Plasmonic-Field Interactions at Nanoparticle Interfaces for Infrared

May 2, 2016 - †Department of Bioengineering and ‡Department of Electric Engineering and Information Systems, ... exhibit microwave transmittance e...
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Plasmonic-Field Interactions at Nanoparticle Interfaces for Infrared Thermal-Shielding Applications Based on Transparent Oxide Semiconductors Hiroaki Matsui,*,†,‡ Shinya Furuta,§ Takayuki Hasebe,∥ and Hitoshi Tabata†,‡ †

Department of Bioengineering and ‡Department of Electric Engineering and Information Systems, The University of Tokyo, 1-3-7 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan § Tomoe Work Co. Ltd, 1-3-6 Namiyoke, Minato-ku, Osaka 552-0001, Japan ∥ Central Customs Laboratory, Ministry of Finance, 5-3-6 Kashiwanoha, Kashiwa, Chiba 277-0882, Japan S Supporting Information *

ABSTRACT: This paper describes infrared plasmonic responses in three-dimensional (3D) assembled films of In2O3:Sn nanoparticles (NPs). The introduction of surface modifications to NPs can facilitate the production of electricfield interactions between NPs due to the creation of narrow crevices in the NP interfaces. In particular, the electric-field interactions along the in-plane and out-of-plane directions in the 3D assembled NP films allow for resonant splitting of plasmon excitations to the quadrupole and dipole modes, thereby realizing selective high reflections in the near- and midinfrared range, respectively. The origins of these plasmonic properties were revealed from electric-field distributions calculated by electrodynamic simulations that agreed well with experimental results. The interparticle gaps and their derived plasmon couplings play an important role in producing high reflective performances in assembled NP films. These 3D assemblies of NPs can be further extended to produce large-size flexible films with high infrared reflectance, which simultaneously exhibit microwave transmittance essential for telecommunications. This study provides important insights for harnessing infrared optical responses using plasmonic technology for the fabrication of infrared thermal-shielding applications. KEYWORDS: transparent oxide semiconductor, nanoparticle, plasmon coupling, near-infrared and thermal-shielding

1. INTRODUCTION Transparent oxide semiconductors (TOSs, such as ITO and doped ZnO) can be induced to exhibit metallic conductivity by doping with defects and/or impurities, allowing for excitations of surface plasmon resonances (SPRs) at dielectric/oxide interfaces.1−6 SPR properties of nanorods and nanowires have also been reported for ITO and doped ZnO.7−9 In addition, localized surface plasmon resonances (LSPRs) can also be produced when confining the collective oscillations of free carriers into nanoparticles (NPs). Investigations concerning nanoplasmonics based on TOSs have received much attention, and are breaking new ground in the area of wide-gap oxide semiconductors. A characteristic property revealed by these studies is that the optical nature of TOSs with infrared (IR) transparency outside the reststrahlen band indicates low-loss plasmonic materials even up to near-IR wavelengths. NP samples on TOSs have shown clear SPRs and LSPRs in the near- to mid-IR range by controlling spectral positions through the careful choice of dopant concentrations.10−13 For optical applications of NPs based on TOSs, the assembled films consisting of In2O3:Sn (ITO) NPs have so © XXXX American Chemical Society

far demonstrated optical enhancements of near-IR luminescence and absorption, which are related to the electric-fields (Efields) induced on the surfaces of the assembled NP films.14,15 Assemblies of Ag and Au NPs can produce high E-fields through plasmon coupling between the NPs in the visible range,16,17 and are utilized in surface-enhanced spectroscopy, waveguides and biological sensors.18−20 The high E-fields localized between NPs are very sensitive to interparticle gaps.21 A gap length down to distances less than the size of a NP causes remarkable enhancements of E-fields.22 Therefore, surfactant- or additive-treated NPs are effective pathways to obtain small interparticle gaps between NPs, which can be developed into one-, two-, and three-dimensional (1D, 2D, and 3D) assemblies of NPs. In contrast to top-down methods, it is not easy to control precisely the interparticle gap because slight variations in distance between NPs are usually created. However, it is beneficial that large-area fabrications at lower Received: January 28, 2016 Accepted: April 25, 2016

A

DOI: 10.1021/acsami.6b01202 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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a positive zeta potential of +31 meV for the NPs using an electrophoresis method, which indicated that NPs had nonaggregated states in the solvent because of electrostatic repulsion. The Sn content in the NPs used in this work was estimated as 4.8% by X-ray fluorescent spectroscopy (XFS). This Sn content (4.8%) provided the electron density of 9.35 × 1020 cm−3 in the NPs, indicating the metallic behaviors of NPs (Figure S4). In general, plasmonic wavelengths of the NPs are strongly dependent on Sn content ( Figure S2a). The NPs with a Sn content of 4.8% showed the lowest peak wavelength at around 1.8 μm. This indicated that the NPs (Sn content: 4.8%) had high plasmonic performance because of the large amounts of electron density (Figure S2b). In addition, direct plasmon excitation on the single NP surface has been further observed using scanning-type TEM (STEM) equipped with electron energy-loss spectroscopy (EELS),13 and is a consequence of homogeneous distributions of Sn atoms in the NP. The above results allowed us to treat an ITO NP as a single metallic nanosphere for the simulations and to discuss the plasmonic-field interaction between NPs. 2.2. Fabrications of Assembled Films of NPs. Assembled NP films were coated on IR-transparent CaF2 substrates using a spincoating method, and were obtained by multiple overglaze of a thin NP film fabricated using a NP concentration of 0.2% in toluene. The spin coating conditions at always involved the following process: 800 rpm (5 s) →1200 rpm (10 s) → 2400 rpm (30 s) → 800 rpm (10 s). An obtained film was heat-treated at 150 °C in air every time in order to evaporate the solvent. 3D NP films with various thicknesses were obtained by repetition of the above coating process (Figure S3). 2.3. Characterizations. The structural properties of samples were investigated by X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS). Structural information at the microscopic scale was investigated using micro-Raman scattering. Local surface and structural information on the samples was evaluated by scanning electron microscopy (SEM) and transmittance electron microscopy (TEM). The thermal-dependent chemical properties of samples were studied by thermogravimetry-differential thermal analysis (TG-DTA) equipped with time-of-flight (TOF) mass spectroscopy. Visible-NIR spectra were measured by a spectrometer from 2600 to 800 nm. Fourier-transform infrared (FTIR) measurements were performed by a spectrometer from 8000 to 1000 cm−1. The EM responses in the microwave range were measured by a dual-focus flat cavity (DFFC: 0.8−15 GHz) and the free-pace method (18−40 GHz). A performance Network analyzer (PNA) and Vector Network analyzer (VNA) were employed for the DFFC and free-space methods, respectively. 2.4. Theoretical Calculations. Optical properties were calculated numerically using the finite-difference time-domain (FDTD) method. The electric-field and charge-vector were also computed at specific peak positions. An ellipsometric measurement of an ITO film was conducted within the visible-to-IR range to obtain the complex dielectric constants.2 The modeled NP films were illuminated with light directed in the Z-direction from the air side. The direction of the electric field was perpendicular to the light and parallel to the Xdirection. Periodic boundary conditions were applied in the X and Y directions, and the top and bottom of the simulated domain in the Zdirection were studied by perfectly matched layer (PML) boundary conditions.15

costs make NP assemblies attractive for industrial development. The plasmon modes from the assemblies of NPs are determined by the collective plasmon resonances (CPRs) that are reasonably attributed to the long-range interactions of LSPRs in the macroscopic assembled films.23 Recently, plasmonic properties on TOS materials have attracted attention for thermal-shielding applications to suppress solar- and radiant-heat in the near- and mid-IR range, respectively.24 In this work, ITO NPs are chosen as a concrete example. Plasmonic responses are dependent on electronic band structures. For example, In2O3, ZnO and WO3 have similar electronic structures with conduction and valence bands consisting of s and p orbitals,25−27 indicating that the plasmonic properties of these materials can be manipulated through the same mechanism. Plasmonic properties of thermalshielding based on ITO NPs can be applied to ZnO and WO3 NPs. The purpose of our study is to apply the plasmonic properties of assembled films of ITO NPs to satisfy recent industry demands for a material with thermal-shielding ability. Their requirements include the fabrication of flexible sheets with high heat-ray reflections, as well as visible and microwave transmissions. To date, the IR optical responses have been investigated mainly in regard to transmittance and extinction spectra of composites and films utilizing oxide semiconductor NPs.28−31 This is because a single ITO NP behaves as a strong absorber (Figure S1). IR-shielding properties by absorption processes have been discussed on the basis of theoretical aspects. Accordingly, no previous paper has reported reflective performances in assemblies of NPs in spite of the desire for thermal-shielding to cut IR radiation not by absorption, but through reflection properties. Plasmonic applications exhibiting a thermal-shielding ability have not been previously studied in detail. Herein, we present the IR plasmonic properties of assembled films of ITO NPs from theoretical and experimental aspects. The surface modifications of NPs are controlled by organic ligands composed of fatty acids in order to limit spatially the interparticle gap. IR reflectance in the assembled NP films is discussed specifically following elucidation of their structure and corresponding plasmonic properties. Above all, we focus on the key role played by assembled films of surface-modified NPs in realizing selective high IR reflectance due to 3D interactions of E-fields along the in-plane and out-of-plane directions in the films. Finally, we consider electromagnetic (EM) responses in the microwave range in relation to electron transport in the assembled NP films. This study provides new insights for enhancement of thermal-shielding ability through plasmonic techniques on TOS materials.

2. EXPERIMENTAL SECTION 2.1. Fabrications of ITO NPs. ITO NPs with different Sn contents were grown using the chemical thermolysis method with various initial ratios of the precursor complexes (C 9 H 2 2 CO 2 ) 3 In and (C9H22CO3)4Sn. The raw materials were purchased from Wako Chemicals (Japan). Indium and tin carboxylates comprised white powders and were heated with a chemical ratio of 95:5 in a flask using a mantle heater to 350 °C without a solvent in a nitrogen atmosphere. The temperature was maintained for 4 h and the mixture was then gradually cooled to room temperature. The resultant mixture produced a pale blue suspension, to which excess ethanol was added to induce precipitation. Centrifugation and repeated washing processes were conducted four times using ethanol, which yielded dried powders of ITO NPs with a pale blue color. Finally, the NP powders were dispersed in a nonpolar solvent of toluene. Furthermore, we confirmed

3. RESULTS AND DISCUSSION 3.1. Structural, Optical, and Chemical Properties. XRD patterns of ITO NPs with Sn contents of 4.8% and 0% are shown in Figure 1a, and indicate broad peaks characteristic of colloidal NPs with a crystalline nature. The patterns were consistent with those of standard cubic bixbyite in the synthesis.32 Furthermore, the patterns indicated that the NPs had no discernible SnO or SnO2 peaks. The lattice parameter (a-axis length) of ITO NPs was 10.15 Å, which was larger than that (10.08 Å) of In2O3 NPs doped with Sn atoms into the NPs. Incorporations of Sn atoms into NPs was also examined using micro-Raman scattering. In2O3 with a cubic structure B

DOI: 10.1021/acsami.6b01202 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) XRD patterns and (b) micro-Raman scattering spectra of ITO NPs with Sn contents of 4.8 and 0%.

belongs to the Ia3 (Th7) space group, showing that Ramanactive peaks are derived from the vibrations with symmetry Ag, Eg, and Tg modes.33 The expected vibration peak (active v1mode) was found at 130.6 cm−1 (Figure 1b), which differed from that (132.4 cm−1) of In2O3 NPs. This was due to the difference in effective mass between In−O and Sn−O pairs. The XRD and Raman analyses revealed the structural properties of the ITO NPs used in this work. Figure 2a shows the SAXS pattern of ITO NPs dispersed in toluene, along with the simulated pattern for an ensemble of a spherical particle with a diameter (D) of 19.5 nm. The SAXS pattern and excellent parameter-fit indicated that the NPs were monodispersed at the ensemble level when the NPs appeared in the TEM image (inset of Figure 2a) and is due to the existence of surface ligands on the NPs, indicating that the peak position in LSPR (ELSPR) of the NPs is dependent on solvent type. Figure 2b shows absorbance spectra of the NPs dispersed in solvents with different refractive indexes (nD); nD is 1.42, 1.39, and 1.37 for cyclohexane, butylacetate, and hexane, respectively. Peak positions of LSPRs were linearly red-shifted with an increase in nD. This provided a refractive index sensitivity (S) of 573 nm RIU−1 (RIU: refractive index unit) and resulted in a figure of merit (FoM = S/Γ) of 1.03 (inset of Figure 2b), as defined by the ratio of the refractive index sensitivity (S) to the line width at half of the peak maximum (Γ). As an additional evaluation, the quality factor (Q = ELSPR/ Γ) was determined as 3.19, which was higher than that for nitrides and Si NPs for IR plasmonics.34,35 The FoM and Q values indicated the plasmonic performance of ITO NPs in this work. The thermal behaviors of the NP samples were investigated by TG-DTA in a N2 atmosphere with a heating rate of 10 °C/ min. The weight loss up to 250 °C might be related to the loss of physically or chemically absorbed water. There was an obvious weight loss in the temperature range 270 to 320 °C due to the generation of organic species identified by m/z peaks at 12 (C) 18 (H2O) 28 (CO) and 44 (CO2, C3H8, C2H4O, etc.) (Figure 3a, b). These chemical species were due to thermal removal of the surface ligands composed of fatty acids on the NPs.

Figure 2. (a) SAXS patterns of ITO NPs dispersed in toluene. Experimental and calculated SAXS spectra are represented by black and red lines, respectively. Inset indicates a TEM image of ITO NPs. (b) Absorbance spectra of ITO NPs dispersed in different solvents. Black, blue, and green indicate cyclohexane, butylacetate, and hexane solvents as different solvents, respectively. Inset represents the extended absorbance spectra around 1.8 μm, providing a plasmon sensitivity (S) of 573 nm/RIU.

FTIR spectra also confirmed the existence of surface ligands on the NPs from the fingerprints of molecular vibrations. An organic ligand on the NP surface consisted of a capric acid with a chemical formula of C10H22O2: CH3(CH2)8C(O)OH, appearing as molecular vibration modes derived from the methyl- and carboxyl-groups. The modes related to the methyl groups were found in the high wavenumber range 3800 to 2500 cm−1. Asymmetric (va) and symmetric (vs) CH3 modes were found at 2968 and 2867 cm−1 (Figure 3c). In addition, va-CH2 was detected at 2934 cm−1. On the other hand, the vibrations of carboxyl groups were found in the low wavenumber range 1800 to 1000 cm−1 (Figure 3d). va and vs COO− modes were observed at 1546 and 1314 cm−1, respectively. These modes appeared instead of a C  O vibration mode at around 1700 cm−1, which represents the distinctive features of carboxyl groups. Furthermore, C−H and C−O modes were observed at 1432 and 1398 cm−1, respectively. These vibration spectra indicated that capric acids combined with the NPs as surface ligand molecules. However, the methyl- and carboxyl-groups peaks were not supported at high temperatures above 350 °C, which indicated that the surface ligands on the NPs were thermally stable below 250 °C because the decomposition temperature of capric acid was in the range 250 to 300 °C ( Figure S5). Accordingly, the existence of the surface ligands on NPs was very helpful in actualizations of narrow interparticle gaps between the NPs. 3.2. Optical Properties and 2D NP Films. The optical properties of monoassembled films of ITO NPs (2D NP films) will now be detailed prior to discussion of multilayered assembled NP films (3D NP films). A surface SEM image of a 2D NP film showed a close-packed structure (inset of Figure C

DOI: 10.1021/acsami.6b01202 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) SAXS pattern of a 2D NP film. Inset represents a surface SEM image. (b) Experimental transmittance (black) and reflectance (red) of a 2D NP film. (c) Simulated transmittance (black) and reflectance (red) of a 2D NP film with a HCP structure. Inset shows a model of a 2D NP film and an E-field distribution when an electric field of light is applied along the X-direction.

gap (L) of 2 nm along the X−Y (in-plane) direction, where L was determined from the TEM image.15 The refractive index (n = 1.437) of the surface ligand on the NP was used as the dielectric medium between the NPs.39 A resonant peak at 2.45 μm was reproduced in transmittance and reflectance spectra, confirming the validity of the modeled 2D NP layer in comparison to the experimental data. The CPR mode was excited because of long-range coherences of E-field interactions between the NPs, as confirmed from the 2D image of the Efield distribution (inset of Figure 4b). 3.3. Optical Properties and 3D NP Films. 3D assemblies of ITO NPs provided a remarkable change in optical properties, which were clearly observed from transmittance and reflectance spectra [Figures 5(a) and 5(b)]. Transmittance with a resonant peak at 2.20 μm decreased to a level close to zero with increasing film thickness. In contrast, reflectance was enhanced at a close proximity of 0.6 in association with the film thickness (Figure 5b). The single peak of a 22 nm-thick 3D NP film was separated into lower and higher wavelengths with the film thickness. (Figure 6a). For a 216 nm thick 3D NP film, two types of peaks (I and II) were positioned 2.13 and 4.02 μm in the near- and mid-IR range, respectively. The thicknessdependent peak separation also appeared in absorbance spectra (Figure S7). The ratio (R/A) of reflectance (R) and absorbance (A) increased quickly to a large value with increasing film thickness (Figure 6c). As a result, the 3D NP films provided reflectance-dominant optical responses.

Figure 3. (a) TG-DTA curves of ITO NP samples in a reducing atmosphere. (b) TOF-Mass spectroscopy combined with TG-DTA. m/z signals at 12, 18, 28, and 44 were detected in the range 27−550 °C. FT-IR spectra of ITO NP samples taken in two wavenumber regions from (c) 3200 to 2500 cm−1 and (d) 1900 to 1000 cm−1.

4a) because spin-coating causes self-organizations of colloidal NPs into a hexagonally close-packed (HCP) structure due to shear and capillary forces on substrates.36,37 The films of assembled NPs provide an interesting insight into the scattering vector (q) of the SAXS intensity. A maximum SAXS peak includes structural information concerning spatial ordering of particles estimated by l = 2π/q with a spatial period (l). The SAXS pattern showed a maximum peak at q = 0.32 nm−1 followed with weak interferences (Figure 4a). This provided an l value of 19.5 nm being close to the edge-to-edge between NPs. For optical responses in the 2D NP film, a resonant peak at 2.64 μm appeared in transmittance, showing the red-shifted resonance wavelength due to a CPR mode compared to those of NPs dispersed in the solvents (Figure 4b).38 Reflectance at the resonant peak was small, indicating that the optical responses were mainly dominated by absorbance. Furthermore, the FDTD simulations were conducted to support the experimental data of the 2D NP film. The modeled 2D NP layer (D = 20 nm) has a HCP structure with an interparticle D

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Figure 6. (a) Resonant wavelengths and (b) reflectance of peaks I and -II as a function of film thickness (bottom horizontal axis) and number of NP layers (upper horizontal axis). Red color indicates simulated results of FDTD simulations. (c) Experimental and simulated R/A ratios evaluated at peak positions related to peak I. R and A indicate reflectance and absorbance, respectively.

the NPs was indispensable for realizing high reflectance from the 3D assembled NP films. The relation between surface ligands and optical properties in the 3D NP films was clarified by the spectral changes after annealing at different temperatures. No interparticle gap has been formed through thermal-removal of surface ligands of NPs.40 Figure 7a shows temperature-dependent reflectance

Figure 5. (a) Experimental and (b) simulated transmittance spectra of 3D NP films. (c) Experimental and (d) reflectance spectra of 3D NP films. (e) Cross-section SEM image of a 96 nm thick film sample. Inset is a FFT pattern. (f) Simulated model of a 3D NP film. The modeled NP sheet was illuminated with light directed in the Z-direction from the air side. The direction of the E-field was perpendicular to the light and parallel to the X-direction.

FDTD simulations were performed to clarify the above optical properties. A cross-section SEM image of a 3D NP film (96 nm-thickness) showed a close-packed structure (Figure 5e). The modeled 3D NP layers are based on a HCP structure with an interlayer distance of 2 nm along the Z- (out-of-plane) direction (Figure 5f). A layer sheet along the in-plane directions employed the 2D NP layer modeled in Section 3.2. The systematic change in the number of NP layers from 1 to 20 was capable of reproducing the experimental data (Figure 5c, d)]. The applied model was satisfactory in describing the optical responses of the 3D NP films. The increase in number of NP layers provided the resonant dips in transmittance and peak separations in reflectance, which matched reasonably well with the experimental data (Figure 6a, b). However, the reflectance for peak I was smaller than that for peak-II in the case of simulations, resulting in a difference of R/A ratio between experimental and simulation data (Figure 6c). As Supporting Information, we note that reflectance decreased with decreasing Sn content in relation to electron density in the NPs (Figure S6). This behavior indicated that the high electron density in

Figure 7. (a) Dependence of reflectance spectra on annealing temperature for a 216 nm thick 3D NP film. (b) SAXS patterns of the 216 nm thick 3D NP film annealed at different temperatures.

spectra taken in an inert atmosphere for a 216 nm thick 3D NP film, and reveal remarkable spectral changes in reflectance as follows. The two resonant peaks at 150 °C were weakened gradually following the change in spectral shape with increasing temperature. In particular, the near-IR reflection at peak-I shifted to longer wavelengths at high temperatures above 300 °C corresponding to the removal of the surface ligands (Figure 8a). The vibrations involving surface ligands also disappeared from the spectra. The removal of surface ligands from the NPs affected the whole reflective performance, which simultaneously demonstrated the importance of interparticle gap in obtaining a high reflectance. SAXS patterns of the 3D NP films also changed with increasing temperature (Figure 7b). 3D NP films annealed below 250 °C showed maximum peaks at around q = 0.3 nm−1 E

DOI: 10.1021/acsami.6b01202 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 8. (a) Dependence of annealing temperature on resonant wavelengths of peaks I and II. (b) Spatial periods (l: (⧫) between NPs and electrical resistivity (ρ: ○) as a function of annealing temperature. (c) TOF-Mass spectroscopy detected at an m/z signal at 44 added as reference data. This reference data was extracted from Figure 2b.

accompanied by interference fringes. This provided l values close to 20 nm, which were consistent with those of the 2D NP film. In contrast, the further increase in annealing temperature changed the peak-shifts to high scattering vectors, resulting in a decrease of l to 15.4 nm at 550 °C (Figure 8b). The annealing effects of 3D NP films were further reflected by the electrical resistivity (ρ) in the films. ρ was in the order of 104 Ω.cm below 250 °C (Figure 8b) because the presence of the surfactant layers on NPs seriously impedes charge transport in assembled NP films. The surface ligands often behave as interparticle insulating layers in NP networks.41 However, this effect was markedly reduced at high temperatures above 350 °C because of the relationship between l and ρ as a function of annealing temperature. The remarkable decrease in ρ was related to coalescence between the NPs through thermal-removal of surface ligands (Figure 8c), which was in agreement with a previous report showing that annealing temperatures above 500 °C are required to achieve film conductivities >10 S/cm42 in the case of ITO NPs. The surface ligands contributed to form small interparticle gaps between the NPs, which shows the origin of the high reflectance in the IR range. The resonant origins of peaks I and II in the reflectance were theoretically examined as a function of interparticle gap between the NPs. Figure 9a shows simulated reflectance spectra of 3D NP layers (20 NP layered model) at different L. Reflectance gradually enhanced with decreasing L. When decreasing L from 10 to 1 nm, peak II exhibited a monotonous red-shift to longer wavelengths, whereas peak I remained almost unchanged (Figure 9b). This suggests a difference in the origin of plasmon resonance between peaks I and II. The localized E-field from each metal NP usually overlaps when metal NPs are closely positioned, and plasmon coupling occurs as a consequence. In the plasmon hybridization model, the plasmon coupling can be categorized into bonding and antibonding states.43 The bonding state provides a red-shift of a resonant peak with decreasing interparticle gap, whereas there is a slight blue-shift of a resonant peak results from the

Figure 9. (a) Simulated reflectance spectra of 3D NP films with different interparticle gaps. (b) Resonant wavelengths of peaks I and II as a function of interparticle gap. (c) Images of electric field distributions. (d) Images of charge vectors at peaks I and II.

antibonding state. The shifts of resonant wavelengths at peaks I and II were close to the antibonding and bonding states, respectively. E-field distributions and their charge vectors were further analyzed at wavelengths of peaks I and II (Figure 9c, d). For the mid-IR reflectance at peak II, a resonant mode consisted of individual dipolar plasmons oscillating in-phase along the direction of incident polarization. The E-fields between the NPs were only localized along the in-plane Xdirection. In contrast, field analysis of the near-IR reflectance at peak I revealed that the dipolar plasmons in the NPs oscillate out-of-phase, resulting in a net dipole moment of nearly zero. Their E-fields interacted with surrounding NPs along the outof-plane and in-plane directions. The mode splitting of plasmon resonances was caused by three-dimensional stacked assemblies of NPs, which produced quadrupole and dipole modes ascribed to peak I and peak II, respectively. These behaviors became naturally pronounced with an increase in film thickness. For the above reason, the differences in reflectance between experimental and simulation data could be explained in terms of a local structure and plasmon mode as follows. The 3D NP films possessed a disordered alignment of NPs as shown by the obscure FFT pattern extracted from the SEM image (inset of Figure 5e). A dipole mode can be strongly observed in precise close-packed NP assemblies. Ideally, each NP is indistinguishable and this should apply to all of its neighbors. Hence, the simulated reflectance in models of 3D NP layers with ideal HCP structures was mainly dominated by the dipole mode. In contrast, the reflectance observed in the experiment was strongly suggestive of the quadrupole mode rather than the dipole mode, indicating the dominant appearance of quadruF

DOI: 10.1021/acsami.6b01202 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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11a). High reflectance with a close proximity of 0.6 was also reproduced (Figure 11a). The shielding effectiveness (SE) of

pole coupling to the symmetry-forbidden nature of dipole coupling and symmetry-allowed quadrupole characteristics.44,45 It is thought that these situations were realized in the 3D NP films with increasing thickness, and that they were related to the inhomogeneous arrangement of NPs. The character of E-field interactions in the 3D NP films was further elucidated from reflectance measurements for light polarized perpendicular (s-polarized) and parallel (p-polarized) to the plane of incidence as a function of the incident angle (θ). Figure 10 shows angular-dependent reflectance spectra for s-

Figure 11. (a) Reflectance spectrum of a 3D NP sheet on a PET substrate. Inset shows a photograph of the fabricated sheet sample made using a roll-coating method. (b) Shielding effectiveness (SB) in the range 0.5 to 40 GHz for a 3D NP sheet (red open circles) and sputtered ITO film (black open circles).

the 3D NP film was almost zero, which largely differed from that of a sputtered 127 nm thick ITO film (Figure 11b). The difference between the two materials concerns electrical conductance (σ) in the films, which was on the order of 1 × 10−5 and 1 × 103 S/cm for the 3D NP film and sputtered film, respectively. If the shielding material is thin, SE can be determined by reflection, as follows:SE = 20log( 2 η0 /2R ), where η02 is μ0 /ε0 (μ0, absolute permeability of the vacuum; and ε0, dielectric constant of the vacuum), and R is the sheet resistivity (= 1/σ).49 The significant obstruction of carrier transport between NPs produced low electrical conductance due to the presence of surface ligands on the NPs, and realized the high microwave transmissions.

Figure 10. Reflectance spectra of a 216 nm thick 3D NP film as a function of incident angle for (a) s- and (b) p-polarized light. All curves are normalized for a clear comparison of spectra. Insets indicate the direction of the electric vector of incident light.

and p-polarizations for a 3D NP film (204 nm-thickness). For an s-polarized light, peaks I and II were observed for all incident angles because the electric vector of the radiation at any incident angle induced electron oscillations in NPs parallel to the plane of the film. In contrast, for p-polarization, peak I only survived with increasing incident angle because of the reduction in peak II. The component of the electric vector excites electron oscillations in NPs normal to the plane of the film sample, and suppresses the field interactions along the in-plane direction. These results revealed that peak II was activated by the field interactions along the in-plane directions. In contrast, the near-IR reflectance at peak I was essential for field interactions along the out-of-plane direction. Accordingly, the near- and mid-IR reflectance of the 3D NP films was derived from the 3D field interactions along the out-of-plane and inplane directions. The film thickness-dependent plasmon splitting was attributed to the formation of field interactions along the out-of-plane direction, leading to the enhanced reflectance in the near-IR range. To obtain high reflectance in the near- and mid-IR range, it is necessary to control quadrupole and dipole modes in cases involving the use of assembled films of ITO NPs, respectively. The knowledge gained in this study can be applied to 3D NP films utilizing inexpensive ZnO and WO3 for large-size coating films of transparent windows.46−48 3.4. Electromagnetic Responses. We briefly report the EM properties of 3D NP films in the microwave range 0.5 to 40 GHz, which is an important frequency range for telecommunications. For microwave measurements, a 250 nmthick 3D NP sheet (40 × 30 cm) was deposited on a flexible sheet of PET using a roll-coating technique (inset of Figure

4. CONCLUSIONS Surface-modified ITO NPs were used to create 3D assembled films with small interparticle gaps due to the presence of surface ligands. This situation induced effective E-field interactions along the in-plane and out-of-plane directions, which caused the splitting of plasmon resonances for the quadrupole and dipole modes. This plasmon coupling induced in the 3D NP films played an important role in producing the high reflectance in the near- and mid-IR range, which was theoretically supported using FDTD simulations that showed agreement with experimental results. In addition, the E-field enhancements between NPs simultaneously caused a remarkable reduction of electrical contacts between the NPs, which contributed to the high microwave transmissions. The plasmonic control in 3D assemblies of NPs represents promising potential for structural and optical designs used to fabricate a flexible thermal-shielding sheet with a reflection-type based on transparent oxide semiconductors. The knowledge gained in this study can be applied to 3D NP films utilizing inexpensive ZnO and WO3 for large-size coating films of transparent windows.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01202. G

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Optical and physical properties of ITO NPs from the viewpoint of theoretical and experimental approaches; absorption spectra and their characteristics of ITO NPs with various Sn contents; detailed fabrications of NP films using a spin-coating method; chemical properties of fatty acids with different numbers of carbons; relationship between reflectance and Sn contents in ITO NPs; and experimental and theoretical absorption properties of 3D NP films (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. and fax: +81-3-58411870. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported in part by a grant-in-aid from the JSPS Core-to-Core Program, A. Advanced Research Network, a grant from the Japan Science and Technology Agency (JST: Astep), and a grant-in-aid for Exploratory Research and Scientific Research (B).

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DOI: 10.1021/acsami.6b01202 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX