Optical and electrical properties of metal-polymer nanocomposites prepared by vapor-phase co-evaporation
Studies of the plasmonic materials remain a hot, active and expanding field of science and technology. Moreover, manufacturing of novel nanocomposites, micro-and nanodevices with specific features require the development of powerful and flexible techniques to control and optimize their structural and physical properties. In this context, co-deposition of polymer molecules and metal atoms from two independent sources, as presented in my work, provides a simple way to manipulate the shape and distribution of metal clusters. This in turn allows the alternations of the optical and electrical properties. Vapor phase co-deposition of Au and Ag simultaneously with various polymers (Teflon, PAMS, Nylon, and PMMA) was successfully used to produce polymer/metal nanocomposites with a wide range of metal volume filling factor. The growth of metallic nanoclusters in polymers is controlled by the strongly contrasting properties of the two materials. This leads to aggregation of the high cohesive metal with the formation of spherical nanoclusters in polymer matrix. The cluster distribution in the polymer matrix, and the size and shape of the individual clusters strongly depend on the condensation coefficient of the metal on the polymer surface during deposition. The condensation coefficient depends on the ratio of the deposition rates of metal and polymer, the type of metal-polymer combination, the substrate temperature and the nucleation density. The optical properties of the composites are strongly related with the microstructure of the composites. The position, intensity and width of the particle plasmon resonance were determined as a function of the metal volume filling factor, cluster size, shape, distribution, intercluster separation, the type of metal and the surrounding dielectric medium. Changes of the microstructure of the composite film upon annealing allowed to demonstrate the effect of interparticle distance and size of the clusters on the plasmonic properties of the composites. This shift of the absorption maximum is mainly caused by an increase in the distance between the clusters by agglomeration during the heat treatment, which leads to a decrease in the number density of the clusters. Bimetallic nanoclusters in polymers exhibit a single plasmon band which is mainly dependent on the alloy composition. For gold/silver bimetallic clusters the position of the band is shifted linearly to longer wavelength with an increase in the gold fraction. Also the electrical properties of nanocomposites consisting of three dimensionally distributed Au or Ag nanoclusters in Teflon and Nylon matrices were investigated. The electrical conductivity was investigated as a function of metal filling factor, and the percolation threshold was found at f = 0.42 and f = 0.32 for Teflon and Nylon composites, respectively. The IV characteristics of Teflon composites containing Au nanoclusters showed different responses for various Au concentrations. Similarly, the resistance versus temperature curves showed different trend for samples with Au concentrations below and above the percolation threshold, respectively. In conclusion, the physical co-deposition of metal and polymer has been found as a promising technique to produce metal-polymer nanocomposites for various applications.