BERKELEY, CA -- A recent experiment in which the movements
of electrons between a conductor and an insulator were recorded on a femtosecond (a
millionth of a billionth of a second) time-scale holds promise for the electronics
industry. Working with a unique femtosecond spectroscopy system, scientists at the U.S.
Department of Energy's Lawrence Berkeley National Laboratory have successfully
demonstrated a technique that could revolutionize the understanding of electron dynamics
at surfaces and interfaces.
Charles Harris, a senior scientist with Berkeley Lab's Chemical Sciences
Division and professor of chemistry with the University of California at Berkeley, led the
experiment which showed that it is possible to observe the dynamics of electrons as they
move across the boundaries where metals and non-metals meet. Results of this work have
been published in the journal Science (Jan. 9, 1998).
"Our findings for a model interface contribute to the fundamental
picture of electron behavior in weakly bonded solids and can lead to better understanding
of carrier dynamics in many different systems, including organic light-emitting
diodes," Harris says.
Performance in computers and other electronic devices depends upon the
degree to which electrons are free to move back and forth across the interfaces between a
conductor and silicon, a semiconductor. At these interfaces, there is an abrupt change in
atomic species and the way in which atoms bond to form a crystal. This change affects the
behavior of electrons which, in turn, affects not only the performance of electronic
devices, but chemical reactivity, magnetism, and other properties as well. Consequently,
understanding the dynamics behind the movement of electrons through interfaces is
considered critical to future technological advances.
Harris and members of his research group may have pointed the way for
future studies by demonstrating an ability to record unprecedented observations of
electron behavior at critical interfaces through the combination of a femtosecond laser
with a high-resolution, time- and angle-resolved spectroscopy technique called
"two-photon photoemission (TPPE)." With this combination, they have to date been
able to study the dynamic behavior of electrons at the interfaces of a metal covered with
a single layer of non-metal molecules and sealed in a vacuum.
Harris and his team start by shining a "pump" pulse of laser
light, as short as 30 femtoseconds in length, and tunable to a desired wavelength, on the
metal to excite or energize an electron, causing it to be emitted into the interface with
the non-metal. This photoelectron is then hit with a second "probe" pulse of
light, causing it to be emitted out of the metal/non-metal interface and into the vacuum.
By recording the photoelectron spectra at different times after each laser pulse and
measuring the angles of emission, Harris and his team are able to track the electron and
determine to what extent it is "de-localized" (free to roam about) or
"localized" (restricted) in its movement.
In their latest round of experiments, the Harris team coated a silver
surface with a layer of alkane molecules. After the silver was irradiated with the pump
pulse, the researchers, through the probe pulse, observed that the electrons are initially
delocalized so that they move freely on the alkane layer parallel to -- but at a fixed
elevation above -- the surface layer of silver atoms. Within a couple of hundred
femtoseconds, however, these electrons become localized within the alkane overlayer as
polarons. A polaron is an electron whose interaction with the atoms in a crystal lattice
creates a deformation (an energy "well") that traps the electron, like a divot
on a fairway can trap a golf ball.
According to Harris, the lattice deformation in which the electron
traps itself is caused by small shifts in the positions of positively charged atomic
nuclei around the negatively charged electrons. After the passage of more than a thousand
femtoseconds, the self-trapped electron is finally able to escape the trap by
quantum-mechanically "tunneling" its way back into the metal.
"The ability to both time- and angle-resolve the dynamics of
electrons at interfaces allows a quantitative determination of the relaxation energies and
lattice displacements associated with the small-polaron self-trapping process," says
Harris. "Time and angle-resolved TPPE is a powerful probe for two-dimensional
electron localization and should be applicable to a wide variety of interfaces."
In addition to studies of electron dynamics at interfaces, the research
techniques developed by Harris and his team could also be relevant to biological studies.
For example, the interplay between localization and delocalization of electrons is thought
to play a major role in the transfer of electrons from one large molecule to another, such
as during the process of photosynthesis. The Harris technique could also be applied to the
study of two-dimensional magnetism, high-temperature superconductivity,
electrical-conductivity in plastics, and other important phenomena in the burgeoning field
of molecular electronics.
Co-authoring the Science paper with Harris were
Nien-Hui Ge, Chung Wong, Robert Lingle, Jr., Jason McNeil, and Kelly Gaffney.
The Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified scientific research and is
managed by the University of California.
