Article pubs.acs.org/JPCC
High-Pressure Electrical Transport Behavior in WO3 Yuqiang Li,† Yang Gao,† Yonghao Han,† Qinglin Wang,† Yan Li,† Ningning Su,† Junkai Zhang,† Cailong Liu,† Yanzhang Ma,‡ and Chunxiao Gao*,† †
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas 79409, United States
‡
ABSTRACT: The high-pressure electrical transport behavior of microcrystalline tungsten trioxides (WO3) was investigated by direct current electrical resistivity measurement and alternate current impedance spectrum techniques in a diamond anvil cell up to 35.5 GPa. Discontinuous changes of electrical resistivity occurred during the pressure induced structure phase transitions at 1.8, 21.2, and 30.4 GPa. The irreversible resistivity reveals that the structure phase transition is not reversible. In addition, the abnormal changes of bulk resistance and transport activation energy at about 3 and 10 GPa are related to the isostructural phase transition reported by previous Raman study. The temperature induced resistivity change indicates that WO3 is a semiconductor from ambient pressure to 25.3 GPa.
1. INTRODUCTION Tungsten trioxide has been proposed as an excellent material for the applications in nanoelectronic and nano-optoelectronic devices due to its potential properties.1 It is also a candidate material for optoelectronics, microelectronics, selective catalysis, and environmental engineering.2 For these applications, a detailed knowledge of its electrical behavior and structural properties is highly desirable. WO3 can exist in several forms: the well-known thermodynamically stable phase of tetragonal, orthorhombic, triclinic, and monoclinic structures. Xu et al.3 performed an X-ray diffraction study on single-crystal WO3 up to 5.7 GPa and found that the triclinic form was transformed into a new monoclinic phase (P21/c) above 0.57 GPa. Their result is distinct from that reported earlier by Salie and Hoppmann.4 Souza-Filho et al.5 carried out Raman measurements of microcrystalline WO3 under high pressure and found a new phase above 1.4 GPa, which began as a mixture of crystalline structures with groups (P21/n) monoclinic and (P1̅) triclinic then transformed gradually to a new variety belonging to the group (P21/c) monoclinic. M. Boulova et al.6 also measured the Raman spectra of microcrystalline WO3 under compression and found two Raman spectral anomalies at about 3 and 10 GPa, and they believed these two anomalies were from two weak structural transitions. At the same time, they found a new high-pressure phase at 22 GPa. This group took XRD study on microcrystalline WO3 and found two first order phase transitions, i.e., from monoclinic phase (P21/c) to monoclinic phase (P21/a) and then to another monoclinic phase (Pm), at about 24 and 31 GPa, respectively.7 In 2003, T. Pagnier et al.8 found a 7-fold coordinated tungsten in compressed WO3 by the first-principle calculation. These results demonstrated an interesting structural change of WO3 under compression; however, the electrical characteristics of WO3 under high pressure remain unknown. In 1959, S. © 2012 American Chemical Society
Sawada and G.C. Danielson reported on the temperature dependent resistivity of single-crystal WO3, which was the earliest report on the electrical property of WO3.9 In 2005, A. Labidi et al. measured the impedance spectrum of WO3 under different density ethanol vapor mixed with dry air in order to understand the mechanisms involved in the detection of ethanol by WO3 sensors.10 Recently, the electrical transport behavior of hexagonal WO3 nanowires was studied, and the back-to-back Schottky barriers with nonlinear and asymmetric I−V properties were found.1 G. A. de Wijs et al. explored the electronic structure changes upon compression and expansion by the first-principle calculation and found that the variation of electronic structure was related to the deformation mechanisms.11 In this article, we present a comprehensive investigation on the electrical transport properties and pressure-induced phase transitions of microcrystalline WO3 using in situ electrical resistivity measurements and impedance spectrum techniques in a diamond anvil cell (DAC) up to a pressure of 35.5 GPa. The abrupt changes of electrical resistivity of WO3 have been found to be related to the pressure induced structural phase transitions. The weak phase transitions investigated by the earlier Raman method are also discussed.
2. EXPERIMENTAL SECTION WO3 powder with purity of 99.998% was obtained from Alfa Aesar Co. The initial structure of the sample was the monoclinic structure (space group P21/n) as confirmed by a powder X-ray diffractometer with Cu−Kα radiation. Figure 1a gives the XRD spectrum. The average particle size of the Received: November 3, 2011 Revised: January 16, 2012 Published: February 1, 2012 5209
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conditions were controlled and maintained stable during the measurements.
3. RESULTS AND DISCUSSION 3.1. Electrical Resistivity under High Pressure. Figure 3 shows the electrical resistivity of the sample as a function of
Figure 1. (a) X-ray diffraction pattern of the WO3 sample at ambient pressure. (b) SEM picture of the WO3 sample.
sample, measured by scanning electron microscope (SEM) as shown in Figure 1b, was about 1.5 μm. A DAC generated the high pressure, and the anvil culet was 300 μm in diameter with a bevel angle of 10°. The applied pressure was determined through the ruby luminescence linear scale.12 A preindented T301 stainless steel with 80 μm in thickness was used as the gasket, and a hole with 110 μm in diameter was drilled at the center of the indentation. To measure the electrical resistivity accurately, the hole was fully covered with compacted WO3 microcrystals, and no pressuretransmitting medium was used. The fabricating method of the microcircuit on the diamond anvil has been reported previously.13,14 Figure 2 shows the microcircuit and the
Figure 3. Pressure dependence of the electrical resistivity of WO3 at room temperature. (a) The electrical resistivity vs pressure below 5 GPa. (b) The electrical resistivity vs pressure above 5 GPa. The arrow heads point out the electrical resistivity anomalies; ■ represents the compression and ○ indicates the decompression.
pressure at room temperature. The resistivity drops sharply in Figure 3a, over an order of magnitude, when the pressure increases from ambient pressure to 1.8 GPa, indicating that WO3 experiences a phase transition from a high resistivity phase to lower resistivity phase. The pressure induced resistivity drop is caused by an increase in carrier concentration, which is related to the increase of the additional energy levels in energy band gap. Under compression, the atoms deviate from the equilibrium position and form many additional energy levels, especially during the phase transition, increasing carrier concentration. Souza-Filho et al.5 reported that the monoclinic form was transformed into a new monoclinic phase (P21/c) at 1.4 GPa, with the sliding of atomic planes and that it was identical with the creation and destruction of stacking disorders.16 The shift observed by Souza-Filho et al.5 can also be confirmed by the sudden decline of resistivity with increasing pressure from another point of view. The present electrical measurement directly reflects the variation of the electron state related to the atomic structure variation under pressure because the transition pressure is related to the structural transition.17 That electrical resistivity decrease during the phase transition indicates that the new monoclinic phase has a smaller band gap compared to the lower pressure form because the symmetry related to the orbital hybridization of O 2p and W 5d induces valence band mixing and leads to a smaller band gap.18 For semiconductors, the electrical resistivity is related to the width of energy band gap, and the conductivity is increased with the decrease of the energy gap. Applied pressure influences the number of electrons in the conduction band and holes in the otherwise filled valence band.17 Between 1.8 and 3.2 GPa, the WO3 remains stable, and no abnormal resistivity change is observed. Above 3.2 GPa, the change in resistivity with increasing pressure is obviously less, which is associated with the report in which the band 1 and band 2 in the Raman spectrum merge into a single component above 3 GPa.6 As a transition metal, W atom contains unfilled d atomic shell and s electron in its exterior shell. The wave function of d electron has a few overlap among adjacent atoms, and the d energy band is narrow. At the same time, the d energy levels for atom are 5-fold degenerate states. The total d bands contain five affiliated bands and have 10N (N indicates
Figure 2. (a) Configuration of a complete microcircuit on a diamond anvil: (1) the Mo electrodes, (2) the Al2O3 layer deposited on the Mo film; A, B, C, and D are the four contact ends of the microcircuit. (b) The cross-section of the diamond anvil cell device.
sectional view of the sample configuration in our DAC. During the measurement, a 1 μA current (I1) was introduced through electrode A and B, while the voltage drop (V1) between D and C was recorded. Then, the current (named I2) was applied from A to D, and the voltage drop (V2) between B and C was recorded. The thickness of the sample under high pressure was determined by measuring the distance between the bottom facets of the two anvils and subtracting the thickness of the diamond anvils, which were measured with an electronic micrometer. The electrical resistivity was determined by the van der Pauw equation15 exp( −πR1dρ) + exp( −πR2dρ) = 1
(1)
where R1 = V1/I1, R2 = V2/I2, d is the thickness of the sample, and ρ is the electrical resistivity. The low temperature condition was obtained by liquid nitrogen, and the temperature was monitored by a standard thermocouple connecting with the anvil. The ac impedance spectroscopy measurements of WO3 were carried out by employing the Solartron 1260 impedance spectrometer equipped with Solartron 1296 dielectric interface with two contact terminals. The frequency ranged from 0.1 Hz to 10 MHz, and the ac voltage amplitude was 0.1 V. Indoor 5210
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K) to room temperature (300 K). The results are plotted in Arrhenius format and shown in Figure 4a. The electrical
atom number) electrons in all, which results in a much higher density of states.11 In contrast, the s energy bands are wide and only contain 2N electrons, accompanying with lower density of states. The abnormal resistivity change at 3.2 GPa is related to the orbital hybridization of O 2p and W 5d and 5-fold degenerate states of d energy. Above 10.6 GPa, as shown in Figure 3b, the slope of the resistivity curve is close to zero, the descending trend of the resistivity becomes much slower, and the resistivity reaches a minimum at 14.5 GPa. In respect to that no structural phase transition has been reported by XRD in the pressure range from 3.8 to 14.5 GPa, the inflection observed at about 10.6 GPa cannot be attributed to a structural phase transition in WO3. M. Boulova et al.6 noticed a split of the Raman spectral band of WO3 at 10 GPa and believed it may correspond to a weak isostructural change as well as the same space group, the same site occupation, and sudden displacements of atoms in general positions at the same time. In terms of the resistivity curve variation with increasing pressure, the inflection should be caused by an electronic phase transition that starts at 10.6 GPa and ends at 14.5 GPa. As pressure increases, the electrical resistivity begins to increase gradually. When pressure increases to 21.2 GPa, the highest electrical resistivity is reached. The pressure point corresponding to the resistivity peak is in good agreement with that reported by high pressure Raman study at which the first order phase transition occurred at about 22 GPa.6 The lower compressibility of this high-pressure phase (P21/a) is related to the compression of the oxygen octahedra themselves through W−O bonds; such a compression leads to a 7-fold coordination for W atoms.7,8 From 21.2 to 30.4 GPa, the electrical resistivity drops slowly and has no abrupt change. There is also no obvious reisitivity change from 30.4 to 33.3 GPa. A degressive trend in reisistivity occurrs after 33.3 GPa. This suggests that another phase transition starts at 30.4 GPa and completes at 33.3 GPa, which is in line with the result from XRD data by P. Bouvier et al. that a phase transition takes place above 31 GPa.7 The pressure point that the discontinuous resistivity occurred has a subtle difference comparing with the pressure of phase transition reported earlier.5−7 Because no pressure transmitting medium is used in our experiment, the pressure across the experimental sample is generally inhomogeneous, and anisotropic stress and shear stresses appear. However, by comparing the transition pressure points reported by earlier papers, the nonhydrostatic effect had little impact on our electrical measurement, and the error of pressure calibrating was less than 1 GPa. The resistivity change in the decompression process is also presented in Figure 3. After releasing the pressure, the resistivity of the sample did not return to its initial value. This means that the sample cannot be retained down to the initial state, at least in resistivity. As the pressure increases, the WO3 crystal will adjust by forming the lattice defects and vacancies during the pressure-induced lattice deformation. Some specific and partly occupied states are evoked by structural defect in the band gap. When pressure returns to ambient conditions, these changed states cannot return to the original states. As a result, the resistivity is not reversible, indicating that the phase transition is not reversible. A similar conclusion was reached by a high pressure XRD experiment.7 For studying the transport mechanism of high pressure phases of WO3, the resistivity measurement was also performed at temperature ranging from liquid nitrogen temperature (77
Figure 4. (a) Reciprocal temperature dependence of the logarithm of resistivity at different pressure. (b) Transport activation energy as a function of pressure; the arrow heads point out the transport activation energy anomalies.
resistivity of WO3 decreases with increasing temperature, indicating that WO3 is a semiconductor from ambient pressure to 25.3 GPa.19 The transport activation energy (Et) of WO3 at given pressure was obtained according to the equation15 ρ = ρ0 exp(E t /2kT )
(2)
where ρ0 is a constant, k is the Boltzmann constant, and T is the temperature. According to eq 2, the Et can be obtained by linearly fitting the plot of ln ρ versus 1000/T in the region from 120 to 200 K where the curves are linear. Its pressuredependent feature in another region of 200−300 K is very close to that in 120−200 K. The activation energy reflects the difference between valence-band edge EV and Fermi level EF, i.e., Et = EF − EV.20 Although the Et does not follow band gap, it remains always less than band gap. It is thus plausible that Et is determined by defect energy levels in the band gap.21 Furthermore, some dangling bonds of W and O would produce a large number of localized-states in the gap, which cannot be treated as defect energy levels, and the electrical transportation at low temperature can only be realized through phononassisted tunneling or hopping between the localized states.20 As shown in Figure 4b, Et initially decreases with increasing pressure, then increases with pressure up to about 3.2 GPa. The electron is evoked more easily because of the relative small activation energy, and the previous discontinuous resistivity also supports this interpretation. M. Boulova et al.6 speculated that the subtle structure change might be an isostructural phase change at about 3 and 10 GPa by a Raman spectrum study. The electronic structure phase transition may lead to an abnormal change in Raman spectrum at about 3 GPa because the changes of electron states have an effect on the mode of vibration. With increasing pressure, the activation energy starts to increase, and the maximum (13.07 meV) appears at 10.6 GPa. From 3.2 to 10.6 GPa, the activation energy keeps increasing linearly, which can be explained as the drop in defect concentration.20 A pressure-induced resistivity drop indicates that the compression plays an important role in changing the energy barrier height, which is related to the electron wave function overlaps for W and O atoms with decreasing atomic distance under increasing pressure. The abnormal activation energy and resistivity reflects the electronic structure phase transition caused by the isostructural phase transition mentioned by Raman study at about 3 and 10 GPa. The activation energy linearly decreases with increasing pressure from 10.6 to 20.7 GPa. Above 20.7 GPa, the activation energy starts to increase slowly and then is constant with pressure between 22.1 and 5211
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Figure 5. Complex impedance plane plots of WO3 microcrystals under different pressure.
The abrupt change of electrical transport property brings about the electronic phase transition. Electronic and ionic conduction processes are two different routes by which electric charge transported in WO3 is implemented.26 Tungsten trioxide is a compound with strong ionic character. On account of impurity and thermodynamic disorder equilibrium, many anion vacancies remain in pure WO3, but the electronic conduction may also contribute to the observed finite conductivity. In a simplified picture, all six W valence electrons are transferred to O atoms, which can accommodate two additional electrons each, leading to the formation of an insulator with stoichiometric WO3.11 Pure tungsten trioxide is an insulator at room temperature and ambient pressure with capacitive behavior from 0.1 to 10 MHz.27 Our previous temperature-induced resistivity indicates that the WO3 becomes a semiconductor under compression, and the semiconductor character remains up to 25.3 GPa. The experimental results also denote that the insulating electronic structure is disrupted at ambient pressure, and a new electronic structure with semiconductor behavior is found between W and O atom under pressure. As shown in Figure 6a, capacitive conduction at the high frequency region and parallel ohmic conduction are presented in the low frequency range. In Figure 6a, the corresponding impedance phases shift changes from −130° to 0° as the frequency decreases from 107 to 104 under different pressures. It remains constant until the frequency comes to 0.1 Hz. With increasing pressure, the impedance magnitude gradually decreases and is almost constant from 0.1 to 104 Hz. From 1.8 to 2.9 GPa, the impedance magnitude decreases over an order of magnitude, at the same time the phase shift is not very evident. For acquiring the time constant under different pressures, the plot of imaginary impedance versus frequency is shown in Figure 6b. The imaginary impedance gradually decreases, and the relaxation peaks shift to a high frequency zone. The grain boundary and the bulk relaxation peaks that occur are displayed separately at different positions.
25.2 GPa. This change is related to the phase transition at 22 GPa found by earlier Raman detection.6 3.2. Variable-Frequency Alternate Current Impedance Spectrum. The impedance data of microcrystalline WO3 at different pressure are presented in Figure 5. The impedance spectrum method is frequently used to detect the bulk, grain boundary, and electrode transport processes of polycrystalline materials with electronic and ionic conduction. Figure 5a gives the impedance spectrum at ambient pressure. The semicircle depicts the transportation of charge carrier in grain inside, and the straight line at the right corner is attributed to the depletion of space charge at grain boundaries.22,23 Comparing with the impedance spectrum obtained under high pressure, the grain boundary effect at ambient condition is different. From ambient to 10.7 GPa, the impedance value decreases over two magnitudes. As shown in Figure 5b−d, only two arcs can be seen, and the electrode contribution is not observed. There is partial overlap between two arcs, which means that there is not only crystal grain contribution but also grain boundary effect under compression. The arc on the right side of the plot is the grain boundary contribution and represents the relaxation process of grain boundary. The arc on the left side, a welldefined semicircle, is the contribution of grain and corresponds to the relaxation process of bulk. Because of the disparity between W and O mass, W atoms move slower.11 Pressure will affect the atom distribution on grain surface and lead to the surface rearrangement, causing a grain boundary effect. The total resistance represents the sum of bulk resistance and grain boundary resistance; its value can be determined by the intercept on the abscissa axis obtained by extrapolating the curves.24,25 The total resistance decreases slowly since 10.7 GPa, indicating the electrical transport behavior is related to the abnormal Raman spectrum about 10 GPa.6 M. Boulova et al. reported an abnormal change at about 10 GPa by the Raman method and suggested that the change was induced by an isostructural phase transition. The displacement of the inequipotential atoms leads to a slow decrease in total resistance during isostructural phase transition. 5212
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process is electronic conduction.27 The simulation of the experimental data and their fitting to obtain optimum parameters allows us to calculate the individual impedance contributions. We extracted the measured bulk resistance Rb and the grain boundary resistance Rgb from fitting experimental data by the ZIEW2 impedance analysis software. Figure 7 represents the variation trend of bulk resistance with increasing pressure.
Figure 7. Rb is plotted as a function of the pressure. The arrowheads (3.8 and 10.0 GPa) point out anomalies of the electrical resistance. Inset: the linear fitting of resistance.
Two changes in slope can be seen at 3.8 and 10.0 GPa, respectively, and the differences in electrical transport behavior can be found. There are some reports that an important variation of the dV/dP derivatives has been observed for some modes above 3 GPa by M. Boulova et al.:6 the wavenumbers for bands 3 and 4 are increasing rapidly with pressure in a high pressure phase, then coming up to a plateau, and even decrease for band 3. They attribute the variation to a weak isostructural phase transition.6 Our results have revealed the discontinuous changes in bulk resistance at 3.8 and 10.0 GPa, indicating that the isostructural phase transitions suggested by M. Boulova et al. are also electronic phase transitions. The bulk resistance remains constant and has no drastic change as pressure increases from 10.0 to 14.5 GPa. This change tendency is in accord with our previous variation trend of resistivity measurement. Some subtle changes of vibration modes and their band half-widths occur around 10 GPa in the Raman spectrum and were believed to be related to a weak isostructural phase.6 The first-principle calculation by T. Pagnier and A. Pasturel also indicated that the W−O distance and the angles of O−W−O change in this pressure region.8 Our data indicates that these changes are related to the local state change of the electron caused by pressure induced electronic structure transition. The pressure effect may bring more oxygen vacancies, which can induce the high ion conductivity in transition metal ceramic oxides such as WO3. Because two different conduction processes (ionic and electronic) coexist in tungsten trioxides, the constant bulk resistance in the pressure region from 10 to 14.5 GPa may derive from the relative balance of two mechanisms. This means that the total contributions of electronic and ionic conduction are definite and nearly invariable. The pronounced discontinuous change of bulk resistance can reflect the abnormal change in the Raman spectrum near 3 and 10 GPa.
Figure 6. (a) Bode diagrams of the WO3 microcrystal as a function of pressure: the left ordinate axis represents the modulus of impedance (○) vs frequency, and the right ordinate axis represents phase shift (★) vs frequency. A single color represents the same pressure. (b) The imaginary part of impedance as a function of frequency under various pressures.
In contrast to the bulk relaxation peaks, the grain boundary relaxation peaks are not obvious. This means that the grain boundary effect decreases, and the scattering effect becomes weak. When pressure increases to 3.9 GPa, the relaxation peaks at the low frequency zone are hardly observed. This can be explained that the compression reduces the height of the grain boundary energy barrier so that the space charges can cross the grain boundary more easily. The disappearance of the grain boundary relaxation peak can also be attributed to an electronic phase transition around 3 GPa. The relaxation peaks shift to the right and become broad at low frequency range with increasing pressure. This can be directly related to a broad distribution of the relaxation time in the grain boundary zone, and in the meantime, it implies a broad distribution of the resistivity. The increase of defect varieties and the change of lattice structure may also lead to the broad distribution of relaxation frequency. There is a reasonable consideration that the sample consists of an infinite number of infinitesimal building blocks. The resistivity of each building block, explained with a statistical scale, may be represented by a Gaussian distribution. The origin of such distributions is rooted in physical or chemical nature. Strong disorder of the electron potential or simply local variations in stoichiometry may be plausible explanations for a ceramic oxide such as WO3.28 The relaxation peaks in the low frequency zone represent the presence of space charges, and the intensity of the relaxation peaks is proportional to the value of the boundary resistance.29 The relaxation frequency corresponds to the imaginary impedance peak and equals the reciprocal of the time constant. The relaxation frequency corresponding to the bulk process increases from 2.0 MHz at 1.8 GPa to 2.8 MHz at 2.9 GPa and to 3.6 MHz at 3.9 GPa. This tendency indicates that the electrical transportation
4. CONCLUSIONS In summary, we performed in situ high-pressure resistivity and impedance spectrum measurements of tungsten trioxides up to 5213
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(20) Xin, H. X.; Qin, X. Y.; Zhu, X. G.; Liu, Y. J. Phys. D: Appl. Phys. 2006, 39, 375−381. (21) Chen, A. L.; Yu, P. Y.; Taylor, R. D. Phys. Rev. Lett. 1993, 71, 4011−4014. (22) Tuller, H. L. Solid State Ionics 2000, 131, 143−157. (23) Wang, Y.; Han, Y. H.; Gao, C. X.; Ma, Y. Z.; Liu, C. L.; Peng, G.; Wu, B. J.; Liu, B.; Hu, T. J.; Cui, X. Y.; et al. Rev. Sci. Instrum. 2010, 81, 013904−1−4. (24) He, L.; Ling, Z. Y. Appl. Phys. Lett. 2011, 98, 242112−1−3. (25) Li, M.; Yang, J.; Snoussi, K.; Li, L. X.; Wang, H. X.; Gao, C. X. Appl. Phys. Lett. 2010, 97, 174101−174101−3. (26) Eder, D.; Kramer, R. J. Phys. Chem. B 2004, 108, 14823−14829. (27) Eder, D.; Kramer, R. Phys. Chem. Chem. Phys. 2006, 8, 4476− 4483. (28) Schmidt, R.; Wu, J.; Leighton, C.; Terry, I. Phys. Rev. B 2009, 79, 125105−1−8. (29) Srinivas, K.; James, A. R. J. Appl. Phys. 1999, 86, 3885−3889.
35.5 GPa. The change in electrical resistivity related to the structure phase transitions of WO3 microcrystalline has been revealed among different monoclinic phases at 1.8, 21.2, and 30.4 GPa. The irreversible electrical resistivity also shows that the structure phase transition is not reversible. WO3 shows a typical semiconductor character between ambient pressure and 25.3 GPa. The electronic phase transitions happening at about 3 and 10 GPa are also found. The activation energy reversion and the discontinuous bulk conduction provide the evidence of the electronic phase transitions caused by isostructural phase transitions.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-431-85168878-601. Fax: +86-431-85168878-602. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2011CB808204), the National Natural Science Foundation of China (11074094, 91014004, 10874053, and 50802033), and the Special Scientific Research Funding for Doctoral Discipline in Higher Education Institutions (200801831007).
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REFERENCES
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