Letters in 2009

Single electrons and single holes have been confined inside a double quantum dot in a carbon nanotube for the first time. A previously unknown type of tunnelling that is analogous to that in the Klein paradox of relativistic quantum mechanics has also been observed.

The first direct measurements of dopant concentrations in arbitrary regions of individual nanowires are reported. Decomposition rates of heterogeneous precursors cause a heavily doped shell to surround an underdoped core. A thermodynamic model relating liquid and solid compositions to dopant fluxes is also presented.

Force spectroscopy allows measurement of reaction rates as a function of the restoring force in molecules that have been stretched or compressed, but at present this approach lacks the temporal and spatial resolution to study systematically the reactivities of small functional groups. A molecular force probe — stiff stilbene — that extends force spectroscopy to the size scale of such reactions has now been reported.

Predicting and controlling the functions in self-organized biomolecular nanostructures is a major challenge in systems biology. Now researchers have developed DNA scaffolds for the topological organization of different enzymes or cofactor-enzyme pairs. The organization of the biomolecules leads to the activation of enzyme cascades that do not occur in non-organized mixtures, and the reactivity of the system can be controlled by the DNA template.

When a semiconductor is subjected to pressure, its mechanical and electrical properties change. Now, the observation of a previously undetected current spike during the nanoscale deformation of gallium arsenide calls for a significant revision of our understanding of nanoscale plasticity.

DNA base pairs are held together by hydrogen bonds. It has now been shown that a scanning tunnelling microscope can be used to measure the strength of hydrogen bonding in such base pairs. These results provide a basis for new types of electronic biosensors and chemosensors.

Metal–insulator–metal electrostatic nanocapacitors can be fabricated in anodic aluminum-oxide nanopores using atomic layer deposition. This approach gives a planar capacitance of up to ∼100 µF cm⁻² — substantially higher than previously reported values for nanostructured electrostatic capacitors.

Silicon nanowires could be central components in electronic and thermoelectric devices, but understanding nanowire surface properties and dopant distribution will be essential for making reproducible high-performance devices. Present methods for determining these parameters are problematic. Now, by using capacitance-voltage analysis, the radial profile and interface state density of silicon-nanowire field-effect transistors have been measured.

Early detection of the protein marker troponin I can reduce the risk of death from heart attacks. A three-dimensional assay based on engineered viral nanoparticles and nickel nanohairs is six to seven orders of magnitude more sensitive than conventional assays.

Molecular-scale switches will be central components in nanoscale electronic devices. Switching in single-molecule junctions has so far been achieved through changes in the conformation or charge state of the molecule. Now, reversible binary switching has been demonstrated by mechanical control of the metal–molecule contact geometry—a mechanism which could form the basis for a new class of mechanically activated single-molecule switches.

Carbon nanotubes and graphene are potential components for nanoscale electronic devices, but power dissipation — a significant issue for high-density electronic circuits — is not fully understood in such materials. Researchers have now mapped the electrically excited phonon populations and the power dissipation pathways in a working carbon nanotube transistor.

A mesoporous silicon double layer with different pore sizes functions as a nanoreactor that can isolate, filter and quantify the kinetics of enzyme reactions in real-time by optical reflectivity. This tiny reactor may be used to rapidly characterize a variety of isolated enzymes in a label-free manner.

Structural DNA nanotechnology offers a powerful route to the dynamic and functional control of specific molecular species. Researchers have now demonstrated a dynamic form of patterning wherein a pattern component is captured between two independently programmed DNA devices. A simple and robust error-correction protocol that yields programmed targets in all cases has also been developed.

Stretching experiments on single molecules offer a unique way to study the fundamental theories of statistical mechanics. Researchers have now shown that entangled calix[4]arene dimers can be used in such experiments as a tuneable model system for investigating the strength of hydrogen bonds on a single-molecule level.

Indium tin oxide (ITO) is widely used as a transparent conducting coating, but it has been difficult to combine electrical conductivity with good optical properties in the visible region. Researchers have now created layers of ITO nanowires that show optimum electronic and optical properties, and have demonstrated their use as fully transparent top contacts for light-emitting devices.

Current-induced forces in atomic wires are shown to be non-conservative, which means that they are able to do net work and to drive atomic-scale motors. Numerical simulations are presented of a motor that turns like a waterwheel when current runs through it

There is a requirement for site-specific and on-demand cooling in a wide array of electronic, optoelectronic and bioanalytical applications. Thermoelectric coolers, fabricated from nanostructured superlattices based on bismuth and tellurium, have now been integrated into state-of-the-art electronic packages in the first demonstration of a viable chip-scale refrigeration technology.

The interplay between atomic and electronic structure, in association with applied mechanical stress, can lead to surprising differences between the atomic arrangements found in nanoscale and macroscopic structures. The spontaneous formation of the smallest possible metal nanotube with a square cross-section has now been observed during the elongation of silver nanocontacts.

A ‘smart dust’ biosensor — composed of tiny particles that can move, sense and process signals — can capture, tag and transport analytes to a detection region. Built from antibody-functionalized microtubules and kinesin motors powered by caged ATP molecules, this small and autonomous sensor could have many applications in biomedicine and biodefence.

A voltage-induced symmetry change in a ferromagnetic material can change its magnetization or magnetic anisotropy, but these effects are too weak to be used in memory devices. Researchers have now shown that a relatively small electric field can cause a large change in the magnetic anisotropy of a few atomic layers of iron. The results could lead to low-power logic devices and non-volatile memory cells.