UC San Diego

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.