Harnessing mechanical deformation : micro/nano-structured materials for optoelectronic and sensing applications

Abstract

This dissertation is comprised of three main sections:(1) the development of a new strain engineering method to introduce spatially varying and controllable strain to two-dimensional (2D) materials and its application in optoelectronics; (2) the integration of nanostructures into indium tin oxide (ITO) for enhanced flexibility ; (3) the fabrication of microneedle electrodes (MNEs) to realize accu- rate and imperceptible long-term monitoring of electrophysiological signals.

The first section demonstrates a controllable and deterministic method for introducing strain to 2D materials using the nanoimprint-induced strain engineering (NISE) technique. By thermally imprinting a monolayer of molybdenum disulfide (MoS2) encapsulated in polyvinyl alcohol (PVA) onto an imprint mold, the method allows the generation of spatially varying and controllable strain patterns in MoS2. The dimensions of the mold and imprint pressure can be precisely designed to introduce different levels of strain magnitudes to the 2D materials. The strain profile is maintained by conforming the 2D layer to the underlying substrate, making it appropriate for practical applications. This method can also be extended to other 2D materials, which paves a new path for the use of strain engineering to tailor 2D materials’ properties. Employing the NISE method, a strain-engineered MoS2 photodetector is fabricated. A shadow mask deposition technique is developed to achieve contamination- and damage-free, and high-resolution metal contact fabrication. Benefiting from controllable strain via NISE, the strained MoS2 devices after NISE show improved photoresponsivity, photocurrent, and detection capability, which could be attributed to the strain-tuned band structure.

The second section elaborates on a simple two-step method for creating highly efficient and flexible ITO electrodes with sub-micron structures. The nanostructured ITO (nano-ITO) is realized using nanoimprint lithography and magnetron sputtering. A thorough experimental study is conducted to demonstrate the excellent flexibility and durability of the nano-ITO even after 2.5% bending strain and 2000 bending cycles. Additionally, numerical simulations are performed to reveal the critical role of nanostructures in the redistribution of stress before crack initiation, the guidance of crack onset positions, and the retardation of crack propagation. Finally, the potential application of nano-ITO in flexible electronics is exploited by fabricating a flexible alternative current electrolumi- nescent display (ACELD) using the flexible nano-ITO electrodes.

The third part presents a compelling and facile method for producing highly conductive, flexible, and ultra-thin MNEs using a three-dimensional (3D) electrodeposition technique. The fabrication process mainly involves magnetron sputtering of ITO on microneedle mold and electrodeposition of metal layer, which is cost-effective for high-volume fabrication. The ultra-thin MNEs exhibit superior mechano-electrical stability, biocompatibility, and comfortability. Compared to the electroplated planar electrodes and commercial Ag/AgCl electrodes, these MNEs have a lower electrode-skin interface impedance (EII) due to their increased surface area and conformal contact with the skin. Furthermore, the MNEs have demonstrated accurate and non-invasive detection capabilities for electrophysiological signals such as EMG and ECG, with a higher signal-to-noise ratio (SNR) than the planar and commercial electrodes, enabling long-term healthcare monitoring and human-robot interaction.