Additive Manufacturing of 3D ZrO₂:Eu³⁺ Luminescent Microstructures

Various metal and metal oxide architectures have been successfully fabricated using the traditional toolbox of planar lithography. Despite having accomplished various hierarchical structures, these methodologies rely on alternating layer-by-layer deposition and etching steps. In other words, the realized architectural designs consist of thin-film stacks lacking a further geometrical complexity. Several additive manufacturing (AM) approaches, i.e., inkjet printing, laser sintering, and direct laser writing (DLW), have demonstrated that the fabrication of compound three-dimensional (3D) architectures is possible. Amid them, DLW has been recognized as a method enabling the reproducible construction of complex structures of arbitrary geometries with sub-micrometer feature sizes. Such microstructures find applications in the fields of, e.g., photonics, microfluidics, or biomedical engineering.<br/><br/>The current bottleneck for DLW is that commercial resins, with scarce exceptions, allow the fabrication of solely organic polymers. In practice, the physicochemical properties of these materials (i.e., chemical and thermal resistance) restrict the range of applications. This situation stimulated the development of novel approaches focusing on either (i.) functionalization of printed structures or (ii.) direct printing using tailor-made resists. In the case of (i.), the surface of the 3D architectures is functionalized, i.e., via electroless plating or atomic layer deposition. In the second example, photoresists are doped with metal-rich salts. Upon thermal treatment of the printed structures, their miniaturized ceramic or metal oxide replicas are yielded. Currently, the range of metals and metal oxides printable with DLW remains relatively limited. The recent efforts in material sciences have resulted in tailor-made photoresists, allowing AM of, e.g., SiOC, ZnO, Ni, TiO₂. Implementing a new range of functional materials will constitute a great leap towards the further extension of the applicability of DLW to the benefit of various areas of engineering.<br/><br/>In this work, we present the manufacturing of well-defined three-dimensional (3D) silicon-free ZrO₂:Eu³⁺ luminescent microstructures of sub-micrometer feature sizes via a DLW process using custom-made organo-metallic acrylic resins. The capabilities of our methodology are showcased with 3D bucky-ball-inspired architectures. 3D structures composed of Eu-doped tetragonal (t-ZrO₂) and monoclinic (m-ZrO₂) zirconia are studied and compared with non-doped t-ZrO₂ and m-ZrO₂. The chemical composition of the structures is determined using Confocal Raman Spectroscopy (Raman). For cross-validation, bulk reference powders are obtained from UV-cured resins characterized with Raman and X-ray powder diffraction (XRD). The optical properties of both 3D architectures and powders are studied in detail with Cathodoluminescence (CL). The 3D-printed ZrO₂:Eu³⁺ architectures are highly efficient phosphors. CL spectra present typical Eu³⁺ transitions, corresponding with red and orange-red emission. We observe that the Eu³⁺ radiative transitions are highly sensitive to the local lattice environment and defects.<br/><br/>We envision the presented material stimulating further studies on the fabrication of phosphor materials via DLW, particularly on 3D microstructures for optical and optoelectronic applications.