Porous silicon (PS) with nondegrading photoluminescence (PL) was prepared by a novel method. For fresh samples, the PL peak intensity is 2 -2.5 times stronger than that in normal PS. Upon exposing PS to ambient air, the PL intensity increases during the first four months and then saturates. No PL degradation is observed for eight months, and the peak position remains unchanged. The same effect can also be achieved by annealing treatments. This PL stability is attributed to the formation of stable Fe-Si bonds on the surface of PS. Exploration of the mechanism provides strong proof of the quantum confinement model of the luminescence of PS. [S0031-9007 (98)06914-2] PACS numbers: 78.55.Mb, 81.05.RmThe discovery of light-emitting porous silicon (normal PS) [1] has made it possible to develop Si-based optoelectronic devices. In recent years, most of the research has been directed at understanding the origin of the luminescence [1-8] and fabricating PS-based light emitters [9][10][11][12]; but the problems of the degradation [2,13,14] and the relatively low efficiency [15] of the photoluminescence (PL) remain unsolved. The PL degradation subsequent to normal PS preparation, accompanied with a blueshift of the peak position, is generally believed to originate from the chemical instability of the PS surface [2,13,14]. Under the attack of oxygen, the Si-H bonds on the surfaces of the Si nanocrystallites will be broken and the quantity of the Si dangling bonds will increase. This variation will ultimately lead to the degradation of the PL intensity [13,14]. On the other hand, deep oxidation will reduce the sizes of the Si nanocrystallites and result in the blueshift of the PL peak position [13,14]. The motivation of this research is to try to construct a stable Fe-Si bond on PS surface to substitute the easily broken Si-H bond and thereby realize the stabilization of the intensity as well as the peak position of the PL of PS.The PS samples reported here were prepared by a hydrothermal method which was first adopted by Chen et al. in preparing normal PS [16], but with a different solution. The initial materials were P͞P 1 -type boron-doped single crystal Si wafers with (111) orientation and ϳ1.5 2.0 V cm resistivity. The solution for hydrothermal treatment was composed of 40% (weight) hydrofluoric acid and 0.3 mol͞l ferric nitrate aqueous solution according to 7:6 (volume) and the treatment was performed at 140 ± C for 50 minutes. We name as-prepared PS samples as iron-passivated PS. For comparison, normal PS samples were also prepared by the hydrothermal method, but here the Si wafers were treated at 170 ± C for 4 hours in a solution of 15 mol͞l hydrofluoric acid.The PL spectra of all these samples were measured with an 850-type visible-ultraviolet spectrophotometer (Hitatchi, Japan) at room temperature, and these results were corrected for the system response. Here the most efficient wavelengths for excitation were adopted as the excitation wavelengths, 320 nm for iron-passivated PS and 280 nm for normal PS. For both fre...
Impact localization is one of the major concerns in maintenance of aircraft structures built from composites. A lot of different techniques like acoustic emission, optimization technique based on system are nowadays investigated to identify impact location. But the method to identify impact position in complex composite structure with high accuracy in real time has not been completely established. In the present work, a new efficient method is proposed for identifying impact location in real and complex composite structure based on array signal processing and Lamb waves. There are two steps in the proposed method. The first step is to estimate the direction- of-arrival (DOA) of impact source using continuous wavelet transform (CWT) and array signal processing; the second step is to estimate the distance between impact source and the coordinate origin based on propagation characteristic of Lamb wave and CWT. The composite panel with stepped thickness of a real and complex aeronautical tank is used to verify the present methodology. The result shows that the present method may identify impact location fast and accurately.
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