Cover story

Vol.2 No.1 January 2007

It is well known that the properties of nanoscale materials can differ significantly from those observed in the bulk substance. For nanoclusters containing just tens or hundreds of atoms, structural defects at the surfaces, edges and corners can dominate the physical characteristics. A striking example is found with molybdenum disulphide: whereas the bulk material is relatively unreactive, nanoclusters are highly effective chemical catalysts for the removal of sulphur from fossil fuels. Now, Flemming Besenbacher and co-workers have used atom-resolved scanning tunnelling microscopy to investigate how the structure of triangular MoS2 nanocrystals varies with their size. They find that some clusters are much more stable than others, and that this stability largely depends on the arrangement of the sulphur atoms along the edges. The catalytic activity is expected to show similar variation, which may have implications for industrial processes. [Article p53 ; News & Views p21 ]

Rare bits

Making a quantum bit or 'qubit' is the starting point in any attempt to make a quantum computer that could, in principle, outperform classical computers on certain tasks by exploiting phenomena such as superposition and entanglement. Qubits have already been demonstrated with a number of systems including superconducting circuits, semiconductor quantum dots and ensembles of electron spins. Bernard Barbara and colleagues now demonstrate a new type of electron-spin qubit that is based on rare-earth ions. Working with erbium ions in a crystal of CaWO4, they report values of the phase coherence time and single qubit figure-of-merit which suggest that rare-earth qubits may be suitable for scalable quantum information processing at liquid helium temperatures. [Letter p39 ]

Poring over ceramics

Ceramics are non-metallic materials that are widely used in applications that involve harsh chemical environments or high temperatures. Now, Julin Wan, Patrick Malenfant and co-workers have shown how the structure of non-oxide ceramics, such as nitrides, can be controlled at the nanoscale. A block copolymer — comprising a sacrificial organic block joined to a hybrid organic–inorganic block — can be self-assembled into either cylindrical or lamellar structures, depending upon the choice of solvent. By burning away the organic sections at high temperatures, ceramic materials retaining the original morphology are formed. Their chemical composition can also be controlled by changing the atmosphere in which they are heated. The surface area of one gram of the porous boron nitride produced in this way is equivalent to five tennis courts and is the highest reported value to date for this material. [Letter p43 ]

Nanotubes target tumours

The increasing use of single-walled carbon nanotubes in a variety of applications means their safety is now under intense scrutiny. New studies in mice show that nanotubes used to target cancer tumours circulate in the blood longer than previously thought. Hongjie Dai, Xiaoyuan Chen and co-workers decorated nanotubes with a water-soluble polymer, a radioactive nuclide for imaging, and specific peptides for targeting the tumours. When injected into mice, the nanotubes accumulated in the tumour, liver and spleen with no apparent negative health effects. Positron emission tomography and the biodistribution data demonstrated the efficacy and safety of nanotubes modified in this way. Although the long-term effects remain unclear, this work sheds new light on the behaviour of single-walled nanotubes in living systems. [Letter p47 ; News & Views p20 ]

Close confinement

Surface effects are thought to play a dominant role in determining the elasticity and other mechanical properties of polymer nanofibres. However, Eyal Zussman and co-workers now argue that this might not be the case. They show that the sudden increase in Young's modulus can be explained by considering how the macromolecular chains in the polymer order themselves, and by comparing the typical size of these supramolecular structures with the diameter of the fibre. Their model suggests that when the size of the regions containing oriented polymer chains is similar to the fibre diameter — in other words, when the supramolecular structure is confined — noticeable variations in both the mechanical and thermal properties are observed. [Article p59 ]

Hot topic

Thermal light emission from carbon nanotubes.

When measuring the resistivity of something it is usually wise to avoid heating the sample with the current, but sometimes hot objects are more interesting. Hongjie Dai and colleagues study light emission from suspended single-walled nanotubes that are biased with a few volts. The voltage accelerates electrons in the nanotube, which then lose energy in the form of light. The tubes are quasi-metallic, and would be expected to lose energy through processes other than radiation. In general it is difficult to detect light directly from just a single nanotube, but well-defined optical emission peaks — that are characteristic of a one-dimensional system — are observed in this study. The hot-electron driven optical emission also lets the authors explore vibrational modes (hot phonons) that are out of thermal equilibrium with the rest of the nanotube. [Letter p33 ]