A Self-Assembly Tale: collective structural and optical phenomena in supraparticles of nanocrystals

Abstract

Since their discovery more than 30 years ago, semiconductor nanocrystals, also known as quantum dots, have attracted increasing interest due to their intriguing optical properties. Just by changing the size of these nanocrystals is in fact possible to tune their photoluminescence emission in the whole visible spectrum. Quantum dots are nowadays employed in several commercial application, such as television and computer displays and luminescent solar concentrators. Furthermore they are forecasted to play an important role in the next generation of solar cells, LEDs and lasers. In most of these applications, quantum dots are in close-packed arrangements which, in some conditions, trigger the emergence of collective properties arising from the interaction between the single quantum dots. In order to study these collective properties, quantum dots can be induced to self-assemble in spherical crystalline structures called supraparticles or supraballs. The supraparticles constitute an ideal model system for these properties and they are easy to produce and manipulate. The purpose of my PhD was to study the collective phenomena emerging from the self-assembly of quantum dots in spherical supraparticles. This study has been performed on two different levels: on one side, I investigate the structural properties associated to the formation of the supraparticles; on the other side, I investigated the optical phenomena emerging from the interaction between the quantum dots or between the composing quantum dots and the optical environment surrounding the supraparticles. As for the structural phenomena, I showed that supraparticles can be formed also from anisotropic semiconductor nanocrystals (while preserving their optical properties) and I performed a detailed investigation on the formation mechanism of the supraparticles. As for the optical properties, I showed that supraparticles composed of quantum dots can be used to produce powders with highly tunable emission for applications as LEDs, and that supraparticles can produce laser light, behaving both as lasing cavity and gain medium at the same time. The findings of my PhD will hopefully give us a better understanding on the intriguing phenomena emerging from the interaction between semiconductor nanoparticles at the nanoscale in close-packed configurations, thus maybe promoting the use of the nanocrystals, and of the supraparticles, in commercial applications such as LEDs and lasers.