Study Of The Synthesis And Photoluminescence Of Quantum Dots With Multiple Components

Semiconductor quantum dots （QDs） have attracted considerable interest over the pastyears because of their unique electronic and optical properties and potential applications, suchas biomedical labeling, optoelectronic devices, and solar cells. As reported in numerouspublications over the last three decades, tuning the size of nanocrystalline semiconductors isone way of adjusting the band gap energy. On the other hand, research on band gapengineering via control of QDs composition, which is achieved by adjusting the constituentstoichiometries of alloyed semiconductors, is still in its infancy. Alloying of twosemiconductors at the nanometer scale produces materials that display properties distinct notonly from the properties of their bulk counterparts but also from those of their parentsemiconductors. As a result, alloyed QDs possess additional properties that arecomposition-dependent aside from the properties that emerge due to quantum confinementeffects. But in the typical process, the starting materials including trioctylphosphine oxide andZn（Et）2are too expensive and toxic. While the current requirements for the synthesis ofsemiconductor QDs mainly aim to the use of easy-to-manipulate, inexpensive, andenvironmentally benign precursors and solvents, necessary for an industrial up-scaling of theprocess. In this paper, we synthesized various QDs through an organic route usingenvironmentally benign precursors and solvents.In the first chapter, we make a brief introduction on the concept of QDs, alloyed QDs,and preparation methods.In the second chapter, Zn_1-xCd_xSe QDs were synthesized through an organic route usingoleic acid as capping agent. Compared with pure ZnSe and CdSe QDs, the Zn_1-xCd_xSe QDscan exhibit higher photoluminescence （PL） efficiency up to68%. The PL properties ofZn_1-xCd_xSe QDs strongly depended on the Cd/Zn ratio in the as-synthesized samples. It wasfound that the luminescence of Zn_1-xCd_xSe QDs revealed a blue-shift with time at low molarratios of Cd/Zn while a red-shift was observed at high molar ratios of Cd/Zn. Thisphenomenon was ascribed to the change of structure of the Zn_1-xCd_xSe QDs from ahomogeneous alloy to a concentration gradient material with increasing the amount of Cd.The Zn_1-xCd_xSe QDs exhibited a mixed structure between ’hexagonal’ and ’cubic’ lattices, which located between those of ZnSe and CdSe QDs. The temporal evolution monitored byabsorption and emission spectra indicated that the growth kinetics and the distribution of Cdand Zn in the resulting ternary Zn_1-xCd_xSe QDs can be sensitive to the molar ratio of Cd/Zn.In the third chapter, a facile organic route has been developed to synthesize CdTe_xSe_1-xQDs using stearic acid as a capping agent. Because of growth kinetics of CdTe and CdSe, themolar ratio of Te/Se enables CdTe_xSe_1-xQDs with various morphologies. With raising theTe/Se ratio, the morphology of the QDs can be adjusted from tetrahedron to tetrapod. This isascribed to the energy difference between wurtzite and the zinc-blende structures, whichdetermines the nucleation and growth processes of the QDs. The diameters and the lengths ofthe branches in the tetrapods varied from4nm to6nm and7nm to20nm, respectively. TheCdTe_xSe_1-xQDs revealed near-infrared （NIR） range （700-800nm） photoluminescence. Forexample, CdTe_xSe_1-xQDs with the molar ratio of Te/Se of0.25reveals a PL peak wavelengthof762nm. The PL properties of the resulting QDs are strongly depended on preparationconditions such as the molar ratio of Te/Se as well as the reaction temperature and time. In thecases of various reaction temperature （120-260°C）, the QDs revealed adjusted PL peakwavelength from visible to NIR range and narrow PL spectra. In addition, even though in thecase of a high Te/Se molar ratio （0.67） used, the CdTe_xSe_1-xQDs revealed improved stabilitycompared with CdTe QDs. Being coated with a composite Cd_yZn_1-yS shell, the PL intensitywas drastically enhanced. The approach described here is utilizable to the fabrication of othersemiconductor QDs with various morphologies.In the fourth chapter, we present the synthesis of CuInS₂QDs through a phosphor-freemethod. The PL properties of CuInS₂QDs synthesized using different Cu/In molar ratioswere investigated. With the reduction of Cu/In molar ratio, the PL peak wavelength shows aclear blue shift by ca.100nm and a noticeable increase in PL intensity. Being coated with aZnS shell on CuInS₂QDs, the resulting core/shell QDs exhibited a dramatic increase of PLefficiency and stability, with the maximum PL efficiency up to39%. This is ascribed to theefficient reduction of non-radiative recombination after surface modification. In addition, thePL peak wavelengths were tuned from670to795nm. The transmission electron microscopyobservation and X-ray diffraction analysis indicated that the CuInS₂and CuInS₂/ZnS QDsrevealed a “dot” shaped morphology and exhibited a wurtzite structure. Time-resolved fluorescence spectroscopy revealed that CuInS₂/ZnS core/shell QDs apparently exhibited aslow decay compared with the CuInS₂cores （214ns for CuInS₂/ZnS QDs and172ns forCuInS₂cores）. Because of the tunable near-infrared range emission, high stability and longlifetimes of the CuInS₂/ZnS QDs, they will be very useful in applications such as solar celland biological imaging.In the last chapter, we summarize our research on colloidal alloyed semiconductor QDsand foresee the possible development of QDs in the future.