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J. Phys. Chem. C 2007, 111, 9110-9115
Improved Photocurrent in Ru(2,2′-bipyridine-4,4′-dicarboxylic acid)2(NCS)2/ Di(3-aminopropyl)viologen/Single-Walled Carbon Nanotubes/Indium Tin Oxide System: Suppression of Recombination Reaction by Use of Single-Walled Carbon Nanotubes Wonjoo Lee,† Jungwoo Lee,† Soo-Hyoung Lee,‡ Jinho Chang,† Whikun Yi,*,† and Sung-Hwan Han*,† Department of Chemistry, Hanyang UniVersity, Haengdang-dong 17, Sungdong-ku, Seoul, Korea 133-791, and DiVision of EnVironmental & Chemical Engineering, Chonbuk National UniVersity, Duckjin-dong, Jeonju 664-14, Korea 561-756 ReceiVed: January 9, 2007; In Final Form: May 1, 2007
This paper reported the use of single-walled carbon nanotubes (SWCNTs) with terminal carboxylic acid groups as an electron-transfer bridge layer for improved photocurrent of viologen/Ru complex-based photoelectrochemical cells. The preparation of SWCNTs on tin-doped indium oxide (ITO) was prepared by spray-coating. The formation of Ru(2,2′-bipyridine-4,4′-dicarboxylic acid)2(NCS)2 [(RuL2(NCS)2)]/di(3aminopropyl)viologen (DPAV)/SWCNTs/ITO was prepared by formation of acid-base complex. RuL2(NCS)2/ DAPV/ITO and RuL2(NCS)2/DAPV/SWCNTs/ITO showed photocurrents of 5.6 and 10.3 nA/cm2 under A.M 1.5 condition (I ) 100 mW/cm2), respectively. The photocurrents of the RuL2(NCS)2/DAPV/SWCNTs/ITO cells in which Ru complexes were attached to carbon nanotubes by only acid-base complex formation were raised to 83.9% in comparison with RuL2(NCS)2/DAPV/ITO due to retardation of recombination reaction in photoelectrochemical cells.
Introduction The organization of functional materials on a surface is an important subject not only in the field of fundamental research but also in applications.1-4 Functional materials with donor/ acceptor complexes on the surface have been used to assemble photoelectrochemical cells, to tailor light-emitting diodes, to fabricate electrochromic devices, and to organize sensor systems.5-8 To generate photocurrent, the photosensitizers are crucial to generate electron-hole pairs, excitons. The excitons can be dissociated at the interface between the sensitizer and electron acceptor materials into a hole and electron-producing free charges. However, the electron transfer from conduction band [or lowest unoccupied molecular orbital (LUMO) electron] to the electrode is hampered by the competing electron-hole recombination process, which degrades the generation of photocurrent. In order to improve photocurrent generation, avoidance of the recombination of the electron-hole species is a very important problem.10-14 Therefore, several concepts to maximize charge separation in photosensitizers have been suggested.10-15 Willner and co-workers10 reported preparation of the carbon nanotubes (CNTs)-CdS nanoparticle system for photoelectrochemical cells.10 Their semiconductor nanoparticles/ CNTs as a hybrid system were successfully constructed on a Au surface, which showed good photocurrent generation effects upon illumination. Recently, we reported the successful formation of a ZnO thin barrier layer on nanoporous TiO2.11,12 The photoelectrochemical solar cell showed a power conversion efficiency of 4.51% under a light illumination intensity of 80 mW/cm2 as thin ZnO films, acting as a barrier layer, reduced * To whom correspondence should be addressed. E-mail: (S.-H.H.)
[email protected] † Hanyang University. ‡ Chonbuk National University.
the recombination reaction. It was efficient electron transport matrices. The presence of ZnO over TiO2 demonstrates an inherent energy barrier between the porous TiO2 electrode and lithium iodide electrolytes. Also, there have been many attempts to fill the pore structures by coating the thin energy barrier layer with a semiconductors or insulator.13-15 However, suppression of the recombination reaction is not yet fully understood in photoelectrochemical cells at the molecular level. In continuation of our research on photoelectrochemical cells to reduce the recombination reaction,11,12 we attempted to prepare Ru(2,2′-bipyridine-4,4′-dicarboxylic acid)2(NCS)2 [RuL2(NCS)2]/di(3-aminopropyl)viologen (DAPV)/single-walled carbon nanotubes (SWCNTs) with terminal carboxylic acid group composite architectures on tin-doped indium oxide (ITO) surface for photoelectrochemical applications. SWCNTs with terminal carboxylic acid groups on ITO were successfully prepared by spray-coating. By the acid-base complex formation reaction, DAPV, as electron bridging material, and RuL2(NCS)2, as organometallic photosensitizer, were successfully constructed on the SWCNTs/ITO surface. If ITO is immersed in a solution containing DAPV for 48 h, self-assembled monolayers (SAMs) of DAPV can be formed on ITO.16 Then, the RuL2(NCS)2/ DAPV/ITO and RuL2(NCS)2/DAPV/SWCNTs/ITO were illuminated and effectively generated photocurrents of 5.6 and 10.8 nA/cm2 under A.M 1.5 condition (I ) 100 mW/cm2) in the presence of I-/I3-/acetonitrile solution. In the presence of SWCNTs, SAM molecular devices showed efficient electrontransfer matrices with suppression of the recombination reaction. Experimental Section Materials. Chemical agents was obtained from Aldrich Co. and used as received. SWCNTs (90% purity) were obtained from Iljin Nanotech (Korea), which were prepared by the arc
10.1021/jp070165v CCC: $37.00 © 2007 American Chemical Society Published on Web 06/07/2007
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Figure 3. Cyclic voltammogram of HPA on modified ITO in 0.1 M HCl at scan rate of 50 mV/s: (a) sequential reduction of HPA; (b) ITO/DAPV/HPA (b), ITO/SWCNTs/HPA (O), and ITO/SWCNTs/ DAPV/HPA (9); (c) ITO/DAPV (b) and ITO/SWCNTs (O).
Figure 1. (a) SEM image of SWCNTs with carboxylic acid/ITO, (b) optical transmittance of SWCNTs with carboxylic acid/ITO, and (c) FT-IR spectra of SWCNTs (O) and SWCNTs with carboxylic acid (b).
Figure 4. UV-vis spectra of ITO/DAPV/RuL2(NCS)2 (b) and ITO/ DAPV/SWCNTs/RuL2(NCS)2 (O).
Figure 2. UV-vis spectra of ITO/DAPV (b) and ITO/SWCNTs/ DAPV (O).
discharge method. Di(3-aminopropyl)viologen (DAPV) was prepared as reported.16,17 The organometallic photosensitizer Ru(2,2′-bipyridine-4,4′-dicarboxylic acid)2(NCS)2 [(RuL2(NCS)2)]
was purchased from Solaronix Co. ITO (10 Ω‚cm) electrode was obtained from Samsung Corning Co. (Korea). High-purity water (Milli-Q, Millipore) was used for all experiments. Preparation of SWCNTs with Terminal Carboxylic Acid. SWCNTs with terminal carboxylic acid were prepared as reported.18-20 SWCNTs were oxidized in a concentrated acid mixture of H2SO4/HNO3 ) 3:1 by volume under ultrasonication for 20∼24 h at 50∼60 °C, to produce shortened SWCNTs with terminal carboxylic acid groups. The resulting solution was
9112 J. Phys. Chem. C, Vol. 111, No. 26, 2007
Figure 5. Photoinduced current generation of RuL2(NCS)2/DAPV/ ITO (b) and RuL2(NCS)2/DAPV/SWCNTs/ITO (O).
Lee et al. acid (HPA, H3PMo12O40) was adsorbed on the amine layers in a 1 mM HPA/0.1 M HCl solution at room temperature for 10 min by formation of an acid-base complex.16,21-23 Then the excess HPA was carefully washed out with 0.1 M HCl solution several times. Cyclic voltammetry was performed with wellcleaned bare ITO in a 0.1 M HCl solution as electrolyte to obtain a baseline as a blank test. Then the cyclic voltammetry of HPA on the ITO electrode was aligned on the baseline, and the concentration of HPA on ITO was obtained from integration of the three reduction peaks. In order to measured photocurrent generation, sandwich-type photoelectrochemical cells were assembled with RuL2(NCS)2/ DAPV/SWCNTs/ITO and Pt-sputtered ITO electrode [electrolyte was 0.1 mol/L LiI and 0.05 mol/L I2 in CH3CN]. RuL2(NCS)2/DAPV/SWCNTs/ITO was illuminated by 100 mW/ cm2 white light with air mass 0 and 1.5 filters as a solar simulator in the presence of a water filter (450 W xenon lamp, Oriel Instruments), and photocurrent was measured with a Kiethley 2400 source meter. Results and Discussion
Figure 6. Action spectra of RuL2(NCS)2/DAPV/ITO and RuL2(NCS)2/ DAPV/SWCNTs/ITO.
filtered by a poly(tetrafluoroethylene) membrane with 100 nm pore size. The filtrate was washed with water by decantation to remove any remaining acid, followed by drying in an oven at 100∼110 °C. SWCNTs with terminal carboxylic acid group on ITO were prepared by spray-coating. Then the SWCNTs/ITO was sintered at 100∼110 °C. Formation of DAPV on SWCNTs/ITO and RuL2(NCS)2 on DAPV/SWCNTs/ITO. DAPV on SWCNTs/ITO and RuL2(NCS)2 on DAPV/SWCNTs/ITO were prepared as reported.16,21 The SAMs of DAPV on SWCNTs/ITO were prepared by immersing the ITO in 2 mM DAPV/0.1 M phosphate buffer (pH 7.0) at 40∼45 °C for 1 min followed by washing with water. The layer of RuL2(NCS)2 as sensitizer was formed by dip-coating DAPV/SWCNTs/ITO in 0.3 mM Ru complex-ethanol solution for 24 h. Uncomplexed Ru complexes were washed out by ethanol. Measurements. SWCNTs thin films were characterized by scanning electron microscopy (SEM, JEOL), optical transmittance (Varian, Cary 100), and Fourier transform infrared (FTIR, ATI Mattson, Genesis series FT-IR). The optical transmittance was measured at room temperature. The formation of DAPV and RuL2(NCS)2 was monitored by UV-vis absorption spectra (Varian, Cary 100) and cyclic voltammetry (BAS 100B, Bioanalytical Systems, Inc.). The UV-vis spectrum and FTIR spectra was measured at room temperature. Electrochemical measurements were performed in a single compartment with a standard three-electrode glass cell. A reference electrode used Ag/AgCl and a counterelectrode used Pt wire. The area of an ITO electrode exposed to the solution was maintained at a constant 0.5 cm2. The aqueous phase measurements were carried out under nitrogen conditions. In order to calculate the concentration of DAPV, phosphomolybdic
Preparation and Characterization of SWCNT Films. CNTs are an attractive material for the application in photosensors, field emission, secondary-electron devices, and optoelectronic devices.18-20 Conductive CNTs usually have high values of conductivity and transmittance.24,25 Such properties of CNTs facilitate electron transfer to electrode.25 The SWCNTs on ITO retain high transparency toward the near-IR part of the electromagnetic spectrum. The network of SWNTs can be tuned for an appropriate thickness and density, thus controllable for an optimal optical transparency. As a result, SWCNT films on ITO can be used as transparent electrodes for a variety of application. SWCNT thin films on ITO were prepared by spray-coating. Figure 1 shows characterization of SWCNTs by SEM, optical transmittance, and FT-IR. Figure 1a shows a SEM image of SWCNTs bundles on ITO. These have diameters of 25∼35 nm, lengths of 200∼600 nm, and thicknesses of 30∼70 nm. The surface morphology of SWCNTs with terminal carboxylic acid groups showed heterogeneous film formation of SWCNTs films on ITO. Figure 1b) shows the wavelength dependence of the optical transparency of SWCNTs on ITO in the visible region. For photovoltaic cells, the optimal tradeoff between transparency and sheet resistance will vary depending on the intrinsic current-voltage characteristics of the device. In the XPS spectrum of SWCNTs, the characteristics C1s peak was observed at 284.6 eV. The binding energies were calibrated by taking the carbon C 1s peak (284.6 eV) as reference. The terminal carboxylic acid groups of the SWCNTs were monitored by FT-IR spectra (Figure 1c): the bands at 1650 and 1725 cm-1 are assigned to CdO stretching mode of quinone groups, while the bands at 1581 and 1725 cm-1 are assigned to CdC double bonds located near the newly formed oxygenated groups.26 CdO stretching frequency of the carboxylic acid group was observed at 1725 cm-1. The carboxylic acid-modified SWCNTs provide a good starting point for the SAMs of DAPV. Formation of DAPV on SWCNTs/ITO and RuL2(NCS)2 on DAPV/SWCNTs/ITO. DAPV on SWCNTs/ITO was prepared by acid-base complex formation reaction. The degree of formation of DAPV layers on the ITO surface is strongly dependent on the soaking time of substrate, temperature, and solvent.16,17,21-23 Especially, the soaking time of substrate in diamine molecular solution has a strong influence upon formation of SAMs of diamine on ITO.16,17,22-23 When ITO substrate
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SCHEME 1: (a) Idealized Scheme of Device Architectures and (b) Energy-Level Arrangement of ITO, SWCNTs, DAPV, and RuL2(NCS)2a
a
Potential vs NHE.
is immersed in diamine solution for less than 1 min, the level of formation of diamine molecules on ITO is very minimal,23 and DAPVs are adsorbed mainly on SWCNTs. Figure 2 shows UV-vis spectra of DAPV/ITO and DAPV/ SWCNTs/ITO, which show adsorption of the viologen moiety with a maximum at 550 nm.16,21 UV-Visible spectra of DAPV/ SWCNTs/ITO with the baseline of SWCNTs/ITO showed the presence of DAPV. The strong UV-vis absorption indicated excellent formation of DAPV layers on the SWCNTs/ITO surface. The absorption amounts of DAPV on SWCNTs/ITO were very similar to those of DAPV on ITO, which indicated that surface concentration of DAPV on SWCNTs/ITO was similar to that of DAPV on ITO. Formation of the second layers of DAPV on SWCNTs with terminal carboxylic acid is a fast process due to the acid-base complex formation reaction and was completed in 1 min. SAMs of DAPV on ITO surface was not formed at 1 min. As a result, diamine of DAPV reacted only to a carboxylic acid of SWNT at 1 min.
Surface concentrations of DAPV were monitored by cyclic voltammetry of HPA adsorbed on the amine layers by formation of an acid-base complex. HPA had good redox properties on ITO. By integration of the reduction peak current, the surface concentration can be calculated. Figure 3 shows cyclic voltammograms of HPA on modified ITO: (a) sequential reduction of HPA; (b) ITO/DAPV/HPA, ITO/SWCNTs/HPA, and ITO/ SWCNTs/DAPV/HPA; and (c) ITO/DAPV and ITO/SWCNTs. Without SWCNT modification on ITO, the surface concentration of HPA on DAPV/ITO was calculated to be 1.13 × 10-10 mol/ cm2 by integration of the reduction peak. The peak at 400 mV is dependent on the pH condition of the electrolytes. The pH dependence suggests the presence of small amounts of [HPMo12O40]2- on the amine functional groups. In the presence of the SWCNTs layers, the surface concentration of HPA on DAPV was on the same order of magnitude of 1.20 × 10-10 mol/cm2. The size of spherical HPA of the Keggin structure was approximately 10∼12 Å,16,17,22,27 and the theoreti-
9114 J. Phys. Chem. C, Vol. 111, No. 26, 2007 cal concentration of the HPA monolayer on a flat surface is 1.7 × 10-10 mol/cm2.22,28 Regarding the surface roughness, the surface concentration of 1.20 × 10-10 mol/cm2 indicated complete monolayer coverage of HPA on DAPV/SWCNTs/ITO. The ratio of HPA/DAPV was 1/3, suggesting that three amine functional groups supported one HPA anion molecule in a regular fashion.16,17,22 As a result, surface concentrations of DAPV/ITO and DAPV/ SWCNTs/ITO are 3.39 × 10-10 and 3.64 × 10-10 mol/cm2, respectively. As an organometallic photosensitizer, RuL2(NCS)2 is also easily incorporated in the DAPV/SWCNTs on ITO by another acid-base complex formation reaction.16,21 Formation of RuL2(NCS)2 on DAPV/SWCNTs/ITO was also monitored by UV-vis spectra. Figure 4 shows the UV-vis adsorption of Ru complexes with maximum absorption at 520 nm,16,21,25 which indicated good formation of RuL2(NCS)2 on DAPV/SWCNTs/ ITO. UV-Visible spectra of RuL2(NCS)2/DAPV/SCWNTs/ITO were measured with the baseline of DAPV/SWCNTs/ITO. The excellent formation of RuL2(NCS)2 on DAPV/SWCNTs/ITO provides a good starting point for photocurrent generation. The surface concentrations of RuL2(NCS)2 on DAPV/ITO and DAPV/SWCNTs/ITO were characterized to give 3.79 × 10-10 and 3.82 × 10-10 mol/cm2, respectively. This was confirmed by the absorbance measurements of the desorbed Ru(II) from DAPV in 1 mM KOH. The surface ruthenium concentrations of the RuL2(NCS)2 on DAPV/ITO and DAPV/SWCNTs/ITO are independently measured by cyclic voltammetry to give 3.51 × 10-10 and 3.68 × 10-10 mol/cm2, respectively. In the presence of DAPV/SWCNTs, the surface concentration of RuL2(NCS)2 is also similar to that of DAPV on ITO. This result is a wellmatched cyclic voltammogram of HPA adsorbed on amine layers (Figure 3). Photoinduced Charge Transfer in Photoelectrochemical Cells. For efficient photoinduced charge transfers, sequential arrangements of LUMO energy level are essential. Scheme 1 show (a) idealized photoinduced charge transfer and (b) idealized energy-level arrangement of ITO, SWCNTs, viologen and RuL2(NCS)2 (potential vs normal hydrogen electrode, NHE). The LUMO of RuL2(NCS)2 is at -0.6 eV,16,21 and the Fermi level of SWCNTs is at 0.2 eV.29 The reduction potential of viologen is -0.4 eV16,21 and effectively bridges the energy gap between RuL2(NCS)2 and SWCNTs/ITO. The proper energylevel alignment will facilitate electron transfers from the LUMO of Ru complex to SWCNTs/ITO electrode. It is expected that, in the presence of a SWCNT layer between DAPV monolayers and ITO electrode, recombination of photoinduced electrons with oxidized sensitizer in addition to I3- in the electrolyte would be decreased due to the energy level cascade. A sandwich-type photoelectrochemical cell was assembled with RuL2(NCS)2/DAPV/SWCNTs on ITO and Pt-sputtered ITO substrate [electrolyte was 0.3 M LiI and 30 mM I2 in CH3CN, 0.09 cm2]. The RuL2(NCS)2/DAPV/SWCNTs films (1.5 × 2.5 cm2) were illuminated by 100 mW/cm2 white light with air mass 0 and 1.5 filters as a solar simulator. As shown in Figure 5, the RuL2(NCS)2 on DAPV/SWCNTs/ITO and DAPV/ ITO system generated outstanding photocurrent upon illumination. The photocurrent increased immediately upon illumination and returned back to its original position when the light is off. The photocurrents of the RuL2(NCS)2 on DAPV/ITO and DAPV/SWCNTs/ITO showed 5.6 and 10.8 nA/cm2, respectively, with similar amounts of Ru sensitizer. As shown in Figure 6, the photocurrent spectra follow the profile of the absorption spectrum of RuL2(NCS)2 (Figure 4), which indicates that it originates from the excitation of the Ru
Lee et al. complex. No photocurrent is generated in the absence of I-/I3-/ CH3CN as electrolyte. The results indicate that the recombination reaction is reduced in the presence of SWCNTs. The generated photocurrent is impressively high, J(λ490) ) 10.3 nA/ cm2 for the RuL2(NCS)2/DAPV/SWCNTs/ITO. The photocurrent was raised to 83.9% in the presence of SWCNTs due to retardation of recombination reaction. Conclusions In conclusion, the RuL2(NCS)2/DAPV/SWCNTs/ITO system was prepared and applied to photocurrent generation. SWCNTs on ITO were prepared by spray-coating. DAPV and RuL2(NCS)2 were easily prepared by acid-base complex formation reaction. The energy levels of RuL2(NCS)2, viologen, SWCNTs, and ITO were well arranged, and the system forms an efficient LUMO level cascade. The photocurrent of RuL2(NCS)2/SWCNTs/ITO samples showed 10.3 nA/cm2 under A.M 1.5 condition (I ) 100 mW/cm2) in the presence of I-/I3-/CH3CN, which were raised to 83.9% in comparison with RuL2(NCS)2/DAPV/ITO without SWCNTs due to retardation of recombination reaction in photochemical cells. Investigations into the application of RuL2(NCS)2/DAPV/SWCNTs to optoelectronic devices are currently underway. Acknowledgment. This work was supported by the Korean Science and Engineering Foundation (ABRL R14-2003-01401001-0) W.L. thanks Brain Korea 21 for the award of financial support. Note Added after ASAP Publication. A corresponding author has been added. This paper was originally posted on June 7, 2007. It was reposted on June 11, 2007. References and Notes (1) Baron, R.; Huang, C.-H.; Bassani, D. M.; Onopriyenko, A.; Zayats, M.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 4010. (2) Yamada, H.; Imahori, H.; Nichimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D. H.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129. (3) Cho, Y.-J.; Ahn, T. K.; Song, H.; Kim, K. S.; Lee, C. Y.; Seo, W. S.; Lee, K.; Kim, S. K.; Kim, D. O.; Park, J. T. J. Am. Chem. Soc. 2005, 127, 2380. (4) Gra¨tzel, M. Nature 2001, 414, 338. (5) Banerjee, I. A.; Yu, L.; Matsui, H. J. Am. Chem. Soc. 2003, 125, 9542. (6) Liu, N.; Chen, Z.; Dunphy, D. R.; Jiang, Y. B.; Assink, R. A.; Brinker, C. J. Angew. Chem., Int. Ed. 2003, 42, 1731. (7) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071. (8) Sortino, S.; Petralia, S.; Conoci, S.; Bella, S. D. J. Am. Chem. Soc. 2003, 125, 1122. (9) Mirkin, C. A. Inorg. Chem. 2000, 39, 2258. (10) Sheeney-Haj-Ichia, L.; Basnar, B.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 78. (11) Mane, R. S.; Lee, W. J.; Pathan, H. M.; Han, S.-H. J. Phys. Chem. B 2005, 109, 24254. (12) Roh, S.-J.; Mane, R. S.; Min, S.-K.; Lee, W.-J.; Lokhande, C. D.; Han, S.-H. Appl. Phys. Lett. 2006, 89, 253512. (13) Kim, S.-S.; Yum, J.-H.; Sung, Y. E. Sol. Energy Mater. Sol. Cells 2003, 79, 495. (14) O’Regan, B. C.; Scully, S.; Mayer, A. C.; Palomares, E.; Durrant, J. J. Phys. Chem. B 2005, 109, 4616. (15) Du¨1rr, M.; Rosselli, S.; Yasuda, A.; Nelles, G. J. Phys. Chem. B 2006, 110, 21899. (16) Hyung, K.-H.; Kim, D.-Y.; Han, S.-H. New J. Chem. 2005, 29, 1022. (17) Hyung, K.-H.; Han, S.-H. Synth. Met. 2003, 137, 1411. (18) Lim, J. K.; Yoo, B. K.; Yi, W.; Hong, S.; Park, H.-J.; Chun, K.; Kim, S. K.; Woo, S.-W. J. Mater. Chem. 2006, 16, 2374. (19) Heo, J. N.; Lee, J. H.; Jeong, T. W.; Lee, C. S.; Kim, W. S.; Jin, Y. W.; Kim, J. M.; Yu, S. G.; Yi, W. K.; Park, S. H.; Oh, T. S.; Yoo, J. B. Appl. Phys. Lett. 2005, 87, 114105.
SWCNTs Improved Photocurrent in Photosensors (20) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. J. Am. Chem. Soc. 2000, 122, 2383. (21) Lee, W.; Hyung, K.-H.; Kim, Y.-H.; Cai, G.; Han, S.-H. Electrochem. Commun. 2007, 9, 729. (22) Oh, S.-Y.; Yun, Y.-J.; Kim, D.-Y.; Han, S.-H. Langmuir 1999, 15, 4690. (23) Oh, S.-Y.; Yun, Y.-J.; Hyung, K.-H.; Han, S.-H. New J. Chem. 2004, 28, 495. (24) Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H.-J.; Dennler, G.; Sariciftci, N. S.; Hu, L.; Gruner, G. Appl. Phys. Lett. 2006, 88, 233506.
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