Excited State Relaxation In CdTe Quantum Dots

With the development of the synthetic technology, the hotspot ofnanomaterials changes from morphology construction to physicalproperties investigation, which aims to clear physical mechanisms basedon nanoscale interaction. At the same time，people attempt to expand thequantum confined effect from nanoscale to mesoscale, and even macroscale.This expectation has become true by chemical mothods. Superlattice orsupramolecule fabrication has been in ascendant. This thesis based onabove two important aspects of nano-science. We study the relationshipbetween excited state relaxation of excitons and quantum confinementeffect. Furthermore, the quantum confinement effect is extended tomesoscale by self-assembly method. Steady and transient state propertiesof quantum dots (QDs) are investigated in both individual and integratingstatus. The specific contents are as follows:In this paper, we prepared the thioglycolic-acid (TGA) passivatedCdTe QDs (TGA-CdTe). It has more remarkable quantum confinement effect.Both absorption and luminescence wavelength are in the visible region,which is preferred in biological and photovoltaic application. Driven bystrong dipole-dipole interaction, the CdTe QDs can form thehyper-branched nanostructures with hierarchical self-assembly process.The building blocks of this nanostructure are pristine CdTe QDs. Thecuring time, pH value of solution and proportion of precipitation agentall affect the morphology of the formed structures. As indicated by theexperimental results, the assembled structures reserve the quantumconfinement effect of pristine QDs. In this point of view, the formednanostructures have both the mesoscale size and nanoscale quantumconfinement effect. Moreover the branched structures have beenexperimentally proved to have better charge separation efficiency. Hence, this structure may have important application prospect in photovoltaicconversion and sensors etc..The main research results are as follows:First, we have successfully synthesized the high-quality TGA-CdTe QDsby environmental and mild hydrothermal-method. Then we characterize themorphology and the steady state optical properties of QDs. We introducethe principle and operation method of the home-built time-resolvedspectroscopy systems, namely, transient absorption and fluorescenceup-conversion. We explore the excited-state relaxation dynamics of QDsand find that the relaxation is strongly influenced by the quantumconfinement effect.Second, we construct the hyper-branched nanostructures byself-assembly method. The preparation parameters are carefullyinvestigated to control the morphology of formed structures. The opticaland TEM experimental results show that the formed hyper-branchednanostructures keep both the unit and the quantum confinement effect ofthe pristine QDs. Since the interval space among pristine QDs reducesdrastically, self-assembled nanostructures may solve the problem of thesmall diffusion radius of carriers in mono-dispersed QDs. Meanwhile,formed structures facilitate the diffusion and transport passageway ofcarriers since pristine QDs connect to each other very well.Third, we construct the exciton-plasmon system, which is constitutedby CdTe QDs and Au nanoparticles. We investigate the influence of localfield effect on the exciton recombination process by femtosecondtime-resolved spectroscopy. The fluorescence spectrum indicates that thequenching become more and more obvious with the increasing doping ratioof Au nanoparticles. The time-resolved spectroscopy clearly shows thatit is the quantum confinement effect that causes this quenching.In this paper, we investigate the excited-state relaxation dynamics of the synthesized TGA-CdTe QDs by the femtosecond time-resolvedspectroscopy. We find that the quantum confinement effect has a strongeffect on the excited state relaxation. It has certain reference valuefor improving the photovoltaic efficiency of photovoltaic devices. Inaddition, the charge is easier separated in hyper-branched nanostructuresthan in the mono-dispersed QDs. Meanwhile, branched morphology canimprove the collection efficiency of the light. Thus this structures mayhave important potential applications in photovoltaic and sensor etc..Finally, we observe the fluorescence quenching phenomenon in CdTe QDs-Ausystem by time-resolved spectroscopy. The result can provides theexperimental information for the mechanism of energy transfer inexciton-plasmon system.