ANHARMONIC PHONON COUPLING AND IN-PLANE HETEROJUNCTION FABRICATION IN 2D IN2SE3

Thesis (Ph.D.), Physics, Washington State UniversityWe study the anharmonic phonon interactions in the single-crystal semiconducting (a) and metal-like (ß) van der Waals In2Se3 layers, through the determination and analysis of temperature-dependent Raman spectra and thermal conductivities, supported by first-principles calculations of phonon band structures. Our results indicate strong lattice anharmonicity in ß-In2Se3 giving rise to significant phonon peak broadening and a suppressed lattice thermal conductivity and reveal that the anharmonic phonon interactions are the main thermal transport-limiting mechanism in both phases. The low thermal conductivity combined with a large electrical conductivity makes the metal-like ß-In2Se3 a potential efficient thermoelectric material. Two-dimensional (2D) van der Waals materials and related heterostructures have shown a wide variety of novel electronic and opto-electronic properties. However, a key challenge in fully realizing their potential is a general lack of manufacturing techniques capable of producing desired heterostructures at a large scale. Here, we demonstrate a highly scalable direct-laser- writing approach to fabricating in-plane heterostructures in two-dimensional In 2 Se 3 . This approach derives from an optically activated solid-solid phase transition that leads to significant changes in local properties (semiconducting vs. metal-like), while preserving the single crystallinity of the local lattice, leading to well-defined heterointerfaces. Carrier transport across in-plane heterojunction devices fabricated by this technique exhibits asymmetric behaviors supported by the presence of interface energy barriers as revealed by Kelvin probe force microscopy. Our numerical modeling of the device characteristics reveals space-charge-limited and injection-limited conduction as the carrier transport mechanisms, in contrast to the standard diode model.Washington State University, Physic