Density Functional Theory Simulations of ... - ACS Publications

ARTICLE pubs.acs.org/JPCC

Density Functional Theory Simulations of Structures and Properties for Ag-Doped ZnO Nanotubes Guoliang Chai,†,‡ Chensheng Lin,† Jinyun Wang,†,‡ Minyi Zhang,†,‡ Jing Wei,†,‡ and Wendan Cheng*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China ABSTRACT: Incorporating silver in ZnO has attracted a lot of interest in recent years to fabricate p-type ZnO, as the naturally occurring ZnO is n-type material due to its native defects such as zinc interstitials and oxygen vacancies. In this work, the structural, electronic, and optical properties of pure and Ag-doped (8, 0) ZnO SWNTs were investigated by using density functional theory (DFT). The optimized structures present buckling side wall surfaces. The configurations with Ag atoms replacing Zn atoms are p-type semiconduction materials while the configuration with Ag atom added out of the surface of ZnO SWNTs is n-type semiconduction material. The optical properties based on dielectric function and absorption coefficient were discussed. It was found that the Ag-doped (8, 0) ZnO SWNTs have absorption in the visible-light region, and the absorption intensity is enhanced with the increase of Ag concentration. Our results indicate that the Ag-doped ZnO nanotubes could have a better performance as a photocatalyst. The calculations of Ag-doped ZnO slabs terminated with (10-10) surfaces are also presented to simulate the crystalline nanotubes synthesized in experiments, and the results are compared with that of ZnO SWNTs.

1. INTRODUCTION In recent years, zinc oxide (ZnO) has been intensively investigated because of its numerous applications. Hexagonal wurtzite structure ZnO (belongs to the space group P63mc) is a wide band gap semiconductor with a large exciton binding energy of 60 meV at room temperature. Moreover, its hardness, chemical stability, optical and piezoelectric properties make it an excellent material. It can be used in ultraviolet (UV) lasers, solar cells, photocatalysts, field emitters, gas sensors, piezo-nanogenerators and the diluted magnetic semiconductors (DMS) for spintronics.1-7 In particular, one-dimensional (1D) ZnO nanostructures such as nanowires and nanotubes have attracted considerable attention because they would have improved applications for their large surface-to-volume ratio, controllable shapes and quantum confinement effects. The 1D ZnO nanostructures have been synthesized by various methods and characterized to have some unique properties.8-12 Theoretically, their structure, stability, electronic, and optical properties have also been investigated.13-17 Doping ZnO with metal ions is a way to modify its electronic and optical performance and improve its applications. Ni-doped ZnO nanowire has shown considerable increase for photoluminescence.18 High-TC ferromagnetism has been achieved by doping ZnO with Mn2þ.19 ZnO is n-type metallic due to the zinc interstitials and oxygen vacancies. To improve its applications, p-type doped ZnO is needed. For a good dopant, it should have appropriate solubility and ionization energy. It also should not form DX or AX centers.20 To get p-type doped ZnO, group-V, group-I A, and group-I B elements have been used as dopants, r 2011 American Chemical Society

respectively.21-26 For group-V elements, N has an appropriate ionization energy to realize p-type doped ZnO. However, it is only favorable in O-poor conditions, which easily produce oxygen vacancy defects and thus make it inefficient. For groupI A and group-I B elements, the doping process is favorable in O-rich conditions, which can suppress the defects in ZnO. Because of the low ionization energy and large diameter of group-I A elements, they easily occupy the interstitial sites in ZnO, which makes them electron donors. Group-I B elements have smaller diameters and larger ionization energies than groupI A elements. Thus, they are better candidates for p-type ZnO doping. In particular, the ionization energy of AgZn is comparable to that of NO, which may make it a good acceptor in ZnO. In this paper, we first calculated the structures and formation energies of Ag-doped ZnO single walled nanotubes (SWNTs) and slabs by using density functional theory (DFT). Then, the electronic and optical properties were calculated with the optimized structures. The photocatalysts are often semiconductors that contain transition-metal or post-transition-metal ions with d0 or d10 electronic configuration as cations and along with VA or VIA group ions as counteranions.27,28 As a semiconductor of this type, ZnO is a low cost and environmentally friendly material with predominant chemical and physical properties, thus it has been widely used as photocatalyst. So, we also discussed the photocatalytic activity of Ag-doped ZnO nanotubes following the part of optical properties. Received: June 14, 2010 Revised: December 6, 2010 Published: February 3, 2011 2907

dx.doi.org/10.1021/jp105446m | J. Phys. Chem. C 2011, 115, 2907–2913

The Journal of Physical Chemistry C

Figure 1. (a) (8, 0) ZnO SWNT in which the Ag atoms can be doped at 1, 2, 3, and 4 sites representing other configurations, and (b) top view of ZnO (10-10) slab in which 1 and 2 sites represent first and second layer doping sites (Agf, Ags) respectively. Gray and red spheres represent Zn and O atoms respectively.

2. CALCULATION DETAILS The ZnO nanotubes synthesized in experiments always grow along the c direction, and their sidewalls are thin films of crystalline ZnO. Fortunately, the structures of this kind of ZnO nanotubes are similar to those of carbon nanotubes due to the hexagonal wurtzite structure of ZnO crystalline. One can get the multiwalled or single-walled ZnO nanotubes whose structures are similar to those of carbon nanotubes by cutting the atoms inside and outside of ZnO crystalline supercell along the c direction. Single-walled ZnO nanotubes can be seen as the thinnest walled ZnO nanotubes whose structures are similar to those of carbon nanotubes. The diameters of ZnO nanotubes synthesized in experiments range from 20 to 100 nm. Although using multiwalled ZnO nanotube as a model is more appreciable to the actual structure, electronic structure calculations are too computationally demanding to be used at the present time. Hence, the models we construct in this theoretical study are single-walled ZnO nanotubes with diameters of about 1 nm. The initial bond lengths in ZnO SWNTs were set to be 1.852 Å according to the bond lengths in ZnO single layers.14 The geometry and chirality of ZnO SWNTs are labeled according to carbon nanotubes.29 The zigzag (8, 0) ZnO SWNT that contains 64 atoms (with two unit cells) was selected as a prototype, which is shown in Figure 1a. Seven other configurations based on this structure were considered for the study of the properties of Ag-doped ZnO SWNTs. The first model was

ARTICLE

obtained by replacing one Zn atom with an Ag atom (Ag1), which corresponds to the Ag concentration of 3.12%. For the configurations with two Ag atoms replacing two Zn atoms, the two Ag atoms can be at 1, 2 or 1, 3 sites, which we named as horizontal (Ag2h) and vertical (Ag2v) configurations, respectively, as shown in Figure 1a. Similarly, the configurations with three Ag atoms at 1, 2, 3 or 2, 3, 4 sites are named as Ag31 and Ag32 configurations. The Ag4 configuration is the one with four Ag atoms at 1, 2, 3, 4 sites. The last configuration is obtained by adding an Ag atom out of the surface of pure ZnO (8, 0) SWNT, which we named as Ago. All the pure and Ag-doped ZnO SWNTs are settled in an orthogonal unit cell. The distance between the nanotubes was set to be 10 Å to avoid the interactions between them. Although the single walled ZnO nanotubes are predicted by previous studies, the synthesized ZnO nanotubes are crystalline nanotubes terminated with (10-10) surfaces until now. Thus, we also calculated properties of Ag-doped ZnO slabs terminated with (10-10) surfaces that represent crystalline nanotubes with infinite diameter. We selected a large slab with 108 atoms as our model, which is shown in Figure 1b. The slab has six atom layers and the upper two layers are relaxed. The distance between the slab and its images is larger than 10 Å to avoid the interactions between them. For one Ag-doped slab, the Ag atom can be in the first layer (Agf), second layer (Ags) or adsorbed on the surface (Aga) as shown in Figure 1b. The ZnO slab with high Ag concentration (Ag3) was also studied. All calculations in this paper are based on density functional theory (DFT). The geometry optimization calculations were carried out in DMol3 code.30 The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) functional form was employed to describe the exchange and correlation interaction.31 Effective core potentials and double numerical plus d-functions (DND) were selected. The forces on all atoms were less than 0.05 eV Å-1. The global orbital cutoff radius was set to be 4.5 Å. Electronic and optical property calculations were carried out in CASTEP code.32 Norm-conserving pseudopotentials and the generalized gradient approximation (GGA) in the Perdew-BurkeErnzerhof (PBE) form were adopted.30 The k-points in the Brilliouin zone were set to 1 1 4 and 1 2 1 for the ZnO SWNTs and slabs according to the Monkhorst-Pack scheme.33 The cutoff energies of Zn, O, and Ag atoms were set to be 600, 900, and 500 eV, respectively. The GGAþU methods were used, and the values of U were set to 4.7 and 2.0 eV for Zn and Ag atoms, respectively. Spin-unrestricted calculations were adopted for the models that have odd number of electrons. The complex dielectric function ε(ω) with the real part ε1(ω) and imaginary part ε2(ω) is always used to describe the linear optical properties of the system. In particular, the imaginary part ε2(ω) represents real transitions between occupied and unoccupied electronic states and it can be written as ε2 ðq f Oû ;hωÞ ¼

2e2 π X jÆΨck j u 3 r jΨvk æj2 δðEck - Evk - EÞ Ωε0 k, v, c ð1Þ

where u is the vector defining the polarization of the incident electric field and k is the reciprocal lattice vector. The superscripts c and v represent the conduction band and valence band, respectively, and ω is the frequency of the incident photon. The real part ε1(ω) of dielectric function can be obtained from ε2(ω) with the Kramers-Kronig relations. Other optical constants 2908

dx.doi.org/10.1021/jp105446m |J. Phys. Chem. C 2011, 115, 2907–2913


ARTICLE

Table 1. Bond Lengths (Å) and Formation Energies (eV) of Ag-Doped ZnO BAg-O Ag1/SWNT

2.192

Ago/SWNT Ag2v/SWNT

2.514 2.152

BAg-Ag

BAg-Zn

Eform 1.579

2.968 2.732

3.463 1.294

Ag2h/SWNT

2.152

2.793

2.045

Ag31/SWNT

2.155

2.921

1.160

Ag32/SWNT

2.163

3.141

1.503

Ag4/SWNT

2.162

3.165

Agf/slab

2.333

2.942

1.610

Ags/ slab

2.245

3.058

2.054

Aga/ slab Ag3/ slab

2.396 2.182

2.834

2.762 1.375

Bulk ZnO

2.297

1.143

2.872

1.257

can be obtained by ε1(ω) and ε2(ω).34 For example, the absorption coefficient can be gained by using the following expression: pffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2Þ RðωÞ ¼ 2ω½ ε1 2 ðωÞ þ ε2 2 ðωÞ - ε1 ðωÞ1=2 Figure 2. Optimized structures of (a) Ag1, (b) Ag2v, (c) Ag31, and (d) Ag4 configurations. Gray, red, and blue spheres represent Zn, O, and Ag atoms, respectively.

3. RESULTS AND DISCUSSION 3.1. Geometry Structures and Formation Energies. Theoretical study of ZnO indicated that the single layer ZnO sheet can be easier to roll up to form a ZnO SWNT than graphite to form single walled carbon nanotubes (SWCNTs).14 Another study showed that the ZnO SWNTs are more stable than ZnO nanowires for small diameters (the number of atoms is smaller than 38 for one unit cell).35 Hence, the (8, 0) ZnO SWNTs (with 32 atoms in one unit cell) constructed in this paper are reasonable. The sidewall surface of optimized geometry structures of pure ZnO SWNTs converges to sawtooth-like structures due to the Zn atoms moving out and O atoms moving in along the direction of diameter. This buckling behavior is common for zigzag polar nanotubes such as BN and SiC nanotubes.36,37 The Ag-doped (8, 0) ZnO SWNTs are distorted compared with the undoped one because the Ag-O bond lengths are longer than the Zn-O bond lengths. For the Ag2v, Ag2h, Ag31 Ag32, and Ag4 configurations, there are bonds between Ag atoms. For the Ago configuration, the Ag atom bonds to O and Zn atoms, respectively. The average bond lengths in these configurations are displayed in Table 1. The bond lengths in ZnO slabs were also displayed for comparison. The formation energies of Ag-doped (8, 0) ZnO SWNTs were calculated to evaluate their stability and that of bulk ZnO with 64 atoms (a Zn atom was replaced by an Ag atom) were also calculated to make comparison. The formation energy38 can be expressed as

Eform ¼ EðZnO : AgZn Þ - EðZnOÞ þ μðZnÞ - μðAgÞ

ð3Þ

where the first two terms on the right-hand are the total energy of ZnO with and without the impurity respectively. In this equation, μ is the chemical potential of Zn and Ag, which depends on the conditions of growth. For the ZnO, the total energy Etot(ZnO)39 can be expressed as ð4Þ μZn þ μO ¼ Etot ðZnOÞ In the O-rich condition, the chemical potential of O is equal to the energy of O in an O2 molecule. So, we can express the

chemical potential of Zn as μZn ¼ Etot ðZnOÞ - μOðO2 Þ

ð5Þ

Similarly, the chemical potential of Ag can be expressed as μAg ¼

1 ½Etot ðAg2 OÞ - μOðO2 Þ 2

ð6Þ

The formation energies are shown in Table 1. The formation energy of Ago is apparently larger than that of other configurations, which indicates that the Ag atoms would be favorable in the Zn sites for Ag-doped ZnO SWNTs. For the configurations with Ag atoms replacing Zn atoms, the formation energy decreases with the increase of Ag concentration. This indicates that high Ag concentration is more stable. The Ag atoms in these configurations bond to each other to form Ag clusters as shown in Figure 2. For the configurations with same Ag concentration, the Ag2v and Ag31 configurations are more stable than the Ag2h and Ag32 configurations. In the following parts, we only consider the properties of the Ag2v and Ag31 configurations for the corresponding Ag concentrations. For the ZnO slabs, an Ag atom is prone to be in the first layer (Agf) rather than the second layer or added on the surface (Ags, Aga). Similar to the case for the ZnO SWNTs, high Ag concentration configuration (Ag3) is more stable with lower formation energy as shown in Table 1. 3.2. Electronic Properties. Before investigating the doping effect of Ag on the optical properties of ZnO SWNTs, we first calculated the density of states (DOS) of pure and Ag-doped (8, 0) ZnO SWNTs. Figure 3a is the calculated density of states (DOS) of pure (8, 0) ZnO SWNT. It can be seen that the upper valence band from -4.0 to 0.0 eV is dominated by O 2p states and the lower valence band from -7.0 to -5.0 eV is dominated by the Zn 3d states. The lowest conduction band is mainly derived from Zn 4s states. Figure 3b-d shows the density of states (DOS) of Ag1, Ag2v, and Ago configurations. The O and Zn states of Ag1 configuration 2909



ARTICLE

Figure 3. Density of states (DOS) of (a) pure ZnO SWNT, (b) Ag1, (c) Ag2v, and (d) Ago configurations.

are similar to that of pure (8, 0) ZnO SWNT. It can be seen that the O 2p states dominated in -4.0 to 0.0 eV and the Zn 3d states dominated in -7.0 to -9.0 eV energy regions. The Zn 4s states dominate in the region from 2.0 to 6.0 eV. The Ag 4d states lie across the Fermi level from about -6.0 to þ1.0 eV. This indicates that the Ag-doped ZnO SWNTs are p-type materials. The DOS structures of Ag2v, Ag31, and Ag4 configurations are similar to that of the Ag1 configuration except that the Ag 4d states are enhanced with increased Ag concentration (the DOS of Ag31 and Ag4 configurations are not given here). From the display of DOS, we can see that the energy difference between the upper Ag 4d states and the low Zn 4s states is about 1.0 eV. For the pure ZnO SWNT, the energy difference between the upper valence band (O 2p states) and lowest conduction band (Zn 4s states) is about 2.0 eV. In this case, the electronic

interband transition from the Ag 4d states and to the Zn 4s states may induce an absorption shift to the visible light region. However, the DOS of Ago configuration is different from that of the other configurations. For the Ago configuration, the O 2p states dominate in -2.0 to -6.0 eV and the Zn 3d states dominate in -8.0 to -10.0 eV energy regions. Zn 4s states lie from 0.0 to 2.0 eV. The Ag 4d states are mainly in the region from -4.0 to -6.0 eV. The Ag 5s states lie across the Fermi level from -0.5 to þ2.0 eV. The Ag 5p states are from 1.0 to 4.0 eV. These show that the Ago configuration is an n-type material. For the ZnO slabs, the DOS of Agf and Aga are displayed in Figure 4a,b. It can be seen that Agf is a p-type material with its Fermi level in the valence band and Aga is an n-type material with its Fermi level in the conduction band. These are similar to that of ZnO SWNTs. 2910



ARTICLE

Figure 4. Density of states (DOS) of (a) Agf ZnO slab and (b) Aga ZnO slab.

)

)

calculated dielectric function of pure and Ag-doped (8, 0) ZnO SWNTs are shown in Figure 5a. Considering optical anisotropy, ε2(ω) under parallel polarization, ε2 (ω), and perpendicular polarization, ε2^(ω), are displayed, respectively. For parallel polarization, the pure (8, 0) ZnO SWNT has four peaks located at about 2.5, 4.5, 6.5, and 10.0 eV. The peak at about 2.5 eV is mainly due to the transition from O 2p to Zn 4s states. The two peaks at 4.5 and 6.5 eV correspond to transitions between the Zn 3d and O 2p states. Finally, the peak at 10.0 eV originates from transitions between Zn 3d and O 2s states. For the Ag1 configuration, the peaks in the 2.0-8.0 eV energy region originate from the Zn 3d to O 2p state transitions and the peak at about 12.5 eV originates from Zn 3d to O 2s states transitions respectively. The peak in the low energy region at about 1.0 eV mainly comes from the electronic interband transition between Ag 4d states and Zn 4s states in the conduction band. The peak positions of the Ag2v, Ag31, and Ag4 configurations are similar to that of the Ag1 configuration except that the peaks are more intense because of higher Ag concentration. However, for the Ago configuration the peak in the low energy region locates at about 1.0 eV originates from Ag 5s to Zn 4s states transitions due to its n-type metallic character. For perpendicular polarization, the peak positions of ε2 (ω) are similar to that of ε2^(ω) whereas the intensity of the peaks are more intense due to optical anisotropy. For the ZnO slabs, there is a peak at about 0.5 eV and a broad peak from 2.5 to 5.0 eV, as shown in Figure 6a. The peak at about 0.5 eV originates from the electronic transition from Ag 4d states to Zn 4s states for Agf and Ag3 configurations while it originates from electronic transitions from Ag 5s to Zn 4s states for Aga configuration. The peak from 2.5 to 5.0 eV originates from the electron transitions from Zn occupied states to O unoccupied states as the case of ZnO SWNTs. To investigate the absorption coefficient of pure and Agdoped (8, 0) ZnO SWNTs, we used the scissors approach to adjust the band gaps. It is known that the DFT calculations often underestimate the bang gap of ZnO. We calculated the band gap

Figure 5. (a) Dielectric function spectra of pure and Ag-doped (8, 0) ZnO SWNTs under perpendicular and parallel polarizations. (b) Absorption spectra of pure and Ag-doped (8, 0) ZnO SWNTs.

3.3. Optical Properties. The optical properties of pure and Ag-doped (8, 0) ZnO SWNTs in this paper are discussed on the basis of the dielectric function and absorption coefficient. The

2911



ARTICLE

ZnO SWNTs and most of the photocatalysts that can work under the visible light region would have improved quantum efficiencies. The photocatalytic activity is also dependent on the exposed surface area of the photocatalyst. For 1D ZnO nanostructures, they have a large surface area to adsorb the degradant, which can also improve the photocatalytic activity. Thus, the Ag-doped ZnO SWNT is a good candidate for photocatalyst.

4. CONCLUSIONS In summary, we studied the structural, electronic, and optical properties of pure and Ag-doped (8, 0) ZnO SWNTs and slabs by using density functional theory (DFT). The configurations with Zn atoms replaced by Ag atoms are p-type semiconduction materials whereas the configuration with Ag atom added on the surface of ZnO SWNT presents n-type semiconduction character. The calculated dielectric function and absorption spectrum show obvious peaks in the visible light region which are due to the electronic transition from doped Ag 4d states to the Zn 4s conduction band for the configurations with Ag atoms replacing Zn atoms and Ag 5s to Zn 4s state transitions for the Ago configuration, respectively. The properties of Ag-doped ZnO slabs are consistent with that of Ag-doped ZnO SWNTs. Especially, the Ag-doped ZnO SWNTs could be a good candidate for photocatalyst for their increased photocatalytic activity in the visible light region. Figure 6. (a) Dielectric function spectra and (b) absorption spectra of Ag-doped ZnO slabs.

’ AUTHOR INFORMATION Corresponding Author

of bulk ZnO. The calculated band gap is 1.133 eV, which is smaller than the experimental band gap of 3.3 eV. Fortunately, previous study has shown that the overall features of the band structure calculated by using DFT methods are almost the same as the band structure calculated by other methods except the underestimated band gap.40 Hence, we can investigate the optical properties of ZnO SWNTs by using scissors to adjust the band gap. The scissors value of 2.167 eV is used for calculating absorption coefficient. The absorption spectra of pure and Agdoped (8, 0) ZnO SWNTs under parallel polarization were displayed in Figure 5b. For the pure (8, 0) ZnO SWNT, there are several peaks from 5.0 to 12.5 eV, which is mainly in the ultraviolet (UV) region. For the Ag-doped ZnO SWNTs, the peak at about 3.0 eV (in the visible light region) emerges, which is due to the electronic transition from dopant Ag 4d states to Zn 4s conduction band states for Ag1, Ag2v, Ag31, and Ag4 configurations and Ag 5s to Zn 4s states transitions for the Ago configuration, respectively. The absorption coefficient increases with the increase of Ag concentration. The results are compared with those for Ag-doped ZnO slabs, which are shown in Figure 6b. The peaks in the UV region locate at about 7.5 and 14.0 eV. Absorption peaks in the visible light region emerge due to the doping of Ag, and the absorption coefficient increases with the increase of Ag concentration. The Fe-doped and Cr-doped ZnO systems also have similar phenomena.41,42 Photocatalysis reactions involve a progress that the photocatalyst generates electron-hole pairs under radiation. The extended absorption region of the Agdoped ZnO SWNTs, especially the emerged absorption in the visible light region would make them have a better performance as a photocatalyst. Because of the wide band gap of ZnO, it mainly absorbs light in the UV region. It is known that ultraviolet light accounts for only 4% of the solar energy. Thus, Ag-doped

*Fax: þ86 591 83714946. E-mail: [email protected].

’ ACKNOWLEDGMENT This investigation was based on work supported by the National Natural Science Foundation of China under project 20773131, and the National Basic Research Program of China (No. 2007CB815307), the Science Foundation of the Fujian Province (No. E0210028), the Knowledge Innovation Program of the Chinese Academy of Sciences and the Funds of Chinese Academy of Sciences (KJCX2-YW-H01). ’ REFERENCES (1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (2) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455. (3) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (4) Bai, X. D.; Wang, E. G.; Ago, P. X.; Wang, Z. L. Nano Lett. 2003, 3, 1147. (5) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659. (6) Wang, X. D.; Song, J. H.; Liu, J.; Wang, Z. L. Science 2007, 316, 102. (7) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; Molnar, S. V.; Roukes, M. L.; Chthelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (8) Sakurai, M.; Wang, Y. G.; Uemura, T.; Aono, M. Nanotechnology 2009, 20, 155203. (9) She, G. W.; Zhang, X. H.; Shi, W. S.; Fan, X.; Chang, J. C.; Lee, C. S.; Lee, S. T.; Liu, C. H. Appl. Phys. Lett. 2008, 92, 053111. (10) Liu, P.; She, G. W.; Liao, Z. L.; Wang, Y.; Wang, Z. Z.; Shi, W. S.; Zhang, X. H.; Lee, S. T.; Chen, D. Appl. Phys. Lett. 2009, 94, 063120. 2912



ARTICLE

(11) Xing, Y. J.; Xi, Z. H.; Xue, Z. Q.; Zhang, X. D.; Song, J. H.; Wang, R. M.; Xu, J.; Song, Y.; S. Zhang, L. D.; Yu, P. Appl. Phys. Lett. 2003, 83, 1689. (12) Tong, Y. H.; Liu, Y. C.; Shao, C. L.; Liu, Y. X.; Xu, C. S.; Zhang, J. Y.; Lu, Y. M.; Shen, D. Z.; Fan, X. W. J. Phys. Chem. B 2006, 110, 14714. (13) Zhou, Z.; Li, Y. F.; Liu, L.; Chen, Y. S.; Zhang, S. B.; Chen, Z. F. J. Phys. Chem. C 2008, 112, 13926. (14) Tu, Z. C.; Hu, X. Phys. Rev. B 2006, 74, 035434. (15) Xu, H.; Zhang, R. Q.; Zhang, X. H.; Rosa, A. L.; Frauenheim, T. Nanotechnology 2007, 18, 485713. (16) Shelly, L.; Elizondo, J. W.; Mintmire, J. Phys. Chem. C 2007, 111, 17821. (17) Hu, J.; Liu, X. W.; Pan, B. C. Nanotechnology 2008, 19, 285710. (18) He, J. H.; Lao, C. S.; Chen, L. J.; Davidovic, D. G.; Wang, Z. L. J. Am. Chem .Soc. 2005, 127, 16376. (19) Kittilstved, K. R.; Agmelin, D. R. J. Am. Chem .Soc. 2005, 127, 5292. (20) Wei, S. H. :: Comp. Mater. Sci. 2004, 30, 337. € ur, U.; Alivov, Ya. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; (21) Ozg€ Doan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. J. Appl. Phys. 2005, 98, 041301. (22) Kim, K. K.; Kim, H. S.; Hwang, D. K.; Lim, J. H.; Park, S. J. Appl. Phys. Lett. 2003, 83, 63. (23) Ryu, Y. R.; Zhu, S.; Look, D. C.; Wrobel, J. M.; Jeong, H. M.; White, H. W. J. Cryst. Growth 2000, 216, 330. (24) Park, C. H.; Zhang, S. B.; Wei, S. H. Phys. Rev. B 2002, 66, 073202. (25) Wardle, M. G.; Goss, J. P.; Briddon, P. R. Phys. Rev. B 2005, 71, 155205. (26) Yan, Y. F.; Al-Jassim, M. M.; Wei, S. H. Appl. Phys. Lett. 2006, 89, 181912. (27) Tsuji, I.; Kato, H.; Kudo, A. Angew. Chem., Int. Ed. 2005, 44, 3565. (28) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295. (29) White, C. T.; Mintmire, J. W. J. Phys. Chem. B 2005, 109, 52. (30) Delley, B. J. Chem. Phys. 2000, 113, 7756. (31) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (32) SeAgll, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. J. Phys.-Condens. Matter 2002, 14, 2717. (33) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. (34) Saha, S.; Sinha, T. P. Phys. Rev. B 2000, 62, 8828. (35) Shen, X.; Allen, P. B.; Muckerman, J. T.; Davenport, J. W.; Zheng, J. C. Nano Lett. 2007, 7, 2267. (36) An, W.; Wu, X.; Yang, J. L.; Zeng, X. C. J. Phys. Chem. C 2007, 111, 14105. (37) Huang, S. P.; Wu, D. S.; Hu, J. M.; Zhang, H.; Xie, Z.; Hu, H.; Cheng, W. D. Opt. Express 2007, 15, 10947. (38) Zhang, S. B.; Northrup, J. E. Phys. Rev. Lett. 1991, 67, 2339. (39) Van de Walle, C. G.; Neugebauer, J. J. Appl. Phys. 2004, 95, 3851. (40) An, W.; Wu, X. J.; Zeng, X. C. J. Phys. Chem. C 2008, 112, 5747. (41) Kim, K. J.; Park, Y. R. J. Appl. Phys. 2004, 96, 4150. (42) Li, L. Y.; Wang, W. H.; Liu, H.; Liu, X. D.; Song, Q. G.; Ren, S. W. J. Phys. Chem. C 2009, 113, 8460.

’ NOTE ADDED AFTER ASAP PUBLICATION This manuscript was originally published on the web on February 3, 2011, with changes to the author affiliations and the Acknowledgment Section. The corrected version was reposted on February 17, 2011.

2913


Density Functional Theory Simulations of ... - ACS Publications

Recommend Documents