Power/Performance Bits: Sept. 21

Catching switches in action
Researchers from SLAC National Accelerator Laboratory, Stanford University, Hewlett Packard Labs, Penn State University, and Purdue University observed atoms moving inside an electronic switch as it turns on and off, revealing a state they suspect could lead to faster, more energy-efficient devices.

“This research is a breakthrough in ultrafast technology and science,” said SLAC scientist Xijie Wang. “It marks the first time that researchers used ultrafast electron diffraction, which can detect tiny atomic movements in a material by scattering a powerful beam of electrons off a sample, to observe an electronic device as it operates.”

The team designed miniature electronic switches made of vanadium dioxide, a prototypical quantum material whose ability to change back and forth between insulating and electrically conducting states near room temperature could potentially be harnessed as a switch for future neuromorphic computing.

Electrical pulses were used to toggle these switches back and forth between the insulating and conducting states while taking snapshots, afterward compiled into a video, that showed subtle changes in the arrangement of their atoms over billionths of a second.

“This ultrafast camera can actually look inside a material and take snapshots of how its atoms move in response to a sharp pulse of electrical excitation,” said Aaron Lindenberg, an investigator with the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC and a professor in the Department of Materials Science and Engineering at Stanford University. “At the same time, it also measures how the electronic properties of that material change over time.”

They found an intermediate state within the material created when it responds to an electric pulse by switching from the insulating to the conducting state.

“The insulating and conducting states have slightly different atomic arrangements, and it usually takes energy to go from one to the other,” said SLAC scientist Xiaozhe Shen. “But when the transition takes place through this intermediate state, the switch can take place without any changes to the atomic arrangement.”

The intermediate state only lasts for a few millionths of a second, but can be stabilized by defects in the material. The team plans to investigate how to engineer these defects in materials to make this new state more stable and longer lasting. This, they say, would allow them to make devices in which electronic switching can occur without any atomic motion, which would operate faster and require less energy.

“The results demonstrate the robustness of the electrical switching over millions of cycles and identify possible limits to the switching speeds of such devices,” said Shriram Ramanathan, a professor at Purdue. “The research provides invaluable data on microscopic phenomena that occur during device operations, which is crucial for designing circuit models in the future.”

Harvesting energy from fingertips
Engineers from the University of California San Diego developed a small energy-harvesting wearable that when applied to the fingertip can generate power from small amounts of sweat and actions like typing.

Even basic actions, like sleeping, generate enough sweat for the device to receive a charge. “Unlike other sweat-powered wearables, this one requires no exercise, no physical input from the wearer in order to be useful. This work is a step forward to making wearables more practical, convenient and accessible for the everyday person,” said Lu Yin, a nanoengineering Ph.D. student at the UC San Diego Jacobs School of Engineering.

Light finger presses, such as typing, generate additional power. “We envision that this can be used in any daily activity involving touch, things that a person would normally do anyway while at work, at home, while watching TV or eating,” said Joseph Wang, a professor of nanoengineering at the UC San Diego Jacobs School of Engineering. “The goal is that this wearable will naturally work for you and you don’t even have to think about it.”

The researchers noted that the fingertips are one of the sweatiest areas of the body, packed with over a thousand sweat glands and constantly perspiring to produce between 100 and 1000 times more sweat than other areas of the body.

“The reason we feel sweatier on other parts of the body is because those spots are not well ventilated,” said Yin. “By contrast, the fingertips are always exposed to air, so the sweat evaporates as it comes out. So rather than letting it evaporate, we use our device to collect this sweat, and it can generate a significant amount of energy.”

This thin, flexible strip can be worn on a fingertip and generate small amounts of electricity when a person’s finger sweats or presses on it. (Credit: UC San Diego Jacobs School of Engineering)

Key to harnessing this finger-sweat power was engineering materials to be both super absorbent and efficient at conversion. It is powered by lactate, a dissolved compound in sweat.

A bioenzyme on the anode oxidizes the lactate, and the cathode is deposited with a small amount of platinum to catalyze a reduction reaction that takes the electron to turn oxygen into water. Once this happens, electrons flow from the lactate through the circuit, creating a current of electricity.

The device is a thin, flexible strip that can be wrapped around the fingertip like a Band-Aid. Underneath the electrodes is a piezoelectric material, which generates additional electrical energy when pressed. The energy is stored in a small capacitor before discharging to other devices.

“The size of the device is about 1 centimeter squared. Its material is flexible as well, so you don’t need to worry about it being too rigid or feeling weird. You can comfortably wear it for an extended period of time,” said Yin.

To test the device, the team had a subject wear the device on one fingertip while doing sedentary activities. From 10 hours of sleep, the device collected almost 400 millijoules of energy, enough to power an electronic wristwatch for 24 hours. From one hour of casual typing and clicking on a mouse, the device collected almost 30 millijoules. Applying it to more fingertips would increase the output.

The team also used it to power small biosensors that included a display, and in the future plan to make it more efficient and durable, including combining it with other energy harvesters. “Our goal is to make this a practical device,” said Yin. “We want to show that this is not just another cool thing that can generate a small amount of energy and then that’s it—we can actually use the energy to power useful electronics such as sensors and displays.”

Tamarind nanosheets
Researchers at Nanyang Technological University Singapore (NTU Singapore), Western Norway University of Applied Sciences, and Alagappa University found a way to process tamarind shells into carbon nanosheets, creating both a useful component of supercapacitors and cutting down on agricultural waste.

“Through a series of analysis, we found that the performance of our tamarind shell-derived nanosheets was comparable to their industrially made counterparts in terms of porous structure and electrochemical properties. The process to make the nanosheets is also the standard method to produce active carbon nanosheets,” said (Steve) Cuong Dang, an assistant professor at NTU’s School of Electrical and Electronic Engineering.

G. Ravi, professor and head of the Department of Physics at Alagappa University, added, “The use of tamarind shells may reduce the amount of space required for landfills, especially in regions in Asia such as India, one of the world’s largest producers of tamarind, which is also grappling with waste disposal issues.”

To create the carbon nanosheets, tamarind shells were washed and dried at 100°C for around six hours before being ground into powder. The powder was then baked in a 700-900°C furnace, in the absence of oxygen, for 150 minutes.

A representation of the experimental process, as well as photographs of the tamarind shell at every step. (Credit: Nanyang Technological University, Singapore)

The resulting tamarind shell-derived nanosheets showed good thermal stability and electric conductivity, which could make them useful for energy storage.

The researchers compared the process favorably against the use of industrial hemp fibers, which require more energy and longer times to process into carbon nanosheets. They plan to explore larger scale production of the carbon nanosheets with agricultural partners and are working on reducing the energy needed for the production process, making it more environmentally friendly, as well as seeking to improve the electrochemical properties of the nanosheets and using other types of fruit skins and shells.