Characterisation on Switching Behavior and Redox-State Fluctuation of Phosphomolybdic acid Molecule Functionalised on Carbon Nanotube
Polyoxometalate (POM) is molecule that displays various properties such as multi-redox, luminescence, catalysis, and magnetism.1)Due to theseremarkable properties, development of super-capacitor, flash-type memory cell, solar cell, light-emitting device, and impulse generator for computing system that contain large number of POM has been made possible.2,3)On the other hand, there are very few reports on single-POM devices,2)thereby limiting further possibledevelopment and application which cannot be attained by largePOM system. Our research group has previously reported the generation of unique random telegraph signal (RTS) due to the redox-state fluctuation of a single phosphomolybdic acid molecule (hereinafter PMo12, which is a type of POM) adsorbed to the sidewall of carbon nanotube (CNT).4)Another research group has performed X-ray adsorption near-edge structure (XANES) analysis on PMo12, and discovered its 24-electron multi-redox property.1)Despite these interesting discoveries, the switching behavior of a single PMo12 has not been examined comprehensively. Motivated by the RTS generation and discovery of 24-electron multi-redox property in PMo12, we fabricated single-PMo12 device in order to study and characterise its switching behavior.A.C. dielectrophoresis was applied to position few CNTs across1μm-gapped electrodes. This device was then soaked in PMo12 solution to allow the formation of “PMo12 junction” between CNT and electrode. When small DC bias was applied to this device, we observed output current response with spontaneous transition between “OFF state” at nearly 0 A and “ON state” at certain maximum conducting current, accompanied by occasional appearance of intermediate states.Marginal histogram of current distribution shows 25 to 26 discrete states, which indicates the existence of single PMo12 in the PMo12 junction that actively provides this spontaneous conductance switching.We consider such phenomenon is caused either by the self-excited vibration of diffused protons, or vibration of chemical reaction in the vicinity of PMo12 junction.