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.

Hot off the presses! Oct 01

The Oct 01 issue of the is now up on Pubget (About ): 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:

• Organics settle down
  - Nature nanotechnology 4(10):607 (2009)
  
• The New Deficit Model
  - Nature nanotechnology 4(10):609-611 (2009)
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• Selling graphene by the ton
  - Nature nanotechnology 4(10):612-614 (2009)
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• It's not just about nanotoxicology
  - Nature nanotechnology 4(10):615 (2009)
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• Our choice from the recent literature
  - Nature nanotechnology 4(10):616-617 (2009)
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• Top down bottom up: Search and destroy
  - Nature nanotechnology 4(10):617 (2009)
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• Nanoelectromechanical systems: Show of strength
  - Nature nanotechnology 4(10):619-620 (2009)
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• Probe microscopy: A closer look at the atoms in a molecule
  - Nature nanotechnology 4(10):620 (2009)
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• Nanomedicine: Sniffing out lung cancer
  - Nature nanotechnology 4(10):621-622 (2009)
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• Nanopatterning: Surfaces feel the heat
  - Nature nanotechnology 4(10):622-623 (2009)
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• Spintronics: Shedding light on nanomagnets
  - Nature nanotechnology 4(10):623-625 (2009)
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• Biomolecular computing: Molecules that reason
  - Nature nanotechnology 4(10):625-626 (2009)
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• Correction
  - Nature nanotechnology 4(10):626 (2009)
  Introduction In the News & Views 'Probing superconductivity at the nanoscale' (Nature Nanotech. 4, 142; 2009), the penultimate sentence should have read: 'red corresponds to a high conductance and a small gap.' Corrected in the HTML and PDF versions, after print: 7 October 2009.
• Correction
  - Nature nanotechnology 4(10):626 (2009)
  Introduction In the News & Views 'Crossing boundaries and borders' (Nature Nanotech. 1, 91–92; 2006), the third sentence of the second paragraph should have referred to Claire Berger. Corrected in the HTML and PDF versions, after print: 7 October 2009.
• Promises, facts and challenges for carbon nanotubes in imaging and therapeutics
  - Nature nanotechnology 4(10):627-633 (2009)
  The use of carbon nanotubes in medicine is now at the crossroads between a proof-of-principle concept and an established preclinical candidate for a variety of therapeutic and diagnostic applications. Progress towards clinical trials will depend on the outcomes of efficacy and toxicology studies, which will provide the necessary risk-to-benefit assessments for carbon-nanotube-based materials. Here we focus on carbon nanotubes that have been studied in preclinical animal models, and draw attention to the promises, facts and challenges of these materials as they transition from research to the clinical phase. We address common questions regarding the use of carbon nanotubes in disease imaging and therapy, and highlight the opportunities and challenges ahead.
• Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective
  - Nature nanotechnology 4(10):634-641 (2009)
  The regulation of engineered nanoparticles requires a widely agreed definition of such particles. Nanoparticles are routinely defined as particles with sizes between about 1 and 100 nm that show properties that are not found in bulk samples of the same material. Here we argue that evidence for novel size-dependent properties alone, rather than particle size, should be the primary criterion in any definition of nanoparticles when making decisions about their regulation for environmental, health and safety reasons. We review the size-dependent properties of a variety of inorganic nanoparticles and find that particles larger than about 30 nm do not in general show properties that would require regulatory scrutiny beyond that required for their bulk counterparts.
• Molecular implementation of simple logic programs
  - Nature nanotechnology 4(10):642-648 (2009)
  Autonomous programmable computing devices made of biomolecules could interact with a biological environment and be used in future biological and medical applications1, 2, 3, 4, 5, 6, 7. Biomolecular implementations of finite automata8, 9 and logic gates4, 10, 11, 12, 13 have already been developed14, 15, 16, 17, 18. Here, we report an autonomous programmable molecular system based on the manipulation of DNA strands that is capable of performing simple logical deductions. Using molecular representations of facts such as Man(Socrates) and rules such as Mortal(X) Man(X) (Every Man is Mortal), the system can answer molecular queries such as Mortal(Socrates)? (Is Socrates Mortal?) and Mortal(X)? (Who is Mortal?). This biomolecular computing system compares favourably with previous approaches in terms of expressive power, performance and precision2, 4, 8, 9, 11, 12, 19. A compiler translates facts, rules and queries into their molecular representations and subsequently oper! ates a robotic system that assembles the logical deductions and delivers the result. This prototype is the first simple programming language with a molecular-scale implementation.
• Single-crystal germanium layers grown on silicon by nanowire seeding
  - Nature nanotechnology 4(10):649-653 (2009)
  Three-dimensional integration and the combination of different material systems are central themes of electronics research. Recently, as-grown vertical one-dimensional structures have been integrated into high-density three-dimensional circuits. However, little attention has been paid to the unique structural properties of germanium nanowires obtained by epitaxial and heteroepitaxial growth on Ge(111) and Si(111) substrates1, 2, despite the fact that the integration of germanium on silicon is attractive for device applications. Here, we demonstrate the lateral growth of single crystal germanium islands tens of micrometres in diameter by seeding from germanium nanowires grown on a silicon substrate. Vertically aligned high-aspect-ratio nanowires can transfer the orientation and perfection of the substrate crystal to overlying layers a micrometre or more above the substrate surface. This technique can be repeated to build multiple active device layers, a key requirement ! for the fabrication of densely interconnected three-dimensional integrated circuits.
• Catalyst preparation for CMOS-compatible silicon nanowire synthesis
  - Nature nanotechnology 4(10):654-657 (2009)
  Metallic contamination was key to the discovery of semiconductor nanowires1, but today it stands in the way of their adoption by the semiconductor industry. This is because many of the metallic catalysts required for nanowire growth are not compatible with standard CMOS (complementary metal oxide semiconductor) fabrication processes. Nanowire synthesis with those metals that are CMOS compatible, such as aluminium2 and copper3, 4, 5, necessitate temperatures higher than 450 °C, which is the maximum temperature allowed in CMOS processing. Here, we demonstrate that the synthesis temperature of silicon nanowires using copper-based catalysts is limited by catalyst preparation. We show that the appropriate catalyst can be produced by chemical means at temperatures as low as 400 °C. This is achieved by oxidizing the catalyst precursor, contradicting the accepted wisdom that oxygen prevents metal-catalysed nanowire growth. By simultaneously solving material compatibility and! temperature issues, this catalyst synthesis could represent an important step towards real-world applications of semiconductor nanowires6, 7, 8, 9, 10, 11.
• Uniform exciton fluorescence from individual molecular nanotubes immobilized on solid substrates
  - Nature nanotechnology 4(10):658-663 (2009)
  Self-assembled quasi one-dimensional nanostructures of -conjugated molecules1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 may find a use in devices owing to their intriguing optoelectronic properties, which include sharp exciton transitions1, 2, 3, 4, 5, strong circular dichroism5, 6, 7, high exciton mobilities8, 9 and photoconductivity10. However, many applications require immobilization of these nanostructures on a solid substrate, which is a challenge to achieve without destroying their delicate supramolecular structure. Here, we use a drop-flow technique to immobilize double-walled tubular J-aggregates of amphiphilic cyanine dyes without affecting their morphological or optical properties. High-resolution images of the topography and exciton fluorescence of individual J-aggregates are obtained simultaneously with polarization-resolved near-field scanning optical microscopy. These images show remarkably uniform supramolecular structure, both along individual nan! otubes and between nanotubes in an ensemble, demonstrating their potential for light harvesting and energy transport.
• Thermochemical nanopatterning of organic semiconductors
  - Nature nanotechnology 4(10):664-668 (2009)
  Patterning of semiconducting polymers on surfaces is important for various applications in nanoelectronics and nanophotonics. However, many of the approaches to nanolithography that are used to pattern inorganic materials are too harsh for organic semiconductors, so research has focused on optical patterning1, 2, 3 and various soft lithographies4. Surprisingly little attention has been paid to thermal5, thermomechanical6, 7 and thermochemical8, 9, 10, 11, 12, 13 patterning. Here, we demonstrate thermochemical nanopatterning of poly(p-phenylene vinylene), a widely used electroluminescent polymer14, by a scanning probe. We produce patterned structures with dimensions below 28 nm, although the tip of the probe has a diameter of 5 m, and achieve write speeds of 100 m s-1. Experiments show that a resolution of 28 nm is possible when the tip–sample contact region has dimensions of 100 nm and, on the basis of finite-element modelling, we predict that the resolution could be! improved by using a thinner resist layer and an optimized probe. Thermochemical lithography offers a versatile, reliable and general nanopatterning technique because a large number of optical materials, including many commercial crosslinker additives and photoresists, rely on chemical mechanisms that can also be thermally activated8, 15, 16.
• Diagnosing lung cancer in exhaled breath using gold nanoparticles
  - Nature nanotechnology 4(10):669-673 (2009)
  Conventional diagnostic methods for lung cancer1, 2 are unsuitable for widespread screening2, 3 because they are expensive and occasionally miss tumours. Gas chromatography/mass spectrometry studies have shown that several volatile organic compounds, which normally appear at levels of 1–20 ppb in healthy human breath, are elevated to levels between 10 and 100 ppb in lung cancer patients4, 5, 6. Here we show that an array of sensors based on gold nanoparticles can rapidly distinguish the breath of lung cancer patients from the breath of healthy individuals in an atmosphere of high humidity. In combination with solid-phase microextraction7, gas chromatography/mass spectrometry was used to identify 42 volatile organic compounds that represent lung cancer biomarkers. Four of these were used to train and optimize the sensors, demonstrating good agreement between patient and simulated breath samples. Our results show that sensors based on gold nanoparticles could form the ! basis of an inexpensive and non-invasive diagnostic tool for lung cancer.
• Monolayer coverage and channel length set the mobility in self-assembled monolayer field-effect transistors
  - Nature nanotechnology 4(10):674-680 (2009)
  The mobility of self-assembled monolayer field-effect transistors (SAMFETs) traditionally decreases dramatically with increasing channel length. Recently, however, SAMFETs using liquid-crystalline molecules have been shown to have bulk-like mobilities that are virtually independent of channel length. Here, we reconcile these scaling relations by showing that the mobility in liquid crystalline SAMFETs depends exponentially on the channel length only when the monolayer is incomplete. We explain this dependence both numerically and analytically, and show that charge transport is not affected by carrier injection, grain boundaries or conducting island size. At partial coverage, that is when the monolayer is incomplete, liquid-crystalline SAMFETs thus form a unique model system to study size-dependent conductance originating from charge percolation in two dimensions.
• Charge-controlled magnetism in colloidal doped semiconductor nanocrystals
  - Nature nanotechnology 4(10):681-687 (2009)
  Electrical control over the magnetic states of doped semiconductor nanostructures could enable new spin-based information processing technologies. To this end, extensive research has recently been devoted to examination of carrier-mediated magnetic ordering effects in substrate-supported quantum dots at cryogenic temperatures, with carriers introduced transiently by photon absorption. The relatively weak interactions found between dopants and charge carriers have suggested that gated magnetism in quantum dots will be limited to cryogenic temperatures. Here, we report the observation of a large, reversible, room-temperature magnetic response to charge state in free-standing colloidal ZnO nanocrystals doped with Mn2+ ions. Injected electrons activate new ferromagnetic Mn2+–Mn2+ interactions that are strong enough to overcome antiferromagnetic coupling between nearest-neighbour dopants, making the full magnetic moments of all dopants observable. Analysis shows that this! large effect occurs in spite of small pairwise electron–Mn2+ exchange energies, because of competing electron-mediated ferromagnetic interactions involving distant Mn2+ ions in the same nanocrystal.
• Golden carbon nanotubes as multimodal photoacoustic and photothermal high-contrast molecular agents
  - Nature nanotechnology 4(10):688-694 (2009)
  Carbon nanotubes have shown promise as contrast agents for photoacoustic and photothermal imaging of tumours and infections because they offer high resolution and allow deep tissue imaging. However, in vivo applications have been limited by the relatively low absorption displayed by nanotubes at near-infrared wavelengths and concerns over toxicity. Here, we show that gold-plated carbon nanotubes—termed golden carbon nanotubes—can be used as photoacoustic and photothermal contrast agents with enhanced near-infrared contrast (102-fold) for targeting lymphatic vessels in mice using extremely low laser fluence levels of a few mJ cm-2. Antibody-conjugated golden carbon nanotubes were used to map the lymphatic endothelial receptor, and preliminary in vitro viability tests show golden carbon nanotubes have minimal toxicity. This new nanomaterial could be an effective alternative to existing nanoparticles and fluorescent labels for non-invasive targeted imaging of molecula! r structures in vivo.

Hot off the presses! Sep 01 Nat Nanotechnol

The Sep 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:

• Recommended reading
  - Nat Nanotechnol 4(9):533 (2009)
  
• Where does nanotechnology belong in the map of science?
  - Nat Nanotechnol 4(9):534-536 (2009)
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• Science from the inside
  - Nat Nanotechnol 4(9):537-538 (2009)
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• Our choice from the recent literature
  - Nat Nanotechnol 4(9):540-541 (2009)
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• Top down bottom up: Digging deeper
  - Nat Nanotechnol 4(9):541 (2009)
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• DNA nanotechnology: Geometric sorting boards
  - Nat Nanotechnol 4(9):543-544 (2009)
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• Nanomedicine: Silence the target
  - Nat Nanotechnol 4(9):544-545 (2009)
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• Carbon nanotubes: A simple approach to superlattices
  - Nat Nanotechnol 4(9):545-546 (2009)
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• Protein-nanoparticle interactions: What does the cell see?
  - Nat Nanotechnol 4(9):546-547 (2009)
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• Nanophotonics: Making the most of photons
  - Nat Nanotechnol 4(9):548-549 (2009)
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• Graphene: Surfing ripples towards new devices
  - Nat Nanotechnol 4(9):549-550 (2009)
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• Molecular electronics with single molecules in solid-state devices
  - Nat Nanotechnol 4(9):551-556 (2009)
  The ultimate aim of molecular electronics is to understand and master single-molecule devices. Based on the latest results on electron transport in single molecules in solid-state devices, we focus here on new insights into the influence of metal electrodes on the energy spectrum of the molecule, and on how the electron transport properties of the molecule depend on the strength of the electronic coupling between it and the electrodes. A variety of phenomena are observed depending on whether this coupling is weak, intermediate or strong.
• Placement and orientation of individual DNA shapes on lithographically patterned surfaces
  - Nat Nanotechnol 4(9):557-561 (2009)
  Artificial DNA nanostructures1, 2 show promise for the organization of functional materials3, 4 to create nanoelectronic5 or nano-optical devices. DNA origami, in which a long single strand of DNA is folded into a shape using shorter 'staple strands'6, can display 6-nm-resolution patterns of binding sites, in principle allowing complex arrangements of carbon nanotubes, silicon nanowires, or quantum dots. However, DNA origami are synthesized in solution and uncontrolled deposition results in random arrangements; this makes it difficult to measure the properties of attached nanodevices or to integrate them with conventionally fabricated microcircuitry. Here we describe the use of electron-beam lithography and dry oxidative etching to create DNA origami-shaped binding sites on technologically useful materials, such as SiO2 and diamond-like carbon. In buffer with 100 mM MgCl2, DNA origami bind with high selectivity and good orientation: 70–95% of sites have individual or! igami aligned with an angular dispersion (1 s.d.) as low as 10° (on diamond-like carbon) or 20° (on SiO2).
• Controlled ripple texturing of suspended graphene and ultrathin graphite membranes
  - Nat Nanotechnol 4(9):562-566 (2009)
  Graphene is nature's thinnest elastic material and displays exceptional mechanical1, 2 and electronic properties3, 4, 5. Ripples are an intrinsic feature of graphene sheets6 and are expected to strongly influence electronic properties by inducing effective magnetic fields and changing local potentials7, 8, 9, 10, 11, 12. The ability to control ripple structure in graphene could allow device design based on local strain13 and selective bandgap engineering14. Here, we report the first direct observation and controlled creation of one- and two-dimensional periodic ripples in suspended graphene sheets, using both spontaneously and thermally generated strains. We are able to control ripple orientation, wavelength and amplitude by controlling boundary conditions and making use of graphene's negative thermal expansion coefficient (TEC), which we measure to be much larger than that of graphite. These results elucidate the ripple formation process, which can be understood in te! rms of classical thin-film elasticity theory. This should lead to an improved understanding of suspended graphene devices15, 16, a controlled engineering of thermal stress in large-scale graphene electronics, and a systematic investigation of the effect of ripples on the electronic properties of graphene.
• Substrate-induced array of quantum dots in a single-walled carbon nanotube
  - Nat Nanotechnol 4(9):567-570 (2009)
  Single-walled carbon nanotubes are model one-dimensional structures1, 2, 3, 4, 5, 6. They can also be made into zero-dimensional structures; quantum wells can be created in nanotubes by inserting metallofullerenes7, by mechanical cutting8, 9, 10 or by the application of mechanical strain11. Here, we report that quantum dot arrays can be produced inside nanotubes simply by causing a misalignment between the nanotube and the 100 direction of a supporting silver substrate. This method does not require chemical or physical treatment of either the substrate or the nanotube. A short quantum dot confinement length of 6 nm results in large energy splittings.
• Plasmonic fluorescent quantum dots
  - Nat Nanotechnol 4(9):571-576 (2009)
  Combining multiple discrete components into a single multifunctional nanoparticle could be useful in a variety of applications. Retaining the unique optical and electrical properties of each component after nanoscale integration is, however, a long-standing problem1, 2. It is particularly difficult when trying to combine fluorophores such as semiconductor quantum dots with plasmonic materials such as gold, because gold and other metals can quench the fluorescence3, 4. So far, the combination of quantum dot fluorescence with plasmonically active gold has only been demonstrated on flat surfaces5. Here, we combine fluorescent and plasmonic activities in a single nanoparticle by controlling the spacing between a quantum dot core and an ultrathin gold shell with nanometre precision through layer-by-layer assembly. Our wet-chemistry approach provides a general route for the deposition of ultrathin gold layers onto virtually any discrete nanostructure or continuous surface, a! nd should prove useful for multimodal bioimaging6, interfacing with biological systems7, reducing nanotoxicity8, modulating electromagnetic fields5 and contacting nanostructures9, 10.
• A quantitative fluorescence study of protein monolayer formation on colloidal nanoparticles
  - Nat Nanotechnol 4(9):577-580 (2009)
  It is now known that nanoparticles, when exposed to biological fluid, become coated with proteins and other biomolecules to form a 'protein corona'1. Recent systematic studies have identified various proteins that can make up this corona, but these nanoparticle–protein interactions are still poorly understood, and quantitative studies to characterize them are few in number. Here, we have quantitatively analysed the adsorption of human serum albumin onto small (10–20 nm in diameter) polymer-coated FePt and CdSe/ZnS nanoparticles by using fluorescence correlation spectroscopy. The protein corona forms a monolayer with a thickness of 3.3 nm. Proteins bind to the negatively charged nanoparticles with micromolar affinity, and time-resolved fluorescence quenching experiments show that they reside on the particle for 100 s. These new findings deepen our quantitative understanding of the protein corona, which is of utmost importance in the safe application of nanoscale obj! ects in living organisms.
• Single europium-doped nanoparticles measure temporal pattern of reactive oxygen species production inside cells
  - Nat Nanotechnol 4(9):581-585 (2009)
  Low concentrations of reactive oxygen species, notably hydrogen peroxide (H2O2), mediate various signalling processes in the cell1, 2. Production of these signals is highly regulated3 and a suitable probe is needed to measure these events. Here, we show that a probe based on a single nanoparticle can quantitatively measure transient H2O2 generation in living cells. The Y0.6Eu0.4VO4 nanoparticles undergo photoreduction under laser irradiation but re-oxidize in the presence of oxidants, leading to a recovery in luminescence. Our probe can be regenerated and reliably detects intracellular H2O2 with a 30-s temporal resolution and a dynamic range of 1–45 M. The differences in the timing of intracellular H2O2 production triggered by different signals were also measured using these nanoparticles. Although the probe is not selective towards H2O2, in many signalling processes H2O2 is, however, the dominant oxidant3, 4, 5, 6. In conjunction with appropriate controls, this prob! e is a powerful tool for unravelling pathways that involve reactive oxygen species.
• Dynamic superlubricity and the elimination of wear on the nanoscale
  - Nat Nanotechnol 4(9):586-591 (2009)
  One approach to ultrahigh-density data storage involves the use of arrays of atomic force microscope probes to read and write data on a thin polymer film, but damage to the ultrasharp silicon probe tips caused by mechanical wear has proved problematic. Here, we demonstrate the effective elimination of wear on a tip sliding on a polymer surface over a distance of 750 m by modulating the force acting on the tip–sample contact. Friction measurements as a function of modulation frequency and amplitude indicate that a reduction of friction is responsible for the reduction in wear to below our detection limit. In addition to its relevance to data storage, this approach could also reduce wear in micro- and nanoelectromechanical systems and other applications of scanning probe microscopes.
• Long vertically aligned titania nanotubes on transparent conducting oxide for highly efficient solar cells
  - Nat Nanotechnol 4(9):592-597 (2009)
  Dye-sensitized solar cells consist of a random network of titania nanoparticles that serve both as a high-surface-area support for dye molecules and as an electron-transporting medium. Despite achieving high power conversion efficiencies, their performance is limited by electron trapping in the nanoparticle film. Electron diffusion lengths can be increased by transporting charge through highly ordered nanostructures such as titania nanotube arrays. Although titania nanotube array films have been shown to enhance the efficiencies of both charge collection and light harvesting, it has not been possible to grow them on transparent conducting oxide glass with the lengths needed for high-efficiency device applications (tens of micrometres). Here, we report the fabrication of transparent titania nanotube array films on transparent conducting oxide glass with lengths between 0.3 and 33.0 m using a novel electrochemistry approach. Dye-sensitized solar cells containing these ar! rays yielded a power conversion efficiency of 6.9%. The incident photon-to-current conversion efficiency ranged from 70 to 80% for wavelengths between 450 and 650 nm.
• A novel magnetic crystal–lipid nanostructure for magnetically guided in vivo gene delivery
  - Nat Nanotechnol 4(9):598-606 (2009)
  Cancer gene therapy requires a safe and effective gene delivery system. Polymer- and lipid-coated magnetic nanocrystals have been used to deliver silencing RNA, but synthesizing these magnetic vectors is difficult. Here, we show that a new nanoparticle formulation can be magnetically guided to deliver and silence genes in cells and tumours in mice. This formulation, termed LipoMag, consists of an oleic acid-coated magnetic nanocrystal core and a cationic lipid shell. When compared with the commercially available PolyMag formulation, LipoMag displayed more efficient gene silencing in 9 of 13 cell lines, and better anti-tumour effects when systemically administered to mice bearing gastric tumours. By delivering an optimized sequence of a silencing RNA that targets the epidermal growth factor receptor of tumour vessels, the intended therapeutic benefit was achieved with no evident adverse immune reaction or untoward side effects.

Hot off the presses! Aug 01 Nat Nanotechnol

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Latest Articles Include:

• Nanotubes keep rolling on
  - Nat Nanotechnol 4(8):465 (2009)
  
• Essential features for proactive risk management
  - Nat Nanotechnol 4(8):467-470 (2009)
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• Designs for living
  - Nat Nanotechnol 4(8):471 (2009)
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• Our choice from the recent literature
  - Nat Nanotechnol 4(8):472-473 (2009)
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• Top down bottom up: East meets northwest
  - Nat Nanotechnol 4(8):473 (2009)
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• Nanoelectronics: From droplets to devices
  - Nat Nanotechnol 4(8):475-476 (2009)
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• Scanning tunnelling microscopy: A DNA sequence scanned
  - Nat Nanotechnol 4(8):476-477 (2009)
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• Force microscopy: On the charge
  - Nat Nanotechnol 4(8):477-478 (2009)
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• Microwave sources: Spin-torque oscillators get in phase
  - Nat Nanotechnol 4(8):479-480 (2009)
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• Diamond nanostructures: Isotopes for nanoelectronic devices
  - Nat Nanotechnol 4(8):480-481 (2009)
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• Carbon nanotubes: Sorted by DNA
  - Nat Nanotechnol 4(8):481 (2009)
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• Carbon nanotube tips for atomic force microscopy
  - Nat Nanotechnol 4(8):483-491 (2009)
  The development of atomic force microscopy (AFM) over the past 20 years has had a major impact on materials science, surface science and various areas of biology, and it is now a routine imaging tool for the structural characterization of surfaces. The lateral resolution in AFM is governed by the shape of the tip and the geometry of the apex at the end of the tip. Conventional microfabrication routes result in pyramid-shaped tips, and the radius of curvature at the apex is typically less than 10 nm. As well as producing smaller tips, AFM researchers want to develop tips that last longer, provide faithful representations of complex surface topographies, and are mechanically non-invasive. Carbon nanotubes have demonstrated considerable potential as AFM tips but they are still not widely adopted. This review traces the history of carbon nanotube tips for AFM, the applications of these tips and research to improve their performance.
• Damping of acoustic vibrations in gold nanoparticles
  - Nat Nanotechnol 4(8):492-495 (2009)
  Studies of acoustic vibrations in nanometre-scale particles can provide fundamental insights into the mechanical properties of materials because it is possible to precisely characterize and control the crystallinity and geometry of such nanostructures1, 2, 3, 4. Metal nanoparticles are of particular interest because they allow the use of ultrafast laser pulses to generate and probe high-frequency acoustic vibrations, which have the potential to be used in a variety of sensing applications. So far, the decay of these vibrations has been dominated by dephasing due to variations in nanoparticle size5. Such inhomogeneities can be eliminated by performing measurements on single nanoparticles deposited on a substrate6, 7, 8, 9, but unknown interactions between the nanoparticles and the substrate make it difficult to interpret the results of such experiments. Here, we show that the effects of inhomogeneous damping can be reduced by using bipyramidal gold nanoparticles with hi! ghly uniform sizes10. The inferred homogeneous damping is due to the combination of damping intrinsic to the nanoparticles and the surrounding solvent; the latter is quantitatively described by a parameter-free model.
• Optical nanocrystallography with tip-enhanced phonon Raman spectroscopy
  - Nat Nanotechnol 4(8):496-499 (2009)
  Conventional phonon Raman spectroscopy is a powerful experimental technique for the study of crystalline solids1, 2, 3, 4, 5 that allows crystallography, phase and domain identification6, 7 on length scales down to 1 m. Here we demonstrate the extension of tip-enhanced Raman spectroscopy to optical crystallography on the nanoscale by identifying intrinsic ferroelectric domains of individual BaTiO3 nanocrystals through selective probing of different transverse optical phonon modes in the system. The technique is generally applicable for most crystal classes, and for example, structural inhomogeneities, phase transitions, ferroic order and related finite-size effects occurring on nanometre length scales can be studied with simultaneous symmetry selectivity, nanoscale sensitivity and chemical specificity.
• Structural transformations in graphene studied with high spatial and temporal resolution
  - Nat Nanotechnol 4(8):500-504 (2009)
  Graphene has remarkable electronic properties, such as ballistic transport and quantum Hall effects1, 2, 3, and has also been used as a support for samples in high-resolution transmission electron microscopy4, 5 and as a transparent electrode in photovoltaic devices6. There is now a demand for techniques that can manipulate the structural and physical properties of graphene, in conjunction with the facility to monitor the changes in situ with atomic precision. Here, we show that irradiation with an 80 kV electron beam can selectively remove monolayers in few-layer graphene sheets by means of electron-beam-induced sputtering. Aberration-corrected, low-voltage, high-resolution transmission electron microscopy with sub-ångström resolution is used to examine the structural reconstruction occurring at the single atomic level. We find preferential termination for graphene layers along the zigzag orientation for large hole sizes. The temporal resolution can also be reduced ! to 80 ms, enabling real-time observation of the reconstruction of carbon atoms during the sputtering process. We also report electron-beam-induced rapid displacement of monolayers, fast elastic distortions and flexible bending at the edges of graphene sheets. These results reveal how energy transfer from the electron beam to few-layer graphene sheets leads to unique structural transformations.
• Measurement of the quantum capacitance of graphene
  - Nat Nanotechnol 4(8):505-509 (2009)
  Graphene has received widespread attention due to its unique electronic properties1, 2, 3, 4, 5. Much of the research conducted so far has focused on electron mobility, which is determined by scattering from charged impurities and other inhomogeneities6, 7. However, another important quantity, the quantum capacitance, has been largely overlooked. Here, we report a direct measurement of the quantum capacitance of graphene as a function of gate potential using a three-electrode electrochemical configuration. The quantum capacitance has a non-zero minimum at the Dirac point and a linear increase on both sides of the minimum with relatively small slopes. Our findings—which are not predicted by theory for ideal graphene—suggest that charged impurities also influences the quantum capacitance. We also measured the capacitance in aqueous solutions at different ionic concentrations, and our results strongly indicate that the long-standing puzzle about the interfacial capaci! tance in carbon-based electrodes has a quantum origin.
• Tunable optical forces between nanophotonic waveguides
  - Nat Nanotechnol 4(8):510-513 (2009)
  The confinement of light in components with nanoscale cross-sections in nanophotonic circuits significantly enhances the magnitude of the optical forces experienced by these components1, 2. Here we demonstrate optical gradient forces between two nanophotonic waveguides, and show that the sign of the force can be tuned from attractive to repulsive by controlling the relative phase of the optical fields injected into the waveguides. The optical gradient force could have applications in optically tunable microphotonic devices and nanomechanical systems.
• Determination of protein structural flexibility by microsecond force spectroscopy
  - Nat Nanotechnol 4(8):514-517 (2009)
  Proteins are dynamic molecular machines having structural flexibility that allows conformational changes1, 2. Current methods for the determination of protein flexibility rely mainly on the measurement of thermal fluctuations and disorder in protein conformations3, 4, 5 and tend to be experimentally challenging. Moreover, they reflect atomic fluctuations on picosecond timescales, whereas the large conformational changes in proteins typically happen on micro- to millisecond timescales6, 7. Here, we directly determine the flexibility of bacteriorhodopsin—a protein that uses the energy in light to move protons across cell membranes—at the microsecond timescale by monitoring force-induced deformations across the protein structure with a technique based on atomic force microscopy. In contrast to existing methods, the deformations we measure involve a collective response of protein residues and operate under physiologically relevant conditions with native proteins.
• Partial sequencing of a single DNA molecule with a scanning tunnelling microscope
  - Nat Nanotechnol 4(8):518-522 (2009)
  The scanning tunnelling microscope is capable of the real-space imaging and spectroscopy of molecules on an atomic scale. Numerous attempts have been made to use the scanning tunnelling microscope to sequence single DNA molecules, but difficulties in preparing samples of long-chain DNA molecules on surfaces, and problems in reproducing results have limited these experiments1, 2, 3, 4, 5, 6. Here, we report single-molecule DNA sequencing with a scanning tunnelling microscope by using an oblique pulse-injection method to deposit the molecules onto a copper surface. First, we show that guanine bases have a distinct electronic state that allows them to be distinguished from the other nucleic acid bases. Then, by comparing data on M13mp18, a single-stranded phage DNA, with a known base sequence7, the 'electronic fingerprint' of guanine bases in the DNA molecule is identified. These results show that it is possible to sequence individual guanine bases in real long-chain DNA ! molecules with high-resolution scanning tunnelling microscope imaging and spectroscopy.
• Evidence of intrinsic ferromagnetism in individual dilute magnetic semiconducting nanostructures
  - Nat Nanotechnol 4(8):523-527 (2009)
  Semiconductors doped with magnetic ions, also known as dilute magnetic semiconductors, are both semiconducting and ferromagnetic. It remains unclear, however, whether this ferromagnetism is intrinsic, as is required for spintronic applications, or is due instead to dopant clustering. Here, we report conclusive evidence for intrinsic ferromagnetism in individual ZnO nanoparticles doped with transition metal ions. Through a simultaneous magnetic and microstructural characterization using electron magnetic chiral dichroism and channelling-enhanced electron energy loss microanalysis, respectively, we show that ZnO nanoparticles have intrinsic ferromagnetism when doped with cobalt, but not when doped with iron.
• Phase-locking of magnetic vortices mediated by antivortices
  - Nat Nanotechnol 4(8):528-532 (2009)
  Synchronized spin-valve oscillators may lead to nanosized microwave generators that do not require discrete elements such as capacitors or inductors. Uniformly magnetized oscillators have been synchronized, but offer low power. Gyrating magnetic vortices offer greater power, but vortex synchronization has yet to be demonstrated. Here we find that vortices can interact with each other through the mediation of antivortices, leading to synchronization when they are closely spaced. The synchronization does not require a magnetic field, making the system attractive for electronic device integration. Also, because each vortex is a topological soliton, this work presents a model experimental system for the study of interacting solitons.