Hot off the presses! Aug 01 Nat Nanotechnol

The Aug 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:

• 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.