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Langmuir-Blodgett Film of a Europium Complex and Its Application in a Silver Mirror Planar Microcavity Yilei Zhao, Dejian Zhou, Chunhui Huang,* and Liangbing Gan State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, People’s Republic of China
Liming Ying and Xinsheng Zhao Department of Chemistry, Peking University, Beijing 100871, People’s Republic of China
Bei Zhang, Yong Ma, Ming Xu, and Ke Wu State Key Laboratory of Mesoscopic Physics, Peking University, Beijing 100871, People’s Republic of China Received October 28, 1997 A planar microcavity using mixed a Langmuir-Blodgett film of a europium complex HDP‚Eu(NTA)4 (where HDP ) cetylpyridinium and NTA ) R-naphthyltrifluoroacetone) and arachidic acid (AA) as fluorescence working material, and vacuum evaporated silver films as reflective mirrors was fabricated for the first time. Using the traditional Langmuir-Blodgett (LB) technique, the mixed Langmuir film of the Eu complex and AA on a water subphase containing 0.20 mM of Eu(ClO4)3 was finely transferred onto a lipophilically pretreated silver mirror to form a multilayer LB film microcavity with a λ/2 optical length (where λ is the emission wavelength). The optical length of the microcavity was measured by a multibeam interference method. The fluorescence spectrum of the λ/2 resonant microcavity coincides with that of the LB film of the complex, and microcavity effects such as fluorescence intensity enhancement and lifetime shortening have been observed.
Introduction Microcavities are optical resonators having an optical length at least one dimension’s size on the order of a wavelength. The effect of optical confinement in one or more dimensions is a rearrangement of the usual freespace spectral mode density. The density of modes at some wavelengths will be increased, whereas at others the density will be decreased. If an optical emitting material is confined into such a cavity, its spontaneous emission will be increased or decreased, depending on the cavity-mode density at the emission wavelength.1,2 Thus the spontaneous emission of the emitting materials could be controlled through the microcavity. Recently, this field is really very attractive because the alteration of spontaneous emission by an optical microcavity is important both in fundamental physics studies on the interaction of matter with vacuum field fluctuations and cavity quantum electrodynamics (CQED) and in exploring potential applications of novel photonic devices such as low threshold lasers, high-efficiency emitting diodes, and large planner display devices.3-5 (1) (a) Brorson, S. D.; Yokoyama, H.; Ippen, E. P. IEEE J. Quantum Electron. 1990, 29, 1492. (b) Ujihara, K.; Nakamura, A.; Manba, O.; Feng, X. P. Jpn. J. Appl. Phys. 1991, 30, 3388. (2) (a) Hulet, R. G.; Hilfer, E. S.; Kleppner, D. Phys. Rev. Lett. 1985, 55, 2140. (b) Goy, P.; Raimond, J. M.; Gross, M.; Haroche, S. Phys. Rev. Lett. 1983, 50, 1903. (c) DeMartini, F.; Innocenti, G.; Jacobovitz, G. R.; Mataloni, P. Phys. Rev. Lett. 1987, 59, 2955. (3) (a) Yokoyama, H. Science 1992, 256, 66. (b) Yokoyama, H.; Nishi, K.; Anan, T.; Nambu, Y.; Brorson, S. D.; Ippen, E. P.; Suzuki, M. Opt. Quantum Electron. 1992, 24, S245. (c) Yamamoto,Y.; Slusher, R. E. Phys. Today 1993, 46, 66. (4) (a) De Martini, F.; Marrocco, M.; Mataloni, P.; Crescentini, L.; Loudon, R. Phys. Rev. A 1991, 43, 2480. (b) Slusher, R. E.; Weisbuch, C. Solid State Commun. 1994, 92, 149.
Despite only one-dimensional optical field confinement, a planar microcavity has the advantages of the ease of cavity construction and analysis, and actually most of the microcavity experiments were carried out with planar cavity structures and the organic planar microcavity is the one whose microcavity effect has been firstly observed.3 The most commonly used optical emitting materials in planar optical microcavities are either organic dyes or some metal complexes such as tris[8-hydroxyquiniline]aluminum.2,6-8 These materials have a relatively wide emission band (typically wider than 100 nm); thus fabrication of such microcavities becomes relatively easy because their optical cavity length can be changed in a wide range. The main disadvantage of these materials is their relatively low fluorescence intensity. In recent years, fluorescence rare earth complexes have been introduced into microcavities, and strong microcavity effects have been observed. The use of fluorescence rare earth complexes as the emitting material in a microcavity may (5) (a) Zhang, B. In Mesoscopic Physics; Yan, S., Gan, Z., Eds. Peking University Press: Beijing, 1995; Chapter 12. (b) Drexhage, K. H. In Progress in Optics; Wolf, E., Ed.; North-Holland: Amsterdam, 1974; Vol. XII, p 163. (6) (a) Dodabalapur, A.; Rothberg, L. J.; Miller, T. M.; Kwock, E. W. Appl. Phys. Lett. 1994, 64, 2486. (b) Yokoyama, H.; Suzuki, M.; Nambu, Y. Appl. Phys. Lett. 1991, 58, 2598. (c) Suzuki, M.; Yokoyama, H.; Brorson, S. D.; Ippen, E. P. Appl. Phys. Lett. 1991, 58, 998. (7) (a) DeMartini, F.; Innocenti, G.; Jacobovitz, R. G.; Mataloni, P. Phys. Rev. Lett. 1987, 59, 2955. (b) DeMartini, F.; Mataloni, P.; Crescentini, L. Opt. Lett. 1992, 17, 1370. (c) Lemmer, U.; Hennig, R.; Guss, W.; Ochse, A.; Pommerehne, J.; Sander, S.; Greiner, A.; Mahrt, R. F.; Bassler, H.; Feldmann, J.; Gobel, E. O. Appl. Phys. Lett. 1995, 66, 1301. (8) (a) Dadabalapur, A.; Rothery, L. J.; Miller, T. M.; Kwoch, E. M. Appl. Phys. Lett. 1994, 64, 2486. (b) Zhang, B.; Zhuang, L.; Lin, Y.; Xia, Z. J.; Ma, Y.; Ding, X. M.; Wang, S. L.; Zhou, D. J.; Huang, C. H. Solid State Commun. 1996, 97, 445.
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have many merits because they have a very high fluorescence efficiency, a very narrow linelike emission band, and excellent fluorescence monochromaticity.9 Particularly, their especially strong absorption in the UV region and semiconductivity make excitation very efficient in thin film devices. However, their sharp linelike emission makes fabrication of the resonant rare earth cavity very difficult because the cavity optical length should be precisely controlled to be the integral number of the half emission wavelength (Nλ/2, where λ is the emission wavelength and N is the natural number). As a result, only the solutions of rare earth complexes are used in the earlier studies.4a,10 These cavities have a relatively long optical cavity length (ca. 2.5λ) or sometimes the cavity length is changeable.10 It is very difficult to obtain the exact λ/2 resonant microcavity (whose microcavity effect being the strongest) of rare earth complexes by using spin coating films or solutions. The Langmuir-Blodgett (LB) technique has the advantages of precise control of the film thickness at the single molecular layer level which may warrant the requirement of the certain emission medium of the microcavity, and indeed strong microcavity effects have been observed from cavities made of LB films of some organic dyes.3b Earlier studies have shown that some strong fluorescence rare earth complexes can form stable Langmuir films at the air/water interface which are readily transferred onto solid substrates to build up good quantity LB films. By fabrication of the strong fluorescence rare earth complexes with the LB technique, strong fluorescence LB films with precisely controlled film thickness have been obtained.11,12 These fluorescence LB films of rare earth complexes may be used as the working materials in λ/2 microcavities. In addition, there is still an argument on whether the alteration of the fluorescence lifetime (spontaneous emission decay time) in planar microcavity terminated by a pair of metal mirrors could be obtained. Drexhage originally suggested that the change of lifetime should be accompanied by the modified emission profile and spectrum of the LB film deposited on a metal mirror, but he did not observe any clear change in emission lifetime.5b Using a solution of an Eudibenzoylmethane complex in a microcavity composed of either metal or dielectric mirrors, DeMartini et al. observed the fluorescence lifetime shortening, but he did not mention the fluorescence enhancement.4a Suzuki and Yokoyama repeated similar experiments on some organic dye containing microcavities terminated by a pair of dielectric mirrors, but they did not observe any measurable lifetime change using the dye solution;3,6 on the other hand, they observed the lifetime change in the microcavities by using the LB film of the dyes as the working material and dielectric mirrors.3 In this paper, we report our successful fabrication of a λ/2 microcavity using the LB film of a europium complex, HDP‚Eu(NTA)4 (whose molecular structure is depicted), as the emitting medium terminated by a pair of silver mirrors. Both the fluorescence enhancement and lifetime (9) Gmlin Handbook of Inorganic Chemistry, 8th ed; System No. 39 Part D3, p 71. (10) Ebina, K.; Okada, Y.; Yamasaki, A.; Ujihara, K. Appl. Phys. Lett. 1995, 66, 2783. (11) Zhou, D. J.; Wang, K. Z.; Huang, C. H.; Xu, L. G.; Li, T. K. Solid State Commun. 1995, 93, 167. (12) (a) Zhou, D. J.; Huang, C. H.; Yao, G. Q.; Bai, J.; Li, T. K. J. Alloys Compd. 1996, 235, 156. (b) Wang, K. Z.; Gao, L. H.; Huang, C. H.; Yao, G. Q.; Yu, A. C.; Zhao, X. S.; Xu, J. M.; Li, T. K. Solid State Commun. 1996, 96, 1075. (c) Huang, C. H.; Wang, K. Z.; Zhu, X. Y.; Xu, Y.; Liu, Y. Q.; Zhang, P.; Wang, X. P. Solid State Commun. 1994, 90, 151. (d) Zhao, Y. L.; Zhou, D. J.; Huang, C. H.; Yao, G. Q. Langmuir 1997, 13, 4060.
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shortening are firstly observed in the on-resonant LB film microcavity simultaneously.
Experimental Section Materials. Arachidic acid (AA) (99%) was purchased from Aldrich. HDP‚Eu(NTA)4 (where NTA and HDP represent β-naphthyltrifluoroacetone and cetylpyridinium, respectively) was synthesized according to the method described in ref 11. Other reagents are all A.R. grade chemicals from Beijing Chemical Factory (Beijing, China). Instrumental Measurements. UV-vis absorption and reflection spectra were recorded on a Shimadazu UV-3100 spectrophotometer; an incident angle of 8° and a BaSO4 white plate reference were used in the reflection spectra measurement. A low-angle X-ray diffractogram was obtained on a Rigaku D/Max3B diffractometer, using Cu KR radiation (λ ) 0.154 nm).13 Fluorescence excitation and emission spectra of the powder complex and LB film were recorded on a Hitachi 850 fluorescence spectrophotometer, using a 430 nm excitation cutoff filter. Excitation and emission bandwidths of 5 nm were employed, and the spectra were corrected for nonlinear instrumental response.14 A computer controlled NIMA Langmuir trough (Model 622) was employed for the LB film deposition. Langmuir films were formed by carefully depositing the CHCl3 solutions (∼1 g/L) onto a water subphase (20 °C, pH ) 5.60, or an Eu(NO3)3 solution (0.2 mM). After the solvent was allowed to evaporate for 20 min, the floating films were compressed at a rate of 10 cm2/min and the surface pressure-area (π-A) isotherms were recorded. The mixed monolayer was transferred onto a hydrophilically pretreated fused quartz plate14 or a lipophillically pretreated silver film at 25 mN/m in Y type at a dipping speed of 5 mm/min. During the Langmuir film transfer, the pressure was always kept within 25 ( 1 mN/m and the transfer ratio was always maintained around unity. The fluorescence lifetime measurement was carried out by exciting the samples with 320 nm light, which was generated from the doubling of the laser output of a PDL-3 dye laser pumped by the second harmonic of a pulsed GCR-4 Nd:YAG laser with 30 Hz repetition and 6 ns pulse width. The laser beam irradiated on the sample in about 1 mm diameter at different pulse energy ranged from dozens of nanojoules to dozens of microjoules. The emission from the Eu-complex medium was collected by a set of focus lenses, dispersed by a 612 nm focal length monochromator with a 1200 grooves/mm grating and detected by a R955 photomultiplier (Hamamatsu). To reduce the laser scattering, a CG550 filter (Schott) was placed in front of the monochromator. The output of the photomultiplier was sent to a 4100 boxcar system (EG&G) for averaging and then transferred to a personal computer via GPIB bus. A HP 54510A digital oscilloscope was employed, and the averaged real-time signal was transferred to a 386 SX/25 computer via GPIB bus in the measurement.15 Silver Mirror Deposition and Surface Chemical Pretreatment. The fused quartz substrates were thoroughly cleaned by using the procedure as described in ref 12, then silver films were coated onto the quartz slides in a Balzers UMS-500 (13) Huang, C. H.; Wang, K. Z.; Xu, G. X.; Zhao, X. S.; Xie, X. M.; Xu, L. G.; Li, T. K.; Xu, Y.; Liu, Y. Q.; Zhu, D. B. J. Phys. Chem. 1995, 99, 14397. (14) (a) Zhou, D. J.; Gan, L. B.; Luo, C. P.; Tan, H. S.; Huang, C. H.; Yao, G. Q.; Zhao, X. S.; Liu, Z. F.; Xia, X. H.; Zhang, B. J. Phys. Chem. 1996, 100, 3150. (b) Zhou, D. J.; Gan, L. B.; Tan, H. S.; Luo, C. P.; Huang, C. H., Yao, G. Q.; Zhang, B. J. Photochem. Photobiol. A: Chem. 1996, 99, 37. (c) Zhou, D. J.; Tan, H. S.; Gan, L. B.; Luo, C. P.; Huang, C. H.; Yao, G. Q. Chem. Lett. 1995, 649. (15) Ying, L. M.; Yu, A. C.; Zhao, X. S.; Li, Q.; Zhou, D. J.; Huang, C. H.; Umetani, S.; Matsui, M. J. Phys. Chem. 1996, 100, 18387.
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ultrahigh vacuum electron beam evaporation system to prepare the microcavity mirrors. The top semireflective silver mirror with thickness of 45.0 nm was firstly deposited onto polished fused quartz substrate (polishing degree 6∇) at a rate of 0.12 nm/s under 2 × 10-6 Torr at 85 °C. After deposition of the LB film onto the semireflective silver mirror, the second silver mirror with thickness of about 100 nm was then deposited onto the LB film using the same rate under 1 × 10-5 Torr at 25 °C. The lower vacuum and substrate temperature are set to prevent the LB film from evaporation. The silver film surface is neither lipophilic nor hydrophilic because of the formation of silver oxide under ambient atmosphere, and this makes deposition of the LB film on silver very difficult. In order to improve the surface property of the silver film, the freshly prepared silver film is soaked into a 0.1 mM water solution of 3-mercaptopropionic acid for 30 min or fumed in a n-propyl mercaptan gas for 20 h at room temperature to obtain a very good hydrophilic or lipophilic surface, respectively. There are no observable changes in both reflection and transmission spectra after the treatment. However, silver film treated by the mercanptopropionic acid solution is easily peeled away from the quartz plate during the LB film transferation; thus the propyl mercaptan gas fumed silver film is selected as the lipophilic substrate in the further LB film deposition.
Results and Discussion Langmuir Films at the Air/Water Interface. In our previous communication, we found HDP.Eu(NTA)4 is an amphiphilic strong luminescence material, whose Langmuir film is readily transferred onto hydrophilic quartz slides to obtain multilayer LB films. However, its isotherm has two condensed regions and strongly depends on the subphase temperature. The condensed region slope is only 1.21 mN m-1 Å-2 and is not stiff enough for transferation.11 This means that the single layer thickness of the LB multilayer may easily change with a small deviation of the outer conditions such as temperature or surface pressure, and this may result in difficulty making a microcavity whose optical length matches the emission wavelength of the complex. To solve the problem, the mixture of HDP.Eu(NTA)4 and AA was used as the filmforming material on a water subphase containing 0.2 mM Eu(NO3)3. The surface pressure vs area (π-A) isotherms of AA and HDP.Eu(NTA)4 on pure water or an Eu(NO3)3 solution are shown in Figure 1. A typical π-A isotherm of AA Langmuir film on pure water has a liquid phase from π ) 0 to 27 mN/m and a solid phase at higher pressure, with condensed region slope and limiting area of 12.1 mN m-1 Å-2 and 22.3 Å2/molecule, respectively. The result is rather similar to that of the earlier studies. However, when it turns onto a subphase containing 0.2 mM of Eu(NO3)3, the liquid phase disappears. The slope of the solid phase enlarges to 19.8 mN m-1 Å-2 and the cross section decreases to 20.2 Å2/molecule. All these indicate that the AA Langmuir film on the Eu(NO3)3 solution is more stable than that on pure water. The π-A isotherm of HDP.Eu(NTA)4 on a pure water is similar to that given in ref 11. The only difference is that the two solid phases are not as clear as those in the reference and distinguish with a turn point around 25 mN/m, which may be due to the different film forming conditions. The π-A isotherm on the 0.2 mM Eu(NO3)3 solution does not change significantly compared with that on the pure water, and the limiting area is around 80 Å2/molecule. The π-A isotherm of the mixed Langmuir film of HDP‚Eu(NTA)4 and AA (molar ratio, 21:100) has a stiff solid phase with a condensed region slope of 13.9 mN m-1 Å-2. Besides, the mixed Langmuir film has an excellent transferability, which is readily transferred onto hydrophilic quartz or lipophilic silver films to obtain good
Figure 1. (1) Pressure vs area isotherms of AA on pure water (‚‚‚), 0.2 mmol/L Eu(NO3)3 solution (s), and the mixture of HDP.Eu(NTA)4 and AA (molar ratio ) 21:100) on 0.2 mmol/L Eu(NO3)3 solution (- - -). (2) Pressure vs area isotherms of HDP‚Eu(NTA)4 on pure water (s) and in 0.2 mmol/L Eu(NO3)3 solution (- - -). Table 1. Major UV Absorption Bands (nm/absorbance) of the Mixture of HDP‚Eu(NTA)4 and AA in CHCl3 Solution and LB Films on Quartz Slides CHCl3 solution mixed LB film (63 layers) mixed LB film (101 layers) pure LB film (50 layers) a
260 (0.228) 260 (0.155) 260 (0.278) 260 (0.386)
267 (0.235) 267 (0.140) 267 (0.250) 268 (0.396)
330 (0.178) 347 (0.034) 345 (0.097) 331 (0.327)a
Reference 10.
quality LB films. So suitable film formation material for the microcavity usage has been obtained. Characterization of the LB Films. The UV-vis spectra of the mixed spreading CHCl3 solution and its LB films on fused quartz substrates are almost identical with that of the pure HDP‚Eu(NTA)4 solution or its LB film (Table 1).11 They all exhibit two major bands, and the one with double peaks at 260 and 267 nm is assigned to π f π* transition and the other weak broad band around 340 nm for is assigned to n f π* transition. It is clear that the incorporation of the AA molecules does not influence the electronic transition of HDP‚Eu(NTA)4 in the absorption spectrum. The low-angle X-ray diffractograms of an 101-layer mixed LB film and a 71-layer AA film on quartz plates are shown in Figure 2. Six diffraction peaks at 2θ, 1.56, 3.12, 4.70, 6.32, 7.92, and 9.40, are observed in the mixed film,
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Figure 2. Low-angle X-ray diffractograms of a 101-layer mixed LB film (lower line) and a 71-layer LB film of AA (upper line) transferred from a 0.2 mmol/L Eu(NO3)3 subphase. Table 2. Reflectivity (R%) at 612 nm and Transmissivity (T%) at 320 nm of the Silver Films Deposited by Thermal Evaporation (A) or Eletron Beam Evaporation (B)a A
B
thickness (Å)
R%
T%
thickness (Å)
R%
T%
416 464 519 569 612 679 726 759
85 85.6 86 86.6 86.5 86 86.3 86.5
64.9 64 57.8 56.3 54.8 51.3 48.8 47.3
445 998
94 98
70 40
Figure 3. (1) Absorption spectra of the silver mirrors with thickness of 445 Å (lower line) and 998 Å (upper line). (2) Reflective spectra of the silver mirrors (upper line, 445 Å; lower line, 998 Å).
a The thickness of the silver film was measured by real time quartz resonic film depth instrument and UV absorption (the absorption coefficient of silver film is 2.44 × 10-3 Å-1 at 265 nm). The results obtained from the two methods are identical. The reflectivity was corrected by a He-Ne laser (632.8 nm) at an incident angle of 8°.
revealing a highly ordered layer structure of the LB film. The peaks are assigned to be the (001), (002), (003), (004), (005), and (006) Bragg diffractions. Thus average doublelayer spacing of 5.63 nm is obtained according to the Bragg equation, and this yields a single-layer thickness of 2.82 nm based on Y-type LB film structure (one double-layer contains two single layers). The single-layer thickness of the mixed LB film is similar to that of the AA film (2.76 nm, obtained from the diffractogram using similar calculation) transferred from the Eu(III) solution subphase. The reason for the similar single-layer thickness is that the sketch of mixed LB films is based on the LB film structure of the AA molecules in which the HDP‚Eu(NTA)4 molecules inserted. The low-angle X-ray diffractograms of three other 101-layer mixed LB films on quartz slides give the same single layer thickness of 2.82 nm, indicating the mixed LB film is very stable and reproducible. Silver Mirror Thickness Selection and Microcavity Fabrication. The reflectivity and transmissivity of the silver mirrors with different thickness deposited by thermal or electron beam evaporation at given wavelengths are listed in Table 2. The reflectivity of the silver film at 612 nm (the emission wavelength of the europium complex) hardly changes with silver thickness ranging from 400 to 800 Å however, it increases with the decreasing of the deposition rate of silver. The transmissivity at 320 nm (the excitation wavelength) decreases from 65% to 47% when the silver thickness increases from 400 to 800 Å sequentially. Taking both the transmissivity at excitation wavelength (320 nm) and reflectivity at emission wavelength (612 nm) into account, the thickness of the
Figure 4. Schematic model of the reflection optical diagram in the multibeam interference method.
semireflective and total-reflective silver mirrors were selected to be 450 and 1000 Å, respectively. The reflection and absorption spectra of two silver mirrors with thickness of 445 or 998 Å are shown in Figure 3. The mixed Langmuir film was transferred on to two 445 Å thick lipophilically pretreated silver films in Y-type to form stepped 38, 42, 46, 50, 54-layer or 40, 44, 48, 52, 56-layer LB films on each substrate. Then a 998 Å thick silver film was deposited onto the mixed LB film surface, and the fabrication of the silver mirror LB film planar microcavities with a serial optical distances was completed. Evaluation of the Optical Length of the Microcavity. Since the flexibility of the alkyl chains in the LB film, and the penetration of the emission light in the silver film (the penetration length here cannot be neglected because the optical microcavity length is very short, ca. λ/2), it would be very difficult to predict the cavity length theoretically. Here we use a multibeam reflective interference method to measure the vertical optical length (D) of the microcavity on a Shimadazu UV-3100 spectrophotometer. The schematic optical interference diagram is shown in Figure 4. When the phase difference of the interference beams equals 2πm (where m is a natural number), the reflective
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Figure 5. Reflective spectra of the stepped 38, 42, 46, 50, and 54-layer (from left to right, respectively) LB film planar microcavities at an incident angle θ ) 8°.
Figure 6. Fluorescence spectra of a λ/2 resonant microcavity (a, 52 layers) and a naked 52-layer mixed LB film (b) under the same excitation energy.
Table 3. Calculated Optical Length Difference (∆L) and the Twice Optical Length (2D) of the Two Serial Microcavities layer no.
∆L
2D
layer no.
∆L
2D
38 42 46 50 54
498.6 537 578 613 645.4
500 539 580 615 648
40 44 48 52 56
500.8 547.8 584.8 610.6 647.6
502.5 550 587 612.7 650
light intensity will arrive the maximum value. Thus the real optical distance (D) of the microcavity can be determined from the resonant wavelength λ in their reflective spectra. The reflective spectra of the microcavities at incident angle θ ) 8° are shown in Figure 5. The layer number of the mixed LB films sandwiched within the two silver mirrors, ∆L (optical distance difference between reflective and refractive light) and 2D values are summarized in Table 3. A linear relationship between the optical length and the number of layers is found from each stepped set of microcavities. After a simple regression of the values, the following two simulated functions are obtained:
2D ) (9.3 ( 0.2)k + (149 ( 11)
(1)
2D ) (9.0 ( 0.5)k + (151 ( 25)
(2)
and
where k is the layer number of the LB film in each stepped microcavity. The two equations are fairly in good agreement with each other within deviation, indicating that the whole process including Langmuir film formation, transferate, and optical cavity fabrication is really reproducible. On the basis of d ) n′ × l (where l and d are the single layer thickness obtained from the low-angle X-ray diffraction and single layer optical length), the refractive index n′ of the mixed LB film is calculated to be 1.65. The constant item 150 in eq 1 or 2 can be considered as twice the light penetration depth in the silver mirrors. If the emission light is resonant within the optical microcavity, the optical distance D should equal to λ/2 (where λ is the emission wavelength, ca. 612 nm). From Table 3, we obtained that the microcavity with 50 (in series 1) or 52 (in series 2) layers of the mixed LB film within the silver mirrors meet such a requirement (e.g., their optical length equals λ/2). Fluorescence Enhancement. The excitation and emission spectra of the mixed LB films on quartz slides are the same as those of the pure HDP‚Eu(NTA)4 LB films. The excitation spectrum of the mixed LB film shows a wide band from 240 to 390 nm with three broad peaks around 260, 296, and 371 nm, just like that of the pure
Figure 7. Fluorescence decay curves of the complex microcrystals, a naked 52-layer mixed LB film, and a λ/2 optical resonant microcavity (52 layers) excited by a 320 nm laser beam at an excitation pulse energy of 8 mJ (from right to left, respectively).
complex LB film.11 The emission spectrum of the mixed LB film shows three characteristic peaks of Eu3+ at 593, 612, and 642 nm, corresponding to 5D0 f 7F1 (magnetic dipole transition), 5D0 f 7F2 (induced electric dipole transition), and 5D0 f 7F3 transitions. Among the three transitions, the 5D0 f 7F2 transition which locates at 612 nm with fwhm (full width at half maximum) of 4 nm being the strongest. All these reflect that the introduction of the AA molecules into the film-formation material does not influence the characteristic fluorescence property of the europium complex. The fluorescence spectra (the 5D0 f 7F2 transition) of a mixed LB film and a λ/2 resonant microcavity with the same layers (52) at the same incident excitation energy are shown in Figure 6. The shape of the spectra is in fairly good agreement with that of DeMartini’s work, where they used a solution of an europium-dibenzoylmethane complex in the microcavity.4a As shown in Figure 6, the fluorescence intensity of the λ/2 resonant microcavity is four times stronger than that of the LB film, indicating that the resonant microcavity has a strong effect on fluorescence enhancement, and this is one of the important effects of resonant microcavity. Fluorescence Lifetime Shortening. The fluorescence emission decay curves of the powderous complex crystal, mixed LB film and λ/2 resonant microcavity are shown in Figure 7. All three decay curves are single exponential. The lifetimes for the powderous crystal, LB film, and microcavity under the same excitation pulse energy of 8 µJ are 239, 125, and 75 ms, respectively. The lifetime of the LB film is shorter than that of the powderous crystal, similar to our earlier studies.11-12 However, it is
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longer compared with that of the λ/2 microcavity. This may be because that microcavity effect also exists to some extent in the LB film, due to the formation of a natural reflective mirror between the interface of two materials. The resonant microcavity has a stronger effect on the fluorescence lifetime shortening than the bare LB film because of the high reflectivity of the silver mirror (i.e., the resonant microcavity has a higher quality factor Q value). The Q value could be calculated from the following equation 4
Q)
πxR1R2 1 - xR1R2
(3)
where R1 and R2 are the reflectivities of the two mirrors at 612 nm. The Q value of the resonant microcavity is calculated to be 76, which is much larger than that of the LB film (2.5); thus its microcavity effect is much stronger. This is in good agreement with our observations. The lifetime of the resonant cavity shortens from 104 to 61 ms when the excitation pulse energy increases from 0.017 to 18 mJ, similar to the observation of Yokoyama et al., where they used a dye solution in a microcavity terminated by a pair of dielectric mirrors.6b If the exciting energy density further increases, the silver film will be destroyed. The lifetime shortening is one of the most important effects of microcavity, and this is the first report on the lifetime shortening from λ/2 resonant microcavity using LB film as the emitting medium terminated by a pair of metal mirrors. Relationship between Fluorescence Intensity and Lifetime and Optical Cavity Length. The relationship between fluorescence intensity and lifetime and optical length of the microcavity excited by the same energy density is shown in Figure 8. It is clear that the λ/2 resonant microcavity has the strongest microcavity effect on the fluorescence property of the europium complex. For example, the fluorescence lifetime of the microcavity decreases with the increasing of the optical cavity length (D) when D is shorter than λ/2 (λ ) 612 nm, the emission wavelength); however, it exhibits an opposite trend when D is longer than λ/2. The λ/2 resonant microcavity has the shortest lifetime among the serial microcavities with different optical lengths. Contrary to the change sequence of the fluorescence lifetime, any deviation from λ/2 optical length results in the fluorescence intensity decreasing. All these reflect that the strongest microcavity effect can be observed when its real optical length meets the half
Figure 8. Relationship between fluorescence lifetime (solid line) and fluorescence intensity (broken line) and the optical length of the planar microcavity under the same excitation energy.
emission wavelength, and our evaluation of the optical cavity length is correct. The lifetime shortening and fluorescence enhancement are two important factors for resonant microcavity. Conclusion λ/2 resonant planar microcavity terminated by a pair of silver mirrors has been successfully fabricated by using the mixed LB film of an europium complex and AA. The real optical cavity length of the microcavity has been measured by a convenient method, UV-vis reflective interference spectrum. Two important microcavity effects, fluorescence intensity enhancement and lifetime shortening, have been observed for the first time from the resonant microcavity terminated by a pair of silver mirrors. Further work is in progress to introduce LB film of terbium complexes into the microcavity, which is expected to obtain the green light emission microcavities. Acknowledgment. The authors wish to thank the Climbing Program (A National Fundamental Research Key Project) and the National Natural Science Foundation of China (29601001) for financial support. LA971176Q
