Self-Pixelation Through Fracture in VO₂ Thin Films

Abstract

Vanadium dioxide (VO 2 ) is an archetypal Mott material with a metal-insulator transition (MIT) near room temperature. In thin films, this transition is affected by substrate-induced strain but, as film thickness increases, the strain is gradually relaxed and the bulk properties are recovered.Epitaxial films of VO 2 on (001)-oriented rutile titanium dioxide (TiO 2 ) relax substrate strain by forming a network of fracture lines that crisscross the film along well-defined crystallographic directions. This work shows that the electronic properties associated with these lines result in a pattern that resembles a "street map" of fully strained metallic VO 2 blocks separated by insulating VO 2 stripes. Each block of VO 2 is thus electronically self-insulated from its neighbors and its MIT can be locally induced optically with a laser, or electronically via the tip of a scanning probe microscope, so that the films behave functionally as self-patterned pixel arrays.

Metallic Diluted Dimerization in VO2 Tweeds

“…The metal/insulator character of the participating phases can be discerned in reflection optical microscopy images according to their different optical density. [ 22,34 ] Thus, the bright contrast (lower optical density) decorating the cracks corresponds to the insulating M1 phase, whereas the dark (higher optical density) rectangular tile‐like regions between the cracks correspond to metallic VO 2 . [ 34 ] This behavior is further confirmed by conducting‐atomic force microscopy (c‐AFM) current maps and I ( V ) measurements, the latter indicating ohmic (metallic) conduction even below room temperature (see Figure S2, Supporting Information).…”

“…[ 22,34 ] Thus, the bright contrast (lower optical density) decorating the cracks corresponds to the insulating M1 phase, whereas the dark (higher optical density) rectangular tile‐like regions between the cracks correspond to metallic VO 2 . [ 34 ] This behavior is further confirmed by conducting‐atomic force microscopy (c‐AFM) current maps and I ( V ) measurements, the latter indicating ohmic (metallic) conduction even below room temperature (see Figure S2, Supporting Information). Reciprocal space mapping around the (−1 −1 2) R reflection indicates that in the 40 and 80 nm‐thick films both the R and M1 phases are fully in‐plane strained (Figure 1c,d), whereas in the 90 nm‐thick film the M1 phase is in‐plane relaxed (Figure 1e).…”

“…By contrast, the tile‐like rectangular regions bounded by these cracks are metallic. [ 34 ] c–e) Reciprocal space maps obtained around the substrate (−1 −1 2) R reflection. For the c) 40 nm‐ and d) 80 nm‐thick films, the in‐plane film components of both the M1 and rutile‐like reflections are vertically aligned with the substrate one.…”

“…rectangular regions bounded by these cracks, are metallic. [33] (c-e) Reciprocal space maps obtained around the substrate (-1 -1 2) R reflection. combination, and the latter appears about 0.7 eV above the π* band edge.…”

“…At a thickness of 90 nm, the unique presence of the M1 peak indicates that the whole volume has evolved to the equilibrium monoclinic phase. Relaxation of the in-plane tensile stress imposed by the substrates proceeds through cracking rather than misfit dislocation, [33][34][35] and Figure 1b illustrates a typical stress-relieving crack pattern in a 60 nm -thick film. The bright contrast decorating the cracks corresponds to the M1 phase, while the dark rectangular tile-like regions between the cracks correspond to metallic VO 2 .…”