Hyeon Suk Shin

The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets

Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.

Two-dimensional hybrid nanosheets of tungsten disulfide and reduced graphene oxide as catalysts for enhanced hydrogen evolution.

Composite materials: Tungsten disulfide and WS2 /reduced graphene oxide (WS2 /rGO) nanosheets were fabricated by hydrothermal synthesis using tungsten chloride, thioacetamide, and graphene oxide (GO) as starting materials. The WS2 nanosheets are efficiently templated on the rGO layer. The WS2 /rGO hybrid nanosheets show much better electrocatalytic activity for the hydrogen evolution reaction than WS2 nanosheets alone.

High yield exfoliation of two-dimensional chalcogenides using sodium naphthalenide

Transition-metal dichalcogenides like molybdenum disulphide have attracted great interest as two-dimensional materials beyond graphene due to their unique electronic and optical properties. Solution-phase processes can be a viable method for producing printable single-layer chalcogenides. Molybdenum disulphide can be exfoliated into monolayer flakes using organolithium reduction chemistry; unfortunately, the method is hampered by low yield, submicron flake size and long lithiation time. Here we report a high-yield exfoliation process using lithium, potassium and sodium naphthalenide where an intermediate ternary Li(x)MX(n) crystalline phase (X=selenium, sulphur, and so on) is produced. Using a two-step expansion and intercalation method, we produce high-quality single-layer molybdenum disulphide sheets with unprecedentedly large flake size, that is up to 400 μm(2). Single-layer dichalcogenide inks prepared by this method may be directly inkjet-printed on a wide range of substrates.

Interaction between metal and graphene: Dependence on the layer number of graphene

The interaction between graphene and metal was investigated by studying the G band splitting in surface-enhanced Raman scattering (SERS) spectra of single-, bi-, and trilayer graphene. The Ag deposition on graphene induced large enhancement of the Raman signal of graphene, indicating SERS of graphene. In particular, the G band was split into two distinct peaks in the SERS spectrum of graphene. The extent of the G band splitting was 13.0 cm(-1) for single-layer, 9.6 cm(-1) for bilayer, and 9.4 cm(-1) for trilayer graphene, whereas the G band in the SERS spectrum of a thick multilayer was not split. The average SERS enhancement factor of the G band was 24 for single-layer, 15 for bilayer, and 10 for trilayer graphene. These results indicate that there is a correlation between SERS enhancement factor and the extent of the G band splitting, and the strongest interaction occurs between Ag and single-layer graphene. Furthermore, the Ag deposition on graphene can induce doping of graphene. The intensity ratio of 2D and G bands (I(2D)/I(G)) decreased after Ag deposition on graphene, indicating doping of graphene. From changes in positions of G and 2D bands after the metal deposition on graphene, Ag deposition induced n-doping of graphene, whereas Au deposition induced p-doping.

Transparent, Flexible Conducting Hybrid Multilayer Thin Films of Multiwalled Carbon Nanotubes with Graphene Nanosheets

We developed a simple, versatile method of integrating hybrid thin films of reduced graphene oxide (RGO) nanosheets with multiwalled carbon nanotubes (MWNTs) via LbL assembly. This approach involves the electrostatic interactions of two oppositely charged suspensions of the RGO nanosheet with MWNTs. This method affords a hybrid multilayer of graphenes with excellent control over the optical and electrical properties. Moreover, the hybrid multilayer exhibits a significant increase of electronic conductivity after the thermal treatment, producing transparent and conducting thin films possessing a sheet resistance of 8 kOmega/sq with a transmittance of 81%. By taking advantage of the conducting network structure of MWNTs, which provides an additional flexibility and mechanical stability of RGO nanosheets, we demonstrate the potential application of hybrid graphene multilayer as a highly flexible and transparent electrode. Because of the highly versatile and tunable properties of LbL-assembled thin films, we anticipate that the general concept presented here offers a unique potential platform for integrating active carbon nanomaterials for advanced electronic, energy, and sensor applications.

Growth of High-Crystalline, Single-Layer Hexagonal Boron Nitride on Recyclable Platinum Foil

Hexagonal boron nitride (h-BN) is gaining significant attention as a two-dimensional dielectric material, along with graphene and other such materials. Herein, we demonstrate the growth of highly crystalline, single-layer h-BN on Pt foil through a low-pressure chemical vapor deposition method that allowed h-BN to be grown over a wide area (8 × 25 mm(2)). An electrochemical bubbling-based method was used to transfer the grown h-BN layer from the Pt foil onto an arbitrary substrate. This allowed the Pt foil, which was not consumed during the process, to be recycled repeatedly. The UV-visible absorption spectrum of the single-layer h-BN suggested an optical band gap of 6.06 eV, while a high-resolution transmission electron microscopy image of the same showed the presence of distinct hexagonal arrays of B and N atoms, which were indicative of the highly crystalline nature and single-atom thickness of the h-BN layer. This method of growing single-layer h-BN over large areas was also compatible with use of a sapphire substrate.

High-quality graphene via microwave reduction of solution-exfoliated graphene oxide

Efficient exfoliation of graphite in solutions to obtain high-quality graphene flakes is desirable for printable electronics, catalysis, energy storage, and composites. Graphite oxide with large lateral dimensions has an exfoliation yield of ~100%, but it has not been possible to completely remove the oxygen functional groups so that the reduced form of graphene oxide (GO; reduced form: rGO) remains a highly disordered material. Here we report a simple, rapid method to reduce GO into pristine graphene using 1- to 2-second pulses of microwaves. The desirable structural properties are translated into mobility values of >1000 square centimeters per volt per second in field-effect transistors with microwave-reduced GO (MW-rGO) as the channel material and into particularly high activity for MW-rGO catalyst support toward oxygen evolution reactions.

Recent advances in layered transition metal dichalcogenides for hydrogen evolution reaction

Hydrogen is considered to be a crucial clean energy for the future. As alternatives to Pt for the catalysis of the H2 evolution reaction (HER), transition metal dichalcogenides (TMDs) have shown great promise. In particular, significant new developments involving 2D layered TMDs have been recently reported. This highlight focuses on recent advances in the synthesis of 2D MoS2 and WS2 sheets, and their performance in the catalysis of the HER.

Highly controllable transparent and conducting thin films using layer-by-layer assembly of oppositely charged reduced graphene oxides

A new approach for the fabrication of reduced graphene oxide (rGO) multilayers which can be used for transparent and conducting thin films was developed. This was achieved by using layer-by-layer (LbL) assembly of positively and negatively charged rGO sheets, which could provide highly controllable thin films in terms of thickness, transmittance, and sheet resistance. In particular, the thickness of the multilayer thin films of rGO was able to be controlled precisely in the subnanometre scale by ∼0.46 nm via simply varying the number of stacking layers. Therefore, this method enabled an excellent control of the rGO multilayers over the optical and electrical properties, which are related to the thickness. Furthermore, we demonstrated the application of the rGO multilayers for an OLED device.

Production of Two-Dimensional Nanomaterials via Liquid-Based Direct Exfoliation.

Tremendous efforts have been devoted to the synthesis and application of two-dimensional (2D) nanomaterials due to their extraordinary and unique properties in electronics, photonics, catalysis, etc., upon exfoliation from their bulk counterparts. One of the greatest challenges that scientists are confronted with is how to produce large quantities of 2D nanomaterials of high quality in a commercially viable way. This review summarizes the state-of-the-art of the production of 2D nanomaterials using liquid-based direct exfoliation (LBE), a very promising and highly scalable wet approach for synthesizing high quality 2D nanomaterials in mild conditions. LBE is a collection of methods that directly exfoliates bulk layered materials into thin flakes of 2D nanomaterials in liquid media without any, or with a minimum degree of, chemical reactions, so as to maintain the high crystallinity of 2D nanomaterials. Different synthetic methods are categorized in the following, in which material characteristics including dispersion concentration, flake thickness, flake size and some applications are discussed in detail. At the end, we provide an overview of the advantages and disadvantages of such synthetic methods of LBE and propose future perspectives.