Penn physicists build and test transistors inside a microscope

In the drive to miniaturize electronics as much as possible, physicists and engineers are beginning to contend with the role of individual atoms when it comes to measuring the performance of a device. How fast or efficiently a nanoscale transistor can transport an electron may rely on atomic features that are at the limits of what can be visualized by even the most advanced microscopes.

Even if the relevant structures can be seen, relating their shape and arrangement to the electrical properties of a working device presents an additional challenge.

Penn physicists Marija Drndić and Charlie Johnson are working to solve this problem. In a study led by Drndić lab member Julio Rodriguez-Manzo and recently published in ACS Nano, the researchers have shown a way to simultaneously measure the performance of graphene transistors while observing their structure on the nanoscale.

The researchers’ technique rests on the ability to sculpt these transistors within the microscope itself.

Graphene, a one-atom-thick lattice of carbon atoms, is looked to as a revolutionary material for electronics, due to its unbeatable thinness and conductivity. Drndić and Johnson had previously collaborated on experiments to see how small variations in the width and shape of graphene ribbons influenced its electrical properties. By visualizing these qualities while the ends of the ribbons were connected to electrodes, they could relate the structural and electrical properties of the ribbons as they changed. 

Rather than light, transmission electron microscopes use a beam of the subatomic particles to visualize structures at the smallest scales. Variations in the amount of electrons that pass through the subject can be used to generate an image. However, by focusing this beam, the electrons have enough energy to cut apart the subject, especially when it is as thin as graphene.

Instead of cutting ribbons of graphene to different thicknesses as in their previous work, the researchers cut a sheet of graphene into two parts: a ribbon and a neighboring terminal, which was also attached to an electrode.  Though the terminal and ribbon do not touch, the former can influence the flow of electrons through the latter by way of an electric field, making what is known as a “field effect transistor.”

“Before,” Drndić says, “we used two electrodes and could not modulate the charge in the graphene ribbon channel. But now, by having a third electrode, what’s known as a gate, we can make a transistor and were able to modulate the channel conductance while still having that structural information.”

Transistors are one of the fundamental components of electronics, as they can amplify electrical signals or switch them between the 0s and 1s used by computers. Understanding graphene-based transistors’ properties on this scale is therefore critical to their adoption in consumer devices.

Further experiments with even more powerful transmission electron microscopes will be necessary to pinpoint individual atoms’ roles in the overall performance of the transistor.