M. Kepenekci, K.-C. Chien, C.-H. Chang
The University of Texas at Austin,
United States
Keywords: bio-inspired structures, moth-eye structures, nanomechanics, nanoindentation, sapphire
Summary:
The nanostructures in nature and their properties have been extensively studied, leading to the development of novel engineered materials with superior optical, surface, and mechanical properties. Broadband antireflection and absorption are achieved through high aspect ratio (AR) nanopillars with sub-wavelength feature sizes [1]. Arrays made of high AR and dense nanopillars are also used to generate self-cleaning superhydrophobic or wicking surfaces [2]. However, there is limited research on the designing of scratch resistant and antireflection nanostructured sapphire surfaces. In previous work, we investigated the mechanical properties of nanostructured surfaces using nanoindentation [3]. However, the trade-off between structure geometry vs mechanical and optical properties is not well-understood. To utilize these structures in areas such as protective windows and IR optics, sensor covers for aerospace, defense, their mechanical robustness should be further investigated to design more robust structures to ensure extended use. Moreover, it is important to further investigate the effect of nanopillar geometry on mechanical properties of nanostructured surfaces, including stiffness, hardness, and modulus. In this work, we examine the effect of pillar geometry on mechanical behavior and antireflection effects of sapphire nanopillars with different period and AR. The nanopillar arrays are patterned via Lloyd’s mirror interference lithography method and transferred to the underlying substrate using inductively coupled plasma reactive ion etching. Nanoindentation is used for mechanical characterization to measure hardness and modulus at different indentation depth. Quasi-static and cyclic nanoindentation test methods are utilized with a conospherical indenter with an indenter tip radius of 10 µm. The antireflection effects are examined by measuring the transmission of the samples via UV-Vis-NIR spectrophotometry. Optical transmission of the nanostructures is modeled using the rigorous coupled-wave analysis (RCWA) method. Mechanical response of the sapphire surface with 460 nm height and 330 nm period nanopillar array is characterized by nanoindentation. The load-depth response of the sample exhibits a smooth loading curve without large pop-ins, which are commonly seen in brittle nanopillars. The result show that the nanostructured sapphire surface has modulus and hardness of 170 GPa and 3.7 GPa, respectively, similar to scratch resistant metals. The optical transmission in visible and NIR region is enhanced due to nanopillars. Nanopillars with smaller period decreases the lower limit of the antireflection while higher nanopillar height enhances transmission in longer wavelength. Broadband optical transmission enhancement requires nanopillar arrays with shorter period and higher height, which increases the pillar AR while mechanical robustness requires lower AR nanopillars. Therefore, co-designing nanopillars for optical and mechanical properties is required. We will present the fabrication details, mechanical and optical test and simulation results. The effect of pillar geometry on antireflection, hardness, modulus, and mechanical robustness of the nanostructured surfaces will be investigated. This work will study the trade-off between mechanical robustness and antireflection effects of these materials. The findings will help design and fabrication of mechanically durable nanostructured surfaces for various applications in protective windows, sensor covers for aerospace, consumer electronics, and renewable energy areas.