Tatsuo Okada

Abstract: Wide-gap semiconductors with nanostructures such as nanoparticles, nanorods, nanowires are promising as a new type of UV photosensor. Recently, ZnO (zinc oxide) nanowires have been extensively investigated for electronic and optoelectronic device applications. ZnO nanowires are expected to have good UV response due to their large surface area to volume ratio, and they might enhance the performance of UV photosensors. In this paper, a new fabrication method of a UV photosensor based on ZnO nanowires using dielectrophoresis is demonstrated. Dielectrophoresis (DEP) is the electrokinetic motion of dielectrically polarized materials in non-uniform electric fields. ZnO nanowires, which were synthesized by nanoparticle-assisted pulsed-laser deposition (NAPLD) and suspended in ethanol, were trapped in the microelectrode gap where the electric field became higher. The trapped ZnO nanowires were aligned along the electric field line and bridged the electrode gap. Under UV irradiation, the conductance of the DEP-trapped ZnO nanowires exponentially increased with a time constant of a few minutes. The slow UV response of ZnO nanowires was similar to that observed with ZnO thin films and might be attributed to adsorption and photodesorption of ambient gas molecules such as O2 or H2O. At higher UV intensity, the conductance response became larger. The DEP-fabricated ZnO nanowire UV photosensor could detect UV light down to 10 nW cm−2 intensity, indicating a higher UV sensitivity than ZnO thin films or ZnO nanowires assembled by other methods.

Abstract: We have succeeded in synthesizing ZnO nanorods by pulsed-laser ablation at comparatively high gas pressures without using a catalyst. The nanorods had an average size of 300 nm and a length of about 6 μm. Stimulated emission was observed from the nanorods at 388 nm by optical pumping. As a catalyst was not used in our method, nanorod growth was not controlled by the vapor-liquid-solid (VLS) mechanism. We found that nanoparticles formed by condensation of ablated particles in the laser ablation plume play an important role in nanorod growth.

Abstract: The direct comparison of the emission characteristics of an extreme ultraviolet (EUV) light between the CO2 and the Nd:YAG laser-produced plasmas (LPP) with a solid tin target is reported. In the case of the Nd:YAG LPP, the conversion efficiency (C.E.) peaked at a laser intensity of about 5×1010W∕cm2 and decreased at higher laser intensity. In the case of the CO2 LPP, the C.E. monotonically increased up to 2×1010W∕cm2, where the C.E. is comparable to the maximum C.E. of the Nd:YAG LPP. The spectral efficiency of the Nd:YAG LPP within the 2% bandwidth around 13.5 nm decreased with laser intensity. The corresponding spectral efficiency of the CO2 LPP was almost constant. This observation indicates the potential of the CO2 laser-produced LPP as the EUV light source for the EUV lithographic systems.

Abstract: The generation of a nano-sized hollow bump array of gold thin film by uniformly spaced melting and inflation of the film induced by a single shot of four interfering fs laser beams is reported. The shape changed from a bump, to a bead on bump, to a standing bead on a hole, and finally to a hole, with the increase in fs laser fluence. The bump height, diameter, and shape as functions of laser fluence are shown. Moreover, the basic shape of the bump changed upon changing the number of interfering beams.

Abstract: Nano-structured ZnO thin films were synthesized by the nanoparticle assisted pulsed-laser deposition in an oxygen background gas. Crystallized and c-axis oriented ZnO nanorods with a size of approximately 300 nm in average diameter and 6 µm in length were grown on sapphire substrates heated at approximately 700°C by the pulsed-laser deposition technique without any catalyst. Strong photoluminescence near the band-gap was observed from nanorods under excitation at 308 nm. The Rayleigh scattering diagnostics of the plume was also conducted, revealing that the nanorods grew from ZnO nanoparticles which formed in the plume and were transported onto the substrate.

Abstract: ZnO nanowire (NW) visible-blind UV photodetectors with internal photoconductive gain as high as G ∼ 108 have been fabricated and characterized. The photoconduction mechanism in these devices has been elucidated by means of time-resolved measurements spanning a wide temporal domain, from 10-9 to 102 s, revealing the coexistence of fast (τ ∼ 20 ns) and slow (τ ∼ 10 s) components of the carrier relaxation dynamics. The extremely high photoconductive gain is attributed to the presence of oxygen-related hole-trap states at the NW surface, which prevents charge-carrier recombination and prolongs the photocarrier lifetime, as evidenced by the sensitivity of the photocurrrent to ambient conditions. Surprisingly, this mechanism appears to be effective even at the shortest time scale investigated of t < 1 ns. Despite the slow relaxation time, the extremely high internal gain of ZnO NW photodetectors results in gain-bandwidth products (GB) higher than ∼10 GHz. The high gain and low power consumption of NW photodetec...

Abstract: ZnO has received much attention over the past few years because it has a wide range of properties that depend on doping, including a range of conductivity from metallic to insulating (including n-type and p-type conductivity), high transparency, piezoelectricity, wide-bandgap semiconductivity, room-temperature ferromagnetism, and huge magneto-optic and chemical-sensing effects. Without much effort, it can be grown in many different nanoscale forms, thus allowing various novel devices to be achieved. We review recent studies of ZnO nanostructures, fabrication, novel device applications, and its potential as an electron-acceptor material in hybrid solar cells. Control of its rich defect chemistry, which is critical for controlling properties but has not been widely addressed in the context of novel applications, is also discussed.

Abstract: We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.

Abstract: One-dimensional (1D) ZnO nanostructures have been studied intensively and extensively over the last decade not only for their remarkable chemical and physical properties, but also for their current and future diverse technological applications. This article gives a comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods. We will cover the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronic, and energy harvesting devices.

Abstract: In recent years, there has been increasing interest in ZnO nanostructures due to their variety of morphologies and availability of simple and low cost processing. While there are still unanswered questions concerning fundamental properties of this material, in particular those related to defects and visible luminescence lines, great progress has been made in synthesis methods and device applications of ZnO nanostructures. In this review, we will provide a brief overview of synthesis methods of ZnO nanostructures, with particular focus on the growth of perpendicular arrays of nanorods/nanowires which are of interest for optoelectronic device applications. Then, we will provide an overview of material properties of ZnO nanostructures, issues related to doping with various elements to achieve either p- or n-type conductivity. Doping to alter optical or magnetic properties will also be discussed. Then, issues related to practical problems in achieving good electrical contacts to nanostructures will be presented. Finally, we will provide an overview of applications of ZnO nanostructures to light-emitting devices, photodetectors and solar cells.