Nanoscale Electrical Characterisation of Functional Electrolyte-Gated Transistors by In-Liquid Scanning Dielectric Microscopy—Exploring Different Operating Regimes

Electrolyte-Gated Transistors (EGTs) based on organic materials are a fundamental building block of transistor-based biosensing¹ and bioelectronic² applications. EGT provides efficient interfaces for translating nanoscale biorecognition events into measurable electrical currents; and together with their favourable characteristics like low working bias, stable operation, high sensitivity to interfacial phenomena, mechanical and biological compatibility with living cells and tissues, and ease of fabrication have resulted in a broad coverage of applications spanning multiple domains. However, despite their extensive usage and interest, the transduction mechanism happening at the interfaces and its operation at the nanoscale is unclear and often debatable. It is partially due to the complex physical and chemical phenomena involved at the interface, but also due to the lack of nanoscale electrical techniques that can probe the interfacial processes in a functional device in an electrolyte environment and help correlate with its global response simultaneously. Towards this goal, we have adapted in-Liquid Scanning Dielectric Microscopy (in-Liquid SDM)^3,4 to characterise nanoscale electrical properties (conductivity and interfacial capacitance) of a functional Electrolyte-Gated Organic Field-Effect Transistor (EGOFET)⁵. The local electrostatic force versus gate voltage transfer characteristics obtained correlate perfectly with the macroscale current-voltage transfer characteristics of the device. Our results can also reveal the presence of minute electrical heterogeneities at the semiconductor/electrolyte interface due to the presence of different phases and materials.<br/>Further. we explore different operating regimes of EGOFET as it transitions from linear to saturation, leading to the onset of pinch-off and other high voltage effects like negative transconductance. The generated mapping of electrical properties down to sub-100 nm resolution in the characteristic space of transistor provide vast information that can aid in the fundamental understanding and optimisation of devices and their interfaces. Lastly. in-Liquid SDM is versatile and can be easily translated to the other systems of EGT.<br/><br/>1. Mulla, M. Y., Torsi, L. & Manoli, K. Electronic biosensors based on EGOFETs. in <i>Methods in Enzymology</i> vol. 642 403-433 (Academic Press, 2020).<br/>2. Kyndiah, A. <i>et al.</i> Bioelectronic Recordings of Cardiomyocytes with Accumulation Mode Electrolyte Gated Organic Field Effect Transistors. <i>Biosens. Bioelectron.</i> 150, 111844 (2020).<br/>3. Gramse, G., Edwards, M. A., Fumagalli, L. & Gomila, G. Dynamic electrostatic force microscopy in liquid media. <i>Appl. Phys. Lett.</i> 101, 213108 (2012).<br/>4. Millan-Solsona, R., Checa, M., Fumagalli, L. & Gomila, G. Mapping the capacitance of self-assembled monolayers at metal/electrolyte interfaces at the nanoscale by in-liquid scanning dielectric microscopy. <i>Nanoscale</i> 12, 20658–20668 (2020).<br/>5. Kyndiah, A. <i>et al.</i> Nanoscale Mapping of the Conductivity and Interfacial Capacitance of an Electrolyte-Gated Organic Field-Effect Transistor under Operation. <i>Adv. Funct. Mater.</i> 31, 2008032 (2021).