The defining characteristic of nanotechnology, where devices and their features are measured in billionths of a meter, is size. But size isn’t the only thing that sets the nano-realm apart. Materials and devices can behave differently – startlingly differently even – at the nano-scale than they do when in larger forms.
Pradeep Sharma, chair and professor of mechanical engineering, has recently co-authored a paper that outlines one such phenomenon and provides a likely explanation for it.
The paper, which was published in a recent issue of the journal Nature Communications, involves a new breed of capacitor built out of nanowires. These devices were constructed in the lab of Rice University’s Pulickel Ajayan and Jun Lou, who led the project represented in the article.
Capacitors are energy storage devices widely used in electronics. While they have the ability to discharge energy very rapidly, their total energy storage ability is typically low.
It’s well known, though, that as capacitors are made smaller, their capacitance, or ability to store an electronic charge, increases per unit area.
So while researchers expected these nanowire devices to have high capacitance, they were surprised to find them storing far more energy than predicted. “The capacitance was almost fifty times larger than it should have been. At that point the question became why,” said Sharma.
Professors Ajayan and Lou approached Sharma, an expert in theoretical and computational modeling of the nanomaterials, to help interpret these experimental results. After detailed quantum mechanical modeling, Sharma and his students realized this higher-than-expected capacitance was a consequence of so-called negative quantum capacitance arising from the interface of the different materials in the nanowires, specifically a metal and a dielectric material.
When compared to classical capacitance, quantum capacitance adds up in such a way that its contribution is very small and usually neglected. On the nanoscale, however, quantum capacitance’s is quite influential. Usually, quantum capacitance is positive. The notable element in this study is that the results can only be explained by invoking the concept of negative quantum capacitance. By combining with classical capacitance, this negative capacitance actually produces the larger-than-expected results.
With the existence of negative quantum capacitance now confirmed, Sharma said that by tailoring the metal-dielectric interface researchers could leverage the phenomenon to create nano-devices with dramatically higher energy capacity.