Chapter 10


Final Conclusions


10.1. Introduction


The electrochemistry of polycrystalline diamond films has been described in detail in these studies. The growing process has been reviewed and characterisation techniques applied to fully describe the samples. To employ diamond films as electrical material the electrical contact is important, development of new contacts is described. Theories have been reviewed to explain the properties of diamond as electrode material. Using simple electrochemical techniques, diamond energy levels and surface structure has been investigated. Finally, diamond films have been employed as heat sinks in the effort of improving and simplifying impedance and ac voltammetric studies. A large amount of results have been obtained. The aim of this chapter is to emphasise the most important conclusions from each step of this fully investigation. Conclusions are presented in the order established in these studies.


10.2. Background


Diamond shows exceptional properties, these have been exploited since simple and inexpensive synthesis methods were achieved. Some applications for diamond are already in the market. A brief review of diamond electrochemistry shows that the majority of studies are related to highly boron doped diamond electrodes in aqueous media. Semiconductor behaviour has not been reported. No full characterisation has been performed for diamond when is used as a heat sink. This strongly suggests a line of research during the studies of the thesis that investigates the diamond electrode behaviour in non-aqueous media and heat sink properties.


10.3. Diamond growth and characterisation


The diamond samples were provided by Dr Latto and Dr May. A brief review of the literature on the diamond synthesis has been presented. Moreover, the system employed to grow diamond has been fully detailed. Different characterisation techniques have been employed to determine the nature and features of the diamond samples provided.


10.4. Electrical contacts to semiconducting diamond


Any material suitable for electrical applications requires an Ohmic contact. Different types of electrical contact have been detailed for diamond. Highly boron doped diamond does not require any special treatment to reach an Ohmic contact. However, moderately boron doped samples require specific electrical contacts. A titanium underlayer contact, not described before in the literature, appears to be the quickest to fabricate and most reliable of the electrical contacts studied.


10.5. Electrochemical theory for diamond electrodes


After a brief review of the standard theories for metals or semiconductors, theory of diamond electrochemistry requires is developed. A new model based on surface states is established that explains the semiconductor behaviour for low doped diamond samples and metal behaviour for highly doped samples. AC impedance theory is developed as an instrument to study in further detail the characteristics of the space charge layer (that shows experimentally the semiconductor properties).


10.6. Electrochemical studies of moderately boron doped diamond on non aqueous electrolyte


Studies of moderately boron doped diamond in non aqueous solvents emphasise the influence of the surface termination in diamond electrochemistry. Surface termination determines the position of the band edges. The shift between hydrogen and oxygen band edges is suggested to be approximately 2.3 V. Graphitic surface states appear to mediate the charge transfer. For a hydrogen terminated diamond surface these electrochemical studies showed features of a non degenerately doped p-type semiconductive material, to the author’s knowledge this is the first time such behaviour is observed for a non-oxidised diamond electrode. Cyclic voltammetries demonstrated that only redox couples of comparable energy to the graphitic surface states show reversible behaviour for oxygen terminated diamond surface samples.


10.7. The influence of the surface preparation on the electrochemistry of boron doped diamond


Benzoquinone can be reduced and oxidised using boron doped diamond electrodes in non aqueous media. The mechanism changes depending on the surface termination of the diamond electrode. When the surface electrode is oxygen terminated the electrochemistry is similar to the one observed at glassy carbon, i.e., two electron reversible reduction processes are observed. However, for hydrogen terminated surface, the electrochemistry points out the presence of a source of protons. This fact suggests that subsurface hydrogen may be implicated in the reduction mechanism. Rigorous tests showed that the solution was not contaminated.


10.8. Photocurrent measurements: a method to characterise surface states in CVD diamond   


Photostudies of highly conductive boron doped diamond in aqueous solution suggest the presence of surface states. The estimated energy for the surface states does not correspond with graphitic states as frequently is reported. A possible answer can be found in boron impurities included during the growth stage.


10.9. Temperature modulated open circuit potential spectroscopy


Using diamond as a heat sink allows the design of a new technique based on relaxation methods (more specifically T-jump) to perform impedance measurements in a simple way and at low cost experiment. This simplicity avoids the limitations of the more sophisticated instruments permitting a wider range of frequencies to be studied by impedance measurements. 


10.10. Temperature modulated ac voltammetry


A new technique has been developed based on the principles of temperature modulation open circuit spectroscopy to perform ac voltammetric measurements. Due to its simplicity, this technique avoids the traditional limitation of more sophisticated equipment and allows ac voltammetries studies in a wide range of frequencies.


10.11. Summary


Diamond displays outstanding properties that make it a very attractive material for many applications. After the development of the CVD methods diamond has begun to be used. Some commercial applications were marketed. However since the first electrochemistry paper on diamond, many researches around the world made an effort to characterise in detail the electrochemical properties of diamond. To the worldwide activity these studies contribute:


·        successful electrochemical theory that indicates surface states give rise to the metal behaviour frequently reported and the semiconductor behaviour rarely reported,


·        electrochemical studies in non aqueous media that allow the emphasis of the dramatic influence of the surface termination for diamond electrodes, in particular non–degenerately doped p-type semiconductor behaviour for low boron doped hydrogen terminated diamond surfaces,


·        significant influence of the surface states in low boron doped oxidised diamond,


·        possible presence of boron impurities that generate surface states or the inner layer hydrogen layer that are important when an electrical contact is provided to diamond but can modify the chemistry at the diamond electrodes,

·        new techniques using diamond as a heat sink to perform impedance measurements and ac voltammetries over a wide range of frequencies.


These studies are just a drop in an immense ocean. Future investigations are required to further characterise the extraordinary properties of diamond electrodes.


10.12. Future work


Further research studies are required to continue on the routes suggested in this thesis:


·        different experiments are required to detail properly the new theory explained,


·        more cyclic voltammograms using different redox couples to characterise in more detail the electronic diamond structure as oxygen or hydrogen terminated,


·        additional analytical techniques (SIMS, XPS, etc) can be used to give more and detailed description of how the changes in the surface termination modify the properties of this electrode material,


·        a much powerful source of energy can be used in the temocps to amplified the signal and allow wider frequency ranges to be explored. Thicker diamond layers will permit magnification of the signal from the gold electrode.