Controlling Traffic on the Electron Highway: Researching Graphene

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In this scanning tunneling microscopy image of a graphene nanobubble, the graphene crystal is distorted and stretched along three main axes. 

In this scanning tunneling microscopy image of a graphene nanobubble, the grapheme crystal is distorted and stretched along three main axes. The strain creates pseudo-magnetic fields far stronger than any magnetic field ever produced in the laboratory.

Stretching Graphene

Stretching graphene offers an alternative path for controlling its properties. When scientists stretch graphene in a specific way, it forms tiny bubbles in which electrons act as if they're actually in a very powerful magnetic field. These bubbles provide scientists with new opportunities for manipulating electron traffic in graphene.

This discovery was also a complete accident. A team at Berkeley LabExternal link happened to be growing a layer of graphene on the surface of a platinum crystal in a vacuum chamber. As researchers tested the graphene, they noticed that its electrons were acting strangely. Rather than moving as they normally do in a smooth continuum, the electrons in the graphene nanobubbles bunched up at very specific energies. When researchers compared their results to what theory suggested, they found that the electrons were behaving as if they were in an ultra-strong magnetic field. However, there was no actual magnetic field present.

With graphene, "often we're chasing after one thing and we find something completely unexpected," Crommie said. 

Pairing with Boron Nitride

When scientists first explored graphene's properties, they placed it on top of silicon dioxide. Because silicon dioxide is a common insulator for electronics applications, it seemed like an ideal match. However, the graphene wasn't reaching its full potential.

James Hone, a Columbia University mechanical engineering professor, recalled thinking, "Is there a layered material like graphene that would be a natural fit?"

Hone's team eventually discovered that graphene works much better when you put it on boron nitride instead. Like graphene, boron nitride can be made only a few atoms thick and has the same honeycomb structure. However, it's an insulator that impedes electrons from moving through it.

They found that putting boron nitride and graphene together can produce a new materialExternal link whose properties are very flexible. This combination is so promising that Alex Zettl from Berkeley Lab joked that his lab is now "Boron Nitride R Us." He commented, "Having the boron nitride influence the graphene is a very powerful tool."

Ordinary light may offer a way to influence electrons in this new composite material. Berkeley Lab scientists have found that they can use light from a simple lamp to create an essential semiconductor device called a "p-n junction."External link P-n junctions have one side that's positive and lacks electrons and another side that's negative with extra electrons. By carefully designing these junctions, engineers can control how and when electrons move between the two sides of a material. They're like the gates that lift up and down at a toll booth.

Scientists realized that if they could put fixed, static charges in the boron nitride in a specific way, they could generate a p-n junction in the nearby graphene. To create the p-n junction, the scientists first prepared the graphene highway to have an excess of electrons, or be an n-type region. Then, by shining a light on the underlying boron nitride, they created a pothole, or p-type region, in the graphene. So with a light pulse and the boron nitride as a mediator, they could "write" p-n junctions – toll gates – into the graphene as needed.

Even after scientists turned off the light, the activation of the boron nitride and its influence on the electron traffic in the nearby graphene, stayed in place for days. The scientists also discovered that they could erase and re-create these junctions, which could be important for designing electronic devices.

Now researchers are using scanning tunneling microscopesExternal link, which use nanometer-sized tips to conduct electricity, to do the same thing with more precision.

Charging Up Empty Spaces in Graphene

Because of its unique structure, graphene remains stable even when scientists punch holes in it. Andrei's team from Rutgers University took advantage of this fact to create an "artificial atom" that influences nearby electrons in the undamaged part of graphene. First, researchers shot helium at graphene on a substrate, knocking out a single carbon atom. They then used a scanning tunneling microscope to apply a positive charge to the substrate under the empty space where the missing atom used to sit. Like a real atom, that positive charge influenced the orbits of electrons in the surrounding graphene. Creating these artificial atoms could be another way that future devices could control electron flow in graphene.

The Future of Graphene

Perhaps the most surprising of these twists and turns is that the future may not lie in graphene at all. As scientists investigated graphene's unique electronic properties, they discovered new extremely thin materials made from elements other than carbon. If a material is only a few atoms thick and has a honeycomb structure, it can demonstrate many of graphene's electronic properties. In fact, scientists have found materials made of silicon, germanium, and tin that act strikingly similar to graphene. Using these materials by themselves or in combination with graphene may offer better characteristics than graphene alone.

In the meantime, scientists will continue to investigate the strange features of this frequently surprising material. As Philip Kim, a Harvard University physics professor said, "[Graphene] always provides you with some new, exciting science that we have not expected."

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