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.