Highly Bendable and Stretchable Electrodes Based on Micro/Nanostructured Gold Films for Flexible Sensors and Electronics

Abstract

DOI: 10.1002/aelm.201500345 of structured gold fi lms (200 nm-thick) transferred onto PDMS and SiO 2 substrates, respectively. To achieve the best adhesion on the receiving elastomers (PDMS/Ecofl ex), they were partially cured prior to the transfer of the structured metal fi lms. It was observed that the presence of bubbles or contaminants at the gold/elastomer interface led to fi lm delamination, resulting in low electrode stretchability. Using the optimized conditions, this simple and inexpensive benchtop method allowed the patterning, structuring, and transferring of thin gold fi lms. The structured fi lms were characterized through electron and optical microscopy before and after the lift-off and the transfer to the receiving substrates. Scanning electron microscopy (Figure 1 D) showed that 20 nm-thick fi lms presented smaller structures than 200 nm ones, because thinner fi lms buckled more readily during shrinking. [ 21 ] Little difference was observed in the morphology for the structured fi lms before lift-off, after lift-off, and after transfer onto elastomer substrates. To confi rm this observation, the surface roughness of the structured fi lms was measured through optical profi lometry. The surface root mean squared roughness was measured to be statistically equal before lift-off, after lift-off, and after transfer to the receiving substrates (Figure S2, Supporting Information). This shows that the fabrication process does not change the physical attributes of the structured fi lms. To test the stretchability of structured Au/PDMS electrodes, their conductivity was tested under strain. The structured electrode resistance was measured in a two-probe setup ( Figure 2 A, bottom inset) at 5% strain increments. Typical I – V curves at 0%, 50%, and 100% strain are shown in the top inset of Figure 2 A. All electrodes remained conductive until the PDMS failed mechanically (110%–130% strain), suggesting that they are excellent candidates for PDMS-based devices. The effect of electrode shape on stretchability was assessed by measuring the conductivity under strain for electrodes with different form factors (defi ned as length-to-width ratio, L / W , Figure 2 A). As L / W increased, the resistance at comparable strains also increased. This is explained by fi lm cracking perpendicular to the stretching axis, which reduces the number of available conductive paths. For electrodes with high L / W , the number of cracks necessary to span the width of the electrode, resulting in a total loss of conductivity, is smaller than for electrodes with low L / W . This is exemplifi ed by Figure 2 A, where the resistance for electrodes with W = 2, 0.8, and 0.8 mm and L / W = 3, 5, and 7.5 at 100% strain is, on average, ≈10%, ≈30%, and ≈100% higher than their initial (relaxed) resistance. Thus, electrodes with large form factors could be useful as strain sensors, while those with small form factors would be robust conductive elements for applications where a constant resistance is required. Simple and inexpensive ways of fabricating electronics on fl exible substrates are in high demand for wearable devices, [ 1 ]