ARTICLE pubs.acs.org/JPCC
Inhibited Long-Scale Energy Transfer in Dysprosium Doped Yttrium Vanadate Inverse Opal Yongsheng Zhu,†,‡ Wen Xu,† Hanzhuang Zhang,‡ Wei Wang,† Sai Xu,† and Hongwei Song*,† †
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China ‡ College of Physics, Jilin University, 2699 Qianjin Street, Changchun 130012, China ABSTRACT: In this article, we first fabricate YVO4:Dy3+ (n = 1.98) inverse opal photonic crystals (PCs) through the polymethylmethacrylate (PMMA) template and the photonic stop bands (PSB) of the PCs controlled on the emissions of yellow (4F9/26H13/2) and blue (4F9/26H15/2) of Dy3+, respectively. Strong modification on steady-state emission spectra and luminescent dynamics are observed at room temperature. It is interesting to observe that the spontaneous emission rates (SER) of the PCs are suppressed as high as 230250% in contrast to the reference grinded powder samples (REF) due to the change of effective refractive index (neff). The more significant, the studies on the temperature-dependent emissions indicate that in the inverse opals, the long-scale energy transfer (ET) among Dy3+ ions and vanadate groups is greatly inhibited, and thus the temperature-quenching of photoluminescence (PL) is considerably suppressed. As a consequence, the luminescent quantum yield of the YVO4:Dy3+ inverse opals increases greater than that of the REF samples. This work shows that inverse opal PCs of RE ions have great application potential in novel devices of lighting and display.
1. INTRODUCTION Since the pioneering works by Yablonovitch1 and John2 in 1987, the optical properties of photonic crystals (PCs) have attracted much attention in the last twenty years because of their giant application potentials on near-zero threshold lasers, waveguides, optical switches, etc.35 Because of their spatial periodicity in dielectric constant on the length scale of the optical wavelength, the periodically electromagnetic modulation created by the periodicity structure of PCs can yield a photonic stop band (PSB). When the transition frequency of excited atoms is inside the PSB, the spontaneous emission inside PCs can be largely modulated. Up to now, a large number of works have been performed on the modification of spontaneous emission by embedding luminescent species in the PCs, including organic dyes,68 quantum dots,911and rare earth (RE) ions.12,13 Many interesting effects have been observed, such as the appearance of photonatom bound states, spectral splitting, enhanced quantum interference effects of spontaneous emission, nonclassic decay, Lamb shift, etc.14 In various modified emitters, organic dyes and quantum dots both demonstrate band emissions, which are often broader than the width of the PSB, and only a part of the emission spectrum can be modified. In contrast, trivalent RE ions having 4f4f inner-shell transitions possess specific advantageous features, such as high luminescence yield, narrow emission line, and long decay time constant,1517 thus is more suitable to examining the PCs effect. On the view of applications, the three-dimensional PC structures can be used for controlling spontaneous emission rate18 and improving the brightness and color purity of RE doped phosphors.19 Especially, the property of space-dependent r 2011 American Chemical Society
emission is key important for the novel device of white light emission diode (LED). Until now, despite the fact that quite a few works have been performed, the modification of the three-dimensional PCs on spontaneous emission of RE ions as well as the other species has not been clarified, For instance, the ET phenomenon is universal and an important key in the process of PL; however, the modification of PCs on ET has not been reported. Yttrium vanadate doped by trivalent europium or dysprosium is a well-known efficient phosphor, which has been applied in lighting and cathode ray tube (CRT) displays.20 The vanadate groups have strong adsorption in the ultraviolet (UV) region and can generate blue emission bands themselves. As Dy3+ ions are doped into the lattices of YVO4, which can efficiently transfer the excited energy to Dy3+, generating yellow (4F9/26H13/2) and blue (4F9/26H15/2) line emissions. Since both the donor and acceptor emissions can be observed and the refractive index of YVO4 is as high as 1.98, it is a suitable system to examine the PCs effect on ET. Herein, high-quantity YVO4:Dy3+ inverse opal PCs by the polymethylmethacrylate (PMMA) template method were first prepared. In the YVO4:Dy3+ inverse opals, we observed not only the large inhibition of PSB on the emission spectrum and spontaneous emission rate (250%) but also, for the first time to our knowledge, the inhibition of a long scale ET process in inverse opal PCs, which leads to the decrease of the Received: September 29, 2011 Revised: December 19, 2011 Published: December 20, 2011 2297
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temperature-quenching of PL. The present work is not only important for understanding the modification nature of the luminescent species insides PCs but also might shed light on new optical devices such as novel white LED devices and PCs sensors.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Yttrium nitrate (Y(NO3)3 3 6H2O) and dysprosium nitrate (Dy(NO3)3 3 6H2O) were received from the National Engineering Research Centre of Rare Earth Metallurgy and Function Materials. Ammonium metavanadate (NH4VO3) was obtained from Tianjing Chemical Factory in China. Methyl methacrylate (MMA), ethanol, and citric acid were purchased from Beijing Chemical Plant (Beijing, P. R. China). All chemicals were used as received. Inverse opal PCs, YVO4:Dy3+ (1 mol %), were prepared by the solgel method with a PMMA latex sphere template technique. First, monodispersed PMMA latex spheres with controllable sizes (∼360 nm and ∼420 nm) were synthesized. Then, a thinfilm template was self-assembled through the vertical deposition process. The colloid suspension (5% solid content) of PMMA microspheres was dropped onto a glass substrate and placed in a 30 °C oven for 3 days. The PMMA 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 40 min at 120 °C to enhance their physical strength. In the preparation of the Y1xDyxVO4 (x = 0.01) precursor sol, stoichiometric amounts of Dy(NO3)3 3 6H2O, Y(NO3)3 3 6H2O, and NH3VO4 were dissolved in ethanol solution. The ethanol solution contained citric acid as the chelating agent. Then, an appropriate amount of nitric acid was dissolved in ethanol. The mixture was stirred for 4 h, forming a transparent 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 one day. Annealing was carried out with slowly elevated temperature (40 °C/h) up to 500 °C for 3 h. Samples prepared by opal templates constructed with PMMA microspheres 360 and 420 nm in diameter were denoted as samples PC1 and PC2, respectively. For comparison, the reference samples were prepared by grinding the samples PC1 and PC2 to destroy the regular 3D ordered structure. 2.2. Measurements. The morphology of the samples was measured with a JEOL JSM-7500 field emission scanning electron microscope (FE-SEM) at an accelerating voltage of 15 kV. The phase structure of the samples were characterized by a X-ray diffractometer, using a monochromatized Cu target radiation resource (λ = 1.54 Å). Ultravioletvisible (UVvis) absorption spectra were measured with a Shimadzu UV-3101PC scanning spectrophotometer with a range of 2001100 nm. The emission spectra were recorded at room temperature using a Hitachi F-4500 spectrophotometer. The luminescent dynamics were pumped by a laser-system consisting of a Nd:YAG pumping laser (1064 nm), the third-order Harmonic-Generator (355 nm), and a tunable optical parameter oscillator (OPO, Continuum Precision II 8000). It was with the pulse duration of 10 ns, repetition frequency of 10 Hz, and line width of 47 cm1. In the measurements of low-temperature emission spectra, the samples were put into a helium gas cycling system in which the temperature varied from 10 to 400 K.
Figure 1. (a) SEM image of the opal template sample; (b, c) SEM image of the high- and low-magnified inverse opal sample; (d) the side-view image of the inverse opal sample.
Figure 2. XRD patterns of different YVO4:Dy3+ samples: (a) PC1, (b) PC2, and (c) REF.
3. RESULTS AND DISCUSSION 3.1. Characterization of YVO4:Dy3+ Inverse Opal. The PC1
and PC2 samples on the glass substrates (1 cm 1 cm), display blue and green color, respectively, when the naked eye is vertical to the samples. While the directions are changed, the color also gradually changes, implying the formation of well-organized PCs. Figure 1ad shows, respectively, SEM images of the PMMA opal template, high- and low-magnified images of the inverse opal PC1, and the side-view image of PC1. Figure 1a shows that a long-range PMMA opal is formed, and the diameter of the PMMA spheres is 360 nm. The PC1 sample yields a long-range ordered hexagonal arrangement of inverse opal with the centerto-center distance of ∼240 nm between the macropores on the (111) planes, which is about 33% smaller than the original size of the PMMA template due to the shrinkage of sphere diameters during calcination. From the side-view, it can be seen that the thickness of the sample PC1 is about 12 μm, indicating that it is a multilayer structure with more than 50 layers. Note that except for the center-to-center distance, the sample PC2 (d ≈ 280 nm) is similar to PC1. Figure 2 shows the X-ray diffraction (XRD) patterns of the PC1, PC2, and REF samples in contrast to the standard card. It can be seen that all of the samples are exactly in agreement with the 2298
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Figure 3. (a) Emission spectra of PC1 and REF samples and transmission spectrum of PC1. (b) Emission spectra of PC2 and REF samples and transmission spectrum of PC 2.
corresponding standard cards (JCPDS 170341 for tetragonal YVO4). No impurity peaks appear, implying that the samples are both YVO4:Dy3+ in pure tetragonal phase. In PC1 and PC2, the board band ranging from 1540 degree comes from the diffraction of the glass substrate. 3.2. Modification of PSB on Emission Spectra and Decay Dynamics. First, the modification of PSB on the steady-state emission spectra of Dy3+ was studied in air. Figure 3a and b, respectively, show the optical transmittance spectra collected from samples PC1 and PC2 and the emission spectra of YVO4: Dy3+ for PC1 and PC2 in contrast to the REF sample in air. From the optical transmittance spectra, it can be seen that the PC1 and PC2 samples both display deep PSBs, centering at ∼482 and ∼583 nm, respectively. Note that as measured from the normal, the 360 and 420 nm PMMA templates display deep and narrow band gaps centered at around 805 and 942 nm. The PSBs of inverse opals shift to the short wavelength side remarkably compared with the PSBs of bare opals, which is due to a lower average refractive index caused by air spheres instead of polymer spheres and shrinkage of spheres’ diameters during calcination. Theoretically, the position of the PSB in face centered cubic (fcc) PCs can be estimated by Bragg’s law of diffraction combined with Snell’s law21 λ¼
2dhkl m
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n2eff sin2 θ
ð1Þ
where λ is the center of the stop band, m is the order of the Bragg diffraction, dhkl is the hkl plane distance, neff is the average refractive index, and θ is the angle from the incident light to the normal of the substrate surface. Assuming air takes 26% of the space in opal templates, then it takes 74% of the space in the inverse opal. According to eq 1, the locations of PSB in PC1 and PC2 are determined to be 490 and 572 nm, respectively, which is consistent with the experimental results. As can be seen for PC1 and PC2, the PSBs overlaps with the emission lines of 4F9/26H15/2 at 487 nm and 4F9/26H13/2 at 576 nm transitions in YVO4:Dy3+, respectively. In all the emission spectra, the 4F9/26HJ (J = 15/2, 13/2, and 11/2) transitions are observed, locating at 487, 576, and 660 nm, respectively. Note that in Figure 3a, the emission spectra are normalized to the 576 nm peak (4F9/26H13/2), which situate relatively further away from the PSB, while in Figure 3b, the emission spectra are normalized to the 487 nm peak (4F9/26H15/2). It can be seen that in PC1, the PL intensity at 487 nm is significantly suppressed in contrast to the REF sample, while in PC2, the intensity at 576 nm is largely suppressed. An inhibition of the light emission
Figure 4. PL decay curves of the PC and REF samples. Left drawing corresponding to the 4F9/26H15/2 transition embedded in PC1and REF. Right drawing corresponding to the 4F9/26H13/2 transition embedded in PC2 and REF. The dots are experimental data, and the solid curves are fitting functions.
from the YVO4:Dy3+ inverse opal has occurred due to the photon trapping caused by Bragg diffraction of lattice planes in the PC1 and PC2 samples.22 The luminescent decay dynamics in the PC1, PC2, and REF samples are compared, as shown in Figure 4. It is obvious that in PCs, the luminescent dynamics are prolonged significantly in contrast to the REF sample, and all the decay curves can be well fitted by a double-exponential function IðtÞ ¼ I1 ð0Þet=τ1 þ I2 ð0Þet=τ2
ð2Þ
where τ1 and τ2 are the slower and faster decay time constants measured at a fixed wavelength, and I1(0) and I2(0) represent the relative contributions of the slower decay and the faster decay components, respectively. The slow decay components (τ1) in different samples all come from the 4F9/26HJ transitions of Dy3+ ions, while the faster decay components (τ2) may originate from the emissions of Dy3+ nearby the defect states and/or the band emissions of vanadate groups. In YVO4 lattices, Dy3+ ions occupy only the sites of the Y3+ ions with the D2d symmetry.23 However, as the Dy3+ ions are nearby defect states, such as lattice defects, surface dangle bonds, or such adsorbed large phonon energy groups, the decay time constant of Dy3+ may become shorter due to the considerable increase of the nonradiative relaxation rate. Note that in the vacuum and at low temperatures, the band emissions originating from vanadate groups are distinguished in PCs and REF samples (see the following text), which overlap with the 4 F9/26HJ line emissions. In order to better understand the modification of PSB on the spontaneous emission rate of the 4F9/26HJ transitions of Dy3+ ions, the decay time constants (in air) in different samples and measured at different wavelengths are listed in Table 1. It can be concluded that (1) the decay time constants vary with monitoring wavelength for all the samples, and the constants measured at 487 nm are always shorter than those measured at 576 nm. Obviously, this behavior is independent of the PCs effect. It is suggested that the variation of decay time constants monitored at different wavelengths comes from the transitions of Dy3+ ions with different local environments or the variation of refractive index with wavelength. (2) In the PCs samples, both the shorter and the longer decay time constants increase remarkably compared to those of the REF sample, implying that the spontaneous emission rates in the PCs are suppressed. The long decay time constants in the PCs increase as high as 230250% in contrast to those of the REF sample, which were seldom observed in the previous literatures.9,24,25 (3) For the PC1 and PC2 inverse opal, 2299
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Table 1. Variation of Decay Time Constants τ1 (Longer) and τ2 (Shorter) with Samples and Emission Wavelengths (λem) λem= 487 nm
λem= 576 nm
REF
τ1 = 55.3 μs
τ1 = 65.5 μs
PC1
τ2 = 3.9 μs τ1 = 126.3 μs
τ2 = 6.3 μs τ1 = 149.4 μs
τ2 = 5.6 μs
τ2 = 13.8 μs
PC2
τ1 = 136.5 μs
τ1 = 151.8 μs
τ2 = 6.8 μs
τ2 = 13.6 μs
samples/decay constants
the decay time constants are nearly independent of the location of PSB. It should be noted that in most of the previous literature, the suppression of spontaneous emission rate in PCs was observed, and usually, it was attributed to the decrease of LDOS in PCs. According to Fermi’s golden rule,26 the rate of spontaneous emission in the weak oscillator-field coupling regime is proportional to the local density of state LDOS electromagnetic modes. The decrease of LDOS should depend strongly on the location of PSB.27 In the present case, the suppression of the spontaneous emission rates in the YVO4:Dy3+ PCs can not be attributed to the decrease of LDOS but to the variation of effective refractive index (neff) in the PCs. The spontaneous emission rate (SER) is the sum of the radiative transition rate and the nonradiative relaxation rate. According to the theory of the virtual-cavity model, the radiative lifetime of the electric dipole transition for the RE ion can be written as τR ≈
1 λ0 2 2 f ðEDÞ 1 ðneff 2 þ 2Þ neff 3
ð3Þ
where, f(ED) is the electric dipole strength, neff is the effective refractive index of the medium, and λ0 is wavelength in vacuum. For the REF sample, the neff is approximately 1.72; the REF sample is similar to the closely packed sample. For the YVO4 inverse opal, neff can be expressed as neff ¼ xnYVO4 þ ð1 xÞnair
ð4Þ
where, x is the volume ratio between YVO4 and air in the inverse opal (x = 0.26). It is easy to obtain that in the inverse opal neff = 1.25. On the basis of eq 3 and supposing f(ED) does not change in different samples, it can be deduced that in the YVO4:Dy3+ inverse opal, the radiative lifetime of the 4F9/26HJ transitions of Dy3+ ions increases nearly 2.6 times over the REF sample, which is nearly consistent with the variation of long decay time constants between the inverse opal and the REF sample. Recently, an accurate description was made for calculating the radiative lifetime of lanthanides in nanocrystals, which takes into account the difference of the average electric field acting in the medium and optical center in nanoparticles.2830 We also calculated the radiative lifetime of Dy3+ ions in PCs and REF samples using the modified model. It shows that the radiative lifetime in inverse opal also increases 2.6 times over the REF sample. Therefore, we can conclude that in the YVO4:Dy3+ inverse opal, the variation of neff does contribute dominantly to the suppression of SER. 3.3. Angle-Dependent PSB and Its Modification on PL. The space-dependent optical property of PSB and its modification on spontaneous emission is an important subject of PC effects. Figure 5 shows the transmittance spectra collected from PC1 and PC2 at different incidence angles with respect to the normal
Figure 5. Transmittance spectra collected at different angles of incidence from the PC1 and PC2. Insets are the theoretical curves of Snell’s law and experimental dates.
Figure 6. Angular dependence of the relative intensity 4F9/26H15/2 (487 nm) transitions of Dy3+ in PC1 and 4F9/26H13/2 (576 nm) transitions in PC2.
direction of the (111) plane. At normal incidence (θ = 0°), the PSBs for PC1 and PC2 are at 482 and 583 nm, respectively, which overlap with the 4F9/26H15/2 and 4F9/26H13/2 emission lines of YVO4:Dy3+, respectively. The central position of the PSB shifts to short wavelengths with the increasing incident angle. In PC1, the central positions of PSB are at 482, 479, 470, 454, and 441 nm as the incident angle increases from 0° to 20°, which make the PL emission overlapping with the red band edge of the stop band at 20°. In PC2, the central positions of PSB are at 583, 580, 569, 550, and 534 nm as the incident angle increases from 0° to 20°, which fit well with Bragg’s law (see the inset of Figure 5.) The PSB becomes broader and shallower as the incident angle increases from 25° to 60°. Figure 6 shows the angle-dependence of the intensity ratio of I(4F9/26H15/2) to I(4F9/26H11/2) in PC1 and the ratio of I(4F9/26H13/2) to I(4F9/26H11/2) in PC2. As the incident angle varies, the location of PSB also changes; however, it situates always far away from the 4F9/26H11/2 transition (∼660 nm) for both the PC1 and PC2 samples. The intensity of 4F9/26H11/2 should not change with the varied incident angle and thus is taken as a reference. It can be seen that as the incident angle of light increases, the relative intensity of the 4F9/26H15/2 at 487 nm for PC1 increases first and approaches at a maximum at θ = 20°, and it then drops again as the incident angle varies continuously from 20° to 60°. As the incident angle is fixed at 60°, the PSB becomes quite weak and is far away from the 4F9/26H15/2 transition, thus the relative intensity at 60° actually reflects the intensity without PSB modification. It is obvious that the emission intensity of 4F9/26H15/2 can be either 63% below or 55% above the 2300
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Figure 7. Dependence of decay time constants on angle of incident light with respect to surface normal in YVO4:Dy3+ PC1 (curves a and c) and PC2 (curves b and d).
Figure 9. Illustration of energy level diagram of Dy3+ ions, emission, annihilation, and ET processes in inverse opal and REF samples. EX, EM, NRET, and RET represent excitation, emission, nonradiative energy transfer, and radiative energy transfer, respectively.
3+
Figure 8. Temperature-dependent PL of YVO4:Dy samples. Inset, the total emission intensity of 4F9/26HJ for Dy3+ ions and vanadate groups as a function of temperature in the PC1 and REF samples.
reference level, depending on the relative position between the emission line and PSB. As the emission line locates on the center of PSB, the emission intensity is suppressed, while as the emission line locates on the edge of PSB, the intensity is enhanced. The result observed in PC2 is similar to that in PC1. For PC2, the relative intensity of 4F9/26H15/2 at 576 nm increases with the increasing angle first, and the maximum appears at θ = 30°, and then it drops again. The emission intensity of 4F9/26H15/2 can be either 47% below or 40% above the reference level. The present result is well in accordance with the theory and similar results were also obtained in the previous literatures.31 Suppression of the emission can readily be understood as being due to the reduction in the number of optical modes available for photon propagation at frequencies within the PSB. The enhancement of the emission is due to the group velocity of light becoming anomalously small near the edge of the stop band. In this case, photons can couple with a local resonance mode and Bragg scatter out of the structure, which greatly enhances the interaction of light with matter. Moreover, this very high density of states near the PSB enhances the coupling of spontaneously emitted photons to the modes.3234 Figure 7 displays the angle-dependence of decay time constants of 4F9/2Σ6HJ for the two inverse opal PCs. It can be concluded that the decay time constants of 4F9/2Σ6HJ are nearly independent of the incident angle of light, for both the shorter and longer ones. This further indicates that the SERs of 4F9/2Σ6HJ do not vary with the location of PSB because while the incident angle varies, the location of PSB shifts significantly. This fact again implies that even in the present high refractive index YVO4: Dy3+ reverse opals (n = 1.98), the SERs of 4F9/2Σ6HJ for Dy3+ ions are not obviously modified by the LDOS electromagnetic modes. Previously, we studied the modification of PSB on the spontaneous emissions of europium complex in weakly PMMA opals (n ≈ 1.6). The experimental result and theoretical calculation both indicated that in spite of the eminent angular redistribution of LDOS, little change occurred in total LDOS.
3.4. Restrained Long-Range ET in the Inverse Opal. ET among vanadate groups is known to be a thermally activated process and, with a rate sufficiently high, occurs between neighbors. Figure 8 shows temperature-dependent emission spectra in the PC1 and REF samples in vacuum. For the REF sample, there are two broad band emissions besides the 4F9/26HJ (J = 15/2 and 13/2) lines, centering at ∼480 nm and ∼700 nm, respectively. They are attributed to the PL of vanadate groups and defect states of oxygen vacancies, respectively. It should be noted that the emissions of vanadate groups in vacuum are more effective than in air, which can be attributed to be suppressed quenching of vanadate groups in vacuum. It can be also seen that for the REF sample, the emission of the vanadate group is most intense at low temperature (∼10 K), and the 4F9/26H15/2 line almost disappears, while the 4F9/26H13/2 line appears on the shoulder of the vanadate group. This implies that even at low temperature, the nonradiative ET from the higher state of vanadate to Dy3+ also happens effectively. The emission of 4F9/26H15/2 is readsorbed by vanadate groups due to resonant ET from Dy3+ to the lower state of vanadate groups and thus quenches at low temperature. As the temperature increases, the luminescence intensity of vanadate decreases quickly because vanadate groups are activated intensively with the rise of temperature. As the temperature is above 300 K, the emission of vanadate groups quenches completely. It is very surprising to observe that for PC1, the temperature-dependent luminescent property is quite different. As the temperature varies from 10 to 400 K, both the line emissions of 4F9/26HJ (J = 13/2 and 15/2) and the band emissions of vanadate groups rarely change. In addition, it can be observed that the central location of the vanadate band blue-shifts to ∼420 nm in contrast to the REF sample, which could be attributed to the suppression of the band emissions from vanadate groups on central of PSB (∼480 nm), and the enhancement of the band emissions on the side of PSB. The inset of Figure 8 displays the total emission intensity of 4 F9/26HJ for Dy3+ and vanadate groups as a function of temperature in the PC1 and REF samples, which further indicates that in PCs, the temperature-quenching of PL is suppressed considerably. In the traditional YVO4:Dy3+ phosphors, the long-term resonant ET among vanadate groups is very effective. Inevitably, luminescent quenching will happen due to the ET from vanadate 2301
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The Journal of Physical Chemistry C groups to defect states, which randomly distribute in the lattices of the phosphors.35 This process is strongly dependent on temperature. In the inverse opals, the long-range ET among vanadate groups should be restrained largely because of the thin thickness of each YVO4:Dy3+ layer (∼20 nm)36 and the existence of a long periodic air cavity (hundreds nanometers) between the two layers. In this case, the ET among vanadate groups can happen only within one YVO4:Dy3+ layer and then the emitted photons are scattered into the air cavity rather than largely captured by the defect states through further long-range ET. A comparison of the excitation, ET, emission, and annihilation processes of YVO4: Dy3+ between reserve opal PCs and the traditional phosphors are illustrated in Figure 9. 3.5. Improved PL Quantum Yield in YVO4:Dy3+ PCs. The external quantum yield (QY) of PL can be obtained from the space integrated spectra in the integrated sphere. The external QY can be calculated as Z Ib ðλÞ Is ðλÞdλ QY ¼ Zemi ð5Þ Is ðλÞ Ib ðλÞdλ exi
where, Ib and Is refer to the spectrum of blank and sample, respectively, λ is wavelength, and emi and exi means integration over the emission and excitation bands. The QY of different samples are deduced to be 13.7%, 12.5%, and 5.6%, respectively, for PC1, PC2, and REF samples. It is very exciting to observe that due to the inhabited long scale ET and nonradiative relaxation, the QY of YVO4:Dy3+ inverse opal PCs is improved greatly over the REF sample. This result shows that three-dimensional (3D) inverse opal PC is a significant device for improving the QY of traditional RE oxide phosphors, which has great application potential in novel LED devices.
4. CONCLUSIONS In summary, 3D-YVO4:Dy3+ inverse opal PCs were successfully prepared by using the PMMA templates. Some novel modification phenomena of PC effects on the spontaneous emission of RE were observed. Our results demonstrate that the temperaturequenching rates of Dy3+ and vanadate groups in the inverse opal was considerably suppressed in contrast to the REF sample due to inhibited long-range ET among vanadate groups and defect states. As a result, the luminescent QY of YVO4:Dy3+ in the inverse opal was remarkably improved. This work is of great significance for understanding the PC effects on spontaneous emissions of RE ions and for novel devices of lighting and display. ’ AUTHOR INFORMATION Corresponding Author
*Tel/Fax: +86-431-85155129. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank the financial support of the National Talent Youth Science Foundation (Grant No. 60925018), the National Natural Science Foundation (Grant Nos. 50772042, 10704073, 20971051, and 10974071), and the Jilin Province Natural Science Foundation of China (No. 20070512).
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