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Hot Microcontact Printing for Patterning ITO Surfaces. Methodology, Morphology, Microstructure, and OLED Charge Injection Barrier Imaging Yoshihiro Koide,§ Matthew W. Such,† Rajiv Basu,† Guennadi Evmenenko,‡ Ji Cui,§ Pulak Dutta,*,‡ Mark C. Hersam,*,† and Tobin J. Marks*,§ Department of Chemistry, Department of Materials Science and Engineering, Department of Physics and Astronomy, and the Materials Research Center, Northwestern University, Evanston, Illinois 60208-3113 Received July 3, 2002. In Final Form: October 21, 2002 A soft lithographic microcontact printing (µCP) procedure is successfully applied for the first time to form densely packed organosilane self-assembled monolayers (SAMs) on the surface of ITO (Sn-doped In2O3) coated glass via a thermally activated deposition process. Hot microcontact printing (HµCP) enables localized transfer with 1.0-40 µm feature sizes of dense docosyltrichlorosilane (CH3(CH2)20CH2SiCl3 ) DTS) monolayer patterns onto ITO, which reacts sluggishly under conventional µCP conditions. X-ray reflectivity measurements yield a thickness of 12.1 ( 0.1 Å and a surface roughness of 2.8 ( 0.1 Å for HµCP printed DTS films, which is well within the range for self-assembled monolayer formation, while the weak reflected intensity from conventionally prepared DTS films indicates a poorly organized monolayer structure. Noncontact mode AFM studies reveal that HµCP creates uniform SAMs over a wide area with excellent line edge resolution, while the original patterns are poorly transferred by conventional µCP, presumably due to the slow Si-O bond formation. Cyclic voltammetry of 1,1′-ferrocenedimethanol solutions using HµCP-derived, DTS SAM coated ITO working electrodes evidences good barrier properties, consistent with dense films. The DTS SAM patterns can be imaged by fabricating organic light-emitting diode (OLED) heterostructures on the patterned ITO. The DTS SAM role as a hole injection blocking layer enables patterned luminescence from the hot contact printed ITO surfaces.

Introduction In the past two decades, insights into the mechanisms of biological templating and self-assembly have stimulated an active interest in nonbiological applications of molecular self-assembly to fabricate two- and three-dimensional structures of submicrometer sizes through chemisorptive interactions of organic molecules in surface patterning1 and electronic2 and optoelectronic device fabrication.3 Microcontact printing (µCP) is a unique variant of selfassembly that utilizes a soft lithographic approach for surface patterning, based on principles of mechanically transferring surface-reactive reagents from an elastomeric stamp to a substrate, and it allows patterning of selfassembled monolayers (SAMs) down to the submicrometer scale.4 Since the first demonstration of µCP by Whitesides and co-workers,5,6 numerous applications have emerged.7 Microcontact printing has several significant attractions: (1) The gentle physical contact between the elastomeric §

Department of Chemistry and the Materials Research Center. Department of Materials Science and Engineering and the Materials Research Center. ‡ Department of Physics and Astronomy and the Materials Research Center. †

(1) Gamota, D., Ong, B., Zhang, J., Eds. Printing an Intelligent Future; Proc. IMAPS Advanced Technology Workshop, Lake Tahoe, NV, 2002, and presentations therein. (b) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (c) Mirkin, C. A., Rogers, J. A., Eds. Emerging Methods for Micro- and Nanofabrication. MRS Bull. 2001, 26, 506, and articles therein. (d) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823 and references therein. (2) Fendler, J. H. Chem. Mater. 2001, 13, 3196 and references therein. (3) van der Boom, M. E.; Zhu, P.; Evmenenko, G.; Malinsky, J. E.; Lin, W.; Dutta, P.; Marks, T. J. Langmuir 2002, 18, 3704. (b) van der Boom, M. E.; Evmenenko, G.; Dutta, P.; Marks, T. J. Adv. Funct. Mater. 2001, 11, 393. (c) Evmenenko, G.; van der Boom, M. E.; Kmetko, J.; Dugan, S. W.; Marks, T. J.; Dutta, P. J. Chem. Phys. 2001, 115, 6722. (d) Cui, J.; Huang, Q.; Wang, Q.; Marks, T. J. Langmuir 2001, 17, 2051. (4) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550.

stamp and the substrate causes minimal surface damage. (2) Soft elastomers accommodate conformal contact with nonplanar surfaces, allowing curved surface coating. (3) Nanoscale patterning is possible on the scale of a few hundred nanometers for a variety of substrate/material combinations. (4) The procedure is relatively effortless, parallel rather than serial in execution, and amenable to rapid prototyping. (5) The instrumentation and materials are economical, the stamp and master mold endure repeated use, and the elastomers are commercially available. In the µCP process, the “ink” reagent forms a SAM upon contacting an appropriate surface, with the metrical aspects of the pattern depending on the relief structure engraved on the stamp surface. For µCP on Au, Ag, Cu, or Pd, an elastomeric PDMS (poly(dimethylsiloxane)) stamp inked with an alkanethiol solution is brought to contact with the substrate to transfer the thiol molecules to those regions of the substrate that contact the stamp. This simple procedure generates remarkably accurate 2-D images on the substrate over relatively wide areas,8 and there is now a fairly complete understanding of the mechanism.4,6-8 µCP has now been demonstrated for a variety of ink/substrate systems such as alkanethiolates on group 11 metals,6-9 alkylsiloxanes on OH-terminated (5) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (b) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219. (6) Xia, Y.; Zhao, X.-M.; Whitesides, G. M. Microelectron Eng. 1996, 32, 255. (b) Xia, Y.; Kim, E.; Whitesides, G. M. J. Electrochem. Soc. 1996, 143, 1070. (7) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker, D.; Kind, H.; Renault, J.-P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel, P.; Stutz, R.; Wolf, H. IBM J. Res. Dev. 2001, 45, 697. (8) Biebuyck, H. A.; Larsen, N. B.; Delamarche, E.; Michel, B. IBM J. Res. Dev. 1997, 41, 159.

10.1021/la020604b CCC: $25.00 © 2003 American Chemical Society Published on Web 12/06/2002

HµCP for Patterning ITO Surfaces Scheme 1 Schematic Diagram of Patterned OLED Emission from an HµCP Patterned ITO Surface, Showing the Role of the DTS SAMs as the Hole Injection Blocking Layers

surfaces,10 and alkylphosphonic acids on the aluminum native oxide surface,11 with all systems transferring patterns on a length scale of g1 µm. For these reasons, it would be of great interest to pattern OH-terminated metal oxide surfaces used in association with optoelectronic device fabrication, for example, silicon oxides, Sndoped In2O3 (ITO), and so forth. To date, SAM pattern formation by µCP on a number of native hydroxyl group covered surfaces has been reported. However, despite its widespread use as a transparent conducting oxide (TCO) electrode for numerous optoelectronic devices, ITO-coated glass had not, at the outset of this work, been successfully patterned by µCP. Conventional ITO surface etching/patterning approaches that are widely used include photolithography combined with halogen acid etching,12 reactive ion etching,13 or plasma etching.14 In an effort to achieve patterned organic light-emitting diode (OLED) luminescence using soft lithography (Scheme 1), we sought to derivatize ITO surfaces with alkyltrichlorosilanes (RSiCl3) using the standard µCP procedure under a variety of conditions: (1) in ambient or in a glovebox; (2) with ink dissolved in various solvents at various concentrations; (3) with stamping for long or short times under varying pressure; (4) with use of a variety of R groups. In addition, the ITO surfaces were cleaned/activated using a variety of procedures: (i) sonicated in acetone and then ethanol; (ii) (9) Moffat, T. P.; Yang, H. J. Electrochem. Soc. 1995, 142, L220. (b) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8, 601. (10) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 11, 9576. (b) Jeon, N. L.; Clem, P. G.; Payne, A. A.; Nuzzo, R. G. Langmuir 1996, 12, 5350. (c) Yan, M.; Koide, Y.; Babcock, J. R.; Markworth, P. R.; Belot, J. A.; Marks, T. J.; Chang, R. P. H. Appl. Phys. Lett. 2001, 79, 1709. (11) Goetting, L. B.; Deng, T.; Whitesides, G. M. Langmuir 1999, 15, 1182. (12) van den Meerakker, J. E. A. M.; Baarslag, P. C.; Scholten, M. J. Electrochem. Soc. 1995, 142, 2321. (13) Oulachgar, E. H.; Xu, Z.; Wang, C.; Zhao, B.; Zhou, X. Semicond. Photonics Technol. 1998, 4, 188. (14) Park, J. Y.; Kim, H. S.; Lee, D. H.; Kwon, K. H.; Yeom, G. Y. Surf. Coat. Technol. 2000, 131, 247.

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sonicated in detergent; (iii) cleaned in an oxygen plasma. Despite these efforts, all the attempts at µCP lithography resulted in irreproducible, poorly resolved or marginally visible luminescent patterns. Unlike DTS SAM formation on native oxide coated Si substrates as reported by Nuzzo et al.,15 the hydrophilic surface of ITO behaves differently and Si-O-In/Sn bond formation is rather sluggish, perhaps because ITO surfaces are poorly nucleophilic, and the formation of dense arrays of chemisorptive bonds to the surface is therefore problematic.16 To achieve high fidelity in transfer of relief patterns, spontaneous Si-O bond formation is essential. Herein we address this problem by increasing the reaction temperature for SAM deposition on ITO surfaces and demonstrate that hot microcontact printing (HµCP) facilitates spontaneous Si-O bond formation and localized delivery of SAMforming RSiCl3 inks down to 1 µm length scales.17 Using the n-docosyltrichlorosilane (DTS) in hexane system as a model “ink”, we present here a full discussion of HµCPderived monolayer microstructure and morphology as determined by a variety of informative physical techniques, control over electronic conduction/charge injection, and accessible topographic dimensions. In the process it will be seen that the electroluminescent response of patterned OLEDs is an excellent SAM contiguity/barrier diagnostic. Since the initial communication of this work,17a a complementary report has appeared describing µCP of long chain phosphonic acids combined with wet etching to achieve ITO patterning on a length scale of ∼2 µm.18 Experimental Section All manipulations of air-sensitive materials were carried out with rigorous exclusion of oxygen and moisture in Schlenk-type glassware on a dual-manifold Schlenk line or in a nitrogen-filled M. Braun Unilab glovebox system. The solvents were distilled before use from appropriate drying agents (sodium benzophenone ketyl, metal hydrides, Na/K alloy) under nitrogen. The solvents toluene and heptane were dried using activated alumina columns according to the method described by Grubbs.19 All chemicals were purchased from Aldrich Chemical Co. and were used without further purification unless otherwise stated. Magnetron sputtered ITO coated glass (rms roughness ) 5.0-10 Å) was supplied by Thin Film Devices Inc. Synthesis of n-Docosyltrichlorosilane, CH3(CH2)20CH2SiCl3. To 16.0 g (10.0 mmol) of 1-docosanol (CH3(CH2)21OH) dissolved in 100 mL of dry toluene under inert atmosphere was added 13.5 g (50.0 mmol) of phosphorus tribromide with stirring, yielding 1-bromodocosane. The reaction mixture was allowed to stir overnight at room temperature. The crude 1-bromodocosane product formed as a precipitate and was collected by filtration. Recrystallization from hexane afforded 19.4 g of 1-bromodocosane as a white powder. To 1.4 g (55 mmol) of magnesium turnings in 100 mL of anhydrous ethyl ether was added dropwise with stirring 19.4 g (48.0 mmol) of 1-bromodocosane. The mixture was stirred for an additional 1.0 h after the halide addition. To a stirred solution of 8.0 g (50 mmol) of freshly distilled SiCl4 in anhydrous ether was then added dropwise the Grignard reagent (15) Finnie, K. R.; Haasch, R.; Nuzzo, R. G. Langmuir 2000, 16, 6968. (16) Donley, C.; Dunphy, D.; Paine, D.; Carter, C.; Nebesny, K.; Lee, P.; Alloway, D.; Armstrong, N. R, Langmuir 2002, 18, 450. (b) Appleyard, S. F. J.; Day, S. R.; Pickford, R. D.; Willis, M. F. J. Mater. Chem. 2000, 10, 169. (c) Milliron, D. J.; Hill, I. G.; Shen, C.; Kahn, A.; Schwartz, J. J. Appl. Phys. 2000, 87, 572. (d) Gardner, T. J.; Frisbie, C. D.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6927. (e) Wilson, R.; Schiffrin, D. J. Analyst 1995, 120, 175. (f) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993, 115, 1193. (17) Portions of this work were communicated in a preliminary fashion: (a) Koide, Y.; Wang, Q.; Benson, D. D.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 11266. (b) Koide, Y.; Marks, T. J. Polym. Mater. Sci. Eng. 2000, 83, 212. (18) Breen, T. L.; Fryer, P. M.; Nunes, R. W.; Rothwell, M. Langmuir 2002, 18, 194. (19) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518.

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prepared above. The mixture was refluxed for 12 h, and the magnesium salt was removed by filtration. The product was purified by vacuum sublimation. Yield, 17.7 g (80%). Anal. Calcd for C22H45SiCl3: C, 59.51; H, 10.21. Found: C, 59.51; H, 10.23.20 Procedures for OLED Device Fabrication. OLED devices were assembled in a glovebox equipped with dual evaporation chambers (one for organic layers and the other for metal cathode layers) using TPD as the hole transport layer (HTL)21 and 5.0 mol % quinacridine (QD) doped tris-8-hydroquinolinato aluminum (Alq3) as the emission/electron transport layer (EML/ ETL).22,23 All commercially available compounds were purified by temperature gradient sublimation at 5 × 10-5 Torr prior to use. TPD (50 nm) hole transport (HTL) and Alq3 (60 nm) electron transport/emissive (ETL/EML) layers were vapor deposited onto the SAM patterned ITO anode using the conventional small molecule method. Vapor deposition was performed at a rate of 3.0 Å/s or less for both layers. Last, an Al cathode layer was thermally deposited (70-100 nm) on top of these two layers. The assembled devices were characterized in the continuous dc mode using a Tektronix PS281 power supply. Luminance was measured using a calibrated Si photodiode confined in a 20 cm × 10 cm black box under a constant N2 gas flow. The I-V curves were recorded simultaneously using a Keithley 2400 current source measurement unit. Fabrication of Elastomeric Stamps. Elastomeric stamps were fabricated using a positive photoresist pattern on a silicon wafer as follows. A 1.0 µm thick film of positive photoresist (Shipley 1045) spin-coated onto an n-type (100) silicon wafer (WaferNet Inc.) was photolithographically patterned and used as a master mold for stamp fabrication. The test pattern consisted of an array of cylinders with cross-sectional diameters ranging from 1.0 to 40 µm with center-to-center distances of 100 µm. The surface of the master (4.0 cm2) was exposed to tridecafluorotetrahydrooctyl-1-trichlorosilane vapor24 prior to casting the poly(dimethylsiloxane) (PDMS) prepolymer (Dow Corning Sylgard 184), which was cured at room temperature for 1.0 h and then at 75 °C for 2.0 h. An elastomeric stamp (1.0 cm × 1.5 cm for the photoresist) was cut out, detached, and used for the soft lithography process. Procedures for Microcontact Printing. All microcontact printing processes were carried out in a glovebox unless otherwise stated. A few drops of a 10 mM DTS solution in dry hexanes was filtered though a 0.22 µm PTFE syringe filter and applied to the surface of the PDMS stamps, which were then lightly spin dried for 30 s at 3000 rpm or allowed to stand for a short time until the surface became visibly dry. ITO coated glass (sheet resistance 20 Ω/square at 10 Å thickness) was cut into 2.5 cm2 pieces, and the pieces were cleaned in an ultrasonic bath with acetone and then with ethanol, for 10 min each. Printing and SAM formation were carried out by manually bringing the inked stamp into contact with the surface and applying a light pressure for varied lengths of time.25 In high-temperature microcontact printing (HµCP), the ITO glass was maintained at the desired temperature by placing it in thermal contact with a hot plate while the print image was transferred. X-ray Reflectivity (XRR) Measurements. XRR studies were performed at the X23B beam line of the National Synchrotron Light Source using a Huber four-circle diffractometer in the specular reflection mode (i.e., the incident angle is equal to the exit angle). X-rays of energy E ) 10 keV (λ ) 1.24 Å) were used for these measurements. The beam size measured 0.35 mm vertically and 2.0 mm horizontally. The DTS SAMs were prepared on the native oxide surfaces of 3 in. × 1 in. × 0.1 in. boron-doped silicon (100) wafers (Semiconductor Processing Inc., Boston) via µCP at room temperature or at 80 °C. Prior to stamping, the (20) See also: Rosenberg, H.; Groves, J.; Tamborski, C. J. Org. Chem. 1960, 25, 243. (21) Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610. (b) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (22) Chen, C. H.; Shi, J. Coord. Chem. Rev. 1998, 171, 161. (23) Mitschke, U.; Ba¨uerle, P. J. Mater. Chem. 2000, 10, 1471. (24) Tate, J.; Rogers, J. A.; Jones, C. D. W.; Vyas, B.; Murphy, D. W.; Li, W.; Bao, Z.; Slusher, R. E.; Dodabalapur, A.; Katz, H. E. Langmuir 2000, 16, 6054. (25) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324.

Koide et al. wafers were cleaned by immersion in boiling Piranha solution (70% H2O2, 30% HNO3scaution!), followed by rinsing with DI H2O, acetone, and methanol. The samples were placed under helium during the measurements to reduce the background scattering from the ambient gas and radiation damage. The scattering experiments were performed at room temperature. The off-specular background was measured and subtracted from the specular counts. Electrochemistry. Cyclic voltammetric measurements were performed with an Epsilon potentiostat (Bioanalytical Systems) in ambient. A three-electrode configuration consisting of a Ag/ AgCl reference electrode, a coiled Pt wire auxiliary electrode, and an ITO (0.25 cm2) working electrode was used. The supporting electrolyte was 1.0 M tetrabutylammonium hexafluorophosphate, and 1,1′-ferrocenedimethanol (10 mM) in distilled water was used as the standard analyte solution. Atomic Force Microscopy (AFM). Topographical measurements of microcontact printed patterns were carried out with a Thermomicroscopes CP Research AFM system, operating under ambient conditions of 20-25 °C and 25-45% relative humidity. Conical Si tips with a typical 10 nm radius of curvature and a 3.2 N/m cantilever force constant were used in noncontact imaging mode. The cantilever was rastered across the surface with a piezotube scanner. The total attractive force between the tip and surface was very low (∼pN), minimizing deformation of elastic materials by the presence of the tip. The system vibrates the cantilever near the resonant frequency (ω), and interatomic forces between the tip and sample change the resonance and, hence, the vibrational amplitude, ω ) [(k - f ′)/m]1/2, where k is the spring constant in free space, f ′ is a spatial force gradient from the sample, and m is the mass. When the cantilever is moved into a regime where the force gradient is positive and increasing, the resonant frequency decreases. As the resonant frequency changes, there is a resulting change in the measured vibrational amplitude. The system responds by moving the sample relative to the tip to restore the set point vibrational amplitude, which provides a topographical sensitivity with subangstrom vertical resolution. The images were flattened to eliminate the curvature caused by the bending motion of the piezotube scanner and deglitched to remove artifacts identified as not being due to surface topography. Glitches most often result from particles dragged by the tip in the fast scan direction or electrical noise. Deglitched points are a composite average of each pixel surrounding them. Scanning Electron Microscopy (SEM). SEM images were acquired using a Hitachi S4500 microscope. The accelerating voltage was set at 3.0 kV and the sample tilt angle at 30°. For EDX spectra, the accelerating voltage was set at 15 kV. X-ray Photoelectron Spectroscopy (XPS). XPS data were collected with an Omicron ESCAPROBE (Omicron NanoTechnology GmbH), equipped with an electron flood gun and a scanning ion gun. The radiation source was monochromatized Al (ΚR) radiation. The source intensity was set at 15 mA (13.3 kV). The detector system consists of a hemispherical energy analyzer, which provides an energy resolution of 0.2 eV.

Results The goal of this work is to better understand the nature of the SAMs deposited in the hot microcontact printing (HµCP) process and, by inference, to better understand the deposition mechanism. The system DTS in hexane was chosen as a model “ink”, and the results are compared and contrasted with the results of conventional µCP using several complementary physical techniques. In the discussion below, we employ X-ray reflectivity, noncontact mode AFM, cyclic voltammetry, and hole injection blocking properties in luminescent organic heterostructures to probe SAM microstructure and morphology. DTS SAM Formation at Room Temperature. ITO surfaces were patterned by conventional µCP using a DTS/ hexane solution under inert atmosphere at room temperature. A typical µCP process is summarized in Scheme 2. The PDMS stamp (Scheme 2A) is “inked” with a hexane solution of DTS (B) and was held manually against the

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Scheme 2. Procedure for µCP ITO Surface Patterninga

a An elastomeric stamp (A) is impregnated with a DTS/hexane solution (B) and printed onto the ITO (C), forming a SAM pattern on the surface (D). A typical OLED device is fabricated by depositing TPD (E: blue), quinacridine doped Alq (E: green), and an aluminum cathode (E: brown), in this order.

substrate with a light pressure for 5 s (C). After the SAM pattern transfer (D), the resulting DTS patterns were imaged by assembling an OLED device (ITO/TPD/5% QD doped Alq3/Al) (E), and the emissive surface was examined under a microscope. Figure 1A shows the original pattern on the shadow mask used for the stamp fabrication. Dark gray areas are regions that will be covered by the DTS SAM and will come into contact with the surface. Figure 1B and C shows digitized optical images of emission from the patterned ITO-based devices prepared using varying stamp-ITO contact times (B, 5 s; C, 60 s). In Figure 1B, the DTS patterned regions create dark green backgrounds, indicating insufficient, nonuniform surface coverage. The occasional dark spots observed in the pixels are likely due to nonemissive defects, commonly observed in OLED devices, and are thought to arise from a deterioration of the aluminum cathode layer.26 When the contact time is increased to 60 s (Figure 1C), the printed region forms a nonemissive background with extremely poor dimensional reproducibility. It is reasonable to speculate that excess quantities of DTS solution contained in the bulk PDMS are discharged upon pressurization and spread uncontrollably across the substrate surface. The role of the DTS SAMs will be discussed later. Using the prolonged contact times at room temperature, the original patterns are significantly disfigured and the contours of the pixels severely distorted. The beauty of the µCP concept is that this procedure transfers only a molecular monolayer of the SAM-forming reagent to the surface, assuming the (26) Scher, M.; Nu¨esche, F.; Berner, D.; Leo, W.; Zuppiroli, L. Adv. Funct. Mater. 2001, 11, 116. (b) Burrows, P. E.; Forrest, S. R.; Zhou, T. X.; Michalski, L. Appl. Phys. Lett. 2000, 76, 2493. (c) Zhang, X.; Jenekhe, S. A. Macromolecules 2000, 33, 2069. (d) Aziz, H.; Popovic, Z.; Hu, N.-X.; Hor, A.-M.; Xu, G. Science 1999, 283, 1900. (e) Aziz, H.; Popovic, Z.; Tripp, C. P.; Hu, N.; Hor, A. Appl. Phys. Lett. 1998, 72, 2642. (f) Smith, P. F.; Gerroir, P.; Xie, S.; Hor, A. M.; Popovic, Z.; Hair, M. L. Langmuir 1998, 14, 5946.

Figure 1. Contact time dependence of the emission from µCP patterned OLED devices (ITO/TPD/5% QD doped Alq3/Al) printed with a conventional µCP procedure. (A) Optical image of the photomask pattern corresponding to the regions shown in parts B and C. (B) Image of SAM-patterned OLED emitting surface printed at 25 °C with a 5 s contact time. (C) Same experiment as in part B except the contact time is increased to 60 s.

SAM formation is essentially instantaneous. However, this is obviously not achieved in the ITO/DTS system under conventional µCP conditions. We hypothesize that the DTS trichlorosilyl functional groups and/or their hydrolysis products do not react spontaneously with the ITO surface hydroxy groups or chemisorbed water to form covalent Si-O-In/Sn linkages. DTS SAM Formation at Higher Temperatures. Given the apparent sluggish Si-O bond formation in ambient temperature µCP, thermal activation was investigated to promote the chemisorption reaction. Accordingly, the ITO substrate was maintained at 50 °C on a thermostated hot plate while the DTS/hexane impregnated stamp was brought into contact. Figure 2 shows the effect of increased SAM transfer temperature on the emission pattern of the anode patterned OLED devices. The original pattern appearing on the shadow mask used for the stamp fabrication is shown in Figure 2A. Figure 2B shows emission from the ITO surfaces printed at 50 °C for 5 s. At elevated printing temperatures, even at short contact times, the DTS SAM uniformity and contrast are markedly improved without sacrificing the edge resolution compared to that of the room temperature process (Figure 1B). This is presumably a consequence of increased SAM packing density (see discussion below);

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Figure 2. (A) Image of the photomask pattern corresponding to the regions shown in part B. (B) Optical images of the emitting surface of the higher temperature (50 °C) µCP patterned OLED device (ITO/TPD/5% QD doped Alq3/Al). The contact time is 5 s.

Figure 4. Optical images of the emitting surface of a HµCP (80 °C) patterned OLED device (ITO/TPD/5% QD doped Alq3/ Al) operated at different bias voltages: (A) at 12 V; (B) at 17 V. The contact time is 5 s. (C) Current-voltage and light outputvoltage curves for the OLED device. The response is corrected for fill factor.

Figure 3. Effect of increased temperature on µCP edge resolution. (A) Optical image of a photomask pattern (an array of 1.0 µm pixels) corresponding to the regions shown in part B. (B) Emitting surface of a higher temperature (50 °C) µCP patterned OLED device (ITO/TPD/5% QD doped Alq3/Al). The contact time is 5 s.

however, non-negligible emission from the SAM-covered region is still obvious. The dimensional resolution is also drastically enhanced, and images as small as 1.0 µm pixel arrays are faithfully reproduced (Figure 3). The images show an array of 1.0 µm diameter pixels on the original photomask (Figure 3A) and emission from the identical pattern (Figure 3B). Even higher precision and image contrast are achieved when the ITO substrate temperature is further raised to

80 °C with the same contact time (5 s). Figure 4 shows the emitting surface of an OLED device operated at 12 V (Figure 4A) and 17 V (Figure 4B) bias voltage. At higher operating bias, the light output from the pixels increases while the printed region provides a uniformly dark background and remains essentially nonemissive. Note that both images in Figure 4 are taken at the same exposure time and that the bright/dark boundary in Figure 4B appears slightly blurred due to the increased brightness. This observation suggests an effective role of the DTS SAM as an ultrathin hole-blocking dielectric layer, which is further discussed below. Uniform coverage of the DTS SAM at high temperatures can be rationalized if monolayer formation occurs instantaneously following the initial stamp-substrate contact. Furthermore, once the ITO surface is masked by the DTS SAM, macroscopic amounts of additional DTS/hexane solution are presumably repelled from the surface. Using this HµCP procedure, both edge resolution and contrast are dramatically improved (Figure 5). Optical micrographs of emitting 2.0 and 10 µm pixel arrays (Figure 5A) and a 1.0 µm pixel

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Figure 6. Plots of the X-ray reflectivity vs wave vector for the DTS SAMs on the native oxide coating of a single-crystal silicon substrate prepared by (a) HµCP (b) and (b) conventional µCP (O).

Figure 5. Optical images of the emitting surface of a HµCP (80 °C) patterned OLED device (ITO/TPD/5% QD doped Alq3/ Al). The contact time is 5 s. (A) 2.0 and 10 µm pixel arrays. (B) 1.0 µm pixel array.

array (Figure 5B) exhibit faithful reproduction of the original patterns. The SAM masked background appears uniformly nonemissive, and negligible pinhole type emission can be detected. The pinhole assay will be further discussed later. When HµCP is attempted at temperatures exceeding 90 °C, the solvent (hexane) in the bulk of the stamp apparently vaporizes rapidly, thereby creating gaseous voids between the stamp and the ITO surface and resulting in the formation of overlapped patterns. X-ray Reflectivity Characterization of Microcontact Printed Films. The deposition temperature dependence of the transferred film thickness and packing density was further characterized by X-ray reflectivity (XRR) measurements. Because of intense interfering background scattering from ITO substrates, useful reflectivity data could not be obtained on overlaying SAMs. Thus, the native oxide covering of (100) single-crystal silicon was used as a model for the ITO surface, to compare and contrast HµCP-derived SAMs from those deposited by conventional µCP. Figure 6 shows film X-ray specular reflectivity data normalized to the Fresnel reflectivity for a HµCP-derived film and for a film deposited by conventional µCP. Fitting of the reflectivity data to the Gaussian-step model gives the film electron density, the thickness of the film, and the film roughness. A detailed description of the XRR data analysis and models can be found elsewhere.3c,27 The XRR data for a HµCP-derived film are characteristic of a welldefined monolayer (Figure 6a). The derived film thickness of 12.1 ( 0.1 Å and the surface roughness, 2.8 ( 0.1 Å, are well within the range expected for DTS monolayer formation.15 Furthermore, the electron density of the HµCP-derived DTS film (0.30 e Å-3) is comparable to that reported for a self-assembled n-octadecyltrichlorosilane

monolayer (0.29 e Å-3).3b,27 In contrast, the XRR data for a film deposited by conventional µCP on a silicon singlecrystal substrate (Figure 6b) show the presence of a relatively thinner film of 8.6 ( 0.3 Å with a surface roughness of 3.6 ( 0.3 Å. The poorly defined structure of the conventional µCP-derived DTS film is likely a consequence of nonuniform surface coverage. This agrees with the experimental results discussed both above and below (vide infra). Attempted Detection of the Residual HCl or Cl Containing Reaction Products by XPS. XPS surface elemental analysis of the DTS SAM assembled ITO was conducted to probe the fate of HCl generated in the reaction of the alkyltrichlorosilane and the surface hydroxy groups. Although the XPS analysis readily detected carbon, oxygen, silicon, indium, and tin, any chlorine was below the detection limits,28 presumably because HCl escapes from the reaction site. Electrochemical Pinhole Assay of the DTS SAM on ITO. Cyclic voltammetry (CV) of 1,1′-ferrocenedimethanol was recorded at DTS SAM masked ITO working electrodes (Figure 7). The continuous DTS SAMs were assembled using a featureless PDMS stamp via HµCP and conventional µCP, and the magnitude of current response at the two electrodes was compared. The large current response measured at the conventional µCP ITO electrode is consistent with a significant exposure of the bare ITO surface to the analyte solution. At the HµCP ITO electrode, however, the current response is considerably diminished, indicative of relatively good surface protection/insulation by the DTS SAM,29,30 although the (27) van der Boom, M. E.; Richter, A. G.; Malinsky, J. E.; Lee, P. A.; Armstrong, N. R.; Dutta, P.; Marks, T. J. Chem. Mater. 2001, 13, 15. (b) Richter, A. G.; Durbin, M. K.; Yu, C.-J.; Dutta, P. Langmuir 1998, 14, 5980. (28) Typical reported detection limits of element-specific XPS measurements range from 10-2 to 10-3 atomic fraction, and variation of the sensitivity range is within a factor of 20. See, for example: Czanderna, A. W., Hercules, D. M., Eds. Ion Spectroscopies for Surface Analysis; Plenum: New York, 1991. (29) Hillebrandt, H.; Tanaka, M. J. Phys. Chem. B 2001, 105, 4270. (30) Malinsky, J. E.; Jabbour, G. E.; Shaheen, S. E.; Anderson, J. D.; Richter, A. G.; Marks, T. J.; Armstrong, N. R.; Kippelen, B.; Dutta, P.; Peyghambarian, N. Adv. Mater. 1999, 11, 227. (b) Malinsky, J. E.; Veinot, J. G. C.; Jabbour, G. E.; Shaheen, S. E.; Anderson, J. D.; Lee, P.; Richter, A. G.; Burin, A. L.; Ratner, M. A.; Marks, T. J.; Armstrong, N. R.; Kippelen, A.; Dutta, P.; Peyghambarian, N. Chem. Mater., 2002, 14, 3054.

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Figure 7. Cyclic voltammetry of a 10 mM 1,1′-ferrocenedimethanol solution in water recorded at a DTS SAM coated ITO working electrode assembled using HµCP (solid line) and conventional µCP (broken line) at the scan rate 500 mV/s.

existence of some pinholes must be responsible for the non-negligible current recorded. AFM Studies of DTS SAM Morphology as a Function of Printing Temperature. The morphologies of transferred DTS SAMs on ITO surfaces formed via HµCP and conventional µCP were imaged using noncontact mode atomic force microscopy. Figure 8A shows a topographical image (top) and the line profile (bottom) of an as-formed 20 µm pixel and the surrounding area created by HµCP. The SAM covered region (outside the circle) appears irregular while the nonstamped region (inside the circle) remains virtually free of detectable DTS coverage. The line profile indicates that the circular feature is surrounded by a 1.5-2.0 nm thick DTS SAM, and although the rms surface roughness of the ITO glass (5.0-10 Å) precludes accurate estimation of the SAM thickness by AFM, no local buildups are recognizable. At the boundary of the circle, the light-colored ring in the topographic image represents a thin solid wall, which can be seen as spikes as high as 4 nm in the line profile. This wall-like structure may be created by diffusion of the DTS/hexane solution toward the boundary of the relief structure engraved on the stamp, producing an area of high local DTS concentration. Consequently, following stamp lift off, the excess DTS molecules, which are excluded from chemisorption by the densely packed SAM, pile up on top of each other, resulting in the observed wall-like structure. Note that this phenomenon may also be a consequence of the capillary flow phenomenon known as the “coffee ring effect”, which causes dense, ringlike deposits around the perimeters of droplets of evaporating solutions.31 With the conventional µCP procedure, however, this wall-like structure is considerably more thick and occupies the majority of the printed area and beyond (Figure 8B). The significantly narrowed SAM-free pixel pattern is positioned near the center of the image, surrounded by a thick, irregular solid structure that reaches 8-15 nm above the baseline, as shown in the line profile. The extremely rough surface implies that no orderly structure is deposited in this area. The pixel is disfigured from its original shape, presumably because of retarded bond formation and a subsequent diffusion of the DTS solution inside the circle. Discussion At room temperature, the conventional µCP process encounters two conflicting requirements: (1) defect-free, dense monolayer formation and (2) achievement of accurate printed image transfer. Apparently, longer contact times benefit the first requirement while the opposite is (31) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827.

Figure 8. (A) Noncontact mode topographical AFM image (top) and the line profile (bottom, measured along the blue broken line) of a 20 µm pixel and the surrounding area created by HµCP of a DTS SAM at 80 °C on an ITO surface. The print duration is 5 s. (B) Noncontact mode topographical AFM image (top) and the line profile (bottom) of a 20 µm pixel and the surrounding area created by conventional µCP of a DTS SAM at 25 °C on an ITO surface. The contact time is 5 s.

true for the second.7,8,15 HµCP addresses these problems by achieving both high packing density, by virtue of thermal activation of the Si-O bond formation, and precision, by reducing ink diffusion using short contact times. Similar reactivity effects are postulated to be operative in the heavily studied RSH-on-gold system.25 A

HµCP for Patterning ITO Surfaces

potential limitation of HµCP, however, is that exposure to higher temperatures will increase the RSiCl3 vapor pressure to some degree, resulting in low resolution due to undesired ink vapor transport to, and deposition on, the nonprinted area.32 Nuzzo and co-workers have shown that µCP patterns of n-octadecyltrichlorosilane (n-C18H37SiCl3, OTS; 10 and 20 mM in hexane) created on Si/SiO2 surfaces at ambient temperature (30 s contact time) form numerous small OTS islands in the unstamped regions.15,33 In accord with the relatively low volatility and mobility of DTS, however, faithful reproduction of the original pattern on the ITO surface argues that minimal direct vapor phase deposition occurs even at 80 °C. Nuzzo also studied thin film growth by DTS on the native oxide surface of Si substrates and concluded that a combination of appropriate contact time and slightly elevated temperatures is beneficial for accurate transfer of the printed image.15 Although these authors also pointed out that suitable humidity improves the creation of a close-packed monolayer, all of the present µCP procedures were carried out under a moisture-free environment in order to prevent adventitious hydrolysis of the trichlorosilyl functional group. DTS SAMs as Insulating Layers. The patterned emission from an OLED device assembled on a DTS SAMmodified ITO surface can be understood if the monolayer serves as a molecular resist/insulator that attenuates hole injection from the ITO anode to the upper organic layers (Scheme 1) so that no emission arising from radiative electron-hole recombination occurs from EML/ETL regions directly above the SAM covered region.34 Given that the present soft lithography leaves no visible ITO transparency changes, the 1.2 nm thick SAM3 clearly impedes hole injection into the TPD layer, rendering the stamped area essentially nonemissive. The capability of alkanethiol (RSH) SAMs on Au, Ag, and Cu substrates to form dense, ordered monolayers and to serve as excellent chemical resists and insulating barriers is well-established,35 although such a capability in the alkyltrichlorosilane/ITO system has not been previously reported. The present hole blocking phenomena are attributed to (1) the highly insulating, barrier nature of C12-C18 alkyl siloxane monolayers, which, according to Vuillaume’s work,36 possess an effective conductivity as low as ∼4.6 × 10-15 S/cm, and (2) anode-hole transport layer interfacial dipoles induced by sufficiently thick siloxane monolayers that hinder hole injection.37 The effect of SAMs of varying dipole moments on the charge injection barrier is ad(32) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324. (33) Jeon, N. L.; Finnie, K.; Branshaw, K.; Nuzzo, R. G. Langmuir 1997, 13, 3382. (34) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv. Mater. 1999, 11, 605. (35) Larsen, N. B.; Biebuyck, H. A.; Delamarche, E.; Michel, B. J. Am. Chem. Soc. 1997, 119, 3017. (36) Vuillaume, D.; Boulas, C.; Collet, J.; Allan, G.; Delerue, C. Phys. Rev. B 1998, 58, 16491. (b) Vuillaume, D.; Boulas, C.; Collet, J. Appl. Phys. Lett. 1996, 69 (11), 1646. (c) Boulas, C.; Davidovits, J. V.; Rondelez, F.; Vuillaume, D. Phys. Rev. Lett. 1996, 76, 4797. (d) Fontaine, P.; Goguenheim, D.; Deresmes, D.; Vuillaume, D.; Garet, M.; Rondelez, F. Appl. Phys. Lett. 1993, 62 (18), 2256.

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dressed in recent articles in which a drop in turn-on voltage is reported as the dipole moment of the adsorbate increases.16a,30,38 It is noteworthy that chain length-dependent electron tunneling effects have also been observed by research groups studying electron transfer between solution redoxactive species and a variety of alkyl SAM grafted electrodes, for example, Au grafted with RSH,39 Si grafted with RSiCl3,40 InP grafted with RSH,41 and ITO grafted with RSiCl3.29 In these cases, a distance-dependent electron tunneling model is invoked to explain the exponential dependence of the tunneling current on the SAM chain length:39b,39c,41 ket ∝ exp(-βn), where 1/β is the characteristic decay length and n is the number of methylene units. The β parameter for the alkane layers is about 1 per CH2.39b,39c Therefore, it follows that, all other things being equal, the tunneling current should decrease exponentially as the SAM chain length increases, which has been demonstrated electrochemically29,39-41 and is consistent with the current findings for a C21 alkyl chain. Conclusions We have demonstrated here that HµCP is a useful variant of the conventional microcontact printing methodology, especially for derivatization of poorly reactive surfaces as exemplified by ITO. It is capable of providing densely packed alkylsiloxane SAMs on selected metal oxide surfaces with features having micrometer-scale dimensions. Densely packed DTS SAMs behave as excellent insulators, as addressed above, and are applicable for nanoscale electronic device fabrication. Micrometerdimension patterned vapor phase ZnO film growth is one such application.10c Acknowledgment. This research was supported by the NSF MRSEC program through the Northwestern Materials Research Center (Grant DMR-0076097). The authors acknowledge the use of the Northwestern MRSEC central facilities, the Keck Interdisciplinary Surface Science facilities, and the EPIC central facilities. M.W.S., R.B., and M.C.H. acknowledge support by the NSF under Grant No. DMR-0134706. G.E. and P.D. are grateful to support by the NSF under Grant No. DMR-9978597. XRR measurements were performed at beamline X23B of the National Synchrotron Light Source, which is supported by the U.S. Department of Energy. LA020604B (37) Le, G. T.; Forsythe, E. W.; Nu¨esche, F.; Rothberg, L. J.; Yan, L.; Gao, Y. Thin Solid Films 2000, 363, 42. (b) Nu¨esche, F.; Li, Y.; Rothberg, L. J. Appl. Phys. Lett. 1999, 75, 1799. (c) Lee, H.-M.; Choi, K.-H.; Hwang, D.-H.; Do, L.-M.; Zyung, T.; Lee, J.; Park, J.-K. Appl. Phys. Lett. 1998, 72, 1998. (d) Hill, I. G.; Gajagopal, A.; Kahn, A.; Hu, Y. Appl. Phys. Lett. 1998, 73, 662. (38) Ganzorig, C.; Kwak, K.-J.; Yagi, K.; Fujihira, M. Appl. Phys. Lett. 2001, 79, 272. (39) Chidsey, C. E. Science 1991, 251, 919. (b) Meller, Cl.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (c) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y.-P. J. Phys. Chem. 1995, 99, 13141. (40) Barrelet, C. J.; Robinson, D. B.; Cheng, J.; Hunt, T. P.; Quate, C. F.; Chidsey, C. E. D. Langmuir 2001, 17, 3460. (41) Gu, Y.; Waldeck, D. H. J. Phys. Chem. 1996, 100, 9573.