3D printing of inorganic nanomaterials by photochemically bonding colloidal nanocrystals

3D printing of inorganic materials with nanoscale resolution offers a different materials processing pathway to explore devices with emergent functionalities. However, existing technologies typically involve photocurable resins that reduce material purity and degrade properties. We develop a general strategy for laser direct printing of inorganic nanomaterials, as exemplified by more than 10 semiconductors, metal oxides, metals, and their mixtures. Colloidal nanocrystals are used as building blocks and photochemically bonded through their native ligands. Without resins, this bonding process produces arbitrary three-dimensional (3D) structures with a large inorganic mass fraction (~90%) and high mechanical strength. The printed materials preserve the intrinsic properties of constituent nanocrystals and create structure-dictated functionalities, such as the broadband chiroptical responses with an anisotropic factor of ~0.24 for semiconducting cadmium chalcogenide nanohelical arrays. A broad challenge for three-dimensional (3D) printing of any material is to ensure good bonding between layers or particles because this will affect many of the properties of the final structure. A typical approach for printing nanoparticles is to use polymer binders, but this brings its own limitations. Li et al . present a method to 3D print concentrated nanocrystal solutions without polymer or monomer additives (see the Perspective by Balazs and Ibáñez). In this approach, nanoparticles are inherently coated in ligands, and then two-photon irradiation is used to locally and selectively form covalent bonds between adjacent nanoparticles. This method allows for micrometer-scale resolution 3D architectures to be directly written without altering the intrinsic properties of the nanocrystals. —Marc S. Lavine Ligand photochemistry of colloidal nanocrystals enables 3D laser direct printing of various inorganic nanomaterials.