New alloys key to efficient energy and lighting


March 22, 2010

Nanowire advances promise improved light-emitting diodes and solar-energy generation

A recent advance by ASU researchers in developing nanowires could lead to more efficient photovoltaic cells for generating energy from sunlight, and to better light-emitting diodes (LEDs) that could replace less energy-efficient incandescent light bulbs. Download Full Image

Electrical engineers Cun-Zheng Ning and Alian Pan are working to improve quaternary alloy semiconductor nanowire materials.

Nanowires are tens of nanometers in diameter and tens of microns in length. Quaternary alloys are made of semiconductors with four elements, often made by alloying two or more compound semiconductors.

Semiconductors are the material basis for technologies such as solar cells, high-efficiency LEDs for lighting, and for visible and infrared detectors.

One of the most critical parameters of semiconductors that determine the feasibility for these technologies is the band gap. The band gap of a semiconductor determines, for example, if a given wavelength of sun light is absorbed or left unchanged by the semiconductor in a solar cell.

Band gap also determines what color of light an LED emits. To make solar cells more efficient, it’s necessary to increase the range of band gaps.

Ideally, the highest solar cell efficiency is achieved by having a wide range of band gaps that matches the entire solar spectrum, said Ning, a professor in the School of Electrical, Computer and Energy Engineering, a part of ASU’s Ira A. Fulton Schools of Engineering.

He said that in LED lighting applications, more available band gaps means more colors can be emitted, providing more flexibility in color engineering or color rendering of light.

For example, different proportions of red, green and blue colors would mix with different white colors. More flexibility would allow white color to be adjusted to suit various situations, or individual preferences.

Similarly, Ning said, detection of different colors requires semiconductors of different band gaps. The more band gaps that are available, the more information can be acquired about an object to be detected. Thus, all of these lighting applications can be improved by having semiconductors with a wide range of band gaps.

The researchers said the hurdle is that every manmade or naturally occurring semiconductor has only a specific band gap.

One standard way to broaden the range of band gaps is to alloy two or more semiconductors. By adjusting the relative proportion of two semiconductors in an alloy, it’s possible to develop new band gaps between those of the two semiconductors.

But accomplishing this requires a condition called lattice constant matching, which requires similar inter-atomic spaces between two semiconductors to be grown together.

“This is why we cannot grow alloys of arbitrary compositions to achieve arbitrary band gaps,” Ning said. “This lack of available band gaps is one of the reasons current solar cell efficiency is low, and why we do not have LED lighting colors that can be adjusted for various situations.”

In recent attempts to grow semiconductor nanowires with “almost” arbitrary band gaps, the research team led by Ning and Pan, an assistant research professor, have used a new approach to produce an extremely wide range of band gaps.

They alloyed two semiconductors, zinc sulfide (ZnS) and cadmium selenide (CdSe) to produce the quaternary semiconductor alloy ZnCdSSe, which produced continuously varying compositions of elements on a single substrate (a material on which a circuit is formed or fabricated).

Ning said this is the first time a quaternary semiconductor has been produced in the form of a nanowire or nanoparticle.

By controlling the spatial variation of various elements and the temperature of a substrate (called the dual-gradient method), the team produced light emissions that ranged from 350 to 720 nanometers on a single substrate only a few centimeters in size.

The color spread across the substrate can be controlled to a large degree, and Ning said he believes this dual-gradient method can be more generally applied to produce other alloy semiconductors or expand the band gap range of these alloys.

To explore the use of quaternary alloy materials for making photovoltaic cells more efficient, his team has developed a lateral multi-cell design combined with a dispersive concentrator.

The concept of dispersive concentration, or spectral split concentration, has been explored for decades. But the typical application uses a separate solar cell for each wavelength band.

With the new materials, Ning said he hopes to build a monolithic lateral super-cell that contains multiple subcells in parallel, each optimized for a given wavelength band. The multiple subcells can absorb the entire solar spectrum. Such solar cells will be able to achieve extremely high efficiency with low fabrication cost. The team is working on both the design and fabrication of such solar cells.

Similarly, the new quaternary alloy nanowires with large wavelength span can be explored for color-engineered light applications.

The researchers have demonstrated that color control through alloy composition control can be extended to two spatial dimensions, a step closer to color design for direct white light generation or for color displays.

The team’s research was initially supported by Science Foundation Arizona and by the U.S. Army Research Office.

For more information, see the research group’s Web site at http://nanophotonics.asu.edu.

Related">http://nanophotonics.asu.edu">http://nanophotonics.asu.edu.

Re... research by Ning and his colleagues has been reported in these articles:

• Pan, R. Liu, M. Sun and C.Z. Ning, Spatial Composition Grading of Quaternary ZnCdSSe Alloy Nanowires with Tunable Light Emission between 350 and 710 nm on a Single Substrate, ACS Nano, http://pubs.acs.org/doi/abs/10.1021/nn901699h

">http://pubs.acs.org/doi/abs/10.1021/nn901699h">http://pubs.acs.org/doi/a...• Pan, R. Liu, M. Sun and C.Z. Ning, Quaternary Alloy Semiconductor Nanobelts with Bandgap Spanning the Entire Visible Spectrum, J. Am. Chem. Soc, 131, 9502 (2009), DOI: 10.1021/ja904137m, http://pubs.acs.org/doi/abs/10.1021/ja904137m

">http://pubs.acs.org/doi/abs/10.1021/ja904137m">http://pubs.acs.org/doi/a...• C.Z. Ning, A. Pan, and R. Liu, Spatially composition-graded alloy semiconductor nanowires and wavelength specific lateral multi-junctions full-spectrum solar cells, Proceedings of 34th PVSC, IEEE, 001492(2009).

Joe Kullman

Science writer, Ira A. Fulton Schools of Engineering

480-965-8122

Students create new futures with Digital Culture Initiative


March 22, 2010

Imagine an undergraduate degree program focused on exploring, creating and adapting the leading edge of digital tools and environments within a highly flexible, multidisciplinary curriculum that allows students choices based on their interests and goals.

Now, place this program within a fully-mediated facility that constantly adapts to the needs of the projects investigated by emerging networks of collaborating students and faculty. Download Full Image

This potential has become reality at the http://herbergerinstitute.asu.edu" target="_blank">ASU Herberger Institute for Design and the Arts with the newly announced http://herbergerinstitute.asu.edu/degrees/digital_culture/" target="_blank">Digital Culture Initiative, a new undergraduate degree emphasis focusing on the investigation and development of digital-based tools and environments that play an ever-increasing role throughout contemporary society. The Digital Culture Initiative enrolls its first class in the fall of 2010 and launches a curriculum that allows students to design a unique educational path to graduation. The institute plans to propose a minor and certificate to complement the degree emphasis.

“Our evolving digital culture allows people to connect in ways that transcend traditional concepts of space and communication,” said Kwang-Wu Kim, dean and director of the Herberger Institute. “The Digital Culture Initiative positions students and faculty to imagine and invent solutions to the new challenges created by our new hybrid reality we face today.”

Undergraduate students are able to pursue the http://herbergerinstitute.asu.edu/degrees/ba_arts/" target="_blank">Bachelor of Arts in the Arts or http://herbergerinstitute.asu.edu/degrees/ba_design/" target="_blank">Bachelor of Arts in Design Studies degrees with an emphasis in Digital Culture. Through a partnership with seven ASU schools and colleges, the curriculum is made up of courses that prepares students with tangible skills in new media as well as skills to be thought leaders in our digital world.

“Digital Culture provides students with a stellar, contemporary liberal arts education that gives them a set of skills that are highly desirable in the workplace over the next 40 years,” said Thanassis Rikakis, director of the Herberger Institute School of Arts, Media and Engineering. “Students explore and create processes and artifacts that shape their daily experience. Interactive creative experiences, portable media, peer networks and content sharing tools, interactive creative experiences and portable media are the core content of the study rather than simply tools for study of traditional content.”

The Digital Culture Initiative offers specially designed courses from across the Herberger Institute’s seven schools as well as the Ira A. Fulton Schools of Engineering, the College of Liberal Arts and Sciences, the W. P. Carey School of Business, the Mary Lou Fulton Institute and Graduate School of Education, the College of Teacher Education and Leadership, and the Walter Cronkite School of Journalism and Mass Communication. Using a customizable http://digitalculture.asu.edu/courses" target="_blank">interactive course and path planner, students are able to map out their path to graduation based on their subject interests and long-term aspirations.

If a student with an interest or background in art and film would like to investigate interactive animation, they can design a multi-year educational path to allow them to graduate with a wide set of core proficiencies that employers like Pixar Animation Studios or Electronic Arts find appealing. If that student finds midway that their interests have changed based on a dance, engineering or education class, they are able to adjust their path to accommodate their new goals.

A state-of-the-art facility for new media work is being developed and is scheduled to open in January 2011. The facility brings under one roof all necessary infrastructure for the development of hybrid, physical-digital cultural systems and experiences in a sustainable educational environment.

For more information on the Digital Culture Initiative, please visit http://herbergerinstitute.asu.edu/degrees/digital_culture/ " target="_blank">herbergerinstitute.asu.edu/degrees/digital_culture/.

Jason Franz

Assistant Director, Strategic Marketing and Communications, Julie Ann Wrigley Global Futures Laboratory

480-727-4072