Exceptional plasticity of silicon nanobridges

Full Electrostatic Control of Nanomechanical Buckling

Buckling of mechanical structures results in bistable states with spatial separation, a feature desirable for sensing, shape configuration, and mechanical computation. Although different approaches have been developed to access buckling at microscopic scales, such as heating or prestressing beams, little attention has been paid so far to dynamically control all the parameters critical for the bifurcation-the compressive stress and the lateral force on the beam. Here, we develop an all-electrostatic architecture to control the compressive force, as well as the direction and amount of buckling, without significant heat generation on micro- or nanostructures. With this architecture, we demonstrated fundamental aspects of device function and dynamics. By applying voltages at any of the digital electronics standards, we have controlled the direction of buckling. Lateral deflections as large as 12% of the beam length were achieved. By modulating the compressive stress and lateral electrostatic force acting on the beam, we tuned the potential energy barrier between the postbifurcation stable states and characterized snap-through transitions between these states. The proposed architecture opens avenues for further studies in actuators, shape-shifting devices, thermodynamics of information, and dynamical chaos.

Crystal orientation-dependent fatigue characteristics in micrometer-sized single-crystal silicon

Repetitive bending fatigue tests were performed using five types of single-crystal silicon specimens with different crystal orientations fabricated from {100} and {110} wafers. Fatigue lifetimes in a wide range between 10⁰ and 10¹⁰ were obtained using fan-shaped resonator test devices. Fracture surface observation via scanning electron microscope (SEM) revealed that the {111} plane was the primary fracture plane. The crack propagation exponent n was estimated to be 27, which was independent of the crystal orientation and dopant concentration; however, it was dependent on the surface conditions of the etched sidewall. The fatigue strengths relative to the deflection angle were orientation dependent, and the ratios of the factors obtained ranged from 0.86 to 1.25. The strength factors were compared with those obtained from finite element method stress analyses. The calculated stress distributions showed strong orientation dependence, which was well-explained by the elastic anisotropy. The comparison of the strength factors suggested that the first principal stress was a good criterion for fatigue fracture. We include comparisons with specimens tested in our previous report and address the tensile strength, initial crack length, volume effect, and effects of surface roughness such as scallops.

Ballistic thermal conductance of a lab-in-a-TEM made Si nanojunction

The thermal conductance of a single silicon nanojunction was measured based on a Lab-in-a-TEM (microelectromechanical systems in a transmission electron microscope) technique and was found to be at least 2 orders of magnitude larger than the ones of long nanowires in the 380-460 K temperature range. The predominance of ballistic phonon transport appears as the best hypothesis to retrieve quantitative predictions despite the geometrical irregularity of the junction. The measurement is based on a MEMS structure including an electrostatic actuator that allows producing nanojunctions with the accuracy based on the resolution of a transmission electron microscope. The thermal conductance is measured by two integrated resistors that are simultaneously heating and measuring the local temperatures at the nearest of the nanojunction. The considerable thermal conductance of short nanojunctions constitutes a new key element in the design of nanosystems and in the understanding of the damaging of mechanical micronanocontacts. This conducting behavior is also paving the way for the development of nanoscale cooling devices as well as of the recent phononic information technology.