Magnetic-Field-Assisted Layer-by-Layer Electrostatic Assembly of

May 14, 2010 - To make them hydrophilic, the substrates were kept in a mixture of NH4OH/H2O2/H2O (1:1:5) at 70 °C for 30 min; they were then washed ...
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Magnetic-Field-Assisted Layer-by-Layer Electrostatic Assembly of Ferromagnetic Nanoparticles Sukumar Dey, Kallol Mohanta, and Amlan J. Pal* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India Received January 15, 2010. Revised Manuscript Received May 4, 2010 We report that layer-by-layer (LbL) electrostatic assembly of Fe3O4 nanoparticles can be supplemented by orienting magnetic domains of the nanoparticles. With the oriented domains of ionic-capped nanoparticles, both magnetic and electrostatic forces of attraction become operative during the LbL deposition process. The magnetic-field-assisted LbL adsorption process has been evidenced by increased electronic absorbance of the films. While atomic force microscopy studies rule out formation of multiple layers during a single adsorption process, magnetic force microscopy images evidence oriented domains in the LbL films. The results show a novel route for LbL deposition of ferromagnetic nanoparticles with oriented magnetic domains in the thin films.

Introduction As modern technologies aim toward miniaturization of electronic, optoelectronic, or magnetic devices, the methods to grow ultrathin films are gaining more importance than previously. Various methods of thin-film deposition, such as thermal evaporation under vacuum, drop-casting, spin-casting, sol-gel technique, Langmuir-Blodgett (LB), and layer-by-layer (LbL) electrostatic self-assembly,1-5 have been chosen to suit one need or another. While some of the above methods form films in a macroscopic scale, the latter two techniques, along with the sophisticated molecular beam epitaxy (MBE) method, deal with films in true molecular scales. In the recent past, the LbL deposition technique has attracted interest due to its applications in diverse materials, such as polyions,6 dyes,7 proteins and enzymes,8 metal or semiconducting nanoparticles,9-13 and so on. Evolved in the 1990s by Decher for inert polyions,1 the technique has flourished and has been practiced by many research groups working with various materials and applications in mind.1-13 The mechanism of the LbL deposition process relies on surface-charge reversal during adsorption of each layer in sequence through electrostatic assembly. Hence, the materials used in this process have more than one *To whom correspondence should be addressed. E-mail: [email protected]. Telephone: þ91-33-24734971. Fax: þ91-33-24732805.

(1) Decher, G. Science 1997, 277, 1232–1237. (2) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117–6123. (3) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309– 4318. (4) Clark, S. L.; Hammond, P. T. Adv. Mater. 1998, 10, 1515–1519. (5) Caruso, F.; Yang, W. J.; Trau, D.; Renneberg, R. Langmuir 2000, 16, 8932– 8936. (6) Onitsuka, O.; Fou, A. C.; Ferreira, M.; Hsieh, B. R.; Rubner, M. F. J. Appl. Phys. 1996, 80, 4067–4071. (7) Das, S.; Pal, A. J. Langmuir 2002, 18, 458–461. (8) He, J.-A.; Samuelson, L.; Li, L.; Kumar, J.; Tripathy, S. K. Langmuir 1998, 14, 1674–1679. (9) Cho, J. H.; Caruso, F. Chem. Mater. 2005, 17, 4547–4553. (10) Cassagneau, T.; Mallouk, T. E.; Fendler, J. H. J. Am. Chem. Soc. 1998, 120, 7848–7859. (11) Hao, E. C.; Lian, T. Q. Langmuir 2000, 16, 7879–7881. (12) Mohanta, K.; Majee, S. K.; Batabyal, S. K.; Pal, A. J. J. Phys. Chem. B 2006, 110, 18231–18235. (13) Lee, D.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2006, 6, 2305–2312.

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charge center in their functional groups. Accordingly, ionic functional groups of the molecules or nanoparticles that in turn form the basis of electrostatic force play a key role in the LbL assembly process. It can be intriguing to introduce an additional force that may supplement the electrostatic adsorption process. An external electric field has been considered for LbL deposition of suitable organic molecules or nanoparticles.14,15 Magnetic domains that are inherently present in some nanoparticles can be another direction in this regard. Here, we chose a ferromagnetic material in its nanoparticle form16-19 and aimed to form a LbL assembly20-23 with extra assistance from oriented magnetic domains (in addition to the usual electrostatic forces). Since the magnetic domains can be oriented with a suitable magnetic field, our target here is to enhance the electrostatic adsorption process through magnetic force of attraction. We show how oriented magnetic domains of magnetite in its nanoparticle form can accelerate the adsorption process of subsequent monolayers in the LbL assembly process.

Materials and Methods Growth of Nanoparticles. Fe3O4 nanoparticles were grown from ferric acetyl acetonate, 1-dodecanediol (DCD), oleic acid, oleylamine, diphenyl ether (DPE), N-(3-aminopropyl) methacrylamide hydrochloride (AMH), methylenebisacrylamide (MBA), (14) Gao, M. Y.; Sun, J. Q.; Dulkeith, E.; Gaponik, N.; Lemmer, U.; Feldmann, J. Langmuir 2002, 18, 4098–4102. (15) Wang, Y.; Wang, X. J.; Guo, Y.; Cui, Z. C.; Lin, Q.; Yu, W. Z.; Liu, L. Y.; Xu, L.; Zhang, D. M.; Yang, B. Langmuir 2004, 20, 8952–8954. (16) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Chem. Mater. 2001, 13, 109–116. (17) Zhu, X. L.; Han, K.; Li, G. X. Anal. Chem. 2006, 78, 2447–2449. (18) Alam, S.; Anand, C.; Ariga, K.; Mori, T.; Vinu, A. Angew. Chem., Int. Ed. 2009, 48, 7358–7361. (19) Wang, X.; Zhou, S. Y.; Lai, Y.; Sun, J. Q.; Shen, J. C. J. Mater. Chem. 2010, 20, 555–560. (20) Suda, M.; Einaga, Y. Angew. Chem., Int. Ed. 2009, 48, 1754–1757. (21) Paterno, L. G.; Soler, M. A. G.; Fonseca, F. J.; Sinnecker, J. P.; Sinnecker, E.; Lima, E. C. D.; Bao, S. N.; Novak, M. A.; Morais, P. C. J. Nanosci. Nanotechnol. 2010, 10, 2679–2685. (22) Gorin, D. A.; Portnov, S. A.; Inozemtseva, O. A.; Luklinska, Z.; Yashchenok, A. M.; Pavlov, A. M.; Skirtach, A. G.; Mohwald, H.; Sukhorukov, G. B. Phys. Chem. Chem. Phys. 2008, 10, 6899–6905. (23) Nakamura, M.; Katagiri, K.; Koumoto, K. J. Colloid Interface Sci. 2010, 341, 64–68.

Published on Web 05/14/2010

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and tetramethylethylenediamine (TED).24 In 20 mL of ferric acetyl acetonate solution (2 mM) in DPE, a mixture of 2.023 g of DCD (10 mM), 1.605 g of oleylamine (6 mM), and 1.694 g of oleic acid (6 mM) were added. The mixed solution was thoroughly degassed by purging N2. The temperature of the solution was raised to 200 C under vigorous stirring conditions to allow the reaction to continue for 30 min. The solution was then refluxed for another 30 min at 250 C. The color of the solution turned blackish, indicating formation of oleic acid-capped Fe3O4 nanoparticles. The product was cooled down to room temperature. It was repeatedly washed in n-hexane and centrifuged at 8000 rpm to separate out the nanoparticles, which were finally dried in vacuum.

Ligand Exchange to Disperse the Nanoparticles in Water. Under ambient conditions, these nanoparticles were redispersed in n-hexane (∼20 mg/mL). A suspension of tetramethyl ammonium 11-aminoundecanoate (TAU) in dichloromethane (20 mg/ mL) was then added. After stirring vigorously for 30 min, the particles were separated by using a magnet. The nanoparticles grown through the above-mentioned protocol had anionic functional groups (-COO- of TAU) as capping agents.24 Since oppositely charged particles were required to continue the LbL deposition process, we also formed nanoparticles with cationic groups on the surface via polymer coating. In brief, a 10 mL solution was prepared with (70 mg) oleic acid-capped Fe3O4 nanoparticles, 36 mg of AMH, 3 mg of MBA as a crosslinker, and 100 μL of TED;25 an optically clear solution was formed with -NH2þ groups on the surface. The solution was taken in a three-neck round-bottom flask and put under an oxygen-free atmosphere by purging N2 for 15 min. Finally, ammonium persulfate solution (3 mg dissolved in 100 μL of water) was injected as a radical initiator to start the polymerization. The polymerization was continued at room temperature for 1 h. The particles were precipitated by adding a few drops of ethanol. The nanoparticles were washed with chloroform and ethanol and finally dissolved in doubly distilled water. Characterization of Nanoparticles. The nanoparticles with both types of capping agents were characterized with electronic absorption spectroscopy and high-resolution transmission electron microscopy (HR-TEM, Jeol JSM 2010). Energy dispersive X-ray analysis (EDXA) was carried out with a scanning electron microscope (SEM), and selected-area electron diffraction (SAED) patterns were recorded with a transmission electron microscope (TEM). Growth of LbL Films. LbL films of the Fe3O4 nanoparticles were grown following standard procedure. Quartz or Æ111æ Si substrates were cleaned through successive sonication in soap solution, water, acetone, and methanol for 15 min each. The cleaned substrates were then dried with hot air. To make them hydrophilic, the substrates were kept in a mixture of NH4OH/ H2O2/H2O (1:1:5) at 70 C for 30 min; they were then washed thoroughly with deionized water. The substrates were dipped in a dispersed solution of cationic-capped nanoparticles for 15 min to adsorb a monolayer of the nanoparticles. To remove electrostatically unbound moieties on the surface, the films were thoroughly washed by dipping the substrates in three separate deionized water baths. A layer of anionic-capped nanoparticles was similarly adsorbed followed by washings in a separate set of water baths. This completed one bilayer of the nanoparticles. The dipping sequence was cycled to obtain a desired number of bilayers. Magnetic Field to Accelerate LbL Assembly. Apart from the conventional LbL deposition, we have aimed to form LbL films with assistance from oriented magnetic domains. For magnetic-field-assisted LbL assembly, we put the monolayer in a (24) Sun, S. H.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. X. J. Am. Chem. Soc. 2004, 126, 273–279. (25) Wei, Y. F.; Jana, N. R.; Tan, S. J.; Ying, J. Y. Bioconjugate Chem. 2009, 20, 1752–1758.

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Figure 1. (a) TEM image of Fe3O4 nanoparticles. Upper and lower inserts show HR-TEM images of amine-capped and acidcapped Fe3O4. (b) Selected area electron diffraction patterns of the nanoparticles. The numbers in the figures indicate the planes for the corresponding diffraction line (JCPDS file number 190629). (c) EDX analysis of the particles obtained from SEM characteristics. The table within the picture shows the atomic weight percent of the elements present in the nanoparticles. 300 mT magnetic field for 15 min, with the field oriented magnetic domains of the nanoparticles perpendicular to the substrate. We then continued with adsorption of the next monolayer. Here the oriented domains of the first monolayer accelerate LbL assembly of the second monolayer. We applied the magnetic field after deposition of every LbL layer (and three washings) and continued for many bilayers. We also studied if the oriented magnetic domains had their effect active over deposition of a number of layers. To do so, magnetic domains were oriented by applying a 300 mT field until deposition of five bilayers; thereafter, usual LbL Langmuir 2010, 26(12), 9627–9631

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Figure 2. Electronic absorption spectra of Fe3O4 nanoparticles dispersed in water. Legend specifies the capping agent on the nanoparticles.

deposition was continued without further application of the magnetic field. Control experiments were carried out by comparing (i) an LbL film deposited without applying any magnetic field and (ii) placing the same film in a magnetic field to orient the magnetic domains of the nanoparticles in the film. This comparison was done for LbL films of different numbers of bilayers. Characterization of the Films. In all the cases, electronic absorption spectra of the thin films were recorded after deposition of every layer of nanoparticles. From atomic force microscopy (AFM) of a scratch on the film, the depth profile and hence the thickness of each of the films were estimated. The AFM topographies also gave the roughness of the surface of the films. Orientation of the magnetic domains in the films was recorded by magnetic force microscopy (MFM) images. The MFM images were recorded with a magnetized tip that recognized magnetic phases of the films. The images were scanned in a phase-lift mode. While MFM studies were carried out with a Veeco CPII instrument, AFM images were recorded with a Nanosurf EasyScan2 apparatus in ambient conditions.

Results and Discussion Characterization of Nanoparticles. Colloidal solutions of the nanoparticles were stable for months without any agglomeration or precipitation. This must be due to the higher zeta potential of the particles (as such, the zeta potential of cationic nanoparticles synthesized through this route is around þ35 mV25). TEM images of the nanoparticles are shown in Figure 1. TEM characteristics of the particles revealed that the size of the particles was reasonably monodispersed. HR-TEM images of the nanoparticles with different capping agents are also shown as insets of the figure. From the HR-TEM images, we have estimated the size of the nanoparticles. The average size of amine-capped and acidcapped particles, which were synthesized in batches, turned out to be 7-8 nm and 6-7 nm, respectively. Figure 1 also shows selected area electron diffraction (SAED) patterns of the two types of particles. Since only the capping agent was different in the two cases, the patterns look mostly identical except some difference in intensities of diffraction lines. The lines were matched with standard data (JCPDS 190629); the (111), (220), (311), and (511) planes of Fe3O4 face-centered cubic lattice are manifested in the diffraction patterns. The (511) plane of acid-capped particles is feebly visible due to the low intensity of the line. Elemental analysis of the particles obtained from EDX analysis, as shown in Figure 1c and a table within, agreed with the expected composition of Fe3O4. Electronic absorption spectra of a dispersed solution of Fe3O4 nanoparticles with two different stabilizing agents have shown the Langmuir 2010, 26(12), 9627–9631

Figure 3. Growth of LbL films of Fe3O4 nanoparticles. Absorption spectra of LbL films of different numbers of layers are shown in (a) and (b). While (a) represents a scenario for usual LbL films without any magnetic field, the case with assistance from oriented magnetic domains is presented in (b). In (c), absorbance at 350 nm as a function of number of bilayers is shown for two different deposition processes. In (b), magnetic domains were oriented up to deposition of five bilayer LbL films.

signature of the particles in the 200-600 nm range (Figure 2). As control experiments, we recorded electronic absorption of the stabilizers alone (TAU and polyacrylate) in solution. While the low-wavelength region below 300 nm appears to be due to organic stabilizers on the surface of the nanoparticles, the low-energy continuum arises due to the nanoparticles themselves. The spectra can hence be used to monitor adsorption of the nanoparticles on the substrates during deposition of LbL films. LbL Assembly of Nanoparticles. Growth of LbL films of nanoparticles can generally be monitored by recording electronic absorption spectrum after deposition of each layer. Figure 3(a) shows such spectra for increasing number of deposited layers. Continuous increase in absorbance of the nanoparticles points toward their deposition in layer after layer thereby forming LbL films. It may be restated that cationic- and anionic-capped nanoparticles were adsorbed in sequence here; electrostatic interaction between one type of charges on the surface and the opposite type on nanoparticles in the dispersed solution was the force behind the LbL assembly process. Magnetic-Field-Assisted LbL Assembly. We then aimed to accelerate the LbL deposition process with the assistance from oriented magnetic domains of the nanoparticles on the substrate. That is, since the nanoparticles were ferromagnetic in nature, we DOI: 10.1021/la101132z

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first oriented the magnetic domains of the nanoparticles deposited as the first layer on a substrate. This was done so that the oriented domains of the nanoparticles can now attract the next layer of nanoparticles through a magnetic force of attraction also (apart from the electrostatic force due to the surface charges). We continued to orient the magnetic domains of the deposited nanoparticles after adsorption of each layer; the assistance in the LbL deposition process due to magnetic force of attraction hence does not fade out during the deposition of subsequent monolayers. We also examined the length scale (in terms of number of layers) up to which the oriented magnetic domains can have some impact on the LbL deposition process. Here, LbL deposition could be supplemented through magnetization of adsorbed nanoparticles, since magnetization in such a material is very strong and the magnetic moment of the material retains its orientation for a quite long time after withdrawal of the magnetic field. These ferromagnetic nanoparticles can be considered to be of single domain as the domain size in these materials is usually very large as compared to the diameter of the nanoparticles. In practice, a layer of cationic-capped nanoparticles was first adsorbed electrostatically on quartz substrates. The substrate with the monolayer was then placed under a 300 mT magnetic field for 15 min, so that the magnetic-domains of the nanoparticles become oriented perpendicular to the plane of the substrate. The substrate was then removed from the magnetic field and dipped in a bath of anionic-capped nanoparticles. Here, while adsorbing a layer of anionic-capped nanoparticles, both electrostatic and magnetic forces become active and lead to adsorption of an increased amount of nanoparticles on the substrate. Though it is expected that the magnetic domains of the nanoparticles adsorbed in the second layer would also become oriented perpendicular to the plane of the substrate, the alignment might not be totally perfect. To enhance the degree of orientation of the magnetic domains, the substrate containing two monolayers was again placed in a magnetic field. The sequence of processes was repeated to deposit a desired number of bilayers of magnetic-field-assisted LbL films of Fe3O4 nanoparticles. In another case, we have examined the number of layers up to which oriented magnetic domains can have some impact on the LbL deposition process. Here, after adsorption of magnetic-fieldassisted LbL films for five bilayers, we continued the deposition process without applying a magnetic field that reinforced orientation of magnetic domains. In both of the cases, that is, LbL deposition (i) without any magnetic field and (ii) with magnetic field for five bilayers and then continuing without the field, electronic absorption was recorded after deposition of every bilayer. The respective bunches of spectra are shown in Figure 3a and b, respectively. Each group of spectra can be combined as a plot of absorbance at a particular wavelength versus number of deposited bilayers. Figure 3c shows such plots for the two types of deposition processes. For the LbL deposition “without any assistance from magnetic field”, the absorbance related to the nanoparticles has grown linearly with the number of deposited LbL layers, implying a uniform adsorption process layer after layer. For the “with magnetic field” case, there are two segments in the plot. The slope of the plot was higher (as compared to the previous case) until seven bilayers; after seven bilayers, the slope became the same as that of the “without any magnetic field” case. It may be recalled that we applied magnetic field after deposition of every layer until five bilayers. This means that the LbL deposition process can be supplemented by orienting magnetic domains of the nanoparticles. The effect of the oriented domains in assisting LbL deposition continued for adsorption of nanoparticles for two more bilayers without application of a magnetic 9630 DOI: 10.1021/la101132z

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Figure 4. Absorbance at 350 nm versus number of LbL bilayers for two types of films: (i) usual LbL films and (ii) placing the film under a magnetic field of 300 mT for 15 min. Inset shows the two spectra for one case, namely, a 13 bilayer LbL film.

field that reinforces the orientation. Afterward, the usual LbL deposition process with only the electrostatic force of attraction continued. LbL Films in a Magnetic Field: Control Experiments. Since electronic absorbance was higher in the “magnetic-fieldassisted” deposition, as shown in Figure 3c, another aspect has to be considered: whether magnetically oriented nanoparticles have a higher absorbance or not. We therefore recorded electronic absorption spectra of the following two films: (i) an LbL film of a particular bilayer deposited without any magnetic field, and (ii) placing the final film in a magnetic field to orient their magnetic dipoles (inset of Figure 4). There was a marginal increase in absorbance in the latter film. Since the amount of material in the two films was exactly the same, the little increase in absorbance should be due to scattering effect as has been discussed elsewhere.26 This however cannot account for the large increase in absorbance in the “magnetic-field-assisted” LbL films, as shown in Figure 3c. In another control experiment, we recorded electronic absorption spectrum of LbL films before and after placing the films in a magnetic field separately for different numbers of deposited LbL layers. A plot of absorbance versus number of bilayers for the two cases is shown in Figure 4. For all the bilayer films, there has been a little increase in absorbance due to orientation of the magnetic domains. This however does not account for the large increase in absorbance in the “magnetic-field-assisted” LbL films, as shown in Figure 3c. Both the control experiments hence rule out the effect of scattering in Figure 3c. The increased absorbance in Figure 3c hence shows that the LbL deposition process was truly supplemented due to orientation of magnetic dipoles. Thickness of a Bilayer. Increased electronic absorption during the magnetic-field-assisted LbL adsorption process, as shown in Figure 3c, manifests either of the two types of filmmorphologies: (i) formation of multiple layers of nanoparticles during each dipping or (ii) compact films with less voids in each monolayer of LbL films. It is imperative to study the morphology of the LbL films formed with the aid of a magnetic field. To do so, we determined the thickness of LbL films deposited with and without a magnetic field. We recorded AFM topography of an intentional scratch on the film made with a very sharp edge so that the thickness of the films could be determined from the depth profile.27 Thickness was measured for films of different numbers of bilayers. Figure 5a and b shows the AFM topography and depth profile, respectively, of a scratch on a 13 bilayer LbL film. Here, the valley and the flat-hill correspond to bare quartz substrate and the film surface, respectively. The difference between (26) Laskar, J. M.; Philip, J.; Raj, B. Phys. Rev. E 2008, 78, 031404. (27) Das, B. C.; Pal, A. J. ACS Nano 2008, 2, 1930–1938.

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Figure 5. (a, b) Typical AFM image of a scratched 13 bilayer LbL film along with the depth profile of the scratch, respectively. The area of the AFM image is 49.5 μm  49.5 μm. (c) Film thickness versus number of bilayers of Fe3O4 films with and without orienting their magnetic domains.

the two gives the thickness of the film at that point. In Figure 5c, we show plots of average thickness obtained from the depth profile versus number of bilayers of adsorbed nanoparticles deposited with and without orienting magnetic domains with a magnetic field. The plot shows that the film thickness does not vary in the two cases. That is, magnetic-field-assisted deposition did not yield multiple layers of nanoparticles during each dipping. It is an interesting LbL film where, due to assistance from a magnetic field, the thickness per bilayer does not change but absorbance increases. Thus, we can conclude that the packing of the nanoparticles in each monolayer must have been better in magnetic-field-assisted films. This is quite possible, since adsorption of nanoparticles on a layer of magnetically oriented nanoparticles is assisted by two types of forces: electrostatic and magnetic. In the usual LbL deposition process (without orienting the magnetic domains), adsorption relies only on electrostatic assembly. Hence, in the case of LbL films by orienting the magnetic domains of the nanoparticles, their packing can be expected to be compact with fewer voids. Magnetic Force Microscopy. To add credence to the thesis that magnetic domains of the nanoparticles become oriented that in turn supplement LbL deposition process, we have carried out MFM measurements of the films. Figure 6 shows MFM images of different LbL films with and without assistance from oriented domains. While Figure 6a shows an MFM image of a standard five bilayer LbL film, Figure 6b is an image of a film that was deposited by orienting the domains after deposition of every layer. In contrast to the former one, the latter image shows a higher alignment implying that the magnetic phases are parallel to each other. A degree of periodicity that appears in Figure 6a may be due to inherent organization occurring from very slow adsorption of nanoparticles. That is, the ferromagnetic nanoparticles, while landing on the substrate, get ample time to orient to some extent. The degree of orientation for magnetic-field-assisted LbL films, as shown in images presented in Figure 6b, is certainly much more than that in Figure 6a, supporting our thesis of magnetic-fieldassisted LbL deposition. We have recorded an MFM image of a 10 bilayer film also, where only the first five bilayers were Langmuir 2010, 26(12), 9627–9631

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Figure 6. Magnetic force microscopy images (phase-lift mode scan) of (a) a 5 bilayer LbL film without any magnetic field, (b) a 5 bilayer film deposited by orienting magnetic domains after adsorption of each layer, (c) a 10 bilayer LbL film without any magnetic field, and (d) a 10 bilayer film where magnetic domains were oriented only for 5 bilayers.

deposited by orienting magnetic domains after deposition of each layer. As a comparison, we recorded an image of a 10 bilayer LbL film without any magnetic field. The image of the latter film is presented in Figure 6c; it looks mostly similar to the image of a five bilayer film. In the other MFM image, namely, Figure 6d, magnetic phases look partially oriented. Images presented in Figure 6b and d, in comparison to Figure 6a and c, respectively, show that the oriented magnetic domains of the nanoparticles on the substrate supplement the LbL deposition process of Fe3O4 nanoparticles. The resulting LbL films of ferromagnetic nanoparticles have their magnetic domains ordered and oriented perpendicular to the substrate.

Conclusions In conclusion, we have shown that LbL assembly of Fe3O4 nanoparticles can be supplemented by orienting magnetic domains of the ferromagnetic nanoparticles. In other words, we have shown that LbL films of ferromagnetic nanoparticles can be deposited via a combined effect of electrostatic and magnetic forces of attraction. That is, the oriented magnetic domains of the nanoparticles on the substrate can supplement the electrostatic adsorption process of the subsequent layers. This proposal has been supported by electronic absorption spectroscopy and thickness deduced from AFM topographies. We have shown that the oriented domains have led to a compact film with higher electronic absorbance. MFM mapping of the films has confirmed that domains become oriented in the LbL films and remain so in the subsequently adsorbed layers. Acknowledgment. The authors acknowledge Sk. Basiruddin of the Indian Association for the Cultivation of Science for the synthesis of nanoparticles. Financial assistance from DST Projects SR/NM/NS-55/2009 and SR/S2/RFCMP-01/2009 are acknowledged. S.D. and K.M. also acknowledge CSIR Fellowship Nos. 9/080(0647)/2009-EMR-I (Roll No. 507031) and 9/080(0491)/ 2005-EMR-I (Roll No.509342), respectively. DOI: 10.1021/la101132z

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