Engineering topics

Lee, Gwan-Hyoung

Publications and source records attributed to Lee, Gwan-Hyoung.

4 records

DOE OSTI2024

Tailoring Interlayer Coupling in Few-Layer MoS 2 with Stacking Configuration

Here we manipulated the stacking configuration of a few-layer MoS 2 to investigate the impact of interlayer coupling on electrical band engineering. By simultaneously synthesizing two distinct stacking types of MoS2 islands, wedding cake (W) and spiral (S), on the same substrate, we explored layer-dependent electrical properties under identical experimental conditions. We used multiple scanning probe microscopy techniques to map local electronic properties with respect to the number of layers, stacking configurations, and local heterogeneities. First-principles calculations verified the role of distinct interlayer coupling in terms of the interlayer distance. Our findings highlight the critical role of interlayer coupling in applications of transition metal dichalcogenides.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Atom-by-atom imaging of moiré transformations in 2D transition metal dichalcogenides

Understanding the atomic-scale mechanisms that govern the structure of interfaces is critical across materials systems but particularly so for two-dimensional (2D) moiré materials. Here, we image, atom-by-atom, the thermally induced structural evolution of twisted bilayer transition metal dichalcogenides using in situ transmission electron microscopy. We observe low-temperature, local conversion of moiré superlattice into nanoscale aligned domains. Unexpectedly, this process occurs by nucleating a new grain within one monolayer, whose crystal orientation is templated by the other. The aligned domains grow through collective rotation of moiré supercells and hopping of 5|7 defect pairs at moiré boundaries. This provides mechanistic insight into the atomic-scale interactions controlling moiré structures and illustrates the potential to pattern interfacial structure and properties of 2D materials at the nanoscale.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

In Situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe 2

Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe 2 ) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2H–T d interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with in situ heating in the transmission electron microscope (TEM). By encapsulating the MoTe 2 with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the T d -to-2H phase transition initiates at phase boundaries at low temperatures (200–225 °C) and propagates anisotropically along the b-axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H-T d -2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Furthermore, our results provide insights on fabricating 2D heterophase devices with atomically sharp and coherent interfaces.

2D materials↗

DOE OSTI2018

Atomically precise graphene etch stops for three dimensional integrated systems from two dimensional material heterostructures

Atomically precise fabrication methods are critical for the development of next-generation technologies. For example, in nanoelectronics based on van der Waals heterostructures, where two-dimensional materials are stacked to form devices with nanometer thicknesses, a major challenge is patterning with atomic precision and individually addressing each molecular layer. Here we demonstrate an atomically thin graphene etch stop for patterning van der Waals heterostructures through the selective etch of two-dimensional materials with xenon difluoride gas. Graphene etch stops enable one-step patterning of sophisticated devices from heterostructures by accessing buried layers and forming one-dimensional contacts. Graphene transistors with fluorinated graphene contacts show a room temperature mobility of 40,000 cm 2 V -1 s -1 at carrier density of 4 × 10 12 cm -2 and contact resistivity of 80 Ω·μm. We demonstrate the versatility of graphene etch stops with three-dimensionally integrated nanoelectronics with multiple active layers and nanoelectromechanical devices with performance comparable to the state-of-the-art.

36 MATERIALS SCIENCE↗