JNN

School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, People’s Republic of China 2 Instrumental Analysis and Research Center, Sun Yat-Sen University, Guangzhou, 510275, China 3 Wuhan National Laboratory for Optoelectronics, and College of Optoelectronic Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan, 430074, China 4 Materials Science and Engineering, National Tsing Hua University 101, Hsinchu, Taiwan 300, R.O.C 5 Center for Advanced Luminescence Materials, Department of Physics, Hong Kong Baptist University, Kowloon Tang, Hong Kong
Luo et al.
ions into ZnO nanocrystals, since energy transfer from the dried in an oven at 40 C and then annealed in air at 150 C for 1 h. host to RE ions has been observed.2–16 However, more than one local environment was found for trivalent RE ions in ZnO nanocrystals, and broadened luminescence 2.2. Measurements lines of Eu3+ were observed due to poor incorporation The morphology, crystal structure, and microscopic strucof Eu3+ ions into the ZnO lattice.10 11 Furthermore, the ture were investigated by transmission electron microscopy near ultraviolet-excited Eu3+ emission was relatively weak (JEOL-3000F), X-ray diffraction (XRD, D/max 2200 v, compared to the broadband green emission from the defect RIGAKU Corp., Cu K , and Raman spectra (Renishaw state of ZnO host and the emission quantum yield has not inVia, 514 nm, UK), respectively. The emission and exciyet been reported. The local symmetry around the lantation spectra were recorded with an F-7000 fluorescence thanide ion and the energy transfer mechanism, are still spectrometer at room temperature. For low temperature not well understood. However, in our work, using a low PL measurement, a 10 K closed-cycle cryostat was used. temperature wet chemical doping technique and chemical For emission quantum yield measurement, the excitation surface modification, we found that the doping efficiency source was a N2 laser (337 nm, 4 ns, 260 J, 10 Hz). can be improved substantially and Eu3+ ion-doped ZnO The quantum yield value for a given sample can be nanocrystals could be excited efficiently by near ultravidetermined by comparing the emission integral intensity olet light which matches well with the emission of GaN and the absorption coefficients with standard phosphors, based LED chips. A sharp and intense red light emission whose quantum yields have been previously determined without defect background was achieved under ultraviolet Delivered by Ingenta to: by absolute measurements.33 The selected quantum yield excitation, due to the highly efficient energy transfer from Hong Kong Polytechnic University 3+ standard was sodium salicylate (NaSal), which has a broad the ZnO host to Eu ions. The emission quantum yield IP : 175.159.5.106 band emission with a maximum at 450 nm and ST = 60% of Eu3+ ions-doped ZnO nanocrystals was measured to be Tue, 19 Jun 2012 04:03:17 at room temperature. It is better suited as a standard, since 31% at room temperature. This is comparable to the comit is readily available and shows a nearly constant quantum mercial red phosphors for white light emitting diodes, like yield up to 380 nm excitation. This method was accuY2 O2 S:Eu3+ , which cannot be effectively excited by blue rate to within 10% reported in Ref. [33]. Utmost care was light. Moreover, the physical and chemical properties of taken to ensure a constant and reproducible position for sulfur complexes are not stable.14 the sample or standard holder and unchanged instrumental In this paper, we present a detailed study on the incorconditions throughout all measurements. The samples and poration, excitation, and emission properties of single standards were thoroughly ground to a fine powder in an crystalline Eu3+ ion-doped ZnO nanocrystals prepared by agate mortar in order to minimize grain size differences. using a co-precipitation method. Moreover, the temperaAll measurements were carried out using a compacted ture dependence of RE luminescence has also been investipowder layer 2 mm thick to prevent insufficient absorpgated, showing that carrier-mediated energy transfer plays tion and back scattering of the excitation radiation. The an important role in the energy transfer process. emission spectra were previously corrected for the spectral response of the photomultiplier and the monochromatic 2. EXPERIMENTAL DETAILS grating. The reflection coefficient r was established by scanning the emission monochromator through the excita2.1. Synthesis and Surface Modification tion wavelength region, and the integral intensities of the spectra were thus obtained. In order to have absolute r First, 8.87 g analytically pure zinc acetate dihydrate value, MgO was used as a reflection standard (r = 0 91 . (Zn (OOCCH · 2H O) was dissolved in 120 ml ethanol,