SPIKY, four-legged crystals no bigger than the average virus are being grown to order in a chemistry lab in California. They are the most complicated nanoparticles that scientists have ever created under controlled conditions.
In the 1980s, nanoscientists learned to grow simple spheres from inorganic materials. Then they graduated to rods and discs. Now a group led by Paul Alivisatos at the University of California, Berkeley, has mastered the tetrapod. The crystals are made of a semiconductor material called cadmium telluride. They grow when cadmium and tellurium are stirred together with organic molecules into a solvent heated to 315 掳C. First small seed crystals form, and then they sprout arms that measure from a few nanometres up to more than 100 nanometres long. The structures can be clearly seen through an electron microscope (see Graphic).
Already, groups are trying to find uses for this quirky nanoparticle. 鈥淲e suddenly have a new piece in our chemistry kit, so now we are going to try and build things with it,鈥� says Alivisatos. His team thinks the tetrapod鈥檚 three-dimensional structure may lend mechanical strength to plastics, while scientists at the University of Cambridge have found that the particles increase the efficiency of experimental organic solar cells. The tetrapods stand upright in the cells鈥� polymer films and the branches act as channels for the current to flow through, they report in an upcoming issue of Nanoletters. The crystals could also be built into networks that might one day form the basis of 3D interconnected computer circuits structured like neural networks in the brain.
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The discovery of the tetrapod was completely unexpected. 鈥淲e were involved in trying to make many different shapes and this one popped out by itself,鈥� recalls Alivisatos. In the two years since then, the team has worked hard to understand how the tetrapods grow. Now they can control the process precisely (Nature Materials, DOI: 10.1038/nmat902).
An organic molecule called n-octylphosphonic acid plays an important role. It sticks to certain faces of the crystal, blocking growth of the structure on those sides. This effect was first used to grow simple rods: when the sides of a crystal are coated in an organic repellent, atoms can only latch onto the ends. Alivisatos thinks the tetrapods acquire their more curious shape because the rods grow out from a central cubic core. By adjusting the ratio of the ingredients in the reaction the team can control the length and thickness of the particles鈥� limbs.
鈥淭he tetrapod shapes are beautiful, but it is the control in the synthesis that impresses me,鈥� says Chris Murray, manager of the nanoscale materials and devices group at IBM鈥檚 T. J. Watson Research Center in Yorktown Heights, New York state.
Alivisatos suggests that there will be more nanoparticles to follow and that crystal growth is enjoying a renaissance. 鈥淭his is an exciting time,鈥� agrees Murray. 鈥淭he challenge in the next 5 years is to get these nanocrystals out of the lab and readily available to the engineering community.鈥�