Kwokkwong Fung

Microstructure and growth of bamboo-shaped carbon nanotubes

Bamboo-shaped carbon nanotubes fabricated by the catalytic growth method with Ni/metallic oxide catalyst are systematically studied using transmission electron microscopy. It is found that except for the nodes the bamboo-shaped carbon nanotubes can be regarded as stacking of truncated conical graphene sheets. Depending on the size of a catalyst, the Ni particles can be encapsulated inside the tubes or at the growth front of the tubes with a plug-shape. A linear relationship between size of the nanotube and the catalyst particle is found. The results suggest that the nanometer-sized Ni catalyst particles are in the liquid state during nanotube growth. A simple model for the growth of bamboo-shaped nanotubes is proposed.

Giant exchange bias and the vertical shifts of hysteresis loops in γ-Fe2O3-coated Fe nanoparticles

We fabricated core/shell-structured Fe nanoparticles, in which the α-Fe core is about 5 nm in diameter and the γ-Fe2O3 shell is about 3 nm thick, and systematically studied their structural and magnetic properties. The magnetic hysteresis (M–H) loops, measured at low temperatures, after the particles were cooled from 350 K in a 50 kOe field, show significant shifts in both horizontal and vertical directions. It has been found that the exchange-bias field can be as large as 6.3 kOe at 2 K, and that the coercive field is also enhanced greatly in the field-cooled (FC) loops. The large exchange bias and vertical shifts of the FC loops at low temperatures may be ascribed to the frozen spins in the shells. A simple model is proposed to interpret the observations.

Passivation of α-Fe nanoparticle by epitaxial γ-Fe2O3 shell

Abstract Nanoparticles of iron prepared by inert gas condensation of plasma evaporated vapour exhibit remarkable resistance to oxidation. They remain rust free in air and in water for years. We have found by transmission electron microscopy and X-ray photoelectron spectroscopy, that all the passivated nanoparticles of iron are covered by an epitaxial shell of γ-Fe 2 O 3 about 4 nm thick. The epitaxial relationship between the γ-Fe 2 O 3 shell and the iron core is (001) γ -Fe 2 O 3 //(001) α -Fe , and [110] γ -Fe 2 O 3 //[100] α -Fe , [ 1 10] γ -Fe 2 O 3 //[010] α -Fe . The passivation of the nanoparticles of iron by an epitaxial oxide can be accounted for by the Caberra–Mott theory of oxidation of metal. The oxide layer grows rapidly at 420 K but slows down dramatically when the layer thickens. When the oxide layer thickens to 4 nm in a few hours, growth virtually stops. The 4-nm epitaxial oxide shell protects the iron core from further oxidation at room temperature.

High-Quality ZnO Nanowire Arrays Directly Fabricated from Photoresists

We report a simple and effective method for fabricating and patterning high-quality ZnO nanowire arrays using carbonized photoresists to control the nucleation site, density, and growth direction of the nanowires. The ZnO nanowires fabricated using this method show excellent alignment, crystal quality, and optical properties that are independent of the substrates. The carbonized photoresists provide perfect nucleation sites for the growth of aligned ZnO nanowires and they also perfectly connect to the nanowires to form ideal electrodes that can be used in many applications of ZnO nanomaterials.

Chemical synthesis of narrowly dispersed SmCo5 nanoparticles

In this article we report a chemical synthetic means for generating a high Ku magnetic material—narrowly dispersed nanoparticles of SmCo5. Using Co2(CO)8 and Sm(acac)3 as the precursors under air-free conditions, we produced SmCo5 nanoparticles according to the procedure reported by Sun et al. [Science 287, 1981 (2000)] but with some modifications. The nanoparticles, with diameters of 6–8 nm, have a SmCo5 composition, as indicated by transmission electron microscopy, electron diffraction, and x-ray photoelectron spectroscopy. The magnetization measurement of the nanoparticles, exhibits superparamagnetism, which is blocked for temperatures below ∼110 K, suggesting Ku to be ∼2.1×106 erg/cm3 for the as-prepared particles.

Catalytic synthesized carbon nanostructures from methane using nanocrystalline Ni

Abstract Carbon nanostructures synthesized with nanocrystalline Ni catalyst from decomposition of methane are investigated by means of transmission electron microscopy (TEM). Two kinds of carbon nanostructures, carbon fibers and bamboo-shaped carbon nanotubes, are observed. The preferential growth direction of graphene sheets depends on the reaction conditions. The bamboo-shaped carbon nanotubes can be obtained only if the reaction temperature is higher than 1000 K, and carbon fibers can be obtained at lower temperatures. The role and state of the catalyst particles are also discussed.

Optimizing nanosheet-based ZnO hierarchical structure through ultrasonic-assisted precipitation for remarkable photovoltaic enhancement in quasi-solid dye-sensitized solar cells

For ZnO hierarchical structures composed of interlaced nanosheets, it has been proved that they are more favorable for electron transportation in the photoanodes of ZnO-based dye-sensitized solar cells (DSCs). Here, we introduce ultrasonic-assisted precipitation for fabricating novel nanosheet-based ZnO hierarchical flowers (HFs) in aqueous solution. With the powerful ultrasound irradiation, these nanosheets on the HFs are not only interlaced and monocrystalline, but also axially oriented, porous and ultrathin. Furthermore, broad channels enclosed by adjacent nanosheets can deeply extend into the inner parts of the HFs. Structural improvements reveal that the specific area of the novel HFs as well as their performances on light-capturing and electron transport have been largely improved compared with those prepared through direct precipitation. Remarkably, when assembled into quasi-solid DSCs, ZnO HF photoanodes show a high conversion efficiency up to 6.19% (under AM 1.5, 100 mW cm−2 illumination), the highest record of quasi-solid ZnO-based DSCs up to now.

On the origin of the giant Hall effect in magnetic granular metals

A large, by a factor of ≈103−104, enhancement of Hall effect in cosputtered magnetic granular metals such as (NiFe)x-(SiO2)1−x near the metal-insulator transition has been termed giant Hall effect (GHE). It is associated with a high resistivity which increases slowly with decreasing temperature. We suggest that particle-size distribution which is singular at zero and quantum size effects may be responsible for these phenomena. From our scaling analysis of the relations between resistivity, and the ordinary and extraordinary Hall effects, it follows that the extraordinary Hall resistivity ϱxys scales with mobility as-ϱxys ∝ μ−y with the exponent γ ≈ 0.5, which is characteristically different from the predictions of both the skew scattering (γ ≈ 1) and the side jump (γ ≈ 2) theories in homogeneous ferromagnets.

Vertically aligned ZnO/amorphous-Si core–shell heterostructured nanowire arrays

We report the synthesis of vertically aligned ZnO/a-Si core-shell nanowire arrays (ZnO nanowires coated with amorphous silicon) through chemical vapor deposition. The core-shell heterostructured nanowires possessed uniform morphology and the thickness of the amorphous silicon shells could be controlled easily by tuning the deposition duration and temperature. The core-shell heterostructured nanowires exhibited enhanced antireflection and absorption performance as well as tunable PL properties. Because the individual ZnO/a-Si nanowires showed p-type characteristics and the ZnO cores were n-type semiconductors, the core-shell nanowires formed p-n junctions naturally.

Long-range exchange coupling between a ferromagnet and an antiferromagnet across a nonmagnetic spacer layer

The antiferromagnet/ferromagnet exchange coupling giving rise to a shifted hysteresis loop has usually been considered an interfacial effect. We show evidence that this exchange coupling between an antiferromagnet (CoO) and a ferromagnet (Ni81Fe19) is long range in nature. Exchange coupling has been observed in tri-layer films consisting of a nonmagnetic noble metal (Ag, Au, and Cu) spacer layer sandwiched between 300 A CoO and 300 A NiFe. The strength of the coupling decreases with increasing spacer layer thickness and vanishes at about 55 A. This suggests that the antiferromagnetic/ferromagnetic exchange coupling is beyond an interfacial effect, and that conduction electrons may be involved in the mediation of the coupling.