Publications by authors named "Hyeon Suk Shin"

75 Publications

Reply to: On the measured dielectric constant of amorphous boron nitride.

Nature 2021 02;590(7844):E8-E10

Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan, South Korea.

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http://dx.doi.org/10.1038/s41586-020-03163-xDOI Listing
February 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

Radio-frequency-transmitting hexagonal boron nitride-based anti- and de-icing heating system.

Nanoscale 2020 Nov;12(42):21895-21900

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

Anti- and de-icing heating systems are used to both prevent the accumulation of ice and to remove it and thus avoid damage. Typically, anti- and de-icing heating systems employ carbon-based materials, metal frames, and bulky ceramic structures. These structures generally lead to the loss of radio-frequency (RF) signals and are also relatively heavy. Therefore, RF equipment such as radar domes (radomes) and antennas require anti- and de-icing systems with high RF transmittance for normal operation. In this work, we fabricated a fluorine-doped tin oxide (FTO) wave pattern covered with hexagonal boron nitride (h-BN) layers (i.e., an h-BN/FTO wave pattern) on a glass substrate for use as an RF-transmitting heating system for anti- and de-icing. The FTO wave pattern and h-BN layer act as the heating element and heat spreader, respectively. The h-BN layer showed a transmittance of approximately 90% for RF waves on glass (X band: 8.2-12.4 GHz) (the 10% loss was attributable to the glass substrate). The differences in the temperatures of the FTO-patterned and non-patterned areas for the h-BN(3.6 nm)/FTO and FTO wave pattern were 19.3 and 25.5 °C, respectively. This means that the h-BN layer improved the heat-spreading performance by 6.2 °C. Furthermore, a de-icing test was performed using the h-BN(3.6 nm)/FTO wave pattern by applying a voltage of 40 V at -20 °C. The ice on the non-patterned area melted within 1 min while that on the FTO-patterned area melted within 30 s. These results suggest that the fabricated h-BN(3.6 nm)/FTO wave pattern for RF-transmitting heating systems is suitable for use with the radomes of drones, unmanned aerial vehicles, aircraft, and spaceships in extremely cold environments.
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http://dx.doi.org/10.1039/d0nr06333aDOI Listing
November 2020

Blue emission at atomically sharp 1D heterojunctions between graphene and h-BN.

Nat Commun 2020 Oct 23;11(1):5359. Epub 2020 Oct 23.

Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, Republic of Korea.

Atomically sharp heterojunctions in lateral two-dimensional heterostructures can provide the narrowest one-dimensional functionalities driven by unusual interfacial electronic states. For instance, the highly controlled growth of patchworks of graphene and hexagonal boron nitride (h-BN) would be a potential platform to explore unknown electronic, thermal, spin or optoelectronic property. However, to date, the possible emergence of physical properties and functionalities monitored by the interfaces between metallic graphene and insulating h-BN remains largely unexplored. Here, we demonstrate a blue emitting atomic-resolved heterojunction between graphene and h-BN. Such emission is tentatively attributed to localized energy states formed at the disordered boundaries of h-BN and graphene. The weak blue emission at the heterojunctions in simple in-plane heterostructures of h-BN and graphene can be enhanced by increasing the density of the interface in graphene quantum dots array embedded in the h-BN monolayer. This work suggests that the narrowest, atomically resolved heterojunctions of in-plane two-dimensional heterostructures provides a future playground for optoelectronics.
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http://dx.doi.org/10.1038/s41467-020-19181-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7585426PMC
October 2020

Ultralow-dielectric-constant amorphous boron nitride.

Nature 2020 06 24;582(7813):511-514. Epub 2020 Jun 24.

Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan, South Korea.

Decrease in processing speed due to increased resistance and capacitance delay is a major obstacle for the down-scaling of electronics. Minimizing the dimensions of interconnects (metal wires that connect different electronic components on a chip) is crucial for the miniaturization of devices. Interconnects are isolated from each other by non-conducting (dielectric) layers. So far, research has mostly focused on decreasing the resistance of scaled interconnects because integration of dielectrics using low-temperature deposition processes compatible with complementary metal-oxide-semiconductors is technically challenging. Interconnect isolation materials must have low relative dielectric constants (κ values), serve as diffusion barriers against the migration of metal into semiconductors, and be thermally, chemically and mechanically stable. Specifically, the International Roadmap for Devices and Systems recommends the development of dielectrics with κ values of less than 2 by 2028. Existing low-κ materials (such as silicon oxide derivatives, organic compounds and aerogels) have κ values greater than 2 and poor thermo-mechanical properties. Here we report three-nanometre-thick amorphous boron nitride films with ultralow κ values of 1.78 and 1.16 (close to that of air, κ = 1) at operation frequencies of 100 kilohertz and 1 megahertz, respectively. The films are mechanically and electrically robust, with a breakdown strength of 7.3 megavolts per centimetre, which exceeds requirements. Cross-sectional imaging reveals that amorphous boron nitride prevents the diffusion of cobalt atoms into silicon under very harsh conditions, in contrast to reference barriers. Our results demonstrate that amorphous boron nitride has excellent low-κ dielectric characteristics for high-performance electronics.
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http://dx.doi.org/10.1038/s41586-020-2375-9DOI Listing
June 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

Spatially controlled lateral heterostructures of graphene and transition metal dichalcogenides toward atomically thin and multi-functional electronics.

Nanoscale 2020 Mar;12(9):5286-5292

Department of Chemistry, Ulsan National Institute of Science & Technology (UNIST), Ulsan 44919, Republic of Korea. and Department of Energy Engineering, Ulsan National Institute of Science & Technology (UNIST), Ulsan 44919, Republic of Korea and Low Dimensional Carbon Material Center, Ulsan National Institute of Science & Technology (UNIST), Ulsan 44919, Republic of Korea and Center for Multidimensional Carbon Materials, Institute of Basic Science (IBS), Ulsan 44919, Republic of Korea.

Edge contacts between two-dimensional (2D) materials in the in-plane direction can achieve minimal contact area and low contact resistance, producing atomically thin devices with improved performance. Particularly, lateral heterojunctions of metallic graphene and semiconducting transition metal dichalcogenides (TMDs) exhibit small Schottky barrier heights due to graphene's low work-function. However, issues exist with the fabrication of highly transparent and flexible multi-functional devices utilizing lateral heterostructures (HSs) of graphene and TMDs via spatially controlled growth. This review demonstrates the growth and electronic applications of lateral HSs of graphene and TMDs, highlighting key technologies controlling the wafer-scale growth of continuous films for practical applications. It deepens the understanding of the spatially controlled growth of lateral HSs using chemical vapor deposition methods, and also contributes to the applications that depend on the scale-up of all-2D electronics with ultra-high electrical performance.
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http://dx.doi.org/10.1039/c9nr10859aDOI Listing
March 2020

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

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

Resonantly hybridized excitons in moiré superlattices in van der Waals heterostructures.

Nature 2019 03 6;567(7746):81-86. Epub 2019 Mar 6.

Department of Physics and Astronomy, University of Sheffield, Sheffield, UK.

Atomically thin layers of two-dimensional materials can be assembled in vertical stacks that are held together by relatively weak van der Waals forces, enabling coupling between monolayer crystals with incommensurate lattices and arbitrary mutual rotation. Consequently, an overarching periodicity emerges in the local atomic registry of the constituent crystal structures, which is known as a moiré superlattice. In graphene/hexagonal boron nitride structures, the presence of a moiré superlattice can lead to the observation of electronic minibands, whereas in twisted graphene bilayers its effects are enhanced by interlayer resonant conditions, resulting in a superconductor-insulator transition at magic twist angles. Here, using semiconducting heterostructures assembled from incommensurate molybdenum diselenide (MoSe) and tungsten disulfide (WS) monolayers, we demonstrate that excitonic bands can hybridize, resulting in a resonant enhancement of moiré superlattice effects. MoSe and WS were chosen for the near-degeneracy of their conduction-band edges, in order to promote the hybridization of intra- and interlayer excitons. Hybridization manifests through a pronounced exciton energy shift as a periodic function of the interlayer rotation angle, which occurs as hybridized excitons are formed by holes that reside in MoSe binding to a twist-dependent superposition of electron states in the adjacent monolayers. For heterostructures in which the monolayer pairs are nearly aligned, resonant mixing of the electron states leads to pronounced effects of the geometrical moiré pattern of the heterostructure on the dispersion and optical spectra of the hybridized excitons. Our findings underpin strategies for band-structure engineering in semiconductor devices based on van der Waals heterostructures.
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http://dx.doi.org/10.1038/s41586-019-0986-9DOI Listing
March 2019

Surface functionalization-induced photoresponse characteristics of monolayer MoS for fast flexible photodetectors.

Nanoscale 2019 Mar;11(11):4726-4734

Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK.

Monolayered, semiconducting molybdenum disulfide (MoS2) is of considerable interest for its potential applications in next-generation flexible, wearable, and transparent photodetectors because it has outstanding physical properties coupled with unique atomically thin dimensions. However, there is still a lack of understanding in terms of the underlying mechanisms responsible for the photoresponse dynamics, which makes it difficult to identify the appropriate device design strategy for achieving a fast photoresponse time in MoS2 photodetectors. In this study, we investigate the importance of surface functionalization on controlling the charge carrier densities in a MoS2 monolayer and in turn the corresponding behavior of the photoresponse in relation to the position of the Fermi-level and the energy band structure. We find that the p-doping and n-doping, which is achieved through the surface functionalization of the MoS2 monolayer, leads to devices with different photoresponse behavior. Specifically, the MoS2 devices with surface functional groups contributing to p-doping exhibited a faster response time as well as higher sensitivity compared to that observed for the MoS2 devices with surface functional groups contributing to n-doping. We attribute this difference to the degree of bending in the energy bands at the metal-semiconductor junction as a result of shifting in the Fermi-level position, which influences the optoelectronic transport properties as well as the recombination dynamics leading to a low dark and thus high detectivity and fast decay time. Based upon these findings, we have also demonstrated the broad applicability of surface functionalization by fabricating a flexible MoS2 photodetector that shows an outstanding decay time of 0.7 s, which is the fastest response time observed in flexible MoS2 detectors ever reported.
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http://dx.doi.org/10.1039/c8nr07655cDOI Listing
March 2019

Author Correction: Planar and van der Waals heterostructures for vertical tunnelling single electron transistors.

Nat Commun 2019 02 25;10(1):987. Epub 2019 Feb 25.

Department of Energy Engineering, Ulsan National Institute of Science & Technology (UNIST), Ulsan, 44919, Republic of Korea.

The original version of this Article contained an error in the spelling of the author Matthew Holwill, which was incorrectly given as Mathew Holwill. This has now been corrected in both the PDF and HTML versions of the Article.
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http://dx.doi.org/10.1038/s41467-019-08910-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6389964PMC
February 2019

Large area patterning of residue-free metal oxide nanostructures by liquid transfer imprint lithography.

Nanotechnology 2019 Jun 15;30(23):235301. Epub 2019 Feb 15.

Advanced Nano-Surface Group, Korea Basic Science Institute (KBSI), 169-148 Gwahak-ro, Yuseong-gu, Daejeon 34133, Republic of Korea.

One-dimensional (1D) and three-dimensional (3D) residue-free metal oxide patterns are directly fabricated over large areas using liquid transfer imprint lithography (LTIL) with an ultraviolet-curable metal oxide precursor resist. A 1D line or pillar array of metal oxides nano-patterns without a residual layer is formed by LTIL and annealing processes. A 3D layer-by-layer nanomesh structure is successfully constructed by repeating the LTIL method without a complex etching process. In addition, it is possible to form a hierarchical structure in which zinc oxide nanowires are selectively grown on a desired zinc oxide (ZnO) seed pattern formed by LTIL via a hydrothermal method. Unlike the pattern fabricated by the conventional nanoimprint lithography method, in the case of the pattern formed by LTIL the residues accumulated between the patterns during the patterning procedure can be removed, and thus it is possible to easily form various types of nanostructures.
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http://dx.doi.org/10.1088/1361-6528/ab077dDOI Listing
June 2019

Planar and van der Waals heterostructures for vertical tunnelling single electron transistors.

Nat Commun 2019 01 16;10(1):230. Epub 2019 Jan 16.

Department of Energy Engineering, Ulsan National Institute of Science & Technology (UNIST), Ulsan, 44919, Republic of Korea.

Despite a rich choice of two-dimensional materials, which exists these days, heterostructures, both vertical (van der Waals) and in-plane, offer an unprecedented control over the properties and functionalities of the resulted structures. Thus, planar heterostructures allow p-n junctions between different two-dimensional semiconductors and graphene nanoribbons with well-defined edges; and vertical heterostructures resulted in the observation of superconductivity in purely carbon-based systems and realisation of vertical tunnelling transistors. Here we demonstrate simultaneous use of in-plane and van der Waals heterostructures to build vertical single electron tunnelling transistors. We grow graphene quantum dots inside the matrix of hexagonal boron nitride, which allows a dramatic reduction of the number of localised states along the perimeter of the quantum dots. The use of hexagonal boron nitride tunnel barriers as contacts to the graphene quantum dots make our transistors reproducible and not dependent on the localised states, opening even larger flexibility when designing future devices.
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http://dx.doi.org/10.1038/s41467-018-08227-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6335417PMC
January 2019

Sphere-to-Multipod Transmorphic Change of Nanoconfined Pt Electrocatalyst during Oxygen Reduction Reaction.

Small 2019 Jan 2;15(2):e1802228. Epub 2018 Nov 2.

School of Energy and Chemical Engineering, UNIST, Ulsan, 44919, South Korea.

An oxygen reduction reaction (ORR) catalyst/support system is designed to have Pt nanoparticles nanoconfined in a nanodimensionally limited space. Holey crumpled reduced graphene oxide plates (hCR-rGO) are used as a carbon support for Pt loading. As expected from interparticular Pt-to-Pt distance of Pt-loaded hCR-rGO longer than that of Pt/C (Pt-loaded carbon black as a practical Pt catalyst), the durability of ORR electroactivity along cycles is improved by replacing the widely used carbon black with hCR-rGO. Unexpected morphological changes of Pt are electrochemically induced during repeated ORR processes. Spherical multifaceted Pt particles are evolved to {110}-dominant dendritic multipods. Nanoconfinement of a limited number of Pt within a nanodimensionally limited space is responsible for the morphological changes. The improved durability observed from Pt-loaded hCR-rGO originates from 1) dendritic pod structure of Pt exposing more active sites to reactants and 2) highly ORR-active Pt {110} planes dominant on the surface.
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http://dx.doi.org/10.1002/smll.201802228DOI Listing
January 2019

AA'-Stacked Trilayer Hexagonal Boron Nitride Membrane for Proton Exchange Membrane Fuel Cells.

ACS Nano 2018 Nov 17;12(11):10764-10771. Epub 2018 Oct 17.

Department of Chemistry, Department of Energy Engineering, Low-Dimensional Carbon Materials Center , Ulsan National Institute of Science and Technology (UNIST) , Ulsan 44919 , Republic of Korea.

Hexagonal boron nitride (h-BN) and graphene have emerged as promising materials for proton exchange membranes because of their high proton conductivity and chemical stability. However, the defects and grain boundaries generated during the growth and transfer of two-dimensional materials limit their practical applicability. Here, we report the fabrication of membrane electrode assemblies using large-area single-oriented AA'-stacked trilayer h-BN (3L-BN), which exhibits very few defects during the growth and transfer, as a proton exchange membrane for use in fuel cell systems. The fuel cell based on AA'-stacked 3L-BN showed a H permeation current density as low as 2.69 mA cm and an open circuit voltage (OCV) as high as 0.958 V; this performance is much superior to those for cells based on Nafion (3.7 mA cm and 0.942 V, respectively) and single-layer h-BN (10.08 mA cm and 0.894 V, respectively). Furthermore, the fuel cell with the AA'-stacked 3L-BN membrane almost maintained its original performance (OCV, maximum power density, and H permeation current density) even after 100 h of an accelerated stress test at 30% RH and 90 °C, while the fuel cells with the Nafion and single-layer BN membranes exhibited severely deteriorated performances. The stability of the cell based on the AA'-stacked 3L-BN membrane was better because the membrane prevented gas crossover and suppressed the generation of reactive radicals during cell operation.
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http://dx.doi.org/10.1021/acsnano.8b06268DOI Listing
November 2018

Efficient Hydrogen Evolution Reaction Catalysis in Alkaline Media by All-in-One MoS with Multifunctional Active Sites.

Adv Mater 2018 May 30;30(20):e1707105. Epub 2018 Mar 30.

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

MoS becomes an efficient and durable nonprecious-metal electrocatalyst for the hydrogen evolution reaction (HER) when it contains multifunctional active sites for water splitting derived from 1T-phase, defects, S vacancies, exposed Mo edges with expanded interlayer spacings. In contrast to previously reported MoS -based catalysts targeting only a single or few of these characteristics, the all-in-one MoS catalyst prepared herein features all of the above active site types. During synthesis, the intercalation of in situ generated NH molecules into MoS sheets affords ammoniated MoS (A-MoS ) that predominantly comprises 1T-MoS and exhibits an expanded interlayer spacing. The subsequent reduction of A-MoS results in the removal of intercalated NH and H S to form an all-in-one MoS with multifunctional active sites mentioned above (R-MoS ) that exhibits electrocatalytic HER performance in alkaline media superior to those of all previously reported MoS -based electrocatalysts. In particular, a hybrid MoS /nickel foam catalyst outperforms commercial Pt/C in the practically meaningful high-current region (>25 mA cm ), demonstrating that R-MoS -based materials can potentially replace Pt catalysts in practical alkaline HER systems.
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http://dx.doi.org/10.1002/adma.201707105DOI Listing
May 2018

Strain-Mediated Interlayer Coupling Effects on the Excitonic Behaviors in an Epitaxially Grown MoS/WS van der Waals Heterobilayer.

Nano Lett 2017 09 28;17(9):5634-5640. Epub 2017 Aug 28.

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

van der Waals heterostructures composed of two different monolayer crystals have recently attracted attention as a powerful and versatile platform for studying fundamental physics, as well as having great potential in future functional devices because of the diversity in the band alignments and the unique interlayer coupling that occurs at the heterojunction interface. However, despite these attractive features, a fundamental understanding of the underlying physics accounting for the effect of interlayer coupling on the interactions between electrons, photons, and phonons in the stacked heterobilayer is still lacking. Here, we demonstrate a detailed analysis of the strain-dependent excitonic behavior of an epitaxially grown MoS/WS vertical heterostructure under uniaxial tensile and compressive strain that enables the interlayer interactions to be modulated along with the electronic band structure. We find that the strain-modulated interlayer coupling directly affects the characteristic combined vibrational and excitonic properties of each monolayer in the heterobilayer. It is further revealed that the relative photoluminescence intensity ratio of WS to MoS in our heterobilayer increases monotonically with tensile strain and decreases with compressive strain. We attribute the strain-dependent emission behavior of the heterobilayer to the modulation of the band structure for each monolayer, which is dictated by the alterations in the band gap transitions. These findings present an important pathway toward designing heterostructures and flexible devices.
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http://dx.doi.org/10.1021/acs.nanolett.7b02513DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5959243PMC
September 2017

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

ACS Appl Mater Interfaces 2017 Aug 9;9(33):27839-27846. Epub 2017 Aug 9.

Department of Chemical Engineering, Pohang University of Science and Technology , Pohang 790-784, Korea.

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.
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http://dx.doi.org/10.1021/acsami.7b05129DOI Listing
August 2017

Imaging of Interlayer Coupling in van der Waals Heterostructures Using a Bright-Field Optical Microscope.

Nano Lett 2017 09 3;17(9):5342-5349. Epub 2017 Aug 3.

Department of Physics and Astronomy, University of Sheffield , Sheffield S3 7RH, United Kingdom.

Vertically stacked atomic layers from different layered crystals can be held together by van der Waals forces, which can be used for building novel heterostructures, offering a platform for developing a new generation of atomically thin, transparent, and flexible devices. The performance of these devices is critically dependent on the layer thickness and the interlayer electronic coupling, influencing the hybridization of the electronic states as well as charge and energy transfer between the layers. The electronic coupling is affected by the relative orientation of the layers as well as by the cleanliness of their interfaces. Here, we demonstrate an efficient method for monitoring interlayer coupling in heterostructures made from transition metal dichalcogenides using photoluminescence imaging in a bright-field optical microscope. The color and brightness in such images are used here to identify mono- and few-layer crystals and to track changes in the interlayer coupling and the emergence of interlayer excitons after thermal annealing in heterobilayers composed of mechanically exfoliated flakes and as a function of the twist angle in atomic layers grown by chemical vapor deposition. Material and crystal thickness sensitivity of the presented imaging technique makes it a powerful tool for characterization of van der Waals heterostructures assembled by a wide variety of methods, using combinations of materials obtained through mechanical or chemical exfoliation and crystal growth.
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http://dx.doi.org/10.1021/acs.nanolett.7b01763DOI Listing
September 2017

Thermodynamically Stable Synthesis of Large-Scale and Highly Crystalline Transition Metal Dichalcogenide Monolayers and their Unipolar n-n Heterojunction Devices.

Adv Mater 2017 Sep 10;29(33). Epub 2017 Jul 10.

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

Transition metal dichalcogenide (TMDC) monolayers are considered to be potential materials for atomically thin electronics due to their unique electronic and optical properties. However, large-area and uniform growth of TMDC monolayers with large grain sizes is still a considerable challenge. This report presents a simple but effective approach for large-scale and highly crystalline molybdenum disulfide monolayers using a solution-processed precursor deposition. The low supersaturation level, triggered by the evaporation of an extremely thin precursor layer, reduces the nucleation density dramatically under a thermodynamically stable environment, yielding uniform and clean monolayer films and large crystal sizes up to 500 µm. As a result, the photoluminescence exhibits only a small full-width-half-maximum of 48 meV, comparable to that of exfoliated and suspended monolayer crystals. It is confirmed that this growth procedure can be extended to the synthesis of other TMDC monolayers, and robust MoS /WS heterojunction devices are easily prepared using this synthetic procedure due to the large-sized crystals. The heterojunction device shows a fast response time (≈45 ms) and a significantly high photoresponsivity (≈40 AW ) because of the built-in potential and the majority-carrier transport at the n-n junction. These findings indicate an efficient pathway for the fabrication of high-performance 2D optoelectronic devices.
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http://dx.doi.org/10.1002/adma.201702206DOI Listing
September 2017

An innovative scheme for sub-50 nm patterning via electrohydrodynamic lithography.

Nanoscale 2017 Aug;9(33):11881-11887

Korea Institute of Machinery and Materials (KIMM), 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 34103, Republic of Korea.

The fabrication of large-area and well-ordered nanostructures using lithographic techniques is challenging. We have developed novel approaches for sub-50 nm nanopatterning using an electrohydrodynamic lithography (EHL) technique by tailoring experimental parameters such as applied voltage, stamp features, filling ratio, and choice of resist film. We obtain a sub-50 nm pattern replica from a master stamp that contains an array of line patterns having 50 nm widths. Moreover, we show that a far-smaller pattern replication than the original pattern size can be readily obtained by carefully adjusting the experimental conditions. Perfect- and much smaller-pattern replicas have been realized from the master stamp with an array of hole patterns having a 400 nm hole size by tuning the filling ratio. We also demonstrate that an array of 30 nm graphene nanoribbons can be easily fabricated by exploring a hierarchical core-shell template structure employing a bilayer resist film via an EHL technique. The proposed minimal-contact patterning method is simple, versatile, and inexpensive and has potential to become a powerful technique for realizing feasible ultrafine nanostructures on a wafer scale.
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http://dx.doi.org/10.1039/c7nr00749cDOI Listing
August 2017

Molecular Beam Epitaxy of Highly Crystalline Monolayer Molybdenum Disulfide on Hexagonal Boron Nitride.

J Am Chem Soc 2017 07 3;139(27):9392-9400. Epub 2017 Jul 3.

SinBeRISE CREATE, National Research Foundation , CREATE Tower, 1 Create Way, Singapore 138602, Singapore.

Atomically thin molybdenum disulfide (MoS), a direct-band-gap semiconductor, is promising for applications in electronics and optoelectronics, but the scalable synthesis of highly crystalline film remains challenging. Here we report the successful epitaxial growth of a continuous, uniform, highly crystalline monolayer MoS film on hexagonal boron nitride (h-BN) by molecular beam epitaxy. Atomic force microscopy and electron microscopy studies reveal that MoS grown on h-BN primarily consists of two types of nucleation grains (0° aligned and 60° antialigned domains). By adopting a high growth temperature and ultralow precursor flux, the formation of 60° antialigned grains is largely suppressed. The resulting perfectly aligned grains merge seamlessly into a highly crystalline film. Large-scale monolayer MoS film can be grown on a 2 in. h-BN/sapphire wafer, for which surface morphology and Raman mapping confirm good spatial uniformity. Our study represents a significant step in the scalable synthesis of highly crystalline MoS films on atomically flat surfaces and paves the way to large-scale applications.
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http://dx.doi.org/10.1021/jacs.7b05131DOI Listing
July 2017

Evidence of Local Commensurate State with Lattice Match of Graphene on Hexagonal Boron Nitride.

ACS Nano 2017 07 22;11(7):7084-7090. Epub 2017 Jun 22.

Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS) , Ulsan 44919, Republic of Korea.

Transition to a commensurate state changes the local symmetry periodicity on two-dimensional van der Waals superstructures, evoking distinctive properties far beyond individual layers. We investigate the morphology of moiré superstructures of graphene on hexagonal boron nitride (hBN) with a low twist angle (≈0°) through moiré fringe analyses with dark field transmission electron microscopy. The moiré fringes exhibit local variation, suggesting that the interaction between graphene and hBN depends on the stacking configuration and that local transition to the commensurate state occurs through the reduced crystalline mismatch (that is, by lattice stretching and twisting on the graphene lattices). This moiré superstructure analysis suggests an inventive method for studying the interaction between stacked van der Waals layers and for discerning the altered electronic and optical properties of graphene on hBN superstructures with a low twist angle, even at low magnification.
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http://dx.doi.org/10.1021/acsnano.7b02716DOI Listing
July 2017

Probing Evolution of Twist-Angle-Dependent Interlayer Excitons in MoSe/WSe van der Waals Heterostructures.

ACS Nano 2017 04 6;11(4):4041-4050. Epub 2017 Apr 6.

Center for Multidimensional Carbon Materials (CMCM), Institute of Basic Science (IBS) , Ulsan 44919, Republic of Korea.

Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe/WSe vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe and WSe layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe and WSe.
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http://dx.doi.org/10.1021/acsnano.7b00640DOI Listing
April 2017

Monolayer optical memory cells based on artificial trap-mediated charge storage and release.

Nat Commun 2017 03 24;8:14734. Epub 2017 Mar 24.

Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK.

Monolayer transition metal dichalcogenides are considered to be promising candidates for flexible and transparent optoelectronics applications due to their direct bandgap and strong light-matter interactions. Although several monolayer-based photodetectors have been demonstrated, single-layered optical memory devices suitable for high-quality image sensing have received little attention. Here we report a concept for monolayer MoS optoelectronic memory devices using artificially-structured charge trap layers through the functionalization of the monolayer/dielectric interfaces, leading to localized electronic states that serve as a basis for electrically-induced charge trapping and optically-mediated charge release. Our devices exhibit excellent photo-responsive memory characteristics with a large linear dynamic range of ∼4,700 (73.4 dB) coupled with a low OFF-state current (<4 pA), and a long storage lifetime of over 10 s. In addition, the multi-level detection of up to 8 optical states is successfully demonstrated. These results represent a significant step toward the development of future monolayer optoelectronic memory devices.
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http://dx.doi.org/10.1038/ncomms14734DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5376667PMC
March 2017

Selective synthesis of pure cobalt disulfide on reduced graphene oxide sheets and its high electrocatalytic activity for hydrogen evolution reaction.

Nano Converg 2016 27;3(1). Epub 2016 Jan 27.

Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan, 689-798 Republic of Korea.

We synthesized single-phase CoS on a large scale by adding graphene oxide of sufficient quantity via the hydrothermal method using cobalt acetate and thioacetamide as precursors; this produced the hybrid of CoS with reduced graphene oxide which exhibited high electrocatalytic activity in the hydrogen evolution reaction.
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http://dx.doi.org/10.1186/s40580-016-0066-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5271118PMC
January 2016

Hexagonal Boron Nitride/Au Substrate for Manipulating Surface Plasmon and Enhancing Capability of Surface-Enhanced Raman Spectroscopy.

ACS Nano 2016 12 6;10(12):11156-11162. Epub 2016 Dec 6.

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS) , UNIST-gil 50, Ulsan 44919, Republic of Korea.

We report on an insulating two-dimensional material, hexagonal boron nitride (h-BN), which can be used as an effective wrapping layer for surface-enhanced Raman spectroscopy (SERS) substrates. This material exhibits outstanding characteristics such as its crystallinity, impermeability, and thermal conductance. Improved SERS sensitivity is confirmed for Au substrates wrapped with h-BN, the mechanism of which is investigated via h-BN thickness-dependent experiments combined with theoretical simulations. The investigations reveal that a stronger electromagnetic field can be generated at the narrowed gap of the h-BN surface, which results in higher Raman sensitivity. Moreover, the h-BN-wrapped Au substrate shows extraordinary stability against photothermal and oxidative damages. We also describe its capability to detect specific chemicals that are difficult to analyze using conventional SERS substrates. We believe that this concept of using an h-BN insulating layer to protect metallic or plasmonic materials will be widely used not only in the field of SERS but also in the broader study of plasmonic and optoelectronic devices.
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http://dx.doi.org/10.1021/acsnano.6b06153DOI Listing
December 2016
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