טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKloper Viktoria
SubjectColloidal Synthesis and Optical Charectarization of
Semiconductor CdTe and CdTe/CdTexSe1-x Core-Alloy
Shell NQDs
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

Our main goal was to suppress fluorescent blinking in CdTe nanocrystal quantum dots. The known approaches for epitaxial shell overgrowth for blinking suppression could not be used due to application utilization reasons. Therefore we developed an inorganic CdTexSe1-x alloy-shell approach for suppressing blinking in nanocrystal quantum dots that has the potential to dramatically improve the utility for application such us single nanocrystal quantum dot tracking of individual molecules in cell biology.

In this work all nanocrystal quantum dots samples were synthesized by a hot injection technique producing colloidal nanoparticles growth with controlled diameters up to 5 nm depending on time evolution. The nanocrystal quantum dots show a high degree of reproducibility and chemical stability over long period of time (years), relatively narrow size distribution (<5%), uniform shape, high quantum efficiency (about 90%), treatable surface, and tunability in the visible and near infra red regimes. The surface of CdTe core nanocrystal quantum dots are chemically and electronically passivated by the epitaxially grown alloy-shell layer using CdTexSe1-x which has close crystalline matching, and by organic surfactants called ligands.

We present optical spectroscopy studies of core and core-alloy shell heterostructures nanocrystal quantum dots. Initially, comprehensive investigation on the influence of the thermally activated processes on the behavior of the photoluminescence of CdTe core and CdTe/CdTexSe1-x core-alloy shell nanocrystal quantum dots was performed. Then, these samples were characterized by a state of the art single dot spectroscopy technique enabling the study of the optical properties of an individual nanocrystal quantum dot.

We demonstrate that the confinement energy in the nanocrystal quantum dots is dependent on the temperature. The coupling with both acoustic and optical phonons temperature and alloy shell growth dependence is also studied. We show that by going from type I to type II configuration there is a change in Huang-Rhys factor.

The photoluminescence from individual CdTe/CdTexSe1-x core-alloy shell nanocrystal quantum dot was then studied by continues wave pumping laser. Single-exciton, biexciton, triexciton, and quadraexciton bands were resolved in the micro-photoluminescence spectrum of a single CdTe/CdTexSe1-x core-alloy shell colloidal quantum dot, revealing nearly blinking-free behavior. Multiexcitons were generated by a sequential filling of electronic shells with the increase of a continuous-wave excitation power, and their probability was evaluated under steady-state conditions. A partial carriers’ delocalization was determined at the core-shell interface, and an exciton binding energy was estimated by a second-order perturbation theory.