Abstract
We demonstrate that arrays of hourglass-shaped nanopillars patterned into crystalline silicon substrates exhibit vibrant, highly controllable reflective structural coloration. Unlike structures with uniform sidewall profiles, the hourglass profile defines two separate regions on the pillar: a head and a body. The head acts as a suspended Mie resonator and is responsible for resonant reflectance, while the body acts to suppress broadband reflections from the surface. The combination of these effects gives rise to vibrant colors. The size of the nanopillars can be tuned to provide a variety of additive colors, including the RGB primaries. Experimental results are shown for nanopillar arrays fabricated using nanoimprint lithography and plasma etching. A finite difference time domain (FDTD) model is validated against these results and is used to elucidate the electromagnetic response of the nanopillars. Furthermore, a COMSOL model is used to investigate the angle dependence of the reflectance. In view of display applications, a genetic algorithm is used to optimize the nanopillar geometries for RGB color reflective pixels, showing that nearly all of the sRGB color space and most of the Adobe RGB color space can be covered with this technique.
Highlights
Among many exciting phenomena exhibited by nanostructured materials, structural coloration is one of the most fascinating
In order to get blue and orange colors, we changed the size of the mask features before the Si etch. This was done in two ways: To make them smaller the descum was run longer than normal such that the resist pillar diameters would shrink in size diametrically, and to make them larger, atomic layer deposition (ALD) of SiO2 was done on a FIJI ALD system to increase the diameter of the SiO2 mask features. 125 cycles of ALD were done with alternating tris(dimethylamino)silane (TDMAS) and O2 gases at 110°C which deposited approximately 10 nm of SiO2
Arrays of each structure are shown in the scanning electron microscope (SEM) images in Fig. 2(a) and the colors they create are shown in Fig. 2(b)
Summary
Among many exciting phenomena exhibited by nanostructured materials, structural coloration is one of the most fascinating. Structural colors can be man-made [5,7,8,9,10] and have unique qualities that may offer advantages over dyes and pigments for use in prints and displays. Because they are typically made from inorganic materials, they generally have superb chemical and mechanical stability which could create immunity to fading and bleaching. These materials lend themselves better to recyclability compared to the organics used in pigments. The scalability of structural colors to extremely small length scales can be utilized to create color pixels as small as the diffraction limit of light [11], allowing for ultra-high resolution and/or microdisplays and prints
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