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THESIS DR. MICHAEL DAENEN - SYNOPSIS

"Thin nanocrystalline MW PE CVD diamond films: nucleation, growth, doping and electrical transport phenomena."

Diamond is an interesting material that is under study for many years. Nowadays, while science evolves towards the small scales, also diamond research has found its way into the “nano” world studying so called nanocrystalline diamond (NCD). An overview of the diamond growth history and the outstanding properties is given in the first chapter. Here is also defined what NCD is and where it is positioned in the broad range of carbon materials.


The second chapter gives an overview of all the important experimental techniques that were applied in this research. Experimental setups for surface and structural characterisation, such as scanning electron microscopy, atomic force microscopy and X-ray diffraction are explained. Also techniques for optical measurements are described such as Raman and transmittance spectroscopy. The preparation techniques, used as seeding before diamond growth, are described for different methods that were used. Among these methods there is also a description of the new monodispersed detonation diamond seeding. In this experimental chapter the chemical vapour deposition technique for diamond growth is explained and the different diamond deposition systems are described as well as the magnetron sputtering system. The in situ analysis methods of the deposition process are also included here. The last part of this chapter describes the setup for electrical characterisation of the doped NCD films. This electrical characterisation is carried out in van der Pauw configuration and involves a resistivity measurement and a Hall effect measurement in a cryostat in the range from 80 K to 400 K.


Chapter three shows the results of my developments in the growth process at IMO, discussed in the context of existing techniques and methodologies developed by other colleagues. It describes the results form the different seeding methods that were described in the experimental setup and it shows the progress that was made over the years. Starting of with mechanical seeding using different powders, the seeding method has evolved towards an ultrasound assisted seeding with diamond powder suspensions and finally was optimised by making use of a monodispersed detonation diamond powder which is obtained by high speed bead milling with zirconia beads. Results for these seeding techniques and the early growth on other substrates are shown including also the use of titanium interlayers. The early growth investigations and the nucleation process itself is then analysed further by looking into nucleation from seeds buried under a TiO2 layer. These buried seeding layers were produced by the monodispersed seeding technique in combination with solution-gel spin coating of thin TiO2 layers that cover the seeding layer. High resolution transmission electron microscopy (HRTEM) techniques combined with electron energy loss spectroscopy (EELS) and energy filtered TEM (EFTEM) made it possible to construct a hypothesis for this nucleation process that is based on the diffusion of carbon from the diamond seeds and formation of sp3 nuclei near the growth interface.


When the seeding methods and early growth of thin NCD films are discussed, chapter four gives an overview of the properties of the grown material. First the structural properties of the grown diamond films are presented. These properties were analysed by Raman and GXRD. From these measurements could be concluded that the NCD films are of good quality and consist mainly of sp3 bonded carbon in the form of diamond. In order to measure optical and electrical properties of the films NCD had to be grown on glass or quartz in order to have insulating and transparent substrates. The results for growth on different sorts of glass and quartz are shown together with optical transmission measurement results. The second part of this chapter is dedicated to boron doping of nanocrystalline diamond, so called B-NCD. Boron doped nanocrystalline diamond is interesting from a fundamental point of view, i.e. incorporation of dopants into the small-sized diamond grains and the influence on the electrical transport properties. For this fundamental research the growth of these materials is studied and electrical characterisation of boron doped NCD (B-NCD) films was carried out. These measurements include van der Pauw resistivity and Hall effect data. Also photocurrent spectroscopy on an unintentionally doped sample shows clearly that boron is incorporated in a grown NCD film in substitunional positions in the diamond lattice as an acceptor.


 The last part of chapter four describes the various cooperations for applications based on NCD or B-NCD films. In the field of biosensors, covalent DNA attachment to hydrogen terminated NCD films was obtained and confirmed by fluorescence microscopy and PCR. Later on this work has evolved into detection of hybridisation and denaturation of the attached DNA with target DNA, detected by impedance spectroscopy and eventually single nucleotide polymorphism sensitivity was achieved by using this technique. Also an ion sensitive transducer based on the pH sensitive properties of hydrogen and oxygen terminated NCD surfaces was developed in a electrolyte-diamond-insulator-silicon (EDIS) structure. The pH sensitivity of the structure was tested by ConCap measurements. It is shown then that the developed seeding technique is very versatile and it makes it possible to coat high aspect ratio structures like AFM tips and neural probes. First results in the attempt to coat stents for implantation are presented and show that the seeding technique makes it possible to grow NCD films on these complex structures. The last application that was looked into was in the field of photoelectrochemistry. B-NCD samples were prepared and tested by Prof. Loh et al. as a molecular platform material for photoelectrochemistry. They have shown that the B-NCD films that were grown outperform some of the commonly used materials in the field such as ITO and FTO as platform. Donor-aceptor molecules were attached to the B-NCD surface and higher photocurrents could be obtained and the results were more stable for B-NCD based donor-acceptor attached molecules than on the ITO and FTO transparent contacts.


Finally the conclusions of this work are presented with some outlook onto further research and developments in the different application fields.