Functionalized Nanoparticle Thin Films

Technology #13155

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Inventors
Professor Robert Cohen
Department of Chemical Engineering, MIT
External Link (cohengroup.mit.edu)
Professor Michael Rubner
Department of Material Science and Engineering, MIT
External Link (web.mit.edu)
Zekeriyya Gemici
Department of Chemical Engineering, MIT
Managed By
Jim Freedman
MIT Technology Licensing Officer - Chemicals, Instruments, Consumer Products
Patent Protection

Coatings

US Patent 8,637,141
Publications
All-Nanoparticle Thin-Film Coatings
Nano Lett., 2006, 6 (10), pp 2305–2312

Applications

This invention might be applied towards the development of anti-reflection coatings, which help to eliminate ghost images and improve the optical transmittance of ophthalmic lenses, flat panel displays, optical instruments, and architectural elements.

Problem Addressed

Existing technologies for anti-reflection coatings can be broadly categorized into two groups. Most commonly used are coatings made up of dense layers of inorganic materials such as MgF2. Due to the relatively low refractive index of the materials used, coatings with thicknesses >200 nm are often required to achieve desired optical properties. These thick and dense coatings not only crack easily, but also exert large film stresses that could alter substrate geometry. They also suffer from high cost due to the vacuum processing required to deposit them.

The primary existing alternative to these inorganic coatings is a class of highly porous single-layer sol-gel coatings. These sol-gel coatings can be deposited using solution-based processes, but the advantages they offer are at least partially offset by their limited optical performance and incompatibility with highly curved substrate geometry.

This invention describes a novel class of thin film anti-reflection coatings that addresses these limitations.

Technology

The class of optical coatings described in this invention consists of nanoparticle or polymer layers with alternating charge. An article being coated is successively dipped in a pair of solutions, each containing either the positively or negatively charged species. This process, which deposits a conformal bilayer of the two materials, can be repeated as necessary to build a stack of multiple bilayers until the desired coating thickness is achieved. The thickness of each layer is a function of its constituent material and the ionic strength of the dipping solution used, which eliminates the need for meticulous process controls.

The refractive index of individual layers of the coating can be controlled by tuning material properties such as the size of nanoparticles included in the layers. Further customization is possible after the assembly process by exposing the coated surface to water or PDMA vapor. The vapor undergoes capillary condensation within the pores of the coating, resulting in local increases in the refractive index. The degree to which condensation occurs at different depths in the coating can be controlled by manipulating the porosity of individual nanoparticle layers, enabling the creation of composition-graded and refractive index-graded anti-reflection coatings.

Advantages

  • High refractive index allows thin and crack-resistant coatings
  • Minimal effect of coating on underlying substrate
  • Reduced manufacturing cost compared to vacuum-deposited coatings
  • Film composition and optical properties tunable throughout thickness