This chapter describes plasmonic responses in In2O3:Sn nanoparticles (ITO NPs) and their assembled ITO NP sheets in the infrared (IR) range. ITO NPs clearly provide resonance peaks related to local surface plasmon resonances (LSPRs) in the near-IR range, which are dependent on electron density in the NPs. In particular, electron-impurity scattering plays an important role in determining carrier-dependent plasmon damping, which is needed for the design of plasmonic materials based on ITO. ITO NPs are mainly dominated by light absorption. However, a high light reflection is observed in the near- and mid-IR range when using assembled NP sheets. This phenomenon is due to the fact that the introduction of surface modifications to the NPs can facilitate the production of electric-field (E-field) coupling between the NPs. The three-dimensional (3D) E-field coupling allows for resonant splitting of plasmon excitations to the quadrupole and dipole modes, thereby obtaining selective high reflections in the IR range. The high reflective performances from the assembled NP sheets were attributed to the plasmon interactions at the internanoparticle gaps. This work provides important insights for harnessing IR optical responses based on plasmonic technology toward the fabrications of IR solar thermal-shielding applications.
- oxide semiconductor
- surface plasmon
- infrared and energy-saving
Plasmonic nanomaterials based on transparent oxide semiconductors (TOSs, such as In2O3, ZnO and SnO2) have received much attention as new optical phenomena with potential applications. In particular, oxide semiconductors with metallic conductivity by doping with intrinsic and/or extrinsic impurities show surface plasmon resonances (SPRs) in the infrared (IR) range [1–5]. Unlike noble metals (silver and gold), SPRs can be controlled by tuning the physical characters of a material [6–8], which provides new possibilities for optical manipulation of light. Studies of nanoplasmonics based on TOSs can break new ground in the areas of oxide semiconductors. A characteristic property cleared by these studies is that the optical nature of TOSs shows a low-loss plasmonic material even up to near-IR wavelengths because of IR transparency outside the reststrahlen band. The band structures on TOSs are simply composed of
SPR excitations on TOSs have been reported on different structures such as nanorods and nanowires [13–15]. In particular, nanoparticles (NPs) of In2O3:Sn and ZnO:Al produce localized surface plasmon resonances (LSPRs), which are strongly generated when confining the collective excitations of carriers into NPs [16, 17]. This makes use of localization of large electric fields in the vicinity of NP surfaces. Thus far, the majority of investigations concerning LSPRs have demonstrated on the noble metal NPs, which have been tailored for use in optical areas as diverse as waveguides and biochemical sensing [18–20]. Recently, In2O3:Sn NPs have launched as nanoplamonic materials. The careful choice of impurity dopants can show clear LSPR peaks in the near-IR range. The assembled films of In2O3:Sn NPs have shown optical enhancements of near-IR luminescence and absorption [21, 22]. These behaviours make use of the electric-fields (
Assembled films of the noble metals have been utilized in surface-enhanced Raman and fluorescence spectroscopies, which are based on high
Plasmonic properties of TOS materials have attracted attention for thermal-shielding applications in order to solar and radiant heat in the near- and mid-IR range, respectively . To date, the composites and films of oxide semiconductor NPs have been studied regard to transmission and extinction spectra in the IR range because optical properties are dominated by absorbance [27–30]. The present thermal-shielding applications have strongly been desired to cut IR radiation not by absorption but through reflection. However, no previous paper has reported reflective performance on doped oxide semiconductor NPs. In addition, plasmonic applications exhibiting a thermal-shielding ability have not been previously studied in detail. The purpose of this chapter is to apply the plasmonic properties for satisfying recent industry demands for a material with thermal-shielding ability. These social requirements include the fabrication of flexible sheets with high heat-ray reflections, as well as visible and microwave transmissions. We use assembled NPs of In2O3:Sn as a concrete example. Plasmonic responses are dependent on electronic structure. For example, In2O3, ZnO and WO3 have similar band structures with conduction and valence levels consisting of
This chapter is organized as follows. In Section 2, we give a description of structural and optical properties of In2O3:Sn (ITO) NPs in the IR range from the viewpoint of local structural analyses. In Section 3, we focus plasmonic responses of ITO NPs from theoretical and experimental approaches, which is not as readily available in the noble metal NPs. To investigate mechanism of plasmonic excitations in ITO, NPs is valuable information for oxide-based plasmonics. Section 4 is devoted to discussion of thermal-shielding based on assembled films of ITO NPs for industrial applications. Above all, we describe plasmonic responses related to the 3
2. Fabrications and structures of ITO NPs
2.1. Fabrications of ITO NPs
In2O3:Sn nanoparticles (ITO NPs) were fabricated using a metal organic decomposition method. Various initial ratios of the metal precursor complexes of (C9H22CO2)3In and (C9H22CO2)4Sn were prepared as starting materials. Indium and tin carboxylates were heated with a chemical ratio of 95:5 in a flask supported by a mantle heater to 350°C. The temperature was maintained for 4 hours, and then the mixture was cooled to room temperature around 30°C. The obtain solutions produced a pale blue suspension, to which excess ethanol was introduced to cause precipitation. Centrifugation and repeated washing processes were carried out several times using ethanol, producing dried powders of ITO NPs with a pale blue colour. As a final step, the NP samples were dispersed in a nonpolar solvent of toluene. A zeta-potential measurement revealed that the NPs showed non-aggregated states in the solvent due to an electrostatic repulsion. The Sn concentration in the NPs in this chapter was measured by X-ray florescence spectroscopy.
2.2. Structural properties of ITO NPs
X-ray diffraction (XRD) measurements clarified that the NPs showed broad peaks characteristic of a colloidal sample with a crystal structure [Figure 1(a)] . The
We further investigated structural properties by scanning-TEM (STEM) combined with electron-energy loss spectroscopy (EELS) . A STEM-EELS technique can easily detect plasmonic response in a single NP. High-angle annular dark field (HAADF) images in Figure 2(a)–(c) cleared that Indium and Sn atoms in the NP were distributed homogeneously, which were consisted with the results of XRD. The EELS spectra at an edge and vacuum region on the STEM-acquired particle image showed a slight spectral difference in energy-loss regions from 1.0 to 0.5 eV [Figure 2(d)]. In Figure 2(e), a differential EELS spectrum had a maximum peak at 0.7 eV that was similar to the optical absorption in the near-IR, which was direct evidence of a LSP excitation on the NP surface as consequence of spatially homogeneous doping of Sn atoms in the NP.
3. Localized surface plasmons in ITO NPs
3.1. Theoretical calculations of optical properties
The absorption and scattering cross sections of a single ITO NP with a diameter (
3.2. Experimental optical properties
An optical absorption of an ITO NP solution (Sn concentration of 5%) was typically examined [Figure 4(a)]. Optical measurements in the IR-range were made at room temperature using a FT-IR spectrometer equipped with a liquid cooled HgCdTe (MCT) detector. An ITO NP solution was confined in an IR-transparent CaF2 holder with an optical thickness of 25 μm, showing that a single absorption peak was located at 1.86 μm because of LSP excitations of ITO NPs. This result was close to the theoretical data. We studied optical quality in plasmon resonances of ITO NPs using Mie theory as follows.
The absorption spectrum was fitted using the classical Mie theory with plasmon damping γ because of ionized impurity scattering derived from Sn impurities in the NPs. The theoretical fitting of an optical absorption (σ) to the experimental data in the quasi-static limit was employed :
The plasma frequency (
In general, plasmon damping
4. Fabrications and structures of ITO NP sheets
Assembled NP sheets were deposited on IR transparent CaF2 substrates by a spin-coating technique. Thick NP sheets were fabricated by way of multiple overglaze of a thin NP film obtained by a NP concentration of 0.2% in toluene. The spin-coating conditions were carried out using the following processes: 800 rpm (5 seconds) → 1200 rpm (10 seconds) → 2400 rpm (30 seconds) → 800 rpm (10 seconds). Fabricated sheet samples were heat-treated at above 150°C in air in order to evaporate the solvent. NP sheets with various thicknesses were obtained by repetition of the above coating sequences.
4.2. Structural evaluations
The assembled sheets of the NPs were evaluated by small-angle X-ray scattering (SAXS), providing an interesting insight into the scattering vector (
5. Infrared optical responses of ITO NP sheets
5.1. Mono-layered NP sheets
The optical properties of a mono-layered ITO NP sheet are shown in Figure 6. Transmittance spectra exhibited a resonant peak at 2.64 μm, which revealed the red-shifted resonant wavelength because of a collective plasmon resonance (CPR) effect compared to those of NPs dispersed in toluene [Figure 6(a)] . On the other hand, reflectance at the resonant wavelength was very small, indicating that the optical responses were mainly dominated by light absorption properties. Furthermore, the finite-difference time-domain (FDTD) simulation was carried out to evidence the experimental results. The modelled mono-layered NP sheet (
5.2. Three-dimensional NP sheets
3D-stacked NP sheets showed a remarkable change in optical properties, which were clearly found on transmittance and reflectance spectra [Figure 7(a) and (b)]. Transmittance with a resonant wavelength at 2.20 μm decreased to a level close to zero with increasing sheet thickness. On the other hand, reflectance was enhanced at a close proximity of 0.6 in terms to the sheet thickness [Figure 8(b)]. The single peak of 22 nm-thick NP sheet was gradually separated into lower and higher wavelengths with the sheet thickness [Figure 8(a)]. We observed two types of resonant peaks (I and II) at 2.13 and 4.02 μm in the near- and mid-IR region on the 216 nm-thick NP sheet, respectively. The ratio of (
FDTD calculations were conducted in order to clear the experimental results. From the cross-section SEM image, the modelled NP layers are based on a 3D HCP structure with an
5.3. Plasmon hybridization and reflectance mechanism
The resonant origins of reflectance of peak-I and peak-II were theoretically examined as a function of interparticle length between NPs. Figure 9(a) exhibits calculated reflectance of NP sheets with different
The character of
The electric vector excites electron oscillations in NPs normal to the plane of the sample, and suppresses the
5.4. Nanoparticle gap and reflectance of ITO NP sheets
The thermal behaviours of the NP samples were investigated by TG-DTA in an 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 270–320°C because of the generation of organic species confirmed by
5.5. Electromagnetic responses in microwave range
EM properties are shortly discussed on a NP sheet in the microwave range 0.5–40 GHz. This range is an important frequency range for telecommunications. Transparent solar-thermal shielding is effective techniques to prevent room heat in order to realize comfortable environment in vehicles. However, it is strongly required for vehicles to transmit EM waves in the microwave range through windows to carry out radio communications such as an Electronic Toll Collection System (ETC) and Information traffic system (ITS). Therefore, it is important to measure EM properties of NP sheets in addition to evaluate optical properties in the IR range.
250 nm-thick NP sheet with an A4 size was fabricated on a PET substrate (thickness: 0.2 mm) using a roll-coating method [inset of Figure 12(a)]. High reflectance with a close proximity of 0.6 was also obtained on a flexible PET substrate [Figure 12(a)]. The shielding effectiveness (SE) of the flexible NP sheet was almost zero, as different from that of a RF sputtered ITO film [Figure 12(b)]. The difference between the two materials related to electrical conductance (σ) in the sheets, which was in the order of 10−5 and 10−3 S/cm for the NP sheet and sputtered film, respectively. If the shielding material is thin, SE is mainly dominated by EM reflection as follows :
ITO NPs were used to obtain assembled NP sheets with small interparticle lengths by the presence of ligand molecules on the particle surfaces. This situation produced effective
Crystallinity and local structures of oxide semiconductor NPs were conducted using ITO NPs by XRD and TEM measurements in Section 2. The plasmonic resonances of ITO NPs were clearly obtained in the near-IR range from the viewpoints of optical and EELS signals. In particular, electron-impurity scattering contributed towards plasmon damping as one of a factor that is absent in metal NPs on the basis of theoretical and experimental approaches, which was discussed in Section 3. In Sections 4 and 5, we described IR plasmonic applications in ITO NP sheets for solar-thermal shielding technology. Above all, the
The above results provided important insights for basic science and practical applications based on plasmonic investigations based on oxide semiconductor NPs. However, plasmonic properties and applications are stand still-points at the present time. Hereafter, it will be needed to study plasmonic phenomena on oxide semiconductor NPs towards new concepts concerning optical manipulations in the IR range.
This research was supported in part by a grant-in-Aid from the JSPS Core-to-Core Program, A. Advanced Research Network, a grand from Toyota Physical and Chemical Research Institute, and for a grant-in-Aid for Exploratory Research and Scientific Research (B).
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