A NEW WAY OF COMMUNICA-TENG Self-powered systems that operate independently, sustainably, and wirelessly, without the use of batteries, have become increasingly common over the past decade. Among these, triboelectric nanogenerators (TENGs) have attracted attention for scavenging mechanical energy from the environment for conversion into electricity using conventional organic materials at low cost. Triboelectric nanogenerators have been integrated into a variety of applications, including sensors and data transmitters. However, TENGs have not previously been explored for applications that combine these two functions, such as selectively identifying signals for delivering information. In a new study, Yu et al. (DOI: 10.1021/acsnano.5b07407) developed a TENG-based, self-powered communication system that translates a series of environmental triggering signals into binary digital signals and transmits this information using electro-optics. The researchers constructed their TENG using a membrane that consists of two triboelectric layers of different materials: a polytetrafluoroethylene film with a deposited copper thin film as a back electrode and a copper thin film deposited on top of a Kapton film. These contact faces were modified with nanofeatures to increase surface roughness and effective surface area for more effective triboelectrification. Sound waves between 1.30 and 1.65 kHz triggered this device to generate electricity, allowing it to act as a band-pass filter. This electrical signal was converted into a binary signal that was, in turn, used to generate an electro-optical signal using a commercial light-emitting diode. Using this system, the researchers successfully transmitted the digital signals “1001” and “0110” wirelessly and without an outside power source. The authors suggest that this type of communication system could have potential for applications in smart cities, smart homes, and password authentication, among others.
through nanopores while monitoring the corresponding fluctuations in ionic or transverse tunneling current to discern nucleotide bases crossing the nanopore. However, the main challenge that prevents this technique from being used for highthroughput, affordable sequencing is the rapid speed at which the DNA chain crosses the nanopore, which prevents the sequence from being discerned at single-base resolution. Although lowering the driving electric field can actively slow DNA’s speed, it also lowers the signal-to-noise ratio. Other methods, such as lowering the temperature, tuning the salt concentration, increasing the solvent viscosity, or functionalizing the nanopore surface for enhanced DNA−surface interactions, provide only passive modulation of the translocation process. In a new study, Liu and Yobas (DOI: 10.1021/acsnano.6b00610) detail a technique that slows DNA translocation across a nanocapillary by modulating its surface charge through an externally applied gate bias. The researchers accomplished this by using a nanofluidic field-effect transistor. Inside this device were two microchannels connected by an alumina nanocapillary whose entire length was surrounded by a gate electrode. After introducing a positive gate bias, the researchers observed a marked slowing of single λ-DNA chains, with translocation speeds reduced by an order of magnitude. The authors suggest that this technology, realized in a conventional semiconductor microfabrication process without advanced lithography, may eventually play a role in electronic single-molecule sequencers.
NANOWIRES SEE THE LIGHT WITH TRANSFER PRINTING Semiconducting nanowires (NWs) have transformed the field of photonics. In particular, significant research has been devoted to their use as nanoscale laser sources, with projected uses as light sources in photonic integrated circuits, optical interconnects, and nanoscale light sensors. However, manipulating, organizing, and transferring these structures has been challenging due to their ultrasmall size and has limited their use. Although researchers have explored a variety of different techniques to manipulate NWs, these methods have had a number of drawbacks, including requiring complex equipment, needing NWs to be in solution, reducing positioning accuracy, preventing transfer between substrates, and not allowing the manipulation of individual nanowires or the combination of different types of nanowires in the same system. Additionally, no system yet exists to manipulate NWs while preserving their lasing emission.
PUTTING THE BRAKES ON DNA TRANSLOCATION Nanopores and nanochannels are a growing focus for research efforts due to their potential for a variety of practical applications, including DNA sequencing. Researchers have made significant progress in driving single DNA chains electrophoretically © 2016 American Chemical Society
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To overcome these hurdles, Guilhabert et al. (DOI: 10.1021/acsnano.5b07752) developed a transfer printing technique that enables the accurate manipulation, transfer, and printing of individual or small bundles of NWs onto different types of substrates while preserving the NW’s lasing emission. The researchers fabricated polydimethylsiloxane (PDMS) microstamps in two shapes, one flat and one tent-shaped. Pressing these onto indium phosphide NWs, the stamps conformed to the NW shape, allowing them to be lifted from surfaces. Pressing the stamp onto the new substrate released the captured NWs. Experiments showed that the flat tip was more effective at printing onto highly adhesive substrates, such as PDMS, whereas the pointed stamp was better at printing onto less adhesive substrates such as silica or gold. The authors suggest that this method could be useful for fabricating tailored nanophotonic devices across multiple fields.
full cells, using it as the anode with a commercial AC double-layer cathode. The resulting cells showed charge−discharge rates comparable to that of the pure AC double-layer capacitor with significantly higher rate capability. Compared with pure AC, the combination particles had up to 12 times higher gravimetric capacity and volumetric capacity. The authors suggest that this new material shows promise for use in asymmetric hybrid capacitors with high-power density and high-energy density characteristics.
THREE ELECTRODES ARE BETTER THAN TWO Research to measure electron transport using transmission electron microscopes (TEM) continues to grow, prompted by the need to better understand transport in ever smaller devices and by a surge of specialized TEM sample holders designed for these measurements. Being able to perform these measurements in situ is particularly attractive because TEM allows a material’s structure and chemistry to be characterized concurrently with its electronic properties. However, thus far, in situ TEM transport measurements have only been performed in two-electrode configurations, either with two static electrodes or by using a movable scanning probe tip as an electrode. In some systems, modulating the conductance by tuning the charge carrier density requires a third electrode that operates like the gate electrode in a field-effect transistor. In a new study, Rodriguez-Manzo et al. (DOI: 10.1021/acsnano.6b01419) report the implementation of a three-terminal transport platform inside of the TEM, enabling more specialized electronic transport measurements. The researchers fabricated this device by nanosculpting a continuous graphene sheet with a focused electron beam by operating the TEM in scanning mode. Using this method, they left a graphene nanoribbon suspended between source and drain electrodes, with a proximal side gate. By varying the dimensions of this device, the researchers were able to demonstrate that the side-gate electrode’s capacity to modulate conductance increases as the graphene nanoribbon width decreases. The authors suggest that this system could be modified to investigate transport phenomena in other nanoscale systems, such as single-molecule transport or metal-toinsulator transitions dependent on channel width or edge structure.
COMBINATION LITHIUM TITANATE/CARBON NANOPARTICLES HAVE THE POWER High-power electrochemical energy storage devices, such as electrochemical capacitors known as supercapacitors, are increasingly being used in a variety of machines. The majority of commercial supercapacitors belong to the class of electrical double-layer capacitors, which are composed of porous carbon, mostly activated carbon (AC), symmetrically in both cathodes and anodes. When compared with lithium-ion batteries, these supercapacitors have advantages, such as longer cycle life, a broader temperature window for efficient operation, and higher power, as well as drawbacks, such as low volumetric energy density, which results in higher cost for the amount of energy stored. Research to improve the properties of lithium-ion batteries has spotlighted a promising anode material, Li4Ti5O12 (LTO), which has minimal volume changes during lithiation and delithiation, leading to excellent cyclic stability. However, its also has drawbacks, including poor electrical conductivity and slower Li+ transport than that in liquid electrolytes. In a new study, Zhao et al. (DOI: 10.1021/acsnano.6b00479) sought to combine the best attributes of porous carbon and LTO by creating AC particles that carry LTO within their pores. The researchers then tested this new material in asymmetric capacitor 3878
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INVESTIGATING ELECTRONIC TRANSPORT IN A TOPOLOGICAL INSULATOR Topological insulators (TIs) are bulk insulators with conducting surface states. The topological surface states (TSSs) in threedimensional (3D) TIs form two-dimensional (2D) electron gases populated by massless Dirac Fermions with spinmomentum locking. Photoemission spectroscopy and electrical transport measurements to investigate the TSSs in 3D TIs, such as BixSb1−x, Bi2Se3, and Bi2Te3, have revealed the presence of isotropic Dirac cones. Highly anisotropic Dirac cones, useful for realizing ideal quantum wires with one-directional spin and electron transport on their surfaces, have been theorized to be present in silver chalcogenides, including Ag2Se and Ag2Te. However, experimental research into electronic transport of anisotropic Dirac Fermions is still at an early stage. In a new study, Kim et al. (DOI: 10.1021/acsnano.5b07368) investigated the electronic transport properties of singlecrystalline β-Ag2Se nanostructures, including nanowires and nanoribbons. After synthesizing these structures using chemical vapor transport, the researchers measured transport properties while applying a magnetic field at different angles. They observed weak magnetoconductance, suggesting the presence of the weak antilocalization effect and the existence of strong spin−orbit coupling. Applying a magnetic field parallel to the nanowire axis caused these devices to exhibit highly periodic magnetoconductive oscillations, implying the existence of surface electronic states. When a magnetic field was applied parallel to the nanowire axis, the researchers observed quantum magnetic oscillations. These observations, combined with first-principles band structure calculations, confirmed the existence of the topological surface states. The authors suggest that further investigation into the electronic transport properties of anisotropic TIs could provide valuable insight into their exotic properties, knowledge that could eventually lead to their possible application in spintronics and quantum information.
quartz substrates using electron beam lithography and reactive ion etching. Experiments using these combined materials demonstrated a high degree of temporally and spatially coherent lasing, with well-defined directional emission. These devices showed high power conversion efficiencies and differential quantum efficiencies on par with epitaxially grown single-crystal semiconductor lasers. Additionally, the authors demonstrate the scalability of the nanofabrication approach to synthesize these devices by creating a 2D pixelated 4 × 4 microlaser array, using it to generate the letters “X”, “N”, “L”, “Z”, and “C” by the directed scanning of the pump beam. Although the device’s longevity was short due to heat-related degradation, the authors suggest that this approach could eventually be used to develop practical perovskite-based lasers.
MAKING POWER COUPLES MORE POWERFUL Plasmonic optical antennas control the luminescence properties of single quantum emitters, providing directional radiation control and enhanced emission rates. Pairing these components efficiently is key to developing quantum plasmonic circuitry and enabling long-range energy transfer between quantum nanoemitters. However, simultaneously maximizing the surface plasmon propagation length (LSPP), the coupling efficiency into the plasmonic mode (β), and the local density of optical states or Purcell factor (FP) remains a major experimental challenge. In a new study, de Torres et al. (DOI: 10.1021/acsnano.6b00287) significantly advance this goal by developing a system to investigate carefully and subsequently to optimize the interactions between fluorescent nanoparticles and surface plasmons on single-crystalline silver nanowires. The researchers used a dualbeam scanning confocal microscope with two sets of mirrors, which enabled independent scanning of the laser excitation and the fluorescence detection. Using this detection scheme, they explored different in- and out-coupling routes for the interaction between polystyrene nanoparticles embedded with fluorescent dye and nanowire surface plasmons to find conditions that maximize LSPP, β, and FP. They found that, in the best case where the excitation is mediated by surface plasmons, coupling efficiencies could be maximized up to 44%, with a 24× lifetime reduction. Taking advantage of their findings, the researchers were able to demonstrate plasmon-mediated fluorescence energy transfer between a set of donor and acceptor nanoparticles
PEROVSKITE FILMS IN PHOTONIC CRYSTALS SEE THE LIGHT Organo-lead halide perovskites have attracted attention as a new class of photovoltaic materials with high conversion efficiencies. Combined with their low-temperature, solution-based synthesis, perovskites are appealing candidate materials for low-cost solar cells. Recently, reported light emission from perovskite thin films has prompted investigation of the optical gain in these materials. Typically, these reports have presented spectral narrowing and nonlinear threshold-like light input−output characteristics as the primary evidence for stimulated emission. However, these findings have not sufficiently demonstrated well-defined, spatially coherent beams, which is a fundamental validation and practical requirement for lasers. In a new study, Chen et al. (DOI: 10.1021/acsnano.5b08153) take a step toward practical perovskite lasers by embedding a solution-processed thin film inside a two-dimensional (2D) photonic crystal resonator. Using CH3NH3PbI3 as the perovskite, the researchers created thin films inside templates patterned into 3879
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separated by 1.3 μm with 17% efficiency. The authors suggest that photodynamics studies using this experimental setup could be useful for developing applications in nanoscale optics, biosensing, and quantum optics.
PEELING BACK THE LAYERS ON A WEAK TOPOLOGICAL INSULATOR Most research on topological insulators (TIs) has focused on strong three-dimensional (3D) TIs, which have robust metallic surface states on all surfaces. In contrast, weak 3DTIs, which have robust metallic surface states on just some surfaces, have received little attention. One recently identified weak 3DTI, Bi14Rh3I9, has been characterized as a layered ionic structure with alternating cationic intermetallic layers and ionic insulating layers. Scanning tunneling microscopy has revealed robust edge states at all step edges in the cationic layer as a topological fingerprint. However, these edge states are significantly below the Fermi level, impeding investigations into electronic transport. In a new study, Pauly et al. (DOI: 10.1021/acsnano.6b00841) overcame this challenge by combining results of density functional slab calculations with scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. The researchers found that n-type doping identified in the intermetallic layer is intrinsically caused by a polar surface that is present after cleavage but is well screened in the bulk material. In contrast, the anionic insulating layer displays a band gap at the Fermi level, whether on the surface at cleavage or in the bulk material, due to iodine desorption. The authors suggest that such a well-screened surface dipole implies that a buried edge state, likely already below a single insulating layer, is located at the Fermi level. As an experimental strategy to move the topologically protected edge state to the Fermi level, they suggest leaving the insulating layer on top and exposing the edge of the underlying intermetallic layer through surface scratching. These findings, they add, offer insight into this recently synthesized material and weak TIs in general.
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