Luminescence properties of ZnO and Eu3+-doped ZnO nanorods

Luminescence properties of ZnO and Eu3+-doped ZnO nanorods

Thin Solid Films 486 (2005) 50 – 52 www.elsevier.com/locate/tsf Luminescence properties of ZnO and Eu3+-doped ZnO nanorods A. Ishizumia,*, Y. Taguchi...

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Thin Solid Films 486 (2005) 50 – 52 www.elsevier.com/locate/tsf

Luminescence properties of ZnO and Eu3+-doped ZnO nanorods A. Ishizumia,*, Y. Taguchia, A. Yamamotoa, Y. Kanemitsua,b a

Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan International Research Center for Elements Science, Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

b

Available online 2 March 2005

Abstract We have studied crystal structure, shape, and photoluminescence (PL) properties of undoped and Eu-doped ZnO nanorods. The transmission electron microscope studies show that the undoped and Eu-doped ZnO nanocrystals have the rod-like shape. Under the high energy excitation above the band-gap energy of ZnO, broad PL band as well as bound-exciton emission were observed in Eu-doped ZnO nanorods. On the contrary, several sharp PL lines were observed when the excitation energy almost coincides with the intra-4f transition energy of Eu3+ ions. The broad PL band is thought to be due to the defects in ZnO nanorods while the sharp PL peaks are attributed to intra4f transitions of Eu3+ ions in ZnO nanorods. D 2005 Elsevier B.V. All rights reserved. Keywords: ZnO; Nanorod; Photoluminescence; Luminescence center

1. Introduction Much attention has been paid to the optical properties of II–VI semiconductor nanocrystals, such as CdSe, CdS, and ZnS, because their optical properties depend strongly on the nanocrystal size and shape [1–4]. High quality II–VI semiconductor nanocrystals also become materials for doping of optically active impurities. The II–VI semiconductor nanocrystals doped with luminescence centers exhibit efficient luminescence even at room temperature [5,6]. Therefore, there are many studies on the fabrication and optical properties of II–VI semiconductor nanocrystals doped with luminescence centers such as transition-metal ions [5–8], rear-earth ions [9,10], and donor–acceptor pairs [11]. The chemical synthesis methods are one of the most useful techniques for the fabrication of II–VI semiconductor nanocrystals doped with luminescence centers [5–7,9,10]. The size and shape of II–VI semiconductor nanocrystals can be controlled by the chemical synthesis methods [2–4]. Recently, in compound semiconductors, oxides and nitrides are expected for electronics and optoelectronics

* Corresponding author. Tel./fax: 81 743 72 6015. E-mail address: ishi[email protected] (A. Ishizumi). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.229

materials, rather than sulfides and selenides. Since ZnO has wide band-gap energy (3.37 eV at room temperature), ZnO nanocrystals are the suitable host materials for the doping of luminescence centers. Furthermore, ZnO is an environmentally friendly material and is expected as the new materials for the future optoelectronic devices. However, the optical properties of impurity-doped ZnO nanocrystals are not known in detail. In this work, we have studied photoluminescence (PL) properties of undoped and Eu-doped ZnO nanorods. Transmission electron microscope (TEM) studies show that both samples have a rod-like shape. The PL properties of undoped and Eu-doped ZnO nanorods are discussed.

2. Experimental Eu-doped ZnO nanorods were fabricated by a microemulsion method, which is one of the chemical techniques for the fabrication of the metal oxide nanocrystals [12,13]. The nanoparticles of hydroxide of Zn and Eu were synthesized as a precursor of ZnO:Eu nanocrystals in the aqueous solution encapsulated within the reverse micelles. The nanoparticles of hydroxides were oxidized by heating and were converted to ZnO:Eu nanocrystals. The size of

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nanocrystals synthesized in the reverse micelles can be usually controlled by changing the ratio of water to surfactant, because the size of reverse micelles depends on the ratio of water to surfactant [14]. However, we did not change the nanocrystal size in this work. For the comparison between the optical properties of undoped and Eu-doped ZnO nanocrystals, undoped ZnO nanocrystal samples were purchased from Sigma-Aldrich Co. The shape of undoped and Eu-doped ZnO nanocrystals were characterized by TEM (JEOL, JEM-3100FEF, 300 kV) measurements. For PL measurements, a He–Cd laser (325 nm) and an Ar+ laser (465.8 nm) were used as the excitation sources. The PL signals from the samples were detected by a photomultiplier tube through a 25-cm monochromator. The signal was amplified by a lock-in amplifier. The spectral sensitivity of the measuring systems was calibrated using a tungsten standard lamp.

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3. Results and discussion Fig. 1(a) and (b) show TEM images of undoped and Eu-doped ZnO nanocrystals, respectively. These TEM images show that undoped and Eu-doped ZnO nanocrystals have the rod-like shape, i.e. they are nanorods. In addition, X-ray diffraction measurements showed that undoped and Eu-doped ZnO nanocrystals have the hexagonal structure, and no diffraction peaks are detected from any other products such as europium oxides. Therefore, we believe that Eu ions are doped in ZnO nanorods [13,15]. Fig. 2(a) and (b) show the PL spectra of undoped and Eu-doped ZnO nanorods, respectively, under 3.81 eV (325 nm) light excitation at 20 K. The emission peak is observed at 3.36 eV in the PL spectrum of ZnO:Eu nanorods, while the several emission peaks are observed around 3.35 eV in the PL spectrum of undoped ZnO nanorods. In the same energy region, similar emission peaks are observed in bulk

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Fig. 2. PL spectra of (a) undoped and (b) Eu-doped ZnO nanorods under 3.81 eV (325 nm) light excitation at 20 K.

ZnO crystals and are assigned to bound excitons [16]. Since the size of Eu-doped nanocrystals is much larger than the exciton Bohr radius of ZnO (1.4 nm), the quantum size effects on the PL spectrum are negligibly small. Therefore, it is considered that the emission peak at 3.36 eV in ZnO:Eu nanorods is related to the bound excitons in ZnO nanorods. In the undoped ZnO nanorods, it was concluded in our previous reports [15] that the highest energy peak is related to the bound excitons, but the origins of lower energy peaks are different from that of the highest energy peak. In Eu-doped ZnO nanorods, the broad band emission is observed around 1.9 eV in addition to the bound-exciton PL near the band edge of ZnO. This broad PL is not observed in undoped ZnO nanorods. By doping of Eu ions

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50 nm Fig. 1. TEM images of (a) undoped and (b) Eu-doped ZnO nanorods.

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4. Conclusions

Acknowledgements

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We have studied PL properties of undoped and Eu-doped ZnO nanorods. In the TEM images of both the samples, rodlike ZnO nanocrystals are observed. In the PL spectra of both the samples, the bound-exciton PL is observed. In the ZnO:Eu nanorods, doped Eu ions exist as the trivalent ions. The ZnO:Eu nanorods exhibit the sharp PL peaks due to the intra-4f transitions of Eu3+ under the low-energy excitation below the band-gap energy of ZnO.

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This work was supported in part by a Grant-in-Aid for Scientific Research (KAKENHI, 14340093) from the Japan Society for the Promotion of Science, The Research Foundation for Opto-Science and Technology, and The Japan Securities Scholarship Foundation.

Fig. 3. PL spectrum of Eu-doped ZnO nanorods under 2.66 eV (465.8 nm) light excitation at 20 K.

References into ZnO nanorods, the low-energy and broad PL band appears. Two different origins of the broad band are considered: the defects or Eu2+ ions in ZnO nanorods. In order to clarify the origin of the broad PL in Eu-doped ZnO nanorods, we tried to measure the PL spectrum of the samples under the low-energy excitation below the bandgap energy. Fig. 3 shows the PL spectrum of ZnO:Eu nanorods under 2.66 eV (465.8 nm) light excitation at 20 K. Several sharp PL lines are observed, and the energies of all the peaks agree well with the energies of intra-4f transitions of Eu3+ ions. The assigned intra-4f transitions are shown in the figure. The broad band emission around 1.9 eV is very weak, because the excitation energy, 2.66 eV, is lower than the band-gap energy of ZnO. The excitation energy almost coincides with the energy of 7 F0–5D2 transition of Eu3+ ions, which is 2.67 eV [17]. The direct excitation of Eu3+ enhances the PL due to Eu3+ ions. The sharp PL lines due to Eu3+ ions are clearly observed under 2.66 eV light excitation, while these are not observed under 3.81 eV light excitation as in Fig. 2(b). This result implies that Eu ions exist in the ZnO nanorods as the Eu3+ ions, rather than the Eu2+ ions. It is believed that the defects are introduced by the doping of trivalent Eu ions into ZnO nanorods. The broad band emission observed in the ZnO:Eu nanorods is attributed to the defects in ZnO nanorods.

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