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Title: The preparation and characterisation of graphene and its analogues
Author: Lin, Liangxu
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2013
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The studies in this thesis give deep insights on the large scale preparation of graphene and the fabrication and properties of novel monolayered quantum dots (QDs). Graphene has received remarkable attention due to its interesting physical and chemical properties. Among various preparations for graphene, the solvothermal deoxidation of graphene oxide (GO) is highly attractive as it potentially offers a relatively economical and scalable manufacturing route for use in industrial applications. Unfortunately, the deep deoxidation of GO and highly dispersable reduced GO (rGO) are difficult to achieve using this approach, although the reasons for this deoxidation remain unclear. This thesis shows that the agglomeration/self-assembly of partially reduced GO (p-rGO) sheets in the solvothermal deoxidation reaction suppresses the deep deoxidation of GO and led to low dispersibility/electrical conductivity of the product. By tuning the surface energy of the solvent to minimize the surface enthalpy of the dispersion, these technical problems can be ameliorated and full deoxidation of GO with high dispersibility and electrical conductivity achieved. In this thesis, an alternative novel and effective route to fabricating graphene QDs (GQDs, lateral size ~ 20 nm) is also described. This technique of delaminating layered structures has also been developed to produce monolayered QDs of boron nitride (BN, lateral size of ~ 10 nm), tungsten disulfide (WS2, lateral size ~ 8-15 nm) and molybdenum disulfide (MoS2, lateral size of ~ 8-20 nm). This has opened up many opportunities in studying these interesting materials with reduced dimensions, with new behaviours and properties emerging from the various QDs. The zigzag edges of GQDs led to the appearance of new band gaps and give strong blue-green luminescence centred at 420 nm wavelength (quantum yield of ~7.6%). In monolayered BN QDs, carbene-replaced zigzag edges, carbon-replaced N vacancy point and BOx- (x = 1 and 2) species added new luminescence at around 425 nm wavelength (quantum yield of ~2.5%). Strong luminescence was created by the reduced dimensions of WS2 and MoS2 monolayered QDs causing them to became direct semiconductors. The reduced lateral dimensions also caused marked quantum confinement effects to arise, such as large blue shifts in absorption features of BN, WS2 and MoS2 monolayered QDs. The formation of monolayered WS2 and MoS2 QDs also led to their valance bands being split by giant spin-orbit coupling effects to a far greater degree than is observed form monolayered sheets. The studies suggest strongly that these features are likely to be tunable with lateral dimensions, which makes the QDs potentially very interesting for applications. Although these uses may include spintronics, optoelectronics and even quantum computing, their application in biology is demonstrated by all the monolayered QDs being used as non-toxic fluorescent labels in confocal microscopy of biological cells.
Supervisor: Allwood, D. ; Zhang, S. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available