J. Phys. Chem. C 2010, 114, 19891–19894
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Preparation and Photoluminescence Properties of Gd2O3:Eu3+ Inverse Opal Photonic Crystals Xuesong Qu,†,‡ Hyun Kyoung Yang,† Byung Kee Moon,† Byung Chun Choi,† Jung Hyun Jeong,*,† and Kwang Ho Kim§ Department of Physics, Pukyong National UniVersity, Korea Busan 608-737, South Korea, Department of Physics, Changchun Normal UniVersity, Changchun 130032, China, and School of Materials Science and Engineering, Pusan National UniVersity, Busan, 609-735, Korea ReceiVed: May 20, 2010; ReVised Manuscript ReceiVed: September 5, 2010
Eu3+-doped Gd2O3 inverse opal photonic crystals were successfully synthesized based on a self-assembly technique and a sol-gel method. The inverse opal Gd2O3:Eu3+ material exhibited a pronounced photonic stop band in the visible region. The effect of the stop band on the spontaneous emission of Eu3+ has been studied. Significant suppression of the emission was observed when the Eu3+ ions’ emission bands overlapped with the photonic stop band, whereas enhancement of the emission intensity was detected when the emission bands appeared at the long wavelength edge of the band gap. I. Introduction Photonic crystals (PCs), with three-dimensional (3D) periodic dielectric structures on an optical length scale have attracted much attention since the pioneering studies by Yablonovitch1 and John2 in 1987. Because of their ability in confining, controlling, and manipulating photons up to 3D, PCs have demonstrated a variety of applications in the area of photonics, such as near-zero threshold lasers, waveguides, optical switches, etc.3-5 According to Fermi’s golden rule, the rate of spontaneous emission in the weak oscillator-field coupling regime is proportional to the density of optical electromagnetic modes around an atom or a molecule within a frequency range corresponding to the spontaneous emission spectrum.6 In PCs with a complete photonic band gap (PBG), spontaneous emission will be inhibited fully because the optical electromagnetic modes do not exist within the PBG frequency range.7,8 That is, radiative properties of excited emitters are determined not only by their internal nature but also by their environment. From the viewpoint of elementary studies, one of the principal basic physical issue of PCs is the modification of the spontaneous emission of excited atoms, whose transition energy is nearby or inside PBG.9 This objective can be approached by imposing a proper photonic energy band structure on the light emitter or by embedding the emitter in a photonic band-gap environment. In the latter case, several experimental studies have been reported and the luminescent emitter are mainly focused on organic dyes molecules, semiconductor nanocrystals, and rareearth ions.10 However, in these experiments, luminescent species were either grafted or coated on the inner surfaces of the PC structures. The distribution is random and may undergo spatial diffusion that cannot be controlled precisely. This problem can be avoided if the whole photonic band material is composed of luminescent material. That is, a 3D luminescent matrix of PBG material could be an alternative mode to investigate the stopband effect. To date, a few examples, such as ZnS:Mn and * To whom correspondence should be addressed. E-mail: jhjeong@ pknu.ac.kr. Tel: +82-51-629-5564. Fax: +82-51-629-5549. † Pukyong National University. ‡ Changchun Normal University. § Pusan National University.
LaPO4, have been reported.11,12 In this work, rare earth ion Eu3+ doped Gd2O3 was prepared based on the polystyrene (PS) template technique. The stop-band effect on the emission characteristics of Eu3+ ions has been investigated. In this system, the Gd2O3:Eu3+ nanomaterials exhibited a rigid and well-defined 3D environment for the luminescence of Eu3+ ions. Moreover, unlike other light emitters, such as quantum dots, trivalent lanthanide rare earth ions possess specific advantages, such as high luminescence efficiencies, narrow emission lines, and a rigidly fixed spectrum location, which is more suitable to examining the PBG effect on spontaneous emission.13 II. Experimental Section Inverse opal PCs, Gd2O3:Eu3+ (5 mol %), were prepared by the sol-gel method combined with a polystyrene (PS) latex sphere template technique. A nearly monodisperse PS latex sphere with an average diameter of 420 and 500 nm was synthesized using surfactant-free emulsion polymerization.14 A thin-film template was self-assembled using the vertical deposition process. The colloid suspension (5% solid content) of PS microspheres was dropped onto a glass substrate and placed in a 40 °C oven for 3 days. The PS colloidal spheres slowly selforganized into highly ordered colloidal arrays on the glass substrate, driven by surface tension of the liquid in the evaporating process. Following deposition, the opals were sintered for 1 h at 100 °C to enhance their physical strength. In the preparation of the Gd2O3:Eu3+ (5 mol %) precursor sol, appropriate amounts of Gd(NO3)3 and Eu(CH3COO)3 were dissolved in a mixture of water and ethanol. The mixture was stirred for 1 h to form a homogeneous solution. The prepared precursor solutions were used to infiltrate into the voids of the opal template through capillary force. After infiltration, the resulting products were dried in air at room temperature for 24 h and then were elevated to 600 °C at a heating rate of 40 °C/h and kept at 600 °C in air for 2 h. Here, samples prepared by opal templates constructed with PS microspheres 420 and 500 nm in diameter were denoted as samples A and B, respectively. For comparison, the nonporous reference sample was prepared and annealed by the same procedure except for not using the PS colloidal as template. X-ray diffraction (XRD)
10.1021/jp104621y 2010 American Chemical Society Published on Web 11/05/2010
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Figure 1. Field emission SEM images of sample A, PS colloidal crystals (A, B), and the inverse opal Gd2O3:Eu3+ sample (C, D).
patterns of the samples were obtained with a Rigaku D/maxrA X-ray diffractometer with Cu KR (λ ) 1.54056 Å) radiation. Field emission scanning electron micrograph (FESEM) images were taken on a Hitachi S-4800 electron microscope. The transmittance spectra were recorded through a UV-vis light spectrophotometer. Fluorescence spectra were recorded on a Hitachi F-4500 spectrophotometer equipped with a 150 W Xearc lamp at room temperature. III. Result and Discussion The PS colloidal crystal templates fabricated in this study exhibited strong opalescence under incident white light, visible evidence of their long-range 3D order. Panels A,B and C,D of Figure 1 show the low- and high-magnified SEM images of the opal PS template and sample A. A long-range ordered hexagonal arrangement of monodisperse PS spheres with a diameter of 420 nm, corresponding to the (111) planes of a facecentered cubic (fcc) structure, is clearly seen in Figure 1A,B. In Figure 1C,D, the inverse opal Gd2O3:Eu3+ exhibits a 3D highly ordered structure comprising interconnected macropores that form an ordered hexagonal arrangement of air spheres, which corresponds to the (111) surface of an fcc structure. It demonstrated that the inverted structure was well preserved by replicating the void space with the resulting Gd2O3:Eu3+ replica. From the edge of Figure 1C, it can be seen that the sample is a multilayer structure (no less than 100 layers), which makes sure the formation of a stop band in the inverse opal sample. Measurements of the center-to-center distance (320 nm) between the macropores on the (111) planes indicated that it was about 23.8% smaller than the original size of the PS template due to the shrinkage during calcinations.10 Here, the SEM image taken from sample B was not shown because it was similar to that of sample A. The center-to-center distance and shrinkage rate in sample B are 370 nm and 26%, respectively. Figure 2a shows the XRD patterns of the inverse opal Gd2O3:Eu3+. According to JCPDS standard cards, it can be identified that the sample
Figure 2. (a) XRD pattern of inverse opal Gd2O3:Eu3+ nanocrystals. (b) EDX spectrum of the Gd2O3:Eu3+ sample.
exhibits a pure cubic crystal structure (PDF file 76-0155) with the characteristic diffraction peaks at (222), (400), (440), and (622). By using the Scherrer formula, a mean particle size of approximately 12 nm was estimated from the width of the peaks. No other phase and free species, except for Gd2O3, was detected,
Gd2O3:Eu3+ Inverse Opal Photonic Crystals
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Figure 3. Transmission spectra of samples A and B. 3+
indicating that the Eu completely came into the Gd2O3 host lattice after calcination. The radius of the Eu3+ ion is quite close to that of Gd3+; it can be readily incorporated into the Gd2O3 framework, substituting one Gd3+ ion. Alternatively, if Eu3+ ions locate outside the lattice of Gd2O3, Eu3+ should exist as the Eu2O3 phase. To identify this case, we compared the luminescence spectrum of the present Gd2O3:Eu3+ sample with that of Eu2O3. As is well-known, Eu3+ could usually be a structure probe supersensitive to the local environment.15 Herein, a distinct difference observed in the charge transfer bands (Eu-O) in Gd2O3:Eu3+ (261 nm) and Eu2O3 (286 nm) give another support that Eu3+ ions enter the Gd2O3 lattice instead of adsorbing on the surface of nanoparticles in the form of Eu2O3. Furthermore, energy-dispersive X-ray (EDX) spectroscopy is also measured to investigate the elemental composition in the Gd2O3:Eu3+ sample. (Figure 2b). The EDX result confirms the presence of gadolinium (Gd), oxygen (O), and europium (Eu) elements in the Gd2O3:Eu3+ sample, and the concentration of Eu3+ in the Gd2O3:Eu3+ sample was determined to be 5.1%. The optical transmittance spectra collected from samples A and B when the incidence is vertical to the (111) plane are shown in Figure 3. The optical stop bands centered at 620 and 715 nm for samples A and B were observed, respectively. Theoretically, the position of the band gap in fcc photonic crystals can be estimated by Bragg’s law of diffraction combined with Snell’s law16
λPBG )
2dhkl 2 n - sin2 θ ) 1.633Dneff m √ eff
neff ) nGd2O3φ + nair(1 - φ) where λPBG is the center of the band gap, m is the order of the Bragg diffraction, dhkl is the hkl plane distance, D is the centerto-center distances of the neighboring macropore, neff is the average refractive index, θ is the angle from the incident light to the normal of the substrate surface, and φ is the volume fraction of Gd2O3:Eu3+. According to the equation above, the values of φ for samples A and B are estimated to 23.2% and 22.3%, respectively. Figure 4 presents the emission spectra of A, B, and a reference sample upon excitation into the charge-transfer band (CTB) of Eu3+ at 262 nm. In all the spectra, the 5D1-7F1 and 5D0-7FJ (J ) 0, 1, 2, 3, 4) transitions are observed, locating at 534, 582, 593, 613, 630, and 707 nm, respectively. Note that the spectra have been normalized to the 534 nm peak (5D1-7F1) because it is situated relatively further away from the observed band gap. It can be seen that the PL intensity in sample A at
Figure 4. Emission spectra of A, B, and the reference sample under the 262 nm excitation.
613 and 630 nm is significantly decreased in contrast to the reference sample. However, the peaks at 652 and 687 nm were enhanced. For sample B, the emission peak at 613 nm, which is attributed to the 5D0-7F2 transition, also decreased compared with the reference sample, but stronger than that in sample A. Because the stop band of sample B lies outside of the range of the emission band, the PL spectrum is unaffected by the PBG effect, and hence, we conclude that the decrease of the 5D0-7F2 transition in sample B is attributed to the change of the local environment surrounding Eu3+ ions. As is well known, the 5 D0-7F2 transition is electric-dipole allowed and its intensity is sensitive to the local structure surrounding the Eu3+ ions. However, the 5D0-7F1 transition is magnetic-dipole allowed, and its intensity shows very little variation with the crystal field strength acting on the Eu3+ ion. Therefore, the intensity ratio of 5D0-7F2 to 5D0-7F1 is closely related to the local environments.15,17 Generally, the larger the intensity ratio of 5D0-7F2 to 5D0-7F1 is, the lower the local symmetry is. In a nonporous reference sample, the number of atoms/ions located at or near the surfaces of the particles is more than that of the inverse opal samples and the site symmetry of Eu3+ ions should be much lower. As a consequence, the emission of the 5D0-7F2 transition in the reference sample is relatively stronger than that in samples A and B. However, for sample A, we consider that photonic stop-band effects should also contribute to the obtained PL spectra besides changes of the local environment. As the center wavelength of the band gap in sample A is 620 nm, it is near the transition of 5D0-7F2 and a suppression of the light emission has occurred due to the photon trapping caused by Bragg diffraction of lattice planes.13,18,19 Note that samples A and B should possess the same local environment because they were fabricated by the same process, with the same inverse opal structure. However, the smaller relative intensity at 613 nm was observed in sample A compared with sample B. This result further confirms that the stop-band structure certainly modified the spontaneous emission of the 5D0-7F2 transition, making the emission frequency of Eu3+ around 613 and 630 nm decreased in sample A. Furthermore, because of the modification effect of the stop band on the emission bands at the long wavelength edge of the stop band, the spontaneous emission of Eu3+ at 630 and 687 nm in sample A was also observed to be enhanced. At the short wavelength edges of the photonic stop band, no change of emission was determined. For the enhancement of emission, it is a more complicated issue. It was usually attributed to the standing wave effect and/or escaping of the diffusion light.20,21 Galstyan et al.20 proposed a model where standing wave effects determined enhancements at the edges of the photonic band gap. At the long wavelengths and short wavelengths, the standing
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wave is, respectively, mainly in the high-index material and in the low-index material. At the long wavelength, a standing wave corresponds to a position in the opal at the equator and in the inverse opal at the pole. At the short wavelength, the light is in the opal mainly at the pole and in the inverse opal at the equator. According to the model, the enhancement of emission may occur at the long wavelength edges of the photonic stop band in the inverse opal. However, the enhancement effects at the short wavelength side were also observed in the inverse opal, and the so-called escape function of diffuse light was attributed to this kind of enhancement.21 In our present work, the enhancement of emission may be attributed to the standing wave effect.20 IV. Conclusions In summary, we have successfully fabricated the inverse opal Gd2O3:Eu3+ and the effect of the stop band upon the spontaneous emission was investigated. By comparing the spectra of the inverse opal with that of the reference sample, the contribution of different local environments to the spectra was observed. Furthermore, the suppression effect on the emission at 613 nm and the enhancement effect at 652 and 687 nm, both induced by the photonic stop band, were confirmed in sample A upon eliminating the influence of the local environment on the emission spectra. We believe that the present work will be valuable for not only the foundational study of the PBG effect but also new optical devices in lighting and display. Acknowledgment. This study was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2007-412J00902), and also, this research was supported by the NCRC (National Core Research Center) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2010-0001-226).
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