magnetite Nanoparticle Multilayers: Preparation and

Oct 25, 2007 - Max-Planck Institute of Colloids and Interfaces, Golm/Potsdam, D-14476, Germany, Faculty of Nano- and Biomedical Technology, Saratov St...
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Langmuir 2007, 23, 12388-12396

Polyelectrolyte/magnetite Nanoparticle Multilayers: Preparation and Structure Characterization D. Grigoriev,*,† D. Gorin,†,‡ G. B. Sukhorukov,§ A. Yashchenok,‡ E. Maltseva,| and H. Mo¨hwald† Max-Planck Institute of Colloids and Interfaces, Golm/Potsdam, D-14476, Germany, Faculty of Nano- and Biomedical Technology, SaratoV State UniVersity, SaratoV, 410012, Russia, Department of Materials, Queen Mary UniVersity of London, E1 4NS, London, U.K., and Federal Institute for Materials Research and Testing, Berlin, D-12489, Germany ReceiVed April 3, 2007. In Final Form: September 5, 2007 Polyelectrolyte composite planar films containing a different number of iron oxide (Fe3O4) nanoparticle layers have been prepared using the layer-by-layer adsorption technique. The nanocomposite assemblies were characterized by ellipsometry, UV-vis spectroscopy, and AFM. Linear growth of the multilayer thickness with the increase of the layer number, N, up to 12 reflects an extensive character of this parameter in this range. A more complicated behavior of the refractive index is caused by changes in the multilayer structure, especially for the thicker nanocomposites. A quantitative analysis of the nanocomposite structure is provided comparing a classical and a modified effective medium approach taking into account the influence of light absorption by the Fe3O4 nanoparticles on the complex refractive index of the nanocomposite and contributions of all components to film thickness. Dominant influence of co-adsorbed water on their properties was found to be another interesting peculiarity of the nanocomposite film. This effect, as well as possible film property modulation by light, is discussed.

1. Introduction In the past two decades, broad perspectives in the application of planar multilayers were demonstrated.1-15 There are many methods for fabrication of nanodimensional multilayered structures for example spin coating, the Langmuir-Blodgett16 approach, and layer-by-layer (L-b-L) adsorption of oppositely charged macromolecules or nanoparticles.1-10,17 Building-up of self-assembled multilayers of polyelectrolytes is a very promising technique of surface modification, which has been used for the formation of hydrophilic and hydrophobic polymeric coatings,10,13,18 sensors,6,12,14 and lithography.15 Considerable progress was achieved especially in the field of investigations of PAH/ PSS planar layers.3-5,9,20-23 For example, in ref 9 the refractive * To whom correspondence should be addressed. Tel: +49-331-5679257. Fax: +49-331-567-9202. E-mail: [email protected]. † Max-Planck Institute of Colloids and Interfaces. ‡ Saratov State University. § Queen Mary University of London. | Federal Institute for Materials Research and Testing. (1) Mo¨hwald, H. Colloids Surf. A 2000, 171, 25. (2) Khomutov, G. B. AdV. Colloid Interface Sci. 2004, 111, 79. (3) Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 210, 831. (4) Decher, G.; Schmitt, J. Prog. Polym. Sci. 1992, 89, 160. (5) Decher, G. Science 1997, 277, 1232. (6) Lee, C.-W.; Park, H.-S.; Gong, M.-S. Sens. Actuators B 2005, 109, 256. (7) Sato, K.; Suzuli, I.; Anzai, J. Langmuir 2003, 19, 7406. (8) Lvov, Y.; Essler, F.; Decher, G. J. Phys. Chem. 1993, 97, 13773. (9) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871. (10) Kommireddy, D. S.; Patel, A. A.; Shutava, T. G.; Mills, D. K.; Lvov, Y. M. J. Nanosci. Nanotechnol. 2005, 5, 1081. (11) Houssam, W.; Schlenoff, J. B. Macromolecules 2005, 38, 8473. (12) Yang, Y.; Yang, X.; Liu, Y.; Liu, Z.; Yang, H.; Shen, G.; Yu, R. J. Photochem. Photobiol. A 2005, 171, 137. (13) Kolasinska, M.; Warszynski, P. Bioelectrochemistry 2005, 66, 65. (14) Kim, J. H.; Kim, S. H.; Shiratori, S. Sens. Actuators B 2004, 10, 241. (15) Tianhong, C.; Hua, F.; Lvov, Y. Sens. Actuators A 2004, 114, 501. (16) Nagy, N.; Dea´k, A.; Ho´rvo¨lgyi, Z.; Fried, M.; Agod, A.; Ba´rsony, I. Langmuir 2006, 22, 8416. (17) An, M.; Hong, J. D. Thin Solid Films 2006, 500, 74. (18) Kolasinska, M.; Warszynski, P. Appl. Surf. Sci. 2005, 252, 759. (19) Kotov, N. A.; Fendler, J. H.; Dekany, I. J. Phys. Chem. 1995, 99, 13065.

indices and thicknesses of individual PAH and PSS layers were determined by ellipsometry in situ on a liquid subphase. The possibility of a more sophisticated variation of the layer composition including embedding of organic or inorganic nanoparticles for fabrication of complex nanostructures was also recently shown.10,14,19,24-30 In general, such nanocomposite materials are very promising due to the manifold of their unique properties: electronic,22,23,27 biomedical,10,31,32 optical,24-26,28-30 etc. For example, nanoparticle layers have been successfully applied for fabrication of super-hydrophilic biocompatible coatings10 and solar cells.19,33 Nanocomposite materials containing layers of magnetic particles have been less investigated, but they have certain perspectives for application, for instance, in areas of new microwave-absorbing materials,30,34 structures for spin electronics, or for fabrication of magnetic data storage media.27-30 (20) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058. (21) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W.G.; Kjaer, K. Macromolecules 1998, 31, 8893. (22) Neff, P. A.; Naji, A.; Ecker, C.; Nickel, B.; v. Klitzing, R.; Bausch, A. R. Macromolecules 2006, 39, 463. (23) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (24) Cho, S.; Lim, H.; Lee, K. S.; Lee, T. S.; Cheong, B.; Kim, W. M.; Lee, S. Thin Solid Films 2005, 475, 133. (25) Lamarre, J.-M.; Yu, Z.; Harkati, C.; Roorda, S.; Martinu, L. Thin Solid Films 2005, 479, 232. (26) Liu, Y.; Rosidian, A.; Lenahan, K.; Wang, Y.-X.; Zeng, T.; Claus, R.O. Smart Mater. Struct. 1999, 8, 100. (27) Skomski, R. J. Phys. Condens. Matter 2003, 15, R841. (28) Mamedov, A.; Ostrander, J.; Aliev, F.; Kotov, N. A. Langmuir 2000, 16, 3941. (29) Mamedov, A. A.; Kotov, N. A. Langmuir 2000, 16, 5530. (30) Correa-Duarte, M. A.; Giersig, M.; Kotov, N. A.; Liz-Marzan, L. M. Langmuir 1998, 14, 6430. (31) Koktysh, D. S.; Liang, X. R.; Yun, B. G.; Pastoriza-Santos, I.; Matts, R. L.; Giersig, M.; Serra-Rodriguez, C.; Liz-Marzan, L. M.; Kotov, N. A. AdV. Func. Mater. 2002, 12, 255. (32) Podsiadlo, P.; Paternel, S.; Rouillard, J.-M.; Zhang, Z.; Lee, J.; Lee, J.-W.; Gulari, E.; Kotov, N. A. Langmuir 2005, 21, 19151. (33) He, J. A.; Mosurkal, R. L.; Samuelson, A.; Li, L.; Kumar, J. Langmuir 2003, 19, 2169. (34) Kim, S.-S.; Kim, S.-T.; Ahn, J.-M.; Kim, K.-H. J. Magn. Magn. Mater. 2004, 271, 39.

10.1021/la700963h CCC: $37.00 © 2007 American Chemical Society Published on Web 10/25/2007

Polyelectrolyte/magnetite Nanoparticle Multilayers

Although the layered nanocomposite materials contained iron oxide particles have, in general, rather high roughness and can therefore only barely be applied for magnetic or optic data storage, a substantial improvement of film morphology may be achieved either by additive of a third stratifying component like montmorrilonite28 or by microwave treatment of polyelectrolyte layer prior to iron oxide particle adsorption30 due to reduction of loose segments of polyelectrolyte. Limitations related to low coercivity of nanoassemblies containing layers of pure magnetite can be overcome by use of particulate layers composed of magnetically hard particles (e.g., Co) under very accurately controlled deposition conditions.28 Thus, the general aim of this study is elaboration of fabrication methods for such films and their following characterization on the nanolevel, as well as feedback from properties to structure improvement. The major goal of the present work is to prove the applicability of the L-b-L method for preparation of polyelectrolyte/iron oxide nanocomposite films with different numbers of particle layers and different alternation of components. The second aim was characterization of the average structure and surface morphology of the synthesized samples on each step of their preparation. AFM, UV-vis spectroscopy, and ellipsometry were used for this purpose. The dependencies of the real part of the complex refractive index, thickness, roughness, and optical absorbance of the films on the layer number were derived, and their interrelations with the nanocomposite structure at L-b-L build-up were discussed using the effective medium approach formalism. Essential kinetic effects caused by water desorption/ adsorption under laser illumination and following long-time sample exposition in humid atmosphere were observed. 2. Experimental Section 2.1. Materials. Poly(sodium 4-styrenesulfonate) (PSS, MW ≈ 70 000), poly(allylamine hydrochloride) (PAH, MW ≈ 70000), polyethylenimin (PEI, MW ≈ 600 000-1 000 000), and sodium chloride were purchased from Sigma. Magnetic fluid from “Berlin Heart”, which is an aqueous suspension of iron oxide nanoparticles (Fe3O4), was employed for fabrication of nanoparticulate layers. The average size of iron oxide nanoparticles measured by dynamic light scattering (DLS, Malvern HPPS5001) was 10 ( 1 nm (PDI ) 0.23) and was in good agreement with the transmission electron microscopy (TEM) data of 8 nm.35 The nanoparticles were stabilized by citric acid and, therefore, had a negative charge (ζ-potential of the nanoparticles in aqueous suspension at pH 6.9 ( 0.2 was -47.4 mV). The water used in all experiments was prepared in a threestage Millipore Milli-Q Plus 185 purification system and had a specific resistivity higher than 18.2 MΩ‚cm. 2.2. Planar Nanodimensional Film Preparation. Chemically cleaned p-type (100) silicon wafers with specific resistivity of 5-10 Ω·cm from Silchem were used as substrate. Before use, the silicon plates were cleaned by a H2O/H2O2/NH3 ) 5:l:l mixture at 70 °C during 15 min (RCA cleaning procedure) and after thorough washing with deionized water were dried in a nitrogen flow. As a result, the silicon wafers were covered by a silicon oxide layer of about 2-3 nm thickness. Due to this layer, the wafer surface became negatively charged in water at neutral pH, and no special chemical treatment was necessary to charge it.36 One precursor layer of PEI was deposited by dipping a substrate in the corresponding polyelectrolyte solutions. Such pretreatment led to higher stability of PAH/iron oxide multilayers deposited later on the same substrate.18 The thickness of the PEI precursor layer was ∼2.5 nm.37 All polyelectrolytes were (35) Gaponik, N.; Radthenko, I. L.; Sukhorukov, G. B.; Rogach, A. L. Langmuir 2004, 20, 1449. (36) Ko¨stler, S.; Delgado, A. V.; Ribitsch, V. J. Colloid Interface Sci. 2005, 286, 339. (37) Protein Architecture. Interfacing Molecular Assemblies and Immobilization Biotechnology; Lvov, Y., Mo¨hwald, H., Eds.; Madison Avenue: New York, 2000; p 394.

Langmuir, Vol. 23, No. 24, 2007 12389 used at a concentration of 2 mg/mL. PAH and PSS solutions were prepared in 0.5 M NaCl. Because the surface charge of the iron oxide nanoparticles dispersed in water at pH 6.9 was negative, PAH, a cationic polyelectrolyte, was used for alternation with these nanoparticles during L-b-L preparation. Adsorption of iron oxide nanoparticles was made from their water suspension with the gross content of 3.22 mg/mL. Multilayers were prepared via L-by-L selfassembly using alternate dipping in a PAH solution and an iron oxide suspension. Each freshly deposited layer was three times washed by Milli-Q water before starting the next step of multilayer build-up. The obtained noanocomposite films were dried at ambient conditions in the flux of dry nitrogen after the defined number of oppositely charged layers was deposited. Then, the samples were placed in the hermetically closed container with inert atmosphere inside for storage between measurements. The following compositions were synthesized. PEI/(Fe3O4/PAH)x, PEI/(Fe3O4/PAH)yFe3O4, and PEI/(Fe3O4/PAH)zPSS. 2.3. Methods. 2.3.1. Ellipsometric Measurements. Two different “Multiskops” from Optrel GbR, Germany, with laser wavelengths λ ) 532 and 632.8 nm were used for ellipsometric measurements. The scheme of this apparatus and the procedure to calculate layer thickness and adsorbed amount was described in detail elsewhere.38,39 In brief, a conventional PCSA (polarizer-compensator-sampleanalyzer) null-ellipsometer was used. A low-capacity laser with the corresponding wavelength (532 or 632.8 nm) serves as light source (beam diameter of about 0.5-1 mm). The angles of incidence of the light, φ, were 60° and 70°. The incidence and reflection arms, as well as the polarizer and analyzer, were motorized and controlled by the computer with high precision. From the nulls of analyzer and polarizer (positions at which a minimal intensity of transmitted light is registered) two ellipsometric angles, ∆ and Ψ, were obtained. Computer-controlled step motors allow very high reproducibility in measured values of ∆ and Ψ, 0.01% and 0.005%, respectively. On the next step, the numerical simulation of ellipsometric angles for the real system under investigation was performed by iteratively changed refractive index, n1, and thickness, δ, of the adsorbed layer until the best agreement between the measured and simulated values of ∆ and Ψ was obtained. This approach was successfully used several times for determination of n1 and δ of the transparent adsorption layers composed of polymers,40 biopolymers,41,42 or polyelectrolytes.9 For the systems under investigation, however, the presence of Fe3O4 nanoparticles in the nanocomposite could cause a remarkable absorbance. In this case, the complex refractive index of the adsorption layer n1 + iκ was applied for numerical simulation of rough ellipsometric data, where the imaginary part of the refractive index, κ, was determined by means of UV-vis spectroscopy and AFM. 2.3.2. Spectrophotometric Measurements. The UV-vis spectra of polyelectrolyte/Fe3O4 nanocomposites on glass substrate were obtained by means of a HP Agilent-8453 (HP, UK) spectrophotometer against a pure substrate plate of the same thickness. 2.3.3. Scanning Force Microspcopy. The AFM images were recorded by a Nanoscope III multimode AFM (Digital Instruments, Inc.) in tapping mode using silicon nitrite cantilevers (spring constant ) 42 N/m). The optimal scan rates were between 0.8 and 0.5 Hz. All measurements were performed in air under ambient conditions. To determine the thickness, the nanocomposite films deposited on Si wafers were scratched using sharp tweezers and the height difference between the scratched and unscratched regions was measured by AFM. The AFM images were processed by the (38) Miller, R.; Fainerman, V. B.; Makievski, A. V.; Kra¨gel, J.; Grigoriev, D. O.; Ravera, F.; Liggieri, L.; Kwok, D. Y.; Neumann, A. W. Encyclopedic Handbook of Emulsion Technology; Sjo¨blom, J., Ed.; Dekker: New York, 2001; p 1. (39) Motschmann, H.; Teppner, R. NoVel Methods to Study Interfacial Layers, Studies in Interface Science; Mo¨bius D., Miller, R., Eds.; Elsevier: Amsterdam, 2001; Vol. 11, p 1. (40) Noskov, B. A.; Akentiev, A. V.; Grigoriev, D. O.; Loglio, G.; Miller, R. J. Colloid Interface Sci. 2005, 282, 38. (41) Benjamins, J.; de Feijter, J. A.; Evans, M. T. A.; Graham, D. E.; Phillips, M. C. Faraday Discuss. Chem. Soc. 1975, 59, 218. (42) Grigoriev, D. O.; Fainerman, V. B.; Makievski, A. V.; Kragel, J.; Wustneck, R.; Miller, R. J. Colloid Interface Sci. 2002, 253, 257.

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Figure 1. AFM surface scans of polyelectrolyte/Fe3O4 nanoparticle multilayers at total layer numbers N ) 1(picture a, only coated by PEI), 2(b), 4(c), 6(d), and 12(e). The area of each visual field corresponds to 0.7 µm × 0.7 µm. Nanoscope III software in order to obtain the surface roughness and composite film thickness.

3. Results and Discussion Typical AFM pictures demonstrating the surface morphology of polyelectrolyte/Fe3O4 multilayers at different stages of their deposition are given in Figure 1. The processing of these pictures

yields two important parameters of the multilayer: its thickness, δ, and its mean roughness, Ra. The latter parameter characterizes the surface inhomogeneity of a multilayer nanocomposite in normal direction to the substrate/multilayer interface and is given in Figure 2 as function of total layer number, N. Generally, this dependence reveals an upward trend with gradually increasing slope. Whereas at the beginning of multilayer formation Ra was

Polyelectrolyte/magnetite Nanoparticle Multilayers

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Figure 2. Mean roughness, Ra, of polyelectrolyte/Fe3O4 nanoparticle multilayers as function of the total layer number, N, and multilayer composition. Filled symbols denote Ra for multilayers with Fe3O4 nanoparticle terminal layer, open symbols denote those with the polyelectrolyte terminal layer. Arrows show the consequence of L-b-L deposition in which the nanocomposites were synthesized.

∼60% of the Fe3O4 particle diameter (N ) 2), for relatively thick multilayers with N ) 12 it reaches the value of 12.1 nm which is nearly the nanoparticle size. This indicates the formation of particle clusters or particle aggregates in the multilayer, which are responsible for further roughness enhancement. Qualitatively, the same tendency is also seen in the AFM pictures presented in Figure 1. The continuation of the above-discussed trend for the Ra ) Ra(N) dependence also for N > 12 should lead to a further increase of the roughness which can then approach values not far from optical wavelengths. Thus, for the considerably thicker nanocomposite the contribution of light scattering to the ellipsometric results can be expected. It is also clearly seen in Figure 2 that if the terminal layer is made of Fe3O4 nanoparticles the value of Ra is significantly higher than in the case of polyelectrolyte. On each step of alternate coating, the deposition of polyelectrolyte leads to a smoothening of the multilayer surface and, therefore, to the decrease of Ra (follow the arrows in Figure 2 starting from the left filled symbol; see also the corresponding figure legend). Qualitatively, this planarization of the surface could be imagined as filling of gapes between neighboring nanoparticles by the polyelectrolyte of opposite charge, PAH. Completely unexpected is, however, the behavior of the roughness after deposition of the second polyelectrolyte layer, PSS, over the first one. Several roughnesses of the corresponding nanocomposites with a terminal PAH/PSS bilayer are given in Figure 2 by open circles. Instead of further smoothing of the surface, a recurring increase of Ra is observed. A possible explanation of this effect could be related to the topography of the already smoothed surface. Since the mean thickness of the PAH layer is approximately four times smaller than the nanoparticle diameter, the surface remains still curved and has some peaks and valleys. Because of the higher curvature in the peaks, the surface charge density also has maxima at these sites. In turn, the inhomogeneity of the surface charge density leads to the adsorption of the next oppositely charged PSS layer predominantly at the same sites. Bulb-shaped patches of PSS are formed there and enhance again the roughness. Moreover, the proposed mechanism could be responsible for the particle cluster formation in general and, therefore, for the increase of Ra(N) also in the case of alternating PAH and Fe3O4 particle layers. Results of AFM thickness determination are presented as function of N in Figure 3a together with the corresponding data

Figure 3. (a) Thickness, δ, of polyelectrolyte/Fe3O4 nanoparticle multilayers as function of total layer number, N. Small filled circles represent the results of AFM measurements. Open and filled squares denote the series obtained from ellipsometric measurements at φ of 60° and 70°, respectively without taking into account the optical absorbance within the nanocomposite. Filled star at N ) 12 denotes the result of evaluation with the complex refractive index of the adsorption layer n1 + iκ. The solid line is a linear fit for points with N e 12. (b) Real part of the complex refractive index, n1, of polyelectrolyte/Fe3O4 nanoparticle multilayers as function of the total layer number, N, without taking into account the optical absorbance within the nanocomposite. Results are given for φ ) 70°. Filled star at N ) 12 denotes the result of evaluation with the complex refractive index of the adsorption layer n1 + iκ.

obtained by ellipsometry neglecting absorbance within nanocomposite. The simultaneously obtained real part of the refractive index, n1, of the nanocomposites is shown in Figure 3b. As one can see (Figure 3a), the agreement between the rough ellipsometric estimation and the completely independent direct method such as AFM is surprisingly good. The absolute values of δ for N e 12, as well as the corresponding linear trends for δ ) δ(N), practically coincide for both data sets. This similarity allows the assumption that for nanocomposite multilayers with N e 12 the influence of optical absorbance on the ellipsometric data is weak. To further support this point, the rough ellipsometric results for the same samples but at two different wavelengths, 532 and 632.8 nm, were compared. Usually, in systems with significant absorbance its influence on the ellipsometric angles ∆ and Ψ depends markedly on the absorption spectrum.43 Then, the experimental dependencies ∆ ) ∆(N) and Ψ ) Ψ(N) measured (43) Kooij, E. S.; Wormeester, H.; Brouwer, E. A. M.; v.Vroonhoven, E.; v. Silfhout, A.; Poelsema, B. Langmuir 2002, 18, 4401.

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kλ )

4πκ λ

(1)

On the other side, using the data on the nanocomposite thickness, δ, obtained independently by AFM, one can express this quantity as

kλ )

Figure 4. (a) Ellipsometric angle, ∆, as function of N. Open symbols and solid line represent the experimental data and best linear fit, respectively, for λ ) 532 nm. Filled symbols and dotted line denote the same but for λ ) 632.8 nm. (b) Ellipsometric angle, Ψ, as function of N. Symbols and lines have the same meanings as in (a).

at different λ should have different slopes. In our case, however, these dependencies are almost parallel in the λ range between 532 and 632.8 nm (Figure 4a and b). Such a behavior can be considered as qualitative evidence of the weak light absorbance in polyelectrolyte/Fe3O4 nanocomposites with N e 12. However, for exact quantitative evaluation of δ and n1 from ellipsometric data, one needs the imaginary part of the complex refractive index, κ. The spectroscopic measurements allow the independent determination of κ for nanocomposites taking into account the values of multilayer thickness obtained by AFM. Absorption spectra of nanocomposites on the glass substrate are given in Figure 5 for several N. Note that until N e 12 the absorbance induced by the presence of Fe3O4 nanoparticles in the nanocomposite remains very weak for both λ ) 532 and 632.8 nm in good agreement with the literature data28 (and practically nonmeasurable at N < 6). Only for N g 12 does the effect become noticeable. The extinction coefficient of the nanocomposite film, kλ (here we use the same definition as in ref 43 by some other authors,44 however, this quantity is called the absorption coefficient) is related to the imaginary part, κ, of the complex refractive index via (44) Tolstoy, V. P.; Chernyshova, I. V.; Skryshevsky, V. A. Handbook of Infrared Spectroscopy of Ultrathin Films; Tolstoy, V. P., Ed.; Wiley: New York, 2003.

2.303A δ

(2)

where A is the absorbance measured for the nanocomposite multilayer with the thickness δ. Then, for N ) 12 and δ ) 40 nm at λ ) 532 nm one obtains from these two relations κ ) 0.046, which indicates again a relatively weak absorbance. Nevertheless, after substitution of the complex refractive index, n1, with κ ) 0.046 in the program for numerical evaluation of δ and n1 from ellipsometric data one gets values deviating from these obtained at the first rough estimation. The corrected δ is 44 nm, ∼9% larger than its estimated value, and the deviation of the exact n1 from its estimation is slightly smaller (by 7%). Differences between the estimated and corrected values for δ and n1 at N ) 12 are visually presented in Figure 3a and b where the exact values are given by filled stars (see also the corresponding figure legends). Unfortunately, the application of the same correction algorithm to the thicker nanocomposites with N ) 22 and 32 led to the unphysicaly high values of κ because of apparent character of ellipsometric angles obtained for these nanocomposites. Indeed, increasing roughness of multilayrs with N > 12 leads to the steadily increasing contribution of light scattered by the sample to the measured parameters ∆ and Ψ. In contrast, for N e 12 the light scattering in the nanocomposite is practically absent because of the very small size of scattering centers compared to the wavelengths used. Only absorbance is responsible here for the deviation of the estimated δ and n1 from their exact values, but because of the weak influence of absorbance on δ even at N ) 12, one can assume a still lower difference between δexact and δest for smaller N, i.e., the dependencies δexact(N) and δest(N) should practically coincide in the range N e 12 and the following qualitative discussion can be based on the observations made for δest(N) without making overly large errors. The thickness, δ, of the nanocomposite increases linearly with increase of N (Figure 3a) with an increment of 2.9 nm independent

Figure 5. Absorbance of nanocomposite films on the glass substrate as function of wavelength. Solid line corresponds to the nanocomposite with N ) 12, dashed line corresponds to those with N ) 22, and dotted line corresponds to those with N ) 32. Inset shows the dependence of absorbance on N measured at λ ) 632.8 nm.

Polyelectrolyte/magnetite Nanoparticle Multilayers

from the chemical nature of the terminal layer (polyelectrolyte or Fe3O4). This thickness behavior reflects the high regularity of the multilayer structure in the direction normal to the substrate. Thus, the doubled increment could be considered as an effective thickness of the polyelectrolyte/ Fe3O4 couple averaged across the whole nanocomposite. The initial point of the dependence δest(N) corresponds to the thickness of the first polyelectrolyte/Fe3O4 bilayer and lies at ∼10 nm. However, from the slope of the line in Figure 3a one derives an average thickness per bilayer of 5.8 nm. The nonlinearity at the beginning of the δ ) δ(N) dependence was already observed for multilayers with included nanoparticles45 and reflects the fact that the first single nanoparticle layer has a structure different from that in the multilayer where the influence of neighboring nanoparticle layers should be taken into account. By analogy with the δ ) δ(N) dependence, n1est and n1exact in the range N e 12 could be also considered as lying very close to each other and the corresponding dependencies on N should be similar. An increase of n1est(N) means then that n1exact(N) is also increased. The observed behavior of the nanocomposite refractive index n1(N) (Figure 3b) indicates changes occurring in the multilayer structure and composition upon N growth. So, in the range N e 6 the absolute values of the refractive index n1 are either almost the same as results reported by refs 9 and 23 for pure polyelectrolyte multilayers or only insignificantly higher in spite of Fe3O4 nanoparticles46 included in the multilayers. This inconsistency may be due to the fact that in our case the Fe3O4 nanoparticle layers alternating with polyelectrolyte cause essentially a higher roughness than in refs 47-50. Because the L-b-L coating is done usually in aqueous medium solvent molecules, i.e., water are also involved in the composition of each consecutively deposited layer being incorporated in the numerous cavities and gapes arising during layer formation. As it was recently established in ref 21, the percentage of water can approach up to 40% at 100% relative humidity even in the case of pure polyelectrolyte multilayers where the roughness is rather low compared to the discussed system. Then one can expect in our case still higher amounts of water included in the polyelectrolyte/Fe3O4 nanocomposites. As a consequence, the values of n1 obtained by ellipsometry should include also a strong contribution from water which opposes the enhancing effect of iron oxide nanoparticles on n1. The effect of embedded water reducing the value of n1, however, should not be constant as N increases. The higher the value of N, the higher the roughness of the nanocomposite (see Figure 2 below and further discussion), which leads to the increasing interdigitation of the neighboring layers in the nanocomposite. Then, the relative amount of the included Fe3O4 nanoparticles increases with N, leading to a steeper growth of the complex refractive index n˜ 1. Even taking into account the corrected values of its real part n1 values one can see the slight increase of the n1(N) slope for 6 e N e 12 (Figure 3b). At the further increase of N, however, the relative changes in n1 should become less and less expressed because the relative contribution of each subsequent Fe3O4 nanoparticle layer decreases comparing (45) Lvov, Y.; Haas, H.; Decher, G.; Mo¨hwald, H.; Mikhailov, A.; Mtchedlishily, B.; Morgunova, E.; Vainshtein, B. Langmuir 1994, 10, 4232. (46) Handbook of Chemistry and Physics, 85th ed.; Lide, D.R., Ed.; CRC Press: Boca Raton, FL, 2004-2005. (47) Gong, H.; Garcia-Turiel, J.; Vasilev, K.; Vinogradova, O. I. Langmuir 2005, 21, 7545. (48) Lowman, G. M.; Buratto, S. K. Thin Solid Films 2002, 405, 135. (49) Paterno, L. G.; Mattoso, L. H. C. Polymer 2001, 42, 5239. (50) Lobo, R. F. M.; Pereira-da-Silva, M. A.; Raposo, M.; Faria, R. M.; Oliveira, O. N., Jr. Nanotechnology 2003, 14, 101.

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with the gradually increasing total thickness. The same could be concluded also regarding behavior of imaginary part of n˜ 1 with an increase of N if one takes into account the dependence of absorbance on N given as an inset in Figure 5. Again, the slope of the light absorbance increases at N > 12, indicating the increase of relative amount of Fe3O4 in the nanoassemblies and then again levels off because of increase of the total nanocomposite thickness as mentioned above. Obviously, the refractive index of pure Fe3O4 can be considered as an absolute upper limit for n1 of the nanocomposite, which in reality can never be approached. This possibility to vary the refractive index of nanocomposite in the broad range from the values typical for the pure polylectrolyte to values almost approaching the refractive index of nanoparticulate Fe3O4 can be potentially used in diverse optical applications of these nanoassemblies. For instance, for the efficacious antireflective coating of the substrates with high value of own refractive index n2 the desired value of nanocomposite refractive index should meet the condition n21 ) n0n2, where n0 is the refractive index of ambient phase. However, the elaboration of the nanocomposite coatings for the optical applications was not the goal of paper at hand where obtained values of refractive indices of various nanocomposites were only used as parameters for the structural characterization of these objects. Further information on the particle packing density and on the structure of the multilayer can be obtained within the framework of the effective medium theory.43,44,51,52 In the general case of a host medium (having dielectric function h) with two types of spherical inclusions with a and b, respectively, the effective dielectric function, eff, of the whole inhomogeneous composite material is expressed by

eff - h a - h b - h ) φa + φb eff + 2h a + 2h b + 2h

(3)

where φa and φb are the volume fractions of components with a and b, respectively. By substituting h ) b one get the Maxwell-Garnett effective medium approximation

φa )

eff - b a + 2b eff + 2b a - b

(4)

whereas φb ) 1 - φa and h ) eff leads to the Bruggeman’s form of this approach

φa )

(32 

1 + a 3 eff(a - b)

(eff - b)

eff

)

(5)

Equations 4 and 5 allow the evaluation of volume fraction of the component denoted by subscript “a” if the quantities a, b, and eff are known. The subscripts “a”, “b”, and “eff” correspond to the Fe3O4 nanoparticle, polyelectrolyte, and nanocomposite multilayer, respectively, and with eq 5 one can thus calculate the volume fractions, φa, of the nanoparticles in the nanocomposite. Ellipsometric and spectroscopic data yield the complex refractive index of the polyelectrolyte/Fe3O4 nanocomposite at N ) 12: n˜ 1) 1.536 + 0.046i. Therefore, eff is also a complex quantity with real and imaginary parts given by n2 - κ2 and 2nκ, i.e., eff ) 2.357 + 0.142i. The complex dielectric function for Fe3O4 (51) Kreibig, U.; Vollmer, M. Optical properties of metal clusters; SpringerVerlag: Berlin, Heidelberg, 1995. (52) Binks, B. P.; Clint, J. H.; Dyab, A. K. F.; Fletcher, P. D. I.; Kirkland, M.; Whitby, C. P. Langmuir 2003, 19, 8888.

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was reported recently by Fontijn et al.53 and can be expressed at λ ) 532 nm as P ) 5.136 + 3.085i. In addition, we have b ≡ PE ) (nPE)2 ) 2.161.9,23 After substitution of all dielectric functions into eq 4, one obtains φP ) 0.084, whereas eq 5 for the same set of input data yields φP ) 0.083. Although both obtained φP values agree well with each other their absolute magnitude seems to be underestimated. The main reason for the obtained too low values of φP is the above-discussed contribution of the water-filled gapes included into the whole nanocomposite structure during L-b-L deposition. To account for the contribution of embedded H2O to the nanocomposite properties, the generalized form of the effective medium approach can be applied. Considering that host medium in eq 3 is composed of polyelectrolyte (h ) PE) and subscripts “a” and “b” correspond now to Fe3O4 nanoparticles and water-filled gapes, respectively, one can express (see Appendix 1) the volume fraction of polyelectrolyte in the nanocomposite φPE as

δlPENPE φPE ≡ 1 - φP - φg ) δ

(6)

where φP ) φa is the volume fraction of Fe3O4 nanoparticles, φg ) φb is the volume fraction of water-filled gapes, δlPE is the thickness of the individual polyelectrolyte layer in the nanocomposite, NPE is the number of polyelectrolyte layers in the nanocomposite, and δ is the thickness of the nanocomposite as a whole. Equation 6 allows the expression of factor φg in the second right-side term of eq 3 via φP and φPE

(

)

P - PE δlPENPE g - PE eff - PE ) φP + 1 - φP eff + 2PE P + 2PE δ g + 2PE (7) Algebraic rearrangement of eq 7 leads to the final form for φP:

φP )

[

g - PE eff - PE + eff + 2PE g + 2PE

][

]

g - PE P - PE δlPENPE g - PE / (8) δ g + 2PE P + 2PE g + 2PE

Applying the latter expression to the nanocomposite with N ) 12, i.e., with NPE ) 6 together with ellipsometric data for δ and literature data for δlPE ) 2.2 nm22 yields a more realistic value for φP ) 0.155 (cf. with data for the Au-nanoparticle fraction reported in ref 43). The corresponding value for φg follows from the definition given in the left part of eq 6: φg ) 1 - φP - φPE ) 0.544 and confirms quantitatively the high water content in the nanocomposite already mentioned above. Moreover, roughness, Ra, that increases with N should lead to the corresponding increase of the gapes volume fraction, φg, with N. If most of them are water-filled gapes, the content of water incorporated in the nanocomposite also should be an increasing function of N. This behavior is qualitatively supported by IR spectra measured for several nanocomposites with progressively increasing N, as shown in Figure 6. Depth of the broad water absorbance peak in the range between 3400 and 3600 cm-1 becomes more and more expressed with N increasing from 12 to 32. Of course, even the most precise eq 8 does not account for some factors influencing φP, such as the inhomogeneous size (53) Fontijn, W. F. J.; van der Zaag, P. J.; Devillers, M. A. C.; Brabers, V. A. M.; Metselaar, R. Phys. ReV. B 1997, 56, 5432.

Figure 6. IR transmittance of nanocomposite films on the glass substrate as function of wavenumber. Solid line corresponds to the nanocomposite with total number of layers N ) 12, dashed line corresponds to those with N ) 22, and dotted line corresponds to those with N ) 32.

distributions of nanoparticles and/or gapes, the fact that a certain fraction of gapes can be filled by gas but not by water, etc. The latter factor should, for example, lead to a still higher value of φP than the one given above. Simple estimation by eq 8 shows that for the case with 30% air-filled gapes the average value of g becomes 1.52 and φP increases up to 0.2, whereas φg decreases until 0.5. For the limit case of only air-filled gapes, one obtains 0.284 and 0.415 for φP and φg, respectively. The value of the latter quantity is in an excellent agreement with data reported recently by Nagy et al.16 for multilayers made of silica nanoparticles. Nevertheless, in all considered examples the magnitude of φP remains quite low, and as a consequence, there is enough space in each polyelectrolyte/Fe3O4 bilayer in the form of gapes to allow significant interdigitation of neighboring bilayers. This, in turn, leads to a more compact structure of the entire nanocomposite. For example, in the case of a nanocomposite with N ) 12, i.e., with six polyelectrolyte layers and six layers of Fe3O4 nanoparticles, the total thickness calculated additively on the basis of data for the first bilayer would yield 6 × 10 ) 60 nm, whereas the real values measured by AFM and ellipsometry are between 40 and 44 nm. During the ellipsometric measurements, interesting kinetic effects of laser radiation on the polyelectrolyte/Fe3O4 nanocomposite properties are observed. The thickness, δ, of the nanocomposite decreases and after ∼20 min almost levels off (Figure 7a). A similar but opposite trend is also observed for the refractive index n1 of the nanocomposite (Figure 7b). To explain these kinetic effects, possible processes taking place in the polyelectrolyte/Fe3O4 nanocomposite upon laser beam illumination should be taken into account. Because the cross-section of the laser beam incident upon the nanocomposite/air interface has an area of some square millimeters and the nominal capacity of the laser applied is 50 mW, the local power density at the site of the light spot is relatively high. Even in the case when only part of the radiation energy is transferred into thermal one and a typical experiment lasts on the order of magnitude of some minutes, the final heating of the nanocomposite in the incidence area may be considerable. Upon heating, first, the water included in the nanocomposites during their preparation has to evaporate, increasing locally the concentration of other components of the multilayer and their contribution to the average value of the local refractive index.

Polyelectrolyte/magnetite Nanoparticle Multilayers

Figure 7. (a) Kinetics of relative changes in the nanocomposite thickness, δ, upon laser illumination during ellipsometry: 1, original sample; 2, as in 1 plus 10 min drying at 70 °C; 3, as in 2 plus 3 days storage at 100% humidity and room temperature. (b) Kinetics of relative changes in the refractive index of nanocomposite n1 upon laser illumination during ellipsometry. Numbers 1, 2, and 3 have the same meanings as in Figure 7a.

Langmuir, Vol. 23, No. 24, 2007 12395

In order to prove the importance of these factors for the observed effects, three subsequent experiments on the same sample were carried out. Initially, the original sample with 12 polyelectrolyte/ Fe3O4 layers was illuminated during 20 min and values of δ and n1 attained at the end of this time were collected (Figure 7a and b, curves 1). Then, the same sample was placed for 10 min in a drying oven at 70 °C and again measured for 10 min (curves 2 in the same figure). Finally, the same sample was stored in a box with 100% humidity during 3 days at room temperature and the same measurements were performed once more (curves 3 in Figure 7a and b). The original sample dried in nitrogen flow during preparation and storage afterward in a hermetical box demonstrates very low and also slowly occurring changes in both control parameters, δ and n1 (curves 1). These effects had already completely vanished after 10 min of exposition in dry atmosphere at 70 °C and subsequent cooling down (curves 2). In contrast, after 3 days humidification (curves 3) the considered effects become most expressed. The presented difference agrees, in principle, with all possible scenarios described above: dehydration, thermal deswelling, or glass-melt transition occurring upon laser radiation. However, taking into account the magnitudes of these three effects, especially in comparison with the magnitudes observed in the present study (about 0.8% for ∆ and 0.15% for Ψ, see Figure 7a and b), one can conclude that neither thermal deswelling nor a transition of the whole film that might be interrupted and then reinforced by light can be responsible for the observed effects. The magnitude of the effect was much higher than observed in this work,54,55 even for the dehydration, which seems to be the most probable explanation for the effects observed in our study the estimation of the magnitude of the effect based on the percentage of the confined water should lead to higher values. Therefore, we suggest that only some topmost layers of the multilayer nanocomposite are involved in the water evaporation process which is responsible for the kinetic effects on the δ ) δ(t) and n1 ) n1(t) dependencies. An unambiguous answer on this question could be given by a set of analogous ellipsometric measurements performed on multilayers of the same composition but with variable N, which are already under way.

4. Conclusions Second, heating could lead to local thermal condensation of the polyelectrolyte component of the multilayer accompanied by a loss of water, as was demonstrated recently for polyelectrolyte microcapsules in ref 54 and, therefore, to a more dense structure of the nanocomposite. This factor also causes the increase of n1 accompanied by a decrease of the average thickness of the multilayer because of a denser arrangement of polyelectrolyte chains inside. Moreover, for some polyelectrolyte systems where a glass-melt transition occurs at a temperature only slightly higher than room temperature, the properties of the system can change drastically already after one heating-cooling cycle.55 If, however, the polyelectrolyte system reveals a stronger component interaction and, as a consequence, can have a higher transition temperature (for example, PAH/PSS multilayers), the fast heating-cooling cycle could probably lead to the intermediate labile state which is still far from equilibrium. In this state any external disturbance (laser light illumination, etc.) can cause significant changes in the system due to its transition either to the thermodynamically stable initial state or to the final one. (54) Ko¨hler, K.; Shchukin, D. G.; Sukhorukov, G. B.; Mo¨hwald, H. Macromolecules 2005, 37, 9546. (55) Mueller, R.; Ko¨hler, K.; Weinkamer, R.; Sukhorukov, G.; Fery, A. Macromolecules 2005, 38, 9766.

The successful fabrication of the polyelectrolyte/Fe3O4 nanoparticle multilayer by the L-b-L self-assembly technique has been shown. The linear increase of the nanocomposite thickness δ with increase of the layer number, N, up to 12 indicates the equality of the adsorbed amounts of Fe3O4 in each nanoparticle layer in this range. A more complex behavior of the refractive index can be qualitatively explained by taking into account the increasing amount of Fe3O4 nanoparticles in the multilayer: at the increase of N, the mean roughnesses, Ra, are increased, causing the subsequent enhanced increase of the total amount of nanoparticles because of the increasing interdigitation of nanoparticle layers. The smoothing (roughening) of the multilayer surface at the deposition of subsequent polyelectrolyte (Fe3O4 nanoparticle) layers leads to the alternating decrease (increase) of Ra compared with the previous layer. On the other hand, significant roughness of subsequent layers is responsible for incorporation of a considerable amount of water in the numerous gapes between layers. Quantitative evaluations of the volume fraction of nanoparticles made on the basis of the MaxwellGarnett and of the Bruggeman effective medium approach yield underestimated values, which do not account for water-filled gapes within the nanocomposite. The modified effective medium approach considered alongside the nanocomposite film thickness

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GrigorieV et al.

variations led to more realistic results coinciding with literature data. The time dependence of the nanocomposite thickness, δ, and its refractive index, n1, as revealed by ellipsometric measurements, are relevant to water evaporation from some topmost layers of the nanocomposite upon laser radiation.

Appendix 1 For an arbitrarily chosen nanocomposite element with area S and thickness δ holds for the volume V:

V ) δS

(A1)

If components of the nanocomposite are polylectrolyte (PE), nanoparticles (P) and water-filled gapes between (g), one can express the same volume also as

V ) VPE + V tP + V tg

V ) VPE + V + V ) t g

δlPENPES

+ NPVP + NgVg

where δlPE is the thickness of individual polyelectrolyte layer in the nanocomposite, NPE, is the number of polyelectrolyte layers in the nanocomposite, NP, VP, Ng, and Vg are the numbers of particles in this nanocomposite element, the volume of individual particle, the number of gapes, and the volume of an individual gape, respectively. Volume fractions of particles φP, gapes φg, and polyelectrolyte φPE in the nanocomposite element are given by

NPVP V tP ) l V δPENPES + NPVP + NgVg

(A5)

φPE )

δlPENPES VPE ) l V δPENPES + NPVP + NgVg

(A6)

where φP + φg + φPE ) 1. Excluding the term NPVP from eqs A4 and A5, one can obtain the following relation

φgδlPENPES + φg

φPδlPENPES + φPNgVg ) NgVg(1 - φg) 1 - φP (A7)

From eq A7, one gets after algebraic rearrangement and reduction

φgδlPENPES ) NgVg(1 - φP - φg) ) NgVgφPE (A3)

φP )

V tg NgVg ) l V δPENPES + NPVP + NgVg

(A2)

Under a simplifying assumption that particles and gapes are both monodisperse, eq A2 can be rewritten as follows t P

φg )

(A4)

(A8)

Finally, eq A8 yields an expression (eq 6) for φPE taking into account eqs A1 and A2:

φPE ) φg

δlPENPES V tg δlPENPES NgVg δlPENPES δlPENPE ) ) ) NgVg V NgVg δS NgVg δ (A9)

Acknowledgment. The work was supported by a project of the European Space Agency (FASES MAP AO-99-052) and the EU project SELECTNANO (STREP No 516922) of the FP6 program. The authors acknowledge the expert technical assistance of Mrs. A. Heilig with the AFM measurements. LA700963H