Article pubs.acs.org/cm
Synthesis of N‑Type Plasmonic Oxide Nanocrystals and the Optical and Electrical Characterization of their Transparent Conducting Films Benjamin T. Diroll,† Thomas R. Gordon,† E. Ashley Gaulding,‡ Dahlia R. Klein,† Taejong Paik,† Hyeong Jin Yun,† E.D. Goodwin,† Divij Damodhar,‡ Cherie R. Kagan,†,‡,§ and Christopher B. Murray*,†,‡ †
Department of Chemistry, ‡Department of Materials Science and Engineering, and §Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: We present a general synthesis for a family of n-type transparent conducting oxide nanocrystals through doping with aliovalent cations. These monodisperse nanocrystals exhibit localized surface plasmon resonances tunable in the mid- and near-infrared with increasing dopant concentration. We employ a battery of electrical measurements to demonstrate that the plasmonic resonance in isolated particles is consistent with the electronic properties of oxide nanocrystal thin films. Hall and Seebeck measurements show that the particles form degenerately doped ntype solids with free electron concentrations in the range of 1019 to 1021 cm−3. These heavily doped oxide nanocrystals are used as the building blocks of conductive, n-type thin films with high visible light transparency.
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INTRODUCTION Widespread adoption of liquid-crystal and light-emitting diode displays has encouraged an extensive search both for new transparent conducting materials and new methods for fabricating transparent electrodes. A small number of doped oxides dominate the commercial market for transparent conducting electrodes, led by indium tin oxide (ITO).1 Despite its utility, the rarity of indium makes ITO potentially expensive, and the most common deposition methods for ITO remain energy-intensive. Many replacement materials have been suggested, including patterned metals,2 metal nanowires,3 carbon nanotubes,4,5 graphene,6 other oxides,7,8 and conductive polymers.9 At the same time, solution-processing methods, in contrast to evaporation or sputtering, are increasingly used to fabricate transparent conducting thin films.4,5,10−13 Separately, the set of nanocrystalline materials exhibiting localized surface plasmon resonances (LSPRs) has substantially diversified from metallic nanoparticles to include heavily doped chalcogenides,14−18 phosphides,19 nitrides,20 oxides,21−25 and silicon nanostructures.26,27 LSPRs arise from the collective oscillation of the free carriers of an individual particle, with the frequency of the plasmon resonance related to several properties of the material, especially the carrier concentration. Transparent conducting oxide nanocrystals (NCs) contribute to both research efforts: wide band gap, high carrier density (>1018 cm−3) oxides show near-infrared (NIR) LSPRs, and they can be deposited into conductive thin films with high visible light transparency using solution-casting methods. In this paper, we explore the controlled synthesis and the optical, structural, and electrical characterization of a family of n-type oxide NCs and their solution-cast thin films. Subtle © 2014 American Chemical Society
differences in the kinetics of precursor decomposition make the direct synthesis of controllably doped colloidal nanocrystals a continuing challenge.28 Nonaqueous, high-temperature syntheses of doped oxide NCs have been reported for a number of oxides using isovalent dopants,29 aliovalent dopants,21,22,24 interstitial dopants, 30 and vacancy-doping of the NC stoichiometry.23 Particularly successful efforts to make highly uniform ternary or doped oxide NCs have focused on finely controlled heating procedures to ensure uniform nucleation and growth.31 Using a general synthesis with little or no modification except for the metal precursors, we synthesized indium-, gallium-, aluminum-, and tin-doped CdO NCs (ICO, GCO, ACO, and TCO, respectively) and tin-doped In2O3 (ITO) NCs. At sufficiently high tin concentrations, the reaction can also be used to make ternary CdSnO3 NCs. Our method using a high-temperature decomposition of metal oleate compounds in the presence of surfactant ligands yields monodisperse, highly crystalline, controllably doped NCs that have high free electron concentrations, as evidenced by the presence of an LSPR in the NIR. Practical devices and electrical measurements require that NCs be formed into a solid state material from a colloidal solution. The literature on using solution-processable oxides to fabricate oxide thin films is extensive, primarily focusing on sol−gel-based approaches.32−36 Sol−gel deposition of oxide films and initially low-temperature processing are frequently coupled with high-temperature annealing (300−800 °C) to Received: May 23, 2014 Revised: July 7, 2014 Published: July 18, 2014 4579
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increase the crystallinity and conductivity of a material.10 By constructing a film from crystalline building blocks, solution processing of NCs offers a potential method for lowering the temperatures used in annealing.37 The synthesis of NCs without ligands38,39 is one method for making conductive thin films, but they often suffer from relatively poor control of grain size and shape. Surfactant-assisted oxide syntheses, conversely, can yield exceptionally monodisperse size- and shape- controlled products.24,40,41 NCs synthesized in hydrophobic solvents are typically stabilized by aliphatic surfactant ligands with polar head groups bound to the particle surface.42 Although these ligands allow high solubility of NCs in organic solvents and enable solutionprocessing techniques, they also form electrically insulating barriers between NCs in the solid state. Where conductivity is desired, postdeposition annealing or ligand exchange is performed to achieve a number of purposes, including film densification, increased crystal grain size, doping of NCs, and/ or removal of organic ligand from the NC surfaces.43 This strategy has been demonstrated for aluminum-doped ZnO (AZO) and ITO NCs,11,44 using annealing in a reducing environment, which is known to generate oxygen vacancies and enhance the conductivity of the materials. Alternatively, oxidative treatment, such as oxygen plasma of In1−xSn NCs, has also been used to make conductive films of ITO.45 Here, we present films annealed in air which show a factor of 106 increased conductivity and yet retain a localized optical plasmon feature and the structural properties of individual NCs. This annealing procedure allows us to use the sensitivity of electrical characterization to determine the film conductivity, carrier type, carrier concentration, and position of the Fermi level relative to the conduction band. The measurements conducted in this work offer a nonoptical assay of NC electronic structure which can be compared with optical characterization methods. Hall and Seebeck measurements of films formed from plasmonic oxide NCs show degenerate doping behavior with free electron concentrations of the orders 1019 to 1021 cm−3.
is associated with this paper. The reaction was then stopped 30 min after this color change and cooled to room temperature. Reactions were performed with synthesis doping concentrations between 0.01 mol % and 25 mol %. The reaction has been scaled successfully to at least 3× (3 mmol) scale of the standard procedure described above with no discernible differences in the quality of the synthesis. Synthesis of CdSe NCs used in quantum dot-sensitized solar cells followed literature procedures.46 The first absorption maximum was located at 638 nm. Titanium dioxide paste for solar cells was prepared as described previously.47 Isolation of NCs. The oxide NCs were washed with toluene and 2-propanol was added to induce flocculation, after which the samples were centrifuged at 8000 rpm for 5 min. This process was repeated three times. NCs were finally resuspended in hexane and centrifuged at 2000 rpm for 1 min to remove aggregated particles and solid metal that precipitated in the reaction. The yield of hexanes-soluble oxide NCs was typically ∼80 mg from a 1 mmol scale reaction, consistent with literature yields of CdO from cadmium carboxylate decomposition.48 An additional precipitation step was added for those samples used in electrical measurements. These samples were dried under vacuum and redispersed in octane for spin-casting. Electron Microscopy. Transmission electron microscopy (TEM) characterization was performed on a JEOL 1400 operating at 120 keV, and high-resolution imaging was performed on a JEOL 2100 operating at 200 keV. Scanning electron microscopy (SEM) was performed on a JEOL 7500F SEM. X-ray Diffraction. Wide-angle X-ray scattering (WAXS) of NCs was performed in reflection geometry using a Rigaku Smartlab diffractometer with Cu Kα X-rays. To perform measurements, samples were deposited into thick films by dropcasting on to glass slides. Small-angle X-ray scattering (SAXS) was performed in transmission geometry using a multiangle X-ray diffractometer (Bruker) with a Cu Kα rotating anode source and two-dimensional wire detector. Optical Spectroscopy. The optical properties of the samples in this paper were studied using transmission ultraviolet, visible, and infrared spectroscopies of both solutions and thin films. Ultraviolet and visible measurements were performed using a Cary 5000 UV−vis-NIR spectrometer, and infrared spectroscopic measurements were performed using a ThermoFisher Continuμm FT-IR. Elemental Analysis. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was conducted on a Spectro Genesis ICP-OES. Solutions for analysis were prepared by dissolving dry NC powders in aqua regia and diluting with 5% (v/v) nitric acid in ultrapure water. Deposition of Thin Films and Annealing. Thin films were deposited by spin-casting concentrated (25−100 mg/mL) octane solutions of NCs at 800 rpm for 30 s followed by 1200 rpm for 30 s. For annealed films, samples on quartz or silicon substrates were annealed under air at 350 °C for 10 min (ramp to 350 °C in 20 s) using a rapid thermal annealer (ULVAC). These steps could be repeated to form thicker films with fewer cracks. Ellipsometry. Variable angle spectroscopic ellipsometry measurements were performed to determine NC film thickness and dielectric function from 0.05 to 0.73 eV using a J.A. Woollam Co. IR-VASE ellipsometer and from 0.73 to 5.90 eV using a J.A. Woollam Co. RC2 ellipsometer. Ellipsometry from
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EXPERIMENTAL METHODS Materials. Oleic acid (90%), cadmium(II) acetylacetonate (≥99.9%), gallium(III) acetylacetonate (99.99%), aluminum acetylacetonate (99%), tin(IV) acetate, selenium (99.99%), and sulfur (99.98%) were obtained from Sigma-Aldrich. Indium(III) acetate (99.99%), sodium sulfide nonhydrate (extra pure), and octadecene (90%) were obtained from Acros Organics. Cadmium oxide (99.99%) was obtained from Strem. ITO sheet (150 nm, 5−15 Ω/sq) was purchased from Delta Technologies. Solvents used in this work were ACS grade or higher, or purified by standard procedures. Warning: cadmium compounds are toxic inhalation hazards and should be handled with care. Syntheses. A table of reaction conditions for many samples used in this work can be found in the Supporting Information. In general, 1 mmol of metal precursor(s) was added to 3−10 mmol oleic acid and 30 mL of 1-octadecene and heated to 120 °C under vacuum for 1 h. Doping is achieved by adding a specified fraction of adatom precursor. The reaction was then heated under nitrogen to 315−320 °C (reflux) for spherical NCs and 300 °C for octahedral NCs. The reaction temperature was maintained for 30 min to 1 h until the reaction changed in color from pale yellow to dark green or brown. A video recording (Supporting Information Video) of this color change 4580
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bimodal population of NCs at early times. With time, the small NCs are consumed and in the case of the larger octahedra, faceting along the (111) crystal planes occurs. Figure 1a−d show representative TEM images of the doped oxide NCs studied in this paper (See also Supporting
3.3 to 0.7 eV was performed using a J.A. Woollam M-2000 ellipsometer. Atomic Force Microscopy. Atomic force microscopy was performed in AC (tapping) mode on a MFP-Bio-3D AFM (Asylum Research). Scans were typically performed over 20 μm × 20 μm area at 0.5 Hz. Hall Measurements. Films for Hall measurements were prepared by two cycles of spin-casting and annealing of NC samples. Hall (and van der Pauw) measurements were performed using two systems: DC Hall measurements were performed using an MMR H-50 Hall measurement system with a fixed 0.5 T magnet. AC Hall measurements were performed using a Lakeshore Model 8404 AC/DC Hall Measurement System with a variable field magnet. Seebeck Measurements. Films for Seebeck measurements were prepared by two cycles of spin-casting and annealing of NC samples. Seebeck measurements were performed using an MMR Seebeck Measurement System (SB-100 coupled with a K-20 temperature controller) from 175 to 375 K against a constantan reference wire with the temperature difference across the sample fixed at approximately 1.5 K.
Figure 1. TEM and HRTEM images of NCs of (a) TCO, (b) ACO, (c) ICO, and (d) octahedral ITO. (e) Powder X-ray diffraction patterns from representative samples of doped oxide NCs. The red lines represent the bulk scattering peaks of rhombohedral In2O3 and the black lines represent the bulk scattering of cubic CdO.
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RESULTS AND DISCUSSION Synthesis of Doped Oxide Nanocrystals. We report the synthesis of monodisperse oxide NCs by a rapid nucleation process at high temperature through the decomposition of metal carboxylate precursors. This represents a variation on one of the most successful strategies for the formation of monodisperse oxide NCs which has been employed to make maghemite,40 magnetite,40,49 and other ferrites50,51 of high crystallinity and uniformity. In our syntheses, we form metal oleate compounds in situ through the reaction of oleic acid with metal salts of organic counterions (commonly acetate or acetylacetonate) at 120 °C under vacuum (∼1 Torr). The formation of oxide NCs occurs after prolonged heating at elevated temperatures (see Experimental Methods for full details). The result is not unlike direct thermal decomposition of metal carboxylates,52 but the surfactant-assisted colloidal synthesis controls the size of the resulting NC grains and allows for colloidal dispersibility. Unlike many nonaqueous syntheses of metal oxide NCs,11,31,53−56 no amines or alcohols, which may induce decomposition of metal carboxylates, are added. During the synthesis, the pale yellow solution darkens as Cd NCs (50−300 nm) precipitate out of solution (See Video 1 and Supporting Information Figures S1 and S2). The likely mechanism for this formation, supported by studies of copper,52,57,58 zinc,59 iron,58,60,61 manganese,59,61 and cadmium carboxylate48 thermal decomposition, is the thermally induced homolysis of MO bonds, resulting in metal reduction. This mechanism is similar to the thermal decomposition of cadmium formate under inert gas flow, which yields both CdO and metallic cadmium in addition to volatile gases.48 Similar processes have been observed for heated cadmium carboxylate solutions62 and other metal−carboxylate-based NC syntheses.63 We also note two observations relevant to this mechanism: increasing the amount of oleic acid in the reaction and lowering the temperature results in substantially longer incubation periods before nucleation. In the second event of the reaction, rapidly following the nucleation of large, insoluble Cd NCs, the solution quickly changes to a dark green or brown, indicating a burst nucleation of soluble oxide NCs. Aliquots taken during a reaction to form ICO spheres at 320 °C and ICO octahedra at 300 °C (Supporting Information Figures S3 and S4) showed a
Information Figure S5). High-resolution TEM shows that the ICO (Figure 1c) and ITO (Figure 1d) particles are single crystals. The images also show that changing the temperature of the synthesis can change the shape of the resulting particles. In the case of both CdO and In2O3, larger octahedral particles can be formed at low dopant concentrations by performing the reactions at temperatures lower than reflux (300 °C), which generates fewer nuclei. Powder X-ray diffraction patterns of ICO, GCO, ACO, and TCO in Figure 1e match the expected pattern from the rocksalt CdO. Similarly, powder diffraction from ITO NCs matches the pattern from bixbyite In2O3. Stoichiometric ternary oxides of cadmium are also known, and at high doping levels (20 mol % in synthesis) of tin, the NCs show a powder diffraction pattern consistent with CdSnO3 (Supporting Information Figure S6). ICP-OES confirmed that this sample had a Cd/Sn ratio of 53:47. We did not, however, observe cadmium stannate (Cd2SnO4), another well-known conducting oxide. Both the average size and the monodispersity of the samples depended on the dopant and host material for given synthetic conditions (i.e., fixed temperature and ligand concentration). SAXS and TEM were used to characterize samples of each doped oxide and the codoped Al- and In-doped CdO (AICO), performed with 5 mol % doping and 5 mmol oleic acid in the synthesis. ACO and GCO NCs were 30−40 nm and more faceted with size dispersions σ > 10%, whereas TCO and ICO (and codoped samples with indium) were progressively smaller and showed σ < 10%, with some of the best samples reaching σ ∼ 5%.24 Samples of ITO NCs consistently showed larger sizedispersion (>15%), as shown in Figure 2. Sizing data suggests a critical role of the dopant precursor decomposition in the nucleation of NCs. Elemental analysis of GCO and ACO samples in particular showed low dopant incorporation (∼1% or lower) concomitant with a larger final particle size and indicative of fewer nuclei suggesting that both size and doping efficiency are linked to the thermal decomposition of the respective metal carboxylates. For those dopants which were more readily incorporated, higher synthesis dopant concen4581
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Figure 2. SAXS patterns (left) and corresponding TEM images (right) of several highly doped oxides. SAXS data are shown in open circles with fits to the data in solid lines. NC diameter and diameter dispersion of the fit are printed next to the curve. The top three curves were collected at a 54 cm sample-to-detector distance; the bottom three curves were collected at a 150 cm distance. Scale bars on TEM images are 30 nm.
Figure 3. Ultraviolet, visible, and infrared extinction spectra of doped oxide NCs dispersed in tetrachloroethylene. Spectra are offset and normalized for clarity. Each grouping represents three samples of the doped oxide NCs, except for the top grouping, which shows the extinction spectra of codoped Al- and In-doped CdO, Ga- and Indoped CdO, and Sn- and In-doped CdO. The starred sample indicates a distinct phase, CdSnO3.
tration yielded smaller final particle sizes for the same reaction conditions. For example, larger octahedral ICO NCs were not obtained at high indium doping concentrations. The IR and visible extinction spectra of ICO, GCO, ACO, TCO, ITO, and codoped CdO NCs are shown in Figure 3 with the elemental doping concentration determined by ICP-OES. Each spectrum shows a LSPR in the IR and band gap absorption in the blue. The position of the LSPR was tuned over a range of 500 meV across the infrared by changing the doping concentration of the synthesis, although there are different ranges of incorporation for each dopant. In particular, we found that the ICO can be synthesized with a NIR plasmon with indium synthesis concentrations as low as 0.01% (plasmon maximum at ∼0.25 eV) to as high as 25% (>0.6 eV) with a monotonic increase in the energy of the plasmon feature. ACO and GCO show LSPRs that are less sensitive to the concentration of dopants in the synthesis due to substantially lower dopant incorporation, reflected in the elemental analysis of the samples. TCO samples showed a large range of LSPR energies by varying the reaction stoichiometry, but as described above, a new phase was produced at high Sn concentrations, which limits the range for comparison. Results for ITO NCs are similar to those resulting from literature procedures.11,21 In addition to the shift in the LSPR feature, higher doping levels in the samples lead to a blue-shift of the band gap absorption, arising from the Burstein−Moss effect,64,65 which results in higher visible transparency for heavily doped samples. Wide band gap oxides show substantially lower loss and higher quality factor (Q = ELSPR/fwhm) LSPR features than many other heavily doped nanocrystalline semiconductors and
features comparable to gold nanorods. The widths of the plasmon features reflect sample heterogeneity and the dielectric and electric properties of the host materials. The sharp lines of the doped oxide NCs of this study suggest a high degree of homogeneity of size, shape, doping level, and dopant distribution.66 Intra- and interband transitions, present in copper chalcogenides and a source of optical loss, are absent in the doped oxide NCs. The intrinsic breadth of single NC LSPRs is largely controlled by the majority carrier mobility in the NC, which dictates the sharpness of Drude oscillators in an effective medium (Supporting Information Figure S8). For CdO doped with 3+ cations, higher doping yielded higher Q plasmon features, with Q as low as 1.4 at less than 1% doping increasing up to more than 6 with doping greater than 10%, comparable in quality to gold nanorods.67 TCO showed the opposite trend, with higher Q LSPRs at lower doping levels (Q = 4.9 for 7.9% Sn) but a very broad LSPR (Q = 2.4) for the CdSnO3 sample. ITO NCs showed lower quality LSPRs than CdO samples with a Q ∼ 2.75 for higher dopant concentrations, similar to literature reports,21 but these are still greater than the values of Q ∼ 1−2 for IR plasmons reported for gold nanoshells and monodisperse chalcogenides.68,69 Size-dispersion only weakly influences the plasmon resonance. Supporting Information Figure S9 shows the NIR extinction spectrum of ITO and ICO samples that were sizeselectively precipitated, revealing almost no change in the resonance line width. Line broadening beyond the intrinsic 4582
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damping of the underlying materials likely arises from subtle shape and doping differences between NCs.24 The optical signatures consistent with LSPRs in colloidal NCs are 2-fold: large extinction cross-section and positive solvatochromism.23,24,68 We have demonstrated that the NIR LSPR of ICO NCs has an extinction coefficient ranging from 107 to 109 M−1cm−1,24 depending on the NC size. This translates to a volumetric extinction coefficients similar to gold NCs70 and more than an order of magnitude greater than CdSe quantum dots at the band-edge.71 The energy of a LSPR is sensitive to the dielectric permittivity of the surroundings according to eq 1:68 E LSPR =
h 4π 2
Ne 2 ε0 me(ε∞ + 2εmedium)
(1) Figure 4. Dielectric properties of a film made from ICO NCs spin-cast on to a quartz substrate. Both the complex dielectric function (top) and the complex index of refraction (bottom) are plotted for reference.
where N is the free carrier concentration, h is Planck’s constant, e is the fundamental unit of charge, me is the effective mass of the carrier (an electron), ε0 is vacuum permittivity, ε∞ is the high-frequency dielectric constant of the material, and εmedium is the dielectric constant of the medium. Supporting Information Figure S10a shows the experimental extinction from the LSPR of ICO NCs in three solvents of varying permittivity. As expected from eq 1, we observe a bathochromic shift of the frequency of localized plasmon with increasing solvent permittivity, cataloged in Supporting Information Figure S10b. On the other hand, this method for identifying the LSPR is less than ideal, particularly for LSPRs in the NIR. The IR dielectric functions for IR-transparent solvents are similar, and the number of such solvents is limited. We note that the dielectric properties of the ligand environment, which may have a strong influence on the LSPR energy, are also ill-defined. Furthermore, large extinction and solvatochromism are not unique to plasmonic resonances. Deploying these pieces of evidence is typically subordinate to assumed, rather than demonstrated, metallic or degenerate character. In fact, doped oxides can show broad absorption features in the visible and IR, which are identified as intraband absorptions.72 Bathochromic shifts have also been observed for other ligand-stabilized materials such as quantum dots73 and carbon nanotubes.74 We endeavor to demonstrate a large array of other physical phenomena to confirm that the large NIR extinction of the doped oxide NCs presented in this paper is indeed a LSPR that arises from degenerate doping of the oxide NCs. Ellipsometric Measurements of Oxide Thin Films. To perform further ellipsometric and electrical measurements on doped oxide NCs, we spin-cast octane solutions of NCs to form films on quartz or thermally oxidized silicon substrates. Figure 4 shows the optical constants of an ICO film, with surface ligand present, determined using spectroscopic ellipsometry. Variable angle spectroscopic ellipsometry data was collected on an ICOcoated quartz substrate from 0.05 to 5.90 eV using two J.A. Woollam Co. spectroscopic ellipsometer systems to cover the large frequency range. Normal incidence transmission intensity data were also collected with the same instruments. The ellipsometry and transmission intensity data was fit simultaneously to determine the film thickness and Kramers−Kronig consistent dielectric function. For reference, both the dielectric function (ε1, ε2) and the refractive index (n, k) of the film are plotted. In this measurement, we resolve a large peak in the imaginary component of the dielectric function which is identified as a LSPR. This very strong optical feature, near 0.3 eV, also generates a small region of “metallic” permittivity in
the real component of the dielectric function in which ε1 is less than 0 (k > n). For metals, the crossover point at which ε1 falls below zero is identified as the plasma frequency: below the plasma frequency, electrons may oscillate resonantly with an excitation source. In this case, it is the strength and low loss of the LSPR that pulls the permittivity below zero over a small frequency range. Although measured over a smaller range, the properties of ITO and TCO films are similar (Supporting Information Figure 11). Optical and Structural Characterization of Annealed Thin films. Although most characterization of plasmonic NCs has focused on optical spectroscopy and modeling of particles in solutions or thin films, many assumptions of spectroscopic modeling can be confirmed using electrical measurements. Unlike optical measurements, measurements of the electrical properties of films provide unequivocal information regarding the carrier type, level of doping, and the position of the Fermi level relative to the transport band. To perform electrical characterization, spin-cast films of NCs were annealed at 350 °C under air for 10 min. Thermal analysis (Supporting Information Figure S12) showed that this treatment removes the organic mass from the NCs (13% of total mass) As cast, ICO films with the original oleic acid ligands show a resistivity ρ ≈ 2 × 108 Ω·cm (Supporting Information Figure S13), between the values measured for gold37 and PbSe75 NCs capped with original ligands. Annealing lowers the resistivity of the oxide thin films to ∼10 Ω·cm or lower, allowing the application of traditional tools of electronic spectroscopy. Several measurements were performed to ensure that the annealed films retained structural and electronic character of the as-synthesized NCs. Figure 5a shows optical extinction of ICO thin films as cast and after annealing. The NIR LSPR feature correlated with the synthesis doping concentration is preserved, although it is broadened and red-shifted due to increased interparticle coupling and higher local dielectric constant. At the same time, mid-IR C−H stretches disappear as the ligands are removed. IR spectroscopy of the carbonyl stretching region shows that both symmetric and antisymmetric oleate binding to cadmium disappear after annealing (Supporting Information Figure S14). Scanning electron microscopy (SEM) images in Figure 5b−d and the AFM height profile (Figure 5e) of an ICO film show that originally smooth void-free layers made using spin-casting 4583
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Figure 5. (a) Normalized extinction spectra of ICO NC films spin-cast on to quartz substrates (top) and annealed at 350 °C for 10 min (bottom). Synthesis doping concentrations are shown above the upper curves. (b) Cross-sectional SEM image of an ICO film as cast and (c,d) SEM images of different ICO samples after annealing. Scale bars are 100 nm (b,c) and 200 nm (d). (e) AFM contour plot of an ICO film after one cycle of spincasting and annealing. (f) SAXS of an ICO NC film as spin-cast (black line) and after annealing (red line) showing the preserved spherical particle form factor.
hole pair concentration created by solar light absorption. Additionally, high doping levels also typically lower carrier lifetime,76 making extraction of photogenerated carriers more difficult. Although there are reports of photogain in noble metal NC films, this effect is observed on resonance with the LSPR.77 The fraction of solar illumination at wavelengths greater than 1500 nm is small, and thus this effect is not expected to contribute substantially to photogeneration of carriers. The results in Figure 6a show negligible photogain for all three samples, despite large differences in conductivity. That is, even the less conductive ITO film behaves as if the number of carriers generated by illumination is negligible. Using a four-point configuration, we measured the sheet resistance of annealed films composed of ICO, TCO, and ITO NCs synthesized with several doping concentrations. These three series of samples demonstrate differences in the valency of the dopant within one host matrix (group III and group IV dopants) and a comparison between host matrices. Using AFM to obtain an average film height d, we obtain the resistivity of the films according to ρ = RSd. The resistivity of the NC films varied from higher than 100 Ω·cm (0.2% ITO) to below 0.01 Ω·cm (ICO), and the results are plotted in Figure 6b. Using our processing conditions, which involve oxidative annealing and measurement under air, the trend in resistivity of the host matrices follows ICO < TCO < ITO. Oxidative annealing is known to diminish the conductivity of ITO, and we observe a particularly large red-shift of the LSPR feature in ITO films upon annealing (Supporting Information Figure S17).78 The choice of a purely oxidative annealing procedure, in contrast to other studies on ITO films,11,79 is the most likely cause for the lower conductivity of ITO NC films compared to those based on the CdO host. Other procedures to raise the conductivity of nanocrystalline thin films, such as reductive annealing or ligand exchange pose problems of chemical compatibility and instability of electrical properties. CdO is particularly susceptible to chemical degradation, and we found that the reducing conditions which are used for ITO11 or AZO44 films reduce CdO to metallic cadmium. In this study, our goal was to
(Figure 5b) become cracked during the annealing process due to the removal of organic ligands (Figure 5c−e). To fill in the gaps created by annealing before measuring our samples, we performed two cycles of spin-casting and annealing on substrates used for electrical measurements.75 This process yields films with standard deviation in height comparable to the diameter of a NC (Supporting Information Figure S15). During the annealing process, the NCs did not sinter and preserved their spherical shape. High-resolution SEM of ICO films (Figure 5d) shows no signs of NC sintering, as do X-ray diffraction techniques. Analysis of the wide-angle X-ray diffraction line width of a TCO sample shows an estimated NC size of 14.3 nm before and after annealing (Supporting Information Figure S16). SAXS measurements of an ICO NC film of show that the particles remain ∼14.1 nm before and after annealing. The ringing features arising from the spherical particle form factor are preserved from the as-made film to the postannealed sample (Figure 5f). Annealed films show reduced interparticle ordering upon annealing and decreased interparticle spacing, inferred from the reduced intensity and slightly shifted location of the first near-neighbor peak at 0.8° 2θ. Electrical Characterization of Annealed Thin films. Heavily doped materials exhibit several characteristic electrical properties including high majority carrier concentration, insensitivity to light or electrostatic stimuli, a small Seebeck coefficient, and, typically, high conductivity. We performed an array of electronic spectroscopies on the NC thin films to verify behavior consistent with the description of their synthesis and plasmonic properties described above. Bottom-contact two-point measurements of conductivity and photoconductivity were performed for ICO (11.8% In), TCO (7.9% Sn), and ITO (9.8% Sn) NCs using prepatterned electrodes (channel W/L = 15) and an AM 1.5 solar simulator as a light source. Although these films are not strongly absorbing through most of the visible, they retain absorption overlapping the solar spectrum in the blue and ultraviolet. Little or no photogain is expected in these materials as the number of free electrons in the material is large compared to the electron− 4584
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films showed a clearly different trend, increasing in resistivity with doping >10% and a fall in electron density at high Sn concentration, due to the formation of CdSnO3. It is important to note that one source of scatter in the results in addition to intrinsic limitations of the technique and sample heterogeneity is subtle changes in the annealing conditions, performed individually on an RTA. The electron densities reported here are generally somewhat lower than those of bulk films, which reach levels >1021 cm−3.8 The composition of the film is useful to remember: when spherical NCs are spin-cast on to a substrate, then annealed, they form glassy films with an inorganic volume fraction of ∼60−70% and therefore a large fraction of the films is void. The samples of ITO and ICO in Figure 6c demonstrate a small increase in the carrier density with an increasing dopant concentration, but the TCO samples show a peak at doping of 7.9% and a decline at higher concentrations. For both the +3 In and +4 Sn dopants in the CdO host and Sn in In2O3, individual dopants contribute a single free electron at the low doping limit. Clear differences in the ionization energy of the dopants are not observed in the optical or electrical measurements of samples measured in this work. At sufficiently high carrier density, depending on the compound, additional dopant atoms may not contribute free electrons, leading to saturation or even decline of the free carrier concentration.8,21,24 This phenomenon explains the relative insensitivity of the free electron concentration to doping percentage that also appears in optical measurements, in which the frequency of the LSPR changes only incrementally (or reverses) at higher doping concentrations. In the case of TCO samples, where the free electron concentration drops substantially for NCs synthesized with higher fractions of tin, the lower carrier density is most likely due to the presence of CdSnO3, which may have a different susceptibility to oxidative annealing. Similar to Hall measurements, thermopower measurements allow a determination of the carrier type, but they are a more sensitive measure of electrical properties of metals and degenerate semiconductors.80 Thermopower, also frequently called the Seebeck coefficient (S), is the magnitude of voltage difference across a sample driven by a temperature difference between two terminals, S = ΔV/ΔT. Figure 6d shows the thermopower of ICO, TCO, and ITO NC films between 175 and 375 K, using the same samples as in Figure 6a. The negative values across all temperatures confirm that electrons are the dominant carrier type. Additionally, temperaturedependent thermopower measurements provide information about the position of the Fermi level relative to the conduction band edge. The temperature-dependence of a heavily doped semiconductor or metal, in which the Fermi level lies at or close to (≤3kBT) the conduction band, is S ∝ T, whereas the temperature-dependence of a semiconducting material is S ∝ T−1.80 The temperature-dependence of the thermopower shown in Figure 6d shows behavior that is typical for metals or degenerately doped semiconductors, with no activation barrier for carrier generation over the temperature range measured, consistent with the Hall carrier density measurements and a mid-IR LSPR in the isolated NCs. Oxide Nanocrystal Thin Films as Transparent Electrodes. Several of the oxides synthesized in this study, particularly ITO, are well-known transparent conducting oxides, and here we evaluate the nanocrystalline thin films produced for electrical measurements as transparent electrodes. The simplest figure of merit for transparent conductors weighs the
Figure 6. (a) Dark current (lines) and photocurrent (open circles) for 11.8%-doped ICO (black), 7.9%-doped TCO (blue), and 9.8%-doped ITO (red). (b) Resistivity of doped NC thin films. (c) Electron densities estimated from Hall measurements of the same thin films. In both (b) and (c), data points are averages from several measurements, typically from three samples, and positive and negative error bars represent the maximum and minimum values obtained, respectively. (d) Temperature-dependent thermopower measurements of ICO (black triangles), TCO (blue squares), and ITO (red circles) NC films.
anneal the films to obtain sufficiently low resistivity to perform electronic spectroscopy but maintain the discrete nature of the NC building blocks. Thus, we eschewed processes, including higher annealing temperatures, that maximize conductivity of thin films at the expense of losing their resemblance to the solution-dispersed NCs. In addition to obtaining the resistivity of the NC films, we performed Hall measurements to determine the majority carrier type, mobility, and carrier concentration in the oxide NC thin films. Both AC and DC Hall measurements were performed on the samples. DC measurements were similar to the AC measurements for higher conductivity films but showed signflipping in the most resistive samples, a problem that is reduced with a variable magnetic field. Our measurements in Figure 6c show a broad consistency with the results expected from optical measurements and elemental analysis: samples show n-type behavior and electron concentrations of the orders of 1019− 1021 cm−3, consistent with substitutional doping in the range of 1−20% of metal atoms and the mid-IR LSPR feature for the dispersed colloids in the mid-infrared. This remains true even for the substantially more resistive samples, as differences in mobility explain much of the scatter of sample conductivities. ITO samples showed similar, lower resistivity at doping >5%, due to mobilities 10−100 times smaller (∼0.01 cm2 V−1 s−1) than those with a CdO host (∼1 cm2 V−1 s−1) or previous reports of ITO NCs annealed under reducing conditions.11 Oxidative annealing conditions eliminate oxygen vacancies and lower the free electron concentration in ITO thin films to the order of 1019 cm−3, below the expected values based on the colloidal solution-phase LSPR feature but with the average slightly increasing with dopant concentration determined by elemental analysis. The average estimated carrier density in ICO also increased slightly with doping concentration. TCO 4585
dx.doi.org/10.1021/cm5018823 | Chem. Mater. 2014, 26, 4579−4588
Chemistry of Materials
Article
transmittance (T) divided by the sheet resistance (RS): T/RS. For this study, we measured the transmission of annealed oxide thin films deposited on to quartz substrates and obtained the sheet resistance from four-point measurements. The appropriate wavelength to evaluate the figure of merit depends on the desired application; the average transmittance in the visible spectrum (1.55−3.10 eV), denoted T, is used here. Before comparing the different compounds, we studied the role of sample thickness on the figure of merit. Thin films may have somewhat different transport properties than the bulk material as they may be prone to interfacial charge trapping or limited charge-transport pathways from film discontinuities, inferred from AFM height profiles (Figures 5e and Supporting Information Figure S15). The additional dielectric interface of a NC layer may also show thickness-dependent optical interference. Both effects are observed in multilayer films of ICO shown in Figure 7. Figure 7a shows the transmittance spectrum in the visible for multilayer films of 5%-doped ICO NC thin films after annealing. Using the sheet resistance for these films, we observed a large increase in ⟨T⟩/RS, shown in Figure 7b, from one layer to two layers and small changes in the figure of merit for thicker films. This finding is consistent with
the notion that a single layer of annealed NCs has a balkanized structure limiting transport but additional spin-casting yields a smooth film with fewer cracks. Following these results, for comparison of different oxides and doping levels, we deposited two layers of each material and compared the figure of merit obtained for each. For reference, we tested a piece of commercially available ITO and found ⟨T⟩/RS = 10−2. The best films made from NCs approach within a factor of 10 the quality of commercial ITO, despite retaining discrete nanocrystalline character. Although the ITO NC films show better transparency, the substantially lower conductivity of ITO films means that they performed quite poorly compared to the ICO and TCO films, especially at low doping levels. Because of the films’ similar transparency and the inverse dependence of the figure-of-merit on RS, the results mirror measurements of the resistivity: ITO films doped >1% show similar figures of merit; TCO samples show a peak at
