Vibrations make electrons jumpBy Kimberly Patch, Technology Research News
Even though humans have been using electricity for more than a century, there are some fairly basic things about the flow of electrons that we haven't figured out yet.
For instance, researchers at the University of California at Santa Barbara and IBM Research have just discovered that the amount of energy a molecule holds in its bonds greatly affects the way it transfers an electron.
The excitation state of a molecule -- literally how much it vibrates -- is a measure of how much energy the molecule holds. The researchers found that adding energy to a molecule of nitrous oxide boosted its willingness to accept an electron.
This step toward a better understanding of electron flow has implications in fields ranging from molecular electronics to biochemistry.
"In a general sense we're improving our understanding of how electron transfer reactions work and how charges are moved between molecules in surfaces," said Alec Wodtke, a chemistry professor at the University of California, Santa Barbara. "We [are starting] to see how the actual motion of the atoms in the molecule influences the likelihood of the electrons to move back and forth," he said
Key to the finding was the unusual way the researchers examined electron transfer, said Wodtke. "What we figured out was a way to study electron transfer reactions [without] the effects of a solvent [in order to] look directly at the influence of the vibration. [We looked] at vibrationally excited molecules interacting with metal surfaces," he said.
This made it easier to see what was happening in the molecule. In solution the molecules of the solvent reorganize according to changes in a charge, which complicates matters, said Wodtke. For example, water molecules have negative and positive ends and, because opposite charges attract, they will point their positively charged ends toward a negative charge. When "an electron hops ... suddenly all those water molecules find themselves seeking out the negative charge that's moved to a new place. All those molecules in the solvent are part of the reaction," he said.
On a metal, instead of whole molecules swinging around, just the electrons on the metal react, said Wodtke. "This makes the interaction of a charge much easier to model," he said.
The researchers used lasers to excite nitrous oxide molecules to high rates of vibration -- about half the energy required to rip the molecule apart. In that state, the bonds that hold together the molecule's nitrogen and oxygen atoms oscillate between stretching almost double in length to compressing to much less than the typical bond length. The researchers put the excited molecules in a vacuum chamber and watched as they approached a metal surface.
"We let them just fly toward the surface [and] when they hit ... we see all this wild stuff happening," said Wodtke. It's like bouncing a ping pong ball off the wall -- the molecule has a very short interaction with the metal, he said. The interaction time is on the order of 100-200 femtoseconds. A femtosecond is one millionth of a billionth of a second. But "during that very short period of time when it comes up close to the metal an electron will hop over from the metal onto the molecule," Wodtke said.
The quick transfer was strange because, like many molecules, nitrous oxide is a poor electron accepter. The experiment showed that "the electron transfer properties [change] very dramatically as the molecule vibrates back and forth, almost as if the molecule changes its chemical identity as it vibrates," said Wodtke. At the point where the atoms' bonds stretched the most, the molecule behaved like an oxygen atom, which is a good electron accepter, he said.
"This is the first clear demonstration in a single-molecule electronic device that the motion of atoms is coupled to the localization of charge on the molecular length scale. [It's] excellent work," said Christopher Chidsey, an associate professor of chemistry at Stanford University.
The work has long-term implications in electronics, where the flow of electrons powers devices, and in biochemistry, were many reactions involve exchanging electrons. "The obvious import is that it will need to be figured into predictions of how devices will work," said Chidsey. "At the very least, it is a new complication that must be factored in when planning very small electronic devices," he added.
In the short term, the discovery could be used to fashion a chemical sensor, said Wodtke. "One can use this phenomenon to probe specifically for trace amounts of chemicals... by pumping a molecule with a laser and letting vibrational influences of electron transfer induce electrical current that could then be detected," Wodtke said.
Wodtke's colleagues in the research were Yuhui Huang of the University of California at Santa Barbara, and Charles T. Rettner and Daniel J. Auerbach of IBM Research's Almaden Research Center. They published their work in the October 6 issue of Science. The research was funded by the U.S. Air Force and Department of Energy.
Timeline: 10 years
TRN Categories: Nanotechnology; Semiconductors and Materials
Story Type: News
Related Elements: Technical paper "Vibrational Promotion of Electronic Transfer," Science, October 6, 2000
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