SYNTHESIS AND CHARACTERIZATION OF CdSe-ZnS CORE-SHELL QUANTUM DOTS FOR INCREASED QUANTUM YIELD

Quantum dots are semiconductor nanocrystals that have tunable emission through changes in their size. Producing bright, efficient quantum dots with stable fluorescence is important for using them in applications in lighting, photovoltaics, and biological imaging. This study aimed to optimize the process for coating CdSe quantum dots (which are colloidally suspended in octadecene) with a ZnS shell through the pyrolysis of organometallic precursors to increase their fluorescence and stability. This process was optimized by determining the ZnS shell thickness between 0.53 and 5.47 monolayers and the Zn:S ratio in the precursor solution between 0.23:1 and 1.6:1 that maximized the relative photoluminescence quantum yield (PLQY) while maintaining a small size dispersion and minimizing the shift in the center wavelength (CWL) of the fluorescence curve. The process that was developed introduced a greater amount of control in the coating procedure than previously available at Cal Poly. Quantum yield was observed to increase with increasing shell thickness until 3 monolayers, after which quantum yield decreased and the likelihood of flocculation of the colloid increased. The quantum yield also increased with increasing Zn:S ratio, possibly indicating that zinc atoms may substitute for missing cadmium atoms at the CdSe surface. The full-width at half-maximum (FWHM) of the fluorescence spectrum did not change more than ±5 nm due to the coating process, indicating that a small size dispersion was maintained. The center wavelength (CWL) of the fluorescence spectrum red shifted less than 35 nm on average, with CWL shifts tending to decrease with increasing Zn:S ratio and larger CdSe particle size. The highest quantum yield was achieved by using a Zn:S ratio of 1.37:1 in the precursor solution and a ZnS shell thickness of approximately 3 monolayers, which had a red shift of less than 30 nm and a change in FWHM of ±3 nm. Photostability increased with ZnS coating as well. Intense UV irradiation over 12 hours caused dissolution of CdSe samples, while ZnS coated samples flocculated but remained fluorescent. Atomic absorption spectroscopy was investigated as a method for determining the thickness of the ZnS shell, and it was concluded that improved sample preparation techniques, such as further purification and complete removal of unreacted precursors, could make this testing method viable for obtaining quantitative results in conjunction with other methods. However, the ZnS coating process is subject to variations due to factors that were not controlled, such as slight variations in temperature, injection speed, and rate and degree of precursor decomposition, resulting in standard deviations in quantum yield of up to half of the mean and flocculation of some samples, indicating a need for as much process control as possible

Junior Recital:James Angell, Guitar

Kemp Recital Hall Sunday Evening April 16, 2000 6:00 p.m

The Family Monitor or A Help to Domestic Happiness (Part 1)

The Family Monitor, or A Help to Domestic Happiness, by John Angell James is a a guide to family life with seven chapters, three of which have to do with the duties of husbands and wives to each other, and the other four concern parents, children, masters and servants. Part 1 includes chapters 1 through 6. This includes the domestic constitution, the mutual duties of husbands and wives, the special duties of husbands and wives, some remarks on the formation of the marriage union, the duties of parents, and the duties of children to their parents. Part 1 goes from the first page to page 107.https://openworks.wooster.edu/motherhomeheaven/1144/thumbnail.jp

The Family Monitor or A Help to Domestic Happiness (Part 2)

The Family Monitor, or A Help to Domestic Happiness, by John Angell James is a a guide to family life with seven chapters, three of which have to do with the duties of husbands and wives to each other, and the other four concern parents, children, masters and servants. Part 2 has chapters 6 to chapter 9. This includes the duties of children and to their parents, on the fraternal duties, the duties of masters and the duties of servants. Part two goes from page 108 to the end of the book.https://openworks.wooster.edu/motherhomeheaven/1145/thumbnail.jp