arXiv subjects

Sunmin Ryu

Publications and source records attributed to Sunmin Ryu.

At least 19 recordsLinked to original sources

arXiv2026

Dimensional Control of Excitonic Interactions in Exfoliated 2D Molecular Crystals

Two-dimensional (2D) materials provide unique opportunities to tailor excited-state properties through reduced dimensionality, altered dielectric screening and layer-dependent structural reconstruction. While such effects have been widely explored in norganic systems, their realization in molecular crystals has been limited by the difficulty of controlling thickness at the atomic scale while preserving crystalline order. Here we show that tetracene and three other molecular crystals can be mechanically exfoliated into mono-, few- or multilayer flakes, while retaining crystalline order. This capability enables new studies of molecular crystals across a well defined thickness range within the same structural organization. Thickness-dependent spectra of these samples reveal how out-of-plane confinement modifies the excited-state energy landscape of tetracene: With decreasing thickness, the Davydov splitting diminishes, the Stokes shift increases, and signatures of more delocalized excitons emerge. Electron diffraction and exciton model-based analyses correlate these trends to changes in molecular packing, intermolecular coupling and dielectric screening. Our results also demonstrate that key features of molecular excitons can be systematically tuned by layer number, extending dimensional control from inorganic 2D materials to molecular crystals.

cond-mat.mtrl-sci↗

arXiv2026

Dual-Mode Exciton Coupling in Epitaxially Registered Organic-Inorganic 2D Heterocrystals

Two-dimensional (2D) heterocrystals comprising molecules and semiconductors can serve as an ideal platform for studying interfacial excitons and for future optoelectronic applications, yet the energy and charge flow across these atomically sharp interfaces remain unclear. In this work, we investigated PTCDA-MoS2 as a prototypical 2D organic-inorganic heterostructure and revealed dual-mode exciton coupling between the constituent crystals. Monolayer-resolved PTCDA molecular crystals were grown on monolayer MoS2 via physical vapor assembly, and their crystallographic details, including the stacking angle, were determined by electron diffraction. Upon the formation of the heterostructures, PTCDA's photoluminescence was completely quenched because of organic-to-inorganic hole transfer, whereas that of MoS2 increased markedly with PTCDA thickness. Using differential reflectance and photoluminescence excitation spectroscopy, we found that the enhancement arises from two distinct mechanisms. Ground-state charge transfer injects holes into MoS2, which suppresses negative trion formation and enhances the radiative recombination of neutral excitons. In addition, resonant energy transfer, enabled by spectral overlap between PTCDA and MoS2, diverts excitation energy from PTCDA to MoS2. Our findings reconcile previously proposed mechanisms and establish a unified framework in which charge and energy transfer cooperate to govern exciton coupling at organic-inorganic interfaces.

cond-mat.mtrl-sci↗

arXiv2026

Activated Migration of Localized Ligand-Field Excitons in Atomically Thin CrCl3

Two-dimensional crystals with densely packed atoms exhibit a range of emerging properties, particularly a wide variety of excitonic behaviors. Thickness-variable layered chromium trihalides with finite surface recombination sites provide an ideal system for understanding how excitons confined in octahedral ligand fields migrate on nanometer length scales, a regime that defies conventional transport probes. In this work, we demonstrate that Cr3+-derived photoluminescence in CrCl3 is spectrally thickness-independent, but its relaxation dynamics are strongly sensitive to thickness and temperature, thereby indicating significant activated migration. A diffusion-coupled surface recombination model reveals an effective out-of-plane diffusivity of 4.5 x 10-6 cm2/s for the ligand-field excitons and a diffusion activation energy of 130 meV. The latter is comparable to the reorganization energy independently estimated from optical Stokes shifts, suggesting that exciton transport is coupled to local lattice relaxation. Furthermore, we show that the relaxation dynamics can be systematically tuned by either enhancing or suppressing surface recombination through controlled surface reactions or encapsulation. This work not only reveals the nanoscopic transport of localized ligand-field excitons but also establishes a spectroscopic transport probe applicable to various 2D materials.

cond-mat.mtrl-sci↗

arXiv2025

Ubiquitous Antiparallel Domains in 2D Hexagonal Boron Nitride Uncovered by Interferometric Nonlinear Optical Imaging

Hexagonal boron nitride (hBN) supports a wide range of two-dimensional (2D) technologies, yet assessing its crystalline quality over large areas remains a fundamental challenge. Both antiparallel domains, an intrinsic outcome of epitaxy on high-symmetry substrates, and associated structural defects have long evaded optical detection. Here, we show that interferometric second-harmonic generation (SHG) imaging provides a powerful, nondestructive probe of lattice orientation and structural integrity in chemical vapor deposition-grown hBN. This approach reveals the ubiquitous formation of antiparallel domains and quantifies their impact on crystalline order. SHG intensity also emerges as a direct optical metric of domain disorder, spanning three orders of magnitude across films produced by ten different growth routes. Correlation with Raman spectroscopy establishes a unified framework for evaluating crystalline quality. Beyond hBN, this method offers a high-throughput route to wide-area structural imaging in various non-centrosymmetric materials, advancing their deployment in electronics, photonics, and quantum technologies.

cond-mat.mtrl-sci↗

arXiv2025

Single-Shot Multispectral Encoding: Advancing Optical Lithography for Encryption and Spectroscopy

Most modern optical display and sensing devices utilize a limited number of spectral units within the visible range, based on human color perception. In contrast, the rapid advancement of machine-based pattern recognition and spectral analysis could facilitate the use of multispectral functional units, yet the challenge of creating complex, high-definition, and reproducible patterns with an increasing number of spectral units limits their widespread application. Here, we report a technique for optical lithography that employs a single-shot exposure to reproduce perovskite films with spatially controlled optical band gaps through light-induced compositional modulations. Luminescent patterns are designed to program correlations between spatial and spectral information, covering the entire visible spectral range. Using this platform, we demonstrate multispectral encoding patterns for encryption and multivariate optical converters for dispersive optics-free spectroscopy with high spectral resolution. The fabrication process is conducted at room temperature and can be extended to other material and device platforms.

physics.optics↗

arXiv2025

Dual-Polarization SHG Interferometry for Imaging Antiparallel Domains and Stacking Angles of 2D Heterocrystals

Optical second-harmonic generation (SHG) enables orientational polarimetry for crystallographic analysis and domain imaging of various materials. However, conventional intensity polarimetry, which neglects phase information, fails to resolve antiparallel domains and to describe two-dimensional heterostructures, which represent a new class of van der Waals-bound composite crystals. In this work, we report dual-polarization spectral phase interferometry (DP-SPI) and establish a generalized SHG superposition model that incorporates the observables of DP-SPI. Antiparallel domains of monolayer transition metal dichalcogenides (TMDs) were successfully imaged with distinction, validating the interferometric polarimetry. From DP interferograms of TMD heterobilayers, the orientation of each layer could be determined, enabling layer-resolved probing. By employing the superposition model, we also demonstrate the photonic design and fabrication of ternary TMD heterostructures for circularly polarized SHG. These methods, providing comprehensive SHG measurements and theoretical description, can be extended to heterostructures consisting of more than two constituent layers and are not limited to TMDs or 2D materials.

physics.optics↗

arXiv2023

Exciton-Sensitized Second-Harmonic Generation in 2D Heterostructures

The efficient optical second-harmonic generation (SHG) of two-dimensional (2D) crystals, coupled with their atomic thickness that circumvents the phase-match problem, has garnered considerable attention. While various 2D heterostructures have shown promising applications in photodetectors, switching electronics, and photovoltaics, the modulation of nonlinear optical properties in such hetero-systems remains unexplored. In this study, we investigate exciton sensitized SHG in heterobilayers of transition metal dichalcogenides (TMDs), where photoexcitation of one donor layer enhances the SHG response of the other as an acceptor. We utilize polarization-resolved interferometry to detect the SHG intensity and phase of each individual layer, revealing the energetic match between the excitonic resonances of donors and the SHG enhancement of acceptors for four TMD combinations. Our results also uncover the dynamic nature of interlayer coupling, as evidenced by the dependence of sensitization on interlayer gap spacing and the average power of the fundamental beam. This work provides insights into how interlayer coupling of two different layers can modify nonlinear optical phenomena in 2D heterostructures.

cond-mat.mtrl-sci↗

arXiv2023

Interfacial-Water-Modulated Photoluminescence of Single-Layer WS$_2$ on Mica

Because of their bandgap tunability and strong light-matter interactions, two-dimensional (2D) semiconductors are considered promising candidates for next-generation optoelectronic devices. However, their photophysical properties are greatly affected by environments because of their 2D nature. In this work, we report that the photoluminescence (PL) of single-layer WS$_2$ is substantially affected by interfacial water that is inevitably present between itself and supporting mica substrates. Using PL spectroscopy and wide-field imaging, we show that the emission signals from A excitons and their negative trions decreased at distinctively different rates with increasing excitation power, which can be attributed to the more efficient annihilation between excitons than trions. By gas-controlled PL imaging, we also prove that interfacial water converts trions into excitons by depleting native negative charges through an oxygen reduction reaction, which renders excited WS$_2$ more susceptible to nonradiative decay via exciton-exciton annihilation. Understanding the roles of nanoscopic water in complex low-dimensional materials will eventually contribute to devising their novel functions and devices.

physics.chem-ph↗

arXiv2022

In-Plane and Out-of-Plane Excitonic Coupling in 2D Molecular Crystals

Understanding the nature of molecular excitons in low-dimensional molecular solids is of paramount importance in fundamental photophysics and various applications such as energy harvesting, switching electronics and display devices. Despite this, the spatial evolution of molecular excitons and their transition dipoles have not been captured in the precision of molecular length scales. Here we show in-plane and out-of-plane excitonic evolution in quasilayered two-dimensional (2D) perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) crystals assembly-grown on hexagonal boron nitride (BN) crystals. Complete lattice constants with orientations of two herringbone-configured basis molecules are determined with polarization-resolved spectroscopy and electron diffraction methods. In the truly 2D limit of single layers, two Frenkel emissions Davydov-split by Kasha-type intralayer coupling exhibit energy inversion with decreasing temperature, which enhances excitonic coherence. As the thickness increases, the transition dipole moments of newly emerging charge transfer excitons are reoriented because of mixing with the Frenkel states. The current spatial anatomy of 2D molecular excitons will inspire a deeper understanding and groundbreaking applications of low-dimensional molecular systems.

cond-mat.mtrl-sci↗

arXiv2022

Photoluminescence Path Bifurcations by Spin Flip in Two-Dimensional CrPS4

Ultrathin layered crystals of coordinated chromium(III) are promising not only as two-dimensional (2D) magnets but also as 2D near-infrared (NIR) emitters owing to long-range spin correlation and efficient transition between high and low-spin excited states of Cr3+ ions. In this study, we report on dual-band NIR photoluminescence (PL) of CrPS4 and show that its excitonic emission bifurcates into fluorescence and phosphorescence depending on thickness, temperature and defect density. In addition to the spectral branching, the biexponential decay of PL transients, also affected by the three factors, could be well described within a three-level kinetic model for Cr(III). In essence, the PL bifurcations are governed by activated reverse intersystem crossing from the low to high-spin states, and the transition barrier becomes lower for thinner 2D samples because of surface-localized defects. Our findings can be generalized to 2D solids of coordinated metals and will be valuable in realizing novel magneto-optic functions and devices.

cond-mat.mtrl-sci↗

arXiv2021

Dual-channel charge transfer doping of graphene by sulfuric acid

Two-dimensional materials represented by graphene and transition metal dichalcogenides undergo charge transfer (CT) processes and become hole-doped in strong mineral acids. Nonetheless, their mechanisms remain unclear or controversial. This work proposes and verifies two distinctive CT channels in sulfuric acids, respectively driven by oxygen reduction reaction involving O2/H2O redox couples and reduction of bisulfate or related species. Acid-induced changes in the charge density of graphene were in-situ quantified as a function of oxygen content using Raman spectroscopy. At acid concentrations lower than 6 M, the former channel is operative, requiring dissolved O2. Above 6 M, the degree of CT was even higher because the former is cooperated with by the latter channel, which does not need dissolved oxygen. The mechanism revealed in this study will advance our fundamental understanding of how low-dimensional materials interact with chemical environments.

cond-mat.mtrl-sci↗

arXiv2021

Extraordinary Photostability and Davydov Splitting in BN-Sandwiched Single-Layer Tetracene Molecular Crystals

Two-dimensional molecular crystals have been beyond the reach of systematic investigation because of the lack or instability of their well-defined forms. Here, we demonstrate drastically enhanced photostability and Davydov splitting in single and few-layer tetracene (Tc) crystals sandwiched between inorganic 2D crystals of graphene or hexagonal BN. Molecular orientation and long-range order mapped with polarized wide-field photoluminescence imaging and optical second-harmonic generation revealed high crystallinity of the 2D Tc and its distinctive orientational registry with the 2D inorganic crystals, which were also verified with first-principles calculations. The reduced dielectric screening in 2D space was manifested by enlarged Davydov splitting and attenuated vibronic sidebands in the excitonic absorption and emission of monolayer Tc crystals. Photostable 2D molecular crystals and their size effects will lead to novel photophysical principles and photonic applications.

cond-mat.mtrl-sci↗

arXiv2021

Redox and Molecular Diffusion in 2D van der Waals Space

Understanding charge transfer (CT) between two chemical entities and subsequent change in their charge densities is essential not only for molecular species but also for various low-dimensional materials. Because of their extremely high fraction of surface atoms, two-dimensional (2-D) materials are most susceptible to charge exchange and exhibit drastically different physicochemical properties depending on their charge density. In this regard, spontaneous and uncontrollable ionization of graphene in the ambient air has caused much confusion and technical difficulty in achieving experimental reproducibility since its first report in 2004. Moreover, the same ambient hole doping was soon observed in 2-D semiconductors, which implied that a common mechanism should be operative and apply to other low-dimensional materials universally. In this Account, we review our breakthroughs in unraveling the chemical origin and mechanistic requirements of the hidden CT reactions using 2-D crystals. We developed in-situ optical methods to quantify charge density using Raman and photoluminescence (PL) spectroscopy and imaging. Using gas and temperature-controlled in-situ measurements, we revealed that the electrical holes are injected by the oxygen reduction reaction (ORR): $O_{2}$ + $4H^{+}$ + $4e^{-}$ $\rightleftharpoons$ $2H_{2}O$, which was independently verified by pH dependence in HCl solutions. In addition to oxygen and water vapor, the overall CT reaction requires hydrophilic dielectric substrates, which assist hydration of the sample-substrate interface. The interface-localized reaction allowed us to visualized and control interfacial molecular diffusion and CT by varing the 2-D gap spacing and introducing defects. The complete mechanism of the fundamental charge exchange summarized in this Account will be essential in exploring material and device properties of other low dimensional materials.

cond-mat.mtrl-sci↗

arXiv2021

Optically Facet-Resolved Reaction Anisotropy in Two-Dimensional Transition Metal Dichalcogenides

Quantifying anisotropy in the chemical reactions of mesoscopic crystals has mostly resorted on the combination of electron microscopy and diffraction. In this work, we established crystal-facet-resolved kinetic measurements of oxidation reactions in 2D transition metal dichalcogenides (TMDs) using optical second-harmonic generation spectroscopy and scanning probe microscopy. We show the in-plane anisotropy of their bond-breaking reactions is governed by their structure and strongly material-dependent among four TMDs. The facet-resolved analysis directly revealed that the reactions proceed fastest (slowest) for chalcogen (metal)-terminated zigzag edges with armchair edges in the middle. The degree of the anisotropy inducing trigonal oxidation patterns was much higher in MoS2 and MoSe2 than WS2 and WSe2. Kinetic Wulff construction based on edge-specific reaction rates verified the material-dependent mesoscopic reaction patterns. We also show that the reactions are initiated at substrate-mediated defects located on the bottom and top surfaces of 2D TMDs.

physics.chem-ph↗

arXiv2021

Optical Imaging of Chemically and Geometrically Controlled Interfacial Diffusion and Redox in 2D van der Waals Space

Molecular motions and chemical reactions occurring in constrained space play key roles in many catalysis and energy storage applications. However, its understanding has been impeded by difficulty in detection and lack of reliable model systems. In this work, we report geometric and chemical manipulation of O2 diffusion and ensuing O2-mediated charge transfer (CT) that occur in the 2D space between single-layer transition metal dichalcogenides (TMDs) and dielectric substrates. As a sensitive real-time wide-field imaging signal, charge-density-dependent photoluminescence (PL) from TMDs was used. The two sequential processes inducing spatiotemporal PL change could be drastically accelerated by increasing the interfacial gap size or introducing artificial defects serving as CT reaction centers. We also show that widely varying CT kinetics of four TMDs are rate-determined by the degree of hydration required for the reactions. The reported findings will be instrumental in designing novel functional nanostructures and devices.

cond-mat.mtrl-sci↗

arXiv2021

Second-Harmonic Young Interference in Atom-Thin Heterocrystals

Optical second-harmonic generation (SHG) is a nonlinear parametric process that doubles the frequency of incoming light. Only allowed in non-centrosymmetric materials, it has been widely used in frequency modulation of lasers, surface scientific investigation, and label-free imaging in biological and medical sciences. Two-dimensional crystals are ideal SHG-materials not only for their strong light-matter interaction and atomic thickness defying the phase-matching requirement but also for their stackability into customized hetero-crystals with high angular precision and material diversity. Here we directly show that SHG in hetero-bilayers of transition metal dichalcogenides (TMDs) is governed by optical interference between two coherent SH fields with material-dependent phase delays using spectral phase interferometry. We also quantify the frequency-dependent phase difference between MoS2 and WS2, which also agrees with polarization-resolved data and first-principles calculations on complex susceptibility. The second-harmonic analogue of Young double-slit interference shown in this work demonstrates the potential of custom-designed parametric generation by atom-thick nonlinear optical materials.

cond-mat.mtrl-sci↗

arXiv2020

Polarized Raman Spectra and Complex Raman Tensors of Antiferromagnetic Semiconductor CrPS4

Chromium thiophosphate (CrPS4), a monoclinic crystal of C_2^3 space group, is a ternary layered semiconductor with an optical bandgap of 1.4 eV and exhibits antiferromagnetism below 36 K. Despite its potential in optoelectronic and magnetic applications, the symmetry of its lattice vibrations has not been systematically studied. In this work, we performed a polarized Raman spectroscopy of bulk CrPS4 using three different excitation wavelengths of 457, 514, and 633 nm. High-quality crystals grown by the chemical vapor transport method were mechanically exfoliated or polished to expose three orthogonal crystallographic facets. Polarized Raman spectra were obtained in parallel and cross configurations by rotating samples about the surface normal to each of the facets. Among 33 Raman active modes (16 A and 17 B) at the Brillouin zone center, 19 (8) peaks observed in the parallel (cross) configuration were assigned as A (B) modes. Complex-valued Raman tensors were determined for 7 major A modes using the angle-dependent Raman spectra of the three facets. The results can also be used in determining the crystallographic orientations of CrPS4 unequivocally.

physics.chem-ph↗

arXiv2020

Interface-Confined Doubly Anisotropic Oxidation of Two-Dimensional MoS2

Despite their importance, chemical reactions confined in a low dimensional space are elusive and experimentally intractable. In this work, we report doubly anisotropic, in-plane and out-of-plane, oxidation reactions of two-dimensional crystals, by resolving interface-confined thermal oxidation of a single and multilayer MoS2 supported on silica substrates from their conventional surface reaction. Using optical second-harmonic generation spectroscopy of artificially stacked multilayers, we directly proved that crystallographically oriented triangular oxides (TOs) were formed in the bottommost layer while triangular etch pits (TEs) were generated in the topmost layer and that both structures were terminated with zigzag edges. The formation of the Mo oxide layer at the interface demonstrates that O2 diffuses efficiently through the van der Waals (vdW) gap but not MoO3, which would otherwise sublime. The fact that TOs are several times larger than TEs indicates that oxidation is greatly enhanced when MoS2 is in direct contact with silica substrates, which suggests a catalytic effect. This study indicates that the vdW-bonded interfaces are essentially open to mass transport and can serve as a model system for investigating chemistry in low dimensional spaces.

cond-mat.mtrl-sci↗