Facile Synthesis of Pd Nanosheets and Implications for Superior Catalytic Activity.

As a member of the 2D materials family, 2D metal nanosheets (metallenes) have received increasing attention due to their intriguing properties distinct from those of graphene and other inorganic 2D nanosheets. However, the synthesis of metallenes is still challenging, owing to the lack of an efficient synthetic approach. Here we present a facile one-pot approach to the controlled synthesis of Pd nanosheets. A key feature of this process is a stepwise reaction using 2,4,6-trichlorophenyl formate (TCPF); TCPF emits carbon monoxide gas, which acts as both a reductant and a surface capping agent, promoting the anisotropic 2D growth of the Pd nanosheets. Photoemission spectroscopy revealed some peculiar features of the surface charge and valence band states due to suppressed electron transfer at the 2D surface. This surface state caused improved catalytic activity for the hydrogen evolution reaction compared to that of bulk Pd.

Electrochemical Li+ Insertion/Extraction Reactions at LiPON/Epitaxial Graphene Interfaces.

Redox reactions of the Li+ insertion/extraction from one to two interlayers of graphene (Gr) on area-defined single-crystalline SiC substrates are investigated using lithium phosphorus oxynitride glass (LiPON) as the solid-state electrolyte. Unlike an organic liquid electrolyte, this glassy electrolyte does not induce a reduction current and excludes the desolvation reaction of Li+. Gr electrodes with less than two Gr layers show a single reduction peak and one or two oxidation peaks below +0.21 V (vs Li+/Li), differing distinctly from those of graphite and multilayer Gr, which display multiple peaks (multiple stage transitions). However, this finding aligns with the conventional understanding that graphite stage structure transitions proceed with stepwise increases or decreases in the number of Gr layers between adjacent Li-inserted interlayers. Cyclic voltammetry measurements indicate the presence of surface capacity due to Li+ adsorption/desorption at the LiPON/Gr interface. Moreover, Li+ insertion and extraction induce different charge transfer resistances at the level of a single interlayer. These sensitive measurements are achieved using high-quality epitaxial Gr and LiPON electrolyte, which prevent the formation of a solid electrolyte interphase and the desolvation reaction of Li+. Similar measurements using bilayer Gr produced by chemical vapor deposition coupled with a Gr transfer method and an ethylene carbonate/dimethyl carbonate liquid electrolyte are not reliable. Thus, the proposed method is effective for electrochemical measurement of Gr electrodes with a controlled number of layers.

Micropipet-Based Navigation in a Microvascular Model for Imaging Endothelial Cell Topography Using Scanning Ion Conductance Microscopy.

Scanning ion conductance microscopy (SICM) has enabled cell surface topography at a high resolution with low invasiveness. However, SICM has not been applied to the observation of cell surfaces in hydrogels, which can serve as scaffolds for three-dimensional cell culture. In this study, we applied SICM for imaging a cell surface in a microvascular lumen reconstructed in a hydrogel. To achieve this goal, we developed a micropipet navigation technique using ionic current to detect the position of a microvascular lumen. Combining this navigation technique with SICM, endothelial cells in a microvascular model and blebs were visualized successfully at the single-cell level. To the best of our knowledge, this is the first report on visualizing cell surfaces in hydrogels using a SICM. This technique will be useful for furthering our understanding of the mechanism of intravascular diseases.

Nanoscale Visualization of Morphological Alteration of Live-Cell Membranes by the Interaction with Oligoarginine Cell-Penetrating Peptides.

The interactions between the cell membrane and biomolecules remain poorly understood. For example, arginine-rich cell-penetrating peptides (CPPs), including octaarginines (R8), are internalized by interactions with cell membranes. However, during the internalization process, the exact membrane dynamics introduced by these CPPs are still unknown. Here, we visualize arginine-rich CPPs and cell-membrane interaction-induced morphological changes using a system that combines scanning ion-conductance microscopy and spinning-disk confocal microscopy, using fluorescently labeled R8. This system allows time-dependent, nanoscale visualization of structural dynamics in live-cell membranes. Various types of membrane remodeling caused by arginine-rich CPPs are thus observed. The induction of membrane ruffling and the cup closure are observed as a process of endocytic uptake of the peptide. Alternatively suggested is the concave structural formation accompanied by direct peptide translocation through cell membranes. Studies using R8 without fluorescent labeling also demonstrate a non-negligible effect of the fluorescent moiety on membrane structural alteration.

S/N Co-Doped Hollow Carbon Particles for Oxygen Reduction Electrocatalysts Prepared by Spontaneous Polymerization at Oil-Water Interfaces.

We herein report that sulfur and nitrogen co-doped hollow spherical carbon particles can be applied to oxygen reduction reaction (ORR) electrocatalysts prepared by calcination of polydopamine (PDA) hollow particles. The hollow structure of PDA was formed by auto-oxidative interfacial polymerization of dopamine at the oil and water interface of emulsion microdroplets. The PDA was used as the nitrogen source as well as a platform for sulfur-doping. The obtained sulfur and nitrogen co-doped hollow particles showed a higher catalytic activity than that of nonsulfur-doped particles and nonhollow particles. The high ORR activity of the calcined S-doped PDA hollow particles could be attributed to the combination of nitrogen and sulfur active sites and the large surface areas owing to a hollow spherical structure.

Nanoscale kinetic imaging of lithium ion secondary battery materials using scanning electrochemical cell microscopy.

To visualize the electrochemical reactivity and obtain the diffusion coefficient of the anode of lithium-ion batteries, we used scanning electrochemical cell microscopy (SECCM) in a glovebox. SECCM provided the facet-dependent diffusion coefficient on a Li4Ti5O12 (LTO) thin-film electrode and detected the metastable crystal phase of LixFePO4.

High-Resolution Electrochemical Mapping of the Hydrogen Evolution Reaction on Transition-Metal Dichalcogenide Nanosheets.

High-resolution scanning electrochemical cell microscopy (SECCM) is used to image and quantitatively analyze the hydrogen evolution reaction (HER) catalytically active sites of 1H-MoS2 nanosheets, MoS2 , and WS2 heteronanosheets. Using a 20 nm radius nanopipette and hopping mode scanning, the resolution of SECCM was beyond the optical microscopy limit and visualized a small triangular MoS2 nanosheet with a side length of ca. 130 nm. The electrochemical cell provides local cyclic voltammograms with a nanoscale spatial resolution for visualizing HER active sites as electrochemical images. The HER activity difference of edge, terrace, and heterojunction of MoS2 and WS2 were revealed. The SECCM imaging directly visualized the relationship of HER activity and number of MoS2 nanosheet layers and unveiled the heterogeneous aging state of MoS2 nanosheets. SECCM can be used for improving local HER activities by producing sulfur vacancies using electrochemical reaction at the selected region.

Chemical Dopants on Edge of Holey Graphene Accelerate Electrochemical Hydrogen Evolution Reaction.

Carbon-based metal-free catalysts for the hydrogen evolution reaction (HER) are essential for the development of a sustainable hydrogen society. Identification of the active sites in heterogeneous catalysis is key for the rational design of low-cost and efficient catalysts. Here, by fabricating holey graphene with chemically dopants, the atomic-level mechanism for accelerating HER by chemical dopants is unveiled, through elemental mapping with atomistic characterizations, scanning electrochemical cell microscopy (SECCM), and density functional theory (DFT) calculations. It is found that the synergetic effects of two important factors-edge structure of graphene and nitrogen/phosphorous codoping-enhance HER activity. SECCM evidences that graphene edges with chemical dopants are electrochemically very active. Indeed, DFT calculation suggests that the pyridinic nitrogen atom could be the catalytically active sites. The HER activity is enhanced due to phosphorus dopants, because phosphorus dopants promote the charge accumulations on the catalytically active nitrogen atoms. These findings pave a path for engineering the edge structure of graphene in graphene-based catalysts.

Visualization of inhomogeneous current distribution on ZrO2-coated LiCoO2 thin-film electrodes using scanning electrochemical cell microscopy.

Cathode surface coating with metal-oxide thin layers has been intensively studied to improve the cycle durability of lithium-ion batteries. A comprehensive understanding of the metal-oxide morphology and the local electrochemical properties is essential for figuring out the metal-oxide coating effect. In this study, scanning electrochemical cell microscopy (SECCM) is used to analyze the surface morphology with high spatial resolution, together with the local electrochemical properties.

Gelatin-Polyaniline Composite Nanofibers Enhanced Excitation-Contraction Coupling System Maturation in Myotubes.

In this study, composite gelatin-polyaniline (PANI) nanofibers doped with camphorsulfonic acid (CSA) were fabricated by electrospinning and used as substrates to culture C2C12 myoblast cells. We observed enhanced myotube formation on composite gelatin-PANI nanofibers compared to gelatin nanofibers, concomitantly with enhanced myotube maturation. Thus, in myotubes, intracellular organization, colocalization of the dihydropyridine receptor (DHPR) and ryanodine receptor (RyR), expression of genes correlated to the excitation-contraction (E-C) coupling apparatus, calcium transients, and myotube contractibility were increased. Such composite material scaffolds combining topographical and electrically conductive cues may be useful to direct skeletal muscle cell organization and to improve cellular maturation, functionality, and tissue formation.

3D electrochemical and ion current imaging using scanning electrochemical-scanning ion conductance microscopy.

Local cell-membrane permeability and ionic strength are important factors for maintaining the functions of cells. Here, we measured the spatial electrochemical and ion concentration profile near the sample surface with nanoscale resolution using scanning electrochemical microscopy (SECM) combined with scanning ion-conductance microscopy (SICM). The ion current feedback system is an effective way to control probe-sample distance without contact and monitor the kinetic effect of mediator regeneration and the chemical concentration profile. For demonstrating 3D electrochemical and ion concentration mapping, we evaluated the reaction rate of electrochemical mediator regeneration on an unbiased conductor and visualized inhomogeneous permeability and the ion concentration 3D profile on a single fixed adipocyte cell surface.

High Speed Scanning Ion Conductance Microscopy for Quantitative Analysis of Nanoscale Dynamics of Microvilli.

Observation of nanoscale structure dynamics on cell surfaces is essential to understanding cell functions. Hopping-mode scanning ion conductance microscopy (SICM) was used to visualize the topography of fragile convoluted nanoscale structures on cell surfaces under noninvasive conditions. However, conventional hopping mode SICM does not have sufficient temporal resolution to observe cell-surface dynamics in situ because of the additional time required for performing vertical probe movements of the nanopipette. Here, we introduce a new scanning algorithm for high speed SICM measurements using low capacitance and high-resonance-frequency piezo stages. As a result, a topographic image is taken within 18 s with a 64 × 64 pixel resolution at 10 × 10 µm. The high speed SICM is applied to the characterization of microvilli dynamics on surfaces, which shows clear structural changes after the epidermal growth factor stimulation.

Improving the electrochemical imaging sensitivity of scanning electrochemical microscopy-scanning ion conductance microscopy by using electrochemical Pt deposition.

We fabricated a platinum-based double barrel probe for scanning electrochemical microscopy-scanning ion conductance microscopy (SECM-SICM) by electrodepositing platinum onto the carbon nanoelectrode of the double barrel probe. The deposition conditions were optimized to attain highly sensitive electrochemical measurements and imaging. Simultaneous SECM-SICM imaging of electrochemical features and noncontact topography by using the optimized probe afforded high-resolution images of epidermal growth factor receptors (EGFR) on the membrane surface of the A431 cells.

Negligible "negative space-charge layer effects" at oxide-electrolyte/electrode interfaces of thin-film batteries.

In this paper, we report the surprisingly low electrolyte/electrode interface resistance of 8.6 Ω cm(2) observed in thin-film batteries. This value is an order of magnitude smaller than that presented in previous reports on all-solid-state lithium batteries. The value is also smaller than that found in a liquid electrolyte-based batteries. The low interface resistance indicates that the negative space-charge layer effects at the Li3PO(4-x)N(x)/LiCoO2 interface are negligible and demonstrates that it is possible to fabricate all-solid state batteries with faster charging/discharging properties.

Nanoscale visualization of redox activity at lithium-ion battery cathodes.

Intercalation and deintercalation of lithium ions at electrode surfaces are central to the operation of lithium-ion batteries. Yet, on the most important composite cathode surfaces, this is a rather complex process involving spatially heterogeneous reactions that have proved difficult to resolve with existing techniques. Here we report a scanning electrochemical cell microscope based approach to define a mobile electrochemical cell that is used to quantitatively visualize electrochemical phenomena at the battery cathode material LiFePO4, with resolution of ~100 nm. The technique measures electrode topography and different electrochemical properties simultaneously, and the information can be combined with complementary microscopic techniques to reveal new perspectives on structure and activity. These electrodes exhibit highly spatially heterogeneous electrochemistry at the nanoscale, both within secondary particles and at individual primary nanoparticles, which is highly dependent on the local structure and composition.

Self-assembly of semiconductor/insulator interfaces in one-step spin-coating: a versatile approach for organic field-effect transistors.

Self-assembly of interfaces is of great interest in physical and chemical domains. One of the most challenging targets is to obtain an optimal interface structure showing good electronic properties by solution-processing. Interfaces of semiconductor/semiconductor, semiconductor/insulator and insulator/insulator have been successfully manipulated to obtain high-performance devices. In this review we discuss a special class of interface, semiconductor/insulator interface, formed by vertical phase separation during spin-coating and focus on the versatile applications in organic field-effect transistors (OFETs). The formation of such an interface can be finished within tens of seconds and its mechanism is related to the materials, surfaces and dynamics. Fascinatingly, such self-assembly could be used to simplify the fabrication procedure, improve film spreading, change interfacial properties, modify semiconductor morphology, and encapsulate thin films. These merits lead to OFETs with high performance and good reliability. Also, the method is very suitable for combining with other solution-processed techniques such as patterning and post-annealing, which leads to facile paper electronics, in situ purification and single crystal formation. Research on this topic not only provides an in-depth understanding of self-assembly in solution processing, but also opens new paths towards flexible organic electronics.

On practical charge injection at the metal/organic semiconductor interface.

We have revealed practical charge injection at metal and organic semiconductor interface in organic field effect transistor configurations. We have developed a facile interface structure that consisted of double-layer electrodes in order to investigate the efficiency through contact metal dependence. The metal interlayer with few nanometers thickness between electrode and organic semiconductor drastically reduces the contact resistance at the interface. The improvement has clearly obtained when the interlayer is a metal with lower standard electrode potential of contact metals than large work function of the contact metals. The electrode potential also implies that the most dominant effect on the mechanism at the contact interface is induced by charge transfer. This mechanism represents a step forward towards understanding the fundamental physics of intrinsic charge injection in all organic devices.

Solution-processed, Self-organized Organic Single Crystal Arrays with Controlled Crystal Orientation.

A facile solution process for the fabrication of organic single crystal semiconductor devices which meets the demand for low-cost and large-area fabrication of high performance electronic devices is demonstrated. In this paper, we develop a bottom-up method which enables direct formation of organic semiconductor single crystals at selected locations with desired orientations. Here oriented growth of one-dimensional organic crystals is achieved by using self-assembly of organic molecules as the driving force to align these crystals in patterned regions. Based upon the self-organized organic single crystals, we fabricate organic field effect transistor arrays which exhibit an average field-effect mobility of 1.1 cm(2)V(-1)s(-1). This method can be carried out under ambient atmosphere at room temperature, thus particularly promising for production of future plastic electronics.