Transistors and information storage devices are getting smaller and smaller. But, to go as small as the nanoscale, scientists must understand how just a few atoms of metals behave when deposited on a surface.
Physicists at the Ames Laboratory are studying the interaction of materials that are promising for use in nanoscale electronics: graphene and different types of metals. The team has discovered that the rare-earth metals dysprosium and gadolinium react strongly with graphene, while lead does not.
Michael Tringides, an Ames Laboratory senior physicist, and colleagues Myron Hupalo, an Ames Laboratory scientist, and Steven Binz, a graduate student in physics, deposited a few atoms of lead or rare-earth metals on the surface of graphene, a one-atom thick layer of carbon. In a process called self assembly, the atoms move on their own and form “islands” or smooth films on graphene. Tringides and the team then used scanning tunneling microscopy to study the islands’ geometry.
These micrographs show dendritic “island” growth of gadolinium on graphene at room temperature. This type of growth indicates a strong bond between gadolinium and graphene.
STM creates realistic images of nanoscale surfaces in fine detail. In the STM, scientists mount a sample and deposit atoms to make nano-islands. Then they use a tiny needle to approach the sample surface. The needle gets close to the sample, as close as one atom-width away, but doesn’t touch the surface, leaving a small gap.
To make measurements, the STM relies on two concepts in quantum mechanics: electrons are both a particle and a wave; and the electrons, acting as waves, “tunnel” through the gap between the sample and the needle tip.
“Tunneling allows current created by the electrons to move from the sample to the STM needle,” says Tringides. “The amount of current is dependent on how far the needle is from the sample, so the STM measures this current and uses it to keep the needle the same distance away from the sample.”
As the needle scans the surface, it encounters dips and peaks -- where the atoms are -- but maintains an equal gap above the sample. The STM then traces the path of the needle up and down to create an image of what the sample surface looks like, representing the size and height of any structures on the surface.
STM images are complemented with data gathered with a different instrument, using scattering of low-energy electrons. In this technique, a beam of electrons is focused on a sample that has islands on a smooth surface. Electrons bounce off the tops of the islands and the substrate. Because the electrons travel different distances, they form interference bands, giving scientists information about the nano-islands’ height and size.
Tringides and his team also collaborate with other research groups that use additional types of characterization techniques, such as X-ray surface scattering, which is able to examine the deeper structures of the islands close to the substrate, and low-energy electron microscopy, which is able to capture real-time movies of how atoms move on surfaces.
In its research into rare-earth islands on graphene, the team used STM to help understand how the atoms diffuse, particularly how quickly.
“In this case, the lead atoms moved quickly when we cooled them down, while the dysprosium moved slowly, even after we heated them up,” says Tringides.
How fast or slow the atoms move and form islands offers insight into how each material interacts, or shares electrons, with the graphene.
“If the atoms move fast, it means you do not have strong interaction,” Tringides says. “It’s like hockey pucks skimming along on an ice rink. There’s little interaction.”
Three different types of materials yield three different types of island growth: (above, top row) lead on a lead surface; (above, bottom row) indium on lead surface; and (right) magnesium on lead surface.
In the case of dysprosium, the slow-moving atoms suggest that the metal reacts strongly with graphene. Gadolinium has an even stronger interaction. The interaction is significant because harnessing the potential of graphene in electronics will require attaching metals to graphene to conduct electricity.
“The hope is that graphene can be used for super-fast transistors,” says Tringides. “Our work is relevant to this because when you put metal on graphene, you want to have very good contact, so the electrical resistance is low.”
Tringides also says that the rare-earth islands on graphene are tiny magnets.
“It turned out that these islands were good nanomagnets on graphene,” Tringides explains. “You have a very high density of nanomagnets. Iron also has a similar high island density. This may be useful in the future for using metals on graphene in computer memory.”
Ames Laboratory theoretical physicists C.Z. Wang and Kai-Ming Ho collaborated on the research, using calculations to confirm the experimental results about the bonds between graphene and the metals studied.
“These findings are interesting for both the fundamental physics and the potential usefulness,” says Tringides. “Whenever you say ‘nano,’ you can make a lot of something in a small size. And that might be very beneficial for something like magnetic computer memory.”
Tringides and his team are also working to understand several other questions about nanosized structures. They are examining the fast and correlated movement of lead atoms when they are placed on silicon at temperatures well below room temperature. At such low temperature the motion should be slow and random, but for a still mysterious reason the motion is extremely fast and the atoms “slide” together like a liquid. The team is also studying how magnesium nano-islands and films can adsorb hydrogen at the nanoscale, which may have implications for macroscale hydrogen storage technologies.
~ by Breehan Gerleman Lucchesi