Study On Heterojunction And Two Photon Absorption Of Colloidal Quantum Dots

Semiconductor colloidal quantum dot (QDs) which is a novel material in nanoscale recently attracts broad research interests. It has a wide range of applications due to its unique optical and electrical properties. The applications include photovoltaic devices based on QDs, QD-LEDs, bio-imaging, hybrid QD photodetector and QDs-Laser. In photovoltaic and photo detection, heterojunction between QDs and other semiconductor (ie, ZnO and graphene) who serves as electron transfer layer plays an important role on their device performances. Determining and understanding the band alignment on the interface of such QDs-ZnO or QDs-gaphene hetero junction can benefit the optimization of devices. Besides, the current cadmium based QDs are toxic to animals, while the non-toxic QDs will be good alternatives for bio-imaging.Herewith, we first focused on the band alignment of QDs-ZnO and QDs-gaphene hetero junctions. UPS and XPS were utilized to determine the valence band offset. Furthermore, a QDs-graphene heterojunction photodetector was realized to understand the energy transfer between them. Last but not least, the two photon absorptions of Zn2+ doped Cu-In-Zn-S QDs were studied. The innovative research results are below:1. Interface band alignment of CdSe QDs and ZnO heterojunction was determined by Ultraviolet photoelectron spectroscopy (UPS). A type Ⅱ hetero junction was formed between CdSe QDs and ZnO layer, in which the valence band offset can be tuned via a control of QD’s size. The interface band alignment between CdSe/ZnS QDs-graphene heterojunction was carried out by X-ray photoelectron spectroscopy (XPS), from which a 1.7eV valence band offset was determined. These results provide hints for optimizing the photovoltaic devices and hybrid QDs-graphene photodetectors.2. The CdSe/ZnS QDs-graphene heterojunction photodetector based on a FET structure was realized via a laser blading technique. The time-resolved PL measurements revealed that the PL lifetime of CdSe/ZnS QDs diminished under back-gate±1V voltage, indicating an efficient energy transfer from QDs to graphene. Further theoretical study confirmed a Forster resonance energy transfer when the fermi level of graphene matches well with the absorbance for QD’s emission, suggesting a possible control of energy transfer rate under back-gate control. This study will help improve the future applications of QDs-graphene heterojunction photodetector.3. Zn2+ doped Cu-In-Zn-S QDs were obtained via two step approach. Two photon induced photoluminescence confirmed the two photon absorption of Cu-In-Zn-S QDs. A highest two photon cross section of 8.109×10-48 cm4 s photon-1 was acquired by the Z-scan technique, where the Zn2+ doping is key to enhance the two photon absorption cross section. Our research provides exprimentl data for two photon bio-imaging application.