Self-Organized Nanostructure Formation for Anti-Reflection Glass Surfaces
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Self-Organized Nanostructure Formation for Anti-Reflection Glass Surfaces Jörn Achtelik, Ricarda M. Kemper, Werner Sievers and Jörg K. N. Lindner University of Paderborn, Department of Physics, Warburger Str. 100, 33098 Paderborn, Germany ABSTRACT Two methods to create biomimetic anti-reflection nanostructures in ordinary glass microscope object slides are presented. One technique is based on a nanosphere lithography process combined with physical vapour deposition of nickel and reactive ion etching (RIE). The other uses plasma induced dewetting of a smooth nickel surface. The amount of reflected light was measured and a method to simulate the reflectivity from an atomic force microscopy (AFM) topography scan of the glass surface is presented. The reflectivity for visible light at normal incidence was reduced to 20-50 % of the original value with both methods and the simulation gives results in good agreement to the measurement. INTRODUCTION Anti-reflective surfaces and structures are common methods to reduce the amount of light reflected by air-glass interfaces. Assuming a refractive index of 1.53 and applicability of Fresnel equations, the reflectivity at perpendicular incidence for a smooth, untreated glass surface is not less than 4.4 % resulting in a need for anti-reflective surface treatments in some applications. Anti-reflective surfaces improve the convenience of spectacles and displays, but they can also be used on solar panels [1], which are usually protected with glass, which may reduce electrical or heat power output. The evolution of nature already solved the problem of surface reflection and emerged sub-wavelength structures on some insect's eyes. A scanning electron microscopy (SEM) scan of an insect's eyes facet boundary is shown in Figure 1.
Figure 1. SEM image of a facet boundary from an insects eye. The sub-wavelength structure is revealed as an ordered array of pillars.
Unlike common human methods like smooth anti-reflection layer systems, the insects eye shows a roughness at a sub-wavelength scale, leading to a graded refractive index profile for the incoming wave, reducing reflectivity of the insect eye's surface and increasing the amount of incoming light [2]. The question arises if these nanostructures are yet optimized and if they can be technically replicated by affordable technologies on large area surfaces. To this end, two approaches to create periodically ordered and irregularly nanostructured glass surfaces, respectively, are studied and compared. Both techniques use self-organization effects and are therefore particularly affordable pattern formation processes as required for energy materials. Different plasma treatment processes are used to transfer the surface pattern into the substrate. EXPERIMENT The two approaches to an anti-reflective nanostructure are different. The first approach is based on ordered nanosphere lithography and consists of five steps. The second approach consists of only three steps to create a self-organized broadband anti-reflective surface for visible ligh
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