URBANA-CHAMPAIGN, Ill. Researchers at the University of Illinois have developed a method of prepping single-crystal silicon wafers for organic molecules, which produce molecular switches with atomic precision.
Wafers from existing silicon manufacturing facilities could be used to achieve switching arrays running at 100 terahertz, the researchers said. By attaching arrays of spinning organic molecules to the surface of a standard silicon wafer, the group demonstrated, in principle, how to fabricate molecule memories running at THz speeds.
"The organic molecules are only attached where a single hydrogen atom was removed, so they spin some as fast as a 100 trillion times a second," said Joseph Lyding, a UI professor of electrical and computer engineering and a researcher at the University of Illinois Beckman Institute for Advanced Science and Technology. "We're working with chemists now who are designing molecules that when attached will act like transistors that can switch at 100 trillion times a second," said Lyding.
Lyding's group removed individual atoms of hydrogen from finished wafers of single crystal silicon, creating "holes" on the silicon surface. The holes create a gradient that attracts any free molecules that happen to float by. By exposing the prepped silicon only to convenient organic molecules, the surface can be automatically populated with arrays of identically spinning memory elements, each emulating a transistor that switches each time it spins.
"If this technology takes hold, we will be returning to the mechanical memories of yesteryear because we are really just building a mechanical relay, but on a nanotechnology scale. It will be read and written to electrically, but the active element is just a very very small mechanical device," said Lyding.
The researchers' first step was to remove the heavy oxidation that's ordinarily applied to a silicon wafer, thus exposing a perfect silicon surface. Then in very high vacuum, Lyding and fellow researchers Mark Hersam and Nathan Guisinger passivated the pure silicon surface with hydrogen forming a single-atom-thick layer of hydrogen strongly bonded to the silicon.
The next step involved using a scanning tunneling microscope to dislodge individual hydrogen atoms to attach the spinning organic molecules. The resulting surface was smooth like pure silicon, except for holes where the individual hydrogen atoms had been removed. In terms of gradients, these holes lure molecules toward their "dangling" bonds, spontaneously self-assembling organic molecules, injected in gas phase, into atomically precise arrays.
Atomic precision in punching out single-atom-size holes in the hydrogen surface was accomplished by a feedback loop from the microscope that controlled the tunneling current. When the single atomic bond was broken from the selected hydrogen atom, the feedback signal instantly cut off the tunneling current to prevent further disturbance to the surface. The microscope can also be programmed to move to the next atom when a bond is broken, so that lines of hydrogen atoms can be removed from the silicon surface.
"We are doing things now like writing two non-parallel lines that get closer and closer together in that way we can gauge just how close we can get lines and still have them remain discrete," said Lyding.
In the end, the researchers impress "templates" atop the silicon surface by scanning the tunneling microscope in chosen patterns. Eventually, they will create arrays of memory or switching elements. But for now the group is trying to perfect procedures. So far, they have tried attaching three organic molecules: norbornadiene, copper phthalocyanine and carbon-60 "buckyballs."
"The advantage of organic molecules is that their ends can be functionalized so that they easily interface with either electronic or nanoscale mechanical devices," said Lyding. The group cautions that it has more work to do to validate its approach before a practical circuit design method can be announced.