"When you grow a film on a solid surface, the molecules of the film tend to interlock with those of the underlying support," said Benjamin Ocko of Brookhaven National Laboratory. "But an underlying liquid surface is not ordered and provides an ideal setting for studying ultrathin states of matter without the complications of the solid support."
Ocko and colleagues deposited a nanometre-thick film of stearic acid - an organic waxlike material that is a common component of cell membranes - onto the surface of a small tray filled with mercury. To check out how the film organized itself on the surface, they used X-rays produced by Brookhaven's National Synchrotron Light Source in combination with a specially developed device that tilted the X-rays onto the surface of the mercury.
As the number of molecules on the surface increased, they formed four distinct patterns. To start with, the molecules lay down on the surface of the liquid, a phenomenon the team says has never been observed before. As more molecules were added, a second layer of prostrate molecules formed on top of the first. But then, with the molecular concentration increasing further, the molecules began to tilt and then to "stand up", forming a densely packed single layer.
The researchers, who reported their work in Science, reckon that it should be possible to choose a film pattern merely by selecting the amount of molecular coverage. In previous studies of stearic molecules deposited onto water, the molecules only exhibited the "standing up" behaviour.
"This work shows that without an underlying lattice, we can control film growth," added Moshe Deutsch of Bar-Ilan University. "By growing other molecules on a liquid support, we will be able to control the size and properties of other films, and thus tailor them for different applications, in particular their use in nanoelectronics and nanosensor technology."