Resumen de Implications of phase coexistence in VO(2) thin-films across the metal-insulator transition
A Metal-insulator transition (MIT) is the ability of some materials to change between metal and insulator electric behaviours as a function of some external stimuli such as temperature, stress, voltage, magnetic field or light. The Fermi level position with respect to the band structure determines one character or the other, and in some materials this band structure is very sensitive to electron-electron correlations. This is the case of some transition metal oxides, which despite having partially filled bands allowing, in theory, metallic conduction, electron-electron interactions split the half-filled band in a lower energy band that is full first and a higher energy sub-band still empty, resembling an insulator.
An archetypical example is vanadium dioxide (VO2), a system with a 3d1 electronic configuration which leads to a first-order MIT happening near room temperature (~68 ºC) with a change in conductivity of several orders of magnitude, accompanied by a structural phase transition (SPT) that occurs simultaneously. This results in a high-temperature state given by a metallic rutile (tetragonal) phase that turns into a semiconductor with monoclinic M1 structure at the low-temperature state. Thus, the electronic and structural elements of the transition in VO2 are tightly entangled, whereby mechanical stress induces lattice deformations in the crystal that distort the surroundings of Àrea de Planificació i Serveis Acadèmics Gestió Acadèmica - Afers Generals i Tercer Cicle Última Actualización: 09/06/2009 Página 4 de 5 V atoms affecting the orbital properties within the unit cell, the lattice’s electrostatic potential field and vibrational modes.
This thesis explores in detail the consequences of in-plane tensile strain in epitaxial VO2 thin films grown on rutile (001)-oriented TiO2 substrates by the pulsed laser deposition (PLD) technique, how the strain relaxes as film thickness increases, and how the strain relaxation itself gives raise to new phases and phase coexistence phenomena of interest. What makes this work novel with respect to other strain studies is the focus on the relation between local properties and global behaviour. As the thesis will show, space-averaged measurements of transport properties or X-ray diffraction (XRD) can miss out on a lot of interesting and important physics that happens on a nanoscopic level, including nano-tweeds or phase coexistence in the form of metal-insulator (M-I) phase boundaries. This work has used various microscopy techniques effective at different scales, from optical microscopy (micron scale) to scanning-probe microscopy (nanoscale) to transmission electron microscopy (atomic scale), yielding a very complete picture of the MIT in strained VO2 films across the different scales.
After a brief introductory chapter contextualizing the thesis, chapter one describes the growth and average structural characteristics of the films, including their evolution due to aging. It was grown a thickness set from ~2 nm up to ~150 nm and surprisingly, the films do not relax with thickness neither conventionally nor progressively, resulting in crack formation, M-I phase separation toward the cracks and strong-strain gradients within the film.
The microstructure far from crack-edges is presented in chapter two. It shows that the strained VO2 regions are not stabilized in the standard high-temperature rutile phase but in a metrically tetragonal Àrea de Planificació i Serveis Acadèmics Gestió Acadèmica - Afers Generals i Tercer Cicle Última Actualización: 09/06/2009 Página 5 de 5 structure that triples the rutile periodicity with monoclinic (2/m) symmetry. Moreover, the orientational variants of this new phase (x3M) coexist forming tweed patterns of few nanometers in size.
Chapter three explores the functional consequences of the coexistence between strain-relaxed insulating phase near the films’ cracks and metallic phase away from them. The peculiar pattern induced by crack formation leads to the concept of “self-pixelation”, whereby each island of VO2 bounded by insulating cracks behaves de facto as a “pixel” whose MIT can be individually triggered independently of the rest of the film.
Finally, the last chapter contains the general conclusions and outlook for future work.
