Basic understanding of photophysical processes at the nanometer scale is a frontier problem in surface physics.
STM-induced photon emission allows local investigations of optical mode excitations and radiative relaxation. We have applied this technique on a well-controlled model system composed of size-monodispersed thiol-covered silver nanospheres deposited onto atomically-flat (111) gold surface under ultra-high vacuum conditions.
STM imaging of this system demonstrate a self-organized close-packed arrangement of the particles. We have been able to study for the first time light emission from individual sphere-shaped particles, electrically isolated from the substrate by a 2-nm gap. Surprisingly, in spite of a homogeneous 2D organization, photon emission exhibits pronounced quenching at particular sites.
A careful analysis of data recorded simultaneously during forward- and backward-scans of the tip clearly shows a correlation of this quenching with mobility of the particle inside its site, relatively to its neighbors. Intensity of light-emission by such weakly-bounded silver spheres is one order of magnitude below the average intensity.
We explain this effect in terms of relaxation of ballistic electrons inside the particles, in these two-barrier tunnel junction.