Academic Seminar by the School of Photovoltaic & Renewable Energy Engineering given by Prof. Efrat Lifshitz (Technion, Israel)

This seminar by Prof. Efrat Lifshitz will give an insight into the research looking at the impact of alloying or/and core/shell hetero-structuring on the physical properties of colloidal quantum dots, examined by specialized magneto-optical methodologies.

Colloidal semiconductor quantum dots (CQDs) are characterized by tunable electronic properties with variation of size, shape and composition. Colloidal techniques facilitate the formation of high-quality CQDs with surface passivation by molecular ligands or/and hetero-structuring. Numerous investigations have explored the optical and electrical properties of these materials, including the study of fluorescence quantum yields, electron-hole exchange interactions, excited state lifetime and polarization, generation of multiple excitons and hot carriers. However, a number of other fundamental issues concerning commonly observed fluorescence blinking, spectral diffusion, carrier intra-band and inter-band relaxations, remain as open questions. Related studies have demonstrated that the issues mentioned are associated with intrinsic properties of the inorganic moiety, such as exciton charging, Auger relaxation and phonon assisted cooling, or with surface and environment effects, such as surface-mediated charge trapping or surface passivation. Surprisingly, not much attention has been paid to the molecular ligands' degrees of freedom, e.g., passivation at selective facets and/or molecular vibrational modes.

The lecture will describe the work in the development of core/shell heterostructure CQDs by addressing the fundamental importance of a close crystallographic and dielectric match at the core/shell interface, facilitated through the introduction of alloyed shells. Heterostructure CQDs with optical activity both in the visible as well as in the near infrared spectral regime, will be presented.  The magneto-optical experiments revealed that a smooth core-shell interface controls the distribution of carriers between core and shell, assisting in increasing the fluorescence quantum efficiency and excited state lifetime.  Furthermore, a soft interface between core and a shell induces a reduction of Auger processes eliminating common fluorescence intermittency, thus, forming blinking-free CQDs.

A wide range of magneto-optical characterization tools will be shown. Among these techniques: linear/circular polarized photoluminescence measured in the presence of an external magnetic field, enabling assignment of angular momentum of an emitting state; and optically detected magnetic resonance to identify carrier trapping sites, exchange interactions and spin dynamics.