Configuration-sensitive molecular sensing on doped graphene sheets

We show by molecular dynamics simulations that configuration-sensitive molecular spectroscopy can be realized on optimally doped graphene sheets vibrated by an oscillatory electric field. High selectivity of the spectroscopy is achieved by maximizing Coulombic binding between the detected molecule and a specific nest, formed for this molecule on the graphene sheet by substituting selected carbon atoms with boron and nitrogen dopants. One can detect binding of different isomers to the nest from the frequency shifts of selected vibrational modes of the combined system. As an illustrative example, we simulate detection of hexanitrostilbene enantiomers in chiral nests formed on graphene.      

Field effects on the statistical behavior of the molecular conductance in a single molecular junction in aqueous solution

We have combined molecular dynamics simulations with first-principles calculations to study electron transport in a single molecular junction of perylene tetracarboxylic diimide (PTCDI) in aqueous solution under external electric gate fields. It is found that the statistics of the molecular conductance are very sensitive to the strength of the electric field. The statistics of the molecular conductance are strongly associated with the thermal fluctuation of the water molecules around the PTCDI molecule. Our simulations reproduce the experimentally observed three orders of magnitude enhancement of the conductance, as well as the temperature dependent conductance, under the electrochemical gates. The effects of the molecular polarization and the dipole rearrangement of the aqueous solution are also discussed.      

<Emphasis Type="Italic">In situ</Emphasis> imaging of layer-by-layer sublimation of suspended graphene

An individual suspended graphene sheet was connected to a scanning tunneling microscopy probe inside a transmission electron microscope, and Joule heated to high temperatures. At high temperatures and under electron beam irradiation, the few-layer graphene sheets were removed layer-by-layer in the viewing area until a monolayer graphene was formed. The layer-by-layer peeling was initiated at vacancies in individual graphene layers. The vacancies expanded to form nanometer-sized holes, which then grew along the perimeter and propagated to both the top and bottom layers of a bilayer graphene joined by a bilayer edge. The layer-by-layer peeling was induced by atom sublimation caused by Joule heating and facilitated by atom displacement caused by high-energy electron irradiation, and may be harnessed to control the layer thickness of graphene for device applications.      

Shape control of CoO and LiCoO<Subscript>2</Subscript> nanocrystals

Shape control of nanocrystals has become a significant subject in materials science. In this work, we describe a convenient way to achieve morphology-controllable synthesis of CoO nanocrystals including octahedrons and spheres as well as LiCoO₂ polyhedrons and spheres. In particular, we explain the formation of CoO octahedrons exposing only high-energy (111) facets using theoretical calculations; these should also be a useful tool for directing future face-controlled preparation of other nanocrystals. More importantly, the as-obtained LiCoO₂ nanocrystals showed different electrochemical performance depending on their morphology, indicating that Li-insertion/deintercalation dynamics might be crystal face-sensitive.      

Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> nanostructures: A new photocatalyst

Uniform colloidal Bi₂S₃ nanodots and nanorods with different sizes have been prepared in a controllable manner via a hot injection method. X-ray diffraction (XRD) results show that the resulting nanocrystals have an orthorhombic structure. Both the diameter and length of the nanorods increase with increasing concentration of the precursors. All of the prepared Bi₂S₃ nanostructures show high efficiency in the photodegradation of rhodamine B, especially in the case of small sized nanodots—which is possibly due to their high surface area. The dynamics of the photocatalysis is also discussed.      

Holed nanostructures formed by aluminum droplets on a GaAs substrate

We have studied the morphology evolution of holed nanostructures formed by aluminum droplet epitaxy on a GaAs surface. Unique outer rings with concentric inner holed rings were observed. Further, an empirical equation to describe the size distribution of the outer rings in the holed nanostructures has been established. The contour line generated by the equation provides physical insights into quantum ring formation by droplets of group III materials on III–V substrates.      

Nanopumping molecules via a carbon nanotube

We demonstrate the feasibility of using a carbon nanotube to nanopump molecules. Molecular dynamics simulations show that the transport and ejection of a C₂₀ molecule via a single-walled carbon nanotube (SWNT) can be achieved by a sustained mechanical actuation driven by two oscillating tips. The optimal condition for nanopumping is found when the tip oscillation frequency and magnitude correlate to form quasi steady-state mechanical wave propagation in the SWNT, so that the energy transfer process is optimal leading to maximal molecular translational motion and minimal rotational motion. Our finding provides a potentially useful mechanism for using an SWNT as a vehicle to deliver large drug molecules.      

Electrocondensation and evaporation of attoliter water droplets: Direct visualization using atomic force microscopy

Working with a biased atomic force microscope (AFM) tip in the tapping mode under ambient atmosphere, attoliter (10⁻¹⁸ L) water droplet patterns have been generated on a patterned carbonaceous surface. This is essentially electrocondensation of water leading to charged droplets, as evidenced from electrostatic force microscopy measurements. The droplets are unusual in that they exhibit a highly corrugated surface and evaporate rather slowly, taking several tens of minutes.      

<Emphasis Type="Italic">In situ</Emphasis> etching for total control over axial and radial nanowire growth

We report a method using in situ etching to decouple the axial from the radial nanowire growth pathway, independent of other growth parameters. Thereby a wide range of growth parameters can be explored to improve the nanowire properties without concern of tapering or excess structural defects formed during radial growth. We demonstrate the method using etching by HCl during InP nanowire growth. The improved crystal quality of etched nanowires is indicated by strongly enhanced photoluminescence as compared to reference nanowires obtained without etching.      

Solvothermal-assisted exfoliation process to produce graphene with high yield and high quality

Monolayer and bilayer graphene sheets have been produced by a solvothermal-assisted exfoliation process in a highly polar organic solvent, acetonitrile, using expanded graphite (EG) as the starting material. It is proposed that the dipole-induced dipole interactions between graphene and acetonitrile facilitate the exfoliation and dispersion of graphene. The facile and effective solvothermal-assisted exfoliation process raises the low yield of graphene reported in previous syntheses to 10 wt%–12 wt%. By means of centrifugation at 2000 rpm for 90 min, monolayer and bilayer graphene were separated effectively without the need to add a stabilizer or modifier. Electron diffraction and Raman spectroscopy indicate that the resulting graphene sheets are high quality products without any significant structural defects.      

Bending-induced conductance increase in individual semiconductor nanowires and nanobelts

Reliable ohmic contacts were established in order to study the strain sensitivity of nanowires and nanobelts. Significant conductance increases of up to 113% were observed on bending individual ZnO nanowires or CdS nanobelts. This bending strain-induced conductance enhancement was confirmed by a variety of bending measurements, such as using different manipulating tips (silicon, glass or tungsten) to bend the nanowires or nanobelts, and is explained by bending-induced effective tensile strain based on the principle of the piezoresistance effect. This article is published with open access at Springerlink.com     

Photoluminescence of CdSe nanowires grown with and without metal catalyst

We present temperature and power dependent photoluminescence measurements on CdSe nanowires synthesized via vapor-phase with and without the use of a metal catalyst. Nanowires produced without a catalyst can be optimized to yield higher quantum efficiency, and narrower and spatially uniform emission, when compared to the catalyst-assisted ones. Emission at energies lower than the band-edge is also found in both cases. By combining spatially-resolved photoluminescence and electron microscopy on the same nanowires, we show that catalyst-free nanowires exhibit a low-energy peak with sharp phonon replica, whereas for catalyst-assisted nanowires low-energy emission is linked to the presence of nanostructures with extended morphological defects.      

Enhanced π-π interactions between a C60 fullerene and a buckle bend on a double-walled carbon nanotube

In situ low-voltage aberration corrected transmission electron microscopy (TEM) observations of the dynamic entrapment of a C₆₀ molecule in the saddle of a bent double-walled carbon nanotube is presented. The fullerene interaction is non-covalent, suggesting that enhanced π-π interactions (van der Waals forces) are responsible. Classical molecular dynamics calculations confirm that the increased interaction area associated with a buckle is sufficient to trap a fullerene. Moreover, they show hopping behavior in agreement with our experimental observations. Our findings further our understanding of carbon nanostructure interactions, which are important in the rapidly developing field of low-voltage aberration corrected TEM and nano-carbon device fabrication.      

Quantum transport properties of chemically functionalized long semiconducting carbon nanotubes

We present a first-principles study of the electronic transport properties of micrometer long semiconducting carbon nanotubes randomly covered with carbene functional groups. Whereas prior studies suggested that metallic tubes are hardly affected by such addends, we show here that the conductance of semiconducting tubes with standard diameter is, on the contrary, severely damaged. The configurational-averaged conductance as a function of tube diameter, with a coverage of up to one hundred molecules, is extracted. Our results indicate that the search for a conductance-preserving covalent functionalization route remains a challenging issue.      

Shape-controlled CuCl crystallite catalysts for aniline coupling

The catalytic activity of crystallites depends mainly upon the arrangement of surface atoms, the number of dangling bonds, and defect site distribution on different crystal planes. Here, we report the shape-controlled synthesis of CuCl crystallites, including tetrahedra, face-centered-etched tetrahedra, tripod dendrites, and tetrapods. These different morphologies of CuCl crystallites expose different proportions of {111} and {110} crystal planes, and materials with a preponderance of {111} crystal planes have better catalytic activity in aniline coupling than those with more {110} planes.      

Energy dissipation and transport in nanoscale devices

Understanding energy dissipation and transport in nanoscale structures is of great importance for the design of energy-efficient circuits and energy-conversion systems. This is also a rich domain for fundamental discoveries at the intersection of electron, lattice (phonon), and optical (photon) interactions. This review presents recent progress in understanding and manipulation of energy dissipation and transport in nanoscale solid-state structures. First, the landscape of power usage from nanoscale transistors (∼10⁻⁸ W) to massive data centers (∼10⁹ W) is surveyed. Then, focus is given to energy dissipation in nanoscale circuits, silicon transistors, carbon nanostructures, and semiconductor nanowires. Concepts of steady-state and transient thermal transport are also reviewed in the context of nanoscale devices with sub-nanosecond switching times. Finally, recent directions regarding energy transport are reviewed, including electrical and thermal conductivity of nanostructures, thermal rectification, and the role of ubiquitous material interfaces.      

Controllable nanostructural transitions in grafted nanoparticle-block copolymer composites

We report a theoretical investigation of self-assembled nanostructures of polymer-grafted nanoparticles in a block copolymer and explore underlying physical mechanisms by employing the self-consistent field method. By varying the particle concentration or the chain length and density of the grafted polymer, one can not only create various ordered morphologies (e.g., lamellar or hexagonally packed patterns) but also control the positions of nanoparticles either at the copolymer interfaces or in the center of one-block domains. The nanostructural transitions we here report are mainly attributed to the competition between entropy and enthalpy.      

Metamaterial-based graphene thermal emitter

A thermal emitter composed of a frequency-selective surface metamaterial layer and a hexagonal boron nitride-encapsulated graphene filament is demonstrated. The broadband thermal emission of the metamaterial (consisting of ring resonators) was tailored into two discrete bands, and the measured reflection and emission spectra agreed well with the simulation results. The high modulation frequencies that can be obtained in these devices, coupled with their operation in air, confirm their feasibility for use in applications such as gas sensing.

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Nanotrains and self-assembled two-dimensional arrays built from carboranes linked by hydrogen bonding of dipyridones

The strong hydrogen bonding ability of 2-pyridones were exploited to build nanotrains on surfaces. Carborane wheels on axles difunctionalized with 2-pyridone hydrogen bonding units were synthesized and displayed spontaneous formation of linear nanotrains by self-assembly on SiO₂ or mica surfaces. Imaging using atomic force microscopy confirmed linear formations with lengths up to 5 μm and heights within the range of the molecular height of the carborance-tipped axles. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users. This article is published with open access at Springerlink.com     

Growth of serpentine carbon nanotubes on quartz substrates and their electrical properties

A simple method for high-yield, chemical vapor deposition (CVD) synthesis of serpentine carbon nanotubes, employing quartz substrates and a molecular cluster catalyst, is described. The growth mechanism is analyzed by controlled addition of nanoscale barriers, and by mechanical analysis of the curved sections. The serpentine structures are used to study the electrical transport properties of parallel arrays of identical nanotubes, which show three-terminal conductance that scales linearly with the number of nanotube segments. This article is published with open access at Springerlink.com