טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentStrassberg Rotem
SubjectMagneto-Optical Properties of CdSe/CdS Core/Shell Colloidal
Nanoplatelets Doped with Magnetic Impurities
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisor Professor Efrat Lifshitz


Abstract

Colloidal synthesis of semiconductor nanocrystals is a solution-based method, which has a significant cost advantage when compared to molecular beam epitaxy and chemical vapor deposition growth techniques. Colloidal synthesis method permits the growth of nanocrystals with controlled size (with diameters or thickness < 10 nm), well-defined shapes such as: quantum dots (QDs), quantum wires (QWs), nanoplatelets (NPLs) etc. and composition. Recently, colloidal semiconductor NPLs having lateral dimensions on the order of tens of nanometers and magic-sized vertical thickness have attracted considerable attention since they offer many advantages; among them are: strongly thickness dependent electronic structure and optical properties, extremely narrow emission bandwidths and large absorption cross-sections. Moreover, with the synthesis of core/shell architectures, the optical properties of NPLs have been shown to be enhanced with respect to the core-only NPLs since these structures have usually better fluorescent quantum yields, more robust optical properties, and permit wave function engineering between the core and the shell. Also, colloidal chemistry enables magnetic doping of semiconductor nanocrystals, providing an alternative and potentially lower-cost route towards magnetically active nanocrystals. This thesis presents an experimental study of the optical and magneto-optical properties of II-VI nanocrystals, and the dependence of these properties on the composition and electronic band-edge structure.

The incorporation of magnetic impurities into semiconductor nanocrystals with size confinement, promotes enhanced spin exchange interaction between photo-generated carriers and the guest spins.  This interaction stimulates new magneto-optical properties with significant merits for emerging spin-based technologies. In this project we elaborate on the direct observation of electron and nuclear spin effects in magnetically doped colloidal NPLs with a chemical formula of CdSe/Cd1-xMnxS, enabled by optically detected magnetic resonance (ODMR) accompanied with magneto-photoluminescence spectroscopy. The host matrix, with a quasi-type II electronic configuration, introduces a dominant interaction between a resident-electron and a magnetic ion. The observations indicate formation of a giant magnetization and a large g-factor of an exciton. Furthermore, the data convincingly presents the interaction between a carrier and nuclear spins of magnetic ions located at neighboring surrounding, with consequent influence on the carrier's spin coherence time. The nuclear spin contribution by the magnetic dopants in colloidal core/shell NPLs is considered here for the first time.