\chapter{Overview} \label{chapter_overview}

\section{Black-Metal Films}

So called black-metal films are the result of deposition of metal elements at a relatively high pressure\footnote{of the order of $10^{-2}$ mbar} or ``bad vacuum''. The films are named due to their high absorbance at visible wavelengths; a sufficiently thick film will appear black to the naked eye. There is a remarkable contrast between such films and metal films deposited under low pressure\footnote{less than $10^{-5}$mbar}, which are typically highly reflective and brightly coloured at comparable thicknesses. It has been established that black-metal films may be prepared in any gas, but when Oxygen is present the resulting films contain tungsten oxides \cite{harris1952} \cite{mckenzie2006}.

% First mentions and early research; Pfund
The formation of black-metal films at high pressure has been known since the early 20th century, with the first papers on the subject published by Pfund in the 1930s \cite{pfund1930}, \cite{pfund1933}. Pfund established the conditions for formation of black-metals \cite{pfund1930}, and showed that the transmission spectrum of metallic black films is almost zero in visible wavelengths, but increases to a plateau in the far infrared \cite{pfund1933}. Pfund's research has focused on the possible applications of black-metal films as absorbing coatings for radiometric devices. 

Subsequent researchers have also focused on determining absorbsion properties of black-metal films as a function of preparation conditions \cite{harris1948} \cite{harris1952}, with the aim of producing selective filters for infra-red detectors.

There have been several attempts to relate the structure of black-metal films to measured optical properties (usually the total reflectivity of the film). Harris et al. have produced experimental results of the transmission of black-metal films from visible wavelengths to the far-infrared. By modelling the films as a layer of metallic strands, acting as ``condensors'', Harris et al. arrived at an expression for the electron relaxation time of [element]-black \cite{}, leading to a a transmission spectrum in good agreement with experimental results.

Mckenzie has established that the presence of oxygen effects the optical and electrical properties of black-metals \cite{mckenzie2006}. Although Mckenzie has been able to use Mie theory for 


% More recent research
More recently, it was shown that black-Au coatings increased the efficiency of thin film solar cells \cite{}. In this study, a simulation approximating an black-Au film as a layer of semi-spherical structures showed plasmonic behaviour which lead to an increase in electric field behind the film. 


% Artificially ``blackened'' thin films
The optical properties of black-metal films have been found to vary when the film is exposed to atmosphere, or heated \cite{donna1993}. This ``degradation'' of the black-metal films is inconvenient for maintaining consistent calibration of devices. Recently there has been interest in artificial ``blackening'' of metal surfaces for optical applications. These ``meta-materials'' offer a promising alternative to the traditional black-metal films, due to the ability to more precisely control properties of the film \cite{sondergaard2012}.


In light of the wealth of previous research, the aims of this project were first to reproduce black-metal films using equipment available at CAMSP, and then employ several techniques for the study of these samples in comparison with other metal films. Most of the existing research has been conducted using optical transmission or reflection spectroscopy techniques. A total current secondary electron spectroscopy experiment was therefore integrated into the sample preparation vacuum chamber, to allow for almost immediate study of prepared samples using this technique. A Variable Angle Spectroscopic Ellipsometer (VASE) has been used in an attempt to relate the optical and structural properties of the samples. We have also used optical transmission spectroscopy in the visible region to characterise the transmission of black-metal films.

\section{Plasmonic Effects}