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

Abstract

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 −3 s −1 ) and nonrecoverable relaxation (creep rate: 9.0 × 10 −8 s −1 ). Several membranes resisted pressures up to 10 kPa without fracturing, indicating that the ultimate biaxial tensile strength of the films was above ∼30 MPa. ■ INTRODUCTIONThin films consisting of ligand-stabilized or cross-linked gold nanoparticles (GNPs) have received considerable scientific attention during the past two decades, and various applications have been demonstrated. For example, transduction elements based on thin GNP films have enabled the fabrication of novel resistive strain gauges, 1−4 touch sensors, 5,6 and chemiresistors. 7 In these sensors the transduction mechanism is based on changes in the interparticle distances, due to either forceinduced strain or sorption-induced swelling. Because the tunneling current between neighboring nanoparticles is exponentially related to their distance, these sensors can afford extremely high sensitivities. Further, nanoparticle networks have great potential for the implementation in next-generation flexible electronics. Very recently, Kotov and co-workers 8 reported on stretchable conductors made from GNP−polyurethane composites enabling electrical tunability of mechanical properties by dynamic self-organization of the nanoparticles under stress.Obviously, the performance of sensors and flexible electronics based on nanoparticle composites critically depends on their specific mechanical properties, e.g., elasticity, viscoelasticity, and ultimate strength. To some extent, these properties have been studied by nanoindentation 9 and forcedeflection measurements employing atomic force microscopes (AFMs). 10−12 In conventional nanoindentation experiments the indenter tip is pressed into the substrate-supported specimen, and the force−distance data are analyzed to extract the material's hardness and reduced elastic modulus. 13 Recently, the micromechanical p...

Freestanding Membranes of Cross-Linked Gold Nanoparticles: Novel Functional Materials for Electrostatic Actuators

“…23 In good agreement with the experimental results, 14 it is based on the assumption of a spherical bulge geometry.…”

“…1−6 Further, these materials enable the costefficient fabrication of devices via laser writing, printing, and self-assembly techniques on various substrates. 7−11 Recently, the mechanical properties of freestanding membranes comprised of ligand-stabilized gold nanoparticles (GNPs), 12,13 cross-linked GNPs, 14 or polymer/GNP composites 15 have been studied in different laboratories using atomic force microscopy (AFM) indentation or micro bulge tests. These investigations showed that GNP membranes are mechanically surprisingly robust.…”

“…17 This was demonstrated in a recent study, in which we investigated the material's elastic and viscoelastic properties via AFM bulge tests and measured a Young's modulus of ∼2.5 GPa for 1,9-nonanedithiol-cross-linked GNP membranes. 14 In principle, the unique mechanical, optical, and charge-transport properties of GNP composites can be tuned by adjusting the particle size and shape or the size and structure of the cross-linker. For example, because charge transport in these membranes relies on thermally activated tunneling, the conductivity can be tuned over several orders of magnitude simply by using different length ADT linkers to tune the interparticle distances.…”

Modeling and Measuring Viscoelasticity with Dynamic Atomic Force Microscopy

“…An increasingly important application of the Atomic Force Microscope (AFM) is the characterization of viscoelastic materials and interfaces, such as cell membranes and tendons [1][2][3][4], polymer blends and composites [5][6][7][8][9][10][11], the liquid-gas and liquid-solid interfaces [12,13], and suspended membranes [14]. The present trend toward higher scanning speeds [15,16] and higher resolution mapping of mechanical properties [17,18] necessitates more rapid tip motion and therefore a larger viscous contribution to the tip-sample force.…”

Gold nanoparticle superlattices: structure and cavities studied by GISAXS and PALS

“…Assemblies of ligand-stabilized noble metal nanoparticles are of great interest because of their unique optical, 1 electrical 2 and mechanical 3,4 properties, which all can be tuned by varying the materials' composition and structural parameters. This provides the opportunity to tailor materials with desired properties for various specic applications like resistive [5][6][7] or optical 8,9 strain and vapor sensing, catalysis, 10 nanoelectromechanical systems, 11,12 and surface enhanced Raman scattering.…”