Graphene nanoribbons are narrow sheets of carbon atoms only one layer thick. Their width, and the angles at which the edges are cut, produce a variety of electronic states, which have been studied with precision for the first time using scanning tunneling microscopy and scanning tunneling spectroscopy. Image: Lawrence Berkeley National Laboratory |
As far back as the 1990s, long before anyone had actually isolated grapheme,
theorists were predicting extraordinary properties at the edges of graphene
nanoribbons. Now physicists at the U.S. Department of Energy’s Lawrence
Berkeley National Laboratory (Berkeley Lab), and their colleagues at the Univ. of California
at Berkeley, Stanford Univ.,
and other institutions, have made the first precise measurements of the “edge
states” of well-ordered nanoribbons.
A graphene nanoribbon is a strip of graphene that may be only a few
nanometers wide. Theorists have envisioned that nanoribbons, depending on their
width and the angle at which they are cut, would have unique electronic,
magnetic, and optical features, including band gaps like those in
semiconductors, which sheet graphene doesn’t have.
“Until now no one has been able to test theoretical predictions regarding
nanoribbon edge-states, because no one could figure out how to see the
atomic-scale structure at the edge of a well-ordered graphene nanoribbon and
how, at the same time, to measure its electronic properties within nanometers
of the edge,” says Michael Crommie of Berkeley Lab’s Materials Sciences
Division (MSD) and UC Berkeley’s Physics Division, who led the research. “We
were able to achieve this by studying specially made nanoribbons with a
scanning tunneling microscope.”
The team’s research not only confirms theoretical predictions but opens the
prospect of building quick-acting, energy-efficient nanoscale devices from
graphene-nanoribbon switches, spin-valves, and detectors, based on either
electron charge or electron spin. Farther down the road, graphene nanoribbon
edge states open the possibility of devices with tunable giant
magnetoresistance and other magnetic and optical effects.
Crommie and his colleagues have published their research in Nature Physics.
The well-tempered
nanoribbon
“Making flakes and sheets of graphene has become commonplace,” Crommie
says, “but until now, nanoribbons produced by different techniques have
exhibited, at best, a high degree of inhomogeneity”—typically resulting in
disordered ribbon structures with only short stretches of straight edges
appearing at random. The essential first step in detecting nanoribbon edge
states is access to uniform nanoribbons with straight edges, well-ordered on
the atomic scale.
Hongjie Dai of Stanford Univ.’s Department of Chemistry and
Laboratory for Advanced Materials, a member of the research team, solved this
problem with a novel method of “unzipping” carbon nanotubes chemically.
Graphene rolled into a cylinder makes a nanotube, and when nanotubes are
unzipped in this way the slice runs straight down the length of the tube,
leaving well-ordered, straight edges.
By “unzipping” carbon nanotubes, regular edges with differing chiralities can be produced between the extremes of the zigzag configuration and, at a 30-degree angle to it, the armchair configuration. Image: Lawrence Berkeley National Laboratory |
Graphene can be wrapped at almost any angle to make a
nanotube. The way the nanotube is wrapped determines the pitch, or “chiral
vector,” of the nanoribbon edge when the tube is unzipped. A cut straight along
the outer atoms of a row of hexagons produces a zigzag edge. A cut made at a
30-degree angle from a zigzag edge goes through the middle of the hexagons and
yields scalloped edges, known as “armchair” edges. Between these two extremes are
a variety of chiral vectors describing edges stepped on the nanoscale, in
which, for example, after every few hexagons a zigzag segment is added at an
angle.
These subtle differences in edge structure have been predicted to produce
measurably different physical properties, which potentially could be exploited
in new graphene applications. Steven Louie of UC Berkeley and Berkeley Lab’s
MSD was the research team’s theorist; with the help of postdoc Oleg Yazyev,
Louie calculated the expected outcomes, which were then tested against
experiment.
Chenggang Tao of MSD and UCB led a team of graduate students in performing
scanning tunneling microscopy (STM) of the nanoribbons on a gold substrate,
which resolved the positions of individual atoms in the graphene nanoribbons.
The team looked at more than 150 high-quality nanoribbons with different
chiralities, all of which showed an unexpected feature, a regular raised border
near their edges forming a hump or bevel. Once this was established as a real
edge feature, the chirality and electronic properties of well-ordered
nanoribbon edges could be measured with confidence, and the edge regions
theoretically modeled.
Electronics at the
edge
“Two-dimensional graphene sheets are remarkable in how freely electrons
move through them, including the fact that there’s no band gap,” Crommie says. “Nanoribbons are different: electrons can become trapped in narrow channels
along the nanoribbon edges. These edge-states are one-dimensional, but the
electrons on one edge can still interact with the edge electrons on the other
side, which causes an energy gap to open up.”
Using an STM in spectroscopy mode (STS), the team measured
electronic density changes as an STM tip was moved from a nanoribbon edge
inward toward its interior. Nanoribbons of different widths were examined in
this way. The researchers discovered that electrons are confined to the edge of
the nanoribbons, and that these nanoribbon-edge electrons exhibit a pronounced
splitting in their energy levels.
A scanning tunneling microscope determines the topography and orientation of the graphene nanoribbons on the atomic scale. In spectroscopy mode, it determines changes in the density of electronic states, from the nanoribbon’s interior to its edge. Image: Lawrence Berkeley National Laboratory |
“In the quantum world, electrons can be described as waves in addition to
being particles,” Crommie notes. He says one way to picture how different edge
states arise is to imagine an electron wave that fills the length of the ribbon
and diffracts off the atoms near its edge. The diffraction patterns resemble
water waves coming through slits in a barrier.
For nanoribbons with an armchair edge, the diffraction pattern spans the
full width of the nanoribbon; the resulting electron states are quantized in
energy and extend spatially throughout the entire nanoribbon. For nanoribbons
with a zigzag edge, however, the situation is different. Here diffraction from
edge atoms leads to destructive interference, causing the electron states to
localize near the nanoribbon edges. Their amplitude is greatly reduced in the
interior.
The energy of the electron, the width of the nanoribbon, and the chirality
of its edges all naturally affect the nature and strength of these nanoribbon
electronic states, an indication of the many ways the electronic properties of
nanoribbons can be tuned and modified.
Says Crommie, “The optimist says, ‘Wow, look at all the ways we can control
these states—this might allow a whole new technology!’ The pessimist says, ‘Uh-oh, look at all the things that can disturb a nanoribbon’s behavior—how are
we ever going to achieve reproducibility on the atomic scale?'”
Crommie himself declares that “meeting this challenge is a big reason for
why we do research. Nanoribbons have the potential to form exciting new
electronic, magnetic, and optical devices at the nanoscale. We might imagine
photovoltaic applications, where absorbed light leads to useful charge
separation at nanoribbon edges. We might also imagine spintronics applications,
where using a side-gate geometry would allow control of the spin polarization
of electrons at a nanoribbon’s edge.”
Although getting there won’t be simple—”The edges have to be controlled,”
Crommie emphasizes—”what we’ve shown is that it’s possible to make nanoribbons
with good edges and that they do, indeed, have characteristic edge states
similar to what theorists had expected. This opens a whole new area of future
research involving the control and characterization of graphene edges in
different nanoscale geometries.”
