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Application of Direct Covalent Molecular Assembly in the Fabrication of Polyimide Ultrathin Films F. Zhang, Z. Jia, and M. P. Srinivasan* Centre for Nanoscale Engineering, Department of Chemical and Biomolecular Engineering, Blk E5, 4 Engineering Drive 4, National University of Singapore, Singapore 117576 Received May 20, 2004. In Final Form: November 22, 2004 Ultrathin films were fabricated using synthesized hydroxyl polyimide (HPI) in a layer-by-layer fashion on amine-terminated substrates of silicon, quartz, and gold. The interlayer linkages were established by using terephthaloyl chloride as a bridging agent to form ester groups between HPI layers. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV-vis absorption spectroscopy, atomic force microscopy, ellipsometry, and electrochemical impedance spectroscopy were employed to study the interfacial chemistry, stepwise growth, morphology, thickness, optical property, and insulation behavior of the assembled film. The films show excellent stability and strength, which can be attributed to the covalent interlayer linkage.
Introduction Ultrathin polymeric films with thicknesses in the range of a few nanometers have found wide applications in areas such as surface modification, sensors, and membrane technology. Well-established techniques to produce such films include Langmuir-Blodgett deposition and alternate adsorption of oppositely charged polyelectrolytes,1-3 both of which are used extensively and studied intensively. Apart from these, layer-by-layer (LBL) depositions of polymers based on coordination,4 charge transfer,5,6 hydrogen bonding,7,8 and physical adsorption9 are also employed as alternatives for polymer film fabrication. All the above-mentioned techniques are able to produce multilayer films with controlled thickness, tailored structure, or desired functionality; however, the films fabricated thus may not be stable or strong enough due to the relatively weak interlayer binding forces. In terms of stability or strength, multilayer polymeric films with covalent interlayer bonding10,11 are believed to be more advantageous since they are robust enough to withstand elevated temperatures, polar solvent attack, mechanical wear and abrasion, etc. Interestingly, there is no published literature concerning noncovalent LBL assembly that has employed ultrasonic treatment in addition to solvent * Corresponding author: (65) 68742171.
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(1) Decher, G. Science 1997, 277, 1232-1237. (2) Sukhorukov, G. B.; Mohwald, H.; Decher, G.; Lvov, Y. M. Thin Solid Films 1996, 285, 220-223. (3) Ladam, G.; Schaaf, P.; Decher, G.; Voegel, J.; Cuisinier, F. Biomolecular Eng. 2002, 19, 273-280. (4) Xoing, H.; Chen, M.; Zhou, Z.; Zhang, X.; Shen, J. Adv. Mater. 1998, 10, 529-532. (5) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385-1387. (6) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1998, 14, 4, 2768-2773. (7) Sun, L.; Kepley, L. J.; Crooks, R. M. Langmuir 1992, 8, 21012103. (8) Wang, L.; Cui, S.; Wang, Z.; Zhang, X. Langmuir 2000, 16, 1049010494. (9) Serizawa, T.; Kamimura, S.; Kawanishi, N.; Akashi, M. Langmuir 2002, 18, 8381-8385. (10) Sun, J.; Wang, Z.; Sun, Y.; Zhang, X.; Shen, J. Chem. Commun.1999, 8, 693-694. (11) Kohli, P.; Taylor, K. K.; Harris, J. J.; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962-11968.
rinsing for effective removal of excess or loosely adsorbed molecules after a deposition step. In contrast, such treatment can be used in covalent LBL assembly.12,13 This attests to the advantage of covalently assembled multilayer films in providing mechanical strength. There are mainly two strategies reported in the literature to fabricate multilayer polymer films with covalent interlayer linkages. One may be termed as indirect or induced strategy. For example, ultraviolet irradiation was used by many groups to induce covalent bond formation in the polyelectrolyte assemblies built from diazoresin and polyanions containing carbonyl or sulfonyl side groups.14-17 Dai and co-workers utilized postdeposition thermal treatment to convert the interlayer ionic bonds to covalent linkages in the electrostatic assembly between a polyamine and a polyacid.18 These two approaches are actually an extension and improvement of electrostatic assembly, which itself is not robust enough against polar solvents, moisture, and high temperatures. Another strategy is direct covalent assembly between polymers containing appropriate functional groups. It has been used to fabricate ultrathin polyamide films via alternate deposition of polyamidoamine (PAMAM)/Gantrez.19 Similarly, ultrathin films with interlayer amide linkages were prepared by the sequential deposition of poly(vinylamine-co-N-vinylisobutyramide) and poly(acrylic acid), whose carboxyl groups were activated by 1-ethyl3-(3-(dimethylamino)propyl)-carbodiimide hydrochloride (EDC) for amidation with the amino groups.20 Major et al. (12) Yang, X.; Shi, J.; Johnson, S.; Swanson, B. Langmuir 1998, 14, 1505-1507. (13) Chan, E. W. L.; Lee, D. C.; Ng, M. K.; Wu, G.; Lee, K. Y. C.; Yu, L. J. Am. Chem. Soc. 2002, 124, 12238-12243. (14) Sun, J.; Wu, T.; Liu, F.; Wang, Z.; Zhang, X.; Shen, J. Langmuir 2000, 16, 4620-4624. (15) Sun, J.; Cheng, L.; Liu, F.; Dong, S.; Wang, Z.; Zhang, X.; Shen, J. Colloids Surf., A 2000, 169, 209-217. (16) Luo, Y.; Li, Y.; Jia, X.; Yang, H.; Yang, L.; Zhou, Q.; Wei, Y. J. Appl. Polym. Sci. 2003, 89, 1515-1519. (17) Lu, C.; Bai, S.; Zhang, D.; Huang, L.; Ma, J.; Luo, C.; Cao, W. Nanotechnology 2003, 14 680-683. (18) Dai, J.; Sullivan, D. M.; Bruening, M. L. Ind. Eng. Chem. Res. 2000, 39, 3528-3535. (19) Liu, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2114-2116. (20) Serizawa, T.; Nanameki, K.; Yamamoto, K.; Akashi, M. Macromolecules 2002, 35, 2184-2189.
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reported a direct covalent LBL deposition of a hydroxylmaleimide-vinyl ether copolymer with interlayer ester linkages.21 In this work, we utilize the strategy of direct covalent LBL assembly to prepare ultrathin polyimide (PI) films. PI films are frequently used as insulating and encapsulating materials in the microelectronics industry because they possess superior mechanical strength, chemical and thermal stabilities, and favorable dielectric properties.22 PI films with thicknesses in the nanoscale may also find potential applications in nanodevices. Several techniques have been used to fabricate such films. For instance, the Langmuir-Blodgett technique involves sequential deposition of an amphiphilic alkylamine salt of poly(amic acid) and subsequent thermal or chemical imidization.23 The electrostatic self-assembly method uses poly(amic acid) as a polyanion to undergo alternate self-assembly with a polycation.24 A separate imidization step is also needed in this method. Another technique is vapor deposition25 in which the film is made by thermal curing of the polyamic acid precursor film obtained from simultaneous evaporation of diamines and dianhydrides at a stoichiometric molar ratio under vacuum. Bitzer et al.26 demonstrated the growth of an ultrathin oligimide film on Si(100)-2 × 1 by reactive coupling of 1,4-phenylene diamine and pyromellitic dianhydride (PMDA). We have fabricated ultrathin oligoimide films on amine-terminated substrates through alternate assembly of PMDA and diamino diphenyl ether.27 Song et al.28 reported multilayer PI film formation by alternate spin coating of PIs bearing acid and base moieties through H bonding. The average thickness of each bilayer was about 3.4 nm. In our method, we synthesize a soluble polyimide with hydroxyl pendant groups and employ it for LBL assembly with a diacid chloride, forming ester groups as interlayer linkages. The covalent cross links and the interlayer phenyl moieties both contribute to the enhanced stability and robustness of the PI film obtained. To the best of our knowledge, this is the first attempt to fabricate ultrathin PI films using covalent LBL assembly with a preformed polymer as a constituent. Experimental Section Materials. 4,4′-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) (Aldrich) was recrystallized from acetic anhydride. Toluene, N-methylpyrrolidone (NMP), and tetrahydrofuran (THF), all from Merck, were distilled before use. Silicon wafers (Wellbond, Singapore) were 0.6 mm thick, p-doped, polished on one side and with a natural oxide layer. Quartz slides were purchased from Achema Co., Singapore. (p-Aminophenyl)trimethoxysilane (APhS) (Gelest) was stored in a desiccator and used without further purification but with special care to minimize exposure to moisture. Terephthaloyl chloride (TC) (Aldrich), 3,3′-dihydroxyl-4,4′-diaminobiphenyl (HAP) (TCI, Japan), acetone (Fisher, HPLC grade), methanol (Merck), chloroform (Merck, HPLC grade), triethylamine (Fluka), and pyridine (Fluka) were all used as received. (21) Major, J. S.; Blanchard, G. J. Langmuir 2001, 17, 1163-1168. (22) Sroog, C. E. Prog. Polym. Sci. 1991, 16, 561-594. (23) Srinivasan, M. P.; Gu, Y.; Begum, R. Colloids Surf., A 2002, 198, 527-534. (24) Mark, R.; Richey, M. D.; Taylor, C. D.; Michaiah, P.; Spencer, C.; Daniela, M.; Michael, M. Langmuir 2001, 17, 8380-8385. (25) Salem, J. R.; Sequeda, F. O.; Duran, J.; Lee, W. Y. J. Vac. Sci. Technol. A 1986, 4, 369-374. (26) Bitzer, T.; Richardson, N. V. Appl. Phys. Lett. 1997, 71, 662664. (27) Zhang, F.; Srinivasan, M. P. Colloids Surf., A (Special issue for the 10th international conference on organized molecular films, Oct 5-10, 2003, Beijing), in press. (28) Song, N.; Wang, Z. Y. Macromolecules 2003, 36, 5885-5890.
Zhang et al. The soluble PI with hydroxyl pendant groups (HPI) was synthesized as follows. A 0.7172 g (3.3 mmol)portion of HAP and 20 mL of NMP were added to a three-necked 100 mL roundbottom flask fitted with a magnetic stirrer, an inlet for nitrogen gas, and a distillation setup. The mixture was stirred under nitrogen purge at room temperature. Upon complete dissolution of HAP, 1.4735 g (3.3 mmol) of 6FDA was added to the solution. After being stirred for 12 h at room temperature, the solution was heated at 200 °C for 6 h. Upon cooling to room temperature, the solution was added dropwise to vigorously stirred methanol. After the mixture was stirred for 2 h, the precipitated polymer was filtered, refluxed in methanol, and dried at 100 °C under vacuum for 12 h. Film Fabrication. The LBL assembly process is shown in Scheme 1. Step 1, Substrate Preparation. Silicon wafers and quartz slides were cleaned with ethanol and deionized (DI) water (with a resistivity of 18.2 MΩ cm), boiled in piranha solution (a 7:3 v/v mixture of concentrated sulfuric acid and 30% hydrogen peroxide) at ca. 100 °C for 0.5 h, rinsed with DI water and methanol, and blown dry with compressed nitrogen. They were subsequently soaked in 5 mM toluene solution of (p-aminophenyl)trimethoxysilane at room temperature for 2.5 h, sonicated in toluene for 5 min, rinsed thoroughly with toluene and methanol, and blown dry with nitrogen. The substrates obtained in this way are surface functionalized with amine groups and suitable for covalent binding of guest species. Step 2, TC Deposition on the Amine-Terminated Substrates. The above-prepared substrates were immersed in 5 mM toluene solution of terephthaloyl chloride with a few drops of triethylamine added. The immersion lasted for 2 h under nitrogen purge with the temperature ramped from 0 to 25 °C. Subsequently, the substrates were sonicated in toluene for 2 min, rinsed vigorously with toluene and acetone, and blown dry with nitrogen. Step 3, HPI Deposition on TC Layer. HPI was deposited from its 5 mM solution in freshly distilled THF, to which a few drops pyridine and triethylamine were added. The deposition proceeded at 45 °C under nitrogen purge for 2 h. After deposition, the Si and quartz slides were sonicated in THF for 2 min, rinsed vigorously with THF and acetone, and blown dry with nitrogen. Step 4, TC Deposition on HPI Layer. The assemblies as obtained from the previous step were immersed in a 5 mM THF solution of TC containing a few drops of pyridine and triethylamine for 2 h under nitrogen purge at 45 °C. They were then sonicated in THF for 2 min, rinsed with THF and acetone, and dried with compressed nitrogen. Steps 3 and 4 were repeated to give a fourbilayer film. Characterization. Fourier Transform Infrared Spectroscopy (FTIR). The spectrum for the synthesized HPI powder was obtained in air on Bio-Rad FTIR model 400 spectrophotometer by accumulating 16 scans at a resolution of 4 cm-1. That for the assembled HPI film on silicon wafer was recorded by accumulating 64 scans at a resolution of 4 cm -1. A transmission-mode configuration was employed with a bare silicon wafer as the background. The sample compartment was purged with nitrogen. Nuclear Magnetic Resonance (NMR). 1H NMR for HPI powder was obtained on Bruker DPX300 FT NMR spectrometer with 5 mm 1H-13C dual probe; deuterated acetone was used as solvent. UV-vis Absorption. UV-visible absorption spectra were recorded on a Shimadzu UV-3101 PC scanning spectrophotometer. X-ray Photoelectron Spectroscopy (XPS). The measurements were made on a Kratos Analytical AXIS HSi spectrometer with a monochromatized Al KR X-ray source (1486.6 eV photons) at a constant dwell time of 100 ms and a pass-energy of 40 eV. The X-ray source was run at a reduced power of 150 W. The pressure in the analysis chamber was maintained at 7.5 × 10-9 Torr or lower during each measurement. Atomic Force Microscopy (AFM). Film surface morphologies were investigated using a Nanoscope III atomic force microscope from Digital Instruments. All images were collected in air using the tapping mode and a monolithic silicon tip. The drive frequency was 330 ( 50 kHz, and the voltage was between 3.0 and 4.0 V. The drive amplitude was about 300 mV, and the scan rate was 0.5-1.0 Hz.
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Scheme 1. Schematic Diagram for the Film Fabrication Process
Scheme 2. Synthesis of HPI from 6FDA and HAP
Ellipsometry. Ellipsometric measurements were performed on a variable angle spectroscopic ellipsometer (M-2000U, Woollam) in air at room temperature. Spectra were acquired over the range of 400-1000 nm at 10-nm intervals and at incidence angles of 65° and 75°. Data analysis was performed using Windows version 3.352 WVASE32 software. Electrochemical Analysis. The electrochemical impedance measurements (EIS) were performed in 0.5 M H2SO4 with an Autolab potentiostat/galvanostat PGSTAT100 and FRA modules both interfaced to a personal computer. A conventional threeelectrode glass cell equipped with a platinum counter electrode and a Ag/AgCl reference electrode was used. The impedance measurements were made at open-circuit potential between 10 mHz and 100 kHz. The amplitude of input sine-wave voltage is 5 mV. Nanoindentation. The tests were performed using a TriboScope nanoindenter (Hysitron) with a Berkovich tip. A loading force of 30 µN was used and loading time was 10 s.
Results and Discussion HPI Characterization. 6FDA and HAP were used as the monomers for the synthesis of HPI following the route shown in Scheme 2. The synthesized HPI can be dissolved in solvents such as NMP, N,N-dimethylacetamide (DMAc), THF, acetone, and so on. The solubility is due to the presence of hexafluoroisopropylidene [-C(CF3)2-] kink in 6FDA molecule, which can decrease the rigidity of the
polymer backbone and the interchain interactions. The enhanced solubility may also originate from the hydroxyl groups giving rise to affinity between the polymer and polar solvents. Additionally, the hydroxyls can facilitate cross-linking of HPI with carboxylic acids or acid chlorides. The FTIR spectrum for the synthesized HPI powder is shown in Figure 1, where a broad band centered at 3415 cm-1 indicates the presence of hydroxyl groups and the characteristic bands attributable to imide functionality are seen at around 1780, 1380, and 720 cm-1. The 1H NMR spectrum of HPI dissolved in acetone (Figure 2) shows a strong single peak at δ ) 9.08 associated with phenolic hydroxyl protons.29 Hydrogen atoms in 6FDA and HAP moieties are observed at δ ) 7.9-8.2 29 and δ ) 7.2-7.5,30 respectively. The NMR spectra also indicate complete imidization of HPI, since no peaks are found at δ ) 5.74 (for -NH) and δ >10 (for -COOH) 31 coming from the unimidized amic acid moieties. Spectroscopy. The ultrathin HPI films were fabricated on substrates bearing surface amine groups obtained by modifying silicon or quartz surfaces with (p-aminophenyl)trimethoxysilane (APhS). APhS is advantageous over other commonly used peers such as (3-aminopropyl)trimethoxysilane since it can deposit uniformly on oxide surfaces and give a high percentage of primary amines according to our previous findings.32 This is beneficial for stable immobilization of species of interest. We further derivatized substrates thus obtained with TC on the basis of the amidation chemistry between amine and acid halide. (Though TC may bind directly to the hydroxylated SiO2 surfaces, deposition of APhS is necessary and important since it has a tridentate structure and can be expected to bind more strongly to the substrate than does TC, giving (29) Akar, A.; Tunca, A.; Talinli, N. Eur. Polym. J. 1995, 31, 9-14. (30) Krishnan, S. P. G.; Vora, R. H.; Veeramani, S.; Goh, S. H.; Chung, T. S. Polym. Degrad. Stab. 2002, 75, 273-285. (31) Sinz, A.; Rudolf, M.; Floris, V. E.; Thawatchai, S.; Suttiporn, C.; Vichai, R. Phytochemistry 1999, 50, 1069-1072. (32) Zhang, F.; Srinivasan M. P. Langmuir 2004, 20, 2309-2314.
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Figure 1. FTIR spectrum for the synthesized HPI powder.
Figure 2. 1H NMR spectrum for the synthesized HPI powder dissolved in deuterated acetone.
rise to better film adhesion to the substrate.) Subsequently, HPI and TC were assembled alternately with ester groups formed between HPI layers. The FTIR spectrum for a fourlayer assembled HPI film on a silicon wafer is shown in Figure 3a, whose hydroxyl band at ca. 3400 cm-1 was markedly reduced compared with that in the spectrum for HPI powder (Figure 1). This suggests that a considerable amount of hydroxyls have been consumed in covalent linking with TC molecules. The FTIR spectrum for HPI film also shows characteristic bands at 1781 and 1373 cm-1 for imide functionality, similar with those in the powder spectrum. However, the carbonyl band for HPI film is seen at 1730 cm-1 (curve A, Figure 3b) while that for the powder sample is at 1725 cm-1 (curve B, Figure 3b); further, a shoulder appears at ca. 1750 cm-1 in the film spectrum. These observations demonstrate that ester bonds were formed between HPI layers in the assembled film. The interfacial reactions involved in the film fabrication process were studied by X-ray photoelectron spectroscopy (XPS) wide scan (Figure 4). The amine-terminated substrates do not show meaningful features between 192 and 208 eV (spectrum A); after deposition of TC, the samples showed a clear peak at around 200 eV (spectrum B) attributed to chlorine, verifying successful immobilization of TC. When HPI was subsequently assembled, the chlorine signal is sharply reduced if not lost completely (spectrum C) due to the acid chloride reaction with the hydroxyl groups from HPI. Upon deposition of TC on the formed HPI layer, the chlorine signal appears again (spectrum D), although it is weak compared with that in spectrum B. This may result from the possibility that some TC molecules have reacted at both acid chloride sites with HPI hydroxyls. (See step 4 in Scheme 1.) However, this will not compromise the covalent tethering of HPI chains due to their random coil conformation, which may not
Figure 3. (a) FTIR spectrum for HPI assembled film. (b) A comparison between FTIR spectra of HPI assembled film (A) and HPI powder (B) in the range 1650-1850 cm-1.
Figure 4. XPS wide scans monitoring the interfacial reaction involved in HPI film fabrication process: (A) APhS substrate; (B) TC derivatized substrate; (C) HPI layer; (D) TC deposition. A magnified region of 192-206 eV in spectrum D is attached.
require a great abundance of binding sites on the underlying surface. Actually, even if all the acid chlorides are available, they may not necessarily and possibly be fully utilized in binding HPI chains due to steric hindrance. Also, the final films obtained exhibited good stabilities and strength against sonication, polar solvent attack, elevated temperatures, and nanoindentation as shown in the last section of this work. This, together with the abovementioned FTIR results, implies that HPI layers are indeed covalently tethered to TC moieties, instead of being simply adsorbed through van der Waals forces. UV-vis absorption spectra of the assembly obtained after each deposition of TC and HPI are shown in Figure 5. By examining the absorptions at each TC or APhS layer (represented by dash and solid curves, respectively), one can easily observe the LBL growth of the assembly. Interestingly, significant changes are seen between the
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Figure 5. UV-vis absorption spectroscopy monitoring the LBL assembly of TC/HPI on an APhS-terminated quartz slide. Digits between any pair of dashed/solid curves denote the number of TC/HPI bilayers. Inset: dependence of the absorbance at 203 and 225.8 nm on the number of HPI layers.
Figure 6. Comparison on UV absorptions of HPI thin film (solid curve) and HPI solution in THF (dashed curve).
absorptions for TC and HPI at all cycles of deposition labeled as 1, 2, 3, and 4, although the most important chromophore in both molecules is a phenyl ring. These changes are probably associated with the coupling between TC moieties and HPI chains. Increase in the absorbance at 203 and 226 nm (attributed to π-π* transition of phenyl rings) against the number of HPI layers (shown in inset of) reveals that the LBL film growth is uniform. A comparison between the UV absorptions of the HPI film and solution is made in Figure 6. The spectrum of HPI in THF shows bands at 273 and 296 nm and a shoulder at 248 nm. The spectrum of the HPI assembled film on a quartz plate displays bands at 203 and 226 nm, which are blue-shifted significantly relative to those of the solution spectrum. The blue shift indicates that the molecules in the assembled film exist in H-aggregate form as opposed to the randomly distributed molecules in solution.33 This type of shift in UV absorption relative to bulk phase is viewed as a typical phenomenon in molecular assembly, similar to that observed in polyimide LB films.34 Morphology. We also examined the change in surface morphology during the film fabrication process. Presented in Figure 7 are the atomic force microscopy (AFM) images of the film surface topography at various deposition steps. The TC layer on APhS substrate is fairly smooth and shows regular domains (image A). This might result from the uniformity of the APhS substrate, which was reported in our previous work.32 Subsequent HPI deposition results (33) McRae, E. G.; Kasha, M. Physical Processes in Radiation Biology; Academic Press: New York, 1964. (34) Lu, T.; Zhu, Y.; Xing, W.; Li, Y.; Ding, M.; Sun, G.; Guo, D. Thin Solid Films 1997, 303, 282-286.
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in some aggregates of polymer chains (image B). When a second layer of TC was deposited, wormlike features are observed on the HPI aggregates (image C). These features are markedly different from polymer aggregates and could result from smaller clusters of TC molecules. They are also different from what is observed in image A probably due to the less populated and more randomized reactive sites provided by HPI than those by APhS substrate. When subjected to next layer of HPI, the wormlike features disappeared completely (image D). A third layer of TC led to reappearance of the wormlike features (image E) quite similar to that obtained in second layer TC deposition. Image F shows a smooth topography obtained after the deposition of the fourth HPI layer as typically observed for polymeric thin films.35 Thus, a smearing of surface features appears to take place with the deposition of every polymer layer. Ellipsometry. Information on variation of film thickness and optical properties with the number of HPI layers was obtained from ellipsometry. Measurements were carried out on films constructed on silicon wafers by recording ellipsometric angles Ψ and ∆, which describe the polarization state of the light beam reflected from the sample surface and are related to the ratio of reflection coefficients for p- and s-polarized light. As observed in Figure 8, samples containing different numbers of HPI layers (labeled as 1, 2, 3, and 4) show evidently different Ψ spectra (solid curves) versus wavelength range measured, confirming that the samples tested do have different optical properties and thickness values. The dashed curves in Figure 8 were obtained from data fitting on the basis of a three-layer optical model as shown in Figure 9a, consisting of a 0.6 mm thick Si layer, a 2.6 nm thick SiO2 layer, and a Cauchy layer whose thickness needs to be fitted. The thickness of 2.6 nm for the SiO2 layer was measured in advance from ellipsometry using a two-layer model (Si/SiO2). It is seen from Figure 8 that the fits between generated and experimental data are good for each sample, with a mean-squared error of less than 5. The fitted multilayer film thickness is plotted against the number of deposited layers and shown in Figure 9b, which demonstrates an approximate linear growth of film thickness with the number of deposited HPI layers, in accordance with the UV-vis absorption spectroscopy results. Thickness of the four-layer HPI film is measured to be 13.2 nm. Taking the presence of interlayer cross-linking agent into consideration, each HPI layer contributes about 3 nm to the total thickness. The fitted refractive indices at 630 nm are found to decrease with the number of layers (from 1.90 at the first layer to 1.71 at the fourth, Figure 9b) and this may suggest that the HPI thin film becomes less compact with increasing number of deposited layers. Electrochemical Impedance Spectroscopy. To highlight the changes of impedance with the number of HPI layers, impedance spectroscopy was recorded in the complex plane (Figure 10), where Z′′ and Z′ represent the imaginary and real parts of the impedance, respectively. Impedance spectroscopy was performed on bare gold electrodes and gold electrodes modified with two-and fourlayer HPI films in the presence of 0.5 M H2SO4. Preparation of HPI films on a gold surface is the same as that on Si or quartz slides, except that the surface modifier for gold is p-aminophenylthiol. All impedance spectra present semicircles whose diameter corresponds to the resistance upon charge transport from the test solution to gold surface (35) Calvo, E. J.; Danilowicz, C.; Lagier, C. M.; Manrique, J.; Otero, M. Biosens. Bioelectron. 2004, 19, 1219-1228.
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Figure 7. Tapping-mode AFM images of the LBL assembly at various steps: (A) first TC layer (on APhS substrate); (B) first HPI layer; (C) second TC layer (on HPI); (D) second HPI layer; (E) third TC layer (on HPI); (F) fourth HPI layer.
Figure 8. Ellipsometric spectra of HPI films with different layers, recorded at an incidence angle of 65°. Numbers represent films with different HPI layers.
(Figure 10). For the gold electrode carrying two HPI layers, the diameter of the semicircle corresponds to a film resistance of around 10 kΩ, which is much higher than that of a bare gold electrode (about 5 kΩ). For the electrode modified with a four-layer HPI film, the diameter of the semicircle increases further to 16 kΩ, indicating a larger film resistance. These EIS results well verify the insulating property of the HPI assembled film and demonstrate its correlation with film thickness. Stability and Mechanical Strength. The four-layer assembled film was exposed to the ambient environment for 2 months and then soaked in THF for 12 h, sonicated
Figure 9. (a) The model established for data fitting. (b) Dependence of film thickness and refractive index at 630 nm on the number of assembled HPI layers.
in THF for 5 min, rinsed with methanol, and blown dry. In a similar fashion, the film was also treated with NMP and 1 M NaOH (aq), respectively. Figure 11 shows the UV absorption spectra before and after the above treatments; not many changes were observed in the intensities of different absorption bands after the various treatments, indicating that the film is durable and able to withstand prolonged polar solvent attack. Indeed, the EIS measure-
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Figure 10. Electrochemical impedance spectra for bare gold electrode and HPI films on bare gold.
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Figure 12. Load-displacement curves for the LBL assembled HPI film and a spin-coated HPI film of similar thickness.
18 nm using ellipsometry. At any load, the assembled film showed a smaller displacement than the spin-coated one. This clearly shows that the former is more resistant against indentation, probably due to its covalent interlayer linkages. All the above observations demonstrate that the HPI film prepared using direct covalent assembly is chemically, thermally, and mechanically robust. Conclusions
Figure 11. UV-vis absorption spectra of the four-layer HPI film on quartz after prolonged exposure to ambient conditions and 12 h of soaking in different polar environments.
ments discussed previously also verify the chemical stability of the films, since they remain intact in the presence of acidic solution. Thermal stability was tested by baking the film (soaked in NMP already) at 300 °C under nitrogen purge for 1 h. The UV-vis absorption spectrum taken after the thermal treatment was virtually indistinguishable from that before thermal treatment (Figure 11) and not plotted separately; the film thickness was measured to be 12.8 nm, roughly the same as before (13.2 nm). All the above observations ably attest to the advantage of covalent LBL assembly in providing good stability of assembled films, considering that little information is available on the stabilities of Langmuir-Blodgett or electrostatic LBL ultrathin films against polar solvent attack and high temperatures. Figure 12 shows the load-displacement curves for the four-layer assembled HPI film and a spin-coated HPI film both subjected to nanoindentation. The spin-coated film was obtained by casting 1 drop of 0.01% (w/v) THF solution of HPI on a cleaned Si wafer and then spinning at 200 rpm for 2 min; the film was subsequently vacuum-dried at 100 °C for 1 h. Its thickness was measured to be around
6FDA-based polyimide containing pendant hydroxyls was synthesized and used to construct ultrathin films on amine-terminated substrates through LBL covalent assembly with terephthaloyl chloride as a cross-linking agent, by establishing ester bonds between HPI layers. Formation of the ester links is confirmed by FTIR and XPS. The stepwise and linear growth of the assembly with number of layers was demonstrated through UV-vis absorption spectra and ellipsometry. From tapping-mode AFM, systematic changes in surface morphologies after various deposition steps were observed. Impedance spectroscopy conducted for films deposited on gold showed a high degree of resistance to interfacial ion transfer, indicating the stability and strength of the assembled film attributable to the covalent interlayer linkage and robust cross-linker. The resulted film exhibited good stabilities toward prolonged aging, polar solvent etching, and thermal treatment at elevated temperatures. Also, it is more resistant than its spin-coated peer against nanoindentation. The layer-by-layer assembly method using covalent links between molecular layers constitutes a very good promise for constructing robust film structures. Acknowledgment. The authors thank the National University of Singapore for providing financial support for this project and a research scholarship for Zhang Fengxiang. Assistance from Mr. Tan Yong Siang (ChBE) for the EIS measurements and from Miss Tan Phay Shing (BioE) for nanoindentation are gratefully acknowledged. LA048741R
