Researchers at Lund University in Sweden grew "nanotrees" out of semiconducting materials, Science News reports. Lars Saumelson and his colleagues spray gold nanoparticles onto nanowire "trunks," just a few microns in length. (In comparison, a human hair is around 100 microns thick.) Exposing the seeded trunk to a mixture of specific gasses causes branches to grow. The trunk and the branches can even be composed of different materials so that the parts have specific functions:

"For instance, in one experiment, the Lund team made trunks out of gallium phosphide and parts of the branches out of gallium arsenide phosphide. The researchers expect combinations of materials such as these to produce a light-emitting diode: The trunk would carry current to the branches, where the gallium arsenide phosphide would convert it into light. Alternatively, the branches could serve as light-harvesting structures, as in a solar cell, which would then shuttle excited electrons into the trunk." Link