Crucial Role of Metal Surface Morphology in Photon Emission from a Tunnel Junction at Ambient Conditions

In this paper, we study light emission from a tunnel contact between the Au film on a glass substrate and the Au-coated tungsten probe of the scanning tunneling microscope at ambient conditions. We investigate the dependence of the intensity of the collected optical signal on the film surface morphology, namely, on geometrical parameters of nanometer-scale gold grains (islands) constituting the film. We reveal that the magnitude of inelastic tunnel current and thus photon emission intensity increase both with the decrease of island height and the increase of island width. We show that the difference in the optical emission intensity could reach 4 orders of magnitude for poly- and monocrystalline (atomically flat) samples. The observed phenomena are explained with dependence of the effective area of a tunnel contact on the grain aspect ratio. The obtained results demonstrate the crucial role of the sample morphology in influencing the efficiency of photon emission from the tunnel junction.