Ultrafast Optical Spectroscopy Techniques applied to colloidal nanocrystals

In this thesis I will describe my experimental work based on ultrafast optical spectroscopy techniques applied to colloidal nanocrystals. Colloidal semiconductor nanocrystals in recent years have attracted a lot of attention in particular in optoelectronic applications, because they present unique optical, electronic and charge carrier transport properties that can be easily modified via colloidal chemical synthesis. In the first chapter of this thesis, I will introduce the basic concepts of nanocrystals and their optical properties, I will consider some simple models to explain the physical properties of semiconductor nanocrystals. I will briefly describe the colloidal chemical synthesis of these nanocrystals and how we can change the optical properties by simply acting on the colloidal chemical synthesis. In the second chapter I will describe the basic concepts of ultra fast optical spectroscopy techniques used in my experimental work. I will describe the basic principles and the experimental set-up of the two main techniques that I used: time-resolved photoluminescence spectroscopy (TR-PL) and transient absorption spectroscopy or also called Pump Probe. These two techniques allow to investigate in a very precise way the main optical properties and dynamics of charge carriers in nanocrystals. In the third chapter I will describe my experimental work based on timeresolved photoluminescence spectroscopy applied to CdSe=CdS core=shell nanocrystals. These nanocrystals are a reference nanocrystals and their optical properties are extensively investigated, however, some aspects are not fully understood, for example, the instability or blinking of the light emission under constant illumination in these nanocrystals is still an unsolved problem that limits the real applications. The particular spectroscopic technique that I have used variable pulse rate photoluminescence spectroscopy revealed the causes of this issue, and will give us a solution to resolve it. In the fourth chapter I will discuss novel nanostructures of CdSe=CdS multi branched shape, in this case octapod shaped nanocrystals consist of eight arms made of CdS grown on a CdSe core, these present a large crosssection for light absorption and efficient charge separation ideally suited for applications of photocatalysis. The optical spectroscopy technique used to investigate the properties of these nanocrystals are the transient absorption spectroscopy. In the fifth chapter I will discuss experimental work on Bi2S3 semiconductor nanocrystals and I will describe related optical techniques used to study the optical and electronic properties. This nanocrystal has excellent properties of optical absorption of solar radiation and can be used for the realization of solar cells. Another important property of Bi2S3 is its nontoxicity that will allow in the future to achieve efficient solar cells and easy disposal and simultaneously not damaging to the environment.