Photoluminescence enhancement of ZnO microrods coated with Ag nanoparticles

By sputtering a layer of 40 nm Ag film on ZnO microrods prepared by the aqueous solution method, we observed photoluminescence enhancement from ZnO microrods coated with Ag nanoparticles, comparing with pure ZnO microrods. The ratio of the enhancement, in particular, reaches 6.6 at 396 nm. The enhancement mechanism is attributed to the special distribution of the silver nanoparticle film, which is helpful for efficiently coupling the energy of electron–hole pairs in ZnO microrods to the local surface plasmons (LSP) of silver particles and scattering the energy of LSP into free space as a form of radiated light. Recently, ZnO has attracted great attention for semiconductor optoelectronic device and ultraviolet (UV) laser device use, due to its wide and direct bandgap of 3.36 eV at room temperature and higher exaction binding energy (60 meV) [1–3], comparing to ZnSe (22 meV), ZnS (40 meV), and GaN (25 meV). But there are also some shortcomings of ZnO, such as the weak fluorescence and comparatively high excitation energy. In recent years, some research groups have concentrated on enhancement of the bandgap emission of ZnO by using the surface plasmons (SP) or localized surface plasmons (LSP) of various noble metals. Lai et al [4] and Ni et al [5] achieved 16-fold and 5.5-fold UV emission enhancement, separately, by coating the ZnO films with certain thicknesses of Al and Ag. You et al [6] sputtered ZnO film on Si(001) substrate which had already been coated with 100 nm Ag film previously, and the UV emission of the composite is found to be greatly enhanced. Cheng et al [7] also have shown 3-fold enhancement of bandgap emission of ZnO by coating the film with a layer of Ag island film. However, the ZnO samples mentioned above are all prepared by RF magnetron sputtering, which imposes high demands on the equipment and the technological parameters 1 Author to whom any correspondence should be addressed. if we want ZnO to be used as the ultraviolet (UV) laser material. In comparison, ZnO microrods can be produced by a comparatively simple method, with a natural cavity [8]. Also the defect caused by the mismatch between the substrate and ZnO can be avoided [9]. Consequently, ZnO microrods have become among the most promising materials for ultraviolet (UV) laser use due to their natural structure and strong ultraviolet emission. But reports on the photoluminescence enhancement of ZnO microrods mediated by the surface plasmon still have not been published. In this paper, we will introduce an Ag nanoparticle/ZnO microrod coupling mode for obtaining the enhancement effect by emitting LSP. The ZnO microrods were prepared in the following two steps [10]. (1) A 10 nm ZnO seed layer was first deposited on quartz substrates by the RF magnetron sputtering method. A commercially supplied ZnO ceramic plate with a purity of 99.99% was used as a target. The background pressure of the vacuum chamber was 1 × 10−4 Pa. A mixed gas of oxygen and argon with a volume ratio of 1:1 was used as the sputtering gas with a total pressure of 3 Pa. The sputtering power and growth temperature were 100 W and 380 ◦C respectively. Then the two substrates coated with 10 nm ZnO film were annealed in the air at the temperature of 750 ◦C in order to form seeds for the microrods. (2) The two samples were placed vertically 0953-8984/08/472202+04$30.00 © 2008 IOP Publishing Ltd Printed in the UK 1 J. Phys.: Condens. Matter 20 (2008) 472202 Fast Track Communication Figure 1. XRD pattern of the ZnO microrods grown on quartz. into the solution, which was mixed with two aqueous solutions of the same volume: 0.01 mol l−1 Zn(CH3COO)2·2H2O and 0.01 mol l−1 C6F12N4, at the temperature of 90 ◦C for 3 h. Then the two substrates growing with ZnO microrods were picked out from the solution, washed in deionized water and