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Posted: Nov 12th, 2012
Sub-nanometer electron-beam engineering of graphene
(Nanowerk Spotlight) The foundation of nanotechnology research on graphene and other materials is the fact that, thanks to recent technological advances, scientists now have tools to observe materials one atom at a time, including sensitive ones such as graphene which is only one atom thick.
"Within graphene research, transmission electron microscopy (TEM) has proven to be an extremely useful and versatile characterization tool," Dr. Mark H. Rümmeli, who leads the Molecular Nanostructures group at the Leibniz Institute for Solid State and Materials Research in Dresden, tells Nanowerk. "However, the electron beam can interact with the sample leading to its modification during the process. This may be an undesirable effect and measures to avoid this do exist. In other cases, however, electron beam–sample interactions can be useful for nanoengineering or nanomanufacturing. It is therefore crucially important to understand how a material responds to the electron beam and the environment inside a TEM."
In new work, Rümmeli and his team, including scientists from Technische Universität Dresden and Wuhan University, have now demonstrated that damage-free sculpting of graphene with condensed electron beams is feasible. As the researchers report in the October 30, 2012 online edition of ACS Nano ("Programmable Sub-nanometer Sculpting of Graphene with Electron Beams"), they developed a technique that does not require any additional thermal or current annealing treatments.
Left panel: Schematic illustration of condensed electron beam 'cutting' or 'sculpting' of graphene. Right panel: Electron beam lithography for the acronym for the Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden - IFW. (Image: Dr. Rümmeli, IFW Dresden)
Based on a better understanding of how electron beams interact with graphene, especially for low energy electrons (accelerated at 80 kV or below), the core finding of this work is the demonstration that electron beams at high acceleration voltages and low acceleration voltages, – above and below the knock-on damage threshold for graphene, respectively – can be used to structure graphene with sub-nanometer precision in a programmable manner. In a previous paper, the team conducted systematic in situ studies to investigate the response of amorphous carbon under 80 keV electron radiation ("Amorphous Carbon under 80 kV Electron Irradiation: A Means to Make or Break Graphene").
In essence, the results of this study provide an electron toolbox to engineer graphene that can ultimately be used for the fabrication of active graphene devices. This has not previously been demonstrated.
As Rümmeli points out, these findings are also attractive as a fabrication route on a larger scale and are an ideal platform to study structure-property relationships with atomic precision.
The need to structure graphene arises because graphene, unlike silicon, lacks an electronic band gap – the gap being an energy range that cannot be occupied by electrons – and therefore has no switching capability; which is essential for electronic applications. Opening an energy gap in graphene's electron energy spectrum is therefore a critical prerequisite for instance for creating graphene transistors (also see our previous Nanowerk Spotlight: "Employing weak interactions to engineer band structures in graphene "). If one is able to structure graphene to a ribbon sufficiently narrow – say <10 nm – one can open an energy gap making it attractive for electronic devices such as nanoscale transistors.
"However" says Rümmeli, "such ribbons are poorly understood and are highly sensitive to their edge structure. Our technology potentially paves the way to not only structure graphene, potential with atomic precision, but also to study structure property relationship with atomic precision. The implications are enormous."
As a proof of concept, the team demonstrated the annealing-free fabrication of graphene nanoribbons and single carbon chain structures inside a TEM. This fabrication process is controlled and reproducible, the sculpting process can be completely tracked, and corrections or adjustments are easy to implement.
"We demonstrate the potential of the technique for routine graphene engineering by preparing graphene constrictions, which upon changing TEM mode can be further engineered using a previously established technique to restructure the graphene nanoribbons to single-atom carbon chains," Rümmeli sums up the team's results.
While the basic science and technology required for this technique have been known, until now no one put the different pieces together to demonstrate the feasibility to combine the different electron energy dependent interactions and provide programmed engineering.
In essence this work pioneers the in situ fabrication of electronic graphene – and other nanomaterials – inside a TEM.
The researchers note that it will be interesting to see just how accurate such electron beam sculpting (electron beam lithography) can be. The resolution will depend on how small a focused electron beam can be and how accurately it can be spatially controlled.
"These days, with advances in spherical aberration and chromatic aberration correction, one can anticipate resolutions toward the Angstrom level," says Rümmeli. "The ultimate goal of course is atomic engineering resolution. i.e. structural engineering of individual atoms."
The challenge for this technique to work in a commercial production environment will be to develop electron beam driven lithography over large areas and reasonable time. However, these issues are not insurmountable.