Nanoscale strain control of oxide thin films

Most materials can be classified either as electric conductors or as insulators. Only a few materials possess the ability to switch their character depending on the conditions to which they are subjected. One of the exotic materials that can reversibly transition between a conducting state and an insulating state is vanadium dioxide (VO₂). In VO₂, the electronic phase transition is directly connected to a structural phase transition from a monoclinic symmetry at low temperatures to a tetragonal, rutile symmetry at high temperatures.

Among the many figurative knobs that can be turned to alter the properties of materials, strain or pressure stands out as a particularly powerful parameter. The strong coupling of the lattice and electronic degrees of freedom of VO₂ was used to mechanically induce the phase transition at room temperature. A controlled amount of pressure was applied along the rutile c-axis of a VO₂ film by a nanosized metallic probe in an atomic force microscope. The phase transition was reversibly triggered at a critical pressure, as witnessed by a sizeable conductivity modulation. The pressure needed to induce the transition decreased with increasing temperature, and the phase coexistence line that separates the insulating and metallic phases was determined in a hitherto unexplored regime.

Furthermore, it was illustrated how four-dimensional scanning transmission electron microscopy (4D STEM) combined with electron energy loss spectroscopy (EELS) could be used to investigate both the structural and the electronic phase transition of VO₂ with nanometer spatial resolution. Thereby, it was possible to image and track the metal-insulator transition of individual domains of VO₂ as they nucleated and grew during heating and cooling cycles. In addition, the samples were analyzed by microfocus X-ray diffraction at various temperatures across the phase transition to quantify the lattice parameters.