J. Phys. Chem. C 2010, 114, 19629–19634
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Characterization of the ZnO-ZnS Interface in THIOL-Capped ZnO Nanoparticles Exhibiting Anomalous Magnetic Properties C. Guglieri and J. Chaboy* Instituto de Ciencia de Materiales de Arago´n, Consejo Superior de InVestigaciones Cientı´ficas and Departamento de Fı´sica de la Materia Condensada, UniVersidad de Zaragoza, 50009 Zaragoza, Spain ReceiVed: August 6, 2010; ReVised Manuscript ReceiVed: October 4, 2010
This work reports an X-ray absorption near-edge structure spectroscopy (XANES) study at the Zn K-edge in ZnO nanoparticles (NPs) capped with different organic molecules. The analysis of the XANES spectrum of NPs capped with dodecanethiol suggests that several Zn atoms bond to the S atoms of the capping molecule. Indeed, the comparison of the experimental XANES spectra and theoretical computations points out the existence of a ZnO-ZnS interface in which both ZnS and ZnO shows a wurtzite structure. These results support that the exotic magnetism observed in these capped ZnO NPs resides at the hybridized p band formed among Zn and the S bonding atoms in this interface, while the rest of the ZnO NPs behaves as a standard ZnO semiconductor. I. Introduction The worldwide research activity at the nanoscale is triggering off the appearance of new, and frequently surprising, materials properties in which the increasing importance of surface and interface effects plays a fundamental role. This opens further possibilities in the developing of new multifunctional materials with tuned physical properties that do not arise together at the bulk scale. In this framework, the possibility of combining ferromagnetism and semiconductor properties is probably one of the most attractive issues because of its implications in the technology of novel magneto-optoelectronic devices.1-4 Accordingly, the research focused in the so-called diluted magnetic semiconductors (DMS), that is, semiconductors doped with ferromagnetic impurities, has recently attracted much attention. However, the origin of the magnetic properties of these systems is still a puzzle as (i) different laboratories have reported incongruous results for seemingly identical materials;5-8 (ii) X-ray magnetic circular dichroism (XMCD) studies performed at the L2,3-edges of the 3d dopants did not see any element specific signature for ferromagnetism at all;9-14 and (iii) in many cases the ferromagnetism can be explained by extrinsic effects.8,15-19 These contradictory results posed serious doubts on the intrinsic nature of the observed high-temperature ferromagnetism in DMS and, consequently, it is still necessary to elucidate if doping of semiconductors with 3d ions would produce a ferromagnetic ground state. On the other hand, recent results have reported that it is possible to induce room-temperature ferromagnetic-like behavior in semiconductor and insulating oxide nanostructures without doping with magnetic impurities.20-23 These new results suggest that the alteration of the electronic configuration of the nanostructures is the responsible of the appearance of the magnetic properties. This is the case of ZnO nanoparticles capped with different organic molecules for which the observation of room temperature ferromagnetism has been recently reported.23 Here, we report a detailed Zn K-edge X-ray absorption near * To whom correspondence should be addressed.
edge spectroscopy (XANES) study of the structural effects induced by capping (with S, O, and N) ZnO NPs. The study of the near-edge part of the absorption spectrum (XANES) is advantageous because of its high sensitivity to the bonding geometry that provides, contrary to extended X-ray absorption fine structure (EXAFS), an unambiguous determination of the coordination polyhedron.24,25 In addition, we have taken advantage of the different backscattering properties of S with respect to both O and N providing specific spectral signatures of the Zn-S scattering contributions to the XANES spectrum. In this way, by comparing the experimental spectra to ab initio calculations of the XANES spectra, we have determined the formation of a well-defined ZnS interface between the oxide NPs and the capping molecules. These results suggest that the observed magnetic properties should critically depend on the details of this interface. II. Experimental and Computational Methods ZnO nanoparticles (NPs) were prepared by sol-gel methods and subsequently capped with three different organic molecules: tryoctylphosphine, dodecylamine, and dodecanethiol, which bond to the particle surface through an O, N, and S atom, respectively.23,26,27 Zn(Ac)2 (5 mmol) was dissolved in dimethyl sulfoxide and the solution was heated and kept at 60 °C under stirring and then a solution of tetramethylammonium hydroxide pentahydate (7.5 mmol) in ethanol was added dropwise. After that, S-capped (hereafter THIOL), O-capped (hereafter TOPO), and N-capped (hereafter AMINE) nanocrystals were precipitated adding, respectively, 7.5 mmol dodecanethiol, tryoctylphosphine, and dodecylamine solutions in heated ethanol to the precursor solution. The products thus obtained were filtered and washed several times with heated ethanol. The recovered powders were allowed to dry at room temperature. The structural characterization, performed by means of X-ray Diffraction (XRD) and transmission electron microscopy (TEM), evidence the formation of hexagonal wurzite ZnO nanoparticles with average size around 20 nm.28 The macroscopic magnetic characterization, whose reliability has been confirmed by an accurate full set of control experi-
10.1021/jp1074303 2010 American Chemical Society Published on Web 11/01/2010
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J. Phys. Chem. C, Vol. 114, No. 46, 2010
Guglieri and Chaboy
Figure 1. Comparison of the experimental Zn K-edge XANES and XMCD spectra ZnO nanoparticles capped with TOPO (green, b), AMINE (blue, O), and THIOL (red, 0). For the sake of comparison, the XANES spectra of both bulk ZnO (black, solid line) and ZnO nanopowder (purple, dotted line) are also shown. In the inset, the ferromagnetic component of the magnetization at 5 K is reported.
ments,18 was performed by using a MPMS Quantum Design SQUID magnetometer. The Zn K-edge X-ray absorption spectroscopy (XAS) experiments were performed at T ) 4.2 K in the transmission mode on the beamline BL39XU of the SPring8 Facility.29 XMCD spectra were recorded by using the helicity-modulation technique under an applied magnetic field of 6 T. For the measurements, homogeneous layers of the powdered samples were made by spreading of fine powders of the material on an adhesive tape. Thickness and homogeneity of the samples were optimized to obtain the best signal-to-noise ratio. We have verified that the XAS spectra of the NPs samples do not show any detectable degradation when subjected to the X-ray beam. Indeed, scans recorded after one year on the same specimens show a perfect coincidence. In all the cases, the absorption spectra were analyzed according to standard procedures and were normalized to the averaged absorption coefficient at high energy to eliminate the dependence of the absorption on the sample thickness. The computation of the Zn K-edge XANES spectra was carried out by using the multiple-scattering code CONTINUUM.30 A complete discussion of the procedure can be found elsewhere.31,32 The potential for the different atomic clusters was approximated by a set of spherically averaged muffin-tin (MT) potentials built by following the standard Mattheis’ prescription. The muffin-tin radii were determined following the Norman’s criterion. During the computations special attention has been paid to determine the best choice for the overlapping factor between the muffin-tin spheres and for the exchange and correlation part of the final state potential.33-35 It should be stressed that no free parameter has been used during the calculations. The theoretically calculated spectra have been directly compared to the experimental XANES spectrum, that is, no fitting procedure has been used. The assessment of the quality of the theoretical computations is based on the correct reproduction of the shape and energy position of the different spectral features and of their relative energy separation and the intensity ratio. In all the cases, the theoretical spectra have been convoluted with a Lorentzian shape function to account for the core-hole lifetime (Γ ) 1.5 eV)36 and the experimental resolution (Γ ) 1 eV).
III. Results and Discussion The comparison of the Zn K-edge absorption spectra of the TOPO-, AMINE-, and THIOL-capped ZnO NPs samples, bulk ZnO, and commercial ZnO nanopowder with particle size
