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 atomic dynamics of the hole in graphene was simulated via a kinetic Monte Carlo method. Probabilities for atomic migration, insertion and ejection were determined by ab-initio calculation. The simulation starts with a predefined hole in a graphene sheet. As it proceeds, the hole grows and the atoms along the edge rearrange themselves. The zigzag configuration is found to dominate the armchair one. Credit: National Center for Electron Microscopy
"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."
Zettl holds joint appointments with Berkeley Lab's Materials Sciences Division (MSD) and the Physics Department at the University of California (UC) Berkeley, where he is the director of the Center of Integrated Nanomechanical Systems. He is the principal author of a paper describing this work which appears in the March 27, 2009 issue of the journal Science. The paper is entitled, "Graphene at the Edge: Stability and Dynamics." Co-authoring this paper with Zettl were Çağlar Girit, Jannik Meyer, Rolf Erni, Marta Rossell, Christian Kisielowski, Li Yang, Cheol-Hwan Park, Michael Crommie, Marvin Cohen and Steven Louie.
In their paper, the authors credit the unique capabilities of TEAM 0.5 for making their movie possible. TEAM stands for Transmission Electron Aberration-corrected Microscope. The newest instrument at Berkeley Lab's National Center for Electron Microscopy (NCEM) - a DOE national user facility and the country's premier center for electron microscopy and microcharacterization - TEAM 0.5 is capable of producing images with half angstrom resolution, which is less than the diameter of a single hydrogen atom.
Said NCEM director Ulrich Dahmen of this achievement with TEAM 0.5, "The real-time observation of the movements of edge atoms could lead to a new level of understanding and control of nanomaterials. With further advances in electron-optical correctors and detectors it may become possible to increase the sensitivity and speed of such observations, and begin to see a live view of many other reactions at the atomic scale."
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.
Movie produced with the TEAM 0.5 microscope shows the growth of a hole and the atomic edge reconstruction in a graphene sheet. An electron beam focused to a spot on the sheet blows out the exposed carbon atoms to make the hole. The carbon atoms then reposition themselves to find a stable configuration. Credit: National Center for Electron Microscopy
"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."
The theorists were enthusiastic about actually being able to see their predictions in action.
Said Cohen, "This work is an excellent example of the power of attacking a fundamental problem through a combination of theory, experiment and cutting edge instrumentation. The instrument is one of the world's best and allows us to see atoms move, the theory allows us to make realistic models, and the experiment was performed through the magic hands of Alex Zettl to ensure that the right measurement was done in the right way."
Said Louie, "As the old saying goes - seeing is believing. The visual verification of the formation and stability of zigzag edges in the live atomic images from TEAM 0.5 is very satisfying. Furthermore, the ability to simultaneously see atomic structure and perform physical measurements, using the kind of set-up that the Zettl group has at NCEM, should greatly accelerate the cycle of discovery, theoretical understanding, applications and further discovery."
For Zettl and his movie-making collaborators, next up they will correlate the atomic dynamics in graphene that they can now observe in real time with such properties as electrical conduction, optical response and magnetism. This will be a major advance towards fully understanding and applying graphene to spintronic technology as well as other electronic and photovoltaic devices.
"While, graphene is particularly exciting, our experimental methods should be applicable to other materials, including other 2-D systems as well," Zettl said. "We are vigorously pursuing these areas of research in collaboration with the theorists and the staff at NCEM."
Said NCEM principal investigator and co-author of this paper, Kisielowski, "The ability to observe the dynamics of single carbon atoms is a dream come true that reaches beyond investigations of graphene. In fact it gets us one step closer to understanding artificial photosynthesis, which is considered to be an ultimate energy technology and is being pursued at Berkeley Lab through the Helios Project."
TEAM 0.5 features state-of-the-art technical advances including an extremely bright electron source, ultra-stable electronics to reduce drift and, perhaps most importantly, the ability to provide optical corrections for spherical aberration (blurring). By making points of light look like disks, spherical aberrations have been the prime limiting factors in the resolution of transmission electron microscopy.
Its ability to correct spherical aberrations makes TEAM 0.5 highly versatile. It can be used for broad-beam "wide-angle" imaging as well as for scanning transmission electron microscopy (STEM), in which the tightly focused electron beam is moved across a sample as a probe. In the STM mode, TEAM 0.5 is capable of performing spectroscopy on one atom at a time — an ideal way to precisely locate impurities in an otherwise homogeneous sample, such as individual dopant atoms in a semiconductor. Aberration correction also enables TEAM 0.5 to produce high resolution images at relatively low electron beam energies. Because of their longer wavelengths, lower energy electrons are more difficult to focus than higher energy electrons. Aberration correction overcomes this problem.
TEAM 0.5 was designed and constructed through a collaboration led by Berkeley Lab and including DOE's Argonne and Oak Ridge National Laboratories, the Frederick Seitz Materials Laboratory of the University of Illinois, and two private companies specializing in electron microscopy, the FEI Company headquartered in Portland, Oregon, and CEOS of Heidelberg, Germany.
The TEAM project is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Zettl's research was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences, and Engineering Division, of the U.S. Department of Energy.
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