Resolving the controversy on the pH sensitivity of diamond surfaces

The pH sensitivity of the surface conductivity of H-terminated diamond has been the subject of great controversy, regarding not only its absolute value but even its sign. In this contribution the effect of the reference electrode on the pH sensitivity is investigated. Thus, the pH response of diamond solution-gate field effect transistors using Pt and Ag/AgCl reference electrodes has been measured. We demonstrate that the Pt electrode potential is itself pH dependent and over-compensates the pH sensitivity of the surface conductivity, which alters significantly the interpretation of the pH sensitivity experiments. If this effect is correctly taken into account the apparent controversy is resolved. The experimental results confirm that the pH sensitivity of the H-terminated diamond surface conductivity does not follow the prediction of the transfer doping model.