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 Collections under which this article appears:Materials Science
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In computer technology, if not in space flight, smaller almost always means faster, cheaper, and better. But researchers at the frontiers of miniaturization, who are fashioning experimental switches and storage devices from single molecules, have outrun their ability to wire these devices together into working systems. At the MRS meeting, scientists described several schemes--including one that uses DNA and another involving electric fields--that could help them link molecular circuitry with nanoscale wires. Although no working molecular devices have been rigged up yet, experts say the approaches could help make dreams of molecular electronics a reality.
"This was really nice work," says Zhenan Bao, an advanced electronics specialist at Lucent Technologies' Bell Laboratories in Murray Hill, New Jersey. "It's the future direction the field [of molecular electronics] will take."
On today's computer chips, the smallest features are 250 billionths of a meter, or nanometers, across. Although that's vanishingly small, the devices can still be made and wired up with photolithography, the industry's workhorse patterning technology, which shines light through stencils to direct the etching of fine features on silicon chips. Molecular-scale devices, however, can measure just a few nanometers in some dimensions, and light simply can't be focused tightly enough to lay out the patterns for the fine wiring needed to connect them. Researchers have to take another tack, such as first making the minuscule wires, and then positioning and connecting them. But although they can already make sufficiently small wires--one technique condenses metal atoms in the pores of membranes to form tiny metal rods--achieving the connections is another matter.
Working with nanorods that have platinum shafts and gold tips, a team of researchers at Pennsylvania State University in University Park led by electrical engineers Theresa Mayer and Thomas Jackson, along with chemists Thomas Mallouk and Michael Natan, has now come up with two speedy ways to solve this problem. One assembly strategy exploits the ability of DNA strands to seek out and bind to strands with matching sequences. The researchers first used small organic molecules called thiols to link single-stranded DNA to the rods' gold tips. Next, they decorated a separate gold surface with single-stranded DNA whose sequences were complementary to those attached to the rods. When the team mixed the rods with a solution containing the DNA-coated gold, the complementary strands bound to one another, linking the rods to the gold surface.
In a second strategy for assembling nanorods, the Penn State team turned to electrical attraction. Here they were linking a pair of electrodes resembling combs, positioned so their teeth interlaced. The researchers insulated these electrodes with a layer of silicon dioxide and placed a tiny gold pad halfway down each tooth, so that the pads on the adjacent teeth lined up in a row. They then immersed the apparatus in a solution containing their tiny metal rods and applied a voltage between the electrodes.
The voltage created an electric field that triggered two very different electrical effects to first attract and then bind the metal nanorods between adjacent gold pads. Initially, the field created a long-range electrical attraction that reeled in the nanorods from afar. The rods then bridged adjacent gold pads, as the full electrode assembly tried to maximize its capacitance, or its ability to store charge. Capacitors store charge as an electric field between two separated conductors harboring opposite charges. In this case, the overall electrode assembly contained two types of capacitors: The first consisted of the large electrodes and the gold pads separated by the insulating silicon dioxide, and the second of adjacent gold pads, separated by the solution.
The small size of the gold pads limited the capacitance of the overall device, which can only store as much charge as its smallest capacitors. But nanowiring lifted this restriction. By linking the gold pads, the nanowires opened a continuous electrical connection between the pads. That disrupted their ability to store charge, allowing the larger capacitors to take over. The bottom line, says Mayer, is that "we can use the field to drive the wires where we want them to be."
That's not all. When the rods were shorter than the gap between the gold pads, the system's tendency to maximize capacitance caused several rods to line up end to end to form a bridge. The researchers then "welded" these rods into continuous wires by enriching the solution with gold ions. The ions filled in the gaps, producing longer rods that could conduct current, as the researchers confirmed.
"It's still somewhat unclear" how these techniques could be adapted for building molecular computers, says Mayer. "These are just baby steps," adds Jackson. But it's the first steps that are the hardest to take.
Protein Patterns for Electronic Devices?
Biological organisms aren't just master builders of soft and squishy organic materials. They also do a pretty decent job of assembling rocklike inorganics--witness the strength of the everyday clam shell or your own bones and teeth. Of course, the secret behind such synthetic feats is the soft, squishy proteins that both clams and people have evolved to help organize inorganic molecules into intricate and useful patterns. Now materials researchers are trying to use the same trick as well.
At the Boston meeting two independent research teams, one led by materials scientist Mehmet Sarikaya of the University of Washington, Seattle, and the other by Angela Belcher, a chemist at the University of Texas, Austin, reported that they had used a laboratory version of evolution to create genetically engineered proteins that could bind to tiny semiconductor and metal particles and assemble them into larger clusters. If the same protein-engineering techniques can generate molecules capable of organizing and patterning a wide variety of materials, proteins could become invaluable tools for crafting transistors, wires, and other electronic devices with components hundreds of times smaller than those on current computer chips.
"The whole idea of merging biology with materials synthesis is very important," says Chad Mirkin, a materials researcher at Northwestern University in Evanston, Illinois. "Organic systems have molecular recognition abilities that have had a long time to evolve. [They] far surpass what we can do readily in the lab."
Both Sarikaya and Belcher hoped to exploit the abilities that the proteins critical to forming bones, shell, and teeth display: They have the selectivity to bind only certain inorganics, seeding and organizing their growth into desired patterns. Abalone, for example, use separate proteins to organize calcium carbonate into different mineral phases: iridescent mother-of-pearl, or aragonite, for the shell's inner layers and rock-hard calcite for the shell's outer surface. Such naturally occurring proteins don't work well with many industrially important materials such as metals and semiconductors, however. So the Washington and Texas researchers decided to see if they could improve matters.
For their part, Sarikaya and his Washington colleagues set out to coax bacterial proteins into binding to gold, which is used widely in the electronics industry. They started with multiporin, a cell membrane protein from the bacterium Escherichia coli that does not bind gold in its natural form. They then cloned the multiporin gene to make millions of copies. From each copy, they snipped out a section coding for a segment of the protein that forms a loop projecting from the E. coli membrane. That's where the protein would bind gold if its chemical makeup allowed it to do so.
To alter the makeup of the loop, the researchers replaced the snipped-out gene segment with random DNA sequences produced by an automated DNA synthesizer. They then introduced the mutated genes back into bacteria, grew the bacteria, exposed them to gold particles, and--in a set of steps analogous to natural selection--they washed off poor binders and regrew the better ones, eventually identifying the colony that did the best job of binding gold. Finally, they purified multiporin from these bacteria and attached the protein to the outer surface of both tiny plastic spheres and flat surfaces in solution. When they then spiked their mixture with a small amount of gold, the protein picked up the flecks, decorating either the outside of the spheres or dotting the surface.
Belcher's group, meanwhile, took a different approach to evolving proteins that could bind to semiconductors, such as zinc selenide and gallium arsenide, which are also widely used in electronics. The team used an off-the-shelf kit containing 109 random DNA sequences, which they inserted into copies of a gene that codes for the outer coat of a bacterial virus called a phage. They then infected bacteria with the modified phages, allowed the phages to multiply, and exposed the viruses to a solution containing semiconductor particles to select the viruses best able to bind the semiconductor.
Thus far, Belcher reported, the technique has worked beautifully. Her team has identified proteins that can discriminate between similar semiconductor alloys, such as gallium-arsenide versus aluminum-gallium-arsenide, and can even discriminate between different faces of the same semiconductor crystal, which have different arrangements of the atoms on the crystal surface. Down the road, she says, her team is planning to pattern the semiconductor-binding proteins on surfaces and use them to nucleate the growth of tiny semiconductor crystals in controlled arrangements. That's just what researchers around the globe are trying to do, in an effort to create ultrasmall transistors and other computing devices. And if Belcher and Sarikaya have their way, proteins may be just the handle they need to get there.
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Collections under which this article appears:Materials Science
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