Relationship between the Magnetic Properties and the Formation of a

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Relationship between the Magnetic Properties and the Formation of a ZnS/ZnO Interface in S‑Capped ZnO Nanoparticles and ZnS−ZnO Thin Films C. Guglieri,† A. Espinosa,‡,# N. Carmona,§ M. A. Laguna-Marco,† E. Céspedes,‡,▽ M. L. Ruíz-González,∥ J. González-Calbet,∥ M. García-Hernández,‡ M. A. García,⊥ and J. Chaboy*,†,○ †

Instituto de Ciencia de Materiales de Aragón, Consejo Superior de Investigaciones Científicas - Universidad de Zaragoza, 50009 Zaragoza, Spain ‡ Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco 28049 Madrid, Spain § Dpto. Física de Materiales, Universidad Complutense de Madrid, 28040 Madrid, Spain ∥ Dpto. Química Inorgánica, Universidad Complutense de Madrid, 28040 Madrid, Spain ⊥ Instituto de Cerámica y Vidrio, CSIC & IMDEA Nanociencia, Cantoblanco 28049 Madrid, Spain ▽ Institute for Science and Technology in Medicine (ISTM), Guy Hilton Research Centre, Keele University, Stoke-on-Trent ST4 7QB, U.K. ○ Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50009 Zaragoza, Spain

ABSTRACT: We have synthesized ZnO nanoparticles capped with butanethiol, octanethiol, and dodecanethiol. In all of the cases, the capped ZnO nanoparticles exhibit ferromagnetic-like behavior up to room temperature whose intrinsic origin has been demonstrated by using both X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS) at the Zn Kedge. Using these tools, we have also determined that the occurrence of ferromagnetism does not critically depend on the details of the synthesis but on the formation of a pristine ZnS/ZnO interface. Within this interface, ferromagnetism is favored in those regions where the local order is closer to wurzite-like ZnO than to w-ZnS. The study of ZnS−ZnO films prepared by cosputtering clearly indicates that increasing the disorder of this interface weakens the onset of ferromagnetic behavior.

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INTRODUCTION Occurrence of high-temperature ferromagnetism in transition metal oxide nanostructures in which the metal is nominally nonmagnetic appears as one of the most intriguing research topics in material science nowadays. However, despite the great body of research performed to date in this type of systems (both doped and undoped),1−4 there is still a strong controversy regarding its origin and whether ferromagnetism is certainly intrinsic. One of the main hindrances to clarify this point comes from the difficulty in obtaining a proper description of the magnetism at the nanoscale. The size reduction implies modifications of the local structure, which are not easy to determine because they are very often constrained to the © XXXX American Chemical Society

surface and/or interface of the bulk-like components of the nanostructured systems. The magnetic properties are critically dependent on local structural details5,6 hidden to macroscopic probes. Thus, most of the claims of evidence of room temperature ferromagnetism (RTFM) were obtained by using standard magnetometry techniques. However, more accurate measurements cast doubts on some of these assignments5,7−9 and point out the need for using more sophisticated characterization tools able to provide atom-specific magnetic properties and establish on firmer grounds the relationship Received: April 4, 2013 Revised: May 20, 2013

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dx.doi.org/10.1021/jp403336z | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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of the TMAH solution and then to 8.7 after the dodecanethiol addition. The samples, listed in Table 1, were prepared in several series, each series using the same starting solution. The first

between the magnetic properties and the local structure of the systems under study. In this way, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) are crucial into determining if the RTFM behavior, as determined from macroscopic magnetometry, is intrinsic or extrinsic.10−13 In the case of doped ZnO, there seems to be a general agreement that there is no a unique explanation for the origin of magnetism in the studied systems but that it is strongly dependent on the structural details of the sample.2,14−16 Thus, recent works highlight the importance of grain boundaries, highly defective regions, and structural distortions into driving this RTFM behavior.11,17,18 It has been also suggested that the magnetism of these doped ZnO systems is strongly influenced by surface capping, being its ferromagnetism is intimately related to the electronic structure of the magnetic impurity ions.19−21 However, avoiding doping ZnO with magnetic dopants but attempting to modify its electronic structure through capping also leads to the observation of ferromagnetism.22 Recently, XMCD measurements performed on ZnO nanoparticles capped with organic molecules and without any 3d doping have shown the existence of an intrinsic ferromagnetic contribution and suggest that it stems from the interface formed between the ZnO core of the nanoparticle and the capping region created by bonding to the organic molecules.6,23 These results are in tune with other claims that ferromagnetism arises from regions of the samples showing a high density of “magnetic interactions”.24 Here, we report a detailed study of the relationship between the magnetic properties and the formation of the aforesaid ZnO/ZnS interface in ZnO nanoparticles capped with butanethiol, octanethiol, and dodecanethiol (which bond to the particle surface through an S atom) as well as on ZnO/ZnS thin films. Our results demonstrate that the modification of ZnO, through the formation of a ZnO/ZnS interface, enables the development of ferromagnetic behavior up to room temperature in both nanoparticles and films. The degree of structural order in this ZnO/ZnS interface is also fundamental to determining the onset of ferromagnetic behavior.

Table 1. Summary of the Studied Samples Indicating the Organic Molecule Used for Capping and tadda sample 20-12C-1 calcinated 20-12C-1 5-12C-2 15-12C-2 30-12C-2 7-12C-3 9-12C-3 12-12C-3 20-4C-4 20-8C-4 20-12C-4 50-4C-4 50-8C-4 50-12C-4

capping agent dodecanethiol12C dodecanethiol12C dodecanethiol12C dodecanethiol12C dodecanethiol12C dodecanethiol12C dodecanethiol12C dodecanethiol12C butanethiol-4C octanethiol-8C dodecanethiol12C butanethiol-4C octanethiol-8C dodecanethiol12C

tadd (min)

ZnO/ZnS weight (±10%)

DXRD (nm)

20

50−50

14 ± 2

20

40−60

15 ± 2

5

35−65

13 ± 2

15

60−40

10 ± 2

30

50−50

12 ± 2

7

10−90

9

10−90

12

12−88

20 20 20

28−72 35−65 31−69

50 50 50

32−68 32−68 25−75

8±3 8±3 14 ± 3 8 ± 32

a

DXRD is the crystalline size determined from XRD, and the ZnO/ZnS weight refers to the relative amount of Zn in both ZnO and ZnS frameworks as determined from XANES (see text for details). Samples are labeled as tadd-nC-batch, where tadd is the time after adding the TMAH and before adding the organic molecule, nC (n = 4, 8, and 12) is the number of carbons of the molecule, and -batch indicates the batch of synthesis.

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synthesis was made by adding the capping agent 20 min after the addition of TMAH (20-12C-1 sample, tadd = 20 min). Besides, part of this sample was placed inside a Petri dish and submitted to thermal treatment at 350 °C for 1 h in a muffle furnace (heating time from 20 °C to 350 °C was 1 h, and the cooling process took 1 h). This process removes the organic part of the samples. We refer to this sample as calcinated-2012C-1. In the second batch of samples, tadd was varied between 0 and 30 min in order to prepare ZnO nanoparticles of different sizes, with tadd = 5 min (hereafter, the 5-12C-2 sample), 15 min (15-12C-2), and 30 min (30-12C-2). Similarly, in a third series the samples 7-12C-3, 9-12C-3, and 12-12C-3 were synthesized. Additionally, to study the influence of the length of the carbon chains on the magnetic properties of the NPs, a new series of samples were prepared (see Table 1) by using different capping organic molecules: butanethiol (tadd -4C-4), octanethiol (tadd -8C-4), or dodecanethiol (tadd -12C-4). It has to be mentioned at this point that among the main drawbacks of the preparation of nanoparticles the reproducibility of each synthesis and the stability of the NPs once prepared stand out. It is well-known that the reaction yield in the preparation of the NPs is low, and therefore, the same process is typically repeated several times to have enough quantity to perform a deep analysis. Synthesis reproducibility has been tested by performing a XAS analysis.

EXPERIMENTAL METHODS ZnO nanoparticles were prepared through a sol−gel method using dimethyl sulfoxide (DMSO, (CH3)2SO, 99.9%, Sigma Aldrich) and absolute ethanol (CH3CH2OH, 99,95%, Panreac) as solvents; zinc acetate dihydrate (Zn(CH3CO2)2·2H2O, 98%, Sigma Aldrich) as precursor; tetramethylamonium hydroxide pentahydrate (TMAH, N(CH3)4OH, 97%, Sigma Aldrich) to perform hydrolysis, and dodecanethiol (CH3(CH2)11SH, 98%, Sigma Aldrich), octanethiol (CH3(CH2)7SH, 98.5%, Sigma Aldrich), and butanethiol (CH3CH2CH(SH)CH3, 98%, Sigma Aldrich) as capping agents. 5 mmol of zinc acetate were dissolved with continuous stirring in DMSO, and the solution was kept at 60 °C. Then, a 7.5 mmol solution of TMAH in ethanol at 60 °C was added dropwise into the previous one. After tadd min of stirring, a 7.5 mmol solution of the capping agent was added all at once. The obtained ZnO nanoparticles that precipitated were filtrated and washed using absolute ethanol at 60 °C to remove unreacted precursors, and the washing process was repeated three times. The purified nanoparticles were then left to dry in air for 2 days. The pH of the initial solution was measured to be 6.3. The pH values of the solutions turned to 9.7 after the addition B



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sake of accuracy, the direction of the applied magnetic field is reversed, and XMCD, now μc = (μ+ − μ−), is recorded again by switching the helicity. The subtraction of the XMCD spectra recorded for both field orientations cancels, if present, any spurious signal. It should be noted that by using this definition of the XMCD, the sign of the signals is hereafter referred to the direction of the total magnetization of each compound. The absorption spectra were analyzed according to standard procedures: the origin of the energy scale, E0, was chosen at the inflection point of the Zn K-edge absorption spectrum of the bulk ZnO reference sample, and the spectra were normalized to the averaged absorption coefficient at high energy. This E0 choice is made to guarantee that all of the spectra are analyzed in a unique energy scale to reveal the existence of any edge-shift associated with the structural modification, i.e., to the modification of the interatomic distances.27,28 We have verified in all of the cases that the recorded spectra are not affected by the long beam-exposure time needed. Moreover, these measurements have been accumulated through a three year period on the same samples and specimens. No modification of both XAS and XMCD spectra has been found on the same specimens measured at the initial run and along a three years period, either in different specimens prepared from the same sample.

Major differences are observed between nominally identical samples from different batches indicating low reproducibility. As a consequence, in this study samples synthesized in the same way but from different batches are clearly labeled as different samples (e.g., 20-12C-2 and 20-12C-4). The stability of the as synthesized ZnO NPs, however, has been proved by the overlapping of successive XAS spectra recorded during the 3 year period. Several thin films were also prepared. A Zn−O−S thin film (ZnS/ZnO -50/50) was prepared by RF cosputtering from ZnO and ZnS 2-inch ceramic targets placed at 15°. The sample was grown using Ar gas (9.1 sccm), on Si(100) and quartz substrates at room temperature. The base pressure provided by the vacuum system was 1 × 10−6 mbar, and the working pressure was 5.2 × 10−3 mbar. The RF power supplied was about 20 W for ZnS and 30 W for ZnO. Under these conditions, the growth rate is nominally the same for both targets, and the samples were grown for 24 min, corresponding to 100 nm of thickness nominally, in agreement with X-ray reflectivity data. A part of the cosputtered samples was annealed in a quartz tube placed inside a furnace and under an O2 controlled flow (5 sccm, 5.0 × 10−3 mbar pressure). In order to investigate the role of ZnO/ZnS interfaces, two ZnO−ZnS multilayers, labeled (ZnO 4 n m /ZnS 4 n m ) 1 0 (+ ZnO4nm) and (ZnO2nm/ZnS2nm)20 (+ ZnO4nm), were prepared.23 The multilayers were fabricated by RF-sputtering on fused silica substrates at room temperature by alternative sputtering, starting with ZnO, and a last additional ZnO4nm layer was deposited on top. Residual chamber pressure was in the 10−7 mbar range. Both ZnO and ZnS layers were prepared using Ar at 5.1 × 10−3 mbar and 20 W. The samples were prepared keeping constant the ZnO and ZnS total thickness and modifying the number of interfaces. X-ray characterization, both diffraction (XRD) and reflectivity (XRR) measurements on the thin-film samples, was made by using a Bruker D8 X-ray diffractometer and Cu Kα radiation. Electron microscopy studies (TEM and HREM) were undertaken using a 200 kV JEOL2000FX transmission electron microscope (TEM) and with a JEOL 3000FEG electron microscope fitted with an Oxford LINK EDS analyzer. The macroscopic magnetic measurements, M(T) and M(H), were recorded by using a commercial Superconducting Quantum Interference Device magnetometer (Quantum Design MPMS5S) in the temperature range from 5 to 350 K. Because of the small magnetization signals of this type of samples, special care was taken to avoid any spurious effect on the magnetic measurements. The reliability of the macroscopic results revealing RTFM behavior was verified by accurate full sets of control experiments.8 Zn K-edge XAS and XMCD experiments were performed at the beamline BL39XU of the SPring8 Facility.25 The experiments were carried out at fixed temperatures, ranging from T = 5 K to ambient and under an applied magnetic field of up 10 T. XMCD spectra of the nanoparticles (NPs) and bulk ZnO and ZnS reference samples were recorded in the transmission mode by using the helicity-modulation technique.26 In this geometry, the sample is magnetized by an external magnetic field applied in the direction of the incident beam, and the helicity is changed from positive to negative at each energy point. The XMCD spectrum is then obtained as the difference of the absorption coefficient μc = (μ− − μ+) for antiparallel, μ−, and parallel, μ+, orientations of the photon helicity and the magnetic field applied to the sample. For the

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RESULTS AND DISCUSSION X-ray diffraction (Figure 1) measurements evidence the formation of ZnO nanoparticles (NPs) with average crystalline

Figure 1. X-ray diffraction patterns of the ZnO nanoparticles capped with thiol and reference samples.

size between 8 and 20 nm. The formation of hexagonal ZnO nanoparticles is further evidenced by the HRTEM images (Figure 2). In addition, energy dispersive X-ray analysis (EDX) excludes the presence of magnetic impurities within the sensitivity (0.01) of this technique. The XRD patterns were indexed on the basis of a ZnO wurtzite type unit cell, and the particle size calculated from the peaks by using the Debye− Scherrer formula are summarized in Table 1. The crystallinity of the samples strongly depends on the synthesis conditions, and in most cases, the XRD patterns show the presence of an amorphous phase coexisting with the crystalline w-ZnO one. In C



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Figure 2. HRTEM and EDX of the 30-12C-2 sample.

Figure 3. (a) Magnetization curves of the dodecanethiol-capped ZnO NPs (12C-2 series) measured at T = 5 K. (b) Thermal dependence of magnetization under an applied field of 1000 Oe. (c) T = 5 K magnetization curves after subtracting the diamagnetic linear background (see text for details). In the inset, details of the low field region, demonstrating the existence of remanence and hysteresis, are shown. (d) The same as panel c for T = 250 K.

samples prevent us from applying the Debye−Scherrer formula to determine the particle size. It should be noted in this respect that upon calcination, most of the organic part of the sample is

some of the samples, the amorphous phase, associated with the organic part of the material, is dominant, as will be evidenced in the XAS analysis, and the low crystallinity of some of the D



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ZnO single crystal reference. As illustrated in Figure 3a, all of the samples exhibit an overall diamagnetic behavior. The diamagnetic component has been determined from the data at 300 K (assuming that at this temperature any paramagnetic contribution is negligible, in view of the thermal dependence of the magnetization in panel Figure 5b), and a linear background from the 300 K curve has been subtracted in all of the samples.6,29 After subtracting this linear background, the magnetization of the dodecanethiol-capped ZnO NPs shows the existence of a ferromagnetic-like (FML) contribution, i.e., the magnetization curves show remanence, coercivity (HC ∼200 Oe), and saturation, in agreement with previous results.6,22 While this ferromagnetic-like signal is saturated at high temperature, it appears to be superimposed on a paramagnetic one at low temperature. The existence of a paramagnetic signal following Curie−Weiss law at low temperature is also observed in thermal dependence of magnetization (Figure 3b). The FML fingerprint is also found after calcination. In Figure 4, the curves corresponding to the 20-12C-1 sample prior to and after the calcination process are shown. After this process, the magnetization is increased by an ∼3 factor with respect to that of the parent compound. This increase can be initially associated with the removal of the organic part of the capping molecules. However, it is necessary to elucidate first if the calcination process actuates also on the crystal structure of the nanoparticle. It should be noted in this respect that the accuracy of the magnetization is limited by the normalization to the mass of the total amount of the sample (ZnO and organic part). Nevertheless, the magnetization after calcination still shows an FML component, and the coercive field remains unvaried, even when the saturation is slightly less marked. The occurrence of this FML component does not depend on the different thiol organic molecules, as illustrated in Figure 4b in the case of the butanethiol-capped 20-4C-4 sample. The FML contribution is maximized in the case of the (ZnO4nm/ ZnS4nm)10 heterostructure, which shows this behavior at both T = 5 K and at room temperature (see Figure 5). The FML component is still present, although less marked, in the case of the (ZnO2nm/ZnS2nm)20 film, while this contribution is not observed in the sample obtained for cosputtering whose magnetization curves approach those obtained in the ZnO single crystal reference. The fact that the nanoparticles and the multilayers present, along with a main paramagnetic response, signs of a ferromagnetic-like contribution, whereas bulk ZnO references and the cosputtered thin film show only paramagnetism suggests that the ferromagnetic-like response is related to the formation of a core−shell structure or interface on ZnO. Moreover, the fact that the FML component is reinforced in the (ZnO4nm/ZnS4nm)10 heterostructure, especially when compared to the (ZnO2nm/ZnS2nm)20 one, suggests that this effect is related not only to the formation of the ZnS−ZnO interface but also to its degree of structural disorder. Clearly, if the observed ferromagnetism is constrained to the interface of the core−shell nanoparticle (and the ZnO/ZnS interface of the multilayers), it is necessary to obtain a precise structural characterization of the systems prior to establishing the precise origin of this magnetic behavior. This task may be further complicated in materials exhibiting low crystallinity, and for this reason, using a local order structural technique is mandatory. In this way, besides XRD and TEM characterization, we have attempted to get a deeper insight into the characteristics of the

removed, and the broad peak at low angles disappears. In turn, a new broad peak appears, centered at 2θ ∼ 30°, indicating the formation of a very disordered ZnS component in the sample. The formation of a ZnS-like component coexisting with a ZnO one (see XANES analysis below) is also evidenced in the observed modification of the shape and broadening of the characteristic wurzite peaks (around 45° and 55°). The structural and magnetic characterization of the (ZnO4nm/ZnS4nm)10 and (ZnO2nm/ZnS2nm)20 multilayers can be found elsewhere.23 The analysis of the X-ray reflectivity spectra recorded on both films indicated smoother ZnO/ZnS interfaces for the (ZnO4nm/ZnS4nm)10 film than for the (ZnO2nm/ZnS2nm)20 one. For the latter sample, an average interface roughness of the order of layer thickness around 2 nm was found. These results pointed toward differences in the morphology of both multilayers, laying (ZnO2nm/ZnS2nm)20 in the regimen between continuous and noncontinuous layers, leading to more disordered ZnO/ZnS interphases. In contrast with the films prepared by sequential sputtering, the XRD patterns of the Zn−O−S sample prepared by cosputtering (ZnS/ZnO -50/50) show no peaks, both as grown and annealed samples, indicating their high degree of structural disorder. Figures 3 and 4 summarize the magnetic properties of the thiol-capped ZnO nanoparticles (only some representative are shown) obtained from macroscopic magnetization measurements (MPMS Quantum Design SQUID magnetometer). The same is shown in Figure 5 for the thin film samples and for a

Figure 4. (a) Magnetization curves of the dodecanethiol-capped 2012C-1 sample recorded at T = 5 K prior to and after calcination. Details of the low field region are given in the lower inset. The upper inset shows a typical TGA curve of the calcination process. (b) Magnetization curves of the butanethiol-capped 20-4C-4 sample measured at both T = 5 K and T = 250 K. Details of the low field region are reported in the inset. E



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Figure 5. (a) Magnetization curves of the (ZnO4nm/ZnS4nm)10 thin film measured at T = 5 K and at room temperature. (b) The same as that described above in the case of the (ZnO2nm/ZnS2nm)20 sample. (c) The same as that described above in the case of the ZnS/ZnO-50/50 sample obtained by ZnS-ZnO cosputtering: AG (as grown) and ANN (after annealing). (d) The same as that described above in the case of a ZnO single crystal.

substitution in an oxygen-terminated surface. On the contrary, they point out that a ZnS shell, showing also wurtzite-like structure, is formed surrounding the ZnO core. Moreover, the differences in the relative intensity of peaks A and B indicate that the details of this ZnO/ZnS core−shell structure, in particular the ZnS/ZnO ratio, depends on the synthesis reaction. Taking into account the absorption profile of bulk ZnS and bulk ZnO, we can clearly see from an inspection of Figure 6 that the ZnS/ZnO ratio is higher for the 5-12C-2 sample than for both 15-12C-2 and 30-12C-2 samples. Estimates of the ZnS/ZnO ratio have been obtained by comparing the experimental XANES spectra to the weighted addition of bulk ZnS and ZnO spectra. The experimental A/B intensity ratio is well reproduced by using a relative weight for the ZnS and ZnO components of 80% and 20% for 5-12C-2, 50% and 50% for 30-12C-2, and 40% and 60% for 15-12C-2 dodecanethiol-capped samples. All of the synthesized nanoparticles show similar XAS profiles independent of the organic molecule used (see Figure 7) but with different A/B ratios, as summarized in Table 1. These results bring us a new insight into the understanding of the observed dependence of the magnetization through the studied series of compounds. The fact that the paramagnetic contribution observed for dodecanethiol-capped samples (Figure 3c) changes for different ZnS/ZnO ratios (Figure 6) whereas the FML contribution remains basically the same (Figure 3d) suggests that the increase of the ZnS shell gives rise to an extra paramagnetic contribution to total magnetization. Thus, when the ZnS component of the nanoparticles prevails over the ZnO, the magnetization at low temperatures is

ZnS−ZnO interface in the synthesized thiol-capped nanoparticles and layered systems by using X-ray absorption spectroscopy at the Zn K-edge. The main reason for using Kedge absorption is its bulk sensitivity. The penetration length of the high energy incoming photons (∼9.6 keV) guaranties that all of the Zn atoms in the material (both at the surface and core of the nanoparticle) are being probed, which is fundamental in order to characterize the expected surface vs bulk effects. The Zn K-edge X-ray absorption spectrum of bulk ZnO (see Figure 6) is characterized by a main absorption peak (B) located at ∼7.5 eV above the absorption edge. In addition, the spectrum shows a positive spectral feature (D1) at ∼18 eV with a double shoulder-like structure at higher energies (D2 ∼ 21 eV; D3 ∼ 26 eV), which lies between the two well-resolved negative dips, C (∼14 eV) and E (∼35 eV). For higher energies, two additional positive resonances, F and G, appear at ∼53 eV and ∼80 eV, respectively, above the absorption edge. The XANES spectra of the dodecanethiol-capped samples (tadd12C-2 series), also shown in Figure 6, resemble that of bulk ZnO. However, they present several notable differences. In all of the cases, the intensity of the main absorption line (peak B) is strongly depressed with respect to that of bulk ZnO. The reduction in the intensity of peak B is coupled to the increase of the intensity of peak A that lies at the lower energy side of the main one (B). The observed experimental spectra are in agreement with previous studies showing the complexity of the bonding between the ZnO nanoparticle and the dodecanethiol molecule.30 The XANES spectra evidence that the molecule does not bond the nanoparticle through a simple S−O F



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dominated by a paramagnetic contribution superimposed onto the ferromagnetic one. In other words, our results indicate that the ZnS does not contribute to FML behavior. This hypothesis has been tested by comparing the behavior of the dodecanethiol-capped 20-12C-2 sample prior to and after the calcination process. The behavior of XANES shows (see Figure 6) that the relative amount of the ZnS component increases after the calcination process, i.e., the calcination process not only removes the organic part of the capping molecules but also actuates as an annealing-like process that enlarges the amount of ZnS showing an ordered wurtzite-like local structure in the shell surrounding the ZnO core. This would explain why the magnetization of the calcinated compound shows (Figure 4) an increase of the PM component with respect to that of the parent 20-12C-2 compound. The Zn K-edge XMCD spectra confirm the presence of separate ZnO and ZnS contributions to magnetization and the fact that the FML behavior is associated only with the ZnO. As shown in Figure 8, the XMCD spectrum of the dodecanethiolcapped 20-12C-1 sample shows a broad peak resulting from the superposition of two single peaks occurring close to the energy at which the main peaks of the XAS spectrum appear. The superposition of two contributions is made clear after calcination. The intensity of the low energy XMCD peak (p1) increases while that of the high-energy one (p2) remains the same. The existence of these contributions has been attributed to the magnetic polarization of the conduction band of Zn in both ZnS and ZnO.6 As shown in Figure 8b, each of the above contributions resembles the XMCD spectra of bulk ZnS and ZnO samples. It should be also noted that the comparison of the XAS and XMCD spectra with that of a Zn metal foil indicates the absence of nonbonded Zn atoms, i.e., Zn segregation in our samples. As shown in Figure 9, a similar behavior is observed for dodecanethiol-capped NPs obtained by varying tadd. The change of the ZnS/ZnO amount ratio is directly reflected in the XMCD spectral shape due to the different weights of the ZnS and ZnO components of the XMCD spectra. Since the observed Zn K-edge XMCD signals do not depend, both in shape and amplitude, on temperature (Figure 9), they can be only due to Pauli paramagnetism or to a ferro(i)magnetic contribution. In this regard, it should be noted that the fact that the XMCD signals do not depend on temperature excludes the occurrence of a Curie−Weiss paramagnetic (CWP) Zn contribution in these orbitals. The dependence of the XMCD intensity with the applied magnetic field should be linear if it is due to Pauli paramagnetism (PP) for the studied range of magnetic fields, while it should depart from linearity if a ferromagnetic contribution is present.23 Accordingly, the observed XMCD signals in both bulk ZnS and ZnO are due to the PP induced by the external magnetic field.23 In contrast, in the case of the 20-12C-1 sample, the dependence of the XMCD spectra with the applied magnetic field shows the coexistence of both types of magnetic behavior. In particular, as shown in the inset of Figure 8, the low-energy peak (p1), ascribed to the ZnSlike component, exhibits a linear XMCD vs H dependence, as expected for a PP contribution, whereas the high-energy component (p2), ascribed to the ZnO, shows a saturation trend suggesting the existence of a ferromagnetic contribution. After calcination, the ZnS component dominates the XMCD spectrum indicating that the calcination process, therefore, increases the structural order in the ZnS shell. Moreover, the linear dependence of the XMCD intensity with the applied

Figure 6. Top: Comparison of the normalized Zn K-edge XAS spectra of bulk ZnO (•) and ZnS (magenta, dashed line) and those of ZnO nanoparticles capped with dodecanethiol. Bottom: Comparison of the normalized Zn K-edge XAS spectra of dodecanethiol-capped ZnO nanoparticles obtained by varying the time at which the capping agent was added after the addition of the TMAH solution (see text for details).

Figure 7. Comparison of the normalized Zn K-edge XAS spectra of the ZnO nanoparticles obtained by using butanethiol (dotted line), octanethiol (dashed line), and dodecanethiol (solid line) and by varying the time at which the capping agent was added after the addition of the TMAH solution.

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Figure 8. (a) Comparison of the normalized Zn K-edge XAS and XMCD spectra of dodecanethiol-capped 20-12C-1 sample prior to (open circles) and after the calcination process (solid circles); samples, recorded at T = 5 K and at H = 10 T. (b) Comparison of the normalized Zn K-edge XAS and XMCD spectra of bulk reference samples, ZnS (blue, ●), ZnO (○) and metallic Zn (red, solid line), recorded at H = 10 T and T = 5 K. (c) Comparison of the XMCD spectra of 20-12C-1 recorded at T = 5 K and at different applied magnetic fields. The inset shows the variation with the applied magnetic field of the integrated XMCD corresponding to peaks p1 and p2 (see text for details). (d) The same as that described above in the case of the calcinated 20-12C-1 sample.

magnetic field agrees with a PP contribution stemming from the ZnS component. Unfortunately, its high-energy tail interferes with the ZnO-like contribution, which makes it difficult to accurately determine the dependence of the ZnO peak with the magnetic field. Nevertheless, the observed evolution could also be compatible with PM + FML contributions. The simultaneous presence of contributions associated with w-ZnO and w-ZnS is verified for all of the measured nanoparticles. Thus, Figure 9 shows the XMCD corresponding to series-2 samples. Dodecanethiol 15-12C-2 and 30-12C-2 behave as in the 20-12C-1 case, i.e., the XMCD spectra show clearly both ZnS and ZnO contributions. In contrast, the 512C-2 sample shows a single main peak resembling that of bulk ZnS, and a only small contribution coming from the ZnO component. This smaller ZnO contribution is clearly concluded from the comparison to bulk ZnS shown in Figure 9b. The XMCD spectra of 5-12C-2 exhibit a positive contribution at ∼7 eV above the edge, where ZnS shows a negative dip, and ZnO presents a positive peak. The modification of the XMCD intensity through the series is in full agreement with the conclusions derived from the analysis of the XAS data. As mentioned above, another parameter playing a significant role in the description of the synthesized nanoparticles is the degree of crystallinity. In this regard, the degree of definition of

peaks A and B as well as the modification affecting the threepeak structure (D1, D2, and D3) at ∼20 eV above the edge in the Zn K-edge X-ray absorption spectrum (Figure 6) is of particular significance. This D-spectral feature is characteristic of the wurtzite-ZnO crystal structure, and it is very sensitive to small modifications of the local structure around Zn.18,30 As shown in Figure 10, the Zn K-edge XANES spectrum of the (ZnO4nm/ZnS4nm)10 heterostructure shows a profile very similar to that of bulk ZnO, in agreement with the high crystallinity and the formation of neat interfaces derived from XRR. In contrast, the spectrum of the Zn−O−S film does not exhibit the characteristic spectral features of w-ZnS or w-ZnO systems but a rounded maximum indicating the amorphous character of the sample. Besides, while a nonzero XMCD signal is found in the case of the heterostructures, no detectable XMCD and thus no hint of saturation was found in the case of the sample made by cosputtering (see bottom panel in Figure 10). These results confirm the need for pristine ZnS−ZnO interfaces to obtain FML behavior in these systems. As for the nanoparticles, despite the fact that in all cases the simultaneous presence of peaks A (more precisely its reinforcement with respect to bulk ZnO) and B indicates the presence of ordered w-ZnS and wZnO regions in the materials, the fact that the three-peak structure (D1, D2, and D3) at ∼20 eV above the edge is illdefined indicates that there is a certain amount of Zn atoms H



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Figure 10. Top: Comparison of both Zn K-edge XANES spectra recorded for ZnO, ZnS, the sample obtained by ZnS-ZnO cosputtering, and of the (ZnO4nm/ZnS4nm)10 thin film. Bottom: Comparison of the XMCD spectra recorded at H = 10 T on the ZnO4nm/ZnS4nm)10 heterostructure (red, ●) and the ZnS-ZnO cosputtered (blue, ○) samples.

series of nanoparticle samples obtained by using butanethiol, octanethiol, and dodecanethiol as further illustrated in Figure 7. The Zn K-edge XANES spectra in Figure 7, indicate low crystallinity of the samples, in agreement with XRD, and a prevailing ZnS component. Accordingly, a dominant paramagnetic contribution in magnetization is measured at low temperatures, as reported in Figure 4.

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Figure 9. (a) Comparison of the normalized Zn K-edge XAS and XMCD spectra of several dodecanethiol-capped NPs in which the ZnS/ZnO ratio varied as a function of tadd. (b) Comparison of the normalized Zn K-edge XAS and XMCD spectra of bulk ZnS (○) and 5-12C-2 samples (●) recorded at T = 5 K and at both H = 6 T and 10 T. (c) Comparison of the XMCD spectra of the 5-12C-2 sample recorded at room temperature and T = 5 K.

SUMMARY AND CONCLUSIONS We have reported here a detailed study on the relationship between magnetic behavior and local structure of ZnO nanoparticles capped with different organic molecules. The XAS spectra recorded along a 3 year period demonstrate the high stability of the synthesized ZnO NPs. On the other hand, the XAS analysis also shows the low synthesis reproducibility since noticeable differences are observed between nominally identical samples. This indicates the need for a careful structural analysis when studying this type of samples. The combined analysis performed by using different characterization tools, including atom-specific XAS and XMCD, demonstrates that the modification of the surface of ZnO nanoparticles through capping with organic molecules enables the development of ferromagnetic behavior up to room temperature. The results indicate that the occurrence of ferromagnetism does not critically depend on the nanoparticle’s crystalline size

that is not forming part of the well-defined w-ZnO and w-ZnS regions of the core and shell23 but shorter-range ordered regions. One can assume that these Zn atoms are localized in the ZnO/ZnS core−shell interface in which the Zn−S and Zn− O bonds are modified with respect to those of the wurtzite structure. These results suggest that the higher crystallinity of the ZnO core would lead to the formation of a highly ordered interface between the ZnO core and the ZnS shell yielding an FML− ZnO contribution. This result is in agreement with the observation of amorphous interlayers in ferromagnetic ZnO films.31,32 Indeed,agreeing results are found for independent I



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(5) Chambers, S. A. Ferromagnetism in Doped Thin-Film Oxide and Nitride Semiconductors and Dielectrics. Surf. Sci. Rep. 2006, 61, 345− 381. (6) Chaboy, J.; Boada, R.; Piquer, C.; Laguna-Marco, M. A.; Garcı ́aHernández, M.; Carmona, N.; Llopis, J.; Ruı ́z-González, M. L.; González-Calbet, J.; Fernández, J. F.; et al. Evidence of Intrinsic Magnetism in Capped ZnO Nanoparticles. Phys. Rev. B 2010, 82, 064411/1−064411/9. (7) Lawes, A.; Risbud, A. S.; Ramrez, A. P.; Seshadri, R. Absence of Ferromagnetism in Co and Mn Substituted Polycrystalline ZnO. Phys. Rev. B 2005, 71, 045201/1−045201/5. (8) Garcı ́a, M. A.; Pinel, E. F.; de la Venta, J.; Quesada, A.; Bouzas, V.; Fernández, J. L.; Romero, J. L.; Martı ́n-González, M. S.; CostaKrämer, J. L. Sources of Experimental Errors in the Observation of Nanoscale Magnetism. J. Appl. Phys. 2009, 105, 013925/1−013925/7. (9) Coey, J. M. D.; Chambers, S. A. Oxide Dilute Magnetic Semiconductors - Fact or Fiction? MRS Bull. 2008, 33, 1053−1058. (10) Barla, A.; Schmerber, G.; Beaurepaire, E.; Dinia, A.; Bieber, H.; Colis, S.; Scheurer, F.; Kappler, J.-P.; Imperia, P.; et al. Paramagnetism of the Co Sublattice in Ferromagnetic Zn1−xCoxO Films. Phys. Rev. B 2007, 76, 125201/1−125201/5. (11) Céspedes, E.; Laguna-Marco, M. A.; Jiménez-Villacorta, F.; Chaboy, J.; Boada, R.; Guglieri, C.; de Andrés, A.; Prieto, C. On the Origin of the Magnetism of Mn-Zn-O Systems: Structural, Electronic, and Magnetic Study of Exotic MnO2−δ/ZnO Thin Films. J. Phys. Chem. C 2011, 115, 24092−24101. (12) Singh, A. P.; Kumar, R.; Thakur, P.; Brookes, N. B.; Chae, K. H.; Choi, W. K. NEXAFS and XMCD Studies of Single-phase Co Doped ZnO Thin Films. J. Phys.: Condens. Matter 2009, 21, 185005/1− 185005/7. (13) Tietze, T.; Gacic, M.; Schütz, G.; Jakob, G.; Brück, S.; Goering, E. XMCD Studies on Co and Li Doped ZnO Magnetic Semiconductors. New J. Phys. 2008, 10, 055009/1−055009/18. (14) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B. Donor Impurity Band Exchange in Dilute Ferromagnetic Oxides. Nat. Mater. 2005, 4, 173−179. (15) Wang, Q.; Sun, Q.; Chen, G.; Kawazoe, Y.; Jena, P. Vacancyinduced Magnetism in ZnO Thin Films and Nanowires. Phys. Rev. B 2008, 77, 205411/1−205411/7. (16) Garcı ́a, M. A.; Ruı ́z-Gonzaĺ ez, M. L.; Quesada, A.; CostaKrämer, J. L.; Fernández, J. F.; Khatib, S. J.; Wennberg, A.; Caballero, A. C.; Martı ́n-Gonzaĺ ez, M. S.; Villegas, M.; et al. Interface DoubleExchange Ferromagnetism in the Mn-Zn-O System: New Class of Biphase Magnetism. Phys. Rev. Lett. 2005, 94, 217206/1−217206/4. (17) Straumal, B. B.; Mazilkin, A. A.; Protasova, S. G.; Myatiev, A. A.; Straumal, P. B.; Schütz, G.; van Aken, P. A.; Goering, E.; Baretzky, B. Magnetization Study of Nanograined Pure and Mn-doped ZnO films: Formation of a Ferromagnetic Grain-boundary Foam. Phys. Rev. B 2009, 79, 205206/1−205206/6. (18) Guglieri, C.; Céspedes, E.; Prieto, C.; Chaboy, J. X-ray Absorption Study of the Local Order around Mn in Mn:ZnO Thin Films: The Role of Vacancies and Structural Distortions. J. Phys.: Condens. Matter 2011, 23, 206006/1−206006/8. (19) Kittilstved, K. R.; Gamelin, D. R. Activation of High-TC Ferromagnetism in Mn2+-Doped ZnO using Amines. J. Am. Chem. Soc. 2005, 127, 5292−5293. (20) Kittilstved, K. R.; Norberg, N. S.; Gamelin, D. R. Chemical Manipulation of High-TC Ferromagnetism in ZnO Diluted Magnetic Semiconductors. Phys. Rev. Lett. 2005, 94, 147209/1−147209/4. (21) Kittilstved, K. R.; Liu, W. K.; Gamelin, D. R. Electronic Structure Origins of Polarity-Dependent High-TC Ferromagnetism in Oxide-Diluted Magnetic Semiconductors. Nat. Mater. 2006, 5, 291− 297. (22) Garcı ́a, M. A.; Merino, J. M.; Fernández Pinel, E.; Quesada, A.; de la Venta, J.; Ruı ́z González, M. L.; Castro, G. R.; Crespo, P.; Llopis, J.; González-Calbet, J. M.; et al. Magnetic Properties of ZnO Nanoparticles. Nano Lett. 2007, 7, 1489−1494. (23) Guglieri, C.; Laguna-Marco, M. A.; Garcı ́a, M. A.; Carmona, N.; Céspedes, E.; Garcı ́a-Hernández, M.; Espinosa, A.; Chaboy, J. XMCD

or the length of the organic molecule (butanethiol, octanethiol, and dodecanethiol) but on the formation of a pristine ZnS− ZnO interface. The fact that all of the samples show similar FML magnetic properties despite the different surface-to-bulk ratios points out that ferromagnetism originates at this interface and not at the bulk-like components of the nanoparticles. The analysis of the XMCD spectra show the coexistence of both Pauli paramagnetism and intrinsic ferromagnetism in the samples. The contribution of the PP to the XMCD stems form the wurtzite-like ZnS and ZnO ordered regions of the sample, while ferromagnetism originates at the interface formed between the ZnS shell and the ZnO core. Moreover, our results demonstrate that within this interface ferromagnetism is favored in those regions of the interface where the local order is closer to w-ZnO than to w-ZnS. The results obtained in the case of thin films indicate that increasing the disorder of this interface weakens the onset of ferromagnetic behavior, which, on the contrary, is favored in the case of pristine ZnS− ZnO interfaces.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address #

Laboratoire Matière et Systèmes Complexes (MSC), UMR 7057, Université Paris Diderot, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Spanish MAT201127573-C04-04, MAT2010-16022, CSD2009-00013, FIS-200806249, CSD2009-00013, MAT2011-27470-C02-02 and MAT2010-09346-E grants, and by the Madrid Government project NANOBIOMAGNET S2009/MAT-1726. We thank the X-ray diffraction service of the ICMM. The synchrotron radiation experiments were performed at SPring-8 (Long Term Proposal No. 2009B0024). The assistance of the BL39XU staff during the SR experiments is acknowledged. C.G. acknowledges the Ministerio de Educación y Ciencia of Spain for a Ph.D. Grant. M.A.L.-M. acknowledges the Ministerio de Ciencia e Innovación of Spain for a Juan de la Cierva grant and the CSIC for a JAE-Doc contract within the Junta para la Ampliación de Estudios program. N.C. acknowledges the financial support of the FSE-MEC, Ramón y Cajal program (ref RYC-2007-01715).

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Relationship between the Magnetic Properties and the Formation of a

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