AVS 58th Annual International Symposium and Exhibition
    Nanometer-scale Science and Technology Division Monday Sessions
       Session NS-MoA

Paper NS-MoA1
Probing the Metal-Insulator Transition of Vanadium Dioxide using Gold Nanoantennas

Monday, October 31, 2011, 2:00 pm, Room 203

Session: Frontiers in Nanophotonics and Plasmonics
Presenter: Davon Ferrara, Vanderbilt University
Authors: D.W. Ferrara, Vanderbilt University
J. Nag, Vanderbilt University
E.R. MacQuarrie, Vanderbilt University
R.F. Haglund, Vanderbilt University
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Vanadium dioxide (VO2­) films and nanostructures in contact with gold (Au) or silver nanostructures can form the building blocks of active metamaterials that can be modulated in response to various stimuli such as the presence of chemical agents, changes in temperature, or irradiation. The semiconducting-to-metal phase transition (SMT) of VO2, — which can be induced thermally (Tc = 68oC), optically, or electrically — leads to a change in the dielectric function of the film. Since the localized surface plasmon resonance (LSPR) of the metal nanoantenna is sensitive to the local dielectric environment, the SMT allows the optical response of the metal::VO2 nanocomposite to be tuned. Thus these Au::VO2 nanocomposites are unique probes of strong-correlation physics because, during the phase transition, the electron-electron interactions in VO2 that drive the SMT are coupled with the plasmonic excitation of the Au nanostructure.

We fabricated arrays of Au nanoparticles (NPs), 180 nm in diameter and 20 nm high on indium-tin-oxide coated glass by electron-beam lithography. Subsequently, the nanoparticle arrays were coated with a 60 nm VO2 film by pulsed laser ablation of vanadium metal targets in 10 mTorr oxygen (O2) background gas, then annealed for 45 minutes at 450oC in 250 mTorr of O2. Using a Peltier heater and thermocouple mounted on a copper sample holder, temperature-dependent extinction of the array was measured using plain VO2 film as a reference to determine the LSPR wavelength and linewidth during the SMT.

The LSPR wavelength of the NPs was 1000 nm in the semiconducting state and approximately 840 nm in the metallic state, thus overlapping the VO2 electronic transitions from the occupied vanadium 3d|| band to the empty 3d­π band centered at approximately 885 nm. As the film undergoes the SMT, the split 3d­­|| bands merge and, with the 3dπ band, form the metallic VO2 conduction band. Since the Au NPs are sensitive to changes in both the real and imaginary parts of the VO2 local dielectric function, they serve as a direct probe of the SMT. The results show a 30% decrease in plasmon dephasing time during the transition due to an increase in carrier-carrier scattering in the VO2. Both Maxwell-Garnett and Bruggeman effective-medium theories predict the decrease in dephasing time during the SMT; however, a linear theory is a more accurate model for the hysteresis in the LSPR wavelength.