Co Cluster Formation Induced by Cu Codoping in Co:ZnO

Article pubs.acs.org/JPCC

Co Cluster Formation Induced by Cu Codoping in Co:ZnO Semiconductor Thin Films Zhiyun Pan,†,∥ Fengchun Hu,†,∥ Shi He,‡ Qinghua Liu,*,† Zhihu Sun,† Tao Yao,† Yi Xie,*,‡ Hiroyuki Oyanagi,†,§ Zhi Xie,† Yong Jiang,† Wensheng Yan,† and Shiqiang Wei† †

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, P. R. China Department of Nanomaterials and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China § Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba 305-8568, Japan ‡

ABSTRACT: Here we report that the occupation sites of Co atoms in ZnO matrix could be effectively tuned by the concentration of Cu codopant. The Co−Cu codoping effect has been revealed by a combination of X-ray diffraction and Xray absorption fine structure spectroscopy techniques for Zn0.95−xCuxCo0.05O (0.005 ≤ x ≤ 0.08) thin films grown by pulse laser deposition at 923 K. Specifically, at the low Cu doping levels (x ≤ 0.02), the Co(+2) ions are substantially incorporated into the ZnO lattice; upon increasing the Cu concentration to 0.03 or higher, partial formation of Co(0) species occurs, and its proportion rises rapidly with the Cu concentration. Further analysis shows that the Cu codopants are precipitated to form Cu(0) metallic phase in all the samples. We propose a competition mechanism between the Co(0)−Cu(0) metallic interactions and the dissolution of Co ions in ZnO to interpret these findings. codopant atoms.14 This theoretical scenario has been supported by subsequent experimental studies on the (Co,Cu)-codoped ZnO systems by means of X-ray spectroscopy and magnetization measurements.17,18 Using X-ray absorption and photoelectron spectroscopy, for example, Kittilstved et al.,19 Shah et al.,20 and Song et al.21 have shown that the doped Co ions are substitutionally incorporated into the ZnO lattice, and by further introducing shallow donors into the systems, the electronic and magnetic properties of Co:ZnO could be controlled.22,23 Whereas, Larde and Liu et al. have reported that depending on preparation conditions Co- or Cu-related clusters or secondary phases could be formed which also mediate the optical and/or electronic properties of ZnO semiconductor.24−26 Many more experimental studies have shown that the existence form of the dopants/codopants in ZnO semiconductor is sensitive to the preparation conditions of the samples, and alteration of the preparation conditions could lead to different atomic occupation characteristics, such as precipitation of TM-related oxides.27−31 In spite of these efforts, the structural nature of dopants in TM-codoped ZnO systems still need to be further clarified. This would be quite helpful for understanding the important issue of how to mediate the functional properties of ZnO-based materials through codoping of TM ions.

I. INTRODUCTION Semiconductor ZnO-based materials have gained substantial research interest for their attractive prospects in blue/UV light emitters, transparent electronics, spintronic devices, chemical and gas sensors, biomedical diagnosis, and solar cells.1−3 To date, the main impediment to the widespread exploitation of ZnO-related materials in optoelectronics and spin functional device applications lies in the difficulty in realization of p-type conductivity and its intrinsic nonferromagnetic character.4−6 Doping of impurity atoms that contribute carriers with special spin and/or charge into the crystal lattice can strongly modify the electronic and band structures of bulk semiconductors4,7−10 and thus opens up a promising pathway for maximizing the utility of ZnO-based materials. Although great progress in modification of semiconductor functionality by doping has been made, the effects of elemental doping on the structural and physical properties of the materials are still in debate,11,12 all of which need in-depth investigations. In particular, doping with transition-metal (TM) elements in ZnO has been intensively investigated for its practical utility in tuning the properties of semiconductor host.11,13 Recently, increasing attention has been put to codoping of double elements such as Cu and Co, primarily because of the possibility of further tailoring the atomic and band structures of semiconductor systems through the synergetic interactions of the codopants.14−16 Lathiotakis et al. have theoretically hypothesized that the substitutional TM atoms at the Zn sites of ZnO could effectively mediate the electronic and/or optical properties of the materials through the interactions between the © 2012 American Chemical Society

Received: October 17, 2011 Revised: January 7, 2012 Published: January 29, 2012 4855

dx.doi.org/10.1021/jp209985n | J. Phys. Chem. C 2012, 116, 4855−4861

The Journal of Physical Chemistry C

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In this work, taking the advantage of the X-ray absorption fine structure (XAFS) spectroscopy that it is sensitive to local environments around a specific atom, we aimed to investigate the atomic interaction and mutual influence between Co and Cu ions codoped in ZnO thin films deposited at substrate temperature of 923 K by the pulse laser deposition (PLD) method. The XAFS analyses at both Co and Cu K-edge were combined with X-ray diffraction results to reveal the local and long-range structural features of the films, respectively. We observed an interesting phenomenon that the existence form of Co ions in ZnO thin films is effectively tuned by the codoped Cu clusters. Our results provide some new insights into the control over dopants occupation form and atomic structure for tuning the functional properties in ZnO-based semiconductor devices.

II. EXPERIMENTAL SECTION The Zn0.95−xCuxCo0.05O thin films with a thickness of about 500 nm were grown on Si(100) substrates by the PLD method. Targets were prepared by the sol−gel technique and sequent calcination treatment. Specially, the starting reagents were chosen using Zn(CH3COO)2·2H2O, Co(CH3COO)2·4H2O, and Cu(CH3COO)2·H2O as precursors, deionized water as solvent, and poly(vinyl alcohol) as stabilizing agent. After mixing at 85 °C for 2 h, the sol was obtained and then dried at 150 °C to get the xerogel. The obtained xerogel was calcined at 1473 K for 2 h in air atmosphere to get the targets with stoichiometric amounts x = 0.005, 0.02, 0.03, 0.04, 0.05, and 0.08. A KrF excimer laser (Lambda Physik LPX 200) with a wavelength of 248 nm was used for the deposition. The repetition rate is 5 Hz, and the laser energy density is 150 mJ/ pulse. The deposition chamber was evacuated to a base pressure lower than 1 × 10−4 Pa. For growing doped ZnO semiconductor thin films with enough crystallinity and less inhomogeneity,13 we deposited the Zn0.95−xCuxCo0.05O thin films at the substrate temperature of 923 K. The X-ray diffraction (XRD) patterns of the samples were recorded using Cu Kα (λ = 0.154 nm) radiation in the 2θ range from 20° to 80°. The Co and Cu K-edge XAFS spectra of Zn0.95−xCuxCo0.05O thin films were measured at the U7C beamline of National Synchrotron Radiation Laboratory (NSRL), China. The storage ring of NSRL was run at 0.8 GeV with a maximum current of 250 mA. A three-pole wiggler with the maximum magnetic field of 6.0 T inserted in the straight section of the storage ring was used. The hard X-ray was monochromatized with Si(111) double crystals. The XAFS signals were collected in fluorescence mode at room temperature. XAFS data were analyzed by UWXAFS3.032 and USTCXAFS3.033 software packages according to the standard data analysis procedures.

Figure 1. XRD patterns for Zn0.95−xCuxCo0.05O thin films with various Cu concentrations: x = 0, 0.005, 0.02, 0.03, 0.04, 0.05, and 0.08.

secondary phases or impurities could be observed. However, for Zn0.95−xCuxCo0.05O samples with Cu concentrations x ≥ 0.02, an additional diffraction peak centered at around 43.3° is apparent. This peak originates from the (111) crystalline face of the face-centered-cubic (fcc) structured Cu, suggesting the formation of metallic Cu in these heavily doped samples. Therefore, the XRD results preliminarily show that the codoped Cu atoms exist in the form of metallic phase in the as-deposited Zn0.95−xCuxCo0.05O thin films with Cu concentrations x larger than 0.02 but could hardly give any definite information on the occupation nature of Co dopants in the samples. Because of the limit of the XRD technique in providing structural information for species with a low content, we used the XAFS technique as a sensitive local-structure probe to further investigate the structural characteristics of Co and Cu dopants in the samples. We first show in Figure 2 the Co Kedge extended-XAFS (EXAFS) oscillation functions χ(k) and their Fourier transforms (FTs) for the Zn0.95−xCuxCo0.05O thin films. As reference, the Co K-edge spectra of Co foil and the Zn K-edge function for ZnO thin film are also shown. Interestingly, although the Co concentration in all the films is the same (0.05), the Co K-edge EXAFS features show a strong relevance to the concentration x of the Cu codopants. For Zn0.95−xCuxCo0.05O thin films with x ≤ 0.02, the χ(k) functions in Figure 2a and their FTs in Figure 2b are very close to those of Zn K-edge of ZnO, presenting two strong peaks located at around 1.55 Å (peak A) and 2.82 Å (peak B) that correspond to the first-nearest Co−O and the second-nearest Co−Zn neighbors, respectively. No extra peak relative to Co-related compounds can be found for the samples with low doping levels of Cu. These results provide strong evidence that all doped Co ions are located substitutionally at the Zn sites for the Zn0.95−xCuxCo0.05O thin films with x ≤ 0.02, in agreement with the XRD results. However, with increasing Cu concentration up to 3% or higher, the EXAFS oscillations of Zn0.95−xCuxCo0.05O show evident changes in shape, especially in the k-region of 5.3−9.6 Å−1, indicative of a different local environment surrounding Co atoms for the heavily Cu-doped films. Seen from the FT curves for the samples with x ≥ 0.03, a new peak C centered at 2.1 Å associated with metallic phase of

III. RESULTS AND DISCUSSION To show the long-range order and to reveal the phase species present in the as-deposited Zn0.95−xCuxCo0.05O thin films, XRD measurements with Cu Kα radiation were performed. The obtained XRD patterns for various Cu concentrations of x = 0, 0.005, 0.02, 0.03, 0.04, 0.05, and 0.08 are shown in Figure 1. In all these patterns, there exhibit two peaks located at 34.5° and 72.8°, corresponding to the (002) and (004) crystalline faces of hexagonal ZnO, respectively, indicating a wurtzite structure with the c-axis preferential orientation for all these thin films.34,35 No diffraction peak coming from Co-related 4856



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Co−Zn coordination are only slightly enlarged than those of ZnO. These results confirm our aforementioned conclusion that all Co ions occupy the Zn sites of ZnO at the low Cu doping levels x ≤ 0.02. Whereas, for the Zn0.95−xCuxCo0.05O film with a higher Cu concentration of x = 0.03, it is readily seen from Table 1 that a considerable proportion of Co metallic species (28%) coexist with the substitutional Co ions. The proportion of the metallic Co species increases sharply from 38% to 75% with increasing the Cu concentration x from 0.04 to 0.08. Therefore, the above EXAFS results at Co K-edge clearly show that codoping of Cu could affect the existence form of Co atoms in the codoped ZnO semiconductor thin films. To understand why the Cu codopants exert such a strong influence on the occupation nature of Co in ZnO, we further examined the structural characteristics of the Cu codopants via a detailed EXAFS analysis. Figure 2c,d displays the EXAFS χ(k) functions and their FTs at the Cu K-edge for Zn0.95−xCuxCo0.05O thin films, with the Cu K-edge data for Cu foil and Zn0.95Cu0.05O thin film presented for comparison. Figure 2c shows that even for the lowest Cu doping level of 0.005, both the shape and the amplitude of χ(k) oscillation spectra are strikingly similar to those for Cu foil. This spectral feature is kept almost unchanged at the higher Cu doping level. The FTs of all the Zn0.95−xCuxCo0.05O thin films in Figure 2d also exhibit three main coordination peaks in the range of 1.5− 4.5 Å, similar to those of Cu foil. The quantitative structural parameters as listed in Table 2, extracted from curve-fitting of the Cu K-edge data for the Zn0.95−xCuxCo0.05O samples, also show very close values of the coordination number and bond length (∼2.54 Å) as compared with those of Cu foil. These results indicate that the coordination environment around the doped Cu atoms in the Zn0.95−xCuxCo0.05O thin films resembles that of Cu foil and basically maintains the fcc structure of crystalline Cu. It is worth mentioning that the spectral features and fitted structural parameters of Cu singly doped Zn0.95Cu0.05O thin film prepared under the same conditions are also analogous to that of Cu foil. These results lead us to conclude that the Cu dopants tend to form metallic phase in ZnO films during the pulsed-laser deposition process under a substrate temperature of 923 K. Summarizing the above XRD and XAFS results, an interesting phenomenon is apparent: the occupation sites of Co ions are strongly dependent on the concentration of the codoped Cu in ZnO, while the codoped Cu atoms always present in the form of fcc metallic phase, even at a lowest doping level of 0.005. These conclusions can be further evidenced by the X-ray absorption near-edge structure (XANES) spectra shown in Figure 3. From the Co K-edge XANES spectra for the samples with lower Cu doping levels (x = 0, 0.005, 0.02) in Figure 3a, we can see that the spectral shape and peak locations are quite similar to the Zn K-edge XANES spectrum of ZnO, suggesting an identical coordination environment of Co and Zn ions. In contrast, even at these low Cu doping levels, the Cu K-edge XANES spectra (Figure 3b) exhibit no noticeable difference relative to that of Cu foil, verifying the fcc metallic structure of the Cu atoms. This is not a surprising result because it is known that the existence form of Cu dopant is strongly sensitive to the preparation methods and/or conditions.26,37 The nonequilibrated and harsh growth conditions of pulsed-laser deposition at 923 K make the Cu codopants tend to form metallic clusters even at a very low concentration, i.e., 0.005. With Cu concentrations x increasing

Figure 2. (a) Co K-edge EXAFS oscillation functions χ(k) and (b) Fourier transform (FT) spectra of k3χ(k) and (c) Cu K-edge EXAFS oscillation functions χ(k) and (d) FT spectra of k3χ(k) for Zn0.95−xCuxCo0.05O thin films. As reference, the Co K-edge spectra of Co foil and Zn0.90Cu0.05Co0.05O thin films deposited at RT and 200 °C, Cu K-edge spectra of Cu foil and Zn0.95Cu0.05O thin film, and Zn K-edge spectra of ZnO thin film have also been shown.

Co cluster appears (Figure 2b). These results suggest that, for the Zn0.95−xCuxCo0.05O with high doping levels x ≥ 0.03, not all the Co ions are incorporated into the wurtzite ZnO matrix but a part of them exist in the form of metallic species. We note that with x increasing from 0.03 to 0.08, the amplitude of peak C is gradually enhanced at the cost of the amplitude decrease of peaks A and B, suggesting an increasingly enlarged proportion of Co metallic species among the Co dopants. To further clarify the structural characteristics of these samples, we quantitatively fitted the main FT peaks (R = 0.7− 3.3 Å) including the first-nearest Co−O and the next-nearest Co−Zn coordination shells. For Zn0.95−xCuxCo0.05O with x > 0.02, the Co−Co coordination was also considered to account for the metallic Co species, where an adjustable parameter p (0 < p < 1) denoting the percentage of metallic Co species was refined in the fitting. The theoretical scattering amplitudes and phase shifts of these coordinations were calculated with the ab initio code FEFF8.36 The best fitting structural parameters are summarized in Table 1. For comparison, the fitting results for ZnO powders are also presented. It is shown that the coordination numbers (N) and the bond lengths RCo−O for the Co−O coordination in the lightly Cu-codoped samples (x ≤ 0.02) are very close to those of ZnO powders, and the bond lengths and the corresponding Debye−Waller factors σ2 for the 4857



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Table 1. Structure Parameters around Co Atoms in Zn0.95−xCo0.05CuxO (0 ≤ x ≤ 0.08) Thin Films and Co Foila sample ZnO x=0 x = 0.005 x = 0.02 x = 0.03

x = 0.04

x = 0.05

x = 0.08

Co foil

bond type Zn−O Zn−Zn Co−O Co−Zn Co−O Co−Zn Co−O Co−Zn (Zn,Co)O Co−O Co−Zn metallic Co Co−Co (Zn,Co)O Co−O Co−Zn metallic Co Co−Co (Zn,Co)O Co−O Co−Zn metallic Co Co−Co (Zn,Co)O Co−O Co−Zn metallic Co Co−Co Co−Co

pb (%)

N

R (Å) ± ± ± ± ± ± ± ±

0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01

σ2 (Å2) 0.0040 0.0075 0.0043 0.0094 0.0044 0.0102 0.0050 0.0094

± ± ± ± ± ± ± ±

4 12 4 12 4 12 4 12

1.97 3.21 1.98 3.22 1.98 3.23 1.99 3.25

0.0002 0.0005 0.0002 0.0002 0.0006 0.0004 0.0005 0.0008

4 12

1.99 ± 0.02 3.26 ± 0.02

0.0084 ± 0.0005 0.0110 ± 0.0008

12

2.54 ± 0.02

0.0075 ± 0.0007

4 12

1.98 ± 0.02 3.25 ± 0.02

0.0070 ± 0.0007 0.0147 ± 0.0008

12

2.54 ± 0.02

0.0095 ± 0.0007

4 12

2.00 ± 0.02 3.26 ± 0.02

0.0069 ± 0.0005 0.0070 ± 0.0008

12

2.54 ± 0.02

0.0094 ± 0.0007

4 12

1.99 ± 0.02 3.24 ± 0.02

0.0064 ± 0.0008 0.0076 ± 0.0004

12 12

2.55 ± 0.02 2.50 ± 0.02

0.0094 ± 0.0005 0.0065 ± 0.0002

28 ± 5

38 ± 5

60 ± 5

75 ± 5 100

a For reference the parameters around Zn atoms in ZnO are also shown. N, R, and σ2 denote the coordination number, bond length, and Debye− Waller factor, respectively. bp is the percentage of Co atoms in the metallic phase.

Table 2. Structure Parameters around Cu Atoms in Zn0.95−xCuxCo0.05O, Cu Foil, and Zn0.95Cu0.05Oa

a

sample

bond type

Cu foil x = 0.005 x = 0.02 x = 0.03 x = 0.04 x = 0.05 x = 0.08 Zn0.95Cu0.05O

Cu−Cu Cu−Cu Cu−Cu Cu−Cu Cu−Cu Cu−Cu Cu−Cu Cu−Cu

N 12 11.0 11.3 11.5 11.6 11.9 12.0 11.7

± ± ± ± ± ± ±

R (Å) 0.3 0.5 0.5 0.6 0.8 0.7 0.6

2.55 2.54 2.54 2.54 2.54 2.54 2.55 2.54

± ± ± ± ± ± ± ±

0.01 0.01 0.02 0.02 0.01 0.02 0.02 0.01

σ2 (Å2) 0.0075 0.0091 0.0086 0.0083 0.0081 0.0080 0.0080 0.0085

± ± ± ± ± ± ± ±

0.0001 0.0002 0.0005 0.0005 0.0002 0.0002 0.0001 0.0002

N, R, and σ2 denote the coordination number, bond length, and Debye−Waller factor, respectively.

the bond length of Co−Co is 2.54 Å for the sample with x = 0.03 and approaches 2.55 Å for the higher Cu doping level samples. This value is notably longer than that of Co foil (2.50 Å) but compatible with that of Cu foil (2.55 Å, Table 2), suggesting that the deoxidized Co atoms favor to interact with Cu atoms and form an fcc-structured metastable Co−Cu alloyalike cluster. Although Co−Cu is not a naturally miscible system at equilibrium,40,41 it is also well-known that the Co− Cu system exhibits a large miscibility gap, and metastable solid solution of both elements could be achieved with the aid of nonequilibrium ultrarapid quenching techniques such as meltspinning, laser ablation, and pulsed laser deposition.42,43 Therefore, the formation of this kind of metastable Co−Cu alloy-alike cluster is possible since the laser deposition along with a high temperature facilitates the intermixing of Co and

to 0.03 or more, the Co K-edge XANES spectral features gradually deviate from those of ZnO (see Figure 3a) in that the peaks are gradually decreased in intensity. This is in agreement with our conclusions deduced from EXAFS analysis that a part Co ions are escaped from their original substitutional sites to form metallic species. Our results on the PLD-prepared (Co,Cu)-codoped ZnO films are contrary to those prepared by mild chemistry methods, for which it is reported that both Co and Cu dopants occupy the Zn sites and the substitutional Cu ions have little influence on the occupation form of Co in ZnO lattice.38,39 Thus, it is interesting to understand why the metallic Cu clusters have such a strong impact on the formation of Co(0) species. Some hints could be found from the quantitative EXAFS results as listed in Table 1. It is noted that 4858



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at the low Cu doping level x ≤ 0.02, most of the Co atoms occupy the substitutional sites of ZnO, we propose a competition mechanism for Co atoms between Cu and ZnO matrix. That is, the atomic interaction between metallic Co and Cu atoms competes with the dissolution of Co ions in ZnO matrix. We describe briefly the scenario of this competition in Figure 4. At the low doping levels of Cu, the distribution of Cu in ZnO thin films is sparse and the interaction between Co and ZnO is dominant. Therefore, the Co ions are basically located at the substitutional Zn sites of ZnO lattice. When the Cu concentration is increased, the particle density of the metallic Cu clusters is increased, resulting in the shortened Cu−Cu cluster distances r and the enlarged Co−Cu interaction areas. If the Cu concentration is increased beyond a critical value, the interacting distance (r) gets finally smaller than the threshold value (r0) for Co alloying on Cu clusters (Figure 4). Thermodynamically, the enlarged interacting areas could lower the formation energy of Co−Cu clustering.37 Kinetically, this atomic interacting process is facilitated by the enhanced mobility of atoms at the high deposition temperature. These scenarios can be further confirmed by the EXAFS results for two reference samples of Zn0.90Cu0.05Co0.05O thin films deposited at lower substrate temperatures of room temperature and 473 K. From Figure 2a,b, it is clear that the spectral features of Zn0.90Cu0.05Co0.05O thin films with lower deposition temperatures are similar to that of ZnO and the characteristic peak for metallic Co in 923 K-deposited Zn0.90Cu0.05Co0.05O thin film is not observed. These results suggest that the Co atoms still occupy the substitutional Zn sites, and there is no discernible interaction between the codopant atoms even at a higher doping concentration of Cu for the samples prepared at lower substrate temperatures. Thus, a relatively high temperature of 923 K improves the mobility of the atoms of Co in the codoped systems and facilitates the intermixing of Co and Cu atoms. All of these factors cooperatively lead to the occurrence of aggregating the deoxidized Co(0) on Cu to form this kind of metastable Co−Cu alloy-alike clusters. Hence, the implication of our results is that they address the possibility of regulating the occupation of TM dopants in ZnO, through which one can intentionally modify the optical, electronic, and magnetic properties of ZnO semiconductor materials. Finally, it is of interest to summarize the relationships of structural characteristics with preparation methods/conditions in (Co,Cu):ZnO systems reported previously. With a standard solid-state reaction method, Lin et al. reported at a low doping concentration of Cu (1.0%) and relatively high calcination temperature of 1173 K, the Cu atoms were incorporated into Co2%:ZnO host and formed a solid solution instead of precipitates.17 This is consistent with our previous studies,16,47 which suggested that the codopant of Cu atoms occupied the

Figure 3. (a) Co K-edge XANES spectra of Zn0.95−xCuxCo0.05O thin films and Co foil. (b) Cu K-edge XANES spectra of Zn0.95−xCuxCo0.05O thin films and Cu foil. The Zn K-edge XANES spectra of ZnO thin film are also displayed in (a) and (b) for comparison.

Cu atoms. Furthermore, we tried to get some structural information from direct observation techniques such as highresolution transmission electron microscopy but failed, mostly because the involved elements of Co and Cu have very similar atomic numbers and lattice spacings. On the basis of above results, we conclude that the presence of metallic Cu phase induces the formation of Co clusters in the deposited ZnO thin films, and through varying the Cu concentrations, the content of the induced metallic Co clusters could be effectively tuned. Next, we qualitatively explain the above interesting phenomenon through a proposed mechanism schematically depicted in Figure 4. It is known that, from the view of ionization energy, the second ionization energy (I2) for Cu, Zn, and Co elements is in this order: Cu (20.34 eV) ≫ Zn (17.89 eV) > Co (17.3 eV).44 Thus, the Cu atoms are energetically much harder to be oxidized to substitute the Zn sites in ZnO lattice than Co, consistent with the fact that the element of Cu has limited thermodynamic equilibrium solubility in ZnO.14,45 Especially under an extremely nonequilibrated growth environment of high substrate temperature, the Cu atoms have a strong tendency to aggregate in the form of reduced valence state. This explains why all the Cu atoms are in the form of metallic clusters in the codoped films under our preparation conditions. Once the metallic Cu clusters are formed, their presence could decrease the formation energy of Co(+2) to Co(0) atoms, making some Co ions overcome the energy barriers and escape from the substitutional sites and be deoxidized (see Figure 4). An analogous phenomenon has also been observed in other TM-doped semiconductor systems.46 Taking into account that

Figure 4. Schematic for the mechanism of deoxidizing substitutional Co ions to Co clusters in ZnO matrix through copper codoping. 4859



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substitutional Zn sites when its concentration was not higher than 4%. As the concentration of Cu is high enough (typically 4−5%), however, the dopant atoms of Cu are hardly incorporated into the ZnO lattice.13 For a physical preparation method, it is recognized that the occupation and property of dopants in ZnO thin films are quite sensitive to the special property of elements and preparation conditions such as substrate temperature, oxygen partial pressure, and substrate materials.13 For example, Chakraborti et al.48 have grown (Co,Cu)-codoped ZnO thin films on sapphire c-plane single crystals at substrate temperature of 500 °C. Similarly, they found separation of metallic Cu nanoparticles within in the ZnO matrix, and the concentration of Cu could influence the magnetic properties of the codoped ZnO thin films. The precipitate of metallic Cu phase at different temperatures between their and our studies might be correlated with the different substrates and laser energy/repetition.13 In short, the formation of secondary phases in ZnO semiconductors is strongly correlated with the preparation methods and conditions, by controlling which ZnO-based semiconductor materials with different structures and properties could be obtained.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Q.H.L.), [email protected] (Y.X.). Author Contributions ∥

REFERENCES

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IV. CONCLUSION Co- and Cu-codoped ZnO semiconductor thin films grown by pulse laser deposition at the substrate temperature of 923 K have been investigated by X-ray absorption fine structure and X-ray diffraction techniques. We found an interesting phenomenon that the occupation sites of Co ions depend heavily on the concentration of the codoped Cu atoms, and the Cu codopants always present in the form of fcc metallic phase. At a low concentration of codoped Cu (x ≤ 0.02), most of the Co ions are substantially incorporated into the ZnO lattice (CoZn). However, as the Cu concentration increases to 0.03 or higher, the metallic phase composed of Co(0) atoms is formed, whose relative proportion increases rapidly with the further increased Cu concentration. An in-depth EXAFS analysis suggests that the precipitated Co(0) and Cu(0) atoms are most likely to form Cu−Co alloy clusters, kinetically facilitated by the enhanced mobility of the metallic atoms at the high deposition temperature of 923 K. These findings could be interpreted by a competition mechanism between the Co(0)−Cu(0) metallic interactions and the dissolution of Co ions in ZnO matrix. Our results suggest that the content of Co ions/clusters in ZnO semiconductor could be effectively regulated by codoping of Cu, providing some new hints for intentionally modifying the functional properties of the wide-band-gap ZnO semiconductors.

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These authors contributed equally to this work.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 11135008, 10905058, 10979041, 10979044, 11079004, and 11105151) and Knowledge Innovative Program of the Chinese Academy of Sciences (KJCX2-YW-N40). The authors thank NSRL and Photon Factory for the synchrotron radiation beamtime. 4860



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Co Cluster Formation Induced by Cu Codoping in Co:ZnO

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