Placed on boron nitride, graphene shows much smaller electric charge fluctuations, shown in red and blue (left) than when mounted on a silicon oxide wafer (right). Credit: Brian LeRoy/Univ. of Arizona |
Graphene may one day replace conventional silicon
microchips, making devices smaller, faster, and more energy-efficient.
In addition to potential applications in
integrated circuits, solar cells, miniaturized bio devices, and gas molecule
sensors, the material has attracted the attention of physicists for its unique
properties in conducting electricity on an atomic level.
Otherwise known as pencil “lead,”
graphene has very little resistance and allows electrons to behave as massless
particles like photons, or light particles, while traveling through the
hexagonal grid at very high speeds.
The study of the physical properties and potential
applications of graphene, however, has suffered from a lack of suitable carrier
materials that can support a flat graphene layer while not interfering with its
electrical properties.
Researchers in the Univ.
of Arizona’s physics department along
with collaborators from the Massachusetts Institute of Technology and the
National Materials Science Institute in Japan have now taken an important
step forward toward overcoming those obstacles.
They found that by placing the graphene layer on a
material almost identical in structure, instead of the commonly used silicon
oxide found in microchips, they could improve its electronic properties.
Substituting silicon wafers with boron nitride, a
graphene-like structure consisting of boron and nitrogen atoms in place of the
carbon atoms, the group was the first to measure the topography and electrical
properties of the resulting smooth graphene layer with atomic resolution.
Under the scanning tunneling microscope, graphene reveals its honeycomb structure made up of rings of carbon atom, visible as small hexagons. The larger hexagons result from an interference process occurring between the graphene and the underlying boron nitride. The scale bar measures one nanometer, or one billionth of a meter. Credit: Brian LeRoy/Univ. of Arizona |
The results are published in the advance online
publication of Nature Materials.
“Structurally, boron nitride is basically the
same as graphene, but electronically, it’s completely different,” said
Brian LeRoy, an assistant professor of physics and senior author of the study.
“Graphene is a conductor, boron nitride is an insulator.”
“We want our graphene to sit on something
insulating, because we are interested in studying the properties of the
graphene alone. For example, if you want to measure its resistance, and you put
it on metal, you’re just going to measure the resistance of the metal because
it’s going to conduct better than the graphene.”
Unlike silicon, which is traditionally used in
electronics applications, graphene is a single sheet of atoms, making it a
promising candidate in the quest for ever smaller electronic devices. Think
going from a paperback to a credit card.
“Using a scanning tunneling microscope, we
can look at atoms and study them,” LeRoy added. “When we put graphene
on silicon oxide and look at the atoms, we see bumps that are about a nanometer
in height.”
While a nanometer may not sound like much, to an
electron whizzing along in a grid of atoms, it’s quite a bump in the road.
“It’s basically like a piece of paper that
has little crinkles in it,” LeRoy explains. “But if you put the
paper, in this case the graphene, on boron nitride, it’s much flatter. It
smooths out the bumps by an order of magnitude.”
LeRoy admits the second effect achieved by his
research team is a bit harder to explain.
“When you have graphene sitting on silicon
oxide, there are trapped electric charges inside the silicon oxide in some
places, and these induce some charge in the overlying graphene. You get quite a
bit of variation in the density of electrons. If graphene sits on boron
nitride, the variation is two orders of magnitude less.”
In his lab, LeRoy demonstrates the first step in
characterizing the graphene samples: He places a tiny flake of graphite on
sticky tape, folds it back on itself and peels it apart again, in a process
reminiscent of a Rorschach Test.
“You fold this in half,” he explained,
“and again, and again, until it gets thin. Graphene wants to peel off into
these layers, because the bonds between the atoms in the horizontal layer are
strong, but weak between atoms belonging to different layers. When you put this
under an optical microscope, there will be regions with one, two, three, four
or more layers. Then you just search for single-layer ones using the
microscope.”
Jiamin Xue, Philippe Jacquod and Brian LeRoy (left to right) with the scanning tunneling microscope they use to study graphene, the thinnest material on Earth. Credit: Patrick McArdle/UANews |
“It’s hard to find the sample because it’s
very, very small,” said Jiamin Xue, a doctoral student in LeRoy’s lab and
the paper’s leading author. “Once we find it, we put it between two gold
electrodes so we can measure the conductance.”
To measure the topography of the graphene surface,
the team uses a scanning tunneling microscope, which has an ultrafine tip that
can be moved around.
“We move the tip very close to the graphene,
until electrons start tunneling to it,” Xue explained. “That’s how we
can see the surface. If there is a bump, the tip moves up a bit.”
For the spectroscopic measurement, Xue holds the
tip at a fixed distance above the sample. He then changes the voltage and
measures how much current flows as a function of that voltage and any given
point across the sample. This allows him to map out different energy levels
across the sample.
“You want as thin an insulator as possible,” LeRoy added.
“The initial idea was to pick something flat but insulating. Because boron
nitride essentially has the same structure as graphene, you can peel it into
layers in the same way. Therefore, we use a metal as a base, put a thin layer
of boron nitride on it and then graphene on top.”
