The structure of GP-zones in an industrial, 7xxx-series Al–Zn–Mg alloy has been investigated by transmission electron microscopy methods: selected area diffraction, conventional and high-resolution ...imaging. Two types of GP-zones, GP(I) and (II) are characterized by their electron diffraction patterns. GP(I)-zones are formed over a wide temperature range, from room temperature to 140–150°C, independently of quenching temperature. The GP(I)-zones are coherent with the aluminum matrix, with internal ordering of Zn and Al/Mg on the matrix lattice, suggested to be based on AuCu(I)-type sub-unit, and anti-phase boundaries. GP(II) are formed after quenching from temperatures above 450°C, by aging at temperatures above 70°C. The GP(II)-zones are described as zinc-rich layers on {111}-planes, with internal order in the form of elongated domains. The structural relation to the η′-precipitate is discussed.
Semiconductor nanowires of III–V materials have generated much interest in recent years. However, the growth mechanisms by which these structures form are not well understood. The so‐called ...vapor–liquid–solid (VLS) mechanism has often been proposed for III–V systems, with a chemically inert, liquid metal particle (typically Au) acting as a physical catalyst. We assert here that Au is, in fact, not inert with respect to the semiconductor material but rather interacts with it to form a variety of intermetallic compounds. Moreover, we suggest that III–V nanowire growth can best be understood if the metallic particle is not a liquid, but a solid‐phase solution or compound containing Au and the group III material. The four materials GaP, GaAs, InP, and InAs will be considered, and growth behavior related to their particular temperature‐dependent interaction with Au.
The growth behavior of III–V semiconductor nanowires and nano‐ trees assisted by Au particles is considered (see Figure). The particle is not inert with respect to the semiconductor, as often assumed, but interacts with it. The growth and morphology of these structures are determined by the temperature‐dependent solid‐phase interaction between the Au particle and each of the III–V materials discussed, which include GaP, GaAs, InP, and InAs.
We report growth of one-dimensional semiconductor nanocrystals, nanowhiskers, in which segments of the whisker with different composition are formed, illustrated by InAs whiskers containing segments ...of InP. Our conditions for growth allow the formation of abrupt interfaces and heterostructure barriers of thickness from a few monolayers to 100s of nanometers, thus creating a one-dimensional landscape along which the electrons move. The crystalline perfection, the quality of the interfaces, and the variation in the lattice constant are demonstrated by high-resolution transmission electron microscopy, and the conduction band off-set of 0.6 eV is deduced from the current due to thermal excitation of electrons over an InP barrier.
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.
We present the growth of homogeneous InAs(1-x)P(x) nanowires as well as InAs(1-x)P(x) heterostructure segments in InAs nanowires with P concentrations varying from 22% to 100%. The incorporation of P ...has been studied as a function of TBP/TBAs ratio, temperature, and diameter of the wires. The crystal structure of the InAs as well as the InAs(1-x)P(x) segments were found to be wurtzite as determined from high-resolution transmission electron microscopy. Furthermore, temperature-dependent electrical transport measurements were performed on individual heterostructured wires to extract the conduction band offset of InAs(1-x)P(x) relative to InAs as a function of composition. From these measurements we extract a value of the linear coefficient of the conduction band versus x of 0.6 eV and a nonlinear coefficient, or bowing parameter, of 0.2 eV. Finally, homogeneous InAs(0.8)P(0.2) nanowires were shown to have a nondegenerate n-type doping and function as field-effect transistors at room temperature.
The structure of the
η′ phase, one of the most important age-hardening precipitates in commercial Al–Zn–Mg alloys, has been studied at the atomic level by means of high-resolution electron microscopy ...(HREM). A structural model of the
η′ phase has been constructed on the basis of the structural characteristics in the observed images and the structure of the
η-MgZn
2 phase. Image simulation of this model shows a good agreement between calculated and experimental images. Comparison of this model with the early existing model on the basis of the X-ray diffraction is also given.
Controllable production of nanometre-sized structures is an important field of research, and synthesis of one-dimensional objects, such as nanowires, is a rapidly expanding area with numerous ...applications, for example, in electronics, photonics, biology and medicine. Nanoscale electronic devices created inside nanowires, such as p-n junctions, were reported ten years ago. More recently, hetero-structure devices with clear quantum-mechanical behaviour have been reported, for example the double-barrier resonant tunnelling diode and the single-electron transistor. The generally accepted theory of semiconductor nanowire growth is the vapour-liquid-solid (VLS) growth mechanism, based on growth from a liquid metal seed particle. In this letter we suggest the existence of a growth regime quite different from VLS. We show that this new growth regime is based on a solid-phase diffusion mechanism of a single component through a gold seed particle, as shown by in situ heating experiments of GaAs nanowires in a transmission electron microscope, and supported by highly resolved chemical analysis and finite element calculations of the mass transport and composition profiles.
Organic-shell-free PbS nanoparticles have been produced in the size range relevant for quantum-dot solar cells (QDSCs) by a vapor aggregation method involving magnetron reactive sputtering. This ...method creates a beam of free 5-10 nm particles in a vacuum. The dimensions of the particles were estimated after their deposition on a substrate by imaging them using
ex situ
SEM and HRTEM electron microscopy. The particle structure and chemical composition could be deduced "on the fly", prior to deposition, using X-ray photoelectron spectroscopy (XPS) with tunable synchrotron radiation. Our XPS results suggest that under certain conditions it is possible to fabricate particles with a semiconductor core and 1 to 2 monolayer shells of metallic lead. For this case the absolute energy of the highest occupied molecular orbital (HOMO) in PbS has been determined to be (5.0 ± 0.5) eV below the vacuum level. For such particles deposited on a substrate HRTEM has confirmed the XPS-based conclusions on the crystalline PbS structure of the semiconductor core. Absorption spectroscopy on the deposited film has given a value of ∼1 eV for the lowest exciton. Together with the valence XPS results this has allowed us to reconstruct the energy level scheme of the particles. The results obtained are discussed in the context of the properties of PbS QDSCs.
Metal-passivated PbS nanoparticles promising to improve carrier transport in quantum-dot solar cells are produced and are characterized layer-by-layer.
