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Abstract

Nanostructured materials, including nanoparticles, nanoplatelets and Metal Organic Frameworks are promising platforms for numerous applications in the field of renewable energy. Inorganic, semiconducting nanoparticles and nanoplatelets, for example, are ideal materials for the design of novel photovoltaic devices due to optoelectronic properties that are tunable through modification of their shape, size and composition. Much attention has been drawn to improving efficiency and device performance through altering the character of the nanomaterials, but the discovery of design rules for optimal device performance is still an open question. Due to challenges in controlling experimental techniques on an atomic scale, as well as numerous combinations of size, shape, composition and surface termination, experimental material design in this field is best complemented by accurate atomistic calculations. The latter can help interpret experiments, predict new materials and offer physical understanding. In this dissertation, we take a tour of three classes of nanomaterials that span different dimensionalities and offer different opportunities for renewable energy material design. First, we seek to understand the collective properties of a thin film of cubic lead sulfide nanoparticles that display quantum confinement in all three directions and are used in solar cell devices. We look at the combined effects of temperature and interactions between nanoparticles, and show that at finite temperature, interacting nanoparticles are dynamical dipolar systems with average values of dipole moments and polarizabilities substantially increased with respect to those of the isolated building blocks. We also present a critical discussion of various results reported in the literature for the dipole moments of nanoparticles. This work has important implications for understanding the nature of charge transport through nanoparticle thin films within a solar cell, as it is the interactions and spacing between nanoparticles that govern the charge transfer behavior. Next, we transition from the collective properties of a nanoparticle film to the optoelectronic properties of individual quasi-two dimensional cadmium selenide nanoplatelets. We use this quasi-2D material, quantum confined in only one direction, to develop a general, predictive computational protocol for calculations of 2D materials. Through building up this framework, we provide an understanding of the optical gap of CdSe nanoplatelets, a main experimental observable essential for photovoltaic devices. Our investigation of the optical gap is completed through disentangling the interplay between three main effects: biaxial strain, quantum confinement, and dielectric contrast between the material and its environment. We present the first calculations for these materials based on many-body perturbation theory, of both the fundamental gap and exciton binding energies, validating models that enable further investigation of larger and more complex systems. We discuss a series of models of quasiparticle energies that allow for comparison with previous theoretical predictions and provide the ability to directly probe the three key effects. These models provide a simple method to estimate the gap of complex nanoplatelets, with potential implications for the search of optimal nanomaterials for photovoltaic devices. Finally, we end with a short investigation of a recently synthesized 3D Metal Organic Framework that, while macroscopic in nature, exhibits a nanoporous structure ideal for gas storage, separation and catalysis. In conjunction with experiment, we show that these MOFs exist in an energetically favorable anti-ferromagnetic state, with the ferromagnetic and non-magnetic spin configurations inaccessible due to structural rigidity. Together, our first principles predictions of the optoelectronic properties of nanoparticles, nanoplatelets and Metal Organic Frameworks, along with experimental characterization and synthesis of similar materials, is expected to help guide the search for optimal nanomaterials for renewable energy devices.

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