pubs.acs.org/Langmuir © 2010 American Chemical Society
Optical Properties of Poly(ferrocenylsilane) Multilayer Thin Films E. Stefan Kooij,*,† Yujie Ma,‡,§ Mark A. Hempenius,‡ G. Julius Vancso,‡ and Bene Poelsema† Solid State Physics, MESAþ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands, and ‡Materials Science and Technology of Polymers, MESAþ Institute for Nanotechnology, University of Twente, P.O.Box 217, 7500AE Enschede, The Netherlands. §Present address: Nanobiophysics, MESAþ and MIRA Institutes, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands †
Received February 21, 2010. Revised Manuscript Received June 30, 2010 Spectroscopic ellipsometry has been used to investigate the optical properties of poly(ferrocenylsilane) polyion multilayer thin films in the visible and near-infrared range of the spectrum. The thin films were deposited using the layerby-layer assembly process. Films with thicknesses of up to 55 nm were fabricated stepwise from polyelectrolyte solutions with a controlled ionic strength. These films allow an accurate characterization of the optical properties of poly(ferrocenylsilane) polyion layers. We show that the complex refractive index can be described by a simple Cauchy model. Refractive index values vary over the spectral range from 1.53 (near-infrared) to 1.8 (ultraviolet).
I. Introduction Poly(ferrocenylsilanes) (PFS), belonging to the class of organometallic polymers,1 consist of ferrocene and silane units in the main chain.2 The presence of skeletal iron and silicon atoms yields characteristics which open up a wide range of application areas.3,4 For example, the metal induces a high resistance to reactive ion etching.5 Combined with a variety of patterning methods, such as microcontact printing, block copolymer self-assembly, or thermal imprinting, PFS films have proven to be very suitable as lithography masks.5-8 The ability to modify the polymers with functional side groups, complemented with the properties of the ferrocene entities, further broadens their scope. Poly(ferrocenylsilane) polyanions and polycations featuring negatively and positively charged side groups, respectively, comprise a special class of polyelectrolytes.9,10 Such polyions can be employed in the layer-by-layer deposition process11,12 to fabricate all-organometallic multilayer thin films13,14 with defined thickness and composition. *To whom correspondence should be addressed. E-mail: e.s.kooij@ tnw.utwente.nl. (1) Nguyen, P.; Gomez-Elipe, P.; Manners, I. Chem. Rev. 1999, 99, 1515. (2) For reviews on poly(ferrocenylsilanes) see: (a) Whittell, G. R.; Manners, I. Adv. Mater. 2007, 19, 3439. (b) Manners, I. Macromol. Symp. 2003, 196, 57. (c) Kulbaba, K.; Manners, I. Macromol. Rapid Commun. 2001, 22, 711. (d) Manners, I. Chem. Commun. 1999, 857. (3) Eloi, J.-C.; Chabanne, L.; Whittell, G. R.; Manners, I. Mater. Today 2008, 11, 28. (4) Schanze, K. S.; Shelton, A. H. Langmuir 2009, 25, 13698. (5) Lammertink, R. G. H.; Hempenius, M. A.; Chan, V. Z.-H.; Thomas, E. L.; Vancso, G. J. Chem. Mater. 2001, 13, 429. (6) Lu, J.; Chamberlin, D.; Rider, D. A.; Liu, M.; Manners, I.; Russell, T. P. Nanotechnology 2006, 17, 5792. (7) Rider, D. A.; Manners, I. Polym. Rev. 2007, 47, 165. (8) Acikgoz, C.; Ling, X. Y.; Phang, I. Y.; Hempenius, M. A.; Reinhoudt, D. N.; Huskens, J.; Vancso, G. J. Adv. Mater. 2009, 21, 2064. (9) (a) Power-Billard, K. N.; Manners, I. Macromolecules 2000, 33, 26. (b) Hempenius, M. A.; Robbins, N. S.; Lammertink, R. G. H.; Vancso, G. J. Macromol. Rapid Commun. 2001, 22, 30. (c) Hempenius, M. A.; Vancso, G. J. Macromolecules 2002, 35, 2445. (d) Wang, Z.; Lough, A.; Manners, I. Macromolecules 2002, 35, 7669. (10) Hempenius, M. A.; Brito, F. F.; Vancso, G. J. Macromolecules 2003, 36, 6683. (11) Decher, G. Science 1997, 277, 1232. (12) Decher, G.; Schlenoff, J. Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Wiley-VCH: Weinheim, 2003. (13) Hempenius, M. A.; Peter, M.; Robins, N. S.; Kooij, E. S.; Vancso, G. J. Langmuir 2002, 18, 7629. (14) Halfyard, J.; Galloro, J.; Ginzburg, M.; Wang, Z.; Coombs, N.; Manners, I.; Ozin, G. A. Chem. Commun. 2002, 1746.
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Another feature of the poly(ferrocenylsilanes), stimulus responsiveness, relates to their redox activity.15 Poly(ferrocenylsilanes) can be reversibly oxidized and reduced by chemical as well as electrochemical means. The degree of charging, controlled for instance electrochemically by the applied voltage, directly influences intra- and intermolecular interactions, including interactions between PFS and solvent. This formed the basis for the electrochemically induce expansion of a cross-linked PFS matrix, used in a switchable photonic crystal.16 Further examples of redox-induced switchable characteristics include electrochromic properties.17-19 Recently, PFS microcapsules were shown to exhibit redox-responsive permeability, simply by adding suitable oxidizing and reducing agents.20,21 For a number of the aforementioned applications, but also to allow characterization and monitoring of PFS structures, a detailed description of the optical properties is highly relevant. For example, accurate determination of film thickness both in situ and ex situ using optical methods, such as ellipsometry,13,19,21-23 is enabled by quantitative knowledge of the complex refractive index and its spectral dependence.24 Although several values of the PFS refractive index at a single wavelength have been used in (15) (a) Foucher, D. A.; Honeyman, C. H.; Nelson, J. M.; Tang, B. Z.; Manners, I. Angew. Chem., Int. Ed. 1993, 32, 1709. (b) Nguyen, M. T.; Diaz, A. F.; Dement'ev, V. V.; Pannell, K. H. Chem. Mater. 1993, 5, 1389. (c) Rulkens, R.; Lough, A. J.; Manners, I.; Lovelace, S. R.; Grant, C.; Geiger, W. E. J. Am. Chem. Soc. 1996, 118, 12683. (d) Wang, X.-J.; Wang, L.; Wang, J.-J.; Chen, T. Electrochim. Acta 2007, 52, 3941. (16) Arsenault, A. C.; Puzzo, D. P.; Manners, I.; Ozin, G. A. Nature Photon. 2007, 1, 468. (17) Nguyen, M. T.; Diaz, A. F.; Dement’ev, V. V.; Pannell, K. H. Chem. Mater. 1994, 6, 952. (18) Kulbaba, K.; MacLachlan, M. J.; Evans, C. E. B.; Manners, I. Macromol. Chem. Phys. 2001, 202, 1768. (19) Peter, M.; Hempenius, M. A.; Kooij, E. S.; Jenkins, T. A.; Roser, S. J.; Knoll, W.; Vancso, G. J. Langmuir 2004, 20, 891. (20) Ma, Y.; Dong, W.-F.; Hempenius, M. A.; M€ohwald, H.; Vancso, G. J. Nature Mater. 2006, 5, 724. (21) Ma, Y.; Dong, W.-F.; Kooij, E. S.; Hempenius, M. A.; M€ohwald, H.; Vancso, G. J. Soft Matter 2007, 3, 889. (22) Ginzburg, M.; Galloro, J.; J€akle, F.; Power-Billard, K. N.; Yang, S.; Sokolov, I.; Lam, C. N. C.; Neumann, A. W.; Manners, I.; Ozin, G. A. Langmuir 2000, 16, 9609. (23) Brouwer, E. A. M.; Kooij, E. S.; Wormeester, H.; Hempenius, M. A.; Poelsema, B. J. Phys. Chem. B 2004, 108, 7748. (24) Tompkins, H. G.; McGahan, W. A. Spectroscopic Ellipsometry and Reflectometry: A User’s Guide; John Wiley & Sons, Inc.: New York, 1999.
Published on Web 08/04/2010
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Figure 1. Chemical structures of water-soluble poly(ferrocenylsilane) polycation (left) and polyanion (right) used in this work.
previous reports,22,25-27 such quantitative data over a broad spectral range are not available, as far as we are aware. Therefore, we set out to investigate the optical properties of layer-by-layer assembled PFS polyelectrolyte films in terms of the complex refractive index.
II. Film Deposition and Ellipsometry Measurements Thin films of water-soluble poly(ferrocenylsilane) polyanions (PFS-) and polycations (PFSþ) were manufactured using layerby-layer assembly from aqueous solutions onto well-defined silicon substrates. Details of the deposition procedure are fully described in previous work.21 The chemical structures of the polyions used in this work are given in Figure 1. The molar mass of the polycation amounted to Mw = 2.91 105 g/mol and Mn = 1.79 105 g/mol, while for the polyanion these values are Mw = 2.65 104 g/mol and Mn = 1.34 104 g/mol.28 Prior to the deposition of the PFS--PFSþ bilayer film, the silicon substrates, coated with a natural oxide layer, were cleaned and subsequently dipped into a solution of polyethylenimine (PEI) to induce a positive surface charge on the substrates. Next, these substrates were alternatingly immersed in aqueous polyanion and polycation solutions (2 mg/mL, varying NaCl concentration), with sufficient rinsing in clean Milli-Q water between and after the various immersion steps. After each deposition cycle, the bilayer stacks were dried in a nitrogen flow; owing to the hydrophobic nature of the PFS main chain, we assume the amount of residual water in the films to be negligible. The optical properties of the layer-by-layer assembled PFS bilayer stacks were studied using a Woollam VASE (variable angle spectroscopic ellipsometer). Measurements were performed typically from 0.8 to 5.1 eV; the photon energy was increased in steps of 0.1 eV. Spectra were collected at three angles of incidence: 65°, 70°, and 75°. Acquisition of the data as well as analysis of the spectra was performed using the WVASE software (J.A. Woollam Co., Inc.). In all cases the Levenberg-Marquardt multivariate regression algorithm was employed for the fitting procedures.29 (25) Espada, L. I.; Shadaram, M.; Robillard, J.; Pannell, K. H. J. Inorg. Organomet. Polym. 2000, 10, 169. (26) Galloro, J.; Ginzburg, M.; Mı´ guez, H.; Yang, S. M.; Coombs, N.; SafaSefat, A.; Greedan, J. E.; Manners, I.; Ozin, G. A. Adv. Funct. Mater. 2002, 12, 382. (27) Arsenault, A. C.; Mı´ guez, H.; Kitaev, V.; Ozin, G. A.; Manners, I. Adv. Mater. 2003, 15, 503. (28) Molar masses of the water-soluble poly(ferrocenylsilane) polyanion and polycation employed in this study were calculated from the molar mass characteristics (Mn, Mw, and polydispersity values) of their direct precursors, which were measured by means of gel permeation chromatography (GPC) in tetrahydrofuran (THF), relative to narrow polystyrene standards. The molar masses of the repeat units of the PFS polyanion and polycation differ slightly from their precursor polymer repeat unit masses. The molar mass values of the polyions were corrected for this. In order to prove that molar mass decline by chain scission did not occur upon converting the precursor chains to the polyions, GPC traces of the polyions were recorded in water and in dimethyl sulfoxide, showing the expected monomodal molar mass distributions.10 (29) (a) Levenberg, K. Q. Appl. Math. 1944, 2, 164. (b) Marquardt, D. W. SIAM J. Appl. Math. 1963, 11, 431.
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Figure 2. Ellipsometry spectra Ψ and Δ as a function of energy for 5 (left) and 10 (right) poly(ferrocenylsilane) polyion bilayer films on PEI-pretreated oxide-coated silicon substrates. The NaCl concentration in the PFS solutions from which the films were deposited was 0.5 M. Experimental spectra were obtained at 65° (squares; dashed line), 70° (circles; dotted line), and 75° (triangles; dashdotted line) incident angle. The solid lines are model calculations as described in the text. The broken lines represent measured spectra of the substrate prior to PFS bilayer deposition.
Typical ellipsometry spectra for films consisting of five- and ten-bilayer stacks are shown in Figure 2; these films were deposited using a NaCl concentration of 0.5 M. As a reference, spectra measured on the predeposited substrates prior to PFS assembly are shown by the broken lines. Analysis of these spectra in terms of a four-layer model (silicon/oxide/PEI/air) indicates that the predeposited PEI layer has a thickness of ∼5 nm. In the further analysis of the PFS bilayer stacks using a five-layer model (silicon/ oxide/PEI/PFS/air), the thickness of the natural oxide and the predeposited layer were taken into account in the model as being constant values obtained from the aforementioned four-layer analysis. The spectra obtained after depositing PFS--PFSþ films are represented by the open symbols in Figure 2. They show a distinct difference with the results prior to the assembly of the bilayer stacks. Generally, Ψ values show a marked increase, while Δ decreases for the five-bilayer sample. For thicker films larger differences are observed as compared to the spectra of the substrates. In the Supporting Information spectra are provided for all PFS thicknesses, which reveal the evolution of spectra with increasing number of PFS bilayers. For the thicker films consisting of more bilayers (right part of Figure 2), a peak appears in the Ψ spectra near 3.8 eV. Also, large variations are observed in Δ in this energy range.30 The latter observations indicate the onset of oscillations in the spectra, which originate from interference of light reflected at the film-substrate and at the film-air surface. It is well-known that for films with increasing optical thickness the oscillations in the spectra become more pronounced, with an increase in number of peaks, which shift to lower energies.24
III. Analysis of the Spectra The primary goal of our present work was to accurately determine the optical properties, i.e., the complex refractive index of PFS thin films as a function of photon energy (or wavelength of (30) Possible values for Δ comprise a full circle and as such range from 0° to 360°; following the standard in ellipsometry, they are plotted on a scale of -70° to 270° in Figure 2.
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the light31). To unequivocally extract the complex refractive index n~ =n þ ik from the experimentally obtained spectra, we followed two different approaches. In both cases, we employed a fitting procedure (using the Levenberg-Marquardt algorithm29), which includes spectra obtained for various film thicknesses. More specifically, we used ten sets of spectra. For each individual sample, the thickness was taken as a fitting parameter, while the same refractive index was taken to describe the optical properties of the PFS film for all samples with different thicknesses. Furthermore, in our model we took into account the natural oxide layer on the silicon substrate (typical thickness 1.5 nm) and the predeposited PEI film (typically 5 nm thick). As such, we solely fitted the parameters which describe the PFS bilayer film and rule out any influence of the substrate and its pretreatment on the optical results. A. Point-by-Point Fitting. In our first approach to extract the optical characteristics from the experimental results, the spectra were fitted in a point-by-point manner. At every energy (or wavelength), the Ψ and Δ values at three different angles of incidence were used as input for the fitting procedure, with the real and imaginary parts of the complex refractive index n~=n þ ik and the thickness of the film as fitting parameters. The latter parameter was obviously taken to be the same for the entire spectrum, while n and k are allowed to vary with energy. If m represents the number of energy values (wavelengths) at which ellipsometric values were measured, there are 6m (3m Ψ values and 3m Δ values) experimental values available, while the number of fitting parameters is limited to 2m þ 1 (m values for both n and k plus the film thickness). The spectra resulting from this point-by-point fitting procedure are shown in Figure 2 by the solid lines. For all angles of incidence and at every energy there is good agreement between the experimental and modeled data. The corresponding refractive index is represented by the symbols in Figure 3. Both the real part and imaginary parts (n and k) show a slight variation with photon energy. For n an increase is observed from 1.56 at the lowest energy of 0.8 eV to ∼1.8 at 5.1 eV. The imaginary part shows a small decline from 0.02 to 0 with increasing energy, followed by a rapid increase to 0.08 at energies exceeding 4.5 eV. Between 2.0 and 3.5 eV the absorption coefficient, i.e., the imaginary part of the refractive index vanishes, indicating that the PFS films are completely transparent in this energy range. The distinct increase of k at energies above 4.0 eV is in good agreement with the generally observed absorption behavior at short wavelengths. Below ∼300 nm the UV-vis absorption typically increases strongly;13,15,17,18,21 see also the Supporting Information where UV-vis spectra of our PFS thin films on glass are given. It has been suggested that the optical absorption in this spectral range arises from electronic transitions localized on the ferrocene groups in the organometallic polymers.32 This threshold wavelength corresponds nicely to the onset of absorption above 4.0 eV, as seen in Figure 3. The thickness values obtained from the fitting procedure described above are plotted in Figure 4 (by the circles) as a function of the number of assembled PFS--PFSþ bilayers. A continuous increase of the PFS-stack thickness is apparent. Two linear growth regimes can be discerned: (i) for the first seven bilayers, the thickness increase per PFS--PFSþ bilayer amounts to ∼5.5 nm, while (ii) further deposition gives rise to thickness increments of 3.5 nm per bilayer. Similar behavior has previously been observed by other means, primarily UV-vis absorption experiments.13 (31) The photon energy E (in eV) is related to the wavelength λ (in nm) via E [eV]= 1240/λ [nm]. (32) Manners, I. J. Inorg. Organomet. Polym. 1993, 3, 185.
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Figure 3. Real and imaginary parts of the complex refractive index n~ = n þ ik as a function of energy/wavelength. The circles represent results obtained from a point-by-point fit, while the solid lines represent the Cauchy function, given by eqs 1 and 2.
However, in many cases an increase of the thickness per bilayer is observed, generally after fewer number of deposited bilayers.22,33 The transition in growth regime may be ascribed to the predeposited PEI layer, which causes the first PFS bilayers to be slightly thicker; for larger numbers of bilayers, the influence of the PEI layer declines. Moreover, in our present experiments the ionic strength was considerably higher as compared to many previous studies. As such, the higher salt concentration may lead to marked differences in the conformation of the PFS molecules, which in turn can affect the growth behavior of the bilayer stacks. B. Cauchy Model. The variation of n and k with energy, as shown in Figure 3, inspired us to follow a second approach to obtain a description of the optical properties of our PFS films. The optical response of many dielectrics, insulators, liquids, and organic species can be modeled using the so-called Cauchy relation.24 In the simplest first-order approximation, the energy (or wavelength) dependence of the real part of the refractive index is given by nðλÞ ¼ A þ
B λ2
nðEÞ ¼ A þ B0 E 2
ð1aÞ ð1bÞ
where λ and E represent the wavelength in micrometers and the energy in electronvolts, respectively. In principle, higher order (33) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319.
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Kooij et al. Table 1. Summary of the Values for the Fit Parameters in the Cauchy Model Given by eqs 1 and 2a n parameter
k value
parameter
value
A 1.518 C 1.07 10-5 B 0.0173 μm2 D 2.27 μm 0.0113 eV-2 D0 1.83 eV-1 B0 a Here, the photon energy E (in eV) is related to the wavelength λ (in μm) via E [eV] = 1.24/λ [μm].
Figure 4. Film thickness as a function of the number of PFS bilayers deposited from 0.5 M NaCl aqueous solutions. The circles and plus symbols refer to the results using the point-by-point fitting routine and the Cauchy model, respectively.
contributions can be included; for the present work these turned out to be negligible. Similarly, the imaginary part was described by an exponential function D ð2aÞ kðλÞ ¼ C exp λ kðEÞ ¼ C expðD0 EÞ
ð2bÞ
The major advantage of such a parametrization of the complex refractive index as a function of energy, as opposed to the pointby-point fitting discussed above, is the limited number of fitting parameters; additionally, it yields a smoothly varying function. Using the Cauchy model, only four parameters quantify the optical response, while for the point-by-point routine n and k at 44 energy values (the symbols in Figure 3) act as fitting parameters, amounting to a total of 88. The real and imaginary parts of the refractive index obtained using the Cauchy model are given by the solid lines in Figure 3. Although there are some differences, overall there is good agreement with the point-by-point results. The real part n increases from 1.53 at low energies to 1.8 at higher energies, while k is effectively zero up to 3.0 eV. For higher energies k increases exponentially to a value of 0.12 at 5.1 eV. The values for the fit parameters in eqs 1 and 2 corresponding to the solid lines in Figure 3 are summarized in Table 1. The spectra calculated using the Cauchy model are nearly identical to those resulting from the point-by-point fit (shown by the solid lines in Figure 2). In fact, considering the finite line thickness, they cannot be distinguished from each other. We now also compare the thickness values obtained using both methods; this is shown in Figure 4. The circles were discussed above, while the plus symbols represent the results obtained using the Cauchy model. As with the spectra, there are hardly any discernible differences in the thickness values, despite the considerably smaller number of fit parameters in the Cauchy model. C. Refractive Index. When we assume that the Cauchy model, together with the parameters in Table 1, is valid for all PFS stacks, ellipsometry spectra can now be analyzed with only the layer thickness as a free parameter. In previous work, in which we used spectroscopic ellipsometry to assess the effective thickness of different PFS films,13,19,23 we employed a single value n=1.687 for the refractive index.34 Although it is not known for which wavelength this value has been determined, it is in line with the results in Figure 3. Additionally, values for the refractive index reported in literature are all in the range of n = 1.6-1.7, depending slightly on (34) This value was determined by V. Z.-H. Chan and R. G. H. Lammertink at the Massachusetts Institute of Technology by profilometry combined with single wavelength ellipsometry.
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the nature of the side groups;22,25-27 these all agree well with our present results. Paquet and co-workers35,36 investigated the optical properties of a large variety of polymetallocenes, mostly polyferrocenes. The studied polymers consisted of an alternating metallocene unit and a main group element (Si, Ge, Sn, P). All considered polymers exhibit high refractive index values, in the same range as our present findings. Moreover, the reported refractive index values for polymetallocenes in general are markedly higher than those of more conventional polyelectrolytes for which typical values around 1.5 are reported.37-39 Interestingly, the higher refractive index of materials consisting of metallocenes occurs without strong absorptions in the visible region. D. Ionic Strength Dependence. Finally, using the Cauchy model and only considering the thickness as a variable, i.e., we now assume the complex refractive index to remain the same (as in Figure 3), we analyzed ellipsometry spectra obtained on PFS bilayer samples deposited from solutions with varying ionic strengths. The results for 5 and 10 PFS--PFSþ bilayer films are shown in Figure 5. Similar to many previous reports in literature on various polyelectrolyte thin films,11,40-45 the thickness of our bilayer stacks increases for higher ionic strengths. Qualitatively, this can be understood by considering that the Debye length decreases for larger salt concentrations. The Debye length governs the spatial extent of the repulsive interactions between like-charged groups along the polymer chains. A low ionic strength corresponds to a larger Debye length and as such gives rise to a more spatially extended conformation of the polymer. Consequently, the thickness increase of the film per deposited bilayer is limited. On the other hand, for higher ionic strength, the repulsive interactions are reduced giving rise to more compact polymers, and as a result more polyelectrolyte molecules are deposited per unit surface area, effectively increasing the thickness of each deposited layer. More quantitatively, the Debye length should depend on the square root of the ionic strength. Therefore, the most obvious choice would be to model the thickness d as a function of the ionic strength I as d µ IR with the exponent R equal to 1/2. However, plotting our data on a double-logarithmic scale (see Figure 5) reveals an exponent R=1/4; the ratio of the prefactors (solid lines in Figure 5 represent fit results) amounts to 1.9, in agreement with the thickness ratio of films consisting of 10 and 5 bilayers. (35) Paquet, C.; Cyr, P. W.; Kumacheva, E.; Manners, I. Chem. Commun. 2004, 234. (36) Paquet, C.; Cyr, P. W.; Kumacheva, E.; Manners, I. Chem. Mater. 2004, 16, 5205. (37) Nabok, A. V.; Hassan, A. K.; Ray, A. Mater. Sci. Eng., C 1999, 8-9, 505. (38) Nolte, A. J.; Rubner, M. F.; Cohen, R. E. Langmuir 2004, 20, 3304. (39) Fujita, S.; Shiratori, S. Thin Solid Films 2006, 499, 54. (40) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160. (41) L€osche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (42) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871. (43) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. (44) Jiang, X.; Chen, Z.; Lu, D.; Wu, Q.; Lin, X. Macromol. Chem. Phys. 2008, 209, 175. (45) Steitz, R.; Leiner, V.; Siebrecht, R.; Klitzing, R. v. Colloids Surf., A 2000, 163, 163.
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The latter may well be related to the fact the deposited bilayer superstructures are not in thermodynamic equilibrium. Owing to slight differences in experimental procedures combined with the kinetically determined structural properties, different growth behavior may be observed.
Figure 5. PFS film thickness for 5 (squares) and 10 (diamonds) bilayers, as a function of the NaCl concentration in the solution from which the bilayer stacks are deposited. The solid lines represent fit results as described in the text.
At present, we do not have an explanation for this value of the exponent. Reviewing the literature on the ionic strength dependent thickness of polyelectrolyte films deposited using layer-by-layer assembly reveals a strong variation of R. In a number of papers, a linear relationship between film thickness and ionic strength is reported for systems consisting of poly(styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH)41,42 and PSS/poly(diallyldimethylammonium) (PSS/PDDA).43,44 In two other reports a square root behavior, i.e., an exponent R = 1/2, is found for PSS/PAH bilayer films.44,45 However, in some cases close examination of the data and, more importantly, replotting on a log-log scale reveals different exponents. In ref 43 the actual exponent for the PSS/ PDDA system is close to 2/3, while in ref 45 the reported square root behavior for PSS/PAH films is much better described using an exponent of 1/3. This shows that for various systems studied by different research groups different behavior is found, indicating that the nature of the polyanions and polycations as well as the experimental conditions has a pronounced influence on the results.
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IV. Conclusions Summarizing, we investigated the optical properties of PFS films deposited using layer-by-layer assembly. The ionic strength strongly influences bilayer thickness, but the primary tool for film thickness control was the number of deposited bilayers. Films with thicknesses up to 55 nm enabled unambiguous determination of the complex refractive index as a function of energy (or wavelength) of the light. The optical characteristics can be described by only four parameters using a Cauchy model. The resulting refractive index values of our layer-by-layer deposited PFS thin films are considerably higher than those typically reported for more common polyelectrolytes. Moreover, the optical properties are in line with previously reported results primarily obtained at single wavelengths in the visible. Finally, for the ionic strength dependent thickness of the PFS films, we found that the system is governed by an exponent of 1/4. Acknowledgment. G.J.V. and M.H. thank the MESAþ Institute for Nanotechnology and The Netherlands Organization for Scientific Research (NWO, TOP Grant 700.56.322, Macromolecular Nanotechnology with Stimulus Responsive Polymers) for financial support. Supporting Information Available: UV-vis absorption spectra for 5 and 10 PFS bilayer stacks as well as ellipsometry spectra (70° incident angle) for all PFS film thicknesses in the range 0-12 bilayers, the latter showing the evolution of Ψ and Δ as a function of energy with increasing film thickness. This material is available free of charge via the Internet at http://pubs.acs.org.
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