Semiconductors have numerous applications in both science and technology. Several methods have been developed to engineer their band gap, which is one of the most important parameters of semiconductors. Here, it is shown that the incorporation of various amino acids into the crystal lattice of copper (I) oxide, akin to the way living organisms incorporate organic macromolecules into minerals during biomineralization, leads to significant shrinkage in the volume of the host unit cell and a strong blueshift in the band gap of up to ≈18%. In examining the potential location of the bio‐organic molecules within the inorganic host's lattice, a very good fit between the proposed model of incorporation and experimental findings is found. The bioinspired phenomenon of band gap widening is thought to be attributable to the void‐induced quantum confinement effect, even though observed in micrometer‐sized crystals. This hypothesis is supported by developing a tight‐binding model that is found to fit well with the experimental data. The outcome of this research could profoundly impact the fields of light‐emitting and spin‐based devices as well as opens up a new bioinspired route to tune the band gap of semiconductors.

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Through controlled biomineralization, organisms yield complicated structures with specific functions. Here, Jania sp., an articulated coralline red alga that secretes high‐Mg calcite as part of its skeleton, is in focus. It is shown that Jania sp. exhibits a remarkable structure, which is highly porous (with porosity as high as 64 vol%) and reveals several hierarchical orders from the nano to the macroscale. It is shown that the structure is helical, and proven that its helical configuration provides the alga with superior compliance that allows it to adapt to stresses in its natural environment. Thus, the combination of high porosity and a helical configuration result in a sophisticated, light‐weight, compliant structure. It is anticipated that the findings on the advantages of such a structure are likely to be of value in the design or improvement of lightweight structures with superior mechanical properties.

Significance Biominerals are extraordinarily intricate and possess superior mechanical properties compared with their synthetic counterparts. In this study, we show that the presence of high-Mg calcite nanoparticles within a low-Mg calcite matrix is a widespread phenomenon among marine organisms whose skeletons are composed of high-Mg calcite. It seems most likely that formation of such a complex structure is possible because of the phase separation that occurs as a result of spinodal decomposition of an amorphous Mg–calcium carbonate precursor and is followed by crystallization. We demonstrate that the basis of such phase separation stems from chemical composition rather than from biological similarities. The presence of high-Mg calcite nanoparticles increases the skeletons’ toughness and hardness.

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