Chibeom Park

Synthesis and properties of molybdenum disulphide: from bulk to atomic layers

Molybdenum disulphide (MoS2) has been one of the most interesting materials for scientists and engineers for a long time. While its bulk form has been in use in conventional industries as an intercalation agent and a dry lubricant for many years, its two-dimensional forms have attracted growing attention in recent years for applications in nano-electronic applications. Specifically, the single layer form of MoS2 shows significant potential as a semiconductor analogue of graphene. These exciting applications are spread over many fields, from flexible and transparent transistor devices, to low-power, high efficiency biological and chemical sensing applications. This Review Article, for the first time, provides a comprehensive overview of the synthesis, structural polytypes, properties, and applications of bulk, few layer, and single layer MoS2.

Catalyst-free Direct Growth of a Single to a Few Layers of Graphene on a Germanium Nanowire for the Anode Material of a Lithium Battery†

Direct growth of a single to a few layers of graphene on a germanium nanowire (Gr/Ge NW; see picture) was achieved by a metal-catalyst-free chemical vapor deposition (CVD) process. The Gr/Ge NW was used as anode in a lithium ion battery. This material has a specific capacity of 1059 mA h g(-1) at 4.0 C, a long cycle life over 200 cycles, and a high capacity retention of 90%.

Copper-vapor-assisted chemical vapor deposition for high-quality and metal-free single-layer graphene on amorphous SiO2 substrate.

We report that high-quality single-layer graphene (SLG) has been successfully synthesized directly on various dielectric substrates including amorphous SiO2/Si by a Cu-vapor-assisted chemical vapor deposition (CVD) process. The Cu vapors produced by the sublimation of Cu foil that is suspended above target substrates without physical contact catalyze the pyrolysis of methane gas and assist nucleation of graphene on the substrates. Raman spectra and mapping images reveal that the graphene formed on a SiO2/Si substrate is almost defect-free and homogeneous single layer. The overall quality of graphene grown by Cu-vapor-assisted CVD is comparable to that of the graphene grown by regular metal-catalyzed CVD on a Cu foil. While Cu vapor induces the nucleation and growth of SLG on an amorphous substrate, the resulting SLG is confirmed to be Cu-free by synchrotron X-ray photoelectron spectroscopy. The SLG grown by Cu-vapor-assisted CVD is fabricated into field effect transistor devices without transfer steps that are generally required when SLG is grown by regular CVD process on metal catalyst substrates. This method has overcome two important hurdles previously present when the catalyst-free CVD process is used for the growth of SLG on fused quartz and hexagonal boron nitride substrates, that is, high degree of structural defects and limited size of resulting graphene, respectively.

Evolution of precipitates in the Nb–Ti–V microalloyed HSLA steels during reheating

Abstract In as-cast slab steel, dendritic Nb-rich (Ti,Nb)(C,N) carbonitrides were observed which have a thermodynamically stable chemistry at lower than 1000 °C. These dendritic carbonitrides were dissolved and then re-precipitated to two kinds of carbonitrides, Ti- and N-rich and Ti- and C-rich (Ti,Nb)(C,N) carbonitrides during reheating.

The critical effect of solvent geometry on the determination of fullerene (C60) self-assembly into dot, wire and disk structures

Geometrically defined C60 self-assembled disks, wires and dots have been systematically obtained via a solution drop-drying process at room temperature; during this process, we discovered that there is a critical correlation between the geometry of the solvent and the final geometry of the self-assembled C60 structure.

Patternable Large‐Scale Molybdenium Disulfide Atomic Layers Grown by Gold‐Assisted Chemical Vapor Deposition

A novel way to grow MoS2 on a large scale with uniformity and in desired patterns is developed. We use Au film as a catalyst on which [Mo(CO)6 ] vapor decomposes to form a Mo-Au surface alloy that is an ideal Mo reservoir for the growth of atomic layers of MoS2 . Upon exposure to H2 S, this surface alloy transforms into a few layers of MoS2 , which can be isolated and transferred on an arbitrary substrate. By simply patterning Au catalyst film by conventional lithographic techniques, MoS2 atomic layers in desired patterns can be fabricated.

A Novel Role of Three Dimensional Graphene Foam to Prevent Heater Failure during Boiling

We report a novel boiling heat transfer (NBHT) in reduced graphene oxide (RGO) suspended in water (RGO colloid) near critical heat flux (CHF), which is traditionally the dangerous limitation of nucleate boiling heat transfer because of heater failure. When the heat flux reaches the maximum value (CHF) in RGO colloid pool boiling, the wall temperature increases gradually and slowly with an almost constant heat flux, contrary to the rapid wall temperature increase found during water pool boiling. The gained time by NBHT would provide the safer margin of the heat transfer and the amazing impact on the thermal system as the first report of graphene application. In addition, the CHF and boiling heat transfer performance also increase. This novel boiling phenomenon can effectively prevent heater failure because of the role played by the self-assembled three-dimensional foam-like graphene network (SFG).

Self-assembled foam-like graphene networks formed through nucleate boiling

Self-assembled foam-like graphene (SFG) structures were formed using a simple nucleate boiling method, which is governed by the dynamics of bubble generation and departure in the graphene colloid solution. The conductivity and sheet resistance of the calcined (400°C) SFG film were 11.8 S·cm–1 and 91.2 Ω□−1, respectively, and were comparable to those of graphene obtained by chemical vapor deposition (CVD) (~10 S·cm–1). The SFG structures can be directly formed on any substrate, including transparent conductive oxide (TCO) glasses, metals, bare glasses, and flexible polymers. As a potential application, SFG formed on fluorine-doped tin oxide (FTO) exhibited a slightly better overall efficiency (3.6%) than a conventional gold electrode (3.4%) as a cathode of quantum dot sensitized solar cells (QDSSCs).

Self‐Crystallization of C70 Cubes and Remarkable Enhancement of Photoluminescence

Light emission from organic molecular systems is one of the essential properties for the development of next-generation cost-effective, light, flexible, and large-scale electronic and optoelectronic devices. Because the light emission mediated by exciton recombination requires a specific energy band gap, highly conjugated organic molecules in the form of crystalline structures have been studied intensively for this purpose, 4] and diverse strategies have been developed for the synthesis of highly conjugated organic and polymeric molecules with geometrically well-defined crystal shapes, by which their optical properties can be modulated. Fullerenes, one of the most popular types of highly conjugated organic molecules, have been studied mainly for their semiconducting and superconducting 9] properties, which result from the discrete HOMO–LUMO band structure as well as the facile doping effect on low-lying energy levels of empty orbitals, into which extra electrons are readily accommodated. In contrast, their optical emission properties have scarcely attracted any attention because of the low exciton population for radiative recombination in fullerene molecules. However, fullerene molecules are still believed to have much potential in terms of their optical properties, as indicated by recent observations of abnormally increased fluorescence from self-aggregated molecules that have intrinsically low emitting power when they exist in an ensemble state in solution or powder phases. For example, Hara and co-workers reported that an asymmetric disulfide compound in which a photoisomerizable azobenzene unit was coupled to a biphenyl fluorophore generally showed negligible fluorescence, but that the photoinduced aggregation of the fluorophores led to remarkably increased fluorescence intensity. The Tang and Park research groups have reported similar phenomena brought about by so-called aggregationinduced emission (AIE) 13] or crystallization-induced emission enhancement (CIEE). 15] In the case of fullerenes, changes in the photophysical properties of C60 and C70 upon aggregation have also been investigated: both absorption and fluorescence spectra were gradually modulated as the degree of aggregation was systematically changed. However, such changes in fluorescence properties were mainly observed for random aggregates, no detailed crystal structures of which are available. Herein we report that C70 molecules undergo selfcrystallization into high-definition cube-shaped crystals in the solution phase at room temperature. As a result of their high crystallinity with sharp edges, C70 cube crystals display enhanced fluorescence. Such fluorescence has not been observed from C70 in either powders or bulk crystals under ambient conditions at room temperature. The synthesis of C70 cube crystals in the solution phase was inspired by recent discoveries of “geometrically defined” fullerene structures obtained through 1) precipitation in a mixture of solvents, 2) drop drying, and 3) vapor transport. In the case of the precipitation method, fullerenes spontaneously aggregate in two-solvent mixtures containing good and poor solvents for fullerenes, and their geometrical structures are determined by the type and shape of the solvents. 23] A drop-drying process with a single good solvent also resulted in various C60 structures, such as spherical particles, wires, and hexagonal disks, the geometries of which were determined selectively according to the type of solvent used. Both processes in the solution phase lead to the inclusion of solvent molecules in the final fullerene crystals; however, solvent-free and highly crystalline C60 hexagonal disks were also obtained by a vapor-transport process. In our current study, we synthesized unprecedented C70 cube crystals by precipitation from a mixture of the good solvent mesitylene and the poor solvent isopropyl alcohol (IPA). We first attempted the precipitation method for the synthesis of C70 cube crystals in a solvent mixture composed of mesitylene and IPA. C70 powder was dissolved in mesitylene, and IPA was slowly added to the resulting solution to form an immiscible liquid–liquid interface. The mixture was then agitated by manual shaking and kept at room temperature without any further agitation for 24 h. A fine black precipitate [*] C. Park, Prof. Dr. H. C. Choi Department of Chemistry and Division of Advanced Materials Science, Pohang University of Science and Technology San 31, Hyoja-Dong, Nam-Gu, Pohang, 790-784 (Korea) Fax: (+ 82)54-279-3399 E-mail: choihc@postech.edu Homepage: http://www.postech.ac.kr/chem/nmrl

Crystallization-induced properties from morphology-controlled organic crystals.

During the past two decades, many materials chemists have focused on the development of organic molecules that can serve as the basis of cost-effective and flexible electronic, optical, and energy conversion devices. Among the potential candidate molecules, metal-free or metal-containing conjugated organic molecules offer high-order electronic conjugation levels that can directly support fast charge carrier transport, rapid optoelectric responses, and reliable exciton manipulation. Early studies of these molecules focused on the design and synthesis of organic unit molecules that exhibit active electrical and optical properties when produced in the form of thin film devices. Since then, researchers have worked to enhance the properties upon crystallization of the unit molecules as single crystals provide higher carrier mobilities and exciton recombination yields. Most recently, researchers have conducted in-depth studies to understand how crystallization induces property changes, especially those that depend on specific crystal surfaces. The different properties that depend on the crystal facets have been of particular interest. Most unit molecules have anisotropic structures, and therefore produce crystals with several unique crystal facets with dissimilar molecular arrangements. These structural differences would also lead to diverse electrical conductance, optical absorption/emission, and even chemical interaction properties depending on the crystal facet investigated. To study the effects of crystallization and crystal facet-dependent property changes, researchers must grow or synthesize crystals of highly conjugated molecules that have both a variety of morphologies and high crystallinity. Morphologically well-defined organic crystals, that form structures such as wires, rods, disks, and cubes, provide objects that researchers can use to evaluate these material properties. Such structures typically occur as single crystals with well-developed facets with dissimilar molecular arrangements. Recently, researchers have proposed several approaches for the vapor and solution phase synthesis of high quality organic crystals with various morphologies. In this Account, we focus on methodologies for the synthesis of various organic- and metal-containing highly conjugated molecular crystals. We also examine the new optical and chemical properties of these materials. In addition, we introduce recent experimental results demonstrating that high crystallinity and specific molecular arrangements lead to crystallization-induced property changes. We believe that the understanding of the crystallization-induced property changes in organic crystals will provide both fundamental knowledge of the chemical processes occurring at various interfaces and opportunities for researchers to take advantage of crystallization-induced property changes in the development of high-performance organic devices.