Forgive the pun, but a new way to get unusual behavior from graphene or europium titanate is a stretch. Literally a stretch, as in taking the material (which is produced in sheets) and stretching it. Stretching is a basic physical technique but applied to unusual materials it sometimes produces unexpected results. In this case two studies, one by Lawrence Berkeley Labs (California, USA) with graphene, and another by researchers at Cornell University (New York, USA) with europium titanate have shown that not only are the properties of the material radically changed, but also their potential application.
Graphene, a one atom thick sheet of pure carbon, is already one of the most promising of nanomaterials. Scientists are continuously expanding the knowledge of this ‘simple’ but remarkable material. One potentially important addition, which is presented in the July 30, 2010 issue of Science [Strain-Induced Pseudo–Magnetic Fields Greater Than 300 Tesla in Graphene Nanobubbles] was discovered – by accident – when in a lab experiment a graphene sheet was grown on the surface of a platinum crystal. A graphene sheet naturally aligns its carbon atoms in a hexagonal pattern. However, this pattern doesn’t align with the triangular shape of the platinum crystal surface. In effect, the underlying triangle shape of platinum crystal pulled or stretched the graphene sheet in three directions, putting it under strain. Here’s where the unexpected and most interesting results occurred: The strain causes the graphene to produce raised triangular bubbles about 4-10 nanometers across.
The bubble formation is interesting, but it was discovered by looking into the nanobubbles spectroscopically with a scanning tunneling microscope that electrons within the bubbles were separated into bands of quantized energy levels – the electrons occupy orbits with discrete energy values, called Landau levels. This is characteristic of electrons in a magnetic field. But there is no magnetic field in the graphene nanobubbles. There is some kind of a pseudo-magnetic field, and a strong one at that, running as high as 300 tesla, which in current laboratory equipment can only be produced in exceptional bursts. By comparison a medical MRI (magnetic resonance imager) operates at less than 10 tesla.
This behavior from stretching graphene was predicted, in theory, just last year. It’s rare that theory and later experiments match so well, so quickly. As Michael Crommie, professor of physics at the University of California Berkeley and lead author of the paper puts it:
“Theorists often latch onto an idea and explore it theoretically even before the experiments are done, and sometimes they come up with predictions that seem a little crazy at first. What is so exciting now is that we have data that shows these ideas are not so crazy,” Crommie said. “The observation of these giant pseudomagnetic fields opens the door to room-temperature ‘straintronics,’ the idea of using mechanical deformations in graphene to engineer its behavior for different electronic device applications.”
[Source: Nanotechnology Today]
The other research, published in Nature August 19, 2010 issue [Editor’s summary: Multiferroics made easy], involves a rather obscure and relatively dull material called europium titanate. Take some of this material, slice it only nanometers thick and place it over a substrate (surface) of another obscure material, dysprosium scandate (also an oxide), and presto: Just like the graphene sheet on a platinum crystal, the thin film of europium titanate is stretched by the crystalline shape of the dysprosium. As a result, it becomes both ferroelectric (electrically charged or polarized) and ferromagnetic (having a permanent magnetic field). Very few materials are both ferroelectric and ferromagnetic and in most of them the effect is quite weak. The europium titanate is stronger than any current material by a factor of 1000, which makes it much more interesting to exploit for both properties.
This new approach to ferromagnetic ferroelectrics could prove a key step toward the development of next-generation memory storage, superb magnetic field sensors and many other applications long dreamed about. But commercial devices are a long way off; no devices have yet been made using this material. The Cornell experiment was conducted at an extremely cold temperature – about 4 degrees Kelvin (-452 Fahrenheit). The team is already working on materials that are predicted to show such properties at much higher temperatures.
Both of these research tracks explore what is loosely called ‘stretching’ – it’s really more like ‘conforming’ as one molecular structure is forced to take on a different shape by another underlying molecular structure – although the effect does tend to pull the target material apart like stretching. What Michael Crommie called it – straintronics might be more accurate (and colorful). Whatever it’s called, straining or stretching nanomaterials is producing some very interesting and potentially very useful new properties that will likely be exploited in the next few years.