Stability, Reactivity, and Constituent Interaction in TiSe2-Based Metastable Misfit Layer Compounds Synthesized from Designed Amorphous Precursors

• Main Author: Merrill, Devin
• Other Authors: Nazin, George
• Language: en_US
• Published: University of Oregon 2016
• Online Access: http://hdl.handle.net/1794/19713

A series of intergrowth compounds with the basic formula [(MSe)1+δ]m(TiSe2)n are reported. The compounds are prepared from modulated elemental reactants and display interesting structural and electronic behavior. Section 1 of this dissertation outlines initial attempts to characterize constituent interaction. The first member of the SnSe based subclass is reported and displays the highest Seebeck coefficient of any m = n = 1 compound reported to date, and a surprising amount of order is observed, compared to previously reported compounds. With properly established deposition parameters, the synthesis was extended to included the m = 2-4 compounds. These compounds display interesting electronic behavior that suggests the band structure shifts considerably as the SnSe block is expanded, affecting the interaction between the constituent layers. The first compound based on BiSe is then reported, suggesting that the Bi structure donates more conduction electrons to the band structure. Targeted substitution through kinetic control is the focus of Section 2, and a family of (PbxSn1-xSe)1+δTiSe2 is reported over the entire range of x, even though a miscibility gap exists in the bulk PbxSn1-xSe system. The resulting alloyed intergrowth compounds also display equal or higher mobility than the end members, suggesting modulation doping could be used to affect transport properties. As a proof of principle, the analogous system based on a BixSn1-xSe constituent was prepared to attempt to systematically affect carrier concentration. It was found that while carrier concentration can be controlled, the evolving structure affects the doping efficiency of the Bi atoms and mobility in the structure. Section 3 outlines attempts to form higher order TiSe2-based heterostructures and the important chemical considerations observed during the preparation of these materials. The 3 component systems in the Pb-Sn-Ti-Se system can be formed at low temperature, with SnSe2 rather than SnSe. While at higher temperatures, topotactic reactions occur, causing rearrangement to the alloyed rocksalt structure. Compounds within the alloy system with m > 1 are presented which show surface segregation of Pb atoms, and a designed experiment suggests this is a thermodynamic effect. This dissertation includes previously published and unpublished coauthored material.