Highly compliant nanometer-thin Au electrodes exploiting the binding to thiol-functionalized polydimethylsiloxane films

Future low-voltage dielectric elastomer transducers (DET) based on nanometer-thin elastomer membranes rely on soft and compliant metal electrodes with reasonable electrical conductivity and sound adhesion to the elastomer. State-of-the-art adhesion promoters, including nanometer-thin Cr/Ti films, form defects for applied areal strains larger than 3% and lead to the stiffness increase of DETs. To generate forces in the Newton range, these lowvoltage DETs have to be stacked to thousands of layers. Herein, we present a compliant electrode, which consists of gold covalently bonded to thiol-functionalized polydimethylsiloxane (SH-PDMS) films. The membranes were fabricated using molecular beam deposition and in situ and/or subsequent ultraviolet light (UV) radiation. Peeloff tests demonstrate the expected strong binding of Au to the SH-PDMS network. The highly stretchable Au/SHPDMS layer withstands strains of at least to 60% without losing the conductivity. Optical micrographs exhibit cracks for strained pure Au and Au/Cr electrodes but not for the Au/SH-PDMS layer. The mechanical properties and adhesion forces of Au/SH-PDMS were extracted by means of atomic force microscopy (AFM) using a spherical Au tip coated with methyl groups (CH₃). The elastic modulus of (12 ± 9) MPa is slightly higher than for 20 nmthin Au/PDMS, but can be tailored by the cross-linking density of Au/SH-PDMS via the UV-irradiation dose. Unloading nanoindentation curves revealed the pull-off forces between the CH₃-functionalized AFM tip and the Au/SH-PDMS layer at the time of separation. For Au/SH-PDMS, the spectral distribution of pull-off forces exhibits repulsive forces with the CH₃ groups of the PDMS network as well as adhesive forces resulting from interactions with the nanometer-size Au clusters. This approach proves the way to homogenously bind gold clusters within the SH-PDMS film. Such compliant electrodes are the prerequisite to fabricate low-voltage DETs stretchable by more than 50%.