Perovskites have emerged as a promising candidate for low-cost production of solar cells. However, the most critical barrier for commercialization of perovskite solar cells (PSCs) is the inadequate stability of the organic metal halide perovskites (OMHPs). The degradation of OMHPs is induced by light, heat, air, and electrical bias. The known degradation pathways involve the oxidation of I^- and Sn²⁺, dissolution of perovskites by moisture, irreversible reactions with water, ion migration, and ion segregation. To improve the stability of OMHPs various methods are adopted, such as additive engineering, perovskite surface treatment, and composition engineering. Surface ligands are used on top of perovskite thin films to passivate the undercoordinated ions leading to improved charge collection efficiency and stability of PSCs. However, not all surface ligands stay at the surface of the perovskite. Some of them penetrate the perovskite layer forming reduced dimensional phases at the surface. This kind of behavior not only alters the electronic nature at the interface, but also negatively affects the stability of the OMHPs compared to surface ligands that remain only at the surface. On the other hand, additives are commonly used to reduce defects in bulk of the perovskites and thus improve their stability. They improve the stability of OMHPs by controlling the morphology of OMHP thin films, improving the thermodynamic stability of Sn²⁺ and I^-, and lowering the ion migration and ion segregation. The stability of OMHPs is also significantly improved by incorporating bulky organic cations into the perovskite composition. Although these routes for improving the stability are optimistic, it is not clear how the surface chemistry of OMHPs and chemical nature of additives or organic cations affects stability.

Surface chemistry of OMHPs can be tuned to control the extent of ligand penetration by changing the composition and processing conditions of OMHPs. To this end, it is important to find out what affects the extent of ligand penetration. We find that the perovskite compositions used in this study have little or no effect on ligand penetration. However, the perovskite film processing conditions have a greater effect on ligand penetration. Using a family of phenethylammonium iodide (PEAI) with different substituents on the benzene ring, we show that the ligand penetration can be affected by type of substituents as well. Stabilizing the perovskite precursors is also important as degraded precursors lead to defective perovskites with poor stability. Here, we show that additives influence the thermodynamic stability of Sn²⁺ and I^- by changing the acidity of the precursor solutions. Using additives with a range of pKa we find that additives with higher pKa provide a more stabilizing chemical environment for Sn²⁺ and I^-. 

It is known that bulky organic cations improve the stability of OMHPs by shielding the metal-halide octahedra from air. However, how the structure of the organic cations affect the air, oxygen, and moisture stability of the OMHPs is not well understood. Using twelve different organic cations we show that the stronger the attractive interactions between the organic cations in two dimensional (2D) OMHPs the higher is the stability. The stability of 2D-OMHP thin films decreases as the orientation of the 2D sheets deviates from planarity with respect to the substrate plane.

Abstract: Over the years, nanomaterials research has advanced towards discovering versatile and readily accessible materials tailored for a diverse range of applications. A comprehensive understanding of materials’ phases and their transformations are integral to this effect to enable better synthetic control as well as the functionalization of nanomaterial properties. Among advanced characterization techniques, the transmission electron microscope (TEM) is a powerful tool that provides direct access to the nanoscale and, therefore, an indispensable tool in studying fundamental materials problems. This dissertation discusses several nanomaterial systems where TEM tools and techniques are utilized to gain a deep understanding of their chemistry. 

This dissertation focuses on structural and phase transformations of nanomaterials using in situ heating in the TEM, which allows direct observation of these dynamic processes. Reported here are studies of the phase transformation and stabilization of the mackinawite phase of iron(II) sulfide nanoplatelets, the structural transformation of gold-catalyzed tin(IV) oxide nanowires into gold core/tin(IV) oxide shell nanowire heterostructures, and finally the interaction between aluminum oxide and lead (at. 17%) lithium alloy proposed for use as a coolant in nuclear fusion reactors. These studies showcase the significance of knowledge of the mechanistic details of phase transformations, with the eventual goal of being able to determine and control structure-property relationships. 

KEYWORDS: phase transformations, nanomaterials, transmission electron microscopy (TEM), in situ TEM

Abstract: The principal concept behind biorefining involves the transformation of lignocellulosic biomass into valuable products and energy resources. Historically, biorefinery strategies for lignocellulosic biomass have primarily focused on improving the conversion of cellulose into ethanol, often neglecting the underutilized lignin component. Lignin consists of phenolic subunits, from which it follows that value-added products can be obtained from lignin depolymerization. Unfortunately, lignin utilization is particularly challenging due to its high structural irregularity and recalcitrance. The goal of this study was to develop an AuPd/Li-Al layered double hydroxide (LDH) bimetallic catalyst for efficient lignin depolymerization, resulting in the production of high-value aromatic compounds. The structural complexity of lignin renders the study of individual reactions in lignin difficult. Therefore, model compounds were used to evaluate catalyst performance. Initially, we prepared AuPd bimetallic nanoparticles with varying molar ratios supported on a basic Li-Al LDH using a sol-immobilization method. Subsequently, we characterized the synthesized catalysts and evaluated them in aerobic oxidation reactions of 1-phenylethanol and simple benzylic alcohols at atmospheric pressure to identify the most effective catalyst configurations. Those catalysts demonstrating promising performance were further examined in the aerobic oxidation of lignin model dimers containing ß-O-4 linkages. Remarkably, these model compounds underwent sequential oxidation, ultimately leading to the cleavage of the ß-O-4 bonds. Subsequently, we evaluated the catalysts in the oxidative deconstruction of ?-valerolactone (GVL) extracted from maple lignin at 120 °C, again using O2 as the oxidant. These results highlight the potential of the AuPd/Li–Al LDH catalyst system as an eco-friendly approach for lignin depolymerization under mild conditions, offering a promising avenue for valorizing lignin in biorefining processes.

Abstract: Understanding materials at their atomic level is important given that the macroscopic properties of a material are intricately linked to its nanoscale structure. This plays a pivotal role in advancing structural materials since their performance is significantly influenced by factors such as composition, and microstructure which consist of different interfaces, crystalline phases, and defects. 

In the automotive and aerospace industries reducing the weight of materials is critical to enhance fuel efficiency without compromising safety and performance. Lightweight aluminum alloys are extensively studied to replace heavier materials in these sectors. This work offers a comprehensive characterization of the evolution of various precipitates within aluminum alloys under laser treatment conditions, aiming to enhance their mechanical properties.

The thesis also delves into understanding the diffusion and dissolution mechanisms of metal nanoparticles on or into metal oxides. Metals like gold, in their bulk form, are traditionally considered chemically inert and inefficient as catalysts. At the nanoscale, however, as the particle size decreases, their catalytic activity towards various reactions significantly increases. Our exploration of these systems under in situ TEM heating has provided valuable insights into the structure-function relationships of these interfaces. This knowledge can be employed in optimizing the production of nanomaterials with enhanced interface properties.

KEYWORDS: Aluminum alloys, precipitate hardening alloys, SLV, TEM, in situ TEM