Tesi

 "Forze di deplezione in soluzioni colloidali: aggregazione guidata dall'entropia"

Relatore: Prof. Sciortino

Data di laurea: settembre 2022

Sommario

La dissertazione si propone lo scopo di indagare perché, in sistemi dove dovrebbero aver luogo solo spinte repulsive, in realtà esista una forza di attrazione, a prima vista inspiegabile. Questa forza, detta forza di deplezione, si manifesta in soluzioni colloidali quando sono presenti anche altre particelle, ed è dovuta all’inesorabile e talvolta apparentemente misterioso aumento dell’entropia. Quest’ultima, comunemente associata al disordine, è in realtà il motore dell’aggregazione spontanea e ordinata di molteplici sistemi.

Dopo aver introdotto e definito i punti fondamentali della questione, è descritto il modello di Asakura e Oosawa, che per primi studiarono le forze entropiche nei colloidi, intuendone la crucialità in fenomeni biologici, chimici e fisici. Si analizza poi più in dettaglio un caso classico di notevole rilevanza, ovvero un sistema composto da sfere dure, di cui due colloidali più grandi e le altre di co-soluti, più piccole e numerose: si spiega perché le due sfere di colloidi sono spinte l’una verso l’altra.

Talvolta l’attrazione entropica tra le particelle colloidali è di tale entità che il sistema può compiere una transizione di fase: può avvenire un’aggregazione spontanea delle particelle in strutture complesse, per esempio nella formazione di gel.

Il controllo delle forze di natura entropica come quelle di deplezione permette di generare sistemi colloidali con caratteristiche molto specifiche di stabilità, instabilità, viscosità, conducibilità: sono pertanto oggetto di numerosi studi ed esperimenti. Conoscere queste forze permette infine di dare una spiegazione teorica a fenomeni tra cui alcune condizioni mediche, come l’aggregazione anomala dei globuli rossi nel sangue.

 "Spatio-temporal exciton tracking using a SPAD camera"

Supervisor: Prof. Dr. Niek van Hulst 

Cosupervisor: Dr. Guillermo Brinatti

Data di laurea: agosto 2024

Abstract

Large-scale and highly-efficient photovoltaic technologies hold great promise for addressing energy scarcity and the global climate crisis. 

Optimizing the conversion of solar energy into electrical energy in photovoltaic devices, especially organic solar cells (OSCs), is crucial for enhancing their efficiency. A key challenge in OSCs is the limited diffusion of energy carriers, such as excitons, which often fail to reach the sites where they can be separated and converted into charges, to be stored as electrical energy. This limitation results in efficiencies significantly below the theoretical limit. To overcome this, it is essential to observe exciton transport with high spatio-temporal resolution following photoexcitation. Such observations would help identify the optimal physical, chemical, and structural properties of organic photovoltaic (OPV) materials, ensuring excitons travel further and faster before decaying, thus improving energy conversion efficiency. 

A recently introduced group of techniques, usually referred to as spatio-temporal microscopy (SPTM), allow the direct observation of the excitons with high spatial and temporal resolution. This is achieved by combining ultrafast spectroscopy techniques together with state of the art microscopy setups. Despite their success, these techniques are far from being universally spread, as their complexity and lack of speed and convenience limit their use to a very small niche of highly specialized research groups. 

To overcome such limitations, this thesis presents a novel method of SPTM for directly and rapidly measuring exciton diffusion using a single-photon avalanche diode (SPAD) camera. SPAD cameras, being still early generation experimental cameras, have never before been used in spatio-temporal applications, but they offer several key advantages: the use of a pixelated detector allows to strongly simplify the experimental setup, as it eliminates the need of beam scanning optics. Additionally, it allows parallel acquisition for all pixels, significantly increasing measurement efficiency, and it enables fast measurements, substantially reducing experimental time. The setup is also robust with respect to environmental fluctuations and misaligning, ensuring accurate and reliable data collection under varying conditions. Furthermore, the SPAD camera allows to achieve high spatial resolution on the nanometer scale and temporal resolution down to hundreds of picoseconds, which is critical for observing fast exciton dynamics. Moreover, the use of a pixelated detector in our setup offers a promising approach to investigating exciton transport, allowing to easily implement different spatio-temporal microscopy approaches, including the well-established diffraction-limited point excitation, together with recently introduced structured illumination approaches. This, together with the high sensitivity of the detector, allows to measure at low fluences, a capability that helps decouple actual exciton diffusion from non-linear processes such as exciton-exciton annihilation. 

This thesis details the experimental work conducted, from the characterization of the SPAD camera and the control electronics, to the design and realization of the experimental setup used for spatio-temporal exciton tracking. Finally, we discuss the obtained results, highlighting the significant improvements in measuring and understanding exciton diffusion.