An increasing number of applications using ultraviolet radiation have renewed interest in ultraviolet photodetector research. Particularly, solar‐blind photodetectors sensitive to only deep UV (<280 nm), have attracted growing attention because of their wide applicability. Among recent advances in UV detection, nanowire (NW)‐based photodetectors seem promising, however, none of the reported devices possesses the required attributes for practical solar‐blind photodetection, namely, an efficient fabrication process, a high solar light rejection ratio, a low photocurrent noise, and a fast response. Herein, the assembly of β‐Ga2O3 NWs into high‐performance solar‐blind photodetectors by use of an efficient bridging method is reported. The device is made in a single‐step chemical vapor deposition process and has a high 250‐to‐280‐nm rejection ratio (∼2 × 103), low photocurrent fluctuation (<3%), and a fast decay time (<<20 ms). Further, variations in the synthesis parameters of the NWs induce drastic changes in the photoresponse properties, which suggest a possibility for tuning the performance of the photodetectors. The efficient fabrication method and high performance of the bridged β‐Ga2O3 NW photodetectors make them highly suitable for solar‐blind photodetection.

The stability of hydrogen in ZnO is studied using hydrogenated nanowires by plasma treatment. Enhanced near band edge UV emission and reduced defect level green emission is observed after hydrogen plasma treatment. Through thermal stability tests, this effect is found to be stable at room temperature and nearly stable up to ~500 K, but begins to deteriorate at higher temperature. The study of the irradiation stability of the hydrogen in ZnO nanowires shows that the hydrogen is stable under an electron beam with an accelerating voltage lower than 5 kV, but is not stable under 10 kV or under an intensive laser beam. The results could benefit the further understanding of the role of hydrogen in ZnO and light-emitting devices based on hydrogenated ZnO.

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ZnO nanowires with strong green emission synthesized by chemical vapor deposition were treated using hydrogen plasma. The effect of hydrogen plasma treatment was studied by means of photoluminescence and photoconductivity. A strong passivation of the green emission and a significant enhancement of the near band edge emission were found after the hydrogen plasma treatment. The conductivity of the nanowires in dark was increased by more than 3 orders of magnitude. The photoconductivity also increased after the hydrogen plasma treatment. The observed changes in the luminescence and photoconductive properties of the ZnO nanowires were likely caused by hydrogen atoms occupying both oxygen vacancies and interstitial sites.

First-principles approaches on reproduction of negative and anisotropic thermal expansion of ceramics were examined. We have tested quasi-harmonic free-energy calculation and molecular dynamics (MD) simulation including lattice change based on the density functional theory. For intuitive understanding, a classical force-field MD was also applied. As a test case, we have chosen cordierite, which is particularly important for recent application for electronic precision components, catalytic converter for automobile exhaust gas, and is known to show negative and anisotropic thermal expansion at a certain region of temperatures. Quasi-harmonic free-energy approach showed appearance of negative expansion of cordierite in certain temperatures, while the anisotropy was not reproduced. We conclude this may be due to lack of anisotropic lattice change throughout the optimization of the unit cell and internal coordinates under hydrostatic pressures at 0 K. On the other hand, MD simulation allowing lattice change showed anisotropy of a-, b-axes and c-axis of cordierite while the negative expansion region in temperature was not found, which may be due to limitation of available size of unit cell. Furthermore, by performing classical force-field MD, we found that local chemical-bond nature is a key in understanding the behavior of the cordierite under finite temperatures. From current simulated results, we propose necessity of intensive works to compare theoretical work with future accessible experiments using single-crystal samples.

Low-voltage, cost-effective, microscopic devices for sensing nonsolar radiation-such as fire-show good spectral selectivity and fast response.

In this report, large-scale vertically aligned ZnO nanowires, with diameter around 75 nm and length around 2-5 μm, were synthesized on a-plane sapphire by a single step chemical vapor deposition method. The XRD pattern of the as-prepared sample showed a strong ZnO (0002) peak and a weak ZnO (0004) peak that indicate good orientation and high crystal quality of the ZnO nanowires. The sample was then treated by hydrogen plasma, without exhibiting obvious structural damage to the nanowires. The photoluminescence spectra of as-prepared and H 2 -plasma-treated samples were then examined. A strong green emission peak (centered at 520 nm) was observed in the PL spectrum of as-prepared sample. In sharp contrast, a significant increase of the near-band edge emission (centered at 380 nm) and a strong decrease of the green emission (centered at 520 nm) were found in the PL spectrum of H 2 -plasma-treated sample. We propose that an efficient passivation of oxygen vacancies by H atoms will cause a drastic decrease of the green emission. More important, it would lead to a significant reduction of surface depletion layer, leading to a great enlargement of total effect area for UV emission. Meanwhile, the significant enhancement of the intensity of UV emission might also attribute to the combined effects of structure-induced waveguide behavior and UV amplified spontaneous emission. It is expected that the enhanced UV emission of vertically aligned ZnO nanowires can be used to improve the performance of UV light emitting devices.

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