REVERSIBLE TUNING OF SILVER QUANTUM-DOT MONOLAYERS THROUGH THE METAL-INSULATOR-TRANSITION

The linear and nonlinear (chi((2))) optical responses of Langmuir mono layers of organically functionalized silver quantum dots were measured as a continuous function of interparticle separation under near-ambie nt conditions. As the distance between metal surfaces was decreased fr om 12 to similar to 5 angstroms, both quantum and classical effects we re observed in the optical signals. When the separation was less than 5 angstroms, the optical second-harmonic generation (SHG) response exh ibited a sharp discontinuity, and the linear reflectance and absorbanc e began to resemble those of a thin metallic film, indicating that an insulator-to-metal transition occurred. This transition was reversible .