Enhancing blue luminescence from Ce-doped ZnO nanophosphor by Li doping
© Shi et al.; licensee Springer. 2014
Received: 1 July 2014
Accepted: 7 September 2014
Published: 10 September 2014
Undoped ZnO, Ce-doped ZnO, and (Li, Ce)-codoped ZnO nanophosphors were prepared by a sol-gel process. The effects of the additional doping with Li ions on the crystal structure, particle morphology, and luminescence properties of Ce-doped ZnO were investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and photoluminescence spectroscopy. The results indicate that the obtained samples are single phase, and a nanorod shaped morphology is observed for (Li, Ce)-codoping. Under excitation with 325 nm light, Ce-doped ZnO phosphors show an ultraviolet emission, a green emission, and a blue emission caused by Zn interstitials. The spectrum of the sample codoped with a proper Li concentration features two additional emissions that can be attributed to the Ce3+ ions. With the increase of the Li doping concentration, the Ce3+ blue luminescence of (Li, Ce)-codoped ZnO is obviously enhanced, which results not only from the increase of the Ce3+ ion concentration itself but also from the energy transfer from the ZnO host material to the Ce3+ ions. This enhancement reaches a maximum at a Li content of 0.02, and then decreases sharply due to the concentration quench. These nanophosphors may promise for application to the visible-light-emitting devices.
78.55.Et; 81.07.Wx; 81.20.Fw
ZnO is an n-type semiconductor material with a wide band gap of 3.3 eV and a large exciton binding energy of 60 meV. Room temperature photoluminescence (PL) spectra from ZnO can exhibit an ultraviolet (UV) emission and possibly one or more visible emissions caused by defects and/or impurities. It has been reported that doping a ZnO host crystal structure with rare earth elements such as Tb, Er, and Ce can lead to excellent luminescence properties[2–4]. Especially, the blue light-emitting Ce-doped ZnO has received particular attention because of its high chemical stability, excellent optoelectronic properties, avirulence behavior, and biological compatibility, resulting in potential applications in the field of visible-light-emitting devices and biological fluorescence labeling. Therefore, a further enhancement of the emission intensity in the blue emission band of Ce-doped ZnO phosphor is highly desirable.
It is well known that Li+ ions, as dopants, even in very small quantities, frequently play an important role in improving the luminescence intensity of phosphors. Gu et al. reported that Li+ doping can enhance the luminescence of Dy-doped ZnO nanocrystals by increasing the recombination probability of electrons and trapped holes. Recently, Chen et al. found that the red-light emission of a Eu-doped CaWO4 phosphor can be increased by using Li+ ions as charge compensators. A similar enhancement of the fluorescence of Pr-doped BaMoO4 phosphors via codoping with Li+ ions was obtained by He et al.. Therefore, the incorporation of Li+ ions into a Ce-doped ZnO phosphor is also expected to enhance the blue luminescence intensity. To investigate this potential enhancement, we prepared samples of undoped ZnO, Ce-doped ZnO, and (Li, Ce)-codoped ZnO nanophosphors by a sol-gel process. Here, we focus on the effect of the variation of the concentration of Li+ ions on the structure, morphology, and luminescence properties of the Ce-doped ZnO phosphor, while the Ce doping concentration was kept at a constant level. Also, we discuss the origin of visible light emission in our samples and propose possible mechanisms to explain the enhanced blue luminescence caused by the codoping with Li+ ions. Our results demonstrate that (Li, Ce)-doped nanophosphors are promising candidates for applications in the field of visible-light-emitting devices.
Undoped ZnO, Ce-doped ZnO, and (Li, Ce)-codoped ZnO phosphors were synthesized by a sol-gel process. Typically, the corresponding starting materials, Zn(CH3COO)2 · 2H2O, Ce(NO3)3 · 6H2O, and CH3COOLi · 2H2O, were mixed according to the nominal stoichiometric ratio (mol ratio, Zn/Ce/Li = (0.996 - x)/0.004/x, 0 ≤ x ≤ 0.04), and were dissolved in a certain amount of deionized water. Each solution was then added to 60 ml of a 1% (w/v) aqueous solution of polyvinyl alcohol acting as a stabilizer, and was stirred for 1 h. The mixtures were aged for 12 h at room temperature and then were heated to 80°C and maintained at this temperature until homogeneous gels had formed. The gels were air-dried at 120°C for 12 h, ground, and preheated at 400°C for 4 h in a muffle furnace. The last step consisted of a final annealing procedure at 550°C for another 4 h in air.
The samples thus obtained were investigated by X-ray diffraction (XRD) with CuKα radiation (λ = 0.15406 nm) in order to identify the individual phases. The particle morphology was analyzed in a Hitachi S4800 (Hitachi, Tokyo, Japan) scanning electron microscope (SEM). PL spectra were recorded on an Edinburgh FLS920 spectrofluorometer (Edinburgh Instruments, Edinburgh, UK) equipped with a 450-W Xe lamp as the excitation light source. The X band electron paramagnetic resonance (EPR) spectra were determined by a Bruker ER-200D-SRC EPR spectrometer (Bruker, Billerica, MA, USA). X-ray photoelectron spectroscopy (XPS) experiments were performed on a Thermo ESCALAB 250XI multifunctional imaging electron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).
Results and discussion
XPS quantitative analysis of Ce 0.004 Zn 0.996-x O:xLi + (x = 0.02) phosphors
Total area (%)
Figure 5iii depicts the EPR spectrum of the Ce0.004Zn0.996 - xO:x Li+ (x = 0.02) sample. In addition to the two signals at g = 1.999 and g = 1.957, two additional signals at g = 2.158 and g = 2.001 are clearly visible. According to literatures[21, 22], the weaker signal at g = 2.001 can be attributed to O2- adsorbed on the sample surface. From the XPS results, we know that Ce3+ and Ce4+ coexist in Ce0.004Zn0.996 - xO:x Li+ (x = 0.02) phosphors. Ce4+ ions are diamagnetic and cannot be detected by the EPR technique. On the other hand, Ce3+ ions are paramagnetic, with spin 1/2, and thus can be detected. It has been reported that Ce3+ ions located in an axisymmetrical field are characterized by the EPR signal pair g∥ = 1.94 and g⟂ = 1.96, while Ce3+ ions in different asymmetric fields give different resonance signals[22–24]. In our work, Figures 1b and5iii confirm the presence of interstitial Li, substitutional Li, interstitial Zn, and oxygen vacancies in the Ce0.004Zn0.996 - xO:x Li+ (x =0.02) crystal, which could destroy the axial symmetry of the field around Ce3+ and lead to Ce3+ ions located in asymmetric sites. Also, Wang et al. reported a broad signal of g = 2.15 for Ce(OH)3, which is a typical EPR signal of Ce3+ in Ce(OH)3 nanorods. Therefore, we ascribe the signal at g = 2.158 to Ce3+ being in an asymmetric field. In addition, the energy difference between the two emission peaks (411 and 446 nm) is about 1,910 cm-1, which is very close to the theoretical difference of about 2,000 cm-1 between the 2F5/2 and 2F7/2 ground state levels of Ce3+. Thus, based on the analysis above, we believe that the blue emissions at about 411 and 446 nm are associated with the respective 2D3/2 → 2F5/2 and 2D3/2 → 2F7/2 transitions of Ce3+.
In summary, undoped, Ce-doped and (Li, Ce)-codoped ZnO phosphors were synthesized using a sol-gel process. The crystal structure, particle morphology, and luminescence properties of the obtained samples were investigated as a function of the content of Li ions. All of the samples only show a single phase. For a Li content of x ≤ 0.005, substitutional Li ions play a predominant role in the (Li, Ce)-codoped ZnO crystal, while interstitial Li ions play a predominant role for x ≥ 0.01. PL spectrum of Ce-doped ZnO consists of a UV emission, a blue emission related to the Zn interstitial, and a green emission assigned to oxygen vacancies. Comparing these results with the PL spectra for (Li, Ce)-codoped ZnO (x ≥ 0.01) phosphors, in addition to the blue emission due to the Zn interstitial (at 463 to 467 nm), the latter exhibit two additional strong blue emissions at 411 and 446 nm ascribed to the Ce3+ ions. This is because incorporating Li ions at a sufficient concentration in Ce-doped ZnO can cause the reduction of Ce4+ to Ce3+. As the Li doping concentration is increased, the intensity of the blue-light emissions related to Ce3+ increases, which results not only from the increase of the Ce3+ ion concentration itself but also from the energy transfer from the ZnO to Ce3+. It reaches maximum at about x = 0.02, and then decreases sharply at a higher doping concentration due to the concentration quench. (Li, Ce)-codoped ZnO phosphors are expected to find potential applications in the field of visible-light-emitting devices.
This project was financially supported by the National Natural Science Foundation of China (nos. 61275147 and 91222110), the Shandong Province Natural Science Foundation of China (nos. ZR2013EML006, ZR2012EMM007, ZR2012AL11, and ZR2010EQ001), the Research Foundation of Liaocheng University (no. 318011311), and the Special Construction Project Fund for Shandong Province Taishan Scholars.
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