Dec 7, 2012
Nanothermodynamics improves surface energy measurements
Researchers in Belgium and France have determined a new value for the surface energy of solid selenium and have found it to be considerably higher than previously thought. Selenium is a key element in nanotechnology and the new result will be important for a variety of applications in which the element is used. These include solar cells, photovoltaic panels, computer memories and antibacterial coatings, as well as light-emitting diodes that rely on cadmium selenide quantum dots. In most of these applications, the material’s surface plays a key role.
The first measurements on selenium’s surface energy were made as long ago as 1971 by M H Lee and produced a figure of about 0.175 J/m2. These experiments relied on extrapolating the surface energy of selenium melt at 20 °C. Lee also tried another technique to determine the surface energy by measuring the contact angle between the selenium surface and liquids with surface energies ranging from 0.028–0.052 J/m2. Unfortunately, he failed because of problems associated with water adsorption or impurities on the solid selenium surface.
Researchers at the University of Mons in Belgium and the Institute of Electronics, Microelectronics and Nanotechnology (IEMN) in France have now repeated these contact-angle experiments in a clean room environment (where temperature and humidity can be strictly controlled). The researchers have also used liquid probes that have a wider range of surface energies (0.047–0.718 J/m2).
“Generally, conventional probe liquids have a liquid surface energy lower than 0.1 J/m2. But by using non-conventional probe liquids like mercury and gallium, we can cover the theoretically predicted solid surface energy value of selenium (of around 0.285 J/m2), which we first calculated using a nanothermodynamic model,” explains team leader Grégory Guisbiers. “We then chose two liquids that have a lower surface energy than selenium (ethylene glycol and de-ionized water) and two liquids that that have a higher surface energy (mercury and gallium).”
Using this technique, the team found a value for selenium’s surface energy of around 0.291 J/m2, which agrees much better with theory. According to the team, the new result will be important for nanotechnology applications in which selenium is widely used. “The surface energy is the energy required to create a new surface and we all know that surface area dominates at the nanoscale,” Guisbiers told nanotechweb.org.
Antibacterial coatings and computer memories
The new, accurate, value of the surface energy of selenium may also help improve antibacterial coatings containing the element, he adds, because it might be used to help bacteria better adhere to these surfaces. It may also help in predicting the phase diagrams of nanoalloys containing selenium, something that will be important for computer memories relying on these materials.
The researchers now plan to look at how the surface energy of selenium varies with temperature. They also hope to further improve the nanothermodynamic model they used by distinguishing between surface atoms and those found at the edges and corners of the surface. “At present, our model differentiates between bulk and surface atoms but it would be much better if it could distinguish between bulk, surface, edge and corner atoms,” said Guisbiers.
The current work is detailed in Applied Physics Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org.