Self-assembly of Periodic, Sub-micron Topography in Organic Semiconducting Thin Films

Certain glassy thin films based on conjugated small molecules are found to spontaneously form periodic patterns during crystallization via thermal annealing. These sinusoidal surface structures are formed perpendicular to the crystal growth direction and originate in the amorphous region within a micron of the crystal front. Previous work has shown that the periodicity of these self-assembled, well-aligned patterns can be tuned from 800 nm to 2,400 nm by varying film thickness and annealing temperature. This phenomenon likely reflects competition between crystal growth and free surface mass transport during the crystalline to amorphous conversion. In contrast to other methods that can induce patterns in organic thin films, this method is capable of producing aligned patterns, without additional anisotropic stress application or substrate patterning. This talk will examine the role of metal-oxide underlayers as a means to engineer the kinetics of film transformation and hence, pattern periodicity. We find that an MoO_x underlayer results in lower crystal growth rate and longer pattern period, suggesting an important role for the film-substrate interaction.