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Title: Functional thin-film transistors based on hybrid materials
Authors: Sun, Zhenhua
Degree: Ph.D.
Issue Date: 2013
Abstract: Functional thin film transistors (TFTs) have attracted increasing interest due to their potential applications in many areas with excellent performance. Such devices can be realized with hybrid materials, which can demonstrate not only the desired properties of their components but also some unique synergistic effect. Therefore, it is important to develop the techniques of tailoring hybrid materials to realize various types of functional TFTs. In this thesis, the applications of functional TFTs are introduced at the beginning. Various TFTs with different functions are described and the photodetectors and memories closely related to this thesis are discussed in details. Their working principles and the current situation of research are reviewed. Then hybrid materials are discussed on their history and classification. Therein, the most important organic-inorganic hybrid materials and the graphene-inorganic hybrid materials which are the rising hotspots are introduced. Their synthesis and applications are reviewed. The reported functional TFTs based on the hybrid materials are listed for reference. The phototransistors and light driving memories based on hybrid materials have been carefully studied by us. In these devices, graphene or polymers acted as conducting media and inorganic components were used to absorb light. We found that the interface properties in the hybrid materials were critical to the performance of optoelectronic devices, which have been optimized in our experiments. Near infrared (NIR) phototransistors based on poly(3-hexylthiophene) (P3HT)/Lead sulfide (PbS) quantum dots (QD) and graphene/PbS QDs hybrid films were fabricated and characterized. The phototransistors based on P3HT/PbS QDs show high photoresponsivity up to 2×10⁴ A/W under NIR of 895nm with low light irradiance. This value is larger than the record of organic phototransistors. Under NIR illumination, the transfer curves shift to high gate voltage horizontally. Therefore the conductivity of the hybrid film changes under NIR light illumination at a constant gate voltage. This behavior is attributed to the photo induced electrons generated in PbS QDs which will induce field effect doping effect in P3HT films. Same effect happens in graphene/PbS QDs hybrid phototransistors which show ultrahigh photoresponsivity of 1×10⁷ A/W. This high value is due to the high carrier mobility in graphene. The devices can be fabricated on flexible substrates and show stable performance. All of the aforementioned devices show fast response and their photoresponse behaviors can be described by a physical mode.
P3HT/Titania (TiO₂) nanoparticles hybrid materials have been investigated. It was found that hole mobility in P3HT can be enhanced by pyridine-capped TiO₂ nanorods. Both the shape of nanoparticles and surface ligand were proved crucial to the enhancement. Characterizations of the hybrid films showed that this enhancement of hole mobility can be attribute to the improved crystallinity of P3HT which may be induced by the self-assembly effect cased by pyridine-capped TiO₂ nanorods. The ultraviolet (UV) phototransistor fabricated based on this kind of hybrid film show much better performance than the devices based on TiO₂ nanoparticles without the surface modification of pyridine. Light driving, multilevel, rewritable and nonvolatile memories were fabricated with CVD grown single-layer graphene decorated with TiO₂ nanodots which are capped with pyridine ligand on the surface. The memory is a TFT with a bottom-gate top-contact structure. The conductivity of the channel can be changed by UV illumination and shows excellent retention property. Since the conductivity can be modulated to various values by carefully controlling the illumination conditions, including the light intensity and illumination time, the device can be used as a multilevel memory. Thus different conductive state can be regarded as nonvolatile information programmed by UV illumination. The change of the conductivity of the channel can be attributed to net positive charges generated in TiO₂ nanodots under UV. The pyridine surface ligand which can trap the photon-induced holes in TiO₂ is critical to the nonvolatile memory effect. Positive gate voltage can be used to erase the excitation states thus make the memory rewritable. This study opens a novel path towards nonvolatile multilevel memories.
Subjects: Thin film transistors -- Materials.
Hong Kong Polytechnic University -- Dissertations
Pages: xxv, 168 leaves : ill. (chiefly col.) ; 30 cm.
Appears in Collections:Thesis

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