Hot off the presses! Jun 01 Nat Nanotechnol

The Jun 01 issue of the Nat Nanotechnol is now up on Pubget (About Nat Nanotechnol): if you're at a subscribing institution, just click the link in the latest link at the home page. (Note you'll only be able to get all the PDFs in the issue if your institution subscribes to Pubget.)

Latest Articles Include:

• The responsibilities of authors
  - Nat Nanotechnol 4(6):331 (2009)
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• How can ab initio simulations address risks in nanotech?
  - Nat Nanotechnol 4(6):332-335 (2009)
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• Are you a responsible nanoscientist?
  - Nat Nanotechnol 4(6):336 (2009)
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• Notes on a scientific scandal
  - Nat Nanotechnol 4(6):337 (2009)
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• Our choice from the recent literature
  - Nat Nanotechnol 4(6):338-339 (2009)
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• Top down bottom up: Blood ties
  - Nat Nanotechnol 4(6):339 (2009)
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• Nanomaterials: Viruses electrify battery research
  - Nat Nanotechnol 4(6):341-342 (2009)
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• Toxicology: Testing in the third dimension
  - Nat Nanotechnol 4(6):342-343 (2009)
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• Nanophotonics: Gradient force shows its potential
  - Nat Nanotechnol 4(6):344-345 (2009)
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• Separation materials: Proteins make for finer filters
  - Nat Nanotechnol 4(6):345-346 (2009)
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• Superconductivity: Flat out
  - Nat Nanotechnol 4(6):346 (2009)
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• Quantum dots: When a barrier is not an obstacle
  - Nat Nanotechnol 4(6):347-348 (2009)
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• Modular construction of DNA nanotubes of tunable geometry and single- or double-stranded character
  - Nat Nanotechnol 4(6):349-352 (2009)
  DNA nanotubes can template the growth of nanowires1, orient transmembrane proteins for nuclear magnetic resonance determination2, and can potentially act as stiff interconnects, tracks for molecular motors and nanoscale drug carriers3. Current methods for the construction of DNA nanotubes result in symmetrical and cylindrical assemblies that are entirely double-stranded2, 4, 5, 6, 7, 8, 9, 10, 11. Here, we report a modular approach to DNA nanotube synthesis that provides access to geometrically well-defined triangular and square-shaped DNA nanotubes. We also construct the first nanotube assemblies that can exist in double- and single-stranded forms with significantly different stiffness. This approach allows for parameters such as geometry, stiffness, and single- or double-stranded character to be fine-tuned, and could enable the creation of designer nanotubes for a range of applications, including the growth of nanowires of controlled shape, the loading and release of! cargo, and the real-time modulation of stiffness and persistence length within DNA interconnects.
• Ultrafast permeation of water through protein-based membranes
  - Nat Nanotechnol 4(6):353-357 (2009)
  Pressure-driven filtration by porous membranes is widely used in the production of drinking water from ground and surface water1, 2, 3. Permeation theory predicts that filtration rate is proportional to the pressure difference across the filtration membrane and inversely proportional to the thickness of the membrane4. However, these membranes need to be able to withstand high water fluxes and pressures, which means that the active separation layers in commercial filtration systems typically have a thickness of a few tens to several hundreds of nanometres5. Filtration performance might be improved by the use of ultrathin porous silicon membranes6 or carbon nanotubes immobilized in silicon nitride7 or polymer films8, 9, but these structures are difficult to fabricate. Here, we report a new type of filtration membrane made of crosslinked proteins that are mechanically robust and contain channels with diameters of less than 2.2 nm. We find that a 60-nm-thick membrane can c! oncentrate aqueous dyes from fluxes up to 9,000 l h-1 m-2 bar-1, which is approx1,000 times higher than the fluxes that can be withstood by commercial filtration membranes with similar rejection properties1, 10, 11. Based on these results and molecular dynamics simulations, we propose that protein-surrounded channels with effective lengths of less than 5.8 nm can separate dye molecules while allowing the ultrafast permeation of water at applied pressures of less than 1 bar.
• Alternating patterns on single-walled carbon nanotubes
  - Nat Nanotechnol 4(6):358-362 (2009)
  Scientific and technological interest in one-dimensional nanomaterials, in particular carbon nanotubes1, 2, is a result of their fascinating properties and their ability to serve as templates for directed assembly. For applications in nanoelectronics it is necessary to create ordered arrays of nanotubes for large-scale integrated circuits, an area in which there has been significant progress3, 4, 5, 6, 7, and to produce controllable patterns on individual nanotubes so that multiple transistors can be fabricated on them, an area where progress has been slower8, 9, 10, 11, 12, 13, 14. Here, we show that judiciously selected crystalline block copolymers can be periodically decorated along carbon nanotubes, leading to amphiphilic, alternating patterns with a period of approx12 nm. In addition, end-functionalization of the block copolymers allowed gold nanoparticles to be periodically attached to the nanotubes. This approach provides a facile technique for the periodic patt! erning of one-dimensional nanomaterials.
• Tunable few-electron double quantum dots and Klein tunnelling in ultraclean carbon nanotubes
  - Nat Nanotechnol 4(6):363-367 (2009)
  Quantum dots defined in carbon nanotubes are a platform for both basic scientific studies1, 2, 3, 4, 5 and research into new device applications6. In particular, they have unique properties that make them attractive for studying the coherent properties of single-electron spins7, 8, 9, 10, 11. To perform such experiments it is necessary to confine a single electron in a quantum dot with highly tunable barriers1, but disorder has prevented tunable nanotube-based quantum-dot devices from reaching the single-electron regime2, 3, 4, 5. Here, we use local gate voltages applied to an ultraclean suspended nanotube to confine a single electron in both a single quantum dot and, for the first time, in a tunable double quantum dot. This tunability is limited by a novel type of tunnelling that is analogous to the tunnelling in the Klein paradox of relativistic quantum mechanics.
• The crossover from two dimensions to one dimension in granular electronic materials
  - Nat Nanotechnol 4(6):368-372 (2009)
  Granular conductors1 are solids comprising densely packed nanoparticles, and have electrical properties that are determined by the size, composition and packing of the composite nanoparticles. The ability to control these properties in two- and three-dimensional granular conductors has made such systems appropriate for use as prototypes for investigating new physics1, 2, 3, 4. However, the fabrication of strictly one-dimensional granular conductors remains challenging. Here, we describe a method for the assembly of nanoparticles into granular solids that can be tuned continuously from two to one dimension, and establish how electron transport evolves between these limits. We find that the energy barriers to transport increase in the one-dimensional limit, in both the variable-range-hopping (low-voltage) and sequential-tunnelling (high-voltage) regimes. Furthermore, in the sequential-tunnelling regime we find an unexpected relationship between the temperature and the vo! ltage at which the conductance becomes appreciable — a relationship that appears peculiar to one-dimensional systems. These results are explained by extrapolating existing granular conductor theories to one dimension.
• Direct measurement of electrical conductance through a self-assembled molecular layer
  - Nat Nanotechnol 4(6):373-376 (2009)
  The self-assembly of organic molecules on surfaces is a promising approach for the development of nanoelectronic devices1, 2. Although a variety of strategies have been used to establish stable links between molecules2, 3, 4, 5, 6, 7, 8, 9, 10, 11, little is known about the electrical conductance of these links. Extended electronic states, a prerequisite for good conductance, have been observed for molecules adsorbed on metal surfaces12, 13, 14, 15, 16. However, direct conductance measurements through a single layer of molecules are only possible if the molecules are adsorbed on a poorly conducting substrate. Here we use a nanoscale four-point probe17 to measure the conductivity of a self-assembled layer of cobalt phthalocyanine on a silver-terminated silicon surface as a function of thickness. For low thicknesses, the cobalt phthalocyanine molecules lie flat on the substrate, and their main effect is to reduce the conductivity of the substrate. At higher thicknesses, ! the cobalt phthalocyanine molecules stand up to form stacks and begin to conduct. These results connect the electronic structure and orientation of molecular monolayer and few-layer systems to their transport properties, and should aid in the rational design of future devices.
• Broadband all-photonic transduction of nanocantilevers
  - Nat Nanotechnol 4(6):377-382 (2009)
  Nanoelectromechanical systems1, 2 based on cantilevers have consistently set records for sensitivity in measurements of displacement3, force4 and mass3, 5, 6 over the past decade. Continued progress will require the integration of efficient transduction on a chip so that nanoelectromechanical systems may be operated at higher speeds and sensitivities. Conventional electrical schemes have limited bandwidth7, 8, and although optical methods9, 10 are fast, they are subject to the diffraction limit. Here, we demonstrate the integration of nanocantilevers on a silicon photonic platform with a non-interferometric transduction scheme that avoids the diffraction limit by making use of near-field effects in optomechanical interactions11. The use of a non-interferometric method means that a coherent light source is not required, making the monolithic integration of optomechanical systems with on-chip light sources feasible. We further demonstrate optomechanical multiplexing of a! n array of ten nanocantilevers with a displacement sensitivity of 40 fm Hz-1/2.
• Trilayer graphene is a semimetal with a gate-tunable band overlap
  - Nat Nanotechnol 4(6):383-388 (2009)
  Graphene-based materials are promising candidates for nanoelectronic devices1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 because very high carrier mobilities can be achieved without the use of sophisticated material preparation techniques1. However, the carrier mobilities reported for single-layer and bilayer graphene are still less than those reported for graphite crystals at low temperatures, and the optimum number of graphene layers for any given application is currently unclear, because the charge transport properties of samples containing three or more graphene layers have not yet been investigated systematically1. Here, we study charge transport through trilayer graphene as a function of carrier density, temperature, and perpendicular electric field. We find that trilayer graphene is a semimetal with a resistivity that decreases with increasing electric field, a behaviour that is markedly different from that of single-layer and bilayer graphene. We show that the! phenomenon originates from an overlap between the conduction and valence bands that can be controlled by an electric field, a property that had never previously been observed in any other semimetal. We also determine the effective mass of the charge carriers, and show that it accounts for a large part of the variation in the carrier mobility as the number of layers in the sample is varied.
• Atomic force microscopy detects differences in the surface brush of normal and cancerous cells
  - Nat Nanotechnol 4(6):389-393 (2009)
  The atomic force microscope is broadly used to study the morphology of cells1, 2, 3, 4, 5, but it can also probe the mechanics of cells. It is now known that cancerous cells may have different mechanical properties to those of normal cells6, 7, 8, but the reasons for these differences are poorly understood9. Here, we report quantitatively the differences between normal and cancerous human cervical epithelial cells by considering the brush layer on the cell surface. These brush layers, which consist mainly of microvilli, microridges and cilia, are important for interactions with the environment. Deformation force curves obtained from cells in vitro were processed according to the 'brush on soft cell model'10. We found that normal cells have brushes of one length, whereas cancerous cells have mostly two brush lengths of significantly different densities. The observed differences suggest that brush layers should be taken into account when characterizing the cell surface b! y mechanical means.