Researchers at the University of Arkansas and the University of Michigan have reported the first use of ultra-low power temperature sensors using graphene-based solar cells. The test is the first hurdle in developing autonomous sensor systems that draw power from multiple sources in the environment — solar, thermal, acoustic, kinetic, nonlinear and ambient radiation.

The end goal is development of multi-modal sensors (incorporating more than one of the above power sources) using the energy-harvesting capability of graphene that can last decades and helps realize the "Internet of Things," in which smart technology is woven into the fabric of daily life.

Ashaduzzaman, a Ph.D. candidate in physics, is first author on the U of A-led paper, but graphene energy harvesters are the brainchild of physics professor Paul Thibado, who began studying graphene's unique properties more than a decade ago and is the corresponding author.

Success depended on overcoming two challenges:

1. Reducing sensor power demand to nanowatts, a billionth of a watt, as opposed to the current standard, which is measured in microwatts (a millionth of a watt) and
2. Powering the sensor using energy harvested from the local environment.

Notably, this system and those expected to follow do not include batteries, which have a limited lifespan, allowing graphene-based energy harvesters to achieve long operational lifetimes — potentially decades.

"Power has to be drawn from the local environment," Thibado explained, "so it's self-powered and autonomous, and it has to have an extremely long operational lifetime to dramatically reduce the total cost of ownership. So set it and forget it."

The U of A team was largely responsible for completing the second challenge above, while the University of Michigan team, led by David Blaauw, a professor of electrical engineering and computer science, was largely responsible for the first. Blaauw is an expert in low-power wireless sensors and embedded systems. He has even designed tiny sensors that can be planted in the wings of a butterfly.

The paper confirms it's possible to create an ultra-low power temperature sensor using graphene-based solar energy.

"We thought if we could remove the power management unit, maybe this sensor system would take an even smaller amount of power," Ashaduzzaman explained. "So that is what we did. Then we connected three sets of solar cells to power the temperature systems directly with three storage capacitors."

Thibado added that "We anticipate building devices that harvest multiple sources of energy within that device."

By making them "multi-modal," intermittent shortages in solar power can be augmented with additional thermal or non-linear power, whatever the case may be.

Thibado foresees the sensors being used in areas and fields where sensors would be useful, but the need to replace batteries would make them labor and cost prohibitive. This could include use in things like agricultural climate monitoring, tracking livestock, wearable fitness monitoring, building alarm systems, predictive maintenance and a wide range of other applications.

The Thibado Group

OVERVIEW OF WORK

The abstract of the paper describes the work in plain language, stating that the researchers "built dozens of graphene-based solar cells, wire bonded them into standard packages and characterized the current-voltage characteristics of each under illumination. Next, solar cells were connected in series to increase the output voltage. Three different sets of solar cells were used to charge three storage capacitors to the voltage levels required by [the] temperature sensor.

"The storage capacitors require only a few minutes to charge, yet power the sensor system for more than 24 hours without recharging. Using storage capacitors also eliminates the need for a typical power management chip and the commonly used rechargeable battery. As a result, one can lower the overall power consumed by the sensor system and significantly extend its useful life."

The paper was published in the Journal of Vacuum Science and Technology B. Co-authors from the U of A also included Syed M. Rahman, Md R. Kabir and James M. Magnum, all doctoral students. Co-authors from the University of Michigan included Hung Do and Gordon Carichner, in addition to Blaauw.

Ashaduzzaman said he has been working on the temperature sensor about a year and a half. He said the next step is to perfect a kinetic energy harvester that harvests energy from the unique vibrational qualities of graphene. This capability will then be joined with the solar sensor, creating a multi-modal sensor. At least, that is the plan.

This innovative work was supported by a $900,000 grant from the WoodNext Foundation.

OTHER COLLABORATORS

NTS Innovations, a company specializing in nanotechnology, owns the exclusive license to develop graphene energy harvesters into commercial products. The company has provided funding for patenting, creating business plans, finding business partners and customer discovery.

NTS Innovations' role over the course of the grant is to engage with customers on acceptance criteria, such as the minimum power levels needed for inclusion in products. Currently, more than 60 parties have expressed interest in testing the technology and working with Thibado and his colleagues to integrate it into their applications.

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About the WoodNext Foundation: The WoodNext Foundation manages the philanthropy of tech innovator and Roku CEO/founder, Anthony Wood, and his wife Susan. Their philanthropic efforts are guided by their overall mission to advance human progress and remove obstacles to a fulfilling life. The WoodNext Foundation makes grants and investments in a variety of areas, including scientific and biomedical research, mental health, homelessness, education, nature conservation, disaster recovery, and economic opportunity, with a focus on addressing root causes.