Smart and Responsive Micro‐ and Nanostructured Materials

Abstract

As high levels of complexity and efficiency are required in advanced optoelectric devices and biomedical systems, there has been growing interest in smart materials. Also known as intelligent or responsive materials, smart materials output controllable and predictable responses to external inputs, which may include mechanical stimuli (stress, strain, pressure), physical stimuli (light, sound, temperature, color), chemical stimuli (changes in concentration, humidity, and pH), and electromagnetic stimuli (electric field, magnetic field, charge injection). Since the output response may also appear in the form of mechanical, physical, chemical, and electromagnetic changes, there can be countless combinations between stimuli and responses, making these materials extremely useful for a wide range of applications including electro-optic and optoelectronic devices such as displays and solar cells, piezoelectric devices, electroactive polymers, electrochromic windows, and artificial muscles. Many smart material-based devices have already been successful in commercialization. In recent years, the fabrication of nanostructures and microstructures have been extensively investigated through both the bottom-up and top-down approaches. Advances in the fabrication techniques have further driven the development of smart materials to a higher level of performance, for example, exhibiting multi-step responses to one stimulus or selectively responding to a stimulus under precisely defined conditions. As a result, novel systems with optical switching and mechanical actuation responding to a variety of stimuli have been developed and used in practical applications. Smart materials have also been employed to design promising biomedicines whose activities can be triggered by molecular recognition and physiochemical changes. Advances in other technical fields have also added momentum to the development of smart materials. For example, the emergence of mobile electronic devices requires energy storage components that are not only efficient but also have unconventional intelligence such as self-healing and self-charging. Recently, 2D nanomaterials have attracted a lot of interest due to their unique physicochemical properties and the transport mechanisms occurring at the sub-nanometer length scale. Paolo Samori and co-workers (article number 1902394) present an overview of the synthesis of the atomically thin membranes with controlled porosity and discuss osmotic power generation and water purification. They provide the outlooks and challenges for 2D membranes. The growth of mobile electronics requires cleaner and more efficient energy storage devices. As representatives, supercapacitors and rechargeable batteries are at the forefront and have advanced into smart multifunctional devices, being self-healable and deformable, self-charged by various energy sources (solar, human motions, and heat). Pooi See Lee and co-workers (article number 1902564) review representative strategies for enhancing the performance of supercapacitors by designing nanostructured electrode materials and the fundamental mechanisms. They discuss the prospects and future directions toward multifunctional smart supercapacitors. The rechargeable battery is facing a limited choice of electrochemical reactions, thus facing its upper bounds. Soojin Park and co-workers (article number 1902499) discuss the emerging problems in the whole range of battery systems, including interfacial control and charge kinetic regulation, structural stability during deformation and self-healing, selective scavenging of functional membranes, and materials for enhancing safety. They suggest possible nanostructured functional materials that have not been investigated and propose potential materials for post-lithium chemistries. The development in this emerging research field takes benefits from multidisciplinary works between material science, chemistry, physics, electrical engineering, energy, and other res