I’ve intended to write about the wonders of graphene and related materials for nanotechnology, both as products and as a basis for building productive nanosystems, but there is so much to say that I didn’t know where to begin. As Rosa reminds me, though, a great virtue of a blog is that you can use a current event as an excuse for starting in the middle, and not worry so much about the order of presentation.
What prompts this post is the current cover of Science, which shows an atomic resolution image of a single sheet of graphene, as seen by a new-generation, transmission electron aberration-corrected microscope (TEAM). (The upper right corner is of this page, by the way, shows gold atoms imaged with a TEAM microscope.) The paper, from the remarkable Zettl group at UC Berkeley and the Lawrence Berkeley National Laboratory, reveals the complex dynamics of atoms at the edge of a graphene sheet, driven by energy from the imaging electron beam itself.
First, a few words about the TEAM microscope.
TEAM meets a Feynman challenge
The TEAM microscope, developed at LBNL, meets a challenge that Richard Feynman proposed in his famous 1959 talk, “There’s Plenty of Room at the Bottom”:
The reason the electron microscope is so poor is that the f-value of the lenses is only 1 part to 1,000; you don’t have a big enough numerical aperture. And I know that there are theorems which prove that it is impossible, with axially symmetrical stationary field lenses, to produce an f-value any bigger than so and so; and therefore the resolving power at the present time is at its theoretical maximum. But in every theorem there are assumptions. Why must the field be symmetrical? I put this out as a challenge: Is there no way to make the electron microscope more powerful?
The TEAM approach does make the electron microscope more powerful, and does indeed so by exploiting asymmetric fields. Why has it taken so long to achieve this? One difficulty is that asymmetric (quadrupole, hexapole, and octupole) correctors must do more than just compensate for the spherical aberration introduced by the symmetric lenses: taken together, they must correct for the asymmetric distortions that they themselves cause. Abandoning symmetry along the axis of the instrument, yet achieving sub-Ångstrom resolution at the end, strikes me as being inherently very, very difficult. I’m uncommonly impressed by the achievement.
(Reading further, I find that requirements have included stabilizing electromagnetic fields to an accuracy of about 100 parts per billion and developing means for extremely precise alignment of the many multipole elements. The TEAM design simultaneously corrects for the chromatic aberration which results from a spread in electron energies.)
Graphene for Nanotechnology, and vice versa
Here are few observations, some of which I hope to expand into discussions later:
- Interest in graphene is exploding because of its unique physical properties and their potential application to nanoelectronic systems, including high-speed digital devices. This potential can be expected to drive forward a wide range of technologies for synthesizing, studying, modifying, and using graphene structures. Atomically precise shapes can be important.
- Carbon nanotubes can be viewed as graphene cylinders, and many of their virtues are also virtues of graphene. For electronic device applications, however, graphene has added promise because graphene devices can merge smoothly into networks of graphene conducting paths, all formed by lithography. Nanotubes, by contrast, must be put in place (not carved from a sheet), and they form less-than-ideal electrical contacts with metal conductors (e.g., Schottky barriers).
- Experiments with multiwall carbon nanotubes, pioneered by the Zettl group, confirm the extraordinarily low sliding friction that had been predicted for analogous nanomechanical bearings (by me, for example). These results, of course, refute the lab-bench legend that attractive intersurface forces imply so called “sticktion” (stickiness + friction), a phenomenon sometimes thought to provide the missing obstacle to the eventual implementation of high-performance nanomechanical systems.
- Graphene, nanotubes, and more complex, curved structures have the potential to serve not only as bearings, but as frameworks and moving parts for intricate mechanical and electromechanical nanosystems.
- Precisely structured graphene sheets with hundreds of atoms have been synthesized by chemical means, and controlled chemical synthesis could potentially make useful components for self-assembled composite nanosystems.
- Intricate, atomically precise graphene structures are of interest for the reasons suggested above, and the synthetic techniques just mentioned suggest that they could be made solution-phase processes guided by mechanosynthetic means (a possible middle-generation technology).
This is a lot to discuss.