Control of Packing Order of Self-Assembled Monolayers of Magnetite

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Control of Packing Order of Self-Assembled Monolayers of Magnetite Nanoparticles with and without SiO2 Coating by Microwave Irradiation Miguel A. Correa-Duarte,† Michael Giersig,‡ Nicholas A. Kotov,*,I,| and Luis M. Liz-Marza´n*,†,⊥ Department of Physical Chemistry and Organic Chemistry, University of Vigo, 36200, Vigo, Spain, Abteilung Physikalische Chemie, Hahn-Meitner Institut, Glienickerstrasse 100, 14109, Berlin, Germany, and Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received May 6, 1998. In Final Form: August 27, 1998 A close packed layer of Fe3O4 and SiO2-coated Fe3O4 nanoparticles can be assembled on silicon and glass substrates modified with polyelectrolytes following the principle of the layer-by-layer self-assembly (Science 1997, 277, 1232). Microwave (MW) treatment of the poly(dimethyldiallylammonium chloride) layer prior to the nanoparticle adsorption results in substantial reduction of the surface roughness of the particulate films. This effect is attributed to reduction of the length of partially desorbed segments of macromolecules protruding into the aqueous phase at a distance of about 70 nm as estimated from force-distance curves. Aggregation of nanoparticles on these segments is responsible for a relatively high degree of disorder in layer-by-layer self-assembled films. A brief MW exposure results in cross-linking of polyelectrolyte chains. This substantially reduces the number of loose segments, and improves 2D packing of nanoparticles. For optimized conditions, the rms roughness, R, of magnetite self-assembled films can be as low as 1.5-3.5 nm. For silica-coated magnetite, initial R of adsorbed films is typically 10-14 nm. It can be reduced to 5.5 nm following the MW treatment of the polyelectrolyte; however, this does not completely prevent the formation of multiparticle 3D aggregates. Further reduction of R to 3.5 nm can be achieved by a brief ultrasonication of the nanoparticulate film, which removes weakly attached particles.

Introduction Magnetic nanoparticles (quantum dots) organized in two dimensions permit the investigation of lateral particleto-particle charge transport2 and nanoscale magnetooptic phenomena.3,4 The fundamental importance of these phenomena is augmented by promising storage characteristics of memory devices from 2D nanoarrays5 and magnetically controlled single-electron devices.6-8 Existing methods of preparation of nanoarrays of magnetic nanoparticles are quite limited and experimentally challenging.5,9-11 Colloid chemistry offers a viable alternative to traditional “physical” methods such as nanolithography, molecular beam epitaxy, and laser patterning. Layer-by-layer self-assembly (LBL) is based on sequential dipping of a substrate in solutions (dispersions) of oppositely charged species1,12-23 and produces multilayer assemblies held together by the combination of attractive electrostatic and dispersive forces. This technique enables a relatively fast buildup of multilayer thickness and easy †

University of Vigo. Hahn-Meitner Institut. I Oklahoma State University. | E-mail: [email protected]. ⊥ E-mail: [email protected]. ‡

(1) Decher, G. Science 1997, 277, 1232-1237. (2) Wiesendanger, R. MRS Bull. 1997, 22, 31-35. (3) Eggers, G.; Rosenberger, A.; Held, N.; Fumagalli, P. Surf. Interface Anal. 1997, 25, 483. (4) Kang, Y. S.; Risbud, S.; Rabolt, J.; Stroeve, P. Langmuir 1996, 12, 4345-4349. (5) Krauss, P. R.; Chou, S. Y. Appl. Phys. Lett. 1997, 71, 3174-3176. (6) Wiesendanger, R.; Bode, M.; Kleiber, M.; Lohndorf, M.; Pascal, R.; Wadas, A.; Weiss, D. J. Vac. Sci. Technol. B. 1997, 15, 1330-1334. (7) Haug, R. J.; Weis, J.; Blick, R. H.; Vonklitzing, K.; Eberl, K.; Ploog, K. Nanotechnology 1996, 7, 381-384. (8) Weinmann, D.; Hausler, W.; Kramer, B. Ann. Phys. Leipzig 1996, 5, 652-695.

automatization of the deposition process. The tradeoff for the facilitation of film preparation is a quite low degree of order manifesting in washed-out interlayer boundaries.1,13,24-26 Therefore, sandwich films prepared by LBL suffer from poor ordering13,27,28 while enabling distinct alteration of properties in the vertical direction at length scale larger than the polyelectrolyte layer thickness (2 nm).26 (9) Bian, B.; Hirotsu, Y. Jpn. J. Appl. Phys., Part 2 1997, 36, L1232L1235. (10) Chou, S. Y. Proc. IEEE 1997, 85, 652-671. (11) Winzer, M.; Kleiber, M.; Dix, N.; Wiesendanger, R. Appl. Phys. A.sMater. Sci. Process. 1996, 63, 617-619. (12) Ferguson, G. S.; Kleinfeld, E. R. Adv. Mater. 1995, 7, 414-416. (13) Keller, S. W.; Kim, H. N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817-8818. (14) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065-13069. (15) Kotov, N. A.; Dekany, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637. (16) Kunitake, T. Macromol. Symp. 1995, 98, 45-51. (17) Laschewsky, A.; Wischerhoff, E.; Bertrand, P.; Delcorte, A. Macromol. Chem. Phys. 1997, 198, 3239-3253. (18) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117-6123. (19) Saremi, F.; Maassen, E.; Tieke, B.; Jordan, G.; Rammensee, W. Langmuir 1995, 11, 1068-1071. (20) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61. (21) Vonklitzing, R.; Mo¨hwald, H. Macromolecules 1996, 29, 69016906. (22) (a) Vonklitzing, R.; Mo¨hwald, H. Thin Solid Films 1996, 285, 352-356. (b) Donath, E.; Walter, D.; Shilov, V. N.; Knippel, E.; Budde, A.; Lowack, K.; Helm, C. A.; Mo¨hwald, H. Langmuir 1997, 13, 52945305. (23) Yoo, D.; Wu, A. P.; Lee, J.; Rubner, M. F. Synth. Met. 1997, 85, 1425-1426. (24) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772-777. (25) Korneev, D.; Lvov, Y.; Decher, G.; Schmitt, J.; Yaradaikin, S. Physica B 1995, 213, 954-956. (26) Kotov, N. A.; Haraszti, T.; Turi, L.; Zavala, G.; Geer, R. E.; Dekany, I.; Fendler, J. H. J. Am. Chem. Soc. 1997, 119, 6821-6832.

10.1021/la9805342 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/02/1998

Control of Packing Order and Surface Density

For high molecular weight polymers, the overall charge of the adsorbed polyelectrolyte exceeds that of the surfaces on which the macromolecules are adsorbed (the charge reversal effect).30,31 This effect and the thermodynamic stabilization of the adsorbed film by short range hydrophobic interactions are responsible for the layer-by-layer self-assembly. When an “overcharged” substrate is in an aqueous solution, ionic dissociation of the polyelectrolyte creates electrostatic repulsion between firmly and loosely adsorbed segments of macromolecules and pushes the latter away from the surface. The parts protruding into the bulk of the solution can serve as anchor points for oppositely charged organic and inorganic species as well as the chains positioned at the solid surface. Natural irregularity of the length of the loose segments translates into a degree of structural disorder of the LBL film. As such, the layers of polymers in LBL of oppositely charged polyelectrolytes were found to interpenetrate each other to a great extent.1,24 For organic-inorganic LBL multilayers, a drastic increase of surface roughness was observed after adsorption of very thin and flexible alumosilicate sheets.26 Precipitation of inorganic particles on the partially detached parts of polymer chains before they can reach the flat substrate surface yields agglomerates that result in unexpectedly high roughness of the films. Substantial decrease of interfacial roughness has been achieved when the propensity of the loose macromolecular segments to get away from the charged surface can be reduced by applying an external electrical field.26 The repulsive forces between loosely and firmly bound macromolecular segments can be offset by attraction to the oppositely charged electrode-substrate, which prevents formation of a “hairy” interface.22b As evidenced by surface plasmon spectroscopy, this greatly improves the uniformity and vertical ordering of the hybrid LBL films.26 Assembly of 2D arrays of Fe3O4 and SiO2-coated Fe3O4 nanoparticles on thin films of polyelectrolytes was used as a model system to investigate (a) methods of improving lateral ordering of layer-by-layer self-assembled films and (b) potential of nanoparticulate magnetic films prepared by this technique. A brief exposure of the polyelectrolyte layer adsorbed on silicon wafers to microwave (MW) irradiation was found to result in a marked improvement of the in-plane ordering of nanoparticles. Under optimal conditions they form a close packed monolayer with 2D domains where topographic variations are less than a half of the particle diameter. The combination of SiO2-coated and uncoated nanoparticles offers a unique opportunity to study collective effects for charge transport, magnetic, and magnetooptic phenomena in 2D arrays. Silica shell forms an insulating layer, which controls electron tunneling between the particles.32,33 Gradual growth of the of primary silica coating by controlled hydrolysis of silica precursors34 also allows for variation of the distance between the cores (27) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370-373. (28) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065-13069. (29) Menager, C.; Cabuil, V. Colloid Polym. Sci. 1994, 272, 12951299. (30) Hong, J. D.; Lowack, K.; Schmitt, J.; Decher, G. In Trends in Colloid and Interface Science; Laggner, P., Glatter, O., Eds.; Dr Dietrich Steinkop: Berlin, 1993; pp 98-102. (31) Sukhorukov, G. B.; Mo¨hwald, H.; Decher, G.; Lvov, Y. M. Thin Solid Films 1996, 285, 220-223. (32) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329-4335. (33) Correa-Duarte, M. A.; Giersig, M.; Liz-Marza´n, L. Chem. Phys. Lett. 1998, 286, 497-501. (34) Donselaar, L. N.; Philipse, A. P.; Suurmond, J. Langmuir 1997, 13, 6018-6025.

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within 1-10 nm, which is likely to affect the pattern of magnetic coupling between the particles. Experimental Section Silicon wafers (Virginia Semiconductors) were used as received. Microscope cover glass slides were cut in ca. 0.7 × 1.5 cm pieces and etched by Ar plasma for 30 s in a Denton Desktop II magnetron sputterer/etcher at 120 mTorr of Ar to remove surface contaminants. A 20% aqueous solution of high molecular weight poly(dimethyldiallylammonium chloride), PDDA, 400 000500 000 (Aldrich), was diluted 20 times to obtain 1% solution which was used for film preparation. Films of medium (200 000350 000) and low (100 000-200 000) molecular weight PDDA for AFM measurements were prepared similarly. FeSO4, FeCl3, HCl, NH4OH, and tetramethylammonium hydroxide (all Aldrich) and sodium silicate solution (27 wt % SiO2, Aldrich) were used as received. Milli-Q deionized water (18 MΩ) was used for all the preparations. The samples for transmission electron microscopy (TEM) were prepared so that a layer of magnetic nanoparticles was deposited only on one side of the carbon coated TEM grid. TEM images were taken with a Phillips CM-12 instrument operating at an acceleration voltage of 120 kV, equipped with a high-resolution lens and 9800 EDAX analyzer. High-resolution images were digitally recorded with a CCD camera. Aqueous dispersions of magnetite nanoparticles were prepared according to Massart’s method:35 20 mL of aqueous 1 M FeCl3 and 5 mL of 2 M FeSO4 in 2 M HCl were added to 250 mL of NH4OH 0.7 M under rapid mechanical stirring. Stirring was allowed to continue for 30 min, and then the black solid product was decanted with the help of a magnet. The sediment was then redispersed in 50 mL of distilled water, and subsequently three aliquots of 10 mL tetramethylammonium hydroxide solution (1 M) were added, again with rapid stirring. Finally, water was added to the dispersion up to a total volume of 250 mL. To homogeneously coat the particles with silica, the method of Philipse et al.36 was used with some modifications taken from our own experience with the silica coating of metal32 and semiconductor33 nanoparticles. First 4 mL of the resulting aqueous ferrofluid was further diluted up to 100 mL, and then 4 mL of 0.54 wt % sodium silicate solution at pH ) 10.5 was added under stirring. The pH was then adjusted to 10 with 0.5 M HCl, and the colloid was stirred for about 2 h and allowed to stand for 4 days. Self-assembly of magnetic nanoparticles started with the adsorption of a layer of PDDA for 2 min followed by thorough washing of a substrate with deionized water in three separate beakers. After drying in air, the substrate was placed in the center of a conventional 600 W (Panasonic) microwave oven producing a required broad range MW spectrum. The power settings were similar to those used previously for cross-linking of polymers and acceleration of organic reactions.37,38 After MW irradiation, the sample was immersed into a dispersion of magnetic nanoparticles for at least 5 min followed by extensive rinsing with deionized water in two beakers. The substrate with the layer of nanoparticles was stored in dust-free environment under ambient conditions. Atomic force microscopy (AFM) images were taken by using a Nanoscope IIIa instrument operating in the tapping mode with standard silicon nitride tips. Typically, the surface was scanned at 2 Hz with 256 lines per image resolution and 1.2-4.0 V setpoint. No filter techniques have been applied to the images presented. Roughness evaluation was performed by using a built-in image analysis software. The force-distance curves were taken in aqueous media in a standard DI liquid cell by using contact mode 200 µm silicon nitride tips with a spring constant of 0.06 N/m. The size of the silicon wafer samples was chosen so that the plastic O-ring is completely underlain by the sample surface in order to avoid contamination of the 18 MΩ water filling the cell caused by the contact with adhesive and metal puck. The force-distance curves were found to depend on positioning of the laser beam on (35) Massart, R. IEEE Trans. Magn. 1981, MAG-17, 1247. (36) Philipse, A. P.; van Bruggen, M. P. B.; Pathmamanoharan, C. Langmuir 1994, 10, 92-99.

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Figure 1. Transmission electron micrographs of uncoated Fe3O4 nanoparticles. The lattice planes observed on TEM images correspond to cubic magnetite, i.e., 4.8 Å for (111) and 2.9 Å for (220) planes, respectively.

Figure 3. UV-vis (a) and near-IR (b) spectra of PDDA-Fe3O4 composite deposited onto a quartz slide. Absorption spectra in UV-vis and near-IR regions were taken with a Hewlett-Packard 8453A and a Perkin-Elmer 2000 spectrophotometer, respectively. The samples were deposited on microscope cover glass slides by LBL technique following by drying in air.

Results and Discussion

Figure 2. Transmission electron micrographs of SiO2-coated Fe3O4 nanoparticles at (a) low and (b) high magnification. The silica shell can be observed as a light area surrounding the darker cores (ca. 3 nm thick). cantilever, and, therefore, all data provided were obtained with one AFM tip without changing the laser alignment. An adjustment of the vertical position of the detector was still necessary possibly due to residual mechanical deformation of the cantilever’s arms. Before taking a measurement on a new sample, the cell and the cantilever were rinsed in situ with a continuous stream of water in the amount of 5 mL.

Nanoparticles. TEM images indicate (Figures 1 and 2) that magnetite nanoparticles are nearly perfect monocrystals though their shape is somewhat irregularsfrom oval to spherical. The measured lattice constants correspond to cubic Fe3O4 (Figure 1), and the average diameter was found to be 12 nm. For silica-coated samples, a homogeneous silica shell about 3 nm thick was observed to be deposited onto every magnetite particle. TEM also shows (Figure 2) that no aggregation occurred during this process, so that no more than one spherical core is included in each composite particle. This result constitutes a marked improvement over the method of Philipse et al.,34,36 which leads to thick coatings through silica deposition in ethanol, but no well-defined silica shells are observed in aqueous solution. The optical properties of all the magnetic nanoparticles used (with or without silica coating) are dominated by a broad featureless absorption tail characteristic of indirect band gap semiconductors39,40 (Figure 3a). Importantly, in the near-IR region (Figure 3b) the Fe3O4-PDDA composite has a window of transparency around 1500 nm. This region is particularly significant for fiber-optic communication technologies and magnetooptic devices. Film Preparation. Magnetic nanoparticles prepared in basic media are negatively charged and therefore are electrostatically attracted to positively charged poly-

Control of Packing Order and Surface Density

Langmuir, Vol. 14, No. 22, 1998 6433

Figure 4. Atomic force microscopy image of uncoated Fe3O4 nanoparticles: (a) without microwave irradiation of the polyelectrolyte layer, (b) after 1200 J (2 s at 600 W) dose of MW, and (c) after 6000 J (10 s at 600 W). Arrows in image a indicate 3D aggregates.

Figure 5. Atomic force microscopy images of a Si substrate: (a) bare Si wafer; (b) Si wafer with a PDDA layer; (c) Si wafer with a PDDA layer after MW treatment (dose ) 12 000 J).

(dimethyldiallylammonium chloride) layer adsorbed on silicon or glass surface. The pH for the most efficient adsorption of the Fe3O4 nanoparticles on polyelectrolyte was found to be 11.9-12.0 and 10.0-11.0 for “naked” and silica-coated particles, respectively. At higher pH values the resulting particle density rapidly decreases (at pH ) 13 no adsorption was observed), whereas for lower pH values the irreversible agglomeration of magnetite dispersions occurs. Atomic force microscopy image of “naked” magnetite film revealed multiple aggregates composed of 3-10 nanoparticles clumped together (Figure 4a). The elevation of the ionic strength and pH, which promotes a higher surface charge of Fe3O4, does not diminish but rather increases the number of particles in aggregates. In a dry state, the surface of the adsorbed polyelectrolyte film (Figure 5a, b) approaches that of underlying substrate, i.e. for silicon wafers variations in surface profile do not exceed 1 nm over the 1 × 1 µm area and no nonuniform thickening of the polyelectrolyte film can be observed. Retaining a similar flatness of the surface is a prerequisite for the preparation of well-ordered films. However, when in solution, the mutual repulsion of strongly charged macromolecular segments results in a “hairy” interface: parts of polyelectrolyte molecules protrude into the solution, being in a dynamic equilibrium between adsorbed and dissolved state. Existence of such detached segments of the polyelectrolyte chain can be seen in the forcedistance curves (Figure 6) measured in aqueous media. As the AFM tip approaches the surface, it experiences a repulsive force at a certain distance from the silicon substrate. The point where the cantilever starts deflecting can be denoted as a first-contact point A. Further advancement toward the surface results in a gradual increase of the deflection, l, until the point of a direct

Figure 6. Force-distance plots for three different surfaces: (a) bare Si wafer; (b) Si wafer with a PDDA high molecular weight monolayer; (c) Si wafer with PDDA after MW treatment (dose ) 12000 W). The left curve in each pair corresponds to approach to the surface (down stroke), and the right curve is removal of the tip (up stroke). The upper inflection point of the plots (point B) corresponds to a direct contact between the tip and Si surface. The lower inflection point (point A) corresponds to the distance at which the tip starts feeling the presence of the surface.

contact between the tip and Si wafer is reached, B, which can be used as a reference point for all the force curves obtained. The difference between the vertical piezo positions, x, in points A and B carries the information of how far away from the physical boundary of the Si wafer the tip-surface interactions can be sensed. Structural characteristics of the surface also manifest in ∂l/∂x derivative related to the “softness” of the interface in each

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Table 1. Force-Distance Curve Characteristics of Si Wafers with a Layer of Regular, High Molecular Weight, PDDA (Si-PDDA), PDDA after Microwave Treatment (Si-PDDA-MW), Medium Molecular Weight PDDA (Si-PDDA-M), and Low Molecular Weight PDDA (Si-PDDA-L) sample

x(A) - x(B) nm

∂l/∂x(A) × 104

∂l/∂x(B) × 104

Si wafer Si-PDDA Si-PDDA-MW Si-PDDA-M Si-PDDA-L

500 570 555 558 535

-6.5 -5.4 -5.9 -5.9 -6.1

-3.6 -3.5 -3.3 -3.5 -3.4

point of the curve: the greater the absolute value of the derivative, the harder the material. Comparison of x(A) - x(B) values for Si wafer with and without a layer of PDDA (Table 1) shows that the effect of polymeric interface extends by 70 nm further than that of native silicon. Since silicon nitride tips carry virtually no charge on their surface, this should be attributed to the contact of the tip with the macromolecular segments effectively extending the interface further into solution. The distance of 570-500 ) 70 nm, describing the effect of a “hairy” interface, corresponds to a ca. 200 monomer unit segment of fully extended PDDA. High molecular weight PDDA typically used for layer-by-layer selfassembly has a molecular weight of 400 000-500 000, and therefore, on average, detached segments amount to ca. 7% of the total number of monomer units in a chain. The length of the detached segment is determined by the probability of desorption. For polymers with shorter chain length, the length of the detached segments should also be shorter. Indeed, progressively smaller x(A) - x(B) values are obtained with the decrease of the molecular weight of PDDA (Table 1). The value of ∂l/∂x in point A is noticeably smaller for silicon bearing polyelectrolyte than for native wafers. The initial part of the force curve corresponds to a compression of the extended macromolecular segments, which cushion the tip approach. The longer the segments, the softer the initial part of the force-distance curves. Oppositely, ∂l/ ∂x values in point B, where the AFM probe is in the direct contact with Si wafer, are very close and are virtually independent of the coating. The “hairy” interface22b,26 and washed-out interlayer boundaries1,13,24-26 associated with it decrease the degree of order in layer-by-layer self-assembled films. Better ordered structures can be obtained after tight cross-linking of the polymer chains, which reduces the tendency of loosely adsorbed segments of the polyelectrolyte to detach from the substrate. Restriction of the mobility of polymer chains in aqueous solutions by using photopolymerizable polyelectrolytes17 with polystyrene pending group gave little improvement of the packing of montmorillonite sheets as evidenced by X-ray diffraction and atomic force microscopy.41 This can be attributed to a fairly low density of interchain connections that can be achieved in the chaotic solid state as opposed to photoreactive LangmuirBlodgett films.42-45 (37) Albert, P.; Holderle, M.; Mulhaupt, R.; Janda, R. Acta Polym. 1996, 47, 74-78. (38) Jacob, J.; Chia, L. H.; Boey, F. Y. J. Appl. Polym. Sci. 1997, 63, 787-797. (39) Dagan, G.; Shen, W. M.; Tomkiewicz, M. J Electrochem. Soc. 1992, 139, 1855-1861. (40) Park, J. H.; Tjeng, L. H.; Allen, J. W.; Metcalf, P.; Chen, C. T. Phys. Rev. BsCondens. Matter 1997, 55, 12813-12817. (41) Kotov, N. A.; Wischerhoff, E. Unpublished results. (42) Aramata, K.; Kamachi, M.; Takahashi, M.; Yamagishi, A. Langmuir 1997, 13, 5161-5167.

Figure 7. Dependence of rms roughness on the microwave dose for (a) uncoated and (b) silica-coated Fe3O4 particles.

Rather indiscriminate participation of various chemical groups in the cross-linking process is expected to provide firmly bound polymer monolayers with a high density of interchain covalent bonds.46 Broad band microwave irradiation (MW) of organic substances was shown to produce large quantities of free radicals by breaking C-H bonds,47 to assist free radical polymerization and to induce cross-linking.37,38 AFM images of a PDDA layer on silicon after MW treatment at power settings 60-600 W were similar to those of untreated surfaces (Figure 5). A little change of the surface relief corresponds most likely to some shrinking of the polymer film caused by cross-linking. The chemical nature of the charged headgroups is likely to be affected only for an extended exposure to MW. Since PDDA is highly charged, moderate MW doses can give a substantial degree of cross-linking, while maintaining a charge density sufficient for adsorption of nanoparticles. The force curves in aqueous media (Figure 6) indicate reduction of the number of partially desorbed segments after MW treatment: x(A) - x(B) and ∂l/∂x(A) parameters become intermediate between those of Si and Si-PDDA and approach the data obtained for short-chain PDDA (Table 1). “Naked” Magnetite Nanoparticles. After adsorption of the magnetite nanoparticles, a marked reduction of the number of aggregates and improvement of packing order of nanoparticles can be seen. (Figure 4b). The gradual reduction of surface roughness of the nanoparticulate layer can be observed with the increase of the dose (Figure 7a). For optimal settings, i.e. 2 s at 600 W, the rms roughness parameter, R, for 1 × 1 µm and 5 × 5 µm areas becomes 1.6 and 3.5 nm, respectively, which is much smaller than the average particle diameter of 12 nm. For doses >6000 J, a rapid decline of magnetite density leading eventually to appearance of only single, well-separated nanoparticles (43) Balasubramanian, K. K.; Cammarata, V. Langmuir 1996, 12, 2035-2040. (44) Noordegraaf, M. A.; Kuiper, G. J.; Marcelis, A. T.; Sudholter, E. J. Macromol. Chem. Phys. 1997, 198, 3681-3697. (45) Seki, T.; Tanaka, K.; Ichimura, K. Adv. Mater. 1997, 9, 561. (46) Delacruz, P.; Delahoz, A.; Langa, F.; Illescas, B.; Martin, N.; Seoane, C. Synth. Met. 1997, 86, 2283-2284. (47) Fujii, T. J. Appl. Phys. 1997, 82, 2056-2059.

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Figure 8. Atomic force microscopy image of silica-coated Fe3O4 nanoparticles: (a) without microwave irradiation of the polyelectrolyte layer; (b) after 4500 J (15 s at 300 W) dose of MW; c and d after 1800 J (3 s at 600 W) followed by ultrasonication. Arrows in image d indicate nanoscale stacking faults. Please note the difference in the veritcal scales for images a, b, and c.

can be observed (Figure 4c), most likely, due to chemical degradation of the thin polyelectrolyte layer and decrease of charge density. Silica-Coated Magnetite Nanoparticles. As expected, the direct assembly of silica-coated magnetic nanoparticles on PDDA results in a rather coarse film with a large number of aggregates and islands of thick multilayer stacks (Figure 8a). Microwave exposure reduces the overall roughness of the films (Figure 7b). Similarly to “naked” magnetite, for overexposed films which received 6000 J or more, the continuous interconnected film breaks apart into 0.5-1 µm domains (Supporting information, Figure S1) and eventually to single particles. Under optimal conditions (15 s at 300 W) the rms roughness of the films was reduced from 13.6 to 5.5 nm, while the range of the topographic features decreased from 348 to 47 nm. Nevertheless, 3D aggregates can still be observed, although in substantially reduced numbers as compared to the untreated films (Figure 8b). Zooming in relatively flat areas in Figure 8b produces images of nanoparticles similar to Figure 4a, which indicates that the surface is completely covered with nanoparticles. The difficulty of producing well-packed films in this case can be attributed to the greater mass of nanoparticles after SiO2 coating, which facilitates aggregation. The problem can be alleviated by a brief ultrasonication of the substrate bearing magnetite films. Loosely bound species are mechanically removed from the surface by the powerful cavitation force.48 AFM shows that the ultrasonic treatment for 30 s produces much smoother films (Figure 8c) with R ) 3.6 nm, which is virtually identical to the roughness of “naked” nanoparticles despite the increased particle size, however, packing imperfections, like the ones marked in Figure 8d, can still be observed. Ultrasoni(48) No change in intensity of excitonic (near band gap) and trapped emission, of SiO2-coated CdS nanoparticles prepared according to ref 33 was observed after 5 min of sonication. Since emission properties of semiconductor nanoparticles are particularly sensitive to surface modification, SiO2 coating is unlikely to be seriously damaged by this procedure.

cation without MW treatment results only in marginal decrease of the surface roughness to R ) 9.5 nm. Conclusions The principle of layer-by-layer self-assembly can be applied to the preparation of monoparticulate films of magnetite and SiO2-coated magnetite nanoparticles. Extensive cross-linking of polyelectrolyte film in the solid state is achieved by a brief exposure of the polyelectrolyte monolayer to microwave irradiation. This results in substantial reduction of agglomeration of nanoparticles on partially desorbed segments of polyelectrolyte chains, which are likely to be the principal source of disorder in layer-by-layer self-assembled films. Overall roughness of nanoparticulate films is diminished and packing order of nanoparticles is greatly improved. This provides essential insight into the mechanism of defect formation in LBL assemblies. The quality of films of magnetic particles is similar or better than those produced by vacuum methods,9-11,49 which opens an opportunity for exploration of their magnetic and magnetooptic properties. Acknowledgment. The authors acknowledge the financial support from NATO, Collaborative Research Grant no. CRG 971167. L.M.L.M. acknowledges the Spanish Consellerı´a de Educacio´n e Ordenacio´n Universitaria (Xunta de Galicia), Project No. XUGA30105A97, for financial support. The authors thank Dr. M. Fisher from Nomadics, Stillwater, OK, for help with IR absorption measurements. Supporting Information Available: Figure S1, showing an AFM image of a film from silica-coated magnetite particles subjected to MW treatment with a dose of 10 000 J (1 page). Ordering and Internet access information is given on any current masthead page. LA9805342 (49) Guo, L. J.; Leobandung, E.; Chou, S. Y. Science 1997, 275, 649651.