Luminescent silica nanotubes and nanowires: Preparation from cellulose whisker templates and investigation of irradiatio
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Luminescent silica nanotubes and nanowires were fabricated from cellulose whisker templates by sol-gel processing. The cellulose templates were removed by calcination at 650 C to generate silica nanotubes with diameters of 15 nm and lengths up to 500 nm. At temperatures of 900 C the core region previously occupied by the cellulose template was closed yielding silica nanowires. Cathodoluminescence spectra of the silica nanotubes and nanowires were measured in the transmission electron microscope during irradiation with 150 keV electrons. A blue emission at 450 nm was observed for the silica nanowires calcined at 900 C. This luminescence was found to be related to defects induced by electron irradiation and was investigated in situ as a function of irradiation dose. The as-synthesized and 650 C calcined nanowires and nanotubes showed a fast decay of the signal. The observed irradiation dose dependent changes in the luminescence spectra will be discussed in terms of defect formation and transformation mechanisms.
I. INTRODUCTION
Within the last 20 years of proceeding progress in nanotechnology a variety of one-dimensional nanoparticles has been produced with ever growing efficiency.1 Silica nanotubes are of particular interest because they have interesting optical and electronic properties, offer several routes to surface modification, and are biocompatible. Potential applications of silica nanotubes include wave guiding in microphotonic devices,2 nanoelectronics,3 bioseparation,4 and biosensing.5 Interestingly, silica nanotubes and nanowires show an intensive photo stimulated blue light emission6–10 establishing them as valuable tools for applications in high-resolution optical probing in scanning near-field optical microscopy, one-dimensional nanoscale luminescence diodes or nanointerconnectors in future integrated optical devices.6 Luminescence from high purity amorphous SiO2 (a-SiO2) has been extensively studied on bulk or thin film samples and luminescence bands have been observed with peak energies at 1.9, 2.2, 2.5, 2.7, 3.1, and 4.3 eV11–13; for a recent review see Skuja.14 The 2.7 eV band in particular has been found characteristic for oxygen-deficient a-SiO2.11 It was attributed to an oxygen-deficient center denoted as ODC II. This defect was found to correlate with the B2 absorption band at 5 eV15 and has been variously assigned to a twofold coordinated silicon,16 a neutral oxygen vacancy,11 or a self a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0224 J. Mater. Res., Vol. 24, No. 5, May 2009
trapped exciton.17 Additionally, it was found that irradiating amorphous silica with ionizing radiation, e.g., electrons,18,19 x-rays,20 or photons21–23 even below the band gap energy (9 eV)24 resulted in radiation damage defects detectable by the appearance of characteristic absorption and emission bands, among them the 2.7 eV band. The irradiation-induced formation of oxygendeficient centers (ODC) in particular was also deduced from an oxygen l
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