YUAN, Qilong
(2020)
Fabrication and applications of graphene-diamond heterostructures.
PhD thesis, University of Nottingham.
Abstract
Carbon, which is a common element, has numerous allotropes existing in forms varying from zero-dimension (0D) to three-dimensions (3D). Diamond and graphene are two typical allotropes, in which the carbon atoms bond together with sp3- and sp2hybridisation to form 3D and 2D structures, respectively. With quick development and maturity in synthesis technologies both in the laboratory and industry, diamond and graphene have attracted much interest in many fields. As both diamond and graphene are formed from carbon atoms, the conversion between hybridisation forms are widely investigated. The early synthesis of artificial diamond is prepared from graphite (sp2hybridisation) by high-pressure high-temperature (HPHT) treatment. The sp3- to sp2hybridisation (graphitisation of diamond) is also observed in the mechanical process of using diamond-plated carbide tools in machining transition metal alloys. Recently, with intensive investigation of graphene, researchers have begun to understand the direct conversion of graphene layers on the diamond surface.
This thesis firstly gives details on the fabrication of graphene on diamond (graphenediamond) heterostructures. Three different kinds of graphene-diamond heterostructures fabricated by different approaches are introduced as follows. 1) Single layered high quality CVD-grown graphene was transferred directly onto the diamond surface. 2) Thermal treatment of the Ni-coated diamond sample, where the surface of diamond will convert from sp3-bonding to sp2-bonding to form graphene layers on the top. 3) By placing diamond on molten Cu, graphene layers will form on the contact surface of diamond.
This thesis also gives details of investigations from three different applications of the fabricated graphene-diamond heterostructures:
1. A diamond detector was fabricated with graphene as the front electrode based on the CVD graphene-diamond heterostructure, which was then characterised. The device shows good Ohmic contact between the graphene and diamond. The device achieves very low dark current and good UV sensitivity. TCAD software is also introduced in this work to simulate the performance of this diamond detector.
2. BasedontheproposedNifilminducedsp3-to-sp2 conversionondiamondsubstrate, a diamond detector was fabricated with graphene layers as electrodes on both sides of the diamond. Density functional theory (DFT) simulations were performed, which demonstrated that the directly converted graphene layer on diamond can achieve Ohmic contact more easily than CVD transferred graphene. Meanwhile, the detector shows good response to the incident X-ray radiation, which indicates a feasible method for the fabrication of an all-carbon detector.
3. The third application reported here is in a graphene-diamond biosensor based on the molten Cu-induced graphene-diamond heterostructure. For traditional graphenebasedbiosensors,thecontactbetweengrapheneandthesubstrateisduetoVanderWaals force, which is not as strong as covalent bonding. As graphene is directly fabricated on the diamond substrate, the contact is based on strong and tight covalent bonding, which ensures high robustness of the graphene-diamond biosensor. When fouled by certain biological detection targets, the performance and sensitivity of the sensor can be fully regeneratedbyasonicationtreatment,whichwouldnothavebeenpossiblefortraditional biosensors. Moreover, the fabricated graphene-diamond biosensor reported in this thesis has high sensitivity and detection range to dopamine.
This thesis concludes with a summary of the results, and outlook on future work proposing improvements in graphene-diamond heterostructure fabrication as well as experimental investigations to enhance the understanding of the materials and device physics of diamond UV detectors and X-ray detectors, and opens more opportunities for the development and commercialisation of the photodetectors based on graphenediamond heterostructures.
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