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Title: Atomic-scale study of electronic defects and ambient degradation in Van der Waals layered black phosphorus
Author: Wentink, Mark
ISNI:       0000 0004 9359 1064
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2020
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Van der Waals heterostructures form a rapidly expanding field of study in physics, chemistry, material science and engineering alike. More and more exfoliable materials are discovered and the techniques to take them apart and build them back up are continuously refined. As stacking techniques become increasingly precise and reliable, potential applications of heterostructures in electronics and opto-electronics are unlocked. Within Van der Waals materials, black phosphorus has the valuable combination of a direct, tunable bandgap and a high charge carrier mobility, making it a good candidate for heterostructure devices. On its own however, black phosphorus suffers from degradation in ambient conditions and a much lower mobility than predicted. Blue phosphorus, a high-pressure allotrope, shows similar promise with further opportunities as precursor to phosphorus nanoribbons and nanotubes, but is predicted to suffer from the same problems. For the materials to be success in nanostructure applications, both these shortcomings will need to be addressed. In the following thesis, I set out to offer a well-rounded study of the properties and structure of black phosphorus in ambient conditions as well as a close examination of apparently inherent point defects at the atomic scale. The ambient degradation of black phosphorus was found to slow down significantly in a low-humidity environment, corroborating the theory that ambient degradation is a multi-step process involving oxygen and water. Chemical analysis of sample produced through both common synthesis pathways showed low concentrations of tin and iodine impurities. Atomic scale defects were studied both through STM and NC-AFM. The dominant defect has a characteristic double-lobed shape. DFT and Tight-binding calculations of tin substitutions are in good agreement with our STM and STS experiments, and find the double lobed shape to arise from a series of hydrogen-like states hosted in a potential well around the negatively charged tin defects. At high pressure, phase transitions between allotropes are found to be slow and exhibit a strong hysteresis, resulting in the coexistence of several phosphorus phases. A sufficient concentration of dopants may be able to stabilise blue phosphorus at ambient pressure.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available