Bridging functional nanocomposites to robust macroscale devices

Hierarchical functional nanocomposites Composite materials are constructed from materials that vary in size. Nanoscale materials have unique properties that may be very useful for developing new types of devices. Begley et al. review synthesis and assembly methods for functional nanocomposites with a focus on potential applications. Some challenges include scaling and ensuring mechanical stability. Combining new developments from a range of disciplines will be key for enabling advanced device concepts. Science, this issue p. eaav4299 BACKGROUND Composites comprising nanoscale particles embedded in a second phase (or matrix) create exciting opportunities to design functional materials with strong coupling between optical, electromagnetic, mechanical, and transport phenomena. Such coupling is strongly enhanced by deterministic control of multiple length scales in a hierarchical structure: for example, the ordering of crystalline nanoscale particles with prescribed shape to form “superlattices,” with controlled particle spacing spanning from tens of nanometers to tens of micrometers and exhibiting emergent collective behavior not hosted in the individual particles. Substantial advances in particle synthesis—encompassing a broad range of compositions, sizes, and shapes—have been combined with equally impressive advances in assembly, creating a virtual “materials design palette” for the generation of materials with targeted responses. The resulting functionalities can advance a broad range of transformative technologies, such as wearable sensors that respond to physiological stimuli, flexible displays, batteries and catalysts with enhanced control over ion and electron transport, etc. However, the use of ordered nanocomposites in such applications has historically been hampered by several related factors: (i) limited pathways to synthesize and pattern such materials over length scales required for devices, (ii) fabrication techniques amenable to the integration of nanocomposites with other materials required to connect or protect functional components, and (iii) limited understanding of the thermomechanical stability of nanocomposites, both as isolated materials and as embedded components. ADVANCES In addition to the rigorous control of particle shape and size, new techniques to control surface chemistry have advanced the ability to fabricate nanocomposites from the “bottom up” and span large distances. These advances expand the scope of surface capping ligands that can be used during self-assembly of colloidal crystals and provide pathways to tailor particle spacing and binding (controlling optical and electronic properties), as well as the mechanical properties of the nanocomposite “matrix.” New inorganic surface chemistries also show tremendous promise for solid-state device integration as a result of added control over particle interactions and the opportunity to broaden the range of solvents and their polarity during synthesis. Therefore, new micro- and macroscale fabrication methods are emerging, many of which exploit established solution processing and lithographic patterning techniques that enable facile integration with other device materials and features. Notable recent examples of bottom-up fabrication routes profit from capillary-driven and other liquid-mediated assembly methods that harness evaporation and wetting behavior. Pairing these approaches with new surface functionalization schemes shows promise for maintaining the deterministic ordering of nanoscale building blocks, which underpins much of their emergent phenomena. At the same time, “top-down” assembly approaches that exploit advances in three-dimensional (3D) printing technology have established bridges from the macroscale down to the nanoscale; direct deposition of nanocomposites has been demonstrated and provides clear pathways for patterning and integration of functional nanocomposites. Similarly, advances in direct writing of nanoparticle-based colloidal inks stand to benefit from parallel successes in the implementation of field-directed assembly during 3D printing. The diversity of material systems that can be targeted using acoustophoretic, electrophoretic, and magnetically directed assembly promises a wide canvas of nanocomposite properties and behavior. Advances in composition and fabrication have enabled the development of more robust nanocomposites, which are amenable to thermomechanical characterization critical to device integration. OUTLOOK Hierarchical fabrication methods to produced patterned features of nanocomposites continue to emerge and set the stage for more rapid and sophisticated integration in devices. This represents a critical step not only in the development of transformative sensors, displays, and batteries, etc., but also in the science and development of the functional materials themselves. The reasons are twofold: First, the ability to pattern nanocomposites and integrate them with other materials takes the nanocomposites “out of the beaker” and enables critical pathways to characterize structure–property relationships and understand the role of particle binding, defects, “grain” boundaries, etc. Second, the integration of nanocomposites into specific device contexts will identify important trade-offs—e.g., functional performance, chemical compatibility, and thermomechanical robustness—that will define essential scientific questions regarding the underlying mechanisms controlling performance. Addressing these questions will lead to new understanding that can be used to identify effective nanocomposite compositions, synthesis techniques, and fabrication pathways. Depiction of a hierarchically structured functional device (embodied as a wearable device for human health). Deterministically ordered nanoparticle assemblies serve as a bridge between nanoscale and microscale features. Subsequent assemblies reflect multimaterial integration enabled by directed mesoscale assembly methods. At the intersection of the outwardly disparate fields of nanoparticle science and three-dimensional printing lies the promise of revolutionary new “nanocomposite” materials. Emergent phenomena deriving from the nanoscale constituents pave the way for a new class of transformative materials with encoded functionality amplified by new couplings between electrical, optical, transport, and mechanical properties. We provide an overview of key scientific advances that empower the development of such materials: nanoparticle synthesis and assembly, multiscale assembly and patterning, and mechanical characterization to assess stability. The focus is on recent illustrations of approaches that bridge these fields, facilitate the design of ordered nanocomposites, and offer clear pathways to device integration. We conclude by highlighting the remaining scientific challenges, including the critical need for assembly-compatible particle–fluid systems that ultimately yield mechanically robust materials. The role of domain boundaries and/or defects emerges as an important open question to address, with recent advances in fabrication setting the stage for future work in this area.