Scientists from the Brookhaven National Laboratory, Harvard University, and Bar-Ilan University in Israel have grown ultrathin films made of organic molecules on the surface of liquid mercury. Their results were reported recently in Science.
Their findings showed a series of new molecular structures that could lead to novel applications in nanotechnology, which involves manipulating materials at the atomic scale.
Ultrathin films are becoming increasingly important for fast-developing applications, such as faster and smaller electronic and magnetic devices, advanced biotechnological membranes, and controlled drug release in the human body. The Brookhaven team is a leader in the field of liquid surface-supported film growth, with expertise gained over the past 20 years.
Films grown on solid surfaces tend to interlock with those of the substrate on a molecular level. This is not possible when films are grown on a liquid as the substrate is not ordered. This situation provides the ideal environment for studying the ultrathin states of matter without the complications of solid supports.
Their first experiments involved depositing nanometre thin layers of stearic acid (a wax-like material and common component of cell membranes) on liquid mercury. Stearic acid is insoluble is mercury and thus floats to the surface.
The researchers used x-rays from the National Synchotron Light source at Brookhaven and a unique instrument for tilting the x-rays to study the films.
They found that four distinct patterns were formed as the molecules were deposited on the surface. At the beginning, when there are few molecules, they tend to try and take up as much space as possible by lying on the surface. As the number of molecules increases, they form a second layer on top of the first.
As more molecules are deposited, they tilt upwards and eventually stand up to make more room for adjacent molecules. They stand up straight only when squeezed by other molecules. This allows dense packing in each layer.
The researchers believe that they may be able to control the film structure via the level of molecular coverage chosen.
These findings are unique. Previous studies performed on water have shown molecules only stand up on the surface.
They hope that future work will show that by growing other molecules on a liquid support, we will be able to control the size and properties of the respective films, and thus tailor them for different applications, such as nanoelectronics and nanosensor technology.