טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentWaldon Eli
SubjectColloidal Synthesis of Small-Sized CdSe and CdSe/ZnS
Core/Shell Semiconductor Quantum Dots
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

Colloidal semiconductor nanoparticles, or quantum dots (QDs), have attracted keen attention among nano-scientists. QDs are interesting due to the strong dependence of their optical and electronic properties on their size. Among them, CdSe and ZnS QDs are extensively studied because of the possibility of their implementation in numerous QD-based devices, such as light emitting diodes (LED), solar cells and biological labeling.

Since QDs have a large volume-to-surface ratio, passivation of QD surface defects is very important for obtaining high quality particles. Hence, there have been numerous studies on improving QD surface passivation, which leads intrinsically to an improvement of the photoluminescence quantum efficiency (PL QE) of the QDs.

Although CdSe QDs with excellent luminescence properties can be prepared, the PL QE drops during processing of the QDs due to a loss of the organic surface ligands. To achieve more stable optical properties, CdSe QDs can be coated with an inorganic semiconductor, which has a band-gap energy that encloses the band-gap energy of CdSe, like e.g. ZnS. Additionally, these CdSe/ZnS QDs usually have a higher PL QE than the uncoated CdSe QDs. However, many of these coating procedures require expensive and pyrophoric chemicals.

In this work we adapt a CdSe/ZnS core/shell QDs synthesis from the literature, which uses less hazardous chemicals, but it only allows the synthesis of brightly luminescent CdSe/ZnS QDs for a small size-range of CdSe QDs. In order to improve this synthesis to be applicable in a broader size-range of CdSe-core sizes, we focus on different reaction parameters, including coating precursor amounts, the types of organic ligands used, the stoichiometric ratios of the ligands with respect to the zinc amount and the temperature program during the coating process. Finally, we were able to enhance PL QE of CdSe/ZnS QDs from below 10 % to above 50 %.