Biomedical subjects

Werner Seifert

Publications and source records attributed to Werner Seifert.

9 recordsLinked to original sources

PubMed2006

Structural properties of <111>B -oriented III-V nanowires.

Controlled growth of nanowires is an important, emerging research field with many applications in, for example, electronics, photonics, and life sciences. Nanowires of zinc blende crystal structure, grown in the <111>B direction, which is the favoured direction of growth, usually have a large number of twin-plane defects. Such defects limit the performance of optoelectronic nanowire-based devices. To investigate this defect formation, we examine GaP nanowires grown by metal-organic vapour-phase epitaxy. We show that the nanowire segments between the twin planes are of octahedral shape and are terminated by {111} facets, resulting in a microfaceting of the nanowires. We discuss these findings in a nucleation context, where we present an idea on how the twin planes form. This investigation contributes to the understanding of defect formation in nanowires. One future prospect of such knowledge is to determine strategies on how to control the crystallinity of nanowires.

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PubMed2006

Au-free epitaxial growth of InAs nanowires.

III-V nanowires have been fabricated by metal-organic vapor-phase epitaxy without using Au or other metal particles as a catalyst. Instead, prior to growth, a thin SiOx layer is deposited on the substrates. Wires form on various III-V substrates as well as on Si. They are nontapered in thickness and exhibit a hexagonal cross-section. From high-resolution X-ray diffraction, the epitaxial relation between wires and substrates is demonstrated and their crystal structure is determined.

Arsenicals↗

PubMed2006

Phase segregation in AlInP shells on GaAs nanowires.

We have studied morphology and phase segregation of AlInP shells on GaAs nanowires. Photoluminescence measurements on single core-shell nanowires indicated variations in the shell composition, and phase segregation was confirmed by cross-sectional scanning transmission electron microscopy on 30 nm thin slices of the wires. It was discovered that Al-rich domains form in the <112> directions where two {110} shell facets meet during growth. We propose that the mechanism behind this phase segregation is a variation in the chemical potential along the circumference of the nanowire together with a difference in diffusion lengths for the different growth species. From the morphology of the core and the shell, we conclude that the side facet growth is temperature dependent forming {112}facets at low growth temperature and {110} facets at high growth temperature.

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PubMed2006

Position-controlled interconnected InAs nanowire networks.

We demonstrate here a method for controlled production of complex self-assembled three-dimensional networks of InAs nanowires on a substrate, based on sequentially seeded epitaxial nanowire structures, or "nanotrees". A position-controlled array of trunk nanowires is first produced using lithographically defined Au particles as seeds. With these wires positioned along the proper crystallographic directions with respect to each other, nanotree branches grow toward neighboring trunks, connecting them together. Finally, we investigate the crystal structure of the interconnected nanotrees, demonstrating that branch growth after the contact with the second trunk has an epitaxial relationship to that trunk.

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PubMed2005

Mass transport model for semiconductor nanowire growth.

We present a mass transport model based on surface diffusion for metal-particle-assisted nanowire growth. The model explains the common observation that for III/V materials thinner nanowires are longer than thicker ones. We have grown GaP nanowires by metal-organic vapor phase epitaxy and compared our model calculations with the experimental nanowire lengths and radii. Moreover, we demonstrate that the Gibbs-Thomson effect can be neglected for III/V nanowires grown at conventional temperatures and pressures.

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PubMed2005

Failure of the vapor-liquid-solid mechanism in Au-assisted MOVPE growth of InAs nanowires.

We report the temperature dependence of the Au-assisted growth of InAs nanowires in MOVPE. Extensive studies of the growth of such nanowires have attributed growth to the so-called vapor-liquid-solid (VLS) mechanism, with a liquid Au-In alloy particle. We assert here that growth is instead assisted by a solid particle and does not occur at all when the particle is a liquid. Thus the temperature range of InAs nanowire growth is limited by the melting of the Au-In alloy. Comparison with growth of InAs nanowires in the same system assisted by a layer of SiO(x) is used to support this conclusion.

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PubMed2005

Growth and optical properties of strained GaAs-GaxIn 1-x P core-shell nanowires.

We have synthesized GaAs-Ga(x)In(1-x)P (0.34 < x < 0.69) core-shell nanowires by metal-organic vapor phase epitaxy. The nanowire core was grown Au-catalyzed at a low temperature (450 degrees C) where only little growth takes place on the side facets. The shell was added by growth at a higher temperature (600 degrees C), where the kinetic hindrance of the side facet growth is overcome. Photoluminescence measurements on individual nanowires at 5 K showed that the emission efficiency increased by 2 to 3 orders of magnitude compared to uncapped samples. Strain effects on the band gap of lattice mismatched core-shell nanowires were studied and confirmed by calculations based on deformation potential theory.

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PubMed2004

Direct imaging of the atomic structure inside a nanowire by scanning tunnelling microscopy.

Semiconductor nanowires are expected to be important components in future nano-electronics and photonics. Already a wide range of applications has been realized, such as high-performance field-effect transistors, bio/chemical sensors, diode logics and single-nanowire lasers. As nanowires have small cross-sections and large surface-to-bulk ratios, their properties can be significantly influenced by individual atomic-scale structural features, and they can have properties or even atomic arrangements with no bulk counterparts. Hence, experimental methods capable of directly addressing the atomic-scale structure of nanowires are highly desirable. One such method is scanning tunnelling microscopy (STM), which, by direct imaging of the atomic and electronic structure of surfaces has revolutionized the perception of nanoscale objects and low-dimensional systems. Here we demonstrate how combining STM with an embedding scheme allows us to image the interior of semiconductor nanowires with atomic resolution. Defect structures such as planar twin segments and single-atom impurities are imaged inside a GaAs nanowire. Further, we image an intriguing GaAs nanowire that is separated into two distinct nanocrystallites along the growth direction of the wire.

Arsenicals↗

PubMed2004

Synthesis of branched 'nanotrees' by controlled seeding of multiple branching events.

The formation of nanostructures with controlled size and morphology has been the focus of intensive research in recent years. Such nanostructures are important in the development of nanoscale devices and in the exploitation of the properties of nanomaterials. Here we show how tree-like nanostructures ('nanotrees') can be formed in a highly controlled way. The process involves the self-assembled growth of semiconductor nanowires via the vapour-liquid-solid growth mode. This bottom-up method uses initial seeding by catalytic nanoparticles to form the trunk, followed by the sequential seeding of branching structures. Each level of branching is controlled in terms of branch length, diameter and number, as well as chemical composition. We show, by high-resolution transmission electron microscopy, that the branching mechanism gives continuous crystalline (monolithic) structures throughout the extended and complex tree-like structures. The controlled seeding method that we report here has potential as a generic means of forming complex branching structures, and may also offer opportunities for applications, such as the mimicking of photosynthesis in nanotrees.

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