The growing demand for wearable devices that can comfortably conform to the contour of the human body has spurred intensive research into innovative materials, structures, and relevant fabrication techniques. These devices require ultrathin and compliant electronic interconnects that can endure repeated bending and stretching without compromising electrical performance. Conductive patterned ultrathin metal films bonded to compliant elastomeric substrates form wrinkled, noncoplanar, or serpentine features exhibiting fracture resistance and guaranteed electrical conductivity under strains. However, preventing mechanical degradation and cracking in ultrathin metallic materials with large or repeated deformations is still challenging. This would lead to the eventual loss of electrical conductivity of the devices in real-world applications where such mechanical loadings are common.

In this study, we developed a novel approach to enhance the electromechanical performance of ultrathin metal films upon large deformations and repeated stretching by incorporating a graphene layer onto the metal film via chemical vapor deposition (CVD), see Figure 1. This approach entails the direct growth of top-quality graphene onto metal networks that have been patterned using photolithography, thereby bypassing the necessity for post-transfer processes of graphene. The resulting graphene-coated metal films, for example, palladium-graphene films, known as PdGr in this study, offer a multifaceted solution to address the key challenges - cracking and mechanical failure - observed in such devices. Compared to bare Pd films, these devices exhibit significantly enhanced fracture toughness, lower rates of crack nucleation, and stable crack propagation.

The experimental results demonstrate that the CVD-grown PdGr thin film networks (TFNs) can accommodate a higher tensile strain when bonded onto a PDMS substrate than Pd TFNs without an abrupt increase in electrical resistance (ε_Pd = 46.0% and ε_PdGr = 73.8% at normalized resistance of (R−R₀)/R₀= 2). PdGr TFNs also demonstrate a fatigue life that is 2- to 30-fold longer than Pd, particularly evident in thinner films subjected to extensive total strain ranges. Figure 2 presents a representative comparison of electromechanical behaviors of Pd and PdGr TFNs. Utilizing the analytical model for thin film electrical conductance degradation, we determined that the observed enhancement can be attributed to reduced crack nucleation and extension due to graphene's high in-plane strength. The fracture toughness of PdGr films is approximately 2.7 times greater than that of bare Pd films, particularly evident in thinner films, attributed to suppression of dislocation movement by graphene. The dependence of the toughening mechanisms and failure modes on the metal thickness is also discussed. As-grown graphene layer plays different roles in enhancing TFNs' fracture resistance, as depicted in Figure 3. Thinner TFNs exhibit a larger surface-to-volume ratio, facilitating more efficient reinforcement by graphene; however, defects like microcracks and rough edges originating during the thin film fabrication eventually led to intergranular fracture, earlier than transgranular fracture in thicker counterparts. 

The presence of graphene in the PdGr films contributes to extrinsic toughening, leading to improved electromechanical stability and fatigue resistance of the flexible interconnects. This paves the way for the development of reliable and high-performance flexible electronic devices. For more details, please read the original version of the manuscript: https://www.nature.com/articles/s41528-024-00300-8.