Date of Award
Doctor of Philosophy
Utilization of solar energy continues to be a prominent topic in our society. The chemical and thermal stability of semiconductors is of great interest due to their widespread appeal and applicability as photoactive materials and in energy conversion devices. However, aside from surface chemistry studies and photo degradation (under continuous illumination), the general chemical reactivity and thermal stability of these materials is poorly understood. In this thesis, we use CdSe and CdS nanorods as model systems to investigate the behavior of II-VI semiconductor nanorods against various conditions of “extreme” chemical and physical stress (acids, bases, oxidants, reductants, heat). We demonstrate CdSe nanorods completely degrade in the presence of acids, but retain their structural and optical properties when subjected to basic or oxidative environments. Reductants, such as n-butyllithium, reduce CdSe to cadmium metal but hydrogen does not. Thermal treatment of both CdSe and CdS nanorods results in annealing, axial melting, and then coalescence of the particles. Axial melting did not depend on the type of inorganic material but on the ligand(s) coating the nanorod surface.
We also explore the synthesis of copper nitride (Cu3N) nanocrystals by nitridation of Cu2O nanocrystals with either ammonia or urea. We characterize the structure, optical properties, and morphology of both oxide and nitride phases using structural, optical and computational methods. Upon exposure to moisture and air, Cu3N decomposes to CuO. Simple thermodynamics calculations explain that the observed concomitant release of NH3 and N2 is due to kinetic factors.
Finally, we investigate the thermal stability of bulk methylammonium lead halide perovskites (CH3NH3PbX3 where X = I, Br, or Cl) to determine the decomposition products. Using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) coupled with a quadrupole mass spectrometer (QMS) and Fourier-Transform infrared (FT-IR) spectrometer, the decomposition temperature and evolved gas products are analyzed. Bulk iodide and bromide decompose by a similar route compared to that of the bulk chloride. For the iodide and bromide, ammonia and methyl halide evolve but the bromide evolves methylamine too. The chloride acts similar to reports in literature with evolution of methylamine and HCl. However, peaks for HCl overlap with the CH3 stretching of methylamine making it difficult to distinguish. Using this data and decomposition pathways, a way to achieve long-term thermal stability may be determined for methylammonium lead halide perovskites for solar cells.
Malinda Denise Reichert
Reichert, Malinda Denise, "Chemical and thermal stability of semiconductors" (2015). Graduate Theses and Dissertations. 14590.