Date on Master's Thesis/Doctoral Dissertation

12-2025

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Physics and Astronomy

Degree Program

Physics, PhD

Committee Chair

Sumanasekera, Gamini

Committee Member

Jasinski, Jacek

Committee Member

Yu, Ming

Committee Member

Mendes, Sergio

Author's Keywords

2D materials; magnetic materials; strain; transport measurements; angle-resolved polarized raman

Abstract

This work investigates the structural, magnetic, thermoelectric, and optoelectronic behavior of layered two-dimensional (2D) materials, with emphasis on Fe3GeTe2, Ni-substituted Fe3GeTe2 alloys, and GeSe. The study aims to clarify how dimensionality, alloying, and external perturbations influence charge transport, magnetic ordering, and light-matter interactions in van der Waals crystals. Single crystals of Fe3GeTe2 and (NixFe1-x)3GeTe2 (x = 0-1) were synthesized by chemical vapor transport and characterized using X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. Angle-resolved polarized Raman spectroscopy resolved symmetry-dependent phonon modes and showed that strain and thickness strongly modify the and vibrations. Electrical transport confirmed metallic conduction in Fe3GeTe2, while thermopower and resistivity reflected diffusive, phonon-drag, and magnon-drag contributions. Magnetotransport measurements identified ferromagnetic ordering with a Curie temperature near 220 K and a strong anomalous Hall effect. Nickel substitution reduced the strength of ferromagnetism, suppressed the anomalous Hall response, and produced a crossover toward paramagnetic behavior. Transport trends across the alloy series were modeled using phonon drag, magnon drag, electron-phonon, and electron-electron scattering mechanisms. GeSe crystals were synthesized in parallel to examine semiconductor transport and photoresponse. Structural and vibrational analyses confirmed high-quality orthorhombic GeSe. Temperature-dependent I-V characteristics indicated Schottky-type behavior dominated by thermionic emission at Ag/GeSe contacts. Under periodic red, green, and blue illumination, GeSe exhibited stable and reproducible photoconductivity, demonstrating its potential for optoelectronic and photodetector applications. Taken together, these results establish connections between lattice structure, magnetic order, and carrier dynamics in low-dimensional materials. The comparative study of Fe3GeTe2, (NixFe1-x)3GeTe2, and GeSe highlights how alloying, strain, and contact effects tune magnetism and transport, providing guidance for future spintronic and photoactive device design.

Download

Share

COinS