My research interests focus primarily on exploiting the exotic physical and chemical properties of strongly correlated systems for novel electronic and energy applications.
My expertise lies in the growth of functional thin films from both chemical and physical methods, microfabrication techniques and material properties characterization, such as XRD, Raman, electrochemistry, magnetism, electrical transport at the micro- and nanoscales and diverse scanning probe techniques.
Novel Materials for Memristive Devices
Memristive devices are capable of changing their resistance state based on history of the applied current and voltage. Typically, this resistive switching effect is non-volatile, i.e., the effect persists after the removal of the electrical estimuli and is induced through ionic migration at the nanoscale. The different resistive states can then be used for information storage and processing, offering key performance features that outperform classical integrated circuit technologies. For instance, the first memristive devices integrated as Resistive-Random Access Memories (RRAM or ReRAM) are starting to be commercialized. In addition, they hold the promise of being a central element in the development of energy-efficient neuromorphic computing architectures. Despite presenting a rich phenomenology, in most of the state-of-the-art memristive devices, a nanometric metallic filament composed of either oxygen vacancies or metallic cations, which grows or shrinks under high enough voltage, is the ultimate responsible of the resistance modulations. These filaments, however, still consume large amount of energy, suffer from stochasticity and produce high non-linearitis. While these are highly desirable to achieve a non-volatile universal memory, they are detrimental for neuromorphic computing schemes, that require the complete opposite, i.e., linear resistance changes.
My research has been focused on exploiting the intrinsic properties of strongly correlated systems to fabricate memristive devices not based on conventional filaments. We recently proposed in collaboration with Prof. Martin Bazant (MIT) lithium titanates, a classic battery material, as a novel memristive device for the first time through a switching mechanism based on an electric-field induced phase separation. In a nutshell, one can control the dynamics and magnitude of the phase separation, and therefore to adjust the non-linearity in the switching process through a fine control of lithium stoichiometry during film processing.
During my PhD thesis at ICMAB-CSIC, we addressed the use of the intrinsic metal-insulator transition in three families of metallic perovskite oxides to induce fast non-volatile reversible transitions to different resistive states upon the application of a voltage. The applied voltage induces oxygen exchange with the ambient by generating oxygen vacancies in the systems and a metal-insulator transition in a volumetric continuous manner. The oxygen exchange was engineered by using an oxygen reservoir layer and by demonstrating the volumetric resistive switching characteristics with excellent performance of the bilayer device and its applicability as an ionic transistor, enhancing scalability and performance of current technologies.
Lithium-based Materials for Solid-State Batteries and Nanoelectronic Devices
Li-ion batteries power every corner of our lives, including electric vehicles revolution ahead of us. Remarkable advances in the understanding and engineering of Li-based materials is the past decades were recently recognized with the Nobel Prize in Chemistry. Two main challenges lie ahead of us. First, current Li-ion batteries use a flammable liquid as the electrolyte, raising safety concerns, which is generally not compatible with metallic Lithium as the anode. This would largely benefit the overall energy density of the final battery. Second, Li-ion batteries have not been been able to keep up with the miniaturization achievements of many other electronic device components.
An alternative gaining momentum in the last 15 years is solid-state batteries (SSBs). SSBs replace the liquid electrolyte by a solid ion-conducting layer with comparable conductivities, that ideally result in a thinner layer that current polymer membrane separator technologies. In addition, solid-electrolytes can be compatible with metallic Lithium. In addition, solid-electrolytes can be processed as thin films, enabling extremely functional thin layers that can be embedded in other microelectronic devices. My research has focused on studying the role of lithiation in amorphous and crystalline thin films based on lithium garnets.
Besides the conventional use of Lithium-based materials for batteries, other physical properties of these materials are attracting interest, expanding the realm of applications of these materials. In particular, I have studied the memristive properties of lithium titanates, a classic battery anode material. Check out my participation in the Nano Explorations seminar at MIT.
Strongly electronic correlated materials
Strongly correlated complex oxides, are a fascinating class of materials in which the interactions between the different degrees of freedom, such as charge, spin, orbital and lattice effects, strongly compete with each other and lead to exotic physical properties. Strongly correlated oxides can show ferroelectricity, multiferroicity, ferromagnetism, metal-insulator transitions, colossal magnetoresistance and high temperature superconductivity, paving the way to a wide range of new functionalities that might enhance the performance of actual devices and bring new opportunities.
Metal-insulator transitions are widely observed in a large variety of materials, specifically in strongly correlated transition metal oxides (TMO). It produces an intrinsic change in the material from an insulator to a metallic state and it can be triggered by different factors, such as pressure, temperature, electric field or concentration changes, among others. Usually, the MIT is accompanied by colossal resistivity changes, even over several orders of magnitude.
My research has been focused on a particular class of strongly correlated systems displaying Metal-Insulator Transitions named perovskite oxides. I investigated the resistive switching properties of three different families of metallic perovskite oxides, cuprates, nickelates and manganites, mainly by means of Scanning Probe Microscopy techniques. I have been researching the influence of Lithium concentration in the Metal-Insulator Transition properties under high electric field in lithium titanates.
Juan Carlos Gonzalez-Rosillo 