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We present firstly the preparation of color tunable Al2Y4O9: Yb, Er inverse opal photonic crystal and influence of photonic bandgap on upconversion emissions of Er3+. The results show that Al2Y4O9: Yb, Er upconversion inverse opals were successfully prepared by a self-assembly technique in combination with a sol-gel method. The intensity of upconversion emission can be tuned by controlling the structure of the inverse opal. Significant suppression of the green or red upconversion emission in the Al2Y4O9: Yb, Er inverse opals was obtained if the photonic band-gap overlaps with the Er3+ ions emission band, resulting in color tunable upconversion Al2Y4O9 phosphors. This research not only realizes the colour modification of upconversion emission, but also opens the opportunity to control the propagation of photons.
©2012 Optical Society of America
1. Introduction
Rare-earth doped upconversion (UC) materials that convert an excitation light of a longer wavelength to an emission light of a shorter wavelength have long been investigated as a visualization technology of infrared and near-infrared light. Much interest has been focused on multi-coloration of UC emission, especially for application to display materials [1–6]. Several approaches to realize colour tunable UC emission have already been proposed. One effective way is to dope samples with three or more kinds of rare earth ions. For example, the controllable red, green, blue and bright white UC luminescence was realized in the Tm/Er/Yb co-doped Lu2O3 nanocrystals [1]. Another method is to adjust the concentrations of rare earth ions. Chen et al have reported that the color of UC emission can be easily tuned by adjusting the concentrations of rare earth ions in triply doping a fluoride glass sample [4]. In some reports, some changes in UC emission intensities of rare earth ions were observed, depending on temperature or the pulse width of the near-infrared excitation, etc [2,3].
Photonic crystals have attracted considerable interest since the concept was first independently proposed by Yablonovitch and John [7,8], which are three dimensional periodic dielectric composites in which the distribution of refractive index varies on the visible wavelength scale. This periodicity in the refractive index may lead to the formation of a photonic band gap. The existence of a photonic band gap can manipulate the spontaneous emission from light-emitting materials embedded in photonic crystals [9–15]. The effect of photonic crystals on the spontaneous emission includes an enhancement of spontaneous emission near the band edge and an inhibition of the spontaneous emission in the stop band region. The rare earth ions with lots of levels can emit in the blue, green and red regions. Therefore, the photonic crystal is a promising tool to realize light emission color tunability of rare earth ions by changing the blue, green or red color. It is well known that the host materials play an important role in the UC emission. Most of host materials that had been developed are halides, oxyhalides and oxysulfides. However, application of these hosts is still greatly limited due to their poor chemical stability and low laser-induced damage threshold. The Al2Y4O9 has high chemical and thermal stability, which is meaningful for practical application of UC emission. As we know, there have been no reports about both preparation and luminescence properties of the Al2Y4O9 inverse opals. In this work, we present both the first preparation of color tunable Al2Y4O9: Yb, Er inverse opal photonic crystal and influence of photonic stop band on UC emissions of Er3+.
2. Experimental details
Inverse opals based on Al2Y4O9: Yb, Er were synthesized by the templating method. For template preparation, the polystyrene (PS) microspheres with different sizes of 220, 400 and 460 nm were used to fabricate opal templates by vertical deposition process. The single size (400 or 460 nm) PS microsphere suspension or mixed microsphere suspension consisted of 400 and 220 nm PS microspheres were added into the glass containers filled with deionized water. Quartz substrates were thoroughly cleaned and immersed in suspension of microspheres. Temperature of template growth was 50 °C. The samples were left undisturbed until growth of the opal template was completed.
In fact, the 2: 1 mol ratio of Yb to Er is advantage to both green and red UC emission from Er3+ [16–18], and the intense upconversion emission of inverse opal can be obtained when the Er3+ concentration is about 5% mol [19]. Therefore, Al2Y4O9: Yb (10% mol), Er (5% mol) precursor sol for infiltrating opal template was prepared by using Al(NO3)3, Y2O3, Yb2O3 and Er2O3 as raw materials. Initially, Y2O3, Yb2O3 and Er2O3 were dissolved in hot nitric acid, followed by evaporation until drying-out. The dry nitrates and Al(NO3)3 were dissolved in ethanol separately, then mixed together. The prepared Al2Y4O9: Yb, Er solutions were used to infiltrate into the voids of the opal templates. Al2Y4O9: Yb, Er inverse opals were obtained by calcining at 850°C in an air furnace.
The UC emission measurement of the inverse opals was carried out on the F-7000 spectrophotometer under a 980 nm excitation. Transmittance spectra were measured by a HITACHI U-4100 spectrophotometer. The microstructures of the opal templates and inverse opals were studied by a PHILIPS XL 30ESEM-TMP scanning electron microscope with secondary electron detection mode at 30 kV and 75 uA. Au conductive coating was applied to the inverse opal samples prior to imaging. X-ray powder diffraction (XRD) data of inverse opal photonic crystal were obtained on Rigaku D/max-2200 diffractometer using monochromatized Cu Kα radiation (λ = 1.542Å), operated at 36kV and 30mA. The 2θ range used was from 5 to 95° in steps of 0.02°, with a count time of 1 s.
3. Results and discussion
Figure 1(a) shows the SEM images of unitary opal templates constructed with single microspheres 400nm in diameter deposited on a quartz substrates. The image shows a fcc arrangement of monodisperse PS microspheres, whose (111) planes are oriented parallel to the underlying quartz substrate. In order to investigate effect of photonic band gap on spontaneous emission of inverse opal, a reference sample is needed. In generally, there are three kinds of methods for preparation of the reference sample. One is using small or large balls to prepare opal template, which results in the photonic band gap of inverse opal is far from the emission wavelengths of the rare earth ions [20]. The other is using films prepared by depositing the precursor solution on a plain glass substrate [21]. The third is using two kinds of balls to self-assemble binary disordered template, leading to formation of disordered inverse opal. In present work, the third method was used to prepare the reference sample. Figure 1(b) shows the SEM images of binary templates constructed with mixed microspheres 220nm and 400nm in diameter. Binary template shows that PS microspheres with two kinds of size were arranged in a completely disorder (random) pattern.
Fig. 1 (a), (b) SEM images of unitary opal templates constructed with polystyrene microspheres 400 nm in diameter and binary templates constructed with polystyrene microspheres 400 and 220 nm in diameter, respectively. (c), (d) SEM images of the IPC-1 and reference sample, respectively.
The Al2Y4O9: Yb, Er inverse opals were fabricated by sacrificial PS microsphere templates method. The Al2Y4O9: Yb, Er inverse opals prepared by unitary opal templates constructed with single size microspheres 400 or 460 nm in diameter were denoted as the IPC-1 and IPC-2, respectively. Figure 1(c) shows the SEM image of the IPC-1. The IPC-1 exhibits a three-dimensional ordered structure comprising interconnected macropores arranged on a fcc lattice. The dark areas within the macropores result from the necking of PS spheres in the opal template. Removal of the PS microspheres by calcination does not destroy the ordered structure of opal template. SEM image taken from the IPC-2 was similar to that of the IPC-1 except macroporous size, so it was not shown. For comparison, disordered reference sample (RS) was also prepared by infiltrating Al2Y4O9: Yb, Er sol into the voids of the binary disordered template constructed with mixed microsphere 220nm and 400nm in diameter, as shown in Fig. 1(d) Based on the SEM results, the center-to-enter distance between the air spheres of the IPC-1 and IPC-2 is about 310 nm and 360 nm, respectively.
Figure 2 shows the XRD patterns of the IPC-1. The orthorhombic phase Al2Y4O9 can be obtained when sintered at 850°C for 5h. The transmission measurement has been widely used for characterizing photonic crystals during the last decade. Compared with the SEM images only providing local area information, the optical transmission spectra will provide large scale information for the inverse opal photonic crystal.
Figure 3 shows the transmission spectra of the IPC-1, IPC-2 and RS, which were all collected at zero incidence angle. The dip in transmission spectra indicate the existence of a partial photonic band gap at the normal direction of the (111) plane. The photonic band gaps for the IPC-1 and IPC-2 are at 551 and 653 nm, respectively, while there was no photonic band gap in the RS. The photonic band gap appearance in the IPC-1 and IPC-2 suggests they have long-length ordered structure. It is well known that the transmittance of photonic band gap depends on quality of photonic crystal [22]. The better the quality of photonic crystal is, the larger the transmittance is. The transmittance discrepancies between IPC-1 and IPC-2 may be probably related to their quality differences of inverse opal. Further investigation will be done in future. The photonic band gap wavelength of Al2Y4O9 inverse opal depends on the center-to-center distance between the air spheres and the volume faction of Al2Y4O9 by the modified form of Bragg’s law:
where λ, D, θ, and f denote the wavelength of the photonic band gap, the average center to center distance between the air spheres, the angle between the incidence light and the normal line of plane and the volume fraction of Yb, Er co-doped Al2Y4O9, respectively. neff, nair (1) and nAYO (1.8) represent average refractive indexes of the inverse opals, air and Al2Y4O9, respectively. The larger the center-to-center distance between the air spheres and the volume faction are, the longer the photonic band gap wavelength is. Based on the photonic band gap position measured by transmittance spectra, the calculated volume faction are 8 and 10% for IPC-1 and IPC-2, respectively, which is lesser than the theoretical volume fraction (26%) of the inverse opal. Therefore, we can change photonic band gap position of inverse opal by volume fraction or microspheres size modulation.UC emission of Er3+ is attractive because it can be conveniently excited by commercial low-cost near-infrared laser diodes. In order to perform the study on the photonic band gap influence on the UC emission in Al2Y4O9 inverse opals, the UC emission of the inverse opals was measured under a 980nm excitation, all fluorescence data were collected at the normal direction of the (111) plane. Figure 4 shows the UC emission spectra of the IPC-1, IPC-2 and RS, respectively. The UC emission bands were at 545 nm, 556, 655, 670 and 680 nm, respectively. All samples show typical Er3+ emission bands, corresponding to the 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions. The bands were at 545, 556 (4S3/2 → 4I15/2), 655, 670 and 680 nm (4F9/2 → 4I15/2), respectively. For the IPC-1, the UC emission at 655 nm (Fig. 4(a)) is not influenced at normal direction since the photonic band gap is centered at 551 nm. In order to reveal the effect of the photonic band gap environment on the 545 and 556 nm UC emissions, the UC emission peak at 655nm is used to normalize the UC emission spectrum of the IPC-1. The 545 and 556 nm emission intensity is significantly reduced in the spectral region of the photonic band gap. For IPC-2, the UC emission at 545 nm (Fig. 4(b)) is not influenced at normal direction since the photonic band gap is centered at 653 nm. In order to investigate the influence of the photonic band gap on the 655, 670 and 680 nm UC emissions, the UC emission peak at 545 nm is used to normalize the UC emission spectrum of the IPC-1I and RS. The 655, 670 and 680 nm UC emission intensity is significantly reduced in the spectral region of the photonic band gap. In fact, the fluorescence intensity is related to the local structure surrounding the rare earth ions, photonic band gap effect, and the ordered degree of opal, etc. In present work, the IPC-1, IPC-2 and reference sample were fabricated by using unitary ordered and disorder binary templates, respectively. Compared with unitary templates, binary templates are only arranged in a completely disorder pattern, others surroundings are identical. Therefore, the contributions of local structure surrounding were the same for all samples. On the other hand, Li et al [23] have reported the luminescence enhancement phenomenon of three-dimensional ordered films of fluorescent latex microspheres. Regardless of the size of the latex spheres, the luminescence of ordered structures always is stronger than that of disordered samples. However, the suppression of UC emission intensity was observed in the ordered inverse opal investigated herein. Therefore, we consider that the photonic crystal effects are responsible for the difference of the UC emission intensity in the three samples. The suppression of UC emission in the inverse opal is due to the photon trapping caused by Bragg reflection of lattice planes.
Fig. 4 Upconversion emission spectrum of (a) the IPC-1 and reference sample, and (b) the IPC-2 and reference sample.
As shown in Fig. 4, the UC emission intensity of Er3+ from the inverse opal is clearly modulated by the photonic band gap, resulting in color tunable up-conversion phosphors with applications in optical integrated devices. Figure 5 shows the CIE chromaticity coordinates of the IPC-I, IPC-2 and RS sample, which were calculated based on the corresponding PL spectra. The CIE coordinates (x, y) of the IPC-1, IPC-2 and RS are (0.4333, 0.4669), (0.3837, 0.4606), and (0.4144, 0.4762), respectively. We successfully achieved the tuning of the UC optical properties of the Al2Y4O9: Yb, Er inverse opal by controlling the structure of inverse opal photonic crystal.
4. Conclusion
Inverse opal photonic crystals of Al2Y4O9: Yb, Er were synthesized by a sol-gel technique in combination with a self-assembly method. Opal template was self-assembled from suspensions of monodisperse polystyrene microspheres with a mean diameter of 400 or 460 nm via a vertical deposition process. After infiltrating the Al2Y4O9: Yb, Er sol into the opal template, the polystyrene microspheres were removed by heat treatment, and Al2Y4O9: Yb, Er inverse opal photonic crystals were formed. The effect of photonic band gap on UC emissions was investigated in the Al2Y4O9: Yb, Er inverse opals. Significant suppression of the red or green upconversion emission was detected if the photonic band-gap overlapped with the Er3+ ions emission band, resulting in color tunable upconversion photonic crystals with applications in solid-state color displays.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51002068), 973 Program (No.2011CB211708), Society Development Foundation of Yunnan Province (2009CC009), Natural Science Foundation of Yunnan Province (2010ZC038), the Postdoctoral Science Foundation of China (20110491759), Education Department Foundation of Yunnan Province (2011Y348), and Open Foundation of Key Lab of Advanced Materials in Rare & Precious and Non-ferrous Metals, Ministry of Education (ZDS2010011B).
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