Gold Nanoparticle Enhanced Charge Transfer in Thin Film Assemblies

Nov 16, 2007 - Copyright © 2007 American Chemical Society ... the porphyrin moiety of the dyad, that is, AuNP|PF, compared to the case of the PF laye...
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Langmuir 2007, 23, 13117-13125

13117

Gold Nanoparticle Enhanced Charge Transfer in Thin Film Assemblies of Porphyrin-Fullerene Dyads Anne Kotiaho,*,† Riikka M. Lahtinen,† Nikolai V. Tkachenko,† Alexander Efimov,† Aiko Kira,‡ Hiroshi Imahori,‡,§ and Helge Lemmetyinen† Institute of Materials Chemistry, Tampere UniVersity of Technology, P.O. Box 541, 33101 Tampere, Finland, Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, and Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan ReceiVed August 16, 2007. In Final Form: September 26, 2007 Photoinduced vectorial electron transfer in a molecularly organized porphyrin-fullerene (PF) dyad film is enhanced by the interlayer charge transfer from the porphyrin moiety of the dyad to an octanethiol protected (dcore ∼ 2 nm) gold nanoparticle (AuNP) film. By using the time-resolved Maxwell displacement charge (TRMDC) method, the charge separation distance was found to increase by 5 times in a multilayer film structure where the gold nanoparticles face the porphyrin moiety of the dyad, that is, AuNP|PF, compared to the case of the PF layer alone. Films were assembled by the Langmuir-Blodgett (LB) method using octadecylamine (ODA) as the matrix compound. Atomic force microscopy (AFM) images of the monolayers revealed that AuNPs are arranged into continuous, islandlike structures and PF dyads form clusters. The porphyrin reference layer was assembled with the AuNP layer to gain insight on the interaction mechanism between porphyrin and gold nanoparticles. Interlayer electron transfer was also observed between the AuNPs and porphyrin reference, but the efficiency is lower than that in the AuNP|PF film. Fluorescence emission of the reference porphyrin is slightly quenched, and fluorescence decay becomes faster in the presence of AuNPs. The proposed mechanism for the electron transfer in the AuNP|PF film is thus the primary electron transfer from the porphyrin to the fullerene followed by a secondary hole transfer from the porphyrin to the AuNPs, resulting in an increased charge separation distance and enhanced photovoltage.

Introduction Size and environment dependent optical and electrical properties of gold nanoparticles motivate the use of these materials in nanoscale assemblies. In terms of optical properties, the interesting characteristics of gold nanoparticles are the surface plasmon band1,2 in the visible range and the optical field enhancement.3 The surface plasmon band is observed in large, metallic nanoparticles, but as the size decreases particles lose their plasmon band feature and become insulators with a distinct band gap.4 In addition to their optical properties, gold nanoparticles have interesting electrical features. Gold nanoparticles are capable of storing charges and participating in charge-transfer reactions.5,6 Gold nanoparticle films are conducting or insulating depending on the size and mutual distance of the metal cores.7 Monolayer protected gold nanoparticles are stable and easy to prepare.8 The protecting thiol layer can be modified with a thiol exchange reaction,9 or gold nanoparticles can be protected * To whom correspondence should be addressed. E-mail: [email protected]. † Tampere University of Technology. ‡ Department of Molecular Engineering, Kyoto University. § Fukui Institute for Fundamental Chemistry, Kyoto University. (1) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Link, S.; El-Sayed, M. A. Int. ReV. Phys. Chem. 2000, 19, 409. (3) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (4) Vinod, C. P.; Kulkarni, G. U.; Rao, C. N. R. Chem. Phys. Lett. 1998, 289, 329. (5) Pietron, J. J.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 5565. (6) Su, B.; Girault, H. H. J. Phys. Chem. B 2005, 109, 11427. (7) Liljeroth, P.; Vanmaekelbergh, D.; Ruiz, V.; Kontturi, K.; Jiang, H.; Kauppinen, E.; Quinn, B. M. J. Am. Chem. Soc. 2004, 126, 7126. (8) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 7, 801-802. (9) Hostetler, M. J.; Green, S. J.; Sokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212.

directly with functional thiols during synthesis. These methods have made it possible to attach chromophores to the surface of gold nanoparticles, thus making the properties of the chromophores adjustable. In chromophore functionalized gold nanoparticles, mainly studied in solutions, the fluorescence of the chromophore can be quenched by either energy10,11 or electron transfer.10 A fluorescence lifetime enhancement of a chromophore has been observed due to a change in the radiative decay rate.12 The details of the interaction mechanism depend on the orientation of the molecular dipole of the chromophore relative to the gold nanoparticle surface,13 on the distance between the gold nanoparticle surface and the chromophore,14 and on the spectral overlap of the emission and absorption spectra of the chromophore and the gold nanoparticles. The vicinity of the gold nanoparticle can prevent electron transfer in a donor-acceptor (e.g., tetrathiafulvalene-anthracene) dyad, thus modifying the fluorescence of the dyad.15 Gold nanoparticle films can be constructed with several methods, for example, Langmuir-Blodgett (LB) and selfassembly.1 Sophisticated solid assemblies show that gold nanoparticles can be an essential part of chromophore containing photovoltaic devices as a conducting array,16,17 as a supporting (10) Thomas, K. G.; Kamat, P. V. Acc. Chem. Res. 2003, 36, 888. (11) Imahori, H.; Kashiwagi, Y.; Endo, Y.; Hanada, T.; Nishimura, Y.; Yamazaki, I.; Araki, Y.; Ito, O.; Fukuzumi, S. Langmuir, 2004, 20, 73. (12) Herna´ndez, F. E.; Yu, S.; Garcı´a, A.; Campiglia, A. D. J. Phys. Chem. B 2005, 109, 9499. (13) Dulkeith, E.; Morteani, A. C.; Niedereichholz, T.; Klar, T. A.; Feldmann, J.; Levi, S. A.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Mo¨ller, M.; Gittins, D. I. Phys. ReV. Lett. 2002, 89, 203002. (14) Ipe, B. I.; Thomas, K. G. J. Phys. Chem. B 2004, 108, 13265. (15) Zhang, G.; Zhang, D.; Zhao, X.; Ai, X.; Zhang, J.; Zhu, D. Chem.sEur. J. 2006, 12, 1067. (16) Lahav, M.; Heleg-Shabtai, V.; Wasserman, J.; Katz, E.; Willner, I.; Du¨rr, H.; Hu, Y.-Z.; Bossmann, S. H. J. Am. Chem. Soc. 2000, 122, 11480. (17) Kim, D.; Carroll, D. L. Appl. Phys. Lett. 2005, 87, 203113.

10.1021/la702535a CCC: $37.00 © 2007 American Chemical Society Published on Web 11/16/2007

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material for complex organization,18 or as an electron-transfer mediator.10,19 The plasmon band absorption can be utilized in photocurrent generation, as photoexcited electrons are transferred from gold nanoparticles to titanium dioxide.20 Local field enhancement due to large gold nanoparticles has been demonstrated by an increase in the photocurrents of porphyrin21 and silicon.22 Controlling the assembly of gold nanoparticles and chromophores to create solid structures is therefore important in terms of existing and future applications. Photoinduced charge separation is a key function in photovoltaic devices. In porphyrin-fullerene (PF) dyads in solutions, the charge separation is fast and reaches quantum yields of unity and, importantly, the recombination of the charge separated state is slower than the charge separation, leading to relatively long lifetimes of the charge separated state.23-27 The remarkable properties of the PF dyads have been utilized in solid selfassembled structures for photocurrent generation.28,29 The charge separation efficiency is enhanced when the porphyrin and fullerene in the dyad are connected by two linkers compared to one linker.30-32 Porphyrin-fullerene dyads with amphiphilic features can be arranged as Langmuir-Blodgett (LB) films. In these organized structures, the donor units lie on the same plane adjacent to the acceptor plane, making it possible to generate the photoinduced charge separated state between the donor and acceptor planes.33 The present study focuses on interactions between molecularly organized porphyrin-fullerene dyads and gold nanoparticles in layered thin films. It is important to investigate the interaction between chromophores and gold nanoparticles in films because their properties in the solid phase are different from those in solutions. Furthermore, solid assemblies are setting the scene for prospective applications of gold nanoparticles and chromophores. Although chromophore-gold nanoparticle interaction has been well studied for chromophores covalently linked to gold nanoparticles in solutions10,12,13,34,35 and in solid hybrid materials,17,36 studies on the interaction between metal nanoparticle (18) Hasobe, T.; Imahori, H.; Kamat, P. V.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fujimoto, A.; Hirakawa, T.; Fukuzumi, S. J. Am. Chem. Soc. 2005, 127, 1216. (19) Barazzouk, S.; Kamat, P. V.; Hotchandani, S. J. Phys. Chem. B 2005, 109, 716. (20) Tian, Y.; Tatsuma, T. J. Am. Chem. Soc. 2005, 127, 7632. (21) Akiyama, T.; Nakada, M.; Terasaki, N.; Yamada, S. Chem. Commun. 2006, 395. (22) Schaadt, D. M.; Feng, B.; Yu, E. T. Appl. Phys. Lett. 2005, 86, 063106. (23) Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. 1996, 100, 15926. (24) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.; Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118, 11771. (25) Armaroli, N.; Marconi, G.; Echegoyen, L.; Bourgeois, J.-P.; Diederich, F. Chem.sEur. J. 2000, 6, 1629. (26) D’Souza, F.; Deviprasad, G. R.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O. J. Am. Chem. Soc. 2001, 123, 5277. (27) Kesti, T. J.; Tkachenko, N. V.; Vehmanen, V.; Yamada, H.; Imahori, H.; Fukuzumi, S.; Lemmetyinen, H. J. Am. Chem. Soc. 2002, 124, 8067. (28) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129. (29) Chukharev, V.; Vuorinen, T.; Efimov, A.; Tkachenko, N. V. Langmuir 2005, 21, 6385. (30) Chukharev, V.; Tkachenko, N. V.; Efimov, A.; Guldi, D. M.; Hirsch, A.; Scheloske, M.; Lemmetyinen, H. J. Phys. Chem. B 2004, 108, 16377. (31) Schuster, D. I.; Cheng, P.; Jarowski, P. D.; Guldi, D. M.; Luo, C.; Echegoyen, L.; Pyo, S.; Holzwarth, A. R.; Braslavsky, S. E.; Williams, R. M.; Klihm, G. J. Am. Chem. Soc. 2004, 126, 7257. (32) Dietel, E.; Hirsch, A.; Eichhorn, E.; Rieker, A.; Hackbarth, S.; Roder, B. Chem. Commun. 1998, 1981. (33) Vuorinen, T.; Kaunisto, K.; Tkachenko, N. V.; Efimov, A.; Lemmetyinen, H. Langmuir 2005, 21, 5383. (34) Gu, T.; Whitesell, J. K.; Fox, M. A. Chem. Mater. 2003, 15, 1358. (35) Deng, F.; Yang, Y.; Hwang, S.; Shon, Y.-S.; Chen, S. Anal. Chem. 2004, 76, 6102. (36) Ostrowski, J. C.; Mikhailovsky, A.; Bussian, D. A.; Summers, M. A.; Buratto, S. K.; Bazan, G. C. AdV. Funct. Mater. 2006, 16, 1221.

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films and chromophore films are rare.37 In the present study, octanethiol protected gold nanoparticle films and double-linked porphyrin-fullerene dyad films are prepared by the LB method. These LB film structures are studied by atomic force microscopy (AFM). Besides the porphyrin-fullerene dyads, also reference porphyrin and fullerene are assembled into multilayer structures with the gold nanoparticles. An essential benefit from the use of the film structures is that properties such as the absorbance and conductivity of each film can be individually controlled during film preparation. From the point of view of the number of synthetic steps, it is an advantage that the chromophores do not require modification with thiols or other linker molecules. In addition, structures are organized in a way that supports unidirectional charge movement, and it is possible to build up complex multicomponent structures. The time-resolved Maxwell displacement charge (TRMDC) method38,39 is employed to study interlayer charge-transfer processes. This method is combined with steady state absorption and fluorescence measurements and the time-correlated single photon counting (TCSPC) method to clarify the interaction type between the gold nanoparticles and the porphyrin-fullerene dyad. Experimental Section Materials. Chloroform (BDH), toluene (Baker), ethanol (Altia), and hexane (Labscan) of analytical grade were used as received. Gold(III)chloride trihydrate (99%), tetraoctylammonium bromide (TOABr, 98%), 1-octanethiol (98.5%), and sodium borohydride (NaBH4, 96%) were purchased from Sigma-Aldrich. Octadecylamine (ODA, 99%) was acquired from Sigma. MilliQ water was derived from a Millipore system. The studied porphyrin-fullerene dyad (denoted in further text as PF dyad) was synthesized by using a previously described procedure.40 The structure of this double-linked dyad is presented in Scheme 1. Porphyrin with tert-butyl groups (P-ref) (Scheme 1) was synthesized as reported40 earlier. Synthesis of diacid fullerene (F-ref), Scheme 1, has been described41 elsewhere. Octanethiol protected gold nanoparticles (AuNPs) were synthesized according to the Brust method.8 TOABr in toluene (1.3 mM, 10 mL) was mixed with gold chloride aqueous solution (0.51 mM, 25 mL) and stirred for 20 min to transfer all the gold to the organic phase. Octanethiol (29 µL) was added to the separated organic phase, and stirring was continued for 20 min to form a complex between the gold and the thiol. The thiol-to-gold ratio was low (1:3), and therefore, at this point of the reaction, no change in the reaction mixture color was observed due to the complexation. In the final step, the gold was reduced by NaBH4 aqueous solution (5.1 mM, 10 mL) during 3.5 h. The dark brown product was separated and washed with water. The product was precipitated from excess ethanol, and the monodispersity of the product was improved by fractional precipitation from ethanol-toluene mixtures. The desired fraction was separated with an ethanol-toluene ratio of 2:1. The core diameter was estimated from TEM images to be approximately 2 nm (Figure S1 in the Supporting Information). The molar mass of the particles can be estimated to be roughly 57 000 g/mol based on a calculation, where the gold atom volume and the surface area occupied by one thiol are known.42 Film Preparation. Surface pressure-mean molecular area isotherm measurements and film deposition were done using LB (37) Choulis, S. A.; Mathai, M. K.; Choong, V.-E. Appl. Phys. Lett. 2006, 88, 213503. (38) Tkachenko, N. V.; Hynninen, P. H.; Lemmetyinen, H. Chem. Phys. Lett. 1996, 261, 234. (39) Tkachenko, N. V.; Vuorimaa, E.; Kesti, T.; Alekseev, A. S.; Tauber, A. Y.; Hynninen, P. H.; Lemmetyinen, H. J. Phys. Chem. B 2000, 104, 6371. (40) Efimov, A.; Vainiotalo, P.; Tkachenko, N. V.; Lemmetyinen, H. J. Porphyrins Phthalocyanines 2003, 7, 610. (41) Camps, X.; Hirsch, A. J. Chem. Soc., Perkin Trans. 1 1997, 1595. (42) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036.

AuNP Enhanced Charge Transfer in PF Dyad Films Scheme 1

5000, LB Minitrough, and Minialternate systems from KSV Instruments. The subphase temperature was set to 18 ( 1 °C by using a thermostat. Samples for photoelectrical measurements were prepared on indium tin oxide (ITO) coated glass plates with a sheet resistance of approximately 10 Ω/square. The plates were cleaned in an ultrasonic bath in acetone as well as in chloroform and plasma etched in a low-pressure nitrogen atmosphere plasma cleaner PDC23G from Harrick. ODA layers were used as insulation between the electrodes and the photoactive layers when preparing samples for the photoelectrical measurements. Typically, ∼11 layers of ODA were deposited before the photoactive layers, and finally the samples were covered with ∼20 ODA layers. Samples for optical measurements were prepared on glass plates cleaned using a standard process43 and plasma etched. Samples for surface characterization were prepared on silicon plates sonicated in acetone and rinsed with isopropanol. Chloroform solutions of the film forming compounds in concentrations less than 1 mM were used for spreading in LB preparation. Spreading solutions were prepared from stock solutions of ODA, PF dyad, AuNPs, or F-ref in chloroform and P-ref in hexane. The subphase was a phosphate buffer containing 0.5 mM Na2HPO4 and 0.1 mM NaH2PO4 in MilliQ water for ODA containing films. With this buffer, a solution pH-value of 7 was maintained. PF dyad film formation has been studied in detail elsewhere;33 the selected (43) Roberts, G. Langmuir-Blodgett films; Plenum Press: New York, 1990.

Langmuir, Vol. 23, No. 26, 2007 13119 Scheme 2

concentration was 10 mol % PF dyad in ODA, and the deposition surface pressure was 15 mN/m with a dip velocity of 5 mm/min. The orientation of the PF dyad relative to the substrate surface was controlled by the dip direction, that is, from water to air or vice versa. P-ref films were prepared44 in 10 mol % concentration in ODA and deposited at a surface pressure of 28 mN/m with a dip velocity of 10 mm/min. F-ref (100%) films were prepared by horizontal dipping (Langmuir-Scha¨fer, i.e., LS method) at a surface pressure of 20 mN/m from a 0.3 mM CdCl2 subphase.45 Pure (100%) AuNP films were prepared by the LS method at a surface pressure of 8 mN/m using MilliQ water as the subphase. Mixed films of 1.3 mol % AuNPs in ODA were deposited at a surface pressure of 7 mN/m with a dip velocity of 5 mm/min. Surface Characterization. Atomic force microscopy (AFM) images of the surfaces were obtained with a Molecular Imaging Pico LE instrument in the tapping mode. Spectroscopic Measurements. Absorption spectra of the films were recorded with a Shimadzu UV-3600 UV-vis-NIR spectrophotometer. Steady-state fluorescence was measured with a Fluorolog 3 Yobin Yvon-SPEX spectrofluorometer with a cooled infrared sensitive photomultiplier (Hamamatsu R2658). The emission spectra were corrected to instrument wavelength sensitivity using a correction spectrum supplied by the manufacturer. Time-resolved fluorescence was measured with a time-correlated single photon counting (TCSPC) system consisting of a PicoHarp 300 controller and PDL 800-B driver. The excitation wavelength was 404 nm from a pulsed diode laser head LDH-P-C-405B. The fluorescence signal was detected with a microchannel plate photomultiplier tube (Hamamatsu R2809U). Time resolution was approximately 100 ps. Photoelectrical Measurements. Photoinduced charge movement was studied with the time-resolved Maxwell displacement charge (TRMDC) method.38,39 The principal configuration of the measurement is shown in Scheme 2. Samples were excited by 10 ns pulses from the second harmonic of a titanium-sapphire laser (adjusted in the wavelength range 410-450 nm) pumped by the second harmonic of a Q-switched Nd:YAG laser at a wavelength of 532 nm. A wavelength of 532 nm was also used for the excitation of the samples. The optically active layers are insulated from the electrodes, and therefore, the photovoltage signal is caused by the photoinduced electron movement perpendicular to the sample plane in the photoactive layers. The amplitude of the photovoltage is proportional (44) Anikin, M.; Tkachenko, N. V.; Lemmetyinen, H. Langmuir 1997, 13, 3002. (45) Vuorimaa, E. Unpublished results.

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Figure 2. Absorption spectra of 100% and 1.3% AuNP monolayers showing a weak surface plasmon band.

Figure 1. Isotherms of the AuNP films: (A) 100% AuNP and (B) mixed films of AuNP in ODA at molar fractions of 0% (i), 1.3% (ii), and 4.3% (iii). to the charge displacement distance and number of charges. The sign of the signal is connected to the direction of electron movement. The decay of the signal depends on the recombination of the charges. The samples have very low conductivities and can be treated as capacitors (100-200 pF). The instrumental time constant is determined by the preamplifier input resistance Rin ) 100 MΩ and the sample capacitance to be roughly 10 ms. In photovoltage measurements, time scales much shorter than this are used. The difference in the work functions of the ITO46 and InGa47 electrodes is roughly 0.5 V, and this difference produces an intrinsic electric field in the sample. An external field can be applied by controlling the value of the external voltage Ubias, which compensates the intrinsic field of the sample when set to -0.5 V.

Results and Discussion Preparation of AuNP Films. The deposition of nanoparticle films at high surface pressures has been successful with the LS method.48,49 The shape of the isotherm is affected by the monodispersity of the gold nanoparticles so that isotherms of monodisperse particles are steep and reach high surface pressures. The LB deposition of nanoparticles is possible at very low surface pressures, and thus, the films have a low density of particles.50 Few reports about LB multilayer deposition have been published.51 One way to improve the deposition of hydrophobic molecules is to mix them with an ideally behaving amphiphilic matrix component with matching molecular height.43 The LB deposition was preferred to the LS deposition in this study because of the possibility for more versatile multilayer sample preparation. Isotherms of pure AuNP films have similar shapes on the MilliQ subphase (figure not shown) and on the phosphate buffer subphase (Figure 1A). Two-phase synthesis in the presence of TOABr has a major positive effect on the formation of Langmuir films of alkanethiol protected gold nanoparticles.52 (46) Park, Y.; Choong, V.; Gao, Y.; Hsieh, B. R.; Tang, C. W. Appl. Phys. Lett. 1996, 68, 2699. (47) Emets, V. V.; Damaskin, B. B. J. Electroanal. Chem. 2000, 491, 30. (48) Huang, S.; Minami, K.; Sakaue, H.; Shingubara, S.; Takahagi, T. Langmuir 2004, 20, 2274. (49) Liljeroth, P.; Vanmaekelbergh, D.; Ruiz, V.; Kontturi, K.; Jiang, H.; Kauppinen, E.; Quinn, B. M. J. Am. Chem. Soc. 2004, 126, 7126. (50) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (51) Zhou, X.; Liu, C.; Jiang, L.; Li, J. Colloids Surf., A 2004, 248, 43.

TOABr is likely to be incorporated into the gold nanoparticles used here because it is not removed during fractional precipitation.53 An increase in the surface area is observed when AuNPs are introduced into an ODA film (Figure 1B), indicating that AuNPs are present on the subphase surface with the ODA molecules. The limiting area for the different compositions of the film can be fitted linearly with the following equation:54 Stot ) xAuNP(SAuNP - SODA) + SODA, where xAuNP is the molar fraction of AuNPs and SAuNP and SODA are the limiting areas of the two components of the film. The fit gives the values SODA ) 19.9 Å2 and SAuNP ) 732 Å2. In a 100% AuNP film, the limiting area is 772 Å2, and thus, quite similar packing of AuNPs can be expected for both of the films. The relative coverage of AuNPs is 33% in a 1.3 mol % film and 62% in a 4.3 mol % film, as calculated from the limiting areas. The coverage would be better for the 4.3 mol % AuNP film, but this film was not very well deposited vertically due to the rigidity of the Langmuir film. Therefore, the 1.3 mol % film was chosen for sample preparation and used in all the optical and photoelectrical measurements unless otherwise stated. Absorption of the 1.3% mixed monolayer is lower than that of a 100% film clearly due to the smaller amount of AuNPs present in the film (Figure 2). The shape of the absorption spectrum with both film compositions, 1.3% and 100%, is essentially the same. This indicates that the environment of the particles is similar in both films, because the plasmon band frequency is inversely proportional to the dielectric constant of the medium surrounding the particles. Preparation and Characterization of AuNP and PF Dyad Multilayer Structures. Absorption spectra of the multilayer structures glass|FP|AuNP and glass|PF|AuNP are simply the sum of the monolayer absorptions of the PF dyad and AuNPs (Figure 3). Charge transfer between porphyrin and fullerene causes the emission to be 400 times less in the porphyrin-fullerene dyad films than in the porphyrin reference films without fullerene.33 Weak fluorescence of the PF dyad is very sensitive to changes in the film preparation, and a quantitative determination of PF dyad monolayer fluorescence is difficult. Porphyrin-fullerene dyads have an exciplex emission band in nonpolar solvents.30 In films, weak porphyrin emission peaks at 660 and 720 nm and the exciplex emission at wavelength 740 nm are observed for porphyrin-fullerene dyads.33 The emission of the PF dyad is quenched by 10-20% at the porphyrin monomer emission (52) Sun, Y.; Frenkel, A. I.; White, H.; Zhang, L.; Zhu, Y.; Xu, H.; Yang, J. C.; Koga, T.; Zaitsev, V.; Rafailovich, M. H.; Sokolov, J. C. J. Phys. Chem. B 2006, 110, 23022. (53) Waters, C. A.; Mills, A. J.; Johnson, K. A.; Schiffrin, D. J. Chem. Commun. 2003, 4, 540. (54) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-gas Interfaces; John Wiley & Sons: New York, 1966.

AuNP Enhanced Charge Transfer in PF Dyad Films

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Figure 3. Absorption spectra of glass|FP (bold line) and glass|FP|AuNP (thin line) films. Similar spectra were also obtained for glass|PF and glass|PF|AuNP films.

Figure 4. Emission spectra of (A) glass|FP (bold line) and glass|FP|AuNP (thin line) and (B) glass|PF (bold line) and glass|PF|AuNP (thin line). Excitation wavelength was 430 nm.

wavelengths in the structures glass|FP|AuNP and glass|PF|AuNP. The effect of the position of the AuNPs relative to the PF dyad is seen at the longer emission wavelengths, in the range of the dyad exciplex emission. The quenching of the exciplex emission is as strong as the quenching of the porphyrin emission (roughly 20%) in the structure glass|FP|AuNP, and the shapes of the emission spectra are practically the same for the structures glass|FP|AuNP and glass|FP (Figure 4A). Relatively stronger quenching of the exciplex emission (roughly 40%) is observed in the structure glass|PF|AuNP, and there is a clear change in the shape of the emission spectrum (Figure 4B) compared to the case of the PF film. Thus, the exciplex formation of the PF dyad might be disturbed by the presence of AuNPs close to the fullerene, whereas the closeness of the AuNPs to porphyrin has only a minor effect on the exciplex formation. Due to the low fluorescence signal of the monolayer samples, the effect of AuNPs on the PF dyad exciplex fluorescence lifetime was not possible to measure with the time-correlated single photon counting (TCSPC) method. The AFM image of a 1.3% AuNP monolayer (Figure 5A) shows that AuNPs form continuous islands with a characteristic height of 2.5-3 nm compared to the smooth ODA matrix. The relative coverage of the AuNPs estimated from the AFM image is 40%, which is in good agreement with the 33% coverage

Figure 5. AFM images of (A) a 1.3% AuNP film, (B) a 10% PF film, and (C) the structure substrate|AuNP|PF.

calculated from the isotherm. The imaged structure confirms that also in a mixed film most of the AuNPs are surrounded by the other particles, which was also indicated by the similar shapes of the absorption spectra of the 100% and 1.3% AuNP films (Figure 2). The AFM image of a PF dyad monolayer (Figure 5B) reveals dyad clusters with a relative coverage of 40% distributed in the ODA matrix. The average height of the PF dyad clusters is 1-1.5 nm compared to the ODA surface. The structure substrate|AuNP|PF has a maximum height of roughly 15 nm (Figure 5C) which corresponds well to the sum of the maximum heights of the AuNPs and PF dyad monolayers (8 and 7 nm,

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Table 1. Fitted Lifetimes and Pre-Exponential Factors of P-ref and P-ref|AuNP Films sample

τ0 (ps)

τ1 (ns)

τ2 (ns)

P-ref 2.0 ( 0.2 6.2 ( 0.1 P-ref|AuNP 205 ( 86 2.0 ( 0.2 6.4 ( 0.1

P 0 P1 P 2 (%) (%) (%)

χ2

28 25

1.0 1.0

24

72 51

respectively). This indicates the actual formation of a PF dyad monolayer on top of the AuNP monolayer. The probability of having a PF dyad layer on top of a AuNP layer is the product of their relative coverages, accordingly 16%. The actual portion of the AuNP|PF structure formed might be higher than the stochastic value because the formation of an ODA layer is more favorable on another ODA layer due to the polarity of this molecule, whereas the PF dyads might prefer the AuNP covered parts of the layer. Optical Properties of the P-ref|AuNP Film Structure. The main interest of this study lies in the interaction between the AuNP film and the porphyrin-fullerene dyad film. Part of this system can be simplified into studying the interaction between the porphyrin reference (P-ref) film and the AuNP film. P-ref presumably organizes in the LB films into monomers perpendicular to the film plane and into dimers parallel to the film plane.44 The P-ref film is in this way different from the PF dyad films, where porphyrins are ideally expected to lie parallel to the film plane. Results obtained with P-ref are thus indicative of the porphyrin|AuNP interaction, but those cannot be fully generalized for the AuNP|PF systems because of the structural differences. The absorption spectrum of the multilayer structure glass|Pref|AuNP is the sum of the monolayer absorptions (Figure S2A in the Supporting Information). The fluorescence emission of P-ref is quenched by 50% in the structure glass|P-ref|AuNP when the porphyrin is excited at the absorption maximum of 422 nm (Figure S2B in the Supporting Information). Fluorescence quenching by energy transfer is in principle possible because the emission spectrum of P-ref overlaps with the absorption spectrum of the AuNPs. The excitation spectrum of the P-ref|AuNP film has a similar shape as the porphyrin absorption spectrum (Figure S2B in the Supporting Information). Fluorescence decay of the P-ref film is biexponential due to differently organized porphyrins.44 Fluorescence decay right after the excitation pulse becomes faster in the P-ref|AuNP structure compared to that in the P-ref film (Figure S3 in the Supporting Information). When the fluorescence decays are fitted exponentially, it can be seen that in the P-ref|AuNP film the two longer fluorescence lifetimes are the same as those in the P-ref film, but the shortest lifetime is in the picosecond range (Table 1). The additional short lifetime might be due to a fast charge (or energy) transfer process. The overlapping area of the P-ref and AuNP films is expected to be low, and that could explain the low percentage of porphyrins participating in the assumed charge-transfer process. The minimum distance from the center of the porphyrins to the surfaces of the gold particles can be approximated to be the length of the octanethiol chain, that is, 10 atoms in the P-ref|AuNP film. With porphyrins covalently linked to gold nanoparticles via 10-18 atoms, the porphyrin fluorescence was strongly quenched by the gold nanoparticles and the fluorescence lifetime was 50-100 ps in benzene.11 In those porphyrin modified gold nanoparticles, energy transfer from the porphyrins to the gold nanoparticles was ascribed to be the main process, since a porphyrin radical cation generated by photoinduced electron transfer was not observed.11 The shortest fluorescence lifetime observed for the P-ref|AuNP film is 200 ps, and thus, the distance

Figure 6. Photovoltage decay of the structures ITO|P-ref|AuNP (i), ITO|AuNP (ii), and ITO|P-ref (iii) at an excitation wavelength of 423 nm and excitation energy density of 0.46 mJ/cm2. In these sample structures, the 100% AuNP film was used.

between some P-ref molecules and AuNPs can be expected to be short, that is, 1-2 nm. In the case of pyrene functionalized gold nanoparticles, a distance of 11 atoms was enough to prevent electron and energy transfers.14 With shorter chains, electron transfer from pyrene to gold was confirmed by observing the pyrene radical cation.14 Thus, the distance between the adjacent films of P-ref and AuNPs is short enough to allow interaction and electron transfer from the AuNPs to the porphyrins. In the case of the F-ref monolayer, no fluorescence was observed,45 and for that reason it was not possible to study the interaction between F-ref and AuNPs by emission measurements. TRMDC Measurements. The 1.3% AuNP films were used for the TRMDC measurements, except for the ITO|P-ref|AuNP structure, where the AuNP films were 100%. Unless otherwise noted, the photovoltage signals were measured with no external voltage applied, that is, Ubias ) 0 V. AuNPs and Porphyrins. In a single film, in the absence of specific donor-acceptor molecules, photovoltage is due to charge migration in the field created by the difference of electrode potentials.38 This field causes electrons to move inside the film in the direction from the ITO to the InGa electrode when no external voltage, that is, Ubias ) 0 V, is applied, and this charge migration results in a negative photovoltage. An example of such response can be seen for the P-ref film (Figure 6). The excitation wavelength of 423 nm corresponds to the absorption maximum of P-ref and is higher in energy than the plasmon band absorption of the AuNP film. Practically no photovoltage is observed for the 100% AuNP film. A strong positive photovoltage emerges in the structure ITO|P-ref|AuNP, indicating electron movement from the AuNP layer to the P-ref layer (Figure 6). The measuring system monitors the charge movement perpendicular to the film plane, and thus, there must be charge separation between the P-ref and AuNP layers. Energy transfer would not produce any photovoltage in this kind of sample structure. The photovoltage of the structure ITO|P-ref|AuNP is virtually independent of the bias voltage, and thus, the charge movement is unidirectional and is not caused by the internal field. Charge transfer was also seen as quenching of the P-ref fluorescence emission by AuNPs (Figure S2B in the Supporting Information) and as faster decay of fluorescence in the P-ref|AuNP film compared to the P-ref film (Table 1). A metallic character has been observed for bare gold nanoparticles with a diameter of more than 1 nm.4 Thus, the ionization potential of the 2 nm gold nanoparticles used in this study is close to the work function of bulk gold,55 5.3-5.5 eV. (55) Lide, D. R., Ed. CRC Handbook of Chemistry and Physiscs, 87th ed., Internet version 2007, http://www.hbcpnetbase.com.

AuNP Enhanced Charge Transfer in PF Dyad Films

Figure 7. Photovoltage of the structure ITO|AuNP|F-ref (i) is higher than that for either ITO|F-ref (ii) or ITO|AuNP (iii) at an excitation wavelength of 434 nm and excitation energy density of 0.22 mJ/ cm2.

Figure 8. Photovoltage of the ITO|PF (i), ITO|PF|AuNP (ii), and ITO|AuNP|PF (iii) films at an excitation wavelength of 432 nm and excitation energy density of 0.27 mJ/cm2. The photovoltage sign depends on the orientation of the PF dyad relative to the AuNP.

The effective work function of the thiol monolayer protected gold nanoparticles, in the metallic size regime, is lower than that in bulk gold due to the charge transfer between sulfur and gold.56 The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of free-base tetraphenylporphyrins such as P-ref are approximately 5.6 and 4.2 eV relative to vacuum, respectively.57 An electron is located on the LUMO and a hole is left on the HOMO after the excitation of P-ref to the singlet excited state. The hole is transferred to a AuNP, leaving a negative charge on P-ref and a positive on the AuNP, which then recombine. A similar mechanism has been observed in blends of free-base porphyrins and polythiophenes, where the photogenerated hole in the HOMO of porphyrin is filled with the electron from the polythiophene valence band.57 Alternatively, it can be thought that as AuNPs are prepared under the strongly reducing conditions, they could be carrying a negative charge,58 which decreases the capability of electron accepting and makes hole accepting more favorable. The hole transfer from the porphyrin to the gold nanoparticle leads to the same final charges as those for electron transfer from the AuNP to the porphyrin, and therefore, the proposed mechanism is concurrent with the experimental results. AuNPs and Fullerenes. To study the interaction between fullerenes and AuNPs, the photovoltage of the structure ITO|AuNP|F-ref was measured. The main absorption bands of F-ref are at 210 and 260 nm, but F-ref has an absorbance of ∼0.01 at the excitation wavelength of 434 nm. The photovoltage signal of the film structure ITO|AuNP|F-ref is negative (Figure (56) Tanaka, A.; Imamura, M.; Yasuda, H. Phys. ReV. B 2006, 74, 113402. (57) Takahashi, K.; Takano, Y.; Yamaguchi, T.; Nakamura, J.; Yokoe, C.; Murata, K. Synth. Met. 2005, 155, 51. (58) Quinn, B. M.; Liljeroth, P.; Ruiz, V.; Laaksonen, T.; Kontturi, K. J. Am. Chem. Soc. 2003, 125, 6644.

Langmuir, Vol. 23, No. 26, 2007 13123

Figure 9. Photovoltage decay of the structure ITO|AuNP|PF (gray bold line) on a double logarithmic scale and fit (black line) with a power decay model.

Figure 10. Photovoltage amplitude dependence on excitation energy density for ITO|PF (triangles) and ITO|AuNP|PF (circles) at an excitation wavelength of 432 nm. Fits are obtained from the saturation model: ITO|PF (dotted line) and ITO|AuNP|PF (solid line).

7) with a photovoltage amplitude ratio of 2.9 at the bias voltages of 0 V and -0.5 V. Thus, the electrons move from the AuNP to F-ref and the movement is dependent on the internal field. When fullerenes are covalently linked with a 12-atom chain to gold nanoparticles, the excited fullerenes transfer energy to the gold cores in solution.59 Carboxyl functionalization of the fullerene causes a change in the redox potentials compared to pristine fullerene,60 but the HOMO level can be approximated to be ∼6 eV, typical for a fullerene film.61 Based on the alignment of the HOMO levels of porphyrin and fullerene, interaction mechanisms between the excited chromophores and AuNPs might be similar for the P-ref|AuNP and AuNP|F-ref film structures. An energy transfer process parallel to charge transfer cannot be excluded. AuNPs and Porphyrin-Fullerene Dyads. The main photoinduced process in the porphyrin-fullerene dyad is the formation of an exciplex followed by electron transfer from the porphyrin to the fullerene moiety.30 Analysis of the porphyrin|AuNP and AuNP|fullerene film properties offers some indications as to what will happen when AuNPs are coupled with the PF dyad. The excitation wavelength of 432 nm overlaps with the Soret band of the PF dyad and is higher in energy than the surface plasmon band of the AuNPs. At this wavelength, the absorbances of the PF dyad and AuNP monolayers are 0.011 and 0.007, respectively. When AuNPs face the porphyrin part of the PF dyad, as in the structure ITO|AuNP|PF, the photovoltage amplitude is enhanced compared to that of the PF dyad film (Figure 8). The photovoltage amplitude, divided by the excitation energy density, is 1480 Vcm2/mJ for ITO|AuNP|PF (Figure 8), whereas it is only 460 Vcm2/mJ for ITO|P-ref|AuNP (Figure 6). (59) Sudeep, P. K.; Ipe, B. I.; Thomas, K.; George, M. V.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. Nano Lett. 2002, 2, 29. (60) Guldi, D. M. http://www.photobiology.com/reviews/6/index.html (accessed July 20, 2007). (61) Ishii, H.; Seko, A.; Kawakami, A.; Umishita, K.; Ouchi, Y.; Seki, K. Mater. Res. Soc. Symp. Proc. 2003, 771, L3.5.1.

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Kotiaho et al.

Table 2. Saturation Fit Parameters for the PF Dyad and AuNP|PF Films wavelength (nm)

U0 (V)

I0 (mJ/cm2)

σ (×10-16 cm2)

A

σ (×10-16 cm2) from absorption

PF

432 532

0.28 ( 0.01

0.72 ( 0.10

6.4

0.011 0.001

7.7 33 0.8 33

AuNP

432 532

0.062 ( 0.009

2.8 ( 0.7

1.6

0.007 0.007

37 37

AuNP|PF

432 532

1.4 ( 0.1 0.59 ( 0.03

0.99 ( 0.11 3.5 ( 0.3

4.6 1.1

0.018 0.008

film

This increase in amplitude indicates better charge separation efficiency in the structure ITO|AuNP|PF compared to ITO|Pref|AuNP. The enhancement of the photovoltage in the structure ITO|AuNP|PF compared to the structure ITO|PF is reasonable, considering that after the excitation the charge separation in the PF dyad yields a negatively and positively charged fullerene and porphyrin, respectively, and that the positive charge then moves from the porphyrin to the AuNPs. When AuNPs face the fullerene part of the PF dyad, in the structure ITO|PF|AuNP, a change in the sign of the photovoltage signal is observed (Figure 8). Therefore, most of the electrons move from the AuNP layer to the fullerene layer of the PF dyad film. The deposition direction and the order of the PF dyad and AuNP films were varied and the qualitative results were reproduced, which rules out the surface roughness of the AuNP films as a reason for the emerging photovoltages. Electron transfer is vectorial in the PF dyad film,33 and the photovoltage amplitude is almost insensitive to the existence of an internal field. This is the case also for the structure ITO|AuNP|PF where the ratio of the photovoltage amplitudes at bias voltages of 0 and -0.5 V is 1.17 (Figure S4A in the Supporting Information). AuNPs thus clearly support the vectorial electron transfer in the PF dyad. The structure ITO|PF|AuNP has the ratio of 0.88 of the photovoltage amplitudes at bias voltages of 0 and -0.5 V, and therefore, the electron transfer is strongly oriented also in this structure (Figure S4B in the Supporting Information). The photovoltage ratio is lower or higher compared to unity depending on the sign of the photovoltage amplitude, because Ubias ) -0.5 V supports electron movement in the direction from ITO to the InGa electrode and thus increases the negative photovoltage amplitude and vice versa for the positive photovoltage amplitude. When the amplitudes at the biasing voltage of -0.5 V are compared for these structures and the difference in excitation energy density is taken into account, it is clear that the charge separation distance is greater in the structure ITO|AuNP|PF than in the structure ITO|PF|AuNP. In the structure ITO|AuNP|PF, after the electron transfer in the PF dyad, charges are separated further by the hole transfer from the porphyrin part of the dyad to the AuNPs. This assumption is also supported by the minor quenching of the exciplex emission of the PF dyad by AuNPs adjacent to the porphyrin moieties of the dyad monolayer (Figure 4A). Even though AuNPs quench the fluorescence emission of porphyrin by 50% in the P-ref|AuNP film, in the case of the AuNP|PF film fluorescence quenching by AuNPs is reduced to 10-20% because of the very efficient fluorescence quenching due to the electron transfer in the dyad. For the structure ITO|PF|AuNP, 40% quenching of the PF dyad exciplex fluorescence (Figure 4B) and the change in the polarity of the photovoltage amplitude compared to the case of the ITO|PF film were seen. These two experimental facts suggest that the electron-transfer process of the PF dyad is affected by the AuNPs adjacent to the fullerene moieties of the dyad monolayer. On the other hand, the photovoltage measurements with F-ref|AuNP films indicate that excited fullerene is capable of accepting

electrons from the AuNPs, a process which might also take place in the ITO|PF|AuNP structure. The photovoltage decay of the PF dyad follows a power law, Uout(t) ∼ t-b with a b value of ∼0.2, except for the very fast recombination right after the excitation.33 Recombination becomes slightly faster with b ∼ 0.3 when AuNPs are introduced adjacent to the porphyrin part of the PF dyad films (Figure 9). Even though this simple power law is suitable for the PF dyad films, for more complex structures it is somewhat inadequate. In structures with a secondary electron donating polymer film adjacent to a PF dyad film, the charge recombination is slower compared to that of the PF dyad film.62 The reason for the slow recombination is that the separated charges, DONOR+|PF-, can migrate farther from each other parallel to the film plane in the donor and fullerene layers.62 Thus, the expected behavior of the recombination in AuNP|PF would be its decreased rate due to the longer charge separation distance compared to the case of the PF dyad. The observed behavior is a boosted rate of recombination, for which a detailed explanation would require information about the back transfer rate of charges between the AuNPs and the PF dyad, and about the charge migration velocity in the AuNP film. Photovoltage saturation was studied by changing the excitation energy density. Saturation of the photovoltage amplitude (Uout) as a function of the excitation density (Iexc) can be fitted with the following equation:33

[

( )]

Uout ) U0 1 - exp -

Iexc I0

where U0 is the saturation photovoltage amplitude and I0 is the saturation excitation density. Saturation parameters for the PF dyad have been reported earlier.33 Saturation of the photovoltage is shown in Figure 10 for the film structures ITO|AuNP|PF and ITO|PF at an excitation wavelength of 432 nm. The saturation intensity is inversely proportional to the absorption cross section (σ), I0 ) hν/σ. The absorption cross section value (Table 2) obtained from the saturation fit for the structure AuNP|PF is very close to the fitted value of the PF dyad. According to this, the absorbing species that creates the photovoltage signal is the same, that is, the PF dyad, in both cases. The absorption cross section of the AuNPs is calculated33 using the surface density of the particles obtained from the isotherm and the absorbance of the film. To gain further insight into the effect of AuNPs on the photovoltage, a wavelength of 532 nm was used for the excitation of the samples. The selective excitation of AuNPs is not possible, but at the excitation wavelength of 532 nm the absorbance of the AuNP film is nonetheless 7-fold compared to the absorbance of the PF dyad. This wavelength is still in the range of the PF dyad Q-band, though the absorbance of the PF dyad is 1/10 of the Soret band absorbance. (62) Vuorinen, T.; Kaunisto, K.; Tkachenko, N. V.; Efimov, A.; Lemmetyinen, H. J. Photochem. Photobiol., A 2006, 178, 185.

AuNP Enhanced Charge Transfer in PF Dyad Films

For ITO|AuNP|PF, as excited at a wavelength of 532 nm, the absorption cross section obtained from the saturation fit is similar to the absorption cross section estimated from the absorbance of the PF dyad (Table 2). This indicates that the PF dyad is the major photoactive component of the structure ITO|AuNP|PF. For small bare gold nanoparticles in solution, excitation with a higher energy than the surface plasmon leads to generation of surface plasmons via excitation of interband transitions.63 There is a continuous increase in the absorption from the red wavelengths to the blue wavelengths, for all sizes of the gold nanoparticles, because of the interband transitions. Interband transitions take place at wavelengths of 432 and 532 nm, but they have no role in creating the photovoltage in the structure ITO|AuNP|PF. The saturation photovoltage corresponds to the photovoltage at the infinite excitation energy density. The saturation photovoltage is 5 times higher for the sample AuNP|PF compared to that for the PF film at the excitation wavelength of 432 nm (Table 2). The increase is rather due to the longer charge separation distance than to the higher absorbance. The ratio of U0 values between the wavelengths 432 and 532 nm is less than could be expected based on the absorption ratio, but if the decay curves with the same amount of incident photons are compared between these two wavelengths, the ratio of photovoltage amplitudes (8) corresponds well to the ratio of PF dyad absorbances (10) (Figure S5 in the Supporting Information). Based on the photovoltage analysis, excitation of the PF dyad leads to charge separation and the charges are further separated by adding a AuNP layer near the porphyrin moiety of the dyad. The longer charge separation distance is also supported by the photovoltage measurements of P-ref|AuNP films, where electron movement from the AuNP layer to the P-ref layer is observed. Emission decay measurements show that for 25% of the P-ref molecules the fluorescence lifetime is reduced to 200 ps by the AuNPs. Only a small portion of the porphyrins is close enough (63) Dulkeith, E.; Niedereichholz, T.; Klar, T. A.; Feldmann, J.; von Plessen, G.; Gittins, D. I.; Mayya, K. S.; Caruso, F. Phys. ReV. B 2004, 70, 205424.

Langmuir, Vol. 23, No. 26, 2007 13125

to the gold nanoparticles to allow interaction, and therefore, the steady-state fluorescence of P-ref is quenched only to half by the AuNPs.

Conclusion Multilayer structures of gold nanoparticles and porphyrinfullerene dyads were assembled successfully. In the structure AuNP|PF, where AuNPs face the porphyrin moiety of the PF dyad, the charge separation of the PF dyad is enhanced by charge transfer from the porphyrins to the AuNPs. When the position of the AuNPs relative to the PF dyad is changed to face the fullerene moiety (PF|AuNP), the charge separation of the dyad is depressed. In the cases of the porphyrin and fullerene references, both films accept electrons from AuNPs after photoexcitation. These results clearly show that adjacent films of AuNPs and PF dyads interact strongly, while the exact mechanism still requires further studies. The observed interaction supports the possibility to couple AuNPs and chromophores into optically and electronically active solid structures. Acknowledgment. A.K. and R.M.L. acknowledge the Academy of Finland (No. 107182) for financial support. The authors are very grateful to Niklaus Baumann and Prof. David. J. Fermı´n (University of Berne, Switzerland) for AFM imaging and Dr. Christoffer Johans (Helsinki University of Technology, Finland) for TEM imaging. Supporting Information Available: TEM image of the gold nanoparticles (Figure S1); absorption, emission, and excitation spectra of P-ref|AuNP films (Figure S2); fluorescence decay of P-ref|AuNP films (Figure S3); effect of bias voltage on photovoltage of ITO|AuNP|PF and ITO|PF|AuNP (Figure S4); and photovoltage decay of ITO|AuNP|PF at excitation wavelengths of 432 and 532 nm (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org. LA702535A