Nanostructured metal coatings let the light through for electrical devices

An array of nanopillars etched by thin layer of grate-patterned metal creates a nonreflective yet conductive surface that could improve electronic device performance. Image courtesy of Daniel Wasserman

Light and electricity dance a complicated tango in devices like LEDs, solar cells and sensors. A new anti-reflection coating developed by engineers at the University of Illinois at Urbana Champaign, in collaboration with researchers at the University of Massachusetts at Lowell, lets light through without hampering the flow of electricity, a step that could increase efficiency in such devices.

The coating is a specially engraved, nanostructured thin film that allows more light through than a flat surface, yet also provides electrical access to the underlying material – a crucial combination for optoelectronics, devices that convert electricity to light or vice versa. The researchers, led by U. of I. electrical and computer engineering professor Daniel Wasserman, published their findings in the journal Advanced Materials.

“The ability to improve both electrical and optical access to a material is an important step towards higher-efficiency optoelectronic devices,” said Wasserman, a member of the Micro and Nano Technology Laboratory at Illinois.

At the interface between two materials, such as a semiconductor and air, some light is always reflected, Wasserman said. This limits the efficiency of optoelectronic devices. If light is emitted in a semiconductor, some fraction of this light will never escape the semiconductor material.

Alternatively, for a sensor or solar cell, some fraction of light will never make it to the detector to be collected and turned into an electrical signal. Researchers use a model called Fresnel’s equations to describe the reflection and transmission at the interface between two materials.

“It has been long known that structuring the surface of a material can increase light transmission,” said study co-author Viktor Podolskiy, a professor at the University of Massachusetts at Lowell.

“Among such structures, one of the more interesting is similar to structures found in nature, and is referred to as a ‘moth-eye’ pattern: tiny nanopillars which can ‘beat’ the Fresnel equations at certain wavelengths and angles.”

Although such patterned surfaces aid in light transmission, they hinder electrical transmission, creating a barrier to the underlying electrical material.

“In most cases, the addition of a conducting material to the surface results in absorption and reflection, both of which will degrade device performance,” Wasserman said.

The Illinois and Massachusetts team used a patented method of metal-assisted chemical etching, MacEtch, developed at Illinois by Xiuling Li, U. of I. professor of electrical and computer engineering and co-author of the new paper. The researchers used MacEtch to engrave a patterned metal film into a semiconductor to create an array of tiny nanopillars rising above the metal film. The combination of these “moth-eye” nanopillars and the metal film created a partially coated material that outperformed the untreated semiconductor.    

“The nanopillars enhance the optical transmission while the metal film offers electrical contact. Remarkably, we can improve our optical transmission and electrical access simultaneously,” said Runyu Liu, a graduate researcher at Illinois and a co-lead author of the work along with Illinois graduate researcher Xiang Zhao and Massachusetts graduate researcher Christopher Roberts.

The researchers demonstrated that their technique, which results in metal covering roughly half of the surface, can transmit about 90 percent of light to or from the surface. For comparison, the bare, unpatterned surface with no metal can only transmit 70 percent of the light and has no electrical contact.

The researchers also demonstrated their ability to tune the material’s optical properties by adjusting the metal film’s dimensions and how deeply it etches into the semiconductor.

“We are looking to integrate these nanostructured films with optoelectronic devices to demonstrate that we can simultaneously improve both the optical and electronic properties of devices operating at wavelengths from the visible all the way to the far infrared,” Wasserman said.

The National Science Foundation and Lam Research supported this work.

University of Illinois at Urbana Champaign

Cooking Up New Nanoribbons to Make Better White LEDs

As the world moves away from incandescent light bulbs, light-emitting diodes (LEDs) are growing in popularity. 

They use significantly less energy and have far longer lifetimes than do the traditional incandescent bulbs, which are being phased out, and they don't contain mercury, as do compact fluorescents. LEDs do have a drawback, however. The phosphors that convert the single color produced by an LED into white light tend to produce a cool, bluish glow instead of the warmer, yellower color most people prefer.

Now scientists from Argonne National Laboratory, Oak Ridge National Laboratory, and the University of Georgia are developing new compounds to create nanoribbons that luminesce in different colors, which they can mix together to make a phosphor that provides a more desirable white light that can be tailored for different uses. Using high-brightness x-rays at several different beamlines at the U.S. Department of Energy Office of Science's Advanced Photon Source (APS), they are figuring out how atoms in the materials are arranged. A better understanding of these crystal structures will help them fine-tune their nanoribbons and make more appealing white phosphors based on LEDs.

The materials combine the rare-earth element europium with aluminum oxide to form europium aluminate nanoribbons. Powders of europium oxide (Eu₂O₃) and aluminum oxide (Al₂O₃) were mixed together with graphite powder and heated in a vacuum chamber to temperatures above 1000° C. Powders heated to between 1200° and 1400° C formed nanoribbons that luminesced orange. Those heated to 1000° to 1200° glowed green. When more aluminum oxide powder was added and the pressure in the vacuum chamber was raised, from 5 Torr to between 10 and 15 Torr, the resulting nanoribbons shone blue.

It is rare, the researchers say, for a single material to be able to cover the spectrum of visible colors, but it could simplify the creation of phosphors that produce desirable colors when excited by a blue or ultraviolet LED. Scientists would simply have to select the right mix of nanoribbons to get the white light they sought. In a separate experiment, the researchers also produced yellow and red luminescent nanoribbons by adding barium to the europium aluminate.

The researchers first used conventional x-ray powder diffraction to determine the crystal structures of the three types of nanoribbon. They compared the patterns they found to several diffraction databases of known materials and discovered the green nanoribbon was a match for strontium aluminate, so they knew it had a similar crystalline shape.

There was no match for the other two types, though, so they turned to the X-ray Science Division (XSD) beamline 11-BM-B at the Argonne Advanced Photon Source to perform high-resolution synchrotron powder diffraction using a single x-ray energy, which provides initial measurements of the spacings between the atomic planes of the crystal. Using a different scattering technique with a wide range of x-ray energies, XSD beamline 34-1D-E, also at the APS, gave them more detailed information, such as the exact angles between different atomic planes and the presence of crystal defects.

Using XSD beamline 20-BM-B at the APS, they performed x-ray absorption near-edge structure measurements, which focused on europium fluorescence and confirmed that the nanoribbons are indeed europium aluminates, and provided their chemical formulas — EuAl₆O₁₀ for the blue and EuAl₂O₄ for the green and orange. The blue nanoribbons, therefore, are a newly discovered compound. While green photoluminescence at room temperature had been seen before from SrAl2O4, the orange color was new as well.

Specifically pinning down the complete atomic structure of the crystal and tying it to the observed behavior of the material is a complex undertaking. The team now has an average picture of the local spatial arrangement of the elements and the oxygen vacancies — areas where a missing oxygen atom affects the electrical behavior of the material. They are now trying to refine that picture using the ChemMatCARS 15-ID-B advanced crystallography beamline at the APS to give theorists enough information to explain cause and effect and suggest possible ways to tweak the materials' luminescence properties.

The researchers also found a result they had not been looking for. They discovered that when they hit an individual nanoribbon with a microfocused x-ray beam, it not only produced the x-ray diffraction patterns they were using for their measurements, but also generated strong visible light emission. That light appeared not only at the spot where the beam struck, but also at the ends of the ribbons, showing the ribbons were acting as waveguides. That ability to route light of different colors means the nanoribbons may help in the creation of circuits inside optical devices, which use light beams to perform their functions.

Source: http://www.aps.anl.gov/Science/Highlights/Content/APS_SCIENCE_20140224.php