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InGaN-based vertical LEDs with acid-modified graphene transparent conductor and highly reflective membrane current blocking layer
The two-dimensionality and structural flatness make graphene films ideal candidates for thin film devices and combination with other semiconductor materials. In this work, vertical light emitting diodes (VLEDs) with highly reflective membrane as current blocking layer and graphene transparent conductive layer have been fabricated and characterized. VLEDs show improved optical output and efficiency droop due to the current spreading effect of the hybrid electrode by preventing current crowding under the top electrode and increasing the internal and external quantum efficiency.
Research creates ‘recipe book’ for building new materials
[Materials Views] By showing that tiny particles injected into a liquid crystal medium adhere to existing mathematical theorems, physicists at the University of Colorado Boulder have opened the door for the creation of a host of new materials with properties that do not exist in nature. The findings show that researchers can create a “recipe book” to build new materials of sorts using topology, a major mathematical field that describes the properties that do not change when an object is stretched, bent or otherwise “continuously deformed.” Published in the journal Nature, the study also is the first to experimentally show that some of the most important topological theorems hold up in the real material world, said CU-Boulder physics department’s Ivan Smalyukh, a study senior author. Once injected into a liquid crystal, the particles behaved as predicted by topology. “Our study shows that interaction between particles and molecular alignment in liquid crystals follows the predictions of topological theorems, making it possible to use these theorems in designing new composite materials with unique properties that cannot be encountered in nature or synthesized by chemists. These findings lay the groundwork for new applications in experimental studies of low-dimensional topology, with important potential ramifications for many branches of science and technology.” The research supports the goals laid out by the White House’s Materials Genome Initiative, Smalyukh said, which seeks to deploy “new advanced materials at least twice as fast as possible today, at a fraction of the cost.”
Tetris-like molecular structure is key to making ultrastable glass
Like a fine wine or aged cheese, ultrastable glass takes a long time to make, needs special conditions and is considered quite valuable. Unfortunately, manufacturers who want to take advantage of the strengths of ultrastable glass don’t have the luxury of waiting hundreds of years for it to develop. While exploring ways to create this valuable material on a shorter timetable, researchers from the University of Wisconsin-Madison have gained some key insights into the bizarre structure of glasses as well as how it affects their properties. “In attempts to work with aged glasses, for example, people have examined amber,” says study coauthor Juan de Pablo, a University of Chicago professor of molecular theory. “Amber is a glass that has been aged millions of years, but you cannot engineer that material. You get what you get.” In many laboratories, scientists use a technique called vapor deposition to create specialized materials. Previous research by another of the new study’s coauthors, Mark Ediger, found that glasses grown in this manner—within a certain temperature range and on a specially prepared surface – are far more stable than ordinary glasses. Ediger determined that in order to achieve this degree of stability, molecules in the glass are arranged in a tightly packed manner like the multi-shaped objects in the popular videogame Tetris.
Changing our material future, layer by layer
Researchers are aiming to develop a new class of materials with remarkable properties using one atom-thick substances such as graphene in a new collaborative project. The proposal, which will involve researchers from the Universities of Manchester, Cambridge, and Lancaster, has been awarded €13.4 million to form a “Synergy Group” by the European Research Council. It will aim to utilize two-dimensional substances, such as graphene, to engineer new types of materials that are just a few atoms thick, but nevertheless have the power to revolutionize the future development of devices such as solar cells, and flexible and transparent electronics. Starting with one atom-thick substances, which possess remarkable properties, the group will focus on ways in which they can be layered up to form “heterostructures.” These heterostructures will still be just a few atoms thick, but will combine the properties of the different two-dimensional materials which comprise them, effectively enabling developers to embed the functions of a device into its very fabric. For example, the research team envisage combining an atomic layer which functions as a sensor, with layers that function variously as an amplifier, transistor, or solar cell, for power generation. The resulting material, still just a few atomic layers in thickness, would be capable of running a whole circuit.
A new charging algorithm could extend battery life for the first all-electric, battery-powered locomotive in the US
Batteries for Norfolk Southern Railway No. 999, just like automotive batteries, are rechargeable until they eventually die. A leading cause of damage and death in lead-acid batteries is sulfation, a degradation of the battery caused by frequent charging and discharging that creates an accumulation of lead sulfate. In a recent study, the researchers looked for ways to improve regular battery management practices. The methods had to be nondestructive, simple, and cheap—using as few sensors, electronics, and supporting hardware as possible while still remaining effective at identifying and decreasing sulfation. “We wanted to reverse the sulfation to rejuvenate the battery and bring it back to life,” says Christopher Rahn, professor of mechanical engineering at Penn State. Rahn, along with mechanical engineering research assistants Ying Shi and Christopher Ferone, cycled a lead-acid battery for three months in the same way it would be used in a locomotive. They used a process called electroimpedance spectroscopy and full charge/discharge to identify the main aging mechanisms. Through this, the researchers identified sulfation in one of the six battery cells. They then designed a charging algorithm that could charge the battery and reduce sulfation, but was also able to stop charging before other forms of degradation occurred. The algorithm successfully revived the dead cell and increased the overall capacity.
Speeding up electronics to light frequencies
Modern information processing allows for breathtaking switching rates of about a 100 billion cycles per second. New results from Ferenc Krausz’s Laboratory for Attosecond Physics of the Max Planck Institute of Quantum Optics, Garching, Germany, and Ludwig-Maximilians-Universität, Munich, could pave the way towards signal processing several orders of magnitude faster. In two groundbreaking complementary experiments a collaboration led by LAP-physicists has demonstrated that, under certain conditions, ultrashort light pulses of extremely high intensity can induce electric currents in otherwise insulating dielectric materials. Furthermore, they provided evidence that the fast oscillations of the electric field instantly alter the electrical and optical properties of the material, and that these changes can be reversed on a femtosecond time scale. This opens the door for signal processing rates reaching the petahertz domain, about 10,000 times faster than it is possible with the best state-of-the-art solid state microchips. The experiments were carried out in close cooperation with the theoretical group of Mark Stockman, Georgia State University.
Delineating the pathways for the site-directed synthesis of individual nanoparticles on surfaces
Although nanoparticles with exquisite properties have been synthesized for a variety of applications, their incorporation into functional devices is challenging owing to the difficulty in positioning them at specified sites on surfaces. To develop a materials-general method for synthesizing nanoparticles on surfaces for broader applications, a mechanistic understanding of polymer-mediated nanoparticle formation is crucial. A Northwestern University group, led by Chad A. Mirkin, has designed a four-step synthetic process that enables independent study of the two most critical steps for synthesizing single nanoparticles on surfaces: phase separation of precursors and particle formation. Using this process, they have elucidate the importance of the polymer matrix in the diffusion of metal precursors to form a single nanoparticle and the three pathways that the precursors undergo to form nanoparticles. Based on this mechanistic understanding, the synthetic process is generalized to create metal (Au, Ag, Pt, and Pd), metal oxide (Fe2O3, Co2O3, NiO, and CuO), and alloy (AuAg) nanoparticles. This mechanistic understanding and resulting process represent a major advance in scanning probe lithography as a tool to generate patterns of tailored nanoparticles for integration with solid-state devices.