Solid-state Superionic Stamping (S4) and Applications
One unique characteristics of ionic transport is that the transport of charge is coupled with mass. And it is this nature that offers a wide range of applications wherein ionic transport is the core. As shown in the figure below, media in which ionic species can move is not only limited to liquids. The materials that support such transport can range from liquid electrolytes to hydrogels, to polymers, and all the way to solids, at the same time, the conductivity range can go from as small as three orders magnitude to several orders. This means that manufacturing processes that are developed based on the transport in liquid media can actually be extended to solids with new advantages associated with those materials.
We have developed a new all-solid, ambient condition electrochemical nanopatterning technique that is capable of generating sub-50 nanometer metallic features is developed. The core of this electrochemical nanoimprint technique is the use of anodic dissolution on a solid electrolyte-metal interface, and the subsequent coupled mass-charge transport in the electrolyte. With such simple yet effective scheme, silver and copper features of 20 nanometers to 10 millimeters resolution with high precision and reproducibility have been demonstrated.
As you can see in the figure, this technique is based on the solid state etching in the metal-solid electrolyte interface. Progressive etching of the metal substrate at its contact points with the pre-patterned electrolyte surface results in replicating the complimentary pattern on the solid electrolyte tool on the metal substrate. Similar to the anodic corrosion in an electrophoretic cell wherein the resulting ions from the oxidation reaction on the anode are carried away by the liquid electrolyte, the metal ions generated by the solid-state etching in this electrochemical nanoimprint migrate through the solid electrolyte tool itself. Unlike the low pattern transfer fidelity issues in electrochemical micro machining (electrophoretic conditions) caused by the un-even current distribution, the etch control in the electrochemical nanoimprint is by contact. As a result, it has superior pattern transfer fidelity and etching control. The figure below shows a typical stamp (left) and the resulting metal features formed by the electrochemical nanoimprint process.
Thanks to the simplicity of such patterning approach, fast turn-around device prototyping is made easy. We have utilized such approach to fabricate novel device designs including optical nanoantennas, metal transistors, and plasmonic and bio sensors. As shown in the figure below.
Current and future developments will be focused on the theoretical analysis of the detailed processes on the metal-solid electrolyte interface and the use of numerical model to optimize the process.
A unique array of applications is enabled by the electrochemical nanoimprint technique because of the plasmonic resonance of the sub-wavelength Ag features produced. Such resonances often times are accompanied by large electromagnetic fields around the Ag features and even higher field concentration in the sharp corners and edges of those features. The use of such enhanced EM-field for sensing purposes has experienced an explosion of interests because chemical detection down to single molecule precision can be achieved with relatively simple spectroscopy equipments, giving it the potential to revolutionize the field of biosensing. Being the metals that have the most prominent resonant response in typical spectral range, silver and gold have received the most research effort in examining how geometrical effect can further enhance their Plasmonic resonance when they are patterned. Through the use of electrochemically nanoimprinted silver features, we have examined the effect of size alone on the EM-field enhancement to Raman scattering of a dye molecule adsorbed on Ag nanofeatures. Current and future effort will be placed on the exploration of novel plasmonic modes in new complex geometries of metallic nanostructures and the their implications on phenomena such as Fluorescence and thermoelectricity.