Sheng Yun Wu

X-ray diffraction and Raman scattering studies on large-area array and nanobranched structure of 1D MoO2 nanorods

One-dimensional (1D) MoO2 nanorods in the form of a large-area array and nanobranched structure were prepared by hot-filament metal?oxide vapour deposition at low and high pressures in atmospheric argon flows respectively. The x-ray diffraction (XRD) patterns of both as-synthesized samples show that the 1D MoO2 nanorods are monoclinic crystals in space group P 21/c. The Raman spectrum of the large-area array of 1D MoO2 nanorods appears to be the same as that of a two-dimensional (2D) MoO2 thin film. The Raman spectrum of the nanobranched structure of 1D MoO2 nanorods showed a downshift and asymmetric broadening of the Raman first-order TO peak when compared with the bulk (q = 0) mode. The Raman shift and broadening were attributed to phonon confinement effect in the 1D nanorods. The in?situ Raman spectra of laser-induced oxidation of the nanobranched structure of 1D MoO2 nanorods demonstrate that they can be oxidized easily and more strongly than the 3D bulk MoO2 powder.

High room-temperature photoluminescence of one-dimensional Ta2O5 nanorod arrays

In this study we analyzed the structural and electronic properties of a new morphological form, one-dimensional (1D) Ta2O5 nanorod arrays, which were synthesized by hot filament metal vapor deposition. Field-emission scanning electron microscopy (FESEM) showed the 1D Ta2O5 nanorods to be arranged in a large-area high-density array about 50 nm wide and approximately 550 nm long. X-ray photoemission spectroscopy (XPS) revealed not only the electronic structures and chemical properties of the 1D Ta2O5 nanorods but also their stoichiometric Ta and O compositions. Photoluminescence (PL) spectra showed intensive green-light, yellow-light and red-light emissions at room temperature. These emissions simultaneously emerged from the trap levels of oxygen vacancies within the Ta2O5 bandgap. The emission results strongly indicate that the 1D Ta2O5 nanorods are good room-temperature visible-light emitters.

Growth mechanism and magnon excitation in NiO nanowalls

The nanosized effects of short-range multimagnon excitation behavior and short-circuit diffusion in NiO nanowalls synthesized using the Ni grid thermal treatment method were observed. The energy dispersive spectroscopy mapping technique was used to characterize the growth mechanism, and confocal Raman scattering was used to probe the antiferromagnetic exchange energy J2 between next-nearest-neighboring Ni ions in NiO nanowalls at various growth temperatures below the Neel temperature. This study shows that short spin correlation leads to an exponential dependence of the growth temperatures and the existence of nickel vacancies during the magnon excitation. Four-magnon configurations were determined from the scattering factor, revealing a lowest state and monotonic change with the growth temperature.PACS: 75.47.Lx; 61.82.Rx; 75.50.Tt; 74.25.nd; 72.10.Di

Direct observation of short-circuit diffusion during the formation of a single cupric oxide nanowire

Short-circuit diffusion was observed in a single CuO nanowire synthesized using a thermal oxidation method. The confocal Raman spectra of a single CuO nanowire permit direct observation of the nature of an individual CuO nanowire. The parameter order obtained from the inverse Raman Bg2 peak linewidth results in the length dependence of the linewidth and a short-circuit diffusion length of 3.3 µm. The observed structural information is also consistent with the energy dispersive x-ray spectroscopic mapping. The results confirm that the growth of CuO nanowires occurs through the short-circuit diffusion mechanism.

Low-temperature phase transformation and phonon confinement in one-dimensional Ta2O5 nanorods

The thermochromic phase transformations of one-dimensional Ta2O5 nanorods have been analyzed at elevated temperatures ranging from 80 to 300 K. The nanorods, grown in a large-area high-density array, are 14–22 nm wide and approximately 500 nm long. The array contained ∼93.5% of the orthorhombic (β) phase and ∼6.5% of the tetragonal (α) phase. Low-temperature X-ray diffraction results showed complex and polymorphic thermochromic phase transformations of the β(001), α(101) and α(103) lattice planes of the nanorods, which incorporate (i) α-to-α (α–α), (ii) α–α–β and (iii) α–β phase transitions. In comparison with the Raman scattering of three-dimensional bulk powder and two-dimensional thin films of Ta2O5, there were concurrent Raman blue- and redshifts in the one-dimensional Ta2O5 nanorods, indicating that the molecular vibrations of the nanorods were confined owing to the reduction of size and dimension.

Oxygen induced strained ZnO nanoparticles: an investigation of Raman scattering and visible photoluminescence

We report the influence of the nanosized effect on the optical properties of non-centrosymmetric ZnO nanoparticles. In this study confocal Raman scattering was employed to investigate the strain effect of the softening A1(LO) phonon mode while controlling particle sizes from 55 ± 1 to 32 ± 1 nm. The observations reveal a positive Poisson ratio between the compressive- and tensile-strain. The intensity ratio of the A1(2LO)/A1(LO) modes exhibits strong size dependence. As the particle sizes decrease further, the ratio decreases rapidly, signaling the short-range electron–phonon coupling effect which confines the electrons and holes within a smaller volume. An energy red-shift in the photoluminescence peak was observed, because of a lowering in the strain of local symmetry at O2− sites caused by excess oxygen, and by strong coupling between the electron and phonon vibration. The variation of the bandgap is very sensitive to the electron–phonon coupling and the distinct size effect of strained ZnO nanoparticles.

In Situ Confocal Raman Mapping Study of a Single Ti-Assisted ZnO Nanowire

In this work, we succeeded in preparing in-plane zinc oxide nanowires using a Ti-grid assisted by the chemical vapor deposition method. Optical spatial mapping of the Confocal Raman spectra was used to investigate the phonon and geometric properties of a single ZnO nanowire. The local optical results reveal a red shift in the non-polar E2 high frequency mode and width broadening along the growth direction, reflecting quantum-confinement in the radial direction.

Spontaneous self-organization of Cu2O/CuO core–shell nanowires from copper nanoparticles

We report on the formation and spontaneous self-organization of Cu(2)O/CuO core-shell nanowires from individual copper nanoparticles. The growth process is interpreted using the results of time-dependent in situ x-ray diffraction. High-resolution transmission electron microscopy is used to observe the intermediate state of pearl-necklace-like aggregates that form a chain-like configuration of Cu(2)O nanoparticles intertwined into nanowires. The existence of an amorphous CuO shell is confirmed by the XANES technique and explained through an intensity simulation using a proposed core-shell nanowire model.

Dense inter-particle interaction mediated spontaneous exchange bias in NiO nanoparticles

We report the finite size effect and nickel vacancy defects in NiO nanoparticles that result in the formation of magnetic phase separation in uncompensated antiferromagnetic NiO-cores with frustrated and disordered spins at the surface NiO-shell. The inter-particle interaction is probed by analyzing the relaxation dynamics measurements. A significant index for the interaction of n derived from the dynamic magnetization is proposed, which paves the way for the examination of the spontaneous exchange bias mechanism and offers insight into the influence of the particle size and defects.

Magnetic Properties of Cluster Glassy Ni/NiO Core–Shell Nanoparticles: an Investigation of Their Static and Dynamic Magnetization

We review the phenomenology of the exchange bias and its related effects in core–shell nanocrystals. The static and dynamic properties of the magnetization for ferromagnetic Ni-core and antiferromagnetic NiO-shell cluster glassy nanoparticles are examined, along with the pinning–depinning process, through the measurement of the conventional exchange bias, and associated with different cooling fields and particle sizes. Two significant indexes for the dipolar interaction n and multi-anisotropic barrier β derived from the dynamic magnetization are proposed, which provide a unified picture of the exchange bias mechanism and insight into the influence of the cooling field.