Visualizing the Electron Wind Force in Nanostructures
Ellen D. Williams
[This is an invited article based on a recently published work by the authors -- 2Physics.com]
Authors: Chenggang Tao, William G. Cullen, and Ellen D. Williams
Affiliation: Materials Research Science and Engineering Center & Department of Physics, University of Maryland, USA
Link to the Williams Lab >>
As electronic devices get smaller and smaller, they are more susceptible to effects of the charge carriers flowing through them. The charge carriers (electrons in metals) can push atoms around by collisions. For some specific types of atomic structures (for example, atomic “steps”, where the surface height changes by one layer of atoms), the scattering force is much stronger than had been thought . These structures are ubiquitous for the surfaces of solid materials, and this becomes very important for nanoscale electronics where surfaces make up a much bigger fraction of the material.
W. G. Cullen (left) and Chenggang Tao (right)
A very careful measurement is needed to directly observe the forces that electrons exert on the atoms of the material which they are passing through. Yet over a long time, the effects of this force accumulate and can lead to failure of wires which connect components in integrated circuits - a process known as electromigration [2, 3]. In our experiment, we carefully created different types of nanoscale structures on top of a very thin wire of silver. One type of structure consists of single-atom high “islands” that contain between 100 and 100,000 atoms. Another type consists of single-atom high “steps” decorated by C60 buckyballs. We then used a scanning tunneling microscope to watch the structures move or change shape when we ran current through the wire. Amazingly, when we changed the direction of the current, we could move the structures back and forth.
The force exerted by the electrons on island edge atoms is up to 20 times larger than previous theoretical calculations had predicted. However, when we decorate the island edge with a chain of C60 molecules (which tend to mildly withdraw electrons locally from the silver atoms, and also change their local configuration) we find that the force is reduced by over a factor of 10. This indicates that the force is very dependent on the local environment of the atoms which comprise the step and island boundaries.
Fig. 1 Schematic of the experimental setup; inset shows STM image of silver wire surface.
The fundamentally interesting idea here is that all the different ways that electrons can move through the wire can be described by how easily an electron can be “transmitted”. Most atomic structures in a solid allow easy transmission, but the defect sites impede the transmission. This results in a local “resistivity dipole” which means that the defect sites have a local resistance. Our measurements detected the motion of atomic-scale surface structures which results from forces exerted by the passing electrons – as the atoms resist the electron flow, they in turn feel a larger “push” from the electrons.
Fig. 2A-B: Island pushed by moving electrons. The current direction is downward, and the island displacement is upward.
Here we have demonstrated that nanoscale surface structures can be moved (and even turned around) using the scattering force from electrons. Further, the scattering force can be significantly reduced by attaching C60 molecules to the structures. On the other hand, a particularly exciting implication is the use of this effect to move atoms around intentionally in nanoelectronic devices, or to harness it to do work . This effect might be used to self-assemble or to create structures that could be cycled through different structures under an alternating current.
Our work was supported by the NSF Materials Research Science and Engineering Center at the University of Maryland, including the use of shared experimental facilities. Additional support was provided by the University of Maryland NanoCenter and the Center for Nanophysics and Advanced Materials.
 Chenggang Tao, W. G. Cullen, E. D. Williams, “Visualizing the electron scattering force in nanostructures”, Science 328, 736–740 (2010). Abstract.
 P. S. Ho and T. Kwok, “Electromigration in metals”, Reports on Progress in Physics, 52, 301 (1989). Abstract.
 H. Yasunaga and A. Natori, “Electromigration on semiconductor surfaces”, Surface Science Reports 15, 205 (1992). Abstract.
 D. Dundas, E. McEniry and T. N. Todorov, “Current-driven atomic waterwheels”, Nature Nanotechnology, 4, 99 (2009). Abstract.