Electron Extraction Dynamics In Quantum Dots Adsorbed With Methyl Viologen

In this graduation thesis, a kind of photochromic molecule and CdSequantum dot have been studied by femitosecond spectroscopy. As therapid development of ultrafast pulse, researchers have found that theultrafast time-resolution can be realized by this technology. And thistechnology provide a helpful tool to investigate physical process whichhappen in a femitosecond or picosecond period, like the photoelectronrecombination, exciton lifetime and so on. All these will play apositive role in scientific research.In this thesis, the pump-probe system is first introduced, includingthe pump-probe and the up-conversion system. First, our laser, chopper,TOPAS, delay line, BBO crystal, spectrometer are introduced in detail.And the mirror,(triangular) prism, half wave plate, polarizing film,and filter should be used proficiently. The laser system we used isthe Solstice system bought from Spectra-Physics, emitting pulses at800nm wavelength,100fs wide, and1kHz. Then through the beamsplitter,20%of the pulses energy is reflected and the resttransmitted. The fraction of the transmitted part is passing throughthe TOPAS system or the BBO crystal to generate the pump light. Andthe fraction of the reflected part is focused on the nonlinear medium(water, sapphire, calcium fluoride) to generate the white light as theprobe light. The up-conversion system is used to probe the dynamics ofthe fluorescence. The principle of the system is sum frequency of thefluorescence and the gate light. The second part is the experimentpart, the static properties of the sample should be probed firstly,including the steady absorption, photoluminescence, excited spectrum,Scanning Electron Microscope, Transmission Electron Microscopy, etc.Then the transient spectra can be obtained using our system, to fullyinvestigate the sample. Our experiment mainly contains two issue. First one is a kind ofphotochromic molecule, spiropyran, which has two tautomers, the ringclosed spiropyran form(SP) and the ring opened merocyanine form(ME).When exposed at the256nm ultraviolet light, a chemical bond of thespiropyran will be broken and the spiropyran will changes to anothertautomer, merocyanine. In our experiment, we conjugate the spiropyranand the fluorescein to enhance the fluorescence of the fluorescein.The second issue is CdSe quantum dot, which is prospective in quantumdot solar cells. When excited at the light beyond the energy gap ofthe quantum dot, the photon electron will be generated andrecombination very soon with the energy lost if there is no chargeseparation process. So we choose a kind of electron quencher, methylviologen, to extract the electrons timely. And another kind ofcore/shell quantum dot, CdSe/CdS/ZnS is performed to be a comparison.Our results demonstrates that there is only one transfer processbetween the CdSe and the methyl viologen. While for the core/shellquantum dot, there are two transfer process. One is from CdS shell tomethyl viologen directly, another is from CdSe core to methyl viologen.