Tuning the bandgap of Pb1-xSnxSe nanocrystals in hot colloidal synthesis
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This thesis presents a convenient oleylamine-based solution phase synthesis process for producing lead tin selenide (Pb 1-x Sn x Se) alloy nanocrystals. Lead selenide (PbSe) nanocrystals have size-tunable optical and electronic properties that make them interesting as a candidate for low-cost, solution-processed photovoltaic devices. However, the narrow intrinsic bandgap of PbSe means that very small nanoparticle size and extreme quantum confinement are required to produce materials with optical band gap at visible wavelengths. Substituting tin for lead to produce Pb 1-x Sn x Se alloy nanocrystals is expected to increase the intrinsic band gap, allowing tuning of the band gap to visible wavelengths at a relatively larger size. Three different kinds of Se precursor for the hot colloidal synthesis were evaluated in this work. I found that a Se precursor prepared by dissolving Se in oleylamine and a strong reducing agent was most effective and enabled simple and fast preparation of Pb 1-x Sn x Se at relatively low reaction temperature. The composition of Pb1-xSnxSe nanocrystals was determined by energy-dispersive x-ray spectrometry (EDS). The optical absorbance of the Pb 1-x Sn x Se nanocrystals was measured by UV-visible absorbance spectrometry. The morphology and crystal structure of the Pb 1-x Sn x Se nanocrystals were characterized by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), and x-ray diffraction (XRD). The nanoparticle composition was tuned over a wide range, while the bandgap varied over a relatively narrow range.