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+\usepackage{lscape} % Needed for landscaping stuff
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{\bf \emph{Keywords:}} surface plasmons, nanostructures, spectroscopy, metallic-blacks \\
{\bf \emph{Supervisers:}} W/Prof. James Williams (UWA), Prof. Sergey Samarin (UWA) \\
+\pagebreak
+\tableofcontents
-%\tableofcontents
+\pagebreak
\section*{Acknowledgements}
-I am extremely grateful for the support offered to me by many individuals during this project.
+I am extremely grateful for the support offered to me by many individuals during this project. There aren't many synonyms for ``Thanks'', so I'm afraid this section may be a little repetitive.
-\section{Introduction}
+Thanks to my supervisors Prof Sergey Samarin and W/Prof Jim Williams for envisioning the project, and their invaluable support throughout the year. I would also like to thank staff members at CAMSP for assisting with the supervision of this project. In particular I am extremely grateful for the help and advice given by Dr Paul Gualiardo during the construction and testing of the Total Current Spectroscopy experiment.
-\section{Overview of Theory}
-Summarise the literature, refer to past research etc
+Thanks to Alexie ??? from CMCA for producing the SEM images which proved a invaluable aid for discussing the structure of the metallic-black films. Thanks to Nikita Kostylev for helping me learn the art of operating the ellipsometer. Thanks to both Prof Mikhail Kostylev and Jeremy Hughes for lending me some samples for ellipsometric analysis. I would also like to endorse the team at J.A Woolam, who provided replacement pins for the ellipsometer alignment detector at no charge after one of the original pins became mysteriously damaged.
-\subsection{Electron Spectra of Solids and Surface}
+Congratulations to Jeremy Hughes who successfully predicted that the emission current of the electron gun was varying periodically less than a quarter of the way through the first period. Condolences to Alexander Mazur, whose theory that the vacuum chamber contained a pulsar proved unfounded.
-In this section, we will first introduce the basic concepts needed to describe the electron spectra of solids. A short description of methods for calculating the electron spectra will be given, and the results shown by these calculations. We will then discuss the electron spectra for the near surface region of solids, compared to the ``bulk'' spectra far from the surface.
+Thanks to all my family and friends for their support and for continuing to put up with my slow descent into madness during the last 12 months.
-\subsubsection{Description of Matter in the Solid State}
+Finally, perhaps as a result of the aforementioned madness, I would also like to thank the various pieces of equipment and inanimate objects which have been crucial to the success of this project. This includes the ellipsometer, the ADC/DAC box, my laptop computer ``Cerberus'', and the two ammeters upon which I relied upon so heavily. Rest in peace Keithly 610B. Your death was not in vain.
-In the simplest models, a solid is represented by an infinite crystalline lattice; a geometrically repeated arrangement of some basis group of atoms. The nuclei of atoms are assumed to remain in fixed positions.
+\pagebreak
-The potential seen by an electron in the lattice is periodic. For a single nuclei, the potential seen by an electron is
+\section{Introduction}
-\subsubsection{Calculation of Electron Spectra}
+The report will be organised as follows; first we will discuss literature relating to surface science and nanostructured thin films in particular. We will then give an overview of the primary experimental techniques employed during this project, before presenting experimental results. Finally, we will discuss stuff.
-\subsubsection{The Near-Surface Region}
+\pagebreak
-In the preceeding sections, solids were assumed to have infinite spatial extent. In practice, any real solid occupies a finite volume in space. Any interactions between a solid and its environment take place at the surface of the solid. As the volume of the solid is decreased, the role of the surface region in determining the behaviour of the solid in its environment is increased.
+\section{Overview}
+In this section we provide an overview of theoretical and experimental literature related to the properties of nanostructured thin films. We also include a summary of the past research which focuses on metallic-black films.
-\subsection{Plasmonics}
+\subsection{Secondary Electron Emission}
+% What is it?
+Secondary electron emission refers to the emission of electrons from a target surface caused by interactions with an incident electron, or more commonly a beam of electrons. Electrons from the incident beam are referred to as primary electons, whilst the term ``secondary electron'' applies to all electrons reflected or emitted from the surface under bombardment.
-\subsection{Metallic-Black Thin Films}
+% First mentions and early research
-\section{Experimental Techniques}
-\subsection{Secondary Electron Spectroscopy}
+% General description of secondary electron spectrum
-Secondary Electron Spectroscopy encompasses a large group of techniques used for studying the electron spectra of surfaces and solids. In these methods a beam of primary electrons is directed at a surface. The interactions between primary electrons and the surface give rise to an energy distribution of electrons elastically and inelastically scattered from the surface. Analysis of the distribution of the scattered ``secondary'' electrons gives information about the electron energy spectrum of the target surface.
+Figure \ref{se_dist.pdf} shows the general shape of an energy distribution of secondary electrons. for a primary electron energy of $E_p$. The narrow peak centred at $E = E_p$ is due to elastically scattered electrons; the width of this peak is determined by the distribution of primary electron energies, as well as the resolution of the detector. The broad peak in the low energy part of the spectrum is due to inelastically scattered electrons.
\begin{center}
\includegraphics[scale=0.40]{figures/se_dist.pdf}
- \captionof{figure}{Simple model of the secondary electron distribution}
+ \captionof{figure}{{\bf Model of Secondary Electron Distribution}}
\label{se_dist.pdf}
\end{center}
-Figure \ref{se_dist.pdf} shows the general shape of the secondary electron distribution for a primary electron energy of $E_p$. The narrow peak centred at $E = E_p$ is due to elastically scattered electrons; the width of this peak is determined by the distribution of primary electron energies, as well as the resolution of the detector. The broad peak in the low energy part of the spectrum is due to inelastically scattered electrons.
+% How real secondary electron spectra depend upon the surface
+
+Real secondary electron distributions also show fine structure imposed upon the inelastic part of simplified spectrum described above. This fine structure is characteristic of the electron spectra of the target surface. Near to the elastic peak, fine structure is caused by energy loss to interband transitions and plasma vibration excitation. The central part of the distribution contains fine structure due to Auger electron emission, and energy losses due to excitation of inner electrons. Fine structure at low energies is due to the structure of empty states of the solid.
+
+% Research of Komolov
+
+\subsection{Plasmons}
+
+% What are they?
+A plasmon is a quasi-particle arising due to charge density oscillations in a solid.
+
+% Early research
+
+\subsection{Metallic-Black Films}
+
+% What are they?
+So called metallic-black films are the result of deposition of metal elements at a relatively high pressure (of the order of $10^{-2}$ mbar). The films are named due to their high absorbance at visible wavelengths; they appear black to the naked eye. There is a remarkable contrast between such films and films deposited under low pressure (less than $10^{-6}$mbar), which are typically highly reflective and brightly coloured.
+
+% First mentions and early research; Pfund
+This phenomenom 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 metallic-blacks \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}. More extensive research on the structural and optical properties of these films by Louis Harris and others during the 1940s and 1950s \cite{harris1948}, \cite{harris1952}, \cite{harris1953}. It has been established that metallic blacks may be prepared in either air or inert gases
+
+
+% Pretty pictures for purpose of this discussion
+% Not really "results", since I didn't make the images
+Secondary Electron Microscope (SEM) images of Au deposited on Si at high and low pressures (in air) were produced at the Centre for Microscopy Characterisation and Analysis (CMCA), UWA. The film imaged on the left (high pressure) appears black at visible wavelengths, whilst the film on the right (low pressure) appears golden yellow.
+
+\begin{center}
+
+
+\begin{tabular}{cc}
+ \includegraphics[scale=0.2]{figures/Au_BLACK_200nm.png} & %\captionof{figure}{Au-Black SEM Image} \label{Au_BLACK_200nm.png} &
+ \includegraphics[scale=0.2]{figures/Au_semi-shiny_1_SEM.png} %\captionof{figure}{Au SEM Image} \label{Au_semi-shiny_1_SEM.png}
+
+ \label{SEM_images}
+\end{tabular}
+
+ \captionof{figure}{{\bf SEM images of Au deposited on Si at $2\times10^{-2}$mbar (left) and $1\times10^{-6}$mbar.} Note that the scales are very similar for both images.}
+
+\end{center}
+
+% Explanation of structure?
+The structural difference between the two films is striking, and yet the exact mechanism behind the formation of the metallic-black film is not well understood. The most widely accepted explanation is that the evaporated metal particles reaching the target surface have insufficient energy to form a regular crystal lattice due to cooling through collisions with the atmosphere \cite{}. As of yet, there is no detailed theoretical description of this behaviour.
+
+% Research by Harris concluding ``condensor'' like structure
+Harris et al. have produced experimental results of the transmission of metallic-black films from visible wavelengths to the far-infrared \cite{}. By modelling the film 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
+Mckenzie has established that the presence of oxygen effects the optical and electrical properties of metallic-blacks \cite{mckenzie2006}.
+
+% Model of structure
+Nanostructured metal films prepared at low pressure are often approximated by an isotropic layer of spherical blobs upon the substrate, or even as a uniform layer with an ``effective'' thickness \cite{}. As the right image in Figure \ref{SEM_images} shows, this is a good representation of the structure of such a film. In contrast, the metallic-black film is highly non-uniform; as a result, detailed characterisation of the properties of such a film is difficult.
+
+
+
+
+% More recent research
+More recently, it was shown that Au-black coatings increased the efficiency of thin film solar cells \cite{}. In this study, a simulation approximating an Au-black 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
+Metallic-black films have proven useful in applications requiring efficient absorption of light, including the. Recently there has been interest in artificial ``blackening'' of metal surfaces in ways which simplify the characterisation of the surfaces for practical applications.
+
+Sondergaard et al. have produced metallic-black surfaces capable of suporting surface plasmon modes \cite{sondergaard2012}. These films exhibit similar optical properties to the previously considered ``evaporated'' metallic-black surfaces.
+
+% What I will be doing with metallic-black films
+This project will employ Total Current Specroscopy, Ellipsometry and Optical Spectroscopy methods to investigate the difference between metallic films deposited at low pressure, and high pressure (metallic-blacks). The production and study of artificially blackened films is beyond the scope of this research.
+
+
+\begin{center}
+ \includegraphics[scale=0.2]{figures/300V-01.jpg}
+ \captionof{figure}{{\bf Au-black film viewed at magnifications of x20000, x50000, x100000 and x200000} (top left, top right, bottom left, bottom right). The film appears non-isotropic, and possibly fractal like upon magnification. This structure has lead some reasearchers to refer to the deposited films as ``smokes'' \cite{}.}
+ \label{300V-01.jpg}
+\end{center}
+
+\pagebreak
+
+\section{Experimental Techniques}
+
+\subsection{Secondary Electron Spectroscopy}
+
+Secondary Electron Spectroscopy encompasses a large group of techniques which exploit secondary electron emission for studying the electron spectra of surfaces and solids. In these methods a beam of primary electrons is directed at a surface. The interactions between primary electrons and the surface give rise to an energy distribution of electrons elastically and inelastically scattered from the surface. Analysis of the distribution of the scattered ``secondary'' electrons gives information about the electron energy spectrum of the target surface.
-Real secondary electron distributions also show fine structure imposed on the inelastic part of the spectrum. This fine structure is characteristic of the target surface. Near to the elastic peak, fine structure is caused by energy loss to interband transitions and plasma vibration excitation. The central part of the distribution contains fine structure due to Auger electron emission, and energy losses due to excitation of inner electrons. Fine structure at low energies is due to the structure of empty states of the solid.
+Techniques of Secondary Electron Spectroscopy can be divided into two classes. Energy-resolved methods are based upon observation of the secondary electron distribution at a fixed primary electron energy. These methods aim to examine specific secondary emission processes which occur within a selected energy interval. The angular distribution of emitted electrons is often also recorded.
+In contrast to Energy-resolved methods, Total Current (or Yield) methods measure the total current of secondary electrons as a function of primary electron energy. The focus of this project has been low energy Total Current Spectroscopy (TCS). While Total Current methods provide less detailed information about secondary emission processes within a solid, they are generally simpler to realise experimentally as they do not require energy analysers, and current measurement may be performed external to the vacuum chamber, using a conventional ammeter.
-\subsubsection{Methods of Secondary Electron Spectroscopy}
+\subsection{Total Current Spectroscopy}
-Techniques of Secondary Electron Spectroscopy can be divided into two classes. Energy-resolved methods are based upon observation of the secondary electron distribution at a fixed primary electron energy. These methods aim to examine specific secondary emission processes which occur within a selected energy interval. In contrast to Energy-resolved methods, Total Current (or Yield) methods measure the total current of secondary electrons as a function of primary electron energy.
+Figure \ref{} shows a simplified schematic for the Total Current Spectroscopy experiments conducted during this study. An electron gun is used to produce the beam of primary electrons. Electrons are emitted from a cathode held at negative potential relative to the target. These electrons are focused into a beam and accelerated onto the target through the electric field produced by a series of electrodes. A detector is used to measure the total current passing through the target.
-The focus of this project has been on low energy Total Current spectroscopy. While Total Current methods provide less detailed information about secondary emission processes within a solid, they are useful for characterisation of the electron structure. Total Current methods are also simpler to realise experimentally, as they do not require energy analysers, and current measurement may be performed external to the vacuum chamber.
-\subsection{Total Current Secondary Electron Spectroscopy}
+If the current incident upon the sample is $I_{\text{total}}$, and the current of secondary electrons scattered from the surface is $I_r$, then the transmitted current $I_t$ is given by:
+
+\begin{align*}
+ I_t &= I_{\text{total}} - I_r
+\end{align*}
-Figure \ref{} shows a simplified schematic for the Total Current Spectroscopy experiments conducted during this study. Electrons are emitted from a cathode held at negative potential relative to the target. The electron beam is focused and accelerated onto the target by the electric field of an electron gun. A detector is used to measure the total current passing through the target.
+Generally $I_{\text{total}}$ is assumed to be independent of initial energy $E$. This assumption is valid if the initial energy is small compared to the accelerating potential of the gun, and the distance of the sample from the gun is sufficiently large.
+In this case, differentiating with respect to $E$:
+\begin{align*}
+ \der{I_t}{E} &= - \der{I_r}{E}
+\end{align*}
+Figure \ref{block_diagram.pdf} is a block diagram of the experimental setup including measurement and control systems external to the vacuum chamber.
+Note that the emission current ammeter is optional, and was used for testing purposes.
-Figure \ref{} is a block diagram of the experimental setup including measurement and control systems external to the vacuum chamber.
+\begin{center}
+ \includegraphics[scale=0.60]{figures/block_diagram.pdf} \label{block_diagram.pdf}
+\end{center}
\subsubsection{Electron Optics}
The electron gun used for this experiment was repurposed from an old Cathode-Ray Oscilloscope (CRO). Figure \ref{} shows a simplified diagram of the electron gun, whilst Figure \ref{} shows a photograph of the gun.
-The full circuit diagram for the electron gun control circuit is shown in Appendix A.
+The full circuit diagram for the electron gun control circuit is shown in Appendix A. %\cite{}
\subsubsection{Automatic Data Acquisition}
-In order to collect data on the large number of planned samples for the study, some form of automation was required. The automated system needed to be able to incrementally set the initial energy by controlling a power supply, and record the total current measured by an ammeter.
+In order to collect data on the large number of planned samples for the study, some form of automation was required. The automated system needed to be able to set the initial energy by adjusting the potential of the cathode relative to the sample, and simultaneously record the total current through the sample.
-The available power supplies at CAMSP featured analogue inputs for external control. This meant that a Digital to Analogue Convertor (DAC) card was needed to interface between the control computer and the power supply. In addition, the available instruments for current measurement at CAMSP produced analogue outputs. As a result, Analogue to Digital Convertors (ADCs) would be required to automate the recording of total current.
+The available power supplies at CAMSP featured analogue inputs for external control. This meant that a Digital to Analogue Convertor (DAC) card was needed to interface between the control computer and the power supply. In addition, the available instruments for current measurement produced analogue outputs. As a result, Analogue to Digital Convertors (ADCs) were required to automate the recording of total current.
Although an external DAC/ADC box was already available for these purposes, initial tests showed that the ADCs on the box did not function. The decision was made to design and construct a custom DAC/ADC box, rather than wait up to two months for a commercial box to arrive. The design of the custom DAC/ADC box is discussed in detail in Appendix B, and the software written for the on-board microprocessor and the controlling computer are presented in Appendix D.
-\subsection{Ellipsometry and Transmission Spectroscopy}
+\subsection{Ellipsometry}
+
+Ellipsometry is an optical technique most commonly used to determine the thickness of multilayered thin films. Ellipsometry can also be used to determine the optical constants and properties of unknown materials.
+
+Essentially, ellipsometry measures the change in polarisation of light reflected from a surface. This change in polarisation can be related to properties of the surface if knowledge of the surface is correctly applied. For a bulk sample, the change in polarisation can be directly related to the optical constants of the material.
+
+A
+
+
+
+\subsubsection{Variable Angle Specroscopic Ellipsometry}
+A single ellipsometric measurement involves recording $r_p$ and $r_s$ at one angle and wavelength. The earliest ellipsometers were
+
+
+A Variable Angle Spectroscopic Ellipsometer at CAMSP has been used to perform a variety of measurements on metallic thin films.,
+
+The VASE
+
+It is also possible to conduct reflection and transmission spectroscopy experiments using the VASE.
+
+
+\subsection{Vacuum Techniques and Sample Preparation}
+
+Both the TCS experiments and the deposition of films must be performed in a vacuum. For convenience and simplicity, a single vacuum chamber at CAMSP has been repurposed to perform both of these tasks. The chamber can be pumped by a molecular turbo pump, backed by a rotaray pump, to a base pressure of $2\times10^{-8}$ mbar, or by the rotary pump alone to a base pressure of $1\times10^{-3}$ mbar. The pressure is monitored using either a pirani or ion gauge (for pressures greater than and less than $10^{-3}$ mbar respectively).
+
+%TODO: Insert graphs of pressure in chamber
+
+Figure \ref{} shows a diagram of the vacuum chamber used both for the creation of nanostructured thin films and their study using TCS. A rotatable sample holder is positioned in the centre of the chamber. One flange of the chamber houses the electron gun used for TCS measurements, whilst the opposite flange contains feedthroughs on which tungsten filament evaporators are mounted. This setup allows for almost immediate study of evaporated films by simple rotation of the sample holder to face the gun.
+
+
+The evaporators consist of a tungsten wire filament attached between two feedthroughs. A piece of a desired metal is folded over the apex of the tungsten wire. The metal can be heated by passing a current through the filament; near the metal's melting point it begins to evaporate. To clean the metal surface and ensure uniform evaporation, this procedure is first performed at low pressure (below $10^{-6}$ mbar) with no sample in the chamber, with the current increased until the metal piece begins to melt and forms a ball on the wire. Figure \ref{} shows an image of an evaporator that has been prepared for use.
+
+This study focused primarily on depositing Au films on an Si substrate, at both high and low pressures. The substrates and sample holders were cleaned in an acetone bath immediately prior to insertion in the vacuum chamber.
+
\section{Experimental Results and Discussion}
\subsection{Ellipsometric Measurements}
+\subsection{Transmission Spectra of Metal Films}
+
+Using the VASE and a commercial spectrometer (OceanOptics) in independent experiments, we obtained transmission spectra for metallic-black and some other metallic films.
+
+
+
\section{Achievements}
+\pagebreak
+
\section*{Appendix A - Electron Gun Control and Current Measurement Circuit}
Figure \ref{} shows the complete electron gun control circuit. The circuit was designed and constructed as part of this project. The design is based upon examples found in \cite{komolov} and \cite{Moore}.
-s
+\subsection*{Electron Optics}
+The electron gun has been recycled from a Cathode Ray Oscilloscope. Figure \ref{} shows a diagram and photograph of the gun. The gun contains a total of 9 electrodes; several electrodes are held at the same potential, as shown in the figure. As shown in figure \ref{}, the electrode potentials are referenced to the cathode, not signal ground. Because of the relatively large distance between the gun and sample (held at ground), this ensures that changes in initial energy do not significantly effect the focusing properties of the gun.
+The optimum potentials of the gun electrodes were determined manually by focusing the gun on an Au film (deposited on Si). I-V curves obtained by measuring current through the sample as a function of initial energy were obtained. The electrode potentials were systematically altered to ensure the curves were as close as possible to the ideal model.
+
+\subsection*{Model of I-V curves}
+The current detected through the sample is due to electrons which have sufficient energy to overcome the potential barrier between the surface and vacuum, entering the conduction band
+
+
+
+\subsection*{}
+
+The filament is surrounded by a conducting cylindrical electrode commonly called the ``Venault''. The potential of the venault has little effect on the focusing properties of the gun, but is largely responsible for determining the current leaving the gun. Figure \ref{} shows I-V curves for several venault settings, including the optimum setting.
+
+\subsection*{Einzel Lens}
+
+%Electrodes passing through the aperture of the venault are accelerated towards the target through six electrodes. The first and last pair of electrodes are at the same potential; the central pair are held at a different potential. Such an arrangement can be considered as an ``Einzel'' or ``zoom'' lens. It can be shown \ref{Moore} that the
+
+\pagebreak
\section*{Appendix B - DAC/ADC Box - Hardware}
\begin{center}
\includegraphics[scale=0.50]{figures/pressure_c.jpg}
\captionof{figure}{Individual digits identified}
- \label{pressure_a.jpg}
+ \label{pressure_b.jpg}
\end{center}
\begin{center}
\includegraphics[scale=0.50]{figures/pressure_d.jpg}
\captionof{figure}{Activated segments (green) for a single digit}
- \label{pressure_a.jpg}
+ \label{pressure_c.jpg}
\end{center}
\section*{Appendix D - Sources of Error}
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