M. Kepenekci, K.-C. Chien, K.-S. Lee, C.-H. Chang
The University of Texas at Austin,
United States
Keywords: multifunctional surfaces, sapphire, silicon, nanoindentation
Summary:
Nanostructured materials have exceptional characteristics, including enhanced optical and wetting performance, finding applications across different fields including displays, photonics, and renewable energy. Through precise adjustments to their structure geometry, surface nanostructures can significantly improve optical transmission by minimizing Fresnel reflection losses, while creating water-repellent and self-cleaning surfaces.1 Despite thorough investigations into their optical and wetting behaviors, the mechanical characteristics of these surfaces remain largely unexplored. This lack of knowledge raises concerns about the long-term durability and wear resistance of surface nanostructures, particularly in applications such as solar panels and touch-screen displays that require extended use. Therefore, further research is required to explore their mechanical properties, including their stiffness, strength, and hardness, and their relationship with the optical and wetting properties. In this work, we investigate the effect of geometry and intrinsic material properties on the mechanical behavior of sapphire and silicon nanopillar arrays with different aspect ratios. Moreover, the optical and wetting properties of sapphire surface nanostructures are examined. For mechanical characterization, nanoindentation is proposed as the testing method, which enables the measurement of both hardness and elastic modulus at nanoscale depths.2 The mechanical response of the samples is measured through quasi-static and cyclic nanoindentation at various depth using a conospherical indenter with a tip radius of 10 µm. The optical transmission of the sapphire sample is examined using a spectrophotometer and its wetting and dust mitigation characteristics are examined via water contact angle tests and using a lunar dust simulant, respectively. Preliminary nanoindentation measurements for sapphire and high aspect ratio silicon, shown in Fig. 1, indicates that the aspect ratio of the pillars determines the failure mode of the pillars under compressive stress. Fig. 1a shows the sapphire sample does not exhibit explicit pop-ins in load-depth graph, which is observed in bulk sapphire,3 hinting an improvement in ductility due to introduction of nanostructures. The hardness of the sample is over 3.5 GPa after maximum indentation depth of 80 nm. In silicon samples, on the other hand, three distinct deformation regimes are observed. In high aspect ratio silicon, as shown in Fig 1c, the first regime is elastic deformation at low indentation depth, the second is bending at intermediate load resulting in multiple pop-ins and the third is densification. In the sapphire sample, the second and third deformation regimes are not observed, showing lower structure-induced ductility in the mechanical response. The maximum hardness of high and low aspect ratio silicon samples is 70 MPa and 1 GPa, respectively. The high aspect ratio silicon sample has two orders of magnitude lower hardness than the sapphire and low aspect ratio silicon samples, but higher resilience due to bending of nanopillars at intermediate loads. We will present detailed nanoindentation results on the depth-dependent stiffness, hardness, and strength of sapphire, and low and high aspect ratio silicon nanopillars and optical and wetting properties of sapphire. The findings will help guide the design and fabrication of mechanically robust nanostructured materials with fine-tuned surface properties for applications in nanophotonics, multifunctional surfaces, and display.