Our choice from the recent literature

Photonic nanostructures: Laser tuning

Nature 462, 633–636 (2009)

Credit: © 2009 NPG

The optical forces from lasers can be used to move, excite and cool nanostructures. Another potentially useful application of laser-induced forces would be to change the optical responses of photonic structures, but such work has proved to be difficult because it requires quite large forces to sufficiently change the structural geometry. Now, a team at Cornell University led by Michal Lipson have designed a device whose optical response changes significantly under relatively weak forces from a laser.

The researchers used electron beam lithography to fabricate a wheel-like silicon nitride structure comprising two concentric rings 30 μm in diameter and just 190 nm thick, attached to a central pedestal by thin spokes (see image). They used a pump laser to apply an optical force to the structure and a weak probe laser to monitor changes in the optical response of the cavity between the rings, which is very sensitive to the distance between rings.

Their results showed that a pumped laser with just 3 mW of power applied enough force to change the distance between the rings by up to 20 nm, corresponding to a large optical resonance shift in the cavity. This response implies that similar devices could act as versatile components in complex optical circuits for future micromechanical systems.

X-ray optics: Hard work pays off

Nature Phys. doi:10.1038/nphys1457 (2009)

Hard X-rays are capable, in theory, of probing the structure, composition and chemical bonding states of materials and biological samples with single-nanometre spatial resolution. However, researchers have not been able to achieve such resolutions in experiments because the extremely short wavelengths involved mean that even the tiniest imperfections on the surface of an optical component can introduce aberrations into the X-ray beam. Now Hidekazu Mimura of Osaka University and co-workers have shown that a technique known as wavefront correction can be used to focus hard X-rays to a spot size of 7 nm.

In experiments at the SPring-8 synchrotron radiation source in Japan, Mimura and co-workers directed a beam of 20 keV X-rays onto a deformable mirror that reflected the beam onto a multilayer focusing mirror, and they measured the intensity profile of the beam reflected by this second mirror. The Japanese team converted these measurements into phase shifts, and then changed the shape of the deformable mirror to compensate for the distortions caused by the two mirrors, repeating the process until they had broken the 10 nm barrier. Mimura and co-workers believe that it should be possible to produce focal spot sizes as small as 1 nm.

Zeolite synthesis: Added value

Angew. Chem. Int. Ed. doi:10.1002/anie.200905214 (2009)

Credit: © 2009 Wiley

Zeolites — microporous crystalline solids with pore diameters of approximately 1 nm — are used as catalysts in many industrial reactions. Such materials can manipulate the selectivity of a catalytic reaction by restricting the size and shape of the molecules that can access and leave the active sites. RTH-type zeolite was discovered in 1995 and was expected to show interesting catalytic properties owing to its unique structure (see image). However, only two examples of this type of zeolite have so far been reported and applications are limited. Takashi Tatsumi and colleagues at Tokyo Institute of Technology have now notably extended the range of RTH-type zeolites and the ways in which they can be synthesized.

The researchers began by incorporating aluminium atoms into the framework of the borosilicate zeolite [B]-RUB-13, which was the first RTH-type zeolite to be reported. They then showed that such zeolites could be prepared without using organic structure-directing agents. Structure-directing agents are often used in the synthesis of zeolites, but their use could be problematic for the development of industrial applications. The key to the synthesis was to use sodium hydroxide along with calcined zeolite crystals as seeds, and water. The resulting RTH-type zeolites were named 'TTZ-1' (Tokyo Tech. zeolite).

The catalytic properties of the new zeolites were tested in the conversion of methanol to alkenes, and were found to be active and selective towards the production of propene — an important chemical for the polymer industry.

Nanowire growth: Solid progress

Science 326, 1247–1250 (2009)

Most applications of semiconductor nanowires require the composition of the nanowire to change along its length so as to build in features such as quantum dots and field-effect transistors. Moreover, the junctions between the different regions need to be abrupt and free from defects. This is routinely done for pairs of iii–v semiconductors such as indium arsenide and indium phosphide, but until now it has not been possible to grow nanowires with abrupt junctions between different group iv semiconductors, such as silicon and germanium. Now Frances Ross and co-workers at IBM, Purdue University and University of California, Los Angeles have grown such nanowires.

Silicon and germanium nanowires are usually grown from gaseous silane (SiH₄) or germane (GeH₄) with the help of a liquid-metal catalyst. The gas is dissociated by the catalyst and the Si or Ge dissolves in the liquid metal, before precipitating out in the form of a nanowire when its concentration reaches a certain value. Changing the gas from SiH₄ to GeH₄ will, therefore, result in a Si–Ge junction. However, the interface between the two regions will be diffuse because large amounts of Si, for example, can be dissolved in the catalyst, and will continue to be included in the growing nanowire after the SiH₄ has been replaced with GeH₄. Ross and co-workers overcome this problem by using solid aluminium–gold alloy nanoparticles as catalysts. The solubility of Si and Ge is very low in this alloy, leading to abrupt junctions in the nanowires.

The definitive versions of these Research Highlights first appeared on the Nature Nanotechnology website, along with other articles that will not appear in print. If citing these articles, please refer to the web version.