Photoluminescence of ZnO nanoparticles and nanorods
Introduction
ZnO is a wide-band-gap semiconductor (3.37 eV) that displays a larger exciton binding energy (∼60 meV, 2.4 times of the room-temperature thermal energy) [1]. The optical properties of nanosized ZnO are substantially different from it of bulk crystalline materials [2], [3], [4], [5], [6], [7], [8], [9], [10]. Changing size of ZnO nanostructures can tune the band-gap and luminescent properties of ZnO in the near ultraviolet and visible regions [11], [12]. In addition, nanostructures emit visible luminescence much stronger than bulk crystals due to the concentration of oscillator strength into just a few transitions [13]. However, the emission properties of ZnO nanostructures in the visible region widely depend on their synthetic method as they are attributable to surface defects [14], [15], [16], [17], [18]. Therefore, the resource of such a luminescence is still unclear. A very simple synthetic method for the preparation of zinc oxide nanoparticles and nanorods are used in this study [19], [20]. This method yields well-crystallized and quasi-spherical nanoparticles. The versatility of this method allows the control of the size and the shape of the ZnO nanoparticles depending on the experimental conditions. We report here the optical properties of ZnO nanoparticles and nanorods. The absorption spectra as well as the emission spectra were recorded in solution.
Section snippets
Experimental details
The syntheses of ZnO nanoparticles and nanorods are performed as described in Refs. [19], [20], [21]. For the preparation of the ZnO nanospheres zinc acetate dihydrate (0.01 M) was dissolved in methanol (125 mL) under vigorous stirring at about 60 °C. Subsequently, a 0.03 M solution of KOH (65 mL) in methanol was added dropwise at 60 °C. The reaction mixture was stirred for 2 h at 60 °C. The resulting solution (stock solution) was concentrated by evaporation of the solvent and heated for different
Results and discussion
The corresponding powder X-ray diffractogram is depicted in Fig. 2. It exhibits the typical size-broadened reflections from wurtzite-type ZnO. Scherer line width analysis of all shown peaks yields particle diameters around 3 nm which is consistent with the TEM images. Such a gradual growth of ZnO nanoparticles upon ageing was also observed and explained [19]. In the corresponding XRD images (Fig. 2b) the 0 0 2 reflection has strongly sharpened up which is consistent with rod formation along the c
Conclusions
In conclusion, we have shown in this study that the strong difference of properties between ZnO nanoparticles and nanorods. The crystalline quality of ZnO nanorods is higher than nanoparticles. After exciting of 325 nm laser, from low temperature PL spectrum the excitonic band are observed in the UV region both for ZnO nanoparticles and nanorods. The excitonic emission band shift from 369 nm for 10 K to 374 nm for 300 K for ZnO nanoparticles, then for nanorods the excitonic band shift from 371 nm for
Acknowledgements
This work was funded by the NSFC (nos. 61307119 and 61235004) and China Postdoctoral Science Foundation (2013M541300).
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