Abstract
Future low-voltage dielectric elastomer transducers (DETs), based on nanometer-thin elastomer membranes, will rely on soft and compliant electrodes with reasonable electrical conductivity and sound adhesion to the elastomer. State-of-the-art adhesion promoters, including nanometer-thin Cr/Ti films, result in defects for applied areal strains larger than 3% and lead to increases in the stiffness of DETs. To generate forces in the Newton range, these low-voltage DETs have to be stacked in thousands of layers. Herein, we present a compliant electrode, which consists of gold bonded covalently 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. Peel-off tests demonstrate the expected strong binding of Au to the SH-PDMS network, with this highly stretchable Au/SH-PDMS layer capable of withstanding strains of at least 60%, without losing conductivity. Optical micrographs show signs of 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 increased slightly against the 20-nm-thin Au/PDMS example, but it can be tailored by the cross-linking density of Au/SH-PDMS via the UV irradiation dose. Unloading nanoindentation curves revealed 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-sized Au clusters. This approach provides the means to bind gold clusters homogenously within the SH-PDMS film. Such compliant electrodes are the prerequisite for fabricating low-voltage DETs that can be stretched by more than 50%.
Highlights
Metal films, including Au, have been deposited on elastomer membranes using various cathodic sputtering, electron beam, and thermal evaporation techniques.[13]. Such conductive layers have obtained the clearance of the Food and Drug Administration (FDA) for a variety of medical implants, but they still involve a number of major challenges
The adhesion layers, given a blue color for the plasma treatment and a gray color for the Cr, have a thermal expansion coefficient that differs from the underlying bulk PDMS
We developed a procedure to realize highly compliant electrodes for Dielectric elastomer transducers (DETs)
Summary
Dielectric elastomer transducers (DETs) are smart devices as they can operate as a sensor, an actuator, an electric energy harvester, and a self-sensing actuator.[1,2,3,4] State-of-the-art DETs are based on elastomer membranes with micrometer thickness and are often fabricated by spin-coating, doctor-blading, or PAD-printing.[5,6] These DETs need to be operated at hundreds of volts to reach strains above 20%.7,8 Recently, 200-nm-thin DETs have been manufactured using molecular beam deposition and operated at 12 V.9. Conventional metal films, dominate the mechanical properties of the stack and prevent the desired DET operation.[12] Currently, the electrodes are based on metallic thin films and on carbon powder or nanotubes, or the implantation of metallic nanometer-sized clusters into the elastomer membrane. Metal films, including Au, have been deposited on elastomer membranes using various cathodic sputtering, electron beam, and thermal evaporation techniques.[13] Such conductive layers have obtained the clearance of the Food and Drug Administration (FDA) for a variety of medical implants, but they still involve a number of major challenges. We show that all the challenges listed above can be mastered for the Au-PDMS system by incorporating a nanometer-thin thiol-functionalized PDMS interlayer In this manner, the Au-based 85-nm-thin compliant electrode forms no cracks for strains as large as 60%, acts as a diffusion barrier and remains highly conductive
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