טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentRubin-Brusilovski Anna
SubjectInfluence of Interfacial Strain, Shape Anisotropy and
Air Exposure on Electronic and Optical Properties
of IV-VI Collodial Nanocrystals
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

Colloidal lead chalcogenide (IV-VI) nanocrystals, such as quantum dots (QDs) and quantum rods (QRs) are of widespread scientific and technological interest, owing to their size tunable energy band gap at the near-infrared optical regime. This thesis presents an experimental and theoretical study of the electronic and optical properties of IV-VI nanocrystals, and the dependence of these properties on the interfacial strain of core/shell QDs, shape anisotropy of colloidal QRs, and exposure to ambient conditions.

The interface in PbSe/PbS core/shell QDs is subject to strain forces due to a 3% crystallographic mismatch between the constituents. The first part of this thesis discusses how the strain profile in PbSe/PbS QDs was simulated using the classical linear elasticity model, under the assumption of spherical-symmetric dot and isotropic materials. The derived strain profile was incorporated into a band structure calculation to evaluate the influence on the electronic band-edges of the core/shell QDs. The electronic energy states evaluated were in close agreement with the absorption edges of various core/shell QDs with different core diameters and shell thicknesses.  Furthermore, the synthesized QDs underwent thermal annealing at various temperatures, thereby creating the alloying interface; consequently, their absorption and photoluminescence spectra exhibited spectral red-shift compared with the untreated samples. The band gap energy red-shift was simulated by the theoretical model, including smoothing potential at the interface. Measurements of the photoluminescence decays indicated an extension of the radiative lifetime after a controlled annealing process, denoting removal of defect quenchers around the core-shell interface. Thus, the study suggests practical means for mitigating interface strain to leverage the quality of core/shell structures.

In the second part of this thesis, the synthesis and structural and optical characterization of PbSexS1-x and PbSe/PbSexS1-xQRs are reported; the QRs have a diameter between 2 nm and 4.5 nm and a length of 10 nm to 38 nm.  The energy band gap of the QRs exhibits a pronounced variation upon the change in diameter and composition, with a minor influence on lengths beyond 10 nm.  The photoluminescence spectrum of the QRs consists of a dominant band, accompanied by a satellite band at elevated temperatures.  The dominant band shows an exceptionally small band-gap temperature coefficient, and negligible extension of the radiative lifetime at cryogenic temperatures, compared with the photoluminescence processes in PbSe QRs and in PbSexS1-x quantum dots with similar band gap energy. A theoretical model suggests the occurrence of independent transitions from a pair of band-edge valleys, located at the L-points of Brillouin zone, related to the dominant and satellite emission processes.  Each valley is four-fold degenerate and possesses a relatively small electron-hole exchange interaction.

Finally, in the third part of this thesis, small-sized PbSe/PbS core/shell colloidal quantum dots with a core diameter of 2-2.5 nm and shell thickness of 0.5-1.0 nm are investigated. The PbSe/PbS core/shell CQDs are chemically stable under time-limited air exposure and have emission quantum efficiency of 60% at room temperature. Theoretical calculations associate these parameters to the small-size regime as well as to a lift of band-edge degeneracy due to slight shape anisotropy.