Doping carbon-nanotube circuits for more reliable, faster, and power-efficient flexible devices

Stanford engineers invent a process to “dope” carbon filaments to improve their electronic performance, paving the way to better bendable digital devices
March 20, 2014

Stanford engineers have developed an improved process for making flexible circuits that use carbon nanotube transistors, a development that paves the way for a new generation of bendable electronic devices (credit: Bao Lab, Stanford University)

Stanford University team has developed a process to create flexible chips using carbon nanotubes (CNTs) that can tolerate electrical noise (rapid fluctuations in voltage).

In principle, CNTs should be ideal for making flexible electronic circuitry. These ultra-thin carbon filaments have the physical strength to take the wear and tear of bending, and the electrical conductivity to perform any electronic task. But flexible CNTs circuits didn’t have the reliability and power-efficiency of rigid silicon chips.

Most silicon chips are based on a type of circuit design that allows them to function flawlessly even when the device experiences electrical noise. However, it is much more challenging to do so with CNT circuits.

Here’s the reason: 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 electrons like a beaded necklace. The first type of semiconductor is called a P-type, the second is called an N-type.

Engineers discovered that circuits based on a combination of P-type and N-type transistors perform reliably even when electrical noise occurs, and they also consume much less power. This type of circuit with both P-type and N-type transistors is called a complementary circuit (the “C” in CMOS). This ideal blend of conductive pathways is achieved by changing the atomic structure of silicon through the addition of minute amounts of useful substances — a process called “doping” (similar to creating alloys).

Doping CNTs to make them electrical-noise-resistant

Doping of single-walled carbon nanotube (SWNT) thin-film transistors (credit: Huiliang Wang et al./PNAS)

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.

A paper by the  Stanford engineers in Proceedings of the National Academy of Sciences explains how they overcame this challenge: they treated CNTs with a chemical dopant they developed known as DMBI, and they used an inkjet printer to deposit this substance in precise locations on the circuit.

“This is the first time anyone has designed flexible CNT circuits that have both high immunity to electrical noise and low power consumption, ” said Zhenan Bao, a professor of chemical engineering at Stanford with a courtesy appointment in Chemistry and Materials Science and Engineering.

(The Stanford 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 outperforms these prior efforts, according to Bao, suggesting that it could be useful for both flexible and rigid CNT circuitry.)

However, Bao has focused her research on flexible CNTs, which compete with other experimental materials, such as specially formulated plastics, to become the foundation for bendable electronics, just as silicon has been the basis for rigid electronics.

As a relatively new material, CNTs are playing catch up to plastics, which are closer to mass market use for such things as bendable display screens. The Stanford doping process moves flexible CNTs closer toward commercialization because it shows how to create the P-N blend, and the resultant improvements in reliability and power consumption.

Although much work lies ahead to make CNTs commercial, Bao believes these carbon filaments are the future of flexible electronics, because they are strong enough to bend and stretch, while also being capable of delivering faster performance than plastic circuitry.

Abstract of Proceedings of the National Academy of Sciences paper

Tuning the threshold voltage of a transistor is crucial for realizing robust digital circuits. For silicon transistors, the threshold voltage can be accurately controlled by doping. However, it remains challenging to tune the threshold voltage of single-wall nanotube (SWNT) thin-film transistors. Here, we report a facile method to controllably n-dope SWNTs using 1H-benzoimidazole derivatives processed via either solution coating or vacuum deposition. The threshold voltages of our polythiophene-sorted SWNT thin-film transistors can be tuned accurately and continuously over a wide range. Photoelectron spectroscopy measurements confirmed that the SWNT Fermi level shifted to the conduction band edge with increasing doping concentration. Using this doping approach, we proceeded to fabricate SWNT complementary inverters by inkjet printing of the dopants. We observed an unprecedented noise margin of 28 V at VDD = 80 V (70% of 1/2VDD) and a gain of 85. Additionally, robust SWNT complementary metal−oxide−semiconductor inverter (noise margin 72% of 1/2VDD) and logic gates with rail-to-rail output voltage swing and subnanowatt power consumption were fabricated onto a highly flexible substrate.