Two papers in a recent issue of Science suggest that graphene is rapidly moving from being “just” a nanotech wonder material to becoming relevant to atomically precise nanotechnologies. Most of the attention that has been focused on graphene has been due to its properties as a one-atom-thick two-dimensional surface. Significant advances in studying and manipulating graphene edges point toward treating graphene nanostructures as atomically precise objects. In the first paper (abstract) scientists at MIT demonstrated a way to use heat to control the edges of graphene nanoribbons. From nanotechweb.org, written by Belle Dumé “Smoothing out graphene ribbon edges” (requires free registration):
As-processed graphene nanoribbons are limited by their edges. This is because even minute deviations from the ideal edge shapes, “armchair” and “zigzag”, can seriously degrade graphene’s exceptional properties. Now, researchers in the US have developed a new method to smooth out nanoribbon edges using a heat treatment that makes most of the resulting edges either zigzag or armchair.
One of the most important challenges in graphene nanoribbon technology is controlling the edge morphology, says lead author Xiaoting Jia, a member of Mildred Dresselhaus’ team at the Massachusetts Institute of Technology. This has proved difficult to do until now. The new technique is an efficient way to transform defective rough edges in the ribbons into atomically smooth ones using “Joule heating”. Here, an electrical current is applied across a suspended graphene nanoribbon inside a high-resolution transmission electron microscope (HRTEM).
With enough heat, the carbon atoms at the edges start to move and reposition themselves either into zigzag or armchair configurations. The electronic properties of the nanowires depend on which configuration the edges are in — the wires are metallic when the edges are zigzag and less conducting when they are armchair-shaped. The researchers observe the transition from rough to smooth edges inside the HRTEM.…
In the second paper (abstract) the atoms were actually observed moving in real time. In a remarkable demonstration of watching individual atoms move in real time, and of the usefulness of that ability, scientists at the University of California at Berkeley and the Lawrence Berkeley National Laboratory (including the 2003 Foresight Institute Feynman Prize winners in the Theory category Marvin L. Cohen and Steven G. Louie) were able to observe carbon atoms moving around the edges of a hole punched in a graphene crystal. They found a specific arrangement of atoms along the edge of the hole called the zigzag configuration to be the most stable arrangement, and calculated that this fact is promising for eventually using nanoscale spintronics in the computer industry. In a Lawrence Berkeley National Laboratory press release “Berkeley Scientists Produce First Live Action Movie of Individual Carbon Atoms in Action“, Lynn Yarris writes:
…Researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), working with TEAM 0.5, the world’s most powerful transmission electron microscope, have made a movie that shows in real-time carbon atoms repositioning themselves around the edge of a hole that was punched into a graphene sheet. Viewers can observe how chemical bonds break and form as the suddenly volatile atoms are driven to find a stable configuration. This is the first ever live recording of the dynamics of carbon atoms in graphene.
“The atom-by-atom growth or shrinking of crystals is one of the most fundamental problems of solid state physics, but is especially critical for nanoscale systems where the addition or subtraction of even a single atom can have dramatic consequences for mechanical, optical, electronic, thermal and magnetic properties of the material,” said physicist Alex Zettl who led this research. “The ability to see individual atoms move around in real time and to see how the atomic configuration evolves and influences system properties is somewhat akin to a biologist being able to watch as cells divide and a higher order structure with complex functionality evolves.”…
Rubbing graphene off the end of a pencil tip and suspending the specimen in an observation grid, Zettl and his colleagues used prolonged irradiation from TEAM 0.5’s electron beam (set at 80 kV) to introduce a hole into the graphene’s pristine hexagonal carbon lattice. Focusing the beam to a spot on the sheet blows out the exposed carbon atoms to create the hole. Since atoms at the edge of the hole are continually being ejected from the lattice by electrons from the beam the size of the hole grows. The researchers used the same TEAM 0.5 electron beam to record for analysis a movie showing the growth of the hole and the rearrangement of the carbon atoms.
“Atoms that lose their neighbors become highly volatile, and move around rapidly, continually repositioning themselves from one metastable configuration to the next,” said Zettl. “Although configurations come and go, we found a zigzag configuration to be the most stable. It occurs more often and over longer length scales along the edge than the other most common configuration, which we called the armchair.”
Understanding which of these atomic configurations is the most stable is one of the keys to predicting and controlling the stability of a device that utilizes graphene edges. The discovery of strong stability in the zigzag configuration is particularly promising news for the spintronic dreams of the computer industry.
Two years ago, co-authors Cohen and Louie, theorists who hold joint appointments with Berkeley Lab’s Materials Sciences Division and UC Berkeley, calculated that nanoribbons of graphene can conduct a spin current and could therefore serve as the basis for nanosized spintronic devices. Spin, a quantum mechanical property arising from the magnetic field of a spinning electron, carries a directional value of either “up” or “down” that can be used to encode data in the 0s and 1s of the binary system. Spintronic devices promise to be smaller, faster and far more versatile than today’s devices because — among other advantages — data storage does not disappear when the electric current stops.
Said Cohen, “Our calculations showed that zigzag graphene nanoribbons are magnetic and can carry a spin current in the presence of a sufficiently large electric field. By carefully controlling the electric field, it should be possible to generate, manipulate, and detect electron spins and spin currents in spintronics applications.”
Said Louie, “If electric fields can be made to produce and manipulate a 100-percent spin-polarized carrier system through a chosen geometric structure, it will revolutionize spintronics technology.”…
Key to observing the individual atoms move along the edge of the graphene hole in real time is an extraordinarily powerful electron microscope called the TEAM 0.5. In a very informative post over at Metamodern, Eric Drexler explains how the TEAM 0.5 meets a challenge made by Richard Feynman in his famous 1959 talk to improve the electron microscope.
—Jim
