IRP Focus: Energy & Sustainability: Research Projects
Perfecting and Simplifying the Fabrication of a New Semiconductor
Jung Han, Dept. of Electrical Eng.
For more than a decade, gallium nitride has been a focus of semiconductor research and a challenging target for materials scientists who seek to exploit its unique properties. The superior energy efficiency of GaN-light-emitted diodes is already displacing conventional lighting technologies, and GaN transistors are predicted to have ubiquitous applications in modernizing our power-grid and electricity infrastructure.
One of the difficulties in fabricating GaN devices is the mechanical toughness and chemical inertness associated with this ceramic-like semiconductor. But a team of Yale engineers led by Electrical Engineering Professor Jung Han is approaching the challenge by employing electrical engineering, applied physics, material science, and surface chemistry and is achieving significant breakthroughs with the material.
Han and colleagues have developed and are improving upon a simple, fast, and inexpensive electrochemical method for creating small, stable gallium nitride films. By drilling nanometer-sized pores into the material’s surface—a technique called “nanoetching”—they weaken and lift off crystalline thin layers within minutes.
Not only are the SEAS engineers simplifying the fabrication of gallium nitride semiconductors, they are perfecting a unique type of “nanoporous” semiconductor, which could open up opportunities for new approaches to the design and manufacture of high-frequency transistors, sensors, and other electronic devices.
Nanotechnology, December 20, 2010, “The fabrication of large-area, free-standing GaN by a novel nanoetching process.”
Developing New Catalysts from Ferroelectric Materials
Eric Altman, Dept. of ChE & EnvE
The role of a heterogeneous catalyst, used in essentially every industrial chemical process, is to speed up a chemical reaction by providing a surface where that reaction readily occurs. By investigating novel surfaces, understanding their properties, and transforming them at the atomic level, the Altman Lab is making headway on developing entirely new categories of catalysts with potential to impact a variety of industries.
Lately the Altman team, working with applied physicists and chemical engineers, has taken on a class of materials called ferroelectrics. Ferroelectrics possess an electric polarization whose direction can actually be switched by applying an external electric field. Switching the material’s polarization direction in turn changes its surface properties, which includes its surface reactivity. A material whose surface reactivity can be altered so easily opens up possibilities for "switchable" catalysts.
Its interest piqued by the potential application of a catalyst based on ferroelectric materials in breaking down nitrogen oxides, an industrial partner has funded the Altman Lab to explore the potential role of ferroelectric materials in catalytic converters. In this case, the engineers are using molecular beam epitaxy to grow thin layers of selected materials that have shown promise in nitrogen oxide reduction on top of oxide ferroelectrics. Key to this work is the nanoscale at which the surfaces are developed: materials fabricated to be only a few atomic layers thick will almost certainly display properties that vary to at least some degree from the properties of the bulk materials; while the bulk materials have many desirable properties, they also have some undesirable ones that the researchers hope will be suppressed when they are atomically thin, especially for those grown on ferroelectric substrates.
In another project involving ferroelectrics, engineers are investigating whether a ferroelectric called lithium niobate could be an ideal substrate for atomic layers of chromium oxide—a reactive material used as a catalyst in a variety of industrial applications. Success in this project could yield a “switchable” catalyst, as the reactivity of the chromium oxide could change in response to switching the electric field of the lithium niobate substrate. The possible applications for these new catalysts are endless, including the one of interest to the lab’s industrial partner: reducing motor vehicle fuel consumption and greenhouse gas emissions by changing the way catalytic converters work.
“Chemistry of Ferroelectric Surfaces,” Advanced Materials, 2010, 22, 2969-2973
“Using Ferroelectric Poling to Change Adsorption on Oxide Surfaces,” J. Am. Chem. Soc., 2007, 129 (50), pp 15684–15689
“Effect of Ferroelectric Poling on the Adsorption of 2-Propanol on LiNbO3(0001),” J Phys Chem C Nanomater Interfaces 111(37):13951-13956 (2007)
Unraveling Microalgae Genetics to Perfect Biofuels
Jordan Peccia, Dept. of ChE & EnvE
Though the idea of producing biofuels from algae is not new, scientists as yet know little about the genes that algae use to make lipids—fats that can be used as biofuel—or how they can be turned off and on under different environmental conditions. As attention to alternative energy sources increases, however, renewed attention is being paid to the genetic makeup of microalgae.
A team of environmental engineers led by Professor Jordan Peccia is using a high-throughput DNA sequencing technology known as 454 pyrosequencing to unravel the genomes of lipid-rich microalgae. There are several goals of their work. If they can determine how the algae ramp up lipid production, and identify those algae that are most capable of producing lipids, the information could be useful in enhancing biofuel production. In addition, the Peccia team is using the technology to create a genetic library of the hundreds of thousands of species of algae in order to identify those that are ripe for genetic modification for biofuel production and those that are pure, lipid-enriched strains.
Because lipid creation is an energy-intensive process, the algae that are the best producers are also the weakest when it comes to surviving in an environment among competitors. So the Peccia team is also researching how to keep one particularly strong lipid producer, S. dimorphus, growing in a reactor contaminated by other algal species. By making genetic libraries of the algae and bacteria that thrive in a reactor as S. dimorphus dies off, they can trace population declines to particular causes and perhaps devise a solution. They are also exploring ways of promoting S. dimorphus growth, such as increasing CO2 content, which many competing algae species cannot tolerate. With information like that, microalgae may soon reach its true potential as a sustainable biofuel.
In the news:
Microbial Diversity: From Genetic Sequencing to Biofuels
Forward Osmosis for Clean Water and Energy
Menachem Elimelech, Dept. of ChE & EnvE
Water and energy are two resources on which all of modern society depends. As demands for each increase, researchers look to alternative technologies that promise sustainability and reduced environmental impact.
Engineers in the lab of environmental and chemical engineering professor Menachem Elimelech have proposed engineered osmosis as the key to addressing not just one resource challenge, but both—providing an answer to the global need for affordable clean water and inexpensive sustainable energy.
The engineers have proposed that the solution to these resource challenges may lie in the design of osmotically-driven membrane systems, capable of producing freshwater from nonpotable sources, including seawater; producing electrical power from naturally occurring salinity gradients; and generating electricity from low-temperature heat sources such as reject heat from thermal processes and conventional power plants.
Their work has already resulted in the Yale spinoff company, Oasys Water, Inc., which estimates that its patented engineered osmosis (EMTM) process will produce drinking water at less than half the cost of current desalination processes by reducing electricity and fuel demands by more than 90 percent.
Environmental Science & Technology
In the news:
Energy Efficient Water Purification Made Possible by Yale Engineers
Engineered Osmosis Holds Promise for Clean Water and Sustainable Energy
Developing Efficient, Commercially Viable Fuel Cells
André Taylor, Dept. of ChE & EnvE; Jan Schroers, Dept. of Mech. Eng, & Mats. Sci.
Fuel cells have been touted as a cleaner solution to tomorrow's energy needs, with potential applications in everything from small portable electronic devices to automobiles to industrial facilities. But one reason fuel cells aren't already more widespread is their lack of endurance. Over time, the catalysts used even in today's state-of-the-art cells break down, inhibiting the chemical reaction that converts fuel into electricity. In addition, only a fraction of the catalyst is properly utilized at any given time, while much of it sits tucked away, unexposed and unavailable for chemical reaction.
In order to produce more efficient fuel cells, chemical and mechanical engineers in the Taylor and Schroers Labs are collaborating to design new catalysts that can increase the active surface area and endurance. Their innovative use of bulk metallic glass nanowires as fuel cell catalysts shows promise for widespread commercial viability in portable electronic devices such as laptop computers and cell phones and in remote sensors.
Bulk metallic glasses are amorphous metal alloys with many potential uses that are synthesized in the Schroers Lab. Nanowires made from it have a novel architecture and outstanding durability, which circumvents the performance problems of current electrochemical devices. The BMG nanowires, fabricated using a facile and scalable nanoimprinting approach, maintain their activity longer than traditional fuel cell catalyst systems. Platinum incorporated into the nanowire architecture boosts the catalyst performance to 2.4 times that of standard technology.
The Yale engineers have shown the effectiveness of the technology in alcohol-based fuel cells, and continue to explore its effectiveness in other types of fuel cells. Ultimately, batteries in electronic devices will be supplemented or replaced entirely by these systems.
Bulk Metallic Glass Nanowires Architecture for Electrochemical Applications, Marcelo Carmo, Shiyan Ding, Golden Kumar, Jan Schroers, and André D. Taylor, ACS Nano.
In the News:
Novel Nanowires Boost Fuel Cell Efficiency
Striving for a Solar Cell Efficiency Record
Minjoo Larry Lee, Dept. of Electrical Eng.
Powering homes with clean, sustainable solar energy has long been an appealing and popular idea, but the shortcomings of current technology still stand in the way of the widespread use of solar cells. Most commercially available solar cells today convert solar energy to electricity at efficiency levels of only between 10 and 29 percent.
Engineers in Larry Lee’s Lab aspire to double that top number in order to make the technology economically viable for widespread use. A record of 43 percent efficiency was achieved just recently. But Lee and his colleagues aspire to get to 60 percent—a level that has, as yet, only been theoretically modeled.
Their work, at the intersection of materials science and electrical engineering, aims to improve the efficiency of solar cells used in conjunction with concentrator photovoltaics. The goal is to develop thumbnail-sized, superefficient solar cells whose effect can be literally magnified by concentrator technology, which acts much as a magnifying glass would to intensify the sunlight shining on the cell.
To get there, Lee and colleagues are meticulously sculpting semiconductor crystals to control the flow of light and electrons to target yellow-green wavelengths—another feat that engineers have yet to achieve. Starting with a single-crystal wafer, they add one atomic layer at a time using molecular beam epitaxy. This careful stacking of indium, gallium, and phosphorous onto a precisely engineered crystalline lattice will enable a solar cell that converts yellow-green and shorter wavelengths of light into electricity, instead of to wasted heat. Lee believes that efficiently capturing this wavelength range is the stiffest barrier to achieving efficiencies of 60 percent.
Designing Functional Proteins that Move Electrons
Corey J. Wilson, Dept. of ChE & EnvE
Caltech theoretical chemist Rudolph A. Marcus won the Nobel Prize in 1992 for his contributions to the theory of electron transfer reactions in chemical systems. But he did not present an instruction manual for replicating them.
Chemical and environmental engineers in the Wilson Lab are integrating experimental and computational modeling approaches to design synthetic electron transfer systems. They began by deconstructing Marcus Theory and translating it into a design algorithm for building proteins that are capable of accepting and donating electrons. To accomplish this, the Wilson Lab modeled protein energetics—analogous to stability analysis of buildings whereby civil engineers use accurate computer models to evaluate designs before they are built—using Yale’s Bulldog high-performance-computing clusters. With blueprints in hand, the experimental nano-scale construction work commenced. To date, the team has succeeded in designing functional proteins that propagate electrons and continues toward the goal of perfecting biological mimics.
The potential applications of the work are numerous and exciting. Synthetic electron transfer systems could eventually be used to generate energy in the human body, for instance by mimicking cellular respiration, or in products such as exterior house paints that could imitate photosynthesis to capture and translate light to power the home.