Synthesis, structural analysis, and optical properties of II-VI and III−VI semiconductor nanocrystals

Synthesis, structural analysis, and optical properties of II-VI and III−VI semiconductor nanocrystals

15/03/2022

seminar room

15:50

Faris Horani, Schulich Faculty of Chemistry, Technion, group of Prof Efrat Lifshitz

Nanoscale semiconductors have captured attention from a wide scientific and technological audience during the past three decades due to their electronic district states, tunable by size, shape, and composition. The properties mentioned inspired a potential implementation of nano-semiconductors in various technologies, including solar energy, light sources, display devices, and biological tagging. Most of the work has been focused on Cd-chalcogenides (the prototypical CdSe) and Pb-chalcogenides. To date, Pb- and Cd-chalcogenide nanocrystals (NCs) can be synthesized with unparalleled control over their size and shape, and their optoelectronic properties are to a large extent well-understood. However, the large-scale deployment of these conventional NCs is severely hindered by restrictions imposed on devices containing toxic elements such as Cd or Pb. This has led to an emergent interest in alternative materials that possess comparable properties and are based on less toxic elements as In-chalcogenides from the III-VI family. This thesis aims to advance the fundamental knowledge on both the synthesis and properties of low toxic In-chalcogenide materials as well as the II-VI materials.

This thesis is divided into three research themes: (1) The development of low toxic III-VI indium sulfide (In2S3) nanocrystals with various phases, morphologies, and compositions, as well as studying their physical and optical properties.[1],[2] This part extensively explores the growth mechanism which governs the formation of chemically and structurally stable In2S3 two-dimensional nanoplatelets and three-dimensional branched structures, called nanoflowers or nanourchins. (2) Low-temperature colloidal production of II-VI nanoplatelet heterostructures with high reproducibility and scale-up opportunities for industrial applications.[3] The presented techniques are alternative pathways that eliminate the requirement for excessive temperatures and/or multistep processes and provide rapid, inexpensive, and scalable procedures with practical advantages. (3) The influence of the surface coating on the optical and optoelectronic properties of colloidal semiconductor nanocrystals.[4] This part reports a thorough study that exposes the influence of surfactants’ dielectric screening on the exciton energy and exciton-phonon coupling in CdSe and CdSe/CdS colloidal QDs and NPLs.

The work in the thesis opens new horizons on the synthesis and design of colloidal II-VI and III-VI nanocrystals with unprecedented sizes, shapes, and heterostructures, in conjunction with understanding their optical and physical properties.

[1] Horani, F.; Lifshitz, E. Unraveling the Growth Mechanism Forming Stable γ-In2S3 and β-In2S3 Colloidal Nanoplatelets. Chem. Mater. 2019, 31 (5), 1784-1793.

[2] Horani, F.; Lifshitz, E. Deciphering the Structural Evolution and Growth Mechanism of 3D β-In2S3 Nanostructures. J. Phys. Chem. C 2019, 123 (50), 30723-30731.

[3] Horani, F.; Meir, I.; Lifshitz, E. Room Temperature Colloidal Coating of II-VI Nanoplatelets with Quantum Dots. J. Phys. Chem. C 2021, 125 (46), 25729–25738.

[4] Meir, I.;* Horani, F.;* Zuri, S.; Lifshitz, E. The tuning of exciton-phonon coupling in colloidal nanocrystals by a dielectric medium. Adv. Opt. Mater. 2022 (Invited paper, In review) (* Equal Contribution).