Dong-Jin Yun

The properties of 2D molybdenum disulfide (MoS ₂ ) strongly depend on its interactions with surrounding materials, making characterization in device‐like configurations essential. In this study, a MoS ₂ sample with a 20‐nm‐thick top AlO _x layer (embedded MoS ₂ ) was used as the dielectric‐semiconductor heterostructure in real devices. Hard X‐ray photoelectron spectroscopy (HAXPES) is the primary analytical technique employed for analyzing the embedded MoS ₂ samples. Compared to conventional X‐ray photoelectron spectroscopy (XPS) using an Al‐Kα source, HAXPES with a Cr‐Kα source offers greater information depth (ID) and broader access to electronic structures. This enables probing of the S KLL, Mo 2p3, and S 1s regions, in addition to the Mo 3d and S 2p regions, at an ID more than twice that of conventional XPS. A combination of Raman spectroscopy, transmission electron microscopy, X‐ray diffraction analysis, and synchrotron‐based HAXPES is used to optimize the monoatomic Ar ion beam sputtering conditions for electronic structure analysis of MoS ₂ , with the validation of the results. Consequently, the optimized HAXPES analysis enables the provision of the intrinsic electronic structure of MoS ₂ free from sputtering damage. It has been demonstrated that the inherent chemical bonding state of MoS ₂ prior to AlO _x deposition is fully preserved even in the embedded MoS ₂ sample.

Colloidal quantum dots (QDs), which exhibit tunable band gaps depending on their size and composition, are widely studied for light-emitting and optoelectronic applications. AgIn _x Ga _{1- x} S ₂ -based QDs are particularly promising due to their pure green emission, high blue absorption, and environmental friendliness. However, a comprehensive understanding of these quaternary QDs remains challenging because of the difficulty in examining their complex compositional structure. Here, we three-dimensionally characterize quaternary QDs (AgIn _x Ga _{1- x} S ₂ -based heterostructured QDs with core/shell and core/shell/shell structures) on the atomic scale using atom probe tomography. We reveal that both the AgIn _x Ga _{1- x} S ₂ /AgGaS ₂ QDs with and without an outer ZnS shell have compositional gradients at their interfaces and elemental inhomogeneity among their cores. Furthermore, an Ag-deficient AgIn _y Ga _{1- y} S ₂ layer is identified on the outer surface of the AgGaS ₂ shell, where the stoichiometric fractions satisfy x » y , arising from differences in the precursor reactivity. Meanwhile, in the AgIn _x Ga _{1- x} S ₂ /AgGaS ₂ /ZnS QDs, the outer ZnS shell evolves into Zn _{1-3/2 x} Ga _x S through a cation exchange process, ensuring structural and chemical compatibility with the inner shell. Our findings uncover the internal architecture and nanoscale elemental distributions of quaternary QDs, providing guidance for the future development of QDs.

Accurate characterization of buried electronic structures in solution-processed bulk heterojunctions (BHJs) is hindered by surface phase separation and spectral overlap between the donor and acceptor components. In this study, a depth-resolved analytical methodology is developed by integrating ultraviolet photoelectron spectroscopy (UPS) with argon gas cluster ion beam (Ar-GCIB) sputtering. This enables precise energy-level analysis of spin-coated P3HT:PCBM71 BHJ films of varying compositions (3:1, 1:1, and 1:3 by weight). Surface-sensitive UPS and reflection electron energy loss spectroscopy (REELS) characterizations reveal that the BHJ film possesses a P3HT-enriched surface that exhibits an electronic structure fully corresponding to that of P3HT but completely distinct from that of PCBM71. For the UPS depth profiles of the BHJ films obtained via Ar-GCIB sputtering, the individual HOMO levels of P3HT and PCBM71 are extracted by deconvolving the UPS spectra using a linear combination method. These values hold for different locations in the vertical direction, thus confirming the robustness of the technique. This approach facilitates the clear delineation of donor–acceptor energy-level alignments, which consistently range from 1.13 to 1.20 eV. These findings demonstrate the efficacy of UPS-Ar-GCIB depth profiling combined with spectral subtraction for non-damaging and accurate electronic structure analysis of complex BHJ systems.

This paper reports improvements made to the performance of single-walled carbon nanotube (SWNT) electrodes by integrating a glycerol-doped PEDOT:PSS (PEGL) layer, resulting in a novel material termed SWGL. The PEGL layer addresses key challenges in SWNT electrodes, including poor adhesion, low work function, and limited catalytic activity, by acting as an adhesive interface, carrier injection layer, and catalyst. Through a series of spin-coating and rinsing processes, SWGL films are prepared, exhibiting significantly improved adhesion to inorganic substrates, enhanced electrical conductivity, and stability under thermal treatment. The electronic structure, morphology, and performance of the SWGL films are characterized using techniques such as X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and scanning electron microscopy, revealing a gradual improvement in conductivity and work function with successive SWNT spin coatings. Optimized SWGL electrodes are evaluated by employing them in organic thin-film transistors (OTFTs) and dye-sensitized solar cells (DSSCs), which show substantial improvements in device performance. Specifically, SWGL-based electrodes exhibit excellent charge injection, reduced interface resistance, and comparable efficiency to conventional electrodes used in OTFTs or DSSCs. This work provides a versatile and cost-effective solution for the development of high-performance SWNT-based electrodes for use in diverse electronic devices.

The distribution of nitrogen in semiconductor devices plays a crucial role in tuning their physical and electrical properties. However, direct observation and precise quantification of nitrogen remain challenging because of analytical limitations, particularly at critical interfaces in silicon-based semiconductors. Although atom probe tomography has emerged as a powerful tool, distinguishing nitrogen from silicon without isotope doping is persistently difficult. In this study, we employ advanced atom probe tomography with an extended flight path under optimized conditions to characterize the three-dimensional nitrogen distribution in actual device structures, including 2- and 5-nm-thick silicon dioxide/silicon oxynitride-based gate dielectrics and a fin-structured three-dimensional device. Our analysis reveals that the nitrogen distribution determines the formation of the nitrogen profile in gate dielectrics, which in turn affects the diffusion of impurities, ultimately impacting the electrical properties and reliability. Our work provides insights into atomic-scale nitrogen behavior, paving the way for advancing next-generation semiconductor devices.

To reliably operate anode-less solid-state Li metal batteries, wherein precipitated Li acts as the anode, stabilizing the interface between the solid electrolyte and electrode is crucial. The interface can be controlled by a metal interlayer on the electrolyte to form a Li alloy buffer that facilitates stable Li plating/stripping, thereby mitigating the loss of physical contact and preventing short circuits. However, the mechanism governing stable Li plating/stripping in the metal interlayer without degrading battery materials remains unclear owing to an incomplete understanding of the dynamic and complex electrochemical reactions in the solid state. Through multiple operando and post-mortem analyses of the Li deposition behavior in the morphology, chemistry and microstructure, a close correlation is found between the Li-metal alloying process with the microstructural evolution and electrochemical performance when Ag, Au, Zn, and Cu interlayers were adopted on the garnet-type solid electrolyte Li_6.5La₃Zr_1.5Ta_0.5O₁₂. The Ag interlayer improved the interfacial stability enabled by Ag-dissolved Li, which inhibited dendritic growth, passing through the phase-separated Li-Ag alloy microstructure, while the other metals did not because of the Li plating at the Li-metal alloy/solid electrolyte interface. This work provides fundamental guidance for material selection and interface design, advancing the development of anode-less solid-state batteries.

High dielectric constant (k) materials have been investigated to improve the performance of dynamic random access memory (DRAM) capacitors. However, the conventional binary oxides have reached their fundamental limit of k < 100. In this study, we investigated alternative ternary oxides, SrTiO₃ (STO) and (Ba,Sr)TiO₃ (BSTO), which were epitaxially grown on SrRuO₃ (SRO) using atomic layer deposition (ALD). The structural compatibility between SRO and STO enables the in situ crystallization of STO during ALD at a low temperature of 300 °C. Consequently, STO on SRO exhibited no film deformation, a common issue during high temperature postdeposition annealing, and maintained superior crystallinity at a thin thickness down to 50 Å. Furthermore, the dielectric constant of STO can be adjusted by modulating its tunable ferroelectric and dielectric properties through Ba doping. BSTO, with a high dielectric constant (k_max:527) achieved at a Ba doping concentration of approximately 50%, displayed a low leakage current density (3.9 × 10^-8 A cm^-2 @ 1 V) and demonstrated excellent reliability of 10¹² cycles in the metal-insulator-metal capacitors. This study proposes a promising alternative to satisfy the extreme EOT required for next-generation DRAM capacitors.

The development of an analytical method for determining the properties of quantum dots (QDs) is crucial for improving the optical performance of QD‐based displays. Therefore, synchrotron‐based X‐ray photoelectron spectroscopy (XPS) is designed here to accurately characterize the chemical and structural differences between different QDs. This method enables the determination of the reason for the minimal differences between the optical properties of different QDs depending on the synthesis process, which is difficult to determine using conventional methods alone. Combined with model simulations, the XPS spectra obtained at different photon energies reveal the internal structures and chemical‐state distributions of the QDs. In particular, the QD synthesized under optimal conditions demonstrates a relatively lower degree of oxidation of the core and more uniformly stacked ZnSe/ZnS shell layers. The internal structures and chemical‐state distributions of QDs are closely related to their optical properties. Finally, the synchrotron‐based XPS proposed here can be applied to compare nearly equivalent QDs with slightly different optical properties.