||A major step toward utilizing inorganic
nanometer-sized crystals as basic building blocks for nanotechnology has
been achieved. In the March 16 issue of the journal Science,
Berkeley Lab scientists report a relatively simple recipe for making
nanocrystals that allows for the all-important control of crystal size and
shape, and can be used to make more than one variety of nanocrystal.
Nanotechnology offers a potential cornucopia of benefits, from palm-sized supercomputers to synthesized antibodies to molecular-scale robots. Such wonders will be constructed from the ground up using nano-sized building blocks. Nanocrystals grown from inorganic materials, including metals and semiconductors, are prime candidates to serve as nanoblocks. Typically under 10 nanometers in diameter, such crystals are larger than molecules, smaller than bulk solids, and frequently exhibit physical and chemical properties somewhere in between. Before nanocrystals can be transformed into nanoblocks, however, researchers will first need a reliable and relatively inexpensive means of growing an assortment of crystals that are a specific size and shape.
Paul Alivisatos, a chemist with a joint appointment to Berkeley Lab’s Materials Science Division (MSD) and UCB’s Chemistry Department, and postdoc Victor Puntes and MSD senior scientist Kannan Krishnan are the coauthors of the Science paper entitled: "Colloidal Nanocrystal Shape and Size Control: The Case of Cobalt."
In that Science paper, the authors describe "size-distribution focusing," a technique in which the processes of crystal nucleation and growth are separated during synthesis to produce nanocrystals that are highly uniform in size. Applying this technique to colloidal inorganic nanocrystals in solutions made up of one or more hot, soap-like films, called surfactants, enabled the researchers to control the shapes of their crystals as well.
"We can now describe a minimum set of requirements to achieve size and shape control of inorganic nanocrystals in general," says Alivisatos, a recognized leader in the field of growing semiconductor nanocrystals.
These minimum requirements were arrived at working with cadmium selenide, a semiconductor from which Alivisatos and his colleagues have recently prepared crystals in a wide assortment of shapes including rods, teardrops, tetrapods and branched tetrapods. For the work reported in Science, the researchers decided to test their strategies on cobalt, a technologically important transition metal.
"Cobalt nanocrystals display a wealth of size-dependent structural, magnetic, electronic, and catalytic properties," Alivisatos says. "In particular, the exponential dependence of the magnetization relaxation time on volume has spurred intensive studies of cobalt nanocrystal synthesis for magnetic storage purposes."
Until now, making magnetic nanocrystals of cobalt has been difficult and required costly size-selective precipitation methods. Alivisatos and his colleagues achieved size and shape control with cadmium selenide by injecting a powder of the semiconductor – the "precursor" – into one or more hot surfactants. When a single surfactant was used, they obtained one-dimensional sphere-shaped crystals. When a binary mixture of surfactants was used, the crystals grew into two-dimensional rods. This is credited to the fact that the two different surfactants, in this case oleic acid and a substance called TOPO, react with the precursor powders in a slightly different manner, causing each crystal to grow in only one direction. The size-distribution focusing technique yielded high uniformity of size.
"The three strategies that we learned from the prototypical cadmium selenide system were used to produce cobalt nanocrystals with high crystallinity, narrow size distributions, and a high degree of shape control," says Alivisatos.
Under the powerful high-resolution microscopes at the Lab's National Center for Electron Microscopy, the magnetic cobalt nanocrystals were observed to spontaneously self-assemble into rods, depending upon the crystal growth control strategies employed. Unlike spherical nanocrystals, nanorods can be stacked and aligned, a real advantage for making magnetic storage devices. It was also observed that, over time, these magnetic particles organized into two- and three-dimensional superstructures, including ribbons of nanorods.
As for the minimum set of requirements for achieving size and shape control of inorganic nanocrystals in general, they are as follows: there must be a suitable precursor that rapidly decomposes at temperatures where the surfactants are stable; two surfactants must be found that "differentially adsorb" to the nanocrystal faces, allowing for rod formation; and one of the surfactants must allow for size-distribution focusing.