טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentZaiats Gary
SubjectCoupling of Air Stable Colloidal Quantum Dots to Plasmonic
Lens
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

Lead chalcogenides (PbSe, PbS, PbTe)  are seminconducting materials with a narrow, direct band gap in the infrared spectral region. When these semiconductors have dimensionality in nano-scale regime in all three dimensions, they are referred as quantum dots. Reduction of the characteristic dimensions confines spatially the charge carriers into a limited volume. This confinement increases inter-band energy gap, which is responsible for the most  of the  fundamental optical  properties of the semiconducting materials. Moreover,  continuous conduction and valence bands become discrete energy levels. Utilization of these effects is important for a wide variety of technological applications, as solar cell, night vision devices and  cancer therapy.  Due to high confinement regime in lead chalcogenides, they are good candidates for many of the mentioned above applications.  However,   successful utilization  of lead chalcogenide quantum dots in practical technological systems  is forestalled by  several obstacles. First of all, they suffer of a very limited stability under ambient conditions. Non-passivated lead chalcogenide colloidal quantum dots suffer of significant deterioration of their optical properties after less than one minute of exposure to air. Another limiting factor is the ability to efficiently collect the emitted photons in specific directions. In this work two methods to overcome the oxidation of lead chalcogenides are examined: (i) Formation of core/shell heterostructure, by cation exchange process. Although this process is a promising passivation route, thick shell might significantly deteriorate the conductivity properties of the nanoparticles, due to charge localization phenomena.  Thus we focus on early stages of cation exchange, in order to achieve thin shell.  (ii) Synthesis in situ atomic layer passivation (with halides). This route is technically much more convenient than core/shell approach and it doesn’t devolve the charge conductivity phenomena. Eventually effect of coupling photoluminescence of air stable PbS colloidal quantum dots to plasmonic lens are examined. We explore the passivation routes and surface plasmon coupling phenomena by using spectroscopical and structural characterization tools, as well as numerical calculations.