Graphene mediated resistive switching and thermoelectric behavior in lanthanum cobaltate

Multifunctional materials are of utmost importance owing to the combination of several functionalities into a single device. We present the multifunctional properties, namely, thermoelectric (TE) for energy harvesting and resistive switching (RS) for memory storage in the LaCoO3 (LCO)-graphene nanocomposite system. The existence of individual phases of LCO and graphene is confirmed from x-ray diffraction and electron microscopy techniques. The x-ray photoelectron spectroscopy measurement reveals the formation of oxygen vacancies in the nanocomposite with the addition of graphene. The bipolar resistive switching behavior observed in a LCO-graphene nanocomposite is explained using space charge limited conduction mechanism and is found to evolve from a trap-limited to a trap-free region with an increase in the graphene volume fraction, which eventually ascribed to the ordering of oxygen vacancies in the nanocomposite system. The endurance plot of the nanocomposite shows a stable RS behavior for consecutive 1000 cycles. On the other hand, the creation of oxygen vacancies in the nanocomposite leads to change in the configurational entropy of charge states of cobalt, which tunes the Seebeck coefficient (alpha). The decrease in alpha leads to an increase in the hopping hole concentration (n(h)) estimated using the classical Heikes formula and is consistent with the increase in the electrical conductivity. The increase in oxygen vacancies leads to point-defect scattering, which further reduces the thermal conductivity of the nanocomposite. The optimized TE parameters show a figure-of-merit of 0.004 +/- 0.000 48 at 300 K. The simultaneous observation of RS and TE properties in LCO-graphene nanocomposite provides new directions for multifunctional materials.