Chul-Ho Lee

Conventional gate-stack scaling reduces dielectric thickness and increases permittivity while largely treating the position and electronic character of the gate-side screening boundary as fixed. Here we show that this boundary can be engineered by converting the surface of the topological insulator Bi ₂ Se ₃ into insulating high-κ BiF ₃ . Position-resolved calculations reveal a gap-opened immediate amorphous-BiF ₃ /crystalline-Bi ₂ Se ₃ interface and a reconstructed gap-closed Bi ₂ Se ₃ -derived state in the adjacent subinterface layer, accompanied by a localized interfacial dipole. Capacitor measurements independently resolve a finite series response consistent with electronic compressibility at this boundary; this response lowers the nominal stack capacitance. Despite this capacitance penalty, MoS ₂ transistors with closely matched BiF ₃ thicknesses and a common BiF ₃ /MoS ₂ channel-side material interface exhibit near-thermionic-limit switching, negligible hysteresis and substantially weaker subthreshold drain-bias dependence than BiF ₃ -only controls. These results identify the position and electronic character of the gate-side screening boundary as additional design variables for transistor electrostatics beyond nominal dielectric capacitance.

In two-dimensional transition-metal dichalcogenides (TMDCs), the local electrostatic field of the chalcogen p -orbitals dictates the crystal field splitting of the transition metal d -orbitals, stabilizing diverse electronic states from semiconducting and semimetallic to exotic topological phases. On-demand substitution of the lattice chalcogens is therefore an efficient route to tuning TMDC electronic phases. Here, we custom-molded van der Waals (vdW) heterostructures by layer-by-layer vapor-phase chalcogen substitution starting from p -type WSe ₂ multilayers into which we locally inscribed insulating WO _x , n -type WS ₂ , and semimetallic WTe ₂ . This spatially precise conversion seamlessly integrated distinct electronic phases, both vertically and laterally, into modular vdW semiconductor heterostructures that are otherwise inaccessible. As a demonstration, we integrated WO _x /WSe ₂ local gate stacks to build ultrascaled metal oxide–semiconductor field effect transistors (MOSFETs), achieving an unprecedented subthreshold swing at subnanometer equivalent oxide thickness.

Controlling interactions between sensor surfaces and target molecules is central to achieving high selectivity and stability in chemical sensing. However, in two‐dimensional (2D) material‐based sensors, unintended adsorption, particularly of water molecules, often disrupts these interactions, degrading selectivity and long‐term stability. Here, we exploit molecular confinement within van der Waals (vdW) gaps of MoS ₂ to engineer sensor–analyte interactions. Ethanol (EtOH) confinement induces electron doping and hydrophobicity, whereas water confinement produces negligible doping and hydrophilicity. This combined modulation suppresses H ₂ O adsorption while promoting selective NO ₂ adsorption, enabling the EtOH‐confined sensor to detect NO ₂ from 200 ppb to 5 ppm, with a maximum response of 48% at 60% relative humidity and a detection limit of 36 ppb. Under highly humid conditions, it exhibits a sevenfold higher NO ₂ response and retains 95% of its initial response after 5 weeks, compared with 44% for H ₂ O‐confined films. First‐principles calculations reveal that molecular confinement regulates sulfur‐vacancy (V _s ) configurations: EtOH favors isolated and lined V _s , which promote NO ₂ adsorption while suppressing H ₂ O binding, whereas H ₂ O stabilizes clustered V _s , which increase H ₂ O affinity. This vdW‐gap confinement provides a design strategy to tune surface chemistry, electronic structure, and defect configurations for selective, humidity‐tolerant, and reliable 2D chemical sensors.

Two-dimensional (2D) semiconductors, with their atomically thin geometry and excellent electrostatic control, are promising channel materials for ferroelectric field-effect transistors (FeFETs). However, integrating high-quality ferroelectric gate stacks onto these ultrathin channels in a manner compatible with modern semiconductor manufacturing, particularly under back-end-of-line (BEOL) constraints, remains a major challenge primarily due to interfacial instability. Here, we demonstrate interface-engineered top-gated 2D FeFETs based on monolayer molybdenum disulfide (MoS2) integrated with ferroelectric hafnium–zirconium oxide (HZO) via atomic layer deposition. An ultrathin high-κ yttrium oxide (Y2O3) interfacial layer serves as both a nucleation and protection layer, enabling conformal HZO growth while preserving a sharp van der Waals interface. All fabrication steps were conducted below 300 °C, fully satisfying BEOL thermal budgets. The resulting devices exhibit intrinsic polarization-driven switching rather than trap-dominant behavior. Notably, they achieve a nearly ideal memory window of 3.6 V, corresponding to ∼90% of the theoretical maximum, under a low operating voltage of ±3.5 V, along with distinguishable nonvolatile switching characteristics. These results establish a CMOS and BEOL-compatible interface engineering strategy for integrating high-performance 2D FeFETs, providing a scalable pathway toward monolithic 3D systems for future embedded memory and logic-in-memory applications.

Photoelectrochemical (PEC) performance of conventional 2D transition metal dichalcogenides (TMDs) in hydrogen evolution reaction (HER) is constrained by the limited selection of metal cations, predominantly MoS ₂ , whose inert basal planes and unstable 1T phases hinder PEC efficiency. High‐entropy TMDs, in which local lattice distortion and charge redistribution occur within a van der Waals layered structure, are expected to overcome these intrinsic limitations by improving catalytic activity, photocarrier dynamics, and phase stability. Here, we demonstrate a wafer‐scale 2D high‐entropy (MoWTaNbRu)S ₂ thin‐film catalyst with distorted 1T phase on p ‐Si photocathode for PEC‐HER. The high‐entropy effect induces substantial electronic redistribution, enhancing the contribution of d ‐orbitals near the Fermi level and optimizing hydrogen adsorption energetics. PEC kinetic analyses, including intensity‐modulated photocurrent spectroscopy, demonstrate that (MoWTaNbRu)S ₂ markedly suppresses the recombination of photogenerated charge carriers, enabling more efficient charge extraction and accelerated interfacial reaction kinetics. Furthermore, the high‐entropy‐driven stabilization of the metastable 1T phase ensures excellent durability of the photocathode. As a result, the (MoWTaNbRu)S ₂ /TiO ₂ / p ‐Si photocathode shows a remarkable photocurrent density and stability for over 100 h, outperforming single‐metal TMDs. This study demonstrates how configurational entropy enhances catalytic activity, photocarrier transport, and phase stability of TMDs, establishing a general design principle for next‐generation PEC catalysts.

To realize in‐memory sensing and computing platforms, it is essential to integrate sensing, computation, and memory functionalities within a single device, enabling energy‐ and time‐efficient vision systems with high perceptual precision. However, achieving such multi‐functional processing capability within a compact device structure remains a major challenge. Here, a reconfigurable vertical photodiode based on α‐In ₂ Se ₃ is presented, a ferroelectric 2D semiconductor. Gradual and reversible modulation of built‐in electric fields at the top and bottom Schottky junctions is achieved through partial out‐of‐plane polarization switching of α‐In ₂ Se ₃ , enabling multi‐level, non‐volatile, and polarity‐tunable photoresponsivity. This allows analog programmability with a high degree of freedom in processing within a two‐terminal metal‐ferroelectric semiconductor‐metal (MFsM) structure, effectively resolving trade‐off between structural compactness and computing versatility. Leveraging these intrinsic characteristics, we demonstrate a versatile sensor‐level perceptual processing framework using a reconfigurable photodiode crossbar array. By exploiting the high‐density spatial integration capability, the system adaptively configures its spatial support to prioritize either suppressing environmental noise for robust feature extraction or preserving fine‐grained details for precise classification, depending on the task requirements. These results lay the foundation for highly scalable and energy‐efficient neuromorphic vision system with high perceptual precision.