Collecting the sun
Sunlight provides us with an endless source of energy and there is little doubt future societies will need to rely, at least in part, on artificial solar power. The reason why not yet every house has a solar panel on its roof is the cost. But instead of covering a whole panel with expensive solar cells, sunlight could be trapped by a much cheaper material that concentrates light. One such concentrator contains dye particles that absorb sunlight and emit it again towards nearby solar cells. This idea has been around for a few decades, but so far has faced a practical hurdle: a large portion of the light emitted by the dye gets absorbed again before it reaches the solar cells.
Researchers at Western Washington University have now tried a new type of dye for solar concentration, namely semiconductor nanocrystals that absorb only ultraviolet light and re-emit it as yellow light, so that it is not reabsorbed by the dye. The nanocrystals are embedded within a transparent polymer sheet. Preliminary results suggest that the material can concentrate sunlight by up to six times.
Because the solar concentrator sheet is transparent, it could be applied as a window coating in future smart building designs.
Bionic spinach
Plants have much more efficient machinery for harvesting sunlight than any human-made solar energy device. However, they use only a small part of the solar spectrum and so scientists at MIT suggest that adding carbon nanotubes, which capture light over a wide spectral range from ultraviolet to near-infrared, could enhance natural photosynthesis.
The team tried this out on spinach plants and found that injecting leaves with nanotubes boosted their "photocurrent" – a measure of their photosynthetic activity – by 30%. (The team also showed that adding nanotubes can turn plant leaves into real-time detectors for environmental pollutants such as nitric oxide.)
Writing in the journal Nature Materials, they conclude that researchers trying to design efficient, biologically inspired light-harvesting devices could get some useful clues from studying the influence of nanoparticles on natural photosynthesis.
A cash machine that strikes back
Thieves will think twice about trying to break into an ATM that spoils and marks all the banknotes when the machine is tampered with.
A team at ETH Zurich in Switzerland, has designed a foil that oozes hot foam when it is ruptured. It has two layers, each with a sealed compartment containing a different chemical, namely hydrogen peroxide and manganese oxide. A mechanical impact breaks the ultrathin barrier between the two compartments so that the chemicals mix and react to produce the foam, which stains banknotes.
The researchers included nanoparticle labels in the manganese oxide compartment so that the banknotes can be forensically traced.
The self-destructing foil may be an attractive alternative to existing electronic security systems, which are expensive and complex.
Nanotube computers
Ever since the birth of the integrated circuit, computers have got steadily smaller, faster and cheaper. Underlying this trend has been the gradual shrinking of the silicon transistor, the basic building block of an electronic chip.
However, the silicon transistor is reaching its physical limit of smallness, beyond which it cannot function properly without overheating. The semiconductor industry is therefore considering different designs that may continue the upward trend in computer functionality, for example transistors made from carbon nanotubes, which operate much more energy efficiently.
In 2013, the first complete nanotube computer was presented by researchers at Stanford. However, the transistors were enormous, comparable to what was state-of-the-art for silicon technology in the 70s.
The Stanford team has caught up a few decades and now produces nanotube transistor circuits with a "channel length" – which defines the transistor size – down to 20 nanometres.
The team has designed a complete working application – infrared sensing – using nanotube circuits that is compatible with fabrication technology introduced for silicon chips in 2010.
Invisible barcodes
Adulterating expensive oils has become a lucrative criminal activity, for example passing off cheap mixtures as extra virgin olive oil or cosmetic oils. One way to combat this new crime wave is to tag oils with invisible, harmless labels that contain information about their origin. Researchers from ETH Zurich in Switzerland have designed microscopic labels that can be mixed into oils and retrieved for testing from small samples using magnets.
Taking inspiration from nature's highly efficient information storage and retrieval system, they use strands of artificial DNA as barcodes. These are attached to iron oxide nanoparticles, so the barcodes can be separated from the oil in a magnetic field, and encased in a protective shell of silica. To read the labels, the silica shell is dissolved by a chemical and the DNA inside is sequenced.
The procedure has been tested on samples of bergamot oil, gasoline and olive oil, demonstrating that only very small concentrations of the labels are required to authenticate the oils and detect any illegal dilution.
The labels will need to pass toxicity tests before they can be used commercially, but this should prove straightforward as both silica and iron oxide are already approved food additives.
