IRP Focus: Surfaces & Nanomaterials: Research Projects
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.
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)
Using the Force of Light to Power Nanomachines
Hong Tang, Dept. of Electrical Eng.
In recent years, members of the Tang Lab have won acclaim for describing the forces of light that can both attract and repel. This force is very weak, but its pressure is powerful enough to push an object of nanometer size and picogram mass, making it a potentially ideal force for controlling nanodevices.
Having found ways to harness this force, the engineers in the Tang Lab at Yale are developing a new class of light-force devices—semiconductors operated by light forces rather than electrostatic energy, by optics not voltage. They’ve identified numerous potential advantages to using light as a control: Photons, unlike electrons, don't interact with each other and so are immune to cross talk; because light has a much greater bandwidth, photonic signals can carry far more bits per second than electronic signals, while dissipating much less power; it's easy to route photons on a chip with an optical nanofiber on the top layer of silicon; and, being in a purely optical domain, impedance is no problem.
By engineering the mechanics and the electromagnetic wave on a chip, the Tang team has found new ways to reduce the loss of energy in both optical and mechanical domains, and has obtained a very high Q factor in both optical and mechanical resonators. This allows them to achieve strong interaction between light and mechanical structures, which is the key to future communications in quantum devices based on photons. Switches and routers equipped with light-force semiconductors, for instance, could relay data much more efficiently and much faster.
Harnessing Optical Forces in Integrated Photonic Circuits
Tunable Optical Coupler Controlled by Optical Gradient Forces, Optics Express
In the news:
IEEESpectrum, Photonics Breakthrough for Silicon Chips
Discover Magazine, Top 100 Stories of 2009, #83: Like Magnets, Light Can Attract and Repel Itself
Scientists Discover Light Force with Push Power
Technology Review, Light Repels Light
Engineering Speedier Silicon Chips with Quantum Dots
Minjoo Larry Lee, Dept. of Electrical Eng.
Imagine holding a crystal in the palm of your hand, perhaps an exotic specimen from the Natural History museum or something as mundane as rock candy. Then you cut it down to half its size over and over again. By the time you’ve halved its size about 20 times, you are left with nanoscale particles so tiny as to be considered "zero dimensional.” When the nanoscale bits are composed of semiconductor materials, they are known as quantum dots, and their properties can diverge widely from larger samples of the same material. One of the most remarkable aspects of quantum dots is their ability to trap or “confine” charge carriers and to convert them into light.
With funding from the Defense Advanced Research Projects Agency, the Lee Group works on a particular type of nanosized semiconductor known as a self-assembled quantum dot (SAQD). While SAQDs have already proven their usefulness in niche applications, the Lee group is striving to integrate them into the multi-billion dollar silicon microchip industry. In 2010, Lee’s team published the first paper describing the successful growth of indium gallium arsenide SAQDs on wafers of gallium phosphide, a silicon-compatible material. Grown inside a molecular beam epitaxy chamber, their SAQDs shape themselves into nanoscale light emitters in a process that resembles water droplets beading up on a well-waxed car.
Having developed the material, engineers on Lee’s team are working to perfect it in a device. They have recently proven that flowing electrical current through the SAQDs leads to light-emitting diode action, and their next goal is to demonstrate a laser using the same materials.
Computing speed has increased dramatically over the years, but still there's a need for faster communication, both on the silicon chip and among chips in a system. One of the most ambitious proposals for boosting microprocessor speed calls for the replacement of electrical communication with optical communication. Lee believes that the new SAQDs from his lab could be the key to enabling this advancement.
Applied Physics Letters
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
Investigating Atomic Precision at the Oxide Interface
Eric I. Altman, Dept. of ChE & EnvE; Sohrab Ismail-Beigi, Dept. of Applied Physics, Dept. of Physics
For many materials scientists, what goes on at the interface between two different materials is where the excitement is. Long gone are the days when electronics developers glued materials together by hand. Today’s materials are sandwiched together with atomic-scale precision – an advancement that has led to the control of exotic solid-state phenomena, such as magnetism and superconductivity at the nanoscale and a promise of applications that will have broad-sweeping impact on the technologies of our time.
The Atomic Scale Design, Control, and Characterization of Oxide Structures Interdisciplinary Research Group at the Center for Research on Innovative Structures and Phenomena (CRISP), led by Yale Professor of Chemical and Environmental Engineering Eric Altman and Associate Professor of Applied Physics Sohrab Ismail-Beigi is investigating the novel chemical, electronic, and magnetic properties that emerge at interfaces between oxides. The group’s work revolves around crystalline oxides –common compounds that can exhibit nearly every possible effect seen in solid-state physics.
Oxide materials range from the very common, such as sand, to esoteric materials that include high temperature superconductors and materials that change from insulators to metals when placed near magnets. Because of their lattice structure, crystalline oxides of different chemical composition can be stacked together, allowing for atomic-scale sandwiching of a variety of materials. What goes on at the interface between materials is of great interest and sometimes surprising, as was the case when researchers found superconducting properties could be displayed between two insulating materials. It can take years to design and grow materials with atomic precision, but CRISP has some of the top “growers” in the field, a team of theorists, and four oxide molecular beam epitaxy (MBE) machines—the sophisticated vacuum systems that grow materials a single atomic layer at a time– as well as state-of-the-art characterization tools uniquely suited to determining the positions and identities of all of the atoms at the interface and their chemical bonding.
The group’s research focuses on designing new materials with unique physical properties; creating new computing, communication, and sensing devices enabled by the novel properties of oxide interfaces; and understanding and manipulating the interactions between electrons that give rise to the novel properties.
In the news:
The Center for Research on Interface Structures and Phenomena
At the Oxide Interface: Where Experimentalists Play
Exploiting the Novel Properties of Bulk Metallic Glass
Themis Kyriakides, Dept. of Biomedical Eng., Yale School of Medicine; Jan Schroers, Dept. of Mech. Eng. & Mats. Sci.
In the field of biomedical implants, device developers rely on materials whose surface properties can be controlled and modified, that can function in nanoscale devices, and that are biocompatible. Having struck on novel methods for exploiting the unique properties of metal alloys known as bulk metallic glasses, mechanical engineers and materials scientists in the Schroers Lab are collaborating with biomedical engineering colleagues in the Kyriakides Lab to expand that materials’ application to medical uses.
They have shown bulk metallic glasses to be superior to other materials used in biomedical implants. BMGs exhibit an excellent combination of properties and processing capabilities desired for versatile implant applications, from stents to bone replacement. Unlike most metals, BMGs have an amorphous structure that yields many advantages: high strength (three times that of steel), elasticity, corrosion resistance, and durability. Most notable, they can be molded like plastics with nanoscale precision and complex geometries.
The Yale Engineering collaborators now are putting the outstanding properties of bulk metallic glasses to the test. So far, they have shown that BMGs can, indeed, be implanted in the human body. And in vitro and in vivo study results indicate that the BMGs are compatible with cell growth and tissue function.
In the news:
New Material for Biomedical Applications Offers Superior Properties
Yale Engineers Revolutionize Nano-device Fabrication Using Amorphous Metals
Developing New Materials at Warp Speed
Jan Schroers, Dept. of Mech. Eng. & Mats. Sci.
In recent decades, high-throughput screening of chemical compounds revolutionized the pharmaceutical industry. Now, Yale engineers are set to revolutionize the understanding and development of materials with a technique they’ve dubbed “high-throughput characterization.”
Mechanical engineering & materials science professor Jan Schroers and his colleagues are known for their work on a class of materials called bulk metallic glasses. In the past, Schroers' five-person team spent a year and $500,000 to develop a single new bulk metallic glass. The process entailed weighing and combining various amounts of metals such as gold, silver, palladium, copper, and silicon, putting the combination into a vacuum, and characterizing the result. Working fast, the team produced one new alloy per day, and produced one combination a day for a year to come up with the one that could be blow-molded as a bulk metallic glass.
Now, with their massively parallel approach that sprays four different metals in various ratios onto a substrate, they can process 750 new combinations in a day. They are systematically creating large composition libraries of materials and characterizing them to find properties of interest. The vast stores of information will enable the engineers to produce stronger experimental data points to compare their theories against, and scale up materials development to a pace not seen before.
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.”
Addressing Key Challenges in Biomaterials
Paul Van Tassel, Dept. of ChE & EnvE
Biomaterials are ubiquitous in modern medicine, from prosthetic limbs to device coatings, and hold promise for future applications like scaffolding for tissue regeneration. Biomaterial design presents a key challenge, however: applications often require materials to be both mechanically rigid (to promote strong cell adhesion) and bioactive (i.e. able to communicate specific cues to contacting cells). To date, achieving one of these features usually required sacrificing the other.
Van Tassel and his group have developed an idea that appears to address this challenge: a thin polymer film biomaterial offering both high rigidity and bioactivity. The approach involves forming the film in the presence of sacrificial nanoparticle templates. The film is first rigidified through a chemical cross-linking procedure; then, the template species are selectively dissolved, leaving behind a pore structure. These pores can then be filled with proteins able to communicate with cells, rendering the film bioactive.
The new approach – part of an ongoing effort toward polymeric thin film biomaterials – achieves mechanical rigidity in conjunction with bioactivity, and, notably, allows for embedding high quantities of biological species without regard to size (previous methods were limited to lower quantities of smaller drug or protein species).
Publications: Advanced Functional Materials