טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentSaraf Meirav
SubjectOptical and Electrical Studies of Semiconductors
Nanocrystals with Various Shapes and Structures
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

Colloidal semiconductor nanocrystals (NCs) exhibit band-gap and luminescence energies that are tunable with their size, due to a quantum size effect (QSE).  These NCs, are useful in various applications, such as: biological tagging, photovoltaic cells and light emitting diodes. The development of colloidal NCs has been extended, recently, from the pure adjustment of the size of the particles to the control of more sophisticated parameters as their shape, composition and structure. This work deals with the synthesis of different NCs varying their composition, shape and structure as well as investigation of their optical and electrical properties. The work included: (1) Synthesis of the following NCs: (a) spherical core and core-shell structures of PbSe and PbSe/PbS  NCs; (b) dots (spherical), rods and tetrapod-shaped NCs of CdTe. (2) Studying the electronic structure of single dot PbSe/PbS NCs by electrical measurements using scanning tunneling spectroscopy (STS). (3) Studying collective effects in a tetrapod-shaped NCs assembly, by optical measurements using photoluminescence (PL) and time resolved photoluminescence (TR PL).

The PbSe/PbS core-shell samples used in this study had a common core radius of 1.5 nm having a PbS shell of a variable width from 0.75 to 2.5 nm. The experimental results showed a decrease of the energy band-gap, as well as a decrease of a few higher energy inter-band transitions, with the increase of the shell thickness, in close agreement with the theoretical prediction, which correlated it with a delocalization of the carriers’ wavefunctions over the entire core-shell structure.  However, the increase of the outer radius of the core-shell NCs has a slightly smaller influence than a simple growth in size of PbSe cores, due to the existence of discontinuity in the effective mass and the dielectric constant at the core-shell interface. 

An assembly of CdTe tetrapod nanocyrstals (TNCs) with two different pod length showed an increase of the relevant contribution of the long-pod constituent on the expense of that of the short-pod partner in the mixed system compared to the pristine ones. This indicates the occurrence of Förster resonance energy transfer (FRET) among the TNCs with different pod length. The optical properties of this assembly were compared with those of a mixture of CdTe dots, which suggests an enhancement of an energy transfer rate among TNCs with respect to a process among adjacent dots, which may be related to the significant absorption cross-section and the large intact contact of the pods within the assembly.