Nano-challenge: Molecular switch
Team lays foundation for a carbon-nanotube-based molecular gate
In these visualizations of a carbon nanotube, the F4-TCNQ molecule in the top image is oriented sideways, blocking electric current. In the bottom image the F4-TCNQ molecule is aligned with the length of the nanotube, which would allow current through—thus, a switch. Among the information technology wonders of the modern world, most are based on one simple question: Is it on or is it off?
Two ORNL researchers, one specializing in theoretical physics, the other in chemistry, have discovered a carbon nanotube-based system that functions like an atom-scale switch. The Computer Science & Mathematics Division researchers believe their work could be laying the foundation for a binary bonanza.
"The semiconductor industry is always working against Moore's Law-the finite capacity of a silicon chip to store information," says Vincent Meunier, the physicist in the team.
He's working with CMSD chemist Bobby Sumpter. Their approach is to perform first-principles calculations on positioning a molecule inside a carbon nanotube to affect the electric current flowing across it.
"The result is an electrical gate at the molecular level," says Vincent. "In other words, in one position, the molecular gate is open, allowing current through. In another position, the gate is closed, blocking the current. We get a molecular gating effect."
The magic molecule is tetrafluorotetracyano-p-quinodimethane, or, more mercifully, F4-TCNQ. Because it is an acceptor, it attracts electric charge. Computer-modeled simulations indicate that a 90-degree difference in the orientation of the F4-TCNQ molecule stationed inside a carbon nanotube determines whether the nanotube lets current through or stops it.
In a silicon chip, the gate is a silicon-oxide barrier within the structure of the chip. In Vincent's and Bobby's model, the gate is within a molecule-the carbon nanotube-that is about one nanometer in size.
"That is about three orders of magnitude smaller than a silicon chip," Vincent says. "That is the possible gain in storing information."
One of the greatest challenges in this basic research is to develop a robust approach to modifying the state of the F4-TCNQ molecule between the on and off possibilities. The flipping mechanism would enable the researchers to rotate the molecule to either a lengthways (on) or sideways (off) position on demand, thereby creating the ultimate nonvolatile memory element working at room temperature.
"There are not many possible ways to move a molecule that is embedded into a nanowire. Here, the trick is to exploit not the properties of the molecule but rather make use of those of the nanowire," Vincent says. In the "on" position, their models have shown that the carbon nanotubes would allow current to pass through them.
In the "off" orientation, the current is reflected back, blocking the current.
Bobby recently completed calculations where small external mechanical strains applied to the external surface of nanowire readily led to the reproducible rotation of the F4-TCNQ molecule, indicating that the gate would be stable and also retain information in a room temperature environment.
The models require enormous computational power, available to the researchers onsite through the National Center for Computational Sciences.
"These are very big calculations using state of the art quantum chemistry methods," says Bobby. "That's the only way we can get it to work."
The two researchers, who split their time between CSMD and the Center for Nanophase Materials Sciences, are enjoying a very successful partnership that incorporates its own interdisciplinary chemistry. Vincent, who is primarily a theoretical physicist, does the physics-based calculations on the concept. Bobby, who is a physical chemist, then applies his practical knowledge of chemistry to the problem.
"We work together," Vincent says. "I wrote the code in transport and Bobby did the physical chemistry. That's one of the strengths of our Computational Chemical Sciences group. It cuts across several areas of research that allows us to apply different approaches to novel ideas. However, it is really the computational infrastructure that allows us to test our ideas."