Although physicists have learned much about single atoms and about solid materials, they have only recently begun to explore the “mesoscopic” world of intermediate-sized objects, such as clusters of atoms. They are developing new tools to study this realm, which reveals the transition from the atomic world to the macroscopic world. One such new tool for probing the surfaces of large molecules or clusters is described in the 28 September PRL. The authors show that scattering electrons from buckminsterfullerene–the spherical molecule of pure carbon–allows simultaneous measurements of several properties, some of which are unavailable by other means. Their paper is also the first report of electron diffraction on “buckyballs.”
Electron microscopes use the wavelike nature of electrons to “illuminate” structures too small to be resolved with the longer wavelengths of normal light waves. Electron diffraction experiments are the atomic-scale equivalent, which physicists have used for many years to study atoms. Andrey Solov’yov, of the Academy of Sciences of Russia in St. Petersburg, and his colleagues have recently shown that electron diffraction of larger objects, such as clusters of atoms, is especially good for probing the surface electrons of the objects. To further develop their technique, the team chose to study buckyballs, because their spherical shell structure puts all 60 atoms and their electrons at the surface.
Solov’yov and his colleagues aimed an 809 eV electron beam at a gas of buckyballs and detected the energies of scattered electrons over a range of positions behind the gas. The most basic electron diffraction effect is identical to that of light traveling through a small hole onto a screen: The light wave interference leads to a bright central spot surrounded by light and dark rings. Just as the hole size determines the spacing of those rings, the buckyball diameter determined the spacing between “electron bright” and “electron dark” areas beyond the gas target. The effective wavelength of the electron beam was shorter than the buckyball diameter but longer than the spacing between carbon atoms. This wavelength was ideal for probing several “plasmons”–collective “sloshing” modes of delocalized buckyball electrons. (This “fluid” of electrons acts almost like water in a bathtub that can slosh back and forth at specific frequencies.) Only one of these modes had been detected in previous optical experiments.
Because is a well-characterized molecule, the team was able to compare their data with theoretical predictions to show the accuracy of the method. Solov’yov says such experiments can also determine other surface properties, such as the charge distribution and polarizability (sensitivity to electric fields). He imagines applying the technique to even more complicated objects, such as protein molecules, in the future.
Jean-Patrick Connerade, of Imperial College in London, says is an “elegant example,” nicely showcasing both the diffraction and the plasmon detection capabilities of the method. He calls the new technique “a very nice tool to tell you what happens on the surfaces of clusters.”