טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentShapiro Arthur
SubjectSynthesis and Characterization of Nanometric Semiconductor
Materials from Families: IV-VI and Perovskites
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

Semiconductor colloidal quantum dots (CQDs) have attracted vast scientific and technological interest throughout the past three decades, due to their unique tuneability of optoelectronic properties by variation of size and composition. However, the nanoscale size brings about a large surface-to-bulk volume ratio, where exterior surfaces have pronounced influence on the chemical stability and on the physical properties of the semiconductor. Therefore, numerous approaches have been developed to gain an efficient surface passivation, including a coverage by organic or inorganic molecular surfactants as well as the formation of core/shell heterostructures. Particularly the focus will be on IV-VI CQDs, such as lead and tin chalcogenides CQDs. Moreover, the achievement of tunable optical properties across a wide spectral range, along with an efficient surface passivation of these CQDs, has significant importance for scientific research and for technological applications. Also, other family of materials has been studied, lead halide perovskite nanocrystals. They have emerged as a new class of semiconductor materials due to their intriguing characteristics (e.g., sharp emission and defect tolerance), attracting great attention in science and engineering community. Recently, a new concept emerged by incorporating magnetic ions' doping or alloying into perovskites materials, inducing variation of the optical and magnetic properties. For this, usually the metal is partially exchanged requiring to overcome the tight binding of the metal ions to its octahedral halide coordination sphere.

In this work two methods to overcome the oxidation of lead and thin chalcogenides and to endow them with a broader optical activity are displayed: (i) the optical activities of PbSe CQDs have been tuned in the NIR (0.75-1.4 μm) and the SWIR (1.4-3 μm) ranges by a one-pot procedure which enabled the growth of relatively large PbSe CQDs (up to 14 nm) exploiting programmable temperature control during the growth process. A better chemical stability of the CQDs with respect to that of PbSe core CQDs was attained by shelling of PbSe by epitaxial layers of PbS, but limited to less than one day. However, air stability of the relatively large PbSe as well as the PbSe/PbS CQDs over a prolonged period of time (weeks) was achieved after a post-synthesis chlorination treatment. (ii) was explored the mechanism for formation of new nanostructures, SnTe/PbTe/SnO2, with a core/shell/shell heterostructure architecture. The new structure was endowed with optical tunability and chemical stability, which lasted more than one month at ambient conditions. The preparation involved a single-step post treatment for the pre-prepared SnTe cores, which simultaneously generated two different consecutive shells, governed by the remarkable Kirkendall effect.

In the field of perovskite NCs the study includes a thorough investigation of the Ni incorporation mechanism via structural, compositional and optical characterizations. The results indicated the essential need for cation-anion co-exchange, enabling control of the Ni concentration from < 1% to about 12%, with uniform distribution across the host lattice. In addition, the doping improved the photoluminescence quantum yields beyond those of the undoped nanocrystals. The observations were corroborated by the density functional theory, confirming that the incorporation of Ni is energetically favorable.