Cu Codoped ZnO Nanocrystals - The

Jul 12, 2012 - Council of Scientific and Industrial Research – Network of Institutes for Solar Energy (CSIR-NISE), New Delhi, India. J. Phys. Chem. ...
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Magnetic Properties of Fe/Cu Codoped ZnO Nanocrystals Ranjani Viswanatha,†,# Doron Naveh,‡,∥ James R. Chelikowsky,§ Leeor Kronik,‡ and D. D. Sarma*,†,⊥,∇ †

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth 76100, Israel § Departments of Physics and Chemical Engineering, Center for Computational Materials, Institute for Computational Engineering and Sciences, The University of Texas at Austin, Austin, Texas 78712, United States ⊥ Council of Scientific and Industrial Research − Network of Institutes for Solar Energy (CSIR-NISE), New Delhi, India ‡

S Supporting Information *

ABSTRACT: Free-standing ZnO nanocrystals simultaneously doped with Fe and Cu with varying Fe/Cu compositions have been synthesized using colloidal methods with a mean size of ∼7.7 nm. Interestingly, while the Cu-doped ZnO nanocrystal remains diamagnetic and Fe-doped samples show antiferromagnetic interactions between Fe sites without any magnetic ordering down to the lowest temperature investigated, samples doped simultaneously with Fe and Cu show a qualitative departure in exhibiting ferromagnetic interactions, with suggestions of ferromagnetic order at low temperature. XAS measurements establish the presence of Fe2+ and Fe3+ ions, with the concentration of the trivalent species increasing in the presence of Cu doping, providing direct evidence of the Fe2+ + Cu2+ ⇌ Fe3+ + Cu+ redox couple being correlated with the ferromagnetic property. Using DFT, the unexpected ferromagnetic nature of these systems is explained in terms of a double exchange between Fe atoms, mediated by the Cu atom, in agreement with experimental observations. SECTION: Physical Processes in Nanomaterials and Nanostructures

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states. While it has been shown that even undoped semiconductor nanocrystals may show magnetism, most probably due to unpaired electrons in surface-related states,10 the magnetic moments are typically too small (∼6 × 10−4 emu/g for 30 nm ZnO nanoparticles) in such cases to be of much practical use, particularly when compared with the large unpaired spins associated with open 3d shell transition-metal ions. Besides the possibility of engineering the band gap to any value, including a gap beyond the visible range allowing for transparent matter, there is another important consequence of obtaining a magnetic nanocrystal of the dimension less than 10 nm. Because the magnetic moment of each such nanocrystal may be manipulated by the application of a magnetic field, these may constitute extremely small-sized magnetic storage bits in a future technology. Despite such exciting possibilities, we, however, note that both extensive doping of transitionmetal ions and obtaining a magnetic state11−13 in such semiconductor nanocrystals have proven to be an even greater challenge than that faced in the case of bulk semiconductors. In order to realize this important, but difficult, goal of attaining magnetic semiconductor nanocrystals, some of us have investigated both experimentally and theoretically a large

lectrical conductivity and magnetic properties in bulk semiconductor lattices are primarily controlled by the incorporation of impurities. Transition-metal impurities have been found to alter optical, magnetic, and other physical properties of the host semiconductor significantly, leading to intense interest in dilute magnetic semiconductors (DMS).1−7 In particular, interest in FM semiconductors has spiked due to their potential as spin-polarized carrier sources and the relative ease of their integration into semiconductor devices.8 Additionally, the idea of magnetically doping wide-band-gap semiconductors holds out the tantalizing possibility of forming transparent magnets with interesting magneto-optical applications. Elucidation of the origin of ferromagnetism in such materials has turned out to be among the most important problems in magnetism to have emerged in several years,2,7 and identification of the critical parameters governing DMS ferromagnetism has been challenging.2,9 It is well-known that the physical properties of semiconductor materials may also be tuned, in the absence of any doping, by changing the grain size in the nanometer regime. Such semiconducting nanocrystals show a nearly continuous tuning of the band gap with decreasing size. This offers the exciting possibility of obtaining magnetic semiconductor nanocrystals with any desired band gap by controlling the size, if we are able to successfully dope semiconductor nanocrystals with transition metal-ions and obtain magnetic © 2012 American Chemical Society

Received: June 7, 2012 Accepted: July 12, 2012 Published: July 12, 2012 2009

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number of doped systems.12−21 It turns out that all of our carefully doped and thoroughly characterized semiconductor nanocrystal systems exhibited purely paramagnetic behavior down to the lowest temperature with indications of small antiferromagnetic interactions,14 belying our expectations. Prompted by a report of a magnetic state of the ZnO bulk semiconductor upon doping with Fe and Cu simultaneously,22 we wished to explore whether this route might be adopted to achieve the even more elusive magnetic, semiconductor nanocrystal. In this Letter, we show that it is indeed possible to dope Fe and Cu individually as well as together in small quantities in free-standing ZnO nanocrystals. Interestingly, individually doped Fe−ZnO and Cu−ZnO systems show no evidence of a ferromagnetic order. In fact, there is clear evidence of antiferromagnetic interactions in such doped systems. Surprisingly, the simultaneously Fe- and Cu-doped systems show a clear signature of ferromagnetism. In order to obtain a clue to this unexpected behavior in terms of microscopic interactions at the atomic level, we have carried out X-ray absorption spectroscopy (XAS) measurements. XAS results clearly show the presence of both Fe2+ and Fe3+ species, the relative concentration being dependent on the presence of Cu dopant, thus providing a direct correlation between electronic structure changes and ferromagnetic coupling using XAS. In order to understand this phenomenon, we employ calculations based on density functional theory (DFT). These calculations explain this phenomenon in terms of Cu-related electronic structure changes that promote a ferromagnetic configuration via a double-exchange mechanism, which is distinct from the earlier suggestion of a double-exchange mechanism based solely on Fe d states.22 Zn1−x−yFexCuyO nanocrystals were synthesized by hydrolyzing stoichiometric amounts of Zn, Cu, and Fe precursors with NaOH. Further details are given in the Methods Section. Typical X-ray diffraction (XRD) patterns for different concentrations of Fe and Cu-doped ZnO nanocrystals are shown in Figure 1a. Comparing the XRD patterns with that of the bulk ZnO crystallizing in the wurtzite phase, we observe that both doped and undoped nanocrystals crystallize in the same phase with similar lattice parameters. Similar to earlier literature reports,12,23 the pattern was simulated (also shown in Figure 1a) by broadening the bulk XRD pattern using the Scherrer formula, and quantitative information on the size of the nanocrystals was obtained. The size of the nanocrystals obtained by this procedure for both doped and undoped samples was found to be 7.7 nm. Nanocrystal size was also confirmed using transmission electron microscopy (TEM), which showed an abundance of nearly spherical particles with an average size of 7.5 nm, in agreement with the values obtained from XRD measurements. A typical high-resolution image of the particle is shown in the inset to Figure 1b. The size distribution of these particles (Figure 1b) is quite broad due to the absence of capping ligands during the growth of the nanocrystals. Selected area electron diffraction (SAED) of the particles confirms a high degree of crystallinity. The various rings that could be indexed to the diffraction planes of ZnO nanocrystals are shown in Figure 1c. Magnetic susceptibility measurements (Figure 2) show the M−H curves at 10 K for various percentages of Fe/Cu-doped ZnO nanocrystals. Shown in inset I of Figure 2 is the M(H) curve of Cu-doped ZnO, exhibiting diamagnetic behavior as expected. The inverse magnetic susceptibility plots as a function of temperature (data not shown) of Fe-doped ZnO nano-

Figure 1. (A) XRD patterns of free-standing ZnO (i) and Fe/Cudoped ZnO nanocrystals (ii−iv), along with the simulated XRD pattern obtained for 7.7 nm (v) particles. The XRD pattern of bulk ZnO (vi) is also shown for comparison. (B) Histogram of the size distribution of these nanocrystals obtained from the TEM images. The inset shows a high-resolution image of the doped nanocrystal. (C) SAED pattern of Fe/Cu-doped ZnO nanocrystals.

particles showed a negative intercept, suggesting the presence of antiferromagnetic (AFM) interactions. Interestingly, the magnetic properties change qualitatively upon codoping with Cu, as shown in the main panel of Figure 2. Attempts at fitting the M(H) curve using the Brillouin function resulted in a poor fit in all regions, suggesting a deviation from the paramagnetic case for all concentrations of Fe and Cu. This suggests that the Fe/Cu-doped ZnO behaves in a qualitatively different way than both individually doped Fe or doped Cu systems and could show signs of FM interactions in the nanocrystal. Importantly, the 5% Fe- and 1% Cu-doped ZnO nanocrystal showed an opening in the hysteresis loop when the M(H) was measured at 2 K, as shown in inset II of Figure 2, whereas all curves measured at higher temperature for the same sample were found to scale with temperature. This provides direct evidence for low-temperature FM coupling in ZnO free-standing nanoparticles when simultaneously doped with Fe and Cu. To test whether the enhancement of FM interaction is directly related to the Cu codoping, we calculated from first principles the total energy of doped Zn38O38 nanocrystals, constructed in a manner described in the Methods Section. For the Cu-free case, we found that the AFM alignment of the Fe spin moments was lower in energy (by ∼275 mRy) than the FM alignment, in agreement with the experimental observation 2010

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Figure 3. XAS of different percentages of Fe/Cu-doped ZnO nanocrystals, where x and y correspond to Fe and Cu composition, respectively. The various scatter plots show the experimental spectra. The solid line shows a typical theoretically calculated spectrum, for both d5 and d6 oxidation states, added in different ratios. The component spectra are shown at the bottom with a thin black line (d6) and a thin red line (d5). The inset shows the variation of d5 concentration as a function of saturation magnetization obtained from Figure 2.

Figure 2. M−H curves of different percentages of Fe/Cu-doped ZnO nanocrystals, measured at 10 K. Inset I shows the M−H curve of Cudoped ZnO, showing diamagnetism. Inset II shows M as a function of H/T for the Zn0.94Fe0.05Cu0.01O sample at different temperatures, showing that the hysteresis loop at 2 K does not scale with temperature.

each case suggests that the doped Fe is present in the lattice with a tetrahedral FeO4 geometry, as both divalent and trivalent species. It is observed that the trivalent Fe component is highest for the case of the 5% Fe- and 1% Cu-doped system, exactly where the saturation magnetization at 10 K is the highest among all compositions investigated by us. This composition also exhibits a ferromagnetic order. In fact, we find a monotonic relationship between the saturation magnetization at 10 K (Figure 2) and the relative contribution of the Fe3+ signal in XAS (Figure 3) for all compositions, as observed in the inset to Figure 3, firmly establishing a correlation between the magnetic properties and the presence of the Fe3+ species. However, it is interesting to note that the average concentration of the dopant does not reflect the actual concentration in each nanocrystal, including cases where there is no Cu or Fe in a particular nanocrystal. Moreover, even in the presence of simultaneously doped Fe and Cu in ZnO nanocrystals, it is quite possible that the Cu dopant is not sufficiently close to a pair of Fe dopants to induce the ferromagnetic interactions. Thus, it is clear that the magnetic properties that we have discussed are indeed an average over many realizations of the dopant concentrations and positions. Interestingly, our work shows that it is still possible to induce ferromagnetism in the average sense in such samples despite statistical variations from one nanocrystal to another. While ref 22 suggested the absence of Cu2+ based on their (unreported) XAS data in the context of the bulk sample, we note that a very low concentration of overall Cu doping along with a lower intrinsic intensity of Cu XAS compared to transition-metal ions, such as Fe, makes it difficult to probe Cu2+ species, if present, as a fraction of the total Cu doping. It is obvious that electron paramagnetic resonance (EPR) will be a more sensitive probe in such cases as the Cu2+ species will be the only one that is EPR-active. Thus, we investigated the EPR of these compounds and obtained a clear Cu2+ signal, as shown

of antiferromagnetic interactions in the Fe-only-doped ZnO nanoparticles. In the presence of Fe and Cu codoping, we envisage redoxlike pairs, Fe2+ + Cu2+ ⇌ Fe3+ + Cu1+, via hopping. This anticipation is fully justified by our experimental results, as shown by probing the valence state of the Fe ions in the ZnO nanocrystals, using XAS measured for several typical Fe/Cu compositions at the Fe L3 edge corresponding to Fe 2p → 3d excitations. The L3 edge of transition-metal ions is known to be dominated by the local electronic structure and, therefore, can provide important information about the transition-metal oxidation state and point-group symmetry.24−27 The obtained spectra are shown in Figure 3 as filled circles of different colors for different compositions. The signals show the L3 edge of Fe to be centered around 712.5 eV, with a prominent shoulder at the 710.9 eV photon energy, the relative intensity of this shoulder depending on the dopant concentrations. The simultaneous presence of these two prominent features at 712.5 and 710.9 eV cannot be explained by the presence of a single oxidation state of Fe. It can only be explained by the presence of both Fe3+, d5 and Fe2+, d6 oxidation states. The variation in the relative intensity of the shoulder at ∼710.9 eV indicates the presence of different percentages of two different components with changing compositions. In order to determine the exact valence, symmetry, and crystal field strengths, we carried out theoretical calculations (see the Methods Section for details) for various crystal field strengths. The calculated spectra for Fe2+ (d6) and Fe3+ (d5) are shown at the bottom of Figure 3 by a thin black line (d6) and a thin red line (d5), respectively. The total spectrum for any given composition was calculated by adding different weights of the component spectra to obtain the best fit with the corresponding experimental spectrum. The good agreement between the experimental spectrum and the calculated result in 2011

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codoped in ZnO system29 or double exchange involving only Fe states.22 This process of double exchange obtained due to the presence of Cu in between two Fe atoms allowing for the highest-energy electrons to delocalize via hopping between Fe and Cu sites is similar to that observed in the literature earlier30 using theoretical ab initio calculations where the empty orbital required for hopping is obtained from the process of an excitation. In this scenario, the total magnetic moment of the two Fe and one Cu together is 7 μB. In the second competing configuration of comparable energy, shown on the right side in Figure 4, the magnetic moment on each atom is the same in magnitude, but the two Fe atoms are coupled antiferromagnetically due to the superexchange mechanism, with the Cu atom moment aligned with one of the Fe atoms. In other words, this AFM state carries the magnetic moment of the Cu atom, 1 μB. One may already anticipate this configuration to be destabilized by the implicit frustration, where the Cu site cannot simultaneously be antiferromagnetically coupled with both Fe sites to lower the total energy; however, this configuration gains from the lowering of the total energy by the superexchangedriven antiferromagnetic coupling of the two Fe sites. Other spin configurations were found to be substantially higher in energy and are ignored here. Thus, the computational results confirm that Cu codoping strongly enhances the relative stability of Fe FM coupling with respect to that of Fe AFM coupling. Again, this is in excellent agreement with the experimental observation. Symmetry- and defect-position-related differences in the relative stability of the FM and AFM phases are well-known and, particularly for codoping of bulk ZnO with Co and Cu, have been studied thoroughly in ref 29. In addition to this doping geometry dependence, clearly, the exact numerical value of the stabilization of the one magnetic phase over the other will also depend on the choice of density functional, residual atomic relaxations, nanocrystal size,31,32 and, for our specific calculations, the choice of the on-site energy, U. Therefore, a complete quantitative comparison with experiment is not attempted here. However, the salient point is that we find consistently that Cu codoping can indeed explain the promotion of FM in the Cu-codoped nanocrystals, whereas in the absence of any Cu, the Fe-doped sample is indeed expected to manifest antiferromagnetic interactions, as observed in the present experiments. In conclusion, we report the synthesis of free-standing Fe/ Cu-doped ZnO nanocrystals using a colloidal method with varying Fe/Cu compositions. XRD patterns suggest the formation of wurtzite nanocrystals with a mean size of ∼7.7 nm. While Fe-doped samples indicate antiferromagnetic interactions and Cu-doped samples are essentially diamagnetic, the magnetic susceptibility curves for these simultaneously doped systems surprisingly suggest possible ferromagnetic interactions as the curves could not be fitted with a Brillioun function. Moreover, at the extremely low temperature of 2 K, an opening of the hysteresis loop is also observed. XAS measurements establish the presence of Fe2+ and Fe3+ in different ratios and expose a correlation between the trivalent species and the promotion of magnetism in these samples. This, in conjunction with the observation of a Cu2+ EPR signal, points to the possible importance of Fe2+ + Cu2+ ⇌ Fe3+ + Cu1+ redox pairs in stabilizing the unique magnetism of the codoped system. Using DFT, the FM nature of these systems is ascribed to a double-exchange mechanism between the Fe atoms, mediated by the Cu atom, while these calculations show

in Figure S1 of the Supporting Information for the optimally doped Fe0.05Cu0.1ZnO sample. In order to understand the role of Cu2+ in bringing about magnetism, we have carried out density functional based calculations, as described in the Methods Section. In the presence of simultaneous doping of Fe and Cu, two prototypical atomistic models, corresponding to “highsymmetry” and “low-symmetry” configurations, were investigated. In the high-symmetry case, the dopants replace nearestneighbor Zn atoms on a plane perpendicular to the c axis, and in the low-symmetry case, the dopants replace second-nearestneighbor Zn atoms on adjacent planes. Two low-energy stable spin configurations were found for both low-symmetry and high-symmetry cases. Carrying out computational calculations on all four configurations, we find that the lowest energy is obtained for the low-symmetry FM coupling case, which is stabilized by about 27 mRy compared to the AFM coupling. Both are shown in Figure 4 for the high-symmetry case, which

Figure 4. Isosurfaces of spin densities in high-spin (left) and low-spin (right) configurations. Red and gray balls represent oxygen and zinc atoms of the ZnO nanocrystal, respectively. Iron and copper atoms are covered by the spin density data, where yellow and cyan, respectively, correspond to isosurface values of ±0.2 × 10−8 μB bohr−3.

lends itself more naturally to visualization. In the first configuration that is shown on the left of Figure 4, the Fe atoms were each coupled antiferromagnetically to the Cu atom and therefore ferromagnetically to each other. The origin of this specific magnetic configuration is easy to understand. Fe2+ ions have a high-spin d6 (e↑2t2↑3e↓1) configuration. The d-electron hopping between the two sites would lower the total energy. Because the Cu d9 configuration (e4t25) can only support hopping via partially filled t2 levels, this requires the spin configuration of the copper site to be necessarily e↑2t2↓3e↓2t2↑2, such that the partially filled t2↑ state of Cu may allow hopping of t2↑ electrons of iron, thereby making the Cu and the Fe antiferromagnetically coupled. Exactly the same process ensures an antiferromagnetic coupling between the Cu site and the second Fe ion, thereby leading to the ferromagnetic coupling between the two Fe sites. This implies that the magnetic moment on each Fe atom is +4 μB, arising from its e↑2t2↑3e↓1configuration, and the magnetic moment on the Cu atom is −1 μB, arising from its e↑2e↓2t2↓3t2↑2 configuration and establishing the antiferromagnetic coupling between Fe and Cu sites. It is important to note here that the origin of the Fe−Fe ferromagnetic coupling is a hopping mechanism between Fe and Cu states, which is a form of the “double-exchange” process28 and not superexchange as suggested earlier for Co− Co ferromagnetic interaction mediated by Cu in Co−Cu 2012

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The Journal of Physical Chemistry Letters that the antiferromagnetic coupling between Fe sites in the absence of codoping is driven by the superexchange mechanism. This opens the road toward rational design of the magnetic properties of an oxide-based nanocrystalline dilute magnetic semiconductor using controlled codoping.

METHODS SECTION A typical synthesis of the Fe/Cu-doped ZnO nanocrystals was carried out by hydrolyzing stoichiometric amounts of Cu(OAc)2·H2O, Fe3(PO4)2, and Zn(OAc)2·2H2O in isopropanol with NaOH solution under ultrasonic agitation forming Zn1−x−yFexCuyO nanocrystals. The detailed synthesis procedure is discussed in the Supporting Information. The percentage of Fe/Cu, as determined by atomic absorption analysis, was found to be close to the stoichiometric amounts added in each of the cases. Nanocrystal structure and size identification of the particles was carried out using X-ray diffraction and transmission electron microscopy. The magnetic properties of these samples were measured using a Quantum Design MPMS XL superconducting quantum interference device magnetometer. X-ray absorption spectra at the Fe L3 edge were measured at the BEAR beamline using the synchrotron radiation source, Elettra, Trieste. All density functional theory calculations were carried out by solving the Kohn−Sham equations using the higher-order finite-difference pseudopotential method,33 as implemented in the PARSEC software.34 An on-site implementation of the LSDA+U approximation35 was used to account for the transition-metal d-orbital correlation. Further details and specific parameters are provided in the Supporting Information. Theoretical 2p→3d XAS spectra were calculated using the Lanczos iterative algorithm of a many-body Hamiltonian, based on a fully coherent spectral function for the (FeO4)6− tetrahedral cluster corresponding to the wurtzite structure. The theoretical approach is discussed elsewhere,36 and details of the calculations and the parameters used are discussed in the Supporting Information.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Details of the synthesis procedure, measurement tools, and discussion of the theoretical methods along with parameters used in the calculations and the EPR spectrum of optimally codoped Fe/Cu ZnO. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

Work in Bangalore was supported by the Department of Science and Technology, Government of India. R.V. and D.D.S. acknowledge the support of the International Centre for Theoretical Physics and Department of Science and Technology under their Users Programs for Synchrotron Radiation. R.V. and D.D.S. thank Debangshu Chaudhuri, Sameer Sapra and the beamline scientists at the BEAR beamline of Elettra, Trieste for help with the X-ray absorption study. Work in Rehovoth was supported by the Minerva Foundation, the Lise Meitner Minerva Center for Computational Chemistry, and the historical generosity of the Perlman family. Work in Texas was supported by the U.S. Department of Energy, Office of Basic Energy Sciences and Office of Advanced Scientific Computing Research (Grant No. DE-FG02-06ER46286 on nanostructures and Grant No. DE-SC0001878 on oxides). Computational resources were provided by National Energy Research Scientific Computing Center (NERSC) and the Texas Advanced Computing Center (TACC) under Grant TG-DMR090026.

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AUTHOR INFORMATION

Corresponding Author

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

Currently at International Centre for Materials Science and New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064. ∥ Currently at the Department of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, U.S.A. ∇ Also at Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, and Department of Physics and Astronomy, Uppsala University, Sweden. Notes

The authors declare no competing financial interest. 2013

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