Hybrid Structures of PbS Quantum Dots and Single Layer Graphene for the Photodetector: Interface and Architecture

Photodetectors that convert light to electrical signals are useful for understanding lots of information containing in the specific wavelength of the light. Depending on the spectral bands, the photodetectors can be used for pollution detection, imaging, bioimaging, telecommunication, chemical analysis, night vision, medical imaging, gas sensing, and astronomy observations. To sense the above-mentioned signals, the photodetectors with good photon absorption and high charge collection efficiency are required. Quantum dots are promising materials for the photodetectors because of their strong light absorption, direct and size tunable band gap properties, but quantum dots have very poor carrier mobility leading to low charge collection efficiency. Two-dimensional materials such as single layer graphene and MoS2 have shown extraordinarily high mobilities, but one or few atoms thickness are ultrathin, resulting in poor photon absorption. As a result, novel strategy to overcome such limitations was demonstrated by integrating hybrid quantum dots and two-dimensional materials. The hybrid structure takes advantage of the synergy between two materials, combining the high mobility of two-dimensional materials for collecting charge efficiently, as well as the strong photon absorption and bandgap tunability of quantum dots for carrier photogeneration. The focus of this dissertation is to understand the hybrid structure of PbS quantum dots and single layer graphene for high performance photodetectors by investigating the interface and structures. We found that the spectral photoresponse of the hybrid structure can be controlled by the size or thickness of PbS quantum dots. Also, we have presented an effective technique to measure the diffusion length of both holes and electrons in the bulk of thick quantum dots. Moreover, we engineered the interface between quantum dots and graphene with pyrene molecules to enhance the coupling between graphene and quantum dots via π- π interactions. In addition, we controlled the carrier transfer from PbS quantum dots to graphene via ZnO electron transporting layer. Finally, we successfully fabricated micron size PbS quantum dot patterning on graphene for integrated photodetector chips. This dissertation demonstrates the fundamental understanding of hybrid structure of quantum dots and graphene. The research results can help to promote technologies and push the limits in hybrid structure of quantum dots and graphene for optoelectronic research area

Photochemical Hydrogen Doping Induced Embedded Two-Dimensional Metallic Channel Formation in InGaZnO at Room Temperature

The photochemical tunability of the charge-transport mechanism in metal-oxide semiconductors is of great interest since it may offer a facile but effective semiconductor-to-metal transition, which results from photochemically modified electronic structures for various oxide-based device applications. This might provide a feasible hydrogen (H)-radical doping to realize the effectively H-doped metal oxides, which has not been achieved by thermal and ion-implantation technique in a reliable and controllable way. In this study, we report a photochemical conversion of InGaZnO (IGZO) semiconductor to a transparent conductor via hydrogen doping to the local nanocrystallites formed at the IGZO/glass interface at room temperature. In contrast to thermal or ionic hydrogen doping, ultraviolet exposure of the IGZO surface promotes a photochemical reaction with H radical incorporation to surface metal–OH layer formation and bulk H-doping which acts as a tunable and stable highly doped n-type doping channel and turns IGZO to a transparent conductor. This results in the total conversion of carrier conduction property to the level of metallic conduction with sheet resistance of ∼16 Ω/□, room temperature Hall mobility of 11.8 cm² V^–1 sec^–1, the carrier concentration at ∼10²⁰ cm^–3 without any loss of optical transparency. We demonstrated successful applications of photochemically highly n-doped metal oxide via optical dose control to transparent conductor with excellent chemical and optical doping stability

Synthesis of TiO₂ Nanoparticle-Embedded SiO₂ Microspheres for UV Protection Applications

Exposure to ultraviolet (UV) radiation induces many serious health issues. Because of serious health concerns, there is an urgent need to develop UV filters with better efficacy and high safety. For this purpose, titanium dioxide (TiO2) nanoparticles are the most desirable materials due to their excellent UV protection abilities. The use of TiO2 as sunscreens has raised some concerns about potential risks due to the formation of TiO2-mediated free radicals. Herein, TiO2 nanoparticles have been successfully embedded in silica (SiO2) microspheres using the emulsion synthesis method. The as-synthesized TiO2 nanoparticles embedded in silica microspheres have shown excellent UV protection ability. TiO2 nanoparticles embedded in silica microspheres suppress the photocatalytic properties compared to bare TiO2 nanoparticles, and these results indicate that TiO2-embedded silica microspheres are promising UV protection materials for sunscreen