טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKigel Ariel
SubjectSynthesis and Optical Characterization of IV-VI
Semiconductor Nanocrystals
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

 

This work focuses on the synthesis, the structural and optical characterization, and the application functionality of PbSe, PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell colloidal quantum dots (CQDs). These materials exhibit unique chemical and physical properties. The utilization of these nanometric scaled materials is associated with quantum size effect. This effect alters the electronic properties of CQDs by splitting the bulk energy bands into discrete states, shifting these transitions to higher energies. The lead chalcogenides are of a special interest due to their tunability in the near infra-red (IR) regime making them useful for various optical and telecommunication applications.

The CQDs were synthesized by a colloidal method which permits the growth of CQDs with controlled diameters of < 10 nm and present a relatively narrow size distribution (<8%). In order to achieve better chemical and electronic passivation PbSe CQDs were covered, by epitaxial growth, with another semiconductor to form a core-shell or core-alloyed shell structures.

A comprehensive investigation of the influence of thermally activated processes on the behavior of the ground-state exciton emission of PbSe CQDs, performed in this work, showed an activation of the dark exciton between 1.4-7K via thermal enhancement of acoustic phonon coupling, and a bright exciton activation between 100-200K, overcoming the dark-bright states splitting. The experiment clarified points about the electronic structure of PbSe semiconductor CQDs, shed light on the controversial theoretical models regarding the magnitude of the dark-bright states splitting and the involvement of other non-radiative relaxations.

The amplified spontaneous emission properties of PbSe CQDs were examined on two different micro-cavity structures. One of the structures which based on CQDs of PbSe embedded in polymer revealed relatively large gain parameter (2.63-6.67 cm-1), which may be attributed to the relatively large sgs. The second structure with PbSe/PbS CQDs embedded between two Bragg reflectors, revealed initial results that show amplified spontaneous spectrum narrowed considerably the free space photoluminescence measurements.

Absorber saturation investigations of the core-shell and core-alloyed shell structures, and the corresponding PbSe core NQDs mentioned above, revealed a relatively large ground state cross section of absorption (sgs = 10-16-10-15 cm2) and proposed a behavior of a "fast" absorber with an effective lifetime (teff = ~ 4.0 ps). These NQDs samples were used as passive Q-switches in eye-safe laser of Er:glass, acting as saturable absorbers.