Publications by authors named "Hu Young Jeong"

129 Publications

Tailored growth of single-crystalline InP tetrapods.

Nat Commun 2021 Jul 22;12(1):4454. Epub 2021 Jul 22.

Department of Energy Science and Center for Artificial Atoms, Sungkyunkwan University (SKKU), Suwon, Republic of Korea.

Despite the technological importance of colloidal covalent III-V nanocrystals with unique optoelectronic properties, their synthetic process still has challenges originating from the complex energy landscape of the reaction. Here, we present InP tetrapod nanocrystals as a crystalline late intermediate in the synthetic pathway that warrants controlled growth. We isolate tetrapod intermediate species with well-defined surfaces of (110) and ([Formula: see text]) via the suppression of further growth. An additional precursor supply at low temperature induces [Formula: see text]-specific growth, whereas the [110]-directional growth occurs over the activation barrier of 65.7 kJ/mol at a higher temperature, thus finalizes into the (111)-faceted tetrahedron nanocrystals. We address the use of late intermediates with well-defined facets at the sub-10 nm scale for the tailored growth of covalent III-V nanocrystals and highlight the potential for the directed approach of nanocrystal synthesis.
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http://dx.doi.org/10.1038/s41467-021-24765-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8298524PMC
July 2021

Reversible Ligand Exchange in Atomically Dispersed Catalysts for Modulating the Activity and Selectivity of the Oxygen Reduction Reaction.

Angew Chem Int Ed Engl 2021 Sep 6;60(37):20528-20534. Epub 2021 Aug 6.

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919, Republic of Korea.

Rational control of the coordination environment of atomically dispersed catalysts is pivotal to achieve desirable catalytic reactivity. We report the reversible control of coordination structure in atomically dispersed electrocatalysts via ligand exchange reactions to reversibly modulate their reactivity for oxygen reduction reaction (ORR). The CO-ligated atomically dispersed Rh catalyst exhibited ca. 30-fold higher ORR activity than the NH -ligated catalyst, whereas the latter showed three times higher H O selectivity than the former. Post-treatments of the catalysts with CO or NH allowed the reversible exchange of CO and NH ligands, which reversibly tuned oxidation state of metal centers and their ORR activity and selectivity. DFT calculations revealed that more reduced oxidation state of CO-ligated Rh site could further stabilize the *OOH intermediate, facilitating the two- and four-electron pathway ORR. The reversible ligand exchange reactions were generalized to Ir- and Pt-based catalysts.
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http://dx.doi.org/10.1002/anie.202108439DOI Listing
September 2021

Low-Temperature and High-Quality Growth of BiOSe Layered Semiconductors Cracking Metal-Organic Chemical Vapor Deposition.

ACS Nano 2021 May 11;15(5):8715-8723. Epub 2021 May 11.

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea.

Ternary metal-oxy-chalcogenides are emerging as next-generation layered semiconductors beyond binary metal-chalcogenides (, MoS). Among ternary metal-oxy-chalcogenides, especially BiOSe has been demonstrated in field-effect transistors and photodetectors, exhibiting ultrahigh performance with robust air stability. The growth method for BiOSe that has been reported so far is a powder sublimation based chemical vapor deposition. The first step for pursuing the practical application of BiOSe as a semiconductor material is developing a gas-phase growth process. Here, we report a cracking metal-organic chemical vapor deposition (c-MOCVD) for the gas-phase growth of BiOSe. The resulting BiOSe films at very low growth temperature (∼300 °C) show single-crystalline quality. By taking advantage of the gas-phase growth, the precise phase control was demonstrated by modulating the partial pressure of each precursor. In addition, c-MOCVD-grown BiOSe exhibits outstanding electrical and optoelectronic performance at room temperature without passivation, including maximum electron mobility of 127 cm/(V·s) and photoresponsivity of 45134 A/W.
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http://dx.doi.org/10.1021/acsnano.1c00811DOI Listing
May 2021

Enhancing Thermocatalytic Activities by Upshifting the d-Band Center of Exsolved Co-Ni-Fe Ternary Alloy Nanoparticles for the Dry Reforming of Methane.

Angew Chem Int Ed Engl 2021 Jul 9;60(29):15912-15919. Epub 2021 Jun 9.

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, Republic of Korea.

Dry reforming of methane (DRM) is a feasible solution to address the reduction of greenhouse gases stipulated by the Paris Climate Agreement, given that it adds value by converting trivial gases, CO and CH , simultaneously into useful syngas. However, the conventional Ni catalyst undergoes deactivation due to carbon coking and particle agglomeration. Here we demonstrate a highly efficient and durable DRM catalyst: exsolved Co-Ni-Fe ternary alloy nanoparticles on the layered perovskite PrBaMn Co Ni O produced by topotactic exsolution. This method readily allows the generation of a larger number of exsolved nanoparticles with enhanced catalytic activity above that of Ni monometallic and Co-Ni bimetallic particles. The enhancement is achieved by the upshift of the d-band center of Co-Ni-Fe relative to those of Co-Ni and Ni, meaning easier charge donation to the adsorbate. Furthermore, the exsolved catalyst shows exceptional stability, with continuous DRM operation for about 350 hours.
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http://dx.doi.org/10.1002/anie.202101335DOI Listing
July 2021

Strain-driven autonomous control of cation distribution for artificial ferroelectrics.

Sci Adv 2021 Apr 28;7(18). Epub 2021 Apr 28.

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

In past few decades, there have been substantial advances in theoretical material design and experimental synthesis, which play a key role in the steep ascent of developing functional materials with unprecedented properties useful for next-generation technologies. However, the ultimate goal of synthesis science, i.e., how to locate atoms in a specific position of matter, has not been achieved. Here, we demonstrate a unique way to inject elements in a specific crystallographic position in a composite material by strain engineering. While the use of strain so far has been limited for only mechanical deformation of structures or creation of elemental defects, we show another powerful way of using strain to autonomously control the atomic position for the synthesis of new materials and structures. We believe that our synthesis methodology can be applied to wide ranges of systems, thereby providing a new route to functional materials.
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http://dx.doi.org/10.1126/sciadv.abd7394DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8081366PMC
April 2021

Cooperative evolution of polar distortion and nonpolar rotation of oxygen octahedra in oxide heterostructures.

Sci Adv 2021 Apr 21;7(17). Epub 2021 Apr 21.

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea.

Polarity discontinuity across LaAlO/SrTiO (LAO/STO) heterostructures induces electronic reconstruction involving the formation of two-dimensional electron gas (2DEG) and structural distortions characterized by antiferrodistortive (AFD) rotation and ferroelectric (FE) distortion. We show that AFD and FE modes are cooperatively coupled in LAO/STO (111) heterostructures; they coexist below the critical thickness ( ) and disappear simultaneously above with the formation of 2DEG. Electron energy-loss spectroscopy and density functional theory (DFT) calculations provide direct evidence of oxygen vacancy ( ) formation at the LAO (111) surface, which acts as the source of 2DEG. Tracing the AFD rotation and FE distortion of LAO reveals that their evolution is strongly correlated with distribution. The present study demonstrates that AFD and FE modes in oxide heterostructures emerge as a consequence of interplay between misfit strain and polar field, and further that their combination can be tuned to competitive or cooperative coupling by changing the interface orientation.
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http://dx.doi.org/10.1126/sciadv.abe9053DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8059930PMC
April 2021

Atomic-layer-confined multiple quantum wells enabled by monolithic bandgap engineering of transition metal dichalcogenides.

Sci Adv 2021 Mar 26;7(13). Epub 2021 Mar 26.

KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul, 02841, Republic of Korea.

Quantum wells (QWs), enabling effective exciton confinement and strong light-matter interaction, form an essential building block for quantum optoelectronics. For two-dimensional (2D) semiconductors, however, constructing the QWs is still challenging because suitable materials and fabrication techniques are lacking for bandgap engineering and indirect bandgap transitions occur at the multilayer. Here, we demonstrate an unexplored approach to fabricate atomic-layer-confined multiple QWs (MQWs) via monolithic bandgap engineering of transition metal dichalcogenides and van der Waals stacking. The WO/WSe hetero-bilayer formed by monolithic oxidation of the WSe bilayer exhibited the type I band alignment, facilitating as a building block for MQWs. A superlinear enhancement of photoluminescence with increasing the number of QWs was achieved. Furthermore, quantum-confined radiative recombination in MQWs was verified by a large exciton binding energy of 193 meV and a short exciton lifetime of 170 ps. This work paves the way toward monolithic integration of band-engineered heterostructures for 2D quantum optoelectronics.
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http://dx.doi.org/10.1126/sciadv.abd7921DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7997527PMC
March 2021

Epitaxial Single-Crystal Growth of Transition Metal Dichalcogenide Monolayers via the Atomic Sawtooth Au Surface.

Adv Mater 2021 Apr 10;33(15):e2006601. Epub 2021 Mar 10.

Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Sungkyunkwan University, Suwon, 16419, Republic of Korea.

Growth of 2D van der Waals layered single-crystal (SC) films is highly desired not only to manifest the intrinsic physical and chemical properties of materials, but also to enable the development of unprecedented devices for industrial applications. While wafer-scale SC hexagonal boron nitride film has been successfully grown, an ideal growth platform for diatomic transition metal dichalcogenide (TMdC) films has not been established to date. Here, the SC growth of TMdC monolayers on a centimeter scale via the atomic sawtooth gold surface as a universal growth template is reported. The atomic tooth-gullet surface is constructed by the one-step solidification of liquid gold, evidenced by transmission electron microscopy. The anisotropic adsorption energy of the TMdC cluster, confirmed by density-functional calculations, prevails at the periodic atomic-step edge to yield unidirectional epitaxial growth of triangular TMdC grains, eventually forming the SC film, regardless of the Miller indices. Growth using the atomic sawtooth gold surface as a universal growth template is demonstrated for several TMdC monolayer films, including WS , WSe , MoS , the MoSe /WSe heterostructure, and W Mo S alloys. This strategy provides a general avenue for the SC growth of diatomic van der Waals heterostructures on a wafer scale, to further facilitate the applications of TMdCs in post-silicon technology.
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http://dx.doi.org/10.1002/adma.202006601DOI Listing
April 2021

A Flash-Induced Robust Cu Electrode on Glass Substrates and Its Application for Thin-Film μLEDs.

Adv Mater 2021 Apr 26;33(13):e2007186. Epub 2021 Feb 26.

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea.

A robust Cu conductor on a glass substrate for thin-film μLEDs using the flash-induced chemical/physical interlocking between Cu and glass is reported. During millisecond light irradiation, CuO nanoparticles (NPs) on the display substrate are transformed into a conductive Cu film by reduction and sintering. At the same time, intensive heating at the boundary of CuO NPs and glass chemically induces the formation of an ultrathin Cu O interlayer within the Cu/glass interface for strong adhesion. Cu nanointerlocking occurs by transient glass softening and interface fluctuation to increase the contact area. Owing to these flash-induced interfacial interactions, the flash-activated Cu electrode exhibits an adhesion energy of 10 J m , which is five times higher than that of vacuum-deposited Cu. An AlGaInP thin-film vertical μLED (VLED) forms an electrical interconnection with the flash-induced Cu electrode via an ACF bonding process, resulting in a high optical power density of 41 mW mm . The Cu conductor enables reliable VLED operation regardless of harsh thermal stress and moisture infiltration under a high-temperature storage test, temperature humidity test, and thermal shock test. 50 × 50 VLED arrays transferred onto the flash-induced robust Cu electrode show high illumination yield and uniform distribution of forward voltage, peak wavelength, and device temperature.
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http://dx.doi.org/10.1002/adma.202007186DOI Listing
April 2021

Synthesis of metallic mixed 3R and 2H NbS nanoflakes by chemical vapor deposition.

Faraday Discuss 2021 Apr 1;227:332-340. Epub 2021 Feb 1.

Institute of Microengineering and Nanoelectronics, Universiti Kebangsaan Malaysia, 43600 UKM, Bangi, Malaysia.

In this work, we report the synthesis and characterization of mixed phase NbS nanoflakes prepared by chemical vapor deposition. The as-grown samples show a high density of flakes (thickness ∼50 nm) that form a continuous film. Raman and X-ray diffraction data show that the samples consist of both 2H and 3R phases, with the 2H phase containing a high concentration of Nb interstitials. These Nb interstitials sit in between the NbS layers to form NbS. Cross-sectional Energy Dispersive Spectroscopy analysis with transmission electron microscopy suggests that the 2H NbS region is found in thinner flakes, while 3R NbS is observed in thicker regions of the films. The evolution of the phase from 2H NbS to 3R NbS may be attributed to the change of the growth environment from Nb-rich at the start of the growth to sulfur-rich at the latter stage. It was also found that the incorporation of Nb interstitials is highly dependent on the temperature of the NbCl precursor and the position of the substrate in the furnace. Samples grown at high NbCl temperature and with substrate located closer to the NbCl source show higher incorporation of Nb interstitials. Electrical measurements show linear I-V characteristics, indicating the metallic nature of the NbS film with relatively low resistivity of 4.1 × 10Ω cm.
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http://dx.doi.org/10.1039/c9fd00132hDOI Listing
April 2021

Enhanced Photoluminescence of Multiple Two-Dimensional van der Waals Heterostructures Fabricated by Layer-by-Layer Oxidation of MoS.

ACS Appl Mater Interfaces 2021 Jan 23;13(1):1245-1252. Epub 2020 Dec 23.

Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Korea.

Monolayer transition metal dichalcogenides (TMDs) are promising for optoelectronics because of their high optical quantum yield and strong light-matter interaction. In particular, the van der Waals (vdW) heterostructures consisting of monolayer TMDs sandwiched by large gap hexagonal boron nitride have shown great potential for novel optoelectronic devices. However, a complicated stacking process limits scalability and practical applications. Furthermore, even though lots of efforts, such as fabrication of vdW heterointerfaces, modification of the surface, and structural phase transition, have been devoted to preserve or modulate the properties of TMDs, high environmental sensitivity and damage-prone characteristics of TMDs make it difficult to achieve a controllable technique for surface/interface engineering. Here, we demonstrate a novel way to fabricate multiple two-dimensional (2D) vdW heterostructures consisting of alternately stacked MoS and MoO with enhanced photoluminescence (PL). We directly oxidized multilayer MoS to a MoO/1 L-MoS heterostructure with atomic layer precision through a customized oxygen plasma system. The monolayer MoS covered by MoO showed an enhanced PL intensity 3.2 and 6.5 times higher in average than the as-exfoliated 1 L- and 2 L-MoS because of preserved crystallinity and compensated dedoping by MoO. By using layer-by-layer oxidation and transfer processes, we fabricated the heterostructures of MoO/MoS/MoO/MoS, where the MoS monolayers are separated by MoO. The heterostructures showed the multiplied PL intensity as the number of embedded MoS layers increases because of suppression of the nonradiative trion formation and interlayer decoupling between stacked MoS layers. Our work shows a novel way toward the fabrication of 2D material-based multiple vdW heterostructures and our layer-by-layer oxidation process is beneficial for the fabrication of high performance 2D optoelectronic devices.
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http://dx.doi.org/10.1021/acsami.0c18364DOI Listing
January 2021

Probing One-Dimensional Oxygen Vacancy Channels Driven by Cation-Anion Double Ordering in Perovskites.

Nano Lett 2020 Nov 28;20(11):8353-8359. Epub 2020 Oct 28.

Department of Materials Science and Engineering, UNIST, Ulsan, 44919, Republic of Korea.

Visualizing the oxygen vacancy distributions is highly desirable for understanding the atomistic oxygen diffusion mechanisms in perovskites. In particular, the direct observation of the one-dimensional oxygen vacancy channels has not yet been achieved in perovskites with dual ion (i.e., cation and anion) ordering. Here, we perform atomic-resolution imaging of the one-dimensional oxygen vacancy channels and their structural dynamics in a NdBaCoO double perovskite oxide. An heating transmission electron microscopy investigation reveals the disordering of oxygen vacancy channels by local rearrangement of oxygen vacancies at the specific temperature. A density functional theory calculation suggests that the possible pathway of oxygen vacancy migration is a multistep route via Co-O and Nd-O (oxygen vacancy) sites. These findings could provide robust guidance for understanding the static and dynamic behaviors of oxygen vacancies in perovskite oxides.
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http://dx.doi.org/10.1021/acs.nanolett.0c03516DOI Listing
November 2020

Ordered Mesoporous Carbons with Graphitic Tubular Frameworks by Dual Templating for Efficient Electrocatalysis and Energy Storage.

Angew Chem Int Ed Engl 2021 Jan 12;60(3):1441-1449. Epub 2020 Nov 12.

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919, Republic of Korea.

Ordered mesoporous carbons (OMCs) have attracted considerable interest owing to their broad utility. OMCs reported to date comprise amorphous rod-like or tubular or graphitic rod-like frameworks, which exhibit tradeoffs between conductivity and surface area. Here we report ordered mesoporous carbons constructed with graphitic tubular frameworks (OMGCs) with tunable pore sizes and mesostructures via dual templating, using mesoporous silica and molybdenum carbide as exo- and endo-templates, respectively. OMGCs simultaneously realize high electrical conductivity and large surface area and pore volume. Benefitting from these features, Ru nanoparticles (NPs) supported on OMGC exhibit superior catalytic activity for alkaline hydrogen evolution reaction and single-cell performance for anion exchange membrane water electrolysis compared to Ru NPs on other OMCs and commercial catalysts. Further, the OMGC-based full-carbon symmetric cell demonstrates excellent performances for Li-ion capacitors.
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http://dx.doi.org/10.1002/anie.202012936DOI Listing
January 2021

Revealing Isolated M-N C Active Sites for Efficient Collaborative Oxygen Reduction Catalysis.

Angew Chem Int Ed Engl 2020 Dec 23;59(52):23678-23683. Epub 2020 Oct 23.

School of Energy and Chemical Engineering/Center for Dimension Controllable Organic Frameworks, Ulsan National Institute of Science and Technology, South Korea.

Single atom catalysts (SACs) are of great importance for oxygen reduction, a critical process in renewable energy technologies. The catalytic performance of SACs largely depends on the structure of their active sites, but explorations of highly active structures for SAC active sites are still limited. Herein, we demonstrate a combined experimental and theoretical study of oxygen reduction catalysis on SACs, which incorporate M-N C site structure, composed of atomically dispersed transition metals (e.g., Fe, Co, and Cu) in nitrogenated carbon nanosheets. The resulting SACs with M-N C sites exhibited prominent oxygen reduction catalytic activities in both acidic and alkaline media, following the trend Fe-N C > Co-N C > Cu-N C . Theoretical calculations suggest the C atoms in these structures behave as collaborative adsorption sites to M atoms, thanks to interactions between the d/p orbitals of the M/C atoms in the M-N C sites, enabling dual site oxygen reduction.
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http://dx.doi.org/10.1002/anie.202008325DOI Listing
December 2020

Gradient tantalum-doped hematite homojunction photoanode improves both photocurrents and turn-on voltage for solar water splitting.

Nat Commun 2020 Sep 15;11(1):4622. Epub 2020 Sep 15.

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919, Republic of Korea.

Hematite has a great potential as a photoanode for photoelectrochemical (PEC) water splitting by converting solar energy into hydrogen fuels, but the solar-to-hydrogen conversion efficiency of state-of-the-art hematite photoelectrodes are still far below the values required for practical hydrogen production. Here, we report a core-shell formation of gradient tantalum-doped hematite homojunction nanorods by combination of hydrothermal regrowth strategy and hybrid microwave annealing, which enhances the photocurrent density and reduces the turn-on voltage simultaneously. The unusual bi-functional effects originate from the passivation of the surface states and intrinsic built-in electric field by the homojunction formation. The additional driving force provided by the field can effectively suppress charge-carrier recombination both in the bulk and on the surface of hematite, especially at lower potentials. Moreover, the synthesized homojunction shows a remarkable synergy with NiFe(OH) cocatalyst with significant additional improvements of photocurrent density and cathodic shift of turn-on voltage. The work has nicely demonstrated multiple collaborative strategies of gradient doping, homojunction formation, and cocatalyst modification, and the concept could shed light on designing and constructing the efficient nanostructures of semiconductor photoelectrodes in the field of solar energy conversion.
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http://dx.doi.org/10.1038/s41467-020-18484-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7493915PMC
September 2020

Intermetallic PtCu Nanoframes as Efficient Oxygen Reduction Electrocatalysts.

Nano Lett 2020 Oct 22;20(10):7413-7421. Epub 2020 Sep 22.

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea.

Nanoframe alloy structures represent a class of high-performance catalysts for the oxygen reduction reaction (ORR), owing to their high active surface area, efficient molecular accessibility, and nanoconfinement effect. However, structural and chemical instabilities of nanoframes remain an important challenge. Here, we report the synthesis of PtCu nanoframes constructed with an atomically ordered intermetallic structure (-PtCuNF/C) showing high ORR activity, durability, and chemical stability. We rationally designed the -PtCuNF/C catalyst by combining theoretical composition predictions with a silica-coating-mediated synthesis. The -PtCuNF/C combines intensified strain and ligand effects from the intermetallic PtCu L1 structure and advantages of the nanoframes, resulting in superior ORR activity to disordered alloy PtCu nanoframes (-PtCuNF/C) and commercial Pt/C catalysts. Importantly, the -PtCuNF/C showed the highest ORR mass activity among PtCu-based catalysts. Furthermore, the -PtCuNF/C exhibited higher ORR durability and far less etching of constituent atoms than -PtCuNF/C and Pt/C, attesting to the chemically stable nature of the intermetallic structure.
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http://dx.doi.org/10.1021/acs.nanolett.0c02812DOI Listing
October 2020

Propagation Control of Octahedral Tilt in SrRuO via Artificial Heterostructuring.

Adv Sci (Weinh) 2020 Aug 25;7(16):2001643. Epub 2020 Jun 25.

Department of Physics Sungkyunkwan University Suwon 16419 Republic of Korea.

Bonding geometry engineering of metal-oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilt of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin-film lattice parameters. In this study, a method to selectively engineer the octahedral bonding geometries is proposed, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering is developed using atomically designed SrRuO/SrTiO (SRO/STO) superlattices. In particular, the propagation of RuO octahedral tilt within the SRO layers having identical thicknesses is systematically controlled by varying the thickness of adjacent STO layers. This leads to a substantial modification in the electromagnetic properties of the SRO layer, significantly enhancing the magnetic moment of Ru. This approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties.
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http://dx.doi.org/10.1002/advs.202001643DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7435247PMC
August 2020

Monolithic Interface Contact Engineering to Boost Optoelectronic Performances of 2D Semiconductor Photovoltaic Heterojunctions.

Nano Lett 2020 Apr 24;20(4):2443-2451. Epub 2020 Mar 24.

KU-KIST Graduate School of Converging Science and Technology, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea.

In optoelectronic devices based on two-dimensional (2D) semiconductor heterojunctions, the efficient charge transport of photogenerated carriers across the interface is a critical factor to determine the device performances. Here, we report an unexplored approach to boost the optoelectronic device performances of the WSe-MoS - heterojunctions via the monolithic-oxidation-induced doping and resultant modulation of the interface band alignment. In the proposed device, the atomically thin WO layer, which is directly formed by layer-by-layer oxidation of WSe, is used as a charge transport layer for promoting hole extraction. The use of the ultrathin oxide layer significantly enhanced the photoresponsivity of the WSe-MoS - junction devices, and the power conversion efficiency increased from 0.7 to 5.0%, maintaining the response time. The enhanced characteristics can be understood by the formation of the low Schottky barrier and favorable interface band alignment, as confirmed by band alignment analyses and first-principle calculations. Our work suggests a new route to achieve interface contact engineering in the heterostructures toward realizing high-performance 2D optoelectronics.
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http://dx.doi.org/10.1021/acs.nanolett.9b05162DOI Listing
April 2020

General Colloidal Synthesis of Transition-Metal Disulfide Nanomaterials as Electrocatalysts for Hydrogen Evolution Reaction.

ACS Appl Mater Interfaces 2020 Mar 9;12(11):13148-13155. Epub 2020 Mar 9.

Physical Chemistry, TU Dresden, Bergstr. 66b, 01062 Dresden, Germany.

The material-efficient monolayers of transition-metal dichalcogenides (TMDs) are a promising class of ultrathin nanomaterials with properties ranging from insulating through semiconducting to metallic, opening a wide variety of their potential applications from catalysis and energy storage to optoelectronics, spintronics, and valleytronics. In particular, TMDs have a great potential as emerging inexpensive alternatives to noble metal-based catalysts in electrochemical hydrogen evolution. Herein, we report a straightforward, low-cost, and general colloidal synthesis of various 2D transition-metal disulfide nanomaterials, such as MoS, WS, NiS, FeS, and VS, in the absence of organic ligands. This new preparation route provides many benefits including relatively mild reaction conditions, high reproducibility, high yields, easy upscaling, no post-thermal annealing/treatment steps to enhance the catalytic activity, and, finally, especially for molybdenum disulfide nanosheets, high activity in the hydrogen evolution reaction. To underline the universal application of the synthesis, we prepared mixed CoMoS nanosheets in one step to optimize the catalytic activity of pure undoped MoS, which resulted in an enhanced hydrogen evolution reaction performance characterized by onset potentials as low as 134 mV and small Tafel slopes of 55 mV/dec.
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http://dx.doi.org/10.1021/acsami.9b21607DOI Listing
March 2020

Phase Instability amid Dimensional Crossover in Artificial Oxide Crystal.

Phys Rev Lett 2020 Jan;124(2):026401

Department of Physics, Sungkyunkwan University, Suwon 16419, Korea.

Artificial crystals synthesized by atomic-scale epitaxy provide the ability to control the dimensions of the quantum phases and associated phase transitions via precise thickness modulation. In particular, the reduction in dimensionality via quantized control of atomic layers is a powerful approach to revealing hidden electronic and magnetic phases. Here, we demonstrate a dimensionality-controlled and induced metal-insulator transition (MIT) in atomically designed superlattices by synthesizing a genuine two-dimensional (2D) SrRuO_{3} crystal with highly suppressed charge transfer. The tendency to ferromagnetically align the spins in an SrRuO_{3} layer diminishes in 2D as the interlayer exchange interaction vanishes, accompanying the 2D localization of electrons. Furthermore, electronic and magnetic instabilities in the two SrRuO_{3} unit cell layers induce a thermally driven MIT along with a metamagnetic transition.
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http://dx.doi.org/10.1103/PhysRevLett.124.026401DOI Listing
January 2020

A General Strategy to Atomically Dispersed Precious Metal Catalysts for Unravelling Their Catalytic Trends for Oxygen Reduction Reaction.

ACS Nano 2020 Feb 6;14(2):1990-2001. Epub 2020 Feb 6.

Department of Chemistry , Korea Advanced Institute of Science and Technology (KAIST) , 291 Daehak-Ro , Daejeon 34141 , Republic of Korea.

Atomically dispersed precious metal catalysts have emerged as a frontier in catalysis. However, a robust, generic synthetic strategy toward atomically dispersed catalysts is still lacking, which has limited systematic studies revealing their general catalytic trends distinct from those of conventional nanoparticle (NP)-based catalysts. Herein, we report a general synthetic strategy toward atomically dispersed precious metal catalysts, which consists of "trapping" precious metal precursors on a heteroatom-doped carbonaceous layer coated on a carbon support and "immobilizing" them with a SiO layer during thermal activation. Through the "trapping-and-immobilizing" method, five atomically dispersed precious metal catalysts (Os, Ru, Rh, Ir, and Pt) could be obtained and served as model catalysts for unravelling catalytic trends for the oxygen reduction reaction (ORR). Owing to their isolated geometry, the atomically dispersed precious metal catalysts generally showed higher selectivity for HO production than their NP counterparts for the ORR. Among the atomically dispersed catalysts, the HO selectivity was changed by the types of metals, with atomically dispersed Pt catalyst showing the highest selectivity. A combination of experimental results and density functional theory calculations revealed that the selectivity trend of atomically dispersed catalysts could be correlated to the binding energy difference between *OOH and *O species. In terms of 2 e ORR activity, the atomically dispersed Rh catalyst showed the best activity. Our general approach to atomically dispersed precious metal catalysts may help in understanding their unique catalytic behaviors for the ORR.
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http://dx.doi.org/10.1021/acsnano.9b08494DOI Listing
February 2020

Atomically dispersed Pt-N sites as efficient and selective electrocatalysts for the chlorine evolution reaction.

Nat Commun 2020 Jan 21;11(1):412. Epub 2020 Jan 21.

Department of Energy Engineering and School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919, Republic of Korea.

Chlorine evolution reaction (CER) is a critical anode reaction in chlor-alkali electrolysis. Although precious metal-based mixed metal oxides (MMOs) have been widely used as CER catalysts, they suffer from the concomitant generation of oxygen during the CER. Herein, we demonstrate that atomically dispersed Pt-N sites doped on a carbon nanotube (Pt/CNT) can catalyse the CER with excellent activity and selectivity. The Pt/CNT catalyst shows superior CER activity to a Pt nanoparticle-based catalyst and a commercial Ru/Ir-based MMO catalyst. Notably, Pt/CNT exhibits near 100% CER selectivity even in acidic media, with low Cl concentrations (0.1 M), as well as in neutral media, whereas the MMO catalyst shows substantially lower CER selectivity. In situ electrochemical X-ray absorption spectroscopy reveals the direct adsorption of Cl on Pt-N sites during the CER. Density functional theory calculations suggest the PtNC site as the most plausible active site structure for the CER.
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http://dx.doi.org/10.1038/s41467-019-14272-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6972710PMC
January 2020

Fabrication and Imaging of Monolayer Phosphorene with Preferred Edge Configurations via Graphene-Assisted Layer-by-Layer Thinning.

Nano Lett 2020 Jan 4;20(1):559-566. Epub 2019 Dec 4.

Department of Physics , Yonsei University , Seoul 03722 , Korea.

Phosphorene, a monolayer of black phosphorus (BP), is an elemental two-dimensional material with interesting physical properties, such as high charge carrier mobility and exotic anisotropic in-plane properties. To fundamentally understand these various physical properties, it is critically important to conduct an atomic-scale structural investigation of phosphorene, particularly regarding various defects and preferred edge configurations. However, it has been challenging to investigate mono- and few-layer phosphorene because of technical difficulties arising in the preparation of a high-quality sample and damages induced during the characterization process. Here, we successfully fabricate high-quality monolayer phosphorene using a controlled thinning process with transmission electron microscopy and subsequently perform atomic-resolution imaging. Graphene protection suppresses the e-beam-induced damage to multilayer BP and one-side graphene protection facilitates the layer-by-layer thinning of the samples, rendering high-quality monolayer and bilayer regions. We also observe the formation of atomic-scale crystalline edges predominantly aligned along the zigzag and (101) terminations, which is originated from edge kinetics under e-beam-induced sputtering process. Our study demonstrates a new method to image and precisely manipulate the thickness and edge configurations of air-sensitive two-dimensional materials.
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http://dx.doi.org/10.1021/acs.nanolett.9b04292DOI Listing
January 2020

Negative Fermi-Level Pinning Effect of Metal/n-GaAs(001) Junction Induced by a Graphene Interlayer.

ACS Appl Mater Interfaces 2019 Dec 6;11(50):47182-47189. Epub 2019 Dec 6.

It is demonstrated that the electric dipole layer due to the overlapping of electron wave functions at the metal/graphene contact results in a negative Fermi-level pinning effect on the region of the GaAs surface with low interface-trap density in the metal/graphene/n-GaAs(001) junction. The graphene interlayer plays the role of a diffusion barrier, preventing the atomic intermixing at the interface and preserving the low interface-trap density region. The negative Fermi-level pinning effect is supported by the decrease of the Schottky barrier with the increase of the metal work function. Our work shows that the graphene interlayer can invert the effective work function of the metal between high and low, making it possible to form both Schottky and Ohmic-like contacts with identical (particularly high work function) metal electrodes on a semiconductor substrate possessing low surface-state density.
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http://dx.doi.org/10.1021/acsami.9b12074DOI Listing
December 2019

Direct Epitaxial Synthesis of Selective Two-Dimensional Lateral Heterostructures.

ACS Nano 2019 Nov 22;13(11):13047-13055. Epub 2019 Oct 22.

Electrical Engineering Division, Engineering Department , University of Cambridge , 9 JJ Thomson Avenue , Cambridge CB3 0FA , United Kingdom.

Two-dimensional (2D) heterostructured or alloyed monolayers composed of transition metal dichalcogenides (TMDCs) have recently emerged as promising materials with great potential for atomically thin electronic applications. However, fabrication of such artificial TMDC heterostructures with a sharp interface and a large crystal size still remains a challenge because of the difficulty in controlling various growth parameters simultaneously during the growth process. Here, a facile synthetic protocol designed for the production of the lateral TMDC heterostructured and alloyed monolayers is presented. A chemical vapor deposition approach combined with solution-processed precursor deposition makes it possible to accurately control the sequential introduction time and the supersaturation levels of the vaporized precursors and thus reliably and exclusively produces selective and heterogeneous epitaxial growth of TMDC monolayer crystals. In addition, TMDC core/shell heterostructured (MoS/alloy, alloy/WS) or alloyed (MoWS) monolayers are also easily obtained with precisely controlled growth parameters, such as sulfur introduction timing and growth temperature. These results represent a significant step toward the development of various 2D materials with interesting properties.
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http://dx.doi.org/10.1021/acsnano.9b05722DOI Listing
November 2019

Order-of-Magnitude, Broadband-Enhanced Light Emission from Quantum Dots Assembled in Multiscale Phase-Separated Block Copolymers.

Nano Lett 2019 10 6;19(10):6827-6838. Epub 2019 Sep 6.

Department of Materials Science and Engineering , Korea Advanced Institute of Science and Technology (KAIST) , 291 Daehak-ro , Yuseong-gu , Daejeon 34141 , Republic of Korea.

Achieving high emission efficiency in solid-state quantum dots (QDs) is an essential requirement for high-performance QD optoelectronics. However, most QD films suffer from insufficient excitation and light extraction efficiencies, along with nonradiative energy transfer between closely adjacent QDs. Herein, we suggest a highly effective strategy to enhance the photoluminescence (PL) of QD composite films through an assembly of QDs and poly(styrene--4-vinylpyridine)) (PS--P4VP) block copolymer (BCP). A BCP matrix casted under controlled humidity provides multiscale phase-separation features based on (1) submicrometer-scale spinodal decomposition between polymer-rich and water-rich phases and (2) sub-10 nm-scale microphase separation between polymer blocks. The BCP-QD composite containing bicontinuous random pores achieves significant enhancement of both light absorption and extraction efficiencies via effective random light scattering. Moreover, the microphase-separated morphology substantially reduces the Förster resonance energy transfer efficiency from 53% (pure QD film) to 22% (BCP-QD composite), collectively achieving an unprecedented 21-fold enhanced PL over a broad spectral range.
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http://dx.doi.org/10.1021/acs.nanolett.9b01941DOI Listing
October 2019

Ultrahigh-current-density niobium disulfide catalysts for hydrogen evolution.

Nat Mater 2019 12 26;18(12):1309-1314. Epub 2019 Aug 26.

Materials Science and Engineering, Rutgers University, Piscataway, NJ, USA.

Metallic transition metal dichalcogenides (TMDs) are good catalysts for the hydrogen evolution reaction (HER). The overpotential and Tafel slope values of metallic phases and edges of two-dimensional (2D) TMDs approach those of Pt. However, the overall current density of 2D TMD catalysts remains orders of magnitude lower (~10-100 mA cm) than industrial Pt and Ir electrolysers (>1,000 mA cm). Here, we report the synthesis of the metallic 2H phase of niobium disulfide with additional niobium (2H NbS, where x is ~0.35) as a HER catalyst with current densities of >5,000 mA cm at ~420 mV versus a reversible hydrogen electrode. We find the exchange current density at 0 V for 2H NbS to be ~0.8 mA cm, corresponding to a turnover frequency of ~0.2 s. We demonstrate an electrolyser based on a 2H NbS cathode that can generate current densities of 1,000 mA cm. Our theoretical results reveal that 2H NbS with Nb-terminated surface has free energy for hydrogen adsorption that is close to thermoneutral, facilitating HER. Therefore, 2H NbS could be a viable catalyst for practical electrolysers.
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http://dx.doi.org/10.1038/s41563-019-0463-8DOI Listing
December 2019

Converting Unstable Imine-Linked Network into Stable Aromatic Benzoxazole-Linked One via Post-oxidative Cyclization.

J Am Chem Soc 2019 Jul 19;141(30):11786-11790. Epub 2019 Jul 19.

School of Energy and Chemical Engineering/Center for Dimension-Controllable Organic Frameworks , Ulsan National Institute of Science and Technology (UNIST) , 50 UNIST , Ulsan 44919 , Republic of Korea.

Efficiently converting unstable linkages into stable linkages is an important objective in the chemistry of covalent organic frameworks (COFs), because it enhances stability and preserves crystallinity. Here, an unstable imine-linked COF was converted into a stable aromatic benzoxazole-linked COF (BO-COF) via post-oxidative cyclization, based on chemistry used to form fused-aromatic ladder-like rigid-rod polymers. The structure of the porous BO-COF was confirmed by transmission electron microscopy, infrared and solid-state nuclear magnetic resonance spectroscopies, powder X-ray diffraction patterns, and nitrogen adsorption-desorption isotherms. The efficient post-treatment of an unstable reversible COF converted it into a stable irreversible COF, which had significantly improved thermal and chemical stabilities as well as high crystallinity. This strategy can be universally applied for the synthesis of stable fused-aromatic COFs, expanding their practical applications.
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http://dx.doi.org/10.1021/jacs.9b05244DOI Listing
July 2019

Paramagnetic Carbon Nanosheets with Random Hole Defects and Oxygenated Functional Groups.

Angew Chem Int Ed Engl 2019 Aug 19;58(34):11670-11675. Epub 2019 Jul 19.

School of Energy and Chemical Engineering/Center for Dimension-Controllable Organic Frameworks, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919, South Korea.

Ordered graphitic carbon nanosheets (GCNs) were, for the first time, synthesized by the direct condensation of multifunctional phenylacetyl building blocks (monomers) in the presence of phosphorous pentoxide. The GCNs had highly ordered structures with random hole defects and oxygenated functional groups, showing paramagnetism. The results of combined structural and magnetic analyses indicate that the hole defects and functional groups are associated with the appearance and stabilization of unpaired spins. DFT calculations further suggest that the emergence of stabilized spin moments near the edge groups necessitates the presence of functionalized carbon atoms around the hole defects. That is, both hole defects and oxygenated functional groups are essential ingredients for the generation and stabilization of spins in GCNs.
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http://dx.doi.org/10.1002/anie.201903226DOI Listing
August 2019

Identifying the structure of Zn-N active sites and structural activation.

Nat Commun 2019 06 13;10(1):2623. Epub 2019 Jun 13.

School of Energy and Chemical Engineering/Center for Dimension-Controllable Organic Frameworks, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST, Ulsan, 44919, South Korea.

Identification of active sites is one of the main obstacles to rational design of catalysts for diverse applications. Fundamental insight into the identification of the structure of active sites and structural contributions for catalytic performance are still lacking. Recently, X-ray absorption spectroscopy (XAS) and density functional theory (DFT) provide important tools to disclose the electronic, geometric and catalytic natures of active sites. Herein, we demonstrate the structural identification of Zn-N active sites with both experimental/theoretical X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Further DFT calculations reveal that the oxygen species activation on Zn-N active sites is significantly enhanced, which can accelerate the reduction of oxygen with high selectivity, according well with the experimental results. This work highlights the identification and investigation of Zn-N active sites, providing a regular principle to obtain deep insight into the nature of catalysts for various catalytic applications.
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http://dx.doi.org/10.1038/s41467-019-10622-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6565687PMC
June 2019
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