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Optical Second Harmonic Generation of Heterogeneous Langmuir films of Hydrophobic Gold Nanoparticles at the Air-Water Interface
Gaëlle Martin-Gassin, Emmanuel Benichou, Guillaume Bachelier, Isabelle Russier-Antoine, Christian Jonin, Stéphane Roux, Olivier Tillement, Michel Broyer and Pierre-François Brevet

We report the observation by optical second harmonic generation of a two dimensional film of hydrophobic gold metal nanoparticles deposited in a Langmuir trough at the air/water interface. The gold particles with a diameter of about 7 nm were functionalized with octadecanethiol and spread at the water surface. The nonlinear optical signal intensity was monitored as a function of the average nanoparticle surface density during a compression-decompression cycle. At all particle surface densities well below the maximum surface density estimated at about 1.4×1016 particles per m2 using a close-compact two dimensional model, the average second harmonic intensity scaled with the square of the average particle surface density. However, large fluctuations of the signal intensity were recorded. A detailed analysis of the intensity histograms showed that these fluctuations could be attributed to both nanoparticle surface density and hyperpolarizability fluctuations. This work not only reveals the heterogeneous nature of the nanoparticle films but also the occurrence of interactions between particles in such two-dimensional systems.

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