• Manufacture graphene films suspended on 1-micron-diameter circular holes by exfoliating bulk graphite
• Mechanical tests were employed on those graphene films by indenting the circular films at the center with AFM (atomic force microscope). Experimental data were analyzed by the model of "center-point load on circular membrane". The results confirmed the excellent mechanical properties of graphene—the strongest material in the world (work published on Science July 2008)
• Build up numerical models (such as finite element simulation, molecular dynamics simulation and ab initio simulation) for further studies on graphene including mechanical properties, etc.

For traditional materials, as we are familiar with, their stress-strain behavior has elastic and plastic regions. When a specimen deforms into plastic region, the plastic deformation will stay as a constant vs. time. However, for the nanocrystalline Cu films we fabricated with grain size of about 40 nm, a phenomena called “plastic strain recovery” or “healing” is found. Our group found this phenomena independently and at the same time, Saif’s group in IL also found the similar phenomena for nanocrystalline Au and Al.

Nanocrystalline (n-) Cu films with average grain size of about 40 nm are fabricated by thermal evaporation. Film thickness is between 100 ~ 500 nm and these free-standing films are sitting on Si wafer with a rectangular window. Mechanical properties of those n-Cu films are characterized by plane-strain bulge test. Yield stress and Young’s modulus are obtained.

(Cross-sectional view of nanoporous Au film on Si wafer)

Nanoporous materials has many potential applications due to their special character: very large surface/volume ratio. For example, if people put a nanoporous gold cantilever into a certain circumstance, such as a gas, the nanoporous structure might be sensitive to the gas so that the surface energy would vary subsequently. The change of surface energy would be indicated by a certain mechanical response. Thus, it is of interest to fabricate specimens with nanoporous structure and characterize their mechanical properties.

(Double-Anchored Freestanding Nanoporous Au Beam)

We fabricate nano Au single crystal beams with the similar process for the nanoporous Au beams. The same nano-indentation test is employed on those beams. Analyzing the data by the same way as for the nanoporous Au beams, we get the information of the modulus of Au along the beam direction, maximum resolved shear stress when slip occurs and etc.

Plane-strain Bulge Test, introduced by Vlassak and Nix, is a powerful approach to test the mechanical properties of nano-scaled thin films. A free-standing sub-micron thick film is deposited on a rectangular window (2a X 2b) which can be manufactured with the standard MEMS techniques on Si wafer. Applied with pressure on the backside, the film deforms under the plane-strain condition.