Nanomaterials
are increasingly gaining the attention of not only
the scientific community, but also the public due
to their unique properties which endear them to new
and exciting ARGONNE,
Ill. (Nov. 25, 2005) — With
the help of the world's most brilliant hard X-ray
beams at the Advanced
Photon Source , scientists have seen for the
first time metal atoms near the surface of a liquid
alloy arrange themselves in alternating layers one
atom thick.
The
research has implications for nanomaterials — materials
made of ultrafine particles in which most atoms lie
at or near the surface — and could lead to such applications
as improved lead-free solder for electronics.
The collaboration, led by physicists from Harvard
University , used X-rays to look at how atoms
of bismuth and tin behave when they form a liquid
alloy. In the bulk material, the two elemental
liquids form a perfectly miscible solution — like
cream mixed with coffee — but near the surface
they separate into atomic layers with alternating
compositions.
"The top atomic layer is mostly bismuth," said
Oleg Shpyrko (currently at Argonne's Center
for Nanoscale Materials , or CNM), the lead author
of the study. "Below that layer, it's mostly tin.
The layers alternate as you get deeper into the material,
but after a few layers they start to mix until it
becomes a pure alloy."
Previous studies have observed the low-surface tension
component in such a mixture form a surface monolayer,
a phenomena known as Gibbs adsorption. However, the
extension of surface effects to sub-surface layers
has never before been observed.
"The demixing we observe is somewhat of a paradox,
since the interactions between the two components
are strongly attractive, not repulsive, as in the
case of immiscible mixtures like oil and water," Shpyrko
said. "Surface demixing was predicted in 1950, but
it eluded experimentalists for more than 50 years."
Shpyrko and collaborators developed an X-ray technique
that allows independent measurements of atomic structure
in the near-surface region of the liquid. The technique
can also determine, and allow the researchers to
compensate for, the effects of temperature-induced
surface waves. These fluctuations can obscure direct
observation of surface structures on minute scales.
The researchers have learned that surface-induced
layering is a quasi-crystalline structure that appears
at liquid-vapor interface of even simplest metallic
fluids, but is apparently absent in dielectric liquids
such as water.
In
the most recent study, Shpyrko and co-workers added
resonant X-ray scattering to obtain element-specific
density profiles, while retaining sub-nanometer spatial
resolution — a method that can be applied to a wide
range of multi-component liquids in the future studies.
"As we learn about a variety of novel nanoscale
materials where most atoms are near the surface,
these and other interfacial effects are expected
to play a dominant role," Shpyrko said.
The study was primarily led by Harvard's Peter Pershan
and his group members Alexei Grigoriev, Reinhard
Streitel and Diego Pontoni. Contributing were Ben
Ocko from Brookhaven
National Lab , Moshe Deutsch from Bar-Ilan
University in Israel, and Binhua Lin and Mati
Meron, who provided beamline support at The University
of Chicago's ChemMat-CARS facility
at the Advanced Photon Source.
The results were reported in Physical
Review Letters [Phys. Rev. Lett. 95, 106103
(2005)] — Dave Jacque
Media contact
|
