Superlattices and Microstructures 46 (2009) 195–204

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Superlattices and Microstructures

journal homepage: www.elsevier.com/locate/superlattices

Large 2D monolayer assemblies of iron oxide nanocrystalsby the Langmuir–Blodgett technique

Matthias Pauly, Benoît P. Pichon ∗, Arnaud Demortière, Julie Delahaye,Cédric Leuvrey, Geneviève Pourroy, Sylvie Bégin-ColinInstitut de Physique et Chimie des Matériaux de Strasbourg, UMR7504 CNRS-ULP-ECPM, 23, rue du Loess, BP 43, 67034 Strasbourg

Cedex 2, France

a r t i c l e i n f o

Article history:

Available online 1 January 2009

Keywords:

Iron oxide nanoparticles

Thin films

Magnetism

Organisation

Langmuir–Blodgett

a b s t r a c t

Densemonolayer assemblies of iron oxide nanoparticles have beenobtained on large areas using the Langmuir–Blodgett technique.The density and ordering of the film were found to dependon the quality of the nanoparticle suspensions, which shoulddisplay monodisperse nanoparticles and no free molecules. Amodification of nanocrystals magnetic properties was noticedbetween the random configuration (powder sample) and theassembly in monolayer (Langmuir–Blodgett). The organisationin film is suggested to induce an enhancement of magneticinteractions.

© 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Nanoparticles exhibit unique properties mainly due to their high surface to volume ratioand quantum size effects [1]. Nowadays, they are considered as building blocks for futurenanotechnological devices and the development of strategies for processing nanoparticles into thinfilms or into 3D architectures has become a challenge [2,3]. Multidimensional assembly of magneticnanoparticles will enable the fabrication of many original and significant devices such as filters,magnetic recording head sensors, electronic logic devices, high-densitymagnetic storagemedia. . . [4–6]. Indeed the physical properties of inorganic nanocrystals self-assemblies at the mesoscopic scalehave been proved to differ from those of isolated nanocrystals and of the bulk materials due todipolar interactions [7–10]. These particle–particle interactions play a significant role in modulatingthe magnetic properties of such assemblies. Thus, synthesizing tailored magnetic nanoparticles and

∗ Corresponding address: IPCMS/GMI, UMR7504 CNRS-ULP-ECPM 23, rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France.

Tel.: +33 3 88 10 70 33; fax: +33 3 88 10 72 47.

E-mail address: benoit.pichon@ipcms.u-strasbg.fr (B.P. Pichon).

0749-6036/$ – see front matter© 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.spmi.2008.11.004

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developing assembling strategies allowing these interactions to be tuned on demand are requiredfor further breakthrough. Furthermore the control of the assembling step and thus of the magneticproperties will allow the building of magnetic-force triggered nanodevices.

Thin films (e.g. arrays) made from nanoparticles are generally obtained by self-organisation [5].However, difficulties arise from the control of the film formation over macroscopic distances (thisbeing mandatory for magnetic recording and for meaningful studies of the film properties) and ofthe interparticle and interlayer spacing. Among the deposition techniques of nanoparticles on solidsubstrates, the Langmuir–Blodgett (LB) technique appears as an efficient method to finely control thelayer thickness, the homogeneity of the film and easy multilayer deposition [5,8,11].

The LB technique has been used to produce two-dimensional arrays with a hexagonal organisationof organically coated nanoparticles of various types [12–16], and in particular ofmagnetite [11,17–21].The use of this technique to obtain ordered arrays of magnetic oxide nanoparticles has been limitedso far, as the homogeneity of the films depends on several parameters: the coating (size ofmolecules),the particle core size, the suspensions stability prior to deposition, the evaporation rate of the solventand the competition between Van der Waals and magnetic interparticle interactions [5,17,22].

Here, we present the formation of a dense monolayer of iron oxide nanoparticles on a largesurface by the Langmuir–Blodgett technique andwe showhow the presence of free organicmoleculesand the particle size polydispersity can influence the film morphology. Finally, the influence of theorganisation in films of iron oxide magnetic nanoparticles on magnetic properties is discussed.

2. Nanoparticle synthesis and film preparation

The iron oxide nanoparticles were synthesized by adapting the procedure of Park et al. [23]. Itconsists of the synthesis of an iron oleate precursor, which is then thermally decomposed in a highboiling solvent into iron oxide nanoparticles covered by oleic acid.

2.1. Synthesis of iron oleate complex (Fe(oleate)3)

10.8 g (40 mmol) of FeCl3·H2O (97%, Aldrich) were dissolved in 60 ml H2O (Milli-Q) and 80 mlethanol. 36.5 g (120 mmol) of sodium oleate (82%, Riedel-de Haën) dissolved in 140 ml hexane weremixed with the iron (III) solution. The resulting biphasic solution was refluxed at 70 ◦C for 4 h. Aftercooling down the solution, the organic phase containing the iron oleate complex was separated,washed three times with 30 ml distilled water to extract salts, dried using MgSO4, and finally hexanewas evaporated. The resulting iron oleate complex was a reddish-brown viscous solution. The finalproduct has been stored at a temperature of 4 ◦C.

2.2. Synthesis of iron oxide nanocrystals

2 g (2.2×10−3 mol) of Fe(oleate)3 and 1.24 g (3.3×10−3 mol) of oleic acid (99%, Alfa-Aesar) wereadded to 20ml octyl ether (97%, Fluka, b.p. 287 ◦C). Themixture was kept under vigorous stirring for 1h to dissolve the reactants. The solution was heated to 287 ◦C with a heating rate of 5 ◦C/minwithoutstirring and was refluxed for 120 min at this temperature under air. The resultant black solution wasthen cooled down to room temperature and the nanocrystals were washed 3 times by addition ofethanol and centrifugation (8000 rpm, 10min.). The nanocrystals could be easily suspended in variousorganic solvents (chloroform, toluene, hexane, dichloromethane, . . . ).

2.3. Langmuir–Blodgett film preparation

Three different suspensions have been used to prepare the films: in a first batch, the as-preparednanoparticles were dispersed in chloroform (suspension 1). A second purification step was applied tothe second batch (suspension 2) by dialysing the as-prepared nanoparticles for three days in toluenein a 25 kDa regenerated cellulose membrane (SpectrumLabs). For the third batch (suspension 3),an additional size selection procedure was applied by Size Selective Precipitation (SSP) [24]: the

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nanoparticles were suspended in hexane at a concentration of 1 mg/ml and precipitated using thesame volume of acetone. This suspension was centrifuged and the precipitate was redispersed inchloroform. This treatment reduces the size polydispersity from 20% to 13%.

Monolayers of iron oxide nanoparticles have been deposited from suspension 1, 2 and 3 ona substrate using the Langmuir–Blodgett technique. The corresponding monolayers were namedsample 1, 2 and 3 respectively. Typically, a small volume (50µl) of a 5mg/ml nanoparticle suspensionin chloroform was spread on the water subphase of the Langmuir trough (KSV 5000, 576 × 150 mm)at room temperature. The area available to the nanoparticles is reduced by compressing the barriers(compression rate: 5 mm/min), and the surface pressure-area isotherm is recorded during the filmcompressionusing aWilhelmyplate. The films formedhavebeen transferred at a pressure of 30mN/mon a hydrophilic siliconwafer (ITME, both sides polished, cleaned in a H2SO4 : H2O2 3:1 solution priorto use) by pulling the substrate out of the water subphase at a rate of 1 mm/min. Films were alsodirectly transferred from the water surface onto a carbon coated Cu-TEM grid using the Langmuir-Schaeffer technique before and after compression.

2.4. Characterization techniques

The suspension and the monolayer of nanoparticles were characterized by transmission electronmicroscopy TEM with a TOPCON model 002B transmission electron microscope operating at 200 kV.Selected Area Electron Diffraction (SAED) patterns were recorded at a camera length of 60 cm. Allthe transferred films were characterized by using a Scanning Electron Microscope (SEM) JEOL 6700Fequipped with a field emission gun (FEG) and operating in the 3–5 kV range. Samples 1, 2 and 3 wereanalyzed as received. No metal was deposited onto the monolayer to obtain a better contrast.

The structure of the film was investigated in situ by Brewster Angle Microscopy (BAM). Brewsterangle microscopy is a technique allowing the observation of monolayers at the air–water interface.The intensity of the reflected light is proportional to the thickness (d) and refractive index (n) ofthe film. Brewster angle microscopy (BAM) pictures have been recorded on a Bam2Plus from NFT[www.nanofilm.de].

ZFC/FC measurements were performed on the powder of nanoparticles and on a monolayer ofnanoparticles deposited on a silicon wafer. Measurements were performed with a SuperconductingQuantum Interference Device (SQUID) magnetometer (Quantum Design MPMS-XL model) between5 K and 300 K under a field of 75 G.

3. Results and discussion

3.1. Nanoparticles characterisation

TEM observations at high magnification of nanocrystals in suspension 1 (without SSP) (Fig. 1(a))and in suspension 3 (after SSP) (Fig. 1(b)) reveal nanocrystals with a quasi spherical shape. Todetermine the particle size distribution and the average diameter, the size of around 400 nanocrystalswas measured from TEM micrographs. The corresponding particle size distributions deduced fromTEM micrographs are given in Fig. 1. The nanocrystals in suspensions 1 and 2, corresponding tosuspensions of as-prepared nanocrystals without and with a second purification step respectively,display an average diameter of 10.2 ± 2.0 nm with a polydispersity index σ of 20%. Although thenanocrystals in suspension 3 display a similar average size (10.8 ± 1.4 nm), their polydispersityindex is lower (σ = 13%) due to the additional size selective precipitation (SSP). For all suspensions,the Selected Area Electron Diffraction (SAED) pattern (Fig. 1(c)) exhibits clearly six diffraction ringsassociated to the (h,k,l) reflections of an iron oxide spinel structure with an intermediate composition(Fe3O4-δ) between those of magnetite (Fe3O4) and maghemite (γ -Fe2O3) [25,26].

3.2. Film structure

The surface pressure-area isotherm recorded after suspension 3has been spread (Fig. 2(c)) suggeststhe formation of a dense monolayer. Indeed the steep increase in pressure and the collapse at a

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Fig. 1. TEM micrographs and particle size distributions of as-prepared nanocrystals in suspension 1 (a) and of nanocrystals

after SSP in suspension 3 (b) and SAED pattern of the nanocrystals (c).

relatively high surface pressure are typical of the formation of a continuous and rigid film of tightpacked nanoparticles. The reported surface pressure-area isotherms obtained from nanoparticleLangmuir films do not generally display such a steep increase in pressure but rather a continuoustransition from liquid to crystalline state [17,19] and the resulting films are generally less dense. Theshape of this isotherm is thus in favour of the formation of a dense Langmuir monolayer.

SEM and TEM micrographs of the film obtained from suspension 3 (Figs. 3(a), (b) and 4(c)) showa dense array of nanoparticles with very few defects. This observation is in good agreement with thesurface pressure area isotherm and suggests an efficient transfer of the monolayer at the air waterinterface onto the substrate. The data shown are representative of the structure observed on thewhole substrate surface, i.e. that a homogeneous and continuous layer of nanoparticles is formed on amacroscopic scale (several cm2). The two rings on the Fast Fourier Transform (FFT) of the TEMpictures(inset of Fig. 3(b)) reveal the local organisation of the nanoparticles. The mean inter-particle distancecalculated from the FFT is 1.9 nm, which is less than twice the length of one oleic acid molecule

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Fig. 2. Surface pressure-area isotherms of (a) sample 1 (• • •), (b) sample 2 (– – –) and (c) sample 3 (——).

Fig. 3. TEM pictures of the film obtained from sample 3 (a, b, c) and sample 2 (d). (a) large view of the monolayer at

Π = 30 mN/m, (b) closer view of the monolayer at Π = 30 mN/m (inset : FFT showing the local organisation of the particles

after compression), (c) particles on thewater subphase before compression atΠ = 0mN/m(inset : FFT of one asembly showing

the hexagonal organisation of the particles), (d) monolayer at Π = 30 mN/m, the arrows show the smaller particles which

connect the domains.

(1.75 nm) [19]. It is certainly due to the compression step andVan derWaals interactionswhich favoura strong interdigitation of the oleate chains between neighbouring nanoparticles.

In contrast to suspensions 1 and 2, the purification and size selection procedures enabled us todetermine precisely the concentration in nanoparticles in suspension 3. Thus, knowing the volumeand the concentration of suspension 3 which was spread on the water surface, it is possible todetermine the average surface occupied by a single nanoparticle at the air water interface at zeropressure by extrapolating the steepest part of the surface pressure-area isotherm to the surface axis.

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Fig. 4. SEM micrographs of the film obtained from : (a) sample 1, (b) sample 2 and (c) sample 3.

Themean area occupied by a nanoparticle in the filmmade from sample 3 is 150 nm2, which is slightlyabove the expected value calculated by assuming an ideal hexagonal superlattice (135 nm2). The slightdifference between theoretical and measured values results certainly from some packing defects asobserved in Fig. 4(c).

Brewster Angle Microscopy pictures have been taken during the compression of the film atdifferent surface pressures (Π = 0, 15, 25 and 37 mN/m). The polarised incident laser beam isadjusted to the Brewster angle before deposition of the nanoparticles, and thus the nanoparticlesappear in bright contrast due to the refractive index difference between the particles and thesubphase. At the beginning of the compression (Π = 0 mN/m, Fig. 5(a)), some bright parts ofthe water surface are covered by an inhomogeneous film of particles and other dark parts remainuncovered. During compression (Π = 15 mN/m, Fig. 5(b)), the bright areas where particles arepresent are getting denser and denser, as indicated by the contrast increase, and the proportionof dark areas where no particles are present is being reduced, until an uniform film is obtained(Π = 25 mN/m, Fig. 5(c)). Finally, parallel brighter lines characteristic of the collapse of the filmcan be observed at Π = 37 mN/m (Fig. 5(d)), which are consistent with the discontinuity seen atthe same pressure on the isotherm. The combination of the BAM technique with TEM gives moreinsights in the formation process of a dense and homogeneous film. The nanoparticles spread on thewater subphase before compression (Fig. 3(c)) are already assembled in quite spherical domains ofapproximately 100 nm with hexagonal organisation, as shown by the FFT of one domain (inset ofFig. 3(c)). It is easily explained by the fact that the nanoparticles are hydrophobic due their oleatecoating, and thus they tend to aggregate and form domains in order to reduce their interaction with

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Fig. 5. Surface pressure-area isotherm of sample 3 and BAM pictures showing the film at different stages of the compression

process : (a) Π = 0 mN/m, (b) Π = 15 mN/m, (c) Π = 25 mN/m, and (d) Π = 37 mN/m.

the water subphase. Van der Waals and dipolar interactions between magnetic nanoparticles alsofavour the formation of domains. These domains are separated by large areas where no particlesare present. During the compression process, the surface occupied by nanoparticles is being reduced,decreasing the free space between the domains. At one point the domains touch each other, whichgives rise to a strong pressure increase. During this last step, the small spherical domains merge intoone large domain, creating a dense layer of nanoparticles. Although this layer covers continuously thewhole surface of the substrate, the local organisation is mainly the same as the organisation in thepre-existing domains, so the final layer has a local hexagonal organisation, but there is no long rangeorganisation (Fig. 4(c)).

It is noteworthy to point that two important conditions must be fulfilled in order to obtaina uniform layer on such a large scale: solution purity and nanoparticle size monodispersity.The nanoparticles synthesised by thermal decomposition of iron oleate are generally washed byprecipitating the particles in ethanol in order to remove the reaction solvent, the unreacted precursorand the excess of oleic acid [21,27,28]. As it has already been noted by other authors [16,28],this simple washing is not sufficient to remove the whole amount of molecules unbound to thenanoparticle surface. Although itmay not be important for some applications, it becomes crucialwhentrying to form Langmuir monolayers on large areas. Fig. 4(a) shows a film formed with suspension 1(sample washed with ethanol), where only very limited and minor parts of the surface of the film arecovered by nanoparticles. As the isotherm displays a strong pressure increase (Fig. 2(a)), it means thatthere is a continuous film formed between the barriers during compression. When the same sampleis dialysed in toluene in order to remove the excess of free organics (either oleic acid, unreacted ironoleate or any by-product formed during the synthesis) (suspension 2), the film obtained is muchmore homogeneous and dense (Fig. 4(b)). This means that when all the free organic molecules arenot removed from the suspension, the Langmuir film on the water subphase is composed of amixtureof nanoparticle domains and molecules filling the gaps between the domains. The increase of thesurface pressure is mainly due to the presence of organic impurities which behave like surfactants atthe air water interface. These organic species consist mainly in oleic acid molecules unbound to thenanoparticles surface, which are well known to form Langmuir monolayers at the air water interface.

Although the washing procedure considerably increases the nanoparticle density on the surface,the structure is still not entirely homogeneous and proceeds in packed domains (Fig. 4(b)). TEMmicrographs of this Langmuir film (Fig. 3(d)) reveal that smaller particles are accumulating at the

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Fig. 6. ZFC/FC curves of the nanoparticle powder (– – –) and the nanoparticles organised on the substrate (–�–).

domains junctions (arrows), preventing the merging of these domains in a continuous monolayerof nanoparticles. As we already showed it in Fig. 3(c), iron oxide nanoparticles coated with oleicacid tends to self assemble into domains at the air water interface before the compression step.With suspension 2, it results in assemblies of the largest particles at the centre, surrounded radiallyby the smallest particles [29]. This domain structure may be explained by magnetic interactionscontributing with Van der Waals interactions to the self-assembling of these magnetic nanoparticles.As magnetic properties are linked to the size of the nanoparticles, such interactions will also dictatethe way nanoparticles with a high polydispersity self-assemble. Thus, stronger interactions betweenthe largest particles tend to assemble them first. The presence of such inhomogeneous domainsresults in a different surface pressure area isotherm (Fig. 2(b)). The surface pressure upon compressiondisplays a bump which may correspond to the rearrangement of these inhomogeneous preformeddomains during the compression step by comparison with homogenous domains (sample 3). Finally,the removal of the excess of organic species and the size selection procedure of the nanoparticlesenable domains to merge and lead to a homogeneous layer (Fig. 4(c)).

3.3. Magnetic properties

The magnetic properties of the nanoparticle powder and the monolayer LB film containing theiron oxide nanocrystals have been studied by SQUID measurements [30]. The susceptibility versustemperature behaviour was measured by a Zero Field Cooled (ZFC) and Field Cooled (FC) experiment.In the ZFC curve, the maximum of the susceptibility corresponds to the blocking temperature (TB)which is related to the particlemagnetic anisotropy energyKaV by the relationKaV = kbTB ln(τ/τ0) ≈

25kbTB, where V is the particle volume, Ka is the magnetic anisotropy energy per volume unit andτ0 ≈ 10−9–10−11 s [31,32].

In Fig. 6, the ZFC/FC curves are reported for the nanocrystal powder (nanoparticles randomlydistributed) and the monolayer film on silicon wafer (the applied magnetic field is parallel to thesubstrate) for sample 3. As expected for powders of small iron oxide nanocrystals with a size of 10.8

nm, the blocking temperature (T(P)B ) is equal to 76 K. From this value and the average diameter of

nanocrystals 〈D〉, the anisotropy constant Ka can be estimated to 4.5 × 105 erg/cm3. This value is ingood agreement with values of Ka reported in other studies [23,31,32]. It is higher than the first-ordermagnetocrystalline anisotropy value of bulk magnetite (K bulk

1 = 1.1 − 1.3 × 105 erg/cm3) [32] andthis enhancement is often associated to surface effects [26,31–33].

M. Pauly et al. / Superlattices and Microstructures 46 (2009) 195–204 203

On the other hand, the ZFC curve corresponding to the LBmonolayer sample exhibits a shift toward

higher temperature (T(LB)B = 87 K). It is known that the increase in magnetic dipolar interactions

induces a shift of the blocking temperature toward higher temperatures [34–36]. Such a result maybe consistent with the formation of a 2Dmonolayer assembly of magnetic nanoparticles whichwouldlead to an increase of magnetic dipolar interactions in a parallel plan to the surface of the film. Thecompression of nanoparticles in a 2D organized monolayer raises their density and so decreasesthe interparticle distance compared to the random disposition in a powder sample. Moreover, inorder to investigate the evolution of the two samples, both ZFC/FC curves have been normalizedto the value of the magnetic susceptibility at T = TB. Below the blocking temperature, both FCcurves present the same quasi-linear evolution. However, for the monolayer sample, the FC curvesexhibits a lower susceptibility value of 1.2 than for the powder sample which is of 1.35. This magneticbehaviour can also be attributed to modifications of interparticle interactions [36,37] and in thiscase, an enhancement of magnetic interactions between the nanoparticles arranged in a monolayer.The above observations suggest a modification of the nanocrystals magnetic properties between therandom configuration (powder sample) and the assembly in a monolayer (Langmuir–Blodgett film).This behaviour has already been observed in similar close-packed two or three dimensional arrays [36,38]. Further investigations on nanocrystal monolayers with weaker magnetic interactions tuned bythe organic coating should lead to a better evaluation of the effect of magnetic dipolar interactions.

4. Conclusion

Iron oxide nanoparticles have been assembled in a monolayer film on large areas. The quality ofthe organisation was found to depend strongly on the particle size distribution and on the lack of freemolecules in the nanoparticles suspension. The organisation of the nanoparticles in films has led to amodification of the magnetic properties by comparison with the powdered sample.

Acknowledgments

We would like to thank J.-L. Gallani for Langmuir Blodgett facilities and A. Derory for magneticmeasurements. M Pauly was supported by the Direction Générale de l’Armement (DGA).

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