Research ArticleMATERIALS SCIENCE

Defect-engineered epitaxial VO2±δ in strain engineering of heterogeneous soft crystals

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Science Advances  25 May 2018:
Vol. 4, no. 5, eaar3679
DOI: 10.1126/sciadv.aar3679
  • Fig. 1 Design schematics of strain engineering at heterogeneous interface via defect-engineered epitaxial strongly correlated oxide crystals.

    (A) Schematics showing the relative crystallographic orientation of the VO2 nanowire and the hexagonal prism of the sapphire crystal. The in-plane growth of the VO2 nanowires on the basal c-plane can be translated to the r-plane, which results in a uniform array of vertically aligned nanowires. (B) Schematics showing the VO2 hybrid material upon the MIT. The nanoforest would experience an abrupt contraction along the c axis at MIT, which can effectively strain the crystal on top (CsPbBr3 in this case). The crystallographic orientation and surface facets of the individual VO2 nanowire are also labeled. (C) A 3D phase diagram of VO2 including the change in temperature, strain, and composition. New phases, such as M2 and T, can be introduced by applying strain or slightly changing the stoichiometric ratio.

  • Fig. 2 Growth and structural analysis of epitaxial VO2±δ array on r-plane sapphire.

    (A) Design schematics of the two-stage growth of the VO2 nanoforest using V2O5 droplets as template by varying the oxygen partial pressure and substrate temperature during the different stages. Insets show SEM images of the V2O5 droplets after the first stage (left), the newly nucleated VO2 nanowire (middle), and the nucleated VO2 nanowires with higher density (right). (B) Side view of the as-grown VO2 nanoforest with the inset being the top view. The top view reveals a rectangular cross section typical for VO2 nanowires. (C and D) Comparison of the VO2 growth without (C) and with (D) V2O5 template and two-stage process. The large difference in the density of nanowires and the decrease in the unwanted in-plane growth can be easily identified. Inset of (D), TEM image and diffraction pattern of a single VO2 nanowire. (E) Pole figure of the as-grown nanoforest. Only one asymmetric pole of (40Embedded Image) of VO2 at χ = 33° can be collected. The asymmetric pole is consistent with the oblique angle of the VO2 nanoforest and the angle between the c-plane and r-plane sapphire. The combining θ-2θ scan at the pole figure geometry shows that only one peak from VO2 exists. a.u., arbitrary units. Scale bars, 10 μm (A, left), 2 μm (A, middle), 5 μm (A, right), 40 μm (B), 10 μm (B, inset), 30 μm (C), 50 μm (D), and 1 μm (D, inset).

  • Fig. 3 Phase transition of defect-engineered epitaxial VO2±δ forest by in situ Raman spectroscopy.

    (A) Binary phase diagram of VO2 with changing stoichiometry and temperature. Oxygen sufficiency and deficiency modulate the phase of VO2 and introduce new phases such as M2 and T. The schematics of the VO2 lattice for each phase and the change of V–V bond are depicted. (B) Magnified SEM image of the VO2 nanoforest showing two kinds of nanowire: horizontally aligned (type A) and vertically aligned (type B). Scale bar, 10 μm. (C) Raman spectra of the two types of nanowires previously described (blue and red) and also the ones grown on SiO2/Si (black) as a reference. The three types of nanowire are in three different phases that can be identified by the V-O Ag vibrational mode. (D) Temperature-dependent Raman spectra of the type B VO2 nanoforest. The as-grown nanowire undergoes a T-M2-R phase transition in the temperature window investigated.

  • Fig. 4 Strain engineering of heterogeneous soft crystals via VO2±δ.

    (A) Schematics showing the structural change of the halide perovksite upon compressive strain. With increasing compressive strain, a phase change (α to β) with the relative octahedral rotation will occur. The phase transition of perovskite will end up with a larger bandgap that is contradictory to the trend of the conventional Pb-X contraction in the α phase. The indexing of the two phases is based on the previous study on MA-based perovskite (50). (B) Optical image of the VO2/CsPbBr3 hybrid material with changing temperature. Despite the existence of the growth layer, the appearance of the metallic domain (marked by letter R) and their propagation with increasing temperature can still be observed. Scale bar, 10 μm. (C) Temperature-dependent PL spectra of the CsPbBr3 crystal grown on VO2 where an abnormal PL shift and peak splitting can be observed at the MIT of VO2. This CsPbBr3 flake under investigation is a small crystallite instead of the large flake shown in (B). (D) Similar temperature-dependent PL spectra of a mixed CsPbBr3−xClx crystal on VO2. The same abnormal PL red shift at the MIT of VO2 can be observed, whereas the PL shift trend returns to normal after the MIT transition, as indicated by the dashed arrows.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/5/eaar3679/DC1

    fig. S1. SEM image of VO2 nanoforest obtained at lower substrate temperature during the second stage of growth.

    fig. S2. SEM image of VO2 nanoforest obtained at a higher substrate temperature during the second stage of growth.

    fig. S3. SEM and TEM image of VO2 nanoforest coated with BaTiO3 by room temperature RF sputtering.

    fig. S4. TEM characterization of the VO2-BaTiO3 core-shell structure.

    fig. S5. Raman spectra of the pristine VO2 nanowire (black) and the VO2 nanowire coated with BaTiO3.

    fig. S6. Optical microscope image of single-crystalline CsPbBr3 flake on VO2.

    fig. S7. Temperature-dependent PL spectra of a small crystallite on a thin (~200 nm in diameter) VO2 nanowire.

    fig. S8. Temperature-dependent PL spectra of a CsPbBr3 flake with larger thickness on a VO2 micron beam.

    fig. S9. Temperature-dependent PL spectra of a VO2 nanowire completely coated with CsPbBr3.

    fig. S10. Temperature-dependent PL spectra of CsPbBrxCl3−x and CsPbBr3 flakes on mica as a control experiment for halide perovskite/VO2 hybrid structure.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. SEM image of VO2 nanoforest obtained at lower substrate temperature during the second stage of growth.
    • fig. S2. SEM image of VO2 nanoforest obtained at a higher substrate temperature during the second stage of growth.
    • fig. S3. SEM and TEM image of VO2 nanoforest coated with BaTiO3 by room temperature RF sputtering.
    • fig. S4. TEM characterization of the VO2-BaTiO3 core-shell structure.
    • fig. S5. Raman spectra of the pristine VO2 nanowire (black) and the VO2 nanowire coated with BaTiO3.
    • fig. S6. Optical microscope image of single-crystalline CsPbBr3 flake on VO2.
    • fig. S7. Temperature-dependent PL spectra of a small crystallite on a thin (~200 nm in diameter) VO2 nanowire.
    • fig. S8. Temperature-dependent PL spectra of a CsPbBr3 flake with larger thickness on a VO2 micron beam.
    • fig. S9. Temperature-dependent PL spectra of a VO2 nanowire completely coated with CsPbBr3.
    • fig. S10. Temperature-dependent PL spectra of CsPbBrxCl3−x and CsPbBr3 flakes on mica as a control experiment for halide perovskite/VO2 hybrid structure.

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