A few months back, there was a buzz about a new, nano-structured anti-reflective material that came out of a development effort at the well-known Rensselaer Polytechnic Institute (RPI). The new material absorbs light so effectively that it was tagged "The New Black" in some articles. In the RPI news report, the material is touted as having a refractive index of 1.05, just barely higher than the index of refraction of vacuum. (Vacuum, as the reference point for the index of refraction, has an index of 1. An index of refraction is the ratio of the speed of light through a given material, to the speed of light through vacuum.) This is a far lower index of refraction than has been previously achieved. The material achieves it by virtue of a surface made up of transparent nanorods, applied at an angle that is oblique to the substrate.
Samsung Electro Mechanical Company (SEMCo) saw the possibilities for these materials and filed patents jointly with RPI in 2005 and 2006 in Korea, the United States, and the European Patent Office for an LED with an omni-directional reflector that uses the new material.
US 20070029561-A1, "Omni-Directional Reflector And Light Emitting Diode Adopting The Same" (Other family members: CN1909256 A, JP2007043045 A, EP1750310 A2)
ABSTRACT:"An omni-directional reflector having a transparent conductive low-index layer formed of conductive nanorods and a light emitting diode utilizing the omni-directional reflector are provided. The omni-directional reflector includes: a transparent conductive low-index layer formed of conductive nanorods and a reflective layer formed of a metal."
The nanorods, you will note, are made of materials that are not merely transparent, but also conductive. In standard LED construction, the reflector normally includes an anti-reflective material, a reflective material, and a conductive layer, energized through electrodes or an array of micro contacts. In the SEMCo/RPI patent, the conductive nanorods provide a two-fold improvement: One, they make the micro contacts unnecessary by providing conductivity directly, combining the functions of the anti-reflective layer and the conductive layer into a single, continuous conductive/anti-reflective layer; and two, they improve on both functions by providing better conductivity and allowing more light to reach the reflective layer. The resulting LED is simpler to make, and far more responsive to an applied voltage.
The SEMCo/RPI patent discusses three different embodiments, each with a different transparent, conductive material. One was made with ITO, one with CIO (CuInO), and one with SiO2 nanorods, on a silicon substrate. All the embodiments are apparently highly efficient and very bright. The embodiment with the most dramatically low index of refraction for the nanorod layer is the embodiment with silicon dioxide nanorods, which the patent states is measured by ellipsometry to be 1.090.
The RPI team was not the first to explore the possibilities of transparent, conductive nanorods for anti-reflection. In March 2005, GE filed US20060207647-A1, "High Efficiency Inorganic Nanorod-Enhanced Photovoltaic Devices", which was published in September 2006. The patent was also filed as CN1855552 A, JP2006261666 A, and EP1703569 A2. This patent discloses the use of silicon nanorods. However, these nanorods are oriented perpendicularly to the substrate, and there is no mention of special anti-reflective properties.
Seoul Opto filed WO2007001099-A1, "Light Emitting Diode Of A Nanorod Array Structure Having A Nitride-Based Multi Quantum Well" in June 2005, and it was published in January 2007. It, too, shows nanorods oriented perpendicularly to the substrate, and does not mention any special anti-reflective properties.
And Dongguk University filed US20070077670-A1, "Super Bright Light Emitting Diode Of Nanorod Array Structure Having InGaN Quantum Well And Method For Manufacturing The Same," which was filed in September 2006, and published only eight months later in April 2007. Again, the nanorods are perpendicular to the substrate, with no mention of anti-reflection.
Both the Dongguk and Seoul Opto applications involve nitride semiconductor systems. In both patents, the active region of the LED is actually comprised of the nanorod or nanowire structures. Both claim high brightness and high efficiency.
Throughout the optical patents, nanostructures are allowing greater manipulation of optical properties, and changing the face of optical innovation.