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Helmut Karl1

1, University of Augsburg, Augsburg, , Germany

VO2 shows a metal-insulator phase transition (MIT) at 68°C which is accompanied by a strong decrease in electrical resistivity, change in optical transmittance and reflectance in the near infrared spectral region. The MIT is also marked by a structural phase transition with large anisotropic and abrupt length change by approximately 1% and 2% when the material transforms from either the insulating monoclinic M1 or mechanical stress stabilized monoclinic M2 to the tetragonal rutile phase R. The large change of the VO2 unit cell volume at the MIT produces in bulk VO2 crystals huge mechanical stress-strain fields leading to severe deterioration of crystal quality.
In this work nanocrystals of VO2 buried in SiO2 were investigated. In contrast to bulk VO2 crystals VO2 nanocrystals can be temperature cycled through the MIT without any signs of fatigue effects. The VO2 nanocrystals were synthesized by sequential ion implantation of the elements vanadium and oxygen followed by a rapid thermal annealing step. We show in this context that chemical phase selective synthesis and positioning of VO2 nanoclusters buried in thermally grown SiO2 on Si-substrates and fused silica substrates can be achieved by ion implantation of stoichiometric fluence ratios of the elements V and O.
The MIT of the VO2 nanocrystals was investigated as a function of temperature by micro-Raman scattering measurements and ellipsometry in the spectral range from UV to the near IR. The temperature dependent dielectric function of the VO2 nanocrystals embedded in the SiO2 matrix was determined by applying a Drude-Lorenz oscillator model and using an effective medium approximation. During temperature cycling through the MIT we observe a huge temperature hysteresis with a width of more than 50 K. This hysteresis is characterized by asymmetric super heating and super cooling around the MIT temperature of 68°C of bulk VO2 material. This can be attributed to the first order phase MIT and the high crystalline quality of the VO2 nanocrystals by very effectively suppressing seeding effects due to lack of metallic and dielectric phase coexistence in the single VO2 nanocrystals. First thermochromic optical devices were fabricated demonstrating high contrast switching at different wavelengths [1,2,3].

[1] T. Jostmeier et al., Thermochromic modulation of surface plasmon polaritons in vanadium dioxide nanocomposits, Optics Express 24, 15 (1016)
[2] T. Jostmeier et al., Optically imprinted reconfigurable photonic elements in a VO2 nanocomposit, Appl. Phys. Lett. 105, 071107 (2014)
[3] J. Zimmer et al., Ion beam synthesis of nanothermochromic diffraction grantings with giant switching contrast at telecom wavelengths, Appl. Phys. Lett. 100, 231911 (2012)

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