Assembled Films of Sn-Doped In2O3 Plasmonic ... - ACS Publications

Article Cite This: ACS Appl. Nano Mater. 2019, 2, 2806−2816

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Assembled Films of Sn-Doped In2O3 Plasmonic Nanoparticles on High-Permittivity Substrates for Thermal Shielding Hiroaki Matsui*,†,‡ and Hitoshi Tabata†,‡ †

Department of Bioengineering and ‡Department of Electrical Engineering and Information Systems, The University of Tokyo, 1-3-7 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

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S Supporting Information *

ABSTRACT: This study presents the infrared (IR) plasmonic responses of assembled films consisting of Sn-doped In2O3 nanoparticles (ITO-NP films) placed on different substrates. The reflectance properties of the NP films are found to depend on substrate permittivity that affects the types of solar thermalshielding. Increasing the substrate permittivity provides high reflectance in the IR range resulting from the plasmon modes and the interference effect in the NP films, as demonstrated through changes in the optical properties by tuning substrate permittivity. In particular, the plasmon modes played an important role in enhancing reflectance in the near-IR range. Induced image charges in the substrates produced field interactions with the plasmon modes in the NP films that are formed along the in-plane and out-of-plane directions, a consideration further verified by the strong image charges induced by metallic substrates consisting of ITO films. On the other hand, the interference effects contributed to the reflectance in the mid-IR range, which was also dependent on substrate permittivity. These results suggest that the plasmon modes and optical interference of the NP films can be modulated by tuning the permittivity of the substrate, thereby contributing to the development of reflective-type solar-thermal shielding in the IR range. This study not only contributes to an understanding of the plasmon modes and the interference effect of the NP films interacting with the substrates but also assists in the growth and development of IR applications based on transparent oxide semiconductor NPs. KEYWORDS: oxide semiconductor, nanoparticle, surface plasmon, infrared, solar-thermal shielding, substrate permittivity, particle interface in the NIR to MIR range,9−12 the resonant wavelengths of which can be easily controlled by free carriers.13−17 In particular, localized SPR (LSPR) modes can confine light to the physical dimensions of the NPs by concentrating it into volumes below the diffraction limit to produce very intensive electric-fields (E-fields) around the NPs.18−20 This near-field enhancement makes oxide semiconductor NPs good candidates to apply in the IR region owing to their lower optical losses compared to noble metals.21 To date, window applications have been reported for In2O3, ZnO, WO3, and VO2-related materials.22−25 Optical properties of nanoparticles are dependent on electronic band structures. For example, doped In2O3 and ZnO have simple band structures with conduction and valence bands consisting of s and p orbitals.26,27 These materials do not have localized states related to d-orbitals in the band gap, resulting in high visible transmittance. Additionally, plasmonic responses of heavily doped In2O3 (ZnO) NPs can be described using a simple

1. INTRODUCTION Solar energy is composed of ultraviolet (UV), visible, and infrared (IR) rays in proportions of 5%, 40%, and 55%, respectively.1 Reflecting films operating in the near-IR (NIR) and mid-IR (MIR) regions are required to prevent heat transfer from sunlight, and their use is promising applications in approaches focusing on the reradiation reduction of indoor heat. In particular, control of not only solar-thermal shielding but also room brightness through windows affected by visible light creates a comfortable environment for humans inside automobiles and buildings. To date, there have been various investigations concerning transparent reflecting films for solar thermal-shielding such as for silica aerogels,2,3 photonic crystals,4,5 and metal oxide multilayers, including ITO− TiN(Au)−ITO and TiO2−Cu−TiO2.6−8 However, recently, maintaining in high transmittance of radiofrequency bands in window applications has become necessary for the prevention of interference in wireless communications. Therefore, optical and electromagnetic control within a wide range of wavelengths from UV to microwaves is desirable. Oxide semiconductor nanoparticles (NPs) based on ZnO:Al and In2O3:Sn (ITO) show surface plasmon resonances (SPRs) © 2019 American Chemical Society

Received: February 14, 2019 Accepted: April 23, 2019 Published: May 2, 2019 2806

DOI: 10.1021/acsanm.9b00293 ACS Appl. Nano Mater. 2019, 2, 2806−2816

Article

ACS Applied Nano Materials

Scheme 1. ITO NPs and ITO Films Fabricated by MOD and PLD Techniques, Respectively, and Fabrication of ITO NP Films by a Spin-Coating Method

However, most studies have focused on the basic optical responses of isolated metal NPs and nanodots. Few reports have appeared that document investigations of the optical responses related to the assembled films of ITO NPs interacting with different substrates for the development of solar thermal applications. In this paper, we explore the effect of a substrate on plasmonic responses in ITO-NP films to advance our previous experimental findings. The nature of the substrates is necessary in optimizing the fabrication of the NP films toward reflectiontype solar thermal shielding. In particular, plasmon modes at the interparticle gaps are important in determining the optical properties of the NP films, thus raising the question of how these plasmon modes in the NP films depend on the nature of the substrate on which the NPs are three-dimensionally assembled to achieve high IR reflectance. We report on the optical responses of the NP films on various substrate materials such as CaF2, Al2O3, YSZ, and TiO2 without supporting their SPR behaviors. Effect of dielectric interfaces on the modifications of plasmon modes of ITO NP films is examined using experimental and theoretical approaches. We shed light on how the dielectric environment created by an asymmetric interface modifies the optical responses of the NP films with further focus on E-field interactions between NPs. The degree of interactions with the substrates is investigated by changing substrate permittivity using thin spacer layers at the subwavelength scale. Finally, we examine the spectral changes of the NP films on metallic ITO substrates to understand the NP film−substrate coupling. As such, this paper provides new insight into the potential use of efficient solar-thermal shielding as a means of plasmonic control of oxide semiconductors.

Drude model, which are expected for efficient and narrow LSPR excitations. In contrast, doped WO3 has optical transitions due to the defect-induced levels formed in the band gap, which produce light absorptions in the visible range.28 Metallic VO2 also shows visible absorptions because of optical transitions related to 3d orbitals.29 These optical absorptions result in weakened and broadened LSPR excitations of NPs in the NIR range. Therefore, NP samples based on doped In2O3 and ZnO are candidate for transparent reflecting films in the IR range. These materials have further advantages for tuning of LSPRs by external control of electrical and optical fields for smart windows.30,31 On the basis of the above considerations, we have investigated the use of transparent reflecting films using metallic ITO NPs with the NIR plasmonic excitations.32 In such work, our concept for obtaining optical reflection in the IR range was derived from strong light scattering emitted from high E-fields at interparticle gaps, which allowed for resonant reflectance from three-dimensionally assembled films of ITO NPs (ITO-NP films). The optical signature of this resonant reflectance was dependent on physical factors, including chemical composition, particle size, and interparticle gap, which markedly affected the resonant wavelengths and their strengths.33 However, certain issues remain as the optical properties of ITO NP films in the IR region are mainly dominated by light absorption. In other words, absorbance is more predominant compared to reflectance, leading to solarthermal shielding with an absorption type. For solutions of these issues, it is further required to understand origins of resonant reflections and absorptions of the ITO NP films in the NIR and MIR range in order to realize reflective-type thermal shielding. In this work, we focus on substrate permittivity as a new approach for tuning of LSPRs in the ITO NP films as follows. Beyond the selection of physical factors for plasmonic control such as composition, size, and interparticle gap, there is a search for external factors influencing the LSPR excitations of metal NPs (e.g., gold and silver), which suggests the employment of supporting substrates for placement of the metal NPs. Presence of substrates is changed dielectric environment around the metal NPs.34,35 Metal NPs are commonly placed on dielectric or metal substrates in ultrathin, multilayered, and nanostructured forms.36−39 Above all, plasmon coupling between the metal NPs and substrates has contributed to enhancing in photocatalytic reactions and solar energy conversions at the interfaces of the metal NPs and semiconductors.40−42 These alternations are strongly related to the dielectric functions of the substrates. Therefore, understanding the role of the supporting substrate of the metal NPs in modifying the plasmonic response is of utmost importance.

2. EXPERIMENTAL SECTION 2.1. Fabrication of ITO NPs and Their Assembled Films. Metallic ITO NPs were synthesized by a chemical thermolysis (metal−organic decomposition: MOD) method with different initial ratios of the precursors complexes In(C 10H 22 O2)3 and Sn(C10H22O2)4. The raw materials were purchased from Wako Chemicals Co. (Japan). Indium and tin carboxylates comprising white powders were mixed with ratio 95:5. The mixed powders were heat-treated at 350 °C using a heating mantle. The heat treatment was performed in a nitrogen atmosphere. The heating temperature was maintained for 4 h, and the mixture was then cooled to room temperature. Excess ethanol was added to the obtained mixture comprising a pale blue suspension 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, NP powders were dispersed in a nonpolar solvent such as toluene. The synthesized ITO NPs were fully terminated by organic ligands (capric acid) of fatty acids to yield a core−shell structure of inorganic and organic materials, which contributed toward spatial separation between the NPs [left in 2807


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ACS Applied Nano Materials Scheme 1]13. The ITO NPs used in this work had an approximate diameter of 20 nm, as confirmed by transmission microscopy. The Sn content in ITO NPs was determined to be 5% by X-ray fluorescent spectroscopy, which induced a high electron density of 9.4 × 1020 cm−3 in the host [Figure S1]. We noted that the optical properties (transmittance and reflectance) of ITO NP films were dependent on electron density [Figure S2]. In this work, we focus on ITO NP samples with the high electron density in an effort to obtain high reflectance in the NIR and MIR range. ITO NP films were deposited on different dielectric substrates such as single crystal substrates of CaF2 (001), Al2O3 (0001), YSZ (001), and TiO2 (001), which were purchased from Crystal Base Co. (Japan). On the other hand, a 220 nm thick polycrystal ITO film was chosen as the metallic substrate. An ITO film (ITO substrate) was fabricated on IR-transparent CaF2 substrates at 450 °C in an oxygen flow of 10−4 Pa by pulsed laser deposition (PLD).43,44 Remarkably high permittivity substrates of Si and GaAs were employed as they have no visible transmittance, which therefore was not suitable for transparent solar thermal shielding applications. The NP films were fabricated by spin-coating at coating conditions of sequential centrifugation at (i) 800 rpm for 5 s, (ii) 1200 rpm for 10 s, and (iii) 1800 rpm and (iv) 800 rpm for 10 s. The obtained films were further heat-treated in air at 150 °C to allow the solvent to evaporate [right in Scheme 1]. Details of structural evaluations of the NP films were reported in Figure S3. The thickness of the NP film was identified from a cross-sectional view of an image obtained from scanning electron microscopy (S-5500; Hitachi Co.). The local structures at the interfaces between the NP films and substrates were evaluated by cross-section transmission electron microscopy (H-9500; Hitachi Co.). 2.2. Characterization. Experimental reflectance, absorbance, and transmittance were measured using a Fourier transform IR (FTIR6000 series; JASCO Co.) spectrometer in the wavenumber range 8000−1000 cm−1 with a resolution of 4 cm−1. The IR optical detector comprised a liquid-nitrogen-cooled mercury−cadmium−telluride (MCT) photovoltaic element. The incident angle of light in the FT-IR spectrometer could be varied from 5° to 70° under linear polarizations (VeeMAX III; PIKEC Technologies). Moreover, the strong back reflection observed when using a high-permittivity substrate was substantially reduced by a back-side prevention layer using Al2O3 films, allowing for more precise examination of the NP films’ optical properties [Figure S4]. Al2O3 films were deposited by electron-beam deposition (EB-680; Eiko Eng. Co.). The complex dielectric functions of ITO-NP films and ITO films were determined by two types of ellipsometric spectroscopes: one is visible to near-IR ellipsometry (M-2000; J. A. Woollam Co.,), and the other is mid-IR ellipsometry (IR-VASE, J.A. Woollam Co.). A wide range of dielectric functions were obtained using both ellipsometric measurements. The dielectric functions of CaF2, Al2O3, YSZ, and TiO2 substrates were also obtained using both ellipsometric spectroscopes. 2.3. Theoretical Model. In an effort to examine the optical properties of ITO NP films from theoretical approaches, we employed the plane-boundary N-multilayer system.45,46 A schematic picture of the system is shown in Figure 1. A light is incident at an air side with a dielectric constant (εair) of 1.0. The dielectric constant and layer thickness of the jth layer are defined as ε̂j and dj (j = 1, 2, ..., N). The complex dielectric constant of each layer is related to its complex refractive index n̂j by ε̂j = n̂j (n̂j = nj + ikj) where nj is the refractive index and kj is the absorption index). Additionally, the semi-infinitely thick dielectric substrate has a dielectric constant ε̂D. When an incident beam of wavelength λ impinges at the multilayer with angle of incidence θ, the reflection and transmission coefficients of the system can be described by the following relations: rp,s =

Figure 1. Schematic diagram of N-multilayer model. rp,s =

(M11 + M12qD)qp + (M 21 + M 22qD)

(2)

where s and p indicate parallel and perpendicular polarized radiations. Herein, we introduce a matrix Mnm that represents an element of the characteristic matrix (2 × 2) of the plane boundary stratified layers. The following Mnm matrix is provided by the experimental parameters of the multilayers as follows: ÄÅ ÉÑ ÅÅ ÑÑ −1 Å Ñ cos( k d ) sin( k d ) Å N Å zj j zj j Ñ ÅÅ ÑÑÑ qj ÑÑ M = ∏ ÅÅÅ ÅÅ ÑÑÑ j=1 Å ÅÅ− iq sin(kzjdj) cos(kzjdj) ÑÑÑ ÅÅÇ j ÑÑÖ (3) where qj = kzj/ε̂j and kzj is the z component of the wave vector in the jth layer. kzj is given in terms of the x component of the wave vector in the substrate kzP by kzj = [(2π /λ)2 εĵ − kzP 2]1/2 with kzP = (2π/ λ)[εP sin2 θ]1/2. kzP is the x-component of the wave vector in air. The reflectance (Rp and Rs) and transmittance (Tp and Ts) are expressed in terms of the Fresnel reflection (rp and rs) and transmission (tp and ts) coefficients, respectively, as follows: R p = |rp|2

and

Tp =

Re[kz D/εD̂ ] 2 |t p| kzP/εp

for p‐polarization (4)

R s = |rs|2

and

Ts =

Re[kz D] 2 |ts| kz P

for s‐polarization

(5)

This concept which we have adopted is based on the assumption that ITO NP films are considered as one layer. In the actual calculation, we employed four layers consisting of air, ITO NP film, spacer, and substrate in order to obtain transmittance (T) and reflectance (R) of ITO NP films. Absorbance (A) was estimated using the following relation: A = 1 − R − T. 2.4. Electric-Field Analysis. Electric-field (E-field) distributions in the ITO NP films were simulated numerically using the threedimensional finite-difference time-domain method (3D Key-FDTD Kagiken Co.). The E-fields and their directions were computed at specific peak positions. The model layer was assumed to have a hexagonally close-packed (HCP) structure because the NP film had a densely stacked layer. The modeled NP layers were illuminated with light directed in the Z-direction from the air side. The E-field was set parallel to the X-direction, and periodic boundary conditions were employed on the X- and Y-directions. The top and bottom regions of the simulated domain in the Z-direction were analyzed using perfectly matched layer (PML) boundary conditions. The E-field distributions formed at the particle interfaces in the NP films are an important clue in clarifying the mechanism of plasmon excitations.

(M11 + M12qD)qp − (M 21 + M 22qD) (M11 + M12qD)qp − (M 21 + M 22qD)

2qp

(1) 2808


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3. RESULTS AND DISCUSSION 3.1. Use of High and Low Permittivity Substrates. Figure 2a shows the real (ε1) and imaginary (ε2) parts of an

Figure 2. Complex dielectric functions of (a) ITO-NP film and (b) ITO film. Real (ε1) and imaginary (ε2) parts are shown by red and black markers.

ITO NP film, as determined experimentally by the ellipsometric spectroscopy. The imaginary part exhibited a maximum peak at 2.20 μm, relative to a change in the real part at the same wavelength. The experimental dielectric function of ITO NP films was similar to that calculated by Maxwell− Garnet effective medium approximation (MG-EMA) theory.47 Use of this optical theory is suitable to quantitatively account for the inhomogeneous size and composition that are inevitably present in real nanoparticle dispersions. That is, the optical properties of ITO NP films could be well described by MG-EMA theory from a macroscopic viewpoint, showing the validity of the experimental results [Figure S6a and Figure S6b]. The experimental dielectric functions of ITO NP films were reasonable as assembled films of nanoparticles. Details of the theoretical analyses using MG-EMA theory are summarized in section 5 of the Supporting Information. Additionally, slight spectral modulations at 3.4 and 7 μm were due to molecular-vibrational peaks of CH2 (CH3) and COO− (C O) in terms of surface ligands on the NPs. These peaks were based on surface enhancements of vibrational resonances of ligand molecules attached on the NP surfaces,48 indicating the presence of strong E-fields at the interparticle gaps. By contrast, the ITO film exhibited a negative ε1 at long wavelengths above 1.31 μm with increasing imaginary ε2 part [Figure 2b]. These phenomena were characteristic of a metallic behavior related to Drude plasma oscillation. Therefore, thee dielectric functions of the NP film showed different dielectric responses depending on measurement wavelength, which affected optical properties (e.g., reflectance and absorbance in the IR region). These experimental dielectric functions were also used to simulate the optical properties of the NP films using the Fresnel-based theoretical model. Figure 3a and Figure 3b show the experimental transmittance spectra of ITO-NP films on different substrates, revealing single resonant peaks at around 2 μm. The transmittance dips were based on the collective plasmon modes from NP assemblies that have been determined by three-dimensionally collective resonances between the NPs as a result of the long-range field interaction.49 The real parts of

Figure 3. (a) Experimental and (b) simulated transmittance spectra of ITO NP films on different substrates. (c) Real parts of CaF2, Al2O3, YSZ, and TiO2 as a function of wavelength. (d) Spectral line widths (fwhm’s) of transmittance spectra as a function of dielectric constant (real part) at 2 μm. The dielectric constant is dependent on substrate type.

the dielectric functions of the substrate in the IR range increased in the order CaF2 < Al2O3 < YSZ < TiO2 [Figure 3c]. The imaginary parts were summarized in Figure S5. Moreover, increased permittivity of the substrate caused a transmittance spectral change from asymmetric to symmetric shapes because of a reduced component at around 4 μm, as evidently provided by a spectral line width defined by the fullwidth half minimum (fwhm) decreasing dramatically with increasing permittivity [Figure 3d]. In particular, the line width was reduced by 24% from 0.62 to 0.47 eV. After such results were reproduced by simulated transmittance, it was confirmed that employing high-permittivity substrates to the NP films resulted in sharpening of the transmittance spectra because of changes in reflectance and absorbance described in the subsequent text. Substrate type affected the spectral shapes of the reflectance of ITO-NP films. With low-permittivity substrate such as such as Al2O3 and CaF2, reflectance spectra remained unchanged [Figure 4a]. Through monitoring of the molecular-vibrational peaks related to the organic ligands at around 3.4 and 7 μm, the entire spectrum in the IR range without back reflection could be examined. Reflectance at peak I and peak II was derived from different plasmon modes, which is discussed later. By contrast, reflectance spectra clearly changed with highpermittivity substrates such as YSZ and TiO2 [Figure 4b and Figure 4c]. For a TiO2 substrate, for instance, the reflectance at peak I in the NIR range increased up to 0.63 with a concomitant decrease in reflectance at peak II in the MIR range. These experimental data were similarly reflected by the 2809


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Substrate-dependent optical changes further appeared with the absorbance of NP films. In particular, the absorbance of the NP film on CaF2 was close to that on Al2O3, and it decreased with high-permittivity substrates such as TiO2 as compared with its reflectance result [Figure 5a−c]. Similar patterns were

Figure 4. Experimental reflectance spectra of ITO-NP films on (a) Al2O3, (b) YSZ, and (c) TiO2. Simulated reflectance spectra of ITONP films on (d) Al2O3, (e) YSZ, and (f) TiO2. Experimental and simulated reflectance spectra of ITO-NP films on CaF2 are provided as reference data and marked by black lines. Figure 5. Experimental absorbance spectra of ITO-NP films on (a) Al2O3, (b) YSZ, and (c) TiO2. Simulated absorbance spectra of ITONP films on (d) Al2O3, (e) YSZ, and (f) TiO2. Experimental and simulated reflectance spectra of ITO-NP films on CaF2 are provided as reference data and marked by black lines.

theoretical simulations [Figure 4d−f]. Thus, the control of substrate permittivity played a role in tuning reflectance in the NIR and MIR range. Optical properties in a film sample can be tuned by the optical interference method in addition to the plasmonic control. It is known that interference effects of light in layered structures produce resonant reflections due to optical interactions between the primary and secondary reflection light.50 The primary and secondary reflection light is defined as reflection waves at the interfaces of air/the ITO NP film and the ITO NP film/substrate [Figure S8]. We will discuss the interference effects on the reflectance at peak I and peak II. The penetration depth of the incident light at peak I into all NP films were below 100 nm because the dielectric constants at peak I comprised the negative ε1 and large ε2 [Table S1]. This indicated that the reflectance at peak II was mainly dominated by the primary reflection light. The interference effect was neglected on the reflectance at peak I. On the other hand, peak II showed greater penetration depths of ∼1000 nm, which were derived from the dielectric constants with high positive ε1 and small ε2. The value of ε1 at peak II of the NP film was similar to those of Si and GaAs, which were higher values than those of the substrates. It is necessary to take into account the interference effect for the reflectance at peak II. The theoretical reflectance at peak II excluding an interference effect was about half that of the experimental reflectance [Tables S2 and S3]. The interference effect became an important factor in producing the reflectance at peak II besides the plasmonic effect. Therefore, the reflectance at peak I was only ascribed to plasmon resonances, while the reflectance at peak II was related to both plasmonic and interference effects. Details of the theoretical estimations for the interference effects are summarized in section 7 of the Supporting Information.

also seen in the simulated spectra [Figure 5d−f]. Accordingly, the high-permittivity substrate decreased the absorbance instead of increased reflectance. The reflectance and absorbance results indicated that substrate permittivity had a marked effect on the optical properties of the NP films, suggesting that the surrounding dielectrics of ITO NP films determined the optical properties. It was noted that the type of substrate did not affect the electron density of ITO NP films. Substrate permittivity determined whether solar-thermal shielding is dominated mainly by light absorption or reflection. The solar-thermal shielding efficiency was evaluated using wavelength-integrated solar transmittance (Tsol) in the IR region expressed by the following equation:51 Tsol =

∫

Φsol (λ) T (λ) dλ

∫ Φsol(λ)

(6)

where Φsol(λ) is the solar irradiation spectrum for air mass 1.5 (the sum at 37 °C above the horizon)52 and T(λ) represents the wavelength-dependent transmittances. In the NIR region, T(λ) mainly determines the shielding efficiency. Figure 6a shows the experimental and simulated Tsol as a function of the permittivity of the substrate, where it was revealed that Tsol exhibited no clear substrate dependence at an approximate value of 0.40. On the contrary, the solar-thermal shielding efficiency by light reflection and absorption was strongly dependent on substrate type, as estimated by the wavelengthintegrated solar reflectance (Rsol) and absorbance (Asol) represented by the following equations: 2810


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Figure 6. (a) Solar transmittance (Tsol) and (b) solar reflectance (Rsol) and absorbance (Asol) as a function of dielectric constant (real part) at 2 μm. (c) Rsol/Asol ratio as a function of dielectric constant (real part) at 2 μm. The dielectric constant is dependent on substrate type. The dotted lines represent simulation results.

R sol =

A sol =

∫ ∫

Figure 7. Reflectance dependence of film thickness for ITO NP films on (a) CaF2, (b) Al2O3, (c) YSZ, and (d) TiO2 substrates. Red and black dots indicate experimental data of peak I and peak II. Red and black lines show simulated data of peak I and peak II.

Φsol (λ) R(λ) dλ

∫ Φsol(λ)

Specifically, the reflectance region at peak I is an important wavelength to cut thermal radiation from solar. Such enhancement contributed to the reflection-type solar-thermal shielding. The use of high-permittivity substrates on the optical response of the NP films cleared difference in origin of optical responses between peak I and peak II in the NP films. In particular, the reflectance at peak II must be considered in terms of interference effects. In the following section, the optical responses of the NP films on TiO2 and CaF2 are mainly discussed. 3.2. E-Field Interactions and Light Polarizations. Two types of reflectance spectra indicated the presence of different optical mechanism, as further examined by the FDTD method. The model NP layers were assumed to have a HCP structure with an interparticle length of 2 nm along the X−Y and Y−Z directions. The number of NP layers (N) was set to N = 15, corresponding to a thickness of 276 nm. FDTD was performed using the complex dielectric functions of the NPs [Figure S8]. The refractive index of capric acid (n = 1.437) was used for the medium between the NPs. Figure 8a shows the reflectance spectra of the NP layers on CaF2 and TiO2 substrates. The FDTD spectra were composed of two reflectance bands in the NIR and MIR regions, corresponding to peak I and peak II in the experimental spectra, respectively, close to the experimental data. Figure 8b and Figure 8c show the E-field distributions at peak positions of peak I (1.9 μm) and peak II (4.0 μm) in the NP layer on a TiO2 substrate. At peak II, strong field enhancements were seen at interparticle gaps along the in-plane directions (X-directions) similar to 2D dipole coupling. By contrast, peak I possessed E-fields along the diagonal directions in the out-of-plane (X−Z) directions in addition to those along the in-plane direction, indicating field interactions comparable to 3D dipole coupling. E-field directions are represented by white arrows in the two figures.

(7)

Φsol (λ) R(λ) dλ

∫ Φsol(λ)

(8)

Figure 6b shows the solar reflectance and absorbance as a function of the permittivity of the substrate. For a low permittivity substrate such as CaF2, the Asol and Rsol values were 0.36 and 0.20, respectively. The ratio of solar reflectance and absorbance (Rsol/Asol) was as low as 0.58, indicating “absorption-type solar-thermal shielding”. By contrast, Rsol gradually increased with decreasing Asol when the permittivity of the substrate increased, as exemplified in TiO2, where Rsol/ Asol ratio was 1.22 to prove that solar-thermal in the NIR range was mainly cut by light reflection [Figure 6c]. Thus, the use of high-permittivity substrates with the NP films was instrumental in facilitating “reflection-type solar-thermal shielding”. The increased substrate permittivity changed the solar-thermal shielding process from absorption to reflection. Figure 7 shows the experimental and simulated reflectance as a function of film thickness, as summarized by the reflectance spectra shown in Figure S6. For all NP films, the reflectance spectra exhibited single resonant peaks at thicknesses below 100 nm. By contrast, two types of resonant reflectance (peak I and peak II) appeared at thick thicknesses above 100 nm. Peak I and peak II were located in the NIR and MIR regions, respectively [Figure S7]. For low permittivity substrates such as CaF2 and Al2O3, the reflectance at peak I was close to that of peak II. However, a change in reflectance of both peaks resulted when using high permittivity substrates such as YSZ and TiO2. The reflectance at peak I was higher than that at peak II with increasing film thickness, which appeared prominently after the resonant splitting to peak I and peak II. This was also supported by the simulated data. 2811


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sample plane. Figure 9a shows the polarized reflectance spectra of the NP films on CaF2 and TiO2 under an incident angle (θ) of 70°. Here, s-polarization of incident light was almost purely polarized “in-plane”, and p-polarized excitation contained polarization components that were both parallel and perpendicular (out-of-plane) with respect to the sample plane. For s-polarization, spectral shapes of θ = 70° at peak I and peak II remained unchanged compared to those of θ = 5° because of the fact that the electric vector of the radiation at any incident angle excited electron oscillations in NPs parallel to the sample plane. Both peak I and peak II were related to the field interactions along the in-plane directions, although the latter was suppressed for p-polarization. Thus, only peak I survived essentially for the field interaction along the out-ofplane direction. Moreover, the reflectance at peak I of the NP film on TiO2 was higher than that on CaF2. In contrast, the suppressed reflectance of peak II must be considered from both microscopic and macroscopic viewpoints. From a microscopic viewpoint, the field interactions based on the plasmonic excitations were small at the interparticle gaps for ppolarization. Besides, the interference effect was also weakened from a macroscopic viewpoint because of different electric directions of incident light between s- and p-polarizations. Therefore, the use of a high permittivity substrate such as TiO2 led to an increase in the field interaction for p-polarization on the reflectance at peak I that operated only on the plasmonic modes. These spectral behaviors were also confirmed by theoretical simulations [Figure 9b]. Substrate permittivity played an important role in increasing the reflectance at peak I that was related to the field interaction along the out-of-plane directions. 3.3. NP Film−Substrate Interaction. The strength of field interaction was sensitive to the change in substrate permittivity. Figure 10a−c shows the experimental reflectance spectra of the NP films on TiO2 covered by low refractiveindex CaF2 spacers with different thicknesses (d). The CaF2 spacers were deposited at thicknesses below the subwavelengths using electron-beam (EB) evaporation. The NP film− substrate interface exhibited a sharp structure, as confirmed by an X-TEM measurement [Figure 10g]. The reflectance at peak I gradually decreased, and peak II increased with spacer thickness (d = 0−120 nm). Both experimental and simulated data agreed remarkably with each other [Figure 10d−f]. The

Figure 8. (a) Calculated reflectance spectra by FDTD of ITO NP layers on CaF2 (black line) and TiO2 (redline). The number of ITO NP layers (N) was set to N = 15, corresponding to a thickness of 276 nm.(b) E-field distributions between ITO NPs along the X−Z plane for ITO NP layers on TiO2 at (b) peak I and (c) peak II. White arrows indicate the directions of E-field interactions.

The difference in E-field distributions between peak I and peak II suggested different plasmon modes. Notably, the E-fields at peak I and peak II of the NP films on TiO2 were close to those on CaF2. FDTD results indicated that the reflectance spectra of the NP films are dependent on the illuminating beam having a polarization component that aligns along the axis normal to the

Figure 9. Reflectance of ITO NP films on CaF2 (black lines) and TiO2 substrates (red lines) under an incident angle of 70° for s- and p-polarized light. (a) and (b) show experimental and simulated spectra, respectively. Schematic pictures indicate the direction of the electric vector of incident light. 2812


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assemblies with collective plasmon modes.57 The image charge model mentioned earlier can be used to explain the optical properties of the NP films interacting with the substrates as follows. The plasmon mode at peak II possessed the E-field along the in-plane direction and was out-of-phase coupled with the image charge induced parallel to the substrate, leading to a decrease in reflectance with increasing substrate permittivity. Of these, the E-field along the in-plane direction was in-phase coupled with the substrate, leading to enhancement in reflectance with increasing substrate permittivity. However, these couplings between the NP film and the substrate weakened the induced image charge in the high permittivity substrate by the introduction of the CaF2 spacers with a low refractive index. Second, the influence of substrate permittivity on optical response was also discussed from a viewpoint of interference effect, which was closely related to the reflectance at peak II. The optical interference is based on coupling between primary and secondary reflection light, relating to light reflections at interfaces of air/the ITO NP film and the ITO NP film/ substrate, respectively. In particular, secondary reflection light is dependent on substrate permittivity. The magnitude of optical interference is due to relationship in refractive index between the NP film and substrate. When using CaF2, the refractive index of the NP film was higher than that of substrate, which sufficiently included the interference effect. In contrast, small difference in refractive index between the NP film and substrate weakened the optical interference in the case of TiO2 [Table S3], resulting in decrease in reflectance at peak II. Therefore, it was indicated that the reflectance at peak II was determined by both plasmonic and interference effects. On this note, the effect of substrates significantly affected the reflectance of NP films from the viewpoints of plasmon and interference effects, which were one of the important factors in developing of transparent reflective-type solar-thermal shielding in the NIR and MIR range. 3.4. Metallic ITO Substrates. Finally, the effect of metallic ITO substrates on the reflectance spectra of the NP films was investigated. Figure 11a and Figure 11b show the experimental and simulated reflectance spectra of the NP films on the metallic substrates, respectively, revealing that dip structures

Figure 10. Experimental reflectance spectra of ITO NP films on TiO2 with different spacer thicknesses of (a) 120 nm, (b) 50 nm, and (c) 20 nm. Simulated reflectance spectra of ITO NP films on TiO2 with different spacer thicknesses of (d) 120 nm, (e) 50 nm, and (f) 20 nm. (g) A cross-section TEM image of an ITO NP film on TiO2 with a 50 nm thick spacer layer. (h) Intensity ratio of peak I and peak II as a function of spacer thickness.

peak I and peak II intensity ratio in both peaks decreased with increasing spacer thickness [Figure 10h], indicating that the presence of spacers between the NP film and the substrate provided the spectral changes of the NP films, as additional evidence of plasmon mode modifications in the NP films interacting with the high substrate permittivity. We first discuss the substrate permittivity from the viewpoints of plasmonic modes. The optical modifications of the NP films resulting from the changes of the substrate permittivity were assessed using metal NPs on dielectric substrates,53 where the plasmon modes are affected by induced surface image charges in the substrate, the magnitude of reflectance depends on the permittivity of the substrate. Specifically, substrates with high permittivity produce a stronger image and larger field interaction than those with low permittivity.54 The plasmon modes of the metal NPs are coupled with the substrate by dipole in-phase or out-of-phase either perpendicular (p-polarization) or parallel (s-polarization) to the sample, respectively.55 The substrate-induced optical properties of the metal NPs were also dependent on the NP−substrate distance56 and were applied to metal NP

Figure 11. (a) Experimental and (b) simulated reflectance spectra of ITO-NP films on ITO with different spacer thicknesses of 0, 20, and 40 nm. (c) Cross-section TEM image of an ITO-NP film on ITO with a 20 nm thick spacer layer. 2813


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ACS Applied Nano Materials appeared at around 3 μm because of light absorptions. Insertion of CaF2 spacers (d = 0, 20, and 40 nm) in the NP film−substrate gap provided a slight red-shift of dip wavelengths. Notably, the interface between the NP film and the substrate was clearly separated at the nanoscale [Figure 11c]. For metal NPs on metallic substrates, substrate-induced modification of optical responses by metal NPs has been particularly strong as surface plasmon polaritons (SPPs) are supported by large amounts of free carriers in the substrates.58,59 ITO showed clear SPPs in the NIR and MIR regions43,60 to readily induce coupling to the plasmon modes of the NP films. Thus, the NP films interacted with the ITO image charge in the metal substrate, which can be understood as a hybridization process between the plasmon modes of the NP films and the SPPs on the metal substrate. Red-shifted peaks of the reflectance dips were related to the coupling strength between the NP films and the substrates and were altered by the insertion of the spacers. The optical response of the NP film on the metallic substrate showed a remarkable change by the insertion of the thin spacer layers different from that on the dielectric substrate, resulting from strength difference in the induced image charge in the substrates. These above results further verified the coupling of the plasmon modes of the NP films to the high permittivity of substrates such as TiO2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone and fax: +81-35841-1870. ORCID

Hiroaki Matsui: 0000-0002-4557-1259 Author Contributions

The manuscript was written through contributions of all authors. 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 TEPCO Memorial Foundation (Grant I2017108-1), a grant from Takahashi Industrial and Economic Research Foundation (Grant 08-003-135), JST A-Step (Grant VP30218088667), and Grant-in-Aids for Scientific Research (B) (Grant 18H01468). This work was also conducted at the AIST Nano-Processing Facility, supported by “Nanotechnology Platform Program” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (Grants 18007844 and 17007592).

4. CONCLUSION We studied the effects of the dielectric properties of substrates on the reflectance of NP films, which was dependent on the nature of the substrate. Increased substrate permittivity resulted in higher reflectance compared to absorbance, thereby contributing toward the development of reflection-type solarthermal shielding. The plasmon modes in the NP films were based on the complex E-field interactions along the in-plane and out-of-plane directions, which could be tuned by changing the permittivity of the substrate. Such behavior could be related to coupling of the plasmon modes between the NP films and the substrate based on the induced image charge in the latter as clarified from the polarization results of incident light and the substrate permittivity modifications. Furthermore, we discussed the interference effects in the NP films in addition to the plasmonic modes. In particular, it was pointed out that the optical interference was related to the enhanced reflectance (peak II) in the MIR range. As a consequence, we identified that the reflectance (peak I) in the NIR range was due only to the plasmon effect. Finally, we demonstrated NP film−substrate coupling using metallic ITO substrates as further evidence of optical interactions between the NP films and substrates. Use of dielectric and metallic substrates supported the notion that the optical responses of NP films are strongly influenced by the permittivity of the substrates and that changing the substrate permittivity is important when considering new fabrications of plasmonic applications using oxide semiconductors.

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analyses of ITO-NP films with different thickness; information concerning dielectric functions of substrates (PDF)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00293. Optical and structural properties of ITO NPs from the viewpoint of theoretical and experimental aspects; absorption spectra, their characteristics, and theoretical 2814


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