In Situ Electron Microscopy Study of the Dynamics of Liquid Flow in Confined Cells.

Confined liquid has attracted great attention due to its potential applications in nanofluidic devices. With the development of liquid-cell transmission electron microscopy (LC-TEM), investigating the behaviors of confined liquid can be realized in real time. However, the dynamics of the liquid layer in liquid cells have not been fully understood. Here, nanoparticles (NPs) adhered to the cell window membranes are used as reference objects to study the flow regime of the liquid layer, which causes cooperative motion of the membranes and the NPs. Two categories of motion behaviors are investigated. One is the contraction of NPs toward the interior viewing area which results from the spreading out of the liquid to the surrounding region, with the bending of the membranes increasing with the loss of liquid in the viewing area. The other motion behavior is the occasional movement of all the NPs in the same direction with the directional movement of the liquid layer. This work offers a new method to study the dynamics of liquids by LC-TEM, the discoveries of which are valuable for understanding the confined liquid dynamics.

Cation/Anion Codoped and Cobalt-Free Li-Rich Layered Cathode for High-Performance Li-Ion Batteries.

Lithium-rich layered oxides have received great attention due to their high energy density as cathode material. However, the progressive structural transformation from layered to spinel phase triggered by transition-metal migration and the irreversible release of lattice oxygen leads to voltage fade and capacity decay. Here, we report a Fe, Cl codoped and Co-free Li-rich layered cathode with significantly improved structural stability. It is revealed that the Fe and Cl codoping can facilitate the Li-ion diffusion and improve the rate performance of the materials. Moreover, the calculations show that the structural stability is enhanced by Fe and Cl codoping. As a result, the Fe and Cl codopant reduces the irreversible release of lattice oxygen, mitigates voltage fade, and improves the first-cycle Coulombic efficiency. This work provides a low-cost, environmentally friendly, practical strategy for high-performance cathode materials.

MnCO3 on Graphene Porous Framework via Diffusion-Driven Layer-by-Layer Assembly for High-Performance Pseudocapacitor.

Diffusion-driven layer-by-layer (dd-LbL) assembly is a simple yet versatile process that can be used to construct graphene oxide (GO) into a three-dimensional (3D) porous framework with good mechanical stability. In particular, the oxygen functional groups on the GO surface are well retained, providing nucleation sites for further chemical reactions to be performed upon. Therefore, such a scaffold should serve as a promising starting material for creating a wide range of 3D graphene-based composites while maintaining a high accessible surface area. Herein, we demonstrate the use of the porous GO macrostructure derived from dd-LbL assembly for the preparation of graphene-MnCO3 hybrid structures. MnCO3 is a newly reported pseudocapacitive material for supercapacitors; however, its electrochemical performance is hampered by its low electrical conductivity and poor chemical stability. Through reaction between KMnO4 and GO during a hydrothermal process, the surface of the porous scaffold was rendered with uniform MnCO3 nanoparticles. With the reduced graphene oxide (rGO) sheets serving as the conductive backbone, the resultant MnCO3 nanoparticles exhibited a capacitance of 698 F g-1 at a charge/discharge current of 0.5 mA (320 F g-1 for the combined rGO and MnCO3 composite). Furthermore, the electrode maintained 77% of its initial capacity even after 5000 cycles of charge/discharge tests at 20 mA.

Diffusion driven layer-by-layer assembly of graphene oxide nanosheets into porous three-dimensional macrostructures.

Despite recent progress in preparing numerous types of nanosheets, it remains a difficult challenge to assemble the tiny building blocks into functional macroscale architectures suitable for practical applications. Here we introduce a diffusion driven layer-by-layer assembly process and demonstrate its application for the construction of graphene oxide sheets into various three-dimensional structures. This process involves complexation of the negatively charged graphene oxide sheets and positively charged branched polyethylenimine at a given interface. We find that the diffusion of branched polyethylenimine molecules allows the complex to continuously grow into foam-like frameworks with tunable porosities. Furthermore, the assembly process is quite robust and can be utilized in various configurations such as to create free-standing architectures with tailored shapes or patterned films on a substrate. With such useful features, we believe that this technique may serve as a valuable tool for the assembly of nanomaterials.

Integration of porous coordination polymers and gold nanorods into core-shell mesoscopic composites toward light-induced molecular release.

Besides conventional approaches for regulating in-coming molecules for gas storage, separation, or molecular sensing, the control of molecular release from the pores is a prerequisite for extending the range of their application, such as drug delivery. Herein, we report the fabrication of a new porous coordination polymer (PCP)-based composite consisting of a gold nanorod (GNR) used as an optical switch and PCP crystals for controlled molecular release using light irradiation as an external trigger. The delicate core-shell structures of this new platform, composed of an individual GNR core and an aluminum-based PCP shell, were achieved by the selective deposition of an aluminum precursor onto the surface of GNR followed by the replication of the precursor into aluminum-based PCPs. The mesoscopic structure was characterized by electron microscopy, energy dispersive X-ray elemental mapping, and sorption experiments. Combination at the nanoscale of the high storage capacity of PCPs with the photothermal properties of GNRs resulted in the implementation of unique motion-induced molecular release, triggered by the highly efficient conversion of optical energy into heat that occurs when the GNRs are irradiated into their plasmon band. Temporal control of the molecular release was demonstrated with anthracene as a guest molecule and fluorescent probe by means of fluorescence spectroscopy.

Self-assembly of two-dimensional nanosheets induced by interfacial polyionic complexation.

Significant progress has been made during the past decade in preparing nanosheets from a wide range of materials, which are actively pursued for various applications such as energy storage, catalysis, sensing, and membranes. One of the next critical challenges is developing a robust and versatile assembly method which allows construction of the nanosheets into functional structures tailored for each specific purpose. An interesting characteristic of nanosheets is that they often behave as charged macromolecules and thus can readily interact with an oppositely charged polyelectrolyte to form a stable complex. In this report, we demonstrate how such a complexation process could be utilized for directing the self-assembly of nanosheets. By confining the nanosheet-polyelectrolyte complexation at air-liquid or liquid-liquid interfaces, the nanosheets are successfully assembled into various mesoscale architectures including fibers, capsules, and films. Furthermore, incorporation of additional components such as nanoparticles or small molecules can be easily achieved for further tailoring of material properties. This novel assembly method opens a pathway to many useful nanosheet superstructures and may be further extended to other types of nanomaterials in general.

Hydration-responsive folding and unfolding in graphene oxide liquid crystal phases.

Graphene oxide is promising as a plate-like giant molecular building block for the assembly of new carbon materials. Its water dispersibility, liquid crystallinity, and ease of reduction offer advantages over other carbon precursors if its fundamental assembly rules can be identified. This article shows that graphene oxide sheets of known lateral dimension form nematic liquid crystal phases with transition points in agreement with the Onsager hard-plate theory. The liquid crystal phases can be systematically ordered into defined supramolecular patterns using surface anchoring, complex fluid flow, and microconfinement. Graphene oxide is seen to exhibit homeotropic surface anchoring at interfaces driven by excluded volume entropy and by adsorption enthalpy associated with its partially hydrophobic basal planes. Surprisingly, some of the surface-ordered graphene oxide phases dry into graphene oxide solids that undergo a dramatic anisotropic swelling upon rehydration to recover their initial size and shape. This behavior is shown to be a unique hydration-responsive folding and unfolding transition. During drying, surface tension forces acting parallel to the layer planes cause a buckling instability that stores elastic energy in accordion-folded structures in the dry solid. Subsequent water infiltration reduces interlayer frictional forces and triggers release of the stored elastic energy in the form of dramatic unidirectional expansion. We explain the folding/unfolding phenomena by quantitative nanomechanics and introduce the potential of liquid crystal-derived graphene oxide phases as new stimuli-response materials.

Surfactant-free water-processable photoconductive all-carbon composite.

Heterojunctions between different graphitic nanostructures, including fullerenes, carbon nanotubes and graphene-based sheets, have attracted significant interest for light to electrical energy conversion. Because of their poor solubility, fabrication of such all-carbon nanocomposites typically involves covalently linking the individual constituents or the extensive surface functionalization to improve their solvent processability for mixing. However, such strategies often deteriorate or contaminate the functional carbon surfaces. Here we report that fullerenes, pristine single walled carbon nanotubes, and graphene oxide sheets can be conveniently coassembled in water to yield a stable colloidal dispersion for thin film processing. After thermal reduction of graphene oxide, a solvent-resistant photoconductive hybrid of fullerene-nanotube-graphene was obtained with on-off ratio of nearly 6 orders of magnitude. Photovoltaic devices made with the all-carbon hybrid as the active layer and an additional fullerene block layer showed unprecedented photovoltaic responses among all known all-carbon-based materials with an open circuit voltage of 0.59 V and a power conversion efficiency of 0.21%. The ease of making such surfactant-free, water-processed, carbon thin films could lead to their wide applications in organic optoelectronic devices.

Graphene oxide sheets at interfaces.

Graphite oxide sheet, now called graphene oxide (GO), is the product of chemical exfoliation of graphite and has been known for more than a century. GO has been largely viewed as hydrophilic, presumably due to its excellent colloidal stability in water. Here we report that GO is an amphiphile with hydrophilic edges and a more hydrophobic basal plane. GO can act like a surfactant, as measured by its ability to adsorb on interfaces and lower the surface or interfacial tension. Since the degree of ionization of the edge -COOH groups is affected by pH, GO's amphiphilicity can be tuned by pH. In addition, size-dependent amphiphilicity of GO sheets is observed. Since each GO sheet is a single molecule as well as a colloidal particle, the molecule-colloid duality makes it behave like both a molecular and a colloidal surfactant. For example, GO is capable of creating highly stable Pickering emulsions of organic solvents like solid particles. It can also act as a molecular dispersing agent to process insoluble materials such as graphite and carbon nanotubes in water. The ease of its conversion to chemically modified graphene could enable new opportunities in solution processing of functional materials.

Graphene oxide: surface activity and two-dimensional assembly.

Graphene oxide (GO) is a promising precursor for preparing graphene-based composites and electronics applications. Like graphene, GO is essentially one-atom thick but can be as wide as tens of micrometers, resulting in a unique type of material building block, characterized by two very different length scales. Due to this highly anisotropic structure, the collective material properties are highly dependent on how these sheets are assembled. Therefore, understanding and controlling the assembly behavior of GO has become an important subject of research. In this Research News article the surface activity of GO and how it can be employed to create two-dimensional assemblies over large areas is discussed.

Visualizing graphene based sheets by fluorescence quenching microscopy.

Graphene based sheets have stimulated great interest due to their superior mechanical, electrical, and thermal properties. A general visualization method that allows quick observation of these single atomic layers would be highly desirable as it can greatly facilitate sample evaluation and manipulation, and provide immediate feedback to improve synthesis and processing strategies. Here we report that graphene based sheets can be made highly visible under a fluorescence microscope by quenching the emission from a dye coating, which can be conveniently removed afterward by rinsing without disrupting the sheets. Current imaging techniques for graphene based sheets rely on the use of special substrates. In contrast, the fluorescence quenching mechanism is no longer limited by the type of substrate. Graphene, reduced graphene oxide, or even graphene oxide sheets deposited on arbitrary substrates can now be readily visualized with good contrast for layer counting. Direct observation of suspended sheets in solution was also demonstrated. The fluorescence quenching microscopy offers unprecedented imaging flexibility and could become a general tool for characterizing graphene based materials.

Construction of evolutionary tree for morphological engineering of nanoparticles.

In addition to chemical composition, the chemistry of nanocrystals involves an extra structural factor--morphology--since many of their properties are size- and shape-dependent. Although often described as artificial atoms or molecules, the morphological control of nanoparticles has not advanced to a level comparable to organic total synthesis, where complex molecular structures can be rationally designed and prepared through stepwise reactions. Here we report a morphological engineering approach for gold nanoparticles by constructing an evolutionary tree consisting of a few branches of independent growth pathways. Each branch yields a string of evolving, continuously tunable morphologies from one reaction, therefore collectively producing a library of nanoparticles with minimal changes of reaction parameters. In addition, the tree also provides ground rules for designing new morphologies through crossing over different pathways.

Remote optical switch for localized and selective control of gene interference.

Near infrared-absorbing gold nanoplasmonic particles (GNPs) are used as optical switches of gene interference and are remotely controlled using light. We have tuned optical switches to a wavelength where cellular photodamage is minimized. Optical switches are functionalized with double-stranded oligonucleotides. At desired times and at specific intracellular locations, remote optical excitation is used to liberate gene-interfering oligonucleotides. We demonstrate a novel gene-interfering technique offering spatial and temporal control, which is otherwise impossible using conventional gene-interfering techniques.

Langmuir-Blodgett assembly of graphite oxide single layers.

Single-layer graphite oxide can be viewed as an unconventional type of soft material and has recently been recognized as a promising material for composite and electronics applications. It is of both scientific curiosity and technical importance to know how these atomically thin sheets assemble. There are two fundamental geometries of interacting single layers: edge-to-edge and face-to-face. Such interactions were studied at the air-water interface by Langmuir-Blodgett assembly. Stable monolayers of graphite oxide single layers were obtained without the need for any surfactant or stabilizing agent, due to the strong electrostatic repulsion between the 2D confined layers. Such repulsion also prevented the single layers from overlapping during compression, leading to excellent reversibility of the monolayers. In contrast to molecular and hard colloidal particle monolayers, the single layers tend to fold and wrinkle at edges to resist collapsing into multilayers. The monolayers can be transferred to a substrate, readily creating a large area of flat graphite oxide single layers. The density of such films can be continuously tuned from dilute, close-packed to overpacked monolayers of interlocking single layers. For size-mismatched single layers, face-to-face interaction caused irreversible stacking, leading to double layers. The graphite oxide monolayers can be chemically reduced to graphene for electronic applications such as transparent conducting thin films.

Chemical synthesis of gold nanowires in acidic solutions.

High aspect ratio gold nanowires with single crystalline surface have long been a missing piece in the toolbox of plasmonics metal nanostructures. Such wires are now made with a room temperature, surfactant assisted chemical synthesis in acidic aqueous solution. The diameters and lengths of the multiply twinned gold nanowires can be tuned by varying the amount of seed particles and acid in the growth solution. Nanowires with diameters around 35 nm and lengths up to 10 micron were made with a low seed concentration in pH approximately 1 solution.

Spontaneous formation of nanoparticle stripe patterns through dewetting.

Significant advancement has been made in nanoparticle research, with synthetic techniques extending over a wide range of materials with good control over particle size and shape. A grand challenge is assembling and positioning the nanoparticles in desired locations to construct complex, higher-order functional structures. Controlled positioning of nanoparticles has been achieved in pre-defined templates fabricated by top-down approaches. A self-assembly method, however, is highly desirable because of its simplicity and compatibility with heterogeneous integration processes. Here we report on the spontaneous formation of ordered gold and silver nanoparticle stripe patterns on dewetting a dilute film of polymer-coated nanoparticles floating on a water surface. Well-aligned stripe patterns with tunable orientation, thickness and periodicity at the micrometre scale were obtained by transferring nanoparticles from a floating film onto a substrate in a dip-coating fashion. This facile technique opens up a new avenue for lithography-free patterning of nanoparticle arrays for various applications including, for example, multiplexed surface-enhanced Raman substrates and templated fabrication of higher-order nanostructures.

Pt nanocrystals: shape control and Langmuir-Blodgett monolayer formation.

We report the synthesis of monodisperse Pt nanocrystals with three different shapes-cubes, cuboctahedra, and octahedra, selectively, with similar sizes of 9-10 nm by a modified polyol process. We found that addition of silver ion enhances the crystal growth rate along 100, and essentially determines the shape and surface structure of the Pt nanocrystals. After the reaction, the silver species can be easily removed by repetitive precipitation giving pure Pt nanoparticles. Two-dimensional arrays of the Pt nanocrystals were assembled by using the Langmuir-Blodgett (LB) method. The particles were evenly distributed on the entire substrate, and their surface coverage and density can be precisely controlled by tuning the surface pressure. The resulting Pt LB layers are potential candidates for 2-D model catalysts as a result of their high surface area and the structural uniformity of the metal nanocrystals.

Crystal overgrowth on gold nanorods: tuning the shape, facet, aspect ratio, and composition of the nanorods.

Electrochemically prepared Au nanorods were used as seeds for the overgrowth of thin shells of gold, silver, and palladium by using a mild reducing agent, ascorbic acid, in the presence of surfactants at ambient condition. The unique crystal facets of the starting nanorods results in anisotropic crystal overgrowth. The overgrowth rates along different crystallographical directions can be further regulated by adding foreign ions or by using different metal reduction methods. This overgrowth study provides insights on how different metal ions could be reduced preferentially on different Au nanorod surfaces, so that the composition, aspect ratio, shape, and facet of the resulting nanostructures can be rationally tuned. These surfactant-stabilized bimetallic Au(core)M(shell) (M=Au, Ag, Pd) nanorod colloids might serve as better substrates in surface-enhanced Raman spectroscopy as well as exhibiting enhanced catalytic properties.

Solution-phase synthesis of single-crystalline iron phosphide nanorods/nanowires.

A solution-phase route for the preparation of single-crystalline iron phosphide nanorods and nanowires is reported. We have shown that the mixture of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP), which are commonly used as the solvents for semiconductor nanocrystal synthesis, is not entirely inert. In the current process, TOP, serving as phosphor source, reacts with Fe precursors to form FeP nanostructures with large aspect ratios. In addition, the experimental results show that both TOP and TOPO are necessary for the formation of FeP nanowires and their ratio appears to control the morphology of the produced FeP structures. A possible growth mechanism is discussed.