This is described in an article ("Synthesis of branched 'nanotrees' by controlled seeding of multiple branching events") in the journal Nature Materials, whose Web edition was published on 02 May. The paper edition should be published in June.
"This opens the possibility of producing ever more complex structures on the nano scale, structures that may offer tremendous potential for applications like solar cells, low-energy lighting, sensors, etc. It is also an example of interdisciplinary collaboration between physics and chemistry, that is, between my research team at Fysicum and the one at Kemicentrum headed by Professor Reine Wallenberg," says Professor Lars Samuelson, solid state physicist at Lund Institute of Technology.
He was the one who came up with the idea of nanotrees. The person who actually produced them is one of his doctoral students, Kimberly A. Dick, who came to Lund from Waterloo University, Canada.
The scientists produce nanowires by first creating tiny nanoparticles of gold and placing them on a semiconducting tray. Then they release reactive molecules that contain the atoms that they want the nanowires to consist of-for instance, to make nanowires of silicon (Si) or of alloyed semiconductors such as indium arsenide (InAs) or gallium phosphide (GaP). The reactive molecules seek out the catalytic gold particles and build crystals on the tiny contact surface directly under the gold. Time determines how long the wires will be. The wires are "baked" at 450-500 degrees centigrade. Typically they will be on the scale of a few micrometers (1,000ths of mm) in length and about 50 nanometers thick (about 100 times thinner than the length).
In a second step new gold particles are then sprayed on the nanowires, and the procedure is repeated. Now new "branches" are grown at sites where the gold particles landed. The number of branches grown is determined by the crystal structure of the trunk. There can be three branches at 60-degree angles, four branches at 90-degree angles, and six branches at 30-degree angles. In this way veritable forests of nanotrees can be created.
In a few experiments the researchers have also tried to repeat the procedure a third time, yielding tiny "twigs" or "leaves" on the branches of the trees.
It remains to be seen what applications this new technology can lead to, but the physicists are talking about the possibility of varying the materials that make up the branches, which is also reported in the article in Nature Materials. By doing so, it would be possible to tailor properties of the trees to convert sunlight to electricity. Conversely, the trees could be used to create efficient light panels for lighting roomsthe analogy with Christmas trees is rather striking. Both of these applications would require extremely inexpensive production methods for nanowires and nanotrees, but the researchers are confident that this will be achievable. However, a great deal of basic research needs to be carried out before that.
"Maybe we can get nanoleaves to replicate plant photosynthesis and extract energy from sunlight," says one of the co-authors, Professor Knut Deppart.
The scientists have already created a spin-off company that will attempt to develop the commercial potential of nanowires, QuMat Technologies AB.
"When MIT Technical Review recently presented "Ten Emerging Technologies That Will Change Your World," nanowires were one of the ten, with references to Harvard, Berkeley, and Lund, and for industrialization to Nanosys in Palo Alto and to QuMat in Lund," says Professor Lars Samuelson, head of the Nanometer Consortium at Lund University.
The scientists at the Nanometer Consortium at Lund University are working together with BTG, a leading patent and technology commercialization company, to develop and commercialize pioneering nanowire technology, a platform that is protected by a strong portfolio of patents.