טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentTilchin Evgeniy
SubjectQuantum Confinement Regimes in CdTe Nanocrystals Probed by
Single Dot Spectroscopy: from Strong Confinement
to Bulk Limit
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

The thesis presents the optical spectroscopy of single and assembled II-VI colloidal semiconductor nanocrystals (NCs). Particularly, in the objective of the current study lies the understanding of the impact of the different types of quantum confinement regimes on the NC’s excitonic properties. Those properties mainly governed by the Coulomb interaction between the photo-generated electron and hole, namely exciton, include its fine structure, non-radiative Auger recombination and hot charge carrier cooling. Since the Coulomb interaction is inversely proportional to the NC volume, the prototype structure was chosen in such a way that only NC size would affect the excitonic properties.

The chosen prototype structure represents various sizes of the CdTe core always passivated with a constant two-monolayer thick CdSe shell and Oleic Acid capping ligand revealing the total NC diameter ranging from 9 to 25.5 nm. Special attention has been paid to choosing the CdTe core size. Even the smallest NC is bigger than 4-5 nm diameter ones usually used in similar studies. Moreover, the thin shell passivation protects the optically active core only chemically without affecting its properties. Additionally, the chosen size range covers the properties of NC that are slightly smaller, and bigger than the bulk exciton Bohr radius (9 nm for CdTe) enabling them to follow after the evolution of excitonic properties from the quantum confinement to the bulk regime.

It has been found that the effective size of exciton grows slower than the actual NC size, revealing the exciton localization inside the semiconductor host matrix. This exciton localization results in homogeneous dielectric surrounding of exciton and prevents its penetration to the NC surface.

Consequently, the exciton is less sensitive to the NCs’ size distribution and the inhomogeneity of its environment. Moreover, the increase of the NC volume immediately suppresses the non-radiative Auger recombination and prolongs the hot exciton’s cooling rate.