Yung Liou

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.

Photoluminescence mechanisms of metallic Zn nanospheres, semiconducting ZnO nanoballoons, and metal-semiconductor Zn/ZnO nanospheres

We utilized a thermal radiation method to synthesize semiconducting hollow ZnO nanoballoons and metal-semiconductor concentric solid Zn/ZnO nanospheres from metallic solid Zn nanospheres. The chemical properties, crystalline structures, and photoluminescence mechanisms for the metallic solid Zn nanospheres, semiconducting hollow ZnO nanoballoons, and metal-semiconductor concentric solid Zn/ZnO nanospheres are presented. The PL emissions of the metallic Zn solid nanospheres are mainly dependent on the electron transitions between the Fermi level (EF) and the 3d band, while those of the semiconducting hollow ZnO nanoballoons are ascribed to the near band edge (NBE) and deep level electron transitions. The PL emissions of the metal-semiconductor concentric solid Zn/ZnO nanospheres are attributed to the electron transitions across the metal-semiconductor junction, from the EF to the valence and 3d bands, and from the interface states to the valence band. All three nanostructures are excellent room-temperature light emitters.

Electrochromic properties of large-area and high-density arrays of transparent one-dimensional β-Ta2O5 nanorods on indium-tin-oxide thin-films

We report on the synthesis, crystalline structure, and electrochromic properties of transparent one-dimensional (1D) orthorhombic (β) Ta2O5 nanorods grown in a large-area high-density array. The transparent 1D β-Ta2O5 nanorod array was synthesized on a conducting indium-tin-oxide thin-film via hot-filament metal-oxide vapor deposition. The array contained ∼1900 β-Ta2O5 nanorods per square micrometer, which were on average, ∼17 nm wide and ∼300 nm long. The good coloration/bleaching cycles, large ion-diffusion coefficient (∼2.35×10−8 cm2/s), and high reversibility (∼79.8%) demonstrate that the 1D β-Ta2O5 nanorods to be a potential electrochromic material for electrochromic devices or smart windows.

Annealing effect on the Fe/Pt multilayers grown on Al2O3 (0001) substrates

We studied the structure and magnetic properties of molecular-beam epitaxy grown 300 A thick Fe/Pt multilayers with different bilayer thickness and annealing temperature. The Fe/Pt multilayers were deposited on 100 A thick Pt buffer layers at 100 °C on Al2O3 (0001) substrates. The structure of as-deposited Fe/Pt films was fcc(111). While the postannealing temperature ⩾400 °C, an additional FePt(100) orientation was observed. A large coercivity range, namely, 200–16 000 Oe can be tuned by varying the bilayer thickness and annealing temperature.

Impact of Nanosize on Supercapacitance: Study of 1D Nanorods and 2D Thin-Films of Nickel Oxide

We synthesized unique one-dimensional (1D) nanorods and two-dimensional (2D) thin-films of NiO on indium-tin-oxide thin-films using a hot-filament metal-oxide vapor deposition technique. The 1D nanorods have an average width and length of ∼100 and ∼500 nm, respectively, and the densely packed 2D thin-films have an average thickness of ∼500 nm. The 1D nanorods perform as parallel units for charge storing. However, the 2D thin-films act as one single unit for charge storing. The 2D thin-films possess a high specific capacitance of ∼746 F/g compared to 1D nanorods (∼230 F/g) using galvanostatic charge-discharge measurements at a current density of 3 A/g. Because the 1D NiO nanorods provide more plentiful surface areas than those of the 2D thin-films, they are fully active at the first few cycles. However, the capacitance retention of the 1D nanorods decays faster than that of the 2D thin-films. Also, the 1D NiO nanorods suffer from instability due to the fast electrochemical dissolution and high nanocontact resistance. Electrochemical impedance spectroscopy verifies that the low dimensionality of the 1D NiO nanorods induces the unavoidable effects that lead them to have poor supercapacitive performances. On the other hand, the slow electrochemical dissolution and small contact resistance in the 2D NiO thin-films favor to achieve high specific capacitance and great stability.

An efficient methodology for measurement of the average electrical properties of single one-dimensional NiO nanorods

We utilized a metal tantalum (Ta) ball-probe to measure the electrical properties of vertical-aligned one-dimensional (1D) nickel-oxide (NiO) nanorods. The 1D NiO nanorods (on average, ~105 nm wide and ~700 nm long) are synthesized using the hot-filament metal-oxide vapor deposition (HFMOVD) technique, and they are cubic phased and have a wide bandgap of 3.68 eV. When the 1D NiO nanorods are arranged in a large-area array in ohmic-contact with the Ta ball-probe, they acted as many parallel resistors. By means of a rigorous calculation, we can easily acquire the average resistance RNR and resistivity ρNR of a single NiO nanorod, which were approximately 3.1 × 1013 Ω and 4.9 × 107 Ω.cm, respectively.

Large-area nanoscale farmland-like surfaces of one-dimensional NbO2 nanorods with multi-growth directions: studies on the purple-blue photoluminescence and low-field electron emissions

We synthesized a scenic morphological form of a large-area NbO2 nanoscale farmland using the hot-filament metal-oxide vapor deposition technique (HFMOVD). The nanoscale farmland is comprised of one-dimensional (1D) NbO2 nanorods arranged in various domains, which grow in multi-directions. Each domain contains ∼620 nanorods per square micrometer and has its own growth direction. The 1D NbO2 nanorods are found to have purple-blue photoluminescence (PL) emissions at room-temperature as well as very low turn-on and threshold fields for field emission (FE). The PL and FE results indicate that the 1D NbO2 nanorods are brilliant light and electron emitters.

Influence of crystal structure on the magnetoresistance of Co/Cr multilayers

Epitaxial Co/Cr multilayers, and single‐crystal Co thin films etc. have been grown on MgO and Al2O3 substrates with Cr and Mo as buffer layers by molecular beam epitaxy technique. From the structure and magnetoresistance studies, we have found that the ferromagnetic anisotropy of resistance (AMR) is strongly influenced by the buffer layer, but with negligible effect due to the variation of the structure of Co films. The AMR of Co film on Cr buffer layer is quite small (0.1%); however, the MR of Co/Cr multilayers is almost one order larger than the AMR of Co film on Cr buffer layer. An enhancement factor of 4 for the MR in Co/Cr multilayers by the interface roughness has been observed. This suggests that the effect due to the spin dependent scattering at the interfacial regions of the superlattice is larger than that due to the spin dependent scattering in the ferromagnetic layers for the MR in the Co/Cr multilayer system.

Room-temperature wide-range photoluminescence and semiconducting characteristics of two-dimensional pure metallic Zn nanoplates

We explore the structural, electronic, photoluminescent, metallic, and semiconducting characteristics of 2D pure Zn-metal nanoplates. The 2D pure Zn-metal nanoplates are synthesized by the hot-filament metal-oxide vapor deposition (HFMOVD) technique. They have an average diameter and thickness of ∼520 and ∼144 nm, respectively. The results of the electronic and crystalline structure studies reveal the 2D nanoplates to be pure Zn hexagonal crystals, which can provide a wide-range photoluminescence from ultraviolet (UV) to red light emissions at room temperature. The measured valence-band and the calculated band-structure of the 2D pure Zn-metal nanoplates verify that the UV and blue light arise from the 3d–sp interband transitions, while the green, yellow, and red lights come from the valence-conduction interband transitions at a bandgap that is only present in the 2D nanoplates. Therefore, the 2D pure Zn-metal nanoplates possess not only metallic, but also semiconducting characteristics.