Nov_Dec_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 7 1 4 TEMPERATURE TUNES 2D ALLOY Researchers at Rice University, Houston, developed a four-material 2D alloy with an optical bandgap that can be tuned by adjusting its growth temperature. Before growing the al- loy—composed of molybdenum, tung- sten, sulfur, and selenium—researchers generated 152 random models. These reveal that the bandgap could be tuned from 1.62 to 1.84 electron volts by vary- ing the growth temperature from 650° to 800°C. After performing these simu- lations, the thermodynamically stable materials were made in a chemical va- por deposition furnace at 50-degree in- crements and tested. Images from Oak Ridge National Laboratory, Tenn., detail the position of each atom and confirm that changes in growth temperature correspond to changes in the way the atoms assemble, as well as in the prop- erties that determine light absorption and emission. These bandgap changes are extremely regular and result in pre- dictable optical properties. Although labs have made 2D ma- terials with two or three components, the researchers believe theirs is the first attempt with four, which allows unprecedented freedom. “With fewer materials, every adjustment you make to change the bandgap turns it into a different material. That’s not the case here,” explains Rice postdoc Chan- dra Sekhar Tiwary. Because the ma- terial can be tuned to cover the entire spectrum of visible light—from 400 to 700-nm wavelengths—the discovery could impact solar cells and light-emit- ting diodes, leading to smaller, more efficient devices. rice.edu . WASH AND WEAR SOLAR CELL Scientists at Riken, Japan, and the University of Tokyo invented an ultra-thin photovoltaic device that provides electricity even after being submerged in water, stretched, or squished. The cell could make its way BRIEF Clean technology company alpha-En Corp., New York, will enter a joint research program with Mercedes-Benz Re- search and Development North America, Sunnyvale, Calif., and Princeton University, New Jersey. The project aims to investigate the use of alpha-En’s highly pure lithiummetal nanorod thin films in next-generation battery technolo- gies with the aim of improving cell performance and battery production for electric vehicles and consumer products. alpha-encorp.com . ENERGY TRENDS Washable, ultra-thin organic solar cells. into the Internet of Things as a compo- nent of washable, wearable electronics. To create the device, an organic solar cell based on a material called PNTz4T was deposited in an inverse architec- ture onto a 1-µm-thick parylene film. The ultra-thin device was then sand- wiched between two identical acryl- ic-base elastomer films, sealing it within a flexible coating that admits light but prevents the infiltration of water and air. Experiments show that the device has an energy efficiency of 7.9%. Based on simulated sunlight of 100 mW/cm 2 , it produces a current of 7.86 mW/cm 2 and a current density of 13.8 mA/cm 2 at 0.57 V. When the device is soaked in water for two hours, its efficiency de- creases by just 5.4%, and after being compressed in half for 20 cycles while subjected to water droplets, the cell maintains 80% of its original efficiency. Past attempts to create photo- voltaics that could be incorporated into textiles have lacked long-term stability in both air and water, energy efficiency, or robustness. The new cell overcomes these obstacles, paving the way toward devices such as health monitors that can be incor- porated into everyday clothing. www. riken.jp/en , www.u-tokyo.ac.jp/en . Subtle changes in growth temperature alter the form of a four-component alloy created at Rice University. Courtesy of Alex Kutana.