J. Phys. Chem. 1993,97, 12835-12841
12835
Assembly of Polyelectrolyte Molecular Films onto Plasma-Treated Glass Yuri LVOV,~ Heinricb Haas, Cero Decher, and Helmuth Miihwald. Institut fiir Physikalische Chemie, Johannes Gutenberg- Universitiit, Welder Weg 1 1 , 55099 Mainz, Fed. Rep. Germany
Alexei Kalacbevt Institut fiir Mikrotechnik, Carl Zeiss Str.18-20, 55099 Maim, Fed. Rep. Germany Receiued: May 12, 1993; In Final Form: August 7 , 1993"
We combine in a unique approach two modern ultrathin film techniques for production of heterostructure films. These are plasma deposition (which permitted one to obtain thin 20-500-A well-defined layers on any solid surface with an active, in our case charged, surface) and layer-by-layer deposition of the polycation/polyanion self-assembly. The latter technique was recently developed in our group and is based on the electrostatic attraction between opposite charges. Alternating adsorption of anionic and cationic polyelectrolytes from their aqueous solution onto charged substrate leads to the formation of multilayer assemblies. The multilayer heterostructure film was monitored by small-angle X-ray reflectivity. Oxygen, methane, argon, and polysiloxane plasma coating were tested as a precursor treatment for polyion assembly. The procedure was successfully accomplished for methane plasma deposition onto a glass substrate with further oxidation and consequent alternate assembling of a layer of polyallylamine (PAH) and polystyrenesulfonate (PSS). The step size of growth for the deposited PAH/PSS pair was 50 f 1 A. Model fitting of the X-ray reflectivity data allowed us to calculate the electron density profiles of plasma-deposited plus polymer-assembled heterofilms and to characterize the thickness and roughness of its composite layers. Furthermore, the combination of methane plasma treatment and polyelectrolyte self-assembly opens a new approach for constructing inorganic/organic nanocomposite films.
1. Introduction Ultrathin organic films are currently gaining interest in many areas such as integrated optics, sensors, friction-reducingcoatings, or surface orientation layers.' Most of these tasks require the preparation of well-defined films composed of molecules with tailor-made properties in a unique spatial arrangement with respect to each other and to the substrate. It is most interesting and challenging to construct films with a supramolecular architecture in which the individual organic molecules are macroscopicallyoriented and/or in which molecules with different functionality can be incorporated into individual layers. The latter task is accomplished in the assembly of multilayer films by applying layer-by-layer deposition methods such as the LangmuirBlodgett (LB) technique or self-assembly techniques based on chemisorption. A new technique for constructing multilayer assemblies by consecutively alternating adsorption of anionic and cationicbipolar amphiphiles and/or polyelectrolytes was recently developed.24 This approach has the advantage that electrostatic attraction between opposite charges is the driving force for multilayer buildup. In contrast to chemisorption techniques596 that require a reaction yield of 100% or lateral cross-linking to maintain a constant surface functional density after each deposition step, no covalent bonds need to be formed. Additionally, an advantage over the classical Langmuir-Blodgett technique is that adsorption processes are independent of substrate size and topology and demand only a suitable surface of a sample. The principle of the multilayer assembly is shown in Figure 1 and is described as follows. A solid substrate with positively charged planar surface is immersed in the solution containing the t Permanent address: Institute of Crystallography, Academy of Science of Russia, Moscow 117333, Russia. t Prcsent address: Plasmachem. GmbH. BodelschwinghStr. 10, D55099 Mainz, FRG. Abstract published in Aduunce ACS Abstrucrs, October 1, 1993.
anionic polyelectrolyte, and a layer of polyanion is adsorbed (step A). Since the adsorption is carried out at relatively high concentration of polyelectrolyte,a number of ionic groups remain exposed to the interface with the solution, and thus the surface charge is reversed. New evidence of the reversal of surface charge upon adsorption of polyelectrolytes has recently been reported.' After rinsing in pure water, the substrate is immersed in the solution containing the cationic polyelectrolyte. Again a layer is adsorbed, but now the original surface charge is restored (step B). By repeating both steps (A, B, A, B, ...) in a cyclic fashion, alternating multilayer assemblies are obtained. We previously reported the construction of alternating multilayer films by using polystyrenesulfonate (PSS) and polyallylamine (PAH).*v9 It was established that films composed of at least 60 layers can be grown and that the thickness of the films increases linearly with the number of layers. Furthermore, the thickness of each individual layer and thus also the total thickness of the film can be adjusted very precisely by changing the ionic strength of the solution from which the polyions are adsorbed. For the case of varying the ionic strength of the PSS solution, it was found that for a concentration of NaCl of 0.0, 1.0, 1.5, and 2.0 mol/L, the average thickness of each anionic/cationic layer pair is fine-tuned to 10.9, 17.7, 19.4, and 22.6 A, respecti~ely.~+~ We believe that in principle all polyelectrolytes should be suitable for the incorporation into multilayer assemblies. The technique operates reliably for poly(viny1 sulfate) potassium salt, polyallylamine hydrochloride, polylysine, polyadenine, polyuracil, and DNA. This technique is strongly dependent on surface charge, and for may surfaces special additional procedures have to be carried out to form the appropriate surface charge. For example, the surface of normal glass is inert regarding adsorption of many polyelectrolytes, and in all of the above-mentioned experiments we used substrates charged by adsorbing in a first step a boladication m ~ n o l a y e r . ~ , ~ The aim of this work is to find an effective method of surface charging for a following self-assembly process and to understand
0022-3654/93/2097- 12835%04.00/0 0 1993 American Chemical Society
Lvov et al.
12836 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
A
B
,...
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insolubility in solvents, high thermostability, and a good adhesion to a wide set of materials. The approach is preferred because (a) a plasma-polymerized coating creates a new surface chemistry, which is independent on an initial surface and therefore can be used for the self-assembly of different substrates, and (b) in combination with self-assembling this leads to the construction of new inorganic/organic nanocomposite films. The investigation of surface charge for plasma-treated/ deposited surface layers presents difficulties, especially because this charge can be a mixture of ionizing groups (such as a carboxilic group) and stable physical charges (enhanced electron density). The self-assembly technique gives one an opportunity to estimate the kind of effective charge (plus or minus) and the relative density of surface charges. Thus, the investigation of self-assembling can yield new information about charged states of a plasmamodified surface. Results of plasma treatment, film assembly, and structural analysis in this work are completely based on X-ray reflectivity data and their fitting to film models. From this evaluation of the X-ray results, we obtain total thicknesses of the films (L),an estimate of their roughness (a), and the thicknesses of pairs of polyelectrolyte layers (d) which will be compared and discussed concerning the conditions of glass plasma treatment. One has to emphasize that the X-ray reflectivity method has not yet been widely used for analyses of plasma-etched and plasmadeposited surface layers. From such experiments, combined with model fitting, one can expect new information about the plasmatreated surface itself. This analysis permits us to control the film growth on the plasma-treated glasses. We found that plasma treatment gives surprisingly perfect films (an oxide layer for treating a glass surface by an oxygen plasma or an organic film for treatment with a methane plasma) with thicknesses of 40-500 A depending on the time of treatment. We elaborated conditions to obtain proper surface charges for further polycation/polyanion assembly.
2. Materials and Methods
1o3
Polystyrenesulfonate (sodiumsalt, M W = 100 000) (PSS)was I
0.50
1.0
1.5
2.0
2.5 3.0 20, deg,
Figure 1. (a, top) Side view schematically depicting the buildup of multilayer assemblies by consecutive adsorption of anionic and cationic polyelectrolytes. For reasons of simplicity, the counterions were omitted in both the chemical structures of the polyelectrolytes and in their oversimplified graphic representations: (1) polystyrenesulfonate, (2) polyallylamine. It is not implied that the symbols used for the polyelectrolytes represent their actual structure in solution or after adsorption. (b,bottom) X-ray reflectivity'curves from PSS/PAH selfassembly films on a boladication-coatedglass: (1) 6 layers of PSS/PAH, (2) 8 layers of PSS/PAH, (3) 10 layers of PSS/PAH.
the resulting structure. To solve the preparation problem, we used a low-temperature plasma treatment which is a very promising technique for surface modification and permits us to functionalize or to charge the surface.IO This approach was already successfully used also in combination with the LangmuirBlodgett technique.I'J2 So, we believe, that the treatment of a solid surface by lowtemperature plasma (glow discharge) can be used to build up the surface charge density required for the self-assembly process. Two main types of plasma treatment of glass substrates were used: (1) plasma etching (activation) with oxygen and argon plasma and (2) plasma deposition (polymerization) using hexamethyldisilazane or methane as the plasma-forming gas with following oxidation by an oxygen plasma. The latter approach yields a plasma-deposited coating with well-defined thickness, which is generally in the range 10-10 000 A. These coatings are characterized by a high degree of cross-linking, high uniformity,
obtained from SERVA, and polyallylamine (hydrochloride, M W = 50 000-65 000) (PAH) was obtained from Aldrich. All polyelectrolytes were used without further purification. The ultrapure water was used for all cleaning steps and as solvent for the adsorption was obtained by reverse osmosis (MilliR O 35TS, Millipore GmbH) followed by ion-exchange and filtration steps (Milli-Q, Millipore GmbH). The resistivity was better than 18 Mn-cm, and the total organic content was less than 10 ppb (according to the manufacturer). Polyelectrolyte films were deposited onto glass slides of size 12 X 38 "2. These substrates were washed for 30 min in an ultrasonic bath a t 60 OC in a solution containing 1% KOH in a mixture of water/ethanol (3:7). After cleaning, the substrates were carefully washed in Milli-Q water. Then the control substrate was immersed in boladication solution for 30 min.*v3 After this procedure, the substrate is positively charged and used for polyelectrolyte deposition beginning with a polyanion. The other samples were subject to plasma treatment, and polycation/ polyanion deposition was done immediately after that. All polyelectrolytes were adsorbed from aqueous solutions containing 0.02 monomol/L of polymer (monomol refers to the molar concentration of monomer residues) and 0.01 M HCI. The adsorption was carried out as follows. The substrates were immersed in 10 mL of the polyelectrolyte solution for 20 min a t room temperature. After deposition of each polyelectrolyte layer, the substrates were rinsed 3 times in pure water. Multilayer assemblies consisting of 4-16 layers of PSS and PAH were prepared by consecutive adsorption of both polymers. Plasma TreatmentProcedure. Plasma treatment was provided using an LE-301 plasma reactor (Leybold AG, FRG) for radio-
Assembly of Polyelectrolyte Molecular Films frequency (RF, 13.56 MHz) discharges, modified by additional attachment for microwave (MW, 2.45 GHz) and electron cyclotron resonance (ECR) plasmas. The device allows uniform treatment of the substrate with a diameter up to 200 mm and operation using each single mode or any desirable combination (RF, MW, ECR). A permanent ring magnet mounted on the outside of the wave guide provides a magnetic strength of 87.5 mT in the region, where the electric field strength of the radial component E,, of the EO,wave reaches its maximum value. With a Langmuir probe, ion densities of 5 X 1O1O~ m and - ~electron temperature of 5 eV at a pressure of 1 X 10-3 mbar have been measured for M W 02dischargeat 1.2 kW. Theabsolute pressure and mass flow rate (for each individual gas) have been measured with a MKS Baratron absolute pressure gauge and mass flow controllers with accuracy of f0.0005 mbar and f 2 sccm. Bias voltage has been measured with accuracy of f 5 V. For the plasma treatment, the following conditions were used: gas pressure, 0.0082 mbar (for oxygen treatment) and 0.0103 mbar for all others; R F power, 98 W; MW power, 600 W; mass flow rate, 60 sccm. (1) Oxygen plasma: 1- or 15-min R F 0 2 plasma. (2) Argon plasma: 15-min R F Ar plasma. (3) Hexamethyldisilazane: 1-min pretreatment by R F 0 2 plasma, then 20 s (a) or 3 min (b) of R F hexamethyldisilazane plasma and final post treatment by R F 0 2 plasma. (4) Methane plasma: 1-min pretreatment by 02 plasma, then 3 min of (a) R F CH4-Ar (plasma mass flow 1:3) or (b) RF MW CH4-Ar (plasma mass flow 1:3) with short posttreatment by O2 plasma. After plasma treatment, samples immediately (within 1 min) were immersed into the polycation solution (assuming a negative charge of the surfaces), and four cycles of polyanion/polycation adsorption were performed. Samples were thendryed in a nitrogen stream, and X-ray reflectivity was recorded. Then this procedure was cyclically repeated. X-Ray Analysis. Small-angle X-ray reflectivity is a convenient and direct method for measuring multilayer buildup and the determination of the film thickness. Measurements were performed with a Siemens D-SO0 powder diffractometer equipped with a graphite monochromator on the detector side using copper K, radiation with a wavelength of 1.54 A. Data were acquired via a DACO-MP interface connected to a personal computer using a step width of 0.01 deg (in 28) and counting intervals of 5 s. The dependence of the SAXR spectra on layer numbers was recorded from a single multilayer specimen on a glass substrate that was dried in a stream of nitrogen between the deposition cycles. The X-ray reflectivity data have been analyzed by using the master formula13.114
+
In eq 1, p o and p are the electron densities of support and film, respectively; Qz= 4u/X sin J is the wave vector transfer along the surface normal z (X = wavelength, 28 = scattering angle); and RFis the Fresnel reflectivity for an ideally sharp interface. For qualitative analysis, we take RFas proportional to Q4 and display the data as R / @ . A uniform film on a substrate can be described as a slab of thickness L and electron density p , with a Gaussian smearing of the interfaces with a roughness parameter u. For the reflectivity, one obtains interfence between waves reflected at the air/film and film/support interfaces. This yields the well-known Kiessig fringes, where the spacing of the minima or maxima is given by AQ = 2u/L (2) From eq 2, the thickness can be calculated to very high precision (taking into account refraction phenomena”, particularly if there are many fringes over the measured Q range. This is true for
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12837 high film thicknessses with a small roughness. In the case of multiple layers of different electron density, any combination of two interfaces contributes to the reflectivity with fringes corresponding to their distance according to eq 2 and intensity proportional to the step in electron density between the interfaces. Differences between the roughness of the interfaces lead to different damping for the individual oscillations. In such cases, it can be necessary to introduce individual smearing parameters to get accurate results. Here we present data for models containing at most two slabs. Thus, we are able to model the basic features of the reflectivity curves, i.e., the frequency of the fringes and the shape of the beating over the whole measured Q range. Deficiencies of the fits to alarge extent are due to these simple models. We tried more refined models, but the introduction of too many parameters makes the results less reliable.
3. Results and Interpretation As seen in Figure la, the complete reversal of surface charge is the crucial factor for a regular stepwise growth of the multilayer films. A stable self-consistent growth requires that surfacecharge density and surface roughness reach the same equilibrium value after each adsorption cycle. In other words, these values can be different for different polyelectrolytes, but fabrication of highquality films and a linear growth are only achieved when the values are identical after adsorption of the same material in a given layer sequence. Film Thickness and Roughness. Figure l b shows X-ray reflectivity curves of PSS/PAH films on a boladication-coated glass. The X-ray curves reveal well-defined Kiessig fringes up to scattering angles 28 = 2.5O (but no Bragg peaks), which proves that polymeric self-assembled films are uniform and flat. This pronounced structure in the measurements enables us to deduce detailed information on the film microstructure. From the oscillation periods already at the first step of analysis, one can easily estimate the thickness of the film,10.*2 and the number of visible oscillations is connected with film roughness. It permits the monitoring of the quality of the growing film and its thickness a t different steps of deposition without sophisticated calculations. The thickness of the PSS/PAH film was found to be proportional to the number of deposited polymeric layers. The thickness of a layer pair is 48 f 1 A, and the surface roughness u = 6 A for the same PSS and PAH solutions, which were used further for plasma-treated samples. This thickness is in good agreement with a value of 50 f 1 8, found in an earlier study of PSS/PAH assembly. The small difference is most likely due to small differences in concentrations of polyanions MnC12 and NaBr salts added into the solutions. 1. Oxygen-Plasma-Treated Glass. Figure 2 shows X-ray reflectivity curves from pure glass and glass treated for 1 and 15 min by oxygen plasma. The X-ray reflectivity curve from pure glass is smooth. But after 1 min of oxygen plasma treatment, one can see well-defined oscillations with the period =2O, which indicates plasma deposition of a layer with thickness of 44.5 f 0.4 A. The surface roughness is surprisingly small (u = 3A). From these data, we cannot judge if the new surface layer is due to surface modification or to film deposition onto the glass surface. The additional experiment, when we in parallel prepared the two plasma-treated glass substrates and then tried to assemble on one of them, proved that the registered layer is due to an oxidation process, and an organic film assembly does not occur. The 15 times prolongation of plasma treatment leads to the formation of a nonuniform layer (the corresponding X-ray curve qualitatively corresponds to an extremely rough layer or outer surface). Attempts to assemble PAH/PSS films onto these three substrates were unsuccessful. X-ray control shows no changes in reflectivity curves after application of six or eight polycation/ polyanion adsorption procedures.
12838 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
Lvov et al.
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-2. X-rayreflectivity(with Frcsnelcorrection1p)venusscattering angle, 28: (1) for pure glass, (2) glass treated 1 min by oxygen plasma plus six cyclesof PAH/PSS deposition,(3) glass treated 15 min by oxygen plasma plus six cycles of PAH/PSS deposition.
t 1
CI
5
0.50 1,O
1.5
2.0
2.5
3.0 3.5 4.0
28,deg. Figure 3. X-ray reflectivity versus scattering angle: (1) pure glass, (2)
after 5 min of HMDS plasma treatment plus six cycles of PAH/PSS deposition, (3) after 10 min of HMDS plasma treatment plus six cycles of PAH/PSS deposition.
2. Argon-Plasm-Treated Glass. The X-ray reflectivity curve from this sample is just a smooth curve without any oscillations, so we cannot given any interpretation. Most likely this plasma treatment does not result in a new layer formation. An attempt to deposit electrolyte layers onto this sample also failed. The comparison between oxygen and argon plasma treatments gives evidence that ion bombardment of a glass surface (which takes place in both plasmas) does in itself not lead to a change of surface chemistry, while active oxygen molecules are able to diffuse into glass substrate and change it. Probably, observed after oxygen plasma treatment, the thin layer exhibits a quartzlike structure, which is thermodynamically favorable. 3. Hexametbyldisilsazane (HMDS)-Plasma-Treated Glass. Figure 3 shows X-ray reflectivity curves from pure glass and glass treated for 20 s (a) and 3 min (b) by H M D S plasma with PAH/PSS layers absorbed above. One can see on the curves from plasma-treated samples well-defined oscillations which correspond to layer thicknesses of 441 f 5 A for the shorter time of treatment and 1040 f 10 A for the longer treatment. The
6
7 8 9 10 11 12 13 Number of depositedlayers, n
Figure 4. (a, top) X-ray reflectivity versus scattering angle: (1) pure glass, (2) after 1 min of methane plasma treatment and adsorption of six alternate layers of PAH/PSS (Le., +6 layers), (3) the same sample after deposition of two additional PAH/PSS layers (Le. +8 layers), (4) +10 PAH/PSS layers, (5) +12 PAH/FSS layers. (b, bottom) Dependence of the PAH/PSS film thickness on the number of deposited polymeric layers, L(n), The step of growth for the PAH/PSS bilayer is d = 48 1 A.
*
surface roughness in both cases is 6 A. In both curves, one can see the beating in the intensity of fringes. The period of beatings is about 2 O which gives the film thickness of =40 A and may correspond to a layer (or bilayer) of adsorbed polymers. Attempts at further assembling PAH/PSS layers onto these substrates were unsuccessful, and X-ray reflectivity measurement shows no changes. So probably in spite of the successful deposition of a first layer we did not reach the main condition for the self-assembly procedure: reversion of the terminal charge after every adsorption cycle. A reason for this may be that the HMDS plasma creates too high a charge density, which could not be converted by polyelectrolyte adsorption. 4. Methane Plasma Treatment. Figures 4 and 5 represent results on coating of glass by methane plasma (treatments a and b) and further PAH/PSS assembling. One can see the Kiessig fringes in X-ray curves obtained from the sample after methane treatment a and six PAH/PSS layer adsorption (Figure 4). After further PAH/PSS pair assembling, the period of the fringes decreased, which corresponds to a proportional increase of the polymeric film thickness. The growth step for PAH/PSS pair deposition is 48 f 1 A (as calculated following eq 2), which coincides with the value obtained for PSS/PAH growth onto the boladication-charged glass surface. So this plasma-deposited layer happens to be appropriate for the further assembly procedure.
The Journal of Physical Chemistry, Vola97, No. 49, 1993 12839
Assembly of Polyelectrolyte Molecular Films
2
109 l L
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1.5
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0.50 1.0
1.5
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2.5
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Figure 5. X-ray reflectivity versus scattering angle: (1) The pure glass, (2) after methane plasma treatment, (3) a sample after methane plasma treatment and deposition of four layers of PAH/PSS (Le., +4 layers PAH/PSS), (4) +6 PAH/PSS layers. For the surface charge density of the boladication glass, we can estimate about one elementary charge on 30-40A2, but for plasma treatment we cannot give the value. It depends on the nature of a plasma layer grown onto a glass surface and on the oxidation process in this layer. It is known16 that during methane plasma treatment a layer of "diamond-like" carbon appears on a surface which we further oxidize. We could not evaluate in the last case the thickness of the underlayer and in the next experiment increased the flow of the methane plasma (treatment 6)togrow a thicker underlayer. Figure 5 demonstrates the results of X-ray reflectivity measurements. After plasma treatment of glass, the initially smooth scattering curve is converted to a well-oscillating curve (the periodicity of fringes, = 0.23O, gives a thickness of the underlayer of 385 f 5 A). The adsorption of the four PAH/PSS bilayers onto this layer resutls in an appearance of remarkable "beating" in the reflectivity curve with a periodicity = 0.9'. The further assembly of PAH/PSS results in a decrease of the beating periodicity. The latter we ascribe to the PAH/PSS film thickness. Calculations yielded values of L = 95, 143, and 205 A, corresponding to +4, +6, and +8 PAH/PSS layers. It means that the growth steps for PAH/PSS in this case is 50 f 5 A, which is less accurate but coincides with the basic growth step value. To confirm this result, we repeated the experiment with another glass slide treated for the same time by the methane plasma and deposited six PAH/PSS layers. The resulting X-ray reflectivity curve shows once more the beatings, and the calculated polymeric film thickness from their periodicity is 146 f 5 A, which corresponds to a three-pairs layer of PAH/PSS with the step 48.5 A. SothePAH/PSS pairself-assemblystepontotheplasmatreated substrate is established as 49 f 1 A. It is of interest to mention that in a decade of our experience in X-ray work we never encountered such a pronounced reflectivity curve as in Figure 5, showing for X-ray radiation the effect of interference with a modulation which is typical for a "visible" wavelength optical experiment. Above, we carried out a reliable but qualitativeanalysisof the heterostructureofthefilms. Further on, we present an example of quantitative model fitting of the electron density profile of the last one, where we analyzed the four reflectivity curves (Figure 5). Model Fittingof the X-rayReflectivity Data. For a quantitative analysis of the four reflectivity curves of Figure 5, we use a twoslab model according to e-q 1 and numberical Fourier analysis. At the first step, we analyze the structureof the methane-plasma-
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Thickness, A Figure 6. (a, top) Experimental X-ray reflectivity (circles) and modeled curve (solid line) for the methane-plasma-depositedlayer with further oxidation. (b, bottom) Electrondensityprofileof thelayer. The thickness of the deposited carbon film is 385 A with an oxidized layer of 20 A. The electron densities normalized to the substrate are 0.59 k 0.05 for the carbon and 1.19 & 0.09 for the oxidized layer. deposited carbon layer (without further film assembly). The reflectivity curve (Figure 6a) shows very regular fringes which can be assigned to a layer of 385 A. For the low-frequency modulation (an obvious convex of the curve), one has to assume a thin slab of about 20-A thickness with one positive and one negative step in electron density. Such a layer could be between substrate and carbon with an electron density higher than that of the substrate or smaller than that of the carbon layer. With a higher electron density, it could be on top of the carbon slab. Trying all three alternatives, we found the best accuracy for the case of a higher electron density layer on top of the carbon layer. This is reasonable, as by oxidation the atoms are thought to migrate into the carbon and increase the electron density (oxygen posttreatment). The electron density profile used for that model is given in Figure 6bwhere the relative electron density of the carbon was found to be 0.59 f 0.05 and for the oxidized layer to be 1.19 f 0.1. Figure 7 shows measured and simulated reflectivity curves for the next sample with a carbon layer and four, six, and eight double layers of polyions adsorbed on top of it. The high-frequency fringes are modulated by beatings of increasing frequency (Figure 7a-c). Additionally, there is a fine structure in terms of the
Lvov et al.
12840 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
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200
400
600
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Figure 8. Electron density profiles of the film heterostructure at different steps of assembly according to our modeling: (a) substrate coated by carbon film (methane plasma treated) plus four PAH/PSS layers, (b) +6 PAH/PSS layers, (c) +8 PAH/PSS layers. For clarity, the abscissas and ordinates of neighboring profiles are shifted.
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Figure 7. Experimental X-ray reflectivity (circles) and modeled curves (solid lints) of PAH/PSS multilayers on methane-plasma-treated precursor. The inserts show the numerical Fourier analysis of the reflectivity curves: (a, top) +4 PAH/PSS, (b, middle) +6 PAH/PSS, and (c, bottom) +8 PAH/PSS layers. Length of carbon slab L1 = 433 A; total length 4 = 536, 586, and 638 A. For the electron densities (normalized tothesubstrate), we f o u n d p h = 0.7 O . l , p ~ , - = 0.55
* *up0.1.to 12Theroughncssofthepolymer/airinterfaceissignificantlyincreased A for curve c. relative intensity of the neighboring peaks. One also finds a loss of structuring going to high Q numbers, especially for the eightlayer case. For the modeling of these curves, we used a two-slab model, one describing the carbon precursor film and one the polymer above. The beatings result from three cosine waves as displayed in Figure 9. They were sufficient to achieve the modeling in Figure 7 (with the phase-independent contributions added) and gave numbers for the thickness of the slabs to very high accuracy.
We found the thickness of the carbon layer to be constant, with A for all three samples, and the thickness of the adjacent polymer to be a multiple of its double-layer thickness of 50 A. Adding a further slab, for example, the electron-rich layer like in Figure 6b, yielded no major improvement of the fit. The Fourier analysis of the reflectivity curves in the upperright corner of the figures yields an autocorrelation function (or Patterson function) of the system with the abscissa as film thickness. Here one finds peaks at identical lengths as used for the two-slab model, with no further signal at any other length. This is an additional hint that two boxes are sufficient for the modeling. For the electron densities, one gets only very rough numbers. Only the relative electron densities are exactly defined. In addition to the restrictions of the model, one reason may be a rather high diffuse scattering, which adds up to the specular part and was not taken into account for our analysis. We derive the electron density of the carbon layer relative to the glass substrate which is consistent with the previous experiment (0.7f O-l),and the electron density for the polymer is smaller than that for the carbon 435
Assembly of Polyelectrolyte Molecular Films (0.55 f O.l), which is within the range expected from the literature.''J8 Looking at the roughness, from Figure 6a we find slightly higher roughness for the support (5-6 A) compared to the other interfaces (4 A). For the curves in Figure 7, the roughness of the air/polymer interface increases at the last step of deposition up to a number as high as 12 A (Figure 7c corresponds to +8 layers of PAH/PSS). This may be the result of an incomplete deposition of the upper polymer layer (which was deposited about 1 week later). In the reflectivity curve, the fast delcine of the beatings and also the rather low intensity of the total length peak in the Fourier analysis make this obvious. Figure 8 gives the calculated electron density profiles along the surface normal of the film. One realizes that, as expected, the plasma-polymerized layer exhibits the same thickness in all cases. Its density is between that of glass and that of the polymer layer. The thickness of the latter scales with deposition cycles, as expected.
4. Conclusions In this work, we demonstrated that plasma treatment enables a charged surface to adsorb polyelectrolyte layers. The process is apparently sufficiently stable against fluctuations in electron density, allowing reproducible self-assembed-filmformation and preparation of nanocomposite films with well-defined structure. We suppose that the mechanism of plasma-induced surface activation for self-assembly concerns the formation of electrone gative groups at the surface. These groups are responsible for contact ion pair formation with a polycation. It was demonstrated consistently that plasma treatment yields a film of uniform thickness and density with surface roughness below 10 A. We could fit X-ray reflectivity measurements with reasonable models, although the pattern is very complex and informative. The latter is probably due to the fact that characteristic slab lengths are rather different and therefore enable beatings to be analyzed. On the other hand, one realizes quantitative differences between measured and fit intensities
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12841 resulting in a large uncertainty of the electron densitim. Still, the order of its value given in Figure 8 is correct and independent of a specific model. It was shown also that X-ray reflectivity measurements, in combination with a model film profile fitting, present a perfect method for investigation of plasma-treated, plasma-deposited, and nanocomposite surface layers.
Acknowledgment. We thank Prof. W. Erfeld (Intitut f i r Mikrotechnik, Mainz) for useful discussions and the "Bundesministerium fiir Forschung und Technologie" and the 'Fond der Chemischen Industrie" for financial support. Y. Lvov thanks the Alexander von Humboldt Foundation (FRG) for supporting his work at the Johannes Gutenberg-UniversitEt Mainz. References and Notes (1) Special issue on organic thin films: Aduanced Materials 1991,3, 3-180. Ulman, A. An Inrroduction to Ultrathin Films, From LungmuirBlodgetr to Self-assembly; Academic Press: Boston, New York, Toronto, 1991;p 440. (2) Decher, G.;Hong, J. D. European Patent 0472 990 A2, 1992. (3) Decher, G.; Hong, J. D. Ber. Bunsenges. P h y ~Chem. . 1991,95,1430. (4) Dechcr, G.; Schmitt, J. Progr. Colloid Polym. Sci. 1992,89, 160. (5) Maoz, R.; Sagiv, J. Lungmuir 1987,3, 1034, 1045. (6) Lee, H.; Kepley, L.; Hong, H.-G.; Mallouk, T. J. Am. Chem. Soc. 1988,110,618. (7) Berndt, P.; Kurihara, K.; Kunitake, T. Lungmuir 1992,8,2486. (8) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992,210/ 211,831. (9) Lvov, Y.;Decher, G.; M6hwald, H. Lmgmuir 1993,9,481. (10) Yasuda, H.Plasma polymerization; Academic Press, 1985;p 410. (1 1) Kalachev, A,; Wegner, G. Macromol. Chemic, Macromol. Symp. 1991,12,229. (12) Kalachev, A.; Mathauer, K.; H i h e , U.; MBwald, H.; Wegner, G. Thin Solid Films 1993,228,307. (13) Haas, H.;Mtjhwald, H. Colloids Surf., in press. (14) Als-Nielscn, J. Phys. Reu. 1986,A140, 376. (IS) Tippmann-Krayer, P.;M(ihwald, H.;Lvov, Y. Lungmuir 1991, 7, 2298. (16) Davica, P.; Martineau, P. Aduanced Materials 1992,I, 729. (17) Russel, T. P. Material Science Reports 1990,5,171. (18) Elbcn, H. Dissertation, Universitat Freiburg, FRG, 1991;p 42.
