Rotation of Graphene on Cu during Chemical Vapor Deposition and Its Application to Control the Stacking Angle of Bilayer Graphene.

Control of the stacking angle (θS) of bilayer graphene (BLG) is essential for fundamental studies and applications of BLG. Especially, the use of chemical vapor deposition (CVD) to grow high-quality BLG requires this control, but methods to achieve it are not available. Here, we found that graphene rotates during the CVD process, and this action can be exploited as a new strategy to control θS. The rotation of graphene was revealed by the population changes of AB-stacked BLG and 30°-twisted BLG upon the growth time change; this change can only be explained by rotation of graphene. The rotation is largely affected by the edge state of graphene which can be tuned by growth temperature. The rotation was observed through experimental results combined with theoretical calculation. The rotation can be blocked or accelerated by controlling the growth temperature, by which highly selective growth of AB-stacked BLG or 30°-twisted BLG can be achieved.

Chemical Vapor Deposition of Edge-on Oriented 2D Conductive Metal-Organic Framework Thin Films.

We demonstrated the synthesis of a conductive two-dimensional metal-organic framework (MOF) thin film by single-step all-vapor-phase chemical vapor deposition (CVD). The synthesized large-area thin film of Cu3(C6O6)2 has an edge-on-orientation with high crystallinity. Cu3(C6O6)2 thin film-based microdevices were fabricated by e-beam lithography and had an electrical conductivity of 92.95 S/cm. Synthesis of conductive MOF thin films by the all-vapor-phase CVD will enable fundamental studies of physical properties and may help to accomplish practical applications of conductive MOFs.

A Platform to Evaluate the Effect of Back Charge Transfer on the Electrical Conductivity of TTF Charge Transfer Complexes: TTF3MCl6 (M = In, Sb).

TTF3MCl6 (M = In, Sb) series were developed for an ideal platform to investigate the effect of back charge transfer of MCl63- on electrical conductivity depending on the metal ions. They were successfully synthesized by a UV light-induced one-pot reaction where TTF oxidation and formation of MCl63- occurred sequentially. In isostructural TTF3InCl6 and TTF3SbCl6, the intermolecular interaction between MCl63- and TTF induces back charge transfer, which were confirmed by the crystal structure and spectroscopic analysis. Despite the similar crystal structure in terms of intermolecular distance, TTF3InCl6 shows 3-orders of magnitude higher electrical conductivity compared to TTF3SbCl6. According to the cyclic voltammograms (CV) and electron spin resonance (ESR) spectra, increased conductivity is because of the higher degree of back charge transfer from MCl63- in TTF3InCl6 compared to TTF3SbCl6, which is due to the lower electronegativity of In, considering that the only difference between the two compounds is the center metal.

Solvent-Effected Coordination Variation of Flexible Ligands to Cu(II) for the Formation of 1D and 2D Secondary Building Units for Metal-Organic Frameworks.

Producing more than one structure from the same set of metal and ligand precursors will grant structural diversity in metal-organic framework (MOF) systems. One of the biggest obstacles of getting structural diversity for the late transition metals is that the coordination mode is pretty much fixed by nature. Herein, we show that two different coordination modes to Cu(II) are possible for flexible ligands containing hydrophilic terminal groups through solvent guidance. It is demonstrated that trans,trans-muconic acid (H2muco) ligands coordinate to Cu(II) to form Cu(II) muconate MOFs having a 1D chain structure and a 2D plane structure in water-rich and DMF-rich water-DMF mixed solvent systems, respectively. It is suggested that the interaction between ligands and solvent is responsible for the selective coordination. A similar result was observed from the attempts using the fumaric acid ligand. Our results provide a new direction to obtain diverse secondary building units for the construction of diverse MOFs.

Anisotropic Electrical Conductivity of a Single-Crystalline Oxo-Bridged Cr4IIIMo2VI Heterometallic Complex.

A new oxo-bridged chromium-molybdenum heterometallic complex, O-CrMoHC ([Cr4(MoO4)2O2(OAc)4(DMF)4]·2DMF), was synthesized by using a simple solvothermal reaction. In this complex, the octahedrally coordinated Cr(III) and tetrahedrally coordinated Mo(VI) metal centers are bridged by oxo ligands. O-CrMoHC has in-plane π-conjugation systems, which are interconnected by noncoordinating DMF molecules. The crystals show anisotropic conductivity with respect to the crystal planes, and theoretical calculations were used to study their origins. The O-CrMoHC single crystals exhibited that a relatively high electrical conductivity with an average value of 5.37 × 10-7 S/cm was observed along the [01-1] direction, but the current level was very low along the [100] direction. This is the first report of anisotropic conductivity observed in the single crystal of a monomeric heterometallic complex.

High Yield Organic Superconductors via Solution-Phase Alkali Metal Doping at Room Temperature.

Alkali metal doping is an essential process for developing organic superconductors. The conventional vapor-phase alkali metal doping, however, frequently suffers from low efficiency and poor reproducibility mainly due to the inhomogeneous reaction between alkali metal vapor and target organic molecule powder. To overcome this issue, here we developed a facile and highly reproducible solution-phase alkali metal doping (SPD) and successfully applied it to prepare potassium-doped fullerene (K3C60) superconductors. Different from the conventional vapor-phase alkali metal doping, the SPD method resulted in almost perfect diamagnetism with an unprecedented high shielding fraction (∼99.5%) with high reproducibility (>80%). It works well with popular commercially available solvents, like ammonia solution in THF, methylamine solution in THF, and even pure THF at room temperature. We believe that our highly facile and efficient SPD approach will be a great help for the finding of next-generation organic superconductors, especially searching for high-Tc organic superconductors.

Highly Conducting and Flexible Radical Crystals.

Together with high conductivity, high flexibility is an important property required for next generation organic electronic components. Both properties are difficult to achieve together especially when the components are crystalline because of the intrinsic high brittleness of organic molecular crystals. We report an organic radical crystal system that has both high flexibility and high conductivity. The crystal consists of 9,10-bis(phenylethynyl)anthracene radical cation (BPEA.+ ) units, and shows flexibility under pressure with high conductivity in ambient condition exhibiting average conductivity of 2.68 S cm-1 when normal linear shape, as well as 2.43 S cm-1 when bent. The structural analysis reveals that both a short π-π distance (3.290 Å) between BPEA.+ units that are aligned along the crystal length direction, and the presence of PF6 - counter ions induce flexibility and high electrical conductivity.

Revealing the Role of Gold in the Growth of Two-Dimensional Molybdenum Disulfide by Surface Alloy Formation.

The formation of Mo/Au surface alloy during Au-assisted chemical vapor deposition (CVD) of MoS2 is confirmed by a series of control experiments. A metal-organic chemical vapor deposition (MOCVD) system is adapted to conduct two-dimensional MoS2 growth in a controlled environment. Sequential injection of Mo and S precursors, which does not yield any MoS2 on SiO2 /Si, grows atomically thin MoS2 on Au, indicating the formation of an alloy phase. Transmission electron microscopy of a cross-section of the specimen confirms the confinement of the alloy phase near the surface only. These results show that the reaction intermediate is the surface alloy, and that the role of Au in the Au-assisted CVD is the formation of an atomically thin reservoir of Mo near the surface. This mechanism is clearly distinguished from that of MOCVD, which does not involve the formation of any alloy phases.

Determining the Facile Routes for Oxygen Evolution Reaction by In Situ Probing of Li-O2 Cells with Conformal Li2O2 Films.

An ongoing challenge with lithium-oxygen (Li-O2) batteries is in deciphering the oxygen evolution reaction (OER) process related to the slow decomposition of the insulating lithium peroxide (Li2O2). Herein, we shed light on the behavior of Li2O2 oxidation by exploiting various in situ imaging, gas analysis, and electrochemical methods. At the low potentials 3.2-3.7 V (vs Li/Li+), OER is exclusive to the thinner parts of the Li2O2 deposits, which leads to O2 gas evolution, followed by the concomitant release of superoxide species. At higher potentials, OER initiates at the sidewalls of the thicker Li2O2. The succeeding lateral decomposition of Li2O2 indicates the preferential Li+ and charge transport occurring at the sidewalls of Li2O2. To ameliorate the OER rate, we also investigate an alternative approach of rerouting charge carriers by using soluble redox mediators. Our in situ probes provide insights into the favorable charge-transport routes that can aid in promoting Li2O2 decomposition.

Synthesis of a Scalable Two-Dimensional Covalent Organic Framework by the Photon-Assisted Imine Condensation Reaction on the Water Surface.

An atomically thin two-dimensional (2D) covalent organic framework (COF) was successfully synthesized via the photon-assisted imine condensation reaction within 1 h from the highly uniform and homogeneous precursor solution layer floating on the water surface. The polarity optimization of the precursor solution was the key step for the successful formation of the high-quality 2D COF because only the precursor solution consisting of polarity-controlled solvents allows ideal floating on the water surface. The polarity-controlled solution not only prohibits the agglomeration of the organic precursors on the water surface but also facilitates the wafer scale and layer number-controllable synthesis of the 2D COF. The resulting 2D COF has a uniform porous structure and highly oriented layered structure along the out-of-plane direction as observed by microscopy analysis and X-ray diffraction, respectively. In addition, we successfully fabricated field effect transistor type polyimine-based COF (pi-COF) electronic devices to demonstrate the prompt electrical responses to photo-exposure and water vapor exposure, suggesting the potential applications of the pi-COF in electrical photodetector or moisture-detector devices.

Transfer-Free, Large-Scale Growth of High-Quality Graphene on Insulating Substrate by Physical Contact of Copper Foil.

High-quality, large-area, single-layer graphene was directly grown on top of a quartz substrate by a low-pressure chemical vapor deposition (CVD) process using Cu vapor as a catalyst. In this process, continuous generation and supply of highly concentrated Cu vapor is the key to the growth of large-scale, high-quality graphene. It was achieved by direct physical contact, or "touch-down," of a Cu foil with an underlying sacrificial SiO2 /Si substrate, and the target quartz substrate was placed on top of the Cu foil, eventually having a quartz/Cu/SiO2 /Si sandwich structure. To establish the reaction mechanism, a test growth was performed without the quartz substrate, which revealed that Cu is diffused through the SiO2 layer of the sacrificial SiO2 /Si substrate to form liquid-phase Cu-Si alloy that emits massive Cu vapor. This Cu vapor catalyzes thermal decomposition of supplied CH4 gas.

Selective Formation of Conductive Network by Radical-Induced Oxidation.

Cd-based coordination networks having channels were formed selectively by using a redox-active aromatic ligand 2,5,8-tri(4-pyridyl)1,3-diazaphenalene (TPDAP, H(+)1(-)). An electron-conductive network having a π-π stacking columnar structure of TPDAP formed in the presence of a trace amount of TPDAP radical (1(•)). In contrast, a nonconductive network having a dimer unit of H(+)1(-) formed in the absence of 1(•). These results suggest the presence of a unique oxidation mechanism of TPDAP induced by formation of H(+)1(-)-1(•) dimer, which was initiated by a trace amount of 1(•). The dimerization increased HOMO level of H(+)1(-) moiety within the dimer to generate further radicals that could not form when H(+)1(-) was well isolated in CH3OH.

Probing Distinct Fullerene Formation Processes from Carbon Precursors of Different Sizes and Structures.

Fullerenes, cage-structured carbon allotropes, have been the subject of extensive research as new materials for diverse purposes. Yet, their formation process is still not clearly understood at the molecular level. In this study, we performed laser desorption ionization-ion mobility-mass spectrometry (LDI-IM-MS) of carbon substrates possessing different molecular sizes and structures to understand the formation process of fullerene. Our observations show that the formation process is strongly dependent on the size of the precursor used, with small precursors yielding small fullerenes and large graphitic precursors generally yielding larger fullerenes. These results clearly demonstrate that fullerene formation can proceed via both bottom-up and top-down processes, with the latter being favored for large precursors and more efficient at forming fullerenes. Furthermore, we observed that specific structures of carbon precursors could additionally affect the relative abundance of C60 fullerene. Overall, this study provides an advanced understanding of the mechanistic details underlying the formation processes of fullerene.

Germanium Silicon Alloy Anode Material Capable of Tunable Overpotential by Nanoscale Si Segregation.

We developed the novel electrode that enables fine control of overpotential by exploiting surface segregation that is the enrichment of one component at the surface of binary alloy. To realize this approach, we controlled the proportion of Si with low Li diffusivity at the surface by annealing the SiGe nanowire in H2 environment at various temperatures. The resulting SiGe nanowires annealed at 850 °C exhibited high reversible capacity (>1031 mA·h·g(-1)), and long cycle life (400 cycles) with high capacity retention (89.0%) at 0.2 C. This superior battery performance is attributed to the remaining unlithiated part acting as support frame to prevent pulverization of anode material, which results from the fine-tuning of overpotential by controlling the degree of Si segregation.

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.

A 3.5 V lithium-iodine hybrid redox battery with vertically aligned carbon nanotube current collector.

A lithium-iodine (Li-I2) cell using the triiodide/iodide (I3(-)/I(-)) redox couple in an aqueous cathode has superior gravimetric and volumetric energy densities (∼ 330 W h kg(-1) and ∼ 650 W h L(-1), respectively, from saturated I2 in an aqueous cathode) to the reported aqueous Li-ion batteries and aqueous cathode-type batteries, which provides an opportunity to construct cost-effective and high-performance energy storage. To apply this I3(-)/I(-) aqueous cathode for a portable and compact 3.5 V battery, unlike for grid-scale storage as general target of redox flow batteries, we use a three-dimensional and millimeter thick carbon nanotube current collector for the I3(-)/I(-) redox reaction, which can shorten the diffusion length of the redox couple and provide rapid electron transport. These endeavors allow the Li-I2 battery to enlarge its specific capacity, cycling retention, and maintain a stable potential, thereby demonstrating a promising candidate for an environmentally benign and reusable portable battery.

Electroluminescence from graphene quantum dots prepared by amidative cutting of tattered graphite.

Size-controlled graphene quantum dots (GQDs) are prepared via amidative cutting of tattered graphite. The power of this method is that the size of the GQDs could be varied from 2 to over 10 nm by simply regulating the amine concentration. The energy gaps in such GQDs are narrowed down with increasing their size, showing colorful photoluminescence from blue to brown. We also reveal the roles of defect sites in photoluminescence, developing long-wavelength emission and reducing exciton lifetime. To assess the viability of the present method, organic light-emitting diodes employing our GQDs as a dopant are first demonstrated with the thorough studies in their energy levels. This is to our best knowledge the first meaningful report on the electroluminescence of GQDs, successfully rendering white light with the external quantum efficiency of ca. 0.1%.

Atomic layer-by-layer thermoelectric conversion in topological insulator bismuth/antimony tellurides.

Material design for direct heat-to-electricity conversion with substantial efficiency essentially requires cooperative control of electrical and thermal transport. Bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3), displaying the highest thermoelectric power at room temperature, are also known as topological insulators (TIs) whose electronic structures are modified by electronic confinements and strong spin-orbit interaction in a-few-monolayers thickness regime, thus possibly providing another degree of freedom for electron and phonon transport at surfaces. Here, we explore novel thermoelectric conversion in the atomic monolayer steps of a-few-layer topological insulating Bi2Te3 (n-type) and Sb2Te3 (p-type). Specifically, by scanning photoinduced thermoelectric current imaging at the monolayer steps, we show that efficient thermoelectric conversion is accomplished by optothermal motion of hot electrons (Bi2Te3) and holes (Sb2Te3) through 2D subbands and topologically protected surface states in a geometrically deterministic manner. Our discovery suggests that the thermoelectric conversion can be interiorly achieved at the atomic steps of a homogeneous medium by direct exploiting of quantum nature of TIs, thus providing a new design rule for the compact thermoelectric circuitry at the ultimate size limit.

High-Conductivity Two-Dimensional Polyaniline Nanosheets Developed on Ice Surfaces.

A new method to develop two-dimensional PANI nanosheets using ice as a removable hard template is presented. Distinctly high current flows of 5.5 mA at 1 V and a high electrical conductivity of 35 S cm(-1) were obtained for the polyaniline (PANI) nanosheets, which marked a significant improvement from previously values on other PANIs reported over the past decades. These improved electrical properties of ice-templated PANI nanosheets were attributed to the long-range ordered edge-on π-stacking of the quinoid ring, ascribed to the ice surface-assisted vertical growth of PANI. The unprecedented advantages of the ice-templated PANI nanosheets are two-fold. First, the PANI nanosheet can be easily transferred onto various types of substrates via float-off from the ice surfaces. Second, PANI can be patterned into any shape using predetermined masks, and this is expected to facilitate the eventual convenient and inexpensive application of conducting polymers in versatile electronic device forms.

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.