Alexei Tkachenko, Htay Hlaing, and Ben Ocko at an experimental end station at the NSLS. Credit: Brookhaven National Laboratory |
As part of the quest to form perfectly smooth single-molecule
layers of materials for advanced energy, electronic, and medical devices,
researchers at the U.S. Department of Energy’s Brookhaven National Laboratory
have discovered that the molecules in thin films remain frozen at a temperature
where the bulk material is molten. Thin molecular films have a range of
applications extending from organic solar cells to biosensors, and
understanding the fundamental aspects of these films could lead to improved
devices.
The study, which appears in Physical Review Letters, is
the first to directly observe “surface freezing” at the buried interface
between bulk liquids and solid surfaces.
“In most materials, you expect that the surface will start to
disorder and eventually melt at a temperature where the bulk remains solid,”
said Brookhaven physicist Ben Ocko, who collaborated on the research with
scientists from the European Synchrotron Radiation Facility (ESRF), in France, and Bar-Ilan
Univ., in Israel. “This is because the
molecules on the outside are less confined than those packed in the deeper
layers and much more able to move around. But surface freezing contradicts this
basic idea. In surface freezing, the interfacial layers freeze before the
bulk.”
In the early 1990s, two independent teams (one at Brookhaven)
made the first observation of surface freezing at the vapor interface of bulk
alkanes, organic molecules similar to those in candle wax that contain only
carbon and hydrogen atoms. Surface freezing has since been observed in a range
of simple chain molecules and at various interfaces between them.
“The mechanics of surface freezing are still a mystery,” said
Bar Ilan scientist Moshe Deutsch. “It’s puzzling why alkanes and their
derivatives show this unusual effect, while virtually all other materials
exhibit the opposite, surface melting, effect.”
Schematic representation of the alkanol monolayer when frozen (left) and melted (right). Credit: Brookhaven National Laboratory |
In the most recent study, the researchers discovered that
surface freezing also occurs at the interface between a liquid and a solid
surface. In a temperature-controlled environment at Brookhaven’s National
Synchrotron Light Source and the ESRF, the group made contact between a piece
of highly polished sapphire and a puddle of liquid alkanol—a long-chain
alcohol. The researchers shot a beam of high-intensity x-rays through the
interface and by measuring how the x-rays reflected off the sample, the group
revealed that the alkanol molecules at the sapphire surface behave very
differently from those in the bulk liquid.
According to ESRF scientist Diego Pontoni, “Surprisingly, the
alkanol molecules form a perfect frozen monolayer at the sapphire interface at
temperatures where the bulk is still liquid.” At sufficiently high
temperatures, about 30 degrees Celsius above the melting temperature of the
bulk alkanol, the monolayer also melts.
The temperature range over which this frozen monolayer exists is
about 10 times greater than what’s observed at the liquid-vapor interfaces of
similar materials. According to Alexei Tkachenko, a theoretical physicist who
works at Brookhaven’s Center for Functional Nanomaterials, “The temperature
range of the surface-frozen layer and its temperature-dependent thickness can
be described by a very simple model that we developed. What is remarkable is
that the surface layer does not freeze abruptly as in the case of ice, or any
other crystal. Rather, a smooth transition occurs over a temperature range of
several degrees.”
Said Ocko, “These films are better ordered and smoother than all
other organic monolayer films created to date.”
Moshe Deutsch added, “The results of this study and the
theoretical framework which it provides may lead to new ideas on how to make
defect-free, single molecule-thick films.”
Funding for this work was provided by the U.S. Department of
Energy’s Office of Science and the U.S.-Israel Binational Science Foundation.
