Electronic life on the edge: Scientists discover the edge states of graphene nanoribbons

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As far back as the 1990s, long before anyone had actually isolated graphene– a honeycomb lattice of carbon just one atom thick– 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 University of California at Berkeley, Stanford University, and other institutions, have made the first precise measurements of the"edge states"of well-ordered nanoribbons.

Agraphenenanoribbon is a strip of graphene that may be only a few nanometers wide (a nanometer is a billionth of a meter). Theorists have envisioned thatnanoribbons, 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 ascanning 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 inNature Physics, available May 8, 2011 in advanced online publication.

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 University'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.

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.

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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. Credit: Hongjie Dai, Stanford University, and Michael Crommie et al, Lawrence Berkeley National Laboratory

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– not the artifact of a folded ribbon or a flattened nanotube– 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.

"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.

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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. Credit: Crommie et al, Lawrence Berkeley National Laboratory

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."

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Experiments settle long-standing debate about mysterious array formations in nanofilms

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(PhysOrg.com) -- Scientists at the California Institute of Technology have conducted experiments confirming which of three possible mechanisms is responsible for the spontaneous formation of three-dimensional (3-D) pillar arrays in nanofilms (polymer films that are billionths of a meter thick). These protrusions appear suddenly when the surface of a molten nanofilm is exposed to an extreme temperature gradient and self-organize into hexagonal, lamellar, square, or spiral patterns.

This unconventional means of patterning films is being developed by Sandra Troian, professor of applied physics, aeronautics, and mechanical engineering at Caltech, who uses modulation of surface forces to shape and mold liquefiable nanofilms into 3-D forms."My ultimate goal is to develop a suite of 3-D lithographic techniques based on remote, digital modulation of thermal, electrical, and magnetic surface forces,"Troian says. Confirmation of the correct mechanism has allowed her to deduce the maximum resolution or minimum feature size ultimately possible with these patterning techniques.

In Troian's method, arbitrary shapes are first sculpted from a molten film by surface forces and then instantly solidified in situ by cooling the sample."These techniques are ideally suited for fabrication of optical or photonic components that exhibit ultrasmooth interfaces,"she explains. The process also introduces some interesting new physics that only become evident at the nanoscale."Even in the land of Lilliputians, these forces are puny at best—but at thenanoscaleor smaller still, they rule the world,"she says.

The experiments leading to this discovery were highlighted on the cover of the April 29 issue of the journalPhysical Review Letters.

The experiments, designed to isolate the physics behind the process, are challenging at best. The setup requires two smooth, flat substrates, which are separated only by a few hundred nanometers, to remain perfectly parallel over distances of a centimeter or more.

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Scanning electron micrograph of solidified protrusions in a 98 nm polystyrene film guided by a remote hexagonal array of cold pins. {Credit: Courtesy of E. McLeod and S. M. Troian, {LIS2T} lab/Caltech.}

Such an experimental setup presents several difficulties, including that"no substrate this size is truly flat,"Troian says,"and even the world's smallest thermocouple is too large to fit inside the gap."In addition, she says,"the thermal gradient in the gap can exceed values of a million degrees per centimeter, so the setup undergoes significant expansion, distortion, and contraction during a typical run."

In fact, all previous studies confronted similar challenges—leading to inaccurate estimates of the thermal gradient and the inability to view the formation and growth of the structures, among other problems."To complicate matters,"Troian says,"all of the previous data in the literature were obtained at very late stages of growth, far beyond the regime of validity of the theoretical models,"Troian says.

The Caltech experiments solved these challenges by reverting to in situ measurements. The researchers replaced the top cold substrate with a transparent window fashioned from a single crystal sapphire, which permitted them to view directly the developing formations. They also used white light interferometry to help maintain parallelism during each run and to record the emerging shape and growth rate of emerging structures. Finite element simulations were also used to obtain much more accurate estimates of the thermal gradient in the tiny gap.

"When all is said and done, our results indicate that this formation process is not driven by electrostatic attraction between the film surface and the nearby substrate—similar to what happens when you run a comb through your hair—or pressure fluctuations inside the film from reflections of acoustic phonons—the collective excitations of molecules—as once believed, Troian explains."The data simply don't fit these models, no matter how hard you try,"she says. The data also did not seem to fit a third model based on film structuring by thermocapillary flow—the flow from warmer to cooler regions that accompanies surface temperature variations.

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(Left) Emergent 3-D protrusions beneath a cold transparent cylindrical mesa (400μm diameter) in a 160 nm polystyrene film subject to a thermal gradient of 240,000 degrees Celsius/cm. (Right) Formations after some have contacted the cold mesa. {Credit: Courtesy of E. McLeod and S. M. Troian, {LIS2T} lab/Caltech}

Troian proposed the thermocapillary model several years ago. Calculations for this"cold-seeking instability"suggest that nanofilms are always unstable in response to the formation of 3-D pillar arrays, regardless of the size of the thermal gradient. Tinyprotrusionsin the film experience a slightly cooler temperature than the surrounding liquid because of their proximity to a cold target. The surface tension of those tips is greater than that of the surrounding film. This imbalance generates a very strong surface force that"pulls"fluid up and"into the third dimension,"she says. This process easily gives rise to large area arrays of dimples, ridges, pillars, and other shapes. A nonlinear version of the model suggests how cold pins can also be used to form more regular arrays.

Troian was initially disappointed that the measurements did not match the theoretical predictions. For example, the prediction for the spacing between protrusions was off by a factor of two or more."It occurred to me that certain properties of thenanofilmto be input into the model might be quite different than those literature values obtained from macroscopic samples,"she notes.

She enlisted the advice of mechanical engineer Ken Goodson at Stanford, an expert on thermal transport in nanofilms, who confirmed that he'd also noticed a significant enhancement in the heat-transfer capability of certain nanofilms. Further investigation revealed that other groups around the world have begun reporting similar enhancement in optical and other characteristics of nanofilms."And voila!… by adjusting one key parameter,"Troian says,"we obtained perfect agreement between experiment and theory. How cool is that!"

Not satisfied by these findings, Troian wants to launch a separate study to find the source of these enhanced properties in nanofilms."Now that our horizon is clear, I guarantee we won't sit still until we can fabricate some unusual components whose shape and optical response can only be formed by such a process."

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Seeing an atomic thickness

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Scientists from NPL, in collaboration with Linkoping University, Sweden, have shown that regions of graphene of different thickness can be easily identified in ambient conditions using Electrostatic Force Microscopy (EFM).

The exciting properties of graphene are usually only applicable to the material that consists of one or two layers of the graphene sheets. Whilstsynthesisof any number of layers is possible, the thicker layers have properties closer to the more common bulkgraphite.

For device applications one- and two-layer graphene needs to be precisely identified apart from thesubstrateand regions of thicker graphene. Exfoliated graphene sheets up to ~100μm in size can be routinely identified by optical microscopy. However, the situation is much more complicated in the case of the epitaxial graphene grown on silicon carbide wafers with a diameter up to 5 inches where the straightforward identification of the graphene thickness is difficult using standard techniques. This research shows that EFM, which is one of the most widely accessible and simplest implementations of scanning probe microscopy, can clearly identify differentgraphenethicknesses. The technique can also be used in ambient environments applicable to industrial requirements.

This work was recently published inNano Letters.

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