Controlling the Growth of Silver Nanoparticles on Thin Films of an n-Type Molecular Semiconductor

Nucleation and growth of silver nanoparticles were studied on the surface of an n-type organic semiconductor (N,N'-bis(n-octyl)dicyanoperylene-3,4:9,10-bis(dicarboximide) (N1400)) as a function of the deposition rate tau and the substrate temperature T-s. Electron tomography was used to probe the bulk diffusion of Ag in the N1400 layers. No Ag nanoparticles (NPs) are formed in the bulk of N1400 even for high substrate temperatures, T-s = 125 degrees C, indicating that Ag diffusion in the organic semiconductor is marginal. The NP distribution on the surface of N1400 is essentially determined by the surface roughness of the N1400 films. A transition in the nucleation mode of Ag NPs on N1400 is evidenced as a function of T-s: for T-s = 50 degrees C, Ag NPs form random patterns, whereas, for T-s 75 degrees C, linear arrays of aligned NPs are observed. Such arrays result from step edge decoration of the N1400 terraces. The surface density of Ag NPs is thermally activated, but the activation energy depends on the structure of the N1400 films: the smaller the crystal size of the N1400 grains, the larger the activation energy.