Surface plasmon enhanced band edge luminescence of ZnO nanorods by capping Au nanoparticles

C. W. Cheng, E. J. Sie, B. Liu, C. H. A. Huan, T. C. Sum, H. D. Sun, H. J. Fan
2010 Applied Physics Letters  
The author observe sixfold enhancement in the near band gap emission of ZnO nanorods by employing surface plasmon of Au nanoparticles, while the defect-related emission is completely suppressed. Time-resolved photoluminescence indicates that the decay process becomes much faster by Au capping. The remarkable enhancement of the ultraviolet emission intensities and transition rates is ascribed to the charge transfer and efficient coupling between ZnO nanorods and Au surface plasmons. The
more » ... smons. The suppression of the green emission might be due to a combined effect of Au surface plasmon and passivation of the ZnO nanorod surface traps. Surface plasmons ͑SPs͒, excited by the interaction between light and electron plasma waves at the metal surface, 1 has attracted intense scientific interest due to its applications in enhancement of the weak physical process, such as the absorption of light in molecules 2,3 and Raman scattering intensities. 4,5 Recently, SPs mediated emission has also been proven as an effective way to improve the quantum efficiency of light emitting materials and light emission diodes ͑LEDs͒. 6,7 Among direct band gap crystals, ZnO has a wide band gap of ϳ3.37 eV and large exciton binding energy of 60 meV at room temperature. Proof-of-concept demonstrations of photonic applications of ZnO nanorods are available such as ultraviolet lasers 8 and LEDs. 9 However, in most cases, a large number of carriers are trapped by defects or impurities inside luminescence centers, resulting in a low efficiency of the light emission. Hence, to obtain highly efficient ultraviolet emission from the near band edge is one of most important issues for photonic applications of ZnO. In the past few years, a number of studies have been conducted to improve the band edge emission from ZnO films and nanostructures by metal capping, and different metals ͑e.g., Ag, Au, Al, and Pt͒ have been used as capping layers. [10] [11] [12] [13] [14] [15] [16] [17] [18] However, there are relatively few publications that address the luminescence dynamics of ZnO/metal structures, and the correlation between SPs and the photoluminescence ͑PL͒ enhancement is in general not well documented. In this letter, we have studied the PL properties of the bare and gold-capped ZnO nanorods. It is found that the band edge emission can be dramatically enhanced, while the defect-related emission is suppressed to the noise level. The intensity ratio of the band gap emission to the defect-related emission can be improved by up to 10 3 times. In addition, the band edge PL decay also becomes faster after the Au capping. A possible excitons coupling mechanism with the metal SPs will be discussed. Vertical aligned ZnO nanorods were grown on GaN/ sapphire substrates through a standard vapor-transport and deposition method as reported elsewhere. 19 The capping Au nanoparticles were deposited on ZnO nanorods using a dc sputtering system ͑JFC-1600, JEOL͒. The coverage of Au particles was controlled by using different sputtering time at a fixed current of 10 mA. PL measurements were performed by excitation from a 325 nm line of a continuous-wave He-Cd laser. For time-resolved PL ͑TR-PL͒ measurements, the sample was excited by 310 nm laser pulses generated from an optical parametric amplifier ͑Coherent TOPAZ™͒ that was pumped using a 100 fs, 1 kHz regenerative amplifier ͑Coherent Legend™͒. The TR-PL data were collected by an Optronis™ Optoscope streak camera combined with a high resolution charge-coupled device camera. All the data were obtained from the same ZnO sample, which was cut into four pieces for PL measurement before Au sputtering. We found that there was nearly no difference in the PL intensity for all four samples before the Au sputtering. Also the PL spectra were recorded at different positions on each sample in order to assure the change in PL intensity is not due to sample nonhomogeneity; we herein present the representative spectra. Moreover, all the PL data were collected under the same conditions ͑light path, excitation power, and acquisition time͒ to exclude any other effect on the PL intensity. Figures 1͑a͒ and 1͑b͒ show the 20°tilted view SEM images of the bare ZnO nanorods and Au/ZnO nanorods with 50 s Au sputtering. The ZnO nanorods have a diameter of 60-70 nm and length of ϳ1 m. Clearly, the bare ZnO nanorod has a smooth surface ͓Fig. 1͑c͔͒, while for the Au/ZnO after 50 s sputtering, Au nanoparticles with diameters of about 5 nm are uniformly distributed on the surface of the ZnO nanorod. Reasonably, the sizes of the Au particles for Au/ZnO with 20 and 100 s sputtering time are respectively smaller and larger than 5 nm. Figure 2 shows the room temperature PL spectra of ZnO nanorods before and after Au sputtering. For the bare ZnO nanorods, a weak near band gap emission at around 378 nm and a broad and intense defect-related emission band centered at ϳ520 nm are observed, which are relatively common features for ZnO nanostructures. After the Au sputtering, it is found that the defect emission disappears for all the samples. Moreover, the band edge emission intensity is variably enhanced with Au sputtering, namely, the Au nanoparticle size. As shown, the enhancement factor reaches a maximum value for the sputter time of 50 s and then decreases. a͒ Electronic addresses: hdsun@ntu.edu.sg and fanhj@ntu.edu.sg. APPLIED PHYSICS LETTERS 96, 071107 ͑2010͒
doi:10.1063/1.3323091 fatcat:z5s22jvbkvawjfelxtnw4hyghq