14658
J. Phys. Chem. C 2007, 111, 14658-14663
Study on Electrophoretic Deposition of Size-Controlled Quinacridone Nanoparticles Hyeon-Gu Jeon, Teruki Sugiyama,† Hiroshi Masuhara,† and Tsuyoshi Asahi* Department of Applied Physics, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ReceiVed: June 4, 2007; In Final Form: July 26, 2007
We have prepared quinacridone (QA) colloids with various particle sizes (25-120 nm) and different crystalline phases by nanosecond laser ablation of its microcrystalline powder dispersed in water and fabricated the prepared colloidal particles into homogeneous films on an indium-tin-oxide electrode by an electrophoretic deposition (EPD) method. The nanoparticle-assembled films consist of closely packed nanoparticles and many nanosized pores. We evaluated the porous nature of the film by scanning electron microscopy observation and by measuring the film thickness dependence of the absorption. It was found that the film morphology is sensitive to the applied bias, and the optimum bias for the QA colloid was determined to be about 10 V/cm. We also demonstrated that organic thin films with different grain sizes and crystalline phases can be fabricated arbitrarily by combining the EPD method with nanoparticle preparation by laser ablation.
Introduction
CHART 1: Molecular Structure of QA
The construction and manipulation of organic nanostructures have been a key issue in research fields of molecular electronics and photonics. Many kinds of nanostructures have been produced by molecular deposition techniques utilizing individual molecules as building blocks, for example, thin films by vacuum vapor deposition and spin-coating, molecular layers by chemical self-assembly methods, and Langmuir-Blodgett films.1-4 A promising alternative approach could be to use nanoparticles as building blocks of nanostructures. Indeed, in the past decade, studies on organic nanoparticles of aromatic and functional dye molecules gradually have been emerging. There are many reports on the nanosize effects for organic materials such as fluorescence emission enhancement,5,6 change in absorption and fluorescence spectra depending on size,7,8 enhanced nonlinear optical responses,9,10 and increased photoconductivity.11,12 Although photoconductive properties are especially interesting in their application to optoelectronic devices such as photodetectors and solar cells,11-15 only a few studies were reported on the electronic and optoelectronic properties of organic nanoparticles as compared to their optical properties. This is mainly owing to the fact that the fabrication of required nanoparticle assemblies on the electrode surface has not been elucidated thoroughly for organic materials. The most popular methods for organic nanoparticle fabrication are reprecipitation,5, 8,16-18 microwave irradiation,9,19 sol-gels,20 and laser ablation.21-26 Size-controlled nanoparticles in the range of 10-300 nm have been obtained from various organic materials such as metallo-phthalocyanines,11,13,23,24 other pigments,17,21 polydiacetylene,27 fullerene,28,29 and dimethylaminostilbazolium tosylate.30 The prepared nanoparticles are typically colloidal dispersion in aqueous or nonaqueous media. Because nanoparticle colloids are usually charged, electrical manipulation should be fruitful for preparing organic nanoparticle assemblies on an electrode. Indeed, electrophoretic deposition (EPD) is a well-known method to construct colloidal films and has been * Corresponding author. Tel.: +81-6-6879-7838; fax: +81-6-6879-7840; e-mail:
[email protected]. † Present address: Hamano Life Science Research Foundation, TRI-305, 1-5-4, Minatojima-minami, Chuo-ku, Kobe 650-0047, Japan.
used in a variety of systems.31 Silica and latex particles,32-34 noble metals,35,36 CdTe,37 and TiO2 nanoparticles38 were deposited electrophoretically from the colloidal suspension. There are also a few reports on organic nanoparticles;39-42 however, no systematic study has been reported on the EPD of organic nanoparticles whose size and polymorphism are regulated. In this paper, we describe the fabrication of quinacridone (QA) (Chart 1) nanoparticle films on an indium-tin-oxide (ITO) electrode by EPD from its aqueous dispersion, which was prepared by laser ablation of QA microcrystalline powder in pure water utilizing a nanosecond YAG laser as a laser source. In the nanoparticle preparation method, a suspension of a target bulk material directly converts into its colloidal solution.21-26 Furthermore, the prepared colloid could be a simple mixture of organic nanoparticles and water since surfactants or organic solvents are not used. EPD is a process where charged colloidal particles move toward and deposit onto an oppositely charged electrode in a dc field.31,43,44 Advanced research on silica and polystyrene particles demonstrated that the deposition speed and morphology of nanoparticle aggregates on the electrode change with applying voltage (strength of the dc field) and electrophoretic deposition time.32-34 We examined here the effect of the applying voltage and deposition time on the morphology and thickness of QA nanoparticle films. By optimizing EPD conditions, we successfully prepared thin films consisting of 3-D packed QA nanoparticles that covered the ITO electrode’s surface uniformly over a 1 cm scale area. On the other hand, EPD has some advantages in the preparation of organic thin films when compared to conventional vacuum deposition and spin-coating methods, by which the grain size and polymorphism of films is hardly controlled in general. EPD would provide nanoparticle films with different grain sizes and crystalline phases by utilizing
10.1021/jp074300f CCC: $37.00 © 2007 American Chemical Society Published on Web 09/18/2007
Size-Controlled Quinacridone Nanoparticles
Figure 1. Experimental setup for the electrophoretic deposition of QA nanoparticles.
nanoparticle colloids having different particle sizes and crystalline phases. In the present work, we prepared QA nanoparticles with different mean sizes in the range of 25-120 nm and with a different absorption spectrum by changing the laser fluence and wavelength. Using the colloidal samples, we demonstrated a variety of QA films having different grain sizes and polymorphisms. Experimental Procedures Preparation of QA Nanoparticle Colloids. QA was purchased from Tokyo Kasei Kogyo Co. Ltd and used without further purification. The detailed procedure of nanoparticle preparation by laser ablation in water has been reported elsewhere.24,25 The micrometer-sized crystalline QA powder (25 or 10 mg) was added into water (100 mL), and the mixture was sonicated for 1 h. The mixture (3.0 mL) was put into a 1 cm × 1 cm × 5 cm quartz cuvette and irradiated by the third harmonic (355 nm) or the second harmonic (532 nm) of the nanosecond YAG laser (7 ns fwhm, 10 Hz repetition rate). The laser beam diameters were about 5.5 and 6.2 mm for 355 and 532 nm lasers, respectively. The laser fluence was changed from 40 to 150 mJ/cm2 using a grand laser prism. Absorption spectra of the supernatant before and after laser irradiation were measured with a UV-vis spectrophotometer (Shimadzu, UV3100PC). For examining the morphology and size of the prepared nanoparticles, a drop of the supernatant after laser irradiation was spread on a surface-modified silicon substrate, dried at 60 °C for 15 min, and then observed using SEM (FEI Strata, DB235-31). The ζ potential of the prepared colloids was measured by a ζ potential analyzer (Malvern Instruments, Zetasizer Nano ZS). Electrophoretic Deposition of Nanoparticles. Figure 1 shows the schematic experimental setup of our EPD cell. ITOcoated glass substrates (CBC Optics, 50 mm × 7 mm × 1.1 mm, 14 Ω/cm2) and stainless steel (SUS) plates (50 mm × 7 mm × 0.3 mm), which were used as an anode and a cathode, respectively, were carefully cleansed by sonication in acetone, ethanol, and deionized water sequentially at room temperature for 30 min. Then, the ITO-coated glass substrate was cleansed in trichloroethylene at 55 °C for 30 min and rinsed sequentially with acetone, ethanol, and deionized water and finally dried by nitrogen gas blowing. A pair of parallel electrodes with 3 mm spacing was set vertically in a cubic glass cuvette (1 cm × 1 cm × 4.5 cm) containing a nanoparticle colloid, and then a constant dc voltage was applied between two electrodes with a dc power supply (KENWOOD, PR18-5A). The voltage was
J. Phys. Chem. C, Vol. 111, No. 40, 2007 14659
Figure 2. Absorption spectra of QA nanoparticle films deposited by applying (a) 1 V, (b) 2 V, (c) 3 V, and (d) 4 V for 20 min and (e) absorption of a QA colloidal solution (1.6 × 10-2 wt %).
changed from 1 to 4 V, and the voltage applying time was varied from 10 to 30 min. After EPD, the ITO substrate was carefully withdrawn from the glass cuvette and dried at room temperature for 1-2 h. Film Characterization. Absorption spectra of the prepared films were measured similarly as stated previously. The thickness of the deposited film was measured with tapping mode atomic force microscopy (AFM, DI, NanoScope III). After making a scratch on the film, the AFM images were measured in an area of 30 µm × 30 µm. The height of the film surface from the substrate was analyzed from the cross-sectional profiles at several different positions. The surface morphology of the prepared film was observed with SEM (FEI Strata, DB235-31) after coating a gold layer (e10 nm thickness) onto the surface with an ionic coater (Eiko, IB‚3). Results and Discussion Formation of QA Nanoparticle Films by EPD. In this section, we describe the details of QA nanoparticle deposition from its colloidal solution by our EPD method. QA is one of the most important classes of organic pigments with a high stability to light and has three polymorphic crystalline phases. In practice, the β-form QA (red-violet) and the γ-form QA (crimson) are well-known as stable phases. A QA colloid with a mean diameter of 70 nm was prepared by irradiating a QAwater mixture (1.0 × 10-2 wt %) with 355 nm nanosecond laser pulses at 80 mJ/cm2 for 30 min. The absorption spectrum is shown in Figure 2. The spectrum with the peak at 580 nm corresponds to a characteristic absorption of the β-form QA nanoparticle.25 The prepared nanoparticle colloid was very stable without any surfactant, and the absorbance of the colloid was decreased by only 7% after 3 days, which is consistent with the high ζ potential (-34 mV) of the colloid particles. The large surface charge of the colloid nanoparticle is very important because EPD takes place under conditions where colloid particles are stably dispersed without an electric field and the particles begin to assemble into a film only upon applying an electric field.31,33,34,43,44 The QA nanoparticle colloidal solution (1.6 × 10-3 wt %) was put into the EPD cell (Figure 1), and a dc voltage was applied between the two electrodes. When the voltage was 3 or 4 V, a density gradient of the QA colloid between two electrodes was observed in a few minutes with the naked eye as a color gradient. The solution near the anode turned deep red, while near the cathode, it turned pale pink. The color gradient became clear with increasing the deposition time, and then a concen-
14660 J. Phys. Chem. C, Vol. 111, No. 40, 2007
Figure 3. SEM images of QA nanoparticle assemblies deposited by applying (A) 1 V, (B) 2 V, (C) 3 V, and (D) 4 V for 20 min. The inset in panel C shows clearly that the prepared homogeneous film consists of closely packed nanoparticles. In panel D, the left part indicates the poorly deposited region corresponding to a stripe in the photograph (Figure 4B).
Jeon et al.
Figure 5. Absorption spectra of QA nanoparticle films deposited from 6.6 × 10-3 wt % QA colloidal solution by applying 3 V for (a) 10 min, (b) 20 min, and (c) 30 min. The spectrum of the solution is given in spectrum d. The inset is the plot of the voltage applying time vs OD at 580 nm for those films.
Figure 4. Photographs of films deposited for 20 min by applying (A) 3 V and (B) 4 V, respectively.
trated QA colloid layer formed near the anode in about 10 min. Just after withdrawing the electrode from the solution, the QA nanoparticle layer was so weakly attached to the electrode that it was easily removed from the substrate surface even by just dipping in water. The particles were more closely packed to each other and tightly bounded to the substrate during the drying process in air. Applying Voltage Dependence of Film Morphology. Figure 2 presents the absorption spectra of nanoparticle films prepared by applying different voltages for 20 min. The films at 3 and 4 V show the same spectrum as the original colloid. At lower voltages, on the other hand, the film absorption is negligibly small, which means that there was quite a small amount of deposited nanoparticles. Figure 3 shows the SEM images of the nanoparticle assemblies on the anode surface. At 1 V, there were small nanoparticle aggregates less than 1 µm on the surface, and at 2 V, the nanoparticles covered a larger area of the surface, but not fully. Nanoparticle films covering the whole
Figure 6. (A) Absorption spectra of QA nanoparticle films with thicknesses of (a) 74 nm, (b) 128 nm, (c) 175 nm, and (d) 203 nm. (B) Relations between the absorbance and the thickness of various films fabricated from different colloids that were prepared by irradiation of a 355 nm laser at (a) 40 (4), (b) 80 (b), and (c) 150 (0) mJ/cm2 fluence. For each, linear fitting is represented by different lines. A solid line (d) presents the slope of a completely packed film.
anode surface were produced exclusively at higher voltages. At 3 V, especially, the film morphology was quite homogeneous, as is discussed later. The effect of the applying voltage on nanoparticle deposition shows that a sufficiently strong electric field is indispensable for film formation on an electrode in EPD.32,43,44 Negatively charged QA colloids are forced to move toward the anode by electrophoretic force, the nanoparticle concentration near the anode surface increases more and more with deposition time,
Size-Controlled Quinacridone Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 40, 2007 14661
TABLE 1: Mean Particle Size of QA Colloids Prepared by Laser Ablation and Laser Irradiation Conditions wavelength (nm)
355
fluence (irradiation time) (mJ/cm2, min) mean particle size (nm)
80(90) + 150(60)a 30
a
532 80(90) 70
40(100) 120
150(60) 25
100(80) 35
80(90) 55
40(100) 110
355 nm laser irradiation at 80 mJ/cm2 fluence for 90 min, followed by irradiation at 150 mJ/cm2 fluence for 60 min.
and then the particles aggregate into a film on the substrate. However, since the colloidal particles were equally charged, they repel each other electrostatically and then do not aggregate on the electrode surface under a weak electric field condition. The mechanism of particle aggregation on the electrode in the EPD method has been investigated for silica and latex particles, and the phenomenon was explained by an electrohydrodynamic process.32,43,44 We considered that a similar process should take place in the case of the QA colloid. Figure 2 shows that the film deposited at 4 V has a larger absorbance than the film deposited at 3 V, which indicates that the former is thicker than the latter. However, the morphology of the former film turned rough, and many stripes formed on the surface as shown in Figure 4. Inhomogeneous aggregates of nanoparticles near the stripes were observed as shown in the SEM image (Figure 3D). One of the possible reasons for the surface roughening was due to the electrochemical reaction of water at the surface of the electrode, on which H3O+ and O2 are generated. At a higher voltage, an excess of the H3O+ ion and O2 induced the lateral convection of water on the surface and/or create small gas bubbles, which should disturb the formation of a homogeneous film by the deposition of nanoparticles onto the electrode.33,41 Consequently, in the case of the QA nanoparticle deposition, the optimum voltage value was about 3 V to prepare a homogeneous thin film where nanoparticles are densely packed. This voltage corresponds to a dc electric field of 10 V/cm, and it is almost the same value as in the case of the polystylene latex, CdTe, silica, and gold colloids.35,37,38 Deposition Time Dependence of Film Thickness. Figure 5 shows the absorption spectra of the nanoparticle films deposited from a QA colloid (6.5 × 10-3 wt %) with a mean size of 70 nm by applying a dc voltage of 3 V for 10, 20, and 30 min. The thicknesses of each film were 85, 110, and 125 nm, respectively. It is clear that the absorbance of the film (i.e., the film thickness) increases with deposition time. However, the dependence shows a saturation behavior, which is mainly owed to the decrease of the concentration of nanoparticles in water.45 The absorbance of the 125 nm film was about a quarter of that of the original colloid in a 1 cm path length cell. This means that 80% of colloid nanoparticles between the electrodes (3 mm spacing) was deposited on the anode surface. Indeed, by using a high concentration colloid (1.6 × 10-2 wt %), we obtained a film thicker than 200 nm in a deposition time of 20 min. We prepared deposited films with different thicknesses ranging from 70 to 200 nm by changing the voltage applying time and the concentration of the colloid and compared their absorption spectra. Figure 6 shows the absorption spectra of four representative films and the relation between the absorbance at the 580 nm wavelength and the film thickness measured by AFM observation. It is clearly confirmed that the absorbance is proportional to the film thickness, which allows us to evaluate the film thickness by absorption measurements. QA Nanoparticle Film with Different Particle Sizes and Crystalline Phases. As mentioned in the Introduction, QA colloid nanoparticles with different sizes and crystalline phases can be produced by laser ablation.21 We prepared differently sized nanoparticles with the same polymorph by changing the
fluence of the 355 nm nanosecond pulses. Table 1 shows the mean size of the prepared colloids under different laser irradiation conditions. The absorption spectra of all colloid particles, regardless of their sizes, are similar to the spectrum in Figure 2, which means that the formed nanoparticles are β-form crystals. Figure 7 shows the photographs and SEM images of the deposited films from different colloids prepared at 40 and 150 mJ/cm2 fluences, respectively. Each film is very homogeneous over the centimeter scale. Furthermore, it is obvious that the grain sizes in the films differ from each other, depending on the particle size. This result means that the grain size of a deposited film can be varied by changing the particle size of the colloid nanoparticles. When looking at the SEM images at higher magnification (inset in Figures 3C and 7), many holes are observed on the film surface, which suggests that deposited films have a porous structure. We evaluated here the porosity of the films from the relation between optical absorption and film thickness. The absorbance at 580 nm increases linearly with the film thickness for each film with different particles sizes, while the slope of absorbance versus thickness depends on the particle size. From the film thickness dependence of absorbance at 580 nm in Figure 6B, we estimated the slopes to be 4.1 × 104, 4.2 × 104, and 4.9 × 104 cm-1 for the films of particle sizes 120, 68, and 30 nm, respectively. We also calculated the slope for a bulk thin film to be 5.8 × 104 cm-1 by using the absorption coefficient of the QA nanoparticle (1.4 × 104 M-1 cm-1) and the density (4.1 × 10-3 mol/cm3) of the β-form QA.21 The solid line in Figure 6B represents the slope for the bulk film, which is steeper than that of the nanoparticle films. This means a smaller density of nanoparticle films. Because the absorption spectrum of the nanoparticle film is in good agreement with that of bulk QA, the molecular packing manner in the nanoparticle should be the same as that of the bulk solid state. Therefore, we can
Figure 7. Photographs (A and B) and SEM images (C and D) of films deposited from the QA colloids prepared by irradiation of 355 nm laser pulses at (A and C) 40 mJ/cm2 and (B and D) 150 mJ/cm2 fluences, respectively.
14662 J. Phys. Chem. C, Vol. 111, No. 40, 2007
Jeon et al.
Figure 8. Absorption spectra of (A) QA nanoparticle colloids prepared by irradiating laser pulses with a 532 nm wavelength at different fluences, from 40 to 150 mJ/cm2 and (B) QA nanoparticle films deposited from each colloid.
Figure 9. SEM images of a film deposited from the QA colloids prepared by a 532 nm laser at (A) 40 mJ/cm2 and (B) 100 mJ/cm2 fluences, respectively.
conclude that the nanoparticle film has many pore sites inside, namely, a porous film formed by 3-D packing of nanoparticles. The discrepancy of the slope, on the other hand, would be ascribed to the deference in film porosity depending on the particle. Thus, we can estimate the volume ratios of the pore to be 30, 26, and 16 vol % for the films of particle sizes 120, 68, and 30 nm, respectively. We also prepared polymorphous QA nanoparticles by 532 nm laser irradiation at different fluences in the range of 40150 mJ/cm2. Figure 8 A shows the absorption spectra of the colloids. The spectral shape at lower fluences is the same as that of nanoparticles prepared with a 355 nm laser excitation, indicating β-form nanoparticles. The peak around 580 nm shifts to shorter wavelengths, and the intensity ratio of the two main peaks (580 and 530 nm) decreases with increasing the laser fluence. This spectral change upon laser fluence can be ascribed to a change in the crystalline phase. Thus, the spectrum at 150 mJ/cm2 shows a characteristic absorption of the γ-form QA nanoparticles,25 and the spectra at intermediate laser fluences are considered to be a mixture of the β-form and γ-form. The particle size, on the other hand, decreases with laser fluence, as is shown in Table 1. By using the colloid samples, QA nanoparticle films were prepared by EPD under the same experimental conditions as described previously. Figure 9 shows the SEM images of the films of QA colloids prepared by 532 nm laser irradiation at 40 and 150 mJ/cm2. As in the case of the QA colloid by 355 nm laser irradiation, we obtained thin films composed of densely packed nanoparticles with different particle sizes that covered the whole anode surface uniformly. It should be pointed out from SEM images of Figures 7D and 9B that the deposited film of 25 nm nanoparticles prepared by 532 nm irradiation shows a similar surface morphology to that of the 30 nm nanoparticles by 355 nm irradiation. On the contrary, the absorption spectrum
of each film differs from one other as shown in Figure 8B. From the absorption spectral shape, we can assign the former film to the γ-form QA and the latter one to the β-form. Furthermore, Figure 8 demonstrates that the absorption spectra of the films deposited from different nanoparticle colloids by 532 nm laser irradiation differ from each other, while the spectral shape is in good agreement with that of the corresponding colloid, respectively. These results confirm that both the crystalline phase and the particle size of the QA nanoparticles remain unchanged in the EPD process. In other words, in the film preparation by EPD, polymorphism of the film and/or its grain size change with varying the crystalline phase and/or the particle size of the colloidal nanoparticles. The control of polymorphism and grain size is very important in practical applications of organic thin films to electronic devices.46,47 The conventional method for controlling polymorphism is to modify the crystalline phase of the prepared films by a post-treatment procedure such as annealing and exposure to the organic solvent vapor.12,48,49 It is well-known, however, that such a post-treatment brings about changes in the film morphology such as the grain size or the roughness at the same time.12,48-51 We can conclude from the present results that the combination of the EPD method with nanoparticle preparation by laser ablation is a very useful method of organic film formation in view of controlling both the crystalline phase and the grain size. It should be noted that the nanoparticle-assembled film has many nanosized pores between particles. It is interesting to remember that this porous structure exhibits a potential application of the nanoparticle-assembled film to organic solar cells as an active layer. The overall solar cell efficiency depends on the efficiency of the charge carrier generation at the donoracceptor interface and on the efficiency of charge collection at the electrodes. Therefore, it is considered that a nanometerscale mixed structure of electron donors and acceptors is indispensable to achieve a high conversion efficiency.14,15,51-53 Such an interpenetrating interface structure of electron donors and acceptors could be constructed easily by introducing domains of a donor (or acceptor) material, such as a π-conjugated polymer, into the nanopore of the nanoparticle-assembled film.42 Moreover, because in the porous film nanoparticles are closely packed to each other, percolating paths for charge migration are considered to be already formed; hence, the charges are translated to the electrodes efficiently. Therefore, the porous nanoparticle film is considered to have a high possibility of improving the solar energy conversion efficiency.
Size-Controlled Quinacridone Nanoparticles Conclusion By combining two useful techniques, laser ablation and EPD, we have successfully demonstrated the fabrication of QA nanoparticle thin films on ITO-coated glass substrates. Their morphology was sensitive to the applying voltage in the EPD process, and the optimum electric field for preparing excellent QA nanoparticle films was confirmed at about 10 V/cm. Their thickness was variable in the range of 70-200 nm by changing the EPD time and concentration of the colloid, and the prepared films were homogeneous over almost the entire substrate surface on a centimeter-scale. SEM observation and absorption measurements showed that the film consists of size-regulated nanoparticles packed closely to each other three-dimensionally and that the film has a porous structure. In addition, we have prepared QA colloids having different particle sizes and crystalline phases and demonstrated that the polymorphism and grain size of EPD films can be controlled by varying the crystalline phase and/or the particle size of the colloidal nanoparticles. The EPD method has been widely used to assemble inorganic nanoparticles or micro-sized latex beads. However, to our knowledge, there is no report on the EPD of size-controlled and crystalline phase-controlled organic nanoparticles smaller than 100 nm. In this paper, we have demonstrated for the first time that the EPD method is very useful and fruitful for assembling organic nanoparticles homogeneously on an electrode surface irrespective of their particle size and polymorphism. We believe that the present nanoparticle-assembly technique is a useful tool to investigate the size dependence of the electronic and optoelectronic properties for organic nanoparticles. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research (KAKENHI) on Priority Areas “Molecular Nano Dynamics (2004-2006)” and by the Korea Science and Engineering Foundation Grant (M06-2004000-10502-0) to H.-G.J. References and Notes (1) Forrest, S. R. Chem. ReV. 1997, 97, 1793. (2) Zhang, X.; Jenekhe, S. A.; Perlstein, J. Chem. Mater. 1996, 8, 1571. (3) Decher, G. Science 1997, 277, 1232. (4) Ohkita, H.; Ogi, T.; Kinoshita, R.; Ito, S.; Yamamoto, M. Polymer 2002, 43, 3571. (5) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. (6) Li, S.; He, L.; Xiong, F.; Li, Y.; Yang, G. J. Phys. Chem. B 2004, 108, 10887. (7) Volkov, V. V.; Asahi, T.; Masuhara, H.; Masuhara, A.; Kasai, H.; Oikawa, H.; Nakanishi, H. J. Phys. Chem. B 2004, 108, 7674. (8) Fu, H.-B.; Yao, J.-N. J. Am. Chem. Soc. 2001, 123, 1434. (9) Nitschke, C.; O’Flaherty, S. M.; Kro¨ll, M.; Doyle, J. J.; Blau, W. J. Chem. Phys. Lett. 2004, 383, 555. (10) Tian, Z.; He, C.; Liu, C.; Yang, W.; Yao, J.; Nie, Y.; Gong, Q.; Liu, Y. Mater. Chem. Phys. 2005, 94, 444. (11) Chen, H. Z.; Jiang, K. J.; Wang, M.; Yang, S. L. J. Photochem. Photobiol., A 1998, 117, 149. (12) Jenekhe, S. A.; Yi, S. AdV. Mater. 2000, 12, 1274. (13) Zhang, X.; Wang, Y.; Ma, Y.; Ye, Y.; Wang, Y.; Wu, K. Langmuir 2006, 22, 344. (14) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45. (15) Hiramoto, M. Electron. Commun. Jpn., Part 2 2006, 89, 13. (16) Horn, D.; Rieger, J. Angew. Chem., Int. Ed. 2001, 40, 4330.
J. Phys. Chem. C, Vol. 111, No. 40, 2007 14663 (17) Kasai, H.; Oikawa, H.; Okada, S.; Nakanishi, H. Bull. Chem. Soc. Jpn. 1998, 71, 2597. (18) Al-Kaysi, R. O.; Mu¨ller, A. M.; Ahn, T.-S.; Lee, S.; Bardeen, C. J. Langmuir 2005, 21, 7990. (19) Baba, K.; Kasai, H.; Okada, S.; Oikawa, H.; Nakanishi, H. Jpn. J. Appl. Phys. 2000, 39, 1256. (20) Spagnoli, S.; Block, D.; Botzung-Appert, E.; Colombier, I.; Baldeck, P. L.; Ibanez, A.; Corval, A. J. Phys. Chem. B 2005, 109, 8587. (21) Tamaki, Y.; Asahi, T.; Masuhara, H. Appl. Surf. Sci. 2000, 168, 85. (22) Tamaki, Y.; Asahi, T.; Masuhara, H. J. Phys. Chem. A 2002, 106, 2135. (23) Tamaki, Y.; Asahi, T.; Masuhara, H. Jpn. J. Appl. Phys. 2003, 42, 2725. (24) Sugiyama, T.; Asahi, T.; Masuhara, H. Chem. Lett. 2004, 33, 724. (25) Sugiyama, T.; Asahi, T.; Takeuchi, H.; Masuhara, H. Jpn. J. Appl. Phys. 2006, 45, 384. (26) Kita, S.; Masuo, S.; Machida, S.; Itaya, A. Jpn. J. Appl. Phys. 2006, 45, 6501. (27) Oikawa, H.; Oshikiri, T.; Kasai, H.; Okada, S.; Tripathy, S. K.; Nakanishi, H. Polym. AdV. Technol. 2000, 11, 783. (28) Fortner, J. D.; Lyon, D. Y.; Sayes, C. M.; Boyd, A. M.; Falkner, J. C.; Hotze, E. M.; Alemany, L. B.; Tao, Y. J.; Guo, W.; Ausman, K. D.; Colvin, V. L.; Hughes, J. B. EnViron. Sci. Technol. 2005, 39, 4307. (29) Georgakikas, V.; Pellarini, F.; Prato, M.; Guldi, D. M.; MelleFranco, M.; Zerbetto, F. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5075. (30) Fujita, S.; Kasai, H.; Okada, S.; Oikawa, H.; Fukuda, T.; Matsuda, H.; Tripathy, S. K.; Nakanishi, H. Jpn. J. Appl. Phys. 1999, 38, 659. (31) Caruso, F. Colloids and Colloid Assemblies; Wiley-VCH: Weinheim, Germany, 2004; Ch. 14, pp 437-464. (32) Hayward, R. C.; Saville, D. A.; Aksay, I. A. Nature 2000, 404, 56. (33) Rogach, A. L.; Kotov, N. A.; Koktysh, D. S.; Ostrander, J. W.; Ragoisha, G. A. Chem. Mater. 2000, 12, 2721. (34) Holgado, M.; Garcia-Santamaria, F.; Blanco, A.; Ibisate, M.; Cintas, A.; Miguez, H.; Serna, C. J.; Molpeceres, C.; Requena, J.; Mifsud, A.; Meseguer, F.; Lo´pez, C. Langmuir 1999, 15, 4701. (35) Bailey, R. C.; Stevenson, K. J.; Hupp, J. T. AdV. Mater. 2000, 12, 1930. (36) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B 1999, 103, 3818. (37) Gao, M.; Sun, J.; Dulkeith, E.; Gaponik, N.; Lemmer, U.; Feldmann, J. Langmuir 2002, 18, 4098. (38) Gu, Z.-Z.; Hayami, S.; Kubo, S.; Meng, Q.-B.; Einaga, Y.; Tryk, D. A.; Fujishima, A.; Sato, O. J. Am. Chem. Soc. 2001, 123, 175. (39) Barazzouk, S.; Hotchandani, S.; Kamat, P. V. AdV. Mater. 2001, 13, 1614. (40) Kutner, W.; Pieta, P.; Nowakowski, R.; Sobczak, J. W.; Kaszkur, Z.; McCarty, A. L.; D’Souza, F. Chem. Mater. 2005, 17, 5635. (41) Jeon, H.-G.; Sugiyama, T.; Masuhara, H.; Asahi, T. Jpn. J. Appl. Phys. 2007, 46, 733. (42) Jeon, H.-G.; Ryo, S.; Sugiyama, T.; Oh, I.; Masuhara, H.; Asahi, T. Chem. Lett. 2007, 36, 1160. (43) Trau, M.; Saville, D. A.; Akay, I. A. Langmuir 1997, 13, 6375. (44) Solomentsev, Y.; Bo¨hmer, M.; Andeson, J. L. Langmuir 1997, 13, 6058. (45) Wong, E. M.; Searson, P. C. Chem. Mater. 1999, 11, 1959. (46) Law, K.-Y. Chem. ReV. 1993, 93, 449. (47) Hadziioannou, G.; van Hutten, P. F. Semiconducting Polymer Chemistry, Physics, and Engineering; Wiley-VCH: Weinheim, Germany, 2000. (48) Dimitrakopoulus, C. D.; Brown, A. R.; Pomp, A. J. Appl. Phys. 1996, 80, 2501. (49) E, J.; Kim, S.; Lim, E.; Lee, K.; Cha, D.; Friedman, B. Appl. Surf. Sci. 2003, 205, 274. (50) Mizuguchi, J.; Rihs, G.; Karfunkel, H. R. J. Phys. Chem. 1995, 99, 16217. (51) Peumans, P.; Uchida, S.; Forrest, S. R. Nature 2003, 425, 158. (52) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers; Oxford University Press: New York, 1999. (53) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693.
