Piezoelectricity and flexoelectricity in 2D transition metal dichalcogenides
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Two-dimensional materials are only on to a few atoms thick, making them the thinnest possible material known to man. Their combination of electrical, optical, and mechanical properties allows for unique electrical devices with a wide range of future applications, from being a post-silicon material option, creating high-speed communication systems, allowing the advancement of flexible electronics, and even creating transparent electronics. Among their amazing characteristics is the coupling of electrical and mechanical properties. Although not unique to 2D materials, electromechanical coupling could be used in 2D materials to create a class of sensors, actuators, and energy harvesters at a scale not previously possible. Specifically, 2D materials could be utilized in flexible, wearable electronics as an energy harvester to convert the motion of the body into electrical energy. In this dissertation, the electromechanical coupling properties known as piezoelectricity and flexoelectricity are studied in 2D materials both to advance the development of 2D materials in general, and to improve the understanding of the relatively novel effect of flexoelectricity. This work focuses on a class of 2D materials known as transition metal dichalcogenides (TMDs), which are semiconducting and intrinsically piezoelectric. To begin, the adhesion between the TMDs and soft substrates is studied. Soft substrates could be used in flexible and wearable electronic systems, so adhesion of TMDs to soft substrates is important. It was found that the adhesions between the TMD molybdenum disulfide and polydimethylsiloxane is roughly 18 mJ m⁻². Next, the out-of-plane electromechanical coupling of molybdenum disulfide and other TMDs was studied. Piezoelectric theory predicts that there should be zero out-of-plane response, but a signal is measured in all TMDs, suggesting the presence of flexoelectricity. The measured effective out-of-plane piezoelectric response is on the order of 1 pm V⁻¹ and the estimated flexoelectric response is on the order of 0.05 nC m⁻¹. Additionally, it was found that the magnitude of the out-of-plane electromechanical response of different TMDs roughly follows a trend predicted by a simple model of flexoelectricity. The work presented in this dissertation provides the first experimental evidence of a flexoelectric effect present in 2D TMDs.