Elastic and Viscoelastic Properties of Cross-Linked Gold Nanoparticles Probed by AFM Bulge Tests

To enable applications of nanoparticle films in flexible electronics, actuators, and sensors, their mechanical properties are of critical concern. Here, we demonstrate that the elastic and viscoelastic properties of covalently cross-linked gold nanoparticles (GNPs) can be probed using AFM bulge tests. For this purpose 30–60 nm thick films consisting of 1,9-nonanedithiol (NDT) cross-linked GNPs (3.8 nm core diameter) were transferred onto substrates with ∼100 μm circular apertures. The resulting freestanding membranes were bulged by applying pressure differences of up to 10 kPa, and the deflection was measured by intermittent contact atomic force microscopy (AFM). Analyzing the pressure-deflection data using the spherical cap model, either by taking into account the peak deflection values or the measured arc profiles of the bulge, yielded 2.3 ± 0.3 and 2.7 ± 0.4 GPa for Young’s modulus, respectively. When cycling the stress–strain measurements at overpressures up to 2.4 kPa, hysteresis was observed and assigned to viscoelastic effects. Creep tests performed at a pressure of 2 kPa revealed both viscoelastic retardation (time constant: 3.3 × 10<sup>–3</sup> s<sup>–1</sup>) and nonrecoverable relaxation (creep rate: 9.0 × 10<sup>–8</sup> s<sup>–1</sup>). Several membranes resisted pressures up to 10 kPa without fracturing, indicating that the ultimate biaxial tensile strength of the films was above ∼30 MPa.