The surface modification of TiO2 nanostructures to incorporate nitrogen and form visible light absorbing titanium oxynitride centers is studied. Anatase TiO2 structures in the 5–20 nm range, formed by a wet chemical technique, were surface modified and the nitridation of the highly reactive TiO2 nanocolloid surface, as determined by X‐ray photoelectron spectroscopy (XPS) studies, is achieved by a quick and simple treatment in alkyl ammonium compounds. The nitriding process was also simultaneously accompanied by metal seeding resulting in a metal coating layer on the TiO2 structures. The structure of the resultant titanium oxynitride nanostructures remains anatase. These freshly prepared samples exhibited a strong emission near 560 nm (2.21 eV), which red‐shifted to 660 nm (1.88 eV) and dropped in intensity with aging in the atmosphere. This behavior was also evident in some of the combined nitrogen doped and metal seeded TiO2 nanocolloids. Electron spin resonance (ESR) performed on these samples identified a resonance at g = 2.0035, which increased significantly with nitridation. The resonance is attributed to an oxygen hole center created near the surface of the nanocolloid, which correlates well with the observed optical activity.
Electron spin resonance and photoluminescence experiments have been performed on freshly etched and oxidized porous silicon. Results indicate the presence of oxygen-related centers (nonbridging oxygen-hole center clusters), which consist of similar core structures in as-made and oxidized porous silicon (PSi) samples. A direct correlation exists between the presence of these centers and a red photoluminescence observed in both freshly anodized and oxidized PSi, suggesting that this emission process is the result of optical transitions in the oxygen-hole centers.
Electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) experiments have been performed on a set of GaN epitaxial layers doped with Mg from 2.5 x 10(18) to 5.0 x 10(19) cm(-3). The samples were also characterized by secondary-ion-mass spectroscopy (SIMS), temperature-dependent Hall effect, and low-temperature photoluminescence (PL) measurements. EPR at 9 QHz on the conductive films reveals a single line with g(parallel to)similar to2.1 and g(perpendicular to)similar to2 and is assigned to shallow Mg acceptors based on, The similarity of the spin density with that found for the number of uncompensated Mg shallow acceptors from Hall effect and the total Mg concentration by SIMS. PL bands of different character are observed from these layers,, including shallow-donor-shallow-acceptor recombination at 3.27 eV from the lowest,doped sample, and, in most cases, broad emission bands with peak energy between 2.9 and 3.2 eV from the more heavily doped films. In addition, several of the films exhibit a weal, broad emission band between 1.4 and 1.9 eV. ODMR at 24 GHz on the "blue" PL bands reveals two dominant features. The first is characterized by g(parallel to),g(perpendicular to)similar to1.95-1.96 and is assigned to. shallow effective-mass donors. The second line is described by similar g tensors as found by the EPR experiments and, thus, is also attributed to shallow Mg acceptors. Although several groups have related the 2.8 eV PL in heavily Mg-doped GaN with the formation of deep donors, no clear evidence was found from the ODMR on this emission for such centers. However, based on the near-midgap PL energy and the observation of the feature assigned to shallow Mg acceptors, the strongest case from magnetic resonance for the existence of deep donors in these films is the isotropic ODMR signal with g = 2.003 found on emission, <19 eV. Possible recombination mechanisms to account for-the ODMR on these "blue" and near-IR PL bands are discussed
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