Surface plasmon-enhanced energy transfer in an organic light ...https://www.researchgate.net/.../Surface-plasmon-enhanced-energy-transfer-in-an-organi...

Surface plasmon-enhanced energy transfer in an organic light-emitting device structure Ki Youl Yang,1 Kyung Cheol Choi1,* and Chi Won Ahn2 1 2

School of Electrical Engineering and Computer Science, KAIST, Daejeon 305-701, Republic of Korea New Technology and Analysis Division, National Nanofab Center, Daejeon 305-701, Republic of Korea * [email protected]

Abstract: We present a surface plasmon-mediated energy transfer based on an organic light-emitting device structure. In order to localize surface plasmons, silver nano clusters were deposited thermally close to the cathode with a 1-nm-thick LiF spacer. It was shown that the surface plasmon formed on the silver nano cluster provides a strong donor decay channel and increases the donor-acceptor dipolar interaction. Thus, photoluminescence results displayed 3.5-fold enhanced acceptor emission intensity, compared with those of sample which has no Ag nano cluster.  2009 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (250.3680) Light-emitting polymers.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

W. L. Barnes, A. Dereux, and T. W. Ebbesen, "Surface plasmon subwavelength optics," Nature 424, 824-830 (2003). S. Nie, and S. R. Emory, "Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering," Science 275, 1102-1106 (1997). N. Félidj, J. Aubard, G. Lévi, J. R. Krenn, A. Hohenau, G. Schider, A. Leitner, and F. R. Aussenegg, "Optimized surface-enhanced Raman scattering on gold nanoparticle arrays," Appl. Phys. Lett. 82, 3095-3097 (2003). P. Anger, P. Bharadwaj, and L. Novotny, "Enhancement and Quenching of Single-Molecule Fluorescence," Phys. Rev. Lett. 96, 11302-11303 (2006). H. Ditlbacher, J. R. Krenn, N. Félidj, B. Lamprecht, G. Schider, M. Salerno, A. Leitner, and F. R. Aussenegg, "Fluorescence imaging of surface plasmon fields," Appl. Phys. Lett. 80, 404-406 (2002). B. P. Rand, P. Peumans, and S. R. Forrest, "Long-range absorption enhancement in organic tandem thin-film solar cells containing silver nanoclusters," J. Appl. Phys. 96, 7519-7526 (2004). E. Ozbay, "Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions," Science 311, 189-193 (2006). S. A. Maier, Plasmonics: Fundamentals and Applications (Springer, New York, 2007). L. Novotny, and B. Hecht, Principles of Nano-Optics (Cambridge University Press, New York, 2006). T. Förster, "Intermolecular energy transference and fluorescence," Annalen der Physik 2, 55-75 (1948). T. Förster, "Transfer mechanisms of electronic excitation," Discuss. Faraday Soc. 27, 7-17 (1959). C. Sönnichsen, B. M. Reinhard, J. Liphardt, and A. P. Alivisatos, "A molecular ruler based on plasmon coupling of single gold and silver nanoparticles," Nat. Biotech. 23, 741-745 (2005). J. N. Anker, W. P. Hall, O. Lyandres, N. C. Shah, J. Zhao, and R. P. Van Duyne, "Biosensing with plasmonic nanosensors," Nat. Mater. 7, 442-453 (2008). M. Hopmeier, W. Guss, M. Deussen, E. O. Gübel, and R. F. Mahrt, "Enhanced Dipole-Dipole Interaction in a Polymer Microcavity," Phys. Rev. Lett. 82, 4118-4121 (1999). D. K. Kim, K. Kerman, M. Saito, R. R. Sathuluri, T. Endo, S. Yamamura, Y. S. Kwon, and E. Tamiya, "Labelfree DNA biosensor based on localized surface plasmon resonance coupled with interferometry," Anal. Chem. 79, 1855-1864 (2007). K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, "Surface-plasmon-enhanced light emitters based on InGaN quantum wells," Nat. Mat. 3, 601-605 (2004). T. D. Neal, K. Okamoto, and A. Scherer, "Surface plasmon enhanced emission from dye doped polymer layers," Opt. Express 13, 5522-5527 (2005). J. Bellessa, C. Bonnand, J. C. Plenet, and J. Mugnier, "Strong Coupling between Surface Plasmons and Excitons in an Organic Semiconductor," Phys. Rev. Lett. 93, 036404 (2004). M. K. Kwon, J. Y. Kim, B. H. Kim, I. K. Park, C. Y. Cho, C. C. Byeon, and S. J. Park, "Surface-PlasmonEnhanced Light-Emitting Diodes," Adv. Mater. 20, 1253-1257 (2008). W.-H. Chuang, J.-Y. Wang, C. C. Yang, and Y.-W. Kiang, "Study on the decay mechanisms of surface plasmon coupling features with a light emitter through time-resolved simulations," Opt. Express 17, 104-116 (2009).

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Received 9 Apr 2009; revised 29 May 2009; accepted 19 Jun 2009; published 24 Jun 2009

6 July 2009 / Vol. 17, No. 14 / OPTICS EXPRESS 11495

21. P. Andrew, and W. L. Barnes, "Förster Energy Transfer in an Optical Microcavity," Science 290, 785-788 (2000). 22. K. Okamoto, I. Niki, A. Scherer, Y. Narukawa, T. Mukai, and Y. Kawakami, "Surface plasmon enhanced spontaneous emission rate of InGaN/GaN quantum wells probed by time-resolved photoluminescence spectroscopy," Appl. Phys. Lett. 87, 071102-071103 (2005). 23. K. Y. Yang, K. C. Choi, and C. W. Ahn, "Surface plasmon-enhanced spontaneous emission rate in an organic light-emitting device structure: Cathode structure for plasmonic application," Appl. Phys. Lett. 94, 173301173303 (2009). 24. P. Andrew, and W. L. Barnes, "Energy Transfer Across a Metal Film Mediated by Surface Plasmon Polaritons," Science 306, 1002-1005 (2004). 25. V. G. Kozlov, V. Bulovic, P. E. Burrows, M. Baldo, V. B. Khalfin, G. Parthasarathy, S. R. Forrest, Y. You, and M. E. Thompson, "Study of lasing action based on Förster energy transfer in optically pumped organic semiconductor thin films," J. Appl. Phys. 84, 4096-4108 (1998).

1. Introduction Surface plasmons (SPs) resonances have a hybrid nature where light waves trapped on the surface of a conductor collectively interact with free electrons. One of the most attractive features of SPs is the capacity to concentrate light in subwavelength structures. Since concentration of light as such can be applied to enhance an electric field, SPs can be used for light-matter coupling and in sensors using nonlinear optical phenomena[1]. Plasmonic applications driven by advances in surface-enhanced Raman spectroscopy[2, 3], plasmonmediated fluorescence emission[4], tip-enhanced fluorescence imaging[5], and long-range absorption enhancement in organic thin films[6] have been reported. For SP excitation, a specific matching method is not required if the fluorophore is very close to a metallic surface. When an incoming wave is incident to the surface of the metal nano particle, electrons are vibrated and polarizability inside the conductor surface is generated by an external field. If the Fröhlich condition is met, maximum polarizability resonance can arise and the near-field both inside and outside the particle can be amplified[1, 7, 8]. The amplified near-field outside particle is associated with enhanced fluorescence molecule excitation, which is related with fluorescence emission and absorbance[4, 9]. SP-enhanced absorbance and emission provide considerable opportunity for improvement of energy transfer (ET) in dye-doped systems. The Förster process is one of the main excitation ET mechanisms between donors and acceptors. Since the distance between the donor and acceptor is closer than λ / 10 in the case of the Förster process, transfer is nonradiative and a resonant dipole-dipole interaction. In a homogeneous environment, the strength of the transfer process depends on the donor-acceptor spacing and spectra overlap between donor emission and acceptor absorption[9-11]. Usually considered as a near-field amplifier, SP-excited absorbance and emission can be directly applied to efficient ET process. This enables efficient ET in OLEDs, dye-doped lasers, and biosensors using fluorescence resonance ET (FRET)[12, 13]. In this manner, plasmonic applications could potentially make an important contribution to more sensitive optoelectronic devices. 2. Experiments: confirmation of the SP effect on the photoluminescence emission

In this paper, we demonstrate that enhanced ET was observed with SP excitation in an Alq3:DCM based fluorescent OLED, fabricated by vacuum thermal evaporation. In order to enhance the Förster process in the donor-acceptor system, the SP wavelength of metal nano cluster should be located on the overlap range between donor emission and acceptor absorption spectra for amplifying the near-field. Among noble metals, since the SP wavelength of Ag clusters is approximately close to 450-550 nm, the Ag nano cluster is suitable for enhancing the ET in the Alq3:DCM layer[14]: the SP peak wavelength becomes longer as the Ag nanocluster size increases and as the spacing narrows[6]. For the localized SP in the OLED structure, a 1 nm LiF film-Ag cluster layer-1 nm LiF film (LAL) structure and an Al metal layer were thermally deposited below the organic layer. The deposition rates for the LiF, Ag cluster and Al cathode were 0.01 nm s-1, 0.01 nm s-1 and 0.5 nm s-1, respectively. All of the deposition processes for the sample fabrication were carried out on a #109875 - $15.00 USD

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Sunicel plus 200 system (SUNIC SYSTEM, Korea). To deposit the Ag nanocluster layer, extra procedures were not used, except thermally evaporation of a very thin mass thickness (thinner than 1-nm-thickness). The formation conditions of nanoclusters, including particle size and spacing, were controlled by the mass thickness; when the mass thickness increases, the particle size becomes larger and the spacing between the particles decreases.

Fig. 1. (a) Absorbance spectra of LiF-Ag cluster-LiF, LiF thin film and Ag film; LiF and Ag film thickness of 2 nm and 50 nm, respectively, (b) TEM Plan view of the top of Ag clusters on the LiF surface. Inset of the left: high resolution image of Ag cluster for confirming crystallites. Inset of the right: overview of Ag cluster on LiF surface. (c) TEM cross-section view of LiFAg cluster-LiF structured Al cathode on the surface of SiN substrate.

A simple method for confirming localized SP is absorbance spectrum measurement [3, 6, 15]. In order to confirm localized SP on the Ag clusters, a specially equipped localized surface plasmon resonance (LSPR) spectroscopy system (Ocean Optics Inc., USA) was employed. The LSPR system was equipped with a tungsten halogen light source (LS-1), a spectrophotometer (USB 400), and an optical probe bundle (R-400-7 UV-visible). The reliable measurement regions of the light source, spectrophotometer and optical probe bundle are 360-2000 nm, 250-1100 nm and 250-850 nm, respectively. Fig. 1(a) shows the absorbance spectra of the LAL structure, LiF 2 nm film, and Ag 50 nm film on Al cathode-coated quartz recorded on a LSPR system[15]. As the optical response of several samples would be radically modified due to the presence of the nearby Al layer on the devices the authors have examined, a thick Al layer underneath several materials was used for the confirmation that the SPs were involved in the as-made devices. While the LiF film and Ag film on Al cathodecoated quartz have no absorbance peak, the LAL structure has a 110 nm wide peak (FWHM) centered at a wavelength of λSP=492 nm. Therefore, it is clear that SPs are localized at the Ag nano cluster layer in LAL structure. In Fig. 1(a) using arbitrary units for absorbance, negative #109875 - $15.00 USD

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y-axis value for the films in this Fig. does not mean that the films have negative absorbance because the absolute absorbance values were not measured; only the arbitral values were measured for confirming the SP-excitation peaks. The evaporated Ag clusters on the LiF surface (Fig. 1(b)) consist of irregular sphereshaped clusters with a randomly-distributed diameter of 3~10 nm, as confirmed from transmission electron microscope (TEM) images showing the top view of the clusters. For confirmation of Ag clusters formation, a field-emission TEM JEM-2100F (JEOL LTD, Japan) was used to get the images. Nano crystallites of Ag clusters are confirmed by a well-defined crystal structure in the high resolution image of the rectangular delineated by a dashed line (Fig. 1(b), inset of the left) and an overview of the Ag clusters distribution on LiF with a surface-coverage of 12.7 % (Fig. 1(b), inset of the right). Fig. 1(c) shows TEM cross-section images of the LAL structure. Since Al and Ag have crystal structure, whereas LiF and the substrate (SiNx) are amorphous, the circle delineated by a dashed line is regarded as an Ag cluster.

Fig. 2. Schematics of samples with different Ag deposition conditions.

In order to confirm the SP effect on the ET process, we examined two types of structure measuring photoluminescence (PL). Both types were fabricated on a 150 nm thick Al film with a quartz substrate[16, 17] (see Fig. 2). Sample 1 structures were formed by depositing a 2 nm LiF thin film and a 30 nm organic layer on top of an Al film. In samples 2-4, LAL structure replaces the LiF film in sample 1. The SP wavelengths of samples 2-4 are 460, 490, and 520 nm respectively, due to the different mass thickness of the deposited Ag cluster layer. As the mass thickness of the cluster layer becomes thicker, the SP resonance wavelength becomes longer, due to the increased particle size and decreased spacing between the particles. An organic layer on the top of a LiF surface in the samples 1-4 was formed by deposition under the same conditions. Through the different types of deposited organic layer, the authors then investigated the SP-excited effect on a donor-only system (Alq3) and donoracceptor system (Alq3:DCM). Organic active layer for donor-only system and donor-acceptor system was a 30-nm-thick Alq3 film and 30-nm-thick 0.7 wt% DCM co-deposited Alq3 film respectively. In previous reports, the effective range between the light emitter and the metal for SP modes-enhanced fluorescence has been observed to be lower than 200 nm[17, 18], and proper distance between the light emitter and the metal nano clusters for SP enhanced-LEDs has been observed to be 50 nm or less[19, 20]. Thus, an organic layer 30 nm thick was deposited for SP-enhanced fluorescence in this study: since the denotation of the samples is confusing, samples of donor-only, acceptor-only, and donor-acceptor systems with different Ag cluster deposition conditions are named and explained in Table 1.

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Table 1. The denotations of samples Systems Donor-only

Deposited layer Alq3 (30 nm thickness)

Acceptor-only

DCM (30 nm thickness)

Donor-Acceptor

Alq3: 0.7 wt% DCM (30 nm thickness)

Denotation Donor-only system 1 Donor-only system 2 Donor-only system 3 Donor-only system 4 Acceptor-only system 1 Acceptor-only system 2 Acceptor-only system 3 Acceptor-only system 4 Donor-Acceptor system 1 Donor-Acceptor system 2 Donor-Acceptor system 3 Donor-Acceptor system 4

Plasmon peak No peak 460 nm 490 nm 520 nm No peak 460 nm 490 nm 520 nm No peak 460 nm 490 nm 520 nm

3. Results: Photoluminescence spectra of donor-only and donor-acceptor systems

Fig. 3. (a) Fluorescence micrographs of donor-only system1 and 3, (b) Photoluminescence (PL) emission results of donor-only system 1-4 ( λexc =266 nm). Inset: PL enhancement ratio of donor-only system 2-4 compared with the donor-only system 1.

Figure 3(a) shows fluorescence micrographs of donor-only system 1 and 3 with Alq3 film at 300K, viewed through a band-pass filter centered at 525±25 nm for donor emission. Fluorescence imaging was carried out by a confocal microscope LSM510 META NLO system (Carl Zeiss, USA), and a 790 nm IR laser (multi-photon excitation) was used for a #109875 - $15.00 USD

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donor-only system excitation. The left side of each image shows no fluorescence emission due to the absence of an organic layer. Fluorescence emission intensity differences between donor-only system 1 and 3 are clearly demonstrated, and typical spectra of all samples are shown in Fig. 3(b). Emission enhancement ratios of donor-only system 2-4 compared with the donor-only system 1 intensity are displayed in the inset of Fig. 3(b), describing 2-fold, 3-fold, and 3.5-fold increased emission of donor-only system 2, 3, and 4 respectively. PL measurements were carried out on an Accent RPM 2000 system (Bio-Rad Laboratories, USA), and the excitation wavelength was 266 nm for both donor-only and donor-acceptor systems. In a donor-only system, since SP resonant modes provide a strong decay channel, the donor decay rate and oscillator strength are increased[21]. The spontaneous emission rate can be more enhanced when the emission wavelength of excitons and the SP wavelength are closer. Since the internal quantum efficiency ( ηint ) is estimated from the PL-enhancement ratio, the Purcell enhancement factor ( Fp (ω ) ) and donor spontaneous emission rate ( τ PL (ω ) ) can reflect the PL intensity:

Fp (ω ) =

krad (ω ) + knon (ω ) + kSP (ω ) τ PL (ω ) 1 − ηint (ω ) = * ≈ . krad (ω ) + knon (ω ) τ PL (ω ) 1 − η *int (ω )

(1)

where η *int and τ *PL are enhanced ηint and decreased τ PL , resulting from the Ag clusters[16, 17, 22]. From the PL results and Eq. (1), the authors predict that the donor decay rate and oscillator strength are increased by SP resonant modes. In addition, it was also shown that singlet exciton lifetime of donor-only system is decreased by Purcell effect, confirmed by time-resolved PL (TRPL) method[23]. Exciton lifetimes measured by the TRPL method were used to estimate the donor emission rates of donor-only systems 1-4, as shown in Table. 2. SP-excitation may be considered as detrimental to the optical efficiency because a large portion of the excitation is coupled to the nonpropagating evanescent wave SPs as opposed to direct emission. The SP energy would be thermally dissipated if metal/ dielectric interface is plane. However, few tens of nanometer-sized clusters can cause the light to scatter, lose momentum, and couple to light radiation[22]. Fluorescence micrographs of donor-acceptor system 1 and 3 are shown in Fig. 4(a). All experimental settings for the images correspond with those for Fig. 3, except the deposited dye and a band-pass filter centered at 650±50 nm. Acceptor emission intensity differences between donor-acceptor system 1 and 3 are clearly visible, and PL spectra of donor-acceptor system with different conditions are shown in the inset of Fig. 4(b). The spectra from samples containing Alq3 ( λEm = 520 nm) and DCM ( λEm = 620 nm) have both donor and acceptor emission characteristics. In the case of donor-acceptor system 3, the acceptor emission intensity is enhanced 3.5-fold, and this enhanced acceptor emission demonstrates that energy is more efficiently transferred from Alq3 to DCM than in the case of donor-acceptor system 1.

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Fig. 4. (a) Fluorescence micrographs of donor-acceptor system 1 and 3, (b) Calculated acceptor emission results of donor-acceptor system 1-4 ( λexc =266 nm). Inset: PL emission result which also includes the donor emission.

4. Discussion: energy transfer rate dependence on SP peak energies Table 2. Energy transfer dependence on the different SP resonance conditions. Estimated amount of transferred energy ΓET SP wavelength (nm) no peak (Sample 1) 460 nm (Sample 2) 490 nm (Sample 3) 520 nm (Sample 4)

Estimated ΓD (normalized to sample1) 1

Absolute value of IDA- fID- IA (normalized to sample 1) 1

1.191

2.805

1.757

3.429

1.828

3.092

To establish the dependence of Förster transfer on SP, the authors calculated the amount of acceptor emission, which arises from the energy transfer from donor, at different SP wavelengths, following the Barnes method introduced in [24]: it is adoptable that the light emission spectra from donor-acceptor system were the superposition of donor and acceptor emission spectra. Thus, if the spectral areas of the 266 nm-excited donor-acceptor, 266 nm-

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excited donor, and 266 nm-excited acceptor emission are I DA , I D , and I A , then the spectral area of the acceptor emission, which arise from the energy transfer from the donor, is I DA − fI D − I A (see Table 2). The correction factor f is determined by the ratio of the donoracceptor and donor-only emission spectral area from wavelengths of 450-500 nm considering the decrease in the Alq3 emission in the donor-acceptor system compared with the donor-only system; given that the amount of acceptor emission in the wavelength range of 450-500 nm is zero, only the donor emission spectrum is subtracted from the spectrum, in which both donor and acceptor spectra were added, using a correction factor. Additionally, since the pump laser matches the Alq3 absorption band but not the DCM, I DA and I D dominate the 266 nm-excited DCM emission I A : as confirmed from Fig. 5(a), 266 nm-excited DCM emission has no dependence on each SP wavelengths and the intensity was approximately 10 times smaller than 532 nm-excited DCM emission. In contrast, this differs from previous research in that the SP-enhanced ET process is a Förster transfer, whereas the Barnes case [24] concerns a radiative energy transfer, as confirmed from the long-lived time-resolved acceptor emission behavior. A relatively long donor decay component compared to that of the acceptor dominates the PL transient of the donor-acceptor system. The Förster transfer is non-radiative, which is in contrast to the radiative transfer and the time-resolved luminescence shows only an acceptor decay component. Thus, this suggests that the energy transfer process quenches the donor molecules, indicating that the donor decay component is not apparent, as confirmed from Fig. 5 (b). The experimental settings for measuring TRPL were described in a recent study[23]; this was detected at the acceptor emission peak. Table 2 displays the estimated donor decay rate and the amount of transferred energy from TRPL results in [23] and PL results in Fig. 4(b), respectively. Both the donor decay rate and the amount of transferred energy were normalized to those of sample 1. From the above table, increased amount of transferred energy ( I DA − fI D − I A ) is not only a consequence of the increased donor emission rate and can be affected by other factors. Theoretically, the ETrate ( Γ ET ) can be expressed as a function of the donor emission rate ( Γ D ), the separation ( R ) between donor and acceptor, Förster radius ( R0 ), and a number of variables:

ΓD 3 Γ c4 (2) [ FD (ω )σ A (ω )d ω ] = D6 R0 6 . 6 4 4 ∫ R 4π ω n R where n , FD (ω ) , and σ A (ω ) are the refractive index of medium, normalized donor emission, and normalized acceptor absorption spectra[21, 25]. Separation between donor and acceptor is equal in all of the samples, due to randomly co-deposited DCM molecules. Γ D , FD (ω ) , and σ A (ω ) depend on the localized SP, since plasmon modes can be associated with donor oscillator strength and acceptor absorbance. Eq. (2) explains that the ET-rates do not simply follow the order of the donor emission rate ( Γ D ). Since FD (ω ) and σ A (ω ) can be modified by SPs, the Förster radius is not constant through the four conditions, and the most appropriate case for ET is not sample 4, which has maximum donor decay rate among all of samples, but sample 3, as confirmed by Table 2. Γ ET =

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Fig. 5. (a) PL emission spectra of the acceptor in acceptor-only system 1-4 excited by 266 nm and 532 nm Nd:YAG laser that excites the acceptor-only system. Samples were prepared as explained in Fig. 2, and a 15-nm-thick DCM layer that acts as an organic active layer were deposited on the sample, (b) Time-resolved photoluminescence detected at the DCM emitting wavelength for donor-acceptor systems 1-4.

Fig. 6. Ratio of the ET rate and the donor emission rate versus wavelength of the resonance mode. The absorption profile of DCM is included for a comparison.

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Since dipole-dipole interaction between donor and acceptor, which is estimated to the Förster radius, can be enhanced in resonance mode[14], the amount of DCM emission, which arises from the energy transferred from donor emission, can be modified at different SP frequencies. Fig. 6 depicts the dependence of the inter-dipolar coupling factor ( Γ = Γ ET / Γ D ) of the emissions from DCM-doped Alq3 donor-acceptor systems 1-4 on the wavelength of the SP resonance: here, authors use the amount of transferred energy in Table 2 as Γ ET . The absorbance of a DCM film in the relevant spectral regime is also displayed for comparison. The inter-dipolar coupling factor increases when the resonance from the tail of the DCM absorbance profile is tuned into the center: this property is corresponds to that already published paper[14]. Therefore, ET rates are not only determined by an enhanced donor emission rate, but also by increased inter dipole interaction (Förster radius) in the SP resonance mode. In summary, the donor decay rates are more enhanced as the SP-excitation wavelengths become closer to the donor emission peak, and the donor-acceptor interaction increases as the localized SP energy is closer to the acceptor absorbance center. Thus, both of the components should be considered to estimate enhanced energy transfer rates for samples. 5. Conclusion

To study the SP-enhanced ET process in an OLED structure, the LAL-coated cathode structure was proposed and Ag cluster-incorporated nano-structures were compared with an Ag cluster-absent structure: a thick Al layer effect on the optical properties is common through all of the structures. Localized SPs act as a strong decay channel and nano structure to allow SPs of high momentum to scatter, lose momentum, and couple to the radiated light. Thus, the transient luminescence properties show a shorter exciton lifetime and CWPL measurements show an increased amount of emission. In addition, donor-acceptor interaction becomes more enhanced, as the SP-resonance mode wavelength moves closer to the absorbance center of the acceptor; this is related to the normalized profile of the acceptor absorbance spectra, which is modified by SP-induced optical effects. In sum, the donor decay rates increase more as the SP-excitation wavelengths becomes closer to the donor emission energy, and donor-acceptor interaction was more enhanced as the localized SP energy becomes closer to the acceptor absorbance center. Consequently, the overall ET rate is determined by both the donor decay rate and the donor-acceptor interaction. The SP-excitation wavelength should be close to both the donor emission and acceptor absorbance peaks. In addition, the time-resolved photoluminescence properties show only the acceptor emission decay component, which suggest the ET process directly quenches the donor molecule, implying that the transfer is non-radiative rather than radiative. In contrast, the relatively long donor decay component compared to that of acceptor dominates the PL transient of the donoracceptor system if the transfer is radiative. Such a device, in which an Ag cluster layer might be added to an OLED structure, possibly by means of thermal evaporation, may open the possibility of increasing the emission efficiency and thus the efficiency of devices such as both a bottom and a top emitting OLED. Furthermore, such a mechanism, in which an Ag cluster might enhance the ET process, may be useful in development of a range of optoelectronic devices and biosensors. Acknowledgements

This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. R11-2007-045-02001-0), and the HRHRP (High Risk High Return Project) funded by KAIST. National Center for Nanoprocess and Equipments (NCNE), Gumi Electronics and Information Technology Research Institute (GERI), Korea Photonics Technology Institute (KOPTI) are also acknowledged for OLED fabrications and PL measurements. The authors acknowledge the help received from S. Yoo for preparing TEM samples.

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