Seminario "Strain effects in vanadium dioxide films with metal-insulator electronic transition"

El próximo viernes 16 de diciembre, a las 11 h, se llevará a cabo el seminario titulado "Strain effects in vanadium dioxide films with metal-insulator electronic transition", a cargo del Dr. José Santiso López, quien realiza sus tareas de investigación en el Instituto Catalán de Nanociencia y Nanotecnología (ICN2) en Barcelona, España. El Dr. Santiso López, invitado por el Dr. Mario C. G. Passeggi (h) es parte del proyecto ULTIMATE-I Ultra Thin Magneto Thermal Sensoring.

El seminario se desarrollará de forma presencial en la Sala Auditorio del IFIS (Planta Baja, Edificio Bernardo Houssay, Güemes 3450).

Prof. Dr. José Santiso (Catalan Institute of Nanoscience and Nanotechnology, ICN2, Barcelona)

Vanadium dioxide (VO 2 ) is an archetypical material showing a metal-insulator (M-I) Mott transition at about T=68C with several orders of magnitude change in the resistivity between the semiconducting low-T M1 phase and the high-T metallic rutile R phase. It has been proposed as a good candidate directly for Resistive-Random Access Memory (RRAM) devices or as complementary selector in combination of a bipolar RRAM device because of its high on/off ratio, fast switching speed and high current density. Although in bulk form the transition is above room temperature either chemical doping or mechanical strain have been demonstrated to bring the transition closer or even below room temperature making it very attractive in different devices. This work studies the structure features across transition of high quality strained epitaxial VO 2 films deposited on isostructural rutile TiO 2 (001) single crystals. Since M-I transition in VO 2 is accompanied by a structural change between monoclinic M1 phase and tetragonal R phase, with substantial variation in cell parameters, the nucleation of the high-T R phase embedded in the low-T M1 phase during transition, is expected to generate local interfacial regions with a large strain. The characterization of the structural features of the films by means of synchrotron X-ray Diffraction upon thermal cycles revealed the formation of local strain effects during the phase transition in competition with the epitaxial strain induced by the growth on the mismatched TiO 2 substrates. Such interphase regions between M1 and R crystal domains are submitted to a uniaxial strain (overlapped to the biaxial in-plane epitaxial strain) that stabilizes other polytypic VO 2 insulating phases (monoclinic M2, and triclinic transitional M3 or T phase) beyond their expected stability regions.[1] Concomitant to the subtle structural distortions a slight rotation of the monoclinic crystal domains occurs to accommodate the elastic energy constraints. The persistence of the coexistence of those polytypic phases even at room temperature turns VO 2 into a complex heterogeneous material. Nonetheless, at conditions where XRD of the films seems to indicate a pure rutile R phase, HRTEM analysis revealed certain degree of heterogeneity at the nanoscale forming an intricate tweed pattern of other polytypes with different arrangement of V-V dimers along the c-axis direction [2], still displaying a metallic character. Altogether these observations evidence that VO 2 micro and nanostructure, and therefore its electrical response, depends on a subtle balance of unconventional strain relaxation mechanisms.

[1] L. Rodríguez, F. Sandiumenge, C. Frontera, J.M. Caicedo, J. Padilla, G. Catalán, J. Santiso, Strong strain gradients and phase coexistence at the metal-insulator transition in VO 2 epitaxial films, Acta Mater. 220 (2021) 117336.

[2] F. Sandiumenge, L. Rodríguez, M. Pruneda, C. Magén, J. Santiso, G. Catalan, Metallic Diluted Dimerization in VO 2 Tweeds, Adv. Mater. 33 (2021) 2004374.