Portland, Ore. - Nanoscale devices have been "self-assembled" in the lab to demonstrate everything from a single-molecule transistor to a computer-in-a-test-tube. But with no way to self- assemble a complex system, they remain laboratory curiosities.
Magnetic self-assembly, argue its inventors at Drexel University (Chicago), could bridge the gap by filling in predefined areas on already fabricated silicon wafers with arrays of molecular-size devices. The technique uses colloidal superparamagnetic nanoparticles to populate specific areas of a wafer with molecular devices at room temperature.
"We are using a beam applied to the bottom of the wafer to deposit magnetic material such as cobalt in the pattern we desire the molecular devices to self-assemble into," said Drexel professor Gennady Friedman. He was assisted by a doctoral candidate, Benjamin Yellen.
Attempts to pattern colloidal nanoparticles, including surface chemistry and electrostatic and surface tension, have succeeded in covering large areas with molecular devices, but not in precise locations. Methods that can selectively pattern nanoparticles and other molecular-size devices in specific areas of a wafer, such as optical trapping methods, are limited to small areas at a time.
To get the best of both worlds, Friedman's team is patterning magnetic material onto wafers just in the areas where molecular devices are needed, thereby achieving selectivity, then dipping the whole wafer in a bath of colloidal nanoparticles to cover large areas. They call it magnetically driven self-assembly.
"We combine the virtues of selective trapping and massive parallel printing by using an array of ferromagnetic traps to direct the self-assembly," said Friedman.Attraction and repulsion
Magnetic fields, say the researchers, act over a longer distance than electrostatic or like forces, since most everything except metal is harmlessly penetrated by magnetism. It's also easy to reverse a magnetic force from attraction to repulsion, thereby adding another dimension to the ease with which patterns can be formed.
For instance, if microwells have been fabricated atop the chip, then the patterned cobalt on the bottom might well attract molecules into them from colloidal nanoparticles awash over the top. Conversely, if a magnetic-field bias is appropriately applied, then the cobalt pattern might well repulse the nanoparticles right out of the microwells. By using both techniques, just the right combination of molecular devices can "decorate" a wafer.
"There would typically be several steps involved, where we dip the wafer into different solutions with different patterns and get just the right molecules decorating just the right places on the wafer," said Friedman. The researchers have already fashioned arrays of microwells over the ends of magnetized islands to form arrays of cobalt islands, placed one at the bottom of each microwell. An external magnetic field was shown to promote colloidal particles to be attracted to, or repelled from, the microwells.
Friedman and his colleagues have embarked on a three-year project to fashion entire nanoscale systems atop wafers using magnetic self-assembly.