Tuning Plasmon Resonance of In2O3 Nanocrystals throughout the

Article pubs.acs.org/cm

Tuning Plasmon Resonance of In2O3 Nanocrystals throughout the Mid-Infrared Region by Competition between Electron Activation and Trapping Hanbing Fang, Manu Hegde, Penghui Yin, and Pavle V. Radovanovic* Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada S Supporting Information *

ABSTRACT: Controlling plasmonic properties of aliovalently doped semiconductor nanocrystals in mid-infrared (MIR) spectral region is of a particular current interest, because of their potential application in heatresponsive devices and near-field enhanced spectroscopies. However, a lack of detailed understanding of the correlations among the electronic structure of the host lattice, dopant ions, and surface properties hampers the development of MIR-tunable plasmonic nanocrystals (NCs). In this article, we report the colloidal synthesis and spectroscopic properties of two new plasmonic NC systems based on In2O3, antimony- and titanium-doped In2O3 NCs, and comparative investigation of their electronic structure using the combination of the Drude−Lorenz model and density functional theory. The localized surface plasmon resonances (LSPRs) lie at lower energies and have smaller bandwidths for Ti-doped than for Sb-doped In2O3 NCs with similar doping levels, indicating lower free electron density. Surprisingly, the Fermi level is found to be higher in Ti-doped In2O3 than in Sb-doped In2O3, suggesting the formation of electron trap states on nanocrystal surfaces, which reduce carrier density without significantly impacting their mobility. Controlling the competition between doping concentration and electron trapping allowed us to generate LSPR in Ti-doped In2O3 nanocrystals deep in the MIR region, and tune the absorption spectra from 650 cm−1 to 8000 cm−1. We also demonstrated the possibility to enhance the intensity of LSPR in these new plasmonic NCs by adjusting the synthesis and post-synthesis treatment conditions. The results of this work allow for an expansion of the tuning range of LSPR of colloidal metal oxide NCs by controlling the electronic structure of aliovalent dopant and charge carrier trapping.

■

resonance.9 Among these nontraditional plasmonic materials, transparent metal oxide NCs doped with aliovalent impurities allow for tuning of LSPR in near-infrared (NIR) to mid-infrared (MIR) range by varying the doping level, and therefore the concentration of free electrons. The theoretical framework for the classical description of plasmonic properties of degenerately doped metal oxides is based on the Drude−Lorentz model, which assumes damped oscillations of free electrons around the lattice sites as a response to an external AC electric field, E⃗ , of an electromagnetic wave. The resulting electric displacement field (D⃗ ) in a doped semiconductor is defined as10

INTRODUCTION Plasmonic nanocrystals (NCs) have been a focus of intense research over the past decade, because of their unique optical properties enabled by the possibility to induce confined collective oscillations of free electrons (plasma oscillations).1−3 The sensitivity of these excited oscillations, known as localized surface plasmon resonance (LSPR), to the local surrounding renders this class of materials interesting as chemical probes and biomedical sensors,3−5 and for energy conversion devices.6 LSPR can also lead to the generation of a strong optical field near the surface of NCs, which is used to enhance signals in modern high-sensitivity spectroscopic techniques, including surface-enhanced Raman spectroscopy (SERS).2,7 Optical fieldenhanced light absorption is also conducive to the application of plasmonic NCs as light concentrators in photovoltaic cells.8 The field of colloidal plasmonic NCs has been dominated by studies of NCs made of noble metals such as gold and silver, because these materials exhibit LSPRs in the visible range, and are chemically stable and easy to prepare. However, they also suffer from significant drawbacks, including high optical losses due to electronic transitions in the visible range, high materials cost, and the inability to tune the concentration of free electrons. Consequently, there has been much interest in developing new types of colloidal NCs with tunable plasmonic © 2017 American Chemical Society

D⃗ = εoptε0E ⃗ −

Ne 2E ⃗ m*(ω 2 + iγω)

(1)

where ε0 is the permittivity of the free space, εopt is the dielectric constant measured in the transparent region of the spectrum of an undoped semiconductor (εopt ≈ n2, where n is the refractive index), ω the frequency of light, m* the effective electron mass, e the electron charge, N the free electron Received: April 1, 2017 Revised: May 15, 2017 Published: May 18, 2017 4970

DOI: 10.1021/acs.chemmater.7b01349 Chem. Mater. 2017, 29, 4970−4979

Article

Chemistry of Materials concentration, and γ the damping rate. The relative dielectric constant (εr) is related to D⃗ through the following expression: ⎛ ⎞ ωp2 ⎟ εr = εopt⎜⎜1 − (ω 2 + iγω) ⎟⎠ ⎝

doped In2O3 (Ce:In2O3) NCs has demonstrated that plasmonic properties such as quality factor and electron mobility can be improved in the same spectral range by eliminating the hybridization of dopant and In orbitals at the bottom of the conduction band and reducing the oxygen vacancy concentration.37 In this work, we report, for the first time, the synthesis and comparative experimental and theoretical investigation of antimony- and titanium-doped colloidal In2O3 NCs, and we demonstrate that they allow for a significant expansion of spectral tunability and characteristics of LSPR. Doping either Sb or Ti leads to a red shift of the LSPR, relative to ITO NCs, with the plasmon band of Ti-doped In2O3 (Ti:In2O3) NCs being tunable deep in the MIR, with the lower limit being below 1000 cm−1. Surprisingly, the density functional theory (DFT) calculations indicate that the Fermi level of Ti:In2O3 lies higher than that of Sb:In2O3, relative to the bottom of the conduction band, suggesting the formation of electron trap states in Ti:In2O3 NCs. The difference in other plasmonic properties of these two systems and the possibility of their manipulation are also discussed. The results of this work demonstrate how the electronic structure of dopant ions can be used in conjunction with the NC defects and surface trap states to design new plasmonic colloidal semiconductor NCs.

(2)

where ωp is the plasma frequency. According to this formulation, the plasma frequency has a square root dependence on the free electron concentration: ⎛ Ne 2 ⎞1/2 ⎟⎟ ωp = ⎜⎜ ⎝ εoptε0m* ⎠

(3)

while the absorption coefficient (α) is linearly dependent on N: α=

εoptωp2 ncω 2τ

=

Ne 2 m*ε0ncτω 2

(4)

where τ is the relaxation (damping) time and c is the speed of light. The Drude−Lorentz theory has been successfully applied to predict and explain the reflectivity of metal oxide thin films, and subsequently the IR absorption of various metal-oxide NCs. Most reported colloidal plasmonic metal oxide NCs, including Sn-doped In2O3 (Sn:In2O3 or ITO),11−14 Sb-doped SnO2 (Sb:SnO2 or ATO),15,16 Al-doped ZnO (AZO),17,18 Indoped CdO (ICO),19 and Nb-doped TiO2,20 have relatively narrow LSPR tuning ranges between NIR and MIR. Achieving and controlling plasmonic properties of colloidal NCs in MIR is of great current interest, because of their potential application in thermal and chemical sensors and near-field enhanced spectroscopies.21−23 Because of the energy range of LSPR, these materials could be particularly promising for applications involving heat harvesting. Different strategies can be applied to extend the tuning range, including direct injection of charge carriers,24 photodoping,25,26 and simultaneous cation−anion codoping,27 but the correlations among the host lattice crystal and electronic structure, electronic structure and properties of a dopant ion, and the resulting plasmonic properties are still relatively poorly understood. Indium oxide is a tremendously important industrial and technological material, and ITO remains the most studied, best understood, and most widely used transparent conducting oxide.28−30 Similarly, In2O3 NCs have also shown promise in electronics,31 photonics,32 and spintronics,33−35 while ITO NCs have emerged as a benchmark for colloidal plasmonic metal oxide NCs.11−13,25 For example, in our recent work on phase-dependent plasmonic properties of ITO NCs, we demonstrated a dramatic difference between the plasma absorption of corundum-type ITO NCs having a rhombohedral crystal structure (rh-ITO) and bixbyite-type ITO NCs having a cubic crystal structure (bcc-ITO).12,36 While bcc-ITO NCs exhibit a strong LSPR transition in the NIR region, rh-ITO NCs show negligible plasmonic effect, despite being prepared under identical conditions. This difference has been attributed to higher concentration of free electrons in bccITO, relative to rh-ITO NCs, because of the smaller donor activation energy. Therefore, indium oxide NCs generally represent an excellent model system for investigating the influence of different dopants on the electronic structure and properties of plasmonic NCs and their potential applications. However, besides ITO NCs, little is known about other colloidal plasmonic systems based on In2O3. A recent report on Ce-

■

EXPERIMENTAL AND THEORETICAL METHODS

Materials. All reagents and solvents were used as received from the manufacturer without further purification. Indium acetylacetonate (In(acac)3, 98%) and titanium(IV) isopropoxide (Ti(O-i-Pr)4, 98%) were purchased from Strem Chemicals. Antimony(III) chloride (SbCl3, 99%), oleylamine (70%), and oleic acid (90%) were obtained from Sigma−Aldrich. Toluene (EMD Chemicals, 99.9%), hexane (Fisher Scientific, 98.5%), and absolute ethanol were used as solvents. Synthesis of Plasmonic In2O3 NCs. The previously developed procedure was used in a modified form to synthesize Sb- and Ti-doped In2O3 NCs.12,38 For the synthesis of Sb:In2O3 NCs, 0.9 g of In(acac)3, 7.2 g of oleylamine, and varying amounts of SbCl3 were mixed in a 100 mL three-neck round-bottom flask under the flow of argon. The mixture was heated to 250 °C and reacted for 1 h with constant stirring at the same temperature. After the reaction, the mixture was cooled to room temperature and precipitated by centrifugation at 3000 rpm for 10 min. The precipitate was washed with ethanol two more times, followed by centrifugation to ensure thorough cleaning of the reaction product. Subsequently, 1.5 mL of oleic acid (OA) was added to the obtained sample, and the mixture was stirred at 90 °C in an oil bath for 30 min, followed by precipitation with ethanol. This dispersion−precipitation procedure was performed three times to remove surface-bound dopant ions and excess oleic acid.39 Some of the final sample was dried and crushed for X-ray diffraction (XRD) measurement, and the remainder was dispersed in toluene for spectroscopic measurements. The same procedure was performed for the synthesis of Ti:In2O3 NCs but with a Ti(O-i-Pr)4 dopant precursor. Characterization and Measurements. XRD measurements were carried out with an INEL XRD diffractometer, utilizing monochromatic Cu Kα radiation (λ = 1.5406 Å). Raman spectra were recorded with a Renishaw 1000 spectrometer, using a Ti-sapphire laser (785 nm) as the excitation source. Transmission electron microscopy (TEM) images were collected with a JEOL Model 2010F microscope operating at 200 kV, and energy-dispersive X-ray spectroscopy (EDX) was employed to characterize the elemental composition of the samples. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo-VG Scientific ESCALab 250 microprobe equipped with a monochromatic Al Kα source (1486.6 eV). Ultraviolet−visible-light−near-infrared (UV-vis-NIR) absorption spectra were recorded with a Varian Cary 5000 UV-vis-NIR spectrophotometer. The samples were prepared by drop-casting a suspension of colloidal NCs on the quartz substrate to minimize the 4971


Article

Chemistry of Materials interference of the solvent vibrational overtones at long wavelengths, and the spectra were collected from 200 nm to 3000 nm. Fourier transform infrared (FTIR) spectra were recorded in the absorption mode with the FTIR Bruker Tensor 37 spectrometer ranging from 400 cm−1 to 4000 cm−1. Colloidal suspensions of NCs in toluene were drop-casted on a pristine KBr pellet to avoid scattering and allow for proper background subtraction. A sealed test tube was utilized for rapid thermal annealing of NCs deposited on the KBr substrate, which was conducted under controlled atmosphere. Quantitative analysis of the intensity of the plasmon band was carried out with an appropriate normalization method based on the phonon vibrations of In2O3. Computational Details. The electronic structure calculations were performed within the plane wave self-consistent field (PWSCF) approximation using Quantum Espresso code.40 The Perdew−Burke− Ernzerhof (PBE) exchange correlation potential was used within the generalized gradient approximation (GGA), and a plane wave cutoff energy of 500 eV. For self-consistent field (SCF) calculations, we used a 6 × 6 × 6 Monkhorst−Pack (MP) k-point grid. For non-selfconsistent field (NSCF) calculations, a denser MP grid of 9 × 9 × 9 was used. As the first step in the calculations, geometrical optimization of all crystal structures was performed. During the optimization process, atoms in the lattice were allowed to move freely along the x-, y-, and zdirections. The equilibrium lattice parameters obtained from the optimization procedure were used for further SCF and NSCF calculations. Indium oxide has a cubic bixbyite structure consisting of a total of 80 atoms in a conventional unit cell and 40 atoms in a primitive bodycentered cubic (bcc) cell. The In atoms are located in two inequivalent sites. One-fourth of In atoms reside in 8b sites (C3v point group symmetry), labeled as In1, and the remaining three-fourths of In atoms reside in less-symmetric 24d sites (C2 point group symmetry), labeled as In2. The Ti and Sb dopants were incorporated into the In2O3 lattice by replacing two In1 atoms, which represents the doping concentration of 6.25 at. %. The calculations were also performed by substituting the d sites with Sb and Ti. However, it was found that doping at b sites is energetically more favorable, in agreement with the existing theoretical results for other relevant dopants.41

tration above ca. 14% in Sb:In2O3 NCs results in the appearance of XRD peaks characteristic for elemental Sb (see Figure S1). The Sb:In2O3 and Ti:In2O3 NCs in Figure 1a were further investigated by Raman spectroscopy, which can provide structural information for crystalline samples. Figure 1b shows the Raman spectra of selected In2O3, Sb:In2O3, and Ti:In2O3 NCs. Four characteristic Raman peaks resulting from the phonon modes of bcc-In2O3 at 307, 366, 497, and 630 cm−1 (labeled as vertical blue lines) are observed for undoped and doped In2O3 NCs.12 These peaks become broader and weaker with increasing doping concentration for both Sb:In2O3 and Ti:In2O3 NCs. Importantly, for doping concentrations above ca. 10%, Raman peaks for Sb:In2O3 NCs vanish completely, while they remain clearly observable for Ti:In2O3 NCs. Although doped In2O3 NCs maintain the bixbyite-type crystal structure, as evidenced from the XRD patterns, weakening, broadening, and eventual disappearance of Raman peaks suggest the local crystal lattice distortion due to the incorporation of dopants into bcc-In2O3 NCs. Figure 2a shows an overview TEM image of 2.8% Sb:In2O3 NCs. These NCs have a quasi-spherical shape, with an average size of 9.9 ± 1.2 nm (Figure 2b). High-resolution TEM images confirmed the single crystalline nature of these NCs. The measured lattice spacing of ca. 2.92 Å corresponds to the {222} plane of bulk bcc-In2O3 (see inset in Figure 2a). At high concentrations of SbCl3 precursor, the NCs self-assemble into flower-like clusters with an average size of ca. 25 nm (see Figure 2c, as well as Figure S2 in the Supporting Information). The lattice fringes with the same spacing are found to extend over several NCs or even the entire cluster (see inset in Figure 2c), indicating that these nanoflowers are formed by oriented attachment of individual Sb:In2O3 NCs. For heavily doped NCs, more Cl− ions are present in the reaction mixture and adsorbed on the NC surfaces, which facilitates the formation of the nanoflowers, because of the anisotropic distribution of localized surface charges and decreased surface protection by the coordinating solvent molecules.12,42 Figure 2d displays an overview TEM image of 7.8% Ti:In2O3 NCs. The Ti:In2O3 NCs exhibit more irregular shapes and noticeably larger size distributions than Sb:In2O3 NCs (Figure 2e). However, no flower-like clusters were observed for heavily doped Ti:In2O3 NCs (see Figure S3 in the Supporting Information). This observation is consistent with our hypothesis that high ionic strength of the reaction mixture is necessary for anisotropic charging of NC surfaces, which leads to the dipole moment formation and oriented NC attachment via dipole−dipole interactions.43 Elemental EDX mapping (Figure 2f) and electron energy loss spectroscopy (EELS) line scans (Figure S4 in the Supporting Information) indicate a homogeneous distribution of Ti dopants among different NCs and within individual NCs, respectively. The oxidation state of dopant ions in Sb:In2O3 and Ti:In2O3 NCs, which is crucial for aliovalent doping and the resulting plasmonic properties, was investigated by XPS. Because of the overlap of the In and Sb peaks in EDX, XPS was also used to confirm the composition of Sb:In2O3 NCs synthesized with different concentration ratios of Sb3+ and In3+ precursors. A representative survey spectrum for 7.3% Sb:In2O3 NCs (synthesized with a precursor ratio of [Sb]/[In] = 0.1) is shown in Figure 3a. Apart from a weak C 1s peak, resulting from the organic molecules adsorbed on the surface of the Sb:In2O3 NCs, Sb, In, and O peaks are clearly observed in the spectrum. Figure 3b shows high-resolution XPS spectra of

■

RESULTS AND DISCUSSION X-ray diffraction (XRD) patterns of typical Sb:In2O3 and Ti:In2O3 NCs having different doping concentrations are shown in Figure 1a, as well as Figure S1 in the Supporting Information. For all samples, the observed diffraction peaks are in good agreement with the bulk bcc-In2O3 reference pattern (vertical red lines), with no evidence of the dopant-related secondary phase formation. Increasing the doping concen-

Figure 1. (a) XRD patterns and (b) Raman spectra of Sb:In2O3 and Ti:In2O3 NCs having different doping concentrations, as indicated in the graphs. The XRD pattern and Raman spectrum of undoped In2O3 NCs are shown with black traces. Vertical lines in panels (a) and (b) represent characteristic peaks for bcc-In2O3. 4972


Article

Chemistry of Materials

Figure 2. (a) TEM image of 2.8% Sb:In2O3 NCs (inset shows a high-resolution TEM image of a single NC). (b) Size distribution histogram for NCs in panel (a), indicating an average NC size. (c) TEM image of 10.6% Sb:In2O3 NCs forming flower-like clusters (inset: high-resolution TEM image of a single nanoflower). (d) TEM image of 7.8% Ti:In2O3 NCs (inset shows a high-resolution TEM image of a single NC). (e) Size distribution histogram for NCs in panel (d), indicating an average NC size. (f) Scanning transmission electron microscopy image and the corresponding EDX elemental maps of 7.8% Ti:In2O3 NCs, as labeled in the panels.

oxidation states, Sb5+ and Sb3+, where Sb5+ can behave as an electron donor (aliovalent dopant), providing two electrons to the conduction band, while Sb3+ is isovalent with In3+ and, hence, is electronically inactive. The difference in the electron binding energy between Sb3+ and Sb5+ is ca. 0.8 eV,16 which is consistent with the contribution of the two oxidation states to the observed 3d3/2 peaks in Figure 3c. Figure 3d shows the deconvolution of the Sb 3d3/2 peaks, taking into account the difference between the binding energy of Sb3+ and Sb5+. The areas of the deconvoluted peaks for Sb5+ are larger than those for Sb3+ by ca. 30%, suggesting the dominant contribution from Sb5+ and the n-type character of Sb:In2O3 NCs. Although the mechanism of Sb3+ oxidation is still under investigation, the presence of Sb5+ dopant has also been demonstrated in the synthesis of Sb:SnO2 NCs using SbCl3 as a precursor.15,16 Possible origins of Sb5+ formation include Sb3+ oxidation by traces of oxygen in the reaction mixture and its disproportionation into Sb5+ and elemental Sb, which was indeed observed as the secondary phase at high doping concentrations. Despite the fact that SbCl5 seems like a more rational choice for aliovalent doping, this reagent is toxic, volatile, and highly corrosive. Furthermore, it acts as a strong oxidizing agent, leading to solvent oxidation and a mixture of dopant oxidation states. On the other hand, in Ti:In2O3 NCs, the dopant ions remain in a 4+ oxidation state, with no evidence of the significant presence of other oxidation states (see Figure S5 in the Supporting Information). Figure 4a shows the band-gap-normalized absorption spectra of Sb:In2O3 NCs having different doping concentrations, as indicated in the graph. The most significant change in the absorption spectrum is a gradual increase in the intensity in the NIR region, corresponding to the onset of the LSPR absorption, with increasing doping concentration. The determined optical band gap for pure In2O3 NCs is ca. 3.69 eV, which is in reasonably good agreement with the value reported in the literature.28 Closer inspection of the spectra in the band-gap region reveals that the onset of band-gap

Figure 3. (a) XPS survey spectrum of 7.3% Sb:In2O3 NCs. (b, c) High-resolution XPS spectra of Sb:In2O3 NCs having different doping concentrations in (b) the In 3d region and (c) the Sb 3d region. (d) Deconvolution of the Sb 3d3/2 peaks in panel (c).

Sb:In2O3 NCs with various doping levels in the In 3d region. The peaks at ca. 452.4 and 444.9 eV are readily assigned to In 3d3/2 and 3d5/2 transitions, respectively. Figure 3c shows the Sb 3d and O 1s peaks for Sb:In2O3 NCs with different doping concentration, as labeled in the graph. The determined binding energies are 540.1 and 531.8 eV for Sb 3d3/2 and 3d5/2, respectively, and 530.9 eV for O 1s.16,44 Given the overlap of Sb 3d5/2 and O 1s peaks, the actual doping concentrations of Sb were determined from the relative intensities of Sb 3d3/2 and In 3d peaks, and the obtained results are listed in Table S1 in the Supporting Information. Importantly, the Sb 3d3/2 band is broad and asymmetric, suggesting a possible superposition of two different peaks. Antimony dopants can exist in two 4973


Article


level and reaction temperature. Compared to the spectrum of undoped In2O3 NCs, intense and broad band is observed from ca. 650 cm−1 to 4000 cm−1 for 2.8% Sb:In2O3 NCs synthesized under identical conditions, which can be attributed to the plasmon resonance of free carriers. The appearance of the broad plasmon absorption for Sb:In2O3 NCs is the signature of the presence of antimony in the 5+ oxidation state. Figure 4c compares the LSPR spectra of Sb:In2O3 NCs having different doping concentrations. The intensity (blue circles) and energy position (red squares) of the band maxima are plotted with respect to the doping concentration in Figure 4d. The peak energy position dependence on the doping concentration is well fit with eq 3 for Sb doping concentration below ca. 11% (red line), indicating the square root dependence of the plasma frequency on the concentration of free electrons, as predicted by the Drude−Lorentz model.10 Based on the optical parameter values for bulk bcc-In2O3 (m* = 0.3m0, εopt ≈ 4.0), the maximum concentration of free electrons in Sb:In2O3 NCs is determined to be ca. 1.35 × 1020 cm−3 for 10.6% Sb:In2O3 NCs. The absorption intensity, on the other hand, obeys linear dependence on the doping concentration, which is consistent with the relationship between the free electron concentration and absorption coefficient (eq 4). Further increase in doping concentration beyond ca. 11% leads to a decrease in both LSPR absorption energy and intensity. This reversal in the trend of the two parameters is largely due to the electron scattering effects by lattice defects. It is interesting to note that the maximum LSPR intensity for this system corresponds to almost completely vanished Raman peaks. This coincidence may be associated with the presence of doubly charged doping sites (Sb5+), which can lead to a more pronounced local lattice distortion, together with a high concentration of free electrons. We compared and contrasted the plasmonic properties of Sb:In2O3 NCs with those of Ti:In2O3 NCs (Figure 5).

Figure 4. (a) Absorption spectra of Sb:In2O3 NCs with different doping concentrations and pure In2O3 NCs, as indicated in the graph. Inset shows Tauc plots of NCs used to determine optical band gaps. (b) FTIR spectra of 2.8% Sb:In2O3 NCs (blue line) and undoped In2O3 NCs (red line) capped with oleic acid. The spectrum of oleic acid is shown for comparison (black line). (c) LSPR absorption bands of Sb:In2O3 NCs in panel (a). (d) LSPR band maximum intensity (blue circles) and energy (red squares) as a function of doping concentration. Red and blue lines are fits to eqs 3 and 4, respectively.

absorption shifts systematically to higher energy with increasing doping concentration, due in part to an increase in the number of electrons accumulating in the conduction band, which is referred to as the Burstein−Moss effect. The optical band gaps of Sb:In2O3 NCs are obtained from linear fits to Tauc plots, as depicted in the inset of Figure 4a (also see Figure S6 in the Supporting Information). To obtain the complementary part of the LSPR absorption of Sb:In2O3 NCs in the MIR, we conducted FTIR spectroscopy measurements. Figure 4b compares the FTIR spectra of 2.8% Sb:In2O3 NCs (blue trace) and pure In2O3 NCs (red trace) treated post-synthetically with oleic acid. The FTIR spectrum of oleic acid is also shown as the reference (black trace). In the spectrum of oleic acid, two sharp bands at 2924 and 2854 cm−1 can be assigned to the asymmetric and symmetric CH2 stretch, respectively, intense peak at 1710 cm−1 can be assigned to the C O stretch, and the band at 1285 cm−1 can be assigned to the C−O stretch.45,46 The peaks at 1462 and 937 cm−1 arise from the O− H in-plane and out-of-plane bending modes, respectively.45,46 In the case of In2O3 and Sb:In2O3 NCs capped with oleic acid, the asymmetric and symmetric CH2 stretching modes experience a slight red shift (ca. 2 cm−1), which can be attributed to the adsorbed configuration of the oleic acid molecules.45 In addition, the disappearance of the CO stretch peak and the appearance of two new peaks at 1556 and 1443 cm−1 assigned to the characteristic asymmetric (COO−) and symmetric (COO−) stretch, respectively, strongly suggest adsorption of oleic acid on the NC surfaces.45 Importantly, four intense peaks at 601, 564, 538, and 441 cm−1 correspond to the In−O phonon modes, which are characteristic of cubic In2O3.47 The peak at 564 cm−1 is used to normalize the spectra of all Sb:In2O3 NCs for quantitative analysis of the intensity of plasmon absorption. The intensity of this peak is proportional to the amount of In−O bonds and is not affected by doping

Figure 5. (a) Absorption spectra of Ti:In2O3 NCs with different doping concentrations and pure In2O3 NCs, as indicated in the graph. Inset shows a magnified view of the onset of the LSPR absorption. (b) Tauc plots of NCs in panel (a) used to determine optical band gaps. (c) LSPR absorption bands of Ti:In2O3 NCs in panel (a). (d) LSPR band maximum intensity (blue circles) and energy (red squares), as a function of doping concentration. Red and blue lines are fits to eqs 3 and 4, respectively. 4974


Article


Figure 6. Calculated band structure (top) and projected density of states (PDOS) (bottom) diagrams of (a) In2O3, (b) 6.25% Sb:In2O3, and (c) 6.25% Ti:In2O3. Fermi levels in parts (b) and (c) (top panels) are indicated with the dashed lines. Contribution from specific orbitals to the total density of states (TDOS) is labeled in the graphs.

have suggested that as few as 3−4 electrons can lead to a collective oscillation of free carriers in semiconductor NCs and the emergence of a plasmon-type absorption band.26,51 The transition between this quantum mechanical and classical phenomenon was suggested to be gradual and driven by the Coulomb repulsion among the excess charge carriers.52 At a Ti4+ doping concentration of 3.6% (blue line), the fwhm increases and the band becomes noticeably asymmetric, which is consistent with the change in the nature of the electron excitation. Further increase in the doping concentration leads to a blue shift of well-defined LSPR band and an increase in its intensity. However, increase in the doping concentration above 8% causes a slight red shift, diminished intensity, and broadening of the LSPR band due to ionized impurity scattering around the Ti4+ dopant, similar to Sb:In2O3 and other semiconductor plasmonic NCs. This reversal is also evident in the red shift of the band edge absorption in Figure 5b and the onset of the plasmon absorption in Figure 5a. Figure 5d plots the correlation between the plasmonic properties and doping concentration of Ti:In2O3 NCs. The absorption intensity increases linearly with increasing Ti4+ doping concentration as expected from eq 3, while the maximum energy follows a square root dependence on the doping concentration (eq 4), because of the corresponding increase in the free electron concentration. The overall match of the absorption band energy and intensity with eqs 3 and 4, respectively, further justifies the assignment of these absorption bands to the collective oscillation of free electrons, as described classically by the Drude−Lorenz model. The concentration of free electrons in Ti:In2O3 NCs ranges from ca. 1.3 × 1019 cm−3 to 6.9 × 1019 cm−3 with the doping concentration range from

Titanium is chosen as an aliovalent dopant ion, because it has a similar size but distinctly different electronic structure from Sb, including much smaller Pauling electronegativity (1.54 for Ti vs 2.05 for Sb). This system also constitutes an interesting comparison with Ce-doped In2O3,37 in which the dopant also exist in a 4+ oxidation state, but has a significantly larger ionic radius than Ti4+. Figure 5a shows the band-gap-normalized absorption spectra of Ti:In2O3 NCs with different doping concentrations. Similar to Sb:In2O3, the absorption intensities of Ti:In2O3 NCs in the NIR region increase gradually with doping concentration, although with an onset at much longer wavelengths (see inset in Figure 5a). This trend is consistent with the formation of lower-lying LSPR in Ti:In2O3 than in Sb:In2O3 NCs. The increased concentration of free electrons introduced by doping with Ti4+ also leads to an increase in the optical band gap of Ti:In2O3 NCs, which is due to the Burstein−Moss effect (Figure 5b). Both the plasmon absorption onset in the NIR and the Burstein−Moss shift of the band-gap absorption indicate the formation of a free electron gas in Ti:In2O3 NCs. The FTIR absorption spectra of Ti:In2O3 NCs normalized to the phonon vibration peak at 564 cm−1 are shown in Figure 5c. These spectra are used for quantitative analysis of the LSPR absorption intensity and energy. At low doping concentrations (ca. 1%), the absorption band maximum is located at ca. 970 cm−1 and has a full width at half-maximum (fwhm) of 670 cm−1 (red line in Figure 5c). As the doping concentration increases the LSPR absorption band broadens and the band maximum shifts to higher energies. The IR absorption of the semiconductor oxide NCs upon the addition of extra charge carriers has previously been associated with discrete intraband transitions.24,48−50 More recent studies 4975


Article


(0.76 Å) and Sb5+ (0.60 Å), and should lead to an enhanced lattice strain and weaker orbital overlap, resulting in stronger electronic damping (for comparison, the ionic radius of In3+ is 0.80 Å). Taken together, the structural and theoretical results suggest that Ti:In2O3 NCs should have a blue-shifted LSPR band and larger absorption bandwidth than Sb:In2O3 NCs. This is in contrast with Ce4+, which has an ionic radius that is similar to that of In3+, reducing the lattice strain and contributing to higher electron mobility and higher Q-factor in Ce:In2O3 than in ITO NCs.37 However, our experimental results are exactly opposite, where the LSPR bands of Ti:In2O3 NCs lie at lower energy, with respect to those of Sb:In2O3 NCs, and exhibit significantly narrower bandwidths. This discrepancy between theoretical prediction and experimental results implies that there are competing mechanisms that reduce doping effectiveness in Ti:In2O3 NCs, without significantly impacting electron mobility. The most plausible explanation for this system is the presence of strain-induced trap states in Ti:In2O3 NCs. Comparison between the Raman spectra indicates that there is effectively less internal structural disorder (e.g., oxygen vacancies) in Ti:In2O3 NCs that would result in ionized impurity scattering, reduced mobility, and LSPR band broadening. The higher degree of long-range order in Ti:In2O3 NCs suggests that a significant fraction of charge carriers is localized in the trap states formed on the NC surfaces, which reduce the carrier concentration while limiting the effect of electronic damping. Lower electronegativity of Ti, relative to Sb, is an important contributing factor for stronger electron trapping in Ti:In2O3 than in Sb:In2O3 NCs. It has been shown that Sn4+ dopants stabilize the extra electrons in ITO NCs, in contrast to photodoped In2O3 NCs.25 Analogously, the higher electronegativity of Sb causes stronger stabilization of the extra electron in Sb:In2O3 than in Ti:In2O3 NCs. More circumstantial evidence of surface strain-induced defect trap states is more irregular NC morphology and a larger NC size distribution of Ti:In2O3, compared to Sb:In2O3 or undoped In2O3 NCs prepared under identical conditions. Note that inhomogeneity in dopant distribution among different NCs can also lead to an increase in the bandwidth of LSPR spectra of both Sb:In2O3 and Ti:In2O3 NCs.57 However, systematic differences in the dopant distribution in these two types of NCs are not likely. We found that Ti4+ dopants, which have ionic radii that are almost identical to that of Sb5+, are largely homogeneously distributed in individual NCs (see Figure S4), suggesting that the aliovalent dopant distribution should not be dramatically different in Sb:In2O3 and Ti:In2O3 NCs. The dependence of LSPR on NC composition, lattice and defect structure, and surface traps offers an abundance of opportunities for manipulating the plasmonic properties of metal oxide NCs in the MIR range. Figure 7a shows LSPR spectra of Ti:In2O3 NCs synthesized with the same starting precursor ratio ([Ti]/[In] = 0.1) at different temperatures. With decreasing reaction temperature the LSPR absorption exhibits a slight blue shift and a significant increase in intensity. This increase in LSPR absorption intensity cannot be ascribed simply to an increase in the free electron density, because of the increased doping concentration. A lower synthesis temperature has been shown to lead to an increase in the concentration of native defects in group 13 oxide NCs,58 ultimately leading to an increase in the electron scattering rate (1/τ). According to eq 4, the LSPR absorption intensity of degenerately doped semiconductors is proportional to 1/τ, due to the critical role of the

1.1% to 7.8%, respectively. Taking into account the average NC volume, we estimate that each 1.1% Ti:In2O3 NC contains ∼4 conduction band electrons. In this regime, it is likely that singlecarrier electronic transitions coexist with the multiple-carrier collective oscillations, as indicated above. To our knowledge, this result matches the lowest number of carriers that has been reported to exhibit a plasmon-like absorption, although in significantly larger NCs.26,51 Several important questions arise from the comparison of the properties of these new In2O3-based plasmonic NCs. What is the origin of such a broad tunability of LSPR absorption of In2O3 NCs aliovalently doped with different ions? What determines the concentration of free electrons and the degree of their delocalization? What is the role of NC surface defects in plasmonic properties of aliovalently doped In2O3 NCs? To address these questions, we performed comparative density functional theory (DFT) electronic structure calculations for geometry-optimized structures of In2O3, Sn:In2O3, Sb:In2O3, Ti:In2O 3 NCs. The band structure diagrams and the corresponding density of states are shown in Figure 6. As evident from Figure 6a, pure In2O3 is a direct band gap semiconductor, having the valence band maximum and the conduction band minimum at the center of the Brillouin zone (Γ point). The calculated band-gap energy at Γ point is ca. 1.15 eV, which is significantly smaller than the optically determined band-gap energy, but generally in good agreement with the previous theoretical results.41,53,54 This discrepancy between experimental and theoretical values is due to well-documented underestimation of the band-gap energy by DFT.55 A large dispersion in the conduction band suggests a low effective mass and high mobility of electrons.53 The density-of-state diagrams for In2O3 also confirm previously reported results that valence band predominantly consists of O 2p and, to a lower extent, In 5p states, while the conduction band has In 5s character.41,56 Sn-doped In2O3 is also a well-characterized system, which retains the large conduction band dispersion and, therefore, the low effective electron mass characteristic for In2O3 (see Figure S7 in the Supporting Information).41,53 The Burstein−Moss shift, which is associated with the carrier population of the conduction band states, contributes mostly to the observed blue shift of the band-edge absorption.53 Importantly, the states originating from Sn dopants do not exhibit significant localization at the conduction band minimum or the Fermi level, suggesting strong hybridization of the Sn4+ dopants with coordinating oxygen ions. The electronic structures of Sb:In2O3 and Ti:In2O3 (Figures 6b and 6c, respectively) were found to be distinctly different from those of Sn:In2O3 or In2O3. Figure 6b shows the calculated electronic band structure and density of states of 6.25% Sb:In2O3. In contrast to Sn:In2O3, Sb:In2O3 shows a smaller conduction band dispersion and has a narrower conduction band width, indicating a lower electron group velocity at the Fermi level. Consequently, the Fermi level lies at lower energy relative to Sn:In2O3, implying a lower conduction band occupancy. Furthermore, Sb 5s states are localized at the Fermi level, suggesting their weak overlap with the oxygen orbitals. Although Ti:In2O3 also has reduced conduction band dispersion, compared to Sn:In2O3 (or In2O3), and strong localization of Ti 3d states in the conduction band, the Fermi level lies at higher energy from the bottom of the conduction band, relative to Sb:In2O3 (Figure 6c). These results imply that Ti:In2O3 should have a larger concentration of free electrons than Sb:In2O3. Furthermore, the ionic radius of six-coordinate Ti4+ (0.61 Å) is smaller than the average radius between Sb3+ 4976



Article

■

CONCLUSIONS In summary, we synthesized two new plasmonic metal oxide nanocrystals (NCs) by doping colloidal In2O3 NCs with Sb and Ti in situ. The two systems have distinctly different electronic structures, allowing for a significant expansion of the plasmonic tunability and properties of In2O3 NCs. Absorption bands associated with the localized surface plasmon resonance (LSPR) of Ti:In2O3 NCs are red-shifted, and significantly narrower and more symmetric, relative to those of Sb:In2O3 NCs having similar doping concentrations. This comparison suggests lower free electron concentration but reduced ionized impurity scattering in Ti:In2O3 NCs, which is consistent with the lower degree of NC lattice disorder. Surprisingly, the relative position of the Fermi level is calculated to be higher in Ti:In2O3 than in Sb:In2O3, suggesting the propensity of free electron trapping in Ti:In2O3 NCs. These trap states can be generated on NC surfaces, because of the strain induced by the significant difference in ionic radius of In3+ and substitutionally doped Ti4+. Although defect trap states also are present in Sb:In2O3 NCs, the extra electrons are less thermodynamically stabilized in Ti:In2O3 NCs, because of the lower electronegativity of Ti, relative to Sb dopants. While surface trap states in NCs could be detrimental for many applications, including photoluminescence or photovoltaics, in this case, they provide yet another pathway to tune LSPR and manipulate the plasmonic properties of metal oxide NCs. There has been a particularly strong interest lately in developing mid-infraredactive plasmonic NCs. Because of the low internal impurity scattering and enhanced surface trapping of the conduction band electrons, the plasmon absorption of Ti:In2O3 NCs is tunable below 1000 cm−1, which is the lowest-lying LSPR reported for aliovalently doped In2O3 NCs. The results of this work have broad implications for the design of plasmonic metal oxide NCs. The ability to simultaneously control surface trapping, defect concentration, and thermodynamic stabilization of free electrons in a given NC host lattice via selection of an aliovalent dopant allows for an expansion of spectral tunability and characteristics of plasmonic metal oxide NCs.

Figure 7. (a) LSPR spectra of Ti:In2O3 NCs synthesized with precursor ratio [Ti]/[In] = 0.1 at different temperatures, as indicated in the graph. (b) LSPR spectra of 7.8% Ti:In2O3 NCs upon rapid annealing at 400 °C (annealing time periods are shown in the graph). (c) LSPR spectra of 7.8% Ti:In2O3 NCs as prepared (black trace), and upon treating in oxygen (red trace) and subsequently in hydrogen (blue trace) atmosphere for 60 s. (d) LSPR spectra of 1.1% Ti:In2O3 (black trace), 7.3% Sb:In2O3 (red trace) and 12.3% Sn:In2O3 (blue trace) NCs, demonstrating tunability throughout the MIR and NIR spectral range.

scattering events in the conservation of electron momentum. Plasmonic properties of metal oxide NCs can also be manipulated post-synthetically. Figure 7b shows LSPR absorption spectra of Ti:In2O3 NCs upon rapid annealing at 400 °C. Annealing for only 60 s increases the intensity of LSPR absorption by a factor of 2. While annealing at a relatively low temperature for such short amounts of time is not sufficient to impact the NC lattice structure, it could release the trapped charge carriers and reduce the density of traps on the NC surfaces, leading to an effective increase in the free electron concentration. A slight increase in the Q-factor with increasing annealing time also attests to the removal of some internal (subsurface) defects, together with surface trap states. The effect of rapid thermal annealing on NC surfaces is also evident through a decrease in the intensity of CH2 vibrational modes, associated with the oleic acid molecules coordinated to NC surfaces (see Figure S8 in the Supporting Information). Annealing in the oxygen atmosphere under similar conditions results in a decrease in intensity and a red shift of the LSPR, which can be restored to its starting state upon equally short thermal treatment in the presence of hydrogen (see Figure 4c). We associate this effect of oxygen atmosphere with the wellknown formation of additional oxide-based surface trap states, including O2−/O−.59 Taken together, these results confirm the importance of dopant electronic structure, and trapping and scattering effects for controlling LSPR, and suggest the promise of Ti:In2O3 NCs as deep MIR-active material for sensing and near-field enhancement processes. Furthermore, the development of these new types of plasmonic NCs allows for tuning of LSPR absorption bands from ca. 650 cm−1 to 8000 cm−1 in In2O3 NCs,12 by changing the nature of dopants, their concentration, and impact on NC growth (Figure 7d).

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01349. XRD patterns, TEM images, XPS and FTIR spectra, elemental analysis data, and band structure calculations (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pavle V. Radovanovic: 0000-0002-4151-6746 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada (No. RGPIN-201567304032) and the University of Waterloo (UW-Bordeaux 4977


Article


TiO2 Nanocrystals with Tunable Infrared Absorption. Chem. Mater. 2013, 25, 3383−3390. (21) Sachet, E.; Losego, M. D.; Guske, J.; Franzen, S.; Maria, J.-P. Mid-Infrared Surface Plasmon Resonance in Zinc Oxide Semiconductor Thin Films. Appl. Phys. Lett. 2013, 102, 051111. (22) Sachet, E.; Shelton, C. T.; Harris, J. S.; Gaddy, B. E.; Irving, D. L.; Curtarolo, S.; Donovan, B. F.; Hopkins, P. E.; Sharma, P. A.; Sharma, A. L.; Ihlefeld, J.; Franzen, S.; Maria, J.-P. Dysprosium-Doped Cadmium Oxide as a Gateway Material for Mid-Infrared Plasmonics. Nat. Mater. 2015, 14, 414−420. (23) Abb, M.; Wang, Y.; Papasimakis, N.; de Groot, C. H.; Muskens, O. L. Surface-Enhanced Infrared Spectroscopy Using Metal Oxide Plasmonic Antenna Arrays. Nano Lett. 2014, 14, 346−352. (24) Shim, M.; Guyot-Sionnest, P. Organic-Capped ZnO Nanocrystals: Synthesis and n-Type Character. J. Am. Chem. Soc. 2001, 123, 11651−11654. (25) Schimpf, A. M.; Lounis, S. D.; Runnerstrom, E. L.; Milliron, D. J.; Gamelin, D. R. Redox Chemistries and Plasmon Energies of Photodoped In2O3 and Sn-Doped In2O3 (ITO) Nanocrystals. J. Am. Chem. Soc. 2015, 137, 518−524. (26) Faucheaux, J. A.; Jain, P. K. Plasmons in Photocharged ZnO Nanocrystals Revealing the Nature of Charge Dynamics. J. Phys. Chem. Lett. 2013, 4, 3024−3030. (27) Ye, X.; Fei, J.; Diroll, B. T.; Paik, T.; Murray, C. B. Expanding the Spectral Tunability of Plasmonic Resonances in Doped MetalOxide Nanocrystals through Cooperative Cation−Anion Codoping. J. Am. Chem. Soc. 2014, 136, 11680−11686. (28) Hamberg, I.; Granqvist, C. G. Evaporated Sn-Doped In2O3 Films: Basic Optical Properties and Applications to Energy-Efficient Windows. J. Appl. Phys. 1986, 60, R123−R159. (29) Rhodes, C.; Franzen, S.; Maria, J.-P.; Losego, M.; Leonard, D. N.; Laughlin, B.; Duscher, G.; Weibel, S. Surface Plasmon Resonance in Conducting Metal Oxides. J. Appl. Phys. 2006, 100, 054905. (30) Hamberg, I.; Granqvist, C. G. Theoretical Model for the Optical Properties of In2O3:Sn Films in the 0.3−50 μm Range. Sol. Energy Mater. 1986, 14, 241−256. (31) Swisher, S. L.; Volkman, S. K.; Subramanian, V. Tailoring Indium Oxide Nanocrystal Synthesis Conditions for Air-Stable HighPerformance Solution-Processed Thin-Film Transistors. ACS Appl. Mater. Interfaces 2015, 7, 10069−10075. (32) Ghodsi, V.; Layek, A.; Hegde, M.; Yildirim, B.; Radovanovic, P. V. Native Defects Determine Phase-Dependent Photoluminescence Behavior of Eu2+ and Eu3+ in In2O3 Nanocrystals. Chem. Commun. 2016, 52, 4353−4356. (33) Farvid, S. S.; Hegde, M.; Radovanovic, P. V. Influence of the Host Lattice Electronic Structure on Dilute Magnetic Interactions in Polymorphic Cr(III)-Doped In2O3 Nanocrystals. Chem. Mater. 2013, 25, 233−244. (34) Tandon, B.; Shanker, G. S.; Nag, A. Multifunctional Sn- and FeCodoped In2O3 Colloidal Nanocrystals: Plasmonics and Magnetism. J. Phys. Chem. Lett. 2014, 5, 2306−2311. (35) Tandon, B.; Yadav, A.; Nag, A. Delocalized Electrons Mediated Magnetic Coupling in Mn−Sn Codoped In2O3 Nanocrystals: Plasmonics Shows the Way. Chem. Mater. 2016, 28, 3620−3624. (36) Hutfluss, L. N.; Radovanovic, P. V. Controlling the Mechanism of Phase Transformation of Colloidal In2O3 Nanocrystals. J. Am. Chem. Soc. 2015, 137, 1101−1108. (37) Runnerstrom, E. L.; Bergerud, A.; Agrawal, A.; Johns, R. W.; Dahlman, C. J.; Singh, A.; Selbach, S. M.; Milliron, D. J. Defect Engineering in Plasmonic Metal Oxide Nanocrystals. Nano Lett. 2016, 16, 3390−3398. (38) Farvid, S. S.; Dave, N.; Radovanovic, P. V. Phase-Controlled Synthesis of Colloidal In2O3 Nanocrystals Via Size-Structure Correlation. Chem. Mater. 2010, 22, 9−11. (39) Dave, N.; Pautler, B. G.; Farvid, S. S.; Radovanovic, P. V. Synthesis and Surface Control of Colloidal Cr3+-Doped SnO2 Transparent Magnetic Semiconductor Nanocrystals. Nanotechnology 2010, 21, 134023.

Collaborative Research Grant). P.V.R. acknowledges the support from the Canada Research Chairs program (NSERC).

■

REFERENCES

(1) Maier, S. A. Plasmonics: Fundamentals and Applications; Springer: New York, 2007. (2) Giannini, V.; Fernandez-Dominguez, A. I.; Heck, S. C.; Maier, S. A. Plasmonic Nanoantennas: Fundamentals and Their Use in Controlling the Radiative Properties of Nanoemitters. Chem. Rev. 2011, 111, 3888−3912. (3) Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G. Nanostructured Plasmonic Sensors. Chem. Rev. 2008, 108, 494−521. (4) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with Plasmonic Nanosensors. Nat. Mater. 2008, 7, 442−453. (5) Hu, M.; Chen, J.; Li, Z.-Y.; Au, L.; Hartland, G. V.; Li, X.; Marquez, M.; Xia, Y. Gold Nanostructures: Engineering Their Plasmonic Properties for Biomedical Applications. Chem. Soc. Rev. 2006, 35, 1084−1094. (6) Sheldon, M. T.; van de Groep, J.; Brown, A. M.; Polman, A.; Atwater, H. A. Plasmoelectric Potentials in Metal Nanostructures. Science 2014, 346, 828−831. (7) Schlücker, S. Surface-Enhanced Raman Spectroscopy: Concepts and Chemical Applications. Angew. Chem., Int. Ed. 2014, 53, 4756− 4795. (8) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205−213. (9) Comin, A.; Manna, L. New Materials for Tunable Plasmonic Colloidal Nanocrystals. Chem. Soc. Rev. 2014, 43, 3957−3975. (10) Fox, M. Optical Properties of Solids; Oxford University Press: New York, 2001. (11) Kanehara, M.; Koike, H.; Yoshinaga, T.; Teranishi, T. Indium Tin Oxide Nanoparticles with Compositionally Tunable Surface Plasmon Resonance Frequencies in the Near-IR Region. J. Am. Chem. Soc. 2009, 131, 17736−17737. (12) Wang, T.; Radovanovic, P. V. Free Electron Concentration in Colloidal Indium Tin Oxide Nanocrystals Determined by Their Size and Structure. J. Phys. Chem. C 2011, 115, 406−413. (13) Capozzi, C. J.; Carson, C. G.; Gerhardt, R. A.; Summers, C. J.; Gilstrap, R. A., Jr. Synthesis of a Nonagglomerated Indium Tin Oxide Nanoparticle Dispersion. Adv. Mater. 2008, 20, 4163−4166. (14) Shanker, G. S.; Tandon, B.; Shibata, T.; Chattopadhyay, S.; Nag, A. Doping Controls Plasmonics, Electrical Conductivity, and CarrierMediated Magnetic Coupling in Fe and Sn Codoped In 2 O 3 Nanocrystals: Local Structure is the Key. Chem. Mater. 2015, 27, 892−900. (15) Nutz, T.; zum Felde, U.; Haase, M. Wet-Chemical Synthesis of Doped Nanoparticles: Blue-Colored Colloids of n-Doped SnO2:Sb. J. Chem. Phys. 1999, 110, 12142−12150. (16) Peters, K.; Zeller, P.; Stefanic, G.; Skoromets, V.; Němec, H.; Kužel, P.; Fattakhova-Rohlfing, D. Water-Dispersible Small Monodisperse Electrically Conducting Antimony Doped Tin Oxide Nanoparticles. Chem. Mater. 2015, 27, 1090−1099. (17) Buonsanti, R.; Llordes, A.; Aloni, S.; Helms, B. A.; Milliron, D. J. Tunable Infrared Absorption and Visible Transparency of Colloidal Aluminum-Doped Zinc Oxide Nanocrystals. Nano Lett. 2011, 11, 4706−4710. (18) Della Gaspera, E.; Chesman, A. S. R.; van Embden, J.; Jasieniak, J. J. Non-Injection Synthesis of Doped Zinc Oxide Plasmonic Nanocrystals. ACS Nano 2014, 8, 9154−9163. (19) Gordon, T. R.; Paik, T.; Klein, D. R.; Naik, G. V.; Caglayan, H.; Boltasseva, A.; Murray, C. B. Shape-Dependent Plasmonic Response and Directed Self-Assembly in a New Semiconductor Building Block, Indium-Doped Cadmium Oxide (ICO). Nano Lett. 2013, 13, 2857− 2863. (20) De Trizio, L.; Buonsanti, R.; Schimpf, A. M.; Llordes, A.; Gamelin, D. R.; Simonutti, R.; Milliron, D. J. Nb-Doped Colloidal 4978


Article

Chemistry of Materials (40) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. Quantum Espresso: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. (41) Chen, Z.; Huang, L.; Zhang, Q.; Xi, Y.; Li, R.; Li, W.; Xu, G.; Cheng, H. Electronic Structures and Transport Properties of n-TypeDoped Indium Oxides. J. Phys. Chem. C 2015, 119, 4789−4795. (42) Narayanaswamy, A.; Xu, H.; Pradhan, N.; Kim, M.; Peng, X. Formation of Nearly Monodisperse In2O3 Nanodots and OrientedAttached Nanoflowers: Hydrolysis and Alcoholysis vs Pyrolysis. J. Am. Chem. Soc. 2006, 128, 10310−10319. (43) Shim, M.; Guyot-Sionnest, P. Permanent Dipole Moment and Charges in Colloidal Semiconductor Quantum Dots. J. Chem. Phys. 1999, 111, 6955−6964. (44) Wang, Y.; Brezesinski, T.; Antonietti, M.; Smarsly, B. Ordered Mesoporous Sb-, Nb-, and Ta-Doped SnO2 Thin Films with Adjustable Doping Levels and High Electrical Conductivity. ACS Nano 2009, 3, 1373−1378. (45) Zhang, L.; He, R.; Gu, H.-C. Oleic Acid Coating on the Monodisperse Magnetite Nanoparticles. Appl. Surf. Sci. 2006, 253, 2611−2617. (46) Wu, N.; Fu, L.; Su, M.; Aslam, M.; Wong, K. C.; Dravid, V. P. Interaction of Fatty Acid Monolayers with Cobalt Nanoparticles. Nano Lett. 2004, 4, 383−386. (47) Zhang, Y.; Li, J.; Li, Q.; Zhu, L.; Liu, X.; Zhong, X.; Meng, J.; Cao, X. Preparation of In2O3 Ceramic Nanofibers by Electrospinning and Their Optical Properties. Scr. Mater. 2007, 56, 409−412. (48) Shim, M.; Guyot-Sionnest, P. n-Type Colloidal Semiconductor Nanocrystals. Nature 2000, 407, 981−983. (49) Liu, W. K.; Whitaker, K. M.; Smith, A. L.; Kittilstved, K. R.; Robinson, B. H.; Gamelin, D. R. Room-Temperature Electron Spin Dynamics in Free-Standing ZnO Quantum Dots. Phys. Rev. Lett. 2007, 98, 186804. (50) Germeau, A.; Roest, A. L.; Vanmaekelbergh, D.; Allan, G.; Delerue, C.; Meulenkamp, E. A. Optical Transitions in Artificial FewElectron Atoms Strongly Confined inside ZnO Nanocrystals. Phys. Rev. Lett. 2003, 90, 097401. (51) Schimpf, A. M.; Thakkar, N.; Gunthardt, C. E.; Masiello, D. J.; Gamelin, D. R. Charge-Tunable Quantum Plasmons in Colloidal Semiconductor Nanocrystals. ACS Nano 2014, 8, 1065−1072. (52) Jain, P. K. Plasmon-in-a-Box: On the Physical Nature of FewCarrier Plasmon Resonances. J. Phys. Chem. Lett. 2014, 5, 3112−3119. (53) Mryasov, O. N.; Freeman, A. J. Electronic Band Structure of Indium Tin Oxide and Criteria for Transparent Conducting Behavior. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 64, 233111. (54) Karazhanov, S. Zh.; Ravindran, P.; Vajeeston, P.; Ulyashin, A.; Finstad, T. G.; Fjellvåg, H. Phase Stability, Electronic Structure, and Optical Properties of Indium Oxide Polytypes. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 075129. (55) Seidl, A.; Görling, A.; Vogl, P.; Majewski, J. A.; Levy, M. Generalized Kohn-Sham Schemes and the Band-Gap Problem. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 53, 3764−3774. (56) Medvedeva, J. E.; Hettiarachchi, C. L. Tuning the Properties of Complex Transparent Conducting Oxides: Role of Crystal Symmetry, Chemical Composition, and Carrier Generation. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 125116. (57) Johns, R. W.; Bechtel, H. A.; Runnerstrom, E. L.; Agrawal, A.; Lounis, S. D.; Milliron, D. J. Direct Observation of Narrow MidInfrared Plasmon Linewidths of Single Metal Oxide Nanocrystals. Nat. Commun. 2016, 7, 11583. (58) Wang, T.; Radovanovic, P. V. In-Situ Enhancement of the Blue Photoluminescence of Colloidal Ga2O3 Nanocrystals by Promotion of Defect Formation in Reducing Conditions. Chem. Commun. 2011, 47, 7161−7163.

(59) van Dijken, A.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. The Kinetics of the Radiative and Nonradiative Processes in Nanocrystalline ZnO Particles Upon Photoexcitation. J. Phys. Chem. B 2000, 104, 1715−1723.

4979


Tuning Plasmon Resonance of In2O3 Nanocrystals throughout the

Recommend Documents