Effect of ammonia gas etching on growth of vertically aligned carbon nanotubes/nanofibers

The etching effect of ammonia （NH3） on the growth of vertically aligned nanotubes/nanofibers （CNTs） was investigated by direct-current plasma enhanced chemical vapor deposition （DC-PECVD）. NH3 gas etches Ni catalyst layer to form nanoscale islands while NH3 plasma etches the deposited amorphous carbon. Based on the etching effect of NH3 gas on Ni catalyst, the differences of growing bundles of CNTs and single strand CNTs were discussed; specifically, the amount of optimal NH3 gas etching is different between bundles of CNTs and single strand CNTs. In contrast to the CNT carpet growth, the single strand CNT growth requires shorter etching time （5 min） than large catalytic patterns （10 rain） since nano dots already form catalyst islands for CNT growth. Through removing the plasma pretreatment process, the damage from being exposed at high temperature substrate occurring during the plasma generation time is minimized. High resolution transmission electron microscopy （HTEM） shows fishbone structure of CNTs grown by PECVD.

Buckled MEMS Beams for Energy Harvesting from Low Frequency Vibrations

Vibration energy harvesters based on the resonance of the beam structure work efectively only when the operating frequency window of the beam resonance matches with the available vibration source.None of the resonating MEMS structures can operate with low frequency,low amplitude,and unpredictable ambient vibrations since the resonant frequency goes up very high as the structure gets smaller.Bistable buckled beam energy harvester is therefore developed for lowering the operating frequency window below 100Hz for the frst time at the MEMS scale.Tis design does not rely on the resonance of the MEMS structure but operates with the large snapping motion of the beam at very low frequencies when input energy overcomes an energy threshold.A fully functional piezoelectric MEMS energy harvester is designed,monolithically fabricated,and tested.An electromechanical lumped parameter model is developed to analyze the nonlinear dynamics and to guide the design of the nonlinear oscillator based energy harvester.Multilayer beam structure with residual stress induced buckling is achieved through the progressive residual stress control of the deposition processes along the fabrication steps.Surface profle of the released device shows bistable buckling of 200μm which matches well with the amount of buckling designed.Dynamic testing demonstrates the energy harvester operates with 50%bandwidth under 70Hz at 0.5g input,operating conditions that have not been demonstrated by MEMS vibration energy harvesters before.