Nanoscale Multimodal Characterization of Operating Electrolyte-Gated Transistors

Abstract

[eng] Electrolyte-gated transistors (EGTs) have emerged as key platforms for transducing and amplifying biological and biochemical signals, making them an integral part of diverse biosensing and bioelectronic applications ranging from single molecule biosensors to neuromorphic devices. Despite being the foundational architecture, the fundamental understanding of the nanoscale electronic and ionic transport governing the device operation remains poor, which hinders further progress in the rational and targeted optimization of devices for various applications. The limitation mainly stems from the lack of characterization methods that can probe ionic and electronic transport processes in operating devices at the nanoscale under electrolyte environments and in application-relevant conditions. Further complexity arises due to diverse molecular design of organic semiconductors that expresses wide variety of coupled electrical and mechanical behaviours. In this thesis, we developed an advanced multimodal characterization method based on in-liquid Scanning Dielectric Microscopy (SDM) that simultaneously probes relevant electrical and mechanical properties at the nanoscale in functional EGTs. The presented method significantly advances an earlier implementation of in-liquid SDM for the same purpose by adding automated data acquisition and analysis functionalities along with technical improvements for the comprehensive characterization of EGTs. We introduced a straightforward and robust approach for data interpretation and representation that effectively eliminated experimental artefacts and the calibration procedures, enabling rapid and accurate analysis. The approach also enabled quantification of the local electric potential directly from raw experimental data, thereby providing direct access to fundamental charge transport parameters such as contact access resistances, inter-and intra-domain charge transport, and anisotropy, which weren't accessible earlier at the nanoscale in operating EGTs. We studied two varieties of EGTs, namely Electrolyte-Gated Organic Field-Effect Transistors (EGOFETs) and Organic Electrochemical Transistors (OECTs), which exhibit distinct operating mechanisms. We first focused on EGOFETs based on a blend of organic semiconducting material diF-TES-ADT and the insulating polymer polystyrene (PS), where we investigated the local electrical properties in various operating regimes, namely sub-threshold, linear and saturation. This investigation reveals that the pinch-off characteristics are dependent on the microstructural property of the organic semiconducting blend, which controls the extension of the pinch-off into the channel region. We also investigated regions consisting of different structural features and their influence on the evolution of local electrical properties, thereby identifying the attributes that lead to inferior charge transport characteristics. Using the developed electric potential mapping mode, we quantified the impact of grain boundaries and contact access resistances and observed that the intrinsic conductivity of individual grains can be as high as 40S/m, whereas the effective device conductivity is a factor of 10 lower, indicating serious charge transport bottlenecks. Therefore, focusing on contact and grain boundary interface engineering would potentially enable reaching electrical characteristics close to that of individual crystalline domains. After that, we focused on OECTs based on ladder-type polymer BBL, where we investigated the coupled electrical and mechanical behaviour across different operating regimes by utilizing the multimodal character of our in-liquid SDM implementation. The spatial variability in the mechanical stiffness in response to applied drain potentials is correlated with the corresponding local electrical properties. A peculiar sudden phase change in mechanical response is observed that coincides with the maximum (positive or negative) transconductance points of the device, where an influx of ions into the polymer affirmed with a simultaneous increase of the effective interfacial capacitance happens. The local electrical mapping is also used to explain device current-voltage characteristics at high doping conditions in the negative transconductance regime. In a nutshell, the thesis provides a detailed understanding of the operating mechanism in a variety of EGTs through a multimodal nanoscale perspective and lays the foundation for further advanced studies.

[spa] Los transistores con dieléctricos líquidos (EGTs) son plataformas bioelectrónicas emergentes que acoplan eficientemente los procesos de transporte iónico y electrónico para permitir tecnologías clave que van desde biosensores electrónicos ultra sensibles hasta dispositivos neuromórficos avanzados. La presencia de un medio electrolítico facilita el acoplamiento iónico-electrónico, aunque, al mismo tiempo, confiere una gran complejidad en la comprensión de la física de estos dispositivos debido a los efectos físicos asociados, como el hinchamiento y ablandamiento de los materiales. En consecuencia, existe una demanda de técnicas avanzadas de caracterización capaces de sondear simultáneamente múltiples propiedades físicas, incluyendo la morfología, el comportamiento mecánico y las propiedades eléctricas, en dispositivos EGT en funcionamiento. Sin embargo, los métodos existentes generalmente se han limitado a investigar una propiedad a la vez, y la disponibilidad de herramientas multimodales, particularmente a escala nanométrica, es escasa. La presente tesis aborda precisamente esta brecha crítica y se centra en el desarrollo de métodos de caracterización multimodales basados en la microscopía de sonda de barrido para sondear simultáneamente las propiedades morfológicas, mecánicas y eléctricas a escala nanométrica en EGTs en funcionamiento. La tesis examina específicamente dos clases de EGTs: los transistores de efecto de campo orgánicos con dieléctricos líquidos (EGOFETs) y los transistores electroquímicos orgánicos (OECTs), que exhiben comportamientos eléctricos y mecánicos acoplados distintos. Además de las importantes contribuciones instrumentales y metodológicas, la tesis ofrece nuevos conocimientos sobre la física de los dispositivos EGOFETs y OECTs. En el caso de los EGOFETs, se dilucida el impacto de la policristalinidad y la anisotropía en las propiedades de transporte de carga, se explora el papel de las resistencias de acceso en serie, y se identifican características únicas de cierre en relación con la firma microestructural del semiconductor orgánico. En el caso de los OECTs, el enfoque se centra en el comportamiento eléctrico y mecánico acoplado, donde se investigan cambios de fase peculiares en puntos característicos de funcionamiento del dispositivo, estableciendo correlaciones entre la conductividad, el hinchamiento y el ablandamiento del polímero. A través de estos esfuerzos, la tesis amplía la comprensión fundamental de los EGTs y sienta las bases para futuros estudios avanzados.