Copper nanomaterials and assemblies for soft electronics

SCIENCE CHINA Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . REVIEWS mater.scichina.com link.springer.com Published online 13 August 2019 | https://doi.org/10.1007/s40843-019-9468-5 Sci China Mater 2019, 62(11): 1679–1708 SPECIAL ISSUE: Celebrating the 100th Anniversary of Nankai University Copper nanomaterials and assemblies for soft electronics 1,2 Yang Feng 1,2,3,4* and Jian Zhu ABSTRACT Soft electronics that can simultaneously offer electronic functions and the capability to be deformed into arbitrary shapes are becoming increasingly important for wearable and bio-implanted applications. The past decade has witnessed tremendous progress in this field with a myriad of achievements in the preparation of soft electronic conductors, semiconductors, and dielectrics. Among these materials, copper-based soft electronic materials have attracted considerable attention for their use in flexible or stretchable electrodes or interconnecting circuits due to their low cost and abundance with excellent optical, electrical and mechanical properties. In this review, we summarize the recent progress on these materials with the detailed discussions of the synthesis of copper nanomaterials, approaches for their assemblies, strategies to resist the ambient corrosion, and their applications in various fields including flexible electrodes, sensors, and other soft devices. We conclude our discussions with perspectives on the remaining challenges to make copper soft conductors available for more widespread applications. Keywords: copper nanomaterials, assemblies, composites, stretchable conductors, soft electronics INTRODUCTION The elastic, soft, and nonplanar electronics inspired by biological systems overcome the fundamental limits imposed by the rigid silicon-based electronics, and expand the horizon of electronics applications unforeseeable in the past [1–3]. These soft electronics enable the more approachable network of internet of things, and allow the future electronics to take more crucial roles in health monitoring, soft robotics, electronic skins and biological sensors [4–7]. Flexible and highly conductive conductors are playing a vital role in these advanced electronics, including wearable electronics [5,8–12], stretchable transistors [13], ultrasensitive and selective non-enzymatic glucose detection [14], flexible solar cells [15], stretchable organic light-emitting diodes (LEDs) [16,17], biosensors or biomimetic sensors [6,18], actuators [19,20], energy harvesting devices [21–28] and so on. One way to realize these soft conductors needs intimate and robust integration of highly conductive metal nanomaterials with mechanically stretchable elastomers. The optimized soft conductors have the ability to be twisted or bent, and easily conform to curvilinear surfaces, or maintain highly conductive characteristics under large strains (>>1%) and recover their initial performance with released stress [29]. The nanomaterials made of noble metals, such as gold or silver, have attracted intense attention as the conductive components in deformable electronics due to their high conductivity and inertness against oxidation [5,6,18,30,31]. However, the use of noble metals in flexible electronics is intrinsically limited by their high cost due to their scarcity on earth. As an alternative to these noble metals, much less expensive copper is receiving increasing interest, and it may act as a potential contender to completely replace noble metals in soft electronic circuits. As a comparison, −8 copper has an electrical resistivity of 1.75×10 Ω m, −8 comparable to that of silver (1.65×10 Ω m) and gold −8 (2.40×10 Ω m), yet is 1000 times more abundant and 100 times less expensive than silver [32]. Despite these advantages, copper suffers from easy oxidation in the ambience, and may lose its conductivity easily. In addition, the morphologies and surface chemistry of copper nanomaterials should be further optimized to enable the proper interface for the integration with elastomers. The ultimate 1 School of Materials Science and Engineering, Nankai University, Tianjin 300350, China National Institute for Advanced Materials, Nankai University, Tianjin 300350, China 3 Tianjin Key Laboratory of Metal and Molecule-Based Material Chemistry, Nankai University, Tianjin 300350, China 4 Tianjin Key Laboratory for Rare Earth Materials and Applications, Nankai University, Tianjin 300350, China * Corresponding author (email: (Zhu J)) 2 November 2019 | Vol. 62 No. 11 © Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019 1679 REVIEWS . . . . . . . . . . . . . . . . . . . . . . . . . . SCIENCE CHINA Materials goal is to achieve copper-based soft conductors with a balance of high conductivity and stretchability, as well as high stability in the atmosphere. To this end, a lot of efforts have been devoted to the synthesis of the copper nanomaterials and their implementation in elastomeric conductors [33–35]. Excited by the advancements in the soft electronics enabled by the copper-based conductors, we intend to summarize the recent progress in this emerging field in the review. The following discussion is divided into six parts. The first part discloses our survey on the recent synthesis methods of zero-, one- and two-dimensional (0D, 1D, and 2D) copper nanomaterials, i.e., copper nanoparticles (CuNPs), copper nanowires (CuNWs), and copper nanoflakes (CuNFs). The second part reveals a variety of techniques to assemble copper nanomaterials into macroscopic soft conductors, including spray coating, spin coating, vacuum assisted assembly and transfer, doctor-blade coating, screen printing, and controlled ink patterning. The third part examines the intrinsic electrical and mechanical properties of copper nanomaterials. The fourth part details various strategies to make copperbased conductors less sensitive to degradation. The fifth part further delves into various electronic applications of copper-based soft conductors, exemplified by stretchable conductors, flexible transparent electrode, solar cells, LEDs, electromechanical sensors and wearable heaters. In the last part, we provide our outlook into the potential future directions to address the remaining challenges in the field of copper-based soft conductors and electronics. SYNTHESIS OF COPPER NANOMATERIALS Copper-based soft conductors are usually prepared by manipulating a proper combination between copper nanomaterials and elastomeric or flexible polymers. The network of copper nanomaterials forms conductive pathways to allow the electrons to hop or tunnel through, while the polymers provide a flexible support to tolerate the mechanical deformation of the conductive networks. In these copper nanomaterials/polymer composites, the quality, surface chemistry, and morphologies of copper nanomaterials play crucial roles in affecting the electric conductivity of soft conductors and their behaviors (...truncated)