arXiv subjects

Hee-Suk Chung

Publications and source records attributed to Hee-Suk Chung.

4 recordsLinked to original sources

arXiv2024

Emergent Topological Hall Effect in Fe-doped Monolayer WSe2

The topological Hall effect (THE) has attracted great attention since it provides an important probe of the interaction between electron and topological spin textures. THE has been considered an experimental signature of the topological spin texture of skyrmions. While THE has been widely reported in chiral magnets, oxide heterostructures, and hybrid systems such as ferromagnet/heavy metal and ferromagnet/topological insulators, the study of monolayer structures is lacking, hindering the understanding of noncollinear spin textures at the atomically thin scale. Here, we show a discernible THE via proximity coupling of Fe-doped monolayer WSe2 (Fe:WSe2) synthesized using chemical vapor deposition on a Pt Hall bar. Multiple characterization methods were employed to demonstrate that Fe atoms substitutionally replace W atoms, making a two-dimensional (2D) van der Waals (vdW) dilute magnetic semiconductor (DMS) at room temperature. Distinct from the intrinsic anomalous Hall effect, we found the transverse Hall resistivity of Fe:WSe2 displaying two additional dip/peak features in the temperature-dependent measurements, consistent with the contribution of THE. The topological Hall effect is attributed to the magnetic skyrmions that emerge from the Dzyaloshinskii-Moriya interactions at the Fe:WSe2 and Pt interface. Our work shows that a DMS synthesized from 2D vdW transition metal dichalcogenides is promising for realizing magnetic skyrmions and spintronic applications.

physics.atom-ph↗

arXiv2024

Defect Passivation of 2D Semiconductors by Fixating Chemisorbed Oxygen Molecules via h-BN Encapsulations

Hexagonal boron nitride (h-BN) is a key ingredient for various two-dimensional (2D) van der Waals heterostructure devices, but the exact role of h-BN encapsulation in relation to the internal defects of 2D semiconductors remains unclear. Here, we report that h-BN encapsulation greatly removes the defect-related gap states by stabilizing the chemisorbed oxygen molecules onto the defects of monolayer WS2 crystals. Electron energy loss spectroscopy (EELS) combined with theoretical analysis clearly confirms that the oxygen molecules are chemisorbed onto the defects of WS2 crystals and are fixated by h-BN encapsulation, with excluding a possibility of oxygen molecules trapped in bubbles or wrinkles formed at the interface between WS2 and h-BN. Optical spectroscopic studies show that h-BN encapsulation prevents the desorption of oxygen molecules over various excitation and ambient conditions, resulting in a greatly lowered and stabilized free electron density in monolayer WS2 crystals. This suppresses the exciton annihilation processes by two orders of magnitude compared to that of bare WS2. Furthermore, the valley polarization becomes robust against the various excitation and ambient conditions in the h-BN encapsulated WS2 crystals.

physics.app-ph↗

arXiv2019

Uniform vapor pressure based CVD growth of MoS2 using MoO3 thin film as a precursor for co-evaporation

Chemical vapor deposition (CVD) is a powerful method employed for high quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD based growth of monolayer molybdenum disulfide (MoS2) by using thermally evaporated thin films of molybdenum trioxide (MoO3) as the molybdenum (Mo) source for co-evaporation. Uniform evaporation rate of the MoO3 thin films provides uniform Mo vapor which promotes highly reproducible single crystal growth of MoS2 throughout the substrate. These high-quality crystals are as large as 95 um and were characterized by scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy and transmission electron microscopy. The bottom gated field effect transistors fabricated using the as grown single crystals show n-type transistor behavior with a good on/off ratio of 10^6 under ambient condition. Our results presented here addresses the precursor vapor control during the CVD process and is a major step forward toward reproducible growth of MoS2 for future semiconductor device applications.

cond-mat.mtrl-sci↗

arXiv2018

Surface-diffusion-limited growth of atomically thin WS2 crystals from core-shell nuclei

Atomically thin transition metal dichalcogenides (TMDs) have recently attracted great attention since the unique and fascinating physical properties have been found in various TMDs, implying potential applications in next-generation devices. The progress towards developing new functional and high-performance devices based on TMDs, however, is limited by the difficulty of producing large-area monolayer TMDs due to a lack of knowledge of the growth processes of monolayer TMDs. In this work, we have investigated the growth processes of monolayer WS2 crystals using a thermal chemical vapor deposition method, in which the growth conditions were adjusted in a systematic manner. It was found that, after forming WO3-WS2 core-shell nanoparticles as nucleation sites on a substrate, the growth of three-dimensional WS2 islands proceeds by ripening and crystallization processes. Lateral growth of monolayer WS2 crystals subsequently occurs by surface diffusion process of adatoms. Our results provide understanding of the growth processes of monolayer WS2 by using chemical vapor deposition methods.

physics.app-ph↗