Angela Sofia
Pereira
a,
Protima
Rauwel
a,
Mário Souza
Reis
b,
Nuno João
Oliveira Silva
c,
Ana
Barros-Timmons
a and
Tito
Trindade
*a
aDepartment of Chemistry and CICECO, University of Aveiro, 3810-193 Aveiro, Portugal. E-mail: tito@ua.pt; Fax: +351 234370084; Tel: +351 234370360
bDepartment of Physics and CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
cInstituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, Departamento de Fisica de la Materia Condensada, Faclutad de Ciencias, 50009 Zaragoza, Spain
First published on 20th August 2008
Surface mediated phenomena are important in controlling the magnetism of nanocrystals. Here, we provide a versatile chemical method which allows the formation of EuS nanocrystals encapsulated in poly(styrene) (PS). These materials are the first example of a polymer based EuS nanocomposite prepared in situ. Besides their practical interest, these materials are of general relevance because they allow fundamental knowledge on the magnetism of EuS, which is commonly used as a model of an insulating ferromagnet, to be acquired. Comparatively to the starting powders of EuS nanocrystals (8 nm), in the final nanocomposites there is a decrease of the blocking temperature TB and a change in the shape of the field-cooled susceptibility curve, indicating that the intensity of dipolar interactions decreases when the nanoparticles are incorporated into PS. Differences in the magnetic properties of the EuS nanocrystals and the EuS/PS nanocomposites are also due to the appearance of a peak in the susceptibility at 60 K, in the latter sample, ascribed to surface phenomena occurring in EuS nanocrystals dispersed in the polymer matrix.
Europium mono-chalcogenides (EuE, E = O, S, Se, Te) with the rock-salt structure have been investigated at least since the 1960s.8 The europium chalcogenides form an important class of magnetic semiconductors, revealing an assortment of magnetic ordering,8 and have frequently served as model Heisenberg magnets.9,10 Research on the effect of particle size and surface nature on the magnetic properties of the europium chalcogenides has become very appealing; for example, pressure studies have shown that the Curie temperature (TC) is very sensitive to both the Eu–Eu distance and the band gap energy.11 Among the europium mono-chalcogenides, EuS has been especially investigated due to its properties as a ferromagnetic semiconductor with a Curie temperature of 16.8 K, an energy gap of 1.6 eV, and a spin-splitting of the gap of 0.36 eV. Lately, nanocrystalline EuS has established its potential as an optomagnetic material.12,13 This is a promising material as a component for the manufacture of devices that can be tuned by material responses to magnetic, electrical, and optical stimuli. This suggests possible optoelectronic applications for EuS nanocrystals in optical isolators, optical circulators, and optical memory.
At present, there is a range of synthetic methods that gives us the ability to prepare high quality inorganic nanoparticles, thus these materials have been recently explored as nanoscopic fillers in polymer matrices in the advancement of functional nanocomposites.14,15 In this regard, a number of polymer nanocomposite particles have been prepared through a variety of chemical methods which include heterophase polymerization,16 heterocoagulation,17 and layer-by-layer self-assembly.18 Heterophase polymerization is by far the most frequently used technique to obtain nanocomposite particles. These can be prepared by carrying out the polymerizationin situ in the presence of the inorganic nanofillers or their precursors.19 Among these methods, miniemulsion polymerization has established itself as an important synthetic strategy to obtain nanocomposites of diverse materials, including glasses, semiconductors, metals and carbon nanotubes as nanofillers.20 In miniemulsion polymerization, particle nucleation takes place, for the most part, within nanometre monomer droplets.20 In view of the fact that organically capped inorganic particles can be dispersed in the monomer after miniemulsification, then each miniemulsion droplet acts as a true nanoreactor. Some characteristics of the miniemulsion polymerization system offer prospective advantages for the encapsulation of nanoparticles, specifically, the capability to nucleate all the droplets containing the inorganic nanoparticles, good control of droplet size and size distribution, and direct dispersion of the nanoparticles within the organic phase.
In the past two decades, considerable research on semiconductor nanocrystals has clearly demonstrated that these particulates show unique size-dependent properties. Along this line of research, the way the collective properties developed from individual nanocrystals to their assemblies have emerged as a less understood issue in solid state science. In this respect, nanocrystals that tend to self-organize into more complex nanostructures are interesting systems of study and this seems to be the case for nanosized EuS.4–6,21 We adopted here a new approach to fabricate assemblies of EuS nanocrystals by promoting their encapsulation in a polymer matrix. Our aim in this study was to investigate the effects of the polymer encapsulation on the magnetic properties of EuS nanocrystals; this is clearly appealing from the academic point of view, in particular when considering that EuS is a common model of a Heisenberg magnet. In addition, by combining EuS nanocrystals with polymers, the resulting material can find further interest for the emerging field of polymer nanodevices. To date this is the first report of the in situ synthesis and magnetic behaviour of morphological well-defined EuS/polymer nanocomposites.
Fig. 1 shows the TEM images of synthetic EuS/OA nanocrystals which have been used in this work as the starting materials to prepare the polymer nanocomposites. Two types of morphology were observed for the EuS nanocrystals and which one is seen depends on the synthesis conditions. Nearly monodisperse EuS nanospheres (d ≈ 8 nm) were obtained when injecting the single-molecule precursor into hot OA while larger EuS nanocubes were prepared by heating the precursor together with the solvent. Assessment of the XRD patterns of EuS/OA nanocrystals revealed the expected rock-salt type crystal structure with all the Bragg reflections indexed to pure EuS (ESI†). This result was further confirmed by selected area electron diffraction (SAED) performed on diverse nanosized EuS samples. The average size of the nanospheres was estimated to be 8 nm by applying the Scherrer equation to the XRD results, which is in fair agreement with TEM analysis.
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Fig. 1 TEM images of oleylamine capped EuS nanospheres (a) and nanocubes (b). |
The assumption that OA is coordinating the EuS nanocrystals’ surfaces is based on the observed hydrophobic characteristics of these particulates, which can be explained because the hydrocarbon chains of OA molecules are directed outwards. As will be described in the next section, these hydrophobic nanocrystals can be easily dispersed in an organic medium, such as the monomer styrene, to perform in situpolymerizations. Confirmation of the presence of OA molecules at the EuS nanocrystals’ surfaces was obtained by means of FT-IR spectroscopy performed on a sample previously washed with a mixture of isopropanol and methanol. The FT-IR spectrum clearly shows (ESI†) diagnostic bands from oleylamine: 3220, 3119 cm−1 (vN–H); 2919, 2849 cm−1 (vC–H); 1466 cm−1 (vC–N).
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Fig. 2 Visible reflectance spectrum of a EuS/PS nanocomposite sample (left); inset shows a micrograph of the as prepared sample, both as a powder and as an aqueous emulsion. UV/VIS spectrum of the original EuS nanocrystals dispersed in toluene (right). |
To prepare the nanocomposites, various parameters have been adjusted to achieve optimal reaction conditions (see Experimental methods section), namely in connection with the type of initiator and the concentration of surfactant. Depending on the experimental conditions, the chemical composition and the particle morphology of the nanocomposite was substantially altered. For example, when KPS was used as the initiator, the EuS nanocrystals invariably dissolved, resulting in loss of color and the final material turned into a film composed of large rice-shaped (12 μm) microparticles (Fig. 3a). This result, combined with those from EDS (energy dispersion spectroscopy), indicate that this sample contains mainly Eu and hardly any S; this suggests that persulfate radicals may be involved in the oxidation of the EuS nanocrystals. However, using AIBN as the initiator, colloidal stable emulsions could be prepared though two distinct particle size populations were observed. As suggested by TEM analysis (Fig. 3b), these two populations correspond to free PS particles and to EuS/PS nanocomposite particles, respectively. Successive centrifugation cycles yielded fractions which became subsequently richer in nanocomposite particles as compared to the starting emulsions. The remaining free polymer particles were analysed by GPC giving a molecular weight of 98500 Da as compared to 154
000 Da, obtained for poly(styrene) prepared using similar conditions but in the absence of EuS nanocrystals. This lower molar mass is thought to be due to chain termination reactions of the growing polymer chains at the surface of the EuS nanocrystals.
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Fig. 3 TEM images of samples obtained after polymerization of styrene in the presence of EuS nanocrystals, using KPS (a) and AIBN (b) as initiators. |
It appears from our results that, in maintaining the same conditions of polymerization, the EuS particles morphology influences the polymer nanocomposite properties. Thus the EuS nanospheres tend to be encapsulated by poly(styrene) yielding round shaped nanocomposite particles, which can be present either as isolated nanocomposite particles (Fig. 4a) or clustered into raspberry-like particles (Fig. 4b). The latter morphology presumably results from the clustering of original EuS/PS nanoparticles and was the most common morphology observed in these nanocomposites. When considering the larger and cubic EuS nanocrystals (Fig. 1), and for similar conditions of miniemulsion polymerization, we could not obtain the polymer encapsulation of the nanoparticles. Instead, TEM analysis showed that the EuS nanophase remained mainly outside the polymer beads (ESI†). This is probably due to the larger size of the nanoparticles (or their assemblies) compared to the monomer droplet dimensions in the reacting system.
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Fig. 4 TEM images of EuS/PS nanocomposite particles: (a) detail of a single spherical particle; (b) set of particles with raspberry-like morphology. |
These observations indicate that ensembles of EuS nanospheres can be coated with poly(styrene) and this might induce changes in the magnetic behaviour of EuS. Therefore, we have decided to focus our attention on the nanocomposites containing the EuS nanospheres (d ≈ 8 nm) and to investigate in detail the magnetism of a selected nanocomposite sample with 5% (w/w) in relation to the Eu content, as determined by ICP analysis. Although we did not find vestiges of europium oxides in the powder XRD of the nanocomposite (ESI†), the oxidation of the EuS surfaces cannot be ruled out, as will be discussed below.
The DC susceptibility curves of the EuS nanocrystals show a transition temperature TC at around 15 K (estimated from the inflection point of the curves) and a thermal irreversibility below Tir = 14 K. The zero-field-cooled (ZFC) curve has a maximum (blocking temperature) at TB = 7.7 K. This TB is higher than that found for 8 nm diameter nanocrystals (∼5 K).7 This may be due to a different intensity of dipolar interactions and/or different nanoparticle anisotropy. The field-cooled (FC) curve tends to saturate at low temperatures, as is usually found in bulk and interacting-nanoparticle systems.23 An inspection of the temperature dependence of the inverse of the magnetic susceptibility shows that above TC the system has two Curie–Weiss regimes: one above ∼50 K with an extrapolated paramagnetic Curie temperature θp of 10.8 K, and other below ∼50 K with an extrapolated θp of 15.7 K. This behaviour is depicted in Fig. 5(a). The ac susceptibility curves show a high temperature region (above TF ≈ 15 K), in which the in-phase susceptibility χ′ is not frequency dependent and the out-of-phase susceptibility χ″ is close to zero. The χ′(T) curve exhibits a maximum at the blocking temperature TB′, which is frequency dependent. In addition, χ″(T) has a frequency dependent maximum TB′′ at around 5 K and a shoulder around 10 K. The existence of dynamics in the studied frequency range places the EuS in the context of nanoparticles/spin-glasses and not in the context of bulk materials. Details on the magnetization data as a function of magnetic field can be found in Fig. 6a, left. The EuS nanocrystals have low coercive fields HC (at 2 K, HC = 90 Oe) and low remanence Mr, this rapidly approaches zero as temperature increases, being zero above TC (Fig. 7). We note that the Mr values were obtained after taking into account the contribution of the remanent field of the superconducting coil.24 If this contribution is ignored, a spurious Mr with negative values for T > 7 K arises (Fig. 7). We remark that this spurious Mr is similar to that found in the literature.25 Therefore, the EuS nanocrystals do not have a magnetization reversal but a canonical behaviour of Mr decrease towards zero as T approaches TC and a zero Mr for T > TC. The Arrot plots shown in Fig. 6a, right, and obtained from the M(H) curves, confirm that the Curie temperature is around 15 K. The gap between the measurement at 2 K and the M2 axis is directly proportional to the demagnetization factor of the sample.
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Fig. 5 DC magnetic susceptibility as a function of temperature, obtained from the ZFC and FC procedures: (a) EuS nanocrystals and (b) EuS/PS nanocomposites. |
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Fig. 6 Magnetization as a function of magnetic field for several temperatures for (a, left) EuS nanospheres and (b, left) EuS nanospheres embedded in poly(styrene). The corresponding Arrot plots are presented in the right-hand panels. |
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Fig. 7 Remanent magnetization as a function of temperature in EuS nanocrystals. |
The DC susceptibility curves of EuS/PS nanocomposite show irreversibility up to a higher temperature than that of EuS nanocrystals (Fig. 5). Characteristic features appear in the (a) 2–20 K and (b) 40–100 K temperature ranges. For that first range, it is possible to observe an increase of the thermal irreversibility below ∼7 K. In particular, the FC curve has no signs of saturation at low temperatures (paramagnetic-like curve), while the ZFC curve has a maximum at TB = 3.9 K. In what concerns the other range (>40 K), we observe a maximum at 60 K (Fig. 6b), probably related to a EuO-like environment located at the surface of the nanocrystals, since EuO has a Tc at 69 K.8 The AC susceptibility curves show a high temperature region Tir > 11 K for which the in-phase susceptibility χ′ is not frequency dependent and the out-of-phase susceptibility χ″ is close to zero. χ′(T) exhibits a frequency dependent maximum at around TB′ = 4 K. On the other hand, χ″(T) has a monotonic decrease to zero as the temperature increases up to 11 K. As shown in Fig. 6b, left, the magnetization as a function of magnetic field for the EuS/PS nanocomposite presents a quite different behaviour to that found for the EuS nanocrystals, which is probably related to an antiferromagnetic arrangement. Also shown in Fig. 6b, right are the Arrot plots for the nanocomposite, that confirm our previous findings based on the DC magnetic susceptibility measurements, that for this material no Curie temperature associated with EuS order is detected.
A comparative analysis of Fig. 5–7, clearly shows that the magnetic properties of the EuS nanocrystals change upon their incorporation into PS. In particular, we note that TB decreases from 7.7 K to 3.9 K. The decrease of TB and the change in the shape of the FC curve indicate that the intensity of dipolar interactions decreases when the nanocrystals are incorporated into PS.26 Therefore, the average interparticle distance increases with their incorporation into PS, pointing out that the nanocrystals are efficiently dispersed in the PS media. Differences in the magnetic properties of the EuS and EuS/PS systems are also due to the appearance of a peak at 60 K. This is ascribed to surface phenomena, namely the existence of an oxidised surface in the EuS nanocrystals of the latter sample, as a result of the reaction conditions employed to synthesize the nanocomposites. The importance of the surface on the magnetic behaviour of EuS nanocrystals has also been recently observed for EuS prepared by a different route which involved the chemical reaction of Eu metal and H2S in liquid ammonia.27
The effect of the polymer encapsulation on the dipolar interactions in nanosized EuS, was further confirmed by analysing the dependence of TB′ with the frequency of the AC field. In this regards, the Néel model applied to superparamagnetism predicts that the dependency of τ = 1/f with TB′ is described by:28
![]() | (1) |
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Fig. 8 Thermal variation of the relaxation time (logτvs. 1/TB′) by linearization of eqn 1 (see text for details). |
[Eu(dtc)3(phen)], elemental analysis (%): found C, 40.65; H, 4.91; N, 8.64; calculated C, 41.74; H, 4.93; N, 9.01. IR and (Raman) major bands (υ/cm−1): 2975 (2977) [υ(C–H)], 1482 (1483) [υ(C–N)], 1000 (994.3) [υ(C–S)] for the dithiocarbamato ligand; 1623 (1624) and 1604 (1602) skeleton vibration of the benzene ring, 852 (843.5) and 730 (720.2) [υ(C–CH)].
The synthesis of EuS nanocrystals was then performed by injecting an oleylamine solution of [Eu(dtc)3(phen)] into hot OA, under a N2 flow. In a typical synthesis, 3 ml of OA was first heated up to 315 °C. In the meantime, a mixture of the precursor (0.25 mmol) in 2 ml of OA was prepared and then injected into the hot OA, the reaction continued at 315 °C over 1 h. The deep purple product was cooled to room temperature and treated with a mixture of isopropanol : methanol (3 : 1) yielding a powder consisting on EuS nanospheres capped with OA molecules. The powders were easily dispersed in non-polar solvents, such as hexane and toluene, due to OA molecules coordinated at the nanocrystals’ surfaces. To alter the morphology of the particles from spherical to cubic nanoparticles, the solution of oleylamine containing the precursor was heated rapidly to 315 °C, over 1 h.
Further studies on the magnetic behaviour of EuS nanocrystals dispersed in a range of polymer matrices, including blends of these materials, will enable other questions to be answered. For example, our preliminary magnetic measurements of EuS/PS blends show a distinct behaviour from the EuS nanocrystals and EuS/PS composites prepared by miniemulsion polymerization. Thus these results strongly advise consideration of the chemical route employed for the nanocomposites preparation when studying the magnetism of EuS nanocrystals in polymers.
Footnote |
† Electronic supplementary information (ESI) available: Powder XRD of EuS and EuS–PS samples; FT-IR spectra for both EuS–OA nanocrystals and their poly(styrene) derivative nanocomposites; TEM image of a nanocomposite sample obtained using EuS nanocubes; histograms of the EuS nanospheres. See DOI: 10.1039/b806499g |
This journal is © The Royal Society of Chemistry 2008 |