Nina Baule¹, Pramod Gupta¹, Lars Haubold¹, Davit Galstyan¹, James Siegenthaler^1,2
1Fraunhofer USA Center Midwest, East Lansing, MI
²Michigan State University, East Lansing, MI
Carbon-based electrochemical sensors for aqueous solutions are typically screen-printed on rigid and flexible substrates. However, most of those electrode materials are poorly suited for non-aqueous solvent electrochemistry due to their binder system. This limitation spurs interest in exploring alternative carbon-based electrode materials. While boron-doped diamond (BDD) electrodes are inert to such conditions, high processing temperatures and complex scalability make them cost prohibitive for low-cost disposable sensing applications. Addressing this, we investigate the viability of a nitrogen-incorporated tetrahedral amorphous carbon (ta-C:N) as an effective solution. Ta-C:N is a highly sp³-bonded carbon n-type semiconductor with electrochemical properties comparable to BDD such as low background current and noise, and good microstructural stability at positive detection potentials. Ta-C:N thin films are synthesized by physical vapor deposition (PVD). As a result, low processing temperature, commercially available, industrial-scale systems and processes are available for potential roll-to-roll production. Here, we have developed and fabricated a ta-C:N 3-in-1 style electrochemical sensor on a rigid silicon substrate. The electrode configuration included a ta-C:N based counter, working, and reference electrode. The amorphous carbon was directly deposited onto the substrate by laser controlled pulsed cathodic vacuum arc (Laser-Arc) and patterned by a standard lift-off photolithography process. While evaluating the same electrode material on flexible polyimide substrates, we observed cracking and delamination after initial electrochemical testing. As a result, further surface engineering was required to withstand such conditions. Hence, this study also investigates different substrate pretreatments and/or interlayers for ta-C:N on polyimide and their effect on the electrical and electrochemical performance of the functional ta-C:N top layer compared to ta-C:N on conductive silicon.