A customizable nanomaterial was developed that combines metallic strength with a foam-like ability to compress and spring back. The material can store and release mechanical energy on the nanoscale, and fits into existing industrial semiconductor processes.
The nanostructures span just a few billionths of a meter in size, and are composed of organic and inorganic molecules. These custom-patterned structures — or pillars — enable more advanced nanoelectromechanical systems (NEMS) in devices that require ultra-small springs, levers, or motors. NEMS technology that could potentially exploit this new material includes ultrasensitive accelerometers, multi-functional resonators, and biosynthetic artificial muscles.
The organic-inorganic hybrid materials exhibit metal-like high strength, but foam-like low stiffness. The unique coupling of mechanical properties accounts for the material's ability to store and release an extraordinarily large amount of elastic energy. That essential elasticity — like the flex and release of a muscle — is constrained by both the chemistry and the structure.
The process of creating the material began with lithography, where a focused beam of electrons carved small pillars (300 nanometers wide and 1,000 nanometers tall) into a polymer called SU-8, a light-sensitive material typically used for micrometer-scale device fabrication. The precise geometry of the lithography process laid the structural foundation for the subsequent infiltration by inorganic elements.
The nanopillar array was placed in a vacuum chamber and an aluminum pre-cursor vapor was introduced — a process called atomic layer deposition (ALD). The precursor naturally soaks into pores in the polymer pillars, a bit like molecular concrete smoothing over cracks and fissures in a sidewalk. Subsequent exposure to water transformed the aluminum precursor into a metal oxide molecule, which strengthens the polymer matrix. The number and duration of these exposures allow researchers to tune the ultimate mechanical properties of the material.
The chemical composition and structure was tested with transmission electron microscopy, which revealed that the spherical aluminum oxide clusters remained chemically discrete, but fully integrated into the nanopillar matrix. The thorough mixing — and in particular, the spherical shape of the metal oxide clusters — contributes to the modulus of resilience. Without the infiltrated nanoscale metal oxide filler, the polymer pillars would be crushed under mechanical strain.
To test that resilience, a nanomechanical tip was run across the sample and was able to gently press down on individual pillars, each one some 200 times thinner than a human hair. The relationship among the elastic mechanical energy, the material's ability to store and release it, and the structural integrity were measured.