Structural and Optical Properties of TiO2 and AgIn₅S₈ Nanoparticles for Optoelectronic Applications

Abstract

The global demand for sustainable energy solutions has intensified research into advanced materials for high-efficiency solar energy conversion. This thesis examines the structural, optical, and functional characteristics of titanium dioxide (TiO₂) nanoparticles (NPs) and AgIn₅S₈ (AIS) quantum dots (QDs), aiming to enhance the performance and reliability of emerging photovoltaic devices. The first part focuses on TiO₂ NPs, utilizing in-situ environmental transmission electron microscopy (ETEM) and scanning transmission electron microscopy (STEM) to monitor their phase evolution under varying thermal and environmental conditions. These experiments revealed the formation of Magnéli phase titanium suboxides (Ti₆O₁₁, Ti₂O₃) and demonstrated reversible oxidation in reduced TiO₂ NPs. Electron Energy Loss Spectroscopy (EELS) confirmed the coexistence of Ti³⁺ and Ti⁴⁺ states, offering insight into changes in electronic structure. Additionally, in-situ heating STEM studies showed that controlled annealing modifies grain boundary formation, enabling targeted defect and structure engineering of TiO₂ for improved electronic behaviour. The second part examines AIS QDs synthesized using a bottom-up colloidal method. Ultrasound-assisted growth produced QDs that were an order of magnitude smaller than those produced without sonication. High-resolution TEM (HR-TEM) verified that most of the sonochemically synthesized QDs were single-crystalline with minimal structural disorder, consistent with low Urbach energy values. Post-synthesis thermal treatment at 200 °C in air caused the QDs to grow to ~35 nm (chemical method) and ~23 nm (sonochemical method), with their band gaps approaching the bulk value of 1.8 eV. Furthermore, Zn doping and ZnS shell formation enabled precise control of optical and electronic properties, achieving band gap tunability up to 2 eV. The non-toxic nature and tunable functionality of AIS QDs make them promising for optoelectronic and biomedical applications. Overall, this research advances understanding of synthesis strategies, phase stability, and defect control in TiO₂ and AIS-based materials, paving the way for the development of efficient, sustainable SCs devices.