\chapter{Introduction}  \label{chapter_introduction}

Interest in the optical properties of nanostructured thin films can be traced back to the 19th century. In the 1857 Bakerian Lecture, Michael Faraday discussed a range of unusual optical effects exhibited by thin films of Au and other metals subjected to different treatements \cite{faraday1857}. The role of metallic nanoparticles of gold in producing the colours of stained glass was first investigated by Garnet in 1904 \cite{garnet1904} using Rayleigh's theory for scattering of light from spherical particles. A few years later, Gustav Mie developed a more complete theory of light scattering for spherical nanoparticles; Mie's theory is still widely used accross many disciplines \cite{mei1908} \cite{wriedt2008}.

Many of the properties investigated by Faraday are now understood in terms of the behaviour of the electron gas in a metal, and in particular the ability for thin metal films to support plasmonic resonance effects. A plasmon is a quasiparticle which describes the collective oscillation of conduction electrons in a metal. The behvaiour of plasmons is well understood in terms of classical models for a free electron gas; in the Lorentz model electrons are treated as damped harmonic oscillators. When light is incident on a metal, the electrons may collectively resonate in response to the sinusoidal forcing field.

Recently there has been much interest in the exploitation of plasmonic effects in nanoscale devices for a bewildering array of applications from improvement of photovoltaic solar cell efficiency \cite{atwater2010,linic2011} to biosensing \cite{maksymov2011}, and even the potential for plasmonic based computing devices \cite{maksymov2010}. %The extremely rapid developement of the field of plasmonics during the last decade has been stimulated by advances in nano-fabrication technology \cite{}.

A fascinating phenomena first observed in the 1930s \cite{pfund1930} is the tendency for metal films deposited in high pressure\footnote{Of the order of $10^{-2}$mbar} atmospheres to appear completely black at visible wavelengths. Such films have found use in a range of optical applications as near perfect black bodies with very low thermal mass \cite{pfund1930,harris1948,harris1952,harris1953}.

Recently, so called ``black metal'' films have been found to enhance the efficiency of thin film solar cells; this is consistent with the presence of plasmonic effects in the films \cite{panjwani2011}. 

\begin{comment}
Just several months ago, an ``artificially blackened'' plasmonic meta-material termed has been produced with similar applications in mind to those of the ``traditional'' black metal films. This technology has largely been motivated by the difficulty of theoretically modelling the traditional Black metal samples. \cite{sondergaard2012}.
\end{comment}

The aim of this project has been the preparation and characterisation of thin metal film samples, with a focus on black metal films. We have also investigated the possibility for plasmonic behaviour in Black metal films. We have prepared samples for subsequent study with both electronic and optical spectroscopic techniques.


The remainder of this report will be organised as follows: in chapter \ref{chapter_overview} we will present an overview of past research into the characterisation of black metal films, followed by a brief explanation of plasmonic behaviour based on the Lorentz model. Chapter \ref{chapter_techniques} will give an overview of the two main experimental techniques employed during this study (Total Current Spectroscopy and Ellipsometry). In chapter \ref{chapter_results} we will present results and discussion of experiments, which will be followed with the conclusions of this study.

\subsubsection*{A Note on Terminology}
There is some possibility for confusion when referring to metal films evaporated at high pressure, which appear black at visible wavelengths, and metal films evaporated at low pressure, which are typically highly reflective. In the remainder of this report, we will refer to the former as ``black metal'' films and the latter simply as ``metal'' films (regardless of whether the film is thick enough for the colour to be visible to the naked eye).

