Nanomechanical characterization of SU8/ZnO nanocomposite films for applications in energy-harvesting microsystems

Integration of piezoelectric zinc oxide (ZnO) nanoparticles with SU8 in the form of photo-patternable nanocomposite films can lead to the development of a new generation of energy-harvesting microdevices. Design of such energy-harvesting micro/nano-systems will require knowledge of the mechanical properties of the SU8/ZnO nanocomposite thin films for various loadings of ZnO. This work presents characterization of mechanical properties of SU8/ZnO nanocomposite films with ZnO concentration varying in the range of 0–25 wt% via quasi-static and dynamic nanoindentation. These films were fabricated using conventional microfabrication steps involving dispersion of ZnO in SU8 by ultrasonication, followed by spin-coating and UV exposure. The elastic modulus obtained via quasi-static nanoindentation varies from ∼6.2 GPa for pristine SU8 to ∼8.8 GPa for SU8/25 wt% ZnO nanocomposite, while hardness varies from 402 MPa to ∼520 MPa for SU8/ZnO nanocomposites in the same range of ZnO wt%. The experimentally-obtained elastic modulus has also been compared with estimates obtained via Eshelby–Mori–Tanaka micromechanics. Storage modulus, loss modulus and loss factor obtained via dynamic nanoindentation tests indicate that the SU8/ZnO nanocomposites exhibit viscoelastic behavior in the studied frequency-range of 10 Hz to 201.5 Hz. Microstructural characterization via scanning electron microscopy and optical characterization via UV–vis spectrometry of the nanocomposites have also been reported. (paper)