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Introduction [top]

The Greer Group research focuses on the problems of unraveling the physical origins of size-dependent strength in nano-scale solids, where the presence of surfaces causes the emergence of unexpected deformation mechanisms in response to mechanical deformation.

Nano-Louvre: overplated 50 nm Au nanopillar.
(image courtesy D. Jang)

It has been shown that when the sample size is reduced not only vertically (i.e. thin films) but also laterally, the mechanical properties of single crystals, for example, drastically differ from those of their bulk counterparts. They are thought to arise from the distinct defect behavior that emerges as a result of reducing material dimensions to the nano-scale and manifest themselves by causing unusual mechanical properties.

These characteristics include avalanche-like stochastic stress-strain signature, size-dependent strength, and tension-compression asymmetry - prevalent only in those structures where the surface area is significantly higher than their volume, i.e. sub-micron scale.

While these studies provide a powerful foundation for the fundamental deformation processes operating in these materials at small scales, they are a far reach from representing real materials, whose microstructure is often complex, containing boundaries and interfaces.

Nano-popsicles: overplated nano-twinned Cu nanopillars.
(image courtesy D. Jang)

In fact, both homogeneous interfaces (grain boundaries, twin boundaries, etc.) and heterogeneous interfaces (phase boundaries, precipitate-matrix boundaries, and free surface) in size-limited features are crucial elements in structural reliability of most modern materials.

Establishing the link between the observed mechanical properties and microstructural evolution remains a grand challenge, and one of my major research goals is in establishing a more quantified, predictive relationship between the competing factors of intrinsic and extrinsic limitations on the overall material properties.

Our key research thrusts lie in the development of innovative experimental approaches that enable us to assess nano-scale mechanical properties, and in subsequent design and fabrication of new, innovative materials with tunable desired properties.

Main research activities [top]

1. Mechanical properties and deformation mechanisms of nano-scale materials
2. Radiation damage tolerance in interface-containing metallic nano structures
3. Materials with controlled microstructural architecture: energy sciences and structural applications
4. Deformation response of vertically aligned carbon nanotube (VACNT) forests
5. Mechanical stability of polymer-embedded Si nanowire arrays for flexible solar panels
6. Innovative materials for low-temperature electronics for space applications
7. Bio-inspired design of damage-tolerant materials
8. Characterization of energy dissipation processes in gravitational measurements in LIGO
9. Transport across grain boundaries in Ceria
10. Mechanical-electrical coupling in graphene-based devices

Stress-strain response of a carbon nanotube foam (CNTs) under compression (courtesy S. Hutchens).

TEM images of nano-twinned Cu nanopillars (courtesy D. Jang).