This Issue

    Cover story

    The superconducting quantum interference device (SQUID) is used to measure magnetic fields in many areas of research and in applications as diverse as medicine and geology. Although improving device performance is the subject of ongoing research, there is also growing interest in the ability of SQUIDs to explore electronics in the quantum regime. Wolfgang Wernsdorfer, Jean-Pierre Cleuziou and colleagues have now made a nanoSQUID, which could have an impact on both these fronts. The nanoSQUID consists of a loop of superconducting aluminium that also contains a single-walled carbon nanotube, which acts as a Josephson junction in two places. The electronic properties of the nanotubes can be tuned by applying voltages to three gates, which makes it possible to control the supercurrent through the loop, and even change its direction. Moreover, it could be possible to detect magnetic changes in single molecules with future devices. [Article p53 ; News & Views p15 ]

    Nanotubes do the twist

    Carbon nanotubes can be metallic or semiconducting depending on the direction in which the graphite sheet has been rolled up to form them. In the past it has been shown that the electronic properties of nanotubes can be changed by applying strain. Now Ernesto Joselevich and co-workers have shown that the application of mechanical torsion — that is, twisting the nanotubes — can lead to much larger changes in conductance. The experiments were performed with multiwalled nanotubes in which the bulk of the current and the torque are carried by the outermost layer. This behaviour could have applications in nanoelectromechanical systems (NEMS). [Letter p36 ]

    Not all viruses are bad

    The tobacco mosaic virus has played a central role in the development of structural biology, microscopy and modern virology. Researchers have now shown that this virus could have applications in nanoelectronics as well. Yang Yang and co-workers decorated the virus with a coat of platinum nanoparticles, embedded it in a polymer and sandwiched the resulting nanostructure between two electrodes. When a voltage was applied, the device displayed an 'on' state that remained stable until the voltage fell below a certain value, resulting in an 'off' state. Although the mechanism underlying this switching action remains unclear, this work represents the first virus-based electronic device. [Article p72 ; News & Views p22 ]

    DNA measures up

    We can learn more about DNA and the processes it is involved in by observing how it changes in size. Existing methods to do this tend to rely on fluorescent or radioactive labels, but there is a demand for simpler techniques. Fanqing Chen and colleagues have shown how the length of a DNA strand can be monitored by attaching it to a gold nanoparticle and measuring how the DNA–gold conjugate scatters light — the wavelength of the plasmon resonance in the conjugate is red-shifted by an amount that depends on the length of the DNA. This 'nanoplasmonic molecular ruler' was used to monitor, in real time, DNA being digested by an enzyme, and to determine the location at which a protein binds to a given DNA strand. [Letter p47 ]

    Sorting out carbon nanotubes

    Carbon nanotubes are promising materials for the construction of nanoelectronic devices. However, progress is perhaps slower than it could be as not all carbon nanotubes are created equal — some conduct electricity better than others. Current synthesis methods make a mixture of metallic and semiconducting nanotubes, and separating them is not straightforward. However, Mark Hersam and co-workers have now developed a scalable method that can sort carbon nanotubes by diameter and electronic type. After being wrapped in a mixture of surfactants, the different nanotubes can be separated by centrifuging them in a liquid that has a density gradient. [Article p60 ; News & Views p17 ]

    Pattern pending

    Various types of one-dimensional nanostructures show great promise as building blocks for advanced semiconductor devices. To enable this, however, we need general fabrication methods that work with these nanostructures. Existing approaches tend to rely on external forces, which make them time-consuming, or linker molecules, which could change the electronic properties of the devices in unexpected ways. Seunghun Hong and colleagues have developed a linker-free method that allows vanadium oxide nanowires and carbon nanotubes to be patterned on a variety of different surfaces, and go on to demonstrate an array of 64,000 nanotube-based junctions. The new approach, which relies on inert surface patterns, is also compatible with existing semiconductor fabrication techniques. [Article p66 ]

    Piezo de resistance

    Silicon nanowires display giant piezoresistance effect

    Most micro- and nanomanipulators are built from piezoresistive materials, which expand or contract under an applied voltage. However, at length scales below 100 nm, surface states may have a pronounced effect on the piezoresistive properties, particularly in semiconductors. Rongrui He and Peidong Yang have explored this question further by measuring piezoresistance in suspended silicon nanowires. The authors find a significant increase in piezoresistivity, compared with bulk silicon, in wires with diameters of less than 300 nm. This 'giant' piezoresistance effect may have applications in high-frequency resonators and other NEMS devices. [Letter p42 ]

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    This Issue. Nature Nanotech 1, v (2006).

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