Portland, Ore.- A research group at the University of Delaware has demonstrated a way to precisely position and control the growth of organic nanowires atop a prepatterned substrate, potentially providing a means of interconnecting future nanocircuits. Until now, there has been no way to interconnect such nanoscale devices as carbon nanotube transistors, quantum dots and molecular-memory arrays.
In a different approach, researchers at Purdue University have used the self-assembling properties of DNA to create nanowires.
University of Delaware professor Thomas Beebe led the team that recently demonstrated nanowires that self-assembled in a thin-film polymer. By stimulating the fledgling wire at one end with a pulse from a scanning-tunneling microscope (STM), the wire self-assembled along the lines of the atomic lattice, yielding nanoscale precision. A barrier, called a corral, stopped the wires' growth at the correct point. This suggests that future devices could be interconnected using self-assembling nanowires that would be stimulated electrically to grow and then terminated with molecular corrals.
"I want to avoid making any fantastic claims," said Beebe. "The new thing that we have done, that has not been done before, is use a molecular corral to confine, isolate and to some extent control how long nanowires are grown." Beebe's team included doctoral candidate Shawn Sullivan, postdoctoral researcher Albert Schnieders and visiting Lincoln University undergraduate student Samuel Mbugua.
Separately, researchers at Purdue University attached magnetic nanoparticles to DNA and then cut the strands into DNA wires. Deoxyribonucleic acid-DNA-has an overall negative charge, but when placed in a solution with magnetic particles that have a positive charge, DNA automatically self-assembles into tiny scaffolds that, in effect, create wires. Those wires can be used to self-assemble electronic devices according to a precise program.
Purdue researchers have previously developed techniques that have allowed strands of DNA to be precisely placed on a silicon chip. Professor Albena Ivanisevic and former Purdue physics graduate student Dorjderem Nyamjav coated DNA with magnetic particles two years ago. Graduate student Joseph M. Kinsella and Ivanisevic then used an enzyme to cut the DNA into wires. Each wire was stretched onto a silicon-oxide surface of up to 35 microns in length but only 2 nanometers wide.
In the case of the University of Delaware team, however, the molecular wires are nanoscale lengths of polymer that are formed along the atomically precise rows of a regular lattice of organic carbon atoms deposited as a thin film atop a wafer. Polymerization leaves the row of carbon atoms bonded with adjacent atoms by virtue of hydrogen bonds that share electrons among the neighbors. The result is a nanometer-thin wire along a straight-line path that ends precisely where the terminating molecular corral was placed.
"We're using the corral as a molecule container," said Beebe. "Our molecule corrals are providing a kind of confinement for a chemical reaction at the step edge of a surface-that is, the edge of the corral."
"The corral itself is a circular structure formed on a surface of carbon atoms-graphite-that is of nanometer dimensions, which we control," said Beebe. "The molecules line up in a well-ordered structure by a process called self-assembly."
As with all polymers, the nanowires are formed by linking millions of repeated organic molecules (mostly carbon and hydrogen). Other groups have fabricated nanoribbon cables by patterning metal lines on a substrate, then pouring a polymer over the lines and removing the combined structure. Beebe's method, however, enables electrically conductive wires to be grown precisely where they are needed atop organic semiconductors.
"Our molecules are oligomers"-a polymer formed by up to four simple monomers," said Beebe, "and just like any other polymer they have structural integrity to the extent that a force or heat will not cause them to break down.
To achieve the precise placement of the nanowires using polymerization, Beebe's group first patterned a wafer with the necessary molecular corrals. Then a layer of pure crystallized carbon (graphite) was deposited, and then an STM was touched at precisely the originating point of each nanowire.
"You use the tip of the scanning-tunneling microscope to initiate a chemical reaction on a terrace outside of the corral, and that reaction will speed up to the edge of the corral and stop, rather than go down into it," said Beebe. "If you give a small initiation pulse, then the reaction will run along the row of molecules until it hits the edge of a corral."
Because the nanowires are so small, the group has yet to confirm their electrical integrity via experiments. But the wires' chemical composition is consistent with that of other conductive polymers.
"I wish we knew exactly what the STM does on the nanoscale to initiate the reaction, but it probably creates a chemical radical-a molecule with an extra electron that is ready to react-because this class of molecules is known for polymerization reactions by a chemical-radical mechanism," said Beebe.
Next, the group will characterize the wires' electrical conductivity by terminating nanowires onto a micron-scale gold pad instead of a nanoscale corral. Then the electrical resistance can be measured between the beginning of the wire at the STM's tip and its termination point on the gold pad.
"We are trying to characterize the nanowires by measuring current and by learning about other mechanisms that control this reaction," said Beebe. "We are hoping the resistance along the rows goes way down, because now the molecules are all hooked together in a conjugated line of bonds-in other words. electrons can freely travel through that line of bonds."
Funding for the work was provided by the Howard Hughes Medical Institute and the National Science Foundation.