Interfacing nanosprings for stability

Researchers in the US have fabricated dense films of nanoscopic springs from silicon. Their approach used seeded Glancing Angle Deposition (GLAD) and marks a putative turning point in the precision engineering and mechanical evaluation of nanoscopic components for a wide range of technologies. Scanning electron microscopy (SEM) and focused ion beam (FIB) tomography revealed the "true" geometry of their nanosprings.

When working towards nanotechnological devices, one of the problems that arises is how to interface different components given the near certainty of a mismatch between materials in terms of differing thermal response and mechanical properties. A compliant material, such as a polymer, sandwiched between two dissimilar surfaces might solve the problem in a range of instances, but not all. The mechanical properties of polymers depend strongly on temperature, even in the case of relatively small temperature changes encountered in the packaging material of a computer processor, for instance. Such polymeric interfaces, albeit compliant, can lead to structural weakness.

Dimitrios Antartis, Ryan Mott, and Ioannis Chasiotis of the department of Aerospace Engineering, at the University of Illinois at Urbana Champaign, Urbana, Illinois, USA, reasoned that nanosprings might be the answer to many of these issues. Such components might effectively overcome the mismatch without compromising strength. It was, they explain, "apparent that an effective interface material should possess the thermal stability and properties of common ceramics or metals, and a microstructure that is designed to provide high compliance and resistance to fracture." In their in-depth study of the mechanical behavior of the building blocks of such interfaces, they selected silicon as an appropriate benchmark material to demonstrate the advantages of their approach.

The team's mechanical tests on their silicon nanosprings using a microelectromechanical system (MEMS) showed that the nanosprings, which had either four or ten turnings, were not necessarily perfect but were springy. Their work quantified for the first time the mechanical stiffness of nanosprings under several loading modes and for different spring geometries. Transmission electron microscopy (TEM) showed how the nanosprings exist as tightly bundled fine fibrils; this structure imparts flaw tolerance and reduces the effective elastic modulus of the individual nanosprings, the team reports. [Antartis et al. Mater Design (2018); DOI: 10.1016/j.matdes.2018.02.017]

The team adds that GLAD has been used in the past to produce films of slanted, straight, or helical micro- or nano-elements of monolithic or hybrid materials not just of silicon, yet there hasn't been a systematic effort to understand the properties of the resulting components. These properties can be controlled by adjusting the rotation speed of the substrate, the deposition rate, and the deposition angle, in ways that were yet unknown. The tests carried out on the team's silicon nanosprings point to limitations but also how they might be overcome. "These fabrication-structure-properties relationships will facilitate the informed design of thin films and interfaces with desirable mechanical behavior that departs from that of bulk metals and ceramics," the team explains.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".