-500°C. The reduction is done under low hydrogen pressure. The reduction initiates at the NiO/YSZ interface, then moves to the center of the NiO grain. At higher temperature the reduction occurs also at the free NiO surface and the NiO/NiO grain boundaries. The growth of Ni is epitaxial on its oxide. Due to high volume decrease, nanopores are formed during reduction. During oxidation, oxide nanocrystallites are formed on the nickel surface. The crystallites fill up the nickel porosity and create an inhomogeneous structure with remaining voids. This change in structure causes the nickel oxide to expand during a RedOx cycle.

Microphotoluminescence studies of tunable wurtzite InAs0.85P0.15 quantum dots embedded in wurtzite InP nanowires

Abstract: We have investigated the effects of strong confinement in wurtzite InAs0.85P0.15 quantum dots embedded in wurtzite InP nanowires using microphotoluminescence spectroscopy. Strain-dependent k . p calculations were used to model the quantum dots, and it was found that the electron effective mass was increased by a factor of 2 and the band gap was increased by 190 meV compared to equivalent quantum dots in the zinc-blende polytype. Measurements indicate that there is a relaxation bottleneck giving rise to an anomalous state filling behavior.

Growth of vertical InAs nanowires on heterostructured substrates

Abstract: We investigate the Au-assisted growth of InAs nanowires on two different kinds of heterostructured substrates: GaAs/AlGaAs structures capped by a 50 nm thick InAs layer grown by molecular beam epitaxy and a 2 mu m thick InAs buffer layer on Si(111) obtained by vapor phase epitaxy. Morphological and structural properties of substrates and nanowires are analyzed by atomic force and transmission electron microscopy. Our results indicate a promising direction for the integration of III-V nanostructures on Si-based electronics as well as for the development of novel micromechanical structures incorporating nanowires as their active elements.

Controlled growth of InAs nanowires on engineered substrates

Abstract: We demonstrate the Au-assisted growth of semiconductor nanowires on different engineered substrates. Two relevant cases are investigated: GaAs/AlGaAs heterostructures capped by a $50 {\rm nm}$-thick InAs layer grown by molecular beam epitaxy and a $2 {\rm \mu m}$-thick InAs buffer layer on Si(111) obtained by vapor phase epitaxy. Morphological and structural properties of substrates and nanowires are analyzed by atomic force and transmission electron microscopy. Our results indicate a promising direction for the integration of III-V nanostructures on Si-based electronics as well as for the development of novel micromechanical structures.

The Titan Environmental Transmission Electron Microscope: Specifications, Considerations and First Results

Abstract: characterization techniques has increased dramatically. In electron microscopy, this demand constitutes an intrinsic challenge as the electron source requires high vacuum to function. Nevertheless, in the 1970’s, transmission electron microscopes (TEMs) were first adapted for use with gases [1]. Such machines are known as

In situ reduction and oxidation of nickel from solid oxide fuel cells in a Titan ETEM

Abstract: Solid Oxide Fuel Cells (SOFC) common technology is based on anode- supported cells composed of nickel-yttria stabilized zirconia (Ni-YSZ) cermet. The nickel is in oxide state (NiO) during SOFC production and is reduced to metallic nickel during the first operation. The microstructure influences the SOFC electrochemical performance as well as its stability for long-term use. Oxidation of the nickel catalyst can occur at high fuel utilization and due to air leakage. The volume change from Ni to NiO can be detrimental for the thin supported electrolyte. In situ transmission electron microscopy enables to acquire further knowledge on the mechanisms behind the reduction and oxidation of nickel in the Ni-YSZ SOFC anode. The in situ reduction and re- oxidation of the FIB prepared TEM lamellae is performed in a FEI Titan equipped with an environmental cell. The reduction started at 400°C under 1.4 mbar of hydrogen. The volume contraction due to NiO reduction is compensated by formation of nanoporosity in the Ni grain. Nanoporosity was previously observed by Waldbillig et al. during ex situ reduction of Ni-YSZ composite TEM lamellae.