Power/Performance Bits: March 25

Stanford researchers invent a process to ‘dope’ carbon filaments with an additive to improve their electronic performance meant to pave the way for bendable digital devices; University of Toronto scientists open a new window into quantum physics with superconductivity in LEDs.


Making flexible carbon nanotube circuits more reliable and efficient
Engineers would love to create flexible electronic devices, such as e-readers that could be folded to fit into a pocket with one such approach involving designing circuits based on electronic fibers known as carbon nanotubes (CNTs) instead of rigid silicon chips — but reliability is essential.

Given that most silicon chips are based on a type of circuit design that allows them to function flawlessly even when the device experiences power fluctuations, at the same time it is much more challenging to do so with CNT circuits.

However, researchers at Stanford have developed a process to create flexible chips that can tolerate power fluctuations in much the same way as silicon circuitry.

Zhenan Bao, a professor of chemical engineering at Stanford said this is the first time anyone has designed flexible CNT circuits that have both high immunity to electrical noise and low power consumption.

 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. (Source: Bao Lab / Stanford Engineering)

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. (Source: Bao Lab / Stanford Engineering)

In principle, the researchers said CNTs should be ideal for making flexible electronic circuitry as 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 until this recent work from the Stanford team, flexible CNT circuits didn’t have the reliability and power-efficiency of rigid silicon chips. Over time, 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 and the second an N-type. Most importantly, the researchers 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 a complementary circuit.

The ideal blend of conductive pathways is created by changing the atomic structure of silicon through the addition of minute amounts of useful substances in a process called doping.

The Stanford engineers overcame this challenge by treating CNTs with a chemical dopant they developed known as DMBI. They used an inkjet printer to deposit this substance in precise locations on the circuit. This marked the first time any flexible CNT circuit had been doped to create a P-N blend that can operate reliably despite power fluctuations and with low power consumption.

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.

Novel approach could pave way for new quantum devices
With the potential to open up a spectrum of new physics as well as devices for quantum technologies, including quantum computers and quantum communication, a team of University of Toronto physicists has proposed a novel and efficient way to leverage the strange quantum physics phenomenon known as entanglement.

The approach would involve combining light-emitting diodes (LEDs) with a superconductor to generate entangled photons. Entanglement occurs when particles become correlated in pairs to predictably interact with each other regardless of how far apart they are. Measure the properties of one member of the entangled pair and you instantly know the properties of the other, the researchers noted. It is one of the most perplexing aspects of quantum mechanics, leading Einstein to call it “spooky action at a distance.”

A usual light source such as an LED emits photons randomly without any correlations and the researchers say they’ve proved that generating entanglement between photons emitted from an LED can be achieved by adding another peculiar physical effect of superconductivity – a resistance-free electrical current in certain materials at low temperatures. This effect occurs when electrons are entangled in Cooper pairs – a phenomenon in which when one electron spins one way, the other will spin in the opposite direction. When a layer of such superconducting material is placed in close contact with a semiconductor LED structure, Cooper pairs are injected in to the LED, so that pairs of entangled electrons create entangled pairs of photons. The effect, however, turns out to work only in LEDs which use nanometer-thick active regions – quantum wells.

Typically quantum properties show up on very small scales – an electron or an atom. Superconductivity allows quantum effects to show up on large scales – an electrical component or a whole circuit. This quantum behavior can significantly enhance light emission in general, and entangled photon emission in particular, the researchers concluded.

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