Date of Award

7-1-2022

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Chemistry

Content Description

1 online resource (xvoo. 184 pages) : illustrations (some color)

Dissertation/Thesis Chair

Jeremy Feldblyum

Committee Members

Marina Petrukhina, Alan Chen, Michael Yeung

Keywords

Semiconductors, Chalcogenides

Subject Categories

Chemistry

Abstract

Metal chalcogenide (MCh) semiconductors have long research history due to their earth abundance, easy synthesizability, and accessible band gap tunability. People have realized their functionality as p-type semiconductors that provide good hole mobility and conductivity within the materials. However, MChs have easier reaction pathway with gas molecules (H2O and O2) compared to other semiconductors such as metal oxides. Also, poor solution processability of all MChs make low-cost thin film fabrication methods hard to achieve. In this thesis, we target to understand both the chemical stability and solution processability of MCh materials. ZnSe based inorganic organic layered hybrids (LHs) were exfoliated by solution-based method the first time. The study suggests under optimum conditions, two materials, ZnSe(butylamine) and ZnSe(octylamine) can be exfoliated down to bilayers. Possible defects of the exfoliated sheets were detected by TEM electron diffraction mapping which originate from the edge of the materials. Further surface analysis by XPS and Raman confirmed the composition change (degradation) at the surface. Also, the extent of degradation is corelated with the intrinsic stability of the materials, which ZnSe(octylamine) that coated with longer organic ligands tend to have slower degradation rate than ZnSe(butylamine). A series of ZnSe-LHs with different types of aliphatic amine ligands coated on ZnSe layers were synthesized next and the chemical stability of all these materials under exposure to atmospheric environment were carefully examined. The result revealed materials with the longest alkylamine show best resistivity toward reacting with gas molecules. To confirm the reaction pathway of ZnSe-LHs with gas molecules and provide an insight into how 2D materials in general can be degraded, H218O and 18O2 isotopic study combined with density function theory calculation provide a H2O initiated degradation pathway of ZnSe-LHs. Dissolution mechanism of pure metals in thiol-amine cosolvent system (alkahest) was studied at the end of the thesis with the aim to synthesize MChs by solution processing method. By mixing ethylenediamine (en) with 2-mercaptoethanol (mer), a wide range of metals can be dissolved well in this semi-eutectic like system that has ionic conductivity. Although mer/en alkahest shows good stability in inert condition, it will form disulfide product when mixing in air. Several tris(ethylenediamine) metal (II) bis(2-hydroxylethanethiolate) crystals have been isolated from this cosolvent system in inert environment. All metals are oxidized into 2+ and coordinated with three neutral en ligands, the counter ions are two deprotonated mer ligands, which show great instability and will be further oxidized upon exposure to air. Also, in this alkahest system, solutions of redox active metals such as Mn, Co and Fe showed reversible color change between inert and air condition. Studies carried out by uv-vis and EPR study showed change of metal oxidation state and potential formation of superoxide ions within the solution when exposed to air.

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