Chien-Ting Wu

Heterostructures of ZnO-Zn coaxial nanocables and ZnO nanotubes

The heterostructures of Zn–ZnO coaxial nanocables and ZnO nanotubes with an average diameter of 30 nm have been synthesized by simple pyrolysis of zinc acetylacetonate. High-resolution transmission electron microscopy analyses reveal that the Zn core and the ZnO sheath of the nanocables have an epitaxial relationship with their longitudinal axis oriented along the 〈001〉 direction. ZnO nanotubes with a wall thickness of 4 nm possess a single-crystal structure and appear to be the extension of the ZnO sheath of the coaxial nanocables. It is suggested that the ZnO nanotubes are formed by partial evaporation of Zn core of the Zn–ZnO coaxial nanocables.

Controlling Steps During Early Stages of the Aligned Growth of Carbon Nanotubes Using Microwave Plasma Enhanced Chemical Vapor Deposition

Vertically aligned carbon nanotubes (CNTs) with controllable length and diameter fabricated by microwave plasma enhanced chemical vapor deposition (MPECVD) are of continuing interest for various applications. This paper describes the role of process gas composition as well as the pre-coating catalytic layer characteristics. It is observed that nucleation of CNTs was significantly enhanced by adding nitrogen in the MPECVD process, which also promoted formation of bamboo-like structures. The very first key step toward growth of aligned CNTs was the formation of high-density fine carbon onion encapsulated metal (COEM) particles under a hydrogen plasma. Direct microscopic investigation of their structural evolution during the very early stages revealed that elongation, necking, and splitting of the COEM particles occurred accompanying the growth of CNTs, such that one of the split portions rode on the top of the growing tube while the remaining one resided on the root. Our results suggest that CNTs grow via the “tip-growth” as well as “root-growth” mechanisms.

On-Chip Fabrication of Well-Aligned and Contact-Barrier-Free GaN Nanobridge Devices with Ultrahigh Photocurrent Responsivity**

In the last two decades, a wide range of semiconductor nanowires have been synthesized and used as building blocks for the development of a new generation of electronic and optoelectronic devices. [1–8] The integration of these nanowires into the thin-film-based microchip has become a critical problem in the practical application of the nanomaterial properties and for industrial manufacture. Single-wire-based devices have been shown to possess novel properties and provide a major platform for fundamental research. [9–13] However, the conventional fabrication of single-wire devices by the ‘‘pick and place’’ method is rather complicated and uneconomic, which is unsuitable for large-scale manufacturing. In contrast, devices based on an ensemble of nanowires are much easier to fabricate, thus reducing the barriers to practical applications. A method based on the bridging concept and showing the potential to directly integrate an ensemble of nanowires onchip was first demonstrated by Haraguchi et al. [14] Different from the traditional ‘‘bottom-up growth and then top-down processing’’, the nanowires are laterally grown across a trench and suspended between two film posts as nanobridges (NBs). [14–26] As the main device architecture and electrodes can be designed and prepared prior to NB growth, nanomaterial deterioration due to the post treatment in the

Enhanced dynamic annealing in Ga ¿ ion-implanted GaN nanowires

Ga+ ion implantation of chemical-vapor-deposited GaN nanowires (NWs) is studied using a 50-keV Ga+ focused ion beam. The role of dynamic annealing (defect-annihilation) is discussed with an emphasis on the fluence-dependent defect structure. Unlike heavy-ion-irradiated epitaxial GaN film, large-scale amorphization is suppressed until a very high fluence of 2×1016 ions cm−2. In contrast to extended-defects as reported for heavy-ion-irradiated epitaxial GaN film, point-defect clusters are identified as major component in irradiated NWs. Enhanced dynamic annealing induced by high diffusivity of mobile point-defects in the confined geometry of NWs is identified as the probable reason for observed differences.

Electronic structure of the carbon nanotube tips studied by x-ray-absorption spectroscopy and scanning photoelectron microscopy

Angle-dependent x-ray absorption near edge structure (XANES) and scanning photoelectron microscopy (SPEM) measurements have been performed to differentiate local electronic structures of the tips and sidewalls of highly aligned carbon nanotubes. The intensities of both π*- and σ*-band C K-edge XANES features are found to be significantly enhanced at the tip. SPEM results also show that the tips have a larger density of states and a higher C 1s binding energy than those of sidewalls. The increase of the tip XANES and SPEM intensities are quite uniform over an energy range wider than 10 eV in contrast to earlier finding that the enhancement is only near the Fermi level.

Deposition of silicon carbon nitride films by ion beam sputtering

Abstract Silicon carbon nitride films have been successfully synthesized at a temperature below 100°C from an adenine(C5N5H5)-silicon-mixed target sputtered by Ar ion beam. The effects of Ar ion sputtering voltage, area ratio of Si to adenine in the mixed target and nitrogen atom addition during deposition on the film growth are investigated. XPS, XRD, and ellipsometry were employed to characterize the composition, chemical bonding, structure, and optical property of the films. The growth characteristic and film properties of the silicon carbon nitride films are also compared with those of the carbon nitride films deposited from an adenine target to elucidate the effect of silicon incorporation. The silicon content of the resultant films increased significantly as the area ratio of Si to adenine increased, whereas a higher Ar ion voltage led to a lower level of silicon incorporation, presumably due to differential sputtering yield of Si and adenine. XPS chemical state analysis revealed multiple bonding structures for every element in the SiCN films, of which possible implications are given. XRD studies showed that only amorphous films for Si-rich SiCN were obtained, while the films with low Si incorporation and deposited at high Ar ion beam voltage contained nanocrystallites. Furthermore, the refractive index of the SiCN films increased with increasing silicon content. The appearance of the nanocrystalline structure in the films led to a reduction in the refractive index.

Blueshift of yellow luminescence band in self-ion-implanted n-GaN nanowire

Optical photoluminescence studies are performed in self-ion (Ga+)-implanted nominally doped n-GaN nanowires. A 50 keV Ga+ focused ion beam in the fluence range of 1×1014–2×1016 ions cm−2 is used for the irradiation process. A blueshift is observed for the yellow luminescence (YL) band with increasing fluence. Donor–acceptor pair model with emission involving shallow donor introduced by point-defect clusters related to nitrogen vacancies and probable deep acceptor created by gallium interstitial clusters is responsible for the shift. High-temperature annealing in nitrogen ambient restores the peak position of YL band by removing nitrogen vacancies.

Quasiquenching size effects in gold nanoclusters embedded in silica matrix

Abstract The surface plasmon energy of gold clusters formed by Ar + ion beam mixing of Au/silica is investigated for the size effect. The redshift with decreasing cluster size is assigned to the ‘spillout’ effect in small clusters ( 5 nm), assisted with annealing treatment. The competition between ‘spillout’ effect and the frequency dependence of interband dielectric function leads to a quasiquenching of the size effects in the optical response.

Mechanism of nanoblister formation in Ga + self-ion implanted GaN nanowires

The formation of voids and bubbles during ion implantation is an important area of material research. Void and bubble formation can result in swelling and embrittlement of metallic or semiconducting materials, and increase catalytic effects in the nanopores of the bubble. Here, we report the observation of metallic nanoblister formation in GaN nanowires under self-ion implantation using a Ga+ focused ion beam. The mechanism of the blister formation was resolved using high-resolution transmission electron microscopy equipped with electron energy loss spectroscopy and plasmon imaging.

Hexagonal-to-Cubic Phase Transformation in GaN Nanowires by Ga+ Implantation

Hexagonal to cubic phase transformation is studied in focused ion beam assisted Ga+-implanted GaN nanowires. Optical photoluminescence and cathodoluminescence studies along with high-resolution transmission electron microscopic structural studies are performed to confirm the phase transformation. In one possibility, sufficient accumulation of Ga from the implanted source might have reduced the surface energy and simultaneously stabilized the cubic phase. Another potential reason may be that the fluctuations in the short-range order induced by enhanced dynamic annealing (defect annihilation) with the irradiation process stabilize the cubic phase and cause the phase transformation.