Next-generation electronic devices, based on micro- and nanoelectromechanical systems, require accurate knowledge and control of material properties at ultra-small scales. As the size of device elements is reduced further and further, down to the nanoscale, mechanical properties in particular can exhibit significant variations from those of their bulk counterparts due to the increase in the surface-to-volume ratio. The inherent effects of proximate surfaces and interfaces alter the properties of the nanosize entities (e.g., nanowires) or those of reduced-scale constituents (e.g., crystallites in nanostructured materials) and modify the mechanical response of the assembly as a whole. For some particular geometries it is possible to deduce the response of the nanosized constituents by testing an assembly at the macroscale. However, it is obviously more desirable to directly probe the local mechanical properties of a nanostructured material.
To address this measurement need, Nanomechanical Properties Group researchers developed a methodology combining contact-resonance atomic force microscopy and scanning tunneling microscopy (CR-AFM and STM) to determine the elastic moduli of materials with nano-scale features that exhibit nano-scale topography. On nanosize granular Au films the elastic modulus at the grain scale was mapped out—with 10 nm spatial resolution—through use of a self-consistent deconvolution of contact-geometry effects in the CR-AFM image. Significant variation in the contact area over granular topography arises as the CR-AFM probe is either in single- or multiple-asperity contact with the surface; such variation was determined through separate STM topographic measurements. In extracting the elastic modulus from CR-AFM measurements on granular surfaces, both the normal and lateral couplings established through multiple-asperity contacts between the tip and the surface were considered. By appropriately considering the change in the contact mechanics during CR-AFM imaging, variations in the elastic modulus were revealed in the intergrain regions as well as across individual grains. The methodology can be extended to other nanoscale structured materials in which consideration of topography-induced artifacts is necessary. The measurements and methodology were published in Nanotechnology 19 (2008) 235701.
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