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Nanoparticles of magnetite (Fe3O4) are envisioned to find used in diverse applications, ranging from magnetic data storage, inks, ferrofluids as well as in magnetic resonance imaging, drug delivery, and hyperthermia cancer treatment. Their magnetic properties strongly depend on their size and morphology, two properties that can be synthetically controlled. Achieving appropriate control under soft chemical conditions has so far remained a challenging endeavor. One proven way of exerting this desired control has been using a biomimetic approach that emulates the proteome of magnetotactic bacteria by adding poly-L-arginine in the co- precipitation of ferrous and ferric chloride. The objective of the work presented here is to understand the impact of this polycation on the formation mechanism of magnetite and, through rational design, to enhance the control we can exert on magnetite nanoparticle size and morphology. We developed a SAXS setup to temporally and structurally resolve the formation of magnetite in the presence of poly-L-arginine in situ. Using analytical scattering models, we were able to separate the scattering contribution of a low-density 5 nm iron structure from the contribution of the growing nanoparticles. We identified that the low-density iron structure is a metastable precursor to the magnetite particles and that it is electrostatically stabilized by poly-L-arginine. In a process analogous to biomineralization, the presence of the charged macromolecule thus shifts the reaction mechanism from a thermodynamically controlled one to a kinetically controlled one. We identify this shift in reactions mechanism as the cornerstone of the proposed mechanism and as the crucial step in the paradigm of this extraordinary nanoparticle morphology and size control. Based on SAXS data, theoretical considerations suggest that an observed morphological transition between spherical, solid, and sub-structured mesocrystalline magnetite nanoparticles is induced through a pH-driven change in the wettability of the nanoparticle surface. With these results, we further demonstrate that SAXS can be an invaluable tool for investigating nanoparticle formation. We were able to change particle morphology from spherically solid particles to sub-structured mesocrystals merely by changing the precipitation pH. Improving the synthesis sustainability by substituting poly-L-arginine with renewable, polysaccharide-based polycations produced at the metric ton scale, we demonstrated that the ability to alter the reaction mechanism of magnetite can be generically attributed to the presence of polycations. Through meticulous analysis and the understanding of the formation mechanism, we were able to exert precise control over particle size and morphology, by adapting crucial synthesis parameters. We were thus able to grow mesocrystals up to 200 nm and solid nanocrystals of 100 nm by adding virtually any strong polycation. We further found a way to produce stable single domain magnetite at only slightly increased alkalinity, as magnetotactic bacteria do it. Thus through the understanding of the biological system, the consecutive biomimetic synthesis of magnetite and the following understanding of the mechanism involved in the in vitro synthesis, we managed to improve the synthetic control over the co-precipitation of magnetite, coming close biomineralization of magnetite in magnetotactic bacteria. Polyanions, in both natural as well as in synthetic systems, have been in the spotlight of recent research, yet our work shows the pivotal influence polycations have on the nucleation of magnetite. This work will contribute significantly to our ability to tailor magnetite nanoparticle size and morphology; in addition, we presume it will provide us with a model system for studying biomineralization of magnetite in vitro, putting the spotlight on the important influence of polycations, which have not had the scientific attention they deserve.
Solar cells based on hybrid perovskites materials have become significantly important among the third generation photovoltaics over the last few years. The first solid state solar cell was reported in 2012. Over the years, the power conversion efficiencies of these devices have increased at a tremendous pace and this has made the perovskite solar cell devices a serious competitor in the well-established market of thin-film and wafer technologies. Over time, a large number of articles on this topic has been published in peer-reviewed journals. The presence of lead in the most efficient hybrid perovskite materials have raised questions about the possible toxicity of these devices and the extent of their environmental impact. Therefore, a lot of research has been devoted to finding alternative perovskite materials with similar or even better opto-electronic properties. An alternative strategy to improve the efficiency of thin film solar cells is to build efficient tandem cells by combining two or more perovskite materials with specifically tailored band gaps. The first step towards the development of perovskite-only tandem solar cells is to identify complementary hybrid perovskite materials with specific band gaps that maximize the efficiency of tandem solar cells. The optimal set of optical gaps for a tandem structure made of two materials is 1.9 eV and 1.0 eV. Since the electronic properties of hybrid perovskites are known to be strongly dependent on the composition and distortion of the crystal lattice, strong focus has been made towards the structure optimisation as well as the calculation of the energy band gaps of the materials using density functional theory (DFT). In an attempt to study the structure-property relationship of these perovskite materials and to find novel perovskite materials for future applications, researchers have employed computational screening procedures to study a large range of these materials by systematic replacement of the cations and anions from the prototypical perovskite. Density functional theory in particular is used as a theoretical tool, because of it’s precision to determine the properties of materials and also it’s computational viability in dealing with complex systems. In this thesis, the main focus is to do a systematic screening of the perovskite materials, of the composition ABX3 again by replacing the A-site, B-site and the X-site elements to find novel materials with band gaps suitable for application in tandem solar cells. As a first step towards contributing to this vibrant field of research, a high-throughput computational screening has been performed by replacing the metal and the halogen in the conventional CH3NH3PbI3 perovskites with homovalent metals and halogens to find materials in the desired range of band gaps that has already been mentioned earlier. This is achieved by performing a geometry optimisation on all the simulated structures followed by calculating their energy band gaps at the semilocal and the hybrid levels of theory. However, it is well known that the rotation of the organic cation CH3NH3 hinders the stability of these devices by the formation of hydrogen bonds between the hydrogen atoms of the cation and the halogens. This causes the materials to degrade under normal temperature and pressure conditions. As an attempt to prevent these devices from being unstable, a next step has been taken where the CH3NH3 cation has been replaced by inorganic cations of similar ionic radius. This is followed by another thorough screening, similar to the previous step. The stability of the materials has been determined by using the empirical Goldschmidt tolerance factor. As a last part of the thesis, a small proportion of the inorganic cation is mixed with CH3NH3 in order to form mixed-halide perovskites. These structures are optimised and their band gaps are calculated using density functional theory in order to predict materials suitable for single junction as well as tandem solar cell devices. It is expected that the contribution made through this thesis will be helpful for the progress of perovskite solar cells in terms of efficiencies and will also allow the community to explore the different properties these materials for further progress and development.