Application Study And Regulation On Recombination Of Photogenerated Carriers For IIB-VIA Quantum Dots

Quantum dots have discrete energy levels similar to atoms.The band gap-offset between the the highest occupied energy state and the lowest unoccupied energy state is regulated by the particle size,which reduces the probability of phonon radiation,leading to slow relaxation of electrons,effectively inhibiting the auger recombination process.In addition,multiple exciton effect has been observed in quantum dots,which is an excellent optical material.However,there are many surface defects in quantum dots,and a recombination central energy level is formed in the forbidden band,which plays an important role in promoting the recombination,and is often dominated by non-radiative recombination,which has disadvantages to the effective utilization of photo-induced carriers.In this paper,the preparation of novel quantum dots with various structures and the study of carrier recombination properties are carried out.It will provide a theoretical basis for the application of quantum dots in the field of solar cells and fluorescence.This work was focused on IIB-VIA semiconductor quantum dots.Core-shell structure Cd_1-XZn_XTe/CdS quantum dots,hierarchical cadmium telluride quantum dots,cobalt ion doped cadmium telluride quantum dots,manganese ion doped cadmium telluride quantum dots,cobalt and manganese ion co-doped cadmium telluride quantum dots,silver ion doped cadmium telluride quantum dots and quantum dots basal material Mn₃O₄ were prepared by wet chemical methods.The effects of different systems on the recombination and transport of photo-induced carrier were studied.Furthermore,they were applied in quantum dot sensitized solar cells and chemical analysis.Combining with the first principle,Cd_1-XZn_XTe/CdS core-shell quantum dot model was designed.The radial distribution of Mulliken populations,LUMO and HOMO for a series of core-shell quantum dots were calculated.Cd_1-XZn_XTe quantum dots and Cd_1-XZn_XTe/CdS core-shell structure quantum dots were successfully prepared by wet chemical methods.In addition,Cd_1-XZn_XTe/CdS quantum dots sensitized titanium dioxide films were prepared,as well.Therefore,type-II core-shell structure quantum dots with electrons and holes spacial separated were obtained,which effectively inhibited the recombination process of photo-induced carriers and improved the photoelectric conversion efficiency of corresponding quantum dot-sensitized solar cells.Effects of chemical components and shell thickness on the photo-induced carrier were also studied.Hierarchical cadmium telluride quantum dots were obtained by self-assembly technique.It not only has the characteristics of original quantum dots,but also possesses new properties caused by the combination of quantum dot units,such as long carrier diffusion distance and better chemical stability.Moreover,the generation,separation and transmission of carriers are not completely in the same face with regard to hierarchical quantum dot structure.Therefore,the lifetime of photo-induced carriers was greatly improved,which is benefit to the separation of photo-induced electrons and holes.Cobalt ion doped cadmium telluride quantum dots,manganese ion doped cadmium telluride quantum dots,cobalt and manganese ion co-doped cadmium telluride quantum dots were prepared by wet chemical methods using the hydropropionic acid as the stabilizer.Doping energy levels were generated in the forbidden band of CdTe quantum dots.Therefore,the recombination process of photo-induced carrier was suppressed for the d-d orbital transition of transition metal is forbidden.Furthermore,the performance of corresponding quantum dot sensitized solar cells were improved,as well.Quantum dots with double fluorescence peak capped by L-cysteine were synthesized through controlling the concentration of silver precursor during preparation process and applied as the fluorescence probe for Hg²⁺detection.The samples were characterized for their structural and morphological properties by means of X-ray diffraction,Transmission electron microscopy and X-ray photoelectron spectroscopy.Results suggested that the double fluorescence emission peak was caused by the transformation of doping type,which was confirmed by excitation density-dependent PL,capacitance-voltage characteristic test and PL decay measurement.The highest PL quantum yield of the as-prepared samples was calibrated to be about 58.7%.The longest fluorescence lifetime of the as-prepared samples was calculated to be about 35.28 ns.Furthermore,the relationship between fluorescence intensity and the concentration of Hg²⁺is established.According to the detection results,there are two linear relations,which is benefit to the extension of detection range.In addition,manganese formate metal organic framework was prepared by using manganese acetate as the precursor.The effects of solvent heat treatment on the microstructure and specific surface area were studied.The hierarchical structure of manganese tetroxide was used as the substrate to deposit quantum dots.In addition,the fluorescence quenching effect of manganese tetroxide on quantum dots was applied to suppress the interference of the fluorescence back ground on the detection signal during chemical analysis and detection of quantum dot.