3D micro/nano printing of functional crystal materials

Abstract

Functional crystal materials comprise a broad category with unique properties originating from their chemical compositions and periodic molecular arrangement. The practical utilization of functional crystalline materials necessitates their high-precision, freeform shaping via cost-effective manufacturing processes. However, most developed approaches have relied on complex, expensive lithographic processes. Although 3D printing techniques provide unprecedented shaping capability in manufacturing, they have not been suitable for crystalline materials due to a lack of crystal engineering strategy. This thesis introduces a novel strategy to incorporate the solution-mediated crystallization pathways into 3D printing, fabricating freestanding, freeform crystalline micro- or nano architectures. The key idea is to exploit a femtoliter ink meniscus to confine and guide either evaporation-driven direct crystallization or solidification of precursor compounds followed by post-treatments in three-dimensions. This strategy, named meniscus-guided 3D printing, can manufacture three classes of functional crystal materials such as metal-organic frameworks (MOF), semiconductor metal oxides (SMOXs), and pre-synthesized nanocrystals, demonstrating microelectronic devices. First, we developed the meniscus-guided 3D printing method to fabricate pure HKUST-1 MOF micro-monolith with a high resolution of < 3 µms. Unlike traditional pelletization or extrusion processes, our layer-by-layer method does not use mechanical force or additives to manufacture the MOF monolith, resulting in retained porosity and surface area. Thus, our 3D-printed HKUST-1 monolith displayed a Brunauer–Emmett–Teller (BET) surface area of 1192 m2/g, superior to the monoliths produced by other manufacturing approaches. A micro humidity sensor composed of the printed HKUST-1 micro-walls was successfully demonstrated. Second, we harnessed meniscus-guided 3D printing to fabricate suspended SMOX nanowires for gas sensor microdevices. This high-precision, damage-free approach enabled the direct integration of individual nanowires on only a 1.5 µm-thick suspended silicon nitride membrane embedding an interdigitated electrode and a micro-heater. The polycrystalline nanowires were acquired by calcination, demonstrating high performance and low power consumption gas sensing. The chemical composition of the printed metal oxide nanowires can be varied by changing metal precursors. This was proved by directly printing twenty-four types of nanowires consisting of six metal oxides and four noble metal dopants and characterizing their gas sensing selectivity. Our method provides a simple and versatile way to produce high-performance electronic nose (E-nose) microdevices. We expect the works presented in this thesis to widen the material option of 3D micro/nano printing, paving the new way for additive manufacturing of microelectronic devices. Another interesting ongoing strategy is using pre-synthesized nanocrystals, including perovskite quantum dots (PeQDs), MOF nanocrystals, and metallic nanoparticles for nano-liquid 3D printing methods. The utilization of 3D printing could potentially make a synergistic effect with conventional semiconducting processing, providing processing simplicity, cost-effectiveness, and design flexibility in microdevice manufacturing, if combined with conventional semiconducting processing.