Fabrication of Rough Polymer Surfaces Exhibiting Anti-reflective Properties

Abstract

We have demonstrated some facile ways to fabricate the large area polymer surfaces with varying roughness followed by studying their anti-reflective properties. One of the approaches is based on electrospun nanofibers deposited on a substrate in an uneven non-woven matrix. This electrospun fabric was used as a master template to fabricate the negative replica of the fibers by soft lithography generating the roughness in polydimethylsiloxane (PDMS) surfaces. The second approach is based on biomimicking of flower petals. Petals are used as a master template to transfer surface features with hierarchical roughness over PDMS surface using replica moulding. As fabricated polymer surfaces with varied roughness have then tested for their anti-reflective properties using UV-VIS spectroscopy over a wide range of wavelengths and angles of incidence of light. These measurements show near zero reflection of patterned PDMS surfaces as compared to planar PDMS. This omnidirectional broadband anti-reflection behaviour of polymer surfaces can be used in wide variety of engineering applications including in solar cells. INTRODUCTIONSurfaces which suppress reflection are known as anti-reflective and are widely used in lot of engineering applications such as optical devices, light emitting diode and solar cells [1-3]. Antireflection can be achieved using thin film coatings however with some limitations like radiation damage due to thermal gradient between different layers, adhesion problems within multiple thin film layers and therefore, cannot be used for a broad range of incident light. Taking inspiration from nature, several studies have been carried out on certain species of moths, as their corneal surface has regular hexagonal arrays of sub-micron structures [4][5]. Further certain species of butterflies have multiscale structures on their wings which exhibits antireflective properties [6][7]. However fabrication of those multiscale structures is either limited by small area constraint or cost associated with use of highly sophisticated micro-and nano-fabrication tools [8][9][10]. In the present work, we have demonstrated a simple approach to fabricate large area anti-reflective polymer surfaces by inducing roughness at different length scale. First approach is based on exploiting the roughness of nanofibers deposited in a non-woven matrix by electrospinning [11][12]. Electrospun nanofiber mat has been used as a master template to fabricate the negative replica of the fibers in polydimethylsiloxane (PDMS) using soft lithography. By varying the fiber morphology from long continuous fibers to only beads, we have fabricated negative PDMS replicas with surface roughness varying over an order of magnitude in length scale. In second approach, biomimicking has been demonstrated as another way to fabricate rough polymer surfaces over a large area [13][14]. Here we have chosen flower petals of a plant, Euphorbia milii, as a master template. Euphorbia milii petal is ultrahydrophobic due to the presence of multiscale surface features lead...