Diameter-Dependent Radial and Tangential Elastic Moduli of ZnO

We show how contact resonance atomic force microscopy (CR-AFM) can be used to accurately determine the radial elastic moduli of [0001] ZnO nanowires with diameters smaller than 150 nm. Using a realistic tip−nanowire contact geometry, we find the radial indentation modulus from CR-AFM data and the tangential shear modulus from friction-type measurements. Both moduli show a pronounced increase when the wire diameter is reduced below 80 nm. The size dependence of the elastic properties can be explained by a core−shell model that accounts for a bulk-like core and an elastically stiffer surface layer. Zinc oxide nanostructures hold tremendous promise for novel and versatile devices due to their intrinsic semiconducting and piezoelectric properties, as well as the amazing structural diversity that can be achieved by current synthesis tech-niques.1 ZnO nanostructures can take the form of nanowires, nanobelts, nanorings, and nanohelices,2 with demonstrated potential for novel nanoscale piezoelectric devices.3 The ability to create such devices with predictable and reproduc-ible operation depends, at least in part, on having knowledg