Flexible electronic devices, like a smartphone that could be folded to fit into a pocket, are on the minds and drawing boards of many engineers these days. One approach being tried involves designing circuits based on flexible electronic fibers called carbon nanotubes, instead of stiff silicon chips.
One challenge is that reliability is a must. Most silicon chips are based on a form of circuit that allows them to work perfectly even when the device undergoes power fluctuations. This is much more difficult to do with carbon nanotubes (CNT) circuits.
Electrical Noise Immunity
In theory, carbon nanotubes should be perfect for making flexible electronic circuitry. The ultrathin carbon filaments boast the physical strength to take the wear and tear of bending, plus sufficient electrical conductivity to perform any electronic task.
However, until this recent work, flexible CNTs circuits were not able to achieve the reliability and power efficiency levels of rigid silicon chips.
Why is that? Engineers have gradually learned that electricity can travel through semiconductors in two different ways. It can jump from positive hole to positive hole, or it can push through a bunch of negative electronic like a beaded necklace. The first type of semiconductor is called a P-type, the second is called and N-type.
Complementary Circuit Doping
The key point here is that engineers discovered that circuits based on a combination of P-type and N-type transistors perform reliably even when power fluctuations occur, and they also consume much less power.
This type of circuit with both P-type and N-type transistors is called complementary circuit. It has taken the last 50 years, but engineers have become adroit at creating this ideal blend of conductive pathways.
They can do this by changing the atomic structure of silicon through the addition of minute amounts of useful substances. It is a process called “doping” that is theoretically similar to what our ancestors did thousands of years ago when they stirred tin into molten copper to create bronze.
The challenge facing the Stanford team was that CNTs are predominately P-type semiconductors and there was no easy way to dope these carbon filaments to add N-type characteristics.
The Stanford engineers overcame this challenge by treating carbon nanotubes with a chemical dopant they developed known as DMBI. To deposit this substance in exact locations on the circuit they used an inkjet printer.
Rigid Carbon Nanotubes
The new process also has some potential application to rigid CNTs. Although other engineers have previously doped rigid CNTs to create this immunity to electrical noise, the precise and finely tuned Stanford process out performs these prior efforts, suggesting that it could be useful for both flexible and rigid CNT circuitry.
Professor Bao’s research focuses on flexible CNTs, which are going up against other experimental materials, like specially formulated plastics, to become the default foundation of bendable electronics, much like silicon has been the basis for rigid electronics.
As a relatively new material, carbon nanotubes are lagging behind plastics, which are closer to mass market use for such things as bendable display screens. The Stanford doping process moves flexible CNTs further toward commercialization since it demonstrates how to create the P-N blend, and the resultant improvements in reliability and power consumption, already present in plastic circuits.
While much work still lies ahead to commercialise CNTs, Bao believes these carbon filaments are the future of flexible electronics. This is because they are strong enough to bend and stretch, while also being capable of delivering faster performance than plastic circuitry.
Huiliang Wang, Peng Wei, Yaoxuan Li, Jeff Han, Hye Ryoung Lee, Benjamin D. Naab, Nan Liu, Chenggong Wang, Eric Adijanto, Benjamin C.-K. Tee, Satoshi Morishita, Qiaochu Li, Yongli Gao, Yi Cui, and Zhenan Bao. Tuning the threshold voltage of carbon nanotube transistors by n-type molecular doping for robust and flexible complementary circuits PNAS, March 17, 2014
_ Photo Credit: Bao Lab, Stanford University_