Synthesis and Characterization of Ternary Selenide Semiconductor Quantum Nanostructures

Abstract

Designed synthesis and characterization of nanomaterials is of key importance for various applications. Recently, the research focus in nanochemistry has shifted to multi-element nanomaterials with enhanced characteristics and/or multi-functionalities. In this respect, IIIIVI semiconductors and doped IIVI semiconductors are representative classes of materials for both the fundamental studies and future applications. This dissertation describes the synthesis of ternary selenide semiconductor quantum nanostructures via Lewis acid-base reactions and their characterization. In particular, size-controlled CuInSe quantum dots (QDs) have been produced and characterized, and their surface engineering for improved photovoltaic characteristics is discussed. In addition, synthesis and characterization of Mn2+-doped CdSe clusters is reported. First, CuInSe QDs, one of the most representative IIIIVI semiconductor nanomaterials, have been synthesized, and their size-dependent properties are examined. The developed preparation method, which utilizes selenocarbamate as Se precursor, can produce monodisperse CuInSe QDs with diameters ranging from 2 nm to 10 nm. The energy band alignment of the QDs is finely tuned for the optimal position corresponding to the effective light absorption and injection of electrons into TiO2 electrodes, and the effect of energy-band engineering of the QDs on their photovoltaic characteristics is investigated. Solar cell fabricated using 4 nm-sized CuInSe QDs, which do not contain any toxic element, exhibits the conversion efficiency of 4.30% under one sun light intensity with an air mass 1.5 G (standard conditions). Second, it has been demonstrated that the photovoltaic characteristics of CuInSe QDs can be significantly enhanced by controlling charge carrier recombination via surface engineering of photoelectrodes. In particular, varying the thickness of ZnS overlayers on QD-sensitized TiO2 electrodes can noticeably improve their conversion efficiency. With thick ZnS overlayers, both interfacial recombination with the electrolyte and non-radiative recombination from the QDs are significantly reduced, while the energetic characteristics of photoanodes are preserved. The best cell yields the conversion efficiency of 8.10%, which is a record for heavy metal-free QD solar cells (Oct., 2015). Finally, the last chapter describes the synthesis and characterization of single-sized Mn2+-doped CdSe clusters. Mass spectroscopy reveals that these clusters can be assigned to Cd13-xMnxSe13 clusters (x = 0, 1, or 2). Despite their small sizes, the doped clusters exhibit characteristics of diluted magnetic semiconductors. Interestingly, they exhibit multiple excitonic transitions with different magneto-optical activities, which can be attributed to fine structure splitting. Magneto-optically active states show a giant Zeeman splitting with g-factors of 81(±8) at 4 K. The results describe a new synthetic method for doped nanomaterials and facilitate understanding of doped semiconductor at the boundary between molecules and quantum dots.