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January 2012

Sculpting the Flow of Light and Matter

Researchers create 3D structured materials for new device applications

Bryce Sadtler

(A) Scanning electron microscopy image of a 3D photonic crystal template partially filled with GaAs by selective area growth. (B) Schematic of a GaAs 3D photonic crystal (blue) with a layer of indium gallium arsenide (red) to create a light-emitting diode.

Collaboration between two Energy Frontier Research Centers has produced the first 3D photonic crystal, a material designed to control the propagation of light, incorporated into an electronically addressable device. The research was a joint effort between the Light-Material Interactions in Energy Conversion EFRC and the Center for Energy Nanoscience.

“This work was conceived to address a long-standing problem in 3D photonic crystals,” says Paul Braun of the Light-Material Interactions in Energy Conversion Center and professor at the University of Illinois at Urbana-Champaign. “Our demonstration of optoelectronically active 3D photonic crystals provides motivation for thinking seriously of designing structures for specific applications.”

Envisioned applications include cloaking devices, high-efficiency lasers, light-emitting diodes (LEDs) and solar cells.

The arrangement of atoms in a solid determines its electronic properties. For example, semiconductors are materials in between insulators and conductors whose electrical conductivity can be precisely tuned by adding small amounts of impurity atoms to the atomic crystal. The electronic transistor, a device that controls the flow of electrical current through a semiconductor, has become a ubiquitous element in modern electronics, including computers, televisions and cell phones.

Now zoom out from the atomic level to the microscale, where 1 micrometer is about 10,000 times larger than an atom, but still 100 times smaller than the width of a human hair. Photonic crystals possessing a periodic, or regularly repeating, structure on the microscale can be used to manipulate the flow of photons, the quantized units of light. Naturally occurring photonic crystals include butterfly wings, peacock feathers and oyster shells, whose periodic microstructure is responsible for their iridescent sheen. The design of synthetic photonic materials to guide the propagation of light through high-quality semiconductors is a crucial step towards producing the novel optoelectronic devices mentioned above.

The difficulty in coupling photonic and electronic materials is that it requires near perfect ordering of the material on two very different length scales, both the atomic and microscale. The EFRC researchers overcame this obstacle by performing selective area growth of the semiconductor material, gallium arsenide (GaAs), through a 3D structured template. Gaseous precursors, that are normally deposited on a planar surface to form a film of GaAs, instead flowed through a porous template consisting of a well-ordered array of silica microspheres (imagine a box of ping-pong balls). The precursor gases flowed through the void spaces between the microspheres and only deposited on the bottom surface, building up the GaAs crystal in between the spheres. See Figure A. This produced a 3D structure with crystalline atomic order and a periodic array of micropores after removal of the template.

To fabricate an LED device from the 3D photonic crystal, the researchers alternated the vapor precursors to build up layers of GaAs and indium gallium arsenide. See Figure B. When electrical current is passed through the device, light is emitted from the structure, which is the first example of an electrically pumped 3D photonic crystal device.

While electronic-photonic coupling in the LED is preliminary, it offers a look into a possible future. "We can now begin to think about the design of structures which optimize both the optical and electronic properties of the resultant material to enable a new class of devices for solid-state lighting and photovoltaics,” says Erik Nelson, a member of the EFRC team.

More Information

Nelson EC, NL Dias, KP Bassett, SN Dunham, V Verma, M Miyake, P Wiltzius, JA Rogers, JJ Coleman, X Li and PV Braun. 2011. "Epitaxial Growth of Three-Dimensionally Architectured Optoelectronic Devices." Nature Materials 10(9):676-681. DOI: 10.10.1038/nmat3071.

Acknowledgments

Fabrication and testing of optoelectronic devices was supported by the Light-Material Interactions in Energy Conversion Center, and design of the optoelectronic devices was supported by the Center for Energy Nanoscience. These two centers are Energy Frontier Research Centers funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. The 3D epitaxy growth process development was supported by the U.S. Army Research Office and optimization of the reactor was supported by the National Science Foundation.

About the author(s):

  • Bryce Sadtler is a Beckman Postdoctoral Scholar at the California Institute of Technology and a member of the Light-Material Interactions in Energy Conversion, an Energy Frontier Research Center. His research interests are in light-driven processes for directing the morphology of inorganic structures and the design of nanoscale materials for energy conversion and storage.

Uniting Light and Electricity

Scanning electron microscopy image of the porous template, filling up from the bottom.

New designs for highly efficient solar cells and other devices that sculpt the flow of light and electricity demand unusual materials. To move light, the material must have nearly perfect ordering of its structure on the micrometer scale. A single micrometer is about 100 times smaller than the width of a human hair. To move electricity, the material must have near perfect ordering at the atomic scale. The challenge is achieving the necessary ordering on both length scales. The scientists overcame this obstacle by using a 3D porous microstructure composed of well-ordered microspheres as a growth template. The basic chemical ingredients for creating the desired material flowed through the voids between the spheres, depositing on the bottom of the template. The material was built in between the spheres. This process resulted in a structure with the needed ordering after removing the template. The researchers demonstrated the utility of this strategy by fabricating a 3D structured light-emitting diode device. Creating these organized materials could lead to new technologies, ranging from more efficient solar cells to cloaking devices. This work was done by the Light-Material Interactions in Energy Conversion Center, led by the California Institute of Technology, and the Center for Energy Nanoscience, led by University of Southern California.

Written by Bryce Sadtler and Kristin Manke

More Information

Nelson EC, NL Dias, KP Bassett, SN Dunham, V Verma, M Miyake, P Wiltzius, JA Rogers, JJ Coleman, X Li and PV Braun. 2011. "Epitaxial Growth of Three-Dimensionally Architectured Optoelectronic Devices." Nature Materials 10(9):676-681. DOI: 10.10.1038/nmat3071.

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.