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Bio-Inspired Routes for Controlling the Structure and Properties of Materials: Reusing proven tricks on new materials

Final Report Summary - BIONICS (Bio-Inspired Routes for Controlling the Structure and Properties of Materials: Reusing proven tricks on new materials.)

In the course of biomineralization, organisms produce a large variety of functional crystals that exhibit fascinating mechanical, optical, magnetic, and other characteristics. Despite limited materials availability, nature can create functional materials and structures with outstandingly appropriate properties. Over the years there have been many attempts not only to study how natural materials are made and to characterize their structure and properties, but also to translate some aspects of these materials into synthetic materilas.
One of the most fascinating strategies used by organisms in forming crystals is the so-called “non-classical” crystallization, i.e. the formation of crystals via an amorphous precursor. This route of crystallization has been shown to have three main impacts on crystals: (i) the short-range order within an amorphous precursor determines the polymorph that forms upon crystallization; (ii) single crystals formed after transformation have curved and rounded shapes rather than clear facets; and (iii) during transformation from the amorphous to the crystalline phase, proteins and organic molecules are trapped within the lattices of the single crystals, leading to lattice strains and enhancing fracture toughness due to crack deflection.
In this ERC project, we emulated these specific strategies used by organisms in forming structural biogenic crystals and applied these strategies to other materials so as to produce materials with new properties and characteristics. Our bio-inspired approach involved the adoption of three specific biological strategies and their application to different smart structural materials.

We have extensively studied the short-range order of the amorphous thin films and showed that they vary structurally as a function of size. Thinner films exhibit a different structure, in comparison to thicker ones. According to different analyses that we performed, these changes arise from the surface of the film. Our theoretical investigation revealed that these atomistic alterations are expected to change the amorphous thin film’s average density, and indeed it was found to vary with the layers' thickness. Moreover, the ERC team has shown that the size of the amorphous thin films has an effect on different density-dependent properties and it was indeed found that the refractive index and dielectric constant of these layers also change with the thin films’ thickness. We believe that the ability to tune one property or another solely by size, according to a specific requirement, can open new possibilities for materials selections and applications, in science and technology.

A remarkable outcome of the growth of crystals via an amorphous precursor is the ability of the organism to obtain unfaceted single crystals with rounded shapes and even crystals that are highly porous. In this ERC project, we developed a novel bio-inspired method of growing single crystals with intricate shapes and morphologies based on a de-wetting mechanism. In particular, a novel route to grow intricately shaped single crystals of gold by exploiting crystallization from a hypoeutectic melt was established. We demonstrated that it is possible to produce micron-sized curved nanoporous gold single crystals and crystals consisting of both whole single crystal and nanoporous single crystal. We have developed a kinetic model that explains this phenomenon and shows that the full crystallization process is faster than the average period between two subsequent nucleation events, a key factor allowing the intricate 3D single crystals of gold to retain their single-crystalline nature. This method allows for the growth of nanoporous single crystals of gold of up to several hundred micrometers in size. We also clearly showed that nanoporous gold single crystals prepared by eutectic composition demonstrate superior thermal stability as compared to their counterpart nanoporous gold prepared by dealloying, which is essential for catalysis. Moreover, we have shown the feasibility of controlling the ligament size of nanoporous gold which opens up a new route to alter its thermal stability and mechanical properties. We believe that this method can also be employed in other crystal systems, thereby opening the door to new technological capabilities.

Over the last several decades it was shown that biogenic CaCO3 is often formed via ACC and is a nanocomposite containing intracrystalline macromolecules within single crystals. The incorporation of such organic molecules leads to the enhancement of fracture toughness of biogenic calcite crystals. In this ERC project, we have shown that such bio-inspired approach allows forming advance composite crystals with tuned properties. The ERC team for the first time reported on a versatile strategy for fabricating crystalline nanohybrid-composite gold crystals via the incorporation of selected amino acids. Fabrication of hybrid organic–metal composites has great importance for a wide range of applications. We could also demonstrate for the first time that specific amino acids can be incorporated into a semiconductor crystalline host, particularly ZnO, just as we have found to occur in biogenic crystals. The amino acids incorporated into ZnO shown to induce lattice distortions that are also accompanied by a modification of the optical band gap of ZnO. We have demonstrated that band gap of ZnO increases upon the incorporation of amino acids. This allowed us to establish a novel band gap engineering approach and elucidate the role of the incorporated amino acids on the band gap modification. We believe that this work will open up a completely new route for tailoring of the band gap of different semiconductors, and especially of those grown from solutions, in addition to the methods known and used today. The ability to tune the band gap is extremely important for sensing, light harvesting, and many other uses.

Overall, we believe that this project opened up exceptionally promising new routes to control the structure and properties of novel synthetic materials.