Room-Temperature Ferromagnetism in Co-Doped In2O3 Nanocrystals

Publication Date (Web): September 16, 2010 .... TUNING THE "d 0 " FERROMAGNETISM IN In 2 O 3 QUANTUM DOTS BY DANGLING BONDS AND VACANCY ...
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J. Phys. Chem. C 2010, 114, 17569–17573

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Room-Temperature Ferromagnetism in Co-Doped In2O3 Nanocrystals Xiuqing Meng, Liming Tang, and Jingbo Li* State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China ReceiVed: July 20, 2010; ReVised Manuscript ReceiVed: August 22, 2010

Co-doped In2O3 nanocrystals showing room-temperature ferromagnetism have been successfully prepared by a simple sol-gel synthesis route. The sample displays a clear ferromagnetism behavior above 300 K. Phase and structure analyses reveal that the nanocrystals are crystallized with Co ions substituted for In ions in the In2O3 matrix, and no trace of secondary phases or clusters is detected. The experimental results are explained theoretically by first-principles calculations based on density functional theory, which indicate that the native ferromagnetic behavior of Co-doped In2O3 could be mainly ascribed to the strong d-d coupling of the magnetic ions. 1. Introduction Diluted magnetic semiconductors (DMSs) have been extensively investigated for potential applications in spintronics with the purpose of realizing a high Curie temperature and large magnetic moments. A large number of transition-metal (TM)doped semiconductor oxides exhibiting room-temperature ferromagnetism, such as TM-doped ZnO,1-3 TM-doped SnO2,4,5 and TM-doped TiO2,6,7 have been reported by various research groups with diverging and controversial magnetic moments.8-13 More recently, TM-doped In2O3-based DMSs with roomtemperature ferromagnetism have been fabricated and studied.14-18 In2O3 is an n-type wide band-gap (3.75 eV) transparent semiconductor with a high electronic carrier concentration and has been widely used as the transparent electrode by alloying with Sn (ITO). In addition, TM-doped In2O3 has been theoretically predicted to be an ideal candidate for room-temperature DMSs.19 However, previous reports mostly focused on In2O3 films, whereas device miniaturization has made it also important to manipulate ferromagnetism on the DMS nanocrystals20-23 because it can combine the properties of spintronics and nanostructures. Moreover, the cost of obtaining ferromagnetic nanocrystals could be lower than that of films, which might be practical and appealing to applications. On the other hand, the identification of key factors governing the ferromagnetism in DMSs has been challenging. Different research groups have reported controversial results for seemingly the same materials. For example, Peleckis et al.24 have found that room-temperature ferromagnetism in Ni-doped In2O3 is carrier induced, whereas Li et al.25 have observed that Ni-doped In2O3 is paramagnetic. Despite that ferromagnetism in Fe-doped In2O3 polycrystalline samples has been reported by Jayakumar et al.,26 Chu et al. and Peleckis et al.27,28 have reported that room-temperature ferromagnetism can be obtained in Fe-doped In2O3 nanocrystals only after magnetic field treatment or codoping. It is, therefore, important to investigate the origin of the ferromagnetism in TMdoped In2O3 nanocrystals, which will be useful to advance the studies on dilute magnetic semiconductors. Currently, only few studies have been carried out for Co-doped In2O3 nanocrystals. Understanding the mechanism of ferromagnetism in Co-doped * To whom correspondence should be addressed. E-mail: jbli@ semi.ac.cn.

In2O3 nanocrystals is, therefore, useful in exploring new areas of dilute magnetic semiconductors. In this paper, we find that room-temperature ferromagnetism in Co-doped In2O3 nanocrystals can be achieved by a sol-gel method. First-principles calculations further confirm that the stable room-temperature ferromagnetism is mainly attributed to the strong d-d coupling of the magnetic ions. 2. Experimental Details The synthesis of Co-doped In2O3 is done using a lowtemperature sol-gel route to yield colloidal crystalline nanocrystals. In a typical Co-doped In2O3 nanocrystal synthesis, 50 mL of absolute ethanol is mixed with 1.8175 g of indium nitrate hydrate (In(NO3)3 · 41/2H2O) and 0.06225 g of cobalt(II) acetate tetrahydrate (Co(Ac)2 · 4H2O) in a three-neck flask under vigorous stirring and refluxing at 60 °C for 3 h. The flask is then cooled to 5 °C and an additional 0.02 mol of C4H13NO is injected into the solution dropwise. The reaction continues for another 30 min, yielding a transparent gel that is then aged overnight and annealed at 500 and 900 °C sequentially under the protection of a nitrogen atmosphere for an hour. Structural and morphological studies are done by means of the X-ray diffraction (XRD) technique using Cu KR irradiation on an 800 W Philips 1830 powder diffractometer and high-resolution transmission electron microscopy (HRTEM) measurements acquired on a Hitachi S-4800 microscope instrument using an accelerating voltage of 15 kV. X-ray photoelectron spectroscopy (XPS) measurement is carried out on a VGESCALAB MK II instrument where Mg KR X-ray (hν ) 1253.6 eV) is used as the emission source. The energy scale of the spectrometer has been calibrated with pure C (Eb ) 284.6 eV) samples, and the pressure in the XPS analysis chamber is 6.2 × 10-7 Pa. The Raman scattering properties are done on a micro-Raman spectrometer using a 514 nm laser line as an excitation source. The magnetization measurements are performed by a Quantum Design superconducting quantum interference device (SQUID) system. First-principles total energy calculations for an In11Co2O24 nanodot are performed using the VASP code based on the spin density functional theory,29 and the surface atoms are passivated by pseudohydrogen atoms. For the exchange and correlation potential, the generalized gradient approximation (GGA)30 is used. The Brillouin zone integration is performed using the Monkhorst-Pack31 2 × 2 × 2 special k-points.

10.1021/jp106767n  2010 American Chemical Society Published on Web 09/16/2010

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Figure 1. XRD patterns for the Co-doped In2O3 nanocrystals.

3. Results and Discussions Figure 1 shows the XRD patterns of the sample, in which all the observed diffraction peaks could be indexed based on the unit cell of a cubic In2O3,32,33 and no secondary phases are found. The lattice parameter a ) 9.6 Å calculated from the X-ray diffractograms (JCPDS no. 6-0416) shows some decrease from the literature data of a ) 10.118 Å34 for In2O3, indicating that Co ions are indeed incorporated into the In2O3 lattice as the ionic radius of Co2+ (0.58 Å) is smaller than that of In3+ (0.80 Å).35,36 Figure 2a shows the TEM image of Co-doped In2O3 nanocrystals, which further confirms that the samples are cubic structured nanocrystals with a size in the range of 50-100 nm. The HRTEM observation is presented in Figure 2b. In this figure, no sign of segregation of impurities or clusters could be detected, indicating that the sample is single-crystalline and free of secondary crystalline phases. Judged from their corresponding selected area electron diffraction (SAED) patterns shown in the inset of Figure 2b, we find that these nanocrystals are single crystals bounded by (100) facets. Moreover, these nanocystals show clear lattice fringes, with an interplanar spacing of ∼4.8 Å, which is smaller than that of the bulk. These deviations are consistent with the XRD analysis and can be attributed to the difference between the ionic radii of Co and In cations. Thus, we conclude that a sizable amount of Co is homogeneously incorporated in the In2O3 matrix. XPS spectral peaks of In 3d5/2 (443.4 eV), In 3d3/2 (450.9 eV), O 1s (529.8 eV), Co 2p3/2 (781.8 eV), and Co 2p1/2 (796.6 eV) are observed in Figure 3. The binding energy of In 3d5/2 is lower than those in bulk In2O3 (444.5,37,38 444.6,39 and 444.7 eV40), suggesting that Co with a smaller ionic radius substituting for In in In2O3 could result in the decreasing of the binding energy of In in In2O3. Similar results are observed in the O 1s state (529.8 eV), which is smaller than that in bulk In2O3 (530.6 and 531.9 eV). The central binding energy of Co 2p1/2 is 796.6 eV, which corresponds to the high spin divalent state of Co2+18 and indicates that Co ions are incorporated in the matrix. The satellite peaks of Co 2p3/2 (781.8 eV) and Co 2p1/2 (796.6 eV) originate from the interferential X-ray of the device. The relative quantitative analysis of each element is completed using the XPS peak area data of different elements and their respective elemental sensitivity factor according to the equation n(E1)/n(E2) ) [A(E1)/S(E1)]/[A(E2)/S(E2)], where n is the number of the atom, Ei is the element i, A is the peak area, and S is the elemental sensitivity factor.41 The sensitive factor S of In 3d5/2, Co 2p3/2, and O 1s is 6, 3.8, and 0.66, respectively. Here, the content ratio of In to Co is 98.1:1.9. That is, the Co concentration in the matrix is estimated to be 1.9%. Previous studies have established Raman spectroscopy as a powerful tool for identifying secondary phases in doped oxide

Figure 2. (a) TEM and (b) HRTEM images of the Co-doped In2O3 nanocrystals. The inset is the corresponding SAED pattern.

semiconducting materials.42 To further exclude the existence of a small amount of secondary phases, which could fall below the XRD detection limit or be overlooked in the local HRTEM studies, we also conducted extensive Raman studies on these nanocrystals. The typical Raman spectra of Co-doped In2O3 as well as pure In2O3 nanocrystals (given as a reference to Codoped In2O3) in the range of 100-800 cm-1 are presented in Figure 4. In the spectrum of In2O3, five Raman scattering peaks locating at 103, 306, 364, 494, and 624 cm-1 are observed, which are in good agreement with the characteristic Raman peaks of cubic In2O3.43-46 The incorporation of Co in In2O3 leads to the peaks’ position shift toward lower frequency compared with pure In2O3; we attribute these shifts to strain induced by Co2+ in the sample. Another possible reason for the energy shifts is due to the broken symmetry in nanocrystals, which also leads to the emergence of the original forbidden transitions; that is, some new peaks compared with bulk material appear. No vibrational mode of impurities, such as Co metal, is observed in the Co-doped In2O3 nanocrystals besides these five peaks, implying the absence of Co-related secondary phases. Magnetization results for the Co-doped In2O3 nanocrystals are shown in Figure 5. The typical magnetization versus temperature (M-T) curve is presented in Figure 5a. The field-

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Figure 4. Raman spectra of the Co-doped In2O3 and the reference sample of In2O3 nanocrystals.

Figure 5. (a) The FC-ZFC M-T curve of the Co-doped In2O3 nanocrystals. (b) M-H curve of the sample taken at 300 K after the necessary background diamagnetic subtraction; the magnetic field used is from 0 up to 0.3 T.

Figure 3. XPS spectra of the Co-doped In2O3 nanocrystals. Panels a-c represent the XPS spectrum of In, O, and Co, respectively.

cooled (FC) and zero-field-cooled (ZFC) magnetization measurements are performed from 5 to 400 K. The FC results are obtained by measuring the magnetic moment of the sample in a magnetic field of 1000 Oe during cooling. The ZFC results are obtained by first cooling the sample to 5 K in zero field and then warming it in the same field as that of the FC measurement. The ZFC magnetization shows a stronger temperature dependence than the FC one below 30 K. The divergence between the FC and ZFC curves indicates that the Co-doped In2O3 nanocrystals are ferromagnetic in the whole temperature range and have a Curie temperature (TC) well above 400 K. A ZFC-FC magnetization curve of pure In2O3 nanocrystals obtained under the same condition as that of Co-doped In2O3 is

shown in the Supporting Information, where no difference between the FC and ZFC is observed, excluding the ferromagnetism in pure In2O3 nanocrystals, as one would expect. Figure 5b shows the magnetization versus applied magnetic field (M-H) curve measured at 300 K after subtracting the diamagnetic background. The well-defined hysteresis loops show that the nanocrystals are clearly ferromagnetic at room temperature. The saturation magnetization (Ms), coercive field (Hc), and remanence magnetization are 0.07 µB/Co, 150 Oe, and 0.0163 µB/Co, respectively. The absence of any detectable traces of secondary phases or clusters from the XRD, HRTEM, and Raman scattering results clearly confirms that the ferromagnetic signal is not produced by secondary phases in the sample. Contamination during sample preparation or annealing also could be ruled out as the experimental conditions are precisely controlled. To further demonstrate that Co-doped In2O3 nanocrystals are favorable for room-temperature ferromagnetism, we investigate

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Figure 6. Spin density (green regions) of FM states for (a) ABQD and (b) BB1QD.

that the magnetic interaction is mainly strong d-d coupling between the magnetic atoms with only little weak p-d coupling between the Co 3d states and O 2p states. The calculated total energy differences between the FM and antiferromagnetic (AFM) configurations (∆E ) EFM - EAFM) are -50 meV for ABQD and 96 meV for BB1QD, respectively, indicating that FM ordering is favorable for ABQD and AFM for BB1QD. It should be noted that the calculations show that the total energy of ABQD is 0.89 eV lower than that of BB1QD, indicating that a pair of Co atoms in In2O3 nanocrystals prefer to form ABQD. Therefore, the observed ferromagnetism should be the intrinsic behavior of Co-doped In2O3 nanocrystals. 4. Conclusions Figure 7. Total density of states (DOS) and local DOS of Co 3d for Co in the In2O3 QD. (a) A Co substitutes at an A-site and (b) a Co substitutes at a B-site. The Fermi level is set to zero.

the magnetic interactions of Co atoms in a small In2O3 quantum dot (QD) by performing first-principles spin-polarized density functional theory calculations. In this study, two In atoms in the QD are replaced by a pair of Co atoms. Figure 6 shows the spin density distributions of ferromagnetic (FM) configurations for two systems: (a) one Co atom substitutes an inner A-site, and the other Co atom substitutes an outer B-site (ABQD) and (b) both of the Co atoms substitute two outer sites (BB1QD). We can see from Figure 6a that almost all of the magnetic moments are distributed within the Co-O or Co-O-H octahedron and are primarily localized at the Co sites. For ABQD, it seems that the magnetization density on the A-site Co is less than that on the B-site Co, indicating that the spin polarization of the inner magnetic atom is weaker than that of the outer one. Indeed, there will be no spin polarization on the A-site Co if we do not consider the magnetic interactions of the two Co atoms (see Figure 7). Similarly, only the nearestneighbor O atoms next to the B-site Co have little spin density, but almost nothing on the O atoms next to the A-site Co. For BB1QD, spin density exists at the O atom joining both of the magnetic atoms and the magnetization density at the B-site is as much as that at the B1-site, indicating that these two outer Co ions have similar spin polarizations although they have different octahedron configurations. The calculations show that the localized magnetic moments of Co and their nearestneighboring O atoms are about 85% and 15% of the total magnetic moments in ABQD, respectively, whereas for BB1QD, they are 80% and 20%, respectively. Therefore, we can infer

In summary, room-temperature ferromagnetism is observed in Co-doped In2O3 nanocrystals with a well-crystallized cubic In2O3 structure and no trace of any secondary phases. Co ions substitute at the In site in these nanocrystals. The strong d-d coupling between magnetic atoms, but weak p-d coupling between the Co 3d states and O 2p states, should be mainly responsible for the observed ferromagnetism in the Co-doped In2O3 nanocrystals. Acknowledgment. J.L. gratefully acknowledges financial support from the “One-Hundred Talent Plan” of the Chinese Academy of Sciences and National Science Fund for Distinguished Young Scholar (Grant No. 60925016). This work is supported by the National High Technology Research and Development Program of China under Contract No. 2009AA034101 and the Postdoctoral Foundation under Contract No. O9T1050000. Supporting Information Available: ZFC-FC curve of pure In2O3 nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Awschalom, D. D.; Flatte, M. E.; Samarth, N. Sci. Am. 2002, 286, 66. (2) Norberg, N. S.; Kittilstved, K. R.; Amonette, J. E.; Kukkadapu, R. K.; Schwartz, D. A.; Gamelin, D. R. J. Am. Chem. Soc. 2004, 126, 9387. (3) Kittilstved, K. R.; Gamelin, D. R. J. Am. Chem. Soc. 2005, 127, 5292. (4) Radovanovic, P. V.; Gamelin, D. R. Phys. ReV. Lett. 2003, 91, 157202. (5) Ogale, S. B.; Choudhary, R. J.; Buban, J. P.; Lofland, S. E.; Shinde, S. R.; Kal, S. N.; Kulkarni, V. N.; Higgins, J.; Lanci, C.; Simpson, J. R.; Browning, N. D.; Sarma, S. D.; Drew, H. D.; Greene, R. L.; Venkatesan, T. Phys. ReV. Lett. 2003, 91, 077205.

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