טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentOsovsky Ruth
SubjectOptical Spectroscopy of a Single and an Ensemble of CdTe
Semiconductor Nanocrystal Quantum Dots
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

This thesis presents optical and magneto-optical spectroscopic studies of CdTe semiconductor nanocrystal quantum dots (NQDs) prepared by chemical colloidal synthesis. The work deals with the study of the optical response of the NQDs in a variety of spatial configurations: 1) as an ensemble of dispersed NQDs; 2) integrated into larger NQDs assemblies, required for practical NQDs-based applications and exhibiting new collective effects arising from interactions between proximal NQDs; 3) by a state-of-the-art single dot spectroscopy technique enabling the study of the optical and physical properties of an individual NQD. The utilization of these nanometric scaled materials is associated with a quantum size effect where the bulk energy-bands are split into discrete electronic states with a tunable band-gap. The CdTe, in particular, shows increasing promise due to its optical activity both in the visible and near-IR spectral regime.

Energy transfer processes in NQDs assemblies could facilitate the creation of novel light-harvesting devices. Förster resonance energy transfer (FRET) investigations of electrostatically mixed and covalently linked CdTe NQDs assemblies, revealed alterations in the photoluminescence (PL) spectrum and a reduced lifetime on the formation of the electrostatic and covalent interactions (from τ = 5.5 ns for the pristine NQDs to τ = 1.2-3 ns), proposing highly efficient FRET processes with rates of kET = 1010-1011 s-1.

The highlight of the research was the establishment of a high-resolution (~1 μm) cryo-magnetic fiber-based confocal microscope setup for unique single dot spectroscopy investigations at extreme conditions of 4 K and under high magnetic fields.  The experimental work included the optical design of the microscope and the development of a sensitive experimental methodology. The microscope was fully characterized and showed reproducibility of the results. For technological implementation as laser materials, multiple-excitons in colloidal NQDs are of a special interest as the groundwork of optical gain devices. By the use of the new single dot spectroscopy facility, for the first time, the signature of four sharp well-resolved multi-excitons emission bands was observed, in the low-temperature μ-PL spectrum of a single CdTe/CdSe core/shell NQD hetero-structure excited with a continuous-wave (cw)-laser. Auger process stimulates fast quenching of the multi-excitons radiation within few picoseconds, inhibiting their detection in time-integrated cw-PL experiments. Our CdTe/CdSe NQDs exhibit an extended PL quantum efficiency of 88% and a reduced Auger process. This investigation supplied information that was previously obscured in ensemble measurements, and under a simple cw-pumping experimental condition, practical for real-life applications.