arXiv subjects

Mansoo Choi

Publications and source records attributed to Mansoo Choi.

At least 19 recordsLinked to original sources

arXiv2026

3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper IV. Key Scientific Mission: Solar-System Small Bodies and Planetary Defense

The baseline 0.2--1.5 $\mu$m observatory provides rapid-response astrometry, visible and near-infrared taxonomy, rotation and phase curves, recovery, and long-arc orbit improvement for near-Earth objects and other small bodies. The instrument study also evaluates calibrated throughput to 2.70 $\mu$m with a 3.0 $\mu$m operational band-edge goal. A reduction to 2.5 $\mu$m remains the formal engineering off-ramp if thermal, detector, cooling, mass, power, or cost constraints require it. The 3.5-meter Segmented-Mirror Robotic Space Telescope does not carry a mid-infrared channel. Coordinated ground-based mid-infrared telescopes provide the thermal fluxes required to infer diameter and albedo, while the space mission supplies contemporaneous reflected-light measurements and observing geometry. The program combines recovery, physical characterization, orbit refinement, and covariance-based hazard assessment. Its CODES dynamics system and OGFinder-to-OpenOrb processing path connect measured astrometry to reproducible orbit solutions and close-approach predictions.

astro-ph.IM↗

arXiv2026

3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper V. Key Scientific Mission: Compact-Object Time-Domain Science

An isolated compact object retains the point-source resolving power of the space-based slitless spectrograph. The baseline wavelength range is 0.2--1.5 $\mu$m. The planning baseline uses $R \simeq 1000$ for broad and faint transient spectra and reserves selectable bands at $R \simeq 5000$ for accretion-disk profiles, velocity structure, and precision line ratios. Broad features can be measured after binning the native $R \simeq 5000$ data to lower resolution. Rapid-response spectroscopy follows gravitational-wave counterparts and kilonovae from hours to days. Repeated spectra of dwarf novae and compact binaries trace accretion state and orbital phase, while uninterrupted imaging of white dwarfs measures pulsation frequencies. The program combines mission-based monitoring with external alerts, including KGMT transient detections. The instrument study must preserve calibrated throughput to 2.70 $\mu$m and evaluate a 3.0 $\mu$m operational band edge, with 2.5 $\mu$m retained as the formal engineering off-ramp. Mid-infrared imaging is not part of the adopted compact-object baseline.

astro-ph.IM↗

arXiv2026

3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper I. Overall Architecture and Scientific Mission

A 3.5-meter segmented-mirror robotic space telescope is under study as a space-based observatory for precision astrophysical observations and rapid-response transient astronomy in the 0.2-1.5 micron wavelength range. The telescope adopts a Cassegrain optical configuration optimized to deliver diffraction-limited performance across a wide, flat focal plane, achieving a Strehl ratio greater than 0.8 at 633 nm. The proposed scientific payload includes a Wide-field Camera (WC), a spectroscopic instrument, and an optional Exoplanet Imaging Coronagraph. The Wide-field Camera (WC) provides multi-wavelength imaging and high-cadence time-series photometry over a field of view ranging from 10'X10' to 30'X30'. The spectroscopic configuration and resolving power remain under study to accommodate the requirements of the principal science programs. An optional Exoplanet Imaging Coronagraph is being investigated for high-contrast imaging of nearby planetary systems, with performance goals extending toward raw contrasts of approximately 10^(-8) and improved post-processed performance. Candidate orbital configurations, including Earth orbit and the Sun-Earth L2 region, are currently being evaluated. Planned investigations include gravitational-wave counterparts, rapidly evolving transients, Type Ia supernova cosmology, direct imaging of exoplanets, and exoplanet atmospheric spectroscopy. Although driven by these core scientific objectives, the observatory is conceived as a general-purpose facility providing open-access observing time to the international scientific community. This paper presents the preliminary architecture, performance goals, and scientific mission of the proposed 3.5-meter space telescope.

astro-ph.IM↗

arXiv2026

3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper II. Key Scientific Mission: Wide-Field Cosmology and Galaxy Evolution

The 3.5-meter Segmented-Mirror Robotic Space Telescope uses an image slicer for all spectroscopic observations. The planning baseline uses $R \simeq 1000$ for the wide survey and retains selectable $R \simeq 5000$ bands for precision line measurements. The central science case is a dense emission-line galaxy redshift survey for baryon acoustic oscillations and redshift-space distortions. Supernova and quasar programs exploit the stability, multiplexing, and repeatability of space operations. The supernova tier measures rest-frame U and near-ultraviolet magnitudes that separate optical twins at subgroup precision to $z \simeq 0.9$--$1.1$ in standard visits and to $z \simeq 1.3$--$1.5$ in ten-hour stacks. Every wide-survey tile receives three spectroscopic orientations, and a joint scene reconstruction uses their different overlap geometries to recover the spectra. The flagship survey covers 100--300 deg$^2$ and targets $10^6$--$3 \times 10^6$ emission-line galaxies. A deep pencil-beam tier and a supernova time-domain tier complement the wide survey. The same observations provide a census of ultra-diffuse and low-surface-brightness galaxies, map intracluster light, and test cold, self-interacting, and fuzzy dark matter through dwarf-galaxy structure and low-mass halo abundance.

astro-ph.IM↗

arXiv2026

3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper III. Key Scientific Mission: Exoplanet Science with a Coronagraph

This volume defines the exoplanet science program enabled by the dedicated high-contrast coronagraph in the baseline science payload of the 3.5-meter Segmented-Mirror Robotic Space Telescope. The observatory architecture incorporates the optical interfaces, wavefront sensing and control, pointing stability, and operations software required for coronagraphic observations from the outset. The observing strategy gives priority to the nearest stellar systems because they provide the most accessible laboratories for planetary exploration and the most likely destinations of future interstellar missions. The diffraction limit sets a reflected-light horizon of roughly 10--15 pc for planets at 1 AU and roughly 50--80 pc for Jupiter analogs. Within those horizons, the telescope can image nearby giant planets, obtain reflected-light spectra of their atmospheres, survey young systems and circumstellar disks, and support the habitability and biosignature programs that larger future missions will pursue. The wide-field imager complements the coronagraph through transit photometry, occurrence-rate statistics, and long-term monitoring of stellar magnetic activity. A systematic census of the nearest stellar neighbors provides a lasting reference for exoplanet science and future space exploration.

astro-ph.IM↗

arXiv2020

Imaging real-time amorphization of hybrid perovskite solar cells under electrical biasing

Perovskite solar cells have drawn much attention in recent years, owing to its world-record setting photovoltaic performances. Despite its promising use in tandem applications and flexible devices, its practicality is still limited by its structural instability often arising from ion migration and defect formation. While it is generally understood that ion instability is a primary cause for degradation, there is still a lack of direct evidence of structural transformation at the atomistic scale. Such an understanding is crucial to evaluate and pin-point how such instabilities are induced relative to external perturbations such as illumination or electrical bias with time, allowing researchers to devise effective strategies to mitigate them. Here, we designed an in-situ TEM setup to enable real-time observation of amorphization in double cation mixed perovskite materials under electrical biasing at 1 V. It is found that amorphization occurs along the (001) and (002) planes, which represents the observation of in-situ facet-dependent amorphization of a perovskite crystal. To reverse the degradation, the samples were heated at 50 oC and was found to recrystallize, effectively regaining its performance losses. This work is vital toward understanding fundamental ion-migration phenomena and address instability challenges of perovskite optoelectronics.

cond-mat.mtrl-sci↗

arXiv2020

Pulsatile therapy for perovskite solar cells

The current utmost challenge for commercialization of perovskite solar cells is to ensure long-term operation stability. Here, we developed the pulsatile therapy which can prolong device lifetime by addressing accumulation of both charges and ions in the middle of maximum power point tracking (MPPT). In the technique, reverse biases are repeatedly applied for a very short time without any pause of operation, leading to stabilization of the working device. The observed efficacies of our pulsatile therapy are delaying irreversible degradation as well as restoring degraded photocurrent during MPPT operation. We suggest an integrated mechanism underlying the therapy, in which harmful deep-level defects can be prevented to form and already formed defects can be cured by driving charge-state transition. We demonstrated the therapy to maintain defect-tolerance continuously, leading to outstanding improvement of lifetime and harvesting power. The unique technique will open up new possibility to commercialize perovskite materials into a real market.

physics.app-ph↗

arXiv2019

Charge transport layer dependent electronic band bending in perovskite solar cells and its correlation to device degradation

Perovskite solar cells (PSCs) have shown remarkably improved power-conversion efficiency of around 25%. However, their working principle remains arguable and the stability issue has not been solved yet. In this report, we revealed that the working mechanism of PSCs is explained by a dominant pn junction occurring at the different interface depending on electron transport layer, and charges are accumulated at the corresponding dominant junction initiating device degradation. Locations of a dominant pn junction, the electric field, and carrier-density distribution with respect to electron-transport layers in the PCS devices were investigated by using the electron-beam-induced current measurement and Kelvin probe force microscopy. The amount of accumulated charges in the devices was analyzed using the charge-extraction method and the degradation process of devices was confirmed by SEM measurements. From these observations, we identified that the dominant pn junction appears at the interface where the degree of band bending is higher compared to the other interface, and charges are accumulated at the corresponding junction where the device degradation is initiated, which suggests that there exists a strong correlation between PSC working principle and device degradation. We highlight that an ideal pin PSC that can minimize the degree of band bending should be designed for ensuring long-term stability, via using proper selective contacts

physics.app-ph↗

arXiv2019

Sparking mashups to form multifunctional alloy nanoparticles

Synthesizing unconventional alloys remains challenging owing to seamless interactions between kinetics and thermodynamics. High entropy alloys (HEAs), for example, draw a fundamentally new concept to enable exploring unknown regions in phase diagrams. The exploration, however, is hindered by traditional metallurgies based on liquid-solid transformation. Vapor-solid transformation that is permissible on pressure-temperature phase diagrams, offers the most kinetically efficient pathway to form any desired alloy (e.g., HEA). Here, we report that a technique called "sparking mashups", which involves a rapidly quenched vapor source and induces unrestricted mixing for alloying 55 distinct types of ultrasmall nanoparticles (NPs) with controllable compositions. Unlike the precursor feed in wet chemistry, a microseconds-long oscillatory spark controls the vapour composition, which is eventually retained in the alloy NPs. The resulting NPs range from binary to HEAs with marked thermal stability at room temperature. We show that a nanosize-effect ensures such thermal stability and mimics the role of mixing entropy in HEAs. This discovery contradicts the traditional "smaller is less stable" view while enabling the elemental combinations that have never been alloyed to date. We even break the miscibility limits by mixing bulk-immiscible systems in alloy NPs. As powerful examples, we demonstrate the alloy NPs as both high-performance fuel-cell catalysts and building blocks for three-dimensional (3D) nanoprinting to construct HEA nanostructure arrays of various architectures and compositions. Our results form the basis of new rules for guiding HEA-NP synthesis and advancing catalysis and 3D printing to new frontiers.

physics.app-ph↗

arXiv2019

Highly durable crack sensor integrated with silicone rubber cantilever for measuring cardiac contractility in culture media

We propose a novel cantilever device integrated with a polydimethylsiloxane (PDMS)-encapsulated crack sensor that directly measures the cardiac contractility. The crack sensor was chemically bonded to a PDMS thin layer to form a sandwiched structure which allows to be operated very stably in culture media. The reliability of the proposed crack sensor has improved dramatically compared to no encapsulation layer. After evaluating the durability of the crack sensor bonded with the PDMS layer, cardiomyocytes were cultured on the nano-patterned cantilever for real-time measurement of cardiac contractile forces. The highly sensitive crack sensor continuously measured the cardiac contractility without changing its gauge factor for up to 26 days (>5 million heartbeats). In addition, changes in contractile force induced by drugs were monitored using the crack sensor-integrated cantilever. Finally, experimental results were compared with those obtained via conventional electrophysiological methods to verify the feasibility of building a contraction-based drug-toxicity testing system.

physics.app-ph↗

arXiv2018

Three-dimensional nanoprinting via charged aerosol focusing

A powerful and flexible method of 3D nano-printing, based on focusing charged aerosol, has been developed. The self-consistent electric field configuration, created with a holey floating mask and used as the scaffold for printing structures, has no restriction as to sizes down to nano-scale. The electric field line is used as a writing tool. Broad material independence opens the way for producing hybrid structures that are essential for electronic devices. The method contains three modes which are complementary: controlled tip-directed 3D-growth printing, the writing mode (that can also produce 3D structures in repeating passages), and the stencil mode that produces wall-like structures of various shapes. Manipulating them gives freedom to manufacture complex 3D designs that we report. The desired morphology of the grown structures is controlled according to a simple phenomenological theory that helps organize the 2D stage motion and the 3D printing process to compete with the 3D printing provided by laser techniques in polymer based material.

physics.app-ph↗

arXiv2018

Decrystallization of CH3NH3PbI3 perovskite crystals via polarity dependent localized charges

Despite soaring performance of organic-inorganic hybrid perovskite materials in recent years, the mechanism of their decomposition at actual operation condition has been elusive. Herein, we elucidated the decrystallization process of CH3NH3PbI3 perovskite crystals via localized charges and identified polarity-dependent degradation pathway by carrying out time-evolution measurements for absorption spectra of perovskite films with underlying different charge transport layers and ab initio molecular dynamics calculations. It was found that the carrier polarity (hole-rich or electron-rich) inside the perovskite films played a critical role in the degradation rate, and polarity-dependent degradation pathway strongly depended on the combination of surrounding gaseous molecules. The hole-rich perovskite films degraded more rapidly in the existence of H2O than the electron-rich one, while the degradation trend became opposite in only-oxygen ambient. Strikingly, the hole-rich one was extremely weak to atmospheric air containing both H2O and O2, whereas the MAPbI3 film with excessive electrons rather stabilized in air. Ab initio molecular dynamics (AIMD) simulation was also done to find the detailed degradation pathway of MAPbI3 under atmosphere for different polarity of localized charge, which are in good agreement with experimental results. Furthermore, X-ray assisted spectroscopic measurements confirmed the production of Pb(OH)I as predicted from the simulation result.

cond-mat.mtrl-sci↗

arXiv2017

Atomistic mechanism for trapped-charge driven degradation of perovskite solar cells

It is unmistakably paradoxical that the weakest point of the photoactive organic-inorganic hybrid perovskite is its instability against light. Why and how perovskites break down under light irradiation and what happens at the atomistic level during the degradation still remains unanswered. In this paper, we revealed the fundamental origin and mechanism for irreversible degradation of hybrid perovskite materials from our new experimental results and ab initio molecular dynamics (AIMD) simulations. We found that the photo-generated charges trapped along the grain boundaries of the perovskite crystal result in oxygen-induced irreversible degradation in air even in the absence of moisture. The present result, together with our previous experimental finding on the same critical role of trapped charges in the perovskite degradation under moisture, suggests that the trapped charges are the main culprit in both the oxygen- and moisture-induced degradation of perovskite materials. More detailed roles of oxygen and water molecules were investigated by tracking the atomic motions of the oxygen- or water-covered CH3NH3PbI3(MAPbI3) perovskite crystal surface with trapped charges via AIMD simulation. In the first few picoseconds of our simulation, trapped charges start disrupting the crystal structure, leading to a close-range interaction between oxygen or water molecules and the compositional ions of MAPbI3. We found that there are different degradation pathways depending on both the polarity of the trapped charge and the kind of gas molecule. Especially, the deprotonation of organic cations was theoretically predicted for the first time in the presence of trapped anionic charges and water molecules. We confirmed that a more structurally stable, multi-component perovskite material(MA0.6FA0.4PbI2.9Br0.1) exhibited a much longer lifespan than MAPbI3 under light irradiation even in 100% oxygen ambience.

cond-mat.mtrl-sci↗

arXiv2016

A low-field temperature-dependent EPR signal in terraced MgO:Mn2+ nanoparticles: an enhanced Zeeman splitting in the wide-bandgap oxide

Mn2+ ion doping is used as an electron paramagnetic resonance (EPR) probe to investigate the influence of low-coordination structural defects such as step edges at the surface of terraced (001) MgO nanoparticles on the electronic properties. Beside the well-known hyperfine sextet of Mn2+ ions in the cubic crystal field of MgO, an additional EPR feature with a striking non-monotonous temperature dependent shift of the g-factor is observed in terraced nanoparticles in the temperature range from 4K to room temperature. By linking the difference in the temperature dependence of the Mn2+ sextet intensity in cubic and terraced nanoparticles with the possible s-d exchange shift and enhanced Zeeman splitting we conclude that the novel EPR feature originates from the loosely trapped charge-compensating carriers at the abundant structural defects at the surface of terraced nanoparticles due to their exchange interaction with neighboring Mn2+ ions.

cond-mat.mes-hall↗

arXiv2016

Trapped charge driven degradation of perovskite solar cells

Perovskite solar cells have shown fast deterioration during actual operation even with encapsulation, but its mechanism has been elusive. We found the fundamental mechanism for irreversible degradation of perovskite materials in which trapped charges regardless of the polarity play a decisive role. A novel experimental setup utilizing different polarity ions revealed that the moisture induced irreversible dissociation of perovskite materials is triggered by charges trapped along grain boundaries. Our finding clearly explained the intriguing observations why light soaking induces irreversible degradation while in the dark, moisture only causes reversible hydration, and why degradation begins from different side of interface for different charge extraction layers. The deprotonation of organic cations by trapped charge induced local electric field is attributed to the initiation of irreversible decomposition.

cond-mat.mtrl-sci↗

arXiv2015

Formation of non-cubic nanoparticles from cubic MgO in intensified self-burning of magnesium

When Mg metal burns in air the resulting rock-salt MgO smoke consists of perfect [100] cubes of about 100 nm. On contrast, we found that intensification of self-burning of Mg micropowder either by injecting it into oxy-hydrogen diffusion flame or under an infrared laser beam switches the growth mechanism producing mostly single-crystalline spheres and terraced nanoparticles. MgO molecule condensation onto primary spherical nanoparticles can account for generation of terraced nanoparticles with regular steps proportional to the nanoparticle size.

cond-mat.mtrl-sci↗

arXiv2014

Electronic modulation of infrared emissivity in graphene plasmonic resonators

Electronic control of blackbody emission from graphene plasmonic resonators on a silicon nitride substrate is demonstrated at temperatures up to 250 C. It is shown that the graphene resonators produce antenna-coupled blackbody radiation, manifest as narrow spectral emission peaks in the mid-IR. By continuously varying the nanoresonators carrier density, the frequency and intensity of these spectral features can be modulated via an electrostatic gate. We describe these phenomena as plasmonically enhanced radiative emission originating both from loss channels associated with plasmon decay in the graphene sheet and from vibrational modes in the SiNx.

cond-mat.mes-hall↗

arXiv2014

Seven, eight, and nine mutually touching infinitely long straight round cylinders: Entanglement in Euclidean space

It has been a challenge to make seven straight round cylinders mutually touch before our now 10-year old discovery [Phys. Rev. Lett. 93, 015505 (2004)] of configurations of seven mutually touching infinitely long round cylinders (then coined 7-knots). Because of the current interest in string-like objects and entanglement which occur in many fields of Physics it is useful to find a simple way to treat ensembles of straight infinite cylinders. Here we propose a treatment with a chirality matrix. By comparing 7-knot with variable radii with the one where all cylinders are of equal radii (here 7*-knot, which for the first time appeared in [phys. stat. solidi, b 246, 2098 (2009)]), we show that the reduction of 7-knot with a set of non-equal cylinder radii to 7*-knot of equal radii is possible only for one topologically unique configuration, all other 7-knots being of different topology. We found novel configurations for mutually touching infinitely long round cylinders when their numbers are eight and ultimately nine (here coined 8-knots and 9-knots). Unlike the case of 7-knot, where one angular parameter (for a given set of fixed radii) may change by sweeping a scissor angle between two chosen cylinders, in case of 8- and 9-knots their degrees of freedom are completely exhausted by mutual touching so that their configurations are "frozen" for each given set of radii. For 8-knot the radii of any six cylinders may be changeable (for example, all taken equal) while two remaining are uniquely determined by the others. We show that 9-knot makes the ultimate configuration where only three cylinders can have changeable radii and the remaining six are determined by the three. Possible generalizations and connection with Physics are mentioned.

math.MG↗