Electrochemical Applications
of CVD Diamond
by
Gustavo Pastor-Moreno
University of
Bristol
School of
Chemistry
Faculty of
Science
A dissertation submitted to
the University of Bristol in accordance with the requirements of the degree of
Doctor of Philosophy in the Faculty of Science
July 2002
CONTENTS
Number of words: 51100
Abstract
Diamond technology has claimed an important role in
industry since non expensive methods of synthesis such as chemical vapour
deposition allow to elaborate cheap polycrystalline diamond. This fact has
increased the interest in the scientific community due to the outstanding
properties of diamond. Since Pleskov published in 1987 the first paper in
electrochemistry, many researchers around the world have studied different
aspects of diamond electrochemistry such as reactivity, electrical structure, etc.
As part of this worldwide interest these studies reveal new information
about diamond electrodes. These studies report investigation of diamond
electrodes characterized using structural techniques like scanning electrode
microscopy and Raman spectroscopy. A new electrochemical theory based on
surface states is presented that explains the metal and the semiconductor
behaviour in terms of the doping level of the diamond electrode. In an effort
to characterise the properties of diamond electrodes the band edges for
hydrogen and oxygen terminated surface are located in organic solvent, hence
avoiding possible interference that are present in aqueous solution. The
determination of the band edges is performed by Mott-Schottky studies. These
allow the calculation of the flat band potential and therefore the band edges.
Additional cyclic voltammetric studies are presented for both types of surface
termination. Mott-Schottky data and cyclic voltammograms are compared and
explained in terms of the band edge localisation. Non-degenerately p-type
semiconductor behaviour is presented for hydrogen terminated boron doped
diamond. Graphitic surface states on oxidised surface boron doped diamond are
responsible for the electrochemistry of redox couples that posses similar
energy. Using the simple redox couple 1,4-benzoquinone effect of surface
termination on the chemical behaviour of diamond is presented. Hydrogen
sublayers in diamond electrodes seem to play an important role for the
reduction of this redox couple modifying completely the mechanism of its
reduction process. Photoelectrochemical studies using 1,4-benzoquinone in
aqueous solution suggest that oxygen terminated surface of diamond is under the
influence of the boron elemental states originated during the growing process.
Diamond used as a heat sink allows developing a couple of new techniques to
perform impedance and ac voltammetry measurements. These techniques do not
depend on electrical components giving the possibility of high frequency
studies without disruption from the instruments. These studies are just a grain
in the dessert, further studies will be required to characterise this amazing
electrode material.
Author’s Declaration
This thesis is an account of
work carried out between June 1999 and July 2002 at the University of Bristol
under the supervision of Dr D. J. Riley.
I declare that the work in
this dissertation was carried out in accordance with the Regulations of the
University of Bristol. The work is original except where indicated by special
reference in the text and no part of the dissertation has been submitted for
any other degree.
Any views expressed in the
dissertation are those of the author and in no way represent those of the
University of Bristol.
The dissertation has not
been presented to any other University for examination either in the United
Kingdom or overseas.
Signed: Date: 25th July 2002
Acknowledgements
Firstly, I would like to thank Dr Jason Riley for
his help, guidance and encouragement during the whole three years.
I would like to thank Dr Paul May for the diamond
samples grown on quartz, Dr James Smith for the free standing diamond samples,
Dr James Petherbridge for his diamond samples grown by MPACVD and Dr Matt Latto
for the boron doped diamond samples by HFCVD.
Thanks to the Interface Analysis Centre (Raman
Spectroscopy and the use of their microscope equipped with a CCD camera) and Dr
Sean Davies (for platinum sputtering and his help to prepare SEM pictures).
I would like to thank Abudinnar Hassan and Trudie
Alder, undergraduates who worked with me on the project.
In addition to the people mentioned above, I would
like to thank all the other members of the electrochemistry lab that they made
my life easier at the beginning of my studies. Thanks to Liz Tull for the
interesting chats whilst eating a chocolate bar during the breaks.
I would like to thank my colleague, Jose Vicente for
his help and friendship during the three years. Also to the support and
understanding during difficult moments of my dear Iris Olivares.
I would like to share the happiness in these moments
with my brother, Francisco Javier Pastor-Moreno because after he recovered from
all his severe health problems he has been source of power when I have felt
down. I would like to thank Francisco Pastor, my father, because he taught me
many things during my life (like how to write the number eight) that have aided
this project. Finally, I would like to
thank Matilde Moreno, my mother, for all her sacrifices that made for us (my
brother and I) and all the care and attentions that she gave (and still gives)
me, especially in the difficult years of my childhood. Nothing is enough when
it comes to her sons. Please, never change Mum! Sincerely, thanks Dad and Mum.
This dream that becomes a reality is as much yours as it is mine and I do not
forget that wherever I am still we are and will be a team!
Agradecimientos
En primer lugar deseo agradecer al Dr Jason
Riley por su ayuda, guia y animo durante estos tres años.
Quiero darle las gracias tambien al Dr Paul May por las muestras de diamante depositadas en cuarzo, al Dr James Smith por las muestras de diamante sin substrato, al Dr James Petherbridge por las muestras sintetizadas usando el reactor MPACVD y al Dr Matt Latto por las muestras de diamante dopado con boro usando el reactor HFCVD.
Gracias
al Centro Interfacial de Analisis (Espectroscopia Raman y el poder usar su
microscopio equipado con una camara CCD) y al Dr Sean Davies (por depositar
platino y su ayuda preparando fotografias SEM).
Quiero
expresar mi agradecimiento tambien a Abudinnar Hassan y Trudie Alder,
estudiantes sin graduar los cuales trabajaron conmigo en el proyecto.
Ademas
de las personas mencionadas ya, deseo expresar mi gratitud a los miembros del
laboratorio de electroquimica que me hicieron mas faciles las cosas en el
comienzo de mis estudios. Gracias a Liz Tull por los interesantes coloquios que
manteniamos durante los descansos comiendo barritas de chocolate.
Tambien
deseo agradecer a mi colega, Jose Viciente su ayuda y amistad durante estos
tres años. Tambien agradecer el animo y
el entendiemiento en los momentos dificiles de mi estimada Iris Olivares.
Deseo
compartir la felicidad en estos momentos con mi hermano, Francisco Javier
Pastor-Moreno porque despues de recupersarse de sus severos problemas de salud
el ha sido para mi una fuente de energia en mis malos momentos. Quiero darle
las gracias tambien a Francisco Pastor, mi padre, por haberme enseñado tantas
cosas durante mi vida (como escribir correctamente el numero ocho) que me han
sido de mucha ayuda en este proyecto. Tambien deseo expresar mi agradecimiento
a Matilde Moreno, mi madre, por todos los sacrificios que ha hecho por nosotros
(mi hermano y yo) y todo el el cuidado y atenciones que me dio (y todavia me
sigue dando), especialmente por todos aquellos dificiles años de mi niñez. Todo
para ella se le hace poco con sus hijos. Por favor, nunca cambies Mama! De
corazon, gracias Papa y Mama. Este sueño que se hace realidad ahora es tan
vuestro como mio y nunca olvido que este donde este seguimos y seguiremos
siendo un equipo!
Abbreviations
AC: Alternate current
CB: Conduction Band
CE: Counter electrode
CV: Cyclic voltammetry
CVD: Chemical vapour deposition
DC: Direct current
EELS: electron energy loss
spectroscopy
FRA: Frequency response
analyser
FWHM: full width half
maximum
HFCVD: Hot filament CVD
HPHT: High temperature high pressure
IPA: Isopropanol (2-propanol)
LEED: Low energy electron diffraction
MPACVD: Microwave plasma assisted CVD
MFC: Mass flow controllers
NHE: Normal hydrogen electrode
p.p.m: parts per million
PTFE: Polytetrafluoroethene
RE: Reference electrode
SBE: Schottky barrier height
s.c.c.m: standard cm3 per minute
SCE: aqueous saturated calomel electrode
SEM: Scanning electron microscopy
SIMS: Secondary ion mass spectroscopy
TBAP: Tetrabutylammoniumperchlorate
TEM: Transmission electron microscopy
Temocps: Temperature modulated open circuit
potential spectroscopy
TiUL: Titanium under layer (contact)
Uv-Vis: Ultraviolet-visible (spectrum)
VB: Valence band
WE: Working elctrode
XPE: X-ray photoelectron spectroscopy
3LM: three metal layer (contact)
Table
of units
|
Symbol |
Meaning |
Units |
Magnitude |
|
ba |
Relates
the changes of potential due to the temperture |
V K-1 |
- |
|
C |
Capacitance |
F, mF |
- |
|
|
heat
capacity |
J K-1m-3 |
- |
|
|
Concentration |
mol dm-3 |
- |
|
C* |
concentration
at equilibrium stage |
mol dm-3 |
- |
|
c |
speed
light in vacuo |
m s-1 |
2.999´108 |
|
D |
diffusion
coefficients |
cm2 s-1 |
- |
|
e |
elementary
charge |
C |
1.60´10-19 |
|
E |
Energy |
J |
- |
|
|
|
1 eV |
1.60´10-19 J |
|
|
applied
potential |
V, mV |
- |
|
F |
Faraday |
F |
96.49´103 C |
|
EF |
Fermi
level energy |
eV |
- |
|
E0 |
Fermi
level energy at surface |
eV |
- |
|
Evac |
vacuum
level energy |
eV |
- |
|
Eg |
band
gap energy |
eV |
- |
|
h |
Planck
constant |
J s |
6.63´10-34 |
|
i |
Imaginary
number |
- |
(-1)0.5 |
|
|
Current |
A, mA |
- |
|
j |
current
density |
A cm-2 |
- |
|
j0 |
Exchange
current density |
A cm-2 |
- |
|
k |
Boltzmann
constant |
J K-1 |
1.38´10-23 |
|
Q |
Surface
charge |
C |
- |
|
R |
gas
constant |
J mol-1K-1 |
8.31 |
|
R |
Resistance |
W |
- |
|
Rct |
Charge
transfer resistance |
W |
- |
|
RW |
Ohmic
resistance |
W |
- |
|
- |
Sheet
resistance |
W cm |
- |
|
T |
Temperature |
K |
- |
|
t |
Time |
s |
- |
|
V |
Applied
potential |
V |
- |
|
Z” |
Imaginary
component of impedance |
W |
- |
|
Z’ |
Real
component of impedance |
W |
- |
|
a |
Transfer
coefficient |
- |
- |
|
dE0 |
Variation
of Fermi level at surface |
eV |
- |
|
dV |
Variation
of the applied potential |
V, mV |
- |
|
e |
Dielectric
constant |
- |
- |
|
e0 |
Permittivity
free space |
C2 N-1 m-2 |
- |
|
k |
Thermal
conducivity |
W m-1 K-1 |
- |
|
|
Double
layer thickness parameter |
cm-1 |
- |
|
f |
Work
function |
eV |
- |
|
Symbol |
Meaning |
Units |
Magnitude |
|
h |
overpotential |
V |
- |
|
c |
Electron
affinity |
eV |
- |
|
j |
Barrier
height |
eV |
- |
|
rD |
Experimental
parameter |
- |
- |
|
s |
Experimental
parameter |
W s- 0.5 |
- |
|
w |
frequency |
s-1, Hz |
- |
Different
common units of pressure are used
1
atmosphere (atm) = 760 Torr
=
760 millimetres of mercury (mm Hg)
=
1.013´105 pascals (Pa)
=
1.013´105 N m-2
=
1.013 bar
=
93.195 pound per square inch (psi)
Table of contents
Abstract
Author’s declaration
Table of tables
Chapter 1: Introduction
1.1. Introduction
1.2. Properties of diamond
1.3. Synthesis of diamond
1.4. Commercial applications of diamond
1.4.1. Thermal applications
1.4.2.
Electronic devices
1.4.3.
Optical windows
1.4.4.
Abrasives
1.5. Electrochemistry of diamond
1.5.1.
Electrochemical applications of diamond
1.5.2.
Electrochemical studies of highly boron doped diamond in aqueous media
1.5.3.
Electrochemical studies of highly boron doped diamond in non aqueous media
1.5.4.
Electrochemical studies of moderately boron doped diamond in aqueous media
1.5.5.
Electrochemical studies of undoped diamond in aqueous media
1.6. Possible lines of investigation
1.7. Summary
1.8. Thesis outline
1.9. References
Chapter 2: Diamond growth and characterisation
2.1. Background
2.1.1.
Introduction
2.1.2.
High-pressure high-temperature technique
2.1.3.
Chemical vapour deposition technique
2.1.4.
Hot filament CVD
2.1.5.
The choice of substrates for growing CVD diamond
2.1.6.
Nucleation
2.1.7.
The CVD diamond film
2.1.8.
The chemistry of CVD diamond growth
2.1.9.
Role of atomic hydrogen in the CVD growth
2.1.10.
In situ doping
2.1.11.
Ex situ doping
2.1.12.
Summary
2.2. Diamond growth
2.2.1.
Introduction
2.2.2.
Gas flow system
2.2.3.
Mass flow controllers
2.2.4.
Dilution
2.2.5.
Deposition chamber
2.2.5.1.
Top flange
2.2.5.2.
Front flange
2.2.5.3.
Bottom flange
2.2.5.4.
Rear flange
2.2.5.5.
Left flange
2.2.5.6.
Right flange
2.2.6.
Substrate heater
2.2.6.1.
Construction of the substrate heater
2.2.6.2.
Operating with the substrate heater
2.2.6.3.
Maintenance
2.2.7.
Filaments
2.2.7.1.
Construction of the filament
2.2.7.2.
Operating conditions
2.2.8.
Substrates
2.2.8.1.
Main features of the substrates
2.2.8.2.
Pre-treatment of silicon surface before growing
2.2.9.
Electrical contacts
2.2.10.
Typical growth conditions
2.3. Diamond characterisation
2.3.1.
Analytical techniques to characterise CVD diamond films
2.3.1.1.
Scanning electron microscopy
2.3.1.2.
Laser raman spectroscopy
2.4. Summary
2.5. References
Chapter 3: Electrical contacts to semiconducting diamond
3.1. Introduction
3.2. Indium/Gallium eutectic electrical
contacts
3.2.1.
Construction of the electrical contact
3.2.2.
Results
3.2.3.
Conclusions
3.3. Silver loaded epoxy resin electrical
contacts
3.3.1.
Construction of the electrical contact
3.3.2.
Results and discussion
3.3.3.
Conclusions
3.4. Gold electrical contacts
3.4.1.
Construction of the electrical contact
3.4.2.
Results and discussion
3.4.3.
Conclusions
3.5. Three layer metal electrical contacts
3.5.1.
Construction of the electrical contact
3.5.2.
Results and discussion
3.5.3.
Conclusions
3.6. Titanium under layer contacts8
3.6.1.
Construction of the electrical contact
3.6.2.
Results and discussion
3.6.3.
Conclusions
3.7. Summary11
3.8. References
Chapter 4: Electrochemical theory for diamond electrodes
4.1. Introduction
4.2. Metal electrochemistry
4.3. Ideal p-type semiconductor
electrochemistry
4.4. Highly doped semiconductors5
4.4.1.
Electrochemistry at boron doped diamond electrode
4.4.1.1.
Hydrogen surface termination on highly doped diamond
4.4.1.2.
Oxygen terminated surface on highly doped diamond
4.5. Surface state mediated electron
transfer
4.5.1.
Contribution to the applied potential
4.5.2.
Contribution of the Helmholtz layer and the space charge region
4.5.3.
Electrical charge at the surface of semiconductor electrode
4.5.4.
The Butler-Volmer Equation
4.5.5.
Schottky Diode
4.5.6.
Model
4.5.7.
Steady state current
4.6. Developing the model
4.6.1.
The case when |jH,0|>>|j|
4.6.2.
The case when |jH,0|<< |j|
4.6.3.
The case when |jSC,0|>>|j|
and j»jH,0
4.6.4.
Considering doping levels
4.7. AC Impedance
4.8. Summary
4.9. References
Chapter 5: Electrochemical studies of moderately
boron doped diamond in non aqueous
electrolyte
5.1.
Introduction
5.2.
Potential distribution across the semiconductor-electrolyte interface
5.3.
Experimental set-up
5.3.1. Electrolyte
solutions
5.3.2. Electrochemical
cells
5.3.3. Counter
electrodes
5.3.4. Reference
electrodes
5.3.5. Potentiostats
5.3.6. The dry box
5.4.
Mott-Schottky plots
5.5.
Cyclic voltammograms
5.6.
Conclusions
5.7.
References
Chapter 6: The influence of
surface preparation on the electrochemistry of boron doped diamond
6.1.
Introduction
6.2.
Experimental setup
6.2.1. Electrolyte solutions
6.2.2.
Growing characteristics of the samples
6.2.3.
Surface sample preparation
6.2.4.
Electrochemical cells
6.2.5.
Counter electrodes
6.2.6.
Reference electrodes
6.2.7.
Glassy carbon electrodes
6.2.8.
Potentiostats
6.2.9.
The dry box
6.3. Cyclic
voltammograms
6.4.
Conclusions
6.5.
References
Chapter 7: Photocurrent
measurements: a method to characterise surface states in CVD diamond
7.1.
Introduction97
7.2.
Experimental set-up7
7.2.1. Electrolyte
solutions
7.2.2. Growing
characteristics of the samples
7.2.3. Surface sample
preparation
7.2.4. Electrochemical
cells
7.2.5. Counter
electrodes
7.2.6. Reference
electrodes00
7.2.7. Faraday cages
7.2.8. Electronic
equipment used in these experiments
7.3.
Uv-vis spectra203
7.4.
Cyclic voltammograms
7.5.
Photocurrent experiments
7.6.
Conclusions
7.7.
References
Chapter 8: Temperature
modulated open circuit potential Spectroscopy
8.1.
Introduction
8.2.
Theory
8.2.1. Relationship between
light intensity and the electrode temperature
8.2.2. Relationship
between the temperature and the open circuit potential
8.3.
Experimental set-up
8.3.1. Electrolyte solutions
8.3.2. Characteristics
of the substrates
8.3.3. Growing
characteristics of the sample
8.3.4. Electrode
construction and cell assembly
8.3.5. Counter
electrodes
8.3.6. Reference
electrodes28
8.3.7. Faraday cages28
8.3.8. Electronic
equipment used in these studies
8.4.
Cyclic voltammograms
8.5.
AC impedance experiments
8.6.
Temperature modulation open circuit potential
8.7.
Conclusions
8.8.
References
Chapter 9: Temperature
modulated ac voltammetry
9.1.
Introduction3
9.2.
Theory
9.2.1. Relationship
between the temperature and the constant applied potential
9.3.
Experimental set-up
9.3.1. Electrolyte solutions
9.3.2. Growing
characteristics of the samples
9.3.3. Working electrode
construction and cell assembly
9.3.4. Counter
electrodes
9.3.5. Reference
electrodes
9.3.6. Faraday cages
9.3.7. Electronic
equipment used in these studies
9.4.
Cyclic voltammograms
9.5.
Temperature modulation ac voltammetry experiments
9.6.
Conclusions
9.7.
References
10.1.
Introduction0
10.2.
Background
10.3.
Diamond growth and characterisation
10.4.
Electrical contacts to semiconducting diamond
10.5.
Electrochemical theory for diamond electrodes
10.6.
Electrochemical studies of moderately boron doped diamond on non aqueous electrolyte
10.7.
The influence of the surface preparation on the electrochemistry of boron doped
diamond
10.8.
Photocurrent measurements: a method to characterise surface states in CVD diamond
10.9.
Temperature modulated open circuit potential spectroscopy
10.10.
Temperature modulated ac voltammetry
10.11.
Summary
10.12. Future work
Appendices
Appendix
A: Diamond growth details
Table of figures
Chapter 1: Introduction
Figure 1.1. Graphite and
diamond structures
Chapter 2: Diamond growth and characterisation
Figure 2.1. SEM picture of a
randomly oriented HFCVD diamond (sample B13)
Figure
2.2. SEM of a “Ballas” diamond (sample B8)
Figure
2.3. Schematic diagram of the physical and chemical process during diamond CVD
Figure
2.4.Triangular diagram of Bachman
Figure
2.5. Schematic diagram of the growth mechanism
Figure
2.6. A schematic diagram of the gas lines which fed the Diamond CVD chamber
Figure
2.7. A picture of the diborane dilution system
Figure
2.8. A picture of the hot filament CVD chamber
Figure
2.9. A picture of the rear view of HFCVD chamber
Figure
2.10. A picture of the substrate heater and the filament
Figure
2.11. A picture of the filament and substrate heater during growing process
Figure
2.12. SEM of a randomly oriented HFCVD diamond film (sample B141a)
Figure
2.13. Cross section SEM picture of a thin diamond film (sample B141a)
Figure
2.14. SEM of a continuous HFCVD diamond film (resolution 10 mm) (sample B117)
Figure
2.15. SEM of a continuous HFCVD diamond film (resolution 5 mm) (sample B117)
Figure
2.16. SEM of a continuous HFCVD diamond film (resolution 2 mm) (sample B117)
Figure
2.17. SEM of a continuous HFCVD diamond film (sample B142a)
Figure
2.18. SEM of an incomplete surface coverage of an edge of a thin diamond film
(sample B142a)
Figure
2.19. SEM of partial growth of a thin diamond film (sample B142a)
Figure
2.20. SEM of industrial undoped MPACVD diamond (resolution 200mm)
Figure 2.21. SEM of
industrial undoped MPACVD (resolution 50 mm)
Figure
2.22. Cross section SEM picture of a free-standing undoped MPACVD diamond
Figure
2.23. SEM of a randomly oriented undoped sample on quartz substrate (sample Q1)
Figure
2.24. SEM of a randomly oriented undoped sample on quartz substrate (sample Q2)
Figure 2.25. SEM of a crack
undoped diamond film on quartz substrate (sample Q3)
Figure 2.26. SEM of a
pinhole in a thin diamond film on quartz substrate (sample Q3)
Figure 2.27. Raman spectrum
of a natural diamond
Figure 2.28. Raman spectrum
of a boron doped diamond film(sample B140a)
Figure 2.29. Raman spectrum of a boron doped diamond film
(sample B140b)
Figure 2.30. Raman spectrum of an undoped diamond film on
quartz substrate (sample Q1)
Chapter 3: Electrical contacts to semiconducting diamond
Figure 3.1. i-V plot for two silver contacts on as
grown moderately boron doped diamond
Figure
3.2. i-V plot for two silver contacts
on as grown low boron doped diamond
Figure
3.3. Metal/semiconductor interface
Figure
3.4. i-V plot for two gold contacts
on as grown and oxidised diamond films (samples B123a and b)
Figure
3.5. i-V plot for two gold contacts
on as grown and oxidised diamond films (samples B128a and b)
Figure
3.6. Schematic diagram of the metal evaporator
Figure
3.7. Schematic diagram of the three layer metal top top contact
Figure
3.8. i-V plot for two 3LM contacts on
a post-annealed diamond film (sample B129a)
Figure 3.9. i-V plot for one 3LM contact and one
gold contact on a post-annealed diamond film (sample B129a)
Figure 3.10. Four point
probe i-V plot for two 3LM contact on
a post-annealed diamond film (sample B129a)
Figure 3.11. Two point probe
i-V plot for two 3LM contact on a
post-annealed diamond film (sample B129a)
Figure 3.12. Plane schematic
view of TiUL contact
Figure 3.13. Perspective schematic
view of TiUL contact
Figure 3.14. Plane schematic
view of double strip under layer contact
Figure 3.15. Perspective
schematic view of double strip under layer contact
Figure 3.16. Optical
microscopy image of the central section taken with a 50 ´ objective lens (sample B147a)
Figure 3.17. SEM picture of
the central section (sample B147a)
Figure 3.18. SEM picture of
an inclusion on the central section of the diamond surface (sample B147a)
Figure 3.19. Optical
microscopy image of the boundary between areas a and b taken with a 20 ´ objective lens (sample B147a)
Figure 3.20. Optical
microscopy image of the boundary between areas a and b taken with a 10 ´ objective lens (sample B147a)
Figure
3.21. SEM image of the boundary between areas a and b (resolution 200 mm) (sample B147a)
Figure 3.22. SEM image of
the boundary between areas a and b (resolution 10 mm) (sample B147a)
Figure 3.23. Optical
microscopy image of the boundary between areas b and c taken with a 10 ´ objective lens (sample B147a)
Figure 3.24. Optical
microscopy image of the boundary between areas b and c taken with a 20 ´ objective lens (sample B147a)
Figure 3.25. Optical
microscopy image of the boundary between areas b and c taken with a 20 ´ objective lens (sample B147a)
Figure 3.26. i-V plot for two TiUL contact on as
grown diamond film (sample B147a)
Figure 3.27. i-V plot for one TiUL contact and one
gold contact on as oxidised diamond film (sample B147a)
Chapter 4: Electrochemical theory for diamond electrodes
Figure 4.1. Schematic
distribution functions on the rate of electron transfer for a metal-solution
interface
Figure
4.2. Current density vs overpotential for a=0.5
Figure
4.3. Current density vs overpotential for a semiconductor
Figure
4.4. Schematic distribution functions on the rate of electron transfer for a
semiconductor-solution interface
Figure
4.5. Schematic distribution functions on the rate of electron transfer for a
heavily doped semiconductor-solution interface
Figure 4.6. Current density
vs overpotential for a heavily doped semiconductor
Figure
4.7. Schematic energy level diagram for diamond
Figure
4.8. Reaction at a hydrogen terminated surface
Figure
4.9. Contact process for an oxygen terminated diamond surface
Figure
4.10. Electron transfer via surface states
Figure
4.11. Forward bias and conventional electrochemical current for an n-type
and/or p-type semiconductor
Figure
4.12. Schematic energetic diagram at positive potential
Figure
4.13. Equivalent circuit for surface mediated transfer with significant
potential drop across the Helmholtz layer
Figure
4.14. Equation 4.51 plotted for |jH,0|>>|j|
Figure
4.15. Equation 4.51 plotted for |jH,0|<<|j|
Figure
4.16. Equation 4.51 plotted for |jSC,0|>>|j|
Figure
4.17. Equation 4.51 plotted for |jSC,0|>>|j| and b=1´1021
Figure
4.18. Equation 4.51 plotted for |jSC,0|>>|j| and b=1´1024
Figure 4.19. Impedance plot for open circuit potentials conditions
Chapter 5: Electrochemical studies of moderately boron doped diamond in non aqueous electrolyte
Figure 5.1. Energy diagram
of the semiconductor-electrolyte interface under equilibrium
Figure
5.2. Energy diagram of an ideally polarisable interface at zero potential
Figure
5.3. Schematic representation of the potential drop and charge across the
semiconductor electrolyte interface under depletion conditions
Figure
5.4. Potential dependence of the band bending for a p-type semiconductor
Figure
5.5. A schematic diagram of an electrochemistry cell
Figure
5.6. Schematic diagram of a counter and platinum wire electrode
Figure
5.7. A schematic diagram of a ferrocene reference electrode
Figure
5.8. A picture of the dry box
Figure
5.9. Mott-Schottky plots for semiconducting hydrogen terminated boron doped
diamond electrode
Figure
5.10. Mott-Schottky plots for semiconducting oxygen terminated boron doped
diamond electrode
Figure
5.11. Proposed energy diagram for the diamond-electrolyte interface for hydrogen
and oxygen terminated samples
Figure
5.12. Cyclic voltammetry recorded at scan rate of 0.1 Vs-1, of
platinum working electrode in 1´10-3 mol dm-3
FeCp2
Figure
5.13. Cyclic voltammetry recorded at scan rate of 0.1 Vs-1, of
platinum working electrode in 1´10-3 mol dm-3
FeCp2*
Figure
5.14. Cyclic voltammetry recorded at scan rate of 0.1 Vs-1, of boron doped diamond electrode. A hydrogen
terminated sample immersed in 1´10-3 mol dm –3
FeCp2
Figure
5.15. Cyclic voltammetry recorded at scan rate of 0.1 Vs-1, of boron
doped diamond electrode. A hydrogen terminated sample immersed in 1´10-3 mol dm-3 FeCp2*
Figure
5.16. Cyclic voltammetry recorded at scan rate of 0.1 Vs-1, of boron
doped diamond electrode. An oxygen terminated sample immersed in 1´10-3 mol dm-3 FeCp2
Figure
5.17. Cyclic voltammetry recorded at scan rate of 0.1 Vs-1, of boron
doped diamond electrode. An oxygen terminated sample immersed in 1´10-3 mol dm-3 FeCp2*
Chapter 6: The influence of surface preparation on the electrochemistry of boron doped diamond
Figure 6.1. Schematic diagram
of glassy carbon electrode
Figure
6.2. Cyclic voltammetry recorded at scan rate of 0.1 Vs-1, of glassy
carbon electrode immersed in 1´10-3 mol dm-3
1,4-benzoquinone
Figure 6.3. Cyclic
voltammetry recorded at scan rate of 0.1 Vs-1, of oxygen terminated,
highly doped diamond electrode immersed in 1´10-3 mol dm-3
1,4-benzoquinone
Figure
6.4. Cyclic voltammetry recorded at scan rate of 0.1 Vs-1, of as
grown condition, highly doped diamond electrode immersed in 1´10-3 mol dm-3 1,4-benzoquinone
Figure 6.5. Cyclic voltammetry recorded at scan rate of 0.1 Vs-1,
of as re-hydrogenated surface, highly doped diamond electrode immersed in 1´10-3 mol dm-3 1,4-benzoquinone
Chapter 7: Photocurrent measurements: a method
to characterise surface states in CVD diamond
Figure 7.1. Detailed diagram of the glass cell used.
Figure 7.2. Schematic diagram of the electrochemical cell in experimental
conditions
Figure 7.3. A schematic diagram of Ag| AgCl reference electrode
Figure 7.4. Uv-vis spectra of 1´10-3 mol dm-3 1,4-benzoquinone
and potassium ferrocyanide
Figure 7.5. Cyclic voltammetry recorded at scan rate of 0.1 Vs-1,
of oxidised surface, highly doped diamond electrode immersed in 1´10-3 mol dm-3 potassium
ferrocyanide
Figure 7.6. Cyclic
voltammetry recorded at scan rate of 0.1 Vs-1, f oxidised surface,
highly doped diamond electrode immersed in 1´10-3 mol dm-3
1,4-benzoquinone in aqueous media
Figure 7.7. Photocurrent
spectra recorded at highly doped diamond electrode (oxidised surface) immersed in 1´10-3 mol dm-3 1,4-benzoquinone
in aqueous solution at applied potentials +0.6 and +0.9 V vs AgCl reference
Figure
7.8. Proposed energy diagram for diamond-electrolyte Interface
Chapter 8: Temperature modulated open circuit potential spectroscopy
Figure 8.1. Schematic diagram of system diamond-gold electrolyte
Figure 8.2. Equivalent circuit
diagram
Figure 8.3. Schematic
diagram of the different stages in the construction of the electrical contact
and cell assembly
Figure
8.4. Detailed diagram of the glass cell used in these studies
Figure
8.5. Experimental temocps display
Figure 8.6. A cyclic voltammetry recorded at gold
(diamond substrate) electrode immersed in 1´10-3 mol dm-3
ferro-ferricyanide aqueous solution. Scan rate 0.1 Vs-1
Figure 8.7. A cyclic
voltammetry recorded at gold (diamond substrate) electrode immersed in 0.1´10-3 mol dm-3 ferro-ferricsulfate
aqueous solution. Scan rate 0.1 Vs-1
Figure 8.8. A cyclic
voltammetry recorded at gold (diamond substrate) electrode immersed in 1´10-3 mol dm-3 ferro-ferricsulfate
aqueous solution. Scan rate 0.1 Vs-1
Figure 8.9. AC impedance of
1´10-3 mol dm-3
ferro-ferricyanide Ac modulation: 10 mV
Figure 8.10. Phase component
vs frequency for 1´10-3 mol dm-3
ferro-ferricyanide
Figure 8.11. Magnitude component
vs frequency for 1´10-3 mol dm-3
ferro-ferricyanide
Figure 8.12. AC impedance of
0.1´10-3 mol dm-3
ferro-ferricyanide Ac modulation: 10 mV
Figure 8.13. Phase component
vs frequency for 0.1´10-3 mol dm-3
ferro-ferricyanide
Figure 8.14. Magnitude
component vs frequency for 0.1´10-3 mol dm-3
ferro-ferricyanide
Figure 8.15. AC impedance of
0.1´10-3 mol dm-3
ferro-ferricsulfate Ac modulation: 10 mV
Figure 8.16. Phase component
vs frequency for 0.1´10-3 mol dm-3
ferro-ferricsulfate
Figure 8.17. Magnitude
component vs frequency for 0.1´10-3 mol dm-3
ferro-ferricsulfate
Figure 8.18. Imaginary vs
real component of temocps for 0.1´10-3 mol dm-3 ferro-ferricsulfate
Figure 8.19. Phase component
vs frequency of temocps for 0.1´10-3 mol dm-3
ferro-ferricsulfate
Figure 8.20. Magnitude component vs frequency of temocps for 0.1´10-3 mol dm-3
ferro-ferricsulfate
Figure
8.21. Diagram of the experimental set up
Figure 8.22. Imaginary vs real component of temocps for 0.1´10-3 mol dm-3
ferro-ferricsulfate. Theoretical and experimental data are shown.
Figure 8.23. Phase component
vs frequency of temocps for 0.1´10-3 mol dm-3
ferro-ferricsulfate. Theoretical and experimental data are shown.
Figure 8.24. Magnitude vs
frequency of temocps for 0.1´10-3 mol dm-3
ferro-ferricsulfate. Theoretical and experimental data are shown.
Chapter 9: Temperature modulated ac voltammetry
Figure 9.1. Equivalent model
circuit
Figure
9.2. Experimental configuration to perform ac voltammetry
Figure 9.3. A cyclic
voltammetry recorded at gold (diamond substrate) electrode immersed in 1´10-3 mol dm-3 ferro-ferricsulfate
aqueous solution. Scan rate 0.1 Vs-1
Figure 9.4. Oxidation peaks
vs concentration of ferro-ferricsulfate
Figure 9.5. Temperature
modulated ac voltammetry recorded at gold (diamond substrate) electrode
immersed in 1´10-3 mol dm-3
ferro-ferricsulfate aqueous solution
Figure 9.6. Temperature
modulated ac voltammetry recorded at gold (diamond substrate) electrode
immersed in 2´10-3 mol dm-3
ferro-ferricsulfate aqueous solution
Figure 9.7. Temperature
modulated ac voltammetry recorded at gold (diamond substrate) electrode
immersed in 4´10-3 mol dm-3
ferro-ferricsulfate aqueous solution
Figure 9.8. Temperature
modulated ac voltammetry recorded at gold (diamond substrate) electrode
immersed in 6´10-3 mol dm-3
ferro-ferricsulfate aqueous solution
Figure 9.9. Temperature
modulated ac voltammetry recorded at gold (diamond substrate) electrode immersed
in 8´10-3 mol dm-3
ferro-ferricsulfate aqueous solution
Figure 9.10. Diagram of the
experimental set up that details the ac and dc components in the system
Figure 9.11. Temperature
modulated ac voltammetry recorded at gold (diamond substrate) electrode
immersed in 4´10-3 mol dm-3
ferro-ferricsulfate aqueous solution Theoretical and experimental data are
shown
Table of tables
Chapter 1: Introduction
Table 1.1. Some of the properties of diamond
Chapter 2:
Diamond growth and characterisation
Table 2.1. Specifications of the mass flow
controllers
Table
2.2. Trace elements in the tantalum wire
Table
2.3. Typical deposition conditions for HFCVD reactor
Chapter 3: Electrical contacts to semiconducting diamond
Table
3.1. Summary of the production process for 3LM contact
Table
3.2. Resistance measurements for 3LM
Table 3.3. Summary of the production process for TiUL contact
Appendices
Table
A.1. Details of the diamond growth runs
