Langmuir 2006, 22, 9431-9435
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Formation of a Perylenetetracarboxylic Diimide Network Film by Post Electrochemical Treatment Suk-ho Kim, Heung Cho Ko,† Bongjin Moon, and Hoosung Lee* Department of Chemistry, Sogang UniVersity, 1-1 Sinsoo-Dong, Mapo-Gu, Seoul 121-742, Korea ReceiVed May 8, 2006. In Final Form: August 17, 2006 Perylenetetracarboxylic diimide (PDI) derivatives bearing two or four peripheral pyrrole pendants (PDI-nPy, n ) 2 or 4) are cross-linkable materials by electro/phototreatment. In this paper, we introduce a new posttreatment technique to produce an insoluble film. Unlike the common solution-phase electrochemical deposition, we first spin-coated PDI-nPy on an electrode and then electrotreated the coated surface in a monomer-free electrolyte solution. This method gives the film a smooth surface with no granules, while the common method induces a rough film with a lot of granules. The post electrochemical treatment also provides a merit of higher resolution in a patterning process on a specific metal electrode. As one of the applications, we carried out an electrochromic study on the posttreated PDI-4Py film. It turned purple (λmax ) 590 nm) and sky blue (λmax ) 797 nm) at 0 and -1.9 V vs Ag/Ag+, respectively. We believe this method will broaden the patterning concept with the desired film morphology and resolution using PDI on a specific electrode.
Introduction In relation to device fabrication for electronic and/or photonic applications, one of the important issues is to develop facile and reliable coating processes with the given materials. This point becomes crucial especially when one of the materials is intractable for the coating process even though its physical characteristics are promising. For example, perylenetetracarboxylic diimide (PDI), which can be used for various photonic and electronic applications,1-14 belongs to this category because the pristine type is insoluble due to the strong π-stacking interactions between the perylene planes. This problem can be overcome by introducing various bulky functional groups at the 1-, 6-, 7-, and 12-positions and/or at the nitrogen positions of PDI.15-17 Another thing to be considered is to guarantee feasibility for various coating processes such as patterning a PDI film on a specific area and/or coating another organic layer on top of a precoated PDI film. One strategy * To whom correspondence should be addressed. E-mail: hlee@ sogang.ac.kr. Phone: 82-2-705-8446. Fax: 82-2-701-0967. † Present address: Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. (1) Gvishi, R.; Reisfeld, R.; Burshtein, Z. Chem. Phys. Lett. 1993, 213, 338. (2) Loewe, R. S.; Tomizaki, K.; Youngblood, W. J.; Bo, Z.; Lindsey, J. S. J. Mater. Chem. 2002, 12, 3438. (3) Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J.; Grimsdale, A. C.; Mu¨llen, K.; MacKenzie, J. D.; Silva, C.; Friend, R. H. J. Am. Chem. Soc. 2003, 125, 437. (4) Lee, S. K.; Zu, Y.; Herrmann, A.; Geerts, Y.; Mu¨llen, K.; Bard, A. J. J. Am. Chem. Soc. 1999, 121, 3513. (5) Lu, W.; Gao, J. P.; Wang, Z. Y.; Qi, Y.; Sacripante, G. G.; Duff, J. D.; Sundararajan, P. R. Macromolecules 1999, 32, 8880. (6) Takahashi, K.; Nakajima, I.; Imoto, K.; Yamaguchi, T.; Komura, T.; Murata, K. Sol. Energy Mater. Sol. Cells 2003, 76, 115. (7) Kudo, T.; Kimura, M.; Hanabusa, K.; Shirai, H. J. Porphyrins Phthalocyanines 1998, 2, 231. (8) Cormier, R. A.; Gregg, B. A. Chem. Mater. 1998, 10, 1309. (9) Gregg, B. A.; Cormier, R. A. J. Phys. Chem. B 1998, 102, 9952. (10) Struijk, C. W.; Sieval, A. B.; Dakhorst, J. E. J.; van Dijk, M.; Kimkes, P.; Koehorst, R. B. M.; Donker, H.; Schaafsma, T. J.; Picken, S. J.; van de Craats, A. M.; Warman, J. M.; Zuilhof, H.; Sudho¨lter, E. J. R. J. Am. Chem. Soc. 2000, 122, 11057. (11) Ko, H. C.; Lim, D. K.; Kim, S.; Choi, W.; Lee, H. Synth. Met. 2004, 144, 177. (12) Langhals, H. Heterocycles 1995, 40, 477. (13) Langhals, H. HelV. Chim. Acta 2005, 88, 1309. (14) Wu¨rthner, F. Chem. Commun. 2004, 14, 1564. (15) Seybold, G.; Wagenblast, G. Dyes Pigm. 1989, 11, 303. (16) Dotcheva, D.; Klapper, M.; Mu¨llen, K. Macromol. Chem. Phys. 1994, 195, 1905. (17) Ford, W. E.; Kamat, P. V. J. Phys. Chem. 1987, 91, 6373.
Figure 1. Chemical structures of PDI derivatives.
that can circumvent this issue is to employ functional groups that can be cross-linked by electro- or phototreatment. For example, there are several reports in which carbazole groups were employed as electrochemically cross-linkable groups.18,19 In our case, we chose pyrrole as a cross-linkable unit and developed several PDI-nPy series with different numbers (n ) 2 or 4) of peripheral pyrrole (Py) units as shown in Figure 1. To prepare an insoluble PDI-networked film, we used two methods.20,21 One is the electrochemical deposition method in which the film is deposited electrochemically from a solution phase.20 In this method, an insoluble film is formed by electrochemical polymerization of the conductive polymer precursors, i.e., Py units. There have been several other examples of PDI-containing films prepared by this method using PDI (18) Garcia-Belmonte, G.; Pomerantz, Z.; Bisquert, J.; Lellouche, J.-P.; Zaban, A. Electrochim. Acta 2004, 49, 3413. (19) Tanimoto, A.; Yamamoto, T. Macromolecules 2006, 39, 3546. (20) Choi, W.; Ko, H. C.; Moon, B.; Lee, H. J. Electrochem. Soc. 2004, 151, E80. (21) Ko, H. C.; Kim, S.; Choi, W.; Moon, B.; Lee, H. Chem. Commun. 2006, 69.
10.1021/la061295i CCC: $33.50 © 2006 American Chemical Society Published on Web 10/03/2006
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Figure 2. Procedure for generating a patterned PDI network film by post electrochemical treatment.
derivatives bearing two or four peripheral oligothiophene pendants.22,23 The other method is to cross-link the peripheral pyrrole units by photoirradiation after spin-coating soluble PDInPy on a substrate, namely, the postphotopolymerization method.21 Concerned with the film morphology, the first method gives a rough surface with a lot of granules, which is a common result of nucleation and growth during the electropolymerization.24-36 On the other hand, a smooth granule-free PDI-networked film can be obtained by the second approach. As an extended work of the postphotopolymerization method, here we report a new method to get a PDI-networked film by “post electrochemical treatment” of a spin-coated PDI film as shown in Figure 2. First, a metal electrode, (i.e., Au/Ti) patterned glass plate is spin-coated with a PDI-nPy solution, and the resulting PDI-nPy-coated film is subjected to electrochemical cross-linking in a “quasi-solid state”. Because only PDI-nPy on top of the metal electrodes will be cross-linked under these conditions, the remaining un-cross-linked PDI-nPy part can be removed by rinsing with an appropriate solvent in the final step, giving a PDI-patterned film. Compared with the common solutionphase electrochemical deposition, we envisioned that the post electrochemical treatment would give a better patterning resolution and film morphology. In this paper, we present that the film prepared by this method indeed has a very smooth surface with no granules and can be deposited on a patterned electrode with a higher resolution than that from the common method. As one of the applications, we also demonstrate the electrochromic behavior of the PDI-networked film prepared by this method. (22) You, C.-C.; Saha-Mo¨ller, C. R.; Wu¨rthner, F. Chem. Commun. 2004, 2030. (23) Chen, S.; Liu, Y.; Qiu, W.; Sun, X.; Ma, Y.; Zhu, D. Chem. Mater. 2005, 17, 2208. (24) Asavapiriyanont, S.; Chandler, G. K.; Gunawardena, G. A.; Pletcher, D. J. Electroanal. Chem. 1984, 177, 229. (25) Downard, A. J.; Pletcher, D. J. Electroanal. Chem. 1986, 206, 139. (26) Marcus, M. L.; Rodroguez, I.; Velasco, J. G. Electrochim. Acta 1987, 32, 1453. (27) Miller, L. L.; Zinger, B.; Zhou, Q. X. J. Am. Chem. Soc. 1987, 108, 2267. (28) Hillmann, A. R.; Mallen, E. F. J. Electroanal. Chem. 1987, 220, 351. (29) Hamnett, A.; Hillman, A. R. J. Electrochem. Soc. 1988, 135, 2517. (30) Hillmann, A. R.; Mallen,E. F. J. Electroanal. Chem. 1988, 243, 403. (31) Hillmann, A. R.; Swann, M. J. Electrochim. Acta 1988, 33, 1303. (32) Bade, K.; Sakova, W. T.; Schultze, J. W. Electrochim. Acta 1992, 37, 2255. (33) Mandic, Z.; Duic, L.; Kovacicek, F. Electrochim. Acta 1997, 42, 1389. (34) Liu, Y.-C.; Tsai, C.-J. Chem. Mater. 2003, 15, 320. (35) Liu, Y.-C.; Huang, J.-M.; Tsai, C.-E.; Chuang, T. C.; Wang, C.-C. Chem. Phys. Lett. 2004, 387, 155. (36) Hwang, B.-J.; Santhanam, R.; Wu, C.-R.; Tsai, Y.-W. J. Solid State Electrochem. 2003, 7, 678.
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Figure 3. Cyclic voltammograms of the PDI derivatives after post electrochemical treatment. The electrolyte solution was acetonitrile/ methylene chloride (1:1 by volume) containing 0.1 M Bu4NPF6. The scan rate was 50 mV s-1.
Experimental Details All chemicals were used as received in the highest commercially available grade unless described otherwise. Acetonitrile (Jin Chemical) and methylene chloride (Jin Chemical) were distilled over calcium hydride/P2O5 and calcium hydride, respectively. Water was distilled and deionized to a level of resistivity greater than 17 MΩ cm. The compounds in Figure 1 were synthesized as reported in the supplementary information of our previous paper.21 A patterned Au/Ti/glass (line width 55 µm, space 45 µm) substrate was prepared by conventional photolithography. Shipley 1805 photoresist (PR) was patterned on a glass slide, Ti/Au (3 nm/60 nm) were deposited by electron-beam evaporation, and the residual PR part was removed by lift-off in acetone. To prepare PDI-nPy-coated electrodes, a droplet of PDI-nPy solution (0.01 wt %) in chloroform was dropped on a glassy carbon (GC) disk electrode (active diameter 3 mm) or the solution was spin-coated at 1500 rpm for 30 s onto a Ti/Au-patterned or an ITOcoated glass (100 Ω/square, Delta-Technology, active diameter 10 mm). The films were gently dried using a heat gun. The thicknesses of all the PDI-nPy-coated films were adjusted to a level of ∼100 nm. Cyclic voltammetric experiments were carried out with an EG&G PAR 362 potentiostat. A three-electrode cell configuration was used for electrochemical treatment. A GC disk or an ITO-coated glass slide was used as a working electrode. Pt mesh or Pt coil was used as a counter electrode. Ag/AgCl (3 M NaCl) or Ag/Ag+ (0.01 M AgNO3) was used as a reference electrode. The reference electrode was fixed with a Luggin capillary. The Pt mesh was cleaned by immersion in a piranha solution (H2O2:H2SO4 ) 1:3 by volume) and rinsed with deionized water. The ITO surface was cleaned in boiling acetone and rinsed with isopropyl alcohol vapor. The electrochemical cells were deaerated by nitrogen bubbling before the experiment. Electronic absorption spectra were recorded on a spectrophotometer (Hewlett-Packard HP8453) with a photodiode array detector. A static electrochromic experiment was carried out using a spectrophotometer (S2000, Ocean Optics), which was interfaced with an AD converting card (ADC-1000, Ocean Optics). All the spectra in the static state were obtained when the current level decayed nearly to zero. The scanning electron microscopy (SEM) images of the film surface were obtained with an ESEM-FEG instrument (Philips XL30) at an acceleration voltage of 5 kV. Atomic force microscopy (AFM) measurements were carried out using a Digital Instrument Nanoscope IV-A in the height mode.
Results and Discussion To carry out post electrochemical treatment on a PDI-nPycoated GC electrode, we set up a three-electrode electrochemical
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Figure 4. (a) Electronic absorption spectra of PDI-nPy films before and after post electrochemical treatment at 1.4 V for 1 min. (b) Electronic absorption ratios of PDI-nPy films before and after post electrochemical treatment as a function of various potentials for 1 min and (c) as a function of treatment time at 1.4 V.
cell with the GC electrode and applied 1.4 V vs Ag/AgCl, which is high enough for pyrrole units to be oxidized,21 for 1 min. The solvent polarity of the electrolyte solution was adjusted by mixing CH3CN and H2O to such a level as to prevent delamination or dissolution of the film without sacrificing the cross-linking efficiency. When a small amount of water was added to the CH3CN solution, the PDI-nPy film became insoluble in the mixed solvent of CH3CN/H2O (13:1 by volume), and an intact PDInetworked film was obtained after the post electrochemical treatment. During the treatment, it was visually evident that the PDI film was partially solvated on the GC electrode surface, presumably due to the increased positive charges at the interface by electrochemical oxidation, but this phenomenon was not significant enough to delaminate the film from the electrodes. After the post electrochemical treatment, the resulting film was rinsed with CH3CN and CH3Cl three times thoroughly to remove any residual soluble species. Figure 3 shows the cyclic voltammograms of the posttreated samples in a monomer-free electrolyte solution. While the PDI-treated GC electrode showed no distinctive redox curve in the potential range, those treated
with PDI-2Py and PDI-4Py revealed broad PDI redox peaks (PDI T PDI- T PDI2-) in the range of -1.9 to -0.7 V vs Ag/Ag+. The higher intensity of redox peaks for PDI-4Py indicates that PDI-4Py, which has twice as many polymerizable Py units, formed a thicker film than PDI-2Py. To determine the cross-linking efficiency of this method, we spin-coated a PDI-nPy film onto an ITO-coated glass and compared the intensities of the absorption spectra of the film before and after post electrochemical treatment (Figure 4a). The absorption spectrum exhibited two distinct bands in the ranges of 400-500 and 500-630 nm. Similar electronic absorption spectra were reported for the other PDI derivatives in the literature.1,10 According to Reisfeld et al.,1 the absorption bands are assigned to two different electronic transitions, i.e., S0-S1 (500-630 nm) and S0-S2 (400-500 nm) transitions. In the S0-S1 transition, two major vibronic transitions from S0 to the 0 and 1 vibronic states of S1 were observed. Figure 4b shows the relative amount of remaining insoluble film after post electrochemical treatment at various applied potentials between 0 and 1.5 V vs Ag/AgCl for 1 min. PDI film,
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Figure 5. AFM images and RMS surface roughness of the PDI-4Py films on GC electrodes: (a) bare GC surface, (b) PDI-4Py spin-cast film surface on GC, (c) cross-linked PDI-4Py film surface formed by spin-casting and subsequent postelectropolymerization in a quasisolid state, (d) cross-linked PDI-4Py film surface formed by the conventional solution-phase electropolymerization (note that the vertical scale is reduced to 1/30 in this case), (e) cross-linked PDI4Py film surface formed by drop-casting and subsequent electropolymerization in a quasi-solid state.
having no Py units, was completely soluble even after post electrochemical treatment so that no absorption was observed after rinsing. On the other hand, in the case of PDI-2Py and PDI-4Py, insoluble moieties were observed after post electrochemical treatment at potentials above ca. 0.8 V for 1 min. After post electrochemical treatment at the potential range of 1.2 and 1.5 V, the absorption intensity indicates that 58-74% of the initial PDI-2Py film and 87-94% of the initial PDI-4Py film remain adhered to the ITO/glass substrates. We also examined the time required for cross-linking the pyrrole units by monitoring the amount of the remaining insoluble species as a function of the duration time of post electrochemical treatment at 1.4 V (Figure 4c). The cross-linking effect was stabilized after post electrochemical treatment for longer than 20 s. These results also show that PDI-4Py can be transformed into an insoluble form more efficiently with higher stability than PDI-2Py. We have also examined the film surfaces on GC electrodes under various conditions using AFM and compared the surface roughness (Figure 5). Figure 5a shows the bare surface of a GC electrode which had been polished with 0.3 µm alumina powder.
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The surface shows unidirectional tracks that had been formed during the polishing. The RMS roughness of this surface is 7.7 nm. When a PDI-4Py solution (0.01 wt %) was spin-cast on this surface and dried, the surface RMS roughness was reduced to 1.7 nm as shown in Figure 5b. When this film was subjected to the post electrochemical treatment at +1.4 V vs Ag/AgCl for 60 s in an electrolyte solution [0.1 M LiClO4 in CH3CN/H2O (13:1 by volume)], the surface RMS roughness slightly increased to 3.8 nm (Figure 5c), which is still a smoother surface than the bare one. In comparison, the film formed under the conventional solution-phase electrochemical polymerization [2 mM PDI-4Py and 0.1 M Bu4NPF6 in CH3CN/CH2Cl2 (1:2 by volume), three scan cycles from 0 to +1.2 V vs Ag/Ag+] was also investigated by AFM (Figure 5d). This film shows remarkably increased surface RMS roughness (112 nm), which is about 30 times larger than that obtained under the post electrochemical treatment (3.8 nm). Note that the vertical scale in Figure 5d is reduced to 1/30. We also investigated the difference between a spin-cast film and a drop-cast film. A drop-cast film on a GC electrode was also subjected to the post electrochemical polymerization under the same conditions used for preparing the film shown in Figure 5c, and the resulting film surface was examined by AFM as shown in Figure 5e. The surface RMS roughness of this film was determined to be 4.6 nm, which is slightly larger than that obtained from the spin-cast sample but still smoother than the bare surface. We also investigated the resolutions in a patterning process by comparing the post electrochemical treatment of the precoated film and the common solution-phase electropolymerization method from a homogeneous solution. A line-patterned Au/Ti/ glass slide substrate was used as an electrode. Figure 6 shows that the post electrochemical treatment gives a much smoother film with higher resolution compared to the common solutionphase electrochemical deposition method. In the common solution-phase electrochemical deposition method, a lot of granules were formed due to the film growth mechanism: nucleation and growth.24-36 Meanwhile, in the post electrochemical treatment, the molecules are not free to move out of the surface. Thus, the morphology cannot be altered significantly from the spin-coated one. Looking at the edge of the electrode area in a sample prepared by post electrochemical treatment, one can observe a clear boundary of the film and that very thin film grew parallel to the surface, reaching out about 2 µm from the edge. However, the sample prepared by the common solutionphase electrochemical deposition method does not show such a
Figure 6. (a-c) SEM images of a PDI-networked film prepared by post electrochemical treatment after spin-coating PDI-4Py: (a) top view, (b) magnified image of (a), and (c) tilted view of (b). (d-f) SEM images of a PDI-networked film prepared by electrochemical treatment in a PDI-4Py solution: (d) top view, (e) magnified image of (d), and (f) tilted view of (e).
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absorption spectrum (λmax ) 590 nm) of the cross-linked PDI4Py at 0 V vs Ag/Ag+ started to change slightly from -1.3 V. As the applied potential was changed stepwise from -1.3 to -1.9 V, the absorbance in the range of 400-640 nm decreased gradually with a concomitant increase of new absorption in the range of 640-850 nm showing a λmax of 797 nm.20 The P(PDI4Py) film turned purple at 0 V and sky blue at -1.9 V.
Conclusions
Figure 7. Static spectroelectrochemistry of P(PDI-4Py): (a) 0, (b) -1.3, (c) -1.4, (d) -1.5, (e) -1.6, (f) -1.7, (g) -1.8, and (h) -1.9 V vs Ag/Ag+. The electrolyte medium was a 0.1 M Bu4NPF6 solution in acetonitrile/methylene chloride (1:1 by volume).
clear boundary and exhibits a relatively thicker film reaching out more than 5 µm from the edge. From this result, it is evident that the post electrochemical treatment gives better resolution in the patterning process. To demonstrate one of the applications using the PDInetworked film prepared by the post electrochemical treatment, we carried out an electrochromic experiment (Figure 7) with a cross-linked film on an ITO/glass substrate. The electronic
Here we demonstrated that a smoother networked PDI film could be obtained by post electrochemical treatment of spincoated PDI-2Py or PDI-4Py on an electrode than those obtained by the common solution-phase electropolymerization. PDI-4Py showed better cross-linking efficiency than PDI-2Py in this process. In the patterning process, this method gives a higher resolution and better morphology with a fine surface compared to the common method of elecropolymerization in a solution phase. In the electrochromic study, the cross-linked PDI-4Py film turned purple (λmax ) 590 nm) and sky blue (λmax ) 797 nm) at 0 and -1.9 V vs Ag/Ag+, respectively. We believe this method will allow another patterning process with the desired film morphology and a good resolution using PDI on a specific electrode. Acknowledgment. This work was supported by a Korea Research Foundation Grant (KRF-2006-005-J02101). H.C.K. thanks the Korea Research Foundation (KRF) for postdoctoral fellowship support (Grant M01-2004-000-20283-0). S.-h. K. was partially supported by the Brain Korea 21 Program. The authors gratefully acknowledge Mr. Chang Hwan Kim for taking AFM images. LA061295I
