1. Introduction

Metals (such as Au and Ag) have been utilized for the majority of plasmonic materials in the visible range. Recently, oxide semiconductors have attracted much attention for use as potential new plasmonic materials. In particular, ZnO: Ga and In2O3: Sn (ITO) are known for use as transparent electrodes due to their metallic conductivity. These oxide semiconductors show surface plasmon resonances (SPRs) in the infrared (IR) range [1, 2]. Propagated SPRs can be excited on metal surfaces using a prism-coupling technique such as a Kretschmann-type attenuated total reflection (ATR) system [3]. Our research group has investigated the optical properties of SPRs excited on ZnO: Ga and ITO film surfaces from the viewpoint of physical characteristics such as field strength and penetration depth [4–6]. On the other hand, subwavelength materials such as nanorods, nanoparticles (NPs), and nanodots are capable of supporting localized surface plasmon resonances (LSPRs), which can be directly excited by incident light in the absence of a prism-coupling method [7, 8]. Above all, LSPRs confined to NPs can lead to light at the nanoscale

when confining the collective oscillations of free electrons into NPs. This LSPR effect further provides strong electric fields (E-fields) on NP surfaces, which contribute to surface-enhanced optical spectroscopy [9]. For example, assembled films consisting of ITO NPs have demonstrated optical enhancements of near-IR luminescence and absorption in the IR range [10, 11]. Therefore, optical studies concerning oxide semiconductor NPs can break new research ground in the area of plasmonics and metamaterials.

2.2 Carrier-dependent plasmon absorptions

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

where k = 2π(εd)

Figure 1.

Figure 2.

57

(c) 1.1 � <sup>10</sup><sup>21</sup> cm�<sup>3</sup> [19].

into the NPs' induced electron density of 6.3 � 1019 cm�<sup>3</sup>

Dot lines indicate theoretical calculations based on the modified Mie theory [19].

TEM images of ITO NPs with electron densities of (a) 6.3 � 1019 cm�<sup>3</sup>

Optical absorptions and TEM images of ITO NPs with different electron densities (ne) were examined (Figure 1). TEM images revealed that all NP sizes (D) were ca. 36 nm (Figure 2(a–c)). This indicates that the systematic change in the absorption spectra is related to the Sn content. Absorption measurements were performed using a Fourier-transform infrared (FT-IR) spectrometer. A value of n<sup>e</sup> was estimated from the absorption spectra by theoretical calculations. The following equation was used to derive absorption intensity (A) from the experimental data [20]:

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

Im <sup>ε</sup>mð Þ� <sup>ω</sup> <sup>ε</sup><sup>d</sup> εmð Þþ ω 2ε<sup>d</sup>

host dielectric constants of toluene, εm(ω) is the particle dielectric function, and R is

Absorption spectra of ITO NPs with different electron densities. Doping with Sn contents of 0.02, 1, and 5%

, 5.7 � 1020 cm�<sup>3</sup>

, and 1.1 � <sup>10</sup><sup>21</sup> cm�<sup>3</sup>

, (b) 5.7 � 1020 cm�<sup>3</sup>

, and

, respectively.

1/2ω/c with c representing the speed of light, ε<sup>d</sup> indicates the

(1)

<sup>A</sup> <sup>¼</sup> <sup>4</sup>πkR<sup>3</sup>

An understanding of plasmon damping is very important in order to achieve high-efficiency LSPRs. A number of plasmonic studies of metal NPs have been devoted to investigating the damping processes of LSPRs. For metal NPs, there are two main damping processes, comprising (i) size-dependent surface scattering and (ii) electronic structure-related inter- and intraband damping [12–15]. The damping processes are closely related to the physical properties of the metals. Therefore, understanding of the damping processes of LSPRs in oxide semiconductor NPs is also important for the control of optical properties. Oxide semiconductor NPs are useful plasmonic materials since their LSPR wavelengths can be widely tuned by electron density in addition to particle size [16–18]. Carrier control of LSPRs indicates that oxide semiconductors have an additional means of tuning the optical properties in a manner that is not as readily available for metal NPs. In particular, carrier-dependent damping is a specific feature of the plasmonic response in oxide semiconductor NPs. Precise elucidation of the carrier-dependent damping process including structural size is required for the optical design of plasmonic materials based on oxide semiconductor NPs.

The purpose of this chapter is to report on the light interactions of size- and carrier-controlled ITO NPs and to discuss their plasmonic applications in the IR range. We introduce size- and carrier-dependent plasmonic responses and provide information for the physical interpretation of optical spectra. A rigorous approach to the analysis of the optical properties allows us to show a quantitative assessment of the electronic properties in ITO NPs. The employments of Mie theoretical calculations, which can describe well the optical properties of metal NPs, are validated in terms of ITO NPs. Finally, we discuss the optical properties assembled films of ITO NPs for solar-thermal shielding.
