Colloidal Fe-Doped Indium Oxide Nanoparticles: Facile Synthesis

Feb 10, 2009 - Chemistry Division, and Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai-400 085, India, and NDE and Science Research ...
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J. Phys. Chem. C 2009, 113, 3600–3606

Colloidal Fe-Doped Indium Oxide Nanoparticles: Facile Synthesis, Structural, and Magnetic Properties Anshu Singhal,*,§ S. N. Achary,§ J. Manjanna,‡ O. D. Jayakumar,§ R. M. Kadam,† and A. K. Tyagi§ Chemistry DiVision, and Radiochemistry DiVision, Bhabha Atomic Research Centre, Mumbai-400 085, India, and NDE and Science Research Centre, Faculty of Engineering, Iwate UniVersity, Morioka 020-8551, Japan ReceiVed: NoVember 6, 2008; ReVised Manuscript ReceiVed: January 5, 2009

We report the preparation, characterization, and magnetic properties of highly crystalline colloidal Fe-doped indium oxide nanoparticles. The nanoparticles have been prepared in high yields by simple one-pot thermal decomposition of indium and iron precursors in hexadecylamine and can be easily dispersed in solvents like chloroform and toluene. Detailed X-ray diffraction, microstructure, and Raman studies reveal that the nanoparticles are single phase cubic bixbyite structure without any parasitic secondary phases. DC magnetization studies as a function of temperature and field indicate that nanoparticles are weakly ferromagnetic at room temperature (RT). This observation is further confirmed by the electron paramagnetic resonance spectra of the samples, which show a distinct ferromagnetic resonance signal at RT. Introduction In recent years, transition element-doped In2O3 magnetic semiconductors have been extensively studied due to their potential applications in spintronic devices.1-10 Among these compounds, Fe-doped In2O3 magnetic semiconductor is very attractive mainly because of its excellent electrical conductivity, high optical transparency, and high solubility of iron element in the In2O3 lattice. Thus, for such systems, a high potential can be foreseen for the development of multifunctional magnetooptoelectronic devices. For practical spintronic applications, diluted magnetic semiconductors (DMS) with Curie temperature (TC) above room temperature are required. The experimental results regarding room-temperature ferromagnetism (RTFM) in the In2O3:Fe system, in thin films, bulk, and polycrystalline samples, however, differ widely with properties of such materials depending highly on the methods of syntheses employed. For example, He et al.5 and Yoo et al.6 have reported RTFM in both bulk and thin film Fe- and Cu-codoped In2O3 films. On the other hand, Peleckis et al.7 and Be´rardan et al.8 have found only paramagnetism in Fe-doped In2O3. Also, Kohiki et al.9 and Ohno et al.10 attributed the observed RTFM to γ-Fe2O3 nanoclusters dispersed in In2O3 lattice. Recently, Jayakumar et al.11 have reported RTFM in polycrystalline (In1-xFex)2O3 samples prepared by gel combustion method and attributed the observed ferromagnetism to the intrinsic defects in the system. Fe-doped In2O3 films prepared by pulsed laser deposition on R-cut sapphire substrate show high temperature ferromagnetism with TC as high as 927 K.12 Chu et al.13 have synthesized Fedoped In2O3 nanocrystals by coprecipitation method, but RTFM in the samples could only be activated by magnetic field treatment. It is clear from the above background that most of the research on In2O3:Fe system is mainly focused on bulk, polycrystalline, or thin film materials generally prepared at high temperatures, * Corresponding author. E-mail: [email protected]. § Chemistry Division, Bhabha Atomic Research Centre. † Radiochemistry Division, Bhabha Atomic Research Centre. ‡ Iwate University.

while reports on nanoparticles are still quite sparse.13 To integrate the DMS into present electronics, low-dimensional structures are required for exploiting the advantages offered by the spin. Thus, the synthesis of transition metal-doped indium oxide nanoparticles with high crystallinity, homogeneous composition, and well-defined particle morphologies with narrow size distribution is of immense technological interest. Further, the colloidal DMS nanocrystals can be easily assembled into three-dimensional close-packed super lattice structures, providing many opportunities related to materials processing and nanoscale engineering for possible spintronic applications. Our motivation in carrying out this study was to synthesize the colloidal Fe-doped indium oxide nanoparticles at relatively low temperatures to ensure that the secondary ferromagnetic phases are not present and study their magnetic behavior. Accordingly, we report herein a simple one-pot high-yield synthesis of highly crystalline and colloidal Fe-doped In2O3 nanoparticles with variable iron content at a temperature as low as 220 °C, their detailed structural characterization, and magnetic properties. Experimental Details All of the experiments were carried out under an argon atmosphere using standard Schlenk techniques. Indium acetylacetonate (99.99%, Aldrich), iron(III) acetylacetonate (99.99%), and hexadecylamine (95%, Merck) were purchased from commercial sources and used without further purification. Synthesis of (In0.95Fe0.05)2O3 Nanoparticles. Hexadecylamine (8.2 g, 33.96 mmol) was taken in a 100 mL three-necked flask and degassed at 100 °C in a vacuum (2 mbar) and by repeatedly flushing with argon for 30 min. Subsequently, solid indium acetylacetonate (0.4 g, 0.970 mmol) and iron(III) acetylacetonate (0.0180 g, 0.051 mmol) were introduced in the flask at 100 °C. The temperature was slowly raised (∼5 °C min-1) to and maintained at 220 °C for 2 h. The reaction mixture was cooled to 70 °C and treated with excess methanol (40 mL) to give a pale yellow precipitate, which was separated by centrifugation at 5000 rpm for 10 min. The resulting nanoparticles could be

10.1021/jp8097846 CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

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Figure 1. (a) Colloidal Fe-doped indium oxide nanoparticles dispersed in chloroform. (b) Plot of intensity of scattered light as a function of time for (In0.9Fe0.1)2O3 nanoparticles.

easily redispersed in organic solvents like chloroform and toluene. The redispersion/precipitation route was repeated twice to remove any unreacted precursors and excess hexadecylamine. Synthesis of (In0.9Fe0.1)2O3 Nanoparticles. A procedure similar to that of the preparation of (In0.95Fe0.05)2O3 nanoparticles was followed by using hexadecylamine (8.2 g, 33.96 mmol), indium acetylacetonate (0.400 g, 0.970 mmol), and iron(III) acetylacetonate (0.038 g, 0.108 mmol). Characterization. Phase purity and structure of the nanoparticles were determined by X-ray powder diffraction data, which were collected on a Philips X’Pert pro X-ray diffractometer using Cu KR radiation (λ 1.5418 Å) at 40 kV and 30 mA. The average crystallite size was calculated from the diffraction line width based on the Scherrer equation: d ) 0.9λ/B cos θ, where λ is the wavelength (in Å) of X-rays, θ is the Bragg angle, and B is the half-maximum line width. Dynamic light scattering (DLS) measurements were performed on a Malvern Autosizer 4800 instrument that employs a coherent Innova Ar ion Laser (514.5 nm) and 7132 digital correlator. Chemical composition of the samples was determined by energy dispersive X-ray analysis using an INCA Energy 250 instrument coupled to Vega MV2300t/40 scanning electron microscope. ICP-AES (inductively coupled plasma atomic emission spectroscopy) analyses were carried out using a Jobin Yvon JY2000 spectrometer. Transmission electron micrographs were obtained using a Hitachi HF-3000F microscope operating at 300 kV. The sample was dispersed in ethanol, and a drop of this solution was applied onto a carbon-coated copper grid. Raman data were obtained using a Horiba Jobin Yvon T64000 spectrometer equipped with an inverted microscope. The DC magnetic measurements were carried out using a superconducting quantum interference device magnetometer (Quantum design, MPMS-XL). EPR spectra were recorded in the temperature range 100-300 K using a Bruker ESP-300 spectrometer operated at X band frequency (9.5 GHz) using 100 KHz modulation frequency. The temperature was varied in the range 100-300 K using variable-temperature accessory Eurotherm B VT 2000. Diphenyl picrylhydrazyl (DPPH) radical was used as reference for the calibration of g factor. Mo¨ssbauer spectra were obtained using a spectrometer operated in constant

acceleration mode in transmission geometry. The source employed is 57Co in rhodium matrix of strength 50 mCi. Results and Discussion Synthesis. Fe-doped In2O3 nanoparticles have been conveniently synthesized by thermal decomposition of indium(III) acetylacetonate and iron(III) acetylacetonate, taken in appropriate molar ratios in hexadecylamine at 220 °C for 2 h. The nanoparticles so obtained could be easily dispersed in organic solvents like chloroform and toluene. Figure 1a shows the photograph of these nanoparticles dispersed in chloroform. The dynamic light scattering (DLS) has been used to determine the stability of the colloid dispersion prepared in chloroform. Accordingly, a 5 wt % solution of (In0.9Fe0.1)2O3 nanoparticles was prepared in HPLC grade chloroform, and the intensity of the light scattered from this solution was measured as a function of time over a period of 6 h. It was observed that the intensity of the scattered light remains almost the same (Figure 1b), confirming that the nanoparticles are well dispersed in the solvent and do not agglomerate. It will be worthwhile to mention here that no significant change in the intensity is observed even after 24 h. Structural Investigation. The phase purity and crystal structure of the nanoparticles prepared have been analyzed by X-ray diffraction (XRD) and Rietveld refinement of the diffraction data using the Fullprof2K software package.14 The XRD patterns (Figure 2) of In2O3 and Fe-doped In2O3 nanoparticles match well with that of the standard cubic bixbyite In2O3 (JCPDS no. 88-2160, space group Ia3). However, for Fe-doped In2O3 nanoparticles, the peaks are slightly shifted to higher 2θ values as compared to those of c-In2O3, which is attributed to decreased lattice spacings due to the presence of smaller Fe3+ (0.645 Å) ions in place of In3+ (0.800 Å). Rietveld profile refinement analysis of XRD data (Figure 2) shows that the lattice parameter a decreased with increase in Fe concentration, confirming the incorporation of Fe into In2O3 lattice. The lattice parameters calculated from the XRD measurements of pure In2O3, (In0.95Fe0.05)2O3, and (In0.9Fe0.1)2O3 nanoparticles are 10.118 (1), 10.103 (1), and 10.052 (1) Å, respectively, which is in agreement with the reported results.11 The full-width at

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Figure 2. Rietveld refined profiles of X-ray diffraction data of In2O3, (In0.95Fe0.05)2O3, and (In0.9Fe0.1)2O3 nanoparticles. The dots represent the observed data, while the solid line through dots is the calculated profile, and vertical tics represent Bragg reflections for the phase. The difference pattern is also shown below the vertical tics in each case.

TABLE 1: EDX Data on Fe-Doped Indium Oxide Nanoparticles average atom % sample

elements

expected

obtained

(In0.95Fe0.05)2O3

Fe In O Fe In O

2.0 38 60 4.0 36 60

1.52 38.71 59.77 3.65 37.08 59.27

(In0.9Fe0.1)2O3

half-maximum (fwhm) from (211), (222), (400), (411), (332), and (431) diffraction peaks has been plotted versus the Bragg diffraction angle following the Williamson-Hall method.15 The result shows an almost negligible strain in pure In2O3 and (In0.9Fe0.1)2O3 nanoparticles (Figure S1, Supporting Information). Thus, peak broadening should be purely due to the reduced particle size. Therefore, the average crystallite size may be estimated from the values of fwhm of the main (222), (400), and (440) diffraction peaks by means of the Scherrer equation. The average crystallite sizes so determined are ∼11, 11, and 7 nm for pure In2O3, (In0.95Fe0.05)2O3, and (In0.9Fe0.1)2O3 nanoparticles, respectively. We would like to point out here that we have carried out a detailed X-ray study on (In0.9Fe0.1)2O3 nanoparticles using the 6 kW rotating Cu anode-based Rigaku powder diffractometer in the angle range 15-90°, in step width of 0.02, and step time of 2 s. We did not see the reflections attributable to any secondary phases, even by using high flux X-rays obtained by rotating anode diffractometer (Figure S2, Supporting Information). The chemical compositions of the nanoparticles were determined by EDX analysis (Figure S3, Supporting Information). The average EDX analyses of In2O3:Fe nanoparticles collected at three different locations on the same samples are shown in Table 1. The results indicate that the samples are homogeneous,

Figure 3. TEM images of (In0.9Fe0.1)2O3 nanoparticles: (a) low magnification image of 7 nm particles; (b) SAED pattern; and (c) HRTEM image of 7 nm nanoparticles. The inset is the high magnification image of a single nanoparticle.

and the cation concentrations observed are in good agreement with the nominal cation concentrations within the experimental error. Additionally, the Fe content in the samples has also been analyzed by ICP-AES (inductively coupled plasma atomic emission spectroscopy). Fe content was found to be 1.87% and 4.25% for (In0.95Fe0.05)2O3 and (In0.9Fe0.1)2O3 nanoparticles, respectively, which is in agreement with EDX data. A more detailed structural characterization of the doped nanoparticles has been done using TEM. The low- and highresolution (HR) TEM images of a representative sample, (In0.9Fe0.1)2O3, are shown in Figure 3. The formation of nearly spherical (In0.9Fe0.1)2O3 nanoparticles with some agglomeration is clearly evident (Figure 3a). The average mean diameter of 7 (6.7 ( 0.3) nm, for the nanoparticles as determined from TEM images, is in good agreement with XRD data. The selected area electron diffraction (SAED) pattern of the nanoparticles is

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Figure 4. Room-temperature Raman spectra of bulk and (In1-xFex)2O3 (x ) 0.0, 0.05, 0.1) nanoparticles.

consistent with the cubic bixbyite structure of In2O3 featuring strong ring patterns assigned to (211), (220), (222), (411), (332), and (431) planes (Figure 3b) and proves the high crystallinity of the nanoparticles. The HRTEM image of the nanoparticles (Figure 3c) shows well-defined lattice planes, indicating the single crystalline nature of the nanoparticles. The lattice spacings of 2.89 and 5.02 Å observed in the HRTEM image correspond to (222) and (200) interplanar distances of c-In2O3, respectively. There are no detectable traces of secondary phases visible in HRTEM micrographs. These observations together with the absence of any additional phase in XRD assignable to either crystalline Fe2O3 or Fe3O4 clearly support the formation of solid solutions of indium(III) and iron(III) oxides in the Fe-doped indium oxide nanoparticles prepared by us. Raman Spectroscopy. Raman spectroscopy is a commonly used technique to study doping effects in doped semiconductor nanocrystals16 as shifts in lattice Raman vibrational energies occur with increasing dopant concentration. Room-temperature Raman spectra of bulk In2O3 and Fe-doped In2O3 nanoparticles are shown in Figure 4. The observed Raman bands around 306, 365, 494, and 628 cm-1 are assigned to phonons associated with the bcc-structured indium oxide.17 From the data in Figure 4, it is clear that all of the prominent peaks of In2O3 are also observed in the Fe-doped In2O3 spectra, but the Raman modes in Fedoped nanoparticles are broad, relatively less intense, and shifted toward lower frequencies as the iron content in the (In1-xFex)2O3 nanoparticles increases from 0 to 0.1. These observations reveal that the local symmetry in the nanoparticles is different from that of bulk, but the crystal structure is the same in both. The Raman mode at 306 cm-1 for (In0.9Fe0.1)2O3 nanoparticles clearly exhibits a downward shift of ∼15 cm-1. This can be related to the decrease in binding energy of In-O bond as a result of substitution of In3+ by Fe3+. Further, no additional Raman modes are observed for (In1-xFex)2O3 nanoparticles as x changes from 0 to 0.1, revealing the absence of any impurity phase.

Figure 5. Magnetization as a function of temperature for (a) (In0.95Fe0.05)2O3 and (b) (In0.9Fe0.1)2O3 nanoparticles in the zero-field cooled (ZFC) and field-cooled (FC) conditions with H ) 100 Oe.

Magnetic Properties. The magnetic properties of Fe-doped In2O3 nanoparticles were analyzed by DC magnetization measurements. Figure 5a and b shows the temperature-dependent magnetization (M vs T) under zero field-cooled (ZFC) and fieldcooled (FC) conditions (H ) 100 Oe) for the (In0.95Fe0.05)2O3 and (In0.9Fe0.1)2O3 nanoparticles, respectively. The curves clearly indicate two types of contribution to the total magnetization: (i) a paramagnetic-like contribution, which shows an increase of susceptibility at lower temperatures, and (ii) a finite ferromagnetic type contribution, which is present even at RT. There is a clear divergence between the FC and ZFC curves around 300 K, indicating that the systems could be weakly ferromagnetic at RT. The steep increase of magnetiztion with decreasing temperatures below 50 K in the magnetization is characteristic of all DMS materials and is probably related to the defect structure and possible fraction of Fe atoms, which are not participating in the long-range ferromagnetic order. Similar behavior has been reported by Jayakumar et al.11 for the polycrystalline Fe-doped In2O3 samples. The ferromagnetism in the sample is further confirmed from the M versus H at room temperature, and the magnetic hysteresis loops for the (In0.95Fe0.05)2O3 and (In0.9Fe0.1)2O3 nanoparticles are presented in Figure 6. The coercive field (HC) and remanence magnetization values at 300 K are 55 Oe, 0.019 emu/g and 30 Oe, 0.027 emu/g for the (In0.95Fe0.05)2O3 and (In0.9Fe0.1)2O3 nanoparticles, respectively. The decrease in coercive field with

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Figure 7. EPR spectra of Fe-doped indium oxide nanoparticles at 100 and 300 K.

Figure 6. Magnetization as a function of applied field (H) at 300 K for (top) In2O3 and (bottom) (In1-xFex)2O3 nanoparticles. Insets expand the lower field region to show the presence of remanence and coercivity.

increasing dopant concentration may be attributed to particle size effects.18 The maximum moment observed at 300 K (H ) 30 kOe) is 0.025 µB and 0.045 µB per Fe ion for (In0.95Fe0.05)2O3 and (In0.9Fe0.1)2O3 nanoparticles, respectively. To ensure that no contamination by magnetic impurities occurred during the synthesis process, the magnetic behavior of undoped In2O3 prepared under similar reaction conditions was recorded. The sample was found to be diamagnetic as expected (Figure 6). It is evident that the observed maximum magnetization is significantly lower than the theoretically expected spin-only value of saturation magnetization for a ferromagnetically ordered system. To investigate the actual reason for the observed ferromagnetism and low value of magnetic moment per Fe ion, we have carried out the EPR and Mo¨ssbauer studies. EPR Studies. EPR spectroscopy is an accessible dopantspecific spectroscopic technique that probes transitions within Zeeman-split ground states of paramagnetic ions. The temperature-dependent changes in the line position, line width, and integrated intensity can be used to obtain information about the long-range magnetic ordering, spin glass behavior, and spin fluctuaton, etc. Thus, to probe the exact electronic configuration and oxidation state of the dopant ion more critically and also to understand the ferromagnetic mechanism of the sample at a microscopic level, EPR measurements were carried out for the Fe-doped indium oxide nanoparticles. Figure 7 shows the EPR

Figure 8. EPR spectra of (In0.9Fe0.1)2O3 nanoparticles in the temperature range 100-300 K.

spectra of (In0.95Fe0.05)2O3 and (In0.9Fe0.1)2O3 at 100 and 300 K. The room-temperature EPR spectrum of (In0.9Fe0.1)2O3 consists of superposition of three overlapping signals, an intense and very broad signal (∆Hpp ) 1550 G) with g ) 2.2655 (signal A), and two relatively weak and narrow signals at g ) 2.0035 (signal B) and g ) 4.3033 (signal C). The broad signal is attributed to ferromagnetic resonance (FMR) arising from the exchange interaction between Fe3+ ions, which would result in the occurrence of long-range ferromagnetic ordering.19-21 Signal B is identified as uncoupled Fe3+ ions at In3+ sites having nearly octahedral coordination,22,23 whereas signal C may be due to Fe3+ ions in rhombic symmetry.24 The EPR spectrum of (In0.95Fe0.05)2O3 at RT shows almost 40% reduction in the intensity of signal A. Thus, in corroboration with magnetic data, EPR results also support the coexistence of ferromagnetism and paramagnetism in Fe-doped In2O3 nanoparticles. Temperature-dependent EPR measurements of (In0.9Fe0.1)2O3 nanoparticles in the temperature range 100-300 K range are presented in Figure 8. As expected for a ferromagnetic signal, the width of signal A increases and signal shifts to lower fields

Colloidal Fe-Doped Indium Oxide Nanoparticles

Figure 9. Room-temperature Mo¨ssbauer spectra for (a) (In0.95Fe0.05)2O3 and (b) (In0.9Fe0.1)2O3 nanoparticles.

with the lowering of the temperature. The observed broadening of the ferromagnetic resonance and a shift of the center of the resonance to the lower fields is due to the presence of a nonhomogeneous local magnetic field, which changes both the resonance field and the line shape of the FMR signals.25 It will be worthwhile to mention here that the temperature-dependent changes in the line width and line position are not observed in the paramagnetic state, that is, for signals B and C, except for their intensities, which show an increase with the lowering of the temperature. To rule out the possibility of formation of impurity phases during the synthesis, which may be responsible for the observation of the room-temperature ferromagnetism, EPR spectra were also recorded for Fe3O4 nanoparticles and commercial R-Fe2O3 sample. In fact, the thermal decomposition of Fe(acac)3 in hexadecylamine (HDA) in the temperature range 230-250 °C is known to give Fe3O4 nanoparticles.26 Accordingly, Fe3O4 nanoparticles (particle size ∼6 nm) were synthesized under the same reaction conditions without adding the indium precursor. For comparison, EPR was also recorded for a commercial sample of R-Fe2O3. The room-temperature EPR spectra of both of the samples are much different from those observed for Fedoped In2O3 nanoparticles. The EPR spectrum of R-Fe2O3 sample shows a very weak signal, while the spectrum for Fe3O4 nanoparticles consists of an intense signal with ∆Hpp ) 1780 G and g ) 2.3676 (Figure S4, Supporting Information). Thus, the presence of Fe2O3 and Fe3O4 clusters as secondary phases can be ruled out. Mo¨ssbauer Spectroscopy. To probe the local magnetic environment prevailing around the Fe sites and also to determine the oxidation state of Fe in the In2O3 matrix, Mo¨ssbauer spectra have been recorded for (In1-xFex)2O3 (x ) 0.05, 0.1) nanoparticles at RT. Figure 9 shows the Mo¨ssbauer spectra recorded for the Fe-doped indium oxide nanoparticles. No magnetic hyperfine pattern is obtained, and the line widths are broad. The fitting of the data indicates the presence of two paramagnetic quadrupole split symmetric doublets. The isomer shifts (IS) for larger absorption quadrupole doublet (signal A) and smaller absorption quadrupole doublet (B) are found to be 0.305 and 0.350 mm s-1 for (In0.95Fe0.05)2O3 nanoparticles. The corresponding values for (In0.9Fe0.1)2O3 nanoparticles are 0.302 (signal A) and 0.303 mm s-1 (signal B). Other fit parameters are given in Table S1, Supporting Information. The isomer shift values clearly indicate that the Fe ion is present in +3 oxidation state in the lattice, and thus the presence of Fe3O4 clusters in the

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Figure 10. Magnetization as a function of applied field (H) at 300 K for colloidal (In0.9Fe0.1)2O3 nanoparticles annealed in oxygen atmosphere.

nanoparticles may be ruled out. Recently, Venkatesan et al.27 have reported room-temperature ferromagnetism in Fe-doped indium-tin-oxide (ITO) films. On the basis of the Mo¨ssbauer spectra obtained at RT, they argue that the observed ferromagnetism in the doped films is due to the presence of the magnetite impurity phase. The Mo¨ssbauer spectra obtained by us do not show any features related to secondary phases such as Fe3O4 or γ-Fe2O3, which are ferromagnetic having inverse spinel structure. The origin of ferromagnetism in transition-metal-doped semiconductors has been controversial and has been explained by different mechanisms such as carrier-mediated interaction.28-30 However, in semiconductors with low carrier densities such as oxides, the novel type of magnetism can be explained by magnetic polaron mechanism.31 In this model, the spins of magnetic dopants incorporated into the semiconductor lattice interact through a donor-impurity band, formed by lattice defects such as oxygen vacancies (VO). Coey et al.32 explain the spin alignment of the 3d transition-metal cations by the coupling of their spins, which are antiparallel to the spin of donor electrons. Because of this coupling, all spins within this expanded orbit are aligned. Because of the overlap of different orbits, an impurity band is formed, aligning a huge number of 3d magnetic spins parallel, resulting in ferromagnetism. In the framework of this theory, not only the dopant concentration but also the number of donor electrons must be quite large to obtain ferromagnetism. Experimentally, there is much indirect evidence that the ferromagnetism is related to defects. For example, Rubi et al.33 demonstrate that the magnetism of Co-doped ZnO powders can be switched reversibly from ferro- to paramagnetic behavior by annealing in either oxygen-poor or oxygen-rich atmosphere, resulting in the generation or annihilation of VO, respectively. Other studies where Co-doped ZnO films are successively treated in reducing and oxidizing atmospheres show similar behavior.34 The magnetism of Co- or Mn-doped ZnO is strongly influenced by surface capping.35 Archer and Gamelin36 have shown that the magnetism of colloidal Ni-doped SnO2 nanoparticles can be activated by interparticle contact, hence grain-boundary defects. From these results, it is clear that not only the dopant concentration but also the defect concentration has to be substantial enough to get a ferromagnetic coupling of the magnetic spins. Nanoparticles possess a lot of such defect

3606 J. Phys. Chem. C, Vol. 113, No. 9, 2009 sites because of their comparatively large surface, which has many defects in the form of unsaturated bonds. The Fe-doped indium oxide nanoparticles prepared by us are highly crystalline as revealed by HRTEM investigations, and surface-bound defects are passivated by the HDA ligand. We believe that due to this fact the defect concentration is low, and hence less number of donor electrons is available in the sample to form extended defect bands, which would align the 3d magnetic spins in large domains. Therefore, only weak ferromagnetism is observed. To corroborate our belief, we have carried out additional magnetic studies on the (In0.9Fe0.1)2O3 nanoparticles, which were annealed in O2 atmosphere for different time periods (4 and 24 h) at 220 °C. As expected, the samples show typical superparamagnetic behavior (Figure 10). Annealed samples were analyzed by XRD studies, finding in both cases that their structures remain unmodified. This supports the idea that only the defects structure of the samples is varied upon annealing. In the case of O2 annealing, the most plausible effect is the annihilation of intrinsic oxygen vacancies, which may further affect the alignment of 3d magnetic spins in large domains, resulting in quenching of ferromagnetism. Conclusions In conclusion, we have prepared dispersible, highly crystalline, and colloidal Fe-doped indium oxide nanoparticles by a simple one-pot thermal decomposition of In and Fe precursors in hexadecylamine in high yields at temperatures as low as 220 °C. Considering the microstructure, EPR, Raman, and magnetic studies together, we believe that Fe element incorporates into the lattice frame of the indium oxide by substituting the position of indium atoms. Magnetic measurements show a weak ferromagnetic behavior of the nanoparticles at room temperature, which results from the low defect concentration. Thus, less number of donor electrons is available in the sample to form extended defect bands, which would align the 3d magnetic spins in large domains. The current synthetic method can also be extended to the large-scale production of other transition-metaldoped indium oxide nanoparticles. Acknowledgment. We thank Drs. A. Asthana and A. Vinu, NIMS, Tsukuba, for providing HRTEM data. Supporting Information Available: Strain data for the pure and Fe-doped indium oxide nanoparticles, Rietveld refined profile of XRD data collected on rotating anode X-ray diffractometer, EDX data for the Fe-doped In2O3 nanoparticles, and EPR data for the Fe2O3 and Fe3O4 nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hong, N. H.; Sakai, J.; Huong, N. T.; Ruyter, A.; Brize´, V. J. Phys.: Condens. Matter 2006, 18, 6897.

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