Self assembly based molecular devices

Planar micro-supercapacitors are emerging as necessary devices for rapid energy sourcing in on-chip circuitry. A severe drawback in micro-supercapacitor is the energy density value, which is limited by the working potential of the electrolyte used in it (typically below 3V). In this regard, the nominee’s group has developed a fabrication recipe for a planar supercapacitor with an operating voltage of 8V, highest among the reported till date. The electrolyte used here is in the form of 1D supramolecular nanofibres, consisting of coronene tetracarboxylate and dodecyl methyl viologen, carrying K+ and Br- ions respectively. The nanofibres are drop-spread across the Ti micro-gap electrodes, wherein the ambient formed TiO2 surface provides the electrochemical stability. The device exhibits typically a capacitance value of 0.2 mF/cm2 under ambient humidity (65% RH), which was enhanced to 9.5 mF/cm2 under 90% RH in the presence of hygroscopic KBr crystallites. Further, it has been shown that dielectrophoresis assisted assembly of the nanofibres along the micro-gap using an external AC voltage of 2V at 500Hz, could again boost the capacitance value by an order and thus, a remarkable value of 388 mF/cm2 has been achieved. Other relevant parameters such as energy and power density, scan rate stability and capacity retention also fall in line making the device overall high performing.

Another aspect studied in connection with the supramolecular fibres is the response to humidity. The fibres are poorly conducting under dry conditions but gain high conductivity when exposed in humid air. This aspect has been exploited in fabricating a sensitive humidity sensor with unprecedented fast response and recovery times. The sensor is demonstrated to work as a breath sensor for monitoring the respiration rate as well as hydration levels in individuals. The fibres have also served as active channel in field effect transistors and from such devices, high mobility values have been derived. These studies clearly bring out the nature of charge transfer interactions responsible for the interesting set of properties observed with this 1D system.