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Going ballistic with electrons

Given the trend in solid-state electronics, where the feature size is expected to shrink every couple of years (Moore's Law), single-molecule transistors (or switches) are certain to be a part of your CPU in the not-so-distant future. In anticipation of this, researchers have been studying single-molecule transistors and switches for a while. However, there are several issues that stand in the way of implementing molecular switches. Mostly, the problems boil down to understanding how electrons flow through molecules. 苏州美睫美甲

To understand this barrier a little better, let's take a quick peek at how electrons flow through a metal (or semiconductor). In these materials, the atoms are arranged in a regular spatial pattern, so the electrons whose movements match that spatial pattern travel more efficiently than those who do not. By match, I mean that electrons with a certain speed, traveling in a certain direction, will move without interruption in a process called ballistic transport. Those electrons with different velocities will collide* with the electrons remaining around the atoms. These electrons lose energy, change direction, and only drift slowly in the direction given by the applied voltage. Normally we don't observe this difference because metallic conductors have a very poor crystalline structure over long distances so no electrons are ballistic for very long. Nevertheless, it is easy to see that ballistic transport is much faster and generates much less heat than non-ballistic transport. It is also clear that single molecule devices will almost certainly need to work with ballistic electrons, otherwise the absorbed energy will eventually destroy them.

To better understand the transport through single molecules, researchers in Germany have modified a form of electron microscopy. In two separate experiments, C60 fullerenes and an organic molecule (3,4,9,10-perylene-tetracarboxylic acid dianhydride, for the chemists hidden amongst us) were evaporated onto an atomically flat, two-atom-thick layer of bismuth that was itself on a silicon substrate. Bismuth and silicon are well matched in their physical and electronic properties, which maximizes the number of ballistic electrons—provided the applied voltage is correct and the electrons are going in the right direction. Non-ballistic electrons lose energy and most of them are trapped in the metallic layer, therefore measuring the current through the silicon amounts to measuring the ballistic transport properties of the molecule-metal-silicon system. Using the tip from a scanning tunneling electron microscope, the researchers were able to direct electrons to specific positions on the surface molecules—essentially probing where injected electrons can match the electronic properties of the molecule and ballistically transfer through to the silicon.

They found that transport through the fullerenes occurs along the carbon-carbon bonds around the surface of the molecule, rather than tunneling straight through the center. Although modeling had predicted this, it was the first experimental evidence that the model was correct. The organic molecule was something of a surprise. The molecule is a flat layer, consisting of a series of interconnected rings with oxygen end groups at each corner. These molecules lay themselves out in a herringbone pattern on the surface of the metal. From this, one might expect that the current should be pretty even over the whole surface. However, ballistic transport is more efficient at the end groups, where the oxygen molecules bend the molecule down towards the underlying metallic layer.

By themselves, the experimental results are that significant finding. The significance, and the reason the work was published in Science, lies in the development of a technique that allows researchers to understand the detailed electron transport properties through molecules and across contacts between molecules and bulk surfaces. Eventually, this should provide insight into many different properties for single molecule switches and wires, such as points of failure, self-assembly, and contact properties.

*By collide, I don't mean two electrons actually hiting each other, rather the field generated by the electrons surrounding the atom slow and change the direction of the drifting electron.