Regulation of Magnetic Behavior and Electronic Configuration in Mn

Apr 19, 2012 - A midgap state with strong O-2p character has been also recognized at the O .... Effect of high manganese substitution at ZnO host latt...
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Regulation of Magnetic Behavior and Electronic Configuration in Mn-Doped ZnO Nanorods through Surface Modifications Linjuan Zhang,†,‡ Jian-Qiang Wang,† Jiong Li,† Shuo Zhang,*,† Zheng Jiang,† Jing Zhou,‡ Jie Cheng,§ Tiandou Hu,‡ Wensheng Yan,§ Xiangjun Wei,† and Ziyu Wu*,‡,§ †

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China ‡ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China § National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, P. R. China S Supporting Information *

ABSTRACT: In this research, Mn-doped ZnO nanorods were synthesized by a solvothermal method and their magnetic behavior was tuned from paramagnetism to ferromagnetism via a change in their surface environment. The structure and electronic configuration of Mn ions were investigated by means of X-ray diffraction and X-ray absorption fine structure to understand the surface-controlled magnetic properties. The surface electronic configuration was studied by Mn L3,2-edge XANES, which were simulated using the ligand field multiplet theory and the ligand-to-metal charge transfer effects. The results pointed out that Mn3+ ions occupy distorted tetrahedronal sites near the surface region and different surface modifications produce changes in the Mn 3d-anion p hybridization strength. A midgap state with strong O-2p character has been also recognized at the O K-edge XANES, and the density of such a state is strongly related to the observed ferromagnetism. This research represents a novel promising route for tuning the magnetic behavior of nano-dilute magnetic semiconductor systems via surface atomic changes. KEYWORDS: dilute magnetic semiconductor, surface modification, X-ray absorption spectroscopy

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samples. According to first principle calculations, the origin of the different magnetic properties of surface and bulk can be associated with states occurring at the surface,13 structural reconstruction,14 etc. Moreover, several recent investigations showed that the RTF could be controlled by surface treatments.15−17 For instance, the sodium bis(2-ethylhexyl) sulfosuccinate (AOT) molecule (S-capped) has been identified as an effective surfactant mediating the RTF in this system.18,19 These findings provide a route to functionalizing this material and point out the presence of an evident surface interaction tuning the RTF in these systems.20−22 Therefore, it is mandatory to characterize the surface interaction and the effect of the surfactant on the electronic configuration of magnetic atoms with reliable experimental data. This knowledge will be useful in explaining the controversial results regarding the magnetic data of this system and also in designing improved materials. In this study, Mn-doped ZnO nanorods with different surface environments were prepared by a simple solvothermal method. The magnetic characterization showed that the naked sample displays a typical paramagnetism. The room-temperature ferromagnetism can be activated by a polyvinylpyrrolidone (PVP) molecule capped and enhanced by a capped sodium bis(2-ethylhexyl)

INTRODUCTION Recent advances in dilute magnetic semiconductors (DMSs) have indicated that electronic control of their spin properties can be used to manipulate magnetic signals.1,2An important issue with these systems is understanding the origin of ferromagnetism. Several theoretical models have been proposed such as a carrier-mediating model,3 a secondary phase model,4 bound magnetic polarons (BMPs),5 etc. However, controversial experimental results have been published, and the underlying mechanism is still unclear. The Mn-doped ZnO system is an ideal DMS candidate and has been intensively investigated because Mn atoms carry the largest magnetic moment among 3d TM atoms. Working parallel to the magnetic data of this system, we found that its magnetic behavior is strongly dependent on the different synthesis approaches and is poorly reproducible. Several magnetic measurements performed on bulk samples show a paramagnetic configuration down to the helium temperature with a very large negative value of the Curie−Weiss temperature, which implies a strong antiferromagnetic (AFM) interaction.6−8 On the other hand, room-temperature ferromagnetism (RTF) was observed in thin films9,10 and in nanostructured samples.11,12 Actually, several experiments showed that the ferromagnetism disappears when nanoparticles are sintered to form a bulk sample at high temperatures. This behavior points out that surface effects may play a significant role in the magnetic behavior of nanostructured © 2012 American Chemical Society

Received: December 7, 2011 Revised: April 18, 2012 Published: April 19, 2012 1676

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sulfosuccinate (AOT) molecule. The X-ray diffraction (XRD) characterization combined with X-ray absorption fine structure (XAFS) data indicates that no secondary phases occur in our samples. Moreover, with a careful analysis of the Mn L3,2-edge and O K-edge X-ray absorption near-edge structure (XANES) data, we show that different surface environments produce a significant change in the Mn 3d-anion 2p hybridization strength. Furthermore, the presence of a midgap state was also detected.

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EXPERIMENTS AND CALCULATIONS

Mn-doped ZnO samples were synthesized by a standard solvothermal method. The raw material included zinc chloride (ZnCl2, 98%), sodium hydroxide (NaOH, 96%), manganese chloride tetrahydrate (MnCl2·4H2O, 99%), sodium bis(2-ethylhexyl) sulfosuccinate (AOT), polyvinylpyrrolidone (PVP), and absolute ethanol without any further purification. To prepare Zn0.95Mn0.05O samples, we dissolved a 0.1 M mixture of 95% ZnCl2 and 5% MnCl2·4H2O and 0.3 M NaOH in 30 mL of absolute ethanol at room temperature with a vigorous magnetic stirring and then transferred the solution into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed, heated automatically to 160 °C, and held at that temperature for 10 h. Finally, the product was washed several times with deionized water and absolute ethanol and subsequently collected after centrifugation and drying. The sample synthesized without surfactant was denoted as naked Zn0.95Mn0.05O. To prepare PVP- or AOT-coated Zn0.95Mn0.05O samples, surfactants were added to the mixture individually with a Zn:surfactant weight ratio of 1:10, before being transferred to the autoclave. The autoclaves were heated automatically to 160 °C and held at that temperature for 10 h, and the same sample was subsequently collected after washing and drying. Crystalline phases of the obtained materials were analyzed by XRD using a D8 Advance X-ray diffractometer with Cu Kα radiation in the angular range (2Θ) from 20° to 80°. Particles sizes and shapes were examined with a JEM-2100 transmission electron microscope (JEOL). All samples maintain a rodlike morphology with lengths of 100−200 nm (see Figure S1 of the Supporting Information). Magnetic properties were characterized by a vibrating sample magnetometer (VSM, RIKIN BHV-50V) at 300 K. The Mn K-edge XAFS spectra were recorded at beamline 1W1B of the Beijing Synchrotron Radiation Facility (BSRF) using a doublecrystal Si(111) monochromator in transmission mode. The electron beam energy of the storage ring was 2.5 GeV, and the maximal stored current was ∼250 mA. The O K-edge and Mn L3,2-edge XANES were measured on beamline U19 at the National Synchrotron Radiation Laboratory (NSRL). The synchrotron radiation from the bending magnet was monochromized with a varied line-spacing plane grating monochromator and refocused with a toroidal mirror. The data were recorded in total electron yield (TEY) mode by collecting the sample drain current under a vacuum better than 5 × 10−5 Pa. The Mn K-edge extended X-ray absorption fine structure (EXAFS) data were analyzed using standard procedures with IFEFFIT.23 According to the photoelectron multiscattering effect in the near-edge range, the XANES was simulated in the framework of the multiple-scattering (MS) theory24 with the FEFF 8.4 code25 considering the structural model with the ideal wurtzite ZnO structure, in which a Mn atom occupies a Zn site as the absorber, with the following lattice parameters: a = b = 3.242 Å, and c = 5.206 Å. Generated by ATOM, FEFF input files work with a set of spherically averaged muffin-tin (MT) potentials, and with the Hedin− Lundqvist model.26,27 A cluster size of ∼7 Å around the central atom was used to achieve an accurate self-consistency field (SCF), while a larger cluster of 8 Å was used to obtain full multiple-scattering calculations. For Mn L3,2-edge XANES calculations, we used the CTM4XAS charge transfer multiplet program, developed by deGroot's team.28−30

Figure 1. Scheme of the split-batch experiment and the dependence of the magnetization at room temperature vs the applied magnetic field (M vs H) for Mn-doped ZnO samples with and without surfactants after subtraction of the diamagnetic background of the sample holder.

behavior is observed in the naked sample. In the PVP-capped sample, a minor hysteresis loop appears in the low-magnetic field range while this sample is substantially characterized by a strong paramagnetism. In the AOT-capped sample, a distinct hysteresis loop with ferromagnetic behavior is observed and its saturation magnetization is 0.02 emu/g with a coercive field (Hc) of 104 Oe. The different magnetic behavior shows that the surfactants play a significant role in tuning the magnetism. Structural Characterization. The XRD patterns of all samples are shown in Figure 2a. To distinguish the tiny change, the intensity was normalized against the highest Bragg peak and plotted on a logarithmic scale. The diffraction pattern and its position match well with those of the standard wurtzite ZnO phase; in particular, the typical diffraction features at 33° and 52° that correspond to (Mn,Zn)Mn2O4 reported by Han et al.31 are lacking. However, it is well-known that XRD needs long-range structural order and has typical impurity detection limits in the range of 10%. As a consequence, XRD alone cannot rule out the presence of an amorphous Mn-correlated phase. Therefore, a careful analysis of the presence of a secondary phase was performed looking at the Mn K-edge that is mainly sensitive to the local structure around Mn ions. In Figure 2b, we compare Mn K-edge XANES spectra of Mn-doped ZnO samples with various surface treatments and standard Mn compounds such as Mn metal, MnO, Mn3O4, ZnMn2O4, and MnO2. All these phases have different spectral patterns. In addition, k-weighted EXAFS oscillations between our Mn-doped ZnO samples and reference ZnMn2O4 and ZnO compounds are shown in Figure 2c. As expected, the spectral shape of the Mn-doped ZnO sample is different from that of the ZnMn2O4 sample but similar to that of the wurzite phase of ZnO. As a consequence, we may claim that in our samples almost all dopants occupy Zn sites in the wurzite ZnO matrix. As shown in Figure 2d, the effect of the different surface changes on the local structure of the dopants could be further

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RESULTS AND DISCUSSION Magnetic Properties. The variations in the magnetization at 300 K under an applied magnetic field of 5 kOe are displayed in the bottom part of Figure 1 (an enlarged image can be seen in Figure S2 of the Supporting Information). A paramagnetic 1677

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Figure 2. (a) X-ray diffraction patterns of Zn0.95Mn0.05O particles with and without different surfactants plotted on a logarithmic scale. Vertical lines indicate the diffraction peaks of the standard wurtzite ZnO phase (JCPDFS Card 89-0511) and those of the standard ZnMn2O4. (b) Comparison of the Mn K-edge XANES of the Mn-doped ZnO sample with and without surfactants, as well as standard Mn-correlated compounds, including Mn metal, MnO, Mn3O4, ZnMn2O4, and MnO2. (c) Comparison of the k-weighted EXAFS χ(k) function at the Mn K-edge for the reference ZnMn2O4 and the three samples, as well as the Zn K-edge EXAFS oscillation for the reference ZnO. (d) Magnification of different Mn K-edge XANES structures among the Zn0.95Mn0.05O samples with and without surfactants.

observed at the Mn K-edge XANES. These XANES spectra show two main changes. (1) The intensity of the white line (peak C) decreases significantly in the capped sample, and (2) for the AOT-capped sample, the valley after feature C almost vanishes and peak D shifts to a low energy. Actually, all features in this energy region mainly originate from photoelectron multiscattering (MS) contributions sensitive to local structural changes around the Mn photo-absorber. They may involve structural rearrangements,32 but also disorder33 and defects.34 In particular, recent theoretical work showed that the type of coordinated atom may have a strong effect on the spectral shape.35 By employing simulations based on the MS theory, we could confirm that these two changes derive from the contribution of the Mn ions bonded to the capping molecule (additional information is available as Supporting Information). It implies that a different surface environment strongly affects the local structure of the dopants located at the surface region. However, a detailed variation is hard to extract from the Mn Kedge XANES because it mainly contains bulk signal and thus conceals the information about the surface. As a consequence, a surface-sensitive characterization is mandatory. Contrary to the bulk-sensitive Mn K-edge XANES, because of the limited escape depth (∼3 nm) of excited photoelectrons collected in the total electron yield (TEY) mode, Mn L3/2-edge XANES spectra are sensitive only to the first few nanometers nearest to the surface. In addition, the transition-metal L3/2edge XANES is dominated by an atomic multiplet, and the spectral shape is sensitive to both ligand field and covalency. This approach offers a deep understanding of surface contributions. In Figure 3a, we compare experimental Mn L3/2-edge XANES spectra of Mn-doped ZnO samples with and without surfactants after a normalization to the highest peak. All spectra are similar, and because of the 2p core hole spin−orbit splitting, the spectra can be divided into two edge regions: a lowerenergy L3-edge and a higher-energy L2-edge. According to a previous investigation,31 the overall spectral shape of all samples indicates a pure Mn3+(d4) valence. Simulations based on the ligand field multiplet theory (LFMT) shown in Figure 3b also

Figure 3. (a) Comparison of experimental Mn L-edge XANES spectra of Mn-doped ZnO samples with and without surfactants. The inset shows the energy diagrams of Mn3+ 3d orbitals with Td and C3v symmetry. (b) Normalized charge transfer multiplet by CTM4XAS calculations of Mn ions for the Mn3+(d4) and Mn2+(d5) ground state in the C3v crystal field without charge transfer. (c) Normalized multiplet structures for a C3v symmetry considering the variation of the parameter T(yz/zx). The other hopping parameters are T(x2−y2,z2) = 4 and T(xy) = 1.

support this conclusion and confirm that Mn3+ ions fit a distorted tetrahedral environment. In addition, two noticeable 1678

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differences (marked with arrows in Figure 3a) that could be recognized show the different effect of surfactants on the electronic configuration of Mn3+. To improve our understanding of the origin of the multiplet changes outlined in the Mn L3/2-edge experimental spectra, a spectrum was simulated considering the ligand-to-metal charge transfer (LMCT) effect. To model such an effect, the ground states of a 3dn ion were described as a linear combination of 3dn and 3dn+1L, where 3dn+1L denotes a configuration with an extra 3d electron transferred from a ligand and the corresponding hole on the ligand orbital. A distorted Td symmetry model (C3v point group symmetry) was used to calculate the ionic multiplet structure, which arises from a slight elongation of the tetrahedron along the c-axis of the ZnO system with the wurtzite structure.36,37 In a C3v symmetry, there are three parameters that determine the positions of the 3d orbitals: Dq, Ds, and Dt. The best overall agreement with the experimental spectra is obtained for the crystal field parameters: 10Dq = −0.5 eV, Ds = 0.01 eV, and Dt = 0.01 eV. The slater integral parameters are 80% of the Hartree−Fock value. If the charge transfer parameters are Δ = 6.5 and U = 2.0, our calculated spectra are similar to those available in the literature.38,39 The strength of the charge transfer configuration was adjusted through the mixing integrals T(i) between 3d4 and 3d5L configurations. These integrals allow for a covalent overlap of different asymmetry components (i). In Td symmetry, the two ligand-to-metal charge transfer integrals T(t2) and T(e) must be considered, while in C3v symmetry, because of slightly distorted coordinates along the c-axis, the three integrals considered included the splitting of the t2 (Td) level into a1 (C3v) and e (C3v) states while the e (Td) level does not split, as described in the inset of Figure 3a. To compare L-edge calculations with experimental spectra, a Lorentzian broadening of 0.2 or 0.4 eV for the L3 or L2 edge, respectively, and a Gaussian broadening of 0.2 eV have been applied to take into account the intrinsic core−hole lifetime and the instrumental resolution, respectively. As shown in the energy diagram, the highest occupied states can be easily affected by ligands. We then adjust the parameter T(yz/zx) until our XANES simulations reproduce the experimental trend versus the addition of surfactants as shown in Figure 3c. It clearly shows that different surface modifications produce changes in the Mn 3d-anion p-hybridization strength. Because the Zn1−xMnxO system exhibits singular midgap ionization levels, which are closely associated with its magnetic properties, an increasing amount of attention has been devoted to the Zn1−xMnxO system. Several pieces of spectral evidence40,41 and an explanation involving a Mn2+/3+ midgap level above the valence band proposed by Gamelin et al. are supported by the time-dependent density of functional (TD-DFT) calculations. Dietl et al. suggested the presence of a Zhang−Rice-like state arising from the strong coupling between holes and local spins.42 In addition, according to the band coupling model, the character of a hole state is crucial for establishing ferromagnetic ground states in the Mn-doped ZnO system.43 Thus, also O K-edge XANES was collected to probe the electronic structure of the conduction band (CB) in this system. Figure 4 shows the normalized O K-edge XANES of all samples. They probe the orbital character of the spectral features of the O-2p unoccupied states in the conduction band and their hybridization with different Zn and Mn orbitals. Comparison of the spectrum of nanostructured samples with the reference spectrum of ZnO shows an evident midgap feature originating from the O-2p hole state at 529.3 eV. It indicates that this hole state has a

Figure 4. Comparison of the O K-edge XANES spectra of Mn-doped ZnO samples and the reference ZnO. The inset shows energy-level diagrams for Mn2+− and Mn3+−O interaction.

strong O-2p character. Thus, the origin of this specific feature can be explained with the scheme suggested by Zunger et al.,44 shown in the inset of Figure 4. The interaction between Mn-3d and O-2p orbitals induces new bonding levels below the valence band maximum (VBM) and antibonding levels inside the band gap. When the energy of Mn 3d levels is higher than that of O-2p levels, i.e., Mn2+-t2 level, this bonding level has strong O-2p character while the antibonding level is mostly composed by Mn-3d orbitals. For Mn3+ ions, the scenario is reversed. Therefore, the midgap feature we observed can be assigned to antibonding levels with a strong O-2p character arising from the interaction between Mn3+-3d and O2−-2p orbitals. Furthermore, a charge redistribution process may occur; i.e., the hole would be transferred to an O atom rather than located around Mn ions. As a consequence, such a hole state is characterized by a much delocalized wave function. This model is in agreement with recent TDDFT calculations showing that the effective radius of this state is much smaller than a free hole in ZnO and larger than a Mn-3d orbital.41 A band coupling model considering p−d and d−d interactions has been considered in describing the magnetic behavior mechanism shown in Figure 5. The competition between the FM

Figure 5. Schematic model considering p−d coupling and d−d coupling in FM and AFM configurations.

and AFM phase and its energy difference can be described as ΔEFM‑AFM = −αmh(Δ1pd + Δ2pd) + Δ1,2 dd , where mh is the additional 1679

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number of holes per Mn ion and Δpd and Δdd are the p−d and d−d coupling parameters, respectively. Within this model, we can see that either the hole density or the p−d coupling increase can enhance the FM phase. From the inset of Figure 4, we may clearly recognize that the variation of the density of hole states modulated by the different surfactant exhibits the same trend as the magnetic property. This behavior is compatible with the robust ferromagnetism observed in the AOT-capped samples.

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CONCLUSIONS In summary, Mn-doped ZnO nanorods with a different surface environment (naked, PVP-capped, and AOT-capped) were prepared by a simple solvothermal synthesis. The magnetic characterization of the naked sample displays a typical paramagnetism.The roomtemperature ferromagnetism can be activated by PVP-capped molecules and enhanced in the AOT-capped sample. From XRD characterization and Mn K-edge XAFS analysis, additional phases have not been detected in all samples. On the basis of the ligand field multiplet theory and the ligand-to-metal charge transfer effects, the contribution of the surfactant on the surface electronic structure has been identified by simulations in the Mn L3,2-edge XANES. Results indicate that different surface modifications induce alteration of the Mn 3d-anion p hybridization strength. In addition, an O-2p hole (midgap state) was also found by O K-edge XANES experiments, a clear indication that a strong electron correlation occurs also in this system. Finally, the results of this investigation clearly point out the role of surface states and of Mn 3d-anion p state coupling in mediating the ferromagnetism of this system.

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ASSOCIATED CONTENT

S Supporting Information *

TEM analysis, magnetic properties, calculated Mn K-edge XAFS data analysis, and reduction of Mn L3,2-edge XANES (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.W.) or [email protected] (S.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the Academy of Sciences (KJCX2-YW-N42), the Key Important Project of the National Natural Science Foundation of China (10734070), the National Natural Science Foundation of China (Grants 11005145 and 20901055), the National Basic Research Program of China (2009CB930804 and 2012CB825800), and the Graduate Student Innovation Fund of NSRL (20090622S). Sincere thanks are due to their support to the staff of beamlines 14W1 of the Shanghai Synchrotron Radiation Facility (SSRF). We acknowledge The-Long Phan for experimental support with standard ZnMn2O4. Thanks are also due to F. M. F. deGroot for his patient and thoughtful guidance.

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REFERENCES

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