Langmuir 2001, 17, 7071-7078
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Transfer of Rodlike Aggregate Phthalocyanines to Hydrophobized Gold and Silicon Surfaces: Effect of Phenyl-Terminated Surface Modifiers on Thin Film Transfer Efficiency and Molecular Orientation Rebecca A. P. Zangmeister, Paul E. Smolenyak, Anthony S. Drager, David F. O’Brien, and Neal R. Armstrong* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received June 4, 2001. In Final Form: August 7, 2001 We present here a study of the effects of surface modification on the efficiency of transfer of ultrathin Langmuir-Blodgett films of rodlike aggregates of the phthalocyanine (Pc) (2,3,9,10,16,17,23,24-octakis((2-benzyloxy)ethoxy)phthalocyaninato)copper (1) to gold and silicon substrates. These surface modifications impact on (i) the molecular orientation of the individual Pcs within the rodlike aggregates, (ii) the coherence of the aggregates (providing for coherence lengths of some Pc rods of over 100 nm), and (iii) the optical anisotropy and the overall uniformity and lack of pinholes in Pc bilayer films, on the 100 µm distance scale. Methyl-terminated and phenyl-terminated surface modifiers were added to both Au and Si surfaces through thiol and silane chemistries, respectively. The phenyl-terminated modifier for Au surfaces, benzyloxyethanethiol, 2, mimics the side chain composition of Pc 1. Both methyl- and phenyl-terminated modifiers produced hydrophobic surfaces, as revealed by high water contact angles. FT-IR spectroscopy and AFM characterization of horizontally transferred bilayer LB thin films of 1 consistently showed higher degrees of coverage on the surfaces with phenyl-terminated modifiers. Reflection-absorption FT-IR of 1-5 bilayer films on modified Au substrates showed that the average tilt angle (ψ) and rotation angle (θ) of the individual Pc chromophores within these films varied with increasing Pc film coverage but that changes to ψ were significantly less on Au surfaces modified with 2. Phenyl-phenyl intermolecular interactions have been hypothesized to lead to the unique formation of rodlike aggregates of 1 and are shown here to be critical in establishing coherent thin films of this material during the transfer of the first bilayer.
Introduction Rodlike self-assembling organic materials have attracted significant attention due to their unique electroand photoactive properties. These properties have been shown to be dependent on both the long-range order and molecular orientation within thin films of these materials, as evaluated in field-effect transistors,1-7 photovoltaics,8,9 and polarized emission light-emitting diodes.10-14 Because of this dependence on order and molecular orientation, * To whom correspondence should be addressed: e-mail
[email protected]. (1) Dyreklev, P.; Gustafsson, G.; Ingana¨s, O. Synth. Met. 1993, 55, 4093-4098. (2) Torsi, L.; Dodabalapur, A.; Rothberg, L. J.; Fung, A. W. P.; Katz, H. E. Science 1996, 272, 1462-64. (3) Lin, Y.-Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE Electron Device Lett. 1997, 18, 606-608. (4) Gundlach, D. J.; Jackson, T. N.; Schlom, D. G.; Nelson, S. F. Appl. Phys. Lett. 1999, 74, 3302-3304. (5) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Adv. Mater. 1997, 9, 42-44. (6) Xu, G.; Bao, Z.; Groves, J. T. Langmuir 2000, 16, 1834-1841. (7) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, W.; Woo, E. P.; Grell, M.; Bradley, D. D. C. Appl. Phys. Lett. 2000, 77, 406-408. (8) Marks, R. N.; Zamboni, R.; Taliani, C. Mater. Res. Soc. Symp. Proc. 1996, 413, 425-430. (9) Videlot, C.; Fichou, D. Synth. Met. 1999, 102, 885-888. (10) Era, M.; Tsutsui, T.; Shogo, S. Appl. Phys. Lett. 1995, 67, 24362438. (11) Dyreklev, P.; Berggren, M.; Ingana¨s, O.; Andersson, M. R.; Wennerstro¨m,; Hjertberg, T. Adv. Mater. 1995, 7, 43-45. (12) Wegner, G.; Neher, D.; Remmers, M.; Cimrova, V.; Schulze, M. Mater. Res. Soc. Symp. Proc. 1996, 413, 23-34. (13) Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Miteva, T.; Neher, D.; Nothofer, H.-G.; Scherf, U.; Yasuda, A. Adv. Mater. 1999, 11, 671-675. (14) Chen, X. L.; Bao, Z.; Sapjeta, B. J.; Lovinger, A. J.; Crone, B. Adv. Mater. 2000, 12, 344-347.
special attention is now being paid to deposition conditions,3-6,8,10,12 mechanical and chemical substrate pretreatment,1,9,11 and substrate morphologies,7,13,14 for each of these materials. Discotic mesophase phthalocyanines, such as compound 1 in Figure 1, are candidates for organic field effect transistor (OFET) and photovoltaic (PV) applications because of the unusually coherent thin films which can be formed from these molecules, owing to their tendency to self-assemble in parallel columnar structures.15 Discotic mesophase materials in general are attractive for these technologies if the coherence length achievable in rodlike aggregates of these materials exceeds the expected separation distance between source/drain (OFET) or anode/cathode (PV) contacts. (Generally a minimum coherence length of 100 nm is required.) Under these conditions, charge mobilities along the rod axis greater than 0.01 cm2 V-1 s-1 are expected.16 In our studies to date we have observed that thin films of 1 contain the Pc oriented nearly “edge-on” to the substrate plane, with coherence lengths in individual rods averaging ca. 40 nm. We have found that a key feature of these molecules which leads to the stiff nature and coherence of each bilayer film is the benzyl termination in the eight side chains, which appears to reinforce the interactions between adjacent Pc rings and between adjacent Pc columns.15,17-19 No other Pc material capable (15) Smolenyak, P. E.; Peterson, R. A.; Nebesney, K.; To¨rker, M.; O’Brien, D. F.; Armstrong, N. R. J. Am. Chem. Soc. 1999, 121, 86288636. (16) van de Craats, A. M.; Warman, J. M. Adv. Mater. 2001, 13, 130-133. (17) Osburn, E. J.; Chau, L.-K.; Chen, S.-Y.; Collen, N.; O’Brien, D. F.; Armstrong, N. R. Langmuir 1996, 12, 4784-4796.
10.1021/la010817l CCC: $20.00 © 2001 American Chemical Society Published on Web 10/02/2001
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Zangmeister et al. Scheme 1. Synthetic Route to Benzyloxyethanethiol (2)
Figure 1. (a) Drawing of CuPc(OCH2CH2OBz)8 (1) and (b) a tapping mode AFM image acquired in solution of a single bilayer of 1 transferred to a HMDS/DPTMDS hydrophobized Si (100) (shown in deflection mode). Several rodlike Pc aggregates are clearly visible whose coherence lengths are in excess of 100 nm, with center-to-center spacing of ca. 2.7-2.9 nm, aligned parallel to the long axis of the compression barriers used in the LB trough (see ref 15 for details of AFM experiments).
of forming a discotic mesophase, held in place by noncovalent interactions, has to date demonstrated such longrange order and thin film coherence. The structural perfection in the individual Pc columns rivals that seen in the rodlike polymers of silicon-phthalocyanines developed by Wegner and co-workers.20,21 Our preliminary experiments with 1 also suggested that phenyl-phenyl interactions between a surface modifier and the Pc assembly played a critical role in the efficient transfer of coherent thin films of these Pc assemblies to metal, silicon/silicon oxide, and transparent conducting oxide substrates.15 Molecular assemblies with phenyl termination are believed to interact through both cofacial and edge-to-face arrangement of the aromatic groups,22-27 which can strongly affect the type and coherence of the resultant molecular aggregate structures.28 We confirm here that such interactions strongly influence the transfer, (18) Smolenyak, P. E.; Osburn, E. J.; Chen, S.-Y.; Chau, L.-K.; O’Brien, D. F.; Armstrong, N. R. Langmuir 1997, 13, 6568-6576. (19) Osburn, E. J.; Schmidt, A.; Chau, L.-K.; Chen, S.-Y.; Smolenyak, P.; Armstrong, N. R.; O’Brien, D. F. Adv. Mater. 1996, 8, 926-928. (20) Tans, S. J.; Geerligs, L. J.; Dekker, C.; Wu, J.; Wegner, G. J. Vac. Sci. Technol. B 1997, 15, 586-589. (21) Wu, J.; Lieser, G.; Wegner, G. Adv. Mater. 1996, 8, 151-154. (22) Janda, K. C.; Hemminger, J. C.; Winn, J. S.; Novick, S. E. J. Chem. Phys. 1975, 63, 1419-1421. (23) Steed, J. M.; Dixon, T. A.; Klemperer, W. J. Chem. Phys. 1979, 70, 4940-4946. (24) Karlstro¨m, G.; Linse, P.; Wallqvist, A.; Jo¨nsson, B. J. Am. Chem. Soc. 1983, 105, 3777-3782. (25) Burley, S. K.; Petsko, G. A. Science 1985, 229, 23-28. (26) Grosel, M. C.; Cheetham, A. K.; Hope, D. A. O.; Weston, S. C. J. Org. Chem. 1993, 58, 6654-6661. (27) Paliwal, S.; Geib, S.; Wilcox, C. S. J. Am. Chem. Soc. 1994, 116, 4497-4498. (28) Whitten, D. G.; Chen, L.; Geiger, H. C.; Perlstein, J.; Song, X. J. Phys. Chem. B 1998, 102, 10098-10111.
ordering, and molecular orientation of Langmuir-Blodgett (LB) films of Pc 1. In this paper we show how the modification of gold surfaces with a new phenyl-terminated thiol (2, Scheme 1), which mimics the side chain composition of these Pcs, leads to a greater efficiency in transfer of 1 to a gold surface, vs the efficiency of transfer seen for a CH3-terminated alkanethiol self-assembled monolayer. Such phenyl surface modifications also influence the orientation of individual Pcs as determined by reflection-absorption IR spectroscopy (RAIRS). Analogous modification of the oxide surfaces of silicon wafers, with a mixture of 1,1,1,3,3,3hexamethyldisilazane (HMDS) and 1,3-diphenyl-1,1,3,3 tetramethyldisilazane (DPTMDS), shows a higher degree of transfer vs the unmodified surfaces. The presence of phenyl groups in the surface modifier leads not only to higher efficiency in transfer but also to more ordered/ optically anisotropic thin films. With the optimized surface modifications schemes described below, coherence lengths in each Pc column of 100 nm or more are now achievable. Experimental Section (2,3,9,10,16,17,23,24-Octakis((2-benzyloxy)ethoxy)phthalocyaninato)copper (1) was synthesized as reported previously.29 Solutions of 1 were prepared using HPLC grade chloroform 99.9% (Aldrich). 1,1,1,3,3,3-Hexamethyldisilazane (HMDS, 97%), 1,3diphenyl-1,1,3,3-tetramethyldisilazane (DPTMDS, 96%), and 1-octanethiol (98.5+%) were purchased from Aldrich and used without further purification. Benzyloxyethanethiol (2) was synthesized from 2-benzyloxyethanol (98%, Aldrich) via two reaction pathways shown in Scheme 1. The one-step process of reacting the alcohol with thiourea (99+%, Aldrich) in the presence of HBr (99+%, Aldrich) was adapted from a method described by Frank and Smith.30 The HBr used in this reaction cleaved a fraction of the alcohol at the benzyl oxygen, producing both benzyl mercaptan and the desired thiol with an overall yield of ca. 50%. The desired product was then separated from the benzyl mercaptan using flash column chromatography with a mobile phase of 80:20 hexanes:ethyl acetate. The two-step process, via the tosylated alcohol in the presence of thiourea and HBr, produced the desired product, also purified with flash chromatography, in a 15% yield. The benzyloxyethanethiol product was characterized using 1H and 13C NMR [1H NMR (CD2Cl2): δ 7.4 (m, 5H, aromatic), 3.8 (s, 2H), 3.7 (t, 2H), 2.6 (t, 2H), 2.5 (s, 1H). 13C NMR (CD Cl ): δ 141.7, 128.9, 128.4, 127.3, 20.1]. 2 2 Au substrates consisted of a ca. 1000 Å Au layer on a titaniumtreated float glass, obtained from Evaporated Metal Films (Ithaca, NY). Au slides were cleaned by immersing in piranha solution (1:4 solution of H2O2:H2SO4) and rinsing successively in water (29) Drager, A. S.; O’Brien, D. F. J. Org. Chem. 2000, 65, 22572260. (30) Frank, R.; Smith, P. J. Am. Chem. Soc. 1946, 68, 2103-2104.
Rodlike Aggregate Phthalocyanines (18 Ω Millipore) and ethanol (200 proof, AAPER Alcohol and Chemical Co.) before submersing in 0.1 mM ethanolic solutions of benzyloxyethanethiol (2) or octanethiol (98.5%, Aldrich). After 24 h immersion times, Au substrates were removed and rinsed with copious amounts of ethanol, to remove unreacted thiols, and were dried in a stream of nitrogen immediately prior to LB film deposition. Si (100) wafers, with a native oxide layer (MEMC Electronic Materials, Inc.), were cleaned by immersing in piranha solution and rinsing with water. Wafer pieces were dried in a stream of dry nitrogen before immersing in solutions of 10% HMDS in CHCl3, 10% DPTMDS in CHCl3, or a mixture of the two (i.e., 5:5:90 v/v/v ratio of HMDS:DPTMDS:CHCl3). Modified substrate surfaces were washed with CHCl3 to remove unreacted silazanes and were dried in a stream of nitrogen immediately prior to LB film deposition. Contact angles of 18 MΩ Millipore water on all modified surfaces were measured using the sessile drop method. Images of multiple 5 µL water droplets on each substrate surface were taken using a Pulnex TM-7CN video camera and Video Snapshot Snappy. Images were converted into tagged image format using corresponding software, and angles were measured using ImagePro Plus 1.3 software (Media Cybernetics). Langmuir-Blodgett films were prepared and transferred using the Schaefer transfer method, as described in earlier publications.15,18 The pressure-area isotherm for these molecules shows a transition from a compact monolayer (ca. 110 Å2/molecule), to a compact bilayer, and then finally a collapse to form long ordered Pc fibers.18,19 Horizontal transfer of the films discussed here took place at the stable bilayer section of the Π-A curve (ca. 30 mN/ cm2). Residual superficial water, associated with the LB film transfer, was removed with a stream of dry nitrogen gas. FT-IR spectra were obtained with a dry-air purged Nicolet 550 spectrometer with a tungsten source and a liquid nitrogen cooled MCT detector. A Au wire grid polarizer (Cambridge Physical Sciences) was used in the thin film transmission and RAIRS experiments. RAIRS spectra were obtained with an FT80 fixed 80° grazing angle accessory (Spectra-Tech). All spectra were collected with an aperture setting of 17 and 0.5 cm-1 resolution and were the summation of 256 individual scans. AFM images were recorded in tapping mode, with the Nanoscope III system (Digital Instruments, Santa Barbara, CA), with the sample immersed in 18 MΩ Millipore water in the standard Digital Instruments solution cell. Oxide sharpened, silicon nitride tips, with nominal force constants of 0.38 N/m were ozone cleaned for ca. 1 h prior to use.15 The porosity of the thiol-modified Au surfaces was probed using the voltammetric oxidation/reduction of ferrocyanide (K4Fe(CN)6) solutions at 3 mM concentration in a degassed aqueous 0.1 M potassium hydrogen phthalate (KHP) electrolyte solution. The modified planar gold electrodes had a surface area of ca. 0.78 cm2. Voltammetry was conducted with a Cypress Systems CS10190 potentiostat. Ferrocyanide was recrystallized multiple times from a supersaturated aqueous solution by adding an equivalent volume of ethanol. The trihydrate product was obtained by equilibrating the ferrocyanide in a desiccator over a saturated solution of sucrose (98+%, Aldrich) and NaCl (98+%, Aldrich). A Kratos AXIS-ULTRA spectrometer (base pressure ca. 5 × 10-9 Torr) was used to collect X-ray photoelectron spectroscopy (XPS) data of modified Si surfaces. The X-ray source was monochromatic Al KR radiation with an approximate analysis area of ca. 300 mm × 700 mm. The photoemission spectra were recorded with a constant analyzer pass energy (20 eV). All spectral peaks were charge shift corrected assuming a C(1s) peak energy for graphitic carbon of 284.3 eV.31
Results and Discussion Water Contact Angles on Modified Au and Si Surfaces. Contact angles of water observed on unmodified and modified Au and Si substrates are listed in Table 1. Contact angles in excess of 95° for both the octanethiol (31) Handbook of Photoelectron Spectroscopy; Perkin-Elmer Physical Electronics Division: Eden Prairie, MN, 1979.
Langmuir, Vol. 17, No. 22, 2001 7073 Table 1. Contact Angles of Water on Modified Surfaces contact angles (deg) Au surface modification none octanethiol benzyloxyethanethiol Si (100) wafer (with native oxide) surface modification none HMDS DPTMDS HMDS:DPTMDS 50%:50%
54 ( 1 101 ( 1 98 ( 1 39 ( 1 62 ( 2 67 ( 1 63 ( 1
and the benzyloxyethanethiol (2) layers on Au (versus the contact angle of ca. 54° on the as-received Au) suggest that a relatively compact layer at the Au surface is formed by both molecules.32 The reaction of HMDS and DPTMDS with the native oxide layer present on Si (100) wafers also results in increased surface hydrophobicity; however, the contact angles on these substrates never exceeded ca. 67°, suggesting that there is still a low coverage of exposed oxide sites even though optimized versions of these treatments produced excellent Pc film quality (see below). Electrochemical Characterization of Modified Au Surfaces. Voltammetric characterization of the modified Au surfaces with aqueous ferrocyanide solutions suggest that modifier 2 produces a less ordered surface layer, with significant small molecule accessibility to the Au substrate. It has been previously shown that short-chain alkanethiol assemblies are more disordered than long-chain thiol assemblies (n g 9), due to differences in alkyl chain interactions and packing densities,33 and that large differences in van der Waals radii of the tail group vs the headgroup in amphiphilic assemblies (as in modifier 2) lead to greater disorder and higher defect densities within a monomolecular layer.34 As shown in Figure 2, the oxidation/reduction process for Fe(CN)64-/3- was effectively blocked by the Au surface modified with octanethiol but produced nearly the same current density on both the clean gold surface and the Au surface modified with 2, over voltammetric sweep rates ranging from 10 mV/s to 1 V/s. The voltammetric data suggest that there is considerable flexibility in these modifying chains which may be critical in obtaining optimum interactions with films of Pc 1 (see below). Pc Transfer to Modified Gold Substrates. Langmuir-Blodgett films of 1 are transferred to solid supports at a point on the Π-A curve where a stable bilayer is formed (surface pressure of ca. 35 mN/m).15 The consistency in the order and coherence of bilayer films, transferred at that point, have been confirmed by AFM characterization of single bilayer films and small-angle X-ray scattering (SAXS) characterization of multilayer films.15 The differences in bilayer film transfer of 1 to clean Au, and Au substrates modified with octanethiol, or benzyloxyethanethiol (2) were examined by RAIRS and AFM. Representative RAIRS spectra are shown in Figure 3 and show the greatest contrast observed between film transfers over three experimental trials. The small IR signal in Figure 3a and the corresponding AFM image are evidence that only small portions of a bilayer of 1 are transferred effectively to this type of surface. The remaining material is left on the LB trough or picked up as highly folded and defective film segments (discussed below), as (32) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (33) Langmuir, I. J. Chem. Phys. 1933. 1, 756-776. (34) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.
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Figure 2. Cyclic voltammetry of the Fe(CN)62-/3- redox couple on (a) (- - -) bare Au electrode, (b) (s) Au electrode with an adsorbed layer of octanethiol, and (c) (- - -) Au electrode with an adsorbed layer of 2 (50 mV/s scan rate).
has been observed by visual inspection and AFM imaging, respectively. Figure 3a shows a corner section of the Pc bilayer film whose dimensions were less than 250 µm × 250 µm and a larger bare Au regionsfeatures typical of transfer to an unmodified Au surface. We hypothesize that film segments, transferred to the unmodified Au surface, interact through nonspecific interactions with carbonaceous contamination.
Zangmeister et al.
Figure 3b reflects the improved transfer of a single bilayer of 1 due to nonspecific hydrophobic interactions with the methyl-terminated octanethiol, although a continuous film is still not achieved, as seen in the corresponding AFM image. Bilayer islands are seen across the surface, with average dimensions of 500 µm × 500 µm. Comparable results are obtained using octadecanethiol; therefore, chain length in the SAM layer cannot by itself be a contributing factor in determining Pc transfer efficiency to a methyl-terminated Au surface. Figure 3c shows the marked improvement in a single bilayer transfer of 1 to the Au surface modified with 2. The transfer efficiency on Au surfaces modified with 2 is consistently a factor of ca. 10 vs the bare gold surface and a factor of ca. 3 vs the gold surfaces modified with CH3terminated SAMs. These estimates are made from AFM imaging and the maximum IR absorbance for each film at 1283 cm-1. The corresponding AFM image to Figure 3c shows a complete bilayer transfer of 1 over a 100 µm × 100 µm area. Images of continuous film features are acquired from scans taken at random over the entire sample, within the physical limits of the imaging scan head (ca. 1 mm2). The entire area of the transferred Pc film is ca. 1 cm2. The scan image shown in Figure 3c also reveals small “ridges” in the bilayer film. This image was specifically chosen to show the maximal corrugation observed in the Pc film. The ridges run from the lower left to the upper right sides of the image, and the average step height of these ridges was 7 ( 2 nm, or approximately 2-3 Pc column diameters.15 Attempts to image these Pc films at the molecular resolution level (as in Figure 1 for Si(100) substrates) on Au surfaces have been unsuccessful to date. We believe this to be mainly due to local roughness of these Au surfaces vs the Si(100) substrates. In previous studies, where the Pc films were deposited on evaporated Au(111) substrates, molecular resolution images were obtainable by STM.15
Figure 3. RAIRS spectra of a single bilayer film of 1 onto (a) a clean Au slide, (b) a Au slide modified with octanethiol, and (c) a Au slide modified with 2. The νPc-O-C (1204, and 1283 cm-1) and out-of-plane bending transitions (δPc-H ) 745 cm-1) have been marked. Corresponding AFM images, shown in the lower half of the figure, are considered representative for transfer to each type of surface and were obtained by tapping mode AFM in an aqueous solution. The majority of the surface shown with (a) is bare Au, with only a small amount of 1 (upper left-hand corner) present on the surface in this region. Larger, islandlike deposits of 1 in (b) are typical for Au substrates that have been hydrophobized and had rms roughness values of ca. 3 nm, while the coherent, conformal deposits in (c) are typical for those obtained on hydrophobized Au surfaces with the addition of phenyl moieties and had rms roughness values of ca. 2 nm.
Rodlike Aggregate Phthalocyanines
Langmuir, Vol. 17, No. 22, 2001 7075 Table 2. Molecular Orientation Angles (in deg) Based on RAIRS Spectra
Figure 4. A pictorial definition of the axes used to characterize the IR (x-y) plane from which the angles ψ (rotation of the x-y plane around the y axis) and θ (rotation of the x-y plane around the x axis) are determined (refs 15 and 41). Rotation φ, about the z-axis, is determined from transmission IR experiments (refs 15 and 37).
The differences in interaction between methyl-terminated Au surface modifiers and phenyl-terminated modifiers are also seen when transfer of the Pc occurs directly from chloroform solutions, by casting or by capillary forces.36 In experiments to be reported elsewhere, we have created patterned thin films of 1 by combining microcontact printing to define a hydrophobic “channel” and electrochemical polymerization of m-aminophenol in the remaining bare gold regions to define hydrophilic “channel walls”. In these experiments methyl-terminated SAM layers on gold, defining the bottom portions of these channels, are not wet effectively by chloroform solutions of 1. The same SAM layers, doped with small percentages of phenyl-terminated alkane modifiers, allow for pinholefree, micron-width Pc films.36 The RAIRS spectra of 1, 3, and 5 bilayer thickness films, transferred to clean gold, octanethiol modified gold, and gold modified with 2, were used to estimate the molecular orientation of individual Pc chromophores within those films. As shown in Figure 4, a coordinate system is defined with respect to the plane of reflection and with respect to orthogonal dipole transitions within a molecule. The coordinate system for the Pc molecule 1, specifically the x-y plane, is defined by using orthogonal absorbance intensities for in-plane stretching vibrations (νPc-O-C ) 1204 and 1283 cm-1) and out-of-plane bending transitions (δPc-H ) 745 cm-1).15,37-41 We can calculate an average tilt angle, ψ, of this x-y plane (and therefore the Pc molecules) with respect to the surface normal and a rotation angle of the x-y plane, θ, around an axis equivalent to the Pc column axis.15 Additional rotation of the Pcs about the z-axis (angle φ) is determined in transmission IR experiments, on substrates such as Si (100) (see below). Values of ψ and θ are listed in Table 2 for 1, 3, and 5 bilayer films (35) Smolenyak, P. E.; Peterson, R. A.; Dunphy, D. R.; Mendes, S.; Nebesny, K. W.; O’Brien, D. F.; Saavedra, S. S.; Armstrong, R. N. J. Porphyr. Phthalocya. 1999, 3, 620-633. (36) Zangmeister, R. A. P.; Armstrong, N. R., manuscript in preparation. (37) Sauer, T.; Arndt, T.; Batchelder, D. N.; Kalachev, A. A.; Wegner, G. Thin Solid Films 1990, 187, 357-374. (38) Chesters, M. A.; Cook, M. J.; Gallivan, S. L.; Simmons, J. M.; Slater, D. A. Thin Solid Films 1992, 210, 538-541. (39) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129-137. (40) Schmidt, V. S.; Reich, R. Ber. Bunsen-Ges. Phys. Chem. 1972, 76, 1202. (41) Debe, M. K. Appl. Surf. Sci. 1982, 14, 1-40.
bilayer films transferred to clean Au 1 bilayer 3 bilayers 5 bilayers bilayer films transferred to ethanethiol modified Au 1 bilayer 3 bilayers 5 bilayers bilayer films transferred to benzyloxyethane thiol modified Au 1 bilayer 3 bilayers 5 bilayers
tilt angle, Ψ
rotation angle, θ
41 ( 3 35.9 ( 0.2 34.1 ( 0.3
8.8 ( 0.8 14.2 ( 0.3 18.3 ( 0.1
39 ( 4 35.3 ( 0.7 34.6 ( 0.3
8.6 ( 0.7 14.7 ( 0.2 18.5 ( 0.4
35 ( 1 32.2 ( 1.0 32.6 ( 0.7
8.5 ( 0.5 14.1 ( 0.3 17.9 ( 0.4
on differently modified gold substrates. In general, ψ decreases by a few degrees with increasing Pc coverage (i.e., the Pcs assume a more edge-on average orientation with respect to the substrate) and the rotation angle θ of the x-y plane more than doubles with increasing Pc film thickness, suggesting a dependence for θ on the Pc packing density. The observed changes in ψ and θ are greater than the average error in these values and are therefore considered significant. The values of ψ on Au surfaces modified with 2 change less with Pc coverage vs the unmodified or CH3-terminated SAM-modified Au surfaces, suggesting less reorganization of those films as Pc coverage increases. Increases in θ with Pc coverage are believed to be due to differences in column packing, as up to six nearest neighbors become available for each Pc column, and hexagonal close packing of the columns becomes dominant. Previous small-angle X-ray scattering experiments have shown that multilayer films (coverages in excess of 5-10 bilayers) of this Pc form columns with hexagonal close packing, as viewed end-on,15 but this packing architecture may not be achieved in single bilayer films. The RAIRS data suggest that other forms of space filling, without interdigitation of the rodlike aggregate Pc columns, are adopted in the single bilayer of 1. In addition, all of the Pc chromophores rotate on average (change θ, but not necessarily ψ) to a new space-filling configuration as coverage is enhanced, as each column is surrounded by up to six nearest column neighbors. Additional smallangle X-ray scattering experiments are underway to explore this change in orientation further. Pc Transfer to Modified Silicon Substrates. Si (100) surfaces with a thin oxide layer are amenable to modification with various silanes and are excellent candidates for both AFM and transmission FT-IR characterization of these Pc aggregate thin films.15 Although a variety of other methyl-terminated silane agents were evaluated initially, HMDS was chosen as a convenient material to create a methyl-terminated oxide/Si surface. Diphenyltetramethyldisilazane (DPTMDS) was selected to introduce phenylterminated groups to the silicon surface and has comparable reactivity to HMDS. XPS C/Si ratios, computed from the ratio of baseline-corrected peak areas for the C(1s) peaks associated with the silane modifier, and the Si(2p) peak associated with the native oxide were 0.41, 0.55, and 0.44 for the HMDS treatment, DPTMDS treatment, and the 50:50 mixture of HMDS:DPTMDS, respectively. Representative 100 µm scale AFM images, of transferred LB films of 1, are shown in Figure 5. Figure 5a, which resulted from transfer of the Pc to an unmodified Si surface, reveals islandlike structures of a bilayer
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Figure 5. Representative AFM images of bilayer transfers of 1 to (a) an unmodified Si oxide surface and (b) a Si oxide surface treated with the 5:5:90 v/v/v ratio of HMDS:DPTMDS:CHCl3 solution. The islandlike deposit of Pc 1 in (a) is typical for substrates which were not properly hydrophobized and had rms roughness values of ca. 11 nm, while the coherent, conformal deposits in (b) are typical for those obtained on properly hydrophobized surfaces and had rms roughness values of ca. 1 nm (less than the column diameter for 1).
film of Pc 1, which typically had dimensions of ca. 50 µm × 100 µm, as torn film segments, and regions where extensive folding of the film occurred.17,18 The rms roughness of such Pc islands is ca. 11 nm. More continuous films of 1 were obtained for Si surfaces modified with either HMDS or DPTMDS, but occasional defects were observable on a typical 100 µm × 100 µm scan region, and the rms roughness values were ca. 3 nm. The best overall film quality was obtained with Si surfaces modified with a 50:50 mixture of HMDS/DPTMDS (the nearly featureless image in Figure 5b). AFM characterization indicates full bilayer coverage and pinhole-free films over scan areas in excess of 100 µm × 100 µm, which appears to be a critical dimension for the various device applications anticipated for these materials.1-7 As with transfer to Au substrates, subtle striations or ridges are evident which indicate the rod-axis direction of the Pc aggregate (proceeding from lower right to upper left), which is parallel to the compression barrier direction on the LB trough.15 Typical heights in these ridges were once again ca. 7 ( 2 nm. Figure 1 shows an AFM image, zoomed in to the 100 nm × 100 nm scale, of a bilayer Pc film like that shown in Figure 5 (Si (100) wafer modified with 50/50 HMDS/ DPTMDS), showing the improved microscopic ordering now obtainable for these Pc aggregates. It can be seen that the macrodomains in Figure 5b consist of large arrays of parallel Pc aggregates with coherence often in excess of 100 nm, column-column spacing of 2.7-2.8 nm, and column orientation parallel to the compression barriers
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Figure 6. Polarized transmission FT-IR spectra of a four bilayer film of CuPc(OCH2CH2OBz)8 on a Si wafer treated with a 5:5:90 v/v/v ratio of HMDS:DPTMDS:CHCl3 solution. Infrared absorbances were measured with the incoming beam polarized along (|) and across (⊥) the Pc column axis. In-plane vibrations, νPc-O-C (1204, 1283 cm-1) and νC-O-C (1103 cm-1), and out-ofplane vibrations, δPc-H ) 745 cm-1, are marked. Table 3. Dichroic Ratios, R, Based on Polarized Transmission FT-IR Spectra Si wafer treatment
1 bilayer film
4 bilayer film
no modification HMDS 100% HMDS:DPTMDS 75%:25% HMDS:DPTMDS 50%:50% HMDS:DPTMDS 25%:75% DPTMDS 100%
1.8 ( 0.8 2.0 ( 0.5 1.7 ( 0.3 4.5 ( 0.8 1.8 ( 0.3 1.1 ( 0.9
2.6 ( 0.3 2.3 ( 0.8 2.4 ( 0.9 3.4 ( 0.8 2.0 ( 0.5 1.9 ( 0.6
on the LB trough. This kind of microscopic near perfection of these aggregates is seldom seen on Si (100) substrates hydrophobized by HMDS or DPTMDS alone. These same films were used for polarized transmission FT-IR investigations of the effects of surface modification on the average molecular ordering within the transferred films of 1 (Figure 6).15,36 We calculated the dichroic ratio, R, defined as the ratio of the absorbances obtained with orthogonal polarized electric fields (R ) A|/A⊥), by monitoring the absolute IR absorption for in-plane vibrations, νPc-O-C (1204, 1283 cm-1) and νC-O-C (1103 cm-1), and for out-of-plane vibrations, δPc-H (745 cm-1). The average anisotropy values are listed in Table 3 for single bilayer and four bilayer films transferred to bare Si and Si wafers modified with HMDS, DPTMDS, and 25:75, 50:50, and 75:25 mixtures of HMDS/DPTMDS. Values of R ) A|/A⊥ for single bilayer films were ca. 2.0 on all substrates, except for the substrates modified with the 50/50 mixture of DPTMDS/HMDS, where dichroic ratios in excess of ca. 4 were observed. For thicker Pc films the dichroic ratios increased, regardless of modification scheme, but were again consistently higher on Si surfaces modified with the DPTMDS/HMDS mixture. The anisotropy values achieved with the HMDS/DPTMDS modified surface treatment rival those achieved with thin films of silicon phthalocyanine polymers, PcPS, shown by Wegner and co-workers.36 Using these dichroic ratios, an estimate of
Rodlike Aggregate Phthalocyanines
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Figure 7. Schematic of a HMDS/DPTMDS modified Si/oxide surface suggesting how the spacing of phenyl groups might assist the transfer of coherent films of 1, through edge-edge interactions between the surface phenyl groups and those at the periphery of the Pc aggregate column. Edge-cofacial interactions, with the same spatial periodicity, would also be possible by slight adjustments in orientation of the terminal phenyl groups in either the Pc aggregate or the surface modifiers.
the average rotation angle, φ (Figure 4), of the Pcsaround the z-axis can be calculated according to established treatments.15,42-43 For the Si (100) substrates treated with the optimum mixture of HMDS/DPTMDS, the average value of φ was 27 ( 2°. Conclusions Optimization of the interactions of compact LB films of Pc 1 have been shown to be critically dependent upon surface modification of the substrate material, especially with regard to the requirement for phenyl groups in the modifier to allow for efficient wetting of the substrate, formation of conformal, ordered films, and even affect the internal ordering (tilt and rotation angles) of the individual Pc chromophores within the rodlike aggregate. While the density and orientation of phenyl-terminated modifiers are presently difficult to ascertain, it is clear that there must be some flexibility in 2 attached to the Au surface, as revealed by the electrochemical studies, even though water contact angles suggest a reasonably nonporous, hydrophobic surface. For phenyl-terminated modifiers on Si/oxide surfaces, however, it is clear that an optimum surface coverage of phenyl groups is needed to effect the most efficient transfer of ordered bilayer films of 1. Previous studies support the idea that there is an ideal number of surface active groups needed to optimize the interaction of an organic film with a modified substrate. Ulman and Scaringe have previously introduced the idea (42) Ulman, A.; Scaringe, R. P. Lanmguir 1992, 8, 894-897. (43) Hooks, D. E.; Fritz, T.; Ward, M. D. Adv. Mater. 2001, 13, 227241.
of “commensurability of intraassembly planes”.42 Their hypothesis includes the idea that even though a multilayer assembly can be formed, organized mainly on the basis of van der Waals interactions, the mismatch of functional groups between adjacent layers can affect the stability of the layered structure and significantly impact the defect density in the resultant thin film. Their discussion focuses mainly on three conditional interactions of distinct layers within a two-dimensional assemblys(i) matching of crosssectional areas of specific functional groups, (ii) the epitaxy of atomic (or molecular) layers, and (iii) matching of the valence bond geometry, the first of which, at least, appears to be important in the transfer of our Pc films. The control that surface functional group spacing exerts on the coherence and molecular orientation of these Pc aggregates is also consistent with the evolving views of large molecule epitaxy on well-defined surfaces, where coincident lattices in the overlayer, relative to the substrate, are routinely observed.43 The density of phenyl moieties per Pc unit in compact LB films of 1 is ca. 1 unit per 50 Å2. This is based on the assumption that there are two phenyl moieties within the Pc that project onto the substrate surface, defined by the column-to-column spacing of ca. 28 Å15 and the Pc-Pc intermolecular spacing of 3.4 Å (see for example the schematic in Figure 7).19 One can compare the density of phenyl moieties found in LB films of 1 to an approximate maximum density of phenyl moieties on the Si oxide surface, modified with the 50/50 mixture of HMDS and DPTMDS. If one approximates the projected areas of the HMDS and DPTMDS molecules to be ca. 20 and ca. 30 Å2,
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respectively, and that an even distribution of the modifying molecules is present over the Si substrate surface, the coverage of phenyl groups at the modified surface ca. one phenyl group per 50 Å2 projected area. This corresponds well with the density of phenyl moieties found in the Pc assembly, i.e., a coincident arrangement of Pcs and phenyl groups is implied, along the Pc aggregate column axis (Figure 7). The fact that Si oxide surfaces modified with 75/25 and 25/75 mixtures of HMDS/DPTMDS or pure DPTMDS did not yield Pc film coherence or IR dichroic ratios as large as with the 50/50 ratio of HMDS/DPTMDS suggests that an optimum spacing of surface phenyl groups is required in order to achieve the highest efficiency film transfer, without loss of long-range order. The importance of these surface modifications to measurements of electronic properties (i.e., field effect mobilities) and to the templated growth of vertical columns of these Pcs on other substrates is now being explored. It is clear that these optimized modifications, based on phenyl-phenyl interactions, are going to be critical in the patterning and polymerization of these molecular
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assemblies and are likely to be general in nature when it comes to producing conformal thin films of molecular materials based on aromatic hydrocarbons of significant size. One recent notable example was reported for pentacene thin films, used as the channel region in OFETs, where the modification of the gate oxide (silica) with pentacene-terminated silane reagents produced a profound improvement in film morphology, conformal coverage, and the resultant electrical properties of this material.3 Work in progress here suggests that similar improvements in electrical and optical properties will be achieved for these Pc materials using the protocols outlined in this paper. Acknowledgment. This research was supported in part by grants from the National Science Foundation (Chemistry, N.R.A., DMR for D.F.O.), and by the Materials Characterization Program, State of Arizona. LA010817L
