Insulating, semiconducting and metallic 2D materials for flexible electronics

Thesis

Two-dimensional (2D) materials have enjoyed significant attention in recent years as new materials for use in flexible electronics. Combining unique electronic properties with unparalleled strain resilience and transparency, they represent ideal candidates in a broad range of different applications. However, though significant progress has been made this field is still in its relative infancy and there is still much to discover. This project details aspects of the synthesis, fabrication, and physical processes and mechanisms that are pertinent to flexible electronics, with a focus on a subset of three materials: metallic graphene, semiconducting tungsten disulphide, and insulating boron nitride. All materials were synthesised by chemical vapour deposition.

'Bulk' heterostructures of the above materials in several different permutations were fabricated on flexible polymer substrates and studied during repeated strain cycling. A mechanism by which strain is accommodated by inhomogeneous debonding from the substrate and a time-dependent relaxation effect were identified and studied. With repeated strain cycling the response of the heterostructures was found to stabilise—an encouraging result for future device work.

Subsequent work explores two disparate flexible devices based on 2D materials. The first of these is an array of flexible photodetectors. Polymer substrates impose severe limitations on processing conditions and necessitated significant modification to existing fabrication techniques. Functional devices were demonstrated and their response to strain studied, revealing a transient enhancement of sensitivity followed by permanent failure. The mechanism behind this effect was explored.

The second device is a MRI and CT compatible cardio-respiratory monitor for use in preclinical imaging—a device which represents a significant improvement over existing technology. Its properties were studied in detail and its functionality confirmed through extensive in vivo testing, and a patent has since been filed. Several prototypes are already in use at the Oxford Institute for Radiation Oncology.

Authors: Tweedie, MEP

• DOI: 10.5287/ora-zvmyp1yx8
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Transition Metal Dichalcogenides; Transparent Electronics; Graphene; Friction; Thin Film; Flexible Electronics; Heterostructure; Two Dimensional Materials; Tungsten Disulphide; MRI; 2D Materials; Photodetector; Spectroscopy
• Subjects: Nanomaterials; Materials Science