Stability of thiol-based self-assembled monolayer functionalized electrodes in EG-OFET-based applications

The surface passivation of thermally deposited Au using n-alkanethiol self-assembled monolayers (SAMs) as insulating monolayers in electrolyte-gated organic-field effect transistor (EG-OFET) and its impact on EG-OFET operation has been clarified. We used three different n-alkanethiols derivates, namely propanethiol (P-SAM), hexanethiol (H-SAM), and octanethiol (O-SAM), with different chain lengths (C₃, C₆ and C₈). The non-uniform distribution of the used SAMs on the inhomogeneous Au surface significantly affects the net capacitance, i.e., the serial capacitance of the double layer (C_DL) and blocking layer (C_BL) capacitance of the functionalized gate. The SAM-functionalized gates were exposed to cyclic electrical stress (forward and reverse) for 128 cycles with gate voltage (V_G) sweep from 0.1 to −0.4 V at constant drain voltage (V_D = -0.4 V) and compared to a bulk-gold reference gate electrode. The registered transfer curves showed increased drain currents that saturated during prolonged cycling. Two figures of merit, i.e., threshold voltage (V_th) and hysteresis, were extracted from the recorded transfer curves, and their responses were studied separately. We found that both V_th and hysteresis increase with cycling. The change is small but constantly increases for the short-chain P-SAM, while for the longer-chain H- and O-SAM, the initial change is more prominent, reaching saturation after approximately 25 cycles. We have investigated the surface roughness of different gate electrodes through Atomic Force Microscopy (AFM) to confirm the packing density of thiol molecules. We also performed 80 h long-term stability data using cyclic voltammetry measurements for each thiol-functionalized electrode. No signs of desorption could be found, as evidenced by XPS. The results are consistent with previously suggested models for electrical transport across such SAMs, confirming that these monolayers restrict faradic processes at the electrodes by passivating the Au surface.