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

Abstract

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(NH3)6 +3/+2, Fe(CN)6 3-/4-, and IrCl6 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.

Boron-doped diamond electrode: synthesis, characterization, functionalization and analytical applications

“…BDD materials are electrically heterogeneous with conductivity and electron-transfer rates varying across the BDD surface. This behavior is also confirmed by microscopic characterization of conductivity and redox activity for both hydrogen- 90 . The extent to which the physicochemical properties of glassy carbon (sp 2 ) and diamond (sp 3 ) carbon electrodes influence the response depends on the mechanistic aspects for the particular redox analyte.…”

Local Imaging of an Electrochemical Active/Inactive Region on a Conductive Carbon Surface by Using Scanning Electrochemical Microscopy

“…It has been reported that the current distribution is observed in SG-TC images for a BDD surface with a Ru(NH3)6 3+ mediator. [5][6][7] Our result is clearly different from the previously reported SECM images for a BDD substrate, because these current distributions for BDD substrates mainly reflect the bulk conductivity distribution, which is caused by the hole density difference derived from the boron concentration. Figure 5(C) indicates the results of line scanning, which results from changing the substrate potential by using 1 mM of the Fe 2+ mediator.…”

“…3,4 This SECM technique has recently been applied to the imaging of local electrochemical activity on carbon materials, such as borondoped diamond (BDD). [5][6][7] The Bard and Swain groups have reported the local electrochemical activity on low boron-doped diamond electrodes attributed to a crystal region and a grain boundary layer. 5 With BDD, the density of holes, which are major charge carriers in BDD, is different for each crystal facet because the boron uptake depends on the crystal growth facets.…”

The Use of Conducting Diamond in Electrochemistry

“…It was postulated that the response could be dominated by a network of electrochemically active NDC inclusions, most probably residing at grain boundaries, within a less-active diamond matrix [100]. Alternate theories questioned the role boron doping density and local DOS played in controlling ET in BDD [71,118]. However, pioneering work with single crystals and electrochemical imaging techniques, the latter where it is possible to probe the electrochemical response on a grain by grain basis, has now irrefutably demonstrated that BDD itself is capable of transferring electrons, albeit at a slightly slower rate (for an outer-sphere redox species) compared to a classical metal electrode.…”

“…SECM SG-TC images (tip size 2 μm) and C-AFM images were also recorded on H-terminated NC BDD thin-film electrodes, grain size 15-20 nm, of different dopant densities in the range of approximately 0 to mid-10 20 B atoms cm −3 [118]. For the undoped NC electrodes, C-AFM revealed a number of conducting pathways, attributed to NDC content at grain boundaries, a conclusion also reached by Figure 5.14 (a) Schematic illustrating SECM SG-TC mode.…”