Surface Charge Transfer Induced Ferromagnetism in Nanostructured

48820 Bilbao, Spain. J. Phys. Chem. C , 2012, 116 (15), pp 8541–8547. DOI: 10.1021/jp211523f. Publication Date (Web): April 2, 2012. Copyright © 20...
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Surface Charge Transfer Induced Ferromagnetism in Nanostructured ZnO/Al Shijian Chen,† Nikhil V. Medhekar,*,† Jose Garitaonandia,‡ and Kiyonori Suzuki† †

Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia Zientzia eta Teknologia Fakultatea, Euskal Herriko Unibertsitatea, 644pk. 48820 Bilbao, Spain

‡

S Supporting Information *

ABSTRACT: The present study reports on the origins of room temperature ferromagnetism in zinc oxide (ZnO)-Al nanoparticles using a combination of X-ray absorption near edge structure (XANES) experiments and density functional theory (DFT) simulations. Our findings reveal that the spontaneous magnetization observed in these systems originates from the adsorption of Al on surfaces of ZnO nanoparticles. Our DFT simulations have identified unique configurations for Al adsorption on ZnO surfaces that lead to a spin-polarized charge transfer to O 2p states in surface and subsurface layers. XANES spectra of the magnetic ZnO/Al nanoparticles provide the necessary experimental evidence for the charge transfer to ZnO surfaces and confirm the origin of ferromagnetic behavior. Our results illustrate a complex interplay between the atomic level interfacial structure and the resulting ferromagnetic ordering in metal-coated semiconductor oxide nanostructures.

1. INTRODUCTION Ferromagnetic semiconductors are a unique class of materials that are being extensively studied for their potential applications in spintronics,1 patterned digital storage media,2 and biomedical diagnostics.3 Among such materials, zinc oxide (ZnO) is a leading candidate due to its ability to sustain ferromagnetism at room temperature.4 Moreover, ZnO can be easily synthesized into a variety of morphologies ranging from thin films to lowdimensional nanostructures such as particles and wires with an average feature size in the range of 10−200 nm.5 As a result, ZnO allows for a greater degree of control over its spindependent electronic properties. Conventionally, room temperature ferromagnetism (RTFM) in ZnO and other semiconductors can be achieved by doping them with transition metals in low concentrations.4 However, the dilute magnetic semiconductors containing transition metals often suffer from clustering of the dopants, which can lead to degradation of their magnetic properties.6 Recent studies have shown that this limitation potentially can be overcome via d0-ferromagnetism where the spin-polarized charge transfer between nonmagnetic elements and crystalline ZnO can be utilized to induce ferromagnetism at room temperatures.7−9 For example, it has been reported that doping thin films of ZnO by 2p light elements (such as C8 or N9) or metals (such as Li10 or Cu11) can give rise to ferromagnetic ordering. More recently, using organic compounds to passivate the surfaces of ZnO nanostructures and nanowires also has © 2012 American Chemical Society

been demonstrated to induce interfacial magnetism in these structures.12−15 One of the recent methods of introducing d0-ferromagnetism in ZnO is by nanoscale alloying of ZnO with Al. RTFM in alloys of ZnO and Al (ZnO/Al) has been reported for sol−gel processed particles,16 mechanically alloyed powders,17 and vapor-deposited thin films.18 ZnO/Al composite nanostructures obtained by these methods typically have an average size in the range of 15−50 nm.16,17 These studies illustrate that the relatively large surface and interface areas of the ZnO/Al nanostructures can be effectively exploited to sustain spontaneous magnetization larger than 25 memu/g at room temperature. While mixing ZnO with Al does provide a costeffective means of achieving RTFM in ZnO, it often leads to poor control over the magnetic properties since the atomic level mechanisms of ferromagnetic ordering are not well understood. Xue et al. have argued that RTFM in the sol−gel processed nanoparticles can be attributed to the oxygen vacancies induced by Al doping,16 whereas our earlier work suggested a possible charge transfer at the nanoparticle interface as the origin of RTFM in mechanically alloyed powders.17 In another study, Ding et al. have attributed RTFM in ZnO/Al thin films to the formation of metallic clusters of Al Received: November 30, 2011 Revised: March 25, 2012 Published: April 2, 2012 8541

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Figure 1. Side view (a) and top view (b) of the relaxed atomic configurations of Al adsorbed on the O-terminated ZnO (0001̅) surface. The three Al adsorption sites can be categorized as top-O (Al adsorbed directly above O in the topmost surface layer), top-Zn (Al adsorbed directly above Zn in the topmost surface layer), and hollow (Al adsorbed above the center of the hexagonal ring in the topmost surface layer). Gray, red, and blue spheres denote Zn, O, and Al atoms, respectively. The distance between the adsorbed Al and the nearest neighboring O is 1.77, 1.90, and 1.86 Å for top-O, top-Zn, and hollow configurations, respectively. Only three top ZnO layers of the supercell are shown for clarity.

in ZnO.18 In light of these studies suggesting different mechanisms for ferromagnetism in nanostructured ZnO/Al systems, a systematic investigation of the atomic level origins of the ferromagnetic behavior is timely. Motivated by these experiments, we build on our earlier work, which demonstrated ferromagnetism in ball-milled nanostructured ZnO/Al.17 Here, we perform a combination of X-ray absorption measurements and first principles density functional theory (DFT) simulations to investigate the origins of RTFM in ZnO/Al nanoparticles. Our simulations and experiments show that the ferromagnetic behavior can be attributed to the spin-polarized charge transfer between Al atoms adsorbed on ZnO surfaces and the oxygen atoms within the first few surface layers. On the basis of our simulations, we have identified energetically favorable Al binding sites on lowindex ZnO surfaces that can give rise to spin-polarized charge transfer. Our measurements of X-ray absorption near edge structures (XANES) provide the evidence for charge transfer between Al and ZnO surfaces and confirm the origin of ferromagnetic behavior. The rest of the paper is organized as follows. In section 2, we describe the details of our DFT simulations and X-ray absorption measurements. Results and discussion on lowenergy Al binding sites that lead to spin-polarized charge transfer to ZnO surfaces, followed by the XANES measurements that provide evidence for such charge transfer, are presented in section 3. Section 4 provides a brief summary and concluding remarks.

size of the nanoparticles is typically in the range of 15−50 nm.16,17 X-ray diffraction and electron microscopy measurements further showed that the particles have a homogeneous hexagonal wurtzite crystal structure, and furthermore, the particles are characterized by the absence of any impurity phases or clusters.16,17 The relatively large size of these particles and their uniform crystalline structure allows us to employ computational methods to model both doping and surface adsorption of Al to investigate the origins of ferromagnetic behavior. Specifically, we considered doping of Al in a bulk ZnO crystal lattice and the adsorption of Al on the following low-energy bulk-terminated ZnO surfaces: O-terminated (0001̅), Zn-terminated (0001), and (101̅0) and (21̅1̅0) nonpolar surfaces.19 In each case, we performed spin-polarized DFT calculations with the simulation package VASP to identify the low-energy configurations, followed by their electronic and spin structures.20−22 In these simulations, electron exchange and correlation were described using the generalized gradient approximation of the Perdew−Burke−Ernzerhof form.21 Core and valence electrons were treated using the projectoraugmented wave potentials supplied with VASP.22 The simulations for bulk Al doping of ZnO and for the adsorption of Al on low-enegy ZnO surfaces were performed within the supercell approach. The supercell for bulk doping of crystalline ZnO consisted of 2 × 2 × 2 wurtzite ZnO unit cells with one Zn atom replaced by Al. For Al adsorption on Oterminated (0001̅), Zn-terminated (0001), and (101̅0) and (21̅1̅0) nonpolar ZnO surfaces, the simulation cell consisted of a 2 × 2 ZnO surface unit cell with nine layers of bulk ZnO forming a slab. The periodic images of the slab were separated by 13 Å of vacuum in the direction normal to the surface. During structural optimization, the supercell vectors and all ionic positions were relaxed using a conjugate gradient

2. DETAILS OF COMPUTATIONAL AND EXPERIMENTAL METHODS A. DFT Simulations. The experimental studies investigating magnetism in nanoparticles of ZnO/Al report that the average 8542

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magnetic moments for Al adsorbed on different ZnO surfaces. We define the adsorption energy Ea as Ea = ET − EZnO − EAl, where ET is the total energy of the supercell containing Al adsorbed on the ZnO surface, EZnO is the total energy of pure ZnO in the same supercell, and EAl is the energy of one Al atom in fcc crystal lattice. With this definition, negative adsorption energy denotes an exothermic reaction, where it is energetically favorable for Al to adsorb on the surface. We find that for the O-terminated ZnO (0001̅) surface, Al has large negative adsorption energies for all three configurations. This can be attributed to an attractive interaction between electropositive Al and electronegative O. With the adsorption energy of −3.73 eV, the hollow site is the most stable Al adsorption site. It is clear from Figure 1 that the top-O site allows Al to bond with only the nearest oxygen (Al−O bond length 1.77 Å), while for top-Zn and hollow configurations, adsorbed Al interacts with the three nearest neighbor oxygen atoms. This results in the adsorption energy for top-O configuration (−1.51 eV) to be much lower than those of top-Zn and hollow configurations. For the case of the Zn-terminated (0001) surface, we find that it is thermodynamically unfavorable for Al to adsorb on the surface. Because of a low difference between the electronegativities of Zn and Al, the adsorption energies are uniformly positive. A similar dependence of the adsorption energy on the local bonding environment is also observed in the case of the nonpolar (1010̅ ) and (211̅ 0̅ ) surfaces, where top-O is the only favorable adsorption site for the (101̅0) surface, while hollow is the only favorable site for the (21̅1̅0) surface. In general, as summarized in Table 1, the binding energies for Al adsorption onto the Oterminated (0001̅) basal surfaces are significantly larger in magnitude than those on the other surfaces examined. Table 1 also shows the net magnetic moment m for each Al adsorption site for different ZnO surfaces. It is clear that the only stable configurations (i.e., with negative adsorption energies Ea) that yield a nonzero net magnetic moment are on O-terminated (0001)̅ surfaces, with magnetic moments of 0.88 μB and 0.71 μB for top-Zn and hollow Al adsorption sites, respectively. On the basis of the spin-polarized total energy calculations, we found that the adsorption of Al in these configurations favors a spin-polarized statethe total energy of the top-Zn and the hollow site is lower than that of the nonspin-polarized state by 20 and 17 meV, respectively. To understand the origin of net magnetic moments, we calculated partial density of states (PDOS) for the adsorbed Al and neighboring O and Zn atoms. Figure 2 shows PDOS for the most stable configuration with Al adsorbed at the hollow site on the O-terminated (0001)̅ surface. It can be seen that Al 3s and 3p electronic states overlap significantly with the 2p states of the nearest-neighbor oxygen near the Fermi level. This strong coupling causes splitting of hybridized Al 3s, Al 3p, and O 2p states near the Fermi level. The spin-up states are fully occupied while spin-down states are unfilled, resulting in a net magnetic moment of 0.71 μB per Al atom. We further find that the magnetic moment is primarily contributed by Al 3s (∼0.175 μB) and 3p (∼0.125 μB) orbitals, with 2p orbitals of each of the nearest-neighbor O contributing a smaller fraction (∼0.08 μB). The distribution of the net magnetic moment between the adsorbent and the surrounding ZnO matrix is also evident from the isodensity profiles for the corresponding spin density distribution as shown in Figure 3a. Apart from the adsorbent, only the nearest and the next nearest oxygen atoms contribute

algorithm until the Hellmann−Feynman forces were less than 0.01 eV/Å. In each case, we employed a 500 eV plane-wave kinetic energy cutoff. A 7 × 7 × 7 and 7 × 7 × 1 Γ-centered Monkhorst−Pack mesh was used to sample the irreducible Brillouin zone for the bulk and slab supercell, respectively. Finally, the magnetic moments on individual atoms were obtained directly from VASP by projecting atomic wave functions onto individual atomic orbitals. B. Sample Preparation and XANES Measurements. We prepared ZnO/Al nanoparticle powders by mixing high-purity ZnO and Al powders in a nominal ratio of 4:1. The details of sample preparation and magnetic measurements are similar to our earlier work.17 The powder mixtures were then ball milled in a high-purity alumina container. After ball milling, the powders were compacted into pellets and subsequently annealed at 10−3 Pa and 923 K. Magnetic characterizations of the samples were performed by vibrating sample magnetometry.17 XANES spectra were obtained at the Soft X-ray beamline of the Australian Synchrotron Facility. The O k-edge XANES spectra were measured by recording the total electron yield from the samples at pressures lower than 5 × 10−5 Pa. The resolutions were set to 0.1−0.2 eV at photon energies of 515−580 eV.

3. RESULTS AND DISCUSSION A. DFT Simulations. First, we investigated the magnetism in pure ZnO. As in the case of experiments,16−18 our DFT simulations revealed no net magnetic moment for bulk ZnO as well as in the slab supercell with low-index ZnO surfaces. It has been suggested that as a consequence of the mechanical ballmilling process, Al can be doped into the ZnO lattice by substituting for Zn ions.23 It is worth noting that our DFT calculations for Al doped in a ZnO crystal at Zn sites also revealed no net magnetic moment. This, in combination with the large surface to volume ratio of nanoparticles, suggests that the origin of the ferromagnetic behavior can possibly be attributed to the interaction between Al and ZnO at the surfaces of nanoparticles. Next, we examined the adsorption of Al on the four low-index surfaces as indicated above. In our simulations of Al adsorption on ZnO surfaces, isolated Al atoms were initially placed in the vicinity of the surface in a random manner to determine low-energy adsorption sites. We found that after relaxation, the Al atom adsorbs in three unique configurations irrespective of the surface orientation. These adsorption sites, in general, can be characterized as top-O (Al adsorbed directly above O in the topmost surface layer), top-Zn (Al adsorbed directly above Zn in the topmost surface layer), and hollow (Al adsorbed above the center of the hexagonal ring in the topmost surface layer). Figure 1 illustrates the three configurations for the case of Al adsorbed on O-terminated (0001)̅ surface. Table 1 shows the adsorption energies and net Table 1. Adsorption Energy Ea and Magnetic Moment m for Al Adsorbed on Low-Energy ZnO Surfaces Obtained Using DFT Calculations top-O

top-Zn

hollow

ZnO surface

Ea (eV)

m (μB)

Ea (eV)

m (μB)

Ea (eV)

m (μB)

(0001̅) (0001) (101̅0) (21̅1̅0)

−1.512 1.247 −0.816 0.770

0.000 0.000 0.000 0.370

−3.208 1.240 0.248 0.003

0.880 0.000 0.000 0.850

−3.731 1.226 0.254 −0.728

0.710 0.000 0.000 0.000 8543

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Al adsorption on ZnO surfaces is qualitatively similar to the doping of the ZnO matrix by the 2p light elements,8,9 where the spin density distribution was characterized by spin-polarized oxygen anions within second and fourth coordination spheres of the dopant atoms. However, spin polarization in ZnO/Al nanoparticles is in contrast to that of semiconductors doped with 3d transition metals,25 where spin is localized in a small tetrahedral region formed by the nearest-neighboring anions of the dopant atom. Next, we show that the long-range ferromagnetic coupling in Al adsorbed on the ZnO (0001̅) surface is primarily mediated by the spin-polarized oxygen sublattice in the surface and subsurface layers. To investigate the magnetic coupling between the adsorbed Al atoms, we considered a supercell twice as large as in the case of the single adsorbent discussed thus far. Taking hollow configuration as again an example, two Al atoms were initially placed in hollow sites separated by 3.3 (nearest neighbor hollow sites) and 6.6 Å (next-nearest neighbor hollow sites), respectively. We found that after complete relaxation, the configuration with two Al adsorbents separated by 6.6 Å favors ferromagnetic couplingthe total energy of the ferromagnetic state is lower by 20 meV than the configuration without spin polarization. Thus, the ZnO/Al nanostructured system is a weak ferromagnet at a low Al adsorption concentration. Figure 4 shows the magnetic coupling between the two Al ions separated by 6.6 Å. As can be clearly seen, the coupling between O 2p states in the surface and subsurface layers mediates the long-range ferromagnetic coupling. It should be noted that no net magnetic moment is obtained when two Al atoms are placed in neighboring hollow sites. After relaxation, Al atoms move closer from an initial distance of 3.3 to 2.72 Å, leading to antiferromagnetic coupling and zero net magnetization. Photoluminescence and Raman spectroscopy studies of ZnO/Al nanoparticles have shown that these nanoparticles are typically characterized by a large concentration of intrinsic vacancy defects,26 in particular, oxygen vacancies due to their relatively low formation energies.27 Our preliminary study on

Figure 2. Partial density of states (PDOS) for Al adsorbed on the Oterminated ZnO (0001̅) surface in the hollow configuration. The PDOS for the nearest-neighboring O (marked 1) and next nearestneighboring Zn (marked 2) are also shown. The atomic positions of atoms marked 1 and 2 are indicated in Figure 3. In all cases, the Fermi level (EF) is set to zero.

to the net magnetic moment, with Zn cations largely remaining unpolarized. Figure 3b shows the charge density difference (computed as the difference between the total charge density and the superposition of charge densities of the ZnO surface and the Al atom) distributed over adsorbed Al and the surrounding ZnO matrix. The charge density difference indicates that there is a net charge transfer from the adsorbed Al to the nearest and next-nearest oxygen. By performing Bader analysis,24 we have estimated the magnitude of the net charge transfer of 2.4 e− from adsorbed Al, of which approximately 2.0 e− and 0.4 e− is shared by the nearest and next-nearest oxygen atoms, respectively. It is evident that the alteration of the electronic structure and the spin structure due to Al adsorption on the surface extend to the second nearest-neighbor oxygen atoms in the ZnO surface and subsurface layer. In this regard, the spin polarization upon

Figure 3. Spin density (a) and electronic charge density difference (b) profiles for Al adsorbed on the O-terminated ZnO (0001̅) surface in the hollow configuration. The yellow color denotes isodensity contours of magnitude 0.0057 Å−3 for (a) and 0.0156 Å−3 for (b). Gray, red, and blue spheres denote Zn, O, and Al atoms, respectively. One and two specify the nearest-neighboring O and next nearest-neighboring Zn to the adsorbed Al atom. Only three top ZnO layers of the supercell are shown for clarity. 8544

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Table 2. Measured Room Temperature Spontaneous Magnetization Ms for ZnO/Al Nanostructures (from Reference 19) sample preparation method magnetization Ms at 300 K (memu/g)

Figure 4. Ferromagnetic coupling between two Al atoms adsorbed on the O-terminated ZnO (0001)̅ surface. The Al atoms are absorbed in the hollow configuration and are separated by 6.6 Å. The yellow color denotes the spin density contours of magnitude 0.0057 Å−3. Gray, red, and blue spheres denote Zn, O, and Al atoms, respectively.

sample A

sample B

sample C

pure ZnO nanoparticles

ZnO/Al nanoparticles, as-prepared 6

ZnO/Al nanoparticles, annealed at 923 K 26

0

samples when ball-milled with Al and subsequently annealed demonstrate a maximum saturation magnetization of 26 memu/g. Furthermore, X-ray diffraction and transmission electron microscopy measurements (not shown here) indicate that the particles in all three samples have a uniformly crystalline structure with an average diameter of 15 nm.17 It is important to note that several studies have suggested that pure ZnO nanostructures can be weakly ferromagnetic due to vacancies and other structural defects;28,29 however, we did not detect any appreciable magnetism in our ball-milled pure ZnO samples. Next, we examined the electronic states of oxygen obtained via XANES spectroscopy experiments at the Australian Synchrotron Facility. Figure 5 presents normalized k-edge

mechanically mixed ZnO/Al nanoparticles has also shown that the spontaneous magnetization is enhanced by annealing under a reduced pressure, which is accompanied by an increase in concentration of oxygen vacancies,17 particularly near the surfaces of nanoparticles. It has been suggested that such structural defects alter the electronic structures of the surrounding ZnO matrix and are therefore likely to give rise to the ferromagnetic behavior. To investigate the role of oxygen vacancies, we have performed spin-polarized DFT calculations for Al adsorption in the vicinity of oxygen monovacancies on O-terminated ZnO (0001̅) as well as (101̅0) and (21̅1̅0) nonpolar surfaces (see the Supporting Information for details). Al adsorption on surfaces with vacancies was modeled by taking out one oxygen atom from the surface ZnO layer and placing Al in the vicinity of the surface. After relaxation, we found that the stable configurations (i.e., with adsorption energy Ea < 0) that yield nonzero net magnetic moments are similar to the case of Al adsorbed on defect-free surfaces. Al adsorbed in top-Zn and hollow sites on O-terminated (0001̅) surface in the vicinity of oxygen vacancy have adsorption energies of −1.757 and 0.575 eV and magnetic moments 0.96 μB and 0.92 μB per Al atom, respectively. In general, the adsorption energies are lower as compared to Al adsorption on defect-free surfaces, since the adsorbent has only two nearest-neighbor oxygen atoms as compared to three on the defect-free surfaces. Nevertheless, the spin-polarized charge transfer to the nearest and next-nearest neighboring oxygen anions leads to enhanced magnetic moments as compared to defect-free surfaces. It is worth noting that in the absence of Al, the surfaces with oxygen vacancies remain nonmagnetic, thus indicating that a spinpolarized charge transfer from Al to ZnO is primarily responsible for the observed ferromagnetism in nanostructured ZnO/Al. B. XANES Experiments. To experimentally confirm our theoretical predictions of spin-polarized charge transfer, we synthesized samples of ferromagnetic ZnO/Al nanoparticles and performed XANES analysis. ZnO/Al nanoparticles were prepared by ball milling high-purity Al and ZnO powders in a nominal mole ratio of 1:4. We selected three samples for our subsequent detailed studies: sample A (pure nanostructured ZnO powders), sample B (as-prepared ZnO/Al nanoparticle powders), and sample C (ZnO/Al nanoparticle powders annealed in vacuum at 923 K). The magnetic properties of these samples measured at room temperature are summarized in Table 2. In agreement with earlier studies investigating magnetism in ZnO/Al,16−18 we found that pure ZnO (sample A) as well as pure Al powders were nonmagnetic, whereas ZnO

Figure 5. Normalized O k-edge XANES spectra for ZnO/Al nanoparticles for sample A (pure ZnO), sample B (as-prepared ZnO/Al nanoparticles), and sample C (ZnO/Al nanoparticles annealed at 923 K). Reduction in intensity of peaks A1 (at 535 eV) and B1 (at 537 eV) as compared with pure ZnO indicates charge transfer to oxygen in ZnO as a result of ball-milling with Al.

XANES spectra for oxygen bound in the ZnO matrix for the three samples. These spectra are typically characterized by five features, viz. A1, B1, C1, D1, and E1 (at absorption energies 535, 537, 542, 545, and 547 eV, respectively), that primarily correspond to the transitions from occupied O 1s states to the O 2pab (along ZnO bilayer) and O 2pc (along c-axis) unoccupied states. It can be observed that for the as-prepared sample (sample B), the intensity of low-energy features A1 and B1 is reduced as compared to the pure ZnO (sample A). This can be understood on the basis of the computed density of states as shown in Figure 2the hybridization of 3sp orbitals of adsorbed Al and 2p orbitals of neighboring O leads to a net charge transfer from Al to O. As a consequence of this charge transfer, there is an increase in the occupation of O 2p states leading to a reduction in A1 and B1 absorption intensities. Upon annealing (sample C), A1 and B1 intensities are further reduced, indicating a greater degree of occupation of 2p states of O in 8545

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diffusion of adsorbed Al to the lowest energy sites with nonzero net magnetic moments (see Table 1). However, when samples are overannealed, it can be argued that the nanoparticle surfaces become coated with adsorbents with maximum possible density, thus leading to an antiferromagnetic coupling and a loss of magnetization. It is important to note that the small size of ZnO nanoparticles surrounded by Al makes it difficult to precisely characterize the structure of all of the surfaces bounding the particles. Moreover, while we have restricted our theoretical simulations only to four low-energy ZnO surfaces, the ballmilling process and subsequent annealing may stabilize other high-energy surfaces with complex structures.32,33 Nevertheless, the mechanism presented herethat is, the spin-polarized charge transfer from adsorbed Al to the near-surface Ogives us an explanation of the possible origins of RTFM in ball-milled ZnO/metal nanoparticles. More detailed investigations can perhaps shed light on the adsorption of metallic charge donors on the high-energy ZnO surfaces.

the ZnO matrix as a consequence of increased charge transfer from adsorbed Al. The reduction in the intensities of the A1 and B1 peaks for as-prepared and annealed samples coincides with a significant enhancement in the saturation magnetization Ms (see Table 2), thus confirming the charge transfer from adsorbed Al to ZnO surfaces as the origin of the ferromagnetic behavior in ZnO/Al nanoparticles. It can also be observed from Figure 5 that in contrast with the low-energy features, the intensity of high-energy features C1−E1 increases for as-prepared and annealed samples. These high-energy features are understood to be associated with the formation of oxygen vacancies.30 Their enhanced intensities therefore confirm that the mechanical milling of ZnO with Al powders can create oxygen vacancies in ZnO and further annealing in vacuum significantly increases their concentration. As our calculations have shown, adsorption of Al near oxygen surface vacancies can also give rise to additional spin-polarized charge transfer and enhance the magnetization of ZnO/Al nanoparticles. In our experiments, we have observed the formation of a trace amount of Al3+ states when adsorbed Al reacts with oxygen liberated during annealing to form alumina. Such a reaction also involves a net charge transfer from Al to O. However, O k-edge XANES spectra for oxygen in alumina typically have an absorption peak below 535 eV,31 which are quite distinct from the characteristic transitions for oxygen in ZnO presented in Figure 5. It should also be noted that our XANES spectra are obtained through total electron yield channel, which are only sensitive to the empty states of oxygen in the surface layer of the particles (typically, less than 4 nm thick). Consequently, the XANES spectra shown in Figure 5 are a direct proof of the charge transfer from Al to ZnO surfaces. Charge transfer from Al to oxygen anions of ZnO can also occur when Al ions are doped in Zn sites in crystalline ZnO as a result of ball milling and subsequent annealing. However, as our calculations have shown, the charge transfer due to the bulk doping of ZnO by Al is not expected to result in a net magnetization of the samples. Finally, the spin-polarized charge transfer from adsorbed Al to the surfaces as the mechanism for ferromagnetism in ZnO/ Al nanoparticles may also explain other experimental observations.16−18 Experiments have shown that the measured saturation magnetization strongly depends on the nominal Al concentration. The magnetization increases initially with Al concentration before reaching a critical value beyond which the samples turn nonmagnetic. As the Al concentration is increased, a greater number of Al atoms become available for adsorption, thereby increasing the degree of charge transfer to the ZnO surface. Our calculations have clearly highlighted the dependence of the magnetic coupling on the surface density of Althe coupling is ferromagnetic when the distance between the neighboring adsorbed atoms is ∼6.6 Å, whereas at a lower distance, the coupling is antiferromagnetic leading to zero net magnetic moment. Thus, there must exist a critical surface density of adsorbed Al beyond which magnetization is lost. A similar behavior is also observed with respect to the annealing temperature, with magnetization increasing with temperature typically until the melting point of Al (∼930 K) beyond which magnetization rapidly reduces. Annealing samples in vacuum can increase the concentration of oxygen vacancies on the surfaces, thereby enhancing magnetization via increased charge transfer from Al adsorbed near vacancy sites. Moreover, elevated temperatures during annealing can enhance surface

4. CONCLUSIONS In summary, we have investigated the origins of RTFM in ZnO/Al nanoparticles using a combination of XANES experiments and first principles DFT calculations. Our calculations demonstrate that the intrinsic ferromagnetism in such systems originates from the adsorption of Al on the (0001̅) surfaces of ZnO nanoparticles. Such adsorption leads to a charge transfer from Al to ZnO surfaces resulting in spinpolarzied Al 3s and 3p electrons and O 2p electrons in the surface and subsurface layers. Moreover, Al adsorption near surface O vacancies also contributes to the net magnetism. Our calculations also show that the coupling interactions between O 2p electrons in surface and subsurface layers can give rise to long-range ferromagnetic coupling in such systems. Furthermore, XANES spectra of the magnetic ZnO/Al nanoparticles provide experimental evidence of the charge transfer from metal to O 2p electrons in ZnO surfaces and confirm the origin of ferromagnetic behavior. The mechanism proposed here also explains observed dependence of magnetization on the Al concentration and annealing temperature. Finally, our study clearly highlights the complex interplay between the atomic structure of ZnO surfaces and adsorbed Al that leads to a spinpolarized charge transfer across the surface. Furthermore, it suggests that the surface coating by metallic elements can be effectively exploited to develop a new class of room temperature ferromagnetic semiconducting materials based on low-dimensional nanostructures (for example, nanotubes34 and nanowires35) with precisely controlled surface and interface structures.

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ASSOCIATED CONTENT

S Supporting Information *

First principles DFT simulations of Al adsorption on lowenergy ZnO surfaces with vacancies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +61 3 9905 1421. Fax: +61 3 9905 4940. E-mail: nikhil. [email protected]. Notes

The authors declare no competing financial interest. 8546

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Smith, K. E.; Learmonth, T.; Glans, P. A.; Schmitt, T.; Guo, J. H. J. Appl. Phys. 2006, 99, 08M111. (31) Jung, R. J.; Lee, J. C.; So, Y. W.; Noh, T. W.; Oh, S. J.; Shin, H. J. Appl. Phys. Lett. 2003, 83, 5226. (32) Sun, S. G.; Zhou, Z. Y.; Tian, N.; Li, J. T.; Broadwell, I. Chem. Soc. Rev. 2011, 40, 4167. (33) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (34) Wu, C. C.; Wuu, D. S.; Lin, P. R.; Chen, T. N.; Horng, R. H. Cryst. Growth Des. 2009, 9, 4555. (35) Wang, Z. L.; Wang, X. D.; Song, J. H. J. Mater. Chem. 2007, 17, 711.

ACKNOWLEDGMENTS We are grateful to the Australian Research Council for its financial support. We also thank Dr. Bruce Cowie of the Australian Synchrotron Facility for his support and useful discussions. Finally, we gratefully acknowledge the support from the Australian National Computational Infrastructure facility through the National Computational Merit Allocation Scheme.

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dx.doi.org/10.1021/jp211523f | J. Phys. Chem. C 2012, 116, 8541−8547