Interrupted-Growth Studies of the Self-Assembly of Intrinsically

Interrupted-Growth Studies of the Self-Assembly of Intrinsically Acentric Siloxane-Derived .... Christina Capacci-Daniel , Karen J. Gaskell and Jennif...
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Langmuir 2003, 19, 10531-10537

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Interrupted-Growth Studies of the Self-Assembly of Intrinsically Acentric Siloxane-Derived Monolayers Milko E. van der Boom,*,† Guennadi Evmenenko,‡,§ Chungjong Yu,‡,§ Pulak Dutta,‡,§ and Tobin J. Marks*,§,| Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel, and Department of Physics and Astronomy, Department of Chemistry, and Materials Research Center, Northwestern University, Evanston, Illinois 60208-3113 Received May 25, 2003. In Final Form: September 17, 2003 The evolution of an intrinsically acentric monolayer formed by self-assembly of a trimethoxysilanefunctionalized, high-β azobenzene-based chromophore from solution onto hydrophilic silicon oxide surfaces shows two distinctly different adsorption steps. The kinetics of the monolayer formation on sodium lime glass and polished silicon were monitored ex situ by a combination of aqueous contact angle measurements, optical spectroscopy, transmission second-harmonic generation (at λ0 ) 1064 nm), and synchrotron X-ray reflectivity. The initial adsorption step from toluene onto the substrate surface at 80 °C is completed within minutes, k1 ≈ 1 × 10-2 s-1, and results in an acentric material having ∼65% of the properties of a fully formed, dense monolayer (i.e., thickness, coverage, optical absorption, wettability, and nonlinear optical response). The second assembly stage is about 2 orders of magnitude slower, k2 ≈ 1.3 × 10-4 s-1, and reaches completion after about 6 h.

Introduction Formation of organized organic materials with controllable electronic, optical, and electrooptical properties is a formidable and complex task.1-11 Organic device-quality functional thin films remain relatively rare despite the tremendous recent progress in this field.12-15 LangmuirBlodgett,16,17 covalent molecular self-assembly,18-32 bipolar * Authors to whom correspondence should be addressed. Email: [email protected] (M.E.v.d.B.); t-marks@ northwestern.edu (T.J.M.). † The Weizmann Institute of Science. ‡ Department of Physics and Astronomy, Northwestern University. § Materials Research Center, Northwestern University. | Department of Chemistry, Northwestern University. (1) Cahen, D.; Hodes, G. Adv. Mater. 2002, 14, 789. (2) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002, 14, 99. (3) Ko¨hler, A.; Wilson, J. S.; Friend, R. H. Adv. Mater. 2002, 14, 2002. (4) Wu¨rthner, F.; Wortmann, R.; Meerholz, K. ChemPhysChem 2002, 3, 17. (5) Functional Organic and Polymeric Materials; Richardson, T. H., Ed.; Wiley: Chichester, U.K., 2000. (6) Yitzchaik, S.; Marks, T. J. Acc. Chem. Res. 1996, 29, 197. (7) Ulman, A. Chem. Rev. 1996, 96, 1533. (8) Molecular Nonlinear Optics - Materials, Physics and Devices; Zyss, J., Ed.; Academic Press: San Diego, CA, 1994. (9) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1990. (10) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic: San Diego, 1991. (11) van der Boom, M. E.; Marks, T. J. Layer-by-Layer Assembly of Molecular Materials for Electrooptical Applications. In Polymers for Micro- and Nanoelectronics; Lin, Q. H. J. C.; Pearson, R. A., Eds.; American Chemical Society: Washington, DC, 2003; in press. (12) van der Boom, M. E. Angew. Chem., Int. Ed. 2002, 41, 3363. (13) Dalton, L. R. Opt. Eng. 2000, 39, 589. (14) Ma, H.; Hen, A. K.-Y.; Dalton, L. R. Adv. Funct. Mater. 2002, 14, 1339. (15) Liu, Z.; Ho, S. T.; Chang, S.; Zhao, Y.-G.; Marks, T. J.; van der Boom, M. E.; Zhu, P. Electrooptic modulators based on intrinsically polar self-assembled superlattices. In Proceedings of SPIE (Linear and Nonlinear Optics of Organic Materials); Eich, M., Kuzyk, M. G., Eds.; SPIE Press: Bellingham, WA, 2002; Vol. 4798, p 163. (16) Schwartz, H.; Mazor, R.; Khodorkovsky, V.; Shapiro, L.; Klug, J. T.; Kovalev, E.; Meshulam, G.; Berkovic, G.; Kotler, Z.; Efrima, S. J. Phys. Chem. B 2001, 105, 5914.

amphiphiles and polyelectrolytes,33 metal-coordination,34-45 and related film transfer techniques46 have been used to integrate various functional molecules in ordered multi(17) Ricceri, R.; Neto, C.; Abbotto, A.; Facchetti, A.; Pagani, G. A. Langmuir 1999, 15, 2149. (18) Baptiste, A.; Gibaud, A.; Bardeau, J. F.; Wen, K.; Moaz, R.; Sagiv, J.; Ocko, B. M. Langmuir 2002, 18, 3916. (19) van der Boom, M. E.; Richter, A. G.; Malinsky, J. E.; Pulak, D.; Marks, T. J.; Lee, P. A.; Armstrong, N. R. Polym. Mater. Sci. Eng. 2000, 83, 160. (20) van der Boom, M. E.; Richter, A. G.; Malinsky, J. E.; Lee, P.; Armstrong, N. A.; Marks, T. J. Chem. Mater. 2001, 13, 15. (21) van der Boom, M. E.; Zhu, P.; Evmenenko, G.; Malinsky, J. E.; Lin, W.; Dutta, P.; Marks, T. J. Langmuir 2002, 18, 3704. (22) van der Boom, M. E.; Evmenenko, G.; Dutta, P.; Marks, T. J. Adv. Funct. Mater. 2001, 11, 393. (23) Evmenenko, G.; van der Boom, M. E.; Kmetko, J.; Dugans, S. W.; Marks, T. J.; Dutta, P. J. Chem. Phys. 2001, 115, 6722. (24) Zhu, P.; van der Boom, M. E.; Kang, H.; Evmenenko, G.; Dutta, P.; Marks, T. J. Chem. Mater. 2002, 14, 4982. (25) Cui, J.; Huang, Q.; Wang, Q.; Marks, T. J. Langmuir 2001, 17, 2051. (26) Lin, W.; Lin, W.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 8034. (27) Li, D.-Q.; Ratner, M.; Marks, T. J.; Zhang, C.; Yang, J.; Wong, G. J. Am. Chem. Soc. 1990, 112, 7389. (28) Cui, J.; Huang, Q.; Veinot, J. C. G.; Yan, H.; Wang, Q.; Hutchison, G. R.; Richter, A. G.; Evmenenko, G.; Pulak, D.; Marks, T. J. Langmuir 2002, 18, 9958. (29) Cui, J.; Huang, Q.; Veinot, J. C. G.; Yan, H.; Marks, T. J. Adv. Mater. 2002, 14, 565. (30) Facchetti, A.; Abbotto, A.; Beverina, L.; van der Boom, M. E.; Marks, T. J.; Pagani, G. A. Chem. Mater. 2002, 14, 4996. (31) Facchetti, A.; Abbotto, A.; Beverina, L.; van der Boom, M. E.; Dutta, P.; Evmenenko, G.; Pagani, G. A.; Marks, T. J. Chem. Mater. 2003, 15, 1064. (32) Facchetti, A.; Abbotto, A.; Beverina, L.; van der Boom, M. E.; Marks, T. J.; Pagani, G. A. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2003, 44, 1171. (33) Roberts, M. J.; Lindsay, G. A.; Herman, W. N.; Wynne, K. J. J. Am. Chem. Soc. 1998, 120, 11202. (34) Flory, W. C.; Mehrens, S. M.; Blanchard, G. J. J. Am. Chem. Soc. 2000, 122, 7976. (35) Doron-Mor, H.; Hatzor, A.; Vaskevich, A.; van der Boom-Moav, T.; Shanzer, A.; Rubinstein, I.; Cohen, H. A. Nature 2000, 406, 382. (36) Fang, M.; Kaschak, D. M.; Sutorik, A. C.; Mallouk, T. E. J. Am. Chem. Soc. 1997, 119, 12184. (37) Neff, G. A.; Page, C. J.; Meintjes, E.; Tsuda, T.; Pilgrim, W.-C.; Roberts, N.; Warren, W. W., Jr. Langmuir 1996, 12, 238. (38) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636.

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Scheme 1 . Schematic Representation of the Self-Assembly of 1-Based Chromophoric Superlatticesa

a Self-assembly is by (i) chromophore deposition from a solution followed by (ii) treatment with octachlorotrisiloxane (2; Si3O2Cl8), resulting in the deprotection of the tert-butyldimethylsilyl-derivatized hydroxyl groups (b) and in the formation of a robust polysiloxane capping layer (∼0.8-nm thick).

layer films, which can be grown on substrates such as silicon, indium-tin oxide-coated glass, and gold. We recently developed an efficient “one-pot” layer-by-layer assembly method for the formation of robust intrinsically acentric siloxane-based superlattices, as shown in Scheme 1.19-24 This straightforward synthetic approach is amendable to automation and involves two alternating solution deposition steps. First, monolayers of high-β chromophores (1) are covalently attached on hydrophilic substrates (step i). Subsequently, the siloxy removal step ii renders the surface hydrophilic, thus, allowing the buildup of a planarization/cross-linking layer derived from octachlorosiloxane (2). The scope of this conceptually new assembly technique was demonstrated by (i) incorporation of distinctly different chromophore building blocks into such thin films20,21 and by (ii) formation of refractive index tunable inorganic-organic “hybrid” materials.22,23 Greater than 80 chromophore + capping layers have been assembled,24,47 resulting in structurally regular submicrometer-thick films. The highly ordered materials exhibit excellent macroscopic nonlinear optical and electrooptic responses (up to χ(2) ≈ 370 pm/V and r33 ≈ 120 pm/V at λ ) 1064 nm, respectively) and have been integrated into frequency doubling devices,48 ultrafast optical switches,49 and prototype electrooptic phase modulators.47,50,51 Furthermore, all data indicate that the (39) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (40) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (41) Hanken, D. G.; Naujok, R. R.; Gray, J. M.; Corn, R. M. Anal. Chem. 1997, 69, 240. (42) Neff, G. A.; Helfrich, M. R.; Clifton, M. C.; Page, C. J. Chem. Mater. 2000, 12, 2363. (43) Hatzor, A.; van der Boom-Moav, T.; Yochelis, S.; Vaskevich, A.; Schanzer, A.; Rubinstein, I. Langmuir 2000, 16, 4420. (44) Hatzor, A.; Moav, T.; Cohen, H.; Matlis, S.; Libman, J.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13469. (45) Soto, E.; MacDonald, J. C.; Cooper, C. G. F.; McGimpsey, W. G. J. Am. Chem. Soc. 2003, 125, 2838. (46) Maoz, R.; Yam, R.; Berkovic, G.; Sagiv, J. Thin Films; Academic Press: San Diego, CA, 1995; Vol. 20. (47) Zhao, Y. G.; Chang, S.; Wu, A.; Lu, H. L.; Ho, S. T.; van der Boom, M. E.; Marks, T. J. Opt. Eng. Lett. 2003, 42, 298. (48) Lundquist, P. M.; Lin, W.; Zhou, H.; Hahn, D. N.; Yitzchaik, S.; Marks, T. J.; Wong, G. K. Appl. Phys. Lett. 1997, 70, 1941. (49) Wang, G.; Zhu, P.; Marks, T. J.; Ketterson, J. B. Appl. Phys. Lett. 2002, 81, 2169.

chromophore layers employed have a near-maximum chromophore packing. This raises the intriguing question as to whether the experimentally observed χ(2) and estimated r33 values represent the maximum possible response or whether the response is reduced because of intermolecular chromophore-chromophore interactions. Efficient transfer of the molecular properties into macroscopic phenomena is often a problematic step in the formation of functional materials. For instance, chromophore-chromophore interactions are thought to significantly limit the maximum possible χ(2) and r33 values in poled polymers and other systems.12,52-56 The introduction of sterically demanding substituents on high-β chromophores or the use of specially designed dendrimer systems reduces undesirable chromophore-chromophore interactions and enhances the r33 values in some poled polymers.12-14,52,53,57 Little is known about the chromophore 1 monolayer deposition process, which is the key step of the assembly scenario presented in Scheme 1. Monolayer formation mechanisms are of great interest, and the pathways involved have been studied extensively for hydro- and fluorocarbon chains.58-89 The formation chemistry and kinetics of various alkanethiol absorption processes on (50) van der Boom, M. E.; Malinsky, J. E.; Zhao, C.-F.; Chang, S.; Lu, W.-K.; Ho, S. T.; Marks, T. J. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2001, 42, 550. (51) Zhao, Y. G.; Wu, A.; Lu, H. L.; Chang, S.; Lu, W. K.; Ho, S. T.; van der Boom, M. E.; Marks, T. J. Appl. Phys. Lett. 2001, 115, 6722. (52) Ma, H.; Chen, B.; Sassa, T.; Dalton, L. R.; Jen, A. K.-Y. J. Am. Chem. Soc. 2001, 123, 986. (53) Shi, Y.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.; Steier, W. H. Science 2000, 288, 119. (54) Wu¨rthner, F.; Yao, S. Angew. Chem., Int. Ed. 2000, 39, 1978. (55) Di Bella, S.; Lanza, G.; Fragela`, I.; Yitzchaik, S.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 3003. (56) Dalton, L. R.; Harper, A. W.; Robinson, B. H. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 4842. (57) Luo, J.; Liu, S.; Haller, M.; Liu, L.; Ma, H.; Jen, A. K.-Y. Adv. Mater. 2002, 14, 1763. (58) Yan, D.; Saunders, A.; Jennings, G. K. Langmuir 2002, 18, 10202. (59) Richter, A. G.; Yu, C.-J.; Datta, A.; Kmetko, J.; Dutta, P. Colloids Surf., A 2002, 198-200, 3. (60) Genzer, J.; Efimenko, K.; Fischer, D. A. Langmuir 2002, 18, 9307. (61) Richter, A. G.; Yu, C.-J.; Datta, A.; Kmetko, J.; Dutta, P. Phys. Rev. E 2000, 61, 607. (62) Heiney, P. A.; Gru¨neberg, K.; Fang, J.; Dulcey, C.; Shashidhar, S. Langmuir 2000, 16, 2651.

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gold have been thoroughly characterized as well.90-96 However, detailed mechanistic film-growth studies on lowsurface-area substrates with structurally more complex, functional building blocks are relatively rare.62,97,98 Monitoring the kinetics of solution-surface reactions of selfassembled, intrinsically acentric monolayers with photonically/electronically functional building blocks has a clear advantage because the dynamics of this intriguing process can be readily followed by both linear and nonlinear optical methods. In this report, mechanistic details of the monolayer self-assembly process followed by chromophore 1 are revealed using a combination of various analytical techniques, including synchrotron X-ray reflectivity (XRR), optical spectroscopy (UV-visible and second-harmonic generation), and advancing aqueous contact-angle (CA) measurements. The interrupted growth study presented here reveals that the assembly of 1 from solution on flat, hydrophilic surfaces (e.g., glass, silicon) proceeds by a two-step uniform growth process, follows Langmuir kinetics, and involves self-organization of the film. (63) Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H.; Basnar, B.; Vallant, M.; Friedbacher, G. Langmuir 1999, 15, 1899. (64) Richter, A. G.; Durbin, M. K.; Yu, C.-J.; Dutta, P. Langmuir 1998, 14, 5980. (65) Bensebaa, F.; Voicu, R.; Huron, L.; Ellis, T. H. Langmuir 1997, 13, 5335. (66) Moon, J. H.; Kim, J. H.; Kim, K.-J.; Kang, T.-H.; Kim, B.; Kim, C.-H.; Hahn, J. H.; Park, J. W. Langmuir 1997, 13, 4305. (67) Durbin, M. K.; Malik, A.; Richter, A. G.; Huang, K. G.; Dutta, P. Langmuir 1997, 13, 3. (68) Zhao, X.; Kopelman, R. J. Phys. Chem. 1996, 100, 11014. (69) Doudevski, I.; Schwartz, D. K. Phys. Rev. Lett. 1999, 60, 14. (70) Resch, R.; Grasserbauer, M.; Friedbacher, G.; Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H. Appl. Surf. Sci. 1999, 140, 168. (71) Vallant, T.; Kattner, J.; Mayer, U.; Hoffmann, H. Langmuir 1999, 15, 5339. (72) Carraro, C.; Yauw, O. W.; Sung, M. M.; Maboudian, R. J. Phys. Chem. B 1998, 102, 4441. (73) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H.; Leitner, T.; Resch, R.; Friedbacher, G. J. Phys. Chem. B 1998, 102, 7190. (74) Doudevski, I.; Hayes, W. A.; Schwartz, D. K. Phys. Rev. Lett. 1998, 81, 4927. (75) Britt, D. W.; Hlady, V. J. Colloid Interface Sci. 1996, 178, 775. (76) Banga, R.; Yarwood, J.; Morgan, A. M.; Evans, B.; Kells, J. Thin Solid Films 1996, 284-285, 261. (77) Woodward, J. T.; Schwartz, D. K. J. Am. Chem. Soc. 1996, 118, 7861. (78) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626. (79) Amar, J. G.; Family, F. Phys. Rev. Lett. 1995, 74, 2066. (80) Banga, R.; Yarwood, J.; Morgan, A. M. Langmuir 1995, 11, 618. (81) Bierbaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11, 2143. (82) Schwartz, D. K.; Steinberg, S.; Israelachvili, J. N.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354. (83) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (84) Tidswell, I. M.; Rabedeau, T. A.; Pershan, P. S.; Kosowsky, S. D. J. Chem. Phys. 1991, 95, 2854. (85) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S. Phys. Rev. B 1990, 41, 1111. (86) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (87) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054. (88) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (89) Mekhalif, Z.; Laffineur, F.; Couturier, N.; Delhalle, J. Langmuir 2003, 19, 637. (90) Bain, C. D.; Troughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (91) Pan, W.; During, C. J.; Turro, N. J. Langmuir 1996, 12, 4469. (92) Shon, Y.-S.; Lee, T. R. J. Phys. Chem. B 2000, 104, 8182. (93) Jung, L. S.; Campbell, C. T. Phys. Rev. Lett. 2000, 84, 5164. (94) Garg, N.; Friedman, J. M.; Lee, T. R. Langmuir 2000, 16, 4266. (95) Tokumitsu, S.; Liebich, A.; Herrwerth, S.; Eck, W.; Himmelhaus, M.; Grunze, M. Langmuir 2002, 18, 8862. (96) Dannenberger, O.; Buck, M.; Grunze, M. J. Phys. Chem. B 1999, 103, 2202. (97) Burtman, V.; Ofir, Y.; Yitzchaik, S. Langmuir 2001, 17, 2137. (98) Yitzchaik, S.; Roscoe, S. B.; Kakkar, A. K.; Allan, D. S.; Marks, T. J.; Xu, Z.; Zhang, T.; Lin, W.; Wong, G. K. J. Phys. Chem. 1993, 97, 6958.

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Figure 1. Advancing aqueous CA measurements versus deposition time for the formation of 1-based chromophore monolayers on glass substrates. The dotted line is drawn as a guide to the eye.

Figure 2. Optical absorption data (UV-vis; λmax ) 580 nm) versus deposition time for the formation of a 1-based chromophore monolayer on clean glass from a toluene solution (2.0 mM, 80 °C). The line through the data points has been fitted to a biexponential model (eq 3), as decribed in the text.

Results and Discussion The deposition study of the chromophore 1-based monolayers was carried out at elevated temperatures (80 °C) from dry toluene solutions in sealed vessels under N2. Under these reaction conditions, it has been observed that densely packed monolayers form from chromophore 1, yielding structurally regular multilayer films (Scheme 1).19,20,22,23 In the present study, we prepared and measured ex situ a series of 1-based monolayers, which were grown on freshly cleaned and dried sodium lime glass and polished single-crystal silicon substrates under identical reaction conditions, except for the length of deposition time, which ranged from 1 min to 24 h. The resulting films strongly adhere to the hydrophilic substrates because they are insoluble in organic solvents and cannot be detached by sonication or by the “Scotch tape decohesion test”.99 The functionalized substrates were stored in sealed vessels under N2 in the dark at room temperature. Aqueous CA measurements (Figure 1), and optical (UV-vis and second-harmonic generation, Figures 2-4) and synchrotron XRR measurements (Figures 5-9) were made within 72 h after monolayer deposition. Previous optical studies using a similar interrupted-growth technique showed that monolayer formation is about 1 magnitude of order faster with similar azobenzene-based chromophores functionalized with highly reactive -SiX3 (X ) Cl, I) groups.24 The formation of HX and the high rate of this assembly process (99) Huang, Z.; Wang, P.-C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. M. Langmuir 1997, 13, 6480.

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Figure 3. Transmission second-harmonic generation response versus deposition time for the formation of a 1-based chromophore monolayer on clean glass from a toluene solution (λ0 ) 1064 nm). The line through the data points has been fitted to a biexponential model (eq 3), as decribed in the text.

Figure 4. Transmission second-harmonic generation response versus absorption at λmax ) 580 nm in the time interval 5 min6.5 h for the formation of a 1-based chromophore monolayer on clean glass from a toluene solution. The line through the data points has been fit by a linear regression.

Figure 5. Representative normalized synchrotron XRR data for three 1-based monolayer deposition times. The solid lines are best fits assuming a Gaussian model (eq 4), as decribed in the text.

is problematic for accurate film-growth studies under the reaction conditions used for multilayer film formation. It is known that the evolution of film properties (i.e., optical absorption, second-harmonic generation, and surface coverage) versus time can be fit to a Langmuir model, which is a common growth mode for monolayers.88,100-102 The rate of surface coverage, Γ, is propor(100) Chen, S. H.; Frank, C. W. Langmuir 1989, 5, 978. (101) Bucher, J. P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979. (102) Sondag-Juethorst, J. A. M.; Schonenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6828.

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Figure 6. XRR-derived film thickness (Å) for the formation from a toluene solution of a 1-based chromophore monolayer on a clean silicon substrate. The film thickness remains constant at ∼12.5 Å for the t ) 12-24 h reaction time. The dotted line is drawn as a guide to the eye.

Figure 7. XRR-derived normalized coverage, θ, versus reaction deposition time for the formation of a 1-based chromophore monolayer from a toluene solution on a clean silicon substrate, assuming a phenomenological biexponential model of film growth (eq 3).

Figure 8. XRR-derived roughness, σ, of silicon substratefilm and film-air interfaces versus deposition time for the formation of a 1-based chromophore monolayer from a toluene solution on a clean silicon substrate.

tional to the uncovered space remaining on the surface in Langmuir kinetics, where

dΓ/dt ) F(1 - Γ)

(1)

Γ ) 1 - e-t/τ

(2)

leads to

Here, F is the adsorption rate and τ is the growth time scale ()1/F). In this model, the adsorbed, noninteracting

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Figure 9. XRR-derived film electron density of the 1-based chromophore monolayer versus reaction deposition time for the formation of a 1-based chromophore monolayer from a toluene solution on a clean silicon substrate.

molecules serve only to hinder molecules in solution from accessing the substrate. Attempts to fit our data to the aforementioned model reveal a deviation from simple Langmuir kinetics at the start of film formation. Plotting ln(1 - Γ) versus time shows that simple Langmuir kinetics are not obeyed over the entire coverage range. The apparent reaction rate gradually decreases with increasing coverage. However, it was found that the surface coverage and our optical data shown in Figures 2, 3, and 7 can be adequately fitted to a phenomenological biexponential model, which takes into account the evolution of the chromophore tilt angle with the coverage.98,103 The adsorption rate constants were then obtained using the equation

Γ ) 1 - Re-t/τ1 - (1 - R)e-t/τ2

(3)

Here, τ1 and τ2 are the corresponding growth time scales for “fast” and “slow” kinetics. Such a biphasic kinetic model with a rapid initial phase, followed by a slower second phase, is not a rarity in the self-assembly of thin films. It is known that monolayer films may assemble via a twostep mechanism: fast adsorption of the building blocks on the surface followed by a much slower reorganization/ ordering process leading to a densely packed film.90-92,94,95 For instance, “fast” and “slow” phases have been reported for (i) self-assembly of alkanethiols on gold,90-92,94,95,104 (ii) quaternization of the pyridine moiety of stilbazole-based chromophores with benzyl chloride-functionalized surfaces,98 and (iii) chemisorption of dicarboxylic acids on GaAs(100).105 The adsorption rate (1/τ) that we estimate from fitting is about 1 × 10-2 s-1 for the “fast” growing mode and is about 1.3 × 10-4 s-1 for the second “slow” mode. Interestingly, van Velzen et al. suggested that the biphasic kinetics for bromomethyldimethylchlorosilane assembly on silicon oxide surfaces might be a result of (i) steric hindrance for molecules approaching the substrate surface caused by surface-bound molecules or (ii) distinctly different hydroxyl groups on the substrate’s surface (i.e., dSi(OH)2 vs tSisOH).103 Geminal hydroxyl groups are expected to be much more reactive than single hydroxyl sites; however, the quantity of possible geminal sites is far less than one-third of the total number of available silanol sites. (103) van Velzen, P. N. T.; Ponjee´, J. J.; Benninghoven, A. Appl. Surf. Sci. 1989, 37, 147. (104) Thomas, R. C.; Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1991, 7, 620. (105) Vilan, A.; Ussyshkin, R.; Gartsman, K.; Cahen, D.; Naaman, R.; Shanzer, A. J. Phys. Chem. B 1998, 102, 3307.

Let us look at the various measurements reported here in greater detail. Advancing aqueous CA measurements reveal that the surface of densely packed monolayers of chromophore 1 are markedly hydrophobic, as evident from the observed maximum CA, θa ≈ 90°, and consist most probably of outward-facing bulky tert-butyldimethylsilyl (TBMDS) groups.20 This surface energy probe is sensitive but only to changes in the groups facing the film-air interface. The large wettability difference of ∆θa ≈ 70° between the hydrophilic glass substrates and the fully functionalized substrates should allow reliable ex situ monitoring of the film formation versus the reaction time. A rapid change in the surface polarity is observed during the first 10 min of the assembly process; the water CA reaches a maximum after ∼6 h, in accord with the formation of densely packed organic films, and then remains constant at ∼90° during the remaining course of the reaction, as shown in Figure 1. Monolayer formation on glass substrates is readily observed by optical spectroscopy and even by the eye as a result of the large extinction coefficient of 1 (i.e., ∈ ) 49 900 in acetone). The deepening of the characteristic film purple color (λmax ) 580 nm) resulting from the uniform substrate coverage with a 1-based monolayer is presented in Figure 2. The plot of the absorption intensity versus the deposition time shows that the highest occupied molecular orbital-lowest unoccupied molecular orbital charge transfer optical absorption of 1 reaches a maximum after ∼6.5 h at 80 °C. No significant shift in the optical absorption maximum is observed during film formation by UV-vis spectroscopy, ruling out formation of large molecular aggregates on the surface. Strong intermolecular chromophore-chromophore interactions are not generally desired because they may significantly limit the maximum possible macroscopic electrooptic response, r33, for an acentric material.12 For decades, electrooptic responses of poled polymers have been limited by the formation of dye aggregates at high chromophore densities.12,52-54,56 The absence of hypsochromic shifts in the present films argues against centrosymmetric H-type chromophore aggregation, which is known to suppress the second-order nonlinear optical responses in certain Langmuir-Blodgett film structures.10,17 Figure 3 shows the deposition kinetics monitored by transmission second-harmonic generation measurements at λ0 ) 1064 nm, which is a sensitive technique for monitoring thin film formation.6,96,98 The second-harmonic 532-nm light output intensity, I2ω, reaches a maximum after ∼6.5 h in good agreement with the advancing aqueous CA and optical absorption data. It appears that the films possess a noncentrosymmetric microstructure even in the early stages of the growth process and that a completely formed film exhibits the maximum possible secondharmonic generation responses, χ(2)zzz ∼ 220 pm/V. This large nonlinear susceptibility is obtained by calibration versus quartz at λ0 ) 1064 nm and indicates a macroscopic electrooptic coefficient of ∼80 pm/V. The latter is estimated using the known relationship: r33 ) -2χ(2)zzz/n4z with the index of refraction, nz ) 1.51 at 1064 nm for 1-based films.51,106 Both UV-vis and second-harmonic generation measurements show a significant increase in absorption and I2ω intensity, respectively, in the time interval 5 min6.5 h. Figure 4 displays the correlation between the maximum second-harmonic intensity versus the observed optical absorption, A, at λmax ) 580 nm in the aforementioned time interval, indicating that a high level of (106) Ashley, P. R.; Cites, J. S. Opt. Soc. Am. Tech. Dig. Ser. 1997, 14, 196.

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Langmuir, Vol. 19, No. 25, 2003

van der Boom et al.

chromophore dipole moment alignment and any possible chromophore interactions is established relatively rapidly (t e 5 min) and remains nearly constant during the final stages of the assembly process. The parallel increase in optical absorption and I2ω can be attributed to the incorporation of additional chromophore units into the film microstructure without greatly affecting the overall microstructural organization. The specular time-dependent XRR studies provide fundamental information on the (i) film thickness, (ii) surface morphology/roughness ()σfilm-air), and (iii) electron density distribution perpendicular to the substrate surface. A physically valid model for the films must be assumed to obtain a plausible electron density profile for each sample normal to the surface by fitting the experimental reflectivity curves while systematically varying the model parameters. The following Gaussian-step model is applied here, in which the regions between the interfaces of the samples in the z direction are assumed to be of uniform density, and the interfaces are modeled as error functions. An organic film will have at least two welldefined interfaces, one between the substrate surface of the single-crystal silicon wafers and the film and one between the film and the air. To fit the observed XRR data, we employ the following relationship:

R(qz) RF(qz)

)

|

1 F0

∫

∂F(z) -izqz e dz ∂z

|

2

(4)

where the wave vector transfer |q| ) qz ) (4π/λ)sinθ is along the surface normal, F0 ()FSi) is the electron density of the substrate, and 〈F(z)〉 is the electron density averaged over the in-plane coherence length of the X-rays (∼1 µm). RF(qz) is the Fresnel reflectivity for an ideally flat substrate surface. This form is valid for θ > 2θc. The fitting parameters include the thickness and electron density of the films and the root-mean-square width of each interface. The details of the XRR-data analysis procedure are described elsewhere.23,62,85,107-109 The density of electrons per unit of substrate area for the monolayer film, Fexp, is calculated using the obtained electron density profiles according to

Fexp )

∫F(z) dz

(5)

where integration is taken over the entire film. The molecular “footprint” can then be calculated from Ne/Fexp, where Ne is the total density of electrons in a single molecule. Typical XRR profiles measured at 10 min (b), 1 h (O), and 24 h (2) are shown in Figure 5. The symbols represent the experimentally observed intensities divided by the Fresnel reflectivity, RF(q), whereas the solid lines are the fitting results as described previously. From the positions of the minima alone, it is evident that the frequency of the reflectivity oscillations increases with time as a result of an increase in the film thickness. Fitting the XRR profiles provides reliable, qualititative information and reveals an initial film thickness of ∼8 Å after a 5-min deposition time, which increases to ∼10.3 Å during the first hour (Figure 6). During the apparent slower second film-growth phase, the film thickness reaches a maximum of ∼12.5 Å (107) Evmenenko, G.; van der Boom, M. E.; Yu, C.-J.; Kmetko, J.; Dutta, P. Polymer 2003, 44, 1051. (108) Als-Nielsen, J. Physica 1986, 140A, 376. (109) Evmenenko, G.; Yu, C.-J.; Kmetko, J.; Dutta, P. Langmuir 2002, 18, 5468.

after a 12-h deposition time. MM2-level calculations indicate a theoretical molecular length of ∼20 Å, suggesting that the molecules are significantly tilted toward the surface. In time, the interplay between entropic (conformational and orientational changes of deposited molecules upon continuing deposition of the film) factors and enthalpic (additional incorporation of chromophore molecules into the film) factors leads to a more preferred alignment of molecules in the direction perpendicular to the substrate surface. The estimated average tilt angle varies from ∼25° after a 5-min to ∼40° after a 12-h deposition time (reasonably assuming that 1 behaves as a rigid rod). A similar time-dependent change in the average tilt angle has been observed for self-assembling adsorption/quaternization of a (dialkylamino)stilbazole chromophore precursor onto benzyl chloride prefunctionalized surfaces.98 Aminoalkyl trialkoxysilanes have a tendency to form oligomeric structures on hydrophilic surfaces,66 which is not observed here. Figures 6-8 show that the 4.5-Å increase in the film thickness is accompanied by only minor fluctuations in the average surface roughness, σfilm-air ∼ 3.1 ( 0.3 Å, which is similar to the typical surface roughness of the polished substrate wafers, σSi-film ∼ 2.6 ( 0.4 Å. In fact, the σfilm-air decreases from ∼3.4 to ∼3.0 Å during this growth period, whereas the apparent substrate roughness increases from 2.2 to 2.7 Å. The electron density, F/FSi, decreases slightly from ∼0.45 to ∼ 0.38 e Å-3 during the film formation, as can be seen in Figure 9. This observation is somewhat inconsistent with uniform growth models,83-86 in which the density of the incomplete film remains close to that of the complete. However, the XRR-derived film coverage (expressed as chromophores/Å2) increases largely during the first few hours of film formation, as shown in Figure 7, meaning that the adsorption of chromophore units continues after that and additional chromophore molecules undergo selfassembly on the substrate surface. We noted previously that the chromophore molecules are significantly tilted from the normal to the surface and that there is a slight decrease of electron density and an increase of the film thickness with the increase in film coverage, all of which can be explained by “re-organization” of the microstructure as film growth progresses. Several other film-growth models75,81,82,87,110-112 have been proposed on the basis of interrupted growth studies. Notably, the XRR-derived surface roughnesses of n-alkyltrichlorosilane-based monolayers on the same high-quality silicon substrates are in the range of 2-4 Å.18 Here, the XRR-derived chromophore density reaches a maximum after ∼6 h and follows the same trend as observed by the present optical and water CA measurements. The maximum observed molecular surface density, Ns ∼ 1.4 × 1014 molecules/cm2 in the present study, is well within the range for these types of building blocks19-22,24,113,114 and is fully in agreement with MM2 analysis of compound 1. Fully formed monolayers of sterically less-demanding n-alkyl chain-based monolayers have much larger surface densities of ∼5 × 1014 molecules/cm2.18 (110) Barrat, A.; Silberzan, P.; Bourdieu, L.; Chatenay, D. Europhys. Lett. 1992, 20, 633. (111) Parikh, A. N.; Liedberg, B.; Atre, S. V.; Ho, M.; Allara, D. L. J. Phys. Chem. 1995, 99, 9996. (112) Rabinovich, Y. I.; Yoon, R.-H. Langmuir 1994, 10, 1903. (113) Facchetti, A.; Abbotto, A.; Beverina, L.; van der Boom, M. E.; Dutta, P.; Evmenenko, G.; Marks, T. J.; Pagani, G. A. Chem. Mater. 2002, 14, 4996. (114) Facchetti, A.; van der Boom, M. E.; Abbotto, A.; Beverina, L.; Marks, T. J.; Pagani, G. A. Langmuir 2001, 17, 5939.

Intrinsically Acentric Siloxane-Derived Monolayers

Langmuir, Vol. 19, No. 25, 2003 10537

Summary and Conclusions

Experimental Section

The present data show that optimal nonlinear optical responses are obtained in fully densified films; there is no decrease or leveling off in the response as a result of chromophore-chromophore interactions at the highest possible chromophore surface densities, in contrast to some poled-polymer systems.53 The lack of chromophorechromophore interactions in the films may reflect the saltlike microstructure or the use of the bulky TBMDS groups. Introduction of TBDMS substituents on constituent chromophore molecules (without affecting β) in poled polymers results in significantly higher r33 values.12-14,53 Various growth mechanisms are believed to be operative in the self-assembly of siloxane and thiol-based thin films. Two commonly observed pathways are “island” and “uniform”-type film growths.63,72,76,80-86 The former proceeds with a nearly constant film thickness and an increasing film average electron density, whereas the latter proceeds with an increasing film thickness while the electron density remains nearly constant. All of the present data indicate that the 1-based monolayer develops in two stages. During the initial growth stage (∆t ≈ 0-5 min) there is a relative fast buildup of a film structure on the surface, which exhibits ∼65-70% of the properties (i.e., thickness, coverage, optical absorption, wettability, and nonoptical responses) of the fully densified monolayer. The rate of the second, slower growth stage is about 2 orders of magnitude less than that of the initial step. The second stage requires ∼6 h under the reaction conditions, follows Langmuir kinetics, and involves absorption of additional chromophores as well as selforganization of the system. Our observations suggest that a uniform growth process may be operative here, which has been observed in other interrupted-growth studies as well.83-86,115 However, one should take into consideration that small variations in XRR-derived electron densities are difficult to determine accurately and that ex situ studies may result in (minor) morphological changes in the films prior to the analytical measurements. It is reasonable that the uniform growth process involves the following mechanism: an initial, relative rapid chromophore physisorption step from dry toluene onto the hydrophilic surfaces takes place (the substrate surfaces are covered by a thin film of physisorbed water),75,83,111,116-119 followed by organization/self-assembly of the film structure and irreversible formation of strong, covalent Si-O-Si linkages.18 This step is accompanied by minor changes in the average chromophore orientation; the chromophores become more tilted with respect to the substrate normal. The integration of additional chromophores to form a densely packed film is a much slower process and requires several hours.

Materials and Methods. The synthesis and characterization of 4-[[4-[N,N-bis[(tert-butyldimethyl-siloxy)-ethyl]amino]phenyl]azo]-1-n-propyl-3-trimethoxysilylpyridinium iodide (1) has been reported previously.20,26 Reagents were purchased from Aldrich Chemical Co. and used as received, unless stated otherwise. Toluene was dried with a Solvtek purification column, and tetrahydrofuran (THF) was distilled under N2 from Na/K alloy. Single-crystal silicon (111) substrates were purchased from Semiconductor Processing Company, Inc. Sodium lime glass and silicon substrates were cleaned by immersion in “piranha” solution (7:3 (v/v) H2SO4/30% H2O2) at 80 °C for 1 h. Caution: piranha solution is an extremely dangerous oxidizing agent and should be handled with care using appropriate shielding. After being cooled to room temperature, the substrates were rinsed with deionized water and then subjected to the following cleaning protocol (1:5:1 (v/v) NH3‚H2O/H2O/30% H2O2 at room temperature, 40 min). They were then washed with deionized water and dried in an oven (125 °C) overnight. The monolayer formation was carried out under an inert atmosphere using either standard Schlenk/cannula techniques or in an N2-filled M. Braun glovebox with H2O and O2 levels < 1 ppm. Advancing CAs were measured on a standard goniometric bench fitted with a Teflon micrometer syringe (Gilmont Instruments, Inc.). UV-vis spectra were recorded with a Cary1E spectrophotometer. Second-harmonic generation measurements were carried out in the transmission mode with a Q-switched Nd:YAG laser operating at λ0 ) 1064 nm, with a pulse width of 3 ns at a frequency of 10 Hz. Details of the instrumentation and calibration procedures can be found elsewhere.98 XRR measurements were carried out with λ ) 1.197 Å on Beamline X23B of the National Synchrotron Light Source at Brookhaven National Laboratory. Details and the data acquisition and analysis procedures are given elsewhere.23 Chromophore Deposition. The freshly cleaned sodium lime glass and silicon substrates were fixed in a Telfon sample holder and fully immersed in a toluene solution of 1 (2.0 mM). The airtight reaction vessels were fully immersed into a temperaturecontrolled oil bath at 80 ( 2 °C. Film growth was quenched after measured time intervals (t ) 1 min-24 h). The substrates (2.5 × 2.5 cm2) were immediately washed with excess dry toluene and THF, sonicated in acetone for 2 min, and then dried at room temperature using a flow of N2. The assembly process can be carried out in a single reaction vessel using standard cannula techniques to transfer the solutions; however, most experiments were perfomed in the glovebox. All samples were characterized ex situ. Experiments were repeated at least two times for each time interval. Figures 1-9 represent averaged data with a variation of e8%.

(115) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (116) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 1215. (117) Le Grange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (118) Brzoska, J. B.; Shahidzadeh, N.; Rondelez, F. Nature 1992, 360, 719. (119) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357.

Acknowledgment. This research was supported by the NSF under Grant DMR-0076077 (NSF MRSEC program through the Northwestern Materials Research Center), ARO/DARPA under Contract No. DAAD 19-001-0368, and by a grant from Sir Harry Djanogly, CBE. XRR measurements were performed at Beamline X23B of the National Synchrotron Light Source, which is supported by the U.S. Department of Energy. We also thank Dr. Dalia Freeman (WIS) for her skillful contributions. M.E.v.d.B. is the incumbent of the Dewey David Stone and Harry Levine career development chair and thanks the Israeli Council of Higher Education for an Alon fellowship. LA034900F