טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentBrumer Gilary Maya
SubjectOptical and Magneto-Optical Studies of Lead Salts
Nanocrystals
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz


Abstract

This work deals with the scientific and technological aspects of PbSe semiconductor nanocrystals quantum dots (NQDs). The utilization of these nanometric scaled materials is associated with the so called quantum size effect. This effect alters the electronic properties of a NQD by splitting the bulk energy bands into discrete states, shifting these transitions to higher energies. The lead chalcogenides in particular are of a special interest due to their tunability in the near IR regime making them useful for various optical and telecommunication applications.

The NQDs are synthesized by a colloidal method which permits the growth of NQDs with controlled diameters of < 10 nm. These NQDs present a high degree of reproducibility, relatively narrow size distribution (<8%), uniform shape, and offer the possibility to treat the surface.  In this nanometer size regime 40-60% of the atoms are located at the surface, leading to the existence of dangling bonds, stoichiometric and structural defects or adsorption of adatoms, therefore a chemical passivation of the surface is a crucial matter. This can be achieved by an epitaxial growth of another semiconductor, which covers the NQD surface, to form a core-shell structure.

            This work focuses on the synthesis, the structural and optical characterization, and the functionality of PbSe, PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell NQDs.  The core-shell and core-alloyed shell structures show chemical robustness over months and years, and band-gap tunability in the near IR spectral regime. Furthermore, these NQDs exhibit high emission quantum efficiency up to 55% and exciton emission band which is narrower than that of the corresponding PbSe NQDs. The optical activity of these materials in the near IR spectral regime, and their structural stability over long periods of time make them very attractive as the active components in a variety of optical applications, such as IR lasers.  

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.

            In addition, the amplified spontaneous emission properties of PbSe NQDs were examined under continuous illumination by a diode laser at room temperature, suitable for standard device conditions.