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Title: Synthesis of graphene-based nanomaterials and their optoelectronic applications
Authors: Lai, Sin Ki
Degree: Ph.D.
Issue Date: 2018
Abstract: Carbon nanomaterials have, for a long history, been intensively applied in electronics and optoelectronics due to their high functionality, stability and the abundance of raw material. They span from the 2D graphene, to the 1D carbon nanotubes and the quasi-0D carbon quantum/nano- dots, including fullerenes. The bandgap in the above nanomaterials vary from zero in the semi-metallic graphene to a bandgap deep into the UV region for the 0D graphene quantum dots. The wide range of bandgap and electrical properties of carbon nanomaterials through dimension reduction may allow selective detection for desirable spectral range of light and optimization in the figures of merit of photodetectors respectively, which make carbon nanomaterials promising for photo-sensing application. In this project, graphene oxide (GO) has been applied to demonstrate broadband photodetection from near infrared to deep ultraviolet. The bandgap of the multi-layer GO nanosheets could be gradually varied by thermal annealing. This was achieved by the removal of oxygen functional groups and restoration of the sp2 hybridization in graphene oxide progressively by controlling the annealing temperature. Electronic properties of the GO could be tuned simultaneously during annealing, which could allow the performance of the photodetectors to be optimized through a trade-off between the magnitude of dark current and carrier mobility, in which the detectivity and response time of the device would be affected respectively. A satisfactory responsivity in the order of mA-1 and fast response time in the range of ms were obtained from the devices with an active layer thickness of ~70 nm.
The graphene quantum dots (GQDs) were chemically functionalized with solution-processable organic conducting polymers. The conducting polymer-GQDs composite showed an enhanced photocurrent by an order of magnitude and also a faster response time as compared to non-functionalized GQDs. Under observation at high magnification, the conducting polymer was revealed to connect the GQDs effectively which could facilitate conduction of charge carriers among the GQDs via the high mobility conducting polymer instead of a greatly hindered electrical transport through hopping of charge carriers among quantum dots in the non-functionalized case. This enables carriers to be harvested more effectively in the composite to give larger photocurrent and faster response time. The heterostructure of large-area graphene with uniformly distributed graphitic carbon nitride nanosheets with ~200 nm in lateral size exhibited synergistic effect for efficient near ultraviolet photodetection and demonstrated a large photocurrent that could also be useful for photoelectrochemcial applications where efficient photocurrent generation is also highly demanded. When heterostructure is adopted, the band alignment between the two components determine the charge transport across the interface. It helps the separation of electron-hole pairs which contribute to more efficient photocurrent generation and thus is of fundamental importance. This is distinctly different from single-component photodetector where photocurrent is usually found only at the metal-semiconductor junction where Schottky barrier may be formed and barrier property is the commonly investigated subject in that case. The charge transfer property at the graphene/carbon nitride interface was systematically analyzed in our devices to elucidate the electrical conduction process for photodetection.
Subjects: Hong Kong Polytechnic University -- Dissertations
Graphene -- Synthesis
Nanostructured materials
Pages: 130 pages : color illustrations
Appears in Collections:Thesis

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