Strain-induced semiconductor to metal transition in the two-dimensional honeycomb structure of MoS2

The electronic properties of two-dimensional honeycomb structures of molybdenum disulfide (MoS₂) subjected to biaxial strain have been investigated using first-principles calculations based on density functional theory. On applying compressive or tensile bi-axial strain on bi-layer and mono-layer MoS₂, the electronic properties are predicted to change from semiconducting to metallic. These changes present very interesting possibilities for engineering the electronic properties of two-dimensional structures of MoS₂.      

Strain effects in graphene and graphene nanoribbons: The underlying mechanism

A tight-binding analytic framework is combined with first-principles calculations to reveal the mechanism underlying the strain effects on electronic structures of graphene and graphene nanoribbons (GNRs). It provides a unified and precise formulation of the strain effects under various circumstances-including the shift of the Fermi (Dirac) points, the change in band gap of armchair GNRs with uniaxial strain in a zigzag pattern and its insensitivity to shear strain, and the variation of the k-range of edge states in zigzag GNRs under uniaxial and shear strains which determine the gap behavior via the spin polarization interaction.      

High performance metal oxide based sensing device using an electrode with a solid/liquid/air triphase interface

The wetting properties of an electrode surface are of significant importance to the performance of electrochemical devices because electron transfer occurs at the electrode/electrolyte interface. Described in this paper is a low-cost metal oxide electrocatalyst (CuO)-based high-performance sensing device using an enzyme electrode with a solid/liquid/air triphase interface in which the oxygen level is constant and sufficiently high. We apply the sensing device to detect glucose, a model test analyte, and demonstrate a linear dynamic range up to 50 mM, which is about 25 times higher than that obtained using a traditional enzyme electrode with a solid/liquid diphase interface. Moreover, we show that sensing devices based on a triphase assaying interface are insensitive to the significant oxygen level fluctuation in the analyte solution.

Open image in new window

     

Short channel field-effect transistors from highly enriched semiconducting carbon nanotubes

Semiconducting single-walled carbon nanotubes (s-SWNTs) with a purity of ∼98% have been obtained by gel filtration of arc-discharge grown SWNTs with diameters in the range 1.2–1.6 nm. Multi-laser Raman spectroscopy confirmed the presence of less than 2% of metallic SWNTs (m-SWNTs) in the s-SWNT enriched sample. Measurement of ∼50 individual tubes in Pd-contacted devices with channel length 200 nm showed on/off ratios of >10⁴, conductances of 1.38–5.8 μS, and mobilities in the range 40–150 cm²·V/s. Short channel multi-tube devices with ∼100 tubes showed lower on/off ratios due to residual m-SWNTs, although the on-current was greatly increased relative to the devices made from individual tubes.      

Layer-by-layer removal of insulating few-layer mica flakes for asymmetric ultra-thin nanopore fabrication

We demonstrate an elaborate method to controllably fabricate ultra-thin nanopores by layer-by-layer removal of insulating few-layer mica flakes with atomic force microscopy (AFM). The fabricated nanopores are geometrically asymmetric, like an inverted quadrangular frustum pyramid. The nanopore geometry can be engineered by finely tuning the mechanical load on the AFM tip and the scanning area. Particularly noteworthy is that the nanopores can also be fabricated in suspended few-layer mica membranes on a silicon window, and may find potential use as functional components in nanofluidic devices.      

A facile tool for the characterization of two-dimensional materials grown by chemical vapor deposition

The metrology of two-dimensional (2D) materials such as graphene, boron nitride or molybdenum disulfide grown by chemical vapor deposition (CVD) is critical for the optimization of their synthesis. We demonstrate the use of film-induced frustrated etching (FIFE) as a facile, scalable method to reveal and quantify structural defects in continuous thin sheets. The sensitivity of the analysis technique to intentionally induced lattice defects in graphene compares favorably to the sensitivity of Raman spectroscopy. A strong correlation between the measured defectiveness and the maximum carrier mobility in graphene emphasizes the importance of the technique for growth optimization. Due to its ease and widespread availability, we anticipate that FIFE will find wide application in the characterization of CVD-synthesized 2D materials.      

One-dimensional ZnO nanostructures: Solution growth and functional properties

One-dimensional (1D) ZnO nanostructures have been studied intensively and extensively over the last decade not only for their remarkable chemical and physical properties, but also for their current and future diverse technological applications. This article gives a comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods. We will cover the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronic, and energy harvesting devices.      

Photothermoelectric effects in single-walled carbon nanotube films: Reinterpreting scanning photocurrent experiments

We revisit the mechanism leading to the photoresponse of locally illuminated single-walled carbon nanotube (SWNT) films deposited on substrates. Our study examines the impact of multiple device parameters and provides many evidences that the position-dependent photocurrent is dominated by photothermoelectric effects. The photoresponse arises from the temperature variations at the metal-nanotube film interfaces, where mismatches of the Seebeck coefficients are measured. Our work also stresses the impact of the substrates, electrode materials and post-thermal treatments on the amplitude and dynamics of the photoresponse. The knowledge gained should guide the future development of photothermoelectric devices and detectors based on SWNTs.      

From proton conductive nanowires to nanofuel cells: A powerful candidate for generating electricity for self-powered nanosystems

The likely goal of nanotechnology is the integration of individual nanodevices into a nanosystem, which includes the nanodevice(s), power harvesting unit, data processing logic system, and possibly wireless communication unit. A nanosystem requires a nanoscale power source to make the entire package extremely small and high performance. The nanofuel and nanobiofuel cells developed here represent a new self-powering approach in nanotechnology, and their power output is high enough to drive nanodevices for performing self-powered sensing. This study shows the feasibility of building self-powered nanosystems for biological sciences, environmental monitoring, defense technology and even personal electronics.      

Fluorophore-photochrome co-embedded polymer nanoparticles for photoswitchable fluorescence bioimaging

Photoswitchable polymer nanoparticles (ps-PNPs) have been constructed by emulsion polymerization through embedding an iridium(III) complex, [Ir(bt)₂(pic)](bt = phenylbenzothiazole, pic = picolinate), as a fluorophore and a diarylethene derivative (DTE) as a photochrome. The ps-PNPs show reversible luminescence modulation with DTE switching using UV light in aqueous solution and even in living cells. Such luminescent modulation only occurred in polymer nanoparticles and not in a simple mixed solution with low concentrations of [Ir(bt)₂(pic)] and DTE. The ps-PNPs displayed good cellular uptake and fast intracellular luminescence modulation by photoswitching, indicating the nanoparticles could be developed as photoswitchable luminescent probes for a diversity of bioimaging applications.      

Highly flexible transparent film heaters based on random networks of silver nanowires

We demonstrate a new concept for the fabrication of flexible transparent thin film heaters based on silver nanowires. Thanks to the intrinsic properties of random networks of metallic nanowires, it is possible to combine bendability, transparency and high heating performances at low voltage, typically below 12 V which is of interest for many applications. This is currently not possible with transparent conductive oxide technologies, and it compares well with similar devices fabricated with carbon nanotubes or graphene. We present experiments on glass and poly(ethylene naphthalate) (PEN) substrates (with thicknesses of 125 μm and extremely thin 1.3 μm) with excellent heating performances. We point out that the amount of silver necessary to realize the transparent heaters is very low and we also present preliminary results showing that this material can be efficiently used to fabricate photochromic displays. To our knowledge, this is the first report of metallic nanowire-based transparent thin film heaters. We think these results could be a useful approach for the engineering of highly flexible and transparent heaters which are not attainable by existing processes.      

An exchange intercalation mechanism for the formation of a two-dimensional Si structure underneath graphene

A two-dimensional (2D) Si film can form between a graphene overlayer and a Ru(0001) substrate through an intercalation process. At the graphene/2D-Si/Ru(0001) surface, the topmost graphene layer is decoupled from the Ru substrate and becomes quasi-freestanding. The interfacial Si layers show high stability due to the protection from the graphene cover. Surface science measurements indicate that the surface Si atoms can penetrate through the graphene lattice, and density functional theory calculations suggest a Si-C exchange mechanism facilitates the penetration of Si at mild temperatures. The new mechanism may be involved for other elements on graphene, if they can bond strongly with carbon. This finding opens a new route to form 2D interfacial layers between graphene and substrates.      

Low-cost synthesis of robust anatase polyhedral structures with a preponderance of exposed {001} facets for enhanced photoactivities

Anatase polyhedral materials with a preponderance of exposed {001} facets have been produced using (NH₄)₂TiF₆ and water as raw materials. The crystallographic structure and the growth mechanism of the anatase TiO₂ product were investigated systematically by XRD (X-ray diffraction), scanning electron microscopy (SEM), TEM (transmission electron microscope), and ultraviolet (UV) visible and photoluminescence spectroscopy. The products exhibited significantly higher activities than commercial P25 titania nanoparticles in the photocatalytic degradation of methylene blue dye. Moreover, the materials have large particle sizes and are very robust, making them suited for practical uses.      

A facile assay for direct colorimetric visualization of lipopolysaccharides at low nanomolar level

We report a facile assay for the rapid visual detection of lipopolysaccharide (LPS) molecules down to the low nanomolar level by taking advantage of the electrostatic interaction between LPS molecules and cysteamine-modified gold nanoparticles (CSH-Au NPs). The large amount of negatively charged groups on the LPS molecules make LPS highly negatively charged. Thus, when modified with cysteamine, the positively charged gold nanoparticles can aggregate in the presence of trace amounts of LPS. The probe is simple, does not require any advanced instrumentation, and the limit of detection (LOD) was determined to be as low as 3.3 × 10⁻¹⁰ mol/L. To the best of our knowledge, it is the most sensitive synthetic LPS sensor reported so far.      

Synthesis of large-area, few-layer graphene on iron foil by chemical vapor deposition

We demonstrate a simple and controllable way to synthesize large-area, few-layer graphene on iron substrates by an optimized chemical vapor deposition (CVD) method using a mixture of methane and hydrogen. Based on an analysis of the Fe-C phase diagram, a suitable procedure for the successful synthesis of graphene on Fe surfaces was designed. An appropriate temperature and cooling process were found to be very important in the synthesis of highly crystalline few-layer graphene. Graphene-based field-effect transistor (FET) devices were fabricated using the resulting few-layer graphene, and showed good quality with extracted mobilities of 300–1150 cm²/(V·s).      

Nanocomposites of CoO and a mesoporous carbon (CMK-3) as a high performance cathode catalyst for lithium-oxygen batteries

A nanocomposite of CoO and a mesoporous carbon (CMK-3) has been studied as a cathode catalyst for lithium-oxygen batteries in alkyl carbonate electrolytes. The morphology and structure of the as-prepared nanocomposite were characterized by field emission scanning electron microscopy, transmission electron microscopy and high resolution transmission electron microscopy. The electrochemical properties of the mesoporous CoO/CMK-3 nanocomposite as a cathode catalyst in lithium-oxygen batteries were studied using galvanostatic charge-discharge methods. The reaction products on the cathode were analyzed by Fourier transform infrared spectroscopy. The CoO/CMK-3 nanocomposite exhibited better capacity retention than bare mesoporous CMK-3 carbon, Super-P carbon or CoO/Super-P nanocomposite. The synergistic effects arising from the combination of CoO nanoparticles and the mesoporous carbon nanoarchitecture may be responsible for the optimum catalytic performance in lithium-oxygen batteries.      

Polarized light microscopy and spectroscopy of individual single-walled carbon nanotubes

Polarized light microscopy (PLM) is used to image individual single-walled carbon nanotubes (SWNTs) suspended in air across a slit opening. The imaging contrast relies on the strong optical anisotropy typical of SWNTs. We combine PLM with a tunable light source to enable hyperspectral excitation spectroscopy and nanotube chirality assignment. Comparison with fluorescence microscopy and spectroscopy confirms the assignment made with PLM. This represents a versatile new approach to imaging SWNTs and related structures.      

Topological to trivial insulating phase transition in stanene

Electronic properties of stanene, the Sn counterpart of graphene are theoretically studied using first-principles simulations. The topological to trivial insulating phase transition induced by an out-of-plane electric field or by quantum confinement effects is predicted. The results highlight the potential to use stanene nanoribbons in gate-voltage controlled dissipationless spin-based devices and are used to set the minimal nanoribbon width for such devices, which is typically approximately 5 nm.

Open image in new window

     

Reshaping the tips of ZnO nanowires by pulsed laser irradiation

Vertically aligned ZnO nanowires have been synthesized by a hydrothermal method. After being irradiated by a short laser pulse, the tips of the as-synthesized ZnO nanowires can be tailored into a spherical shape. Transmission electron microscopy revealed that the spherical tip is a single-crystalline piece connected to the body of the ZnO nanowire, and that the center of the sphere is hollow. The growth mechanism of the hollow ZnO nanospheres is proposed to involve laser-induced ZnO evaporation immediately followed by re-nucleation in a temperature gradient environment. The laser-irradiated ZnO nanowire array shows hydrophobic properties while the original ZnO nanowire array shows hydrophilicity. The as-grown ZnO nanowire arrays with hollow spherical tips can serve as templates to grow ZnO nanowire arrays with very fine tips, which may be a good candidate material for use in field emission and scanning probe microscopy.      

Sequential assembly of metal-free phthalocyanine on few-layer epitaxial graphene mediated by thickness-dependent surface potential

Due to strong interactions between epitaxial graphene and SiC(0001) substrates, the overlayer charge density induced by the interface charging effect is much more attenuated than that of exfoliated graphene on SiO₂. We report herein a quantitive detection of the charge properties of few-layer graphene by surface potential measurements using electrostatic force microscopy (EFM). A minor difference in surface potential is observed to mediate a sequential assembly of metal-free phthalocyanine (H₂Pc) on monolayer, bilayer and trilayer graphenes, as demonstrated by scanning tunneling microscopy (STM). In order to understand this, we further executed density functional theory (DFT) calculations which showed higher adsorption energies for Pc on thinner graphenes. In this case, we attribute the unique growth behavior of Pc to its variable adsorption energies on few-layer graphene, and in turn the layer charge variations from the viewpoint of energy minimizations. This work is expected to provide fundamental data useful for related nanodevice fabrications.