Self-Organization Characteristics of Soluble Pentacene on Wettability

Jan 11, 2010 - due to the exactness and simplicity of the patterns. Especially, we systematically studied the dependence of the self-organization beha...
0 downloads 0 Views 2MB Size
Download PDF
J. Phys. Chem. C 2010, 114, 2329–2333

2329

Self-Organization Characteristics of Soluble Pentacene on Wettability-Controlled Patterned Substrate for Organic Field-Effect Transistors Hwa Sung Lee,† Donghoon Kwak,† Wi Hyoung Lee,† Jeong Ho Cho,‡ and Kilwon Cho*,† Department of Chemical Engineering/Polymer Research Institute, Pohang UniVersity of Science and Technology, Pohang, 790-784, Korea, and Department of Organic Materials and Fiber Engineering, Soongsil UniVersity, Seoul, 156-743, Korea ReceiVed: September 25, 2009; ReVised Manuscript ReceiVed: NoVember 25, 2009

Solution-processed self-patterning of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS_PEN) has been achieved by wettability control on a dielectric surface. This is useful for the fabrication of arrayed patterns due to the exactness and simplicity of the patterns. Especially, we systematically studied the dependence of the self-organization behavior of soluble TIPS_PEN on pattern size and solvent evaporation rate using a circular geometry. With fast evaporation rates during solution-casting, the patterned deposits developed a low degree of crystalline microstructure and ringlike morphological (coffee-staining) structures, which were induced when evaporation-driven flow in a solution droplet was dominant. In contrast, long evaporation times produced TIPS_PEN deposits with well-ordered crystalline and dotlike morphological structures resulting from dominant diffusion-driven flow. Solvents with moderate evaporation rates allowed control over the conditions in which the morphological and crystalline structures of the semiconductor deposit formed, producing enhanced control over the electrical performance of organic field-effect transistors. 1. Introduction During the past 2 decades, organic field-effect transistors (OFETs), produced via a combination of low-cost and largearea solution processing, have emerged as competitors of inorganic-based devices.1–4 For low-cost and large-area applications, in particular, efficient patterning is crucial for development. Several innovative patterning schemes, such as micromolding,5 inkjet printing,6–9 screen printing,10 soft lithography,11,12 and self-organization processes,3,4,13,14 have been used to address manufacturing challenges associated with solution processing. Among these schemes, solution-based self-organization, using a selective wetting/dewetting process, holds great potential for assembling micro- and nanoscale patterns of solution-processed materials due to the high-throughput, low-cost, and simplicity of the process. Stebe and co-worker reported the assembly of colloidal particles on hydrophobic patterned surfaces created using soft lithography.15 They demonstrated that solution evaporation on wettability-patterned surfaces provides a highly efficient means of tailoring the geometry of particle distributions. Chabinyc and co-workers reported OFETs fabricated by selective dewetting using a combination of wax-resist patterning and dip coating.3 Bao and co-workers reported the solution-assisted assembly of organic semiconducting crystals on patterned substrates with a variety of self-assembled monolayers (SAMs).16 They also used a solvent wetting/dewetting process with a crystal suspension. However, these studies have major drawbacks. The irregular shape of the patterned deposits and the residue of material left in undesired regions give rise to poor device performance and the need for an additional removal process, respectively. It is particularly important to fabricate patterned organic semiconductor films with uniform morphology and desired

molecular orientation, because charge carrier transport in organic electronic devices is strongly related to both macroscale morphological structure and crystalline microstructure of a film.17 In a recent study of the drying process of droplets in the inkjet printing system, we demonstrated that deposit structures and device performance were found to be strongly dependent on drying conditions, such as solvent evaporation rate, solution concentration, and surface wettability of the substrate.6 In the present study, we investigated the self-organization behavior of a solution-processable organic semiconductor, 6,13bis(triisopropylsilylethynyl) pentacene (TIPS_PEN), using two strategies. First, well-aligned patterns of the organic semiconductor were obtained on an inclined substrate with patterned wettability. The advantage of this method is its simplicity, as it involves only surface patterning and drop-casting. Second, we systematically studied the dependence of the self-organization behavior of soluble TIPS_PEN upon pattern size and solvent evaporation rate, using a circular geometry, by controlling the solution evaporation rate and characterizing the relationship between these properties and the electrical performance of the FETs. To control the evaporation rate of the solution dissolving TIPS_PEN, several solvents with different boiling points (BPs) were used, namely, chloroform (CF), toluene (Tol), chlorobenzene (CB), 1,2-dichlorobenzene (DCB), and 1,2,4-trichlorobenzene (TCB). The crystalline microstructure and film morphology of the TIPS_PEN deposits were analyzed by two-dimensional grazing incidence X-ray diffraction (2D GIXD), optical microscopy (OM), field-emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). The relationship between these properties and the electrical performance of devices was also investigated.

* To whom all correspondence should be addressed. E-mail: kwcho@ postech.ac.kr. † Pohang University of Science and Technology. ‡ Soongsil University.

For the fabrication of the OFETs, a highly doped p-Si wafer with a 300 nm thick thermally grown oxide layer was used as the substrate. The wafer served as the gate electrode, while the

2. Experimental Section

10.1021/jp909227b  2010 American Chemical Society Published on Web 01/11/2010

2330

J. Phys. Chem. C, Vol. 114, No. 5, 2010

Lee et al.

TABLE 1: Physical Properties of Organic Solvents and the Electrical Characteristics of Devices Fabricated with TIPS_PEN Deposits solvent BP (°C) mobility (cm2/(V s)) on/off current ratio threshold voltage (V)

CF

Tol

CB

DCB

TCB

61.2 0.0006 1.4 × 103 -23.9

110.6 0.0095 3.8 × 104 -19.3

131.2 0.052 1.8 × 106 -10.6

180.5 0.011 6.4 × 104 -9.6

214.4 0.0053 4.9 × 104 0.9

oxide layer acted as the gate insulator. Prior to treatment of the silicon oxide layer surface, the wafer was cleaned in piranha solution (70 vol % H2SO4, 30 vol % H2O2) for 30 min at 100 °C and washed with copious amounts of distilled water. To fabricate the solvophobic surface, the coupling agent octadecyltrichlorosilane (ODTS, Gelest) was deposited with ordering in the phase of the alkyl chains.18 Surface wettability was adjusted between solvophobic and solvophilic by exposure to UV/ozone (253.7 nm) through a shadow mask for 10 min. The variation of surface energy and wettability was determined by measuring the contact angle of several solvents using a contact angle meter (Kru˝ss BSA 10) (see Figure S1 in the Supporting Information). 6,13-Bis(triisopropylsilylethynyl) pentacene (TIPS_PEN) was synthesized, purified, and characterized following the procedure reported by John E. Anthony et al.19 To investigate the effects of solvent evaporation rate on the TIPS_PEN deposits, we selected solvents characterized by several boiling points (BPs) (see Table 1). The patterned TIPS_PEN films were obtained by dropping 3.0 wt % TIPS_PEN solution droplets of each solvent. The morphologies of TIPS_PEN deposits were characterized by polarized optical microscopy (Axioplan, Zeiss) and fieldemission scanning electron microscopy (Hitachi S-4200) with an accelerating voltage of 8 kV. To investigate the crystalline structure of the deposits, two-dimensional grazing-incidence X-ray diffraction (2D GIXD), and X-ray diffraction (XRD) measurements were performed using the 4C2, 8C1, and 10C1 beamlines at the Pohang Accelerator Laboratory, Korea. The field-effect mobilities of the TIPS_PEN deposits were measured using a top-contact OFET. Source and drain electrodes (Au) were evaporated through shadow masks to minimize damage to the patterned TIPS_PEN deposits. To study the current-voltage characteristics of the prepared devices, the OFETs were operated in the accumulation mode by applying a negative gate bias. The source electrode was grounded, and the drain electrode was negatively biased. All the results were obtained at room temperature under ambient conditions in a dark environment using the Keithley 2400 and 236 source/ measure units.

cases of CF, Tol, and CB, all patterns were well-defined on the solvophilic regions treated with UV light. However, the TIPS_PEN deposits from DCB and TCB tended to shrink into the wettable regions due to the long evaporation time of these solvents, which have high BPs (Figure 1b). The BPs of the solvents are listed in Table 1. OM images of the TIPS_PEN deposits created by the different solvents are shown in Figure 2a and confirm the formation of well-defined patterns for all solvents. However, solvents with high BPs, such as DCB and TCB, resulted in shrunken TIPS_PEN deposits unlike the low BP solvents, as shown in Figure 1b, which may have been due to a long solution drying time. The evaporation behavior during the drying process played a vital role in controlling the deposit morphology and distribution of solute in deposits. In a pinned evaporating droplet, there are two main flows, evaporation-driven flow and diffusiondriven flow.20–22 The evaporation-driven flow is radially directed, from the droplet center to the contact line, to compensate for

3. Results and Discussion Figure 1a shows a schematic of the patterning process. First, an octadecyltrichlorosilane (ODTS)-treated solvophobic surface was patterned by ultraviolet (UV) light through a shadow mask to the solvophilic surface. Subsequently, a TIPS_PEN solution was dropped on the inclined (angle, 30°) substrate that had been patterned with selective wettability, and then the substrate was maintained horizontally right after the solution dropping. By variation of the wettability and inclination of the surface, the solution was deposited only onto UV-treated areas, as shown in Figure 1a. As the solvent evaporated, patterned TIPS_PEN deposits were obtained in the desired geometry (Figure 1b). The features fabricated by this process were circles with diameters ranging from 50 to 2000 µm, adequate for display applications. The obtained patterns depended on the solvent used. For the

Figure 1. (a) Schematic of procedures for surface wettability control and organic semiconductor patterning. (b) Photographs of the patterned TIPS_PEN deposits for various solvents after drying. All substrates are 2 cm × 2 cm in size.

Self-Organization of Soluble Pentacene

Figure 2. (a) OM images of TIPS_PEN deposits with various pattern sizes produced by various solvents. Polarized OM images representatively show the deposits with 100 µm size features. (b) Evaporationdriven flow velocity in an evaporating droplet for the 200 µm pattern size. The equation used and parameters estimated from experiments are shown in the Supporting Information. (c) Height profiles of TIPS_PEN deposits for 100 µm pattern size.

the difference in evaporation rate and volume change across the drop. On the other hand, diffusion-driven flow is directed toward the droplet center to equilibrate the solute concentration gradient that is set up between the periphery and interior of the droplet. Usually, when evaporation-driven flow is much faster than the equilibration induced by diffusion-driven flow, most of the dissolved material is transported to the contact edge and then deposited, mainly at the periphery of the drop (a ringlike deposit).20,23 This phenomenon is known as the “coffee-stain” effect. In contrast, if the diffusion of the dissolved molecules is much faster than the evaporation-driven flow, the solute has a uniform concentration throughout the solution at all times in a solution droplet. Therefore, the final pattern is deposited mainly in the droplet center (a dotlike deposit).20,24 To investigate the formation mechanism of the TIPS_PEN deposit during the evaporation process, we calculated the evaporation-driven flow velocity (V) in an evaporating droplet (Figure 2b; see the Supporting Information for a description of the calculation). From the calculated results, we confirmed that not only did V rapidly increase as the contact edge approached, but V for a low BP solvent (Tol) was much higher than that of a high BP solvent (TCB). (Calculation of V for the CF case was not possible due to the extremely fast drying time. However, we are convinced that V for the CF case was much higher than that of the Tol case.) These results indicated that the evaporation rate increased as the edge region of the droplet approached and

J. Phys. Chem. C, Vol. 114, No. 5, 2010 2331

Figure 3. 2D GIXD patterns for TIPS_PEN deposits with 100 µm size patterns produced by various solvents. The schematic diagrams of the TIPS_PEN molecular packing structures, based on the 2D GIXD patterns, are shown. Rectangular symbols represent the pentacene ring in the TIPS_PEN molecule.

that the evaporation-driven flow decreased with increasing solvent BP. For these reasons, ring- and dotlike structures formed for the CF and Tol solutions with low BP and for the DCB and TCB solutions with high BP, respectively, as shown in the height profiles in Figure 2c. On the other hand, CB, with a moderate BP, formed a tablelike structure due to a counterbalancing of the evaporation-driven and diffusion-driven flows in the droplet. To fabricate an ideally shaped TIPS_PEN deposit, equilibration between flows induced during the drying process in a droplet is important. In the CF and Tol cases with low BPs (evaporation-driven flow dominant), a ringlike structure that marked the perimeter of the droplet was observed. A dotlike structure deposited in the center was observed for the DCB and TCB cases with high BP (diffusion-driven flow dominant). Therefore, solvents with an adequate BP, such as CB, should be selected for formation of the optimal shape such as a tablelike structure, as shown in Figure 2c. In Figure 2a, we could confirm additional important information that optical contrast in the polarized OM (POM) images for CF with a very fast evaporation time was not observed, indicating the absence of crystalline order on the micrometer scale. On the other hand, droplets produced from CB, DCB, and TCB, with slow evaporation times, yielded TIPS_PEN crystals with clear optical contrast in the POM images, indicating the presence of uniaxial molecular ordering in a crystal.25 To precisely confirm the molecular ordering and crystalline nature of the TIPS_PEN deposits, 2D GIXD was performed for the 100 µm pattern (Figure 3). The diffraction pattern for CF contained not only a (001) reflection corresponding to a c-axis of 16.8 Å in the TIPS_PEN unit cell but also an intense (011)

2332

J. Phys. Chem. C, Vol. 114, No. 5, 2010

Lee et al.

Figure 4. (a) Output characteristics of a FET produced from CB (VG ) 0 to -40 V, step, -10 V). The inset represents an OM image of the device. (b, c) Transfer characteristics and their slopes, respectively, with various solvents at a fixed VD of -40 V. (d) Variation of fieldeffect mobilities with respect to the solvents. All devices were fabricated using TIPS_PEN deposits with 100 µm size patterns.

peak along the qz (out-of-plane) axis.6,25 In addition, the reflection peaks in the qz and qxy directions showed highly scattered patterns along the Debye rings. Scattered rings implied that the patterned deposits had an unfavorable orientation of TIPS_PEN molecules stacked in a direction perpendicular to the surface of the pentacene ring, and a serious crystal mismatch anddislocationintheverticalandlateraldirections,respectively.18,26,27 However, as the solvent BP increased, the scattering of reflection peaks was reduced, and several reflection spots in the direction of qz at a given qxy appeared. The results indicate that the crystal perfection and ordering of the TIPS_PEN molecules was enhanced in systems with increasing solvent BPs and that relatively well-ordered 3D crystals of the TIPS_PEN film formed in both the vertical and lateral directions (schemes of crystals are shown in Figure 3),18,26,27 because the organization of TIPS_PEN molecules was facilitated by the slow evaporation time. Whereas, lower BP solvents produced faster transport of the materials due to the fast evaporation, insufficient molecular organization, and short crystallization times during the drying process. To determine the electrical characteristics in the patterned TIPS_PEN deposits based on the various solvents, the fieldeffect mobilities of devices fabricated using 100 µm patterns of the semiconductor were measured using a top-contact FET geometry. Figure 4a shows the typical output characteristics of a representative CB-based device. The device displayed wellbehaved p-type transistor characteristics, including a linear regime and a saturation regime.28 Parts b and c of Figure 4 show the transfer characteristics as a function of VG at VD) -40 V. The electrical properties calculated in the saturation regime (VD ) -40 V) of each FET using various solvents are shown in Figure 4d and Table 1. We confirmed that the on/off current ratios and field-effect mobilities of the devices improved with increasing solvent BP. However, DCB- and TCB-based devices, with high solvent BPs, showed dramatically decreased mobility, although the TIPS_PEN deposits had better crystalline microstructures than those of the CB-based device (Figure 3). In addition, we confirmed a positive shift of the threshold voltage with increasing BP. The threshold voltage of a transistor is

Figure 5. FE-SEM images of TIPS_PEN deposits, 100 µm in size, produced from various solvents. The enlarged images are shown, respectively, on the right side.

influenced by several factors, including the interface states of semiconductor/dielectrics, trap states in the bulk, and the surface potential of gate dielectrics.29,30 In the devices examined here, it is possible that the increased molecular ordering and crystalline nature of the TIPS_PEN deposits produced using solvent with higher BPs may have affected the trap states in the bulk and shifted the threshold voltage.31 However, further investigations are necessary to clarify this threshold voltage shift. The decrease in electrical performance for the DCB and TCB cases is an interesting result because it is usually known that enhanced molecular ordering and crystal perfection facilitate charge carrier transport in the channel region of an active layer due to more efficient π-orbital overlapping, which enhances device performance.6,32 To determine the cause of the above trend, we analyzed the surface morphology of 100 µm patterned TIPS_PEN deposits using FE-SEM (Figure 5). The device performance of FETs was also dramatically influenced by the morphological macrostructure of the organic semiconductor, because morphological continuity plays a critical role in determining charge carrier transport.32 Therefore, the fabrication of deposits with both uniform morphology and desired molecular orientation is an important challenge. As shown in Figure 5, the deposit from CF had an obvious ringlike structure resulting

Self-Organization of Soluble Pentacene from the fast evaporation-driven flow, as described above in Figure 2. On the other hand, the TIPS_PEN deposits in the DCB and TCB cases were the dotlike structures due to the equilibration induced by diffusion-driven flow. However, empty regions between the large crystals in TIPS_PEN deposit were found for the cases of DCB and TCB with extremely slow evaporation rates (or drying times), which may have been due to the participation of most TIPS_PEN molecules in the formation of the large crystals. The empty regions prevented the formation of the continuous channel region necessary for charge carrier transport in an active layer.17 Device performance, therefore, decreased compared with the CB case, even though crystal perfection and molecular ordering were improved, as confirmed by 2D GIXD (Figure 3). In the case of CB, a more uniform morphology (tablelike structure) of the patterned deposit appeared, and a sufficiently crystalline microstructure was obtained, even though the crystallinity was inferior to that observed for the DCB and TCB cases. Therefore, TIPS_PEN deposits formed from CB had the highest device performance. Namely, optimization between the morphological macrostructure and crystalline microstructure of the patterned TIPS_PEN deposits could result from a balance in the drying (evaporation-/diffusiondriven) flow velocities via control of the evaporation rate in an evaporating solution drop. 4. Conclusions In conclusion, solution-processed self-organization of organic semiconductor patterns was achieved by both wettability patterning and drop-casting on an inclined dielectric surface. The method is considerably useful for the fabrication of arrayed patterns because of the exactness and simplicity of the arrays, involving only surface patterning and drop-casting. In particular, we systematically demonstrated that the balance between crystalline microstructure and morphological macrostructure of a TIPS_PEN deposit, which is critically important for high device performance, could be achieved through the control of the flows in the evaporating solution droplet by tuning the solvent evaporation rate. In the cases of CF and Tol, with fast evaporation rates, device performance decreased due to the low degree of crystallization in the deposit microstructure resulting from insufficient crystallization time during the drying process. In contrast, DCB and TCB required too long an evaporation time, which limited device performance by producing a discontinuous morphology despite the presence of well-ordered crystalline structures. Therefore, by choosing a solvent (CB) with an adequate evaporation rate, we could clarify the relationship between the morphological and crystalline structures of the TIPS_PEN deposit and thus obtain better FET electrical performance. Our results provide an efficient technique for the fabrication of arrays of organic electronics using a solution process and suggest an excellent method for improving device performance. Acknowledgment. This work was supported by Grant F0004021-2009-32 from the Information Display R&D Center under the 21st Century Frontier R&D Program, Creative Research Initiative-Acceleration Research (Grant R17-2008-02901001-0), and Pohang Acceleratory Laboratory for providing the synchrotron radiation source at the 4C2 and 10C1 beamlines used in this study.

J. Phys. Chem. C, Vol. 114, No. 5, 2010 2333 Supporting Information Available: Variations of SAM thickness, surface energy, and wettability changes for ODTStreated substrates, OM images of arrays of the TIPS_PEN deposits, AFM height and phase images of TIPS_PEN deposits, XRD patterns for TIPS_PEN deposits, and calculation of evaporation-driven flow velocity in a droplet. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sekitani, T.; Takamiya, M.; Noguchi, Y.; Nakano, S.; Kato, Y.; Sakurai, T.; Someya, T. Nat. Mater. 2007, 6, 413. (2) Newman, C. R.; Frisbie, C. D.; da Silva, D. A.; Bredas, J. L.; Ewbank, C. P.; Mann, K. R. Chem. Mater. 2004, 16, 4436. (3) Chabinyc, M. L.; Wong, W. S.; Salleo, A.; Paul, K. E.; Street, R. A. Appl. Phys. Lett. 2002, 81, 4260. (4) Liu, S.; Briseno, A. L.; Mannsfeld, S. C. B.; You, W.; Locklin, J.; Lee, H. W.; Xia, Y.; Bao, Z. AdV. Funct. Mater. 2007, 17, 2891. (5) Menard, E.; Meitl, M. A.; Sun, Y.; Park, J.-U.; Shir, D. J.-L.; Nam, Y.-S.; Jeon, S.; Rogers, J. A. Chem. ReV. 2007, 107, 1117. (6) Lim, J. A.; Lee, W. H.; Lee, H. S.; Lee, J. H.; Park, Y. D.; Cho, K. AdV. Funct. Mater. 2008, 18, 229. (7) Noh, Y.-Y.; Zhao, N.; Caironi, M.; Sirringhaus, H. Nat. Nanotechnol. 2007, 2, 784. (8) Volkman, S. K.; Molesa, S.; Mattis, B.; Chang, P. C.; Subramanian, V. Mater. Res. Soc. Symp. Proc. 2003, 771, 391. (9) Katz, H. E. Chem. Mater. 2004, 16, 4748. (10) Garnier, F.; Hadjlaoui, R.; Yasser, A.; Srivastava, P. Science 1994, 265, 1684. (11) Chabinyc, M. L.; Salleo, A.; Wu, Y.; Liu, P.; Ong, B. S.; Heeney, M.; McCulloch, I. J. Am. Chem. Soc. 2004, 126, 13928. (12) Meitl, M. A.; Zhui, Z.-T.; Kumar, V.; Lee, K. J.; Feng, X.; Huang, Y. Y.; Adesida, I.; Nuzzo, R. G.; Rogers, J. A. Nat. Mater. 2006, 5, 33. (13) Minari, T.; Kano, M.; Miyadera, T.; Wang, S. D.; Aoyagi, Y.; Seto, M.; Nemoto, T.; Isoda, S.; Tsukagoshi, K. Appl. Phys. Lett. 2008, 92, 173301. (14) Salleo, A.; Arias, A. C. AdV. Mater. 2007, 19, 3540. (15) Fan, F.; Stebe, K. J. Langmuir 2004, 20, 3062. (16) Briseno, A. L.; Aizenberg, J.; Han, Y.-J.; Penkala, R. A.; Moon, H.; Lovinger, A. J.; Kloc, C.; Bao, Z. J. Am. Chem. Soc. 2005, 127, 12164. (17) Lim, J. A.; Lee, H. S.; Lee, W. H.; Cho, K. AdV. Funct. Mater. 2009, 19, 1515. (18) Lee, H. S.; Kim, D. H.; Cho, J. H.; Hwang, M.; Jang, Y.; Cho, K. J. Am. Chem. Soc. 2008, 130, 10556. (19) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am. Chem. Soc. 2001, 123, 9482. (20) Li, G.; Graf, K.; Bonaccurso, E.; Golovko, D. S.; Best, A.; Butt, H.-J. Macromol. Chem. Phys. 2007, 208, 2134. (21) Deegan, R. D. Phys. ReV. E 2000, 61, 475. (22) Sommer, A. P.; Rozlosnik, N. Cryst. Growth Des. 2005, 5, 551. (23) Bonaccurso, E.; Butt, H.-J.; Hankeln, B.; Niesenhaus, B.; Graf, K. Appl. Phys. Lett. 2005, 86, 124101. (24) Stupperich-Sequeira, C.; Graf, K.; Wiechert, W. Math. Comput. Modell. Dyn. Syst. 2006, 12, 263. (25) Kim, D. H.; Lee, D. Y.; Lee, H. S.; Lee, W. H.; Kim, Y. H.; Han, J. I.; Cho, K. AdV. Mater. 2007, 19, 678. (26) Roe, R.-J. Methods of X-Ray and Neutron Scattering in Polymer Science; Oxford University Press: New York, 2000. (27) Lee, W. H.; Kim, D. H.; Jang, Y.; Cho, J. H.; Hwang, M.; Park, Y. D.; Kim, Y. H.; Han, J. I.; Cho, K. Appl. Phys. Lett. 2007, 90, 132106. (28) Payne, M. M.; Parkin, S. R.; Anthony, J. E.; Kuo, C.-C.; Jackson, T. N. J. Am. Chem. Soc. 2005, 127, 4986. (29) Pernstich, K. P.; Haas, S.; Oberhoff, D.; Goldmann, C.; Gundlach, D. J.; Batlogg, B.; Rashid, A. N.; Schitter, G. J. Appl. Phys. 2004, 96, 6431. (30) Yoon, M. H.; Kim, C.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 12851. (31) Gundlach, D. J.; Royer, J. E.; Park, S. K.; Subramanian, S.; Jurchescu, O. D.; Hamadani, B. H.; Moad, A. J.; Kline, R. J.; Teague, L. C.; Kirillov, O.; Richter, C. A.; Kushmerick, J. G.; Richter, L. J.; Parkin, S. R.; Jackson, T. N.; Anthony, J. E. Nat. Mater. 2008, 7, 216. (32) Park, Y. D.; Lim, J. A.; Lee, H. S.; Cho, K. Mater. Today 2007, 10, 46.

JP909227B