Seminar on In-situ Nanomechanics of Crystalline Nanowires

Crystalline nanowires are not only important building blocks for a broad spectrum of applications such as flexible and stretchable electronics, but also an ideal platform to study fundamental nanomechanics. In this talk, I will start with recent advances in in-situ TEM nanomechanical testing based on microelectromechanical systems (MEMS), including high-temperature testing, high-strain-rate testing, and displacement-controlled testing with feedback control. Next, I will discuss several recent studies using MEMS-based in-situ TEM testing: 1) Effect of free surface and internal boundaries on the plasticity mechanisms (twinning vs. slip). We observed competition of twinning and slip in single-crystalline metallic nanowires – dependent on the cross-sectional shape of the nanowire – which was attributed to the change of surface energy associated with each mechanism. We also found that twin boundaries (either one or five) along the nanowire length direction can cause interesting recoverable plasticity and Bauscinger effect. 2) Hydrogen embrittlement. We probed hydrogen embrittlement using metallic nanowires as a model system. We found increasing yield strength and brittle failure with the presence of hydrogen, which was attributed to the hydrogen-induced suppression of dislocation nucleation at the free surface. 3) Brittle to ductile transition. We found that the crystalline Si nanowires under tension are brittle at room temperature but exhibit ductile behavior with dislocation-mediated plasticity at elevated temperatures. We revealed that unconventional ½<110>{001} dislocations become activated with increasing temperature. 4) Anelasticity in nanowires and 3D nanolattices. We observed strong anelastic behaviour in single-crystalline nanowires and more recently in 3D ultrathin nanolattice structures. Experiments and theoretical studies found that the origin of the anelasticity is stress gradient driven point defect diffusion. I will conclude the talk with challenges and perspectives of in-situ nanomechanical testing.