Herein is reported a detailed study of the luminescence properties of nanostructured Si using X-ray excited optical luminescence (XEOL) in combination with X-ray absorption near-edge structures (XANES). P-type Si nanowires synthesized via electroless chemical etching from Si wafers of different doping levels and porous Si synthesized using electrochemical method are examined under X-ray excitation across the Si K-, L(3,2) -, and O K-edges. It is found that while as-prepared Si nanostructures are weak light emitters, intense visible luminescence is observed from thermally oxidized Si nanowires and porous Si. The luminescence mechanism of Si upon oxidation is investigated by oxidizing nanostructured Si at different temperatures. Interestingly, the two luminescence bands observed show different response with the variation of absorption coefficient upon Si and O core-electron excitation in elemental silicon and silicon oxide. A correlation between luminescence properties and electronic structures is thus established. The implications of the finding are discussed in terms of the behavior of the oxygen deficient center (OCD) and non-bridging oxygen hole center (NBOHC).
[Show abstract][Hide abstract] ABSTRACT: Core-shell structured silicon nanowires (Si NWs) were obtained by coating Si NWs with an HfO2 layer. Enhanced photoluminescence (PL) and a slightly decreased PL lifetime are achieved by HfO2 coating. Furthermore, the sensing stability is strongly improved. The improvement of PL properties is interpreted in terms of surface passivation and the Purcell effect.
[Show abstract][Hide abstract] ABSTRACT: A novel heterojunction white light emitting diode (LED) structure based on an array of vertically aligned surface-passivated p-type porous Si nanowires (PSiNWs) with n-type amorphous In-Ga-Zn-O (a-IGZO) capping is introduced. PSiNWs were initially synthesized by electroless etching of p-type Si (100) wafers assisted by Ag nanoparticle catalysts and then surface-passivated by thermal oxidation. The nanowires synthesized by metal-assisted electroless etching were found to have longitudinally varying nanoporous morphologies due to differences in the duration of exposure to etching environment. These PSiNWs were optically active with orange red photoluminescence consisting of dark red to yellow emissions attributable to quantum confinement effects and to modified band structures. The LED structures emitted visible white light while exhibiting rectifying current-voltage characteristics. The white light emission was found to be the result of the combination of dark red to yellow emissions originating from the quantum confinement effect within the PSiNWs and green to blue emissions due to the oxygen-deficiency-related recombination centers introduced during the surface oxidation.
[Show abstract][Hide abstract] ABSTRACT: Chemical imaging, electronic structure and optical properties of ZnO/CdS nano-composites have been investigated using scanning transmission X-ray microscopy (STXM), X-ray absorption near-edge structure (XANES) and X-ray excited optical luminescence (XEOL) spectroscopy. STXM and XANES results confirm that the as-prepared product is ZnO/CdS core/shell nanowires (NWs), and further indicate that ZnS was formed on the surface of ZnO NWs as the interface between ZnO and CdS. The XEOL from ZnO/CdS NW arrays exhibits one weak ultraviolet (UV) emission at 375 nm, one strong green emission at 512 nm, and two broad infrared (IR) emissions at 750 and 900 nm. Combining XANES and XEOL, it is concluded that the UV luminescence is the near band gap emission (BGE) of ZnO; the green luminescence comes from both the BGE of CdS and defect emission (DE, zinc vacancies) of ZnO; the IR luminescence is attributed to the DE (bulk defect related to the S site) of CdS; ZnS contributes little to the luminescence of the ZnO/CdS NW arrays. Interestingly, the BGE and DE from oxygen vacancies of ZnO in the ZnO/CdS nano-composites are almost entirely quenched, while DE from zinc vacancies changes little.
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