Optical Properties of Quantum Dots with a Core–Multishell Structure

In the last decade, colloidal semiconductor nanocrystals (quantum dots) have been not only studied fundamentally but also applied in photovoltaics, optoelectronics, and biomedicine. Beginning with simple approaches to the deposition of protective shells, e.g., ZnS on CdSe cores, searches for ways to increase the quantum yield of photoluminescence of quantum dots have resulted now in the development of new types of quantum dots characterized not only by record high extinction coefficients but also by high photoluminescence quantum yields. In this work, the optical properties of core–multishell quantum dots have been analyzed. These quantum dots have been specially designed to reach the maximum possible localization of excited charge carriers inside luminescent cores, which makes it possible to reach a photoluminescence quantum yield close to 100%. Core–multishell quantum dot samples with a shell thickness of 3–7 monolayers have been fabricated. Changes in the characteristics of optical transitions in such quantum dots with an increase in the number of layers of the shell have been studied. The effect of the thickness of the shell on the optical properties of prepared quantum dots has been analyzed. In particular, analysis of photoluminescence lifetimes of such quantum dots has revealed a possible alternative mechanism of radiation of core–multishell quantum dots based on the slow charge carrier transfer from the excited outer layer of the CdS shell to the CdSe core.