Record-setting flexible phototransistor

Developed by UW electrical engineers, this unique phototransistor is flexible, yet faster and more responsive than any similar phototransistor in the world. Photo: Jung-Hun Seo

Inspired by mammals' eyes, University of Wisconsin-Madison electrical engineers have created the fastest, most responsive flexible silicon phototransistor ever made.

The innovative phototransistor could improve the performance of myriad products — ranging from digital cameras, night-vision goggles and smoke detectors to surveillance systems and satellites — that rely on electronic light sensors. Integrated into a digital camera lens, for example, it could reduce bulkiness and boost both the acquisition speed and quality of video or still photos.

Developed by UW-Madison collaborators Zhenqiang "Jack" Ma, professor of electrical and computer engineering, and research scientist Jung-Hun Seo, the high-performance phototransistor far and away exceeds all previous flexible phototransistor parameters, including sensitivity and response time.

The researchers published details of their advance this week in the journal Advanced Optical Materials.

Like human eyes, phototransistors essentially sense and collect light, then convert that light into an electrical charge proportional to its intensity and wavelength. In the case of our eyes, the electrical impulses transmit the image to the brain. In a digital camera, that electrical charge becomes the long string of 1s and 0s that create the digital image.

While many phototransistors are fabricated on rigid surfaces, and therefore are flat, Ma and Seo's are flexible, meaning they more easily mimic the behavior of mammalian eyes.

"We actually can make the curve any shape we like to fit the optical system," Ma says. "Currently, there's no easy way to do that."

One important aspect of the success of the new phototransistors is the researchers' innovative "flip-transfer" fabrication method, in which their final step is to invert the finished phototransistor onto a plastic substrate. At that point, a reflective metal layer is on the bottom.

"In this structure — unlike other photodetectors — light absorption in an ultrathin silicon layer can be much more efficient because light is not blocked by any metal layers or other materials," Ma says.

While many phototransistors are fabricated on rigid surfaces, and therefore are flat, Ma and Seo's are flexible, meaning they more easily mimic the behavior of mammalian eyes.

The researchers also placed electrodes under the phototransistor's ultrathin silicon nanomembrane layer — and the metal layer and electrodes each act as reflectors and improve light absorption without the need for an external amplifier.

"There's a built-in capability to sense weak light," Ma says.

Ultimately, the new phototransistors open the door of possibility, he says.

"This demonstration shows great potential in high-performance and flexible photodetection systems," says Ma, whose work was supported by the U.S. Air Force. "It shows the capabilities of high-sensitivity photodetection and stable performance under bending conditions, which have never been achieved at the same time."

University of Wisconsin-Madison 

New Energy Storage Capabilities Between the Layers of Two-Dimensional materials

Drexel University researchers are continuing to expand the capabilities and functionalities of a family of two-dimensional materials they discovered that are just a few atoms thick, but have the potential to store massive amounts of energy. Their latest achievement has pushed the materials storage capacities to new levels while also allowing for their use in flexible devices.

About three years ago, Dr. Michel W. Barsoum and Dr. Yury Gogotsi, professors in Drexel’s College of Engineering, discovered atomically thin, two-dimensional materials -similar to graphene- that have good electrical conductivity and a surface that is hydrophilic, or can hold liquids. They named these new materials “MXenes,” which hearkens to their genesis through the process of etching and exfoliating atomically thin layers of aluminum from layered carbide “MAX phases.”  The latter also discovered at Drexel about 15 years ago by Barsoum

Since then, the pair, and their team of materials scientists, have forged ahead in exploring the potential uses of MXenes. Their latest findings are reported in the Sept. 27 issue of Science. In their piece entitled “Cation Intercalation and High Volumetric Capacitance of Two-dimensional Titanium Carbide,” Gogotsi and Barsoum along with Drexel researchers Maria Lukatskaya, Olha Mashtalir, Chang Ren, Yohan Dall’Angese and Michael Naugib and Patrick Rozier, Pierre Louis Taberna and Dr. Patrice Simon from Université Paul Sabatier in France, explain how MXenes can accommodate various ions and molecules between their layers by a process known as intercalation.

Intercalation is sometimes a necessary step in order to exploit the unique properties of two-dimensional materials. For example, placing lithium ions between the MXene sheets makes them good candidates for use as anodes in lithium-ion batteries. The fact that MXenes can accommodate ions and molecules in this way is significant because it expands their ability to store energy.

“Currently, nine MXenes have been reported by our team, but there are likely many more that will be discovered - the MXene-and-ion combinations that have been tested to date are by no means an exhaustive demonstration of the material’s energy storage capabilities,” said Gogotsi, who is also director of the A.J. Drexel Nanotechnology Institute. “So even the impressive capacitances that we are seeing here are probably not the highest possible values to be achieved using MXenes. Intercalation of magnesium and aluminum ions that we observed may also pave the way to development of new kinds of metal ion batteries.”

Barsoum and Gogotsi’s report looks at intercalation of MXenes with a variety of ions, including lithium, sodium, magnesium, potassium, ammonium and aluminum ions. The resulting materials show high energy storage capacities and present another avenue of research in this branch of materials science.

“Two-dimensional, titanium carbide MXene electrodes show excellent volumetric super capacitance of up to 350 F/cm3 due to intercalation of cations between its layers,” Barsoum said. “This capacity is significantly higher than what is currently possible with porous carbon electrodes. In other words, we can now store more energy in smaller volumes, an important consideration as mobile devices get smaller and require more energy”

The researchers also reported on using MXene “paper” electrodes, instead of conventional rolled powder electrodes with a polymer binder. The flexibility of this paper suggests MXenes may also be useful in flexible and wearable energy storage devices, which is another major area of ongoing research at Drexel in collaboration with Professor Genevieve Dion’s Shima Seiki Haute Technology Laboratory.

Source: http://www.drexel.edu/now/news-media/releases/archive/2013/September/MXenes-Science/#sthash.8rtZ168M.dpuf