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Microstructural and optical properties of europium-doped zinc oxide nanowires
Journal of Physics and Chemistry of Solids 74 (2013) 1733–1738
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Microstructural and optical properties of europium-doped zinc oxide nanowires S.A. Al Rifai a,n, B.A. Kulnitskiy b a b
Voronezh State University, Universitetskaya Place 1, Voronezh 394006, Russian Federation Technological Institute for Superhard and Novel Carbon Materials (TISNCM), 7а Centralnaya Street, Moscow, Troitsk 142190, Russian Federation
art ic l e i nf o
a b s t r a c t
Article history: Received 21 April 2013 Received in revised form 25 June 2013 Accepted 28 June 2013 Available online 6 July 2013
Single-crystal Eu3+-doped wurtzite ZnO micro- and nanowires were synthesized by chemical vapor deposition. The nanostructures grew via a self-catalytic mechanism on the walls of an alumina boat. The structure and properties of the doped ZnO were characterized using X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning and transmission electron microscopy, and photoluminescence (PL) methods. A 10-min synthesis yielded vertically grown nanowires of 50–400 nm in diameter and several micrometers long. The nanowires grew along the 7 [0001] direction. The Eu3+ concentration in the nanowires was 0.8 at.%. The crystal structure and microstructure of were compared for Eu3+-doped and undoped ZnO. PL spectra showed a red shift in emission for Eu3+-doped (2.02 eV) compared to undoped ZnO nanowires (2.37 eV) due to Eu3+ intraionic transitions. Diffuse reflectance spectra revealed widening of the optical bandgap by 0.12 eV for Eu3+-doped compared to undoped ZnO to yield a value of 3.31 eV. Fourier-transform infrared spectra confirmed the presence of europium in the ZnO nanowires. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. oxides B. vapor deposition D. luminescence
1. Introduction One-dimensional nanostructures have attracted increasing attention because of their unique optical properties. Zinc oxide is a very important semiconductor with a wide bandgap and large exciton binding energy. The synthesis of 1D single-crystal ZnO nanostructures is of interest owing to their promise for nanoscale optoelectronic devices and gas sensors. Rare earth (RE)-doped semiconductors have been extensively studied for potential use in integrated optoelectronic devices such as visible (blue, green, and red) and infrared luminescent devices. In particular, doping of zinc oxide with optically active ions, such as RE ions, allows the fabrication of devices that can emit light of different wavelengths from the UV to the visible range [1–5]. The unique electronic, optical, and chemical properties of RE oxides make them useful in a variety of diverse applications such as laser materials, phosphors, catalysis, and magnetic devices. One of the potential applications of europium-doped nanowires is as waveguides in nanophotonic devices, similar to the use of Er-doped silica fibers in optoelectronics [6]. The luminescence of Eu3+ is particularly interesting because the major emission band is centered near 612 nm (red), corresponding to one of the three primary colors. However, it is difficult to obtain effective europium ion concentrations in zinc oxide nanostructures. The relatively large radius and charge mismatch
n
Corresponding author. Tel.: +7 9204214048; fax: +7 4372208363. E-mail addresses: [email protected], [email protected] (S.A. Al Rifai).
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.06.019
between trivalent RE ions and divalent zinc ions hampers successful Eu3+ incorporation in the ZnO lattice. Luo et al. prepared Eu-doped ZnO nanocrystals using a sol–gel method [1]. The UV lasing efficiency was enhanced and the lasing threshold decreased in Eu-doped ZnO nanocrystals at room temperature, which was attributed to a passivation mechanism. In the present study, we prepared Eu3+-doped ZnO nanowires via a vapor–liquid–solid (VLS) mechanism that activated europium ions for successful doping. 2. Experimental High-purity metallic zinc and europium oxide (Eu2O3) powder at a Zn/Eu2O3 molar ratio of 4:1 were placed in an alumina crucible at the center of a quartz tube (2 cm in diameter). The tube was inserted in a horizontal tube furnace. All experiments were carried under normal atmospheric pressure. The furnace was heated to 1050–1100 1C over a period of 30 min and was kept at this temperature for a further 10 min. Nitrogen was used as the vapor carrier gas at a flow rate of 160–170 sccm with a controlled oxygen impurity flow (30 sccm). This procedure yielded white wool-like material at the ends of the alumina boat. For comparison, undoped ZnO nanowires were prepared under similar experimental conditions without Eu2O3 powder. X-Ray diffraction (XRD) patterns were recorded on a DRON-4 instrument (Cu Kα emission, 2θ range 30–801, step 0.11) and were compared to the electronic PDF2 library. Transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and
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energy-dispersive X-ray spectrometry (EDX) were performed on a Jeol JEM-2100 transmission electron microscope operated at 200 kV. Specimens for TEM were prepared by sonicating assynthesized samples in isopropanol for 15 min. A few drops of the resulting suspension were placed on a TEM grid. The sample morphology was observed by scanning electron microscopy (SEM; JSM-6380LV). Photoluminescence (PL) measurements were carried out on an automatic spectrometer. Samples were excited by UV radiation from a nitrogen laser (λmax ¼337 nm). The luminescence of the sample was recorded with a photomultiplier detector (R928P, Hamamatsu, Japan, with a C4900-51 energy source) operating in photon counting mode and a diffraction monochromator (MDR-23). Excitation light fluxes were automatically controlled by a PC. PL measurements were obtained at 77 K. Diffuse reflectance spectra (DRS) were recorded in the range 300–800 nm at a medium scan rate with a 0.5-nm slit at room temperature (Shimadzu 2450 PC spectrophotometer equipped with an integrating sphere). Reflectance measurements were converted to absorption spectra using the Kubelka–Munk function. The sample composition was characterized by Fourier-transform infrared (FTIR) spectroscopy at room temperature in the range 550– 4000 cm–1 using a VERTEX 70 instrument. ZnO samples were mixed with KBr at a ratio of 5:100 and the background spectrum for KBr was subtracted from sample spectra.
3. Results and discussion SEM images reveal that the morphology and crystal structure differ strongly for Eu3+-doped and undoped ZnO. Fig. 1 shows that well-aligned Eu-doped nanowires formed on the alumina crucible. In contrast, undoped ZnO yielded nanotetrapods in different parts of the quartz tube, particularly at 20 cm from the metallic zinc (Fig. 2). TEM images for Eu-doped ZnO (Fig. 3a,b) show that the nanowires are single crystalline and grew in the 7[0001] direction. As revealed by the SAED pattern and HRTEM analysis (Fig. 3c, d), nanowires grew along the direction of the ZnO c-axis. The SAED pattern obtained for one leg of a tetrapod-like undoped ZnO nanostructure can be indexed as hexagonal ZnO (Fig. 4c), indicating that these ZnO nanostructures are single crystalline with growth for each tetrapod leg along the 7[0001] c-axis (Fig. 4b). An EDX spectrum (Fig. 5) confirmed the presence of europium (0.8 at.%) in the doped nanowires. Europium ions incorporated in the ZnO lattice behave as active centers, influencing the morphology and nanostructure. Europium has a larger ionic radius than zinc, so Eu3+ incorporation leads to expansion of the ZnO lattice. We assume that self-assembled tetrapods formed via a catalystfree vapor–solid (VS) growth mechanism in different parts of the quartz tube. Catalyst-assisted VLS mechanisms usually yield ZnO nanowires of similar size and a uniform orientation [11]. Some researchers believe that catalyst doping of the nanowires occurs as contamination [10], whereas others have suggested that it evaporates during the growth process. However, convincing experimental evidence is lacking. Zhu et al. proposed a three-stage growth
model in which VLS and vapor–solid growth compete with each other [11]. In our case, Eu3+ ions partly influence nanowire growth as a catalyst. However, TEM analysis showed that the nanowire tips are free of any catalyst particles (Fig. 3b). Therefore, we suggest that europium plays two roles during synthesis. First, it is crucial for growth initiation by absorbing Zn vapor. Second, it precipitates ZnO surface drops, which not only provide templates for further growth but also guide the growth orientation, leading to nanowire alignment and successful Eu3+ incorporation in the ZnO lattice. In the initial growth stage, Zn vapor can be easily melt into europium particles to form a liquid alloy. After supersaturation, Zn reacts with oxygen, precipitating out a very thin layer of drops with catalyst particles. Thus, the growth is governed by a VLS mechanism. In the case of undoped ZnO, for which there is no europium to act as a catalyst, we observed self-assembled growth of tetrapods via a VS mechanism. The crystal structure of the samples was characterized by XRD (Fig. 6). The XRD patterns show only peaks corresponding to hexagonal wurtzite ZnO (space group C6mc). No diffraction peaks for Eu2O3 can be detected. All the peaks for the doped sample are shifted to lower 2θ angles and show broadening. High-resolution XRD data for (1 0 0), (1 0 1), and (2 0 1) peaks are shown in Fig. 7a–c. The shift in peak position to lower 2θ angle and the increase in lattice parameters can be attributed to doping with Eu3 + cations, which are larger (effective ionic radius 0.95 Å) than Zn2+ (0.74 Å). Mohanty et al. ascribed this peak shifting and broadening to lattice mismatch and distortion and crystal strain [2]. In any case, the XRD results confirm europium doping in ZnO nanowires. We calculated the lattice constants for doped and undoped samples using the following equations: 2 2 1 4 h þ kh þ k L2 ¼ ⋅ þ 2 ð1Þ 2 2 3 a c d a2 ¼
4 A1 B2 A2 B1 ⋅ 2 3 ðB2 =d2 ÞðB1 =d
h 2 k 2 l2 Þ
h 1 k 1 l1
c2 ¼
A1 B2 A2 B1 2
2
ðA1 =dh2 k2 l2 ÞðA2 =dh1 k1 l1 Þ
;
ð2Þ
ð3Þ
where d is the interplanar distance, h and k are Miller indices, А¼h2+hk+k2, and B¼ L2. In the case of our hexagonal system, we used a pair of diffraction peaks with different Miller indices. For the ideal ZnO wurtzite structure, the parameters are a¼ b¼0.3249 nm and c¼ 0.5206 nm. For our undoped sample we obtained a ¼b ¼0.3256 nm and c¼0.5266 nm. Europium doping increased these parameters to a ¼b¼ 0.3291 nm and c¼0.5311 nm. Differences in the crystal field surrounding RE ions result in slight shifts of their luminescence peaks or differences in their relative intensity. In this case, local changes in the ZnO environment are caused by Eu3+ addition, which would lead to a modified local crystal field. The PL of different ZnO nanostructures is one of their most interesting and important properties. PL spectra (Fig. 8)
Fig. 1. SEM images of Eu3+-doped ZnO nano- and microwires.
S.A. Al Rifai, B.A. Kulnitskiy / Journal of Physics and Chemistry of Solids 74 (2013) 1733–1738
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Fig. 2. SEM images of undoped ZnO nanotetrapods.
Fig. 3. (a,b) TEM images, (c) SAED pattern, and (d) HRTEM image of Eu3+-doped ZnO nanowires.
Fig. 4. (a,b) TEM images and (c) SAED pattern for a ZnO tetrapod.
Fig. 5. EDX spectrum for Eu-doped ZnO nanowires.
confirmed our predictions for Eu3+ doping in nanowires. We observed two strong peaks, one in the UV region (3.21 eV) confirming high-quality ZnO nanowires and one in the red–orange range (2.02 eV), for the doped sample. The UV emission originates from excitonic recombination corresponding to near-band-edge emission of ZnO [15], indicating good ZnO nanostructure quality. The luminescence of europium ions is significantly influenced by the host crystal lattice because electronic transitions involve
only electron redistribution within the 4f layer. The higherintensity peak centered at 612 nm corresponds to the 5D0-7F2 transition that occurs via dipole electric force [5–7] and is more intense than the peak at 586 nm. The small peak for 5D0-7F1 is attributed to a magnetic dipole transition. The probability of the magnetic dipole transition 5D0-7F1 is mainly independent of the host matrix and the electric-dipole-allowed 5D0-7F2 transition is strongly influenced by the local structure and site asymmetry
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around the Eu3+ ion. Besides, the transition 5D0-7F1, which occurs via magnetic dipole, indicates that Eu3+ occupies locations without an inversion center. From emission spectral analyses it was possible to predict that Eu3+ ions are located at sites of higher symmetry. The dominant red emission for the 5D0-7F2 transition indicates inversion of the antisymmetric crystal field around Eu3+, which is advantageous in improving the color purity of red phosphors [17]. Moreover, XRD results confirmed that Eu3+ ions
Fig. 6. XRD patterns for Eu3+-doped and undoped ZnO confirm a hexagonal wurtzite ZnO structure.
were successfully incorporated in ZnO lattice sites, providing further support for the emission characteristics. We suggest that the peaks observed at 680 and 692 nm may be related to transitions from 5D0 to 7F3 and 7F4, respectively [12,13]. Compared to the doped sample, ZnO tetrapods showed only strong luminescence in the green–yellow region (522 nm). We attribute this to ionized oxygen vacancies, which can decrease the UV emission efficiency. Oxygen vacancies and zinc interstitial defects are the most probable reasons for the green emission of
Fig. 8. PL spectra for Eu3+-doped and undoped ZnO measured at 77 K.
Fig. 7. Magnifications of peaks in the XRD patterns for Eu3+-doped and undoped ZnO. (a) The main (1 0 0) peak shows a shift to lower 2θ angle on Eu3+ doping. (b) The (1 0 1) peak and (c) the (2 0 1) peak.
S.A. Al Rifai, B.A. Kulnitskiy / Journal of Physics and Chemistry of Solids 74 (2013) 1733–1738
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Fig. 9. (a) Diffuse reflectance spectra for Eu3+-doped and undoped ZnO samples. (b) Kubelka–Munk conversion.
Fig. 10. (a) FT-IR spectra of doped and undoped ZnO samples. (b) Magnification of the range 550–700 cm–1.
the undoped sample, which was grown under a flow of oxygen. Surface band bending revealed that chemisorbed oxygen played a crucial role in the recombination process for this emission. DRS is more convenient for characterizing unsupported nanomaterials than UV-Vis absorption spectroscopy since it takes advantage of the enhanced scattering in nanocrystalline materials. Our results reveal that europium doping of ZnO nanostructures increased and shifted the optical DRS peaks (Fig. 9a). The optical direct bandgap evaluated according to the well-known energy– exponential relation (Fig. 9b) was 3.31 eV for Eu-doped ZnO and 3.19 eV for ZnO tetrapods. This difference can result from variations in carrier concentrations and carrier scattering by microstructural defects, grain boundaries, and ionic impurities. It can also be explained in terms of lattice distortion due to incorporation of larger Eu3+ ions in the ZnO lattice. We suggest that widening of the bandgap is associated with the Moss–Burstein effect [8,18], which occurs when electron carrier concentrations exceed the density of states for the conduction band edge, a process related to degenerate doping in semiconductors. The Fermi level for Eu-doped ZnO lies between the conduction and valence bands. As the doping concentration is increased, electrons
populate states within the conduction band, which shifts the Fermi level to higher energy. In the case of degenerate doping, the Fermi level lies inside the conduction band. In our case of Eu-doped ZnO, an electron from the top of the valence band can only be excited into a conduction band above the Fermi level since all the states below the Fermi level are occupied. The Pauli exclusion principle forbids excitation into these occupied states. Thus, we observe an increase in the apparent bandgap. This slight increase in the bandgap can be explained in terms of a change in the nature and strength of the crystalline potential by addition of Eu3+ dopant ions, including the effect of their 4f electrons on the crystalline electronic states. Widening of the optical bandgap is due to filling of states near the bottom of the conduction band. The quality and composition of different ZnO nanostructures were characterized by FTIR spectroscopy at room temperature in the range 550–4000 cm–1 (Fig. 10). The average transmittance in the visible range was 97% for both samples. An absorption band at 557 cm–1 characteristic of wurtzite hexagonal ZnO (Zn–O stretching vibration) [9] was observed for both samples. The peak at 2400 cm–1 is assigned to CO2 molecules on the surface of the doped sample. The band at 2800–2950 cm–1 for the doped
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sample is due C–H stretching. The absorption at 3600–3800 cm–1 is ascribed to hydroxyl vibrations on the surface of the doped ZnO. The change in position for the ZnO absorption band reflects perturbation of the Zn–O–Zn network by presence Eu3+ [14–19]. Therefore, the FTIR results confirm that Eu3+ ions occupy Zn sites in the ZnO matrix, as observed from XRD measurements. 4. Conclusions 3+
Well-aligned epitaxial Eu -doped ZnO nanowires were grown by VLS on an alumina boat. Eu3+ doping of ZnO increased the lattice parameters because of the larger size of Eu3+ cations. The nanowires were single crystalline in the 7[0001] growth direction. The Eu3+ concentration in the nanowires was 0.8 at.%, which is sufficient to be of use in the fabrication of different optical devices. Red luminescence related to Eu3+ intraionic transitions was observed. The bandgap for ZnO widened from 3.19 to 3.31 eV on Eu doping, which can be attributed to the Moss–Burstein effect, in particular the influence of Eu3+ impurity ions on crystalline electronic states.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
L. Luo, L. Gong, Y.F. Liu, et al., Opt. Mater. 32 (2010) 1066. P. Mohanty, B. Kim, J. Park, Mater. Sci. Eng. B 138 (2007) 224. L. Chen, J.H. Zhang, et al., Opt. Express 16 (2008) 11796. C.-S. Chen, et al., Trans. Nonferrous Met. Soc. China 17 (2007) 704–707. T.S. Barros, B.S. Barros, et al., Mater. Sci. Forum (2008) 591–593. P. Hidalgo, E. Libert, et al., J. Phys. Chem. C 113 (2009) 17200–17205. Y.P. Du, Y.W. Zhang, L.D. Sun, C.H. Yan, J. Phys. Chem. C 112 (2008) 12234–12241. A.A. Dakhel, Opt. Mater. 31 (2009) 691–695. L. Feng, A. Liu, et al., Mater. Charact. 61 (2010) 128–133. L. Chen, S. Wu, Y. Yin, J. Phys. Chem. C 113 (2009) 21572–21576. G. Zhu, Y. Zhou, S Wang, et al., Nanotechnology 23 (2012) 055604. Z.L. Wang, Nano Today 5 (2010) 540–542. A. Ishizumi, Y. Kanemitsu, Appl. Phys. Lett. 86 (2005) 253106. J. Kaur, P. Kumar, et al., Int. Nano Lett. 3 (2013) 4. S.C. Lyu, Y. Zhang, H. Ruh, et al., Chem. Phys. Lett. 363 (2002) 134–138. M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113–116. M. Zhong, G. Shan, Y. Li, G. Wang, Y. Liu, Mater. Chem. Phys. 106 (2007) 305. Y. Ortega, P. Fernández, J. Piqueras, J. Nanosci. Nanotechnol. 10 (2010) 502–507. S. Das, A.A. Reddy, S. Ahmad, N. Nagarajan, G.V. Prakash, Chem. Phys. Lett. 508 (2011) 117–120.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Microstructural and optical properties of europium-doped zinc oxide nanowires S.A. Al Rifai a,n, B.A. Kulnitskiy b a b
Voronezh State University, Universitetskaya Place 1, Voronezh 394006, Russian Federation Technological Institute for Superhard and Novel Carbon Materials (TISNCM), 7а Centralnaya Street, Moscow, Troitsk 142190, Russian Federation
art ic l e i nf o
a b s t r a c t
Article history: Received 21 April 2013 Received in revised form 25 June 2013 Accepted 28 June 2013 Available online 6 July 2013
Single-crystal Eu3+-doped wurtzite ZnO micro- and nanowires were synthesized by chemical vapor deposition. The nanostructures grew via a self-catalytic mechanism on the walls of an alumina boat. The structure and properties of the doped ZnO were characterized using X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning and transmission electron microscopy, and photoluminescence (PL) methods. A 10-min synthesis yielded vertically grown nanowires of 50–400 nm in diameter and several micrometers long. The nanowires grew along the 7 [0001] direction. The Eu3+ concentration in the nanowires was 0.8 at.%. The crystal structure and microstructure of were compared for Eu3+-doped and undoped ZnO. PL spectra showed a red shift in emission for Eu3+-doped (2.02 eV) compared to undoped ZnO nanowires (2.37 eV) due to Eu3+ intraionic transitions. Diffuse reflectance spectra revealed widening of the optical bandgap by 0.12 eV for Eu3+-doped compared to undoped ZnO to yield a value of 3.31 eV. Fourier-transform infrared spectra confirmed the presence of europium in the ZnO nanowires. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. oxides B. vapor deposition D. luminescence
1. Introduction One-dimensional nanostructures have attracted increasing attention because of their unique optical properties. Zinc oxide is a very important semiconductor with a wide bandgap and large exciton binding energy. The synthesis of 1D single-crystal ZnO nanostructures is of interest owing to their promise for nanoscale optoelectronic devices and gas sensors. Rare earth (RE)-doped semiconductors have been extensively studied for potential use in integrated optoelectronic devices such as visible (blue, green, and red) and infrared luminescent devices. In particular, doping of zinc oxide with optically active ions, such as RE ions, allows the fabrication of devices that can emit light of different wavelengths from the UV to the visible range [1–5]. The unique electronic, optical, and chemical properties of RE oxides make them useful in a variety of diverse applications such as laser materials, phosphors, catalysis, and magnetic devices. One of the potential applications of europium-doped nanowires is as waveguides in nanophotonic devices, similar to the use of Er-doped silica fibers in optoelectronics [6]. The luminescence of Eu3+ is particularly interesting because the major emission band is centered near 612 nm (red), corresponding to one of the three primary colors. However, it is difficult to obtain effective europium ion concentrations in zinc oxide nanostructures. The relatively large radius and charge mismatch
n
Corresponding author. Tel.: +7 9204214048; fax: +7 4372208363. E-mail addresses: [email protected], [email protected] (S.A. Al Rifai).
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.06.019
between trivalent RE ions and divalent zinc ions hampers successful Eu3+ incorporation in the ZnO lattice. Luo et al. prepared Eu-doped ZnO nanocrystals using a sol–gel method [1]. The UV lasing efficiency was enhanced and the lasing threshold decreased in Eu-doped ZnO nanocrystals at room temperature, which was attributed to a passivation mechanism. In the present study, we prepared Eu3+-doped ZnO nanowires via a vapor–liquid–solid (VLS) mechanism that activated europium ions for successful doping. 2. Experimental High-purity metallic zinc and europium oxide (Eu2O3) powder at a Zn/Eu2O3 molar ratio of 4:1 were placed in an alumina crucible at the center of a quartz tube (2 cm in diameter). The tube was inserted in a horizontal tube furnace. All experiments were carried under normal atmospheric pressure. The furnace was heated to 1050–1100 1C over a period of 30 min and was kept at this temperature for a further 10 min. Nitrogen was used as the vapor carrier gas at a flow rate of 160–170 sccm with a controlled oxygen impurity flow (30 sccm). This procedure yielded white wool-like material at the ends of the alumina boat. For comparison, undoped ZnO nanowires were prepared under similar experimental conditions without Eu2O3 powder. X-Ray diffraction (XRD) patterns were recorded on a DRON-4 instrument (Cu Kα emission, 2θ range 30–801, step 0.11) and were compared to the electronic PDF2 library. Transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and
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S.A. Al Rifai, B.A. Kulnitskiy / Journal of Physics and Chemistry of Solids 74 (2013) 1733–1738
energy-dispersive X-ray spectrometry (EDX) were performed on a Jeol JEM-2100 transmission electron microscope operated at 200 kV. Specimens for TEM were prepared by sonicating assynthesized samples in isopropanol for 15 min. A few drops of the resulting suspension were placed on a TEM grid. The sample morphology was observed by scanning electron microscopy (SEM; JSM-6380LV). Photoluminescence (PL) measurements were carried out on an automatic spectrometer. Samples were excited by UV radiation from a nitrogen laser (λmax ¼337 nm). The luminescence of the sample was recorded with a photomultiplier detector (R928P, Hamamatsu, Japan, with a C4900-51 energy source) operating in photon counting mode and a diffraction monochromator (MDR-23). Excitation light fluxes were automatically controlled by a PC. PL measurements were obtained at 77 K. Diffuse reflectance spectra (DRS) were recorded in the range 300–800 nm at a medium scan rate with a 0.5-nm slit at room temperature (Shimadzu 2450 PC spectrophotometer equipped with an integrating sphere). Reflectance measurements were converted to absorption spectra using the Kubelka–Munk function. The sample composition was characterized by Fourier-transform infrared (FTIR) spectroscopy at room temperature in the range 550– 4000 cm–1 using a VERTEX 70 instrument. ZnO samples were mixed with KBr at a ratio of 5:100 and the background spectrum for KBr was subtracted from sample spectra.
3. Results and discussion SEM images reveal that the morphology and crystal structure differ strongly for Eu3+-doped and undoped ZnO. Fig. 1 shows that well-aligned Eu-doped nanowires formed on the alumina crucible. In contrast, undoped ZnO yielded nanotetrapods in different parts of the quartz tube, particularly at 20 cm from the metallic zinc (Fig. 2). TEM images for Eu-doped ZnO (Fig. 3a,b) show that the nanowires are single crystalline and grew in the 7[0001] direction. As revealed by the SAED pattern and HRTEM analysis (Fig. 3c, d), nanowires grew along the direction of the ZnO c-axis. The SAED pattern obtained for one leg of a tetrapod-like undoped ZnO nanostructure can be indexed as hexagonal ZnO (Fig. 4c), indicating that these ZnO nanostructures are single crystalline with growth for each tetrapod leg along the 7[0001] c-axis (Fig. 4b). An EDX spectrum (Fig. 5) confirmed the presence of europium (0.8 at.%) in the doped nanowires. Europium ions incorporated in the ZnO lattice behave as active centers, influencing the morphology and nanostructure. Europium has a larger ionic radius than zinc, so Eu3+ incorporation leads to expansion of the ZnO lattice. We assume that self-assembled tetrapods formed via a catalystfree vapor–solid (VS) growth mechanism in different parts of the quartz tube. Catalyst-assisted VLS mechanisms usually yield ZnO nanowires of similar size and a uniform orientation [11]. Some researchers believe that catalyst doping of the nanowires occurs as contamination [10], whereas others have suggested that it evaporates during the growth process. However, convincing experimental evidence is lacking. Zhu et al. proposed a three-stage growth
model in which VLS and vapor–solid growth compete with each other [11]. In our case, Eu3+ ions partly influence nanowire growth as a catalyst. However, TEM analysis showed that the nanowire tips are free of any catalyst particles (Fig. 3b). Therefore, we suggest that europium plays two roles during synthesis. First, it is crucial for growth initiation by absorbing Zn vapor. Second, it precipitates ZnO surface drops, which not only provide templates for further growth but also guide the growth orientation, leading to nanowire alignment and successful Eu3+ incorporation in the ZnO lattice. In the initial growth stage, Zn vapor can be easily melt into europium particles to form a liquid alloy. After supersaturation, Zn reacts with oxygen, precipitating out a very thin layer of drops with catalyst particles. Thus, the growth is governed by a VLS mechanism. In the case of undoped ZnO, for which there is no europium to act as a catalyst, we observed self-assembled growth of tetrapods via a VS mechanism. The crystal structure of the samples was characterized by XRD (Fig. 6). The XRD patterns show only peaks corresponding to hexagonal wurtzite ZnO (space group C6mc). No diffraction peaks for Eu2O3 can be detected. All the peaks for the doped sample are shifted to lower 2θ angles and show broadening. High-resolution XRD data for (1 0 0), (1 0 1), and (2 0 1) peaks are shown in Fig. 7a–c. The shift in peak position to lower 2θ angle and the increase in lattice parameters can be attributed to doping with Eu3 + cations, which are larger (effective ionic radius 0.95 Å) than Zn2+ (0.74 Å). Mohanty et al. ascribed this peak shifting and broadening to lattice mismatch and distortion and crystal strain [2]. In any case, the XRD results confirm europium doping in ZnO nanowires. We calculated the lattice constants for doped and undoped samples using the following equations: 2 2 1 4 h þ kh þ k L2 ¼ ⋅ þ 2 ð1Þ 2 2 3 a c d a2 ¼
4 A1 B2 A2 B1 ⋅ 2 3 ðB2 =d2 ÞðB1 =d
h 2 k 2 l2 Þ
h 1 k 1 l1
c2 ¼
A1 B2 A2 B1 2
2
ðA1 =dh2 k2 l2 ÞðA2 =dh1 k1 l1 Þ
;
ð2Þ
ð3Þ
where d is the interplanar distance, h and k are Miller indices, А¼h2+hk+k2, and B¼ L2. In the case of our hexagonal system, we used a pair of diffraction peaks with different Miller indices. For the ideal ZnO wurtzite structure, the parameters are a¼ b¼0.3249 nm and c¼ 0.5206 nm. For our undoped sample we obtained a ¼b ¼0.3256 nm and c¼0.5266 nm. Europium doping increased these parameters to a ¼b¼ 0.3291 nm and c¼0.5311 nm. Differences in the crystal field surrounding RE ions result in slight shifts of their luminescence peaks or differences in their relative intensity. In this case, local changes in the ZnO environment are caused by Eu3+ addition, which would lead to a modified local crystal field. The PL of different ZnO nanostructures is one of their most interesting and important properties. PL spectra (Fig. 8)
Fig. 1. SEM images of Eu3+-doped ZnO nano- and microwires.
S.A. Al Rifai, B.A. Kulnitskiy / Journal of Physics and Chemistry of Solids 74 (2013) 1733–1738
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Fig. 2. SEM images of undoped ZnO nanotetrapods.
Fig. 3. (a,b) TEM images, (c) SAED pattern, and (d) HRTEM image of Eu3+-doped ZnO nanowires.
Fig. 4. (a,b) TEM images and (c) SAED pattern for a ZnO tetrapod.
Fig. 5. EDX spectrum for Eu-doped ZnO nanowires.
confirmed our predictions for Eu3+ doping in nanowires. We observed two strong peaks, one in the UV region (3.21 eV) confirming high-quality ZnO nanowires and one in the red–orange range (2.02 eV), for the doped sample. The UV emission originates from excitonic recombination corresponding to near-band-edge emission of ZnO [15], indicating good ZnO nanostructure quality. The luminescence of europium ions is significantly influenced by the host crystal lattice because electronic transitions involve
only electron redistribution within the 4f layer. The higherintensity peak centered at 612 nm corresponds to the 5D0-7F2 transition that occurs via dipole electric force [5–7] and is more intense than the peak at 586 nm. The small peak for 5D0-7F1 is attributed to a magnetic dipole transition. The probability of the magnetic dipole transition 5D0-7F1 is mainly independent of the host matrix and the electric-dipole-allowed 5D0-7F2 transition is strongly influenced by the local structure and site asymmetry
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around the Eu3+ ion. Besides, the transition 5D0-7F1, which occurs via magnetic dipole, indicates that Eu3+ occupies locations without an inversion center. From emission spectral analyses it was possible to predict that Eu3+ ions are located at sites of higher symmetry. The dominant red emission for the 5D0-7F2 transition indicates inversion of the antisymmetric crystal field around Eu3+, which is advantageous in improving the color purity of red phosphors [17]. Moreover, XRD results confirmed that Eu3+ ions
Fig. 6. XRD patterns for Eu3+-doped and undoped ZnO confirm a hexagonal wurtzite ZnO structure.
were successfully incorporated in ZnO lattice sites, providing further support for the emission characteristics. We suggest that the peaks observed at 680 and 692 nm may be related to transitions from 5D0 to 7F3 and 7F4, respectively [12,13]. Compared to the doped sample, ZnO tetrapods showed only strong luminescence in the green–yellow region (522 nm). We attribute this to ionized oxygen vacancies, which can decrease the UV emission efficiency. Oxygen vacancies and zinc interstitial defects are the most probable reasons for the green emission of
Fig. 8. PL spectra for Eu3+-doped and undoped ZnO measured at 77 K.
Fig. 7. Magnifications of peaks in the XRD patterns for Eu3+-doped and undoped ZnO. (a) The main (1 0 0) peak shows a shift to lower 2θ angle on Eu3+ doping. (b) The (1 0 1) peak and (c) the (2 0 1) peak.
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Fig. 9. (a) Diffuse reflectance spectra for Eu3+-doped and undoped ZnO samples. (b) Kubelka–Munk conversion.
Fig. 10. (a) FT-IR spectra of doped and undoped ZnO samples. (b) Magnification of the range 550–700 cm–1.
the undoped sample, which was grown under a flow of oxygen. Surface band bending revealed that chemisorbed oxygen played a crucial role in the recombination process for this emission. DRS is more convenient for characterizing unsupported nanomaterials than UV-Vis absorption spectroscopy since it takes advantage of the enhanced scattering in nanocrystalline materials. Our results reveal that europium doping of ZnO nanostructures increased and shifted the optical DRS peaks (Fig. 9a). The optical direct bandgap evaluated according to the well-known energy– exponential relation (Fig. 9b) was 3.31 eV for Eu-doped ZnO and 3.19 eV for ZnO tetrapods. This difference can result from variations in carrier concentrations and carrier scattering by microstructural defects, grain boundaries, and ionic impurities. It can also be explained in terms of lattice distortion due to incorporation of larger Eu3+ ions in the ZnO lattice. We suggest that widening of the bandgap is associated with the Moss–Burstein effect [8,18], which occurs when electron carrier concentrations exceed the density of states for the conduction band edge, a process related to degenerate doping in semiconductors. The Fermi level for Eu-doped ZnO lies between the conduction and valence bands. As the doping concentration is increased, electrons
populate states within the conduction band, which shifts the Fermi level to higher energy. In the case of degenerate doping, the Fermi level lies inside the conduction band. In our case of Eu-doped ZnO, an electron from the top of the valence band can only be excited into a conduction band above the Fermi level since all the states below the Fermi level are occupied. The Pauli exclusion principle forbids excitation into these occupied states. Thus, we observe an increase in the apparent bandgap. This slight increase in the bandgap can be explained in terms of a change in the nature and strength of the crystalline potential by addition of Eu3+ dopant ions, including the effect of their 4f electrons on the crystalline electronic states. Widening of the optical bandgap is due to filling of states near the bottom of the conduction band. The quality and composition of different ZnO nanostructures were characterized by FTIR spectroscopy at room temperature in the range 550–4000 cm–1 (Fig. 10). The average transmittance in the visible range was 97% for both samples. An absorption band at 557 cm–1 characteristic of wurtzite hexagonal ZnO (Zn–O stretching vibration) [9] was observed for both samples. The peak at 2400 cm–1 is assigned to CO2 molecules on the surface of the doped sample. The band at 2800–2950 cm–1 for the doped
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sample is due C–H stretching. The absorption at 3600–3800 cm–1 is ascribed to hydroxyl vibrations on the surface of the doped ZnO. The change in position for the ZnO absorption band reflects perturbation of the Zn–O–Zn network by presence Eu3+ [14–19]. Therefore, the FTIR results confirm that Eu3+ ions occupy Zn sites in the ZnO matrix, as observed from XRD measurements. 4. Conclusions 3+
Well-aligned epitaxial Eu -doped ZnO nanowires were grown by VLS on an alumina boat. Eu3+ doping of ZnO increased the lattice parameters because of the larger size of Eu3+ cations. The nanowires were single crystalline in the 7[0001] growth direction. The Eu3+ concentration in the nanowires was 0.8 at.%, which is sufficient to be of use in the fabrication of different optical devices. Red luminescence related to Eu3+ intraionic transitions was observed. The bandgap for ZnO widened from 3.19 to 3.31 eV on Eu doping, which can be attributed to the Moss–Burstein effect, in particular the influence of Eu3+ impurity ions on crystalline electronic states.
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