Hot off the presses! May 01 Nat Nanotechnol

The May 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:

• May the force be with you
  - Nat Nanotechnol 4(5):271 (2009)
  
• Mind the gap revisited
  - Nat Nanotechnol 4(5):273-274 (2009)
  
• Small differences
  - Nat Nanotechnol 4(5):275 (2009)
  
• Our choice from the recent literature
  - Nat Nanotechnol 4(5):276-277 (2009)
  
• Top down bottom up: Magnetic attraction
  - Nat Nanotechnol 4(5):277 (2009)
  
• Nanoelectronics: Oxides offer the write stuff
  - Nat Nanotechnol 4(5):279-280 (2009)
  
• Nanopatterning: What diffraction limit?
  - Nat Nanotechnol 4(5):280 (2009)
  
• DNA nanotechnology: A nanomachine goes live
  - Nat Nanotechnol 4(5):281-282 (2009)
  
• Nanowires: Keeping track of dopants
  - Nat Nanotechnol 4(5):282-283 (2009)
  
• Graphene production: From nanotubes to nanoribbons
  - Nat Nanotechnol 4(5):283 (2009)
  
• Mechanochemistry: Force probes in a bottle
  - Nat Nanotechnol 4(5):284-285 (2009)
  
• An electric current spike linked to nanoscale plasticity
  - Nat Nanotechnol 4(5):287-291 (2009)
  The increase in semiconductor conductivity that occurs when a hard indenter is pressed into its surface has been recognized for years1, 2, 3, 4, 5, and nanoindentation experiments have provided numerous insights into the mechanical properties of materials. In particular, such experiments have revealed so called pop-in events, where the indenter suddenly enters deeper into the material without any additional force being applied; these mark the onset of the elastic–plastic transition6, 7, 8, 9, 10, 11. Here, we report the observation of a current spike—a sharp increase in electrical current followed by immediate decay to zero at the end of the elastic deformation—during the nanoscale deformation of gallium arsenide. Such a spike has not been seen in previous nanoindentation experiments on semiconductors1, 2, 3, 4, 5, and our results, supported by ab initio calculations, suggest a common origin for the electrical and mechanical responses of nanodeformed gallium arse! nide. This leads us to the conclusion that a phase transition is the fundamental cause of nanoscale plasticity in gallium arsenide12, and the discovery calls for a revision of the current dislocation-based understanding of nanoscale plasticity6, 7, 8, 9, 10, 11.
• Nanotubular metal–insulator–metal capacitor arrays for energy storage
  - Nat Nanotechnol 4(5):292-296 (2009)
  Nanostructured devices have the potential to serve as the basis for next-generation energy systems that make use of densely packed interfaces and thin films1. One approach to making such devices is to build multilayer structures of large area inside the open volume of a nanostructured template. Here, we report the use of atomic layer deposition to fabricate arrays of metal–insulator–metal nanocapacitors in anodic aluminium oxide nanopores. These highly regular arrays have a capacitance per unit planar area of approx10 microF cm-2 for 1-microm-thick anodic aluminium oxide and approx100 microF cm-2 for 10-microm-thick anodic aluminium oxide, significantly exceeding previously reported values for metal–insulator–metal capacitors in porous templates2, 3, 4, 5, 6. It should be possible to scale devices fabricated with this approach to make viable energy storage systems that provide both high energy density and high power density.
• Tunnelling readout of hydrogen-bonding-based recognition
  - Nat Nanotechnol 4(5):297-301 (2009)
  Hydrogen bonding has a ubiquitous role in electron transport1, 2 and in molecular recognition, with DNA base pairing being the best-known example3. Scanning tunnelling microscope images4 and measurements of the decay of tunnel current as a molecular junction is pulled apart by the scanning tunnelling microscope tip5 are sensitive to hydrogen-bonded interactions. Here, we show that these tunnel-decay signals can be used to measure the strength of hydrogen bonding in DNA base pairs. Junctions that are held together by three hydrogen bonds per base pair (for example, guanine–cytosine interactions) are stiffer than junctions held together by two hydrogen bonds per base pair (for example, adenine–thymine interactions). Similar, but less pronounced effects are observed on the approach of the tunnelling probe, implying that attractive forces that depend on hydrogen bonds also have a role in determining the rise of current. These effects provide new mechanisms for making s! ensors that transduce a molecular recognition event into an electronic signal.
• A molecular force probe
  - Nat Nanotechnol 4(5):302-306 (2009)
  Force probes1 allow reaction rates to be measured as a function of the restoring force in a molecule that has been stretched or compressed. Unlike strain energy2, approaches based on restoring force allow quantitative molecular understanding3 of phenomena as diverse as translation of microscopic objects by reacting molecules4, 5, 6, crack propagation7, 8 and mechanosensing9. Conceptually, localized reactions offer the best opportunity to gain fundamental insights into how rates vary with restoring forces, but such reactions are particularly difficult to study systematically using microscopic force probes10, 11, 12, 13, 14. Here, we show how a molecular force probe, stiff stilbene, simplifies force spectroscopy of localized reactions. We illustrate the capabilities of our approach by validating the central postulate of chemomechanical kinetics15—force lowers the activation barrier proportionally to the difference in a single internuclear distance between the ground an! d transition states projected on the force vector—on a paradigmatic unimolecular reaction: concerted dissociation of the C–C bond.
• Three-dimensional imaging of short-range chemical forces with picometre resolution
  - Nat Nanotechnol 4(5):307-310 (2009)
  Chemical forces on surfaces have a central role in numerous scientific and technological fields, including catalysis1, 2, thin film growth3 and tribology4, 5. Many applications require knowledge of the strength of these forces as a function of position in three dimensions, but until now such information has only been available from theory2. Here, we demonstrate an approach based on atomic force microscopy that can obtain this data, and we use this approach to image the three-dimensional surface force field of graphite. We show force maps with picometre and piconewton resolution that allow a detailed characterization of the interaction between the surface and the tip of the microscope in three dimensions. In these maps, the positions of all atoms are identified, and differences between atoms at inequivalent sites are quantified. The results suggest that the excellent lubrication properties of graphite may be due to a significant localization of the lateral forces.
• Dopant profiling and surface analysis of silicon nanowires using capacitance–voltage measurements
  - Nat Nanotechnol 4(5):311-314 (2009)
  Silicon nanowires are expected to have applications in transistors, sensors, resonators, solar cells and thermoelectric systems1, 2, 3, 4, 5. Understanding the surface properties and dopant distribution will be critical for the fabrication of high-performance devices based on nanowires6. At present, determination of the dopant concentration depends on a combination of experimental measurements of the mobility and threshold voltage7, 8 in a nanowire field-effect transistor, a calculated value for the capacitance, and two assumptions—that the dopant distribution is uniform and that the surface (interface) charge density is known. These assumptions can be tested in planar devices with the capacitance–voltage technique9. This technique has also been used to determine the mobility of nanowires10, 11, 12, 13, but it has not been used to measure surface properties and dopant distributions, despite their influence on the electronic properties of nanowires14, 15. Here, we m! easure the surface (interface) state density and the radial dopant profile of individual silicon nanowire field-effect transistors with the capacitance–voltage technique.
• Direct measurement of dopant distribution in an individual vapour–liquid–solid nanowire
  - Nat Nanotechnol 4(5):315-319 (2009)
  Semiconductor nanowires show promise for many device applications1, 2, 3, but controlled doping with electronic and magnetic impurities remains an important challenge4, 5, 6, 7, 8. Limitations on dopant incorporation have been identified in nanocrystals9, raising concerns about the prospects for doping nanostructures9, 10. Progress has been hindered by the lack of a method to quantify the dopant distribution in single nanostructures. Recently, we showed that atom probe tomography can be used to determine the composition of isolated nanowires11, 12. Here, we report the first direct measurements of dopant concentrations in arbitrary regions of individual nanowires. We find that differences in precursor decomposition rates between the liquid catalyst and solid nanowire surface give rise to a heavily doped shell surrounding an underdoped core. We also present a thermodynamic model that relates liquid and solid compositions to dopant fluxes.
• Phonon populations and electrical power dissipation in carbon nanotube transistors
  - Nat Nanotechnol 4(5):320-324 (2009)
  Carbon nanotubes and graphene are candidate materials for nanoscale electronic devices1, 2. Both materials show weak acoustic phonon scattering and long mean free paths for low-energy charge carriers. However, high-energy carriers couple strongly to optical phonons1, 3, which leads to current saturation4, 5, 6 and the generation of hot phonons7. A non-equilibrium phonon distribution has been invoked to explain the negative differential conductance observed in suspended metallic nanotubes8, while Raman studies have shown the electrical generation of hot G-phonons in metallic nanotubes9, 10. Here, we present a complete picture of the phonon distribution in a functioning nanotube transistor including the G and the radial breathing modes, the Raman-inactive zone boundary K mode and the intermediate-frequency mode populated by anharmonic decay. The effective temperatures of the high- and intermediate-frequency phonons are considerably higher than those of acoustic phonons, ! indicating a phonon-decay bottleneck. Most importantly, inclusion of scattering by substrate polar phonons is needed to fully account for the observed electronic transport behaviour.
• A DNA nanomachine that maps spatial and temporal pH changes inside living cells
  - Nat Nanotechnol 4(5):325-330 (2009)
  DNA nanomachines are synthetic assemblies that switch between defined molecular conformations upon stimulation by external triggers. Previously, the performance of DNA devices has been limited to in vitro applications. Here we report the construction of a DNA nanomachine called the I-switch, which is triggered by protons and functions as a pH sensor based on fluorescence resonance energy transfer (FRET) inside living cells. It is an efficient reporter of pH from pH 5.5 to 6.8, with a high dynamic range between pH 5.8 and 7. To demonstrate its ability to function inside living cells we use the I-switch to map spatial and temporal pH changes associated with endosome maturation. The performance of our DNA nanodevices inside living systems illustrates the potential of DNA scaffolds responsive to more complex triggers in sensing, diagnostics and targeted therapies in living systems.