Kwang Seob Jeong

2D heterostructures have promising potential for applications in mid‐infrared (MWIR) photodetection but often suffer from transfer‐induced interfacial defects and limited micrometer‐scale active areas. In this study, PtSe _x thin films are synthesized via a scalable thermally assisted conversion (TAC) method that enables precise stoichiometry control and carrier‐type conversion from Se‐deficient n ‐type to Se‐rich p ‐type phases. Structural and spectroscopic analyses confirm layered structures and defect‐induced mid‐gap states that improve infrared absorption and facilitate bandgap engineering. Functional 2D/2D photodetectors are fabricated via a two‐step TAC approach that leads to vertically stacked n‐PtSe _1.7 /p‐PtSe ₂ junctions. The top n ‐type layer is rapidly synthesized without disturbing the underlying p ‐type film because n ‐type PtSe _1.7 forms within tens of seconds, whereas the formation of p ‐type PtSe ₂ requires several minutes. This kinetic selectivity preserves pristine interfaces, yielding reproducible rectification and robust MWIR photoresponse with a specific detectivity of 3.8 × 10 ⁹ Jones across centimeter‐scale active areas. This study establishes PtSe _x as a scalable material platform, with the transfer‐free growth approach and uniform film quality highlighting its strong potential for wafer‐scale integration of layered heterojunction devices.

Coherent hybridization between distinct material systems can fundamentally reshape the optical response, yet its quantitative influence on nonlinear susceptibility has remained largely unexplored. Here, we demonstrate that coherent coupling between CdSe-based quantum dots and monolayer MoS2 governs second-harmonic generation (SHG), giving rise to both a pronounced non-monotonic distance dependence and orders of magnitude enhancement in SHG intensity. By systematically tuning the interfacial separation, we uncovered a robust non-monotonic evolution of the SHG. This behavior cannot be explained by previously suggested incoherent mechanisms such as energy transfer or charge transfer, which predict strictly monotonic scaling and fail to account for the dramatic amplification observed. First-principles calculations reveal substantial conduction-band wave function delocalization across the heterogeneous interface, providing microscopic evidence of coherent electronic hybridization. Guided by this insight, we developed a rigorous intermaterial coherent hybridization (ICH) model in which the SHG response is governed by the mixing angle of hybridized eigenstates. The ICH model accurately reproduces both the non-monotonic distance dependence and the large enhancement of SHG without invoking dominant incoherent processes, establishing a deterministic hybridization framework for second-order nonlinear susceptibility at material interfaces. These results identify coherent electronic hybridization as a universal route for engineering a nonlinear optical response beyond intrinsic material limits, offering a new paradigm for interfacial nonlinear photonics.

Analyzing the nanocrystal (NC) size, morphology, and crystal structure is essential for understanding their fundamental properties. Here, we report NC-TEM Analyzer, a browser-assisted Python workflow for scale-calibrated transmission electron microscopy (TEM) image analysis of NCs. Demonstrated are scale-bar calibration, morphology-aware particle detection, spherical-dot diameter measurement, rotated-rectangle length, width, and aspect-ratio measurement for rod-like or platelet-like particles, and FFT-based lattice-spacing analysis from user-selected high-resolution TEM regions. Automatic candidates can be accepted, rejected, or geometrically corrected by the user, enabling human-in-the-loop validation. The software exports annotated images, histograms, summary statistics, CSV files, and structured metadata for reproducible comparison of nanocrystal samples.

Occupying discrete electronic states of a conduction band with electrons by tuning non‐stoichiometry, a phenomenon known as self‐doping, provides a powerful platform for exploring higher quantized states of a colloidal quantum dot (CQD). Here, we report three tunable intraband transitions in mid‐wavelength infrared to long‐wavelength infrared and very‐long‐wavelength infrared (λ >15 µm) arising from broken‐degeneracy in the conduction band of quantum dots. We demonstrated the three electronic transitions by using FT‐infrared absorption, FT‐infrared photoluminescence, and FT‐infrared photocurrent spectroscopy. In analogy with the atomic p‐orbital, the quantum dot exhibits a p‐like confined energy level in the conduction band (CB) arising from spatial quantization. Interestingly, the electronic transitions with the same angular momentum quantum number between 1P _e states, below 25 meV, are successfully detected by terahertz spectroscopy, suggesting that the 1P _e states of CQDs are not strictly orthogonal.

After displays, short- and mid-wave infrared (IR) sensing is seen as the next frontier for optoelectronic applications of colloidal semiconductor nanocrystals (NCs). Considerable research efforts have focused on two material platforms: lead (Pb) and mercury (Hg) chalcogenides. Although impressive progresses have brought this technology to the development of focal plane arrays, the presence of heavy metals is often recognized as a critical barrier to broader commercial adoption. This challenge has generated a push toward new synthetic pathways enabling greener IR-active NC materials. However, in practice, each material platform tends to create parallel, independent fields, thus mitigating the benefits of already established developments. In this perspective, we discuss how the Pb/Hg-based IR sensor legacy can benefit the emergence of III-V and silver chalcogenide platforms by offering genuine and critical discussions about the limitation of each kind of material. Finally, we propose some development strategies for the emergence of next generation greener NC-based IR imagers.

As demand for sustainable and biocompatible technologies grows, low-toxicity mid-infrared materials, such as silver chalcogenides, have attracted significant interest. Herein, we report mid-wavelength infrared tunable Ag ₂ Te colloidal quantum dots through a post-growth method starting from short-wavelength infrared Ag ₂ Te colloidal quantum dots. Using the synthesized Ag ₂ Te colloidal quantum dots, we successfully fabricate a photodetector covering the full mid-wavelength infrared spectral range (3–5 μm) with an onset wavelength extending to 6.9 μm. At 78 K, the photodetectors exhibit a photoresponse time of 230 ns (rise) and 576 ns (fall). Responsivity varies from 1.9 × 10 ^-3 A W ⁻¹ at 0.02 V to 1.1 A W ⁻¹ at 0.5 V, depending on the applied bias, and the specific detectivity of the device is 1.2 × 10 ⁹ Jones at 0.02 V. The measured noise-equivalent temperature difference of 0.3 K enables us to reliably distinguish temperature variations between 37 °C and 40 °C, directly enabling the diagnosis of fever-level body temperatures.

Rod-shaped tellurium microcrystals are promising mid-infrared photonic materials and a prototypical chiral (Weyl) semiconductor, yet intrinsic band-edge relaxation at room temperature is often masked by hot-carrier and photothermal pathways under above-gap excitation. Here, we employ mid-infrared asynchronous and interferometric transient absorption (MIR AI-TA) spectroscopy with frequency-comb pulses centered near the band-edge (380 meV) to directly interrogate two band-edge resonances. The time- and frequency-resolved AI-TA data reveal a fast component characterized by a time constant of 1-2 ps and a long-lived component with a time constant of ∼50 ps, common to both resonances. A global spectro-temporal model assigns the fast component to phonon-assisted redistribution of valence holes coupled to recovery of pump-induced Peierls distortion, while the slow component corresponds to band-edge electron-hole recombination. These results demonstrate that MIR AI-TA can quantitatively disentangle coupled electronic and structural dynamics in narrow-bandgap semiconductors.

Nanoparticle incorporation to supramolecular assemblies is essential for designing hybrid nanostructures with tailored optical and structural properties. However, understanding the interactions that govern the attachment and formation of such composites remains a challenge, particularly when complex structures are involved. In this study, we explore the fabrication of quantum dot (QD)/J-aggregate of tetrakis(4-sulfonatophenyl)-porphyrin (H₂TPPS₄) composites, where electrostatic interactions between cysteamine-functionalized QDs and negatively charged J-aggregate of H₄TPPS₄ with L-alanine play a key role in their formation. By systematically varying QD concentrations, we examine how QD loading influences the structure and attachment pattern of the composites. Quantitative transmission electron microscopy (TEM) image analysis and three-dimensional (3D) TEM tomography were employed to provide detailed insights into the interparticle spacing, thickness distribution, and 3D morphology of the QD/J-aggregate composites. The results show that higher QD concentrations lead to multilayered structures with decreased interparticle spacing, and TEM tomography reveals the helical arrangement of QDs on the H₄TPPS₄ with L-alanine framework. This work emphasizes the critical role of advanced imaging techniques and quantitative analyses in understanding the evolution of nanoparticle assemblies, opening new possibilities for the design of advanced hybrid nanostructures.