Numerical investigation of carbon nanotube thin-film composites and devices
Gupta, Man Prakash
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Carbon nanotubes (CNTs) are known for their exceptional electrical, thermal, mechanical, optical, and chemical properties. With the significant progress in recent years on synthesis, purification and integration challenges, CNT network/array based thin-film transistors (TFTs) are likely to play a critical role as the building blocks of future electronics. CNT-TFTs can find applications in flexible, transparent and energy-efficient circuits, e-displays, solar cells, RFID tags, e-paper, touch screens, implantable medical devices and chemical/bio/optical sensors. CNTs in CNT-TFTs are deposited on low thermal conductivity substrates which can impede the heat dissipation resulting in high temperature. The excessive self-heating in CNT-TFTs can degrade the electrical and thermal performance and could potentially lead to failure of the devices. Therefore, the issues related to operational reliability of CNT-TFTs arising from the self-heating effects need to be examined and studied. In the present work, a computational approach is developed and employed to study the electrical and thermal transport in CNT-TFTs. The modeling framework can predict the current and temperature profile of CNT network/array and the supporting structure. The model is validated against the experimental results. In case of CNT network TFTs, the computational method allows us to examine the role of various device parameters such as network morphology (i.e., network density, CNT junction topology, and CNT length and alignment distribution) and channel geometry (i.e., channel length and width) on heat dissipation and thermal reliability. The simulation results help interpret experimental data and provide the quantitative information about the thermal boundary conductances at CNT junctions and CNT-substrate interfaces in CNT-TFTs. The findings suggest that the structure of CNT junctions on substrate can become very critical in CNT network TFTs as the lack of contact with the substrate at these junctions can lead to junction temperatures hundreds of degrees higher than the rest of the device, which will severely deteriorate the performance of these devices. High-field breakdown study of CNT network TFTs is also conducted which provides guidelines for the design and optimization with respect to aforementioned parameters in order to enhance the performance and reliability. Dense CNT arrays are preferred for better electrical performance in CNT array TFTs, but they also experience electrostatic and thermal cross-talk which can adversely affect the device performance. These effects have been studied in details. The role of trap charges in CNT array TFTs is also investigated to understand and mitigate hysteresis. Lastly, CNT-liquid crystal composites are studied using dissipative particle dynamics (DPD) technique with the aim to understand how the CNT concentration in composite affects the alignment of liquid crystals and to explore the method of CNT alignment using liquid crystals.