At the boundary between conductor and insulator
Using a new technique, scientists have studied the dynamics of electrons at the interface between a conductor and an insulator. They observed processes lasting only a few femtoseconds. The electronics industry in particular will be able to benefit from the results. Charles Harris, a scientist in the Department of Chemistry at Lawrence Berkeley National Laboratory and a professor at the University of California, Berkeley, conducted the experiment, which observed the dynamics of electrons as they crossed the metal-nonmetal boundary (Science, ed dated January 9, 1998).
Harris says: "Our findings on a model of the interface contribute to the general picture of the behavior of electrons in solids and can lead to a better understanding of charge carrier dynamics in many different systems, including organic light-emitting diodes."
The performance of computers and other electronic devices depends on how easily electrons can move back and forth across the interface between a conductor and the semiconductor silicon. At these interfaces there is an abrupt change in the types of atoms and the way in which atoms bond to form a crystal. This change affects the behavior of the electrons, and this in turn determines not only the performance of electronic devices, but also chemical reactivity, magnetic properties, and more. Therefore, understanding the dynamics behind electron motion across interfaces is considered crucial for future technical advances.
Harris and the members of his research group may have paved the way for future studies by demonstrating how hitherto impossible observations of electron behavior at the critical interfaces can be made. They used a combination of a femtosecond laser and a spectroscopy technique called two-photon photoemission (TPPE) that allows high-resolution time and angle measurements. This combination allowed the researchers to study the dynamic behavior of electrons at the interfaces of a metal covered with a single layer of nonmetallic molecules and placed in a vacuum.
An experiment begins with a laser flash lasting only 30 femtoseconds, the wavelength of which can be freely selected. The light excites an electron in the metal, causing it to be emitted into the non-metal transition region. A second flash of light then knocks the photoelectron out of the sample and into the vacuum. Harris and his team recorded the spectra of the electrons at different times after each laser pulse and measured the exit angles. This allowed them to track the particle's path and determine to what extent it was "delocalized" (free to move) or "localized" (constrained) in its movements.
In the latest set of experiments, the scientists coated a silver surface with a layer of alkanes. After the silver was irradiated with the first flash, the researchers observed through the second light pulse that the electrons were initially delocalized. The electrons moved freely in a plane of the alkane layer that was parallel to the surface of the silver atoms at a fixed distance. But within a few hundred femtoseconds, these electrons became stationary in the alkane coating as polarons. A polaron is the combination of an electron and a perturbation it causes in the crystal lattice that creates an energy well into which the electron falls, making it much more immobile. After more than 1000 femtoseconds, the trapped electron can finally escape the trap it created by quantum mechanically tunneling back into the metal.
"The ability to analyze the electron dynamics at the interfaces in terms of both time and angle allows quantitative determination of the relaxation energies and lattice shifts associated with polaron formation," explains Harris."The TPPE … is a powerful tool for the two-dimensional localization of electrons and should be applicable to a variety of interfaces."
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