Microstructures And Properties Of Different Surface-terminated Boron-doped Diamond Films

Boron-doped diamond film is a perspective material with perfect physical and chemical properties. Some electronic and electrochemical devices have been developed by use of it. Hydrogen-terminated and oxygen-terminated boron-doped diamond films exhibit different electronic and electrochemical behaviors, which affect the developed devices dramatically. So, it is important to elucidate the microstructures, properties and conducting mechanisms of the two surface-terminated boron-doped diamond films. This research field is a frontier of the surface science and the application of carbon materials.By means of experiment and first principles methods, the electronic structures and electrochemical behaviors of the two surface-terminated boron-doped diamond films were investigated. The results can be divided into two parts. On the aspect of experimental study: Firstly, by means of X-ray photoelectron spectroscopy and scanning probe microscopy, the detailed electronic structures of hydrogen-terminated and oxygen-terminated diamond films were obtained. The results indicated that for the hydrogen-terminated surface, it exhibits a p-type conductivity with upward surface energy band bending and, there are empty states near the valence band maximum in band gap. In the case of oxygen-terminated one, it exhibits an insulator with downward surface band bending and, there is a wide and clean band gap; Secondly, using cycle voltammetry and AC impedance spectroscopy, the clear structures of the electrode/solution interfaces were measured. It indicated that the electrochemical window corresponding to the hydrogen-terminated boron-doped diamond film electrode is narrower and its film resistance and capacitance are negligible, exhibiting a metal-like character. While, the oxygen-terminated one has a high film resistance and capacitance; Thirdly, by means of their electronic structures, the electrochemical behaviors of the two surface-terminated boron-doped diamond film electrodes were interpreted. The above results are beneficial to the development of the electronic and electrochemical devices with high quality according to the different properties of different surface-terminated boron-doped diamond films. On the aspect of theoretical study: Firstly, according to the reaction heat and the lowest total energy, the most stable geometries of the surface chemisorbed hydrogen, boron and hydrogen atoms in the subsurface were deduced. The results showed that the most stable structure of the surface chemisorbed hydrogen is mono-hydrogenated 2×1 reconstructed surface. In the subsurface, the boron atoms substitute the carbon atoms and the hydrogen atoms exist at the bond-centered site between a boron atom and a carbon atom, or two carbon atoms; Secondly, the different conducting mechanisms corresponding to different electronic performances were discussed. The results indicated that the surface chemisorbed hydrogen atoms are necessary, but insufficient for a p-type conductive layer. It is also for the hydrogenated diamond films to expose to the air to adsorb the activated molecules. Moreover, when the density of hydrogen atom is comparable to that of the boron atom, the diamond surface exhibits an insulator. When the density of hydrogen atom is two times higher than that of boron atom, the diamond surface exhibits a n-type conductivity. It is impossible for the hydrogen atoms in the subsurface to act as shallow acceptors. These results are useful to improve proper surface-treatment methods to obtain the boron-doped diamond films with good surface properties.