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High-Temperature Ferromagnetism of Hybrid Nanostructure Ag-Zn0.92Co0.08O Dilute Magnetic Semiconductor Tao Yao,† Wensheng Yan,*,† Zhihu Sun,† Zhiyun Pan,† Bo He,† Yong Jiang,† He Wei,† Masaharu Nomura,‡ Yi Xie,§ Yaning Xie,| Tiandou Hu,| and Shiqiang Wei*,† National Synchrotron Radiation Laboratory, UniVersity of Science and Technology of China, Hefei, Anhui 230029, People‘s Republic of China, Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan, Department of Nanomaterials and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China, and Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: October 29, 2008; ReVised Manuscript ReceiVed: December 29, 2008
Hybrid-nanostructure Ag-Zn0.92Co0.08O dilute magnetic semiconductors synthesized by a solvothermal method show that the magnetic property of the nanorods is changed from paramagnetism to room-temperature ferromagnetism when 2% Ag is codoped into Zn0.92Co0.08O nanorods. The detailed analysis of the X-ray absorption fine structure spectra at both Co and Ag K-edge reveals that a substantial part of Ag atoms exists in the tetrahedral interstitial sites of (Zn, Co)O matrix in the Ag(2%)-Zn0.92Co0.08O. We suggest that the interstitial Ag atoms, together with Ag nanoparticles, play a most important role in mediating the hightemperature ferromagnetism of Ag(2%)-Zn0.92Co0.08O, due to the strong hybridization between the Co 3d states and the spin-split impurity band at the Fermi level. 1. Introduction Diluted magnetic semiconductors (DMSs) have recently attracted considerable research interest due to their promising applications in the next-generation multifunctional “spintronic” devices.1,2 For practical spintronics applications, ferromagnetic DMSs with Curie temperatures (TC) greatly exceeding room temperature will be required. Since the initial theoretical prediction3 and experimental observation4 of the room-temperature ferromagnetism in ZnO-based DMSs, (Zn, Co)O has been regarded as one of the most important DMS materials and has stimulated considerable research efforts. For the requirement of device miniaturization, nanostructures are potentially ideal functional components for nanoelectronic and optoelectronic devices, and lots of efforts have been dedicated to realizing highTC ferromagnetism nanoscale ZnO-based DMSs.5,6 To interpret the origin of the magnetic properties in DMSs, a number of magnetic interaction mechanisms have been proposed, including direct superexchange, indirect superexchange, carrier-mediated exchange, double-exchange mechanism, and bound magnetic polaron (BMP) model.7 However, the microscopic origin of the high-TC ferromagnetism (FM) in wide-band-gap DMSs is still poorly understood, which hampers their functionality in device applications. The development of such an understanding has emerged as one of the most important challenges in modern magnetism. Considerable reports have suggested that the observed high-temperature FM is intimately relevant to the relative location of Fermi level and impurity
band.8,9 For instance, it has been found that FM is favored in n-type Co2+:ZnO codoped with shallow donors, while an inverse correlation is observed for Co2+:ZnO codoped with p-type dopants such as N,10 because of their different relative positions of Fermi level and impurity band. More recently, it has been shown that metal-semiconductor hybrid nanostructure provides a unique opportunity to yield insight into the mechanism of hightemperature FM since metallic decoration may tune the Fermi level of semiconductors due to the electron transfer between metal and semiconductor.11,12 It has been known that the magnetic and electronic properties of DMSs are sensitively dependent on the environment around the dopants.13-15 Therefore, it is highly important to carry out careful structure studies to elucidate the microscopic origin of FM in DMSs. In this work, a comprehensive structural study on AgxZn0.92Co0.08O hybrid nanostructures using X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM) is performed, and the results are correlated to the magnetic properties of the material. Different from the substitution of Co in ZnO lattice, the Ag dopant exists in two forms: Ag metal nanoparticles and tetrahedral Ag interstitial. Co K-edge XANES analysis reveals the transfer of electrons from Ag-interstitialderived impurity band to Co 3d states. This charge transfer favors the enhancement of ferromagnetism in Ag(2%)Zn0.92Co0.08O hybrid structure, providing experimental evidence for the spin-split impurity band model of ferromagnetism. 2. Experimental Section
* Authors to whom correspondence should be addressed. E-mail:
[email protected] (S.Q.WEI);
[email protected] (W.S.YAN). † National Synchrotron Radiation Laboratory, University of Science and Technology of China. ‡ High Energy Accelerator Research Organization (KEK). § Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China. | Chinese Academy of Sciences.
Synthesis. The Ag-Zn0.92Co0.08O hybrid nanorods are synthesized through a solvothermal method. In a typical procedure, a certain percentage of Zn(Ac)2 · 2H2O and Co(Ac)2 · 4H2O was dispersed in ethanol with stirring. A well-controlled amount of 0.1 mol/L AgNO3/ethanol solution was added to the above suspension, and then 1 mol/L NaOH solution in ethanol was
10.1021/jp809560s CCC: $40.75 2009 American Chemical Society Published on Web 02/06/2009
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added drop by drop with agitation at room temperature. Subsequently, the mixture was sealed in 20 mL Teflon lined stainless steel autoclaves to heat at 120 °C for 2 h. When the reactions were completed, the precipitates were collected by filtration, washed with deionized water and ethanol several times, and then dried at ambient conditions. Characterization. The powder X-ray diffraction (XRD) patterns of the samples were recorded using Cu KR (λ ) 0.154 nm) radiation in the 2θ range from 20° to 80°. The morphologies and microstructures of the samples were investigated by HRTEM. XPS measurements were performed on an ESCALAB 250 spectrophotometer with Al KR radiation. The magnetizations were measured with a superconducting quantum interference device at 300 K under magnetic fields. The Co and Ag K-edge XAFS spectra of Agx-Zn0.92Co0.08O samples were measured at the BL-12C beam line of Photon Factory, High Energy Accelerator Research Organization (PF, KEK) and 4W1B of Beijing Synchrotron Radiation Facility (BSRF), respectively. The storage ring of PF was operated at 2.5 GeV with the maximum current of 400 mA. The storage ring of BSRF was run at 2.2 GeV with a maximum current of 200 mA. A double-crystal Si (111) monochromator was used as a monochromator. The XAFS signal of Co K-edge was collected in a fluorescence mode, and Ag K-edge XAFS data were recorded in transmission mode with ionization chambers at room temperature. XAFS data were analyzed by UWXAFS3.01416 and USTCXAFS3.01517 software packages. 3. Results and Discussion Figure 1a displays the XRD patterns of Zn0.92Co0.08O and Ag(2%)-Zn0.92Co0.08O samples. For Zn0.92Co0.08O nanorod, only diffraction peaks corresponding to the hexagonal wurtzite structured ZnO (JCPDS file no. 36-1451) can be observed. When Ag is codoped into Zn0.92Co0.08O nanorods, new diffraction peaks (marked with an asterisk) corresponding to face-centered-cubic (fcc) metallic silver (JCPDS file no. 04-0783) appear in the XRD pattern, while no other Co-metal-related diffraction peaks can be observed within the detection limit. A careful study of the lattice parameters reveals that the cell dimension of the Ag(2%)-Zn0.92Co0.08O is larger than that of Zn0.92Co0.08O. We found that although the c parameter (5.2070 Å) remains almost invariable, the lattice parameter a (3.2557 Å) of Ag(2%)-Zn0.92Co0.08O expands compared with that (3.2521 Å) of Zn0.92Co0.08O. This expansion of lattice parameter a may be attributed to possible interstitial coordination of some larger cations.18 More detailed structural information will be discussed below. Figure 1b shows the Co 2p core level XPS of Ag(2%)-Zn0.92Co0.08O hybrid nanostructure and Zn0.92Co0.08O nanorod. Obviously, both of the samples show the same XPS spectra which are characterized by a Co 2p3/2 peak at 780.7 eV, accompanied by a shake-up satellite peak at 785.8 eV, and a Co 2p1/2 peak at 796.2 eV (∆E ) 15.5 eV). These are significantly different from those of metallic Co and Co nanoclusters,19 confirming the XRD results on the absence of metallic Co in our samples. From the TEM images (Figure 2a, b), we can clearly find the formation of the Zn0.92Co0.08O nanorods and the metal-semiconductor Ag(2%)-Zn0.92Co0.08O hybrid nanostructures. The metallic Ag nanoparticles (highlighted by circles) are attached to the top of Zn0.92Co0.08O nanorods (Figure 2c) with 12 nm diameter and 100 nm length. A representative HRTEM image (Figure 2c) of individual hybrid nanostructure also shows the uniform lattice structure and high crystal quality of the ZnCoO nanorod with preferential growth along the [001] direction (c axis), further indicating that the
Figure 1. (a) XRD patterns of the Ag(2%)-Zn0.92Co0.08O hybrid nanostructure and Zn0.92Co0.08O nanorods. Peaks marked by (*) are indexed to the fcc silver. The lines on the x axis correspond to the peak positions for the pure ZnO. (b) Co 2p core level XPS of Agx-Zn0.92Co0.08O with given Ag and Co contents.
dopant is evenly distributed without the coexistence of any perceptible microstructure. All above results suggest that the metal-semiconductor Agx-Zn0.92Co0.08O hybrid nanostructures with high structural perfection have been prepared successfully. Figure 3 displays the Co K edge XANES spectra of the Zn0.92Co0.08O and Ag(2%)-Zn0.92Co0.08O powders along with the reference spectra of standard Co foil and CoO. Both of the Zn0.92Co0.08O nanorod and Ag(2%)-Zn0.92Co0.08O hybrid nanostructure exhibit quite similar XANES features, in which four main peaks A1 (7705 eV), B1 (7720 eV), C1 (7733 eV), and D1(7765 eV) can be observed. Evidently, spectral features of the Agx-Zn0.92Co0.08O powders are clearly different from those of CoO and Co foil, suggesting the absence of Co clusters and the cobalt oxides. In addition, comparing the onset of the XANES spectra for the Agx-Zn0.92Co0.08O powders with that of the CoO, we can obtain that the Co ions exist in the form of the +2 state in the Agx-Zn0.92Co0.08O samples,20 consistent with the XPS results mentioned above. Hence, it can be concluded that there is no evidence of Co metal throughout the powders. In fact, from the viewpoint of the chemical reaction, the ethanol solvent at 120 °C has a very low reduction activity,21 and it is very hard for Co ions to be reduced to metallic clusters. Also, considerable efforts to prepare the Zn1-xCoxO DMSs through the solvothermal method have proved the nonexistence of Co cluster.6,22 Thus, the XANES results, in conjunction with the
High-Temperature Ag-Zn0.92Co0.08O
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Figure 4. Magnetization vs magnetic field curves of the Ag(2%)-Zn0.92Co0.08O and Zn0.92Co0.08O nanorods recorded at room temperature.
Figure 2. (a) and (b) TEM images showing various Zn0.92Co0.08O nanorods and Ag(2%) –Zn0.92Co0.08O metal-semiconductor hybrid nanostructures. (c) Individual and HRTEM images of the enlarged hybrid nanostructure.
Figure 5. (a) Fourier-transformed spectra and EXAFS oscillation functions k3χ(k) (inset) obtained at the Co K-edge for Ag(2%)-Zn0.92Co0.08O, Zn0.92Co0.08O, and Zn K-edge for ZnO powders. (b) Ag K-edge FT-EXAFS spectra for Ag(2%)-Zn0.92Co0.08O and the reference metal Ag. The inset is the sketch map of Ag atoms within the tetrahedral interstitial sites of (Zn, Co)O lattice with substitutional Co around it. The empty symbols in (a) and (b) show the fitting results. Figure 3. Co K-edge XANES spectra of Agx-Zn0.92Co0.08O samples, Co foil, and CoO powders. The inset displays the comparison of the intensity for the pre-edge peak A1 where the background has been subtracted.
XPS results, clearly indicate that Co ions in the samples are indeed Co2+ valence, without forming Co-metal clusters in Agx-Zn0.92Co0.08O. The magnetizations M of the Zn0.92Co0.08O and Ag(2%)-Zn0.92Co0.08O samples were measured at room temperature as a function of the applied magnetic field (Figure 4). It can be seen that Zn0.92Co0.08O nanorods show paramagnetic (PM) properties, consistent with the intrinsic PM nature of (Zn, Co)O DMS without charge doping.23,24 However, as for the Ag(2%)-Zn0.92Co0.08O hybrid nanostructure, the well-defined hysteresis loop with the saturation magnetic moments (MS) of 1.12 A · m2 · kg-1 can be observed. This indicates the obvious ferromagnetic behavior of the Ag-codoped Zn0.92Co0.08O hybrid nanostructure. The underlying physics will be discussed later. In order to investigate the correlation between the local structure and the observed magnetic behavior, the Co K-edge
EXAFS oscillation functions k3χ(k) and their Fourier transform (FT) spectra are displayed in Figure 5a. As references, the Zn K-edge function of ZnO powder is also plotted. The inset of Figure 5a manifests that the oscillation characteristics of the Agx-Zn0.92Co0.08O samples are quite similar to those of ZnO. The Zn0.92Co0.08O sample also exhibits two coordination peaks at 1.60 and 2.84 Å corresponding to the Co-O and Co-Zn neighbors, respectively, in the FT, suggesting that Co ions in Zn0.92Co0.08O are substitutionally doped into the ZnO host. Upon Ag codoping to Zn0.92Co0.08O, a new coordination peak B located at 2.16 Å, besides the same position and magnitude of peaks A (1.60 Å) and C (2.84 Å), is apparent in the FT spectrum. The present peak B may be induced by several possibilities, i.e., (1) Co-Ag alloy and (2) cation interstitial (Coi, Zni, or Agi) sites. However, the position of the Co-Ag coordination peak in Co-Ag alloy is 2.72 Å,25 which is much larger than the position 2.16 Å of peak B, excluding the possibility of Co-Ag alloy. In view of the changed magnetic behavior and the FT spectra when Ag is doped in the system, we consider that the appearance of peak B is most likely correlated with the Ag
3584 J. Phys. Chem. C, Vol. 113, No. 9, 2009 addition. Likewise, we have performed the fitting considering three cases, the Co, Zn, and Ag ions located in the interstitial site. We have found that the spectral shapes for the cases of Co and Zn are the same as each other due to the close atomic numbers and similar characteristics of scattering photoelectrons between Co and Zn scatters, Therefore, we only show the fitted spectra of Co ions in the interstitial sites. It can be seen that this structural model gives a poor fitting quality as displayed by empty blue triangles in Figure 5a. From the quantitative analysis of Co and Ag K-edge EXAFS, we found that the FT spectra of Ag(2%)-Zn0.92Co0.08O can only be fitted well when we assign Ag ions at tetrahedral interstitial sites (shown in the inset of Figure 5b), rather than Co or Zn ions at the interstitial sites. In the fitting process, we quantitatively fit the three characteristic peaks A, B, and C using three coordination shells including Co-O, Co-Ag, and Co-Zn pairs. The curve-fitting result is shown as empty red circles in Figure 5a. The obtained best-fit value of the atomic distance RCo-Ag is 2.30 Å, which agrees very well with the interatomic distance between substitutional Co and tetrahedrally coordinated interstitial Ag, suggesting that part of the Ag ions exists in the tetrahedral interstitial sites in the Ag(2%)-Zn0.92Co0.08O sample. Accordingly, we ascribe peak B in the Co K-edge FT of Ag(2%)-Zn0.92Co0.08O to the CoZn-Agi coordination. To further examine the possibility of the existence of tetrahedral interstitial Ag in the Ag(2%)-Zn0.92Co0.08O sample, we measured the Ag K-edge EXAFS spectrum and show the Fourier-transformed k3χ(k) function along with that of metal Ag in Figure 5b. The presence of substitutional Ag can be readily excluded since two peaks at 1.60 and 2.84 Å associated with the Ag-O and the Ag-Zn coordinations are invisible within the EXAFS detection limit. The FT for the Ag(2%)-Zn0.92Co0.08O sample shows a primary peak at 2.68 Å corresponding to Ag-Ag pair, indicating that Ag atoms mostly exist in a metallic Ag phase, in agreement with our results of XRD and HRTEM. However, a shoulder peak at 2.15 Å observed for Ag(2%)-Zn0.92Co0.08O cannot be simply attributed to the nonlinearity of the phase shift function of Ag scatters, because the valley between the shoulder and main peak is obviously deeper than that of Ag foil. In addition, the position of this peak is close to that of peak B in the Co K-edge FT which has been ascribed to the CoZn-Agi coordination. Moreover, the Ag K-edge FT can only be quantitatively fitted well when the Agi-CoZn, Agi-O, and metal Ag-Ag coordination are taken into account, as displayed by empty circles in Figure 5b. The extracted interatomic distances, RAg-Co ) 2.31 ( 0.01 Å and RAg-O ) 2.32 ( 0.02 Å, are correspondingly close to Ag-O and Ag-Zn distances when Ag is located in the tetrahedral interstitial site of ZnO. This suggests that besides the main form of metallic Ag, a part of Ag atoms occupies the tetrahedral interstitial site of ZnO, confirming our earlier conclusion drawn from Co K-edge EXAFS analysis. As concluded from the above results, codoping Ag into Zn0.92Co0.08O nanorods does not produce any secondary phase with room-temperature ferromagnetism, but it can change the magnetic behavior of the nanorods from PM to FM. This suggests that the Ag species plays an important role in mediating the magnetic interactions. We note that the Agi ions introduced into Zn0.92Co0.08O could cause a shallow donor impurity band near the conduction band in ZnO. This is similar to that of group-I elements (Li, Na, or K) doped into ZnO matrix which prefer to form the shallow donors,26 due to their analogous electronic configuration. Furthermore, it is well-known that when metal nanoparticles come in contact with a semiconductor
Yao et al.
Figure 6. Schematic band structure for Zn0.92Co0.08O nanorods, and Ag-Zn0.92Co0.08O hybrid nanostructure with Co 3d impurities and Ag interstitial donor impurity band (adapted from Coey et al., with permission).
nanostructure, the Fermi levels of the two systems equilibrate. Often such charge equilibration between the semiconductor and metal nanostructures in contact often drives the Fermi level close to the conduction band edge of the semiconductor, with electrons transferring to and accumulating in the semiconductor due to the difference in their work functions.11,27,28 As a result, this enhances the hybridization strength between Co 3dV and the Agiderived shallow impurity bands at the Fermi level. This strengthened hybridization can be confirmed experimentally by a comparison between the Co K-edge XANES spectra of Zn0.92Co0.08O and Ag(2%)-Zn0.92Co0.08O as shown in the inset of Figure 3 previously. Although their XANES features are similar in the postedge region, we can find the slight difference concerning the pre-edge peak A1 (7705 eV) which is assigned to the transition of Co 1s core electron to the hybridized orbital of Co 3d and O 2p states. It can be seen that the intensity of peak A1 is relatively lower for Ag(2%)-Zn0.92Co0.08O than for Zn0.92Co0.08O as shown in the inset of Figure 3, implying that the Co 3d unoccupied density of states is decreased, which can be interpreted by the electron transfer from the Agi donor band. From the above analysis, the central hypothesis of the spinsplit impurity band model (BMP) theory,8,10 namely, that high TC of dilute ferromagnetic oxides requires a donor-derived impurity band and its hybridization with unoccupied 3d states at the Fermi level, appears to be satisfied experimentally for the Ag(2%)-Zn0.92Co0.08O hybrid nanostructure. In the current case, the donor-derived impurity band arises from the insertion of interstitial Ag. At the same time, as discussed above, the presence of a schottky barrier between the Ag nanoparticle and the Zn0.92Co0.08O nanorod drives the Fermi level close to the conduction band edge of the Zn0.92Co0.08O semiconductor, which can enhance the hybridization strength between Co 3dV and the Agi-derived shallow impurity bands at the Fermi level. The band structure can be schematically described in Figure 6. For Zn0.92Co0.08O sample, the Fermi level (EF) is located in the partially occupied Co 3dV states within the 2p(O)-4s(Zn) gap of ZnO, while the Fermi level of Ag(2%)-Zn0.92Co0.08O (EF) is shifted toward the conduction band by the Agi-derived impurity bands as well as the interaction between the metal Ag and semiconductor. In addition, from the results of the EXAFS, we estimate that about 0.5% Ag appears in the interstitial sites. According to ref 29, critical concentration of defects (δcrit) for the ferromagnetic ordering can be obtained from the relation γ3δcrit ≈ 4.3, where the radius of the orbital γ ) r(m/m*), where r is the dielectric constant, m is the electron mass, and m* is the effective mass of the donor electrons. For Co:ZnO, m/m*
High-Temperature Ag-Zn0.92Co0.08O ) 3.57 and r is in the range of 4∼37. Then we can obtain that δcrit is approximately 0.15%, which is much lower than our concentration of interstitial defect (Agi). This leads us to conclude that the high-temperature ferromagnetic behavior arises only when 3d states of the transition element hybridize with the spin-split impurity band states at the Fermi level, which can be used to well explain the Ag-induced high-TC ferromagnetism in the Ag(2%)-Zn0.92Co0.08O hybrid nanostructures. 4. Conclusion In summary, Agx-Zn0.92Co0.08O hybrid nanostructures are synthesized through a chemical method. HRTEM and XRD results showed the formation of crystalline nanorods with Ag nanoparticles attached at the surface and cobalt dopants incorporated into the wurtzite structure ZnO. Magnetization measurement has demonstrated that the codoping of Ag and Co in ZnO nanostructures changes the magnetic properties from paramagnetism to room-temperature ferromagnetism. Combining Co and Ag K-edge EXAFS analysis, we have revealed that besides the main metallic Ag phase, there is a substantial part of interstitial Ag atoms existing in the Ag(2%)-Zn0.92Co0.08O. These interstitial Ag and metal Ag play crucial roles in the enhancement of ferromagnetism due to the effective hybridization between Co 3dV states and Ag-interstitial-induced donor impurity band at the Fermi level. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 10605024, 10635060, 10725522, 20621061, and 20701036). The authors would like to thank KEK-PF and BSRF for the synchrotron radiation beam time. Shishen Yan (Shandong University) is acknowledged for his valuable help in magnetization measurement. References and Notes (1) Awschalom, D. D.; Flatte, M. E. Nat. Phys. 2007, 3, 153. (2) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; Molnar, S. V.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (3) Dietl, T.; Ohno, H.; Matsujura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019. (4) Ueda, K.; Tabata, H.; Kawai, T. Appl. Phys. Lett. 2001, 79, 988. (5) Yuhas, B. D.; Fakra, S.; Marcus, M. A.; Yang, P. D. Nano. Lett. 2007, 7, 905.
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3585 (6) Chu, D. W.; Zeng, Y. P.; Jiang, D. J. J. Phys. Chem. C 2007, 111, 5893. (7) Liu, C.; Yun, F.; Morkoc, H. J. Mater. Sci.: Mater. El. 2005, 16, 555. (8) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B. Nat. Mater. 2005, 4, 173. (9) Reed, M. J.; Arkun, F. E.; Berkman, E. A.; Elmasry, N. A.; Zavada, J.; Luen, M. O.; Reed, M. L.; Bedair, S. M. Appl. Phys. Lett. 2005, 86, 102504. (10) Kittilstved, K. R.; Liu, W. K.; Gamelin, D. R. Nat. Mater. 2006, 5, 291. (11) Yadav, H. K.; Sreenivas, K.; Guptaa, V. Appl. Phys. Lett. 2007, 90, 172113. (12) Qiu, X. Q.; Li, L. P.; Tang, C. L.; Li, G. S. J. Am. Chem. Soc. 2007, 129, 11908. (13) Criadoa, G. M.; Segura, A.; Sans, J. A.; Homs, A.; Porres, J. P.; Susini, J. Appl. Phys. Lett. 2006, 89, 061906. (14) Yan, W. S.; Weng, W. X.; Zhang, G. B.; Sun, Z. H.; Liu, Q. H.; Pan, Z. Y.; Guo, Y. X.; Xu, P. S.; Wei, S. Q.; Zhang, Y. P.; Yan, S. S. Appl. Phys. Lett. 2008, 92, 052508. (15) Liu, T.; Xu, H.; Chin, W. S.; Yang, P.; Yong, Z. H.; Wee, A. T. S. J. Phys. Chem. C 2008, 112, 13410. (16) Stern, E. A.; Newville, M.; Ravel, B.; Haskel, D.; Yacoby, Y. Physica B 1995, 208, 117. (17) Zhong, W. J.; Wei, S. Q. J. Chin. UniV. Sci. Technol. 2001, 31, 328. (18) Risbud, A. S.; Spaldin, N. A.; Chen, Z. Q.; Stemmer, S.; Seshadri, R. Phys. ReV. B 2003, 68, 205202. (19) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer: Eden Prairie, MN, 1992; p 83. (20) Griffin, K. A.; Pakhomov, A. B.; Wang, C. M.; Heald, S. M.; Krishnan, K. M. Phys. ReV. Lett. 2005, 94, 157204. (21) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci., Pure Appl. Chem. 1979, 13, 727. (22) Zhang, X. L.; Qiao, R; Qiu, R.; Li, Y.; Kang, Y. S. J. Phys. Chem. A 2007, 111, 4195. (23) Jayakumar, O. D.; Gopalakrishnan, I. K.; Kulshreshtha, S. K. AdV. Mater. 2006, 18, 1857. (24) Sati, P.; Deparis, C.; Morhain, C.; Schafer, S.; Stepanov, A. Phys. ReV. Lett. 2007, 98, 137204. (25) Faraci, G.; Pennisi, A. R.; Balerna, A.; Pattyn, H.; Koops, G. E. J.; Zhang, G. I. Phys. ReV. Lett. 2001, 86, 3566. (26) Orlinskii, S. B.; Schmidt, J.; Baranov, P. G.; Hofmann, D. M.; Donega´, C. M.; Meijerink, A. Phys. ReV. Lett. 2004, 92, 047603. (27) Wood, A.; Giersig, M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, 8810. (28) Shan, G. Y.; Xu, L. H.; Wang, G. R.; Liu, Y. C. J. Phys. Chem. C 2007, 111, 3290. (29) Pan, F.; Song, C.; Liu, X. J.; Yang, Y. C.; Zeng, F. Mater. Sci. Eng., R 2008, 62, 1.
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