Polymer Films with Embedded Metal Nanoparticles

Composite materials, that is, materials that develop by mixing two or more basic constituents, are a topic of materials science with dramatically increasing interest [1–4]. Applications of fiber composite materials in particular have increased enormously as a result of their mechanical properties. Apart from these materials, applications of nanostructured composite materials, in which one or both of the basic components are structured in the nanometer region, are still in their early stages.

The large number of applications of thin films based on polymerized hydrocarbons has led to many methods for producing this kind of film. These deposition technologies can be classified in different ways:

A prerequisite for correlating nanostructure with electrical and optical properties is an extensive determination of the nanostructure itself. The nanostructure of polymer films with embedded metal nanoparticles was defined in Chap. 1 as the lateral and vertical particle size and shape distribution. However, the film surface, inner interfaces and substrate interface must also be considered. This means that, in addition to information about size, shape and distribution of embedded particles, details of the inner interfaces (e.g., formation of particle shells, changes in polymer structure), the film surface, the interface between film and substrate, and the structure of the basic elements (e.g., grain boundaries within the metal particles) are also needed to determine the nanostructure.

Structural changes affecting nanoparticles embedded in an insulating matrix material are generally defined as changes in the size and shape distributions of the embedded particles. This includes changes in the inner and outer interfaces, or the chemical structure of the matrix material. The simplest way to bring about nanostructural change is thermal treatment of the films.

The electrical direct current conductivity of a polymer film with embedded nanoparticles varies between the conductivity of the insulator material (e.g., for plasma polymers б _po = 10⁻¹⁴−10⁻¹⁶Ω⁻¹ cm⁻¹, see Sect. 2.1.3) and the bulk metal (e.g., for silver QA_g = 6.3 × 10⁵ Ω⁻¹ cm⁻¹, and for gold σ _Au = 4.8 × 10⁵ Ω⁻¹ cm⁻¹ [414]). Because this variation ranges over 20 orders of magnitude and no unified description of electrical transport processes exists for all possible metal proportions, different models are assumed for the d.c. electrical conductivity in each of the three structural regions: the metallic range, the percolation range and the insulating range (Fig. 5.1).

The optical properties of metal nanoparticles differ substantially from the optical properties of bulk materials. Under the influence of an electric field, there is a plasmon excitation of electrons at the particle surface. This resonance which takes place at a certain energy of the incident light results in optical absorption, the so-called plasma resonance absorption. The phenomenon of plasma resonance absorption will be described briefly to give a basis for understanding the present chapter. Comprehensive reviews of the optical properties of metal nanoparticles are given in [5–7,479].