Crystal growth and band engineering of transition metal chalcogenide systems

Metal chalcogenides are a diverse class of materials with a wide range of chemical and physical properties, owing to their compositional diversity and structural flexibility. Their covalent rather than ionic character gives rise to narrow band gap. In addition, their electronic properties, including band gaps, are often tunable, which make them suitable for a variety of applications in optoelectronics, spintronics, and even thermoelectrics. This highlights the need for scalable high-quality crystal synthesis and following characterization. While recent interest in metal chalcogenides has driven extensive studies on materials like WTe2 and Fe(Se,Te), many remain underexplored. In response, my doctoral research set off with a relatively well-known Fe(Se,Te) system, specifically FeSe0.5Te0.5, motivated by predicted topological superconductivity relevant to quantum computing applications. Here, I explore various synthetic conditions to produce high-quality single crystals, which can be judged by the sharpness of superconducting transition (∆Tc) and the critical temperature (Tc). Concurrently, I investigated understudied systems Ag3AuSe2 and Ag3AuTe2. They are naturally occurring stoichiometric minerals with the same chiral cubic structure type. These semiconductors are predicted to have small band gaps, and this raised the question of whether closing these gaps could induce exotic physical phenomena such as band inversion and topologically insulating or conducting states. To address this question, I synthesized Ag3AuSe2 and Ag3AuTe2, and determined the band gaps experimentally and computationally using optical measurements and DFT simulations, which revealed strain-induced gap tunability. Two approaches are explored to achieve strain: chemical substitution (alloying Ag3AuSe2 with Ag3AuTe2) and mechanical compression. The results agree with the DFT predictions in which Ag3AuSe2 gap opens, while Ag3AuTe2 gap closes, under compressive strain. During this examination, I also synthesized cubic crystals of Ag3AuSe2, Ag3AuTe2, and Ag3Au(Se,Te)2 solid solutions for the first time by slow-cooling from the melt. They have micron-scale chiral domains, which hold promise for spintronic and optoelectronic applications, leveraging their nonlinear optical properties. My experience in solid-state synthesis developed over the course of my thesis work was instrumental in my final project on synthesizing large, high-quality crystals of WTe2, motivated from the experimentally realized quantum spin Hall (QSH) effect. I synthesized millimeter- to centimeter-scale crystals using the flux growth method, addressing the narrow solubility range for W-Te. I found that the crystal quality of WTe2, which was assessed using the residual resistivity ratio (RRR), depends on a complex number of synthetic parameters like reagent form, heating profile, and careful post-process steps. This work provides a glimpse into both well-established and underexplored metal chalcogenide systems and demonstrates my approaches to addressing key questions in each project. The methods developed in this work are expected to broadly apply to other similar systems and I hope this contributes to the expansion of the material library for future exploration.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Materials Science & Engr

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Won, Juyeon

Contributors dc:contributor

• Shoemaker, Daniel P.
• Schleife, André
• Zuo, Jian-Min
• Mahmood, Fahad

• Metal chalcogenide
• band engineering
• semiconductor

Statement dc:rights

• Copyright 2025 Juyeon Won

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129307

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129307