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Langmuir 1998, 14, 5594-5602
Fabrication of Monolayers Containing Internal Molecular Scaffolding: Effect of Substrate Preparation Mark D. Mowery, Henning Menzel,† Mei Cai, and Christine E. Evans* Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109-1055 Received March 24, 1998. In Final Form: July 17, 1998 The nanoscale design and fabrication of monolayer assemblies is becoming increasingly important for research areas ranging from adhesion to chemical sensors. The formation of molecular scaffolding within a single molecular layer by linking adjacent molecules provides an important means for the nano- to microscale fabrication of such interfacial assemblies. Unfortunately, key factors in the design and fabrication of these internally linked monolayers are often overlooked due to direct analogy with the often studied n-alkyl monolayer systems. In this investigation, the impact of substrate preparation on the resulting monolayer structure is compared for n-alkyl (C18) and internally linked polydiacetylene monolayer assemblies formed on evaporated, sputtered, and colloidal gold surfaces. Polydiacetylene monolayers are fabricated by the spontaneous assembly of diacetylene-containing disulfides followed by photoinduced polymerization. The resulting polydiacetylene monolayers exhibit systematic variations in the degree of polymerization, alkyl chain crystallinity, advancing contact angle, and electron-transfer inhibition with substrate preparation. By all these measures, both the short- and long-range order of these polymerized monolayers are observed to increase substantially on evaporated gold substrates. In contrast, the n-alkylbased monolayers formed under identical conditions show minimal structural variation. Moreover, surface pretreatment is demonstrated to have a significant impact on the long-range order for both the n-alkyl and polydiacetylene monolayers. These experimental observations implicate domain size as a significant parameter in the fabrication of polydiacetylene monolayers, while exhibiting little or no impact on the apparent structure of n-alkyl-based monolayer assemblies. Ultimately, the successful fabrication of monolayer structures containing internal molecular scaffolding is made possible by the judicious choice of substrate preparation conditions.
Introduction In the past decade, spontaneously organized monolayers formed from thiol or disulfide compounds on gold surfaces have been extensively studied.1 The most exhaustive studies have been centered on n-alkyl and ω-functionalized assemblies, where a wide range of monolayer interfaces are fabricated by manipulating the chain length and terminal functionality.1-3 More recently, studies have begun to incorporate internal functionality within the monolayer structure including chromophores and electroactive species.4-7 When these internal functionalities behave in concert, an internal molecular scaffolding is created within the monolayer framework. Individual molecules within the monolayer are interconnected by interaction mechanisms including π-stacking,8,9 hydrogen * To whom correspondence should be addressed: Fax: 734 6474050. E-mail,
[email protected]. ‡ Permanent Address: Institut fu ¨ r Makromolekulare Chemie, Universita¨t Hannover, Am Kleinen Felde 30, 30167 Hannover, Germany. (1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991 and references therein. (2) Bain, C. H.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (3) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437 and references therein. (4) Cheng, J.; Saghi-Szabo, G.; Tossell, J. A.; Miller, C. J. J. Am. Chem. Soc. 1996, 118, 680. (5) Campbell, D. J.; Herr, B. R.; Hulteen, J. C.; Van Duyne, R. P.; Mirkin, C. A. J. Am. Chem. Soc. 1996, 118, 10211. (6) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E.; Hussey, C. L. Langmuir 1998, 14, 124. (7) Yu, H. Z.; Shao, H. B.; Luo, Y.; Zhang, H. L.; Liu, Z. F. Langmuir 1997, 12, 55774. (8) Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L. R.; Small, J. F.; Newton, M. D.; Feldberg, S. W.; Chidsey, C. E. D. J. Am. Chem. Soc. 1997, 119, 10563.
bonding,6,10 dipole coupling,11 and covalent attachment.12-17 Extension of these specific interactions laterally within the monolayer makes possible the design of interfacial structures with novel optical, electronic, and structural properties.4-17 Fabrication of self-assembled monolayers with such interconnected structures presents some unique challenges. The nature of these challenges is not always apparent when considered from the perspective of the wellstudied n-alkyl, ω-terminated structures. Indeed, fabrication strategies based on direct analogy to simple n-alkyl systems can often mislead, causing important parameters in film preparation to be neglected. As illustrated in Figure 1A, three regions contribute to the interactions that determine the overall structure of n-alkyl-based monolayers: (I) sulfur headgroup interactions with the gold surface; (II) van der Waals interactions between the alkyl chains; (III) interactions between the terminal functional groups. In contrast, the formation of internal molecular scaffolding within the monolayer structure can create five distinct organizing regions. As (9) Dhirani, A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (10) Clegg, R. S.; Hutchinson, J. E. Langmuir 1996, 12, 5239. (11) Evans, S. D.; Goppert-Berarducci, K. E.; Urankar, E.; Gerenser, L. J.; Ulman, A. Langmuir 1991, 7, 2700. (12) Peanasky, J. S.; McCarley, R. L. Langmuir, 1998, 14, 113. (13) Mowery, M. D.; Evans, C. E. J. Phys. Chem. 1997, 101, 8513. (14) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Ha¨ussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (15) Kim, T.; Chan, K. C.; Crooks, R. M. J. Am. Chem. Soc. 1997, 119, 189. (16) Kim, T.; Crooks, R. M.; Tsen, M.; Sun, L. J. Am. Chem. Soc. 1995, 117, 3963. (17) Kim, T.; Ye, Q.; Sun, L.; Chan, K. C.; Crooks, R. M. Langmuir 1996, 12, 6065.
S0743-7463(98)00331-X CCC: $15.00 © 1998 American Chemical Society Published on Web 08/27/1998
Monolayers Containing Internal Molecular Scaffolding
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Figure 2. (A) Schematic diagram of the structure and polymerization of diacetylene monolayers on surfaces with a high ratio of step to terrace sites. (B) Polymerization in diacetylene monolayers is dictated by strict spatial requirements (a ) 4.7-5.2 Å, b ) 3.4-4.0 Å) and is topochemical in nature (a ≈ e). Polymerization across step sites is greatly hindered because the spatial tolerances are exceeded (c and d).
Figure 1. Schematic depicting the monolayer spatial regions contributing to structural order in (A) n-alkyl and ω-functionalized monolayers (a ≈ 2.18 Å and b ) 2.36 Å)31 and (B) internally functionalized monolayers with the terminal functional group represented by Y.
illustrated in Figure 1B, the headgroup and terminal group interactions remain (regions I and III), the new scaffolding region is incorporated, and the alkyl chain interactions are now divided into a spacer and tail region (regions IIspacer and IItail). In this new architecture, interactions between neighboring molecules that form the scaffolding region may significantly contribute to the overall monolayer structure. In this case, van der Waals interactions between alkyl chains are not the only factor driving intermolecular interactions in the spontaneous assembly process. In fact, structure within the alkyl regions may be affected by intermolecular interactions within the scaffolding region.11 In contrast with the n-alkyl assemblies, the alkyl region between the substrate and the scaffolding (region IIspacer) resides in a more constrained environment than the alkyl tail region (region IItail). As a consequence, these two alkyl regions may behave quite differently and cannot be assumed to have identical structure. Thus, the design and fabrication of molecular scaffolding within monolayer assemblies cannot be considered as a simple modification on n-alkyl-based structure and must be evaluated in light of all the components contributing to the overall structure. The impact of substrate preparation on the structure of these more complex monolayer assemblies is one case
in point. In the fabrication of n-alkyl-based self-assembled layers on gold, the deposition conditions are rarely considered.18,19 Given the consistency of monolayer structures obtained under a wide range of conditions, the structure of the monolayers does not appear to be significantly affected by these variations.1-3,20 As a result, substrate fabrication conditions often remain overlooked when fabricating monolayers with internal structure. In this paper, we examine the impact of common surface preparation methods on the fabrication of monolayer structures containing internal molecular scaffolding. The specific system of interest here utilizes diacetylenecontaining disulfides to create spontaneously assembled monolayers (Figure 2). Molecular scaffolding is formed by covalent attachment between adjacent molecules, initiated by photoinduced polymerization. As shown in Figure 2, the expected spatial constraints for polymerization are considerable,21,22 offering a rigorous test of surface effects. However, this system is also unique, in that no significant expansion or contraction of the monolayer is expected upon polymerization.21,22 As a result, the polymerization process itself is not expected to significantly alter the monolayer structure. Extensively studied after the first report by Wegner,23 the feasibility of polymerizing self-assembled diacetylenes has been recently demonstrated.13-17 However, the role of substrate preparation conditions on the formation of these poly(18) Guo, L.-H.; Facci, J. S.; McLendon, G.; Mosher, R. Langmuir 1994, 10, 4588. (19) Creager, S. E.; Hockett, L. A.; Rowe, G. K. Langmuir 1992, 8, 854. (20) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (21) Schott, M. W.; Wegner, G. In Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, J., Ed.; Academic Press: Orlando, FL, 1987. (22) Lando, J. B. In Polydiacetylenes; Bloor, D., Chance, R., Eds.; Nijhoff: Dordrecht, The Netherlands, 1985. (23) Wegner, G. Z. Naturforsch. 1969, 24B, 824.
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merized monolayers has been largely overlooked. Recent studies indicating preferential polymer formation in the interstitial regions of sputtered gold films provide evidence that substrate topography is an important factor in the fabrication of these polymerized monolayers.13 In this study, polymer formation and characterization of the resulting monolayer will be assessed under varying substrate conditions. In all cases, direct comparison is shown with monolayers formed using octadecane thiol and, therefore, containing no internal molecular scaffolding. Experimental Methods Monolayer Fabrication. Three procedures for gold film preparation are assessed in this study: high-temperature evaporation, room-temperature sputtering, and surface-confined colloids. Evaporated and sputtered films are prepared directly on a mica surface, while the colloids were attached to silicon substrates. Evaporated gold films were prepared using a custombuilt ultrahigh vacuum (UHV) thin film deposition system. The mica (ASTM V-2; Asheville-Schoonmaker Mica Co.) was cleaved on both sides immediately before insertion into the chamber where they were suspended 21 cm above the source using a tantalum mask and degassed at 380 °C for 12 to 24 h with two 500 W halogen lamps positioned approximately 8 cm below the substrate. The temperature was subsequently decreased to 200 °C, and gold (99.99%) was evaporated from a K-Cell (Oxford Instruments) onto the mica at a rate of 0.03 Å/s to a final thickness, as measured by a quartz crystal microbalance (Leybold Inficon Inc.), of approximately 2000 Å. The pressure during deposition was less than 1 × 10-7 Torr. After deposition, the substrates were annealed for 3 h at the deposition temperature and then allowed to cool to room temperature (P ) 2 × 10-10 Torr). Finally, the chamber was backfilled with dry nitrogen and the substrates were removed. Detailed surface morphology studies indicate that this procedure is expected to yield atomically flat terraces over distances of 100 to several hundred nanometers.24 Sputtered gold substrates were prepared using an electrical discharge deposition system (Denton Desk II). Films of 2000 Å gold (g99.99%) were sputtered onto freshly cleaved mica. Although literature studies indicate that this room-temperature deposition yields grain sizes of 10-50 nm, the flat crystalline terraces on these surfaces are typically less than 10 nm.25-28 A portion of the gold films were subsequently cleaned by UV irradiation with a mercury-vapor pen lamp (model 11SC-1; Spectronics Corp.) for 1 h followed by rinsing in ethanol.29-32 The colloidal gold monolayers were prepared on silicon substrates following a previously reported procedure.33 A layer of 20 nm colloidal gold (Sigma) was surface attached using (3-mercaptopropyl)methyldimethoxysilane (Fluka). Gold colloids in this size range, synthesized by AuCl4- reduction, have been shown to consist of decahedra and cuboctahedra with flat facets from 2 to 8 nm in diameter.34 Consistent with previous reports, a single layer thickness of incomplete surface coverage was obtained.33 (24) (a) Liu, Z. H., Brown, N. M. D. Thin Solid Films 1997, 300, 84. (b) Buchholz, S.; Fuchs, H.; Rabe, J. P. J. Vac. Sci. Technol., B 1991, 9, 857. (c) DeRose, J. A.; Thundat, T.; Nagahara, L. A.; Lindsay, S. M. Surf. Sci. 1991, 256, 102. (d) Putnam, A.; Blackford, B. L.; Jericho, M. H.; Watanbe, M. O. Surf. Sci. 1989, 217, 276. (e) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45. (f) Salmeron, M.; Kaufman, D. S.; Marchon, B.; Ferrer, S. Appl. Surf. Sci. 1987, 28, 279. (25) Golan, Y.; Margulis, L.; Rubinstein, I. Surf. Sci. 1992, 264, 312. (26) Schonherr, H.; Vansco, G. J. Langmuir, 1997, 13, 3769. (27) Butt, H.-J.; Seifert, K.; Bamberg, E. J. Phys. Chem. 1993, 97, 7316. (28) Butt, H.-J.; Mu¨ller, T.; Gross, H. J. Struct. Biol. 1993, 110, 127. (29) Ishida, T.; Tsuneda, S.; Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 4638. (30) Ron, H.; Rubinstein, I. Langmuir 1994, 10, 4566. (31) Takahagi, T.; Nagai, I.; Ishitani, A.; Kuroda, H.; Nagagawa, Y. J. Appl. Phys. 1998, 64, 3516. (32) King, D. E. J. Vac. Sci. Technol., A 1995, 13, 1247. (33) Grabar, K. C.; Brown, K. R.; Keating, C. D.; Stranick, S. J.; Tang, S. L.; Natan, M. J. Anal. Chem. 1997, 69, 471. (34) Mulvaney, P.; Giersig, M. J. Chem. Soc., Faraday Trans. 1996, 92, 3137.
Mowery et al. Synthesis of the 15,9-diacetylene disulfide followed the method recently reported by our laboratory.35 Further purification of the disulfide was accomplished by chromatography using silica gel and hexanes/dichloromethane (4:1) as the solvent. The resultant disulfide was a white crystalline material that showed a single spot by thin-layer chromatography (TLC) (hexanes/DCM 4:1) and no spurious peaks by 1H NMR. Rigorous light control was maintained throughout synthesis, solution storage, and monolayer fabrication. In contrast with their thiol counterparts, diacetylene-containing disulfides are very robust and can be stored for extended periods (>12 months) without spurious polymerization or degradation. Preparation of the monolayer films was accomplished by immersing the gold substrates in a 1 mM chloroform solution of the 15,9-diacetylene disulfide or octadecane thiol immediately following gold deposition, unless otherwise noted. After 40 to 48 h of equilibration, substrates were removed and rinsed extensively with chloroform (Aldrich, >99%) and deionized water (model UV Plus Milli-Q, Millipore; >18 MΩ) and dried under nitrogen. The resulting diacetylene monolayers were subsequently polymerized under nitrogen for 5 min with a low-intensity UV lamp (model UVG-11; Ultra-Violet Products Inc.; λ ) 250-260 nm) at a distance of 2 cm. Control experiments using n-alkane thiol monolayers indicated no measurable alterations in monolayer properties upon UV irradiation in this manner.13 Raman Spectroscopy. Resonance Raman spectra were obtained using an imaging system consisting of a microscope objective (10×, 0.25 numerical aperature), a spectrograph (Holoscope f/1.8; VPT; Kaiser Optical System), and a chargecoupled device detector (TK1024AB, Photometrics). Excitation was accomplished using the 632.8 nm line from a He-Ne laser (model 05-LHP-991, Melles Griot) at an incident power of 10 mW. The detector was cooled with liquid nitrogen to -110 °C. Spectra were calibrated using emission lines of known wavelength from a neon lamp.36 Fourier Transform Infrared (FTIR) Spectroscopy. Grazing-angle FTIR spectra were obtained using a nitrogen-purged Nicolet 550 Magna IR spectrometer with a liquid nitrogen cooled MCT detector. By use of a specular reflectance accessory (Spectra-Tech Inc.), p-polarized light was incident on samples at 85° with respect to normal. An average of 1024 scans is reported in all cases with referencing against an unmodified gold film. A fresh gold film is utilized for each series of measurements due to reference surface contamination with time that leads to bias in the measured spectrum. All spectra were collected with 2 cm-1 resolution, except the colloidal gold surfaces where 4 cm-1 resolution was necessary. Contact Angle Measurements. Contact angles were measured using the standard tilted plate method in an enclosed system. A drop of liquid was placed on the sample, which was subsequently tilted until just before the drop moves. The angle was then measured using a custom-built instrument. The hexadecane measurements were carried out under ambient conditions, and the water contact angles were measured in a humidity-controlled chamber. Electrochemistry. Cyclic voltammetry experiments were conducted using a standard three-electrode cell with a double junction Ag/AgCl reference electrode (saturated KCl internal solution) and a coiled platinum wire counter electrode. All measurements were performed at room temperature (22 ( 1 °C) with a potentiostat (CV-27; Bioanalytical Systems) and XY recorder (model 7035B, Hewlett-Packard) using an inert elastomer O-ring to define the working electrode area (0.95 cm2). All solutions were prepared immediately prior to use using ultrahigh purity water (Milli-Q UV Plus, Millipore, >18 MΩ). Atomic Force Microscopy (AFM). AFM images of the substrate surfaces were acquired in tapping mode under ambient conditions (Nanoscope III, Digital Instruments Inc.). To facilitate direct comparison of cross sections, all surface images were measured using a single silicon cantilever with integral tip (spring constant ) 20-100 N/m). Images were obtained by oscillating (35) Mowery, M. D.; Evans, C. E. Tetrahedron Lett. 1997, 38, 11. (36) Striganov, A. R.; Sventitsku, N. S. Tables of Spectral Lines of Neutral and Ionized Atoms; Plenum Data: New York, 1979; Vol. 4, pp 101-184.
Monolayers Containing Internal Molecular Scaffolding
Figure 3. Representative cross sections of the substrates investigated in this paper. All images were acquired using tapping-mode atomic force microscopy under identical conditions. The vertical scale in the bottom portion of the figure is magnified 10×. The colloidal Au cross section was chosen to show a defect site to illustrate the colloidal Au thickness. the cantilever slightly below its resonance frequency (typically 200-300 kHz) and raster scanning across the surface. The same scanning parameters (setpoint and free vibration amplitude) were used for all substrates.
Results and Discussion The influence of substrate preparation conditions on the fabrication of self-assembled monolayers with internal molecular scaffolding is examined using a photopolymerizable diacetylene disulfide,
[CH3(CH2)nCtC-CtC(CH2)mS-]2 where n ) 15 and m ) 9. Although the spatial constraints for polymerization in diacetylene compounds are considerable (Figure 2), previous studies have demonstrated the feasibility of fabricating these polymeric structures within self-assembled monolayers.13-17 In this study, formation of the conjugated polymer backbone, indicative of internal molecular alignment, is monitored spectroscopically under varying surface conditions. By use of resonance Raman spectroscopy, polymer formation is directly measured by the appearance of the characteristic delocalized double and triple bonds associated with backbone formation. The polymerization process itself is not expected to significantly expand or contract the monolayer structure, and as a result, minimal perturbation in the overall structure of the monolayer is expected upon polymerization. Substrate preparation conditions were chosen for this study to represent the range of conditions often encountered in monolayer fabrication. Representative cross sections of tapping-mode AFM images of these surfaces are illustrated in Figure 3. While the cross-sectional
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images are undoubtedly a convolution of surface topography and AFM tip shape, these images qualitatively illustrate the long-range surface roughness examined here. Previous studies have assessed the detailed surface morphology of gold films under similar preparation conditions. From these studies, atomically flat regions for the high-temperature evaporated gold films are expected to be greater than 100 nm.24 In contrast, the sputtered gold films are prepared at room temperature and deposited at a higher kinetic energy. As a result, the atomically flat regions are expected to be less than 10 nm.26-28 Surface-attached colloidal gold represents a new range of substrates presently being utilized for selfassembled monolayer fabrication. Consistent with previous reports,33 the colloidal substrate is characterized by an incomplete monolayer coverage of apparently spherical gold features with the height consistent with the initial colloidal structures (∼20 nm). Due to the convolution with tip shape, individual colloids were not visible using AFM imaging under these conditions.33 High-resolution transmission electron microscopy studies have shown that gold nanoparticles in this size range are most commonly decahedra and cuboctahedra with atomically flat facets from 2 to 8 nm in diameter.34 In addition to initial surface preparation, evaluation of the impact of surface contaminants is examined by comparison of sputtered gold substrates upon direct immersion from the sputterer and after cleaning by a UV/EtOH pretreatment. Although commonly presumed to be free of surface contamination on direct removal from a commercial instrument, gold films have been shown to suffer from contamination.1,24e,37 This difficulty is likely due to adsorption of low molecular weight species and may result from cleaving mica in air.1,24e,37 This problem is often alleviated for evaporated films by a preliminary heating procedure and by the hightemperature deposition conditions.24 Finally, all studies simultaneously examine monolayers formed under identical conditions using octadecane thiol. In this way, direct comparison is possible between n-alkyl-based monolayers and those containing an internal molecular scaffolding. Polymerization. The spatial constraints for polymerization in diacetylene compounds are considerable, requiring precise registry and spatial proximity of adjacent monomers (Figure 2). However, no significant expansion or contraction of the monolayer structure is expected upon polymerization. As a result, initiation of covalent attachment between adjacent monomers is not anticipated to significantly perturb the alkyl chain crystallinity. The change in alkyl chain order upon polymerization is assessed using external reflection infrared spectroscopy. Consistent with previous studies,13-17 the unpolymerized 15,9-diacetylene on the evaporated gold film has a strong asymmetric C-H stretching band at 2919 cm-1 indicative of a highly crystalline trans structure.20 The C-H symmetric stretch at 2850 cm-1 also shows the highly crystalline structure consistent with a well-ordered monolayer.20 Upon polymerization, no change in transition frequency is observed but the intensity of the asymmetric stretch decreases while the symmetric stretch intensity exhibits a small increase.13-17 Thus, although no significant change in crystalline order is observed by IR, the tilt and/or twist of the alkyl chains is apparently altered during the polymerization process. This result is consistent with the change in hybridization expected at two positions for each monomer upon polymerization. With change from (37) (a) Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116. (b) Trapnell, B. M. W. Proc. R. Soc. London 1953, A218, 566. (c) Smith, T. J. Colloid Interface Sci. 1980, 75, 51. (d) Emch, R.; Nogami, J.; Dovek, M. M.; Lang, C. A.; Quate, C. F. J. Appl. Phys. 1989, 65, 79.
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sp to sp2 hybridization, an internal stress may be created above and below the polymer backbone. However, this perturbation appears to be relatively modest, and the overall crystallinity within the alkyl regions is not significantly affected upon polymerization. The polymerization process itself is monitored using resonance Raman spectroscopy. The presence of the conjugated polymer backbone in polydiacetylene monolayers affords an increased cross section, allowing the direct measurement of polymer formation. Monolayer assemblies composed of octadecanethiol adsorbates or unpolymerized diacetylenes containing no extended conjugation do not display detectable Raman scattering under the conditions utilized in this study. Moreover, the surface roughness chosen for these studies is not sufficient to expect significant surface enhancement in the Raman signal with the possible exception of the colloidal assembly. As a result, the intensity of the resonance Raman signal is expected to be largely proportional to the amount of polymer present within the monolayer. Resonance Raman measurements of the polymerization process in crystals and Langmuir-Blodgett (LB) multilayer assemblies is often limited by fluorescence. However, previous studies on LB monolayers demonstrate minimal fluorescence interference.38,39 If the polymer absorbance properties are similar on gold substrates, excitation at 632 nm is expected to yield selective measurement of the more well-ordered blue phase of the polymer.38 Unfortunately, direct comparison of resonance Raman and absorption measurements of the polymer form on gold was not feasible due to spectral overlap arising from the substrate plasmon bands.40 This convolution renders the measurement of the polymer form intractable by absorption spectroscopy. As a result, resonance Raman is used here as a sensitive and direct measurement of the polymerization process. Moreover, the resonance nature of this measurement ensures that only the conjugated polymer backbone region is measured. Resonance Raman spectra of the polymerized assemblies (Figure 4) display peaks attributable to alkene (1460 cm-1) and alkyne (2084 cm-1) stretches which are assigned to vibrational transitions arising from the polymer backbone.41,42 These stretching transitions are considerably lower than those expected for isolated double or triple bonds (1620 and 2260 cm-1, respectively).43 This diminution of peak frequency in the polymerized assemblies is consistent with delocalization in the polymer backbone region.43 The peaks occurring at 705 cm-1 and between 1100 and 1400 cm-1 are not conclusively assigned in the literature but are conjectured to arise from the in-plane stretching vibrations of the polydiacetylene backbone and the rocking and wagging modes of the methylene units adjacent to the backbone, respectively.44-46 (38) Miyano, K.; Maida, T. Phys. Rev. B 1986, 33, 4386. (39) Burzynski, R.; Prasad, P. N.; Biegajski, J.; Cadenhead, D. A. Macromolecules 1986, 19, 1059. (40) Menzel, H.; Mowery, M.; Cai, M.; Evans, C. Submitted for publication in J. Phys. Chem. B. (41) Bloor, D.; Preson, F. H.; Ando, D. J.; Batchelder, D. N. In Structural Studies of Macromolecules by Spectroscopic Methods; Iven, K. J., Ed.; John Wiley & Sons: London, 1976. (42) Batchelder, D. N.; Bloor, D. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley Heydon: London, 1984. (43) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. In The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, CA, 1991. (44) Angkaew, S.; Wang, H.-Y.; Lando, J. B. Chem. Mater. 1994, 6, 1444. (45) Bower, D. I.; Maddams, W. F. The Vibrational Spectroscopy of Polymers; Cambridge University Press: Cambridge, 1989. (46) Tieke, B.; Bloor, D.; Young, R. J. J. Mater. Sci. 1982, 17, 1156.
Mowery et al.
Figure 4. Raman spectra of 15,9-PDA monolayers on various gold substrates. The integration time was 10 min for all samples except the colloidal gold sample which was integrated for 60 min.
The intensities of the alkene and alkyne stretches provide a relative measure of the formation of well-ordered polymer domains. The influence of surface preparation conditions on the polymerization of these diacetylene monolayers is illustrated in Figure 4. Increased signal intensity indicates an obvious propensity for more efficient formation of the blue-phase polymer on the evaporated gold substrates. The degree of polymer formation on the cleaned sputtered substrate is diminished but remains significant. Additionally, the monolayers formed by direct immersion of the sputtered substrates are characterized by decreased signal intensity compared to cleaned sputtered surfaces. Thus, the cleaning procedure is shown to significantly affect the polymerization process. Although no contaminants were expected upon direct removal from the sputterer, clearly surface cleanliness cannot be presumed under common sputtering conditions. These surface contaminants may lead to misalignment of diacetylene groups, decreasing the degree of polymerization. For the special case of surface-attached colloidal gold, diacetylene polymerization is observed in small but measurable quantities (inset of Figure 4). The decrease in intensity caused by the incomplete surface coverage of the colloids is partially offset by a 6-fold increase in integration time relative to measurements on other gold substrates. The small peaks associated with the polymer backbone on colloidal gold surfaces suggest a greatly diminished degree of polymer formation compared to both the cleaned sputtered and evaporated surfaces. However, to our knowledge, this represents the first formation of diacetylene polymers on colloidal gold. This experimental correlation between surface preparation and the presence of blue-phase polymer may be examined in terms of the spatial constraints on polymerization. Since the presence of step sites may limit the polymer length (Figure 2), it is instructive to compare the length of the polymer chain to the atomically flat domain size under these varying surface preparation conditions. Unfortunately, measurements of the polymer chain length within these monolayer films are technically very challenging and have yet to be accomplished. However, a lower
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Table 1. FTIR Peak Positions in the Methylene Stretching Region for Octadecanethiol and Polydiacetylene Monolayers on Different Substratesa monolayer C18
15,9-PDA
substrate
νa (cm-1)
fwhm (cm-1)
νs (cm-1)
fwhm (cm-1)
colloidal Au sputtered Au (direct) sputtered Au (cleaned) evaporated Au colloidal Au
2920 2918 2918 2919 2924 (2924) 2921 (2921) 2921 (2921) 2919 (2919)
12 12 11 12 20 (20) 19 (18) 17 (16) 11 (9)
2852 2850 2850 2850 2854 (2854) 2852 (2852) 2852 (2852) 2850 (2852)
9 9 9 8 13 (14) 14 (14) 13 (12) 13 (14)
sputtered Au (direct) sputtered Au (cleaned) evaporated Au
a Values for the unpolymerized monolayers are shown in parentheses. The peak positions and widths are reported with an error of (1 cm-1 and (2 cm-1, respectively, as determined by replicate measurements from at least three films (with the exception of colloidal Au which was determined for two films).
value for the length of the polymer chain can be estimated from the predicted conjugation length. On the basis of Kuhn’s model,47 a delocalization length of ∼50-100 (2550 monomer units) is calculated for blue-phase absorption at 630 nm.48 Only the blue-phase polymer is monitored under these resonance Raman conditions, and the conjugation length can be estimated from the monomer spacing (Figure 2B) as ∼10 to 25 nm. It is important to note that these values provide only approximate conjugation lengths due to the simplified nature of Kuhn’s model, and the polymer length will always be greater than the conjugation length. Nonetheless, these values allow the semiquantitative comparison with the expected atomically flat domain size as a function of preparation method. As discussed earlier, gold films evaporated under hightemperature conditions are expected to have atomically flat domain sizes greater than 100 nm.24 As a result, atomic steps on the evaporated gold surface are not expected to impede the formation of blue-phase polymer. In contrast, the atomically flat terraces of less than 10 nm that are expected on the sputtered gold substrates26-28 are near the order of magnitude predicted for the polymer chain length. In this case, the domain size on the gold surface may constrain the polymer chain length, leading to a decreased formation of blue-phase polymer. Finally, the facet size of 2-8 nm cited for the colloidal gold34 is likely to limit the chain length of polymer films and little blue-phase polymer is expected. This trend predicted for polymerization behavior is in general agreement with the decreased resonance Raman intensity measured as a function of surface preparation conditions (Figure 4). The simultaneous determination of surface morphology and polymer chain length needed to confirm this relationship are presently underway in our laboratory. However, this comparison indicates the important possibility of using the surface domain size as a template for polymerization in these self-assembled monolayers. Alkyl Chain Crystallinity. Structure within the alkyl regions of polydiacetylene and n-alkyl monolayers is measured using grazing-angle Fourier transform infrared spectroscopy. In bulk compounds, the symmetric and asymmetric methylene C-H stretching vibrations are highly sensitive to the structural environment of the alkyl chains. The asymmetric and symmetric methylene stretching transitions occur at 2918 and 2950 cm-1, respectively, for highly crystalline systems and increase in frequency systematically with decreasing order.20 (47) Kuhn, H. Fortsch. Chem. Org. Naturst. 1959, 17, 404. (48) Tanaka, H.; Thakur, M.; Gomez, M. A.; Tonelli, A. E. Polymer 1991, 32, 1834-1840.
Likewise in single-layer assemblies, a monolayer of highly ordered alkyl chains in an all-trans conformation should exhibit methylene stretching vibrations near those of crystalline systems, with disorder induced by gauche defect sites resulting in a systematic increase in vibrational frequencies.20 For a monolayer formed with octadecane thiol, the alkyl region has been shown to be highly crystalline and is expected to be homogeneous with the possible exception of regions near the terminal methyl group.1 In contrast, the polydiacetylene monolayers have two distinct alkyl regions above and below the polymer backbone (regions IItail and IIspacer). Both regions will be probed in the external reflection measurement, however, leading to a convolution of the tail and spacer regions. Peak positions in the methylene stretching region are shown in Table 1 as a function of monolayer composition and substrate preparation. The asymmetric and symmetric stretching frequencies for the octadecanethiol monolayer are consistent with previous reports for alkanethiol monolayers and indicate the presence of a highly crystalline environment.20 Indeed, only a modest variation in the crystallinity of alkyl chains in the C18 monolayer is observed for this broad range of substrate conditions. This observation is consistent with the fortuitous registry distance between next-nearest neighbor methylene units and atomic step heights on gold surfaces. As illustrated in Figure 1A, integral increments of gold atomic step heights (2.36 Å)49 are in near correspondence with vertical spatial increments between n and n + 2 methylene units in an all trans geometry (2.18 Å at 30° tilt).50 In this way, the resultant compensation between the monolayer and the substrate may account for the minimal impact of domain size and substrate preparation. Likewise, the full width at half-maximum (fwhm) height of the asymmetric and symmetric stretching bands is quite constant with surface preparation, indicating no significant increase in the range of ordered states. In addition, no differences in peak frequencies for the n-alkyl monolayer are observed upon cleaning of the sputtered gold substrate. In contrast to the octadecanethiol monolayers, the polydiacetylene monolayers exhibit a broader range of peak frequencies for the methylene stretching transitions across the range of substrates. With the unpolymerized values shown in parentheses, it is clear that the polymerization process (49) Watanabe, M. O.; Kuroda, T.; Tanaka, K.; Sakai, A. J. Vac. Sci. Technol., B 1991, 9, 924. (50) The vertical distance between next-nearest neighbor methylene units was calculated from known bond lengths and angles (CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; The Chemical Rubber Publishing Co.: Boston, 1990) assuming an adsorbate tilt angle of 30° from normal.
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Figure 6. Average advancing contact angles for octadecanethiol and polydiacetylene monolayers on various substrates. The error bars indicate the standard deviation for at least three separate measurements of a single monolayer. Hexadecane contact angles on colloidal gold monolayers resulted in complete wetting of the surface.
Figure 5. Grazing-angle FTIR spectra of octadecanethiol and 15,9-PDA monolayers on sputtered gold, evaporated gold, and colloidal gold substrates. The sputtered and evaporated gold spectra were taken with a resolution of 2 cm-1 and the colloidal gold spectra with a resolution of 4 cm-1. Monolayer spectra on the direct and cleaned sputtered gold surfaces are indistinguishable.
has little affect on the alkyl chain crystallinity. However, substrate preparation appears to have a significant impact on the peak frequency as well as the range of ordered states indicated by the fwhm. Polymerized monolayers on evaporated gold show transition frequencies indicative of a highly crystalline structure, whereas transitions on colloidal substrates suggest a more disordered, liquidlike structure. Similar to the n-alkyl monolayer, no significant difference in the polymerized monolayer is observed upon surface cleaning of the sputtered gold substrate. However, the fwhm of the methylene transitions in the polymerized monolayers is significantly broader than their n-alkyl counterparts, indicating a wider range of ordered states. These results are consistent with the lack of internal compensation due to registry that was feasible for their n-alkyl counterparts. Indeed, the registry length of the diacetylene internal groups is not integral units of step height, and as a result, variations in surface height are expected to create misalignment of neighboring diacetylenes, disrupting the alkyl chain ordering. In addition to frequency and fwhm, the peak shape of these transitions provides further insight into the alkyl chain structure on various substrates (Figure 5). Octadecanethiol monolayers exhibit a relatively constant and symmetric peak shape for both methylene transitions at all substrate conditions. In contrast, both the asymmetric and symmetric stretching bands for the polydiacetylene monolayers exhibit distinct asymmetry in peak shape for all substrates. This deviation toward the high frequencies is most clear for the asymmetric band. On the evaporated gold substrate, the asymmetric transition appears to consist of two distinct transitions; one highly crystalline and the other more disordered. On sputtered and colloidal gold, the less ordered component increases resulting in a shift in peak frequency of the entire transition. Although the symmetric methyl Fermi resonance transition at 2936 cm-1 complicates this portion of the spectrum, the polymerized monolayers appear to contain regions of highly
crystalline alkyl chains together with a more disordered fraction. This observation may arise from the formation of highly crystalline domains with significant gauche defects in the interstitial regions between domains. The apparent correlation between alkyl chain crystallinity and the degree of polymerization measured by resonance Raman is consistent with this hypothesis. However, distinct regions of methylene crystallinity may also arise from different structural ordering within the tail and spacer regions of the monolayer. While this latter hypothesis is supported by recent studies of dipole-coupled monolayers,11 the exact origin of these crystallinity variations remains to be resolved. Nonetheless, comparison of n-alkyl and polymerized monolayers indicates that the presence of molecular scaffolding significantly affects the alkyl chain crystallinity. Wettability. Further characterization of film quality is accomplished by measurement of the contact angle of a drop of solvent on the monolayer surface. Although this measurement assesses a large lateral portion of the monolayer film, it provides a good indication of structural order in the outermost region of the assembly.1 Higher contact angles are observed for highly ordered, crystalline monolayer structures due to the interfacial region comprised of well-ordered methyl groups. With diminishing structural order more methylene groups become exposed to the solvent, systematically decreasing the contact angle.1 Through investigations of hexadecane and water contact angles on polydiacetylene and octadecanethiol monolayers as a function of substrate topography, the effect of surface roughness on the monolayer quality and structure of the air/monolayer interfacial region is evaluated. Figure 6 depicts the advancing contact angles of water and hexadecane on polydiacetylene and octadecanethiol monolayers as a function of substrate preparation. The contact angles for water indicate that, with the exception of the colloidal gold substrate, octadecanethiol and polydiacetylene monolayers show similar wetting behavior for all substrates. The seemingly significant drop in water contact angle for colloidal gold may be misleading due to the incomplete coverage of colloidal gold particles on these surfaces, leaving a considerable fraction of the bare silica/ adhesive layer exposed. On the basis of the total wetting of the colloidal surface by hexadecane, much of the contact angle measures interaction with the (3-mercaptopropyl)-
Monolayers Containing Internal Molecular Scaffolding
methyldimethoxysilane adhesive layer. In contrast to the inherent nonwetting nature of water on alkyl-based surfaces, hexadecane contact angle measurements provide a more rigorous measure of changes in the outermost structural order. Figure 6 illustrates a clear dependence of the hexadecane contact angles on substrate conditions for both n-alkyl and polymerized assemblies, with the angles increasing with decreasing long-range surface roughness. In addition, the octadecanethiol monolayers display higher contact angles than polydiacetylene monolayers on cleaned and evaporated surfaces, indicating slightly higher structural order in the interfacial region for the n-alkyl system. Cleaning the substrate appears to slightly affect the quality of only the octadecanethiol monolayers, presumably by removing contaminants and increasing adsorbate coverage. While the contact angles of hexadecane on octadecanethiol and polydiacetylene monolayers reveal a significant dependence on long-range surface roughness, methylene crystallinity measured by FTIR shows no dependence on substrate roughness for the n-alkyl assemblies. This disparity may be attributable to offsets in vertical registry for the sputtered substrate that exposes methylene groups to the contacting solvent at the interfacial region.1 In this case, contact angles for both types of assemblies would decrease on rougher substrates, but internal compensation of registry (vide supra) would largely maintain methylene chain crystallinity for the n-alkyl assembly. Collectively, these wettability measurements demonstrate good overall film quality for both n-alkyl and polydiacetylene monolayers with some substrate dependence. Long-Range Order. The impact of substrate preparation on long-range structural order is assessed using heterogeneous electron transfer measurements. Comparison of spatial defects and overall film permeability is accomplished for octadecanethiol and polydiacetylene monolayers as a function of substrate preparation conditions. The degree of electron transfer inhibition is an indication of long-range order in monolayer systems and is evaluated by the shape and magnitude of the cyclic voltammetric current response. As illustrated in Figure 7, a bare gold substrate exhibits classical peak-shaped curves consistent with unimpeded diffusion-limited transfer of ferricyanide ions to the gold surface. Increased blocking is shown for both C18 and polydiacetylene on the sputtered gold substrate, with the plateau in current response indicative of radial diffusion to small, widely spaced defect sites.51,52 Further inhibition of electron transfer is observed upon cleaning of the sputtered gold substrate, with the C18 layer predominantly showing tunneling exemplified by the exponential current increase in the cyclic voltammogram.20 This significant decrease in electron transfer after surface pretreatment demonstrates that, to a large extent, the defect density observed for sputtered substrates is accounted for by adventitious contaminants on the surface prior to monolayer adsorption. The polydiacetylene monolayer also exhibits diminished response upon contaminant removal, although not to the same extent as the n-alkyl monolayer. The greater response for the polydiacetylene assemblies compared to octadecanethiol is attributed to a higher number of defect sites and/or greater monolayer permeability on the cleaned, sputtered surfaces. This dissimilarity in blocking behavior between n-alkyl and polymerized monolayers on the sputtered gold substrate is consistent with the difference in the crystallinity of (51) Amatore, C.; Saveant, J.-M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39. (52) Chailapakul, O.; Crooks, R. M. Langmuir, 1993, 9, 884.
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Figure 7. Cyclic voltammetric current response vs applied potential for octadecanethiol and 15,9-polydiacetylene monolayers as a function of substrate. The solution is 1.0 mM in Fe(CN)6- and 1.0 M in KCl. The sweep rate is 100 mV/s and the temperature is 22 ( 1 °C.
alkyl chains measured by FTIR. For the evaporated gold substrates, the inhibition of the polymerized monolayer to electron transfer improves significantly and is nearly indistinguishable from the octadecanethiol assembly. Although the exact role of the polymer backbone in mediating the electron-transfer process is not presently understood, these results clearly demonstrate the considerable blocking capabilities of these polymerized assemblies. These heterogeneous electron-transfer results are mirrored by the measurement of small ion transport by capacitance. Monolayers on the direct sputtered substrate exhibit considerable permeability to small ions, with capacitance values for the polydiacetylene more than twice that of the C18 (78 and 34 µF/cm2, respectively). Upon removal of surface contaminants, both layers show a distinct increase in small ion blocking behavior. Similar to the ferricyanide permeability, surface cleaning affects the polydiacetylene monolayer to a lesser extent than the C18 layer (12 and 2 µF/cm2, respectively). Films on the evaporated gold substrate exhibit a significant decrease in capacitance for the polydiacetylene monolayer and no
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measurable change in the C18 layer (1 and 2 µF/cm2, respectively). In both monolayer systems, excellent blocking for small ions is observed on the evaporated gold substrate. Conclusions Substrate preparation conditions are demonstrated to play an important role in the fabrication of polydiacetylene monolayers. Comparison of common preparation methods for gold substrates, including high-temperature evaporation, room-temperature sputtering, and surface-attached colloids, indicates significant differences in monolayer structural properties. Both alkyl chain crystallinity and wettability measurements exhibit increased ordering on the high-temperature-evaporated gold substrates. Moreover, long-range order, as measured by heterogeneous electron transfer and interfacial capacitance, is significantly enhanced on the evaporated surfaces. These observations are in direct contrast with their n-alkyl counterparts, which show no significant variation chain crystallinity or long-range order with substrate preparation. This distinction may be due, in large part, to a fortuitous alignment for the n-alkyl chains with integral step heights on gold surfaces and the increased disruption in alkyl chain alignment expected for the diacetylene-
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containing monolayers. Moreover, a general correlation is observed between the atomically flat domain size previously measured for these preparation conditions and estimates of the polymer length. Atomically flat regions on the evaporated gold surface are not expected to impede polymerization, leading to the highest conversion to the blue-phase polymer. In contrast, atomically flat domains may limit the polymer chain length on sputtered and colloidal gold surfaces, consistent with the lower conversion to the blue-phase polymer on these substrates. As this hypothesis is central to implementing surface control of the polymerization process, detailed studies of the relationship between the surface morphology and the polymerization process are presently underway. In addition to the importance for the successful fabrication of polydiacetylene monolayers, it is clear that the substrate must be carefully considered for all monolayers containing internal molecular scaffolding. Acknowledgment. The authors acknowledge financial support from the National Institute of General Medical Sciences, National Institutes of Health (GM52555-01 A1). H.M. thanks the Fulbright Commission for a travel grant. LA980331D
