Power/Performance Bits: June 16

Lighting up graphene

A team of scientists from Columbia University, Seoul National University, and Korea Research Institute of Standards and Science demonstrated an on-chip visible light source using graphene 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 James Hone, professor of mechanical engineering at Columbia Engineering. “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.”

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 Young Duck Kim, a postdoctoral research scientist at Columbia Engineering.

This movie clip schematically shows the light emission from graphene and radiation spectrum engineering by strong optical interference effect. Vibration of graphene during light emission is due to the flexural mode of graphene at high temperature. (Source: Myung-Ho Bae/KRISS)

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 group is currently working to further characterize the performance of these devices to see 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.

Origami battery                                                          

Origami, the Japanese art of paper folding, can be used to create beautiful birds, frogs and other small sculptures – and, perhaps, batteries. Binghamton University engineer Seokheun “Sean” Choi used the techniques to develop an inexpensive, bacteria-powered battery made from paper.

The battery generates power from microbial respiration, delivering enough energy to run a paper-based biosensor with nothing more than a drop of bacteria-containing liquid. “Dirty water has a lot of organic matter,” Choi says. “Any type of organic material can be the source of bacteria for the bacterial metabolism.”

The method should be especially useful to anyone working in remote areas with limited resources. Indeed, because paper is inexpensive and readily available, many experts working on disease control and prevention have seized upon it as a key material in creating diagnostic tools for the developing world.

Origami batteries like this one, developed by Binghamton University researcher Seokheun Choi, could one day power biosensors for use in remote locations. (Source: Jonathan Cohen/Binghamton University)

“Paper is cheap and it’s biodegradable,” Choi says. “And we don’t need external pumps or syringes because paper can suck up a solution using capillary force.”

While paper-based biosensors have shown promise in this area, the existing technology must be paired with hand-held devices for analysis. Choi says he envisions a self-powered system in which a paper-based battery would create the microwatts required to run the biosensor.

Choi’s battery, which folds into a square the size of a matchbook, uses an inexpensive air-breathing cathode created with nickel sprayed onto one side of ordinary office paper. The anode is screen printed with carbon paints, creating a hydrophilic zone with wax boundaries. The total cost? Five cents.

Flexible PRAM

Phase change random access memory (PRAM) is one of the strongest candidates for next-generation nonvolatile memory for flexible and wearable electronics. In order to be used as a core memory for flexible devices, the most important issue is reducing high operating current. The effective solution is to decrease cell size in sub-micron region as in commercialized conventional PRAM. However, the scaling to nano-dimension on flexible substrates is extremely difficult due to soft nature and photolithographic limits on plastics.

Recently, a team at the Korea Advanced Institute of Science and Technology (KAIST) developed the first flexible PRAM enabled by self-assembled block copolymer (BCP) silica nanostructures with an ultralow current operation (below one quarter of conventional PRAM without BCP) on plastic substrates. BCP is the mixture of two different polymer materials, which can easily create self-ordered arrays of sub-20 nm features through simple spin-coating and plasma treatments. BCP silica nanostructures successfully lowered the contact area by localizing the volume change of phase-change materials and thus resulted in significant power reduction. The ultrathin silicon-based diodes were integrated with phase-change memories (PCM) to suppress the inter-cell interference, which demonstrated random access capability for flexible and wearable electronics.

Low-power nonvolatile PRAM for flexible and wearable memories enabled by (a) self-assembled BCP silica nanostructures and (b) self-structured conductive filament nanoheater. (Source: KAIST)

Another way to achieve ultralow-powered PRAM is to utilize self-structured conductive filaments (CF) instead of the resistor-type conventional heater. The self-structured CF nanoheater originated from unipolar memristor can generate strong heat toward phase-change materials due to high current density through the nanofilament. The researchers’ methodology showed that sub-10 nm filament heater, without using expensive and non-compatible nanolithography, achieved nanoscale switching volume of phase change materials, resulted in the PCM writing current of below 20 uA, the lowest value among top-down PCM devices. In addition, due to self-structured low-power technology compatible to plastics, the research team was able to fabricate a flexible PRAM on wearable substrates.