Introduction
NEMS: Higher and higher
Phys. Rev. Lett. 97, 087203 (2006)
Making devices that operate at room temperature and atmospheric pressure is a challenge in many areas of science, with many sensitive pieces of equipment only working at cryogenic temperatures or under vacuum conditions. However, physicists at Berkeley in the US have now built a nanoelectromechanical system (NEMS) that can detect mass with attogram (10-18 g) resolution under ambient conditions.
The NEMS approach uses electric currents to drive and detect the motion of tiny mechanical structures such as beams and cantilevers. In the Berkeley device, Alex Zettl and co-workers suspend a carbon nanotube across a trench in a silicon-based substrate and connect it to source and drain electrodes on either side of the trench.
ROBERT J. A. RAMIREZ
Zettl and colleagues monitor how the current through the nanotube varies as signals of different frequency are applied to the drain and a gate electrode at the bottom of the trench. They find that the device has a fundamental frequency of 1.3 GHz, which is a record for a NEMS device. Moreover, when a small amount of iron is deposited onto the nanotube, the resonance frequency changes significantly, so the device could be used as a highly sensitive mass detector.
Molecular electronics: Do the twist
Nature 442, 904–907 (2006)
Exploiting the electronic properties of single molecules in computing devices is one of the main goals of nanotechnology. However, for this to happen we need to understand the transport of electrons through molecules in fine detail. Researchers at Columbia University in the US have taken a major step in this direction by showing that the conductance of certain molecules depends on their shape.
Latha Venkataraman and colleagues start by breaking a gold wire in a solution containing the molecules to form metal–molecule–metal junctions. The first challenge in such experiments is to make sure that there is just one molecule between the electrodes. It is also important to ensure that the properties of the molecules, not the electrodes, are being measured. Recently, the Columbia team showed that the use of amine groups (which contain nitrogen atoms) to link the molecules to the electrodes overcame many of the problems encountered in such experiments.
Now, they have measured the conductance of a variety of molecules that contain two rings of six carbon atoms (known as phenyl groups) linked together by a single carbon–carbon bond. The simplest of these molecules is essentially flat, but on adding different atoms onto the carbon rings the molecule twists to different angles about the carbon–carbon bond. Venkataraman and colleagues show that the conductance of the molecule decreases as the twist angle increases. Indeed, the conductance varies with the square of the cosine of the angle, as had been predicted by theorists.
Biotechnology: Axon, ax-off
Conventional techniques for detecting and controlling electronic signal transfer in a neuron use micropipettes. However, multiplexing this process is difficult. Now, Charles Lieber and colleagues at Harvard University in the US have made integrated neuron–silicon nanowire transistor arrays to detect, stimulate and even halt neuronal signal transfer.
Numerous device architectures were studied, but the essential structure consists of the neuron body with its axon and dendrites stretched over an array of silicon nanowire transistors. A polylysine pattern directs the dendrite and axon growth along the nanowire array during cell growth and ensures successful neuron–nanowire electrical contact. Measurements are made by stimulating the neuronal action potential at the cell body with a microelectrode and detecting the signal at a given nanowire–neuron junction. Growing the axon or dendrite across a series of nanowires permits the measurement of both signal speed and distortion. Moreover, setting the voltage of one of the nanowires in the series above a certain threshold completely blocks signal propagation.
The importance of this work lies in its evident scalability. Successful growth and signal detection of a neuronal axon across 50 nanowire elements suggests the potential of these devices for designing real-time cellular arrays and for drug testing.
Molecular switching: Let there be light
J. Am. Chem. Soc. 128, 12404–12405 (2006)
Azobenzenes are organic molecules that can be switched between two discrete shapes when you shine light on them. The more stable shape (the linear or trans form) is converted into a less stable arrangement (the bent or cis form) when the molecules are exposed to short-wavelength light (
380 nm). This process is reversed by illumination with light of a longer wavelength (450 nm) or by thermal relaxation.
The switching of azobenzene molecules is usually studied by observing the collective behaviour of a large number of them in solution. Now, Hagan Bayley and Sandra Loudwig of Oxford University have studied the switching of a single azobenzene molecule. When attached to the inside of a protein pore and illuminated with 330-nm light, the molecule can repeatedly flip between the cis and trans forms. It was found that the current generated by ions flowing through the nanopore can have one of two discrete values, and that this current alters in response to the shape of the azobenzene.
Furthermore, the molecule could be fixed in either of the two forms by simply turning off the light at the appropriate time, which means that the system could form the basis of a digital molecular switch.
Toxicology: Food for thought
Nanotechnology 17, 4668–4674 (2006)
There are mixed reports on the toxic and non-toxic effects of carbon nanotubes in living systems. In a departure from most other findings, researchers at the Chinese Academy of Sciences now show that a solution of multiwalled carbon nanotubes (MWNT) can stimulate the growth of a single-celled organism (Tetrahymena pyriformis). This freshwater protozoan is commonly used in laboratory toxicological research and health-risk assessments.
Ying Zhu and colleagues incubated the protozoan with MWNT solutions of different concentrations for up to 48 hours. In a yeast-extract culture medium, the MWNTs stimulated growth of the protozoan by threefold in a concentration-dependent manner. When the yeast medium was replaced with filtered pond water, MWNTs inhibited the growth of the protozoans. Atomic force microsope images and thermogravimetric analysis revealed the formation of conjugates between the nanotubes and peptone — a major food component in the yeast extract. Based on imaging of peptone labelled with a fluorescent tag, Zhu and colleagues suggest that the MWNTs serve as an efficient carrier of this foodstuff into the protozoans, and thereby stimulate their growth.
This work provides new insights into the importance of culture-media components in understanding how carbon nanotubes may affect living systems.
Nanostructures: Say it with flowers
Appl. Phys. Lett. 89, 071113 (2006)
© 2006 AIP
After several years of controversy, there is now a general consensus that the bandgap of indium nitride — a group-III nitride semiconductor — is between 0.7 and 0.9 eV. Its attractive electron-transport properties and narrow bandgap make this material a promising candidate for the production of novel ultrahigh-frequency optoelectronic devices for terahertz communications.
Producing nanosized InN structures raises the possibility of making devices whose operation is based on quantum effects. However, InN nanostructures grown from either indium chloride or indium oxide by metal–organic chemical vapour deposition (MOCVD) often contain unwanted impurities. Now, Ting-Ting Kang and colleagues from the Chinese Academy of Sciences in Beijing have grown hexagonal InN 'nanoflowers' by a 'self-catalysis' method that does not require either of these precursors or any foreign catalysts. In this process, hydrogen gas is deliberately introduced during MOCVD growth to promote the formation of metallic indium, which acts as a catalyst for the formation of InN. Although hydrogen is known to hinder the growth of InN by MOCVD processes, careful control of its flow rate allows the synthesis to proceed.
These results shed new light on the growth mechanism of InN by MOCVD, and will be important for the synthesis of novel nanometre-scale InN device structures.
Conjugated polymers: No loose ends
Adv. Mater. 18, 2461–2465 (2006)
Dendrimers are highly branched molecules with structures that are somewhat reminiscent of trees. A number of discrete arms emanate from a central core and each one branches in a regular repeating fashion. The outer ends of these branches can be decorated with molecules of a different type from those that comprise the rest of the structure. These terminal groups often dominate the properties of the ensemble because they sit on the surface.
Rigoberto Advincula and co-workers from the University of Houston in the US have shown how the carbazole groups at the ends of the branches of a dendrimer can be linked to one another to form an outer shell with interesting optoelectronic properties. This not only results in a polycarbazole conjugated electronic system (an alternating sequence of single and double carbon–carbon bonds), but also makes the dendrimer much more rigid and slightly more compact. Atomic force microscope images of the dendrimers reflect this change; before crosslinking, they adopt a pancake-like shape, whereas after the reaction, they appear more egg-shaped.
This work highlights how molecular crosslinking can be used to reinforce dendrimers, and demonstrates how conjugated systems can be built simultaneously into these nanostructures.
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.
