Leakage current, self-clearing and actuation efficiency of nanometer-thin, low-voltage dielectric elastomer transducers tailored by thermal evaporation

Abstract

The low-voltage operation is the key challenge for dielectric elastomer transducers (DET) to enter the application field of medically approved actuators or sensors, such as artificial muscles or skin. Recently, it has been successfully shown that the reduction of the elastomer film thickness to a few hundred nanometers allows for the DEA operation reaching 6 % strain using only a few volts. Molecular beam deposition (MBD) enables us to tailor elastomer films with low defect level. Combined with in situ spectroscopic ellipsometry, MBD is a unique method to reliably deposit polydimethylsiloxane (PDMS) thin films with true nanometer precision. The homogenous cross-linking of the PDMS film has been in situ realized by curing through ultraviolet (UV) radiation during deposition. We present the successful tailoring of the elastomer membrane’s elastic modulus down to a few hundreds of kPa by varying the UV-irradiation density. Atomic force microscopy (AFM) nano-indentation reveals homogeneously polymerized membranes. An adhesion layer of thiol-functionalized PDMS is applied to localize gold particles of the electrode layer to prevent diffusion into the nanometer-thin elastomer film and to reduce the leakage current. The understanding of leakage currents of such nanometer-thin elastomer films is crucial to preserve the unique actuation efficiency for DETs in low-voltage operation. Leakage currents are determined for a 200 nm-thin DEA as low as 10-3 A/m2 at applied electric fields of about 80 V/μm just before local breakdown events occur. Known as self-clearing, the vaporization of local defects enables to regain the functionality of the DET with subsequent reduced leakage current. AFM is utilized for the characterization of these DET low-voltage nanostructures regarding their vertical strain and actuation efficiency. A strain-to-voltage-squared (s/V2) ratio of 755 %/kV2 for a single-layer 500 nm-thin DEA is acquired - by far the highest reported (s/V2)-value for thin-film DEAs. A two-layer DET nanostructure is compared to a single layer DET with doubled elastomer film thickness to evaluate the repeatedly discussed stiffening electrode effect. This occurs when DET nanostructures are stacked above hundreds of times, the major challenge remaining to realize biomimetic DET with forces and compliance close to the natural muscles.