Plasmonic-Field Interactions at Nanoparticle Interfaces for InfraredThermal-Shielding Applications Based on Transparent OxideSemiconductorsHiroaki Matsui,*,†,‡ Shinya Furuta,§ Takayuki Hasebe,∥ and Hitoshi Tabata†,‡

†Department of Bioengineering and ‡Department of Electric Engineering and Information Systems, The University of Tokyo, 1-3-7Hongo, 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 re-sponses in three-dimensional (3D) assembled films ofIn2O3:Sn nanoparticles (NPs). The introduction of surfacemodifications to NPs can facilitate the production of electric-field interactions between NPs due to the creation of narrowcrevices in the NP interfaces. In particular, the electric-fieldinteractions along the in-plane and out-of-plane directions inthe 3D assembled NP films allow for resonant splitting ofplasmon excitations to the quadrupole and dipole modes,thereby realizing selective high reflections in the near- and mid-infrared range, respectively. The origins of these plasmonicproperties were revealed from electric-field distributionscalculated by electrodynamic simulations that agreed well withexperimental results. The interparticle gaps and their derived plasmon couplings play an important role in producing highreflective performances in assembled NP films. These 3D assemblies of NPs can be further extended to produce large-size flexiblefilms with high infrared reflectance, which simultaneously exhibit microwave transmittance essential for telecommunications. Thisstudy provides important insights for harnessing infrared optical responses using plasmonic technology for the fabrication ofinfrared thermal-shielding applications.

KEYWORDS: transparent oxide semiconductor, nanoparticle, plasmon coupling, near-infrared and thermal-shielding

1. INTRODUCTION

Transparent oxide semiconductors (TOSs, such as ITO anddoped ZnO) can be induced to exhibit metallic conductivity bydoping with defects and/or impurities, allowing for excitationsof surface plasmon resonances (SPRs) at dielectric/oxideinterfaces.1−6 SPR properties of nanorods and nanowires havealso been reported for ITO and doped ZnO.7−9 In addition,localized surface plasmon resonances (LSPRs) can also beproduced when confining the collective oscillations of freecarriers into nanoparticles (NPs). Investigations concerningnanoplasmonics based on TOSs have received much attention,and are breaking new ground in the area of wide-gap oxidesemiconductors. A characteristic property revealed by thesestudies is that the optical nature of TOSs with infrared (IR)transparency outside the reststrahlen band indicates low-lossplasmonic materials even up to near-IR wavelengths. NPsamples on TOSs have shown clear SPRs and LSPRs in thenear- to mid-IR range by controlling spectral positions throughthe careful choice of dopant concentrations.10−13

For optical applications of NPs based on TOSs, theassembled films consisting of In2O3:Sn (ITO) NPs have so

far demonstrated optical enhancements of near-IR lumines-cence and absorption, which are related to the electric-fields (E-fields) induced on the surfaces of the assembled NP films.14,15

Assemblies of Ag and Au NPs can produce high E-fieldsthrough plasmon coupling between the NPs in the visiblerange,16,17 and are utilized in surface-enhanced spectroscopy,waveguides and biological sensors.18−20 The high E-fieldslocalized between NPs are very sensitive to interparticle gaps.21

A gap length down to distances less than the size of a NPcauses remarkable enhancements of E-fields.22 Therefore,surfactant- or additive-treated NPs are effective pathways toobtain small interparticle gaps between NPs, which can bedeveloped into one-, two-, and three-dimensional (1D, 2D, and3D) assemblies of NPs. In contrast to top-down methods, it isnot easy to control precisely the interparticle gap because slightvariations in distance between NPs are usually created.However, it is beneficial that large-area fabrications at lower

Received: January 28, 2016Accepted: April 25, 2016Published: May 2, 2016

Research Article

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© 2016 American Chemical Society 11749 DOI: 10.1021/acsami.6b01202ACS Appl. Mater. Interfaces 2016, 8, 11749−11757

costs make NP assemblies attractive for industrial development.The plasmon modes from the assemblies of NPs aredetermined by the collective plasmon resonances (CPRs)that are reasonably attributed to the long-range interactions ofLSPRs in the macroscopic assembled films.23

Recently, plasmonic properties on TOS materials haveattracted attention for thermal-shielding applications tosuppress solar- and radiant-heat in the near- and mid-IRrange, respectively.24 In this work, ITO NPs are chosen as aconcrete example. Plasmonic responses are dependent onelectronic band structures. For example, In2O3, ZnO and WO3have similar electronic structures with conduction and valencebands consisting of s and p orbitals,25−27 indicating that theplasmonic properties of these materials can be manipulatedthrough the same mechanism. Plasmonic properties of thermal-shielding based on ITO NPs can be applied to ZnO and WO3NPs. The purpose of our study is to apply the plasmonicproperties of assembled films of ITO NPs to satisfy recentindustry demands for a material with thermal-shielding ability.Their requirements include the fabrication of flexible sheetswith high heat-ray reflections, as well as visible and microwavetransmissions. To date, the IR optical responses have beeninvestigated mainly in regard to transmittance and extinctionspectra of composites and films utilizing oxide semiconductorNPs.28−31 This is because a single ITO NP behaves as a strongabsorber (Figure S1). IR-shielding properties by absorptionprocesses have been discussed on the basis of theoreticalaspects. Accordingly, no previous paper has reported reflectiveperformances in assemblies of NPs in spite of the desire forthermal-shielding to cut IR radiation not by absorption, butthrough reflection properties. Plasmonic applications exhibitinga thermal-shielding ability have not been previously studied indetail.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 organicligands composed of fatty acids in order to limit spatially theinterparticle gap. IR reflectance in the assembled NP films isdiscussed specifically following elucidation of their structureand corresponding plasmonic properties. Above all, we focus onthe key role played by assembled films of surface-modified NPsin realizing selective high IR reflectance due to 3D interactionsof E-fields along the in-plane and out-of-plane directions in thefilms. Finally, we consider electromagnetic (EM) responses inthe microwave range in relation to electron transport in theassembled NP films. This study provides new insights forenhancement of thermal-shielding ability through plasmonictechniques on TOS materials.

2. EXPERIMENTAL SECTION2.1. Fabrications of ITO NPs. ITO NPs with different Sn contents

were grown using the chemical thermolysis method with various initialratios of the precursor complexes (C9H22CO2)3In and(C9H22CO3)4Sn. The raw materials were purchased from WakoChemicals (Japan). Indium and tin carboxylates comprised whitepowders and were heated with a chemical ratio of 95:5 in a flask usinga mantle heater to 350 °C without a solvent in a nitrogen atmosphere.The temperature was maintained for 4 h and the mixture was thengradually cooled to room temperature. The resultant mixtureproduced a pale blue suspension, to which excess ethanol was addedto induce precipitation. Centrifugation and repeated washing processeswere conducted four times using ethanol, which yielded dried powdersof ITO NPs with a pale blue color. Finally, the NP powders weredispersed in a nonpolar solvent of toluene. Furthermore, we confirmed

a positive zeta potential of +31 meV for the NPs using anelectrophoresis method, which indicated that NPs had nonaggregatedstates 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 Sncontent ( Figure S2a). The NPs with a Sn content of 4.8% showed thelowest peak wavelength at around 1.8 μm. This indicated that the NPs(Sn content: 4.8%) had high plasmonic performance because of thelarge amounts of electron density (Figure S2b). In addition, directplasmon excitation on the single NP surface has been further observedusing scanning-type TEM (STEM) equipped with electron energy-lossspectroscopy (EELS),13 and is a consequence of homogeneousdistributions of Sn atoms in the NP. The above results allowed usto treat an ITO NP as a single metallic nanosphere for the simulationsand to discuss the plasmonic-field interaction between NPs.

2.2. Fabrications of Assembled Films of NPs. Assembled NPfilms were coated on IR-transparent CaF2 substrates using a spin-coating method, and were obtained by multiple overglaze of a thin NPfilm fabricated using a NP concentration of 0.2% in toluene. The spincoating conditions at always involved the following process: 800 rpm(5 s) →1200 rpm (10 s) → 2400 rpm (30 s) → 800 rpm (10 s). Anobtained film was heat-treated at 150 °C in air every time in order toevaporate the solvent. 3D NP films with various thicknesses wereobtained by repetition of the above coating process (Figure S3).

2.3. Characterizations. The structural properties of samples wereinvestigated by X-ray diffraction (XRD) and small-angle X-rayscattering (SAXS). Structural information at the microscopic scalewas investigated using micro-Raman scattering. Local surface andstructural information on the samples was evaluated by scanningelectron microscopy (SEM) and transmittance electron microscopy(TEM). The thermal-dependent chemical properties of samples werestudied by thermogravimetry-differential thermal analysis (TG-DTA)equipped with time-of-flight (TOF) mass spectroscopy. Visible-NIRspectra were measured by a spectrometer from 2600 to 800 nm.Fourier-transform infrared (FTIR) measurements were performed bya spectrometer from 8000 to 1000 cm−1. The EM responses in themicrowave range were measured by a dual-focus flat cavity (DFFC:0.8−15 GHz) and the free-pace method (18−40 GHz). A perform-ance 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 calculatednumerically using the finite-difference time-domain (FDTD) method.The electric-field and charge-vector were also computed at specificpeak positions. An ellipsometric measurement of an ITO film wasconducted within the visible-to-IR range to obtain the complexdielectric constants.2 The modeled NP films were illuminated withlight directed in the Z-direction from the air side. The direction of theelectric field was perpendicular to the light and parallel to the X-direction. Periodic boundary conditions were applied in the X and Ydirections, and the top and bottom of the simulated domain in the Z-direction were studied by perfectly matched layer (PML) boundaryconditions.15

3. RESULTS AND DISCUSSION

3.1. Structural, Optical, and Chemical Properties. XRDpatterns of ITO NPs with Sn contents of 4.8% and 0% areshown in Figure 1a, and indicate broad peaks characteristic ofcolloidal NPs with a crystalline nature. The patterns wereconsistent with those of standard cubic bixbyite in thesynthesis.32 Furthermore, the patterns indicated that the NPshad no discernible SnO or SnO2 peaks. The lattice parameter(a-axis length) of ITO NPs was 10.15 Å, which was larger thanthat (10.08 Å) of In2O3 NPs doped with Sn atoms into theNPs. Incorporations of Sn atoms into NPs was also examinedusing micro-Raman scattering. In2O3 with a cubic structure

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belongs to the Ia3 (Th

7) space group, showing that Raman-active peaks are derived from the vibrations with symmetry Ag,Eg, and Tg modes.33 The expected vibration peak (active v1-mode) was found at 130.6 cm−1 (Figure 1b), which differedfrom that (132.4 cm−1) of In2O3 NPs. This was due to thedifference in effective mass between In−O and Sn−O pairs.The XRD and Raman analyses revealed the structuralproperties 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 aspherical particle with a diameter (D) of 19.5 nm. The SAXSpattern and excellent parameter-fit indicated that the NPs weremonodispersed at the ensemble level when the NPs appearedin the TEM image (inset of Figure 2a) and is due to theexistence of surface ligands on the NPs, indicating that the peakposition in LSPR (ELSPR) of the NPs is dependent on solventtype. Figure 2b shows absorbance spectra of the NPs dispersedin 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-shiftedwith an increase in nD. This provided a refractive indexsensitivity (S) of 573 nm RIU−1 (RIU: refractive index unit)and resulted in a figure of merit (FoM = S/Γ) of 1.03 (inset ofFigure 2b), as defined by the ratio of the refractive indexsensitivity (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 fornitrides and Si NPs for IR plasmonics.34,35 The FoM and Qvalues indicated the plasmonic performance of ITO NPs in thiswork.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 lossof physically or chemically absorbed water. There was anobvious weight loss in the temperature range 270 to 320 °Cdue to the generation of organic species identified by m/z peaksat 12 (C) 18 (H2O) 28 (CO) and 44 (CO2, C3H8, C2H4O,etc.) (Figure 3a, b). These chemical species were due tothermal removal of the surface ligands composed of fatty acidson the NPs.

FTIR spectra also confirmed the existence of surface ligandson the NPs from the fingerprints of molecular vibrations. Anorganic ligand on the NP surface consisted of a capric acid witha chemical formula of C10H22O2: CH3(CH2)8C(O)OH,appearing as molecular vibration modes derived from themethyl- and carboxyl-groups. The modes related to the methylgroups were found in the high wavenumber range 3800 to 2500cm−1. Asymmetric (va) and symmetric (vs) CH3 modes werefound at 2968 and 2867 cm−1 (Figure 3c). In addition, va-CH2was detected at 2934 cm−1. On the other hand, the vibrations ofcarboxyl groups were found in the low wavenumber range 1800to 1000 cm−1 (Figure 3d). va and vs COO− modes wereobserved at 1546 and 1314 cm−1, respectively. These modesappeared instead of a C O vibration mode at around 1700cm−1, which represents the distinctive features of carboxylgroups. Furthermore, C−H and C−O modes were observed at1432 and 1398 cm−1, respectively. These vibration spectraindicated that capric acids combined with the NPs as surfaceligand molecules. However, the methyl- and carboxyl-groupspeaks were not supported at high temperatures above 350 °C,which indicated that the surface ligands on the NPs werethermally stable below 250 °C because the decompositiontemperature of capric acid was in the range 250 to 300 °C (Figure S5). Accordingly, the existence of the surface ligands onNPs was very helpful in actualizations of narrow interparticlegaps between the NPs.

3.2. Optical Properties and 2D NP Films. The opticalproperties of monoassembled films of ITO NPs (2D NP films)will now be detailed prior to discussion of multilayeredassembled NP films (3D NP films). A surface SEM image of a2D NP film showed a close-packed structure (inset of Figure

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

Figure 2. (a) SAXS patterns of ITO NPs dispersed in toluene.Experimental and calculated SAXS spectra are represented by blackand 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 hexanesolvents as different solvents, respectively. Inset represents theextended absorbance spectra around 1.8 μm, providing a plasmonsensitivity (S) of 573 nm/RIU.

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4a) because spin-coating causes self-organizations of colloidalNPs into a hexagonally close-packed (HCP) structure due toshear and capillary forces on substrates.36,37 The films ofassembled NPs provide an interesting insight into the scatteringvector (q) of the SAXS intensity. A maximum SAXS peakincludes structural information concerning spatial ordering ofparticles estimated by l = 2π/q with a spatial period (l). TheSAXS pattern showed a maximum peak at q = 0.32 nm−1

followed with weak interferences (Figure 4a). This provided anl value of 19.5 nm being close to the edge-to-edge betweenNPs. For optical responses in the 2D NP film, a resonant peakat 2.64 μm appeared in transmittance, showing the red-shiftedresonance wavelength due to a CPR mode compared to thoseof NPs dispersed in the solvents (Figure 4b).38 Reflectance atthe resonant peak was small, indicating that the opticalresponses were mainly dominated by absorbance. Furthermore,the FDTD simulations were conducted to support theexperimental data of the 2D NP film. The modeled 2D NPlayer (D = 20 nm) has a HCP structure with an interparticle

gap (L) of 2 nm along the X−Y (in-plane) direction, where Lwas determined from the TEM image.15 The refractive index (n= 1.437) of the surface ligand on the NP was used as thedielectric medium between the NPs.39 A resonant peak at 2.45μm was reproduced in transmittance and reflectance spectra,confirming the validity of the modeled 2D NP layer incomparison to the experimental data. The CPR mode wasexcited because of long-range coherences of E-field interactionsbetween the NPs, as confirmed from the 2D image of the E-field distribution (inset of Figure 4b).

3.3. Optical Properties and 3D NP Films. 3D assembliesof ITO NPs provided a remarkable change in optical properties,which were clearly observed from transmittance and reflectancespectra [Figures 5(a) and 5(b)]. Transmittance with a resonantpeak at 2.20 μm decreased to a level close to zero withincreasing film thickness. In contrast, reflectance was enhancedat 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 wasseparated into lower and higher wavelengths with the filmthickness. (Figure 6a). For a 216 nm thick 3D NP film, twotypes of peaks (I and II) were positioned 2.13 and 4.02 μm inthe near- and mid-IR range, respectively. The thickness-dependent 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 filmthickness (Figure 6c). As a result, the 3D NP films providedreflectance-dominant optical responses.

Figure 3. (a) TG-DTA curves of ITO NP samples in a reducingatmosphere. (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 wavenumberregions from (c) 3200 to 2500 cm−1 and (d) 1900 to 1000 cm−1.

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

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FDTD simulations were performed to clarify the aboveoptical properties. A cross-section SEM image of a 3D NP film(96 nm-thickness) showed a close-packed structure (Figure5e). The modeled 3D NP layers are based on a HCP structurewith an interlayer distance of 2 nm along the Z- (out-of-plane)direction (Figure 5f). A layer sheet along the in-plane directionsemployed the 2D NP layer modeled in Section 3.2. Thesystematic change in the number of NP layers from 1 to 20 wascapable of reproducing the experimental data (Figure 5c, d)].The applied model was satisfactory in describing the opticalresponses of the 3D NP films. The increase in number of NPlayers provided the resonant dips in transmittance and peakseparations in reflectance, which matched reasonably well withthe experimental data (Figure 6a, b). However, the reflectancefor peak I was smaller than that for peak-II in the case ofsimulations, resulting in a difference of R/A ratio betweenexperimental and simulation data (Figure 6c). As SupportingInformation, we note that reflectance decreased with decreasingSn content in relation to electron density in the NPs (FigureS6). This behavior indicated that the high electron density in

the NPs was indispensable for realizing high reflectance fromthe 3D assembled NP films.The relation between surface ligands and optical properties

in the 3D NP films was clarified by the spectral changes afterannealing at different temperatures. No interparticle gap hasbeen formed through thermal-removal of surface ligands ofNPs.40 Figure 7a shows temperature-dependent reflectance

spectra taken in an inert atmosphere for a 216 nm thick 3D NPfilm, and reveal remarkable spectral changes in reflectance asfollows. The two resonant peaks at 150 °C were weakenedgradually following the change in spectral shape with increasingtemperature. In particular, the near-IR reflection at peak-Ishifted to longer wavelengths at high temperatures above 300°C corresponding to the removal of the surface ligands (Figure8a). The vibrations involving surface ligands also disappearedfrom the spectra. The removal of surface ligands from the NPsaffected the whole reflective performance, which simultaneouslydemonstrated the importance of interparticle gap in obtaining ahigh reflectance.SAXS patterns of the 3D NP films also changed with

increasing temperature (Figure 7b). 3D NP films annealedbelow 250 °C showed maximum peaks at around q = 0.3 nm−1

Figure 5. (a) Experimental and (b) simulated transmittance spectra of3D NP films. (c) Experimental and (d) reflectance spectra of 3D NPfilms. (e) Cross-section SEM image of a 96 nm thick film sample. Insetis a FFT pattern. (f) Simulated model of a 3D NP film. The modeledNP sheet was illuminated with light directed in the Z-direction fromthe air side. The direction of the E-field was perpendicular to the lightand parallel to the X-direction.

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

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

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accompanied by interference fringes. This provided l valuesclose to 20 nm, which were consistent with those of the 2D NPfilm. In contrast, the further increase in annealing temperaturechanged the peak-shifts to high scattering vectors, resulting in adecrease of l to 15.4 nm at 550 °C (Figure 8b). The annealingeffects of 3D NP films were further reflected by the electricalresistivity (ρ) in the films. ρ was in the order of 104 Ω.cm below250 °C (Figure 8b) because the presence of the surfactantlayers on NPs seriously impedes charge transport in assembledNP films. The surface ligands often behave as interparticleinsulating layers in NP networks.41 However, this effect wasmarkedly reduced at high temperatures above 350 °C becauseof the relationship between l and ρ as a function of annealingtemperature. The remarkable decrease in ρ was related tocoalescence between the NPs through thermal-removal ofsurface ligands (Figure 8c), which was in agreement with aprevious report showing that annealing temperatures above 500°C are required to achieve film conductivities >10 S/cm42 inthe case of ITO NPs. The surface ligands contributed to formsmall interparticle gaps between the NPs, which shows theorigin 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 gapbetween the NPs. Figure 9a shows simulated reflectancespectra of 3D NP layers (20 NP layered model) at different L.Reflectance gradually enhanced with decreasing L. Whendecreasing L from 10 to 1 nm, peak II exhibited a monotonousred-shift to longer wavelengths, whereas peak I remainedalmost unchanged (Figure 9b). This suggests a difference in theorigin of plasmon resonance between peaks I and II. Thelocalized E-field from each metal NP usually overlaps whenmetal NPs are closely positioned, and plasmon coupling occursas a consequence. In the plasmon hybridization model, theplasmon coupling can be categorized into bonding andantibonding states.43 The bonding state provides a red-shiftof a resonant peak with decreasing interparticle gap, whereasthere is a slight blue-shift of a resonant peak results from the

antibonding state. The shifts of resonant wavelengths at peaks Iand II were close to the antibonding and bonding states,respectively. E-field distributions and their charge vectors werefurther analyzed at wavelengths of peaks I and II (Figure 9c, d).For the mid-IR reflectance at peak II, a resonant modeconsisted of individual dipolar plasmons oscillating in-phasealong the direction of incident polarization. The E-fieldsbetween the NPs were only localized along the in-plane X-direction. In contrast, field analysis of the near-IR reflectance atpeak I revealed that the dipolar plasmons in the NPs oscillateout-of-phase, resulting in a net dipole moment of nearly zero.Their E-fields interacted with surrounding NPs along the out-of-plane and in-plane directions. The mode splitting of plasmonresonances was caused by three-dimensional stacked assembliesof NPs, which produced quadrupole and dipole modes ascribedto peak I and peak II, respectively. These behaviors becamenaturally pronounced with an increase in film thickness. For theabove reason, the differences in reflectance between exper-imental and simulation data could be explained in terms of alocal structure and plasmon mode as follows. The 3D NP filmspossessed a disordered alignment of NPs as shown by theobscure FFT pattern extracted from the SEM image (inset ofFigure 5e). A dipole mode can be strongly observed in preciseclose-packed NP assemblies. Ideally, each NP is indistinguish-able and this should apply to all of its neighbors. Hence, thesimulated reflectance in models of 3D NP layers with idealHCP structures was mainly dominated by the dipole mode. Incontrast, the reflectance observed in the experiment wasstrongly suggestive of the quadrupole mode rather than thedipole mode, indicating the dominant appearance of quadru-

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

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

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pole coupling to the symmetry-forbidden nature of dipolecoupling and symmetry-allowed quadrupole characteristics.44,45

It is thought that these situations were realized in the 3D NPfilms with increasing thickness, and that they were related tothe inhomogeneous arrangement of NPs.The character of E-field interactions in the 3D NP films was

further elucidated from reflectance measurements for lightpolarized 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-

and p-polarizations for a 3D NP film (204 nm-thickness). Foran s-polarized light, peaks I and II were observed for all incidentangles because the electric vector of the radiation at anyincident angle induced electron oscillations in NPs parallel tothe plane of the film. In contrast, for p-polarization, peak I onlysurvived with increasing incident angle because of the reductionin peak II. The component of the electric vector exciteselectron oscillations in NPs normal to the plane of the filmsample, and suppresses the field interactions along the in-planedirection. These results revealed that peak II was activated bythe field interactions along the in-plane directions. In contrast,the near-IR reflectance at peak I was essential for fieldinteractions along the out-of-plane direction. Accordingly, thenear- and mid-IR reflectance of the 3D NP films was derivedfrom the 3D field interactions along the out-of-plane and in-plane directions. The film thickness-dependent plasmonsplitting was attributed to the formation of field interactionsalong the out-of-plane direction, leading to the enhancedreflectance in the near-IR range. To obtain high reflectance inthe near- and mid-IR range, it is necessary to controlquadrupole and dipole modes in cases involving the use ofassembled films of ITO NPs, respectively. The knowledgegained in this study can be applied to 3D NP films utilizinginexpensive ZnO and WO3 for large-size coating films oftransparent windows.46−48

3.4. Electromagnetic Responses. We briefly report theEM properties of 3D NP films in the microwave range 0.5 to 40GHz, which is an important frequency range for tele-communications. For microwave measurements, a 250 nm-thick 3D NP sheet (40 × 30 cm) was deposited on a flexiblesheet of PET using a roll-coating technique (inset of Figure

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

the 3D NP film was almost zero, which largely differed fromthat of a sputtered 127 nm thick ITO film (Figure 11b). Thedifference between the two materials concerns electricalconductance (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 bedetermined by reflection, as follows: η= RSE 20log( 2 /2 )0 ,where η0

2 is μ0 /ε0 (μ0, absolute permeability of the vacuum;and ε0, dielectric constant of the vacuum), and R is the sheetresistivity (= 1/σ).49 The significant obstruction of carriertransport between NPs produced low electrical conductancedue to the presence of surface ligands on the NPs, and realizedthe high microwave transmissions.

4. CONCLUSIONSSurface-modified ITO NPs were used to create 3D assembledfilms with small interparticle gaps due to the presence of surfaceligands. This situation induced effective E-field interactionsalong the in-plane and out-of-plane directions, which causedthe splitting of plasmon resonances for the quadrupole anddipole modes. This plasmon coupling induced in the 3D NPfilms played an important role in producing the high reflectancein the near- and mid-IR range, which was theoreticallysupported using FDTD simulations that showed agreementwith experimental results. In addition, the E-field enhancementsbetween NPs simultaneously caused a remarkable reduction ofelectrical contacts between the NPs, which contributed to thehigh microwave transmissions. The plasmonic control in 3Dassemblies of NPs represents promising potential for structuraland optical designs used to fabricate a flexible thermal-shieldingsheet with a reflection-type based on transparent oxidesemiconductors. The knowledge gained in this study can beapplied to 3D NP films utilizing inexpensive ZnO and WO3 forlarge-size coating films of transparent windows.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b01202.

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

Figure 11. (a) Reflectance spectrum of a 3D NP sheet on a PETsubstrate. Inset shows a photograph of the fabricated sheet samplemade using a roll-coating method. (b) Shielding effectiveness (SB) inthe range 0.5 to 40 GHz for a 3D NP sheet (red open circles) andsputtered ITO film (black open circles).

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Optical and physical properties of ITO NPs from theviewpoint of theoretical and experimental approaches;absorption spectra and their characteristics of ITO NPswith various Sn contents; detailed fabrications of NPfilms using a spin-coating method; chemical properties offatty acids with different numbers of carbons; relation-ship between reflectance and Sn contents in ITO NPs;and experimental and theoretical absorption properties of3D NP films (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: hiroaki@ee.t.u-tokyo.ac.jp. Tel. and fax: +81-3-5841-1870.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was supported in part by a grant-in-aid from theJSPS Core-to-Core Program, A. Advanced Research Network, agrant from the Japan Science and Technology Agency (JST: A-step), and a grant-in-aid for Exploratory Research and ScientificResearch (B).

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