Chapter 9
Conclusions
These studies investigated
the use of boron doped polycrystalline diamond as an electrode material.
Diamond is a unique material with many extreme properties. Reliable,
inexpensive methods of synthesis may be used to make diamond for a wide range
of applications.
Pure diamond is a very good
electrical insulator but the material may be doped with boron to produce
electrodes with semiconducting or semimetallic properties. The use of diamond
electrodes has been proposed for many electrochemical applications but the
existing literature focuses on highly doped electrodes and does not fully
explain the mechanism of the electron transfer across the diamond/electrolyte
interface.
9.2 Growth and Characterisation
CVD apparatus has been
assembled which can deposit boron doped polycrystalline diamond films with a
wide range of doping levels from 130,000 p.p.m. to 6 p.p.m.
(corresponding to doping levels from 2 ´ 1022 B atoms cm‑3
to 1 ´ 1018 B atoms cm‑3
†).
SIMS, SEM and optical microscopy have been used to show the films to be continuous and of high quality.
Diamond films have been
deposited selectively onto patterned silicon wafers. The use of patterned
silicon wafers with a partial titanium covering provided a novel method to
fabricate boron doped diamond films with good Ohmic contacts.
9.3 Electrical Contacts
A variety of methods were used to make electrical contacts to the diamond films. For highly doped diamond films used over a limited potential range, simple silver dag contacts were sufficient to provide a linear response. Low doped films required more sophisticated contacts to avoid the formation of Schottky barriers.
Despite suggestions in the
existing literature, evaporated gold contacts were found to be insufficient to
provide Ohmic contacts. Three layer metal (3LM) contacts are a popular way of
forming contacts to diamond. However, 3LM contacts were found to be difficult
to fabricate and it proved impossible to prepare a good Ohmic contact in a
standard laboratory environment. Titanium underlayer (TiUL) contacts provided a novel solution to the problem. They were
relatively simple to fabricate and gave a linear response.
9.4 Standard Electrochemical Theory
Metal electrodes exhibit a
symmetrical response to applied potential. Oxidation and reduction peaks can
both be seen in cyclic voltammograms taken with metal electrodes.
p-type semiconductor electrodes are expected to show oxidative peaks, as there will be a forward bias at positive overpotential. The rate of reduction at negative overpotential should reach a limiting value where the total current density is equal to the equilibrium cathodic current density.
High doping levels can lead to degenerate doping conditions where the semiconductor will exhibit metallic behaviour.
An expression may be derived
to relate the overpotential to the steady state current for surface-state
mediated charge transfer at p-type semiconductors.
9.5 Electrochemistry of Highly and Moderately
Doped Diamond Films
Diamond films with high to
moderate doping levels (over 1020 cm-3) exhibited
metallic behaviour. Oxidation and reduction peaks could be seen in cyclic
voltammograms of several well known redox couples.
The surface termination of
boron doped diamond electrodes was an important factor in the electrode
kinetics. For oxygenated polycrystalline diamond electrodes two time constants
were observed in impedance studies of a simple redox reaction. The result
suggests that the electron transfer process was mediated by surface states, in
agreement with a model proposed by van de Lagemaat 128 in
single crystal studies.
Diamond samples were grown
for a number of studies by French and German collaborators. An undergraduate
project studied the possibility of detecting copper (Cu) underpotential
deposition (UPD) of diamond in acidic environments.
9.6 Electrochemistry of Low Doped Diamond
Films
At low doping levels, boron
doped polycrystalline diamond films exhibited semiconductor behaviour and the
electrochemistry was dependant on surface termination.
Titanium underlayer (TiUL) contacts did not require treatment of
the sample after the diamond growth step. Ohmic contacts could therefore be
made to hydrogen terminated diamond electrodes.
Oxygen terminated diamond
samples exhibited irreversible behaviour with an absence of a reverse reductive
peak. Hydrogen terminated samples were more reversible. Both forward and
reverse peaks were visible in cyclic voltammograms. Mott-Schottky plots for
oxidised diamond samples showed a flatband potential corresponding to a surface
state at an energy level at Ev = 1.7 eV.
9.7 Electrochemical Theory for Boron Doped
Diamond Films
The standard theories for
metal or semiconductor electrochemistry, as outlined in chapter 4, are not
sufficient to provide a model for the electrochemistry of boron doped
polycrystalline diamond films.
The mechanism of charge
transfer must be considered. The presence of surface states explains the
difference in behaviour between hydrogen and oxygen terminated diamond films.
A relationship between the
current density and applied potential has been derived. This agrees with the
experimental results, which show that metallic behaviour predominates in highly
doped diamond samples, while semiconductor behaviour is seen in low doped samples.
The surface state model may
be applied to the AC impedance studies and an expression obtained that predicts
the two time constants that have been seen in the experimental results.
9.8 Intensity Modulated Photocurrent
Spectroscopy
IMPS results have been
obtained for boron doped polycrystalline diamond electrodes. Values for wmax have been obtained for the
Fe2+/3+ redox couple. The values of wmax are independent of DC
potential and the choice of excitation wavelength (470 nm or 430 nm).
The surface state model
developed for the AC impedance of the diamond electrodes has been extended to
cover intensity modulated photocurrents. The results suggest that charge
transfer occurs via a surface state.
9.9 Possible Future Work
This study has shown the electrochemical
behaviour of boron doped diamond to be dependent on the boron doping level. At
low doping levels, semiconductor behaviour has been detected.
Further investigations into
the effect of concentration on the redox species could be performed to test the
theory that has been developed (see section 6.6).
Future work on the
electrochemistry of the films could look at a greater number of redox couples
and electrolyte systems. In particular, the couples could be selected to cover
a greater range of energy levels (as shown in figure 7.1).
In addition to the aqueous
electrochemistry of diamond, the material should prove useful for non-aqueous
electrochemical studies. Preliminary results show the diamond samples to
perform well in organic electrolytes.
Electrochemical studies of
highly oriented CVD diamond samples could provide an insight into the effect of
the polycrystalline nature of the electrodes on the surface state mediated
electron transfer.
Further study into the
photoelectrochemistry of boron doped diamond could be performed. In particular
more photocurrent spectroscopy could be performed to gain an insight into the
surface states. Further IMPS work could be performed using a tuneable laser to
replace the LEDs as the light source.
A number of applications for
diamond electrodes have already been investigated, including: the detection of
trace metals in various electrolytes; and the use of diamond in biological
systems. The use of diamond electrodes for a wide range of electroanalytical
and electrosynthetic reactions could be investigated.
The electrode geometry could
be investigated. This study has only used macroscopically flat electrodes. CVD
diamond may be deposited on a range of substrate geometries and so other
electrode designs could be fabricated.
The diamond deposition
process could be further studied. Surface analysis using techniques such as:
SIMS profiling; Raman and photoluminescence spectroscopy at a range of
different excitation wavelengths; and IR spectroscopy would provide a more
detailed characterisation of the films.
In addition to boron doping
recent work has shown that it may be possible to sulphur dope diamond. Studies could be performed to see if sulphur
doped polycrystalline diamond acts as a n-type semiconductor at low doping
levels. Interesting studies could also
be performed by co-doping diamond with more than one dopant. Electrochemical
studies could investigate diamond doped with varying quantities of boron,
nitrogen and sulphur.
Studies could also
investigate the effect of modifying the surface of the diamond with either
halogens or organic species.