Harnessing solid ionics for nanomanufacturing

Capable of transporting mass and charge simultaneously, liquid electrolytes are the core of many technologies such as energy conversion and numerous manufacturing processes. Sharing the same coupled mass-charge transport properties with their liquid counterpart, solid-state ionic materials have gained numerous interests in their transport properties and the discovery of new materials that exhibit this coupled mass-charge transport phenomenon since the 1890's when Nernst discovered the ionic transport in refractory oxides. Not until a few decades ago has research begun to take advantage of this unique property for applications in areas such as high performance batteries, fuel and electrolysis cells, chemical sensors, as well as thermoelectric converters [1-3]. More recently, several physical phenomena that are coupled with this unique transport in solid electrolytes have started to strike curiosity in quantitative understanding the underlying mechanism and the interest in exploiting them to develop novel applications. The dissolution of metal on its interface with a solid electrolyte, the-order-of-magnitude increase in electrical conductivity while a junction is bridged by an ionic species, the induced strain (as high as 20%) of the solid electrolyte matrix when allowing intercalation of mobile ions, and the thermal energy carried by ionic species while migrating through solid matrix of an ionic crystal are phenomena that have potential in revolutionizing the field of nanopatterning, transistors, sensing and actuation, and energy conversion with the enhancement of these properties at nano-scale. As illustrated in figure 1, these aspects of the behavior of solid ionic materials stem from the coupled charge-mass transport and will be the main theme of this research proposal.

    With the thorough investigations and understandings in the kinetics of a metal-solid electrolyte interface that have been established through the past few decades, there has only been very little work on developing a patterning technique that is based on controlled corrosion of metal on its interface with a solid electrolyte. Similar to its liquid counterpart whose electrolytic behavior has long been exploited for electrochemical machining, solid electrolytes have the same ability to dissolve metal substrate upon contacting a metallic surface whose electron potential is altered. Superior to liquid electrolytes whose flow can affect electrical current distribution and can cause non-uniform erosions of materials, solid electrolytes are ideal candidates for stamping-type of patterning due to its structural integrity. One of the main goals of this research proposal is to develop a stamping-type technique based on the dissolution of metal-solid electrolyte interface and the coupled charge-mass transport in the solid electrolyte. This approach has high potential in creating a new patterning technique that will revolutionize micro-/nano-patterning of metal.  

 In fulfilling the need for faster electronics, numerous achievements in the field of nanoelectronics have been made for the past few decades in light of the new properties materials exhibit at nanoscale. An example is the quantized conductance of nano-scale metal wires and junctions that is seen when lateral dimension of the junction is close the to Fermi wavelength of the electrons transmitting through these nano-features; under this condition, the ballistic nature of the electron transport give rise to a higher conductance than that of its bulk form. Conventionally, fabricating metallic nano-scale metallic wires and junctions has relied on lithographic methods and liquid state electrolytic growth control which are expansive, difficult, and not environmental friendly. Finding a new way of producing these nano-structures in a reversible manner to either complement or replace existing methods is the key to taking the full advantage of the use of nano wires and junctions for constructing integrated electronics and nano-scale devices. As another main goal of this research proposal, the methodology of using solid electrolyte to generate nano-scale metallic wires and junctions will be explored.  

    In the field of micro-\nano-electromechanical systems, the pursuit of actuation with high range of motion, high speed, as well as low energy consumption has been one of the main-stream research areas of intensive study. Materials that change their dimensions upon external stimuli, such as electrical potential, have long been candidates for actuation purposes. Some of the best known examples include piezoelectric and electrostrictive ceramics, who have been well studied and employed to construct actuators with a wide rang of performance. While the use of piezoelectric materials for actuator is well-developed and off-the-shelf piezoactuators are easily accessible, the use of high voltage for driving and limited range of motion have called for a search in new materials that can be developed into actuation mechanisms for applications where piezoactuation is impractical. Recently attracted much attention, the large strain experienced by solid electrolytes such as AgxV2O5, AgxTi2S, etc. upon intercalation of other ionic species is a new candidate for constructing actuators. Its low activation energy for ionic migration and enhanced ionic transport properties in the space charge layer give the solid compound traits such as lower driving energy and fast response that are ideal for actuators. The third main objective of this research proposal is to develop an actuation mechanism by exploiting the control strain obtained upon regulated ionic intercalation and de-intercalation.