| AVS 63rd International Symposium & Exhibition | |
| Scanning Probe Microscopy Focus Topic | Tuesday Sessions |
| Session SP-TuP |
| Session: | Scanning Probe Microscopy Poster Session |
| Presenter: | Christina McGahan, Vanderbilt University |
| Authors: | C.L. McGahan, Vanderbilt University S. Gamage, Georgia State University J. Liang, Tianjin University, China B.G. Cross, Georgia State University R.E. Marvel, Vanderbilt University R.F. Haglund, Vanderbilt University Y. Abate, Georgia State University |
| Correspondent: | Click to Email |
For the past decade, scattering-scanning near-field optical microscopy (s-SNOM) has been employed to image the coexisting metallic and insulating domains in single-crystal nanorods and platelets of vanadium dioxide (VO2) during the insulator-to-metal phase transition. In virtually all studies, the coexisting domains appear as alternating stripes perpendicular to the cR (growth axis) of the nanocrystals and extending from one side of the beam to the other.
We employed s-SNOM with a laser wavelength of λ=10.7 µm and polarized far-field optical microscopy to examine a single VO2 microcrystal decorated with gold (Au) plasmonic dipole antennas. Metallic and insulating domains can be easily distinguished during the thermal phase transition in VO2 using s-SNOM due to the large dielectric contrast between metallic and insulating VO2 at that wavelength. Plasmonic dipole antennas are positioned on the crystal, designed to be resonant at the s-SNOM probe wavelength to allow simultaneous probing of the pattern of coexisting phases of VO2 and the nanorod plasmon.
We observe a novel herringbone pattern of phase coexistence, seen in cracked epitaxial thin films but never in single crystals, in a VO2 single crystal which is large enough that the phase coexistence is not constrained by high aspect-ratio geometry. The herringbone pattern is altered by the presence of ferroelastic strain domains that form to relieve stress and can nucleate metallic domains. These ferroelastic domains are imaged with polarized far-field optical microscopy. Though the local dielectric environment of the crystal changes during the phase transition, as indicated via s-SNOM, the plasmon resonance frequency of Au nanoantennas atop the crystal does not change in response to the growth of metallic domains. This indicates that the metallic domains nucleate in the bulk of the single crystal, beyond the range of the plasmon field, which only penetrates tens of nanometers below the crystal surface but in range of the s-SNOM due to the penetration depth of 10.7 µm laser light. The domain pattern is insensitive to the locations and orientations of the resonant Au antennas because the field, as determined through simulations, is not high enough to induce the VO2 phase transition. Simulations indicate that a bowtie antenna has sufficient field to locally switch VO2 from insulating to metallic, enabling localized induction of the phase transition near the surface of the VO2.