Z. R. Dai

Side-by-side silicon carbide–silica biaxial nanowires: Synthesis, structure, and mechanical properties

Elevated-temperature synthesis has been used to generate side-by-side biaxially structured silicon carbide–silica nanowires. The axial growth direction approaches [311] for nanowires with a high density of microtwins and is [211] for defect-free nanowires. The structure of these nanowires, their cross-sectional shape, and their structural transformation between a biaxial and coaxial configuration have been studied by transmission electron microscopy. The Young’s modulus of the biaxially structured nanowires was measured to be 50–70 GPa depending on the size of the nanowire.

Exchange-coupled FePt nanoparticle assembly

We have produced exchange-coupled FePt nanoparticle assemblies by chemical synthesis and subsequent thermal annealing. As the interparticle distances decrease by tuning the annealing conditions, interparticle interactions change from dipolar type to exchange type, and the magnetization reversal mechanism switches from rotation controlled to domain-nucleation controlled. With increasing annealing temperature, the coercivity first increases due to improved chemical ordering, and then drops significantly, resulting from excessive interparticle exchange coupling. For the samples exhibiting exchange coupling, both the remanence ratio and coercive squareness increase.

Lead oxide nanobelts and phase transformation induced by electron beam irradiation

β-PbO2 nanobelts, with a rectangular cross section, a typical length of 10–200 μm, a width of 50–300 nm, and a width-to-thickness ratio of 5–10, have been successfully synthesized by simple elevated evaporation of commercial PbO powders at high temperature. The PbO2 nanobelts are enclosed by top surfaces ±(201) and side surfaces ±(101) and their growth direction is [010]. Each PbO2 nanobelt is found to have a large polyhedral Pb tip at one of its ends, suggesting the growth is dominated by a vapor–liquid–solid mechanism. Electron beam irradiation of the PbO2 nanobelts results in the phase transformation from PbO2 to PbO and finally to Pb.

Mechanical and electrostatic properties of carbon nanotubes and nanowires

Nano-scale manipulation and property measurements of individual nanowire-like structure is challenged by the small size of the structure. Scanning probe microscopy has been the dominant tool for property characterizations of nanomaterials. We have developed an alternative novel approach that allows a direct measurement of the mechanical and electrical properties of individual nanowire-like structures by in situ transmission electron microscopy (TEM). The technique is unique in a way that it can directly correlate the atomic-scale microstructure of the nanowire with its physical properties. This paper reviews our current progress in applying the technique in investigating the mechanical and electron field emission properties of carbon nanotubes and nanowires.

Shapes, multiple twins and surface structures of monodisperse FePt magnetic nanocrystals

Shapes, surface atomic arrangement and structural evolution induced by annealing of monodisperse FePt nanocrystals, synthesized by a solution phase chemical procedure, have been studied by high-resolution transmission electron microscopy. The as-synthesized FePt nanocrystals display dominantly a truncated octahedron shape enclosed by flat {1 0 0}, stepped {1 1 1} and zig-zag {1 1 0} facets. The Marks decahedron FePt nanocrystals and the icosahedron based multiply twined FePt nanocrystals are identified in the as-synthesized particles. An improved structural model has been proposed for the multiply twined nanocrystals. After annealing, the {1 1 0} facets disappear and a regular cuboctahedron becomes the dominant shape. Surfaces of the FePt nanocrystals show no reconstruction but with some atomic steps and kinks. 2002 Elsevier Science B.V. All rights reserved.

Synthesis, chemical ordering, and magnetic properties of FePtCu nanoparticle films

FePtCu nanoparticles with varying composition were prepared by the simultaneous polyol reduction of platinum acetylacetonate and copper bis(2,2,6,6-tetramethyl-3,5-heptanedionate) and the thermal decomposition of iron pentacarbonyl. As prepared the particles had a fcc structure with an average diameter of 3.5 nm and were superparamagnetic. Heat treatment of the self-assembled films at temperatures above 550 °C transformed the particles from the fcc to the L10 phase, give in-plane coercivities as high as 9000 Oe. X-ray diffraction revealed that the Cu remained in the films and the presence of an extra peak, indicating a second phase was present. Consistent with two or more phases, the magnetic hysteresis curves could be decomposed into a hard component (Hc>5000 Oe) and a soft component (Hc<2000 Oe). Unlike our earlier results for Ag in FePt, adding Cu to FePt did not lower the temperature required for phase transformation from the fcc to the L10 phase.

The crystal structure and growth direction of Cu2S nanowire arrays fabricated on a copper surface

We examine the crystal structure and growth direction of Cu2S nanowire arrays grown from copper surfaces at room temperature. By using X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction (ED) techniques, the monoclinic Cu2S nanowires are shown to grow preferentially along the c-axis, characterized by a layered structure. This result may shed light on the unusual growth mechanism of the Cu2S nanowires.

Structures of Oxide Nanobelts and Nanowires

We have recently reported the synthesis of one-dimensional nanobelt structures of ZnO, SnO2, In2O3, CdO, Ga2O3, and PbO2 by evaporating the desired commercial metal oxide powders at high temperatures (Science (2001), 291, 1947). The as-synthesized oxide nanobelts are pure, structurally uniform, single crystalline, and most of them free from dislocations. The beltlike morphology appears to be a unique and common structural characteristic for the family of semiconducting oxides. In the present article, we focus on the twin and stacking fault planar defects found in oxide nanobelts and nanowires although they are rarely observed. Some interesting and unique growth morphologies are presented to illustrate the roles played by surface energy and kinetics in growth. It is shown that the surfaces of the oxide nanobelts are enclosed by the low-index, low-energy crystallographic facets. The growth morphology is largely dominated by the growth kinetics.

Nanocomposite (Nd, Dy)(Fe, Co, Nb, B)5.5/α-Fe multilayer magnets with high performance

The structural and magnetic properties of rare-earth nanocomposite magnets of a (Nd,Dy)(Fe,Co,Nb,B)5.5 single layer and a (Nd,Dy)(Fe,Co,Nb,B)5.5/α-Fe multilayer prepared by sputtering and heat treatment have been investigated. Incomplete exchange coupling behaviour is observed in the Ti-buffered NdDyFeCoNbB + 40 wt%Fe single layer magnet. After annealing, the laminated nanostructure consisting of mutually dispersed soft and hard phases results in remanence enhancement. The optimum magnetic properties of Jr = 1.11 T, μ0iHc = 0.88 T and (BH)max = 192 kJ m−3 are achieved in the multilayer magnet annealed at 550 C for 30 min. The designed multilayer film with thinner soft magnetic layers may favour the formation of more ideal nanostructures for exchange coupling between softand hard-magnetic phases. Experimental researches on nanostructured exchange-coupled magnets have been performed since 1988 [1]. Taking advantage of high coercivity and high magnetization contributed by hardand soft-magnetic components, respectively, high remanence and a large maximum energy product would be obtained if a full exchange coupling existing between the grains of the two phases in nanocomposite magnets, as predicted based on micromagnetic calculation [2–4]. However, up to now, the energy products of the rare-earth nanocomposite magnets prepared by means of rapid quenching and mechanical alloying have been much lower than the theoretical expectation, due to difficulties in controlling the nanostructures [5–8]. 5 Also at: International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China. 6 Author to whom any correspondence should be addressed. Recently, some studies on exchange coupling were carried out for nanostructured CoSm/FeCo and PrCo/Co multilayers prepared by sputtering and subsequent heat treatment [9, 10]. The magnetic properties of exchange-coupled α-Fe/Nd–Fe–B multilayer magnets were investigated by Shindo [11] and observations for Nd–Fe–B/Fe/Nd–Fe–B trilayers were reported by Parhofer and Yang et al [12–14]. In our previous study [15], we investigated the magnetic properties of nanocomposite multilayer magnets of NdDyFeCoNbB/M (M = Co, Fe65Co35) on Ti-buffered Si substrates prepared by sputtering and subsequent heat treatment. In comparison with the single layer, the remanence of the multilayer magnets was found to increase noticeably. In this letter, we report the structural and magnetic properties of nanocomposite (Nd,Dy)(Fe,Co,Nb,B)5.5/Fe multilayer magnets synthesized 0022-3727/03/170067+04

Synthesis, Self-assembly and Magnetic Properties of FePtCu Nanoparticle Arrays

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