Spatially Heterogeneous Electrical and Electrochemical Properties of Hydrogen-Terminated Boron-Doped Nanocrystalline Diamond Thin Film Deposited from an Argon-Rich CH₄/H₂/Ar/B₂H₆ Source Gas Mixture

Spatially heterogeneous electrical and electrochemical properties were observed for hydrogen-terminated, boron-doped nanocrystalline diamond thin-film electrodes by conducting-probe atomic force microscopy (CP-AFM) and scanning electrochemical microscopy (SECM). CP-AFM was used to simultaneously map the topography and electrical conductivity of the film as a function of the boron-doping level and bias voltage. The electrode was characterized by areas of high electrical conductivity separated by regions of low conductivity. The fraction of a particular film's area that exhibited high conductivity remained invariant with the polarity and magnitude of the bias voltage, while the current flow at any of the conductive regions increased proportionally with the bias voltage indicative of metal-like electronic properties. The fraction of highly conductive area increased with the doping level. One possible cause for the differential conductivity across a film is a nonuniform distribution of boron dopant, which leads to a nonuniform charge carrier concentration. SECM was also used to spatially interrogate the electrochemical activity of the diamond film using three redox systems: Ru(NH 3 ) 6 +3/+2, Fe(CN) 6 3-/4-, and IrCl 6 2-/3- . Isolated regions of high electrochemical activity were found for all three systems, and interestingly, the active regions on a given thin-film electrode were different for each. It is concluded that current flow through this doped nanocrystalline film occurs through a fixed number of conductive sites and all of these sites appear to be electrochemically active for the redox systems tested. The results have important implications for understanding how boron-doped diamond functions as an electrochemical electrode.