Nanostructured Metal Coatings Let the Light Through

An array of nanopillars etched by thin layer of grate-patterned metal creates a nonreflective surface that could improve electronic device performance. (Source: D. Wassermann)

An array of nanopillars etched by thin layer of grate-patterned metal creates a nonreflective surface that could improve electronic device performance. (Source: D. Wassermann)

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 UML, 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 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. At the interface between two materials, such as a semi­conductor and air, some light is always reflected. 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 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  Viktor Podolskiy, at the UML. “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 team used a patented method of metal-assisted chemical etching, MacEtch. 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.

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. (Source: U. of I.)

Reference: R. Liu et al.: Enhanced Optical Transmission Through MacEtch-Fabricated Buried Metal Gratings, Adv. Mater., online 8 December 2015; DOI: 10.1002/adma.201505111

Links: Mid-IR Photonics Group (D.Wassermann), Electrical and Computer Engineering, University of Illinois at Urbana Champaign U. of I.Multiscale Electromagnetics Group (V. A. Podolskiy), Dept. of Physics and Applied Physics, University of Massachusetts Lowell UML

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