Zinc oxide (ZnO) 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 techniques. ZnO nanostructures can take the form of nanowires, nanobelts, nanorings, and nanohelices, with demonstrated potential for novel nano-scale piezoelectric devices. The ability to create such devices with predictable and reproducible performance depends on having knowledge of the mechanical properties of ZnO nanostructures.
Measurements of the normal and tangential elastic moduli of nanowires.
The small dimensions of these nanostructures raise serious challenges for experimental measurements of properties relevant to device applications. An example is that there are conflicting reports of the elastic modulus (the proportionality constant relating stress and strain quantifying the resistance to elastic deformation) and its dependence on wire diameter for ZnO nanowires (NWs): elastic moduli reported so far span the range 30 GPa to 250 GPa and are often found to have no specific size dependence. There is thus a clear need for different and more accurate ways to measure the elastic properties of NWs.
To address this measurement need, NIST Nanomechanical Properties Group researchers employed contact-resonance atomic force microscopy (CR-AFM) and AFM-based friction-type measurements to determine the radial indentation modulus and in-plane shear modulus of ZnO NWs with diameters in the range 250 nm to 25 nm. In CR-AFM measurements the changes in resonance frequency of an AFM cantilever are measured as a probe attached to the cantilever is brought into contact with a structure, in this case  ZnO NWs and single-crystal Si reference surfaces. The changes in frequency are related to the contact stiffness of the probe-structure interface, and knowledge of the contact geometry thence provides a measurement of modulus. Key features of the NW work, which extends the measurement capabilities of CR-AFM and improves the accuracy of modulus measurements, were the use of an elliptical contact area analysis appropriate to a spherical probe-cylindrical NW contact geometry and the use of dual reference surfaces, here Si (100) and Si (111), to eliminate uncertainty associated with probe modulus. Both indentation modulus and shear modulus increased significantly with decreasing ZnO NW diameter, from the values of bulk ZnO at 250 nm to factor of two increases at 25 nm. A core-shell model, consisting of a stiffened 12 nm shell coaxial with a bulk-like core, was used to describe the increase.
The CR-AFM methodology demonstrated modulus measurements with about 10% precision (experimental uncertainty) and 3% accuracy (the modulus values are traceable to the elastic constants of Si). Use of the technique will enable optimization of the processing methods for NWs and development of advanced devices incorporating these and other nano-scale entities.
The measurements and methodology were published in “Diameter-dependent Radial and Tangential Elastic Moduli of ZnO Nanowires,” G. Stan, C.V. Ciobanu, P.M. Parthangal, and R.F. Cook, Nano Letters 7 (2007) 3691-3697