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- Investigation of electro-optical properties of gold colloidal suspensions.

Auteur(s): Coursault D.(Corresp.), Blanc C., Nobili M., Dozov I., Robert Antoine, Lacroix Lise-marie, Viau Guillaume, Slyusarenko Kostyantyn

Conference: Integrating devices and materials: A challenge for new instrumentation in ICT IC1208 Joint 3rd Man (Santarem, PT, 2014-03-20)


Résumé:

We investigate electro-optical properties of suspensions of gold anisometric particles. Gold nanoparticles exhibit localized surface plasmon properties with a transverse mode and longitudinal mode in case of ellipsoidal shape. The transverse mode remains maximum in the visible range but the longitudinal one is red shifted and its resonance strongly depend on the aspect ratio of the nanoparticles. Among others, we thus expect highly tunable properties at optical frequencies under the external electric field. In our experimental setup, the electrodes are placed outside the sample. In this configuration, they do not interact with the suspension. We work with two kinds of particles: gold nanorods (GNRs) dispersed in water with an aspect ratio AR ≈10 (Φ=10nm) and gold nanowires (GNWs) dispersed in hexane with an AR≈1000 (Φ=1.5nm). We observed alignment of both GNRs and GNWs along the applied electric field due the intrinsic conductivity of the particles. Time responses of GNRs to the electric field are of the order of the millisecond and of more than 10 ms for GNWs. The saturated voltage is 4 times smaller in this last case compared to GNRs. Those results are in agreement with a change in the aspect ratio. More surprising, dichroism and spectroscopy measurements reveal, in both cases, a positive dichroism in the optical visible range. On the contrary, we expect a negative one (ie absorption higher for polarization perpendicular to the electric field) while in this frequency range, we are sensitive to the transverse mode of the plasmon band. This suggests that this mode is less sensitive to polarization than the longitudinal one . Concerning GNWs, a longitudinal plasmon band is not yet evidenced. To the best of our knowledge, numerical simulations have only predicted their presence for wire of similar length but larger diameter. Similarly to carbon nanotubes, we observe their progressive assembly into fibers strictly oriented along the electric field. Those fibers appear highly birefringent between crossed polarisers. Their properties and the way to control their growth are under investigation.