Charge Transfer in Ferecrystalline Compounds ([MSe]1+δ)m(NbSe2)n (M = Pb and Sn) and VSe2(m)-GeSe2(n)
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Date
2015-08-18
Authors
Alemayehu, Matti
Journal Title
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Publisher
University of Oregon
Abstract
The stabilization mechanism of misfit layer compounds has been debated in the literature for the past two decades with one group claiming entropy as the main driving force and another group suggesting charge transfer. The inability to synthesize higher orders of misfit layer compounds has prohibited scientists from thoroughly investigating this concept. However, the modulated elemental reactant technique has allowed researchers in the Johnson lab to access metastable products that are otherwise inaccessible. With ideal ferecrystal constituents, such as a metal and a semiconductor with sufficiently large differences in electrical properties, the stabilization mechanism of these compounds can be investigated.
This work focuses on investigating charge transfer in compounds of ([MSe]1+δ)m(TSe2)n where M = Pb and Sn and T = Nb. As a starting point, a comparative study between a ferecrystal and its misfit layer analogue, ([SnSe]1.16)1(NbSe2)1, was performed. Electrical property differences mainly resulted from the turbostratic disorder present in the ferecrystal. In addition, with the ability to make several m and n combinations of the ferecrystals, the concept of charge transfer was investigated. Charge transfer was observed to increase with increasing number of PbSe layers in ([PbSe]1.14)m(NbSe2)1. Similar studies were conducted for an isostructural compound: ([SnSe]1.16)m(NbSe2)1. In the case of ([SnSe]1.16)m(NbSe2)1, the extent of charge transfer was lower than that of ([PbSe]1.14)m(NbSe2)1. The data confirm that charge transfer plays an important role in the stabilization of these compounds.
To achieve the goals of material by design, both interlayer interaction and influence of interfaces need to be well understood. As the modulated elemental reactant (MER) technique allows the formation of different combinations of m and n including inorganic isomers, ferecrystals are the perfect candidate for investigating the influence of interfaces on charge transfer.
By pushing the limits of the MER technique, new heterostructures of VSe2(m)-GeSe2(n) were synthesized. This is the first GeSe2 based intergrowth with a crystal structure different from a typical rock salt structure. Hence, the versatility of the method allows the synthesis of superlattices from building blocks of a variety of structures.
This dissertation includes previously published and unpublished co-authored material.
Description
Keywords
Charge Transfer, Ferecrystals, Inorganic isomers, Polytypes, Transition metal dichalcogenides, van der Waals Heterostructures