Langmuir 1996, 12, 5109-5113
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Neutron Reflectivity Investigations of Self-Assembled Conjugated Polyion Multilayers G. J. Kellogg, A. M. Mayes,* W. B. Stockton,† M. Ferreira, and M. F. Rubner Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307
S. K. Satija Reactor Radiation Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received March 25, 1996. In Final Form: August 6, 1996X Neutron reflectivity has been used to study the organization of self-assembled multilayers of sulfonated polyaniline and polyallylamine. Films were prepared by the sequential adsorption of polycations and polyanions from dilute aqueous solutions. Scattering contrast was achieved by selective deuteration of the blocks of bilayers at varying intervals along the film. The multilayer structure was found to be preserved over 40 bilayer depositions, but with an internal organization which decays monotonically away from the substrate. These results suggest that the observed interfacial widths are primarily due to the accumulation of defects as the bilayers are deposited.
Introduction Molecular-scale processing of polymers holds great promise for next-generation optoelectronic device fabrication. Waveguides, light emitting diodes, and anisotropic conductors have been prepared via Langmuir-Blodgett and related molecular assembly schemes.1 Major shortcomings of these methods are that they are typically laborintensive, relatively inefficient, and difficult to scale-up. An alternate self-assembly method was recently developed whereby oppositely charged polyions are sequentially adsorbed from dilute aqueous solutions.2 The method has the advantage of being easily automated and could be readily adapted to industrial-scale processing. It has been shown that such sequential bilayer adsorptions result in monotonic growth of the total thickness of the adsorbed film and that solution pH and ionic strength may be used to manipulate the individual layer thicknesses.2-6 However, the characterization methods generally usedsX-ray reflectivity and UV-vis adsorptions usually yield no information about the internal structure of the film: while the overall thickness as a function of number of layers is highly reproducible, these measurements do not reveal whether the resultant film contains well-defined molecular layers or to what degree these layers interpenetrate. (Decher and co-workers have recently reported that a different system, a repeating ABAC multilayer, exhibits an X-ray reflectivity Bragg peak indicative of a superlattice.) Neutron reflectivity with selective deuteration of blocks of layers has been † X
Current address: 3M Center, Austin, Texas 78726-9000. Abstract published in Advance ACS Abstracts, October 1, 1996.
(1) Rubner, M. F.; Scotheim, T. A. In Conjugated Polymers; Bredas, J. L., Silvey, J. L., Eds.; Kluwer Academic Publishers: Boston, MA, 1991; p 363 and references therein. Rubner, M. F. In Polymers for Electronic and Photonic Applications; Wong, C. P., Ed.; Academic Press: Boston, MA, 1993; p 601 and references therein. (2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. Lvov, Y.; Haas, H.; Decher, G.; Mohwald, H.; Kaladev, A. J. Phys. Chem. 1993, 97, 12835. Lvov, Y.; Decher, G.; Mohwald, H. Langmuir 1993, 9, 481. (3) Schmitt, J.; Grunewald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Losche, M. Macromolecules 1993, 26, 7058. (4) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772. (5) Ferreira, M.; Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 806. (6) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107. Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115.
S0743-7463(96)00285-5 CCC: $12.00
used to probe the internal structure of films.3 For the system of sulfonated polystyrene (SPS) and polyallylamine (PAH), a multilayer structure was reported with a root mean square interfacial width of σ ) 19 ( 1 Å and component layer thicknesses of dSPS ≈ 34 Å and dPAH ≈ 19 Å. These results are consistent with some chain-chain interpenetration. The uniformity in bilayer thickness and interfacial widths, beyond the first few deposited bilayers, suggests that no degradation of the multilayer organization occurs with successive depositions. This is consistent with the interpenetration of polymer chains being the primary source of the observed interfacial widths. While SPS and PAH provide a good model system, these nonconjugated polymers are not electronically active. Real devices will need to be made from highly-conjugated polymers, where the conjugation length and planarity of the backbone can be expected to affect the optoelectronic properties of the films.1 In this study we report the neutron reflectivity measurements of self-assembled films fabricated through the sequential adsorption of polyallylamine and sulfonated polyaniline (SPAn). SPAn is of interest because it is a highly-conjugated, amphoteric macromolecule, where the magnitude and sign of charge may be manipulated through the control of the dopant concentrations and pH of solutions. PAH is a common polycation which provides the opposite charge necessary for building films. Films of this type are electrically active, having an electrical conductivity of ∼10-3 S/cm. We also note that the issue of the interpenetration of chains in self-assembled polyelectrolyte multilayers, electronically active or not, is still largely unresolved. The degree of interpenetration is critically important for understanding the electronic properties of thin films built from conjugated polymers. Experimental Details Polished silicon substrates 75 mm in diameter and 2.5 mm thick7 were immersed in a bath of H2SO4/H2O4 for 30 min and subsequently rinsed with deionized water. The surfaces of the wafers were then hydroxylated by boiling in NaOH/H2O2 for 1 h. The surfaces were subsequently treated with a monofunctional (7) Semiconductor Processing Co., Boston, MA.
© 1996 American Chemical Society
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Figure 1. Neutron reflectivities and fits (-) for films with (a) eight deuterated bilayers at the substrate, (b) eight deuterated bilayers in the middle of the film, (c) eight deuterated bilayers at the interface, and (d) three groups of eight deuterated bilayers located at the substrate, interior of the film, and at the air interface. The insets show the profiles used to generate the modeled reflectivities. coupling agent, (4-aminobutyl)dimethylmethoxysilane, for 1 h, to promote the adhesion of a layer of sulfonated polystyrene; this SPS layer provided the charges necessary to adsorb the first layer of PAH.6 The substrates were stored in deionized water until multilayer deposition had begun. Perdeuterated aniline monomer was obtained from Cambridge Isotopes. The SPAn and deuterated SPAn (SPAn-d-7) with an estimated molecular weight Mw ≈ 30 000 were synthesized inhouse following a procedure previously reported.8 These were maintained in metastable 0.005 M aqueous solutions adjusted to pH ) 3.5 with methanesulfonic acid (CH3SO3H), a dopant which partially-protonates the backbone of SPAn and reduces its net negative charge. The other polyelectrolyte, polyallylamine hydrochloride, was purchased from Aldrich with Mw ≈ 50- 65000. This was used in a 0.005 M aqueous solution adjusted to pH ) 3.5 with HCl. Multilayer fabrication was performed by dipping the treated substrates in the PAH solution for 15 min, followed by rinsing with deionized water. The PAH-coated substrates were subsequently dipped in SPAn or SPAn-d-7 solution for 15 min and again rinsed with deionized water. This sequence was repeated until the desired number of layers had been deposited. To investigate the multilayer organization, eight bilayer blocks were selectively contrasted at varying depths within a series of films by substituting SPAn-d-7 for SPAn. Four 40-bilayer systems were investigated with the following labeling schemes, counting from the substrate up: 8D/32H (A), 16H/8D/16H (B), 32H/8D (C), and 8D/8H/8D/8H/8D (D), where D denotes bilayers containing SPAn-d-7 and H denotes unlabeled bilayers. Samples A through C allow us to determine the structure at the interfaces between labeled and unlabeled blocks, while sample D integrates the labeled regions of the other three samples into a single film serving as a test of the reproducibility of fabrication and the consistency of our analysis. The periodic spacing of the labeled blocks in sample D should also give rise to observable Bragg reflections if the film is sufficiently well ordered. Neutron reflectivity measurements were performed on the BT-7 instrument at the National Institute of Standards and Technology research reactor in Gaithersburg, MD. The experi(8) Yue, J.; Epstein, A. J. J. Am. Chem. Soc. 1990, 112, 2800.
mental setup and sample alignment procedures are described in previous publications.9,10 Neutrons of wavelength λ ) 2.37 Å and energy resolution ∆λ/λ ≈ 0.01 were obtained from reflection of the white beam from a graphite monochromator. Reflectivity profiles were obtained by rotating the sample θ and detector 2θ degrees with reference to the incident beam, with angular resolution ∆θ /θ ≈ 0.02 determined by the incident defining slits. Reflectivity measurements were made to angles of at least θ ) 0.8° or qz ) 0.07 Å-1 ()4π sin θ/λ) in neutron momentum transfer. Background intensities were obtained by offsetting 2θ by +0.25°. After background subtraction, the reflectivity was obtained by normalizing the net intensity by the main beam intensity for identical slit conditions. To fit the reflectivity data, scattering length density (b/v) profiles were generated by modeling the films as adjacent layers of an assumed thickness di and (b/v)i value, with error-function interfaces between blocks of differing scattering length. The interfaces are characterized by a root mean square roughness σi, such that σi describes the interface between layers i - 1 and i. The theoretical reflectivity for a model profile was calculated using the Parratt formalism11 and evaluated against the experimental data. Model parameters di, (b/v)i, and σi were modified iteratively until a best fit to the data was achieved. In practice, it was found that simple slabs corresponding to deuterated or nondeuterated blocks were sometimes insufficient to give good fits to the reflectivity. In these cases, “composite slabs” of two adjacent interdiffuse blocks were used, especially at the air/film interface, to model structures not well described by error-function interfaces. For these films, the quoted roughnesses are derived from error-function fits to the composite interfaces used in the model.
Results and Discussion Figure 1a shows the measured reflectivity as a function of qz from sample A, which contains an eight bilayer block of SPAn-d-7/PAH adjacent to the substrate. Oscillations in the reflectivity data arise from interferences between
Self-Assembled Conjugated Polyion Multilayers
neutrons that are reflected from the surface of the film, the internal interface created by incorporating the isotopically labeled block, the interfaces between the film and the substrate’s native SiO2 layer as well as the interface between the SiO2 layer and silicon. The oscillations decay rapidly as a function of increasing angle θ (qz). Qualitatively, this result implies significant surface roughness. The solid line in Figure 1a represents the best fit to the data, corresponding to the scattering length density profile shown in the inset. Two observations can be made on examining this profile. First, the deuterated block is clearly distinguishable as a region of high scattering length density next to the substrate, showing that the deuterated material has remained localized in the portion of the film where it was deposited. (Note that the thin SiO2 layer has been incorporated into the profile at the Si surface.) Second, the interface between this region and the unlabeled portion of the film is quite broad when compared to the typical width of the substrate interface, σi ) 21 ( 7 Å vs σs ≈ 3-5 Å.12 The reflectivity results for the differently-labeled films are summarized in parts b-d of Figure 1, which show the results for samples B-D. Films B and C, in which one labeled block is placed either in the middle of the film or at the surface, respectively, show markedly different reflectivities, illustrating that the major features are due to the location of the labeled bilayers. These reflectivities also become smooth rapidly with increasing qz, showing that the internal interfaces and film/air interface are broad. Film D, which is composed of periodically arranged labeled blocks, additionally exhibits a first-order Bragg peak at qz ≈ 0.026 Å-1, corresponding to a block-spacing of L ≈ 264 Å; the expected value is L ) 240 Å from our labeling scheme. From this we draw the important conclusion that the multilayer structure induced by alternately depositing labeled and unlabeled blocks has largely been preserved. The insets of parts b-d of Figure 1 display the scattering length density profiles used to generate the model reflectivities. A number of conclusions can be drawn from observation of these profiles. First, the films show surface roughnesses which are nearly the same, the average value being σs ) 48 ( 9 Å. This is evident in the raw data as well, since the range in qz over which oscillations decay is about the same for all samples. Second, the film thicknesses derived from the fits are self-consistent in the range of 575-628 Å. This translates to a bilayer thickness range of 14.4-15.7 Å, in good agreement with previously reported values for this system.6 Observed thickness differences between samples might be attributed to small variations in the solution pH or polyelectrolyte concentration during film fabrication. Third, the b/v levels and widths of the deuterated blocks and undeuterated blocks are generally consistent from sample to sample. (Some observed variation in the b/v profiles might arise from differences in salt content.) All samples appear to show a broadening and flattening of the deuterated bilayer blocks, combined with a broadening of the internal interfaces, as the distance from the substrate increases. These results can be seen best in Figure 2 in which the b/v profiles are superposed for all four samples: sample D is indeed a composite of samples A-C, to within the certainty afforded by our fits. Further consistency of the results can be seen by comparing the fitted scattering length densities to what (9) Anastasiadis, S. H.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F. J. Chem. Phys. 1990, 92, 5677. (10) Mayes, A. M.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F. Macromolecules 1992, 25, 6523. (11) Parratt, L. G. Phys. Rev. 1954, 95, 359. (12) Russell, T. P. Mater. Sci. Rep. 1990, 5, 171.
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Figure 2. Superposition of the three model profiles in parts a-c of Figure 1 (- - -). The superposition captures the structure of the film with three groups of deuterated bilayers (s).
is expected based on the known composition of the film components. The fitted block scattering lengths, (b/v)i, cluster around two values, as expected from our labeling scheme. We may assume that the structure (number density and thickness) of the bilayers is the same whether deuterated or nondeuterated, the only difference being the scattering lengths of SPAn and SPan-d-7. If this is the case, then the difference in the scattering length densities of the large deuterated and nondeuterated blocks should be
(b/v)D - (b/v)H ) nSPAn [dSPAn/(dSPAn + dPAH)][bSPAn-D - bSPAn-H] (1) where nSPAn is the number density of SPAn or SPAn-d-7, dSPAn is the thickness of the SPAn or SPAn-d-7 component of each bilayer, dPAH is the thickness of the PAH component, and bSPAn-D and bSPAn-H are the scattering lengths of SPAn-d-7 and SPAn, respectively. The ratio of the thicknesses determines the fractional composition of the bilayers. Assuming equal thicknesses for the components and a number density consistent with a SPAn mass density of 1 gm/cm3, we expect
(b/v)D - (b/v)H ≈ 8.5 × 10-7 cm-2
(2)
The experimental value can be obtained best near the substrate, where the model profiles are least smeared. It can be readily seen from parts a and d of Figure 1 that this value is nearly the same as that calculated above. Though the difference in the scattering length density between the deuterated and nondeuterated blocks is as we expected, the overall scattering length density is uniformly higher than what is expected by ∆(b/v) ≈ 1.0 × 10-6 cm-2. This may be attributed to the presence of HCl counterions (from the PAH solutions) and CH3SO3dopant counterions. It is not possible to calculate the concentrations of multiple unknowns on the basis of a single offset value. We can attempt to quantify the depth-dependent structure by plotting the interfacial widths derived from the fits as a function of their position within the films. The error bars for these parameters are large, on the order of 10-50% of the value, but all of the values at a given depth in the various films lie well within these error bars of one another. With this in mind, we display in Figure 3 the average interfacial width (roughness) obtained by averaging the values at a given depth and propagating the fitted errors. Though the errors are significant, the trend is nonetheless clear, showing a roughly linear increase in roughness with increasing distance from the substrate σi ) (16 + 0.7n) Å, where n is the number of
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Figure 3. Average widths of interfaces between deuterated blocks and nondeuterated blocks of bilayers or film/air interface (value at 40 bilayers). The solid line is a linear fit showing that the width of the internal interfaces increases with number of adsorbed bilayers and that this width accurately reflects the roughness at the film/air interface.
the bilayer interface counted up from the substrate. Thus, we see an increase in the apparent roughness of the internal interfaces from 22 to 38 Å, moving from 8 bilayers to 32 bilayers. The surface roughnesses observed are consistent with the atomic force microscopy (AFM) measurements performed on sample D. Figure 4 shows an AFM measurement which indicates a top surface roughness of σs ≈ 60 Å, in reasonable agreement with the reflectivity profiles. The lateral length scale of the surface corrugations appears to be 100-1000 Å. The surface roughness and the widths of the internal interfaces can be compared to those observed for the selfassembled multilayer films of sulfonated polystyrene/
Kellogg et al.
polyallylamine (SPS/PAH) on silicon using X-ray and neutron reflectivity.3 In this system, the widths of the internal interfaces were found to be about 19 Å with little change as a function of the distance from the substrate. The free surfaces of the films were reported to have a roughness σs ≈ 13-15 Å for a 48 bilayer film, consistent with the observation of up to 25 thickness oscillations (Kiessig fringes) in the X-ray reflectivity. Our SPAn/PAH films show much broader internal interfaces and a greater surface roughness; preliminary X-ray reflectivity measurements show only one to two Kiessig fringes. At least two interpretations can be made from the larger observed internal interfacial and surface roughnesses seen in SPAn/PAH. If interpenetration of chains is responsible for the widths of these interfaces, this interpenetration would have to occur over more than a single bilayer: bilayer thicknesses are ≈15 Å while all interfacial widths are >20 Å. It is difficult to imagine how interpenetration or diffusion would result in broader interfaces farther from the substrate. An alternative interpretation would be that the interfaces are actually very rough, or corrugated, rather than being diffuse. Since the microscopic roughness at the silicon surface is expected to be no greater than 5 Å,12 this internal roughness would have to be introduced during the deposition of the bilayers. The surface roughness of the films (σs ) 48 ( 9 Å) supports this latter interpretation. This interpretation still leaves open the issue of interpenetration. If the linear fit in Figure 3 were to be literally believed, it would suggest an intrinsic width on the order of 16 Å, which is approximately the thickness of a bilayer. This would lead to the conclusion that interpenetration over the bilayer length scale is probable. Given the magnitude of the error bars in the figure though, this interpretation is highly speculative. Whether the basic structural unit of the film is individual layers, an
Figure 4. AFM image of Film D showing a corrugated surface with roughness of σs ≈ 60 Å.
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intimately mixed bilayer, or bilayers with some interdiffusion cannot be determined conclusively from these results. If the interfacial widths are dominated by roughness, we must ask why this system is so much rougher than the SPS/PAH system described above. We suspect that two factors could be responsible. First, highly-conjugated SPAn is relatively stiff; hence, this system lacks the relaxation mechanisms inherent to flexible polymer chains which could smooth any irregularities from uneven layer deposition. Second, SPAn solutions are metastable and prone to aggregation. Aggregate particles could provide larger irregularities which then accumulate with the increasing number of depositions. Such aggregate particles could explain the highly-corrugated surface seen with AFM. In conclusion, we have shown through neutron reflectivity that the system SPAn/PAH self-assembles in such a way as to maintain significant multilayer organization for films as thick as 40 bilayers. The internal interfaces of these films are quite broad with widths increasing approximately linearly with distance from the substrate. We interpret the observed interfacial widths and surface roughness as the result of defects introduced during
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deposition; not only would the roughness due to defects be propagated through the film as new layers are deposited, but new defects with each deposition would increase the roughness with increasing thickness. Similar conformal roughness phenomena have been observed in multilayered structures prepared by vapor deposition methods.13 Off-specular scattering measurements, not performed in this study, offer a means to further investigate the degree of roughness conformality exhibited by these systems.14 With improved control over the film deposition process to reduce defect incorporation, this study suggests that reproducible manufacture of thinfilm heterostructures containing electronically-active polyelectrolytes is feasible. Acknowledgment. This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-9400334. LA960285M (13) Savage, D. E.; Kleiner, J.; Schimke, N.; Phang, Y.-H.; Jankowski, T.; Jacobs, J.; Kariotis, R.; Lagally, M. G. J. Appl. Phys. 1991, 69, 1411. Kortright, J. B. J. Appl. Phys. 1991, 70, 3620. (14) Sinha, S. K.; Sirota, E. B.; Garoff, S.; Stanley, H. B. Phys. Rev. B 1988, 38, 2297. Pynn, R. Phys. Rev. B 1992, 45, 602.
