7812
Langmuir 2005, 21, 7812-7822
Covalent Molecular Assembly of Oligoimide Ultrathin Films in Supercritical and Liquid Solvent Media Sreenivasa Reddy Puniredd and M. P. Srinivasan* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576 Received April 8, 2005. In Final Form: June 14, 2005 An ultrathin film of oligoimide has been fabricated on amine-modified substrates of silicon and quartz through alternate layer-by-layer (LBL) assembly of pyromellitic dianhydride (PMDA) and diaminodiphenyl ether (DDE), with interlayer links established by covalent bonds. The assembly was formed in supercritical carbon dioxide (SCCO2) and in solution (dimethyl acetamide, DMAc), and the imidization reaction was performed by thermal and chemical methods, in benzene and in the supercritical medium. X-ray photoelectron and UV-visible absorption spectroscopies, atomic force microscopy (AFM), and ellipsometry were employed to study the interfacial chemistry, growth, morphology, and thickness of the assembled film. XPS analysis confirmed the sequential deposition of PMDA and DDE through formation of amic acids. At each deposition step, surface functionalities for the assembly of the next layer were generated. The interfacial chemical reaction was almost complete in the SCF (supercritical fluid) medium, as compared to the conversions observed in conventional assembly. Both the PMDA and DDE molecules were assembled in an organized manner, resulting in uniform surface morphology. Uniform film growth was revealed from the increase of UV absorption intensity and film thickness. The overall growth and quality of the films in SCF medium were greater than that for films formed in DMAc. The results of this novel study show that an environmentally friendly solvent can be used to obtain mechanically robust and thermally stable ultrathin films with little loss of material during the imidization step. In contrast to conventional deposition of the molecular layers that utilizes liquid solvents, use of SCCO2 avoids solvent effects and posttreatment for solvent removal, while ensuring facile transport during contact.
1. Introduction Ultrathin films with thicknesses in the nanoscale have found potential applications in nanodevices, surface modification, sensors, and membrane technology.1-4 These films are typically made using techniques such as Langmuir-Blodgett (LB) deposition and electrostatic selfassembly. LB films, which are multilayers formed on a solid substrate by transferring monolayers from an airwater interface,5 are not mechanically robust due to the weak interlayer bonding force, which is usually the van der Waals interaction. In electrostatic self-assembly, spontaneous sequential adsorption of oppositely charged materials is carried out from dilute (and most often aqueous) solutions on charged surfaces.6 Electrostatic assembly, although providing interactions that are stronger than van der Waals, may still not be robust enough to make a mechanically strong film. In particular, LB and electrostatic self-assembly techniques may require introduction of weaker species into the film, which needs special effort to be removed to avoid adverse impact on film properties. In LB assembly, the long-chain alkyl* Corresponding author. Tel: +65-68742171. Fax: +6567791936. E-mail:
[email protected]. (1) Swalen, J. D. Some emerging organic-thin-films technologies. In Organic Thin Films: structure and applications; Frank, C. W., E.; American Chemical Society: Washington, DC, 1998; p 2. (2) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, J.; Murray, R.; Pease, R. F. Langmuir 1987, 3 (6), p 932. (3) Petty, M. C. Biosens. Bioelectron. 1995, 10 (12), p 129. (4) Nicolini, C.; Erokhin, V.; Ram, M. K. Supramolecular Organic Layer Engineering for Industrial Nanotechnology. In Nano-Surface Chemistry; Rosoff, M., Ed.; Marcel Dekker: New York, 2002; p 141. (5) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (6) Mark, R.; Richey, M. D.; Taylor, C. D.; Michaiah, P.; Spencer, C.; Daniela, M.; Michael, M. Langmuir 2001, 17, 8380.
amines, which are often introduced for imparting amphiphilicity to poly(amic acid) for LB deposition, will constitute a chemical impurity that requires removal.7 Likewise, the polycation introduced as the counterpart of poly(amic acid) for electrostatic self-assembly may undermine film properties if it is not removed.8 In terms of stability or strength, multilayer ultrathin films with covalent interlayer bonding9,10 are believed to be more advantageous since they are robust enough to withstand elevated temperatures, polar solvent attack, mechanical wear and abrasion, etc. Bitzer et al. used vapor deposition as an alternative for thin film formation and demonstrated the growth of an ultrathin oligoimide film on Si (100) by reactive coupling of 1, 4-phenylene diamine and pyromellitic dianhydride, which were sequentially dosed on the substrate under ultrahigh vacuum and showed that oligoimide chains formed upon imidization at 200 °C stand upright on the substrate and bond to the silicon substrate via Si-(NH)-C linkages.11 The disadvantage of this method is that care must be taken to ensure a 1:1 ratio in the flux of monomers, as otherwise, unreacted monomers might be trapped in the film. Recently Fengxiang et al. fabricated ultrathin oligoimide films on amineterminated substrates through alternate assembly of PMDA and DDE12 and also reported a covalent process in which ultrathin films were fabricated in a layer-by-layer fashion on amine-terminated substrates of silicon surface (7) Sasaki, T.; Fujii, H.; Nishikawa, M. Jpn. J. Appl. Phys. (I) 1992, 31, 632. (8) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (9) Sun, J.; Wang, Z.; Sun, Y.; Zhang, X.; Shen, J. Chem. Commun. 1999, 8, 693. (10) Kohli, P.; Taylor, K. K.; Harris, J. J.; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962. (11) Bitzer, T.; Richardson, N. V. Appl. Phys. Lett. 1997, 71, 662. (12) Zhang, F.; Srinivasan, M. P. Colloids Surf. A 2005, 257-258, 295.
10.1021/la0509302 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/19/2005
Molecular Assembly of Oligoimide Ultrathin Films
and demonstrated that these films are better than spin coated films in terms of stability or strength.13 This attests to the advantage of covalently assembled multilayer films in forming higher quality films. Among the drawbacks of the present conventional process for covalent molecular self-assembly of ultrathin films is the necessity of a solvent rinse after each deposition step. If deposition is conducted in the vapor state, the participation of solvent will be minimized in the fabrication process, and unused monomers can be swept away by the gas together with the residual solvent. It may also be useful to use a gas in the supercritical phase in order to exploit the advantageous properties of the supercritical state. Among the possible carrier fluids, carbon dioxide may be the one of choice due to its environmentally benign properties and relatively low cost. Recently, thin film deposition techniques from supercritical carbon dioxide (SCCO2) have received much attention because of the unique nature and properties of materials in the supercritical state. The density and, hence, solvating power of supercritical fluids is tunable, allowing a degree of control which is not available in conventional solvents. SCCO2 is inexpensive, environmentally benign, and permits easy recovery of solutes. It has been used recently for the formation of nanoparticles,14 and self-assembled structures of block copolymers,15 selfassembled monolayers of alkanethiols on gold surfaces,16 as well as for many other colloidal applications,17 and as the carrier in impregnation and polymer synthesis.18 It has been also used for dissolution and deposition of thin perfluoropolyether films on solid substrates.19,20 Mechanical, thermal, and chemical stability in thin films is promoted to a large extent by the presence of aromatic moieties and imide bonds. The well-known polyimide is a good example of this characteristic. Conventionally, polyimides have been prepared by a two-step method of synthesis that involves formation of a soluble polyamic acid precursor, and subsequently, imidization is carried out thermally or chemically to convert the precursor to robust polyimide. Thermal imidization is carried out at temperatures in the range 250-300 °C, whereas chemical imidization is performed with mixtures of aliphatic carboxylic acid anhydrides (dehydrating agent) and tertiary amines (that catalyze the cyclodehydration) in an aromatic solvent, such as benzene. Srinivasan et al. have conducted the chemical imidization of LangmuirBlodgett (LB) films of polyamic acid-n-octadecylamine salt (13) Zhang, F.; Jia, Z.; Srinivasan, M. P. Langmuir 2005, 21, 3389. (14) (a) Clarke, M. J.; Harrison, K. L.; Johnston, K. P.; Howdle, S. M. J. Am. Chem. Soc. 1997, 119, 6399. (b) Ji, M.; Chen, X.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631. (c) Parra, J. L.; Maza, A.; Holmes, J. D.; Bhargava, P. A.; Korgel, B. A.; Johnson, K. P. Langmuir 1999, 15, 6613. (d) Reverchon, E. J. Supercrit. Fluids. 1999, 15, 1. (e) Cason, J. P.; Thompson, J. B.; Roberts, C. B. J. Phys. Chem. B 2000, 104, 1217. (15) (a) Fulton, J. L.; Pfund, D. M.; Romack, T. J.; Combes, J. R.; Samulski, E. T.; DeSimone, J. M.; Capel, M. Langmuir 1995, 11, 4241. (b) McClain, J. B.; Betts, D. E.; Canelas, D. A.; Samulski, E. T.; DeSimone, J. M.; Londono, J. D.; Cochran, H. D.; Wignall, G. D.; Chillura-Martino, D.; Triolo, R. Science 1996, 274, 2049. (c) Buhler, E.; Dobrynin, A. V.; DeSimone, J. M.; Rubinstein, M. Macromolecules 1998, 31, 7347. (16) Weinstein, R. D.; Yan, D.; Jennings, G. K. Ind. Eng. Chem. Res. 2001, 40, 2046-2053. (17) Johnston, K. P.; DaRocha, S. R. P.; Holems, J. D.; Jacobson, G. B.; Lee, C. T.; Yates, M. Z.; Psathas, P. Colloids in Supercritical Carbon Fluids: Fundamentals and Applications. Proc. 5th Int. Symp. Supercrit. Fluids; Atlanta, GA, 2000. (18) Gallymov, M. O.; Vinokur, R. A.; Nikitin, N. N.; Said Galiv, E. E.; Khokhlov, A. R.; Yaminsky, I. V.; Schaumburg, K. Langmuir 2002, 18, 6928. (19) Henon, F. E.; Camaiti, M.; Burke, A.; Carbonell, R. G.; DeSimone, J. M.; Piacenti, F. J. Supercrit. Fluids 1999, 15, 173. (20) Novick, B. J.; Carbonell, R. G.; DeSimone, J. M. Proc. 5th Int. Symp. Supercrit. Fluids; Atlanta, GA, 2000.
Langmuir, Vol. 21, No. 17, 2005 7813 Scheme 1. Molecular Structure of Materials Used
in supercritical carbon dioxide, which constituted a replacement for benzene as the solvent vehicle for transporting the acetic anhydride and pyridine to the polyimide precursor. The reaction is completed in a much shorter time than that required for imidization in benzene. Use of supercritical medium also obviated the need for post-imidization rinse and drying.21 The advantageous solvating and transport properties of matter in the supercritical state is particularly suitable since it is a nonpolar, aprotic solvent,22 similar to DMAc, which is typically used for polyimide precursors. In addition, the dehydrating agent and catalyst for the imidization process, i.e., acetic anhydride and pyridine, respectively, are completely miscible with SCCO2.23 This further ensures the compatibility of the SCCO2 with the imidization process. To the best of our knowledge, ours is the first effort to conduct covalent LBL assembly in a supercritical medium. Our aim in this work is to demonstrate the feasibility of constructing robust, covalently linked molecular multilayers using SCCO2 as the medium. We have compared these films with those assembled in a conventional solvent medium. The comparison is further extended to carrying out the imidization reaction by various methods. 2. Experimental Section 2.1. Materials. Pyromellitic dianhydride (PMDA), 4, 4′diaminodiphenyl ether (DDE), and p-aminophenyltrimethoxysilane (APhS) were purchased from Aldrich, Fluka, and Gelest, respectively, and used as received. Their molecular structures are shown in Scheme 1. Toluene (Merck) was distilled and stored over molecular sieves, and pyridine, benzene (both from Fluka), carbon dioxide (SOXAL, Code P40J Purified grade,
