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Magnetism of O-Terminated ZnO(0001) with Adsorbates En-Zuo Liu* and J. Z. Jiang International Center for New-Structured Materials (ICNSM), Zhejiang UniVersity and Laboratory of New-Structured Materials, Department of Materials Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China ReceiVed: April 22, 2009; ReVised Manuscript ReceiVed: July 15, 2009
On the basis of first-principles total-energy calculations, the magnetic property of O-terminated ZnO(0001) surfaces adsorbed with different molecules (NH3, H2S, and H2O) is studied. NH3 is adsorbed above Zn sites. A magnetic moment distribution occurs because of the formation of N 2p holes with minority spin states in NH3, which results from a charge transfer from NH3 to the ZnO(0001) surface. Furthermore, the ZnO(0001) surface adsorbed with NH3 prefers ferromagnetism. On the other hand, due to the decomposition of H2S on the ZnO(0001) surface, magnetic moments appear that mainly exist on the S atom and its neighboring atoms, whereas there are no magnetic moments in ZnO(0001) adsorbed with H2O. The interaction between molecules and the ZnO surface results in the unsaturated outer shells of the N atom in NH3 and the S atom in H2S, and as a result, magnetism of the molecule-adsorbed ZnO(0001) surface comes into being. Results indicate that the electronegativity of the atoms in the molecules and at the surface may be an important factor for the origin of magnetic moments and may provide a general explanation for the occurrence of ferromagnetic properties in molecule-enwrapped oxide nanoparticles. I. Introduction Zinc oxide (ZnO)-based diluted magnetic semiconductors (DMSs) are promising candidates for spintronic materials.1 Since the early work on high-temperature ferromagnetism (FM) in ZnO-based DMSs,2 extensive studies on the magnetic properties of ZnO-based DMSs have been reported.3-10 However, the origin of the magnetic properties of ZnO-based DMSs still remains a controversial issue. It seems that the magnetic behavior of ZnO-based DMSs is strongly dependent on the preparation methods used and is poorly reproducible. By now, the consensus that has been reached is that transition-metal (TM)-doped ZnO (x < 0.1) samples with less density of defects are intrinsically paramagnetic.3,4 As a result, defects (O vacancy (VO), Zn vacancy, interstitial Zn, n-type doping, p-type doping, etc.) were suggested to induce high-temperature FM in TMdoped ZnO.8-17 Recent research indicates that surface effects can induce FM of ZnO-based nanoparticles.18-21 ZnO nanoparticles both without and with TM dopants display room-temperature FM, which may come from the exchange interactions between unpaired electron spins arising from VO at the surfaces of the nanoparticles18 and the interaction between the ZnO surface and molecules on it,19 respectively. For ZnO nanoparticles enwrapped with other molecules, there must be a new mechanism that is different from the magnetic impurity induced FM.19,20 Thus, it is very interesting to investigate the interaction between the ZnO surface and adsorbates, which will be very helpful in understanding the controversial results about the magnetic properties of ZnO-based DMSs. To explain the origin of magnetic moments in ZnO nanoparticles enwrapped with molecules in atomic scale, in this study, through first-principles total-energy calculations, we investigate O-terminated ZnO(0001) adsorbed with different * To whom correspondence should be addressed. E-mail:
[email protected] (E.-Z.L.),
[email protected] (J.Z.J.).
molecules (NH3, H2S, and H2O). Results indicate that NH3 is adsorbed above Zn sites on O-terminated ZnO(0001) and there is an attractive interaction between H in NH3 and O atoms at the surface. ZnO(0001) adsorbed with NH3 exhibits FM. Because of the decomposition of H2S on the ZnO(0001) surface, magnetic moments appear that mainly exist on the S atom and its neighboring atoms, whereas there are no magnetic moments in ZnO(0001) adsorbed with H2O because H2O does not decompose on ZnO(0001). An explanation to the origin of the magnetic properties is given from the point of view of electronic structures. II. Method and Models We have performed total-energy and electronic structure calculations using the projector-augmented wave22 (PAW) formalism of density functional theory (DFT) as implemented in the Vienna ab initio Simulation Package (VASP).23 An energy cutoff of 400 eV for plane-wave expansion of the PAWs is used. The exchange-correlation functional is approximated with the generalized gradient approximation proposed by Perder, Burke, and Ernzerhof (PBE).24 Because ZnO(0001) is very stable and has implications for the use of the material in gas sensing and catalytic applications,25 here, we study the adsorption of molecules (NH3, H2S, and H2O) on O-terminated ZnO(0001). ZnO(0001) is modeled by periodically repeated slabs containing nine ZnO planes and separated by a vacuum region of more than 15 Å. We use a two-dimensional unit cell containing four Zn and four O per ZnO plane; 3 × 5 k points are sampled in the surface Brillouin zone. Calculations are performed in a spinunrestricted manner. For geometry optimization, all the internal coordinates are relaxed until the Hellmann-Feynman forces are less than 0.01 eV/Å. III. Results In our calculations, pure ZnO(0001) has no magnetic moment. Molecules (NH3, H2S, and H2O) are initially put at three
10.1021/jp9037304 CCC: $40.75 2009 American Chemical Society Published on Web 08/18/2009
Magnetism of O-Terminated ZnO(0001) with Adsorbates
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Figure 1. Top view and side view of the relaxed atomic configurations with NH3 on O-terminated ZnO(0001) at different positions. Green, red, white, and blue spheres are Zn, O, H, and N, respectively.
TABLE 1: Adsorption Energies Ea (eV) of the Molecules on ZnO(0001) and the Magnetic Moments M (µB) Appearing in ZnO(0001) Adsorbed with the Molecules in Different Configurations. d1 and d2 Indicate the Distances (Å) of Atoms 1 and 2, As Labeled in Figures 1 and 4, above ZnO(0001) H 2S configuration
NH3 configuration A1
A2
B1
A3
B3
H 2O configuration D1
A4
B4
Ea -0.15 -0.21 -0.10 -0.17 -0.16 -1.22 -0.21 -0.21 M 0.00 0.59 0.00 0.00 0.00 1.76 0.00 0.05 d1 2.38 2.71 3.23 3.09 3.32 2.21 2.40 2.96 d2 2.77 2.18 2.78 2.19 3.31 0.95 2.12 3.23
adsorption sites with N, S, and O atoms above Zn atoms (A), above O atoms (B), and above the center of the hexagonal ring (C) at the ZnO(0001) surface, respectively. A. ZnO(0001) Adsorbed with NH3. When NH3 is adsorbed on ZnO(0001), two types of initial atomic configurations, with H or N atoms of NH3 closer to the surface, are considered. After relaxation, three stable atomic configurations are revealed, as shown in Figure 1. Configurations A1 and A2 are both adsorbed at site A on ZnO(0001), with H further away and closer to ZnO(0001) relative to the N atom, respectively. In configuration B1, NH3 is adsorbed at site B with H closer to ZnO(0001). Table 1 shows the adsorption energies of molecules on ZnO(0001) with different configurations. The adsorption energy of a molecule on ZnO(0001) is defined by Ea ) ET - ES - EM, where ET is the total energy of ZnO(0001) with a molecule in a supercell, ES is the total energy of pure ZnO(0001) in the same supercell, and EM is the total energy of the isolated molecule. A2 is the most stable configuration of the three. There is a magnetic moment of about 0.59 µB in configuration A2. The electronic configuration with the magnetic moment is more stable by 11 meV than the randomly polarized state. This may give an explanation to the experimental results reported by Garcia et al.20 We have also used more k points (5 × 7) in the surface Brillouin zone. The adsorption energy of NH3 in configuration A2 is 0.22 eV, and there is a magnetic moment of about 0.45 µB in configuration A2. Thus, the k-point grid (3 × 5) for the integration of the surface Brillouin zone is sufficient. It should be noted that, when NH3 adsorbs at site B with H atoms further away from the ZnO surface, magnetic moments also come into being.26 However, the configuration is metastable. If NH3 adsorbs at the position with a slight departure from the metastable site, NH3 diffuses into site A in configuration A2. In configuration A2, there is an attractive interaction between H and O atoms because H is electropositive and O is electronegative. The NH3 molecule is tilted, that is, the distances from
Figure 2. PDOS of N 2p, H, O 2p, and Zn 4s (red and black lines) and 4p (green and blue lines) electrons in configuration A2.
Figure 3. Isosurface (0.4 × 10-3 e/Å3) of the spin density distribution of the ZnO(0001) surface with (a) one and (b) two NH3 molecules. (c) The corresponding atomic configuration of (b).
the three H atoms to their respective nearest-neighbor O atoms are not equal. The distance between H2 and O3 is smallest (2.18 Å). Such interaction between NH3 and the ZnO surface may cause a redistribution of electrons, which could result in the occurrence of magnetic moments in A2. It can be seen from the analysis of the partial density of state in configuration A2. Partial densities of states (PDOS) in configuration A2 are shown in Figure 2. Figure 3a shows the isosurface of the spin density distribution of A2. For ZnO(0001) adsorbed with NH3, the magnetic moments mainly distribute on N and neighboring O atoms. When NH3 is adsorbed on ZnO(0001), electrons of N and H are both spin-polarized, as shown in Figure 2. N 2p, H s, and O 2p electrons couple at the Fermi level, and N 2p electrons are delocalized. There is a charge transfer from N to O atoms because of the attractive interaction between H in NH3 and O atoms at the surface. Because of the electron transfer, the long pairs on N are partially unpaired and there are a number of N 2p holes with minority spin states in the N valence band. To make sure that the system has magnetic moments, that is, 2p holes exist with minority spin states instead of both spin states, there must be a critical number of holes.21 The mechanism that 2p holes induce magnetic moments is similar to the mechanism of the O surfaces of ceramic oxides proposed by Gallego et al.27 On the other hand, the distribution of the magnetic moments on N and O atoms (Figure 3a) is anisotropic
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Figure 4. Relaxed atomic configurations with H2S and H2O on O-terminated ZnO(0001) at different positions. Green, red, white, and yellow spheres are Zn, O, H, and S, respectively.
and mainly along the Z direction. This indicates that the magnetic states are mainly associated with the long pair p electrons on N, which hybrids with O 2p electrons. This is consistent with the DOS analysis above. The net spins in N and O atoms can further induce spin polarization of electrons of the surrounding and sublayer Zn and O atoms. The spin polarization of the surrounding atoms may induce a long-range FM coupling between the spins of N atoms. We have further studied the configuration with two NH3 adsorbed on ZnO(0001). The electronic configurations with spin polarization (FM) and without spin polarization are both considered. FM is more stable by 16 meV than the electronic configuration without spin polarization. The spin distribution and atomic configuration are shown in Figure 3b,c, respectively. Thus, the ZnO(0001) surface adsorbed with NH3 may exhibit ferromagnetic properties. B. ZnO(0001) Adsorbed with H2S and H2O. Figure 4 shows the relaxed atomic configurations with H2S and H2O on ZnO(0001). When H2O and H2S are put at sites A and C, respectively, the relaxed atomic configurations are A3 and A4 with H atoms pointing to O atoms at the ZnO(0001) surface and O and S atoms nearly above the Zn atoms at ZnO(0001). When H2O (H2S) is put at site B, the relaxed atomic configuration is B3 (B4) with one O-H (S-H) bond perpendicular to the ZnO(0001) and pointing to O atoms at ZnO(0001). We did not find magnetic moments on all the four configurations. However, experimental results suggested that ZnO nanoparticles enwrapped with molecules containing S exhibit FM-like behavior, whereas ZnO nanoparticles with molecules containing O possess paramagnetism.20 This difference between experimental and theoretical results further triggers our attention for both H2S and H2O adsorbates because O and S are both group VIA elements. After a survey of the literature, experimental results suggested that thiol and H2S on ZnO(0001) decompose and produce ethylene or H atoms and adsorbed sulfur atoms.28,29 However, H2O is not likely to decompose on the ZnO surface. This may be the reason for the different behaviors of S- and O-containing molecules. Thus, the configurations with decomposed H2S and H2O are further studied. In our calculations, H2S prefers decomposing into one S and two H atoms by more than 0.6 eV, whereas the configuration with H2O adsorbed on ZnO(0001) is energetically more stable by more than 0.3 eV than the configuration with the decomposed H2O on ZnO(0001). Table 1 shows the adsorption energies of molecules on ZnO(0001) with different configurations. It is interesting that one magnetic moment (1.76 µB) appears in configuration D1, as shown in Table 1, and the electronic configuration with the magnetic moment is more stable by 0.25 eV than the randomly polarized state. A detailed study about the origin of the magnetic moment is required. Next, we will investigate the origin of the magnetic moments in ZnO(0001) adsorbed with H2S and H2O. Partial densities of
Figure 5. PDOS of H, O 2p, and Zn 4s (red and black lines) and 4p (green and blue lines) electrons in configuration B4.
Figure 6. PDOS of S 3p, O 2p, and Zn 4s (red and black lines) and 4p (green and blue lines) electrons in configuration D1.
states (PDOS) of configurations B4 and D1 are shown in Figures 5 and 6, respectively. For ZnO(0001) adsorbed with H2O (configuration B4), the electron of the H atom is coupled with O 2p electrons. There is a little charge transfer between H and O atoms. As a result, the electrons of H and O atoms are slightly spin-polarized (0.05 µB in configuration B4), which can be seen from the PDOS of H and O 2p electrons. We also find that the adsorption of H2O on ZnO(0001) has little affect on the electronic states of the Zn atom on ZnO(0001). For H2Sadsorbed ZnO(0001) (configuration D1), the magnetic moments mainly come from the S atom and the surrounding O atoms (Figure 7a). For isolated S atoms, the electrons of S 3p states are spin-polarized, which is more stable by 0.82 eV than the electronic configuration without spin polarization. After being adsorbed on ZnO(0001), S 3p states are coupled with O 2p states and delocalized around the Fermi level. As a result, the spinpolarized S 3p states induce the spin polarization of O 2p states. H atoms from H2S are adsorbed on O atoms at the ZnO(0001) surface, and as a result, the surface is passivated. This can be
Magnetism of O-Terminated ZnO(0001) with Adsorbates
J. Phys. Chem. C, Vol. 113, No. 36, 2009 16119 the H2S molecule decomposes and H from H2S forms a bond with O at the surface. S atoms bring magnetic moments and induce the spin polarization of neighboring atoms at the surface. The results obtained here give an explanation to the ferromagnetic properties in molecule-enwrapped oxide nanoparticles. V. Conclusion
Figure 7. (a) Isosurface (0.4 × 10-3e/Å3) of the spin density distribution in configuration D1. (b) The atomic configuration of ZnO(0001) with 1 ML coverage of H2S.
seen from the PDOS of sublayer Zn atoms, whose valence band maximum moves down and below the Fermi level. Furthermore, we investigated the ZnO(0001) surface with 1 ML (monolayer) coverage of H2S (Figure 7b). Results indicate that the system has a magnetic moment of 0.16 µB, more stable by 4 meV than the electronic configuration without spin polarization, which suggests that ZnO(0001) enwrapped with H2S may exhibit FM. IV. Discussion On the basis of the present calculations, a H2O molecule cannot induce FM on the ZnO(0001) surface, whereas ZnO(0001) surfaces with H2S and NH3 have FM. This may give some explanation to the experimental results reported by Garcia et al.20 It should be noted that the ferromagnetic-like behavior may be related to many other factors, such as coverage, different surface orientation, etc. Here, we only give some insight on the phenomenon from the point of view on O-terminated ZnO(0001) surfaces with molecules adsorbed on them. Recent DFT calculations revealed that the unpaired p electrons of O atoms at ZnO(0001j) are the origin of FM of pure ZnO nanoparticles.21 Wang et al.30 also reported magnetic moments arising from the unpaired 2p electrons at O sites surrounding Zn vacancies in undoped ZnO thin films. The p magnetism associated with uncompensated charge in ionic oxides seems to be a general phenomenon.27 In our study, it can be seen that adsorbates can result in magnetic moments on ZnO(0001) through the mechanism that adsorbates (NH3, etc.) interact with the ZnO(0001) surface. A charge transfer between the adsorbates and atoms at the surface occurs. As a result, the electron outer shells of the adsorbates are unsaturated, 2p holes with minority spin states emerge, and the system is spin-polarized. The magnetic moments of the ZnO surface adsorbed with molecules come from the unsaturated p electrons (p holes) of surface atoms. This is similar to the mechanism proposed by Sanchez et al.21 It is noted that O-terminated ZnO(0001j) is unstable compared with O-terminated ZnO(0001). As a result, the magnetism of pure ZnO nanoparticles may depend on the preparation conditions. In the case of O-terminated ZnO(0001) adsorbed by molecules, the electronegativity of the atoms in the molecules is an important factor for the origin of magnetic moments. Because the electronegativity value of N is lower than that of O at the surface, charge transfers from NH3 to O-terminated ZnO(0001) occur, and as a result, magnetic moments on N and its surrounding atoms come into being. For H2O, there is no charge transfer between H2O and ZnO(0001), for they have the same element oxygen. As a result, there is no magnetic moment in H2O-adsorbed ZnO(0001). On the other hand, for H2S, because the electronegativity value of S is further lower than that of O,
In conclusion, on the basis of first-principles total-energy calculations, we find that NH3 is adsorbed above Zn sites on O-terminated ZnO(0001) and there is an attractive interaction between H in NH3 and O atoms. The charge transfer from NH3 to ZnO(0001) results in 2p holes with minority spin states of the N atom in NH3, and a magnetic moment distribution comes into being. Because of the decomposition of H2S on the ZnO(0001) surface, magnetic moments appear that mainly exist on the S atom and its neighboring atoms, whereas there are no magnetic moments in ZnO(0001) adsorbed with H2O. We conclude that the electronegativity of the atoms in molecules adsorbed on oxide nanoparticles and at the surface may be an important factor for the origin of magnetic moments, which is associated with the charge transfer between molecules and the oxide nanoparticles’ surface. The explanation for the occurrence of ferromagnetic behavior of the molecule-adsorbed ZnO surface could be a general mechanism for the magnetism in moleculeenwrapped oxide nanoparticles. Acknowledgment. Calculations were carried out at the Shanghai Supercomputer Center. Financial support from the National Natural Science Foundation of China (Grant Nos. 50425102, 50601021, 50701038, 60776014, 60876002, and 10804096), China Postdoctoral Science Foundation (Grant No. 20070421158), Zhejiang University-Helmholtz cooperation fund, the Ministry of Education of China (Program for Changjiang Scholars and the Ph.D. research funding), the Department of Science and Technology of Zhejiang Province, and Zhejiang University is gratefully acknowledged. References and Notes (1) Ohno, H. Science 1998, 281, 951. (2) Ueda, K.; Tabata, H.; Kawai, T. Appl. Phys. Lett. 2001, 79, 988. (3) Yin, S.; Xu, M. X.; Yang, L.; Liu, J. F.; Rosner, H.; Hahn, H.; Gleiter, H.; Schild, D.; Doyle, S.; Liu, T.; Hu, T. D.; Takayama-Muromachi, E.; Jiang, J. Z. Phys. ReV. B 2006, 73, 224408. (4) Sati, P.; Deparis, C.; Morhain, C.; Scha¨fer, S.; Stepanov, A. Phys. ReV. Lett. 2007, 98, 137204. (5) Shi, T.; Zhu, S.; Sun, Z.; Wei, S.; Liu, W. Appl. Phys. Lett. 2007, 90, 102108. (6) Zhao, Z. W.; Tay, B. K.; Chen, J. S.; Hu, J. F.; Lim, B. C. Appl. Phys. Lett. 2007, 90, 152502. (7) Lin, Y.-H.; Ying, M.; Li, M.; Wang, X.; Nan, C.-W. Appl. Phys. Lett. 2007, 90, 222110. (8) Naeem, M.; Hasanain, S. K.; Kobayashl, M.; Ishida, Y.; Fujimorl, A.; Buzby, S.; Shah, S. Nanotechnology 2006, 17, 2675. (9) Sharma, V. K.; Varma, G. D. J. Appl. Phys. 2007, 102, 056105. (10) Venkatesan, M.; Fitzgerald, C. B.; Lunney, J. G.; Coey, J. M. D. Phys. ReV. Lett. 2004, 93, 177206. (11) MacManus-Driscoll, J. L.; Khare, N.; Liu, Y.; Vickers, M. E. AdV. Mater. 2007, 19, 2925. (12) Hsu, H. S.; Huang, J. C. A.; Chen, S. F.; Liu, C. P. Appl. Phys. Lett. 2007, 90, 102506. (13) Deka, S.; Joy, P. A. Appl. Phys. Lett. 2006, 89, 032508. (14) Venkatesan, M.; Stamenov, P.; Dorneles, L. S.; Gunning, R. D.; Bernoux, B.; Coey, J. M. D. Appl. Phys. Lett. 2007, 90, 242508. (15) Patterson, C. H. Phys. ReV. B 2006, 74, 144432. (16) Liu, E.-Z.; He, Y.; Jiang, J. Z. Appl. Phys. Lett. 2008, 93, 132506. (17) He, Y.; Sharma, P.; Biswas, K.; Liu, E. Z.; Ohtsu, N.; Inoue, A.; Inada, Y.; Nomura, M.; Tse, J. S.; Yin, S.; Jiang, J. Z. Phys. ReV. B 2008, 78, 155020. (18) Sundaresan, A.; Bhargavi, R.; Rangarajan, N.; Siddesh, U.; Rao, C. N. R. Phys. ReV. B 2006, 74, 161306.
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