Publications

2023

2022

1. Accepted

   Challenges, Opportunities, and Prospects in Metal Halide Perovskites from Theoretical and Machine Learning Perspectives

   Myung, Chang Woo, Hajibabaei, Amir, Cha, Ji-Hyun, Ha, Miran, Kim, Junu, and Kim, Kwang S.

   Advanced Energy Materials 2022

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   Metal halide perovskite (MHP) is a promising next generation energy material for various applications, such as solar cells, light emitting diodes, lasers, sensors, and transistors. MHPs show excellent mechanical, dielectric, photovoltaic, photoluminescence, and electronic properties, and such intriguing physical and chemical properties have drawn attention recently. However, there exists a chasm between the successful applications of MHPs and theoretical understandings. The difficulty arises from the intrinsic properties of MHPs, including structural disorder, ionic interactions, nonadiabatic effects, and composition diversity. Machine learning (ML) approaches have shown great promise as a tool to overcome the theoretical obstacles in many fields of science. In this Perspective, we overview the pending theoretical challenges from experiments and propose promising ML approaches, including ab initio ML potentials, materials design/optimization models, and data mining strategies. Possible roles and piplines of ML frameworks are highlighted to close the gap between experiment and theory in MHPs.

2. Mater. Chem. Frontiers

   Upconversion and multiexciton generation in organic Mn(ii) complex boost the quantum yield to > 100%

   Jana, Atanu, Myung, Chang Woo, Sree, Vijaya Gopalan, and Kim, Kwang S.

   Materials Chemistry Frontiers 2022

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   Highly efficient, low-cost, and eco-friendly fluorescent bulk materials showing the quantum confinement effect with both upconversion (UC) and multiexciton generation (MEG) are promising for optoelectronic devices. Yet, these combined phenomena have not been realized in bulk organic–inorganic single crystals (SCs). MEG by low-energy photons remains a critical challenge for generating multiexcitons. Herein, we report non-toxic, zero-dimensional (0D) bulk organic–inorganic hybrid, green light-emitting SCs of [Me3NPh]2MnBr4 (1) (Ph: phenyl), which show both UC and MEG along with a long lifetime (400 \mus). This is supported by many-body theory predicting a large exciton binding energy (483 meV), upon excitation by band-gap energy (2.62 eV) photons. The MEG in 1 contributes to the photoluminescence (PL) quantum yield (QY) of up to 189%, the highest among any 0D hybrid or other single crystals. Our findings will pave the way to design and synthesize lead-free 0D hybrid materials having UC and MEG properties, improving the performances of solar cells, LEDs, and other optoelectronic devices.

3. Adv.Ener.Mater.

   Al-Doping Driven Suppression of Capacity and Voltage Fadings in 4d-Element Containing Li-Ion-Battery Cathode Materials: Machine Learning and Density Functional Theory

   Ha, Miran, Hajibabaei, Amir, Kim, Dong Yeon, Singh, Aditya Narayan, Yun, Jeonghun, Myung, Chang Woo, and Kim, Kwang S.

   Advanced Energy Materials Aug 2022

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   Abstract The anion redox reaction in high-energy-density cathode materials such as Li-excess layered oxides suffers from voltage/capacity fadings due to irreversible structural instability. Here, exploiting density functional theory (DFT) as well as fast simulations using the universal potential/forces generated from the newly developed sparse Gaussian process regression (SGPR) machine learning (ML) method, the very complicated/complex structures, X-ray absorption near-edge-structure (XANES) spectra, redox phenomena, and Li diffusion of these battery materials depending on charging/discharging processes is investigated. It is found that voltage/capacity fadings are strongly suppressed in 4d-element-containing cathodes by Al-doping. The suppressed fadings are discussed in view of the structural and electronic changes depending on charged/discharged states which are reflected in their extended X-ray absorption fine structure and XANES spectra. According to crystal orbital Hamilton populations (COHP) and Bader charge analyses of Li1.22Ru0.61Ni0.11Al0.06O2 (Al-LRNO), the Al-doping helps in forming Ni-Al bonding and hence strengthens the bonding-orbital characteristics in Al-O bonds. This strengthened Al?O bonding hinders oxygen oxidation and thus enhances structural stability, diminishing safety concerns. The Al-doping driven suppression of capacity fading and voltage decay is expected to help in designing stable reversible layered cathode materials.

4. ELSEVIER

   Electrocatalytic CO2 reduction reaction over group 15 bismuth and antimony film electrodes: What makes difference?

   Jang, Hye Ji, Maeng, Ju Young, Kim, Young Jun, Yoon, Ilsun, Myung, Chang Woo, Rhee, Choong Kyun, and Sohn, Youngku

   Journal of CO2 Utilization Oct 2022

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   Electrocatalytic CO2 reduction reaction (EC CO2 RR) activities and products are known to be highly dependent on the nature of electrode materials. Herein, Bi and Sb (in group 15 with n-1d10ns2np3) electrodes were chosen and evaluated for EC CO2 RR under various conditions of different applied potentials, electrolytes, concentration, and UV light. Major products were observed to be formate and H2, and minor products included CO. Bi film electrode showed high Faradaic efficiency (FE) of ~94% and the selectivity of 95% for formate. Overlayer vanadium on Bi further enhanced the FE of formate. However, Sb film exhibited much higher FE of H2 than that of formate. UV light was observed to promote formate and CO productions. The FEs of formate and H2 were discussed with exposed crystal facets and density of states near the Fermi level. The very unique detailed study provides valuable information on the development of Bi and Sb-based electrodes for EC CO2 RR of producing value-added carbon products.

5. Nature

   Tracking single adatoms in liquid in a transmission electron microscope

   Clark, Nick, Kelly, Daniel J., Zhou, Mingwei, Zou, Yi-Chao, Myung, Chang Woo, Hopkinson, David G., Schran, Christoph, Michaelides, Angelos, Gorbachev, Roman, and Haigh, Sarah J.

   Nature Jul 2022

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   Single atoms or ions on surfaces affect processes from nucleation1 to electrochemical reactions2 and heterogeneous catalysis3. Transmission electron microscopy is a leading approach for visualizing single atoms on a variety of substrates4,5. It conventionally requires high vacuum conditions, but has been developed for in situ imaging in liquid and gaseous environments6,7 with a combined spatial and temporal resolution that is unmatched by any other method—notwithstanding concerns about electron-beam effects on samples. When imaging in liquid using commercial technologies, electron scattering in the windows enclosing the sample and in the liquid generally limits the achievable resolution to a few nanometres6,8,9. Graphene liquid cells, on the other hand, have enabled atomic-resolution imaging of metal nanoparticles in liquids10. Here we show that a double graphene liquid cell, consisting of a central molybdenum disulfide monolayer separated by hexagonal boron nitride spacers from the two enclosing graphene windows, makes it possible to monitor, with atomic resolution, the dynamics of platinum adatoms on the monolayer in an aqueous salt solution. By imaging more than 70,000 single adatom adsorption sites, we compare the site preference and dynamic motion of the adatoms in both a fully hydrated and a vacuum state. We find a modified adsorption site distribution and higher diffusivities for the adatoms in the liquid phase compared with those in vacuum. This approach paves the way for in situ liquid-phase imaging of chemical processes with single-atom precision.

6. Adv.Ener.Mater.

   Al-doping driven suppression of capacity and voltage fadings in 4d-element containing Li-ion-battery cathode materials: machine learning and density functional theory

   Ha, Miran, Hajibabaei, Amir, Kim, Dong Yeon, Singh, Aditya Narayan, Yun, Jeonghun, Myung, Chang Woo*, and Kim, Kwang S.*

   Advanced Energy Materials Jul 2022

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   The anion redox reaction in high-energy-density cathode materials such as Li-excess layered oxides suffers from voltage/capacity fadings due to irreversible structural instability. Here, using density functional theory (DFT) along with the fast simulations using the universal potential/forces generated from our newly developed sparse Gaussian process regression (SGPR) machine learning (ML) method, we investigated the very complicated/complex structures, XANES spectra, redox phenomena and Li diffusion of these battery materials depending on charging/discharging processes. We found that voltage/capacity fadings are strongly suppressed in 4d-element-containing cathode by Al-doping. The suppressed fadings are discussed in view of the structural and electronic changes depending on charged/discharged states which are reflected in their EXAFS and XANES spectra. According to crystal orbital Hamilton populations (COHP) and Bader charge analyses of Li1.22Ru0.61Ni0.11Al0.06O2 (Al-LRNO), the Al-doping helps in forming Ni-Al bonding and hence strengthens the bonding-orbital characteristics in Al-O bonds. This strengthened Al-O bonding hinders oxygen oxidation and thus enhances structural stability, diminishing safety concerns. The Al-doping driven suppression of capacity fading and voltage decay would help in designing stable reversible layered cathode materials.

7. Adv.Ener.Mater.

   Challenges and Opportunities in Metal Halide Perovskites from Machine Learning Perspectives

   Myung, Chang Woo Myung, Hajibabaei, Amir, Cha, Ji-Hyun, and Kim, Kwang S.

   Adv. Energy Mater. Jul 2022

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   Metal halide perovskite (MHP) is a promising next generation energy material for numerous applications, such as solar cell, light emitting diode, laser, sensor, transistors and more. MHPs show excellent mechanical, dielectric, photovoltaic, photoluminescence, electronic properties that provide ideal platform for various device applications, and such intrigu- ing physical and chemical properties draw many attentions from theory as well. Unfortunately, however, there exists a chasm between the continuing experimental success of MHPs and theoretical understanding because of its disordered structures, compositions, solution-based synthesis routes, ionic interactions and non-adiabatic effects. As these chal- lenges have become major bottlenecks against further development and application of MHPs, it is timely to consider fresh perspectives to circumvent these challenges. In this regard, machine learning (ML) approaches showed a great promise to overcome many challenges and bottlenecks in many fields of science. In this Perspective, we overview some of experimental challenges in MHPs that require detailed theoretical insights and understanding. We propose some of promising ML schemes, including ab initio ML potentials, materials design tools, natural language process (NLP) models, and data mining strategies. Possible roles and pipelines of ML frameworks are highlighted that could close the gap between experiment and theory and could establish systematic MHPs research directions in future

8. Phys.Rev.Lett.

   Prediction of a Supersolid Phase in High-Pressure Deuterium

   Myung, Chang Woo, Hirshberg, Barak, and Parrinello, Michele

   Phys. Rev. Lett. Jan 2022

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   Supersolid is a mysterious and puzzling state of matter whose possible existence has stirred a vigorous debate among physicists for over 60 years. Its elusive nature stems from the coexistence of two seemingly contradicting properties, long-range order and superfluidity. We report computational evidence of a supersolid phase of deuterium under high pressure (p>800 GPa) and low temperature (T<1.0 K). In our simulations, that are based on bosonic path integral molecular dynamics, we observe a highly concerted exchange of atoms while the system preserves its crystalline order. The exchange processes are favored by the soft core interactions between deuterium atoms that form a densely packed metallic solid. At the zero temperature limit, Bose-Einstein condensation is observed as the permutation probability of N deuterium atoms approaches 1/N with a finite superfluid fraction. Our study provides concrete evidence for the existence of a supersolid phase in high-pressure deuterium and could provide insights on the future investigation of supersolid phases in real materials.

2021

1. Phys.Rev.B

   Sparse Gaussian process potentials: Application to lithium diffusivity in superionic conducting solid electrolytes

   Hajibabaei, Amir, Myung, Chang Woo, and Kim, Kwang S.

   Phys. Rev. B Jun 2021

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   For machine learning of interatomic potentials a scalable sparse Gaussian process regression formalism is introduced with a data-efficient on-the-fly adaptive sampling algorithm. With this approach, the computational cost is effectively reduced to those of the Bayesian linear regression methods while maintaining the appealing characteristics of the exact Gaussian process regression. As a showcase, experimental melting and glass-crystallization temperatures are reproduced for Li 7 P 3 S 11, Li diffusivity is simulated, and an unchartered phase is revealed with much lower Li diffusivity which should be circumvented.

2. Two Liquid–Liquid Phase Transitions in Confined Water Nanofilms

   Pourasad, Saeed, Hajibabaei, Amir, Myung, Chang Woo, and Kim, Kwang S.

   May 2021
3. Adv.Mater.

   Anharmonicity-Driven Rashba Cohelical Excitons Break Quantum Efficiency Limitation

   Myung, Chang Woo, and Kim, Kwang S.

   Advanced Materials May 2021

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   Abstract Closed-shell light-emitting diodes (LEDs) suffer from the internal quantum efficiency (IQE) limitation imposed by optically inactive triplet excitons. Here, an unrevealed emission mechanism of lead halide perovskites (LHPs) APbX3 (A = Cs/CN2H5; X = Cl/Br/I) that circumvents the efficiency limit of closed-shell LEDs is explored. Though efficient emission is prohibited by optically inactive J = 0 in inversion symmetric LHPs, the anharmonicity arising from stereochemistry of Pb and resonant orbital-bonding network along the imaginary A+\ldotsX−(T1u) transverse optical (TO) modes, breaks inversion symmetry, introducing disorder and Rashba–Dresselhaus spin-orbit coupling (RD-SOC). This results in bright cohelical and dark antihelical excitons. Many-body theory and first-principles calculations show that the optically active cohelical exciton is the lowest excited state in organic/inorganic LHPs. Thus, RD-SOC can drive to achieve the ideal 50% IQE by utilizing anharmonicity, much over the 25% IQE limitation for closed-shell LEDs.

4. Ener.Environ.Sci.

   Tuning metal single atoms embedded in NxCy moieties toward high-performance electrocatalysis

   Ha, Miran, Kim, Dong Yeon, Umer, Muhammad, Gladkikh, Vladislav, Myung, Chang Woo, and Kim, Kwang S.

   Energy Environ. Sci. May 2021

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   Noble nanoparticle (NP)-sized electrocatalysts have been exploited for diverse electrochemical reactions, in particular, for an eco-friendly hydrogen economy such as water splitting. Recently, minimal amounts of single atoms (SAs) are exploited to maximize the active surface area and to tune the catalytic activity by coordinating the SAs in defect sites of N-doped graphene (GN). For the hydrogen evolution reaction (HER) and oxygen evolution/reduction reactions (OER/ORR), we show high-performance 3d–5d transition metal (TM) SA catalysts using density functional theory (DFT) along with machine learning (ML)-based descriptors. We explore the stability and activity of TM–GN from the view of structure/coordination, formation energy, structural/electrochemical stability, electronic properties, electrical conductivity, and reaction mechanism, which have not been seriously explored yet. Among various –NnCm moieties, the –N2C2 moieties tend to be more easily formed and show higher electrochemical catalytic performance and longer durability (without aggregation/dissolution) compared with the widely studied pure –C4/C3 and –N4/N3 moieties. We found that some TM(SA)s favor a new OER/ORR mechanism, completely different from any known mechanism. The ML-based descriptors showing super HER/OER/ORR performances better than those of bench-mark noble metal catalysts are assessed. In the N2C2 templates, Ni/Ru/Rh/Pt show low HER overpotentials. Here, the H adsorption sites are shared by both the metal and C (not N), which was undiscussed in most of the previous literature where the H is attached on top of a metal atom. Low OER overpotentials are found for Pt/Ni–N2C2, Ni/Pd–C4, and Rh–N4, while low ORR overpotentials are found for Ir/Rh-N4, Pd–C4, Ru–N3C1 and Ni/Pd/Pt–N1C3. The present findings should help in designing high-performance SA catalysts for other various electrocatalytic reactions such as the ammonia evolution reaction.

2020

1. Nano Res.

   Enhanced photoluminescence quantum yield of MAPbBr3 nanocrystals by passivation using graphene

   Park, Youngsin, Jana, Atanu, Myung, Chang Woo, Yoon, Taeseung, Lee, Geungsik, Kocher, Claudius C., Ying, Guanhua, Osokin, Vitaly, Taylor, Robert A., and Kim, Kwang S.

   Nano Research May 2020

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   Diminishing surface defect states in perovskite nanocrystals is a highly challenging subject for enhancing optoelectronic device performance. We synthesized organic/inorganic lead-halide perovskite MAPbBr3 (MA = methylammonium) clusters comprising nanocrystals with diameters ranging between 20–30 nm and characterized an enhanced photoluminescence (PL) quantum yield (as much as ~ 7 times) by encapsulating the MAPbBr3 with graphene (Gr). The optical properties of MAPbBr3 and Gr/MAPbBr3 were investigated by temperature-dependent micro-PL and time-resolved PL measurements. Density functional theory calculations show that the surface defect states in MAPbBr3 are removed and the optical band gap is reduced by a 0.15 eV by encapsulation with graphene due to partial restoration of lattice distortions.

2. J.Phys.Chem.C

   Machine Learning for Predicting the Band Gaps of ABX3 Perovskites from Elemental Properties

   Gladkikh, Vladislav, Kim, Dong Yeon, Hajibabaei, Amir, Jana, Atanu, Myung, Chang Woo, and Kim, Kwang S.

   The Journal of Physical Chemistry C Apr 2020

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   The band gap is an important parameter that determines light-harvesting capability of perovskite materials. It governs the performance of various optoelectronic devices such as solar cells, light-emitting diodes, and photodetectors. For perovskites of a formula ABX3 having a non-zero band gap, we study nonlinear mappings between the band gap and properties of constituent elements (e.g., electronegativities, electron affinities, etc) using alternating conditional expectations (ACE)—a machine learning technique suitable for small data sets. We also compare ACE with other machine learning methods: decision trees, kernel ridge regression, extremely randomized trees, AdaBoost, and gradient boosting. The best performance is achieved by kernel ridge regression and extremely randomized trees. However, ACE has an advantage that it presents its results in a graphic form, helping in interpretation. The models are trained with the data obtained from density functional theory calculations. Different statistical approaches for feature selection are applied and compared: Pearson correlation, Spearman’s rank correlation, maximal information coefficient, distance correlation, and ACE. A classification task of separating metallic perovskites from nonmetallic ones is solved using support-vector machines with the radial basis function kernel.

3. npj Compt.Mater.

   Efficient electron extraction of SnO2 electron transport layer for lead halide perovskite solar cell

   Kim, Junu, Kim, Kwang S., and Myung, Chang Woo

   npj Computational Materials Apr 2020

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   SnO2 electron transport layer (ETL) has been spotlighted with its excellent electron extraction and stability over TiO2 ETL for perovskite solar cells (PSCs), rapidly approaching the highest power conversion efficiency. Thus, how to boost the performance of ETL is of utmost importance and of urgent need in developing more efficient PSCs. Here we elucidate the atomistic origin of efficient electron extraction and long stability of SnO2-based PSCs through the analysis of band alignment, carrier injection, and interfacial defects in the SnO2/MAPbI3(MA = CH3NH3+) interface using unprecedentedly high level of first-principles calculations at the PBE0 + spin-orbit-coupling + dispersion-correction level for all possible terminations and MA directions. We find that Sn-s orbital plays a crucial role in carrier injection and defect tolerance. SnO2/MAPbI3 shows favorable conduction band alignments at both MAI- and PbI2-terminations, which makes the solar cell performance of SnO2/MAPbI3 excel that of TiO2/MAPbI3. Different electron transfer mechanisms of dipole interaction and orbital hybridization at the MAI- and PbI2-terminations indicate that post-transition metal (sp valence) oxide ETLs would outperform transition metal (d valence) oxide ETLs for PSCs.

4. Small

   Unfolding the Influence of Metal Doping on Properties of CsPbI3 Perovskite

   Kajal, Sandeep, Kim, Junu, Shin, Yun Seop, Singh, Aditya Narayan, Myung, Chang Woo, Kim, Jin Young, and Kim, Kwang S.

   Small Methods 2022/04/25 2020

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   Low-temperature α-phase stabilization using HI or zwitterions in cesium lead iodide (CsPbI3) endures the metastable phase properties but is thermally unstable. Doping with a small amount of heterovalent metals (i.e., Bi3+, Sb3+) in CsPbI3 has been assumed to stabilize the α-phase, while here this assumption is challenged. It is demonstrated that heterovalent metal ion doping stabilizes ?-CsPbI3 at low temperatures without replacing the Pb2+ cations, while divalent cations (i.e., Ba2+, Sr2+, and Sn2+) doping stabilizes the α-CsPbI3 by replacing the Pb2+ cations. This finding is demonstrated by both theoretical and experimental results. It is also found that the divalent cations stabilize α-CsPbI3 films, making thermally stable at high temperatures, whereas heterovalent metal-doping stabilizes ?-CsPbI3 films, making metastable. The doping influence on crystal grains and the chemical composition of thin films is discussed. In particular, the charge dissociation kinetics for the Sr doped thin film are much enhanced than α-CsPbI3 and Ba doped thin films, also the initial results of the fabricated perovskite red-light-emmiting diode suggests that the Sr-doped thin films would be more suitable for the device fabrication. These findings will guide a way for further development in thermally and air-stable optoelectronic devices.

5. App.Catal.B:Envir.

   Immiscible bi-metal single-atoms driven synthesis of electrocatalysts having superb mass-activity and durability

   Harzandi, Ahmad M., Shadman, Sahar, Ha, Miran, Myung, Chang Woo, Kim, Dong Yeon, Park, Hyo Ju, Sultan, Siraj, Noh, Woo-Suk, Lee, Wanggeun, Thangavel, Pandiarajan, Byun, Woo Jin, Lee, Seong-hun, Tiwari, Jitendra N., Shin, Tae Joo, Park, Jae-Hoon, Lee, Zonghoon, Lee, Jae Sung, and Kim, Kwang S.

   Applied Catalysis B: Environmental 2022/04/25 2020

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   Properties of metal elements can be modified by alloying. Given that catalytic efficiencies are often maximized using metal single-atoms (SAs), two immiscible metals can improve the catalytic activity when they can be present as non-agglomerated dual SAs. Here, we report yet-unexplored synthesis of high-performance electrocatalysts utilizing immiscibility of Cu/Ru bimetal atoms. In the synthesized electrocatalyst (Cu/Ru\char64GN), both Cu-SAs and Ru-SAs are dispersed on N-doped graphitic-matrix (GN), while other Cu-SAs are bridged to Ru nanoparticles (NPs) surface via nitrogen. It shows superior mass-activity with outstanding catalytic performance and durability for hydrogen evolution reaction (HER), outperforming commercial Pt20wt%/C. Cu-SAs along with Ru-NPs enhance electric conduction or charge-transfer rate of GN-templates for fast kinetics. Owing to Cu-Ru immiscibility, the coordination of Cu-SAs by N atoms which bridge to Ru-NPs surface, introduces new active sites and brings about long-term stability (over 600 h in acidic media) by preventing aggregation of Ru-NPs.

2019

1. J.Mater.Chem.A

   A thermally stable, barium-stabilized α-CsPbI3 perovskite for optoelectronic devices

   Kajal, Sandeep, Kim, Gi-Hwan, Myung, Chang Woo, Shin, Yun Seop, Kim, Junu, Jeong, Jaeki, Jana, Atanu, Kim, Jin Young, and Kim, Kwang S.

   Journal of Materials Chemistry A 2022/04/25 2019

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2. Adv.Ener.Mater.

   Single Atoms and Clusters Based Nanomaterials for Hydrogen Evolution, Oxygen Evolution Reactions, and Full Water Splitting

   Sultan, Siraj, Tiwari, Jitendra N., Singh, Aditya Narayan, Zhumagali, Shynggys, Ha, Miran, Myung, Chang Woo, Thangavel, Pandiarajan, and Kim, Kwang S.

   Advanced Energy Materials 2022/04/26 2019

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3. Adv.Ener.Mater.

   High-Performance Hydrogen Evolution by Ru Single Atoms and Nitrided-Ru Nanoparticles Implanted on N-Doped Graphitic Sheet

   Tiwari, Jitendra N., Harzandi, Ahmad M., Ha, Miran, Sultan, Siraj, Myung, Chang Woo, Park, Hyo Ju, Kim, Dong Yeon, Thangavel, Pandiarajan, Singh, Aditya Narayan, Sharma, Pankaj, Chandrasekaran, Selvaraj Selva, Salehnia, Foad, Jang, Ji-Wook, Shin, Hyeon Suk, Lee, Zonghoon, and Kim, Kwang S.

   Advanced Energy Materials 2022/04/26 2019

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   The most efficient electrocatalyst for the hydrogen evolution reaction (HER) is a Pt-based catalyst, but its high cost and nonperfect efficiency hinder wide-ranging industrial/technological applications. Here, an electrocatalyst of both ruthenium (Ru) single atoms (SAs) and N-doped-graphitic(GN)-shell-covered nitrided-Ru nanoparticles (NPs) (having a Ru-Nx shell) embedded on melamine-derived GN matrix {1: [Ru(SA)+Ru(NP)\char64RuNx\char64GN]/GN}, which exhibits superior HER activity in both acidic and basic media, is presented. In 0.5 m H2SO4/1 m KOH solutions, 1 shows diminutive ?negative overpotentials? (?? = |?| = 10/7 mV at 10 mA cm?2, lowest ever) and high exchange current densities (4.70/1.96 mA cm?2). The remarkable HER performance is attributed to the near-zero free energies for hydrogen adsorption/desorption on Ru(SAs) and the increased conductivity of melamine-derived GN sheets by the presence of nitrided-Ru(NPs). The nitridation process forming nitrided-Ru(NPs), which are imperfectly covered by a GN shell, allows superb long-term operation durability. The catalyst splits water into molecular oxygen and hydrogen at 1.50/1.40 V (in 0.1 m HClO4/1 m KOH), demonstrating its potential as a ready-to-use, highly effective energy device for industrial applications.

4. Nat.Commun.

   Superb water splitting activity of the electrocatalyst Fe3Co(PO4)4 designed with computation aid

   Sultan, Siraj, Ha, Miran, Kim, Dong Yeon, Tiwari, Jitendra N., Myung, Chang Woo, Meena, Abhishek, Shin, Tae Joo, Chae, Keun Hwa, and Kim, Kwang S.

   Nature Communications 2022/04/26 2019

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   For efficient water splitting, it is essential to develop inexpensive and super-efficient electrocatalysts for the oxygen evolution reaction (OER). Herein, we report a phosphate-based electrocatalyst [Fe3Co(PO4)4\char64reduced-graphene-oxide(rGO)] showing outstanding OER performance (much higher than state-of-the-art Ir/C catalysts), the design of which was aided by first-principles calculations. This electrocatalyst displays low overpotential (237 mV at high current density 100 mA cm−2 in 1 M KOH), high turnover frequency (TOF: 0.54 s−1), high Faradaic efficiency (98%), and long-term durability. Its remarkable performance is ascribed to the optimal free energy for OER at Fe sites and efficient mass/charge transfer. When a Fe3Co(PO4)4\char64rGO anodic electrode is integrated with a Pt/C cathodic electrode, the electrolyzer requires only 1.45 V to achieve 10 mA cm−2 for whole water splitting in 1 M KOH (1.39 V in 6 M KOH), which is much smaller than commercial Ir-C//Pt-C electrocatalysts. This cost-effective powerful oxygen production material with carbon-supporting substrates offers great promise for water splitting.

2018

1. J.Mater.Chem.A

   Organic cation steered interfacial electron transfer within organic–inorganic perovskite solar cells

   Javaid, Saqib, Myung, Chang Woo, Yun, Jeonghun, Lee, Geunsik, and Kim, Kwang S.

   Journal of Materials Chemistry A 2022/04/26 2018

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   Methylammonium lead-iodide (MAPbI3, MA: CH3–NH3) interfaced with rutile TiO2 is widely used in photovoltaic devices. These devices utilize the electron transfer from MAPbI3 to TiO2, which may not be explained solely by the band structures of the two bulk materials. To elucidate the interface dynamics and its impact on the electron transfer process, we have studied the interfacial features of a TiO2/MAPbI3 system. First principles calculations and ab initio molecular dynamics simulations show that the rotational freedom of MA present within the bulk is considerably suppressed due to interaction of MA with the TiO2 substrate, highlighting orientationally ordered MA at the interface. The optimized interface structure shows the C–N axis of MA titled towards the TiO2 surface so as to maximize the interaction between N-attached H and underlying O. The very short O⋯H⋯N distance with very large hydrogen bonding energy identifies short strong hydrogen bonding (SSHB) as the origin of structural re-organization at the interface. As for the electronic structure, this proton sharing between MA and TiO2 has a critical impact on the energy level alignment at the interface, thus driving the electron transfer process from MA to TiO2. Indeed, significant reduction in the electron transfer barrier is observed for the energetically optimal interface configuration which promotes the electron transfer across the interface and photovoltaic properties.

2. J.Mater.Chem.A

   La-doped BaSnO3 electron transport layer for perovskite solar cells

   Myung, Chang Woo, Lee, Geunsik, and Kim, Kwang S.

   Journal of Materials Chemistry A 2022/04/26 2018

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   Due to the photoinstability and large hysteresis of the TiO2 electron transport layer (ETL) in perovskite solar cells (PSCs), the search for novel electron transport materials has emerged. Herein, using first principles calculations, we unveiled the key factor for the ideal band alignment between La-doped BaSnO3 (LBSO) and methyl ammonium (MA) lead iodide perovskite (MAPbI3). The (PbI2-, MAI-terminated)CH3NH3PbI3/(SnO2-, BaO-terminated)LaxBa(1−x)SnO3 interface formed a stable “all-perovskite”heterostructure with large binding energy. The selective band alignment of the conduction band was easily manipulated by La-doping on the Ba site due to the band gap renormalization (or shrinkage) caused by doped electrons and La3+ dopant. In addition, the MAI-terminated MAPbI3/LBSO interface exhibited proton transfer (BaO-terminated) and strong hydrogen bonding (SnO2-terminated) between MA and oxygen anion. LBSO presenting high mobility, photostability, and structural stability would help develop the next generation ETL materials for PSCs.

3. J.Mater.Chem.A

   A highly hydrophobic fluorographene-based system as an interlayer for electron transport in organic–inorganic perovskite solar cells

   Javaid, Saqib, Myung, Chang Woo, Pourasad, S., Rakshit, Bipul, Kim, Kwang S., and Lee, Geunsik

   Journal of Materials Chemistry A 2022/04/26 2018

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   Degradation of perovskite halide materials under humid conditions is one of the major hurdles in the commercialization of organic–inorganic perovskite solar cells. Herein, we studied the interface between highly hydrophobic fluorographene (FGr) and cubic methylammonium lead iodide (MAPbI3, MA: CH3–NH3) by employing density functional theory (DFT)-based simulations. We demonstrate that the adsorption of FGr on MAPbI3 results in the formation of a stable interface with appreciable binding energy (∼0.4 eV per Pb atom). Thorough assessment of energy-level alignment indicates that the FGr/MAPbI3 interface has desirable properties with regard to the electron transfer (hole blockage) process. These results underscore the potential of using FGr as an interlayer for electron transport between a perovskite layer and an electron transfer medium (such as TiO2) as well as a moisture blocker for achieving high perovskite stability by perfect waterproofing. The future research study towards the integration of hydrophobic FGr or electronically optimized partially fluorinated graphene-based systems within perovskite halide photovoltaic devices may pave the way for stable and efficient solar cell technologies.

4. ACS Ener.Lett.

   Rashba–Dresselhaus Effect in Inorganic/Organic Lead Iodide Perovskite Interfaces

   Myung, Chang Woo, Javaid, Saqib, Kim, Kwang S., and Lee, Geunsik

   ACS Energy Letters Jun 2018

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   Despite the imperative importance in solar cell efficiency, the intriguing phenomena at the interface between perovskite solar cell and adjacent carrier transfer layers are hardly uncovered. Here we show that PbI2/AI-terminated lead iodide perovskite (APbI3; A = Cs+/ methylammonium (MA)) interfaced with the charge transport medium of graphene or TiO2 exhibits a sizable/robust Rashba–Dresselhaus (RD) effect using density functional theory and ab initio molecular dynamics (AIMD) simulations above the cubic-phase temperature. At the PbI2-terminated graphene/CsPbI3(001) interface, ferroelectric distortion toward graphene facilitates an inversion breaking field. At the MAI-terminated TiO2/MAPbI3(001) interface, the enrooted alignment of MA+ toward TiO2 by short-strong hydrogen bonding and concomitant PbI3 distortion preserve the RD interactions even above 330 K. The robust RD effect at the interface even at high temperatures, unlike in bulk, changes the direct-type band to indirect-type to suppress recombination of the electron and hole, thereby letting these accumulated carriers overcome the potential barrier between perovskite and charge transfer materials, which promotes the solar cell efficiency.

5. Nat.Energy

   Multicomponent electrocatalyst with ultralow Pt loading and high hydrogen evolution activity

   Tiwari, Jitendra N., Sultan, Siraj, Myung, Chang Woo, Yoon, Taeseung, Li, Nannan, Ha, Miran, Harzandi, Ahmad M., Park, Hyo Ju, Kim, Dong Yeon, Chandrasekaran, S. Selva, Lee, Wang Geun, Vij, Varun, Kang, Hoju, Shin, Tae Joo, Shin, Hyeon Suk, Lee, Geunsik, Lee, Zonghoon, and Kim, Kwang S.

   Nature Energy Jun 2018

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   Platinum is the most effective electrocatalyst for the hydrogen evolution reaction in acidic solutions, but its high cost limits its wide application. Therefore, it is desirable to design catalysts that only require minimal amounts of Pt to function, but that are still highly active. Here we report hydrogen production in acidic water using a multicomponent catalyst with an ultralow Pt loading (1.4 μg per electrode area (cm2)) supported on melamine-derived graphitic tubes (GTs) that encapsulate a FeCo alloy and have Cu deposited on the inside tube walls. With a 1/80th Pt loading of a commercial 20% Pt/C catalyst, in 0.5 M H2SO4 the catalyst achieves a current density of 10 mA cm−2 at an overpotential of 18 mV, and shows a turnover frequency of 7.22 s−1 (96 times higher than that of the Pt/C catalyst) and long-term durability (10,000 cycles). We propose that a synergistic effect between the Pt clusters and single Pt atoms embedded in the GTs enhances the catalytic activity.

6. ACS Appl.Ener.Mater.

   Ambient-Stable Cubic-Phase Hybrid Perovskite Reaching the Shockley–Queisser Fill Factor Limit via Inorganic Additive-Assisted Process

   Yoon, Taeseung, Kim, Gi-Hwan, Myung, Chang Woo, Kajal, Sandeep, Jeong, Jaeki, Kim, Jae Won, Kim, Jin Young, and Kim, Kwang S.

   ACS Applied Energy Materials Nov 2018

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   Additive-assisted organic–inorganic perovskite materials have attracted substantial attention as photovoltaic light absorbers which lead to outstanding power conversion efficiency. Here we report an easy and effective fabrication of cubic-phase perovskite with an inorganic molecule additive like hydrazinium chloride (N2H5Cl, to be denoted as HZCl). We predict that this inorganic cation of N2H5+, which can substitute for the organic A-site in the perovskite structure, can tune Fröhlich polaron properties by controlling the interaction strength and the number of proton coordinations to halide. This prediction is experimentally demonstrated with an optimized perovskite device with 2% N2H5Cl additive, which exhibits an unprecedented 85% fill factor (FF) with the highest value close to the Shockley–Queisser limit. An extra power conversion efficiency (PCE) of 2.3% and a fill factor (FF) efficiency of 14% are boosted. These optimized performances by additive effects lead to a new approach based on the theoretical calculation toward an improved performance of the perovskite solar cell.

7. Adv.Ener.Mater.

   A New Perspective on the Role of A-Site Cations in Perovskite Solar Cells

   Myung, Chang Woo, Yun, Jeonghun, Lee, Geunsik, and Kim, Kwang S.

   Advanced Energy Materials 2022/04/25 2018

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   As the race toward higher efficiency for inorganic/organic hybrid perovskite solar cells (PSCs) is becoming highly competitive, a design scheme to maximize carrier transport toward higher power efficiency has been urgently demanded. In this study, a hidden role of A-site cations of PSCs in carrier transport, which has been largely neglected is unraveled, i.e., tuning the Fröhlich electron?phonon (e?ph) coupling of longitudinal optical (LO) phonon by A-site cations. The key for steering Fröhlich polaron is to control the interaction strength and the number of proton (or lithium) coordination to halide ions. The coordination to I? alleviates electron?phonon scattering by either decreasing the Born effective charge or absorbing the LO motion of I. This novel principle discloses low electron?phonon coupling in several promising organic cations including hydroxyl?ammonium cation (NH3OH+), hydrazinium cation (NH3NH2+) and possibly Li+ solvating methylamine (Li+∙∙∙NH2CH3), on a par with methyl?ammonium cations. A new perspective on the role of A-site cations could help in improving power efficiency and accelerating the application of PSCs.

2017

1. ACS Nano

   Two-Dimensional Excitonic Photoluminescence in Graphene on a Cu Surface

   Park, Youngsin, Kim, Yooseok, Myung, Chang Woo, Taylor, Robert Anthony, Chan, Christopher C. S., Reid, Benjamin P. L., Puchtler, Tim J., Nicholas, Robin J., Singh, Laishram Tomba, Lee, Geunsik, Hwang, Chan-Cuk, Park, Chong-Yun, and Kim, Kwang S.

   ACS Nano Mar 2017

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   Despite having outstanding electrical properties, graphene is unsuitable for optical devices because of its zero band gap. Here, we report two-dimensional excitonic photoluminescence (PL) from graphene grown on a Cu(111) surface, which shows an unexpected and remarkably sharp strong emission near 3.16 eV (full width at half-maximum ≤3 meV) and multiple emissions around 3.18 eV. As temperature increases, these emissions blue shift, displaying the characteristic negative thermal coefficient of graphene. The observed PL originates from the significantly suppressed dispersion of excited electrons in graphene caused by hybridization of graphene πand Cu d orbitals of the first and second Cu layers at a shifted saddle point 0.525(M+K) of the Brillouin zone. This finding provides a pathway to engineering optoelectronic graphene devices, while maintaining the outstanding electrical properties of graphene.

2. ACS Appl.Mater.Inter.

   Graphene and Graphene Analogs toward Optical, Electronic, Spintronic, Green-Chemical, Energy-Material, Sensing, and Medical Applications

   Rezapour, M. Reza, Myung, Chang Woo, Yun, Jeonghun, Ghassami, Amirreza, Li, Nannan, Yu, Seong Uk, Hajibabaei, Amir, Park, Youngsin, and Kim, Kwang S.

   ACS Applied Materials & Interfaces Jul 2017

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   This spotlight discusses intriguing properties and diverse applications of graphene (Gr) and Gr analogs. Gr has brought us two-dimensional (2D) chemistry with its exotic 2D features of density of states. Yet, some of the 2D or 2D-like features can be seen on surfaces and at interfaces of bulk materials. The substrate on Gr and functionalization of Gr (including metal decoration, intercalation, doping, and hybridization) modify the unique 2D features of Gr. Despite abundant literature on physical properties and well-known applications of Gr, spotlight works based on the conceptual understanding of the 2D physical and chemical nature of Gr toward vast-ranging applications are hardly found. Here we focus on applications of Gr, based on conceptual understanding of 2D phenomena toward 2D chemistry. Thus, 2D features, defects, edges, and substrate effects of Gr are discussed first. Then, to pattern Gr electronic circuits, insight into differentiating conducting and nonconducting regions is introduced. By utilizing the unique ballistic electron transport properties and edge spin states of Gr, Gr nanoribbons (GNRs) are exploited for the design of ultrasensitive molecular sensing electronic devices (including molecular fingerprinting) and spintronic devices. The highly stable nature of Gr can be utilized for protection of corrosive metals, moisture-sensitive perovskite solar cells, and highly environment-susceptible topological insulators (TIs). Gr analogs have become new types of 2D materials having novel features such as half-metals, TIs, and nonlinear optical properties. The key insights into the functionalized Gr hybrid materials lead to the applications for not only energy storage and electrochemical catalysis, green chemistry, and electronic/spintronic devices but also biosensing and medical applications. All these topics are discussed here with the focus on conceptual understanding. Further possible applications of Gr, GNRs, and Gr analogs are also addressed in a section on outlook and future challenges.

3. ACS Appl.Mater.Inter.

   Anomalous Ambipolar Transport of Organic Semiconducting Crystals via Control of Molecular Packing Structures

   Park, Beomjin, Kim, Kyunghun, Park, Jaesung, Lim, Heeseon, Lanh, Phung Thi, Jang, A-Rang, Hyun, Chohee, Myung, Chang Woo, Park, Seungkyoo, Kim, Jeong Won, Kim, Kwang S., Shin, Hyeon Suk, Lee, Geunsik, Kim, Se Hyun, Park, Chan Eon, and Kim, Jin Kon

   ACS Applied Materials & Interfaces Aug 2017

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   Organic crystals deposited on 2-dimensional (2D) van der Waals substrates have been widely investigated due to their unprecedented crystal structures and electrical properties. van der Waals interaction between organic molecules and the substrate induces epitaxial growth of high quality organic crystals and their anomalous crystal morphologies. Here, we report on unique ambipolar charge transport of a “lying-down”pentacene crystal grown on a 2D hexagonal boron nitride van der Waals substrate. From in-depth analysis on crystal growth behavior and ultraviolet photoemission spectroscopy measurement, it is revealed that the pentacene crystal at the initial growth stage have a lattice-strained packing structure and unique energy band structure with a deep highest occupied molecular orbital level compared to conventional “standing-up”crystals. The lattice-strained pentacene few layers enable ambipolar charge transport in field-effect transistors with balanced hole and electron field-effect mobilities. Complementary logic circuits composed of the two identical transistors show clear inverting functionality with a high gain up to 15. The interesting crystal morphology of organic crystals on van der Waals substrates is expected to attract broad attentions on organic/2D interfaces for their electronic applications.

2015

1. ACS Nano

   Graphene Edges and Beyond: Temperature-Driven Structures and Electromagnetic Properties

   Hyun, Changbae, Yun, Jeonghun, Cho, Woo Jong, Myung, Chang Woo, Park, Jaesung, Lee, Geunsik, Lee, Zonghoon, Kim, Kwanpyo, and Kim, Kwang S.

   ACS Nano May 2015

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   The atomic configuration of graphene edges significantly influences the various properties of graphene nanostructures, and realistic device fabrication requires precise engineering of graphene edges. However, the imaging and analysis of the intrinsic nature of graphene edges can be illusive due to contamination problems and measurement-induced structural changes to graphene edges. In this issue of ACS Nano, He et al. report an in situ heating experiment in aberration-corrected transmission electron microscopy to elucidate the temperature dependence of graphene edge termination at the atomic scale. They revealed that graphene edges predominantly have zigzag terminations below 400 \,^∘C, while above 600 \,^∘C, the edges are dominated by armchair and reconstructed zigzag edges. This report brings us one step closer to the true nature of graphene edges. In this Perspective, we outline the present understanding, issues, and future challenges faced in the field of graphene-edge-based nanodevices.