IRP Focus: Surfaces & Nanomaterials: Research Projects

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.

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.

Publications:

“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)

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.

Publications:

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

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.

Publications:

Applied Physics Letters

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.

Publications:

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

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.

Publications:

Science, Advanced Materials

In the news:

The Center for Research on Interface Structures and Phenomena

At the Oxide Interface: Where Experimentalists Play

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.

Publications:
JOM, Nature

In the news:
New Material for Biomedical Applications Offers Superior Properties
Yale Engineers Revolutionize Nano-device Fabrication Using Amorphous Metals

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.

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.

Publications:

Nanotechnology, December 20, 2010, “The fabrication of large-area, free-standing GaN by a novel nanoetching process.”

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