Using Small Particles to Reap Colossal Benefits

June 2, 2014

Because energy conservation is such a prevalent concern today, it is highly desirable to develop functional materials for energy efficiency applications. Researchers at UCSB have created a unique compound semiconductor embedded with metallic nanostructures in the hopes of tackling a range of energy efficiency problems. The artist’s view of this semiconductor structure, as grown by CEEM researcher Dr. Hong Lu using MBE, is shown in Figure 1.

    Figure 1. Artist's concept of nanometer-size metallic wires and metallic
particles embedded in semiconductors. Credit: Peter Allen, UCSB

This semiconductor design has been employed in a few different energy-related applications, such as multi-junction solar cells and high efficient thermoelectrics. The highest efficiency of photovoltaics achieved today is to use a multi-junction structure created by stacking semiconductor layers, each with its own absorption wavelength. An important problem, however, is how to achieve good electrical connections between the layers. Embedding metallic nanostructures at the junction can greatly enhance the tunneling current, resulting in an increased efficiency in a multi-junction solar cell.

Thermoelectrics aims to convert thermal energy to electrical energy. Since more than 50% of the energy produced in the U.S. is wasted as heat, thermoelectric technology can make a significant contribution to renewable energy sources. An ongoing research effort at CEEM is to utilize these nanostructures to contribute desired electrical carriers but suppress unwanted heat conduction, thus improving the material’s thermoelectric figure of merit.

In a new work, CEEM researchers Hong Lu, Peter Burke and Professor Arthur Gossard collaborated with Physics department researchers Daniel Ouellette, Justin Watts, Benjamin Zaks, Professor Mark Sherwin and Dr. Sascha Preu (at the time a Humboldt fellow and now a professor at University of Darmstadt), and discovered new optical properties of this semiconductor structure. As detailed in their recently published work in Nano Letters, the structure consists of a gallium antimonide (GaSb) matrix embedded with a variety of erbium antimonide (ErSb) nanostructures. These nanostructures exhibit the surface plasmon resonance (SPR) phenomenon, which is the oscillation of electrons at a metal surface excited by light. When light hits the surface of a metal, electrons begin to resonate; that is, they move back and forth from their equilibrium positions and oscillate at the same frequency as the light.

This property will help to preserve optical information at the nano-level, allowing the utilization of the speed and data capacity of photons and the compactness of electronics for information processing. By controlling the size, shape and orientation of the nanostructures, they are able to use the semiconductor to manipulate light in the invisible infrared/terahertz range. The highly conductive nanostructures can polarize electromagnetic radiation in a broad range, helping to filter and to define images with infrared and terahertz light signatures. The researchers have filed a patent application for a polarizer that can be integrated into infrared and terahertz devices.

The researchers believe that this is a new way to design semiconductor materials and structures, and they are looking for ways to apply the plasmonic effect in energy devices.

Editor’s Note: This is an edited version with assistance from Pavithra Rajesh, of an article originally written by Sonia Fernandez, Public Affairs, UCSB.

To read the full research paper, click here.

Author and publication details:
Hong Lu, Daniel G. Ouellette, Sascha Preu, Justin D. Watts, Benjamin Zaks, Peter G. Burke, Mark S. Sherwin, and Arthur C. Gossard.Self-Assembled ErSb Nanostructures with Optical Applications in Infrared and Terahertz.” Nano Lett., 2014, 14 (3), pp 1107–1112.

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