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Quantum tunneling expands the boundaries of self-powered sensors

Quantum tunneling expands the boundaries of self-powered sensors

Photomicrograph of the quantum tunnel sensor chipset and the corresponding Fowler-Nordheim tunnel barriers. Credit: Chakrabartty Lab, McKelvey School of Engineering, University of Washington, St. Louis

Shantanu Chakrabarti̵

7;s lab is working to create sensors that can operate with the least amount of energy. His lab has been so successful in building smaller and more efficient sensors that they have encountered an obstacle in the form of a basic law of physics.

Sometimes, however, when you hit something that seems like an impenetrable obstacle, you just have to turn to quantum physics and go through it. This was done by Chakrabarti and other researchers from the McKelvey School of Engineering at the University of Washington in St. Louis.

The development of these self-powered quantum sensors by Chakrabartty’s laboratory, Professor Clifford W. Murphy in the Department of Systems and Electrical Engineering at Preston M. Green, was published online on October 28 in the journal Nature Communications.

The roadblock that inspired this study is the threshold effect.

“Imagine there is an apple hanging on a tree,” Chakrabarti said. “You can shake the tree a little, but the apple doesn’t fall. You have to give it enough tug to shake the apple.” This pulling is similar to threshold energy. “This is the minimum amount of energy needed to move an electron over a barrier.” If you can’t move the electron across the barrier, you can’t create current.

But the natural quantum mechanical phenomenon moves electrons through barriers all the time. The research team took advantage of this to create a self-powered device that could operate on its own for more than a year with little initial energy input.

Here’s how it’s built:

The device is easy and cheap to build. Only four capacitors and two transistors are needed.

From these six parts, the Chakrabartty team built two dynamic systems, each with two capacitors and a transistor. Capacitors hold a small initial charge, about 50 million electrons each.

They added a transducer to one of the systems and connected it to the property they were measuring. In one application, the team measured the surrounding micromovement using a piezoelectric accelerometer, a type of transducer that converts mechanical energy (such as the movement of molecules in the air) into electrical signals.

Here’s what you need to know:

Quantum physics. At least some of the more unusual properties of subatomic particles, especially tunneling.

Imagine a hill, Chakrabarti said. “If you want to get to the other side, you have to physically climb the hill. Quantum tunneling is more like crossing a hill.”

The beauty of this, he said, is that when a hill has a certain shape, you get many unique, dynamic properties that can last for years.

In this case, the “hill” is actually a barrier called the Fowler-Nordheim tunnel barrier. It is located between the plate of the capacitor and the semiconductor material; with a thickness of less than 100 atoms.

By building the barrier in a certain way, Chakrabarti said, “you can control the flow of electrons. You can make it relatively slow, up to one electron every minute, and still keep it reliable.” At this speed, the dynamic system has been operating as a timer – without any batteries – for more than a year.

Here’s how it works:

A small piezoelectric accelerometer was connected to the sensor to measure the movement of the environment. The researchers mechanically shook the accelerometer; then its motion is transformed into an electrical signal. This signal changes the shape of the barrier, which, thanks to the rules of quantum physics, changes the speed at which electrons pass through the barrier.

To understand what happened, the process must be read as something like a backward Rube Goldberg machine.

The probability of a certain number of electrons passing through the barrier is a function of the size of the barrier. The size of the barrier is determined by the energy produced by the piezoelectric transducer, which in turn is determined by the magnitude of the acceleration – how much it has shaken.

By measuring the voltage of the sensor capacitors and counting how many electrons are missing, Darshit Mehta, a PhD student in Chakrabarti’s laboratory and lead author of the article, was able to determine the total energy of acceleration.

Of course, in order to be used in practice, these extremely sensitive devices are likely to move around – on a truck, monitoring the ambient temperature when driving the cold chain of vaccines, for example. Or in your blood by monitoring your glucose.

That is why each device is actually two systems, a sensor system and a reference system. In the beginning, the two are almost identical, except that the sensor system is connected to a sensor, while the reference system is not.

Both systems are designed so that the electrons are tunneled at the same speed, designed to exhaust their capacitors identically if there were no external forces.

Because the sensor system was affected by the signals it received from the transducer, its electrons were tunneled at different times by the reference system. After the experiments, the research team reported the voltage in both the sensors and the capacitors of the reference system. They used the difference in the two voltages to find the true measurements from the converter.

For some applications, this end result is sufficient. The next step for Chakrabarti’s team is to overcome the computational challenge of more accurately recreating what happened in the past – how exactly were the electrons affected? When did an electronic tunnel pass through the barrier? How long did the tunnel take?

One of the goals of Mehta’s doctoral thesis is to use multiple devices to restore the past. “All the information is stored on the device, we just have to come up with intelligent signal processing to resolve this,” Chakrabarti said.

After all, these sensors promise everything from continuous monitoring of glucose levels in the human body to the eventual recording of nerve activity without the use of batteries.

“Currently, the platform is common,” Chakrabarti said. “It just depends on what you’re connecting to the device. As long as you have a converter that can generate an electrical signal, it can power our data logger sensor on its own.”

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More info:
Darshit Mehta et al., Autonomous analog sensor data logger based on Fowler-Nordheim dynamic systems, Nature Communications (2020). DOI: 10.1038 / s41467-020-19292-w

Provided by the University of Washington at St. Louis

Quote: Quantum tunneling expands the boundaries of self-powered sensors (2020, November 17), retrieved on November 17, 2020 from https://phys.org/news/2020-11-quantum-tunneling-limits-self-powered- sensors.html

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