Abstract
The nanomechanical properties of ultrathin and nanostructured films of rigid electronic materials on soft substrates are of crucial relevance to realize materials and devices for stretchable electronics. Of particular interest are bending deformations in buckled nanometer-thick films or patterned networks of rigid materials as they can be exploited to compensate for the missing tensile elasticity. Here, we perform atomic force microscopy indentation experiments and electrical measurements to characterize the nanomechanics of ultrathin gold films on a polydimethylsiloxane (PDMS) elastomer. The measured force-indentation data can be analyzed in terms of a simple analytical model describing a bending plate on a semi-infinite soft substrate. The resulting method enables us to quantify the local Young’s modulus of elasticity of the nanometer-thick film. Systematic variation of the gold layer thickness reveals the presence of a diffuse interface between the metal film and the elastomer substrate that does not contribute to the bending stiffness. The effect is associated with gold clusters that penetrate the silicone and are not directly connected to the ultrathin film. Only above a critical layer thickness, percolation of the metallic thin film happens, causing a linear increase in bending stiffness and electrical conductivity.
Highlights
Integration of advanced microelectronic sensor and actuator technology into devices with soft and stretchable mechanical properties is a major challenge for electronic materials science and device engineering.[1,2] Low-invasive biomedical implants,[3,4] soft robotics,[5] or electro-mechanical energy harvesters,[6] all rely on deformable electronic devices that are compliant to a mechanically demanding environment while maintaining their electronic functionality
In an atomic force spectroscopy experiment, we push the tip against the surface and measure how the force increases as a function of the sample indentation
Our work demonstrates how Atomic force microscopy (AFM) indentation experiments can be performed and interpreted to investigate the nanomechanics of hard nanometer-thick metallic films on soft elastomer substrates
Summary
Integration of advanced microelectronic sensor and actuator technology into devices with soft and stretchable mechanical properties is a major challenge for electronic materials science and device engineering.[1,2] Low-invasive biomedical implants,[3,4] soft robotics,[5] or electro-mechanical energy harvesters,[6] all rely on deformable electronic devices that are compliant to a mechanically demanding environment while maintaining their electronic functionality. Recent research demonstrated how structural engineering at the micro- and nanoscale permits to combine such hard materials with soft and elastic substrates, resulting in overall stretchable properties.[1] The progress relies on the fact that hard inorganic materials can be bent with small forces, when patterned into ultrathin layers or nanowires.[7] Such bending deformations can be exploited to compensate tensile strain during device stretching Examples of this approach are stretchable serpentine conductor lines,[8] buckled conducting thin films,[9] a kirigami,[10] or island-based conducting network structures.[11,12] So far, development and optimization of the stretchable surface patterns has been based on the analogy to their macroscopic counterparts and empirical findings. The voltage is measured on the opposite side as in the standard van der Pauw setup.[35,36]
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