Effect of Power Density on the Electrochemical Properties of Undoped Amorphous Carbon (a-C) Thin Films

Undoped a-C thin films were deposited with varying power density from 10 to 25 W/cm² using unbalanced closed-field magnetron sputtering (CFUBMS). The effect of power density on the physical and electrochemical properties was investigated by experimental characterization methods and atomistic simulations. XPS indicated that the films were composed mostly of sp²-bonded carbon (55–58 at.%) with a small amount of oxygen (8–9 at.%) in the surface region. The films appeared completely amorphous in XRD. The I_D/I_G ratio obtained by Raman spectroscopy indicated an increase from 1.76 to 2.34 with power density. The experimental and simulated data suggested a possible ordering and/or clustering of the sp² phase with power density as the cause of the improved electrical properties of the a-C films. The electrochemical properties of a-C were between those of glassy carbon and tetrahedral amorphous carbon with potential windows ranging from 2.77 to 2.93 V and double-layer capacitance values around 0.90 μF cm⁻². Electron transfer for Ru(NH₃)₆ ^3+/2+ and FcMeOH^+1/0 was reversible whereas that for IrCl₆ ^2−/3− was quasi-reversible. Peak potential separation of dopamine and oxidation potential of ascorbic acid decreased with power density, correlating with the structural and electrical changes of the films. The a-C thin films deposited by CFUBMS are inherently conductive and their physical properties can be adjusted by varying the deposition parameters to a wide range of electrochemical applications.