Super Nanostructures Based On The Self-assembling CdTe Quantum Dots: Optical Properties Of Steady And Transient State

In recent years, considerable work has been done onⅡ-Ⅵ semiconductornano-materials because of their unique physical and optoelectronic properties. Insemiconductor materials, the behavior of excitons has specific changes because ofsize induced quantum confinement effect, which makes great difference of physicaland chemistry properties as comparing with bulk semiconductor materials. Thus,various researches and applications of semiconductor nanomaterials have beensuspected and investigated in recent decades. As the requirement of application andinvestigation, high quality semiconductor materials and details of their opticalproperties have been becoming the focus of nano-research field.In this thesis, thioglycolic acid (TGA) was used as the surface stabilizer tosynthesize the hydrosoluble TGA-CdTe quantum dots (QDs), the quantum yields ofwhich can exceed50%. Then, three dimension TGA-CdTe nano super-structures,nanowires(NW), hyper-branched nanostructures(HCN), are constructed based on thedipole-dipole interaction drived self-assembling process. TEM investigation indicatesthat the supter-structure are built by uniformed quantum dots. Thus, the opticalproperties of origina quantum dots are inherited well in super-structures, which areconfirmed by fluorescence microscopy experiments. By analysing the steady andtransient state spectrum in detail, we investigate their optical properties. Theirexcellent photoelectric properties illustrate the TGA-CdTe nano-materials are verypromising for application in optoelectronic devices.The main results of this research are as follows:Firstly，in line with the concept of cost reduction and environmental protection,the high quality hydrosoluble TGA-CdTe quantum dots are prepared by thehydrothermal method，and perform the characterization for the samples. Then, we calculate the fluorescence quantum yields for the TGA-CdTe QDs, and discuss themechanism of QDs growth. And then, as basic building blocks, TGA-CdTe QDs cansystematically reorganize into multi-dimensional nano-architectures (for example,nanowires, nanobelts, hyperbranched CdTe nanostructures and so on.), and studythe effect of the environment factors on the self-assembly process and themorphology of the nanomaterials, by controlling the size of the original QDs, thewater bathing temperature, and the QDs to acetonitrile ratio. Finally, from the crystalphase structure and steady-state fluorescence spectroscopy of self-assemblednanomaterials, we found that the nanomaterials inherit many good properties oforiginal QDs.Secondly, First of all, introduce the two types of ultrafast time-resolved methods,which are transient absorption and fluorescence up-conversion, and use them toinvestigate the exciton dynamics about the TGA-CdTe nanomaterials. AboutTGA-CdTe QDs, the bleach recovery of the1S transition shows initial rise, but thefluorescence up-conversion signal for the1S transition is slower rise time. Then, wetake a attention on the HCN, multi-exciton interaction happen in the HCN, forexample, multi-exciton recombination is suppressed effectively, which make theexcitons separate easily and the electrons transfer conveniently.Thirdly, the binary system, which is gotten by mixing TGA-CdTe QDs and Aunanoparticles, is studied that is about the fluorescence of QDs quenched by Aunanoparticles. And we explain the reason of quenching process by the time-resolvedspectrum of the binary system.In summary, by both steady state and time-resolved spectroscopy on the samples,we found that, comparing with the short carriers diffusion length and the poorcharge separation efficiency in the mono-dispersed QDs, the hyper-branchednanostructures can provide an intrinsic percolated structures and thus allowing thecarriers to reach the electrodes before recombination. In addition, the structure ofmulti-branches helps it collect sunlight more easily. So we think the HCN is expectedto become one of the important structure materials, which are applied in a newgeneration of optoelectronic devices. Finally, we explain the process of fluorescence quenching about the system, which is composed of TGA-CdTe QDs and Aunanosphere by time-resolved spectroscopy, that provides the basis for improving theefficiency of energy transfer about the binary system.