Hole doping can control the conductivity of diamond either through boron substitution, or carrier accumulation in a field-effect transistor. In this work, we combine the two methods to investigate the insulator-to-metal transition at the surface of nanocrystalline diamond films. The finite boron doping strongly increases the maximum hole density which can be induced electrostatically with respect to intrinsic diamond. The ionic gate pushes the conductivity of the film surface away from the variable-range hopping regime and into the quantum critical regime. However, the combination of the strong intrinsic surface disorder due to a non-negligible surface roughness, and the introduction of extra scattering centers by the ionic gate, prevents the surface accumulation layer to reach the metallic regime.

Nanocrystalline diamond is an excellent material for the fabrication of Multi Electrode Arrays used to monitor the activity of biological cells and tissues. Yet, the overall performances in terms of background noise, electrochemical activity and transparency for fluorescence detection, are difficult to optimize. The aim of this study is to obtain an orientative guide on how to reach a good compromise between the competing properties. For this purpose, several samples of diamond films were produced under a variety of synthesis conditions, to be employed in the fabrication of amperometric Multi Electrode Arrays. After the fabrication, the samples were characterized from a structural, electrical, electrochemical and optical point of view in order to find possible correlations with the parameters adopted in their production.