Self-assembly: Polymerizing particles

Science 329, 197–200 (2010)

Credit: © 2010 AAAS

New functional materials can be created by organizing nanoscale building blocks, such as nanoparticles, into precise structures. Assemblies of inorganic nanoparticles can, for example, exhibit collective properties that rely on interactions between the individual species. An appealing strategy for preparing these ensembles is self-assembly, but such organization of nanoparticles in a controllable and predictable manner is difficult to achieve. Eugenia Kumacheva, Michael Rubinstein and colleagues at the University of Toronto and the University of North Carolina at Chapel Hill have now shown that the self-assembly of metal nanoparticles can behave in a similar way to well-known polymerization processes; this allows the shape of the resulting nanostructures to be predicted.

The researchers synthesized gold nanorods with arrowhead-shaped ends that were coated with polystyrene molecules. Dispersed in solution, the rods linked up end-to-end to form chains connected by reversible, non-covalent bonds. This self-assembly could be controlled by reducing the solubility of the polystyrene through changes to the composition of the solvent, allowing linear, branched and cyclic chains to be created.

By examining the growth of the chains, Kumacheva and colleagues found that the self-assembly could be described using the kinetics and statistics of step-growth polymerization. This allowed the architecture and aggregation number of the nanorod structures to be predicted.

Cancer detection: Nanoparticles heat up

Nanomed. Nanotechnol. Biol. Med. doi:10.1016/j.nano.2010.06.007 (2010)

Several imaging methods, such as mammography and magnetic resonance imaging, are used to detect cancer at present, but they lack sensitivity and are expensive. Thermography — a method that detects temperature differences on the skin surface when tumours are present — has been proposed for breast cancer, but has low sensitivity for deep and small tumours. Moreover, it is difficult to distinguish tumours from natural 'hot spots' that result from, for example, inflammation. Now Israel Gannot and co-workers at Tel Aviv University show that magnetic nanoparticles can mark tumours for improved detection using thermography.

The idea essentially relies on targeting magnetic nanoparticles to tumours using antibodies, and heating up the tumour using alternating magnetic fields. Changes in the skin temperature above the tumour are measured using an infrared camera and the data are processed using an algorithm to determine the presence of the tumour and estimate its location and depth. Gannot and colleagues tested this by embedding an electrical heat source in an aqueous gel, or by embedding an agar gel containing magnetic nanoparticles inside a piece of pig skin, to simulate a magnetically targeted tumour inside a tissue. The proposed algorithm detected the embedded tumour with high sensitivity but this depended on the tumour depth and the amount of power it emitted. Thermal power of 0.5 W (which is shown to be equivalent to a tumour of 0.5 mm) could be detected as deep as 14 mm below the tissue surface, suggesting potentially huge improvements compared with other methods of cancer detection.

Superconductivity: Mind the gap

Nature Mater. 9, 550–554 (2010)

Credit: © 2010 NPG

Shell effects are familiar in the electronic structure of atoms, and have also been seen in nuclei and clusters of atoms. Now, for the first time, physicists have observed effects owing to degenerate energy levels or shells in superconducting nanoparticles, confirming a number of recent theoretical predictions. The effects were seen in experiments with tin nanoparticles by Sangita Bose, of the Max Planck Institute for Solid State Research in Stuttgart, and co-workers. However, there was no evidence for these effects in experiments with lead nanoparticles.

Bose and co-workers grew tin and lead nanoparticles on a boron nitride/rhodium substrate and used a scanning tunnelling microscope to probe their properties. The nanoparticles had heights in the range of 1–35 nm and were cooled to a temperature of about 1 K to make them superconducting.

The researchers measured how the superconducting gap — the energy needed to split the electron pairs responsible for the superconductivity — varied with the size of the nanoparticle. They found that the gap in tin nanoparticles oscillated as a function of size, reaching values of 60% higher than the gap for bulk tin. Similar oscillations were not observed for lead because it has a much shorter superconducting coherence length.

Solar cells: In hot pursuit

Science 328, 1543–1547 (2010)

Semiconductor photovoltaic cells are usually most efficient under light with the same energy as that of the cell's electronic bandgap. Less-energetic light is not absorbed, whereas more-energetic light creates electron–hole pairs that quickly shed their excess energy by emitting lattice vibrations, or phonons. If, instead, these charges could be extracted before they relax, while they are still 'hot', efficiencies could be greatly increased. Now, Xiaoyang Zhu, Eray Aydil, David Norris and colleagues from the University of Minnesota and the University of Texas at Austin have observed hot-electron transfer from a light-absorbing semiconductor to an electrode.

The team observed this transfer from one or two monolayers of PbSe nanocrystals resting on a substrate made of the commonly used electron acceptor, titania. The small nanocrystal size caused an energy-level spacing that was large relative to phonon energies, which reduced cooling rates. Furthermore, PbSe excited states are spatially large, increasing the chance of electron transfer into the substrate. When light from a red laser was pulsed onto the monolayers, they reflected a blue second-harmonic signal, which was very sensitive to the presence of any electric field generated by charge transfer.

By studying the second-harmonic time dynamics and temperature dependence, the team concluded that hot carriers were being transferred to titania. The effect is expected to be relevant to other semiconductor nanocrystals and conducting substrates and, if combined with a minimization of energy loss in the conducting substrate, may enable highly efficient hot-carrier solar cells to be made.