Research Article  Open Access
Jiangni Yun, Zhiyong Zhang, Tieen Yin, "Effect of Intrinsic Point Defect on the Magnetic Properties of ZnO Nanowire", The Scientific World Journal, vol. 2013, Article ID 541496, 6 pages, 2013. https://doi.org/10.1155/2013/541496
Effect of Intrinsic Point Defect on the Magnetic Properties of ZnO Nanowire
Abstract
The effect of intrinsic point defect on the magnetic properties of ZnO nanowire is investigated by the firstprinciples calculation based on the density functional theory (DFT). The calculated results reveal that the pure ZnO nanowire without intrinsic point defect is nonmagnetic and ZnO nanowire with , , , , or Zn_{O} point defect also is nonmagnetic. However, a strong spin splitting phenomenon is observed in ZnO nanowire with defect sitting on the surface site. The Mulliken population analysis reveals that the oxygen atoms which are close to the defect do major contribution to the magnetic moment. Partial density states calculation further suggests that the appearance of the halfmetallic ferromagnetism in ZnO nanorod with originates from the hybridization of the O2p states with Zn 3d states.
1. Introduction
Recently diluted magnetic semiconductors (DMSs) have attracted much attention due to their potential application in novel spintronic devices. Among these DMSs, low dimensional ZnObased DMSs have been found to be promising for nanometer scale optomagnetics devices, optoelectronics devices, and biotechnology [1–15]. Many investigations have been reported regarding the intrinsic ferromagnetism in undoped onedimensional ZnO nanomaterials [9–15]. However, these reports demonstrate controversial results. For example, Xing et al. [9] have reported that the room temperature ferromagnetism (RTFM) in undoped ZnO nanowire can be attributed to the large population of Vo, since they can initiate defectrelated hybridization at the Fermi level and establish a longrange ferromagnetic ordering. Ghosh et al. [10] have found that V_{Zn} was responsible for the ferromagnetic behaviour in pure ZnO nanowire. Until now, the exact mechanism of intrinsic magnetism in undoped ZnO nanowire is still controversial. On the other hand, to reveal the origin of intrinsic magnetism in undoped ZnO, many theoretical calculations have been performed to study the roles of intrinsic point defects in ZnO [16–23]. These calculation results have made important contributions to the understanding of the roles of defects in ZnO and enriched the underlying knowledge of ZnO significantly. However, most of these calculations focused on the formation energies and transition levels [17, 18, 20, 23], which could not be detected directly in experiments. To date, the comprehensive investigation about the effect of intrinsic point defect on the magnetic properties of ZnO nanowire is lacking.
In this paper, we perform the firstprinciples calculation based on the density functional theory (DFT) to investigate the effect of intrinsic point defect on the magnetic properties of ZnO nanowire. The method we use offers the advantage of costefficient alternative to conventional ab initio methods in quantum chemistry. It gives results of a quality comparable to or even better than MP2 for a cost that is on the same order as HartreeFock. A further advantage of DFT is that it can be significantly more efficient than even traditional SCF theory. Also the robust electron ensemble DFT approach can be used for systems with partial occupancies.
From the results obtained from this paper, a better understanding of the mechanism of intrinsic magnetic properties in undoped ZnO nanowire with sufficient details can be estimated, and some helpful instructions can be provided for the growth of low dimensional ZnObased DMSs. These results also imply a promising way to synthesize undoped ZnO nanowire with ferromagnetism and shed light on the fabrication of ZnObased nanometer scale magnetic devices.
2. Computational Details
As shown in Figure 1, the ZnO nanowire is generated from a supercell of bulk wurtzite ZnO along the direction. It has a diameter of 1.0 nm, corresponding to 96 atoms per unit cell. A vacuum region of about 16 Å is set to avoid the interaction of the nanowire with its image. For the ZnO nanowire with V_{Zn}, V_{O}, O_{Zn}, or Zn_{O} defect, there are three inequivalent defect positions, which are denoted by , , , and as shown in Figures 2(a), 2(b), 2(d), and 2(e), respectively, whereas, there are two inequivalent Zn_{i} and O_{i} defect positions, which are denoted by  and  as shown in Figures 2(c) and 2(f), respectively.
(a)
(b)
(a)
(b)
(c)
(d)
(e)
(f)
All the calculations are performed by using the CASTEP software package [24]. The interaction between nuclei and electrons is approximated with Vanderbilt ultrasoft pseudopotential [25] and the Perdew and Wang 91 parametrization [26] is taken as the exchangecorrelation potential in the generalized gradient approximation (GGA). Plane wave basis with kinetic energy cutoff of 420 eV is used to represent wave functions. The Brillouin Zone integration is approximated using the special points sampling scheme of MonkhorstPack [27] and points grid is used. In order to obtain a stable structure, full relaxation is performed by using the BFGS algorithm [28] to minimize energy with respect to atomic position. The spinpolarized calculations are adopted to properly describe the electronic structure and magnetic properties of the constructed ZnO nanowire models. Each calculation is considered converged when the maximum rootmeansquare convergent tolerance is less than eV/atom.
With the settings described earlier, we have performed test calculations for bulk ZnO with wurtzite structure to verify the accuracy of the computational method. The obtained geometry optimization crystallographic parameters of wurtzite ZnO were Å, Å, and , which are in good agreement with the experimental values [29], Å, Å, and . We also tested the total energies and electronic structures of the considered defects model and the results indicate that the 96atom supercell is sufficient for the present calculations.
3. Results and Discussion
3.1. Structural Stability
As shown in Figure 2, there are inequivalent defect positions in ZnO nanowire with intrinsic point defect. To determine the stable configurations for each intrinsic point defect, the relative energy is calculated by comparing the total energy values of different configurations, with the ground state energy taken as the reference value. From the calculated relative energy listed in Table 1, it is evident that the total energy of V_{Zn} at site is about 0.12 and 0.04 eV smaller than that at the and sites, respectively, and thus, the is the most stable site for V_{Zn} defect. Similar to V_{Zn}, the V_{O}, O_{Zn}, and Zn_{O} the also prefered at the , , and sites, respectively. While for the Zn_{i} and O_{i} defect, they are prefered at the and sites, respectively. Therefore, the magnetic properties of ZnO nanowire with intrinsic point defects are calculated based on the , , , , , and configurations. On the other hand, the and configurations are nearly degenerate in total energy, and the total energy of them is smaller than that of the configuration. This clearly suggests that the V_{Zn} defect shows selectivity of site occupancy and prefers sitting on the surface site.

3.2. Magnetic Properties
In this part, the electronic structures of ZnO nanowire without and with different intrinsic point defect will be discussed and compared with each other. Four indicators will be used to reveal the effect of intrinsic point defect on the magnetic properties of ZnO nanowire, which are the band structure (BS), total density of states (DOS), partial density of states (PDOS), and Mulliken population analysis. Each of these tools can demonstrate some aspects of structure features. For comparison, the BS, total DOS, and PDOS of ZnO nanowire without intrinsic point defect are calculated first and the results are shown in Figure 3.
(a)
(b)
As shown in Figure 3, the spinup and spindown DOSs of pure ZnO nanowire are completely symmetrical, indicating that the pure ZnO nanowire is nonmagnetic. Moreover, the ZnO nanowire has a direct band gap at the relatively high dispersion along the direction. The direct gap of 1.86 eV is in good agreement with that of the firstprinciples calculation by Yang et al. [30].
The total DOSs for spinup and spindown electrons of the ZnO nanowire with V_{Zn}, V_{O}, Zn_{i}, O_{i}, O_{Zn} or Zn_{O} point defect are plotted in Figure 4. Similar to the pure ZnO nanowire, the ZnO nanowire with V_{O}, Zn_{i}, O_{i}, O_{Zn}, or Zn_{O} point defect also is nonmagnetic. However, a strong spin splitting phenomenon is observed in the ZnO nanowire with V_{Zn} defect. The spinup and spindown DOSs are asymmetrical near the top of valence bands.
By analysis of the BS of spinup and spindown electrons of the ZnO nanowire with V_{Zn} defect shown in Figure 5, it is found that the Fermi level moves from the spinup to the spindown band. The system shows halfmetallic behavior in which the spinup and spindown electrons possess a Fermi surface, in agreement with the previous report [21]. In addition, as shown in Figure 5(b), three defectrelated acceptor energy levels appear near the top of the spindown valence bands. The upper two acceptor energy levels are deep and highly localized. The lower one is partially occupied by the electrons and is highly localized. This result reveals that the hole of the acceptor level cannot be excited easily into the valence bands, and the V_{Zn} defect has little contribution to the ptype electrical activity of ZnO. Our conclusion confirms the experiment results that V_{Zn} defects play minor role in ptype conductivity of ZnO [31].
(a)
(b)
By further analysis of the PDOS shown in Figure 6, it can be found that the metallic spindown DOS near the Fermi level is mainly composed of and Zn 3d states. In particular, states do major contribution to the magnetic moment. This suggests that the appearance of the halfmetallic ferromagnetism in ZnO nanorod with V_{Zn} originates from the hybridization of the states with Zn 3d states.
Furthermore, the calculated Mulliken population analysis of the ZnO nanowire with V_{Zn} defect is listed in Table 2. Evidently, the O atoms around the V_{Zn} make major contribution to the observed magnetic moment. Magnetic moment of 0.562, 0.556 and 0.557 is from each of the three O atoms in the basal plane and 0.102 from the O atom in the axial plane. The residual magnetic moment is from the neighboring Zn atoms of the V_{Zn}.

Structurally, at V_{Zn} in ZnO, there are four O atoms with dangling bonds pointing towards the vacancy site, where the dangling bonds feature states. When this system is neutral, each dangling bond holds 1/2 electrons and interacts with each other to lower the total energy of the whole system. In this case, because of the tetrahedral structure of the four O atoms, the interaction between the dangling bonds results in hybrid orbital splitting. Therefore, the O atoms around the V_{Zn} make major contribution to the observed magnetic moment.
4. Conclusions
In conclusion, the effect of intrinsic point defect on the magnetic properties of ZnO nanowire is investigated by the firstprinciples calculation based on the DFT. The calculated BS, DOS, PDOS, and Mulliken population analysis results reveal that the pure ZnO nanowire without intrinsic point defect is nonmagnetic and ZnO nanowire with V_{O}, Zn_{i}, O_{i}, O_{Zn}, or Zn_{O} point defect also is nonmagnetic. However, a strong spin splitting phenomenon is observed in ZnO nanowire with V_{Zn} defect sitting on the surface site. At V_{Zn} in ZnO, there are four O atoms with dangling bonds pointing towards the vacancy site. Because of the tetrahedral structure of the four O atoms, the interaction between the dangling bonds results in hybrid orbital splitting.
Acknowledgments
This work is supported by the Natural Science Basic Research Plan in Shaanxi Province of China (Program no. 2011JQ8034), the Science and Technology Star Project of Shaanxi Province (2013KJXX24), the Open Foundation of Key Laboratory of Photoelectronic Technology of Shaanxi Province (Grant no. ZS11009), the Science Foundation of Northwest University (nos. 10NW08, PR10069), and the NWU Graduate Innovation and Creativity Funds (YZZ12098). The authors also acknowledge the support of the Scientific Research Program Funded by Shaanxi Provincial Education Department (Program no. 11JK0831).
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Copyright © 2013 Jiangni Yun et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.