4. Infrared applications for solar-thermal shielding

#### 4.1 High reflections in the IR range

ð4Þ

ð5Þ

ð6Þ

. The

where k is the incoming wave vector and L are integers representing the dipole, quadrupole, and higher multipoles of the scattering. In the above equations, a<sup>L</sup> and b<sup>L</sup> are represented by the following parameters, composed of the Riccati-Bessel

Here, m ¼ n~=nm, where n~ ¼ nR þ inI is the complex refractive index of the metal and n<sup>m</sup> is the refractive index of the surrounding medium. Additionally, x = kmr, where r is the radius of the particle. It should be noted that k<sup>m</sup> = 2π/λ<sup>m</sup> is defined as the wavenumber in the medium rather than the vacuum wavenumber. Peak positions of absorption spectra of ITO NPs were estimated using the full Mie theory

(black line in Figure 7(b)). The dielectric constants were taken from the ellipsometric data of an ITO film with an electron density of 1.0 � <sup>10</sup><sup>21</sup> cm�<sup>3</sup>

described using Mie theory in the quasi-static limit.

estimated peak positions remained almost unchanged with particle sizes below 120 nm and then slightly redshifted to longer wavelengths with particle sizes above 120 nm. That is, ITO NPs with particle sizes below 40 nm had no high-order plasmon mode and were mainly dominated by light absorptions. These results differed largely from those of metal NPs. LSPR properties of ITO NPs could be fully

(a) Absorption spectra of ITO NPs with different sizes comprising (a) 10 nm, (b) 20 nm, and (c) 36 nm. (d) LSPR peak energy as a function of particle size. A black line represents using Eqs. (4)–(6) [25].

functions ψ<sup>L</sup> and χ<sup>L</sup> [26]:

Nanocrystalline Materials

Figure 7.

62

Recently, plasmonic properties on oxide semiconductors have attracted much attention in the area of solar-thermal shielding. The purpose of our study is to apply the plasmonic properties of assembled films of ITO NPs. To date, IR optical responses have been investigated with regard to transmittance and extinction spectra of composites and films using oxide semiconductor NPs. IR shielding properties by transmittance and absorption properties have mainly been discussed [27–30]. Reports concerning reflective performances in assemblies of NPs have yet to appear in spite of the desire for thermal shielding to cut IR radiation, not by absorption, but through reflection properties.

Assemblies of Ag and Au NPs can produce high E-fields through plasmon coupling between NPs in the visible range and are utilized in surface-enhanced spectroscopy [31, 32]. The high E-fields localized between NPs are very sensitive to interparticle gaps [33]. A gap length down to distances less than the size of a NP causes remarkable enhancements in E-fields. Surfactant- and additive-treated NPs are effective strategies that can be employed to obtain small interparticle gaps between NPs, which can be developed into one-, two-, and three-dimensional assemblies of NPs [34]. In particular, optical applications based on NPs have the benefit of large-area fabrications with lower costs to make NP assemblies attractive for industrial development.

In this section, we report on the plasmonic properties of assembled films comprising ITO NPs (ITO NP films) and their solar-thermal applications in the IR range [35]. Both experimental and theoretical approaches were employed in an effort to understand the plasmonic properties of the NP films. The IR reflectance of the NP films was analyzed on the basis of variations in particle size and electron density. The investigation focused in particular on E-field interactions in order to determine how the NP films affected high IR reflectance. This behavior is discussed in terms of the physical concept of plasmonic hybridization, which further clarified the importance of interparticle gaps for high IR reflectance.

#### Figure 8.

(a) Reflectance spectra of ITO NP films with different electron densities of 1.1 <sup>10</sup><sup>21</sup> cm<sup>3</sup> (○), 8.7 <sup>10</sup><sup>19</sup> cm<sup>3</sup> (□), and <sup>&</sup>lt; 1019 cm<sup>3</sup> (Δ). (b) Reflectance as a function of NP film thickness of ITO NP films with different electron densities.

Figure 8(a) shows reflectance spectra of ITO NP films with different electron densities. The assembled ITO NP films were deposited on IR-transparent CaF2 substrates through a spin-coating technique. The spin-coating conditions comprised sequential centrifugation at (i) 800 rpm for 5 s, (ii) 2400 rpm for 30 s, and (iii) 800 rpm for 10 s. The fabricated NP films were then thermally treated at 150°C in air to evaporate the solvent. Reflectance was enhanced with increasing electron density and reached a value of ca. 0.6 in the NP film with <sup>n</sup><sup>e</sup> = 1.1 <sup>10</sup><sup>21</sup> cm<sup>3</sup> . Additionally, reflectance was dependent on film thickness (Figure 8(b)). Reflectance gradually increased with increasing film thickness and was then saturated in film thicknesses above 200 nm. As a result, it is necessary to use NPs with high electron density in order to obtain NP films with high IR reflectance.

Figure 9(a) shows reflectance spectra of ITO NP films with different particle sizes. Reflectance gradually increased with increasing particle size, which was dependent on NP film thickness (Figure 9(b)). That is, increasing in particle size contributed to obtain high IR reflectance. Highly efficient solar-thermal shielding played an important role in controlling electron density and particle size. We found that the high IR reflectance was closely related to plasmon coupling between the NPs in the NP films as follows.

#### 4.2 Electric field distributions

Figure 10(a) shows experimental and theoretical absorption spectra of ITO NPs dispersed in toluene. The theoretical data was simulated using the finite-difference time-domain (FDTD) method and was close to the experimental data. We observed the formation of a strong electric field (E-field) on the NP surface (inset of Figure 10(a)). The relationship between the E-field and photon energy was further investigated, as shown in Figure 10(b–d). The E-field on the NP surface increased with increasing photon energy. A high E-field was obtained at an LSPR peak position of 1.8 μm. The LSPRs of ITO NPs produced the strong E-field on the NP surface.

We evaluated the optical properties of ITO NP films from the viewpoint of electrodynamic simulations based on the finite-difference time-domain (FDTD) method (Figure 11(a)). The modeled NP layer was assumed to have a hexagonally closepacked (HCP) structure with an interparticle distance (r) of 2 nm along the in-plane

#### Figure 9.

(a) Reflectance spectra of ITO NP films with different particle sizes of 36 nm (○), 20 nm (□), and 10 nm (▵). (b) Reflectance as a function of NP film thickness of ITO NP films with different particle sizes.

(x-y) and out-of-plane (y-z) directions (Figure 11(b and c)). The modeled sample was illuminated with light directed in the z direction from the air side. The E-field was parallel to the x direction. The refractive index (real part: 1.437) of capric acid was used for the medium between the NPs. The dielectric functions of the ITO NPs were obtained from the parameter fitting for the absorption spectra. Figure 11(a) shows the reflectance spectra of ITO NP layers with different particle sizes (D) of 10, 20, and

(a) Simulated reflectance spectra of ITO NP layers at different particle sizes (D). A number of NP layer (N) was set to N = 20 NP layers. (b) and (c) indicate structural diagrams of a simulated NP layers along the inplane (x-y) and out-of-plane (x-z) directions, respectively. The modeled structure was assumed to have a HCP structure with an interparticle distance (r) of 2 nm and was illuminated with light directed in the z direction

(a) Absorption spectra of ITO NPs: Experimental (open circles) and simulated data (solid line). Inset indicates an electric field distribution on the NP surface obtained by the FDTD simulation. Cross-section field

Surface Plasmons in Oxide Semiconductor Nanoparticles: Effect of Size and Carrier Density

distributions at 1.2 μm (b), 1.5 μm (c), and 1.8 μm of the NPs.

DOI: http://dx.doi.org/10.5772/intechopen.86999

from the air side. The E-field was parallel to the x direction.

Figure 10.

Figure 11.

65

Surface Plasmons in Oxide Semiconductor Nanoparticles: Effect of Size and Carrier Density DOI: http://dx.doi.org/10.5772/intechopen.86999

#### Figure 10.

Figure 8(a) shows reflectance spectra of ITO NP films with different electron densities. The assembled ITO NP films were deposited on IR-transparent CaF2 substrates through a spin-coating technique. The spin-coating conditions comprised sequential centrifugation at (i) 800 rpm for 5 s, (ii) 2400 rpm for 30 s, and

(iii) 800 rpm for 10 s. The fabricated NP films were then thermally treated at 150°C in air to evaporate the solvent. Reflectance was enhanced with increasing electron density and reached a value of ca. 0.6 in the NP film with <sup>n</sup><sup>e</sup> = 1.1 <sup>10</sup><sup>21</sup> cm<sup>3</sup>

Additionally, reflectance was dependent on film thickness (Figure 8(b)). Reflectance gradually increased with increasing film thickness and was then saturated in film thicknesses above 200 nm. As a result, it is necessary to use NPs with high

Figure 9(a) shows reflectance spectra of ITO NP films with different particle sizes. Reflectance gradually increased with increasing particle size, which was dependent on NP film thickness (Figure 9(b)). That is, increasing in particle size contributed to obtain high IR reflectance. Highly efficient solar-thermal shielding played an important role in controlling electron density and particle size. We found that the high IR reflectance was closely related to plasmon coupling between the NPs in the NP films as

Figure 10(a) shows experimental and theoretical absorption spectra of ITO NPs dispersed in toluene. The theoretical data was simulated using the finite-difference time-domain (FDTD) method and was close to the experimental data. We observed the formation of a strong electric field (E-field) on the NP surface (inset of Figure 10(a)). The relationship between the E-field and photon energy was further investigated, as shown in Figure 10(b–d). The E-field on the NP surface increased with increasing photon energy. A high E-field was obtained at an LSPR peak position of 1.8 μm. The

We evaluated the optical properties of ITO NP films from the viewpoint of electrodynamic simulations based on the finite-difference time-domain (FDTD) method (Figure 11(a)). The modeled NP layer was assumed to have a hexagonally closepacked (HCP) structure with an interparticle distance (r) of 2 nm along the in-plane

(a) Reflectance spectra of ITO NP films with different particle sizes of 36 nm (○), 20 nm (□), and 10 nm (▵).

(b) Reflectance as a function of NP film thickness of ITO NP films with different particle sizes.

electron density in order to obtain NP films with high IR reflectance.

LSPRs of ITO NPs produced the strong E-field on the NP surface.

follows.

Figure 9.

64

4.2 Electric field distributions

Nanocrystalline Materials

.

(a) Absorption spectra of ITO NPs: Experimental (open circles) and simulated data (solid line). Inset indicates an electric field distribution on the NP surface obtained by the FDTD simulation. Cross-section field distributions at 1.2 μm (b), 1.5 μm (c), and 1.8 μm of the NPs.

#### Figure 11.

(a) Simulated reflectance spectra of ITO NP layers at different particle sizes (D). A number of NP layer (N) was set to N = 20 NP layers. (b) and (c) indicate structural diagrams of a simulated NP layers along the inplane (x-y) and out-of-plane (x-z) directions, respectively. The modeled structure was assumed to have a HCP structure with an interparticle distance (r) of 2 nm and was illuminated with light directed in the z direction from the air side. The E-field was parallel to the x direction.

(x-y) and out-of-plane (y-z) directions (Figure 11(b and c)). The modeled sample was illuminated with light directed in the z direction from the air side. The E-field was parallel to the x direction. The refractive index (real part: 1.437) of capric acid was used for the medium between the NPs. The dielectric functions of the ITO NPs were obtained from the parameter fitting for the absorption spectra. Figure 11(a) shows the reflectance spectra of ITO NP layers with different particle sizes (D) of 10, 20, and

directions. Figure 13 shows the E-field distributions along the x-z directions. For peak-I, the E-field between the NPs was strongly localized along the x direction when an electric field of light was applied along this direction. In contrast, peak-II displays E-fields along the diagonal directions in the x-z plane in addition to those along the x direction. A difference in the E-field of peak-I and peak-II was clearly found. The FDTD simulations revealed that the two types of reflectance peaks had different mechanisms of plasmon excitations. Therefore, it was indicated that different E-field distributions between the NPs played an important role in producing

Surface Plasmons in Oxide Semiconductor Nanoparticles: Effect of Size and Carrier Density

Optical properties of carrier- and size-dependent LSPRs were investigated using dopant-controlled ITO NPs. From systematic correlations between LSPR excitations and electron density, plasmon damping of ITO NPs was closely related to electronimpurity scattering, which was effective with high n<sup>e</sup> values greater than 10<sup>20</sup> cm<sup>3</sup>

That is, the role of electron carriers in ITO NPs could enhance LSPRs with simultaneous damped plasmon excitations. Changes in particle size also affected the LSPRs in ITO NPs. Increasing particle size altered the magnitude and peak splitting of the resonant reflectance, which covered a wide IR range. As a result, the carrier and size control of ITO NPs led to high solar-thermal shielding. The origin of the high IR reflectance of ITO NP films was clarified by electrodynamic simulations (FDTD). We found that the E-field distributions between the NPs along the in-plane and outof-plane directions played key roles in producing the high IR reflectance. Control of electron carrier and particle size revealed important aspects that should be considered in the area of structural design when fabricating thermal-shielding materials.

This research was supported in part by a grant from JST A-Step (No. VP30218088667) and for Grant-in-Aids for Scientific Research (B)

The authors declare no competing financial interest.

.

the IR reflectance in the IR range.

DOI: http://dx.doi.org/10.5772/intechopen.86999

5. Conclusion

Acknowledgements

(No. 18H01468).

67

Conflict of interest

Figure 12.

SEM images of an ITO NP film along the in-plane (a) and out-of-plane (b) directions.

36 nm. The number of the NP layer was set to the N = 20 NP layer. Reflectance clearly enhanced with increasing particle size, which appeared as a result of threedimensional assemblies of ITO NPs, and it was suggested theoretically that increasing particle size contributed to the reflective-type thermal shielding in the IR range.

Plasmon coupling between NPs produces large enhancements of E-fields at interparticle gaps. We typically investigated the E-field distributions at peak-II (0.60 eV) and peak-I (0.208 eV) for a 20 NP layer with D = 36 nm. Figure 12(a and b) shows SEM images of ITO NP films (D = 36 nm) along the in-plane and out-ofplane directions, revealing that the NPs had close-packed structures along both

#### Figure 13.

Images of the E-field distributions and charge vectors at peak-I and peak-II along the x-z directions. Regions delimited by white circles were positioned in the respective bottom parts. An E-field was applied along the x direction. Light was incident along the z direction from the air side.

Surface Plasmons in Oxide Semiconductor Nanoparticles: Effect of Size and Carrier Density DOI: http://dx.doi.org/10.5772/intechopen.86999

directions. Figure 13 shows the E-field distributions along the x-z directions. For peak-I, the E-field between the NPs was strongly localized along the x direction when an electric field of light was applied along this direction. In contrast, peak-II displays E-fields along the diagonal directions in the x-z plane in addition to those along the x direction. A difference in the E-field of peak-I and peak-II was clearly found. The FDTD simulations revealed that the two types of reflectance peaks had different mechanisms of plasmon excitations. Therefore, it was indicated that different E-field distributions between the NPs played an important role in producing the IR reflectance in the IR range.

#### 5. Conclusion

36 nm. The number of the NP layer was set to the N = 20 NP layer. Reflectance clearly

dimensional assemblies of ITO NPs, and it was suggested theoretically that increasing particle size contributed to the reflective-type thermal shielding in the IR range. Plasmon coupling between NPs produces large enhancements of E-fields at interparticle gaps. We typically investigated the E-field distributions at peak-II (0.60 eV) and peak-I (0.208 eV) for a 20 NP layer with D = 36 nm. Figure 12(a and b) shows SEM images of ITO NP films (D = 36 nm) along the in-plane and out-ofplane directions, revealing that the NPs had close-packed structures along both

Images of the E-field distributions and charge vectors at peak-I and peak-II along the x-z directions. Regions delimited by white circles were positioned in the respective bottom parts. An E-field was applied along the x

direction. Light was incident along the z direction from the air side.

enhanced with increasing particle size, which appeared as a result of three-

SEM images of an ITO NP film along the in-plane (a) and out-of-plane (b) directions.

Figure 12.

Nanocrystalline Materials

Figure 13.

66

Optical properties of carrier- and size-dependent LSPRs were investigated using dopant-controlled ITO NPs. From systematic correlations between LSPR excitations and electron density, plasmon damping of ITO NPs was closely related to electronimpurity scattering, which was effective with high n<sup>e</sup> values greater than 10<sup>20</sup> cm<sup>3</sup> . That is, the role of electron carriers in ITO NPs could enhance LSPRs with simultaneous damped plasmon excitations. Changes in particle size also affected the LSPRs in ITO NPs. Increasing particle size altered the magnitude and peak splitting of the resonant reflectance, which covered a wide IR range. As a result, the carrier and size control of ITO NPs led to high solar-thermal shielding. The origin of the high IR reflectance of ITO NP films was clarified by electrodynamic simulations (FDTD). We found that the E-field distributions between the NPs along the in-plane and outof-plane directions played key roles in producing the high IR reflectance. Control of electron carrier and particle size revealed important aspects that should be considered in the area of structural design when fabricating thermal-shielding materials.

#### Acknowledgements

This research was supported in part by a grant from JST A-Step (No. VP30218088667) and for Grant-in-Aids for Scientific Research (B) (No. 18H01468).

#### Conflict of interest

The authors declare no competing financial interest.

Nanocrystalline Materials
