Rb2Ti2O5 at the nanoscale: characterization and charge transport

Over the last century, the energy storage industry has continued to evolve, adapt, and innovate in response to changing energy requirements and advances in technology. Currently, energy storage systems are available for various large-scale applications and are classified into different types. In particular, electrical energy storage systems (EESS) in terms of electrochemical capacitors (ECs) and batteries have demonstrated great potential. In the last decades, research has focused on the development of all aspects of EES, electrolyte material among them. The perovskite-derived material Rb2Ti2O5 (RTO), has been recently found to display very interesting dielectric and conduction properties, showing an equivalent relative permittivity up to 10^9 at room temperature, high ionic conductivity, and very low electronic conductivity. The remarkable properties of this material make it interesting for applications, in particular for its use as a solid electrolyte for the realization of advanced energy storage systems. Here, I present the first approach to study RTO at the nanoscale, in order to characterize its properties and investigate the charge transport. Having a lamellar structure, RTO is a promising candidate for micro-mechanical exfoliation to obtain thin samples. In this work, first, a review of the previous results obtained for bulk RTO crystals is described in the State of the art section, then, a detailed study of the physical and electrical properties of RTO nanocrystals is presented. Samples are obtained after micro-mechanical exfoliation through scotch-tape technique. Characterization of the exfoliated flakes is carried out by performing AFM (Atomic Force Microscopy) and Raman spectroscopy measurements. Devices are fabricated in cleanroom by photolithographic process, e-beam evaporation, and hot pick-up transfer technique. Finally, electrical measurements are performed to extract information on the charge transport in RTO nanocrystals. Nanocrystals are feasibly obtained through scotch-tape exfoliation technique. The already known hygroscopic character of the material is also found in exfoliated RTO. Water absorption tends to deteriorate the crystals over time, but it is responsible for the modification of the RTO Raman spectrum and for the ionic conduction. The conductivity of nanocrystals is comparable to that of bulk RTO and in agreement with the results of previous works. This work represents the first approach to the study of Rb2Ti2O5 at the nanoscale. Exfoliated nanocrystals have shown similar properties to those of bulk RTO, concerning water absorption and electrical conduction. The obtained results strengthen the theory about the cruciality of water in determining the properties of this material. Further investigation will be needed in order to determine quantitative relations between the water content inside the material and its physical and electrical properties and to fully understand the charge transport in RTO.