Graphene: Against the grain

Nature 469, 389–392 (2011)

Credit: © 2011 NPG

Graphene — a one-atom-thick flat sheet of carbon — can be grown on the metre scale at present. However, such large-scale films are not single crystals, and the honeycomb lattice of the carbon atoms is disrupted by defects and grain boundaries. These grain boundaries are expected to influence the electronic and mechanical properties of the material, but analysing the grain structure of graphene is challenging, not least because there is a 100,000-fold difference between the size of the grains and the size of the atoms that make up the grain boundaries. David Muller and colleagues have now shown that transmission electron microscopy (TEM)-based techniques can be used to characterize both graphene grains and their grain boundaries.

The researchers — who are based at Cornell University, Oregon State University, Brigham Young University and the Kavli Institute at Cornell for Nanoscale Science — were able to image the location and determine the atomic number of every atom at a grain boundary with the help of aberration-corrected annular dark-field scanning TEM. The atomic-scale imaging revealed that different grains were principally connected through pairs of carbon pentagons and heptagons. Moreover, by using dark-field TEM — a diffraction-sensitive imaging technique — the team could quickly map the location, shape and lattice orientation of hundreds of grains and boundaries (see image; the different colours correspond to different lattice orientations).

Muller and colleagues also correlated their analysis of graphene grain structure with scanning probe and transport measurements, which revealed that the presence of the grain boundaries significantly decreases the mechanical strength of the material but has only a limited effect on its electronic properties.

Spintronics: Another tool in the toolkit

Science 330, 1645–1648 (2010)

For spintronic devices to manipulate electron spin, they require the ability to isolate 'up' spins from 'down'. Although this can be done by passing an electronic current through a spin filter (often built with ferromagnets), moving charges involves an energy cost. Marius Costache and Sergio Valenzuela have now experimentally demonstrated a method to generate spin polarization with no net charge transport.

The researchers — who are based at the Campus Universitat Autònoma de Barcelona and ICREA in Spain, and at Harvard University — demonstrated a spin ratchet consisting of a nanoscale superconducting island connected to two regular metal contacts, each with a different resistance. In analogy to a hand-held ratchet, an oscillatory voltage applied across the device resulted in a unidirectional flow of spins in a direction that could be set by a series of applied electrical and magnetic fields. The ratchet effect applied only to spin, and not charge.

By using ferromagnetic contacts with known spin polarization, Costache and Valenzuela measured the efficiency of their spin ratchet to be approximately 50%. And because the device operates on a single electron at a time, it may be useful for spin-based quantum computing as well as in basic physics research.

Porous materials: An exotic approach

Phys. Rev. Lett. 106, 023401 (2011)

Positronium is a short-lived exotic atom that contains an electron and its antiparticle, a positron, bound together. It has a lifetime of about 140 ns and decays into two gamma rays when the electron and positron annihilate each other. One way to make positronium is to direct a beam of positrons at a porous material: the positrons capture electrons from the material and the resulting positronium atoms diffuse into the pores, where they remain trapped until they decay. Physicists are interested in improved methods for the production of positronium because it can be used in a wide range of fundamental experiments, and also as a stepping stone for the production of antihydrogen beams. However, measurements on the positronium can also tell us more about the porous material used to make these exotic atoms in the first place.

David Cassidy and colleagues at the University of California Riverside and San Diego State University have now used laser spectroscopy to measure the 1s–2p transition in positronium atoms trapped in nanoporous silicon. They find that trapping the atoms reduces the linewidth of this transition by a factor of about two compared with measurements made in vacuum, and also increases the energy separation of the 1s and 2p levels. The results are consistent with a pore size of about 5 nm in the silica. The AEGIS collaboration at CERN has also explored positronium in a variety of other porous materials including Vycor, metal organic frameworks and aerogels (J. Phys. Conf. Ser. 199, 012009; 2010).

Nanobiotechnology: Nano boost for tomatoes

Proc. Natl Acad. Sci. USA doi: 10.1073/pnas.1008856108 (2010)

Understanding the impact of engineered nanomaterials on plants and the environment is a major challenge in nanotechnology. Two years ago, for example, Mariya Khodakovskaya of the University of Arkansas and colleagues showed that single-walled carbon nanotubes can lead to the enhanced growth of tomato plants. Now Khodakovskaya and colleagues at Arkansas have explored the effects of nanoparticles on plants in greater detail using a combination of genetic, photothermal and photoacoustic techniques.

The Arkansas team started by growing tomato plants in growth media that contained one of four forms of carbon — active carbon, few-layer graphene, single-wall nanotubes and multiwall nanotubes. The multiwall nanotubes had the biggest impact on the growth of the plants so the team subjected roots, leaves and fruit from these plants to further analysis, and compared the results with a control sample that did not contain any nanotubes after both had been growing for ten days.

Photothermal and photoacoustic techniques revealed the presence of the nanotubes in the roots, leaves and fruit to the level of single nanoparticles and cells. The genetic analysis revealed effects not seen in the control sample, such as activation of a number of stress-related genes, including the gene for tomato water-channel protein. The activation of this gene had a significant impact on the germination of the seeds and the growth of the seedlings.