Colloidal Chromium-Doped In2O3 Nanocrystals as Building Blocks for

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2008, 112, 17755–17759 Published on Web 10/28/2008

Colloidal Chromium-Doped In2O3 Nanocrystals as Building Blocks for High-TC Ferromagnetic Transparent Conducting Oxide Structures Shokouh S. Farvid, Ling Ju, Matthew Worden, and Pavle V. Radovanovic* Department of Chemistry, UniVersity of Waterloo, 200 UniVersity AVenue West, Waterloo, Ontario N2L 3G1, Canada ReceiVed: September 03, 2008; ReVised Manuscript ReceiVed: October 09, 2008

Colloidal free-standing Cr3+-doped In2O3 nanocrystals were synthesized in oleylamine from indium (III) and chromium (III) acetylacetonate precursors. The nanocrystals were treated with trioctylphosphine oxide to remove surface-bound dopant ions and ensure internal doping. The lattice resolved transmission electron microscopy images reveal that nanocrystals are faceted and highly crystalline, with no evidence of a secondary phase formation. The average doping concentration estimated with energy dispersive X-ray spectroscopy at the single nanocrystal level agrees with the average doping concentration from the analogous nanocrystal ensemble measurement. Ligand-field electronic absorption spectroscopy suggests that Cr3+ dopants are preferentially substituted for In3+ ions in their trigonally distorted octahedral (b) sites in In2O3 nanocrystals. Nanocrystalline films, prepared under mild conditions using colloidal Cr3+-doped In2O3 nanocrystals as building blocks, exhibit robust room temperature ferromagnetism. Structural and compositional analyses combined with the ligand-field spectroscopy indicate intrinsic ferromagnetism in this material. The ability to rationally synthesize and manipulate a new form of transition-metal-doped In2O3 nanocrystals opens up new opportunities for spintronics applications and may provide a framework for understanding the origin of ferromagnetism in transparent conducting oxides. Introduction Multifunctional materials of reduced dimensionality have become an increasingly active research area in chemistry and physics at the nanoscale. Semiconductor spin-electronics (spintronics), for example, relies on the mutual interactions of electron spins and charges in semiconductor materials, enabling a more efficient information manipulation and an introduction of new functionalities into electronic devices.1 Nanostructured diluted magnetic semiconductors (DMSs)2 having high ferromagnetic phase transition temperatures (TC) have been identified as promising materials for this emerging technology.3 Transparent conducting oxides (TCOs), such as ZnO, TiO2, and SnO2 have attracted particular interest as host lattices for high-TC DMSs, due to their stability, electrical conductivity, and optical transparency.4-7 Among the technologically most important TCOs is In2O3, which has been employed in batteries, solar cells, electrodes, displays, and sensors.8,9 In2O3 is an n-type widebandgap (∼3.75 eV) semiconductor with both high charge carrier density and mobility.8-10 These characteristics make it especially promising host lattice for the preparation of charge carrier-mediated transparent magnetic semiconductors11 and for the fabrication of integrated opto-spintronic devices. The origin of ferromagnetism in magnetically doped semiconductor materials has long been under debate, due in part to the difficulties in demonstrating a direct correlation between the dopant speciation at the nanoscale and the macroscopically observed magnetic properties.12 Rationally designed synthetic * To whom correspondence [email protected].

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methods for the preparation of particular semiconductor nanocrystalline systems doped with selected paramagnetic transitionmetal impurities provide an opportunity to establish such a correlation. Specifically, the solution phase preparations of colloidal nanocrystals (NCs) are performed under mild conditions in comparison to typical solid-state synthetic methods, and allow for a uniform diffusion of reactants during the nanocrystal growth. Because of small sizes of free-standing NCs, the formation of secondary phases within individual NCs is not likely. Furthermore, colloidal NCs can be used as building blocks for the bottom-up assembly of functional devices13 and may enable microscopic understanding of transition-metal dopant spin ordering and the origin of ferromagnetism in semiconductors.5,12 While the syntheses of colloidal In2O3 NCs with cubic bixbyite crystal structure have been reported,14,15 to our knowledge there have been no reports on the controlled synthesis of colloidal transition-metal-doped In2O3 NCs. Here we report the first synthesis and characterization of free-standing colloidal Cr3+-doped In2O3 (Cr3+:In2O3) NCs and demonstrate roomtemperature ferromagnetism in thin films fabricated from these NCs. Chromium (III) ion (d3 system with total spin quantum number of S ) 3/2) is chosen as a kinetically inert species with a relatively large magnetic moment (3.87 µB calculated spinonly magnetic moment) and a high affinity for six-coordinate sites16 if substituted for In3+ ions in In2O3. It is also isoelectronic with In3+ eliminating the considerations associated with charge compensation and Coulomb repulsion. There have been a few conflicting reports recently about the magnetic properties of bulk Cr3+:In2O3.17-19 Some researchers have suggested intrinsic  2008 American Chemical Society

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charge carrier-mediated ferromagnetic ordering of chromium dopants,17,18 while others have reported paramagnetic behavior.19 These differences reflect the sensitivity of DMS properties with the preparation conditions, fueling a debate about the origin of magnetic behavior in this class of materials. The methodology reported in this letter allows for the preparation of Cr3+:In2O3 NCs that can be fully dispersed in organic solvents yet can be large enough for single NC analysis. Furthermore, chromium compounds and related solid-state phases do not form or precipitate under the described synthetic conditions. Combined ligand-field electronic absorption spectroscopy and a single NC characterization provide a direct correlation between chromium dopant speciation in individual NCs and the magnetic properties of the corresponding Cr3+:In2O3 nanocrystalline films. Experimental Methods Synthesis of Cr3+-Doped In2O3 Nanocrystals. Colloidal Cr3+:In2O3 NCs were synthesized by applying a modified procedure developed for preparation of pure In2O3 NCs.14 In a typical preparation of Cr3+:In2O3 NCs, 4 mmol of indium (III) acetylacetonate (In(acac)3) and 0.2 mmol of Cr(acac)3 (5 at % of chromium with respect to indium) were added to 48 mmol of oleylamine in a two-neck round-bottom flask. The reaction mixture was slowly heated to 250 °C and refluxed at that temperature for 7 h under argon. The obtained viscous suspension was then cooled to room temperature. The resulting NCs were precipitated and washed three times with ethanol to remove unreacted precursors and intermediates. The obtained green precipitate was added to melted trioctylphosphine oxide (TOPO) and heated at 150 °C for 1 h, followed by precipitation with ethanol. This procedure was repeated three times to ensure the removal of NC surface-bound dopant ions.7 TOPO-capped NCs were finally dispersed in hexane or toluene, giving completely clear colloidal suspensions. The described synthetic methodology allowed us to obtain In2O3 nanocrystals with average diameters ranging from ca. 4 to 17 nm by changing the reaction temperatures from 200 to 300 °C, respectively. We also performed a control experiment using the same procedure but without indium precursor in the reaction mixture. Synthesis of Nanocrystalline Cr2O3. Nanocrystalline Cr2O3, the most common oxide of chromium, was prepared by a different method20 in order to compare its structural and magnetization properties with the analogous properties of Cr3+: In2O3 NCs. Nanocrystalline Cr2O3 was prepared by calcination of a dried gel of Cr(OH)3 precipitate. The Cr(OH)3 was precipitated from a solution of 0.05 M CrCl3 · 6H2O with 0.18 M NaOH. The resulting gel was filtered and washed several times with deionized water. The obtained amorphous Cr(OH)3 · xH2O precursor was dried overnight at 60 °C, and calcined in air for 3 h at 350 °C. Characterization and Measurements. The transmission electron microscopy (TEM) and scanning TEM (STEM) images and the energy dispersive X-ray (EDX) spectra were obtained with JEOL 2010F microscope operating at 200 kV. The elemental analysis was performed with Varian Liberty Series II inductively coupled plasma atomic emission spectrometer (ICP-AES). The XRD patterns were recorded with Bruker AXS D8 powder diffractometer using CuKR radiation (λ ) 1.5418 Å) for free-standing nanocrystals and with PANalytical X’Pert PRO MRD for nanocrystalline films. The electronic absorption spectra were collected with a Varian Cary 5000 UV/vis/NIR spectrophotometer, and the magnetization was measured with the physical property measurement system (PPMS, Quantum Design) in ACSM mode. For magnetization measurements, the

Figure 1. (a) Overview TEM image of 2.7% Cr3+:In2O3 NCs. (b) A lattice resolved TEM image of a typical Cr3+:In2O3 NC showing the {222} facet. (c) STEM image of Cr3+:In2O3 NCs showing the areas over which the single NC (orange rectangle) and the background (bluegreen rectangle) EDX spectra were collected. (d) EDX spectra of the single 2.5% Cr3+:In2O3 NC (orange trace) and the background (bluegreen trace), corresponding to STEM image in panel c (inset: magnified Cr peak).

colloidal NCs were spin-coated at least 10 times on the clean accurately weighed sapphire substrates, followed by mild annealing at 350 °C for 1 min between consecutive coatings. The final nanocrystalline films were weighed again on an analytical balance in order to determine the magnetization per unit mass of the samples. All samples were handled identically under carefully controlled magnetic contamination-free conditions. Results and Discussion A low-resolution TEM image of Cr3+:In2O3 NCs is shown in Figure 1a. The NCs have nearly spherical shapes, and the average diameter of ca. 13 nm. Figure 1b shows a high resolution TEM image of a typical NC having high crystallinity and well-defined faceting with no evidence of chromium segregation within the NC. The average NC lattice spacing of 2.92 Å corresponds to the {222} lattice plane of In2O3 (JCPDS no. 06-0416). The average concentration of Cr3+ dopants with respect to all In3+ sites in In2O3 NCs was accurately determined with ICP-AES to be 2.7 at % (In1.946Cr0.054O3). Spatially resolved chemical analysis at the nanoscale has been shown to be a valuable tool for gaining new insights into the nature of dopant ion distribution in magnetic semiconductors.12 Figure 1c shows a STEM image of representative NCs analyzed by EDX spectroscopy. The single NC and the background EDX spectra shown in Figure 1d were collected over the areas marked with the orange and blue-green rectangles, respectively. Indium and chromium can be readily detected in the NC, while no signal is recorded from the background spectrum. Magnified chromium peak is shown as an inset in Figure 1d. The concentration of Cr3+ dopants was estimated to be 2.5 at % based on the observed signal intensities. The average chromium concentration in 30 different randomly selected NCs was ca. 3.5%, which is in very

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Figure 2. XRD pattern of free-standing 2.7% Cr3+:In2O3 NCs (green trace), and the nanocrystalline film (red trace) prepared from the same NCs. Blue lines are the reflections of bulk cubic In2O3.

good agreement with the doping concentration determined from EDX measurements performed under identical conditions on a large number of NCs (Figure S1, Supporting Information). Even though the uncertainty in EDX concentration determinations can be significant for low doping levels, this agreement indicates that Cr3+ is predominantly doped in In2O3 NCs and does not tend to form a separate phase outside of the NCs. Importantly, Cr(OH)3, Cr2O3, or other chromium-related solid-state phases cannot be formed or precipitated under the given synthetic conditions, based on the control preparation performed using only Cr(acac)3 in the reaction mixture. Figure 2 shows the X-ray diffraction (XRD) patterns of freestanding 2.7% Cr3+:In2O3 NCs (green trace) and a film prepared from the same NCs (red trace) collected over 10 h. The XRD patterns confirm that these nanostructures have a cubic bixbyite crystal structure characteristic of bulk In2O3 (blue vertical lines, JCPDS no. 06-0416). No secondary phases were detected either in the NCs or in the nanocrystalline film. The average NC size estimated from broadening of the XRD peaks using the Debye-Scherrer equation is 13 nm, which agrees well with the average size determined from TEM measurements. The XRD spectrum of the nanocrystalline film implies no preferential orientation of the nanocrystals deposited on the substrate. Indium oxide crystallizes the cubic bixbyite, or C-type rare earth sesquioxide, crystal structure.21 The bixbyite structure can be derived from the fluorite structure by removing one-fourth of the anions and slightly offsetting the positions of the remaining anion sites. A unit cell of indium oxide has a total of 80 atoms, including 32 In3+ cation sites. The In3+ cations reside in two nonequivalent six-coordinate sites (Figure 3a). One-fourth of In3+ cations reside in a slightly trigonally compressed octahedral coordination (b sites, D3d point group) having six equidistant oxygen neighbors at 2.18 Å. Three-fourths of In3+ cations are located in highly distorted octahedral (d) sites, with three different oxygen distances of 2.13, 2.19, and 2.23 Å. Figure 3b shows an electronic absorption spectrum of colloidal 2.7% Cr3+:In2O3 NCs. The band gap transition of the host In2O3 NCs is observed as a broad shoulder at ∼32 500 cm-1. The band gap energy is estimated from the absorption spectrum to be ca. 30 250 cm-1 (∼3.75 eV) (Figure S2, Supporting Information), similar to the band gap energy of bulk In2O3.8 Since the Bohr radius of In2O3 is ca. 1.3 nm,8 which is significantly smaller than the average radius of the NCs obtained in this work, strong quantum confinement effects are not expected. The absorption spectrum of the concentrated suspension of the same NCs reveals two transitions at ∼16 250 cm-1 and 21 300 cm-1 which can be readily assigned to 4A2 gf4T2 g (F) and 4A2 gf4T1 g (F) ligand-field transitions, respectively, of pseudo-octahedral Cr3+ coordinated with six O2- ions.16 The 4A2 gf4T1 g (F) transition is observed as a shoulder due to tailing of the In2O3 NC band gap transition into the visible region. A small but observable shoulder at 14 400 cm-1 can be assigned

Figure 3. (a) Schematics of nonequivalent b and d indium cationic sites (In) with respect to oxygen (O) and oxygen vacancy (VO) sites in the cubic bixbyite In2O3. (b) Electronic absorption spectrum of colloidal 2.7% Cr3+:In2O3 NCs, recorded at 300 K. Ligand-field transitions of Cr3+ are collected on 100 times concentrated NC suspensions (inset: photograph of colloidal 2.7% Cr3+:In2O3 NCs dispersed in toluene).

to the 4A2 gf2T2 g, 2Eg doublet (Figure 3, inset). Trigonal b sites, approximated to the ideal octahedral geometry in Figure 3a, are similar to the Al3+ sites in Al2O3, which have C3V point group.22 Very similar Cr3+ ligand-field transition energies and band shapes to those in Figure 3 are observed for the analogous transitions in Cr3+:Al2O3.22 The examples of Cr3+ in the coordination environment corresponding to In2O3 d site are much more difficult to identify. A reduction in symmetry of octahedral Cr3+ however results in additional splittings in the Cr3+ energy level diagram16,23 and an increase in the absorption intensity.24 From the similarity of the Cr3+ spectra in In2O3 and Al2O3, and the lack of additional spectral features in Figure 3 we conclude that Cr3+ ions are internally doped in In2O3 NCs, and preferentially substitute In3+ cations in their b sites. Coincidentally, these sites have also been suggested to be the main Sn4+ doping sites in indium-tin-oxide.21 The magnetization properties of 2.7% Cr3+:In2O3 NCs measured from 5 to 300 K are shown in Figure 4. Free-standing NCs show only paramagnetism at all temperatures (Figure 4a, green squares). Nanocrystalline films fabricated from the same NCs exhibit a rapid magnetization (M) saturation and a small hysteresis coercivity, indicating ferromagnetic ordering. Hysteresis loops for nanocrystalline films at 5 and 300 K are shown in Figure 4a as red circles and squares, respectively. A decrease in magnetization and hysteresis coercivity with temperature is observed. The temperature dependence of the ferromagnetic saturation magnetization (Ms) is plotted in Figure 4b. The ferromagnetic phase transition is not observed indicating TC above 300 K. The saturation magnetization per dopant ion in the nanocrystalline film was estimated based on the unit mass magnetization and the elemental composition of the sample. The NCs anchor a significant amount of TOPO ligands and the occluded solvent molecules, which must be taken into account when calculating the magnetic moment per Cr3+ in nanocrystalline Cr3+:In2O3 structures. From the phosphorus ICP-AES analysis, we determined the amount of TOPO ligands associated with the NCs in the nanocrystalline film after additional drying. The mass of the dried Cr3+:In2O3 NCs in the film was then

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Letters owing to the spins at nanoparticle surfaces.20,27 Nanocrystalline Cr2O3 has a distinctly different magnetic behavior from Cr3+: In2O3 nanocrystalline film, showing no ferromagnetism at room temperature, and only superparamagnetic behavior at low temperatures (Figure S4, Supporting Information). Taken together, our results suggest the intrinsic ferromagnetism in Cr3+: In2O3 nanocrystalline structures fabricated from colloidal Cr3+: In2O3 NCs as building blocks. Conclusions

Figure 4. (a) M vs H data for free-standing 2.7% Cr3+:In2O3 NCs measured at 300 K (green squares), and the corresponding nanocrystalline films measured at 5 K (red circles) and 300 K (red squares). All loops are corrected for diamagnetic contribution. (b) Temperature dependence of Ms for Cr3+:In2O3 nanocrystalline films.

corrected for this amount of TOPO. This corrected mass, which represents the upper limit mass of bare 2.7% Cr3+:In2O3 NCs is used to determine the magnetization per dopant ion. The saturation magnetization of the nanocrystalline film in Figure 4 is estimated to be 0.17 Bohr magneton (µB) per Cr3+ at 300 K and 0.27 µB/Cr3+ at 5 K. The origin of ferromagnetism in nanocrystalline TCOs is of great fundamental and practical importance. Long-range magnetic ordering in nanocrystalline Co2+:ZnO,7 Ni2+:ZnO,5 and Ni2+:SnO26 prepared from analogous colloids has been attributed to an increase in the magnetic domain volumes through interparticle electronic coupling and to the generation of charge carriers through the formation of interfacial defects. The observation of ferromagnetism in Cr3+:In2O3 nanocrystalline film is consistent with the previous findings, implying that the interfacial defect-induced charge carrier mediation of the dopant spin ordering is a general property of nanocrystalline transition-metal-doped TCOs properly prepared from colloidal NCs. While the magnetic moment per Cr3+ varies from sample to sample, it is roughly 5-10 times smaller than the values reported for Cr3+:In2O3 thin films (the highest reported value is 1.5 µB).17 The variation in the Cr3+ magnetic moments and their smaller values in comparison to those in Cr3+:In2O3 thin films17 can be associated with a difference in interactions between NCs in particular nanocrystalline films and the presence of TOPO on NC surfaces.5,6 It has been suggested that charge carriers arising from interfacial oxygen vacancies may also be responsible for the observation of ferromagnetism in pure nanocrystalline oxides.25 This effect is not evident in In2O3 nanocrystalline films prepared under the conditions described in this work (Figure S3, Supporting Information). The magnetization observed in pure oxide dielectrics, such as HfO2 thin films, has also been associated with magnetic contamination resulting from handling the substrates with inadequate tools.26 We handled identically all of our samples (doped and undoped) in a carefully controlled nonmagnetic environment. It should be also noted that nanocrystalline antiferromagnetic materials, such as Cr2O3, can generate spontaneous magnetic ordering at high temperatures

In summary, we reported the first synthesis of colloidal freestanding Cr3+:In2O3 NCs and demonstrated a direct correlation between the average doping concentration determined from the single NC and ensemble NC analyses. This correlation combined with the lattice resolved TEM and the dopant specific ligandfield spectroscopic data demonstrates internal Cr3+ doping in these NCs. Ligand-field electronic absorption spectroscopy suggests that Cr3+ is preferentially substituted for In3+ ions in their b sites in In2O3 NCs. Nanocrystalline films fabricated from colloidal Cr3+:In2O3 NCs exhibit ferromagnetism with TC higher than 300 K, consistent with Cr3+ magnetic moment ordering mediated by charge carriers originating from interfacial NC defects. The control of size and composition of colloidal Cr3+: In2O3 NCs and of their assembly into nanocrystalline films will allow us in the future to study the effect of spinodal decomposition attributed by some authors as the potential origin of ferromagnetism in DMSs.12 The future studies will also explore other dopants in colloidal In2O3 NCs, and the microscopic origins of ferromagnetism in these multifunctional materials, which should lead to an improvement in their magnetic and electrical properties for potential spintronic applications at the nanoscale. Acknowledgment. This work was supported by NSERC (Discovery and RTI grants) and the University of Waterloo. P.V.R. thanks Canada Research Chairs Program for their support. Supporting Information Available: EDX area spectrum of a large number of Cr3+:In2O3 NCs, absorption spectra of In2O3 NC bandgap transition, magnetization data of nanocrystalline In2O3 and Cr2O3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (b) Fert, A. Angew. Chem., Int. Ed. 2008, 47, 5956. (2) Furdyna, J. K. J. Appl. Phys. 1988, 64, R29. (3) (a) Ohno, H. Science 1998, 281, 951. (b) Chambers, S. A. Mater. Today 2002, 5, 34. (4) (a) Chambers, S. A. Surf. Sci. Rep 2006, 61, 345. (b) Matsumoto, Y.; Murakami, M.; Shono, T.; Hasegawa, T.; Fukumura, T.; Kawasaki, M.; Ahmet, P.; Chikyow, T.; Koshihara, S.; Koinuma, H. Science 2001, 291, 854. (c) Ogale, S. B.; Choudhary, R. J.; Buban, J. P.; Lofland, S. E.; Shinde, S. R.; Kale, S. N.; Kulkarni, V. N.; Higgins, J.; Lanci, C.; Simpson, J. R.; Browning, N. D.; Das Sarma, S.; Drew, H. D.; Greene, R. L.; Venkatesan, T. Phys. ReV. Lett. 2003, 91, 077205. (d) Ueda, K.; Tabata, H.; Kawai, T. Appl. Phys. Lett. 2001, 79, 988. (e) Yuhas, B. D.; Zitoun, D. O.; Pauzauskie, P. J.; He, R.; Yang, P. Angew. Chem., Int. Ed. 2006, 45, 420. (f) Fitzgerald, C. B.; Venkatesan, M.; Douvalis, A. P.; Huber, S.; Coey, J. M. D.; Bakas, T. J. Appl. Phys. 2004, 95, 7390. (5) Radovanovic, P. V.; Gamelin, D. R. Phys. ReV. Lett. 2003, 91, 157202. (6) Archer, P. I.; Radovanovic, P. V.; Heald, S. M.; Gamelin, D. R. J. Am. Chem. Soc. 2005, 127, 14479. (7) Schwartz, D. A.; Norberg, N. S.; Nguyen, Q. P.; Parker, J. M.; Gamelin, D. R. J. Am. Chem. Soc. 2003, 125, 13205.

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J. Phys. Chem. C, Vol. 112, No. 46, 2008 17759 Gupta, A.; Cao, H.; Parekh, K.; Rao, K. V.; Raju, A. R.; Waghmare, U. V. J. Appl. Phys. 2007, 101, 09N513. (19) Berardan, D.; Guilmeau, E.; Pelloquin, D. J. Magn. Magn. Mater. 2008, 320, 983. (20) Banobre-Lopez, M.; Vazquez-Vazquez, C.; Rivas, J.; LopezQuintela, M. A. Nanotechnology 2003, 14, 318. (21) Gonzalez, G. B.; Cohen, J. B.; Hwang, J.-H.; Mason, T. O.; Hodges, J. P.; Jorgensen, J. D. J. Appl. Phys. 2001, 89, 2550. (22) McClure, D. S. J. Chem. Phys. 1962, 36, 2757. (23) (a) Dubicki, L.; Hitchman, M. A.; Day, P. Inorg. Chem. 1970, 9, 188. (b) Keeton, M.; Fa-Chun Chou, B.; Lever, A. B. P. Can. J. Chem. 1971, 49, 192. (24) Decurtins, S.; Gudel, H. U.; Neuenschwander, K. Inorg. Chem. 1977, 16, 796. (25) Sundaresan, A.; Bhargavi, R.; Rangarajan, N.; Siddesh, U.; Rao, C. N. R. Phys. ReV. B 2006, 74, 161306. (26) Abraham, D. W.; Frank, M. M.; Guha, S. Appl. Phys. Lett. 2005, 87, 252502. (27) Winkler, E.; Zysler, R. D.; Vasquez Mansilla, M.; Fiorani, D. Phys. ReV. B 2005, 72, 132409.

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