Influence of the Host Lattice Electronic Structure on Dilute Magnetic

Jan 8, 2013 - The effect of the host lattice structure on the spectroscopic and magnetic properties of Cr3+-doped In2O3 nanocrystals is reported...
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Influence of the Host Lattice Electronic Structure on Dilute Magnetic Interactions in Polymorphic Cr(III)-Doped In2O3 Nanocrystals Shokouh S. Farvid, Manu Hegde, and Pavle V. Radovanovic* Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada S Supporting Information *

ABSTRACT: The effect of the host lattice structure on the spectroscopic and magnetic properties of Cr3+-doped In2O3 nanocrystals is reported. The influence of the dopant ions on the nanocrystal growth allows for the solution-phase stabilization and separation of doped colloidal In2O3 nanocrystals having different crystal structures − stable cubic phase (bcc-In2O3) and metastable rhombohedral (rh-In2O3) phase − and comparative study of the electronic structure and magnetic properties of Cr3+ in both polymorphs. Investigations by a range of complementary spectroscopic techniques, including Raman, X-ray absorption and magnetic circular dichroism spectroscopies, revealed that the change in the In2O3 phase leads to distinctly different electronic structure of Cr3+ dopants, associated with a different nature of the substitutional doping sites and different electronic structure of the nanocrystal host lattice. Nanocrystalline films prepared from colloidal nanocrystals exhibit ferromagnetism at room temperature, although the average magnetic moment of Cr3+ in rh-In2O3 is an order of magnitude smaller than that in bcc-In2O3 samples. This difference in magnetization is associated with wider band gap of rh-In2O3 nanocrystals, which prevents effective hybridization of the defect donor band, as a mediator of the Cr3+ magnetic exchange interactions, and the Cr3+ 3d states at the Fermi level. The results of this work demonstrate that a change in the defect and electronic structures of the same semiconductor host lattice by nanocrystal phase control in solution allows for tuning of the magnetic properties of diluted magnetic semiconducting oxides. KEYWORDS: nanocrystal doping, colloidal nanocrystals, diluted magnetic semiconducting oxide, transparent conducting oxide, indium oxide, chromium(III), ferromagnetism, electronic structure, polymorphism, phase transformation

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INTRODUCTION Diluted magnetic semiconductors (DMSs) continue to captivate the researchers in different science and engineering fields because of the richness of their magnetic and optical properties,1 and the associated technological promise, particularly in the areas of spintronics and photonics.2,3 The range of the observed phenomena is even wider in reduced dimensions because of the size, anisotropy, and surface area dependence of the electronic structure and properties of DMSs. The recent reports of unusual phenomena in DMS nanostructures include the different mechanisms of dopant incorporation relative to the analogous bulk systems,4−7 an enhancement of the photoexcitation-generated dopant-carrier exchange fields due to spatial confinement in colloidal DMS nanocrystals,8 orientation-dependent exchange interactions in DMS nanowires,9 and interfacial defect-mediated ferromagnetic ordering of dopant ions in DMS nanocrystalline films and aggregates.10−13 Following the first report of room temperature ferromagnetism in cobalt-doped anatase TiO2 thin films,14 the attention has turned to investigations of the possibility of ferromagnetism in doped transparent conducting oxides, and the origin of magnetic interactions in these diluted magnetic semiconducting oxides (DMSOs).15−18 Wide band gap semiconducting oxides © XXXX American Chemical Society

containing a few atom percent (at%) of paramagnetic dopant ions which can be ferromagnetically ordered at or above room temperature could be an attractive class of materials for integrated spintronic devices due to their transparency, relatively high n-type conductivity, and mechanical robustness and durability. However, the true origin of the observed ferromagnetism in such magnetically doped oxide lattices has been intensely debated in the chemistry and physics communities for over a decade, fueled by inconsistent reports and often controversial conclusions in the literature.19−23 The dependence of the magnetic properties of DMSOs on the preparation method, synthesis conditions and sample treatment is evident in case of transition metal-doped In2O3, which is the focus of this work. Room temperature ferromagnetism has been reported for Cr-, Fe-, Co-, and Ni-doped In2O3 bulk and thin films,23−26 and Fe- and Cr-doped In2O3 nanocrystals.11,27 On the other hand, Berardan et al.28 have shown that bulk Cr-, Mn-, Fe-, Ni-, and Cu-doped In2O3 samples are intrinsically paramagnetic. They observed ferromagnetism only for dopant concentrations exceeding the percolation threshold, which was Received: October 14, 2012 Revised: December 17, 2012

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polymorphs of In2O3 NCs have different electronic structure,34,41 which could in principle allow for manipulation of the electronic structure and magnetic properties of dopant ions substitutionally incorporated in different phases of the same host lattice. In this article, we compare the spectroscopic properties and magnetic behavior of Cr3+ dopants in colloidal In2O3 NCs having corundum- and bixbyite-type crystal structure. Chromium(III) is chosen as a dopant ion because of its strong preference for six coordinate sites and its documented kinetic stability.42 Although several experimental studies of Cr3+-doped bcc-In2O3 have been reported,11,24,26,31,43 to the best of our knowledge, there has been no detailed spectroscopic and magnetic investigation of Cr3+ in rh-In2O3. Consistent with the previous observations for different DMSO NC systems, the presence of Cr3+ ions in the reaction mixture critically determines the growth kinetics and size of the resulting colloidal NCs, allowing for the stabilization of metastable rhIn2O3 phase for higher Cr3+ precursor concentrations. The difference in symmetry and electronic structure of the cation sites in the two polymorphs results in distinctly different spectroscopic properties of substitutional Cr3+ ions. Importantly, the magnitude of the ferromagnetic interactions in Cr3+doped In2O3 (Cr3+:In2O3) nanocrystalline films is also dependent on the structure of the corresponding NC building blocks. Observation of the higher magnetic moment per Cr3+ in bcc-In2O3 compared to that in rh-In2O3 samples is attributed to the difference between the electronic band structure of bccIn2O3 and rh-In2O3 NC host lattices, as well as the difference in the electronic configuration of Cr3+in these two phases. This work demonstrates the possibility of tuning magnetic interactions in DMSO NCs by controlling their crystal structure and size in solution. Furthermore, better understanding of the correlation between the electronic structure of the host lattice and dopant ions, and the corresponding magnetic properties helps elucidate the mechanism of ferromagnetic ordering in DMSOs.

linked to the formation of secondary phases. A powerful argument against the prospect of ferromagnetism in DMSOs is that doping concentrations typical for ferromagnetic DMSOs are generally significantly lower than those required for any conventional description of the ferromagnetic exchange interactions among paramagnetic centers in oxides.29 Despite the lack of widely accepted theoretical framework for the explanation of dilute magnetic ordering, and the concerns about the secondary phase formation, a number of studies have presented convincing evidence of intrinsic room temperature ferromagnetism in DMSOs.10,11,26,30 Because of these studies, a consensus about the importance of defect states in facilitating magnetic exchange interactions among paramagnetic dopant ions has begun to form. For example, ferromagnetic ordering in nanocrystalline films prepared under mild conditions from colloidal DMSO NCs has been associated with extended interfacial structural defects (grain boundaries), which have been correlated with oxygen vacancies.10−12 Coey et al.16 have proposed that ferromagnetic exchange coupling in DMSOs is mediated by shallow donor electrons, arising from the presence of oxygen vacancies. These donor electrons have been proposed to form bound magnetic polarons, which overlap to generate a spin-split donor impurity band. According to this model reaching high Curie temperature (TC) requires hybridization and charge transfer from a donor-based impurity band to unoccupied 3d states at the Fermi level. A possible role of lattice defect states in mediating dopant ferromagnetic ordering in DMSOs requires a systematic study of the influence of the structure and size of colloidal NC building blocks on the magnetic properties of DMSO nanocrystalline films. Furthermore, the technological promise of DMSOs has generated significant interest in finding new means of manipulating magnetization and dopant-carrier interactions in these materials. The demonstrated approaches include controlling structural disorder in thin film samples,30 oxygen environment during the synthesis and in the postsynthesis treatment,24,26,31 and the type of ligands on DMSO NC surfaces.32 In addition to their functional properties discussed above, transparent conducting oxides are polymorphic materials, which could be stabilized in a variety of possible crystal structures. This structural versatility provides an opportunity for using phase transformation to tune electronic structure of and magnetic interactions in DMSOs. However, the extreme temperatures and pressures needed to induce phase transformation are often not compatible with dopant incorporation or retention in the host lattice. In2O3 crystallizes in two known forms: bixbyite (bcc-In2O3, bcc = body-centered cubic) as a stable phase and corundum (rh-In2O3, rh = rhombohedral) as a metastable phase.33−35 Rhombohedral phase is traditionally obtained by high-temperature and/or high-pressure treatment of cubic In2O3. The bcc-In2O3 phase has two crystallographically unique cation sites − the d site with C2 symmetry, and the b site with S6 or C3i symmetry.36 The cation sites in rh-In2O3 are equivalent and have C3v symmetry, similarly to those in corundum Al2O3.37 In both In2O3 polymorphs, cations and anions are six- and four-coordinated, respectively.34 Our recent studies have shown that metastable rh-In2O3 can be stabilized in colloidal NCs below ca. 5 nm in size, because of the surface energy or surface stress contributions.38 This size-dependent phase stabilization allows for the formation of high-energy rh-In2O3 phase by controlling NC growth kinetics via reaction temperature, synthesis time, or the presence of dopant impurities in solution.5,38−40 The two

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EXPERIMENTAL SECTION Chemicals. Indium acetylacetonate (In(acac)3, 98%) and chromium acetylacetonate (Cr(acac)3, 97.5%) were purchased from Strem Chemicals. Oleylamine (70%) and trin-octylphosphine oxide (TOPO; 90%) were purchased from SigmaAldrich. The solvents, including absolute ethanol, toluene (99.98%, EMD Chemicals), and hexane (99.9%, Fischer Scientific) were used as received. Synthesis of Cr3+-doped In2O3 NCs. The synthesis of Cr3+:In2O3 NCs was performed according to the previously reported procedure.11,38 In a 100 mL three-neck round-bottom flask, 4 mmol of In(acac)3 was mixed with 48 mmol of oleylamine and varying amounts of Cr(acac)3. The mixture was degassed and heated to 250 °C, and the reaction was allowed to proceed in the argon atmosphere for 1 h under constant stirring. The reaction mixture was then slowly cooled to room temperature, yielding a viscous suspension of colloidal NCs. The obtained NCs were collected by precipitation with ethanol and centrifugation, and subsequently washed three times with ethanol. The separation of rh-In2O3 and bcc-In2O3 NCs in the samples prepared at intermediate temperatures was achieved by size-selective precipitation. In this procedure, viscous suspension of NCs was first centrifuged at 3000 rpm for 5 min, leading to the precipitation of large In2O3 NCs having bixbyite-type crystal structure. Small NCs with corundum-type structure were B

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described in our previous publication.9 XMCD measurements were performed on ferromagnetic samples, which were prepared by dry pressing of a ferromagnetic nanocrystalline film (vide infra) onto a silicon nitride sample holder window. The sample holder was mounted on a magnetic ring (H ≈ 0.15 T) allowing for a parallel orientation of the magnetic field with respect to the X-ray beam propagation. A selected area of the sample was imaged at room temperature with sequential photon energies at the Cr L2,3-edge (565.0 to 610.0 eV), using left and right circularly polarized (LCP and RCP, respectively) beams. The collected image stacks were first aligned using Jacobson Analysis software, and the image analysis and the extraction of the spectra were performed by using Axis 2000 software. The theoretical simulations of the Cr L2,3-edge LCP and RCP X-ray absorption spectra were performed using Charge Transfer Multiplet program for X-ray Absorption Spectroscopy (CTM4XAS).46 The software includes multiplet and charge transfer effects, as two key components for calculating the spectra. CTM4XAS program is based on a semiempirical approach, and the spectra were calculated for C3i point group symmetry of Cr3+ ion, as a possible doping site in bcc-In2O3 (vide infra). This assumption is also reasonable given that we were predominantly interested in the difference between the absorption of photons having opposite helicity (ρ− and ρ+), or sign of XMCD at L3 and L2 edges, which is largely insensitive to the variations of symmetry in a particular coordination. The specific input parameters, such as crystal field splitting (10Dq), were obtained from the optical absorption and MCD spectra. The optical absorption spectra of Cr3+:In2O3 NCs were collected with a Varian Cary 5000 UV−vis-NIR spectrophotometer. The spectra were collected for NCs in colloidal form, using 1 cm path-length quartz cuvettes, and deposited on strain-free quartz substrates. UV−vis magnetic circular dichroism (MCD) spectra were collected in the Faraday configuration with Jasco J-815 spectropolarimeter, using a variable-field (0−7 T) magneto-optical cryostat (SM4000−8, Oxford) equipped with a variable-temperature (1.5−300 K) insert. MCD measurements were taken on NCs deposited on high quality strain-free quartz substrates mounted in the magneto-optical cryostat. The magnetization measurements were taken with the Physical Property Measurement System (PPMS, Quantum Design) in ACMS mode. For magnetization measurements of free-standing NCs, the colloidal NC samples were precipitated with ethanol, dried, and loaded into sample capsules. For magnetization measurements of nanocrystalline films, the colloidal NCs were spin-coated multiple times on the sapphire substrates previously treated in aqua regia, followed by mild annealing at 300 °C for 1 min between consecutive coatings. Special precaution must be taken when preparing and handling aqua regia. The aqua regia mixture was prepared by mixing concentrated nitric and hydrochloric acids in 1:3 (v/v) ratio. The mixture was made by slowly adding nitric to hydrochloric acid in a Pyrex glass container. The preparation and subsequent substrate treatment must be performed in an efficient, properly shielded fume hood, whereas wearing personal protection clothing (labcoat and appropriate gloves) and face shield. The resulting mixture is a fuming yellow-to-orange solution which must be used immediately after preparation. The substrates cut in suitable sizes were sonicated in aqua regia in a sealed glass container, and subsequently washed multiple times with deionized water and ethanol. Aqua regia mixture was

subsequently precipitated from the supernatant by the addition of 20 mL of ethanol and centrifugation for ca. 10 min. All NC precipitates were then heated in melted TOPO, followed by precipitation with ethanol. This procedure was repeated three times to ensure the removal of surface-bound dopant ions.13 The obtained NCs were dispersed in nonpolar organic solvent, such as hexane or toluene. Nanocrystal Characterization. Transmission electron microscopy (TEM) imaging was performed with a JEOL2010F microscope operating at 200 kV. The specimens were prepared by dropping dilute toluene suspensions of colloidal NCs on copper grids with lacey Formvar/carbon support films purchased from Ted Pella. The average doping concentrations of NCs were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) and energy dispersive X-ray (EDX) spectroscopy. The results of these analyses were in very good agreement. Powder X-ray diffraction patterns were collected with an INEL powder diffractometer equipped with a position-sensitive detector, using monochromatic Cu Kα radiation. Raman spectra were recorded at room temperature with a Renishaw 1000 Raman spectrometer using He−Ne laser as the excitation source (λ = 632.8 nm). The samples were excited using ca. 10% of the maximum laser output power of 40 mW. The spectrometer was calibrated using a silicon standard. Spectroscopic and Magnetic Measurements, and Data Analysis. X-ray absorption spectroscopy (XAS) measurements were performed at the Canadian Light Source (CLS) synchrotron facility. Indium L3-edge X-ray absorption near-edge structure (XANES) spectra were collected at the Soft X-ray Microcharacterization Beamline (SXRMB, 06B1−1), operating in the energy range 2.1−10 keV. XANES data were simultaneously recorded using fluorescence and total electron yield methods. Chromium K-edge XAS measurements were performed at the Hard X-ray MicroAnalysis (HXMA) beamline (06ID-1), capable of collecting the data from 5 to 40 keV. The Si (111) double crystal was used as a monochromator, and the harmonic rejection was achieved by detuning the crystals by 60%. Chromium K-edge energy calibration (5989 eV) was carried out by using chromium foil. For Cr2O3 reference, the spectra were recorded in the transmission mode using three ionization chambers. The first and second ionization chambers were used to monitor the incident and transmitted X-ray intensities, respectively, and the third one was used together with the Cr foil standard to provide internal calibration for the Cr K-edge position. For Cr3+:In2O3 NC samples, the spectra were collected in the fluorescence mode with a 32-element germanium array detector. In this configuration, the samples were oriented at ca. 45° relative to the incident beam, and the fluorescence signal was detected at 90° with respect to the incident beam. The processing and analysis of the data were described in more detail elsewhere.44 Briefly, extended X-ray absorption fine structure (EXAFS) data analysis was performed using the Cherokee and RoundMidnight codes from the Multiplatform Applications for the XAFS software.45 Structural parameters, such as the coordination number (N), bond length (R), and the Debye−Waller factor (σ2) were obtained from the analysis of the EXAFS spectra. Chromium L2,3-edge X-ray absorption and magnetic circular dichroism (XMCD) measurements were performed in scanning transmission X-ray microscopy (STXM) configuration at Soft X-ray Spectromicroscopy (SM) beamline (10ID-1). Details about the experimental procedure and data analysis were C

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these histograms are shown in Figure S1 (see the Supporting Information). Undoped In2O3 NCs prepared under identical conditions have an average NC size of 8.6 nm and a relatively narrow size distribution of ca. 20%.5 The doped NCs exhibit bimodal size distribution, with an average NC size shifting to smaller values with increasing Cr3+ starting concentration. For [Cr3+]/[In3+] = 0.2 the average NC size is only 3.0 nm, and NCs again have a narrow size distribution. XRD patterns of the same NC samples are shown in Figure 1e. The XRD pattern of Cr3+:In2O3 NCs prepared with [Cr3+]/[In3+] = 0.05 (purple trace) can be assigned to bulk bcc-In2O3 (black lines, JCPDS 06−0416), with a small contribution from rh-In2O3 (orange lines, JCPDS 21−0406), evident by the appearance of a small shoulder at 2θ ≈ 32.5 degrees. As the concentration of the Cr3+ precursor increases the XRD peaks corresponding to rh-In2O3 begin to emerge. For the precursor mixture [Cr3+]/[In3+] = 0.10 (green trace) broad peaks corresponding to rh-In2O3 phase become comparable to the peaks of bcc-In2O3 phase. For [Cr3+]/[In3+] = 0.20 (red trace) only NCs with the corundum crystal structure can be identified. XRD peak broadening, which follows the change in NC phase with increasing starting concentration of Cr3+, is consistent with a decrease in the average NC size. Taken together, TEM and XRD data indicate that increasing concentration of Cr3+ in the reaction mixture leads to a decrease in NC size, which is accompanied by the stabilization of metastable rh-In2O3 phase. The nanocrystal phase dependence on the dopant precursor concentration for Cr3+:In2O3 NCs was confirmed by X-ray absorption spectroscopy. L3-edge XANES spectroscopy can probe the local electronic structure of the In3+ host lattice cations,48 which makes it an excellent complementary technique for studying the phase transformation of In2O3 NCs as a function of doping concentration. Indium L3-edge spectra of typical rh-In2O3 and bcc-In2O3 NCs, and Cr3+:In2O3 NCs synthesized with different [Cr3+]/[In3+] precursor concentration ratios in the reaction mixture are shown in Figure 2. The spectra of bcc-In2O3 NCs (black trace) show a

neutralized and discarded immediately after the use. The nanocrystalline films were then weighed on an analytical balance in order to determine the magnetization per unit mass of the sample. All samples were handled using strictly nonmagnetic tools.

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RESULTS AND DISCUSSION In our previous studies involving different dopant ions in transparent conducting oxide NCs, we have shown that the presence of dopants significantly influences NC growth, phase stabilization and dopant incorporation.5,39 Similar observations were made in this study. We prepared Cr3+:In2O3 NCs with the starting dopant precursor concentration ratio (i.e., [Cr3+]/ [In3+]) varying from 0.05 to 0.20. The size distribution histograms of NCs synthesized with different starting concentration of Cr3+ precursor in the reaction mixture are shown in Figure 1a−d. The TEM images corresponding to

Figure 2. Indium L3-edge XANES spectra of bcc-In2O3 NCs (black trace, bottom), rh-In2O3 (aqua blue trace, top), and Cr3+:In2O3 NCs synthesized with [Cr3+]/[In3+] ratio of 0.05 (purple), 0.10 (green), and 0.20 (red). Inset: magnified absorption edge region. Figure 1. (a−d) Size distribution histograms of Cr3+:In2O3 NCs synthesized with different ratios of chromium and indium precursor concentrations ([Cr3+]/[In3+]): (a) 0.05, (b) 0.10, (c) 0.15, and (d) 0.20. The histograms were constructed by measuring the sizes of 200 NCs in each sample. (e) XRD patterns of Cr3+:In2O3 NCs synthesized with precursor concentration ratios as indicated in the graph. The vertical lines are XRD peak positions of bulk bcc-In2O3 (black) and rhIn2O3 (orange).

broad structured shoulder at 3735 eV (sh1) and a narrow shoulder centered at ca. 3745 eV (sh2). In the presence of Cr3+ in the starting mixture, sh1 narrows down while sh2 becomes broader. Moreover, the centers of gravity of both transitions gradually shift to lower energy with increasing initial Cr3+ concentration, suggesting a change of the electronic structure of In3+ in NCs. At high [Cr3+]/[In3+] values (e.g., 0.2), the D

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XANES spectrum becomes indistinguishable from that of undoped rh-In2O3 NCs (aqua blue trace). These results confirm at the molecular level that the change in crystal structure from bcc-In2O3 to rh-In2O3 occurs in the presence of dopant ions, and that substitutional doping sites have different local environment in the two phases. Similar results have been obtained for Sn4+ dopant ions in In2O3 NCs,39 suggesting that dopant impurities, regardless of their nature and electronic structure, can be used to manipulate the size and crystal structure of In2O3 NCs. The speciation (electronic structure and symmetry) of dopant ions and the interactions between dopants and In2O3 NC host lattice were studied by different spectroscopic and structural techniques. For these studies, the nanocrystals with bcc-In2O3 structure in the reaction products were separated from those with rh-In2O3 structure based on their size difference (vide supra), using a size-selective precipitation method. Typical TEM images and XRD patterns of bcc-In2O3 and rh-In2O3 NCs separated by size-selective precipitation are shown in Figure S2 (see the Supporting Information). Separation of these two polymorphs makes it possible to study relevant properties of Cr3+ doped in both bcc-In2O3 and rh-In2O3 NCs. The average doping concentrations of NCs synthesized with different starting ratios of [Cr3+] to [In3+] are summarized in Table 1. The doping concentration was

Figure 3. (a) Raman spectra of bcc-In2O3 NCs (black trace) and Cr3+:In2O3 NCs synthesized with different ratios of chromium and indium precursor concentrations ([Cr3+]/[In3+]): 0.05 (purple trace), 0.10 (green trace), 0.15 (blue trace), and 0.20 (red trace). (b) Raman shift (blue) and fwhm (red) of the dominant ν1 transition as a function of Cr3+ doping concentration in bcc-In2O3 NCs.

Table 1. Doping Concentrations of Cr3+ in In2O3 NCs for Different Precursor Concentration Ratios Cr3+ doping concentration (n(Cr)/n(In+Cr)) Precursor ([Cr3+]/[In3+])

bcc-In2O3 NCs [%]

0.05 0.10 0.15 0.20

3.0 3.5 4.7

the same samples in Figure 1e. In the presence of Cr3+ dopants Raman peaks shift to lower energies and become noticeably weaker and broader. This behavior is consistent with that of Fe3+- and Sn4+-doped bcc-In2O3 NCs, for which Raman peaks also shift to lower energies and become weaker and broader with increasing doping concentration.27,39 For [Cr3+]/[In3+] = 0.20 (Figure 3a, red trace), the peaks related to bcc-In2O3 can no longer be detected. Figure 3b plots the Raman shift and the full width at half-maximum (fwhm) of the dominant ν1 peak as a function of Cr3+ doping concentration in bcc-In2O3 NCs. The fwhm and the energy at the maximum of the Raman peaks were determined from the baseline-corrected spectra. The shifts of the Raman peaks observed in this work indicate substitutional incorporation of Cr3+ ions in In2O3 NCs and the concomitant change in frequency of the NC host lattice phonon modes. The broadening and weakening of the Raman spectra is a result of a decrease in the phonon correlation lengths due to local crystal lattice disorder caused by the dopant incorporation and/or decrease in NC size. To directly obtain quantitative information about the local environment of chromium sites, we conducted a systematic Cr K-edge XAS study of Cr3+:In2O3 NCs having different phase and doping concentrations. Normalized overview Cr K-edge Xray absorption spectra of Cr3+:In2O3 NCs and Cr2O3 reference are shown in Figure 4a. A comparison between absorption edge energies for the spectra of Cr3+:In2O3 NCs and Cr2O3 reference (Figure 4a, inset) confirms that chromium ions have 3+ oxidation state in In2O3 NCs. The observed XANES transitions of Cr3+ in In2O3 NCs and Cr2O3 reference are distinctly different (Figure 4a, inset), further indicating the absence of Cr2O3 in Cr3+:In2O3 NC samples. Figure 4b shows the kweighted Cr K-edge EXAFS spectra of the same samples. The spectra were Fourier transformed from k-space to R-space in the region of 2.6−15.0 Å−1. The resulting pseudoradial

rh-In2O3 NCs [%] 9.5 14.0 20.0

determined by ICP-AES and reported as at% (n(Cr)/n(In +Cr)) in NCs. The results in Table 1 convincingly show that Cr3+ dopants are incorporated to a larger extent in rh-In2O3 than in bcc-In2O3 NCs for the same starting concentrations of the dopant precursor. This phenomenon has been associated with the fact that both rh-In2O3 and Cr2O3 have corundum crystal structure.5 The consequence of this structural similarity is that dopant ions bind to the NC surface sites for the amount of time comparable to the NC growth rate, allowing for an effective incorporation into the NC host lattice. By using repeated TOPO exchange procedure, which has proven to be effective for different doped oxide NCs,13 we have demonstrated effective removal of surface-bound dopant ions and the internal doping of Cr3+ in In2O3 NCs (see Figure S3 in the Supporting Information). Cubic In2O3 phase can be identified by the characteristic Raman spectrum. Raman spectra of cubic and rhombohedral In2O3 NCs are compared in Figure S4 (see the Supporting Information). The spectra of commercial bcc-In2O3 powder and bcc-In2O3 NCs are essentially identical (see Figure S4 in the Supporting Information), and exhibit the Raman shifts of ca. 132 (ν0), 307 (ν1), 366 (ν2), 496 (ν3), and 629 (ν4) cm−1.38,49 Figure 3a compares the Raman spectra of pure bccIn2O3 and Cr3+:In2O3 NCs prepared with different starting concentrations of Cr 3+ . No additional Raman modes corresponding to Cr2O3 or other chromium-based secondary phases were observed, in agreement with the XRD patterns of E

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Cr2O3. Fourier-filtered EXAFS spectra of the second shell (Cr− In and/or Cr−Cr), corresponding to the R-space region of 2.0−3.0 Å, are shown in Figure 4d. Contrary to the spectrum of Cr2O3, the back scattering amplitude increases for Cr3+:In2O3 NCs in the k-range 9.0−15.0 Å−1. A likely origin of this increase at high k is higher back scattering amplitude from neighboring indium atoms compared to the chromium atoms, reconfirming the substitutional incorporation of Cr3+ in both rh- and bccIn2O3 NCs. The electronic structure of Cr3+ in many complexes and minerals has been extensively studied by ligand field electronic absorption and emission spectroscopies.42 In most lattices, Cr3+ adopts quasi-octahedral coordination, which gives many Cr3+ containing minerals and gemstones various shades of colors.53,54 Of particular historical importance were the spectroscopic studies of ruby37 − alumina doped with Cr3+ ions − which was the first known laser in the visible region. Substitutional doping sites in rh-In2O3, in which Cr3+ sits in trigonally distorted octahedral coordination of oxygen ions and has C3v point group, are similar to those in ruby. Because bccIn2O3 has two unique cation sites with C3i and C2 symmetry, known as b and d sites, respectively,36 substitutional transition metal dopants are expected to have distinct spectroscopic properties depending on the sites they occupy. The atomicscale theoretical simulations have suggested that when dopant cation is smaller than the host lattice cation, the C2 symmetry (d) site is preferentially occupied by the dopant, while when dopant cation is larger than the host cation, the C3i (b) site becomes energetically favorable doping site.36 Ionic radius of Cr3+ in octahedral coordination is 0.62 Å, which is smaller than the ionic radius of In3+ in octahedral environment (0.81 Å).55 According to these theoretical results Cr3+ is expected to preferentially occupy C2 symmetry site in bcc-In2O3. However, structural study of Sn4+-doped bcc-In2O3 indicated that Sn4+, which also has a smaller ionic radius than In3+ (r(Sn4+) = 0.71 Å), has a strong preference for the higher symmetry site (C3i).55 The ligand field spectroscopic studies of Cr3+ could provide some insights into its site preference in bcc-In2O3 NCs. In the ideal octahedral ligand field the ground state of free Cr3+ ion (4F) splits into three terms (Figure 5) giving rise to two

Figure 4. X-ray absorption spectra of Cr2O3 (olive), 20.0% Cr3+:rhIn2O3 NCs (red), and 3.0% Cr3+:bcc-In2O3 NCs (purple). (a) Overview Cr K-edge absorption spectra (inset: magnified XANES region), (b) k-weighted Cr K-edge EXAFS spectra, (c) Fourier-filtered EXAFS spectra for the Cr−O (first) shell, and (d) Fourier-filtered EXAFS spectra for the Cr−In and/or Cr−Cr (second) shell.

distribution functions are shown in Figure S5 (see the Supporting Information). The first peak centered at ca. 1.5 Å is due to electron back scattering from the first coordination shell, and is associated with Cr−O bonding. The inverse Fourier transform of the first shell (R = 1.1−2.0 Å) of the pseudoradial distribution functions yields the Fourier-filtered EXAFS spectra, which are shown in Figure 4c. The Fourierfiltered curves were fitted in the same k-range assuming a single Cr−O shell (Figure 4c, dashed lines). Table 2 summarizes the Table 2. Results of the Fitting of EXAFS Spectra for the First (Cr−O) Shell sample 3+

3.0% Cr :bcc-In2O3 20.0% Cr3+:rh-In2O3 Cr2O3 a

N

R(Cr−O) [Å]

σ2 (Å2)a

ρ (%)b

6 6 6

2.04 ± 0.05 2.08 ± 0.02 1.99 ± 0.02

0.020 0.015 0.016

8.24 2.45 6.36

Debye−Waller factor. bWeighted residual factor.

structural parameters for the Cr−O shell obtained from the curve-fitting. A simple comparison of the k-weighted EXAFS spectra of Cr2O3 and Cr3+:In2O3 NCs reveals a qualitative difference, which indicates a different local environment of chromium atoms in Cr2O3 and In2O3 lattices. Fitting results for Cr2O3 reference suggest that Cr3+ is coordinated with 6 oxygen atoms, with an average Cr−O bond distance of 1.99 ± 0.02 Å. These results are in good agreement with the crystallographic data, from which the average Cr−O bond length in Cr2O3 was also determined to be 1.99 Å.47 The average bond distance for the In−O shell in both cubic and rhombohedral In2O3 polymorphs was reported to be 2.17−2.18 Å.50−52 The bond distances obtained from EXAFS analysis (R(Cr−O), Table 2) show that the average Cr−O bond distances in Cr3+:In2O3 NC samples are longer than those in Cr2O3, but shorter than the In−O bond distances in undoped In2O3. These differences are due to the substitution of Cr3+ for In3+ in In2O3, which leads to an increase in the average Cr−O bond distances compared to

Figure 5. Splitting pattern of six-coordinate Cr3+ ion in different symmetries, as defined by the corresponding point groups (indicated in the graph).

characteristic spin-allowed transitions, and a spectrum that consists of two broad bands in the visible region.42 The lowerenergy transition, 4A2g→4T2g, is known as the U band, and higher energy transition, 4A2g→4T1g, as the Y band.42,53,54,56,57 These d−d transitions are formally parity-forbidden by the Laporte selection rule, which can be relaxed by lowering the symmetry of the Cr3+ site, or through vibronic coupling. The weak and narrow absorption features, which are often observed just below the energy of the 4A2g→4T2g band, are due to the components of the spin-forbidden transitions 4A2g→2Eg, 2T1g, which are known as R,R′ lines, respectively.42,53,56,57 These F

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not readily observable in the room temperature absorption spectrum. The assignments of the observed features are summarized in Table 3. Specifically, a sharp peak at ca.

spin-forbidden bands gain intensity primarily through the spin− orbit coupling with the 4T2g state, enabled by the vibrational modes.58 Similarly, a set of narrow bands which can be observed on the low energy side of the Y band corresponds to split spin-forbidden 4A2g→2T2g transition, and is traditionally labeled as B lines.53,56,57 In distorted octahedral coordination, U and Y bands are subject to further splitting due to the lifting of the 4T state degeneracy. In trigonally distorted octahedral fields (e.g., C3v), 4T2g state splits into 4E and 4A1 terms, and 4T1g splits into 4E and 4A2 terms (Figure 5). As symmetry further decreases to C2 point group, 4E terms split into B terms.59 The ligand-field electronic absorption spectrum of the concentrated suspension of 9.5% Cr3+:rh-In2O3 NCs, collected at 300 K, is shown in Figure 6a. The band centered at ca. 16

Table 3. Assignments of Cr3+ Features in Absorption and MCD Spectra sample 3.0% Cr3+:bcc-In2O3

9.5% Cr3+:rh-In2O3

energy (cm−1)a 14 440, 15 080, 15 540 16 300 20 700 21 300 14 440, 15 000 16 500 20 100 23 000

assignment

label

A2→2E, 2T1 4 A2→4T2 4 A2→2T2 4 A2→4T1 4 A2→2E, 2T1 4 A2→4T2 4 A2→2T2 4 A2→4T1

R,R′ U B Y R,R′ U B Y

4

a

The energy values are reported as a center of gravity of the corresponding bands due to the broad nature of these electronic transitions.

14 440 and a shoulder at 15 000 cm−1 are assigned to R,R′, and similar features around ca. 20 100 cm−1 to B transitions. More pronounced feature is the band at ca. 28 200 cm−1, which is barely detected as a shoulder at the onset of the band gap absorption. We assign this band to the charge transfer transition (most likely involving oxygen-to-chromium charge transfer), on the basis of the similar transitions involving Cr3+ in oxide bulk and nanocrystal host lattices.12,53 This charge transfer band overlaps on the low energy side with the spin-allowed 4A2→4T1 transition, giving it a characteristic asymmetric shape and broadening. The overall MCD intensity associated with Cr3+ dopants increases with increasing magnetic field strength owing to the Zeeman splitting of the ground and excited states of Cr3+ ions. Figure 6c plots MCD intensity at different energies (indicated with the corresponding symbols in Figure 6b) as a function of the applied magnetic field. The MCD intensity saturates at high fields, consistent with the C-term MCD behavior. These data were fit to the spin-only Brillouin function (eq 1) for the spin state S = 3/2, using the established value for Lande g-factor of 1.96.60 Ms =

⎡ ⎛ ⎛ g μ H ⎞⎞ 1 NgμB ⎢(2S + 1)coth⎜⎜(2S + 1)⎜ B ⎟⎟⎟ 2 ⎢⎣ ⎝ 2kBT ⎠⎠ ⎝ ⎛ g μ H ⎞⎤ − coth⎜ B ⎟⎥ ⎝ 2kBT ⎠⎥⎦

Figure 6. (a) 300 K electronic absorption spectrum of colloidal 9.5% Cr3+:rh-In2O3 NCs. The spectrum of the same colloidal suspension concentrated by a factor of 100 is also shown. (b) 4.5 K MCD spectra of the same sample collected in a variable magnetic field (1−7 T, bottom to top). (c) The magnetic field dependence of the MCD intensities at the spectral positions indicated by the corresponding symbols in b. The black lines are fits to the Brillouin function.

(1)

In this expression N, μB, and k are the number density of dopant centers, the Bohr magneton and the Boltzmann constant, respectively, T is the temperature, and H is the applied magnetic field strength. The spin-only Brillouin function provides a good fit to the experimental data as a result of the absence of the orbital angular momentum in the 4 A2 ground state of Cr3+. For the spectral position at 28 209 cm−1 (red squares) the best fit slightly deviates from the experimental data at high magnetic fields. This deviation can be associated with the contribution from the charge transfer transition involving NC host lattice. The agreement between the Brillouin function fit and the MCD intensity of the Cr3+ ligand field transitions demonstrates at the molecular level the presence of isolated substitutionally incorporated paramagnetic Cr3+ ions in In2O3 NCs. These conclusions were confirmed by

500 cm−1 and the broad shoulder at ca. 23 000 cm−1 are assigned to the U and Y transitions, respectively, of pseudooctahedral Cr3+. The band gap transition of the host rh-In2O3 NCs is observed as a broad shoulder at ca. 37 500 cm−1.38 The band gap energy was determined from the onset of the absorption spectrum to be ca. 31 400 cm−1 (3.9 eV). Figure 6b shows 4.5 K MCD spectra of the same NCs collected in different magnetic fields from 1 to 7 T, in 1 T increments. The main features of the absorption spectrum also appear in the MCD spectra, and are analogously assigned (i.e., U and Y bands). Furthermore, MCD spectra reveal the features that are G

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U band at ca. 14 440, 15 080, and 15 540 cm−1. These transitions can be assigned to spin-forbidden R,R′ bands, and have different structure from the analogous transitions in Cr3+:rh-In2O3 NCs. This difference may arise from distinctly different symmetry and/or different ligand field strength experienced by Cr3+ in the two NC structures, and it is interesting in the context of identifying Cr3+ doping site in bccIn2O3 NCs. For instance, weaker ligand field strength in bccIn2O3 NCs could result in enhanced mixing of the doublet states with 4T2(F), leading to additional splitting of R,R′ bands even if Cr3+ occupies C3i (b) site, which has a very similar symmetry to the doping site in rh-In2O3 (C3v). The shift of the U and Y bands for Cr3+ in bcc-In2O3 to lower energies would generally also indicate weaker ligand field strength, at least in the approximation of an ideal octahedral coordination. However, in reduced symmetry the red shift of the U and Y bands is also consistent with a stronger distortion of Cr3+ in bcc-In2O3 relative to rh-In2O3 NCs, as illustratively shown in Figure 5. This explanation is in agreement with similar Cr−O bond distances calculated from the EXAFS analysis (Table 2), which suggest similar average ligand field strength of Cr3+ in the two phases. Similarly, if the difference in R,R′ band splitting originates from a different symmetry, it would suggest that Cr3+ likely occupies the low-symmetry d site (C2 point group) in bcc-In2O3 NCs. The study of Cr3+ in diopside mineral, in which Cr3+ resides in the C2 symmetry site, by polarized electronic absorption spectroscopy has revealed three sharp spinforbidden transitions superimposed on the U band, similarly to the MCD spectra of 3.0% Cr3+:bcc-In2O3 NCs in this study.54 Preferential occupancy of the d site by Cr3+ in bccIn2O3 NCs would be consistent with the fact that Cr3+ has smaller ionic radius than In3+. Although we currently favor this latter scenario, further investigations involving complementary spectroscopic techniques and analysis are necessary to definitively infer the nature of the doping sites in bcc-In2O3 NCs. More important for this study is the MCD transition at ca. 24 000 cm−1, which overlaps on its red side with the Y band and on the blue side with the band gap absorption. Following the same arguments as for Cr3+:rh-In2O3, we assign this band to the charge transfer transition. This band follows the red shift of the band gap absorption of bcc-In2O3 relative to rh-In2O3 NCs, justifying its assignment to the charge transfer involving the NC host lattice. The MCD intensities at different spectral positions were fit to spin-only Brillouin function using g = 1.96 (Figure 7c). In contrast to Cr3+:rh-In2O3 NCs, the fits to the MCD ligand field (U and R,R′) band intensities as a function of H show some deviation from the experimental data. This deviation suggests different effective g-factor (see Figure S8 in the Supporting Information) or different nature of the excited states of Cr3+ dopant sites in bcc-In2O3 from those in rh-In2O3 NCs. The magnetic susceptibility data for freestanding 3.0% Cr3+:bcc-In2O3 NCs (see Figure S9 in the Supporting Information) are also fit fairly well with the Brillouin function using the same parameters, confirming the presence of isolated paramagnetic Cr3+ ions within NCs. The 300 K magnetic hysteresis loops for nanocrystalline films prepared using 3.0% Cr3+:bcc-In2O3 and 20.0% Cr3+:rh-In2O3 NCs as building blocks are shown in Figure 8a. In contrast to the paramagnetic character of free-standing NCs in colloidal and powder forms (see also Figure S10 in the Supporting Information), the films prepared from the same NCs exhibit some ferromagnetic ordering. The saturation magnetic moments (Ms) were determined from the hysteresis loops to be

magnetic susceptibility measurements (see Figure S6 in the Supporting Information). Furthermore, very similar MCD spectra were obtained for 20.0% Cr3+:rh-In2O3 (see Figure S7 in the Supporting Information), suggesting substitutional Cr3+ incorporation even at high doping concentrations. The absorption spectra of Cr 3+ :bcc-In 2 O 3 NCs are qualitatively similar to those of Cr3+:rh-In2O3 NCs. Figure 7a

Figure 7. (a) 300 K electronic absorption spectrum of colloidal 3.0% Cr3+:bcc-In2O3 NCs. The spectrum of the same colloidal suspension concentrated by a factor of 100 is also shown. (b) 4.5 K MCD spectra of the same sample collected in a variable magnetic field (1−7 T, bottom to top). (c) The magnetic field dependence of the MCD intensities at the spectral positions indicated by the corresponding symbols in b. The black lines are fits to the Brillouin function.

shows an electronic absorption spectrum of 3.0% Cr3+:bccIn2O3 NCs. The U and Y ligand field bands are observed at ca. 16 300 and ca. 21 300 cm−1, respectively, and a small shoulder at the low energy side of the U band (ca. 14 400 cm−1) corresponds to the formally spin-forbidden R,R′ bands. The characteristic feature at ca. 32 500 cm−1 is associated with the band gap absorption of the host bcc-In2O3 NCs. The band gap energy was estimated to be ca. 29 300 cm−1 (3.6 eV), in good agreement with the band gap energy reported for bulk bccIn2O3.38 Broadening, intensity and resolution of the Cr3+ absorption bands allows for the extraction of the limited amount of information from the absorption spectra. MCD spectra can greatly aid in the detection and assignment of hidden or weak Cr3+ transitions in the absorption spectrum. The 4.5 K MCD spectra collected in variable magnetic fields (Figure 7b) clearly show the additional features, similarly to Cr3+:rh-In2O3 NCs (see Table 3 for summary). Significantly, these spectra reveal three narrow peaks under the red tail of the H

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Figure 9. (a) Cr L2,3-edge X-ray absorption spectra of 3.0% Cr3+:bccIn2O3 nanocrystalline film collected using circularly polarized ρ− (blue) and ρ+ (red) photons in STXM configuration. (b) Calculated Cr L2,3-edge circularly polarized X-ray absorption spectra (ρ−, blue; ρ+, red) for C3i symmetry. The spectra were calculated using CTM4XAS package.

nanocrystalline films. The same analysis at different regions of the specimen revealed that Cr3+ is distributed homogeneously throughout the sample, further ruling out dopant segregation as the origin of the observed ferromagnetism. The observations of intrinsic ferromagnetism in Cr3+:In2O3 nanocrystalline films are consistent with the previous reports of ferromagnetism in DMSO films prepared from colloidal NCs.10−13 The long-range magnetic ordering in these samples has been associated with the existence of structural defects, such as grain boundaries, at the NC interfaces, which play a key role in mediating magnetic ordering of the dopant ions. Furthermore, oxygen vacancies have been proposed to be a key component of these grain boundaries.12 The studies of bulk bcc-In2O3 and other types of bcc-In2O3 nanostructures have also invoked the role of oxygen vacancies.24,26,31,43 Philip et al.26 have shown that magnetic properties of Cr3+:bcc-In2O3 thin films can be tuned by adjusting the defect (oxygen vacancy) concentration. They observed that Cr3+:bcc-In2O3 thin films exhibit ferromagnetism when the samples are highly oxygen deficient and conductive, but are otherwise paramagnetic. Xing et al.31 have also found a strong correlation between ferromagnetism and oxygen deficiency in Cr3+:bccIn2O3 nanostructures prepared by the chemical vapor transport method. Recent theoretical study has suggested that the energy of the oxygen vacancy formation in In2O3 decreases rapidly with decreasing distance from the surface.61 Consequentially, the concentration of oxygen vacancies, which form shallow donor states, increases by approaching the surface. This type of surface segregation is a general phenomenon which occurs at materials interfaces and extended structural defects, such as grain boundaries and dislocations.61 An increased concentration of oxygen vacancies in the interfacial (grain boundary) region in DMSO nanocrystalline films prepared from colloidal NCs likely plays a key role in mediating magnetic exchange interactions of dopant ions. A decrease in the average size of colloidal NCs should lead to nanocrystalline films with comparatively higher magnetization, based on an increased surface-to-volume ratio.62 Considering that correlation alone, Cr3+:rh-In2O3 nanocrystal-

Figure 8. (a) Magnetization data recorded at 300 K for the nanocrystalline films prepared from 3.0% Cr3+:bcc-In2O3 NCs (solid squares) and 20.0% Cr 3+ :rh-In2 O3 NCs (open circles). (b) Concentration and phase dependence of the saturation magnetization of Cr3+:In2O3 nanocrystalline films.

0.24 and 0.014 μB/Cr3+ for 3.0% Cr3+:bcc-In2O3 and 20.0% Cr3+:rh-In2O3 films, respectively. The magnetic moments per dopant ion were quantified based on the unit mass magnetization and the elemental composition of the sample. The average magnetic moment as a function of Cr3+ doping concentration in these two host NC structures is plotted in Figure 8b. Although the net magnetic moment varies somewhat among different samples, Ms of 3.0% Cr3+:bcc-In2O3 is at least 10 times higher than the saturation magnetization measured for typical Cr3+:rh-In2O3 films (Figure 8b). XMCD spectroscopy is a powerful complementary technique for studying magnetization, and inferring the origin of ferromagnetism at the molecular level. It measures the difference in absorption of ρ− and ρ+ photons by a specific element, and as such is a direct consequence of its magnetization. We performed XMCD measurements by STXM imaging using LCP and RCP X-ray beam. This method allows for the spectroscopic detection of magnetization with high spatial resolution. Figure 9 shows (a) experimental and (b) calculated Cr L2,3-edge absorption spectra of a Cr3+:bcc-In2O3 nanocrystalline film collected using ρ− and ρ+. A STXM image of the sample corresponding to the spectra in Figure 9a is shown in Figure S11 (see the Supporting Information). The theoretical spectra agree reasonably well with the experimental spectra, although it is not possible to discern the site symmetry and the electronic structure details from the existing data alone. More importantly, the calculated spectra represent very well the difference between the absorption of LCP and RCP photons for both L2 and L3 edges (sign of XMCD), confirming the origin of magnetization in this sample. These results strongly suggest room-temperature magnetic exchange interactions of Cr3+ dopant ions in In2O3 I

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line films are expected to have higher net magnetic moment, because of the smaller sizes of constituent NC building blocks. The experimental observations in Figure 8 are, however, exactly opposite. Considering the interplay between two factors, concentration of dopants and donors (i.e., oxygen vacancy defects), Coey et al.16 have constructed the general magnetic phase diagram for DMSOs. According to this prediction, ferromagnetism can occur when the donor concentration reaches or exceeds the percolation threshold needed to form bound magnetic polarons, whereas the dopant concentration remains below the percolation threshold (ca. 25%).16,18 Increase in the dopant impurity concentration above this threshold leads to antiferromagnetic interactions. Although the doping concentrations in rh-In2O3 NCs are below the percolation limit, the higher doping level in these NCs could potentially be the cause of reduced average magnetic moment per Cr3+ because of antiferromagnetic coupling within the groups of nearestneighbor dopants. To test the influence of doping concentration on the magnetization of Cr3+:rh-In2O3 nanocrystalline films, we prepared the films from rh-In2O3 NCs with low Cr3+ doping concentration (ca. 5%), and obtained results similar to those in Figure 8b (see Figure S12 in the Supporting Information). This experiment indicates that the difference between Cr3+ magnetization in the two nanocrystalline phases cannot be related to antiferromagnetic interactions alone. Furthermore, the observation that Cr3+ magnetic moment is not strongly sensitive to the doping concentration in rh-In2O3 NCs suggests that phase segregation can also be ruled out as the origin of ferromagnetism in this material, consistent with our previous report and our detailed structural and spectroscopic analysis in this work. According to the proposed magnetic phase diagram, to attain high TC in DMSOs having the optimal concentrations of oxygen vacancies and early transition metal dopant ions, hybridization and charge transfer from the donor impurity band to unoccupied 3d states at the Fermi level is required (Figure 10a).16 Density functional theory calculations have suggested

rh-In2O3 phase and/or the quantum confinement in smaller rhIn2O3 NCs.38 Widening of the band gap of 9.5% Cr3+:rh-In2O3 NCs reduces the interactions between the conduction band and the empty 3d states of Cr3+. This phenomenon prevents efficient hybridization between the donor band and the 3d states at the Fermi level, as depicted in Figure 10b. The different electronic structure of Cr3+ dopants relative to the NC host lattice band structure is evident from the blue shift of the MCD charge transfer transition in Cr3+:rh-In2O3 with respect to Cr3+:bcc-In2O3 NCs. Furthermore, a study of Sn4+-doped bcc- and rh-In2O3 (ITO) NCs have revealed that bcc-ITO NCs have significantly higher concentration of free electrons in the conduction band, because of the lower donor activation energy associated with the narrower band gap of bcc-In2O3 NCs.15 Taken together, these results provide an evidence of the correlation between dilute magnetic ordering and the position of the dopant ions relative to the Fermi level in the two phases of the same TCO material. Other mechanisms of ferromagnetic ordering in DMSOs in general, and Cr3+:In2O3 in particular, have also been proposed. Notably, according to the carrier-induced ferromagnetism model,63 the free electrons introduced by external n-type doping cause a partial occupancy of the Cr3+ resonance levels within the conduction band, leading to the stabilization of the ferromagnetic configuration. While the difference in magnetization of the In2O3 NC polymorphs could also be explained in the context of this model using the above electronic structure arguments, on the basis of the well-documented correlation between the extended structural defects and the magnetic properties of In2O3 and other DMSO nanocrystalline and structurally disordered thin films,10−13,30,31,43 it is more likely that the mechanism of long-range ordering in these materials involves the defect states. Although Cr3+:bcc-In2O3 has been predicted to be ferromagnetic at room temperature, at least for relatively small separation between Cr3+ centers,63 the observed average magnetic moments per Cr3+ in bcc-In2O3 nanocrystalline films is still relatively weak in comparison to the spin-only magnetic moment of the isolated Cr3+ ion (3.87 μB), similarly to other reports of Cr3+:bcc-In2O3 treated under analogous conditions.24,31,43 This could be related to relatively weak hybridization between the impurity band and Cr3+ (3d) states, low density of empty 3d states at the Fermi level, and/or low concentration of interfacial defect donors owing to relatively large sizes of bcc-In2O3 NC building blocks. Optimizing these parameters provides a path for the rational design of dilute ferromagnetic oxides, using colloidal NCs as building blocks.

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CONCLUSIONS In summary, a comprehensive study was carried out on Cr3+ doped into In2O3 NCs having different crystal structures. The polymorphism of TCO NCs is generally enabled by controlling their size. This size control can be achieved in the presence of dopant ions in the reaction mixture, which inhibit the NC growth, leading to the stabilization of metastable phases below critical NC size. To the best of our knowledge, there has been no prior study of Cr3+ doped into metastable rh-In2O3, or systematic investigation comparing the spectroscopic and magnetic properties of dopants in different phases of the same TCO NCs. A combination of different spectroscopic measurements and analyses indicates a distinctly different electronic structure of Cr3+ ions in bcc- and rh-In2O3 NCs, which is partly associated with a different electronic band structure, particularly band gap energy, of the NC host lattices.

Figure 10. Schematics depicting a plausible influence of the electronic structure of (a) Cr3+:bcc-In2O3 and (b) Cr3+:rh-In2O3 nanocrystalline films on their magnetic properties. Increase in the band gap of rhIn2O3 reduces the hybridization between Cr3+ (3d) states and the donor impurity states at the Fermi level, causing a decrease in the conduction band splitting and ferromagnetic interactions. The positions of the impurity-based bands are arbitrary and are shown here only for phenomenological purposes.

that the Cr3+ t2 levels in bcc-In2O3 lie below the bottom of the conduction band,63,64 enabling e-derived states ((3d3)↓ band) to cross the Fermi level in the impurity band. From the analysis of the optical absorption spectra we showed that the band gap of Cr3+:bcc-In2O3 NCs is smaller than that of Cr3+:rh-In2O3 NCs by ca. 0.3 eV, owing to the larger band gap energy of bulk J

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and Ontario Research Fund (ORF-RI 204782). P.V.R. thanks NSERC (Canada Research Chair) and Ontario Ministry of Economic Development and Innovation (Early Researcher Award) for partial support of this work. S.S.F. and M.H. acknowledge Canadian Light Source (CLS) for Graduate Travel Awards. S.S.F. thanks Waterloo Institute for Nanotechnology for the WIN Graduate Research Fellowship. The research described in this article was partly performed at CLS, which is supported by the NSERC, NRC, CIHR, the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan. We thank Drs. Ning Chen (HXMA), Jian Wang (SM), and Yongfeng Hu (SXRMB) for their assistance at the respective beamlines.

These differences are particularly interesting and important in the context of the magnetic properties of nanocrystalline films, assembled from the colloidal NCs as building blocks. It has been well-established that colloidal DMSO NCs are generally paramagnetic, and that ferromagnetic interactions can be turned on upon preparing nanocrystalline films from these NCs, which has been attributed to the formation of extended structural defects (or grain boundaries) associated with NC interfaces. The formation of such grain boundaries is likely accompanied by the generation of oxygen vacancies which act as donor states that can facilitate long-range ordering of the dopant ions. Although a decrease in size of colloidal NCs usually leads to stronger magnetization of these structurally disordered films owing to the larger surface-to-volume ratio and density of interfacial defects, the films prepared from Cr3+:rhIn2O3 NCs, which are smaller than Cr3+:bcc-In2O3 NCs, have significantly smaller average magnetic moment. This surprising result is predominantly attributed to the difference between the electronic band structure of bcc-In2O3 and rh-In2O3 NCs. A plausible explanation is that widening of the band gap of rhIn2O3 NCs reduces an interaction between the conduction band and the empty Cr3+ (3d) states, which is necessary for ferromagnetic ordering. The mechanism and nature of the dopant-defect interactions in this process is still not sufficiently understood, and will continue to be an area of rich research opportunity. The results of this work emphasize the role of the host lattice electronic structure in controlling the magnetic properties of DMSOs, and open the door for manipulating the dilute magnetic interactions in this class of materials by phase transformations of NCs in solution. Furthermore, these results could be instrumental for the design of new high-T C ferromagnetic semiconductors. Colloidal synthesis methods and the size-structure dependence allow for accessibility and separation of different NC phases, as well as tuning of their electronic structure, enabling a convenient engineering of the complex functionalities of TCO NCs.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of Cr3+:In2O3 NCs for different precursor concentration ratios; TEM images and XRD patterns of NCs obtained by size-selective precipitation; Cr3+ ligand field electronic absorption spectra before and after TOPO treatments; Raman spectra of bcc- and rh-In2O3 powder and NCs; Fourier transform EXAFS spectra of Cr2O3 reference and Cr3+:In2O3 NCs; additional magnetization data for freestanding Cr3+-doped bcc-In2O3 and rh-In2O3 NCs and nanocrystalline films; MCD spectra of 20.0% Cr3+:rh-In2O3 NCs; Brillouin fitting of the MCD data for 3.0% Cr3+:bcc-In2O3 NCs; STXM images corresponding to XMCD spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSERC (Discovery and RTI grants), Canada Foundation for Innovation (CFI-LOF 204782) K

dx.doi.org/10.1021/cm303317t | Chem. Mater. XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/cm303317t | Chem. Mater. XXXX, XXX, XXX−XXX