Contrary to the defect that causes the green emission, the one that causes the yellow emission is not found at the surface. To add to the green and yellow emission, there was orange-red emission that also comes out. The intense emission that can be seen in ZnO nanosheets was believed to temporarily result from the dislocation at the surface. The orange red emissions resulted from the oxygen interstitials. There are many methods that are being used in combination with photoluminescence to get the source of the defect emissions in ZnO.
Electron paramagnetic resonance (EPR) spectroscopy has been used frequently to identify paramagnetic defects.EPR is useful in the study of ZnO defects. A combination of photoluminescence and positron annihilation spectroscopy is also useful in the study of defects in ZnO (Liu et al pp. 230). Stimulated emission in ZnO nanostructures Studies have shown there is a possibility of ZnO producing amplified spontaneous emissions when optically pumped lasing is applied to it. There is amplified spontaneous lasing for self organized ZnO fibres.
Researchers have also showed that lasing happens in various structures such as nanowires, nanoribbns and tetracombs. The coherent coherent result in the nanostructures of ZnO or nanostructured films may be achieved through two fundamental ways. In the first method multiple reflections bring about coherent feedback from the final sides of the nanostructure. The emission of UV increases at the edges of the nanowire while green emissions come from all over the nanowires. In method two the coherent feedback comes from several dispersed events (Liu et al pp. 230). Random lasers have the coherent feedback coming from recurring dispersing events.
The wavelength of the emissions is bigger than the scatter size. The lasing is dependent on the area of excitation and this has been displayed in ZnO polycrystalline films, nanowires and nanorods (Liu et al pp. 236). Polycrystalline thin films have random lasing in ZnO. Others with random lasing include ZnO powder films, nanoneedles, nanorods and nanowires. The threshold of lasing is affected by crystallinity and the structure of the film. Strain also determines the characteristics of lasing.
A major sign of random lasers is that it is possible to observe stimulated emission from every any direction and the measured spectra has its mode structure displaying angular dependence. Researchers have also reported about lasing with ZnO nanowires whereby each of the wires from a Fabry Perot reasonator surrounded by reflecting faces. After that, many other studies have reported of stimulated emissions from many different ZnO nanostructures. Studies have reported about lasing in tetrapod nanostructures, nanoribbons, nanocombs, whiskers, nanowires, nanocoral reefs, nanofibers and microtubes.
Some measurements were experimented on nanostructure ensembles but stimulated emissions emanating from each nanostructure were seen and well as those nanostructure with low density (Sajjad 2011, pp. 56). This was evidence that there was no possibility of getting feedback from multiple random scattering. In certain cases for example where there were specific nanoribbons and nanowires cavity identification is very clear and studies have also shown lasing with nanostructures have very intricate morphologies.
Again, stimulated emission was not just obtained for those nanostructures whose fabrication had been done at hot temperatures through deposition of vapor. It was also done for those nanostructures made at reduced temperatures using aqueous solutions (Liu et al pp. 231). Studies show that when there is poor crystallinity very high thresholds of lasing can be achieved in ZnO films. It is also possible to get stimulated emission in nanostructures that have varying times of decay and different defect emissions.
However, if the quality of crystals is poor and there are high cavity loses then it cannot be attained. Nanostructure dimensions, conditions of experimentation and the quality of the cavity will therefore determine the threshold of lasing.
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