טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAmel Alina
SubjectSynthesis and Caracterization of CdSe/CdTe Core Shell
Semiconductor Quantom Rods
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

This work is focusing on creating core-shell nanocrystals by controlling their size and shape. CdSe/CdTe core shell type II nanorods were selectively chosen for the present study due to their optical activity in the visible and near infra-red regimes. Band-edges of the constituents have staggered configuration and thus electrons and holes are spatially separated. In addition the long-lived states in these type II semiconductors make them attractive for photovoltaic applications. The investigated nanorods were synthesized through colloidal approach, and the shape was tuned by ligands, having different affinity to certain crystal faces. Profound fundamental chemical studies were performed on the nanocrystals, to confirm the stoichiometric composition of the samples and to determine the exact location of each component inside the nanorod. For this purpose we performed transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy and energy dispersive spectroscopy measurements. TEM measurements yielded the samples diameters and lengths, while chemical investigation revealed the locations of each atom; Se was found mostly in the centre, Te on the tips and along the length of the nanorods, and Cd was present everywhere. Absorption spectroscopy showed discrete energy band gaps in CdSe core, as well as decrease of a few higher energy inter-band transitions in core-shell with the increase of the shell thickness. Photoluminescence measurements of the core-shell, showed an unusual emission behavior by exhibiting two main peaks with an interesting dependence between them. These peaks were investigated further by photoluminescence excitation, temperature effects and time resolved photoluminescence. We assumed that one of the peaks (A) was attributed to core nanorods that were coated with a thin layer shell, while the other peak (B) represented core nanorods that were coated with a thick layer shell. Over time both types of the nanorods combined into one, creating high quality nanorods with thick layer shell. Other calculations as full width at half maximum and Stokes shift strengthen our hypothesis. Moreover, lifetime measurements were performed on these nanorods, and the results also strengthen the assumption that was previously presented, by displaying short/long radiative lifetime for peak A and B respectively. Finally, the samples were characterized by Raman spectroscopy, in order to learn more about their inner structure and their electron-phonon coupling behavior.