Significant Suppression of Spontaneous Emission in SiO 2 Photonic Crystals Made with Tb 3+ -Doped LaF 3 Nanoparticles

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1 J. Phys. Chem. C 2007, 111, Significant Suppression of Spontaneous Emission in SiO 2 Photonic Crystals Made with Tb 3+ -Doped LaF 3 Nanoparticles Marie Aloshyna, Sri Sivakumar, Mahalingam Venkataramanan, Alexandre G. Brolo,* and Frank C. J. M. van Veggel* Department of Chemistry, UniVersity of Victoria, P. O. Box 3065, Victoria, British Columbia, Canada V8W 3V6 ReceiVed: NoVember 2, 2006; In Final Form: January 12, 2007 SiO 2 inverse opals made with LaF 3 :Tb 3+ nanoparticles were fabricated. The spontaneous emission lifetime of the 5 D 4 f 7 F 4 transition of Tb 3+ ions at 542 nm embedded in the SiO 2 inverse opal structure was determined. In contrast to an average lifetime of 2.8 ms in photonic crystals when the photoluminescence of the Tb 3+ envelop was located outside the photonic stop band, a lifetime of 4.0 ms was observed when the Tb 3+ photoluminescence fully overlaps with the photonic stop band in the inverse opal. The obtained results demonstrate that the photonic structure affects the spontaneous emission significantly, resulting in a 40% increase of the luminescence lifetime of Tb 3+ ions. In other words, the density of optical states is reduced at the emission frequency of Tb 3+ (i.e., 542 nm) which reduces the radiative rate constant. Introduction Photonic crystals have attracted great interest since the concept was first proposed independently by Yablonovitch 1 and John. 2 These long-range ordered structures possess a periodic modulation of the refractive index (RI) (or dielectric constant) on the length scale of the optical wavelength. This periodicity may lead to the formation of a photonic band gap (PBG). The PBG inhibits the propagation of light within a particular wavelength range that satisfies the Bragg condition through the material. There is a wide potential for applications of three-dimensional (3D) photonic structures with a complete PBG in the visible and near-infrared (near-ir) ranges of the electromagnetic spectrum. 3,4 According to theoretical calculations, 5,6 it is expected that inverse opal structures can be used as attractive candidates for the realization of 3D PBG in the optical and near- IR range. The spatial periodicity of optical properties of photonic structures opens new possibilities for creation and development of optoelectronic devices. One of such interesting possibilities is the modification of the spontaneous emission of photoluminescent guests embedded in a photonic crystal. 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. 7 In photonic crystals with a complete PBG, spontaneous emission will be inhibited fully because the optical electromagnetic modes do not exist within the PBG frequency range. 8,9 The effect of complete inhibition of spontaneous emission has not been yet demonstrated experimentally. However, there have been experimental reports about the effects of the photonic structure with an incomplete PBG (defined as a photonic stop band) on * To whom correspondence should be addressed. Telephone: +1 (250) (A.G.B.); +1 (250) (F.C.J.M.v.V.). Fax: +1 (250) (A.G.B.); +1 (250) (F.C.J.M.v.V.). agbrolo@uvic.ca (A.G.B.); fvv@uvic.ca (F.C.J.M.v.V.). the rate of spontaneous emission of photoluminescent guests, such as fluorescent dyes and quantum dots. The emission band of fluorescent dyes is often broader than the width of the photonic stop band, and suppression of only a part of the emission spectrum is generally observed In contrast, the application of light emitters with narrow emission band, such as quantum dots and lanthanide ions, could potentially maximize the photonic crystal effect. Suppression of the spontaneous emission by 50% was observed in the case of quantum dots. 13,14 Emissions from lanthanide ions involve f-orbital electrons and are much narrower than those observed from organic molecules. The quantum yield of lanthanide emission is high in the absence of quenching, and photobleaching effects, commonly observed for fluorescent dye molecules, are not observed. The bandwidth of a spontaneous lanthanide emission is also much narrower than the photonic stop band of typical inverse opal structures. Only a few experimental works have been devoted to studies of photoluminescence properties of lanthanide ions embedded in SiO 2 photonic crystals Photoluminescence properties of Er 3+ ions in the silica and silicon sections of a Si-infiltrated SiO 2 colloidal photonic crystal were studied by varying temperature and the excitation wavelength, but an effect of the photonic stop band was not reported. 15 In another report, 16 some changes in emission intensities and lifetimes of Er 3+ ions in SiO 2 opal matrix and nanocomposites depending on the concentration of guests and element composition were observed, but the influence of the photonic structure on the photoluminescence properties of Er 3+ ions in nanocomposites and SiO 2 opal matrix was not studied. Photoluminescence emission of Eu 3+ ions embedded in SiO 2 opal matrix was modulated (suppressed or enhanced) depending on the relative overlap between photonic stop band and Eu 3+ emission. 17 In another work, 18 certain modification of green emission (lower intensity and less spectral features) of Tb 3+ ions in the presence of a photonic stop band was observed due to a SiO 2 opal lattice. These observations do not necessarily lead to the conclusion that they are a result of a reduced density of optical states (DOS) but could in principle also be obtained /jp067244b CCC: $ American Chemical Society Published on Web 02/20/2007

2 4048 J. Phys. Chem. C, Vol. 111, No. 10, 2007 Aloshyna et al. by leaky modes in directions not experimentally accessed. However, these experimental studies 17,18 do not quantify the magnitude of the variation of the spontaneous emission lifetimes of infiltrated lanthanide ions due to the photonic stop band. In this paper, the results of the photonic effect on the spontaneous emission of Tb 3+ ions embedded in inverse opals are presented. To our knowledge, this paper is the first work in which an increase of 40% of the spontaneous emission lifetime of lanthanide ions in an inverse opal structure was experimentally demonstrated. To maximize the effect of the photonic effect on the spontaneous emission of Tb 3+ ions, a high quantum yield is desirable. As we have observed earlier, 19 the luminescence efficiency of the lanthanide ions is greatly improved when they are doped in LaF 3 nanoparticles and diluted in a silica sol-gel matrix since the LaF 3 nanoparticles allow for significant reduction of concentration quenching and provide an efficient shielding of the lanthanide ions from the quenching effects of the environment. Three types of samples were fabricated. The PC I sample is a SiO 2 inverse opal made with LaF 3 :Tb 3+ nanoparticles. In this case, the photoluminescence of 5 D 4 f 7 F 4 transition of Tb 3+ ions is within the photonic stop band. The control PC II sample is also a SiO 2 inverse opal made with LaF 3 :Tb 3+ nanoparticles. However, the photoluminescence of the 5 D 4 f 7 F 4 transition of Tb 3+ ions is far from the photonic stop band, and the control sample III is disordered silica made with LaF 3 :Tb 3+ nanoparticles. The structure of the fabricated photonic crystals was checked by scanning electron microscopy (SEM). The optical properties of samples PC I and II were studied by angular-resolved optical transmission measurements. The experimental angular dependence of the optical transmission minima position was compared with the theoretical estimation of Bragg diffraction of light from the different planes of the photonic crystal. The luminescence lifetime was measured for all samples. Experimental Section Materials. La(NO 3 ) 3 6H 2 O, Tb(NO 3 ) 3 6H 2 O, citric acid, NaF, tetraethyl orthosilicate (TEOS), aqueous NH 4 OH (28-30%), and concentrated HCl were all received from Aldrich and used as received. Milli-Q water with resistivity greater than 18 MΩ cm was used in all our experiments. Suspensions of 10 wt % 400 and 200 nm polystyrene (PS) microspheres were purchased from Bangs Laboratories. These microspheres were diluted to a 3 wt % concentration with Milli-Q water. The quartz substrates (75 25 mm 2 ) were obtained from Chem Glass and cut into mm 2 pieces. Preparation of LaF 3 :Tb 3+ Nanoparticles. The preparation of the citrate-stabilized LaF 3 :Tb 3+ nanoparticles (doped at 5 at. % with respect to the total amount of lanthanide ions) and their incorporation into the silica sol-gel matrix have been described in our previous work. 19 Preparation of Silica Sol Precursor. Briefly, 50 mg of the citrate-stabilized nanoparticles were dissolved in 1.5 ml of water, which was then mixed with 3 ml of TEOS and 7.8 ml of ethanol. The ph of the mixture was adjusted to 2 with dropwise addition of 0.1 N HCl and left to stir for 24 h at room temperature to get a clear sol. Fabrication of SiO 2 Inverse Opals Made with LaF 3 :Tb 3+ Nanoparticles (PC I and PC II). The quartz substrates used for fabrication of photonic crystals were first cleaned extensively by exposing them to chromic acid for 12 h followed by thorough rinsing with copious amounts of Milli-Q water. The cleaned substrates were dried at 120 C for 1 h. The procedure for fabrication of photonic crystals consisted of three steps. 20 First, the colloidal PS thin films from polystyrene colloids of 400 (for sample PC I) and 200 nm (for sample PC II) were formed on quartz substrates by vertical convective self-assembling method (at 60 C, suspension of 3% PS colloids, final drying at 105 C for 5 min). Second, the prepared polystyrene films were infiltrated with silica sol doped with LaF 3 :Tb 3+ nanoparticles diluted in anhydrous ethanol with 1:6 ratio. The substrate was place on the bottom of a 25 ml beaker, and the sol was added by letting it flow gently from the side of the beaker, by using a syringe. The whole opal was immersed by the sol for 5 min. After every infiltration step, the remaining sol was carefully removed with a syringe, followed by drying in air for about 20 min. The infiltration was repeated three times. Finally, the SiO 2 inverse opal photonic crystals (PC I and PC II) were obtained after a thermal treatment consisting of slow heating during 5 h to 600 C followed by annealing during3hat600 C and slow cooling during 5htoroom temperature. 21 Preparation of Disordered Silica Made with LaF 3 :Tb 3+ Nanoparticles (Sample III). Silica sol made with LaF 3 :Tb 3+ nanoparticles diluted 1 to 6 with anhydrous ethanol was disposed on quartz slide and treated in the same thermal conditions as photonic crystals (PC I and PC II). The silica was scratched from the quartz slide and ground to collapse the long-range order. Some of this powder was mixed with KBr and pressed into a pellet for convenient measurements. Characterization. SEM observations were performed with a Hitachi S3500N scanning electron microscope operating at 15 kv. Angular-dependent optical transmission spectra were measured using a 150 W quartz halogen illuminator (Dolan Jenner Fiber-Lite Series Model 180) as a light source and a USB2000 UV-vis miniature fiber optic spectrometer (Ocean- Optics). The sample could be rotated about its vertical axis, allowing the collection of optical transmission spectra at different angles with respect to the surface normal. Two objectives (RMS 10 Olympus) were used, one to illuminate the sample with a well-focused white beam of 1 mm diameter and the other to collect the transmitted light. The optical transmission was determined as the ratio of the intensity I(λ) of a light beam passing through the sample on quartz slide to the intensity I 0 (λ) of the beam passing through the reference (quartz slide without sample), T(λ) ) I(λ)/I 0 (λ) 100. Luminescence analyses were done using an Edinburgh Instruments FLS 920 luminescence system. Lifetime and emission analyses were done by exciting the samples at 488 nm with a 10 Hz Q-Switched Quantel Brilliant, in which the third harmonic of the Nd:YAG laser pumps the optical parametric oscillator (OPO), with a tunable optical range from 410 to 2400 nm (full width at half-maximum (fwhm) ) 5 ns). The emissions were collected by using a red-sensitive Peltier-cooled Hamamatsu R928P photomultiplier tube (PMT), with a photon-counting interface. All emissions in the visible region were measured with a 1 nm resolution. All spectra were corrected for the detection sensitivity. Decay curves were measured with a 0.01 ms lamp trigger delay for the R928P PMT. The Edinburg Instruments F900 deconvolution software package was used to fit the lifetime decays. The lifetime decays were fitted by a threeexponential function based on the equation I(t) I 0 3 ) B 0 + A i exp(-t/τ i ) (1) i)1 Intensities down to 1% of the initial intensities were included in these lifetime analyses. For all fittings, the χ 2 was in the range

3 Spontaneous Emission Suppression in SiO 2 Crystals J. Phys. Chem. C, Vol. 111, No. 10, Figure 2. Optical transmission spectra of PC I at different incident angles with respect to the surface normal of the sample and photoluminescence spectrum of LaF 3:Tb 3+ nanoparticles (5%) in silica sol diluted in ethanol (λ exc ) 488 nm). 30% in comparison to the original size of the polystyrene spheres. A similar level of shrinking was observed for other SiO 2 inverse opal structures reported in the literature. 22 The optical transmission spectra of the PC I sample, measured at different incident angles with respect to the surface normal, are presented in Figure 2. A dip at 551 nm with a shoulder at 478 nm in transmission minimum is observed for normal incidence (0 ). An estimation of the position of the photonic stop band can be obtained from the Bragg condition for (hkl) planes: 23 λ hkl ) 2d hkl n eff 1 - sin 2 r hkl (3) Figure 1. SEM images of SiO 2 inverse opals made with LaF 3:Tb 3+ nanoparticles: (A) PC I and (B) PC II. The inset in A is a digital micrograph of PC I, showing a clear green iridescence. of All the calculations were based on duplicate lifetime measurements on several spots of the same samples, and the reported values were within an error of 10%. The average lifetimes were estimated using expression (2): τ av ) A 1 τ A 2 τ A 3 τ 3 (2) A 1 τ 1 + A 2 τ 2 + A 3 τ 3 where τ 1, τ 2, and τ 3 are the component decay times and A 1,A 2, and A 3 are weighed amplitudes. Results and Discussion The fabricated inverse opals PC I and PC II are thin films of air spheres in silica. In Figure 1, the SEM images of PCs I and II are presented. As can be seen from the inset of Figure 1A, although the fabricated photonic crystals still contain some defects and cracks, the areas of uniform long-range order extend over a few tens of micrometers. The diameters of voids in the SiO 2 inverse structure were about 300 and 160 nm for PC I and PC II, respectively. The hexagonal arrangement of air spheres, which corresponds to the (111) surface of a fcc structure with high filling is clearly seen in Figure 1. The dimensions of the domains with a well-ordered fcc structure were typically around 3 3 µm 2. The voids of the inverse opal structure had shrunk by about where λ hkl is the optical stop band center, d hkl is the spacing between (hkl) planes, n eff is an effective refractive index of the sample (for SiO 2 inverse opal structure: the refractive index of silica is 1.455, and the filling factor of silica was assumed to be 0.26 considering well-ordered fcc structure of prepared photonic crystals (see Figure 1A); hence, n eff ) 0.74n air n silica ) 1.12), r hkl is the internal angle between the wave vector and the [hkl] direction, and θ is the external angle between the incident light beam and the normal of the (111) inverse opal surface. The internal angle r 111 is given by Snell s law: n air sin θ ) n eff sin r 111 (4) It has been demonstrated that materials with fcc ordering may not present photonic stop bands only from the (111) plane but also from the (200), (220), and (311) planes and other planes with either all-even or all-odd h, k, l indices. 24 These extra contributions arise from the random orientation of the crystal planes relative to surface normal, and photonic crystals may exhibit photonic stop bands from at least two sets of planes in the UV-vis region. The ratio of the wavelength position of photonic stop bands from (200) and (111) planes is ,25 Therefore, the shoulder in the transmission at normal incidence (at 478 nm), presented in Figure 2, can be assigned to a photonic stop band from the (200) reflections. To confirm this assignment, the angular dependences of the Bragg resonances from (111) and (200) planes for inverse opal structure with 300 nm diameter of voids were calculated using eq 3. The value of r 200 was calculated using r 200 ) r 111, where is the angle between the (200) and (111) planes. 23 These two calculated dependences were compared with the experimental one in Figure 3.

4 4050 J. Phys. Chem. C, Vol. 111, No. 10, 2007 Aloshyna et al. Figure 3. Angular dependence of experimental optical transmission minimum position (full circles) and the calculated minima positions associated with Bragg resonances from (111) planes (open triangles) and (200) planes (open squares). Figure 4. incidence. Optical transmission spectra of PCs I and II at normal The shift of photonic stop band position to the shorter wavelength from 551 to 483 nm in the range of 0 to 40 is in good agreement with Bragg diffraction from (111) sets of planes, whereas the shift of the stop band position to the longer wavelength from 483 to 507 nm in the range of 40 to 60 corresponds well to the diffraction from (200) sets of planes. Thus, the experimental angular dependence of optical transmission minimum for PC I can be summarized as follows: at normal incidence the spectrum exhibited two features. The first one was the minimum at 551 nm caused by the (111) sets of planes, and the second one was a shoulder at 478 nm due to the (200) sets of planes (Figure 2). As the incident angle relative to the surface normal was increased, the minimum corresponding to the (111) sets of planes moved to shorter wavelength, whereas the shoulder corresponding to the (200) sets of planes shifted to longer wavelength. When the angle θ reached 40, these two minima approach each other and overlap. At angles larger than 40, the minima continue to move in opposite directions, causing a broadening of the photonic stop band. As can be seen in Figure 2, the depth of the transmission minima of PC I is about 25%. The fwhm is about nm and depends on the incidence angle. The attenuation in transmission at the center of the photonic stop band in comparison with its edges is about 1.6 times. The remaining transmission of about 40% in the center of the photonic stop band is caused mainly by the reflections of the sample, substrate surfaces, and light scattering from the surface and defects. The photoluminescence of the 5 D 4 f 7 F 4 transition of Tb 3+ ions in solution, the main peak centered at 542 nm, with a fwhm of 11 nm, should be located inside the photonic stop band for PC I at all incident angles, as shown in Figures 2 and 3. According to eq 3, the stop band position for Bragg diffraction from (111) planes of PC II at normal incidence should occur at 290 nm, which is out of the photoluminescence of the 5 D 4 f 7 F 4 transition of Tb 3+ ions. Figure 4 compares the optical transmission of the PC I and PC II samples. No photonic stop band was observed in the optical transmission spectrum of PC II at normal incidence in the wavelength region of the 5 D 4 f 7 F 4 transition of Tb 3+ ions. The photoluminescence decays corresponding to the 5 D 4 f 7 F 4 transition of Tb 3+ ions embedded in PC I were compared with those of PC II and the disordered sample (sample III). These decay curves are shown in Figure 5A (the decay curve of the disordered materials coincides with PC II, data not shown). All decays could well be fitted with a three-exponential function. The decay time components for PC I were 4.30 (75%), Figure 5. (A) Photoluminescence decay curves of PCs I (black line) and II (gray line) at an observation angle of 30, λ ex ) 488 nm. (B) The average lifetime of LaF 3:Tb 3+ nanoparticles at different observation angles with respect to surface normal in PC I (full circles), PC II (open squares), and sample III (open triangles) (24%), and 0.14 ms (2%); for PC II, 3.00 (79%), 1.18 (19%), and 0.15 ms (2%); and for sample III, 3.08 (82%), 1.29 (17%), and 0.25 ms (1%). Because of the complex behavior of the lifetimes, which can be explained by the different constants of quenching of Tb 3+ ions depending on their location in the core or in the layer close to surface of the LaF 3 shell, 25 the average lifetime defined by expression 2 in the Experimental Section was used to evaluate the changes in lifetimes between the samples. The average lifetime obtained for PC I was 4.0 ms. This value was around 40% larger than the 2.8 and 2.9 ms obtained for PC II and sample III, respectively. The PCs and the disordered silica samples were fabricated under the same conditions; hence,

5 Spontaneous Emission Suppression in SiO 2 Crystals J. Phys. Chem. C, Vol. 111, No. 10, the nonradiative contributions were the same for all samples. Therefore, the lifetime changes between the samples were due to the changes in the radiative lifetime. According to theoretical calculations which have been done for perfect photonic crystals with high dielectric contrast, 8,26 the formation of photonic band gaps changes the density of optical states: DOS is zero in the PBG region. In the case of the photonic systems with lower dielectric contrast which do not exhibit a complete PBG but only a photonic stop band, the influence of changes in DOS was also proposed on the radiative properties of active materials embedded in photonic structure when the resonant wavelength of these materials matches the frequencies in the photonic stop band in one propagation direction. 26 The change in DOS influences the lifetime of light emitters inside the photonic crystals, 9,27 depending on the reciprocal location of their photoluminescence spectrum and the photonic stop band. For inverse fcc structure, the full PBG is known to be located between the 8 and 9 bands when the effective refractive index is larger than 2.9 and a lower frequency photonic stop band is also observable. 26 In addition, for typical inverse opal crystals with realistic low effective index contrast (n eff ) 2), the influence of the photonic band gap region on the density of optical states has already been shown theoretically. A photonic stop band of the fcc inverse structure was calculated as the remnant of a full band gap along the 111 direction. 28 Therefore, the increase of lifetime of spontaneous emission from 5 D 4 to 7 F 4 transition of Tb 3+ ions embedded in PC I can be explained by the reduction in the DOS available for photon propagation. Our transmission spectra show that the emission frequency of the Tb 3+ ions is close to the minimum of the photonic stop band in the case of the PC I sample, whereas, for PC II, in which the photonic stop band and the spontaneous emission from the 5 D 4 f 7 F 4 transition of Tb 3+ ions did not overlap, the photonic effect was absent. In this case, the emission rests uninfluenced and the experimental lifetime was very close to the lifetime of the sample with a disordered structure. Therefore, the results we obtained support the conclusions of Busch and John about the influence of the photonic structure on active infiltrated materials. 26 Similar results on quantum dots were observed experimentally. 13,14 The alteration of the DOS in the photonic structure was explained to influence the quantum dot emission and time decay as well. The angular dependence of photoluminescence decays of these three samples is presented in Figure 5B. The lifetimes of Tb 3+ ions in PC I, as well as in PC II and sample III, were almost the same for the whole range of observation angles. The 5 D 4 f 7 F 4 emission line of Tb 3+ was much narrower than the wavelength region of the photonic stop band, and the overlap between the photonic stop band and the emission band was practically the same at all angles of observation. Thus, the photonic mode density did not change and the emitted photons were influenced by the photonic effect in the same way at all angles of observation. The observed effect on modulation of the lanthanide ions emission in PCs can find future application in optoelectronic devices, such as light-emitting diodes (in principle, the photonic effect could allow the choice of emission lines and different combinations of colors by inhibition of some emission lines), and low-threshold lasers. Conclusions In summary, an air sphere SiO 2 inverse opal with photonic stop band at 551 nm made with LaF 3 :Tb 3+ nanoparticles was fabricated. The emission of the 5 D 4 f 7 F 4 transition of Tb 3+ ions embedded in inverse opal structure was located in the center of the photonic stop band. It has been shown that the luminescence lifetime of the Tb 3+ ions in this inverse opal is increased by 40% compared to the luminescence lifetime of the Tb 3+ ions in an inverse opal with stop band out of the emission region of 5 D 4 f 7 F 4 transition of Tb 3+ ions and in a disordered control sample. This increase in the luminescence lifetime can be explained by the suppression effect on spontaneous emission of Tb 3+ ions due to an incomplete photonic band gap or photonic stop band. Acknowledgment. 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