Electrical and Electrochemical Properties of Nitrogen-Containing Tetrahedral Amorphous Carbon (ta-C) Thin Films

Tetrahedral amorphous carbon (ta-C) is a diamond-like carbon (DLC) material comprised of a mixture of sp2 (∼40%) and sp3-bonded (∼60%) carbon domains. The physicochemical structure and electrochemical properties depend strongly on the sp2/sp3 bonding ratio as well as the incorporation of impurities, such as hydrogen or nitrogen. The ability to grow ta-C films at lower temperatures (25-100 °C) on a wider variety of substrates is a potential advantage of these materials as compared with diamond films.;In this project, the basic structural and electrochemical properties of nitrogen-incorporated ta-C thin films will be discussed. The major goal of this work was to determine if the ta-C:N films exhibit electrochemical properties more closely aligned with those of boron-doped diamond (sp 3 carbon) or glassy carbon (amorphous sp2 carbon). Much like diamond, ta-C:N thin-film electrodes are characterized by a low background voltammetric current, a wide working potential window, relatively rapid electron-transfer kinetics for aqueous redox systems, such as Fe(CN) 6-3/-4 and Ru(NH3)6+3/+2 , and weak adsorption of polar molecules from solution. For example, negligible adsorption of methylene blue was found on the ta-C:N films in contrast to glassy carbon; a surface on which this molecule strongly adsorbs. The film microstructure was studied with x-ray photoelectron microscopy (XPS), visible Raman spectroscopy and electron-energy loss spectroscopy (EELS); all of which revealed the sp2-bonded carbon content increased with increasing nitrogen.;The electrical properties of ta-C:N films were studied by four-point probe resistance measurement and conductive-probe AFM (CP-AFM). The incorporation of nitrogen into ta-C films increased the electrical conductivity primarily by increasing the sp2-bonded carbon content. CP-AFM showed the distribution of the conductive sp2-carbon on the film surface was not uniform. These films have potential to be used in field emission area. The heterogeneous electrochemical properties were studied with scanning electron microscopy (SECM). If was found that more electrically conducting sites were isolated by less conducting or even insulating sites. Consistent with the heterogeneous electrical properties, heterogeneous electrochemical activity was observed for different aqueous redox analytes.;Flow injection analysis with electrochemical detection (EC-FIA) of dopamine and norepinephrine were carried out using the N-ta:C films. Detection figures of merit for these biomolecules were determined. These biomolecules can be detected stably and reproducible in PBS buffer at constant potential by N-ta:C film. The N-ta:C films provide good limits of detection for these biomolecules.;N-ta:C films show great promise in the electroanalytical field. Compared with the diamond, the growth condition is much milder. These N-ta:C films are commercially available and could be produced in large scale. All of these virtues make N-ta:C films excellent choices for electroanalytical measurements.