We are interested in the mechanical and thermomechanical properties of nanostructures and nanostructured materials, in particular the elastic modulus and thermal expansion coefficient. These properties can be relevant for applications including thermal interfaces in automotive waste heat recovery applications, for which the modulus influences the stress state of the bonded heat exchanger and thermoelectric materials.  Another application is for phase change materials and devices, for which the modulus and CTE influence void formation and other failure modes.  The mechanical properties of nanofilms and nanostructures can be exceptionally difficult to isolate.

We have developed a technique for measuring mechanical properties using a silicon MEMS resonator diving board structure and a laser vibrometer.  The resonators are fabricated from silicon-on-insulator (SOI) substrates using lithography and etching techniques in the Stanford Nanofabrication Facility.  The sample material is grown or deposited using catalysis or other facilitating materials.  The resonant frequency is measured by subjecting the resonator to background white noise vibrations and measuring the mechanical response using the laser vibrometer.  Comparative measurements with and without the nanostructured film are used to isolate the shift in resonant frequency that is induced.  We also measure the q-factor, or ringdown, to determine the impact of the nanostructured material on energy absorption by the resonator.

In addition to developing these unique resonator measurements for nanostructures, we have been using and extending existing AFM indentation techniques to determine the local modulus in nanomaterials.  These methods use a compact sphere at the tip of an Atomic Force Microscope tip and measure the force-displacement curve, which is governed by both modulus of the sample in the near-surface region.  The AFM and resonator approaches are complimentary by focussing on highly localized (AFM) and in-plane (resonator) components, respectively, and are enabling unprecedented detail in the characterization of anisotropy and nonhomogeneity in the mechanical properties.  When combined with thermal and electrical property measurements, we are able to provide a more comprehensive data set in support of applications ranging from thermoelectric waste heat heat recovery to phase change nanodevices.

Resonator SEM
silicon resonators of varying length before sample growth
MEMS Resonator Schematic
schematic of resonator structure depicting both mechanical and thermal characterization
Resonator Fabrication
resonator fabrication procedure starting from silicon-on-insulator (SOI) substrates
AFM mechanical
complimentary mechanical characterization using an atomic force microscopy tip (see Gao et al. 2012)