2, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Naturally occurring responsive systems such as folding and unfolding in self-assembled DNA bundles prove natural designs are hierarchical, with structures and property on multiple scales through interactions of subunits or building blocks. Mimicking these designs in fabrication of active materials requires a clear picture of energy landscaping that governs local interactions such as hydrogen bonding, van der Waals interactions, dipole-dipole interaction, capillary forces, etc, which will provide correct thermodynamic end points as well as facile kinetics for precise control of hierarchical structure for responsive functions. To date, fabrications of active and responsive nanostructures have been conducted at ambient pressure and largely relied on these specific chemical or physical interactions. Using our recently developed stress-induced assembly (SIA) as an artificial actuator, we can emulate natural folding and unfolding processes to explore energy landscaping that govern local interactions. Through SIA, we can design new classes of active materials with controlled structure and function and investigate new properties resulting from the folding and unfolding processes. We show that under a hydrostatic pressure field, the unit cell dimension of a 3D ordered nanoparticle arrays can be manipulated to reversibly shrink and swell during compression and release of pressure, allowing precise tuning of interparticle symmetry and spacing, ideal for controlled investigation of distance-dependent energy couplings and collective chemical and physical property such as surface plasmon resonance. Moreover, beyond a threshold pressure, nanoparticles are forced to contact and sinter, forming new classes of chemically and mechanically stable 1-3D nanostructures that cannot be manufactured by current top-down or bottom-up methods. Depending on the orientation of the initial nanoparticle arrays, 1-3D ordered nanostructures (Au, Ag, etc.) including nanorods, nanowires, nanosheets, and nanoporous networks can be fabricated. The SIA method mimics embossing and imprinting manufacturing processes and opens exciting new avenues for the study of responsive behaviors of active materials during compression (folding) and pressure release (unfolding). Exerting stress-dependent control over the structure of nanoparticle or building block arrays provides a unique and robust system to understand collective chemical and physical characteristics of nanocrystal superlattices.