Francesco Calavalle - ESR 13, July 2018 - June 2021
NanoGUNE, San Sebastián, Spain
The ferroelectric copolymer P(VDF-TrFE), has attracted, in recent years, a great interest in scientific research for modern electronics applications, such as energy harvesting for self-powered wearable devices, biomedical sensors and nonvolatile memories. Lots of efforts have been spent in the development of fabrication procedures that could enhance the electromechanical performance of this material. Electrospinning has been proposed as an efficient and low-cost solution for the production of polarized polymeric nanofibers, ready for the integration in devices without post-poling process required. In this thesis, I employ Scanning Probe Microscopy techniques to provide an accurate investigation of the electromechanical properties of single P(VDF-TrFE) electrospun nanofibers. I find that electrospinning does not induce ferroelectric polarization of the fibers, but leads to the accumulation of space charges in the material. I argument that such a space charge gives rise to an apparent ferroelectric response at the macroscopical scale due to the electret effect. Further, after dissipation of the space-charges, with the implementation of high voltage Switching Spectroscopy-PFM, I could demonstrate that poling processes at the level of a single nano-fiber are possible and that the observed piezoelectric performance is comparable to the thin-film behavior. Therefore, I predict that elestrospun fibers will show improved behavior in electromechanical devices once complete poling of fiber networks is achieved.
Personal Training Committee
Main Supervisor: Luis Hueso, NanoGUNE
Co-supervisor: Floriana Lombardi, Chalmers
Mentor: Amaia Zurutuza, GRA SEMI
At GRA SEMI, Spain (December 2018/January 2019) for training in the growth and integration of materials in industrial environments,
At Chalmers, Sweden (September-October 2019 and remotely in Spring 2021) for training in low temperature electrical measurements,
At ETH, Switzerland (February 2020) for training on delamination and stacking of 2D materials.
Advanced hybrid organic spintronic devices with small spin-orbit coupling
Objectives: The objective of my project is to explore hybrid organic spintronic devices in which to perform spin transport and manipulation beyond simple spin valves. In the first place, with the aim of exploring the spin transport mechanisms in organic materials with small spin-orbit coupling, I will fabricate double organic spin hot-electron transistors in which the spin current can be manipulated by an external out-of-plane magnetic field. For the proof-of-principle devices, I will use C60 fullerene as well as metal-organic molecules such as CuPc and Alq3. The observation of the effect of the spin precession will be the first in this discipline. I will then merge the spin transport with other organic functionalities, such as photoconductivity or the photoelectric effect. In particular, I will explore the concept of self-powered hybrid organic spin valves-photovoltaic cells. You will fabricate multi-layered organic devices in which exciton-to-carrier separation occurs at the organic homo-interface. The charge carrier will be utilized for powering the device autonomously, but also for performing spin manipulation. Here we will focus again on the molecules named above (making use of the p- and n-type conductivities for creating natural energy barriers) together with some polymers such as P3HT and N2200.