Cooperative phenomena in artificial solids made from silver quantum dots: The importance of classical coupling

Recent work has shown that metal quantum dots can be treated as "artificial atoms" and crystallized into "artificial solids" that have electronic properties that can be tuned by controlling interparticle coupling through the application of pressure. The interactions between the nanocrystals in such artificial solids are classified as either dipole or exchange.(1) Dipole (including many-body) coupling between nanocrystals is treated in a classical manner using an effective medium model that permits the calculation of both the linear and nonlinear optical spectra of the solid as the interparticle separation is continuously varied. We find agreement between the classical model and experimental linear reflectance data until the crystallites are separated by less than 10 Angstrom, after which they sharply diverge. We find that the classical model fails to predict the overall experimental trend in second-harmonic intensities.