OCTOBER 14 [Evanston, IL] Nathaniel Stern, assistant professor of physics and astronomy in the Weinberg College of Arts and Sciences at Northwestern University, received the 2014 Northwestern-Argonne Early Career Investigator Award for Energy Research for his proposal to investigate the use of monolayer semiconductor quantum dots to improve solar efficiency. The research could produce a solar cell that improves efficiency, while minimizing size and material inputs to reduce costs. Stern’s funding will total $100,000 over three years.
Presented jointly by Argonne National Laboratory and the Institute for Sustainability and Energy at Northwestern, the award honors a scientist working collaboratively between Argonne and Northwestern on research relating to energy production or use.
“I’m honored to receive this award,” said Stern. “As a physicist, I’m trying to develop new environments, new systems, that change how we look at and manipulate energy,” said Stern.
Il Woong Jung, an assistant scientist in the Center for Nanoscale Materials (CNM) at Argonne, will be Stern’s counterpart at the research lab.
“I’m very excited to be part of this collaboration. There has been tremendous effort on improving efficiency in solar cells, but without much improvement in simplicity or cost. This proposal can lead to important breakthroughs in improving efficiency, while not losing focus on keeping it economical,” said Jung.
Stern’s research is inspired by the capability of materials to exhibit different physical properties at different length scales.
A well-known example of this phenomenon is exhibited by graphene, a two-dimensional sheet of carbon atoms that is highly durable and an efficient conductor of electricity (whereas bulk structure carbon itself – for example, the brittle graphite used in pencils, is not). Although graphene is a great conductor of energy, it is not capable of absorbing it efficiently.
Stern proposes working with a material that is conducive to absorbing energy – isolated sheets of the three-dimensional crystalline molybdenum disulfide, or MoS2.
Reducing three-dimensional bulk crystals into two-dimensional nanoscale monolayers alters their optical properties and improves their ability to absorb light, which is especially important for solar technologies. Stern speaks to the advantage of a material of this scale that has such high absorption properties.
“All of a sudden we have materials absorbing light that are barely there,” he said, a sign of the material’s versatility and applicability in a variety of settings. For example, less than 1nm thick of MoS2 absorbs the same amount of incident sunlight as 50nm of silicon, an industry standard for solar cell materials.
Adapting established mechanisms for manipulating bulk crystals, Stern will reduce the dimensionality of MoS2even further by patterning the layers into quantum dots, or very small nanocrystals which can then be packed into a single, versatile, two-dimensional monolayer with controllable optical properties.
“The layers are very thin, yet remarkably strong, so you can imagine covering anything with them,” said Stern. This thinness translates to less material, which could mean reduced costs for building solar cells, potentially resulting in negligible raw material cost for large-scale solar projects.
Uniquely, Stern proposes placing the monolayer quantum dots directly on a solar cell’s electrode, the region that converts optically excited electrons into usable electricity. Placing the single layer, semiconductors directly on the electrode further reduces transport loss, greatly increasing efficiency of electricity production in the solar cell. By using highly-conductive sheets of graphene for the electrical contacts, the thinness of the monolayer semiconductor quantum dots and proximity to the graphene electrode in Stern’s design reduce the size of the solar cells to the fundamental atomic limits.
Stern’s approach – combining atomic layers of diverse composition – advances an emerging trend in nanoscience to assemble hybrid devices integrating different materials with distinct, highly-tuned physical properties into a unified device ideally suited for its application.
“The discovery and mastery over physical interactions in new materials and environments, such as reduced dimensionality, expands the available toolbox for technological innovation,” said Stern.
The collaboration with Il Woong Jung, an expert in nanofabrication technology, and Argonne National Laboratory, is an integral part of this research.
“To carry out this research, we need the state of the art equipment and expertise with processing that Jung and Argonne provide,” says Stern.
The user facility at the Center for Nanoscale Materials houses a nanofabrication cleanroom with electron beam lithography, focused ion beam and other nanopatterning, deposition, etching and metrology tools for the fabrication of the proposed devices.
Continues Stern, “this unique combination of resources at CNM coupled with their openness for diverse collaborations with academic researchers are the special ingredients that make Argonne such a valuable partner for advancing science and engineering.”
Before coming to Northwestern, Stern was the Richard C. Tolman prize Postdoctoral Fellow at the California Institute of Technology. He earned his Ph.D. in physics in 2008 at the University of California, Santa Barbara, where as a Hertz Fellow, he studied spintronics. Stern has also recently been named a Department of Energy (DOE) Early Career Researcher for his research probing coherent states of light and matter in two-dimensional semiconductors. He is currently an Alfred P. Sloan Research Fellow at Northwestern.
Il Woong Jung received his PhD in electrical engineering from Stanford University in 2007. His research has been on photonic microsystems for applications in telecommunications, adaptive optics, imaging, microscopy, and sensing. His current research focuses on the manipulation of light-matter interactions at the nanoscale, specifically using nano- and micro- electromechanical systems (NEMS/MEMS) to control the optical properties of photonic nanostructures.
Over the next three years, Stern and Jung will work to create the two-dimensional monolayers, take experimental measurements, and create the highly efficient hybrid devices of MoS2 and graphene.
Funding for the award is provided jointly by Argonne and the Institute for Sustainability and Energy at Northwestern University (ISEN).
Past winners include Fengqi You, Assistant Professor of Chemical and Biological Engineering at Northwestern in 2013, Lynn Trahey, Materials Scientist at Argonne in 2012, Emily Weiss, the Clare Booth Assistant Professor of Chemistry at Northwestern in 2011, and Adilson Motter, Professor of Physics and Astronomy in the Weinberg College of Liberal Arts and Sciences at Northwestern in 2010.