Date of Graduation

12-2015

Document Type

Thesis

Degree Name

Master of Science in Chemistry (MS)

Degree Level

Graduate

Department

Chemistry & Biochemistry

Advisor

Colin D. Heyes

Committee Member

Bill Durham

Second Committee Member

Jingyi Chen

Keywords

Pure sciences; Applied sciences; Colloids; Fluorescence; Nanotechnology

Abstract

CIS/ZnS core/shell QDs are an important class of nanomaterials for optoelectronic, photovoltaic and photoluminescence applications. They consist of lower toxicity materials than the prototypical II-VI Cd-based QDs and show long fluorescence lifetimes, which generates prospective in biological imaging applications. It is vital to develop reproducible synthetic methods for this new class of nanomaterials in order to maintain small sizes with high QYs. CIS core QDs have been shelled with ZnS at various temperatures from 90-210°C for reaction times ranging from 20-140 minutes to examine the role of thermodynamics and kinetics on the shell growth. Using HR-TEM and ICP-MS, it was observed that, rather than growing a ZnS shell onto the cores (as observed for II-VI QDs), ion-exchange occurs, leading to negligible size change at temperatures up to 210°C. Adding Zn via this ion-exchange mechanism leads to an increase in their QY, primarily by increasing the average radiative time through removing surface defects during the exchange process. The Zn concentration and QY is maintained if the shelling temperature is 210°C, but if the temperature is 190°C or lower, Zn is removed over 1-2 hours, although QY is maintained.

After a second injection of the ZnS shelling precursors, both the temperature and time of the reaction have a significant effect on the QY and structural properties. At 210°C, the second injection leads to a significant decrease in the QY, although the Zn concentration is maintained. On the other hand, if the temperature is 190°C or lower, the second injection does not lead to a decrease in the QY. In fact, with time, it can lead to an even higher QY than a single injection at 210°C, even though the Zn concentration drops to almost zero.

Recent reports of CIS/ZnS synthesis have varied within the 190-210°C range. The results in this thesis show that differences in the kinetics of the ion-exchange reactions, alloying between the core and shell, lattice self-purification of CIS and restructuring of the surface between 190°C and 210°C all play crucial roles, and may explain the differences reported in CIS/ZnS optical and structural properties in the literature.

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