Size Dependence of Defect-Induced Room Temperature

Article pubs.acs.org/JPCC

Size Dependence of Defect-Induced Room Temperature Ferromagnetism in Undoped ZnO Nanoparticles Xiaoyong Xu,†,‡ Chunxiang Xu,*,† Jun Dai,† Jingguo Hu,‡ Fengji Li,§ and Sam Zhang§ †

State Key Laboratory of Bioelectronics and School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China ‡ School of Physics Science and Technology, Yangzhou University, Yangzhou 225002, China § School of Mechanical and Aerospace Engineering, Nanyang Technological University, 639798 Singapore S Supporting Information *

ABSTRACT: We report the intrinsic room-temperature ferromagnetism in undoped ZnO nanoparticles with different sizes synthesized by a wet chemical method at different temperatures. Electron paramagnetic resonance, X-ray photoelectron spectroscopy, and photoluminescence measurements demonstrate clearly the singly charged oxygen vacancies are the main defects, and the relative occupancy of that decreases with increasing sizes and annealing temperatures. Importantly, a direct correlation between the ferromagnetism and the relative concentration of the singly charged oxygen vacancies is established, which suggests that the singly charged oxygen vacancies play a crucial role in modulating ferromagnetic behaviors. Moreover, the size-dependent ferromagnetism can be manipulated conveniently by changing of the surface−volume ratio, which is in favor of future electronic and spintronic application. analyses.24 Meanwhile, RTFM has also been found in other some undoped oxides such as HfO2,26 TiO2,27 and Al2O3,28 and this so-called d0 ferromagnetism was usually attributed to the point defects. Currently, the objective to clarify the clear defect origin and unambiguous coupling mechanism behind RTFM in undoped ZnO inspires extensive experimental22−24 and theoretical interests;29,30 however, it is very difficult to establish a direct link between the magnetization and defects due to complexity of defect states in ZnO. As a result, the nature of the defect-induced ferromagnetism, as an important controversial topic, needs more delicate investigations to address further. In addition, it is increasingly evident that d0 RTFM is often observed only in thin film and nanostructures and exhibits notable surface effects,31−33 which is suggested to contact with superficial defects.22,28 Therefore, nanoparticles (NPs) with the high surface−volume ratio are expected to be an excellent model for studying the defect-related ferromagnetism because most of the defects must exist near the surface of NPs.28 This implies that the size of NPs should strongly affect the observed ferromagnetism. To our knowledge, however, there is no report on a systematic study on the size dependence of the magnetic properties in pure ZnO NPs. In this paper, we prepared undoped ZnO NPs with different sizes that show clear size-dependent RTFM resulting from defects. With the help of photoluminescence (PL), electron paramagnetic resonance (EPR), and X-ray photoelectron

1. INTRODUCTION Following the theoretical prediction of room-temperature ferromagnetism (RTFM) in Mn-doped ZnO,1 ZnO-based diluted magnetic semiconductors (DMSs) have been studied intensively because of their promising applications in spintronic devices tuning simultaneously charge and spin.2−9 So far, the diverse magnetic properties have been observed experimentally in ZnO-based DMSs.10−13 Correspondingly, several different theoretical models have also been developed to explain the coupling mechanism responsible for the observed RTFM.14−16 However, the conclusive origin of RTFM has yet to be uncovered, and the mechanism behind the magnetic ordering is still under debate. Particularly, some outstanding works17−19 revealed no evidence for ferromagnetic order of the active doped transition metal atoms in Co- and Mn-doped ZnO systems. Thus, this even questions whether the ferromagnetism is intrinsic or due to extrinsic origins including magnetic metal clusters, secondary phases or contaminants, etc. In recent years, some startling discoveries of RTFM in nonmagnetic doped20,21 and undoped ZnO22−25 systems indicated that the magnetic properties may be not exclusively related to the presence of the magnetic ions but strongly mediated by the point defects, such as oxygen vacancies (VO),22 Zn vacancies (VZn),23 and Zn interstitials (Zni).24 For example, Herng et al.21 found that both oxygen vacancies and Cu impurities are essential to the ferromagnetism in Cu-doped ZnO films by using soft X-ray magnetic circular dichroism (SXMCD). Zhang et al. attributed the observed RTFM in ZnO granular films to Zni based on the room-temperature photoluminescence and high-temperature X-ray diffraction © 2012 American Chemical Society

Received: December 30, 2011 Revised: March 12, 2012 Published: March 27, 2012 8813

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Figure 1. TEM (scale bar: 20 nm) and HRTEM (scale bar: 5 nm) images of samples (a, d) S1, (b, e) S2, and (c, f) S3. The insets show the corresponding diameter distributions and electron diffraction patterns.

in X-band with magnetic field modulation at 9.86 GHz, and the masses of all the samples for EPR were the same of 0.02 g.

spectroscopy (XPS) results, we illustrated that the singly charged oxygen vacancies (V+O) located near the surface are crucial for RTFM in our samples. Moreover, our results also provide a probability to modulate such a defect-induced RTFM by controlling the size of nanoparticles.

3. RESULTS AND DISCUSSION ZnO NPs synthesized at different temperatures of 20, 35, and 50 °C are denoted as S1, S2, and S3, respectively. The TEM images of the three samples in Figures 1a−c reveal the formation of almost monodispersed NPs with nearly spherical shape and narrow diameter distribution. The average diameters determined from analysis of ∼500 NPs were 3.67, 4.52, and 6.30 nm for S1, S2, and S3, respectively. The subsequent HRTEM images and the selected area diffraction pattern (SAED) (inset) shown in Figures 1d−f confirm the high crystallinity of the particles and the presence of amorphous phase, in which individual particles are highlighted by white circles. The clear lattice fringes with 0.28 nm as illustrated corresponds well with that of d-spacing of (100) planes of wurtzite ZnO. Figure 2a shows XRD patterns of the samples S1, S2, and S3. All the peaks correspond to the wurtzite structure of ZnO. No other diffraction peaks from impurities and residues are detected, indicating that the synthesized samples are pure ZnO without impurity phase within the resolution. Sizedependent XRD broadening is also observed in these samples, implying the increasing of the average particle diameters as the synthesized temperature increases. Moreover, the mean size of ZnO nanoparticles can be estimated by Debye−Scherrer’s formula, and the results are comparable with those determined from analysis of ∼500 NPs in HRTEM images (see Supporting Information). The composition and chemical states of samples are further investigated by XPS analysis. Figure 2b shows a representative broad scan survey spectrum of S2. Evidently, there only the elements of Zn, O, and C are presented. It further excludes the presence of any other impurities. Especially, the absence of the feature peaks at 778.1, 706.6, and 640.8 eV reveals that there are no impurities of Co, Fe, and Mn elements. The inset of Figure 2b presents the XPS spectrum of the Zn 2p core level, and the peak positions of Zn 2p3/2 and Zn 2p1/2 locate at 1021.9 and 1045 eV, respectively. A spin−orbital splitting of 23.1 eV confirms that Zn is present as Zn2+.7 The analysis of

2. EXPERIMENTAL METHODS ZnO NPs were prepared through a wet chemical route described by Schwartz et al.,34 and their sizes were adjusted by changing synthetic temperatures. In a typical preparation, 5.4 g of 25% N(CH3)4OH (TMAH) solution dissolved in 30 mL of absolute ethanol (EtOH) was added dropwise at about 2 mL/min to a 90 mL solution of 0.1 M Zn(CH3COO)2 in dimethyl sulfoxide (DMSO) under vigorous stirring at 20, 35, and 50 °C. The resulting nanocrystals can be precipitated from DMSO by addition of ethyl acetate. After removal of the supernatant, the nanocrystals were resuspended in 100 mL of EtOH. The nanocrystals could be precipitated again by adding heptane and were washed several times with EtOH by centrifugation. The powders were obtained by evaporating the solvent from the concentrated EtOH suspension of nanocrystals in an oven at 60 °C to be used for X-ray diffraction (XRD), XPS, EPR, and magnetic as well as optical measurements. The morphologies and microstructures of the samples were characterized by transmission electron microscopy (TEM, JEM-2100) and high-resolution TEM (HRTEM) equipped with selected area electron diffraction (SAED). The crystal structures of the samples were investigated by X-ray diffraction (XRD) using a Shimadzu XRD-7000 diffractometer with Cu Kα radiation (λ = 1.54 Å). The chemical compositions and states in the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Kratos, Axis-ULTRA) with monochromatic Al Kα (1486.71 eV) radiation (15 kV and 10 mA). The magnetic properties of samples were measured by a vibrating sample magnetometer (VSM) integrated in a physical property measurement system (PPMS-9, Quantum Design). The photoluminescence (PL, F4600) measurements were performed at the room temperature (RT) with a Xe lamp emitting at 325 nm as excitation source. Electron paramagnetic resonance spectra (EPR, A300-10/12) were recorded at RT 8814

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as remanence (Mr), coercivity (Hc), and saturation magnetization (Ms); moreover, they are much more significant at the low temperature of 5 K. The observed Hc, Mr, and Ms are 82.6 Oe, 1.6 × 10−4 emu/g, and 1.5 × 10−3 emu/g, respectively, at 300 K. At 5 K, they increase to 220.2 Oe, 3.8 × 10−4 emu/g, and 2.2 × 10−3 emu/g, respectively. The inset of Figure 3b exhibits the dividable zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of sample S2 in the temperature range 5−300 K, revealing that the Curie temperature of the sample is above room temperature. Furthermore, no blocking temperature occurs in this temperature range, indicating that such a reproducible RTFM is of intrinsic nature rather than due to ferromagnetic impurities. More importantly, it can also been found that Ms decreases monotonically with the size of the NPs increasing in Figure 3a. This size dependence of RTFM supports further the grain boundary effect and the surface effect of magnetic behaviors in ZnO thin film reported recently by Straumal35 and Li33 et al. In previous research publications, many point defects including VO,22 VZn,23 and Zni24 were proposed respectively to be responsible for the ferromagnetic response present in the undoped ZnO. There is still no definite agreement on the defect-origin of RTFM in undoped ZnO, although the point defects have already been accepted generally as an important role in triggering magnetic order. Shalish36 and Zhang37 et al. observed respectively that size reduction always causes comparatively more defects to be closer to the surface in ZnO nanowires and quantum dots based on a linear relationship between the visible PL intensity and the surfaceto-volume ratio. Therefore, we believe that this size-dependent ferromagnetic behavior should be able to give an effective clue both to understand the defect origin of RTFM and to realize its controllability in undoped ZnO NPs. In order to clarify the origin of RTFM existing in ZnO NPs, XPS, PL, and EPR measurements were performed to identify the point defects in above three samples. Figures 4a−c show the normalized high-resolution XPS scan of O 1s core level

Figure 2. (a) XRD patterns of samples S1, S2, and S3. (b) XPS survey scan spectrum of representative sample S2. The inset shows the highresolution XPS spectrum of Zn 2p3/2 and 2p1/2 regions.

XPS is consistent with the above XRD and HRTEM results, indicating the incorporation of any extrinsic magnetic impurities can be ruled out. The magnetization versus magnetic field (M−H) curves of the samples S1, S2, and S3 were measured at RT. The ferromagnetic contribution can be observed under the diamagnetic background (see Supporting Information). Figure 3a shows M−H curves after subtracting the diamagnetic signals of the samples and the holders. It can be seen that all samples exhibit clear hysteresis loops, suggesting unambiguously reproducible RTFM. Figure 3b gives the M−H curves of the sample S2 measured at 300 and 5 K after subtracting the diamagnetic background. The clear hysteresis loops observed at 300 and 5 K show the typical features of ferromagnetism, such

Figure 3. (a) RT M−H loops of samples S1, S2, and S3. (b) M−H loops measured at 300 and 5 K of representative sample S2. The inset shows ZFC and FC curves at 700 Oe field of sample S2.

Figure 4. Experimental and fitted curves for normalized O 1s XPS spectra of samples (a) S1, (b) S2, and (c) S3. 8815

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fitted by two Gaussian functions for the samples S1, S2, and S3. The lower binding energy peak located at 530.92 eV (Oa) is attributed to O2− ions in wurtzite structure of hexagonal ZnO.38 The higher binding energy peak located at 532.47 eV (Ob) can be assigned to O2− ions in the oxygen-deficient regions within the matrix of ZnO.39 The decrease of electron charge density in the VO region results in less screening of the O2− 1s electrons from their nucleus, which raises the effective nuclear charge and the binding energy of an O2− 1s electron.40 Thus, the intensity of the Ob relative area may be associated with the concentration of VO. From Figure 4, it can be seen that the Ob peak is always present; moreover, its relative area decreases as the size of the NPs increases. This indicates that there are some oxygen vacancies in all three samples, and its concentration decreases with decreasing surface-to-volume ratio. Interestingly, the variation of the Ob relative area is consistent with that of the Ms in these three samples. Moreover, among the oxygen vacancies with different charged states, only the singly charged oxygen vacancies have possibility to contribute to magnetization due to cancellation of paired electron spins in the doubly charged and neutral oxygen vacancies. So, it might be assumed that the singly charged oxygen vacancies may be linked with the RTFM in ZnO NPs, which also coincides with some previously reported results.41−43 To further verify this proposal, the more convincing analyses of defect states were performed with PL and EPR. Figure 5a

quantum confinement effect. More importantly, the relative intensity of green emission decreases monotonously with increasing size. This quantum size effect on PL properties of ZnO NPs was also observed by Zhang et al.;37 moreover, they identified V+O located predominantly near the surface of NPs as the defect-centers of green emission. So, the reduction of the green emission illustrates that the relative concentration of the corresponding V+O is significantly decreased with the surface-tovolume ratio decreasing. In addition, EPR which can directly characterize paramagnetic defects was used to identify the defect states in samples. Figure 5b depicts the EPR results measured at RT for the samples S1, S2, and S3. During the EPR measurement, the same instrument parameters and the same sample masses of 0.02 g were performed. It can be seen that a single EPR signal appears in all samples, which suggests that all samples possess the same type of paramagnetic defects. According to previous EPR studies, such an EPR signal is believed usually be one of the characteristics of ferromagnetism in oxide nanostructures,44,45 which may originate from paramagnetic V+O with an unpaired electron.46 More interestingly, the intensity of EPR signal becomes weaker with increasing size, which corresponds to the intensity-change of defect-related visible emission. Herein, it is indicated that the visible emission may indeed directly associated with the paramagnetic V+O detected by EPR, and two correspondingly decreasing intensities of visible emission and EPR signal can jointly reflect the relative concentration of the singly charged oxygen vacancies decreases with increasing size, which is in agreement with the above XPS result. Figure 6 compares clearly the intensity of EPR signal (IEPR), the relative intensity of visible emission (IVis/IUV) in PL spectra,

Figure 6. Variations of Ms, IEPR, IVis/IUV, and IOb/IOa versus surface-tovolume for samples S1, S2, and S3.

the relative intensity of Ob (IOb/IOa) in XPS spectra, and Ms versus the surface-to-volume ratio of NPs. They show the similar and linear variation trends. Thus, combining with the above analyses, the two insightful points can be revealed. First, the observed reproducible RTFM in our samples is apparently related to V+O; moreover, its intensity is tunable by controlling the concentration of V+O. Second, the observed RTFM exhibits near linear size dependence because most of V+O exist near the surface, and its relative occupancy can be modulated linearly by the surface-to-volume ratio. These results are consistent with the view recognized by several pervious studies22,29,32 that the surface and size are very important for defect-induced RTFM in pure metal oxides. And they can further explain why this d0 RTFM is often found only in low-dimensional nanostructures or thin film with high surface-to-volume ratio.

Figure 5. (a) RT PL spectra normalized to the UV emission and (b) RT EPR spectra of samples S1, S2, and S3.

illustrates the RT PL spectra normalized to the ultraviolet (UV) emission for the samples S1, S2, and S3. All the samples exhibit a weak UV emission peak and a defect-related green emission peak. The UV emission is well-known to be associated with the radiative recombination of electrons from conduction band with holes from valence band. The green emission is generally attributed to the recombination of electrons trapped in V+O with photoexcited holes. As shown in Figure 5a, with the size of NPs increasing, the peaks of both UV and green emissions red-shift to the positions with longer wavelengths because of the reduced 8816

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vacancies, which indicates that V+O indeed plays an important role in triggering magnetism in ZnO. However, it does not exclude other types of defects like cation vacancies23 as the possible origins of long-range ferromagnetic ordering. Understanding the relation between complicated defects and magnetism needs further attention and is presently under investigation in our laboratory.

The sequential oxygen annealing was carried out for 1 h in a high-purity oxygen atmosphere at different temperatures to further clarify the role of oxygen vacancies in triggering the magnetic property. Figure 7a shows the M−H curves deducted

4. CONCLUSIONS In summary, well crystalline ZnO nanoparticles with different sizes were synthesized by a simple wet chemical method. The intrinsic room-temperature ferromagnetism is observed in all samples and exhibits clearly a monotonous size dependence. Several techniques, including HRTEM, XRD, XPS, PL, and EPR, were used to eliminate magnetic contaminations and to systematically investigate the defect states. The results show that the singly charged oxygen vacancies located mainly near the surface play an important role in modulating ferromagnetism in undoped ZnO nanoparticles. Moreover, the magnetization value can be tuned by changing the relative concentration of singly charged oxygen vacancies, which is affected strongly by the size and annealing condition. These findings not only get further insight into the defect origin of room-temperature ferromagnetism observed in undoped oxide semiconductors but also contribute to achieve the tunable room-temperature ferromagnetism.

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Figure 7. (a) RT M−H loops and (b) EPR spectra before and after oxygen annealing at 600 and 800 °C. The inset shows the corresponding RT PL spectra.

ASSOCIATED CONTENT

S Supporting Information *

HRTEM images and corresponding diameter distributions of samples S1; RT M−H curves of samples S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

diamagnetic signal of S2 before and after oxygen annealing at different temperatures of 600 and 800 °C. After annealing of 600 °C, the Ms and Hc decrease obviously. When the annealing temperature is improved to 800 °C, the Ms and Hc disappear almost completely. The evolution of normalized PL spectra after oxygen annealing at different temperatures was shown in the inset of Figure 7a. The unmovable UV emission peaks centered at around 364 nm indicate that the variation of visible emission peaks is not because of the quantum size effect but because of the change of defect states induced by annealing. Generally, it is accepted that the green emission with wavelength less than 550 nm originates from singly charged oxygen vacancies, while the yellow−red emissions with longer wavelengths are typically attributed to the excess oxygen on the ZnO surface.47−49 Thus, from the change of visible emissions, it can be suggested that the oxygen annealing compensates VO and even promotes the formation of oxygen interstitials (Oi) at the surface. Meanwhile, Figure 7b shows that the EPR signal wears off and then vanishes as the annealing temperature increases from 600 to 800 °C, showing explicitly that the number of V+O is obviously reduced by oxygen annealing. Therefore, the coincident evolution between Ms and the relative concentration of VO+ is confirmed again by the postannealing study. In other words, the observed RTFM is closely associated with V+O in our samples. The F+ center exchange mechanism, as an extended bound magnetic polarons (BMP) mechanism, was proposed to explain the relevant RTFM induced by oxygen vacancies in some oxides without transition metal doping.50−53 In this mechanism, the F+ centers, basically the electrons in the singly occupied oxygen vacancies lying deep in the gap, can favor a ferromagnetic ordering. Our result shows a qualitative link between the ferromagnetism and the singly charged oxygen

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-025-83790755; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Grants 11104240 and 60976002), “973” Program (Grant 2011CB302004), and MOE (Grant 20110092130006).

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REFERENCES

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