arXiv subjects

Young Yong Kim

Publications and source records attributed to Young Yong Kim.

14 recordsLinked to original sources

arXiv2025

Defect migration in supercrystalline nanocomposites

Supercrystalline nanocomposites (SCNCs) are nanostructured hybrid materials with unique emergent functional properties. Given their periodically arranged building blocks, they also offer interesting parallelisms with crystalline materials. They can be processed in multiple forms and at different scales, and crosslinking their organic ligands via heat treatment leads to a remarkable boost of their mechanical properties. This study shows, via X-ray and in-situ scanning transmission (STEM) electron microscopy analyses, how each of these processing steps plays a distinct role in the generation, migration, interaction and healing of supercrystalline defects. Pressing of SCNCs into bulk pellets leads to a distortion of the otherwise fcc superlattice, while emulsion-templated self-assembly yields supraparticles (SPs) with stacking faults and size-dependent symmetries. Interestingly, heat treatment at the same temperatures as those applied for the organic crosslinking has significant effects on planar defects. Stacking faults migrate and get healed, as also confirmed via molecular dynamics simulations, and inter-supercrystalline 'grain' boundaries undergo structural changes. These rearrangements of defects at the supercrystalline scale (tens of nm) in nanocomposites with such remarkable mechanical properties (compressive strength of 100-500 MPa) provide new insights into the formation and evolution of ordered assemblies of functionalized nanoparticles.

cond-mat.mtrl-sci↗

arXiv2023

Infrared-induced ultrafast melting of nanostructured platinum films probed by an X-ray free-electron laser

Understanding melting in metals is a hot topic of present research. This may be accomplished by pumping the system with infrared (IR) laser radiation, and probing it with hard X-rays produced by an X-ray Free-Electron Laser (XFEL). In this work we studied nanostructured polycrystalline thin films of platinum that were illuminated by IR radiation of increasing fluences. We characterized the structural response as well as the nucleation and propagation of the liquid phase as a function of time delay between the IR pump and X-ray probe. We observed partial melting of the samples for IR fluences higher than 200 mJ$\cdot$cm$^{-2}$. To fit the contribution of the liquid phase to the scattering pattern in platinum we applied a model of liquid metal. The two-temperature model simulations were performed to understand the solid-state fraction of the sample heating process as a function of time delay and fluence.

cond-mat.mtrl-sci↗

arXiv2022

High energy-resolution transient ghost absorption spectroscopy

We demonstrate the measurement of ultrafast dynamics using ghost spectroscopy and a pump-probe approach with an optical pump and a short-wavelength radiation probe. The ghost spectroscopy approach is used to overcome the challenge of the strong intensity and spectrum fluctuations at free-electron lasers and to provide high -spectral resolution, which enables the measurement of small energy shifts in the absorption spectrum. We exploit the high resolution to explore the dynamics of the charge carrier excitations and relaxations and their impact on the photoinduced structural changes in silicon by measuring the variation of the absorption spectrum of a Si(100) membrane near the silicon L2,3 edge and the accompanying edge shifts in response to the optical illumination.

physics.optics↗

arXiv2022

Statistical analysis of hard X-ray radiation at PAL-XFEL facility performed by Hanbury Brown and Twiss interferometry

Hanbury Brown and Twiss interferometry experiment based on second-order correlations was performed at PAL-XFEL facility. The statistical properties of the X-ray radiation were studied within this experiment. Measurements were performed at NCI beamline at 10 keV photon energy in various operation conditions: Self-Amplified Spontaneous Emission (SASE), SASE with a monochromator, and self-seeding regimes at 120 pC, 180 pC, and 200 pC electron bunch charge, respectively. Statistical analysis showed short average pulse duration from 6 fs to 9 fs depending on operation conditions. A high spatial degree of coherence of about 70-80% was determined in spatial domain for the SASE beams with the monochromator and self-seeding regime of operation. The obtained values describe the statistical properties of the beams generated at PAL-XFEL facility.

physics.acc-ph↗

arXiv2022

High-resolution absorption measurements with free-electron lasers using ghost spectroscopy

We demonstrate a simple and robust high-resolution ghost spectroscopy approach for x-ray and extreme ultraviolet absorption spectroscopy at free-electron laser sources. Our approach requires an on-line spectrometer before the sample and a downstream bucket detector. We use this method to measure the absorption spectrum of silicon, silicon carbide and silicon nitride membranes in the vicinity of the silicon L2,3-edge. We show that ghost spectroscopy allows the high-resolution reconstruction of the sample spectral response using a coarse energy scan with self-amplified spontaneous emission radiation. For the conditions of our experiment the energy resolution of the ghost-spectroscopy reconstruction is higher than the energy resolution reached by scanning the energy range by narrow spectral bandwidth radiation produced by the seeded free-electron laser. When we set the photon energy resolution of the ghost spectroscopy to be equal to the resolution of the measurement with the seeded radiation, the measurement time with the ghost spectroscopy method is shorter than scanning the photon energy with seeded radiation. The exact conditions for which ghost spectroscopy can provide higher resolution at shorter times than measurement with narrow band scans depend on the details of the measurements and on the properties of the samples and should be addressed in future studies.

physics.ins-det↗

arXiv2021

Impact of the Ligand Shell on Structural Changes and Decomposition of All-Inorganic Mixed-Halide Perovskite (CsPbX3) Nanocrystals under X-Ray Illumination

We show that the decomposition of caesium lead halide perovskite nanocrystals under continuous X-ray illumination depends on the surface ligand. For oleic acid/oleylamine, we observe a fast decay accompanied by the formation of elemental lead and halogen. Upon surface functionalization with a metal porphyrin derivate, the decay is markedly slower and involves the disproportionation of lead to Pb0 and Pb3+. In both cases, the decomposition is preceded by a contraction of the atomic lattice, which appears to initiate the decay. We find that the metal porphyrin derivative induces a strong surface dipole on the nanocrystals, which we hold responsible for the altered and slower decomposition pathway. These results are important for application of lead halide perovskite nanocrystals in X-ray scintillators.

cond-mat.mtrl-sci↗

arXiv2021

Spatially resolved fluorescence of caesium lead halide perovskite supercrystals reveals quasi-atomic behavior of nanocrystals

We correlate spatially resolved fluorescence (-lifetime) measurements with X-ray nanodiffraction to reveal surface defects in supercrystals of self-assembled caesium lead halide perovskite nanocrystals and study their effect on the fluorescence properties. Upon comparison with density functional modelling, we show that a loss in structural coherence, an increasing atomic misalignment between adjacent nanocrystals, and growing compressive strain near the surface of the supercrystal are responsible for the observed fluorescence blueshift and decreased fluorescence lifetimes. Such surface defect-related optical properties extend the frequently assumed analogy between atoms and nanocrystals as so-called quasi-atoms. Our results emphasize the importance of minimizing strain during the self-assembly of perovskite nanocrystals into supercrystals for lighting application such as superfluorescent emitters.

cond-mat.mtrl-sci↗

arXiv2021

Influence of Contacts and Applied Voltage on a Structure of a Single GaN Nanowire

Semiconductor nanowires (NWs) have a broad range of applications for nano- and optoelectronics. The strain field of gallium nitride (GaN) NWs could be significantly changed when contacts are applied to them to form a final device, especially considering the piezoelectric properties of GaN. Investigation of influence of the metallic contacts on the structure of the NWs is of high importance for their applications in real devices. We have studied a series of different type of contacts and influence of the applied voltage bias on the contacted GaN NWs with the length of about 3 to 4 micrometers and with two different diameters of 200 nm and 350 nm. It was demonstrated that the NWs with the diameter of 200 nm are bend already by the interaction with the substrate. For all GaN NWs, significant structural changes were revealed after the contacts deposition. The results of our research may contribute to the future optoelectronic applications of the GaN nanowires.

cond-mat.mes-hall↗

arXiv2021

High spatial coherence and short pulse duration revealed by the Hanbury Brown and Twiss interferometry at the European XFEL

Second-order intensity interferometry was employed to study the spatial and temporal properties of the European X-ray Free-Electron Laser (EuXFEL). Measurements were performed at the soft X-ray SASE3 undulator beamline at a photon energy of 1.2 keV in the Self-Amplified Spontaneous Emission (SASE) mode. Two high-power regimes of the SASE3 undulator settings, i.e. linear and quadratic tapering at saturation, were studied in detail and compared with the linear gain regime. The statistical analysis showed an exceptionally high degree of spatial coherence up to 90% for the linear undulator tapering. Analysis of the measured data in spectral and spatial domains provided an average pulse duration of about 10 fs in our measurements. The obtained results will be valuable for the experiments requiring and exploiting short pulse duration and utilizing high coherence properties of the EuXFEL.

physics.optics↗

arXiv2021

Single Alloy Nanoparticle X-Ray Imaging during a Catalytic Reaction

The imaging of active nanoparticles represents a milestone in decoding heterogeneous catalysts dynamics. We report the facet resolved, surface strain state of a single PtRh alloy nanoparticle on SrTiO3 determined by coherent x-ray diffraction imaging under catalytic reaction conditions. Density functional theory calculations allow us to correlate the facet surface strain state to its reaction environment dependent chemical composition. We find that the initially Pt terminated nanoparticle surface gets Rh enriched under CO oxidation reaction conditions. The local composition is facet orientation dependent and the Rh enrichment is non-reversible under subsequent CO reduction. Tracking facet resolved strain and composition under operando conditions is crucial for a rational design of more efficient heterogeneous catalysts with tailored activity, selectivity and lifetime.

cond-mat.mtrl-sci↗

arXiv2020

An advanced workflow for single particle imaging with the limited data at an X-ray free-electron laser

An improved analysis for single particle imaging (SPI) experiments, using the limited data, is presented here. Results are based on a study of bacteriophage PR772 performed at the AMO instrument at the Linac Coherent Light Source (LCLS) as part of the SPI initiative. Existing methods were modified to cope with the shortcomings of the experimental data: inaccessibility of information from the half of the detector and small fraction of single hits. General SPI analysis workflow was upgraded with the expectation-maximization based classification of diffraction patterns and mode decomposition on the final virus structure determination step. The presented processing pipeline allowed us to determine the three-dimensional structure of the bacteriophage PR772 without symmetry constraints with a spatial resolution of 6.9 nm. The obtained resolution was limited by the scattering intensity during the experiment and the relatively small number of single hits.

physics.bio-ph↗

arXiv2018

Ghost Imaging at an XUV Free-Electron Laser

Radiation damage is one of the most severe resolution limiting factors in x-ray imaging, especially relevant to biological samples. One way of circumventing this problem is to exploit correlation-based methods developed in quantum imaging. Among these, there is ghost imaging (GI) in which the image is formed by radiation that has never interacted with the sample. Here, we demonstrate GI at an XUV free-electron laser by utilizing correlation techniques. We discuss the experimental challenges, optimal setup, and crucial ingredients to maximize the achievable resolution.

physics.optics↗

arXiv2018

Towards Time-Resolved Atomic Structure Determination by X-Ray Standing Waves at a Free-Electron Laser

We demonstrate the structural sensitivity and accuracy of the standing wave technique at a high repetition rate free-electron laser, FLASH at DESY in Hamburg, by measuring the photoelectron yield from the surface SiO2 of Mo/Si multilayers. These experiments open up the possibility to obtain unprecedented structural information of adsorbate and surface atoms with picometer spatial accuracy and femtosecond temporal resolution. This technique will substantially contribute to a fundamental understanding of chemical reactions at catalytic surfaces and the structural dynamics of superconductors.

cond-mat.mtrl-sci↗

arXiv2018

Evidence of a first-order smectic -- hexatic transition and its proximity to tricritical point in smectic films

Experimental and theoretical studies of a smectic-hexatic transition in freely suspended films of 54COOBC compound are presented. X-ray investigations revealed a discontinuous first-order transition into the hexatic phase. Moreover, the temperature region of two phase coexistence near the phase transition point diminishes with film thickness. The coexistence width dependence on film thickness was derived on the basis of the Landau mean-field theory in the vicinity of the tricritical point (TCP). Close to TCP the surface hexatic ordering penetrates anomalously deep into the film interior.

cond-mat.soft↗