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 O(2) or H(2)O. 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.
The authors recently succeeded in growing two-dimensional ZnO nanowalls on sapphire substrates using high-pressure pulsed laser deposition (PLD) without any catalysts. Depending on the PLD growth conditions and the composition of the target, ZnO nanowalls with thickness of tens of nanometers and dimension of several micrometers were synthesized reproducibly. Most of the nanowalls were vertically epitaxial on the c-cut sapphire substrates with a preferred c-axis orientation as confirmed with X-ray diffraction and transmission electron microscopy. The room temperature photoluminescence spectrum of such a ZnO nanowall exhibited a strong intrinsic UV emission and a week defect-related visible emission. It was found that the ZnO nanowalls showed stable field emission properties with low threshold field and a big field enhancement factor. Photocurrent measurements also indicated that these ZnO nanowall films showed a high sensitivity to UV light, which can be used as a UV photodetector.
The fluorescence characteristics of different Ce3+:Er3+-codoped fluoride host glasses, such as fluorozirconate and fluoroindate glass, are reported. It is shown that Ce3+ codoping into Er3+ doped fluoride glasses resulted in a quenching of Er3+ ions from the I11/24 to the I13/24 state, and the branching ratio for the Er3+ I11/24→I13/24 transition increased from 0.20 to over 0.80 by codoping 2.0 mol % Ce3+. Further, the fluorescence quantum yield at 1.55 μm was also significantly improved. A Ce3+:Er3+-codoped fluorozirconate fiber laser operating at 1.55 μm band with 980 nm excitation has been realized for the first time, which clearly indicates the effectiveness of Ce3+ codoping.
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