Jae-Ung Lee

Two-dimensional (2D) transition-metal dichalcogenides are attractive for nanoelectronics and energy harvesting, where thermal transport critically impacts device reliability, performance, and energy efficiency. Here, we establish hierarchical defect engineering in monolayer MoS2 by combining growth-programmed mesoscale grain boundaries with He+-irradiation-induced atomic vacancies and quantify how these multiscale defects govern the in-plane thermal conductivity (κ) and its temperature dependence. By integrating opto-thermal Raman thermometry and variance-reduced Monte Carlo simulations, we decouple the distinct, temperature-dependent roles of these multiscale defects in phonon scattering. We reveal that while grain boundaries impose an approximately temperature-insensitive suppression by limiting long-mean-free-path phonons, vacancies specifically target high-frequency phonons, thereby dominating scattering at elevated temperatures. This complementary behavior enables a synergetic 85% suppression of κ at an ambient temperature (Ta) of 50 K, from 42.24 to 6.25 W m–1 K–1, confirming broadband phonon blocking. At Ta = 300 K, vacancy-induced scattering overrides grain boundary effects, causing the κ of highly defective single- and polycrystalline samples to converge. Our work provides fundamental design rules for tuning phonon transport in 2D materials and for deploying them in thermal management and thermoelectric applications.

Monolayer transition-metal dichalcogenides (TMDCs) host tightly bound excitons with unique valley pseudospin properties, establishing them as an emerging material platform for nanophotonics and quantum technologies. Exciton-exciton interactions modify light-matter coupling and significantly affect the formation of exciton complexes. Here, we employ a top-down nanofabrication technique to manipulate interexcitonic interactions in WS₂ monolayers through lateral confinement. By restricting the motion of excitons in confined two-dimensional (2D) spaces, interexcitonic interactions are significantly modified, resulting in strong biexcitonic emission in nanodots smaller than 100 nm that is obscured in pristine monolayers. Moreover, we demonstrate selective optical excitation of valley pseudospins for excitonic quasiparticles in confined monolayers. Our work highlights the role of spatial confinement in excitonic behavior in 2D systems and provides new insights into the development of future photonic and valleytronic devices with low-dimensional platforms.

Point defects play a crucial role in determining the properties of atomically thin semiconductors. This work demonstrates the controlled formation of different types of defects and their comprehensive optical characterization using hyperspectral line imaging (HSLI). Distinct optical responses are observed in monolayer semiconductors grown under different stoichiometries using metal‐organic chemical vapor deposition. HSLI enables the simultaneous measurement of 400 spectra, allowing for statistical analysis of optical signatures at close to a centimeter scale. The study discovers that chalcogen‐rich samples exhibit remarkable optical uniformity due to reduced precursor accumulation compared to the metal‐rich case. The utilization of HSLI as a facile and reliable characterization tool pushes the boundaries of potential applications for atomically thin semiconductors in future devices.

Mesoscopic-scale stacking reconfigurations are investigated when van der Waals films are stacked. We have developed a method to visualize complicated stacking structures and mechanical distortions simultaneously in stacked atom-thick films using Raman spectroscopy. In the rigid limit, we found that the distortions originate from the transfer process, which can be understood through thin film mechanics with a large elastic property mismatch. In contrast, with atomic corrugations, the in-plane strain fields are more closely correlated with the stacking configuration, highlighting the impact of atomic reconstructions on the mesoscopic scale. We discovered that the grain boundaries don`t have a significant effect while the cracks are causing inhomogeneous strain in stacked polycrystalline films. This result contributes to understanding the local variation of emerging properties from moiré structures and advancing the reliability of stacked vdW material fabrication.

Mesoscopic‐scale stacking reconfigurations are investigated when van der Waals (vdW) films are stacked. A method to visualize complicated stacking structures and mechanical distortions simultaneously in stacked atom‐thick films using Raman spectroscopy is developed. In the rigid limit, it is found that the distortions originate from the transfer process, which can be understood through thin film mechanics with a large elastic property mismatch. In contrast, with atomic corrugations, the in‐plane strain fields are more closely correlated with the stacking configuration, highlighting the impact of atomic reconstructions on the mesoscopic scale. It is discovered that the grain boundaries do not have a significant effect while the cracks are causing inhomogeneous strain in stacked polycrystalline films. This result contributes to understanding the local variation of emerging properties from moiré structures and advancing the reliability of stacked vdW material fabrication.

MoS₂ nanoscrolls that have inner core radii of ∼250 nm are generated from MoS₂ monolayers, and the optical and transport band gaps of the nanoscrolls are investigated. Photoluminescence spectroscopy reveals that a MoS₂ monolayer, originally a direct gap semiconductor (∼1.85 eV (optical)), changes into an indirect gap semiconductor (∼1.6 eV) upon scrolling. The size of the indirect gap for the MoS₂ nanoscroll is larger than that of a MoS₂ bilayer (∼1.54 eV), implying a weaker interlayer interaction between concentric layers of the MoS₂ nanoscroll compared to Bernal-stacked MoS₂ few-layers. Transport measurements on MoS₂ nanoscrolls incorporated into ambipolar ionic-liquid-gated transistors yielded a band gap of ∼1.9 eV. The difference between the transport and optical gaps indicates an exciton binding energy of 0.3 eV for the MoS₂ nanoscrolls. The rolling up of 2D atomic layers into nanoscrolls introduces a new type of quasi-1D nanostructure and provides another way to modify the band gap of 2D materials.