Optical Properties Nanocomposite Composed of Ag Nanoparticles Embedded in a DLC Film

Abstract

The results of theoretical and experimental investigation of the optical absorption spectra of thin films of nanocomposite materials composed of silver nanoparticles embedded in a diamond like carbon (DLC) matrix are presented. The optical characteristics of DLC-Ag nanocomposite are considered within the framework of the effective medium approximation. Fundamental properties of nanostructured materials are currently excessively studied because of their potential application in numerous fields such as electronic devices, opto-electronics, optics, tribology, biotechnology, human medicine and others. The noble metal nanoparticles embedded in a matrix such as amorphous carbon have been studied intensively (Montiel-Gonzalez Z et al. Thin Solid Films 519:5924–5932, 2011), since such type of nanocomposites are expected to exhibit interesting optical property so called surface plasmon resonance and are promising materials for developing the elemental base of different physics devices. Amorphous hydrocarbon coatings, often called diamond-like carbon (DLC) coatings are nowadays used in many applications due to low friction coefficient and high hardness and also corrosion resistance, chemical inertness, high electrical resistivity, infrared-transparency, high refractive index (Robertson, Mater Sci Eng R Rep 37:129–281, 2002). In this work, the optical absorption of DLC-Ag films was experimentally and theoretically investigated. The experimental absorption spectra of DLC based silver nanocomposites deposited by reactive magnetron sputtering on fused silica were compared with the simulated ones. Role of the host matrix and silver nano-particles on the tunable effective refractive index of the films was analyzed. Using Mie theory it was researched that absorption of the nanocomposite thin films is sensitive to both refractive index and extinction coefficient parameters of the DLC host and in the evaluation of the absorption of the film it is necessary to take into account the spectral dependence of the complex refractive index of DLC. Reduction in size of the metal particles results in blue-shift of the surface plasmon resonance peak but spectral position of the absorption peak is more sensitive to the dielectric constant of DLC, than to the radius of nanoparticles. The absorption peaks related to the surface plasmon resonance are red-shifted with the increase in Ag volume fraction and this shift may be attributed to the increased electromagnetic interaction between the particles and change in the effective permittivity of the DLC-Ag nanocomposite (Yaremchuk et al. Physica Status Solidi (a) 211:329–335, 2014). In Fig. 71.1 experimental and calculated optical absorption spectra of DLC-Ag thin film with 20 % Ag concentration are depicted. The optical absorption peak due to the surface plasmon resonance of Ag particles centered at around 530 nm was observed. Strong absorption peak centered at around 480 nm we address to the nanoparticles with a radius of 8.5 nm (calculation 1, see Fig. 71.1) and peak at around 580 nm – to the nanoparticles with a radius of 30 nm (calculation 2, Fig. 71.1). The approximation curve consists of these two peaks and provides reasonable fit of the experimental curve. Application of the extended Maxwell–Garnett effective theory provides good fit of the experimental optical absorption spectra of the DLC-Ag films with the silver nanoparticle concentration up to 20 %. The difference between the Maxwell-Garnett theory prediction and the experimental results is that a surface plasmon resonance is broader than the computed one as a result of wide distribution of the particle size consequently resulting in the broadening of the experimental plasmon resonance peak.