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James, Tyler S S (2024) Optoelectronic van der Waals heterostructure devices and the growth of C-doped hBN by MBE. PhD thesis, University of Nottingham.

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Abstract

By encapsulating self-assembled layers of optically-active organic molecules between atomically thin insulating layers, van der Waals heterostructures can be fabricated which incorporate these encapsulated molecules between two graphene electrodes. The thin, transparent insulating layers allow the optical properties of the molecules to be investigated through optical excitation, and also for the molecules to be excited by charge carriers tunnelling between the electrodes. The lateral size of these tunnelling devices can be reduced by ∼ 1000× by replacing the graphene electrodes with carbon nanotubes, opening a route for lithography-free nanometre-scale electronic devices. Alternatively, to produce such devices at an industrial scale, the controlled growth of large-area, high-quality 2D materials is required, which requires the use of techniques such as molecular beam epitaxy.

The research in this work details the fabrication and measurement of hybrid molecular-2D material van der Waals tunnel diodes. Newly-developed fabrication processes for producing van der Waals heterostructures which incorporate self-assembled monolayers of organic molecules deposited onto ultra-thin hBN layers are presented. These fabrication processes were employed to fabricate a range of van der Waals heterostructures which included monolayers of free base phthalocyanine (H2Pc) and perylene tetracarboxylic diimide (PTCDI). Heterostructure devices that encapsulated H2Pc between graphene electrodes and hBN tunnel barriers were observed to exhibit triplet-mediated photon upconversion of 120 meV of the zero-phonon (0-0) optical transition (1.72 eV) at cryogenic temperatures (6±2 K). Electroluminescence measurements of van der Waals tunnelling diodes which incorporate monolayers of both H2Pc and PTCDI within a single device are presented which exhibited an additional emission peak not associated with either of the individual molecules.

The fabrication and measurement of van der Waals tunnel diodes which employ individual multi-walled carbon nanotubes (MWCNTs) as electrodes are presented. A novel fabrication process which combines van der Waals transfer techniques with atomic force microscopy (AFM) to facilitate the deterministic transfer and controlled manipulation of individual MWCNTs. This fabrication process was employed to generate a series of progressively more complex MWCNT-based heterostructures which were investigated using electrical measurements. Individual MWCNTs were measured through transfer onto pre-formed contacts. Junctions formed from pairs of crossed MWCNTs were fabricated and measured. MWCNT/hBN/MWCNT tunnelling diodes were fabricated with a range of hBN barrier thicknesses. Analysis of these devices reveals a range of tunnelling phenomena.

Finally, hBN layers were grown on HOPG using molecular beam epitaxy (MBE) with an isotopically pure B10 source and were characterised using a range of AFM imaging techniques. The effect of substrate features and growth time on the morphology of the grown layers was investigated. hBN multi layers were observed to exhibit a contact potential difference of±25meV compared to the monolayer hBN, which is indicative of alternative layer stacking. The morphological effects of carbon doping during the growth of hBN layers on HOPG were investigated using AFM and reveal a potential C-hBN alloy grown via MBE.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Beton, Peter H. Khlobystov, Andrei
Keywords:	Nanotubes, 2D materials, graphene, hBN, organic molecules, nanoscale devices, MBE, molecular beam epitaxy
Subjects:	Q Science > QC Physics > QC501 Electricity and magnetism
Faculties/Schools:	UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID:	77260
Depositing User:	James, Tyler
Date Deposited:	23 Jul 2024 04:40
Last Modified:	23 Jul 2024 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/77260

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