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The design and implementation of micro-/nano-structures as active components has become an increasingly popular scheme for the development of electronic devices and systems in pursuit of unprecedented device performance and unique system functionality. In particular, cooperative actuation of active structures assembled in parallel results in emerging concepts of electromechanical devices and systems with tunable characteristics enabled by exploiting all the degrees of design freedom available to the active components. Engineering such micro-/nano-structures requires precise and scalable fabrication with resolution down to the nanometer-scale, often beyond the capabilities of conventional processing techniques. Exploring these opportunities could contribute substantially to the understanding of fundamental physical phenomena and the upsurge of novel device structures and concepts, as well as pushing the frontier of nanoscale fabrication and metrology.
In this thesis, challenges and opportunities in engineering active micro-/nano-structures for electromechanical actuation are explored through two case studies, a tunneling nanoswitch and an acoustically-active surface, which aim to present paradigms for high-performance electromechanical devices and systems designed based on cooperating active micro-/nano-structures.
The tunneling nanoswitch utilizes molecules as active nanoscale springs. An ensemble of such molecular springs takes the form of a self-assembled molecular layer sandwiched between ultra-smooth bottom electrodes and an active top nanoparticle contact. This nanoswitch operates by electromechanical modulation of the current tunneling through the nanometer-scale molecular switch gap. This unique mechanism enables the device to demonstrate a low turn-on voltage (under 3 V) and a short delay (2 ns) simultaneously, which are among critical challenges facing nanoelectromechanical (NEM) switches. Significantly, the molecular layer and the top nanoparticle contact serve as two degrees of design freedom with which to independently tailor static and dynamic device characteristics, thereby enabling a path towards sub-1-V switching in the GHz regime for electromechanical logic.
The acoustically-active surface depends on widely distributed, microstructured piezoelectric transducers as active components. One example of such an acoustic surface is a PVDF film embossed with active micro-domes in an array. Existence of these freestanding micro-domes actuating in parallel significantly enhances the acoustic performance and allows our acoustic surface to achieve a unit-area sensitivity of 0.2075 mPa/(V∙cm2), which well outperforms existing flexible loudspeakers. The acoustic response can be further improved by engineering the profile and dimensions of the micro-domes. In addition, coordinating actuation of the micro-domes based on adaptive amplitude and phase control could enable directional audible sound generation, currently unavailable based on standalone commercial loudspeakers. The outstanding acoustic performance, attractive features (wide-area, thin, flexible, low-cost and even transparent), and unique functionality make the acoustic surface promising for broad emerging application scenarios.
Creating high-performance active structures in these case studies has motivated our development of novel processing techniques, including scalable manipulation of nanomaterials and low-cost, high-precision micro-embossing of polymer thin films. The highly uniform, mechanically tunable molecular junctions and the wide-area, phased acoustic micro-transducer array provide promising platforms for scientific studies of fundamental physical phenomena and innovations in diverse electronic devices and systems. |
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