Tag Archives: Eric Pop

Courtesy of graphene: world’s thinnest light bulb

Columbia University’s (US) School of Engineering and Applied Science is trumpeting an achievement with graphene, i.e., the world’s thinnest light bulb. From a June 15, 2015 Columbia Engineering news release (also on EurekAlert),

Led by Young Duck Kim, a postdoctoral research scientist in James Hone’s group at Columbia Engineering, a team of scientists from Columbia, Seoul National University (SNU), and Korea Research Institute of Standards and Science (KRISS) reported today that they have demonstrated — for the first time — an on-chip visible light source using graphene, an atomically thin and perfectly crystalline form of carbon, as a filament. They attached small strips of graphene to metal electrodes, suspended the strips above the substrate, and passed a current through the filaments to cause them to heat up.

“We’ve created what is essentially the world’s thinnest light bulb,” says Hone, Wang Fon-Jen Professor of Mechanical Engineering at Columbia Engineering and coauthor of the study. “This new type of ‘broadband’ light emitter can be integrated into chips and will pave the way towards the realization of atomically thin, flexible, and transparent displays, and graphene-based on-chip optical communications.”

The news release goes on to describe some of the issues associated with generating light on a chip and how the researchers approached the problems (quick answer: they used graphene as the filament),

Creating light in small structures on the surface of a chip is crucial for developing fully integrated “photonic” circuits that do with light what is now done with electric currents in semiconductor integrated circuits. Researchers have developed many approaches to do this, but have not yet been able to put the oldest and simplest artificial light source—the incandescent light bulb—onto a chip. This is primarily because light bulb filaments must be extremely hot—thousands of degrees Celsius—in order to glow in the visible range and micro-scale metal wires cannot withstand such temperatures. In addition, heat transfer from the hot filament to its surroundings is extremely efficient at the microscale, making such structures impractical and leading to damage of the surrounding chip.

By measuring the spectrum of the light emitted from the graphene, the team was able to show that the graphene was reaching temperatures of above 2500 degrees Celsius, hot enough to glow brightly. “The visible light from atomically thin graphene is so intense that it is visible even to the naked eye, without any additional magnification,” explains Kim, first and co-lead author on the paper.

Interestingly, the spectrum of the emitted light showed peaks at specific wavelengths, which the team discovered was due to interference between the light emitted directly from the graphene and light reflecting off the silicon substrate and passing back through the graphene. Kim notes, “This is only possible because graphene is transparent, unlike any conventional filament, and allows us to tune the emission spectrum by changing the distance to the substrate.”

The ability of graphene to achieve such high temperatures without melting the substrate or the metal electrodes is due to another interesting property: as it heats up, graphene becomes a much poorer conductor of heat. This means that the high temperatures stay confined to a small “hot spot” in the center.

“At the highest temperatures, the electron temperature is much higher than that of acoustic vibrational modes of the graphene lattice, so that less energy is needed to attain temperatures needed for visible light emission,” Myung-Ho Bae, a senior researcher at KRISS and co-lead author, observes. “These unique thermal properties allow us to heat the suspended graphene up to half of the temperature of the sun, and improve efficiency 1000 times, as compared to graphene on a solid substrate.”

The team also demonstrated the scalability of their technique by realizing large-scale of arrays of chemical-vapor-deposited (CVD) graphene light emitters.

Yun Daniel Park, professor in the Department of Physics and Astronomy at Seoul National University and co-lead author, notes that they are working with the same material that Thomas Edison used when he invented the incandescent light bulb: “Edison originally used carbon as a filament for his light bulb and here we are going back to the same element, but using it in its pure form—graphene—and at its ultimate size limit—one atom thick.”

The group is currently working to further characterize the performance of these devices—for example, how fast they can be turned on and off to create “bits” for optical communications—and to develop techniques for integrating them into flexible substrates.

Hone adds, “We are just starting to dream about other uses for these structures—for example, as micro-hotplates that can be heated to thousands of degrees in a fraction of a second to study high-temperature chemical reactions or catalysis.”

Here’s a link to and a citation for the paper,

Bright visible light emission from graphene by Young Duck Kim, Hakseong Kim, Yujin Cho, Ji Hoon Ryoo, Cheol-Hwan Park, Pilkwang Kim, Yong Seung Kim, Sunwoo Lee, Yilei Li, Seung-Nam Park, Yong Shim Yoo, Duhee Yoon, Vincent E. Dorgan, Eric Pop, Tony F. Heinz, James Hone, Seung-Hyun Chun, Hyeonsik Cheong, Sang Wook Lee,    Myung-Ho Bae, & Yun Daniel Park. Nature Nanotechnology (2015) doi:10.1038/nnano.2015.118 Published online 15 June 2015

This paper is behind a paywall.

Two final notes: there was an announcement earlier this year (mentioned in my March 30, 2015 post) that a graphene light bulb would be in stores this year. Dexter Johnson notes in his June 15, 2015 post (Nanoclast blog on the IEEE [International Institute of Electrical and Electronics Engineers] website) that the earlier light bulb has a graphene coating. You may want to check out Dexter’s posting about the latest light bulb achievement as he also includes an embedded video illustrating how Columbia Engineering’s graphene filament works.

Graphene and its grain boundaries

Most folks who follow the graphene scene are familiar with the honeycomb structure (hexagonal network) shown in diagram after diagram but I imagine there’s more than one of us who didn’t realize that defects can occur at the boundaries, from the Jan. 15, 2012 news release on EurekAlert,

When graphene is grown, lattices of the carbon grains are formed randomly, linked together at different angles of orientation in a hexagonal network. However, when those orientations become misaligned during the growth process, defects called grain boundaries (GBs) form. These boundaries scatter the flow of electrons in graphene, a fact that is detrimental to its successful electronic performance.

The Jan. 14, 2013 University of Illinois Beckman Institute news release written by Steve McGoughey, which originated the item on  EurekAlert, provides insight into the problem and its solution,

Beckman Institute researchers Joe Lyding and Eric Pop and their research groups have now given new insight into the electronics behavior of graphene with grain boundaries that could guide fabrication methods toward lessening their effect. The researchers grew polycrystalline graphene by chemical vapor deposition (CVD), using scanning tunneling microscopy (STM) and spectroscopy for analysis, to examine at the atomic scale grain boundaries on a silicon wafer. They reported their results in the journal ACS Nano.

“We obtained information about electron scattering at the boundaries that shows it significantly limits the electronic performance compared to grain boundary free graphene,” Lyding said. “Grain boundaries form during graphene growth by CVD, and, while there is much worldwide effort to minimize the occurrence of grain boundaries, they are a fact of life for now.

“For electronics you would want to be able to make it on a wafer scale. Boundary free graphene is a key goal. In the interim we have to live with the grain boundaries, so understanding them is what we’re trying to do.”

Lyding compared graphene lattices made with the CVD method to pieces of a cyclone fence.

“If you had two pieces of fence, and you laid them on the ground next to each other but they weren’t perfectly aligned, then they wouldn’t match,” he said. “That’s a grain boundary, where the lattice doesn’t match.”

Their analysis showed that when the electrons’ itinerary takes them to a grain boundary, it is like, Lyding said, hitting a hill.

“The electrons hit this hill, they bounce off, they interfere with themselves and you actually see a standing wave pattern,” he said. “It’s a barrier so they have to go up and over that hill. Like anything else, that is going to slow them down. That’s what Justin was able to measure with these spectroscopy measurements.

“Basically a grain boundary is a resistor in series with a conductor. That’s always bad. It means it’s going to take longer for an electron to get from point A to point B with some voltage applied.”

In the paper, the researchers were able to report on their analysis of the orientation angles between pieces of graphene as they grew together, and found “no preferential orientation angle between grains, and the GBs are continuous across graphene wrinkles and Si02 topography.” They reported that analysis of those patterns “indicates that backscattering and intervalley scattering are the dominant mechanisms responsible for the mobility reduction in the presence of GBs in CVD-grown graphene.”

The researchers work is aimed not just at understanding, but also at controlling grain boundaries. One of their findings – that GBs are aperiodic – replicated other work and could have implications for controlling them, as they wrote in the paper: “Combining the spectroscopic and scattering results suggest that GBs that are more periodic and well-ordered lead to reduced scattering from the GBs.”

“I think if you have to live with grain boundaries you would like to be able to control exactly what their orientation is and choose an angle that minimizes the scattering,” Lyding said.

Here’s a citation and link for the article,

Atomic-Scale Evidence for Potential Barriers and Strong Carrier Scattering at Graphene Grain Boundaries: A Scanning Tunneling Microscopy Study by Justin C. Koepke, Joshua D. Wood, David Estrada, Zhun-Yong Ong, Kevin T. He, Eric Pop, and Joseph W. Lyding in ACS Nano, Article ASAP DOI: 10.1021/nn302064p Publication Date (Web): December 13, 2012

Copyright © 2012 American Chemical Society

The article has not been published in print and it is behind a paywall.

Finger pinches today, heartbeats tomorrow and electricity forever

Devices powered by energy generated and harvested from one’s own body have been of tremendous interest to me. Last year I mentioned some research in this area by Professor Zhong Lin Wang at Georgia Tech (Georgia Institute of Technology) in a July 12, 2010 posting. Well, Wang and his team recently announced that they have developed the first commercially viable nanogenerator. From the March 29, 2011 news item on Physorg.com,

After six years of intensive effort, scientists are reporting development of the first commercially viable nanogenerator, a flexible chip that can use body movements — a finger pinch now en route to a pulse beat in the future — to generate electricity. Speaking here today at the 241st National Meeting & Exposition of the American Chemical Society, they described boosting the device’s power output by thousands times and its voltage by 150 times to finally move it out of the lab and toward everyday life.

“This development represents a milestone toward producing portable electronics that can be powered by body movements without the use of batteries or electrical outlets,” said lead scientist Zhong Lin Wang, Ph.D. “Our nanogenerators are poised to change lives in the future. Their potential is only limited by one’s imagination.”

Here’s how it works  (from Kit Eaton’s article on Fast Company),

The trick used by Dr. Zhong Lin Wang’s team has been to utilize nanowires made of zinc oxide (ZnO). ZnO is a piezoelectric material–meaning it changes shape slightly when an electrical field is applied across it, or a current is generated when it’s flexed by an external force. By combining nanoscopic wires (each 500 times narrower than a human hair) of ZnO into a flexible bundle, the team found it could generate truly workable amounts of energy. The bundle is actually bonded to a flexible polymer slice, and in the experimental setup five pinky-nail-size nanogenerators were stacked up to create a power supply that can push out 1 micro Amp at about 3 volts. That doesn’t sound like a lot, but it was enough to power an LED and an LCD screen in a demonstration of the technology’s effectiveness.

Dexter Johnson at Nanoclast on the IEEE (Institute of Electrical Engineering and Electronics) website notes in his March 30, 2010 posting (http://spectrum.ieee.org/nanoclast/semiconductors/nanotechnology/powering-our-electronic-devices-with-nanogenerators-looks-more-feasible) that the nanogenerator’s commercial viability is dependent on work being done at the University of Illinois,

I would have happily chalked this story [about the nanogenerator] up to one more excellent job of getting nanomaterial research into the mainstream press, but because of recent work by Eric Pop and his colleagues at the University of Illinois’s Beckman Institute in reducing the energy consumed by electronic devices it seems a bit more intriguing now.

So low is the energy consumption of the electronics proposed by the University of Illinois research it is to the point where a mobile device may not need a battery but could possibly operate on the energy generated from piezoelectric-enabled nanogenerators contained within such devices like those proposed by Wang.

I have a suspicion it’s going to be a while before I will be wearing nanogenerators to harvest the electricity my body produces. Meanwhile, I have some questions about the possible uses for nanogenerators (from the Kit Eaton article),

The search for tiny power generator technology has slowly inched forward for years for good reason–there are a trillion medical and surveillance uses–not to mention countless consumer electronics applications– for a system that could grab electrical power from something nearby that’s moving even just a tiny bit. Imagine an implanted insulin pump, or a pacemaker that’s powered by the throbbing of the heart or blood vessels nearby (and then imagine the pacemaker powering the heart, which is powered by the pacemaker, and so on and so on….) and you see how useful such a system could be.

It’s the reference to surveillance that makes me a little uneasy.