Abstract
Spinodal decomposition, the spontaneous phase separation process of periodic lamellae at the nanometer scale, of correlated oxide ((Ti, V)O2) systems offers a sophisticated route to achieve a new class of mesoscale structures in the form of self-assembled superlattices for possible applications using steep metal–insulator transitions. Here, we achieve the tunable self-assembly of (Ti, V)O2 superlattices with steep transitions (ΔTMI < 5 K) by spinodal decomposition with accurate control of the growth parameters without conventional layer-by-layer growth. Abrupt compositional modulation with alternating Ti-rich and V-rich layers spontaneously occurs along the growth direction because in-plane lattice mismatch is smaller in this direction than in other directions. An increase in the film growth rate thickens periodic alternating lamellae; the phase separation can be kinetically enhanced by adatom impingement during two-dimensional growth, demonstrating that the interplay between mass transport and uphill diffusion yields highly periodic (Ti, V)O2 superlattices with tunable lamellar periods. Our results for creating correlated (Ti, V)O2 oxide superlattices provide a new bottom-up strategy to design rutile oxide tunable nanostructures and present opportunities to design new material platforms for electronic and photonic applications with correlated oxide systems.
Highlights
Superlattices, structures with periodic blocks on the nanometer scale, have realized unique properties unachievable with the use of a single layer via structural and electronic confinement effects[1,2,3,4]
This spinodal decomposition strongly depends on the crystallographic anisotropy that emerges from the anisotropic lattice parameters of a rutile oxide with tetragonal symmetry; compositional modulation of the lamellar structure preferentially appears normal to the c-axis rather than the a-axis because the lattice mismatch between TiO2 and VO2 is smaller along the a-axis (~1.5 %) than the c-axis (~3.7 %)
Depending on the growth temperature Tg, the degree of the phase separation was significantly modulated at a constant laser pulse frequency (f = 7 Hz), target composition (Ti:V = 0.5:0.5), and oxygen partial pressure (10 mTorr)
Summary
Superlattices, structures with periodic blocks on the nanometer scale, have realized unique properties unachievable with the use of a single layer via structural and electronic confinement effects[1,2,3,4]. Since TiO2/VO2 superlattices are uniquely composed of 3d1-correlated functional materials coupled with isostructural 3d0 insulators, they are predicted to have electrical and optical properties that can be tuned by external stimuli to provide a new platform for thin-film devices (i.e., half-metallic VO2 slabs[20] and tunable metamaterials[19,21]). Spinodal decomposition occurs below 830 K if atomic diffusion is thermally activated[22] This spinodal decomposition strongly depends on the crystallographic anisotropy that emerges from the anisotropic lattice parameters of a rutile oxide with tetragonal symmetry; compositional modulation of the lamellar structure preferentially appears normal to the c-axis rather than the a-axis because the lattice mismatch between TiO2 and VO2 is smaller along the a-axis (~1.5 %) than the c-axis (~3.7 %). The formation of periodic Ti-rich and V-rich lamellae normal to the a-axis is the only option to minimize the elastic strain energy at the interfaces between two rutile phases, resulting in the generation of well-ordered superlattices 22,23
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