Vanadium dioxide (VO2) has been drawing attention since its metal-insulator transition (MIT) with several orders of magnitude resistivity change at 341 K was discovered.[1,2] The MIT is accompanied with the abrupt first order structural phase transformation from a metallic tetragonal rutile (R) phase (P42/mnm), to an insulating monoclinic (M1) phase (P21/C). Up to now, most studies on epitaxial VO2 thin films used Al2O3 and TiO2 single crystal substrates to control film orientation.[3,4]
However, VO2-based devices are restricted to low cost and size-limited single crystal substrates, as well as on the integration compatibility with the current Si-based technology. Direct deposition of VO2 on glass or Si substrates with a native amorphous silicon dioxide layer leads to a polycrystalline film with predominant (011)M1 orientation, whereas VO2 is favorably grown (010)M1-oriented on a buffer layer of Pt(111) on Si substrate.
The challenge is how to direct VO2 film orientation on Si or even arbitrary substrates at will. Oriented growth is not only of interest for the fundamental study of the MIT mechanisms, but also for potential applications for next-generation transistors, memory metamaterials, sensors, and novel hydrogen storage technology. Recently, various metal oxide films were grown by epitaxy on oxide nanosheets on glass and Si substrates.[12–15] Oxide nanosheets span a wide range of crystal lattices and 2D symmetric structures, allowing for new possibilities to tailor the important structural parameters and properties of thin films.
In the present work, Ti0.87O2 (TO) and NbWO6 (NWO) nanosheets were identified to direct the orientation of VO2 thin films.
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In 1999 Prof. Dr. Ir. G. (Gertjan) Koster did his PhD on “Artificial layered complex oxides by pulsed laser deposition“. In that same year, he moved to the US to join the Kapitulnik-Geballe-Beasley (KGB) group at the Geballe Laboratory for Advanced Materials, Stanford University. In 2007, he joined the Inorganic Materials Science group, MESA+ institute for nanotechnology, University of Twente, where since July 2014 he has been associate professor (adjunct hoogleraar). Additionally, he was a visiting professor at UBC, Vancouver in 2014 and currently is a guest professor at JSF, Ljubliana, Slovenia. His research focuses on the structure-property relation of atomically engineered complex (nano)materials, especially thin film ceramic oxides. For the thin film synthesis, he developed the first time-resolved RHEED-system, operating at high pressures up to 100 Pa during pulsed laser deposition. Current research includes the growth and study of artificial materials, the physics of reduced scale (nanoscale) materials, metal-insulator transitions and in situ spectroscopic characterization.’