Micro-Aperture Controlled Nucleation and Solid-Phase Homoepitaxy of Single-Crystal rac-BINAP Thin Films

Abstract

Crystalline organic semiconductors can exhibit improved charge transport compared with amorphous films, but their use in thin-film devices is limited by the difficulty of controlling nucleation, grain boundaries, and crystal orientation. This thesis investigates controlled crystallization in thin films of rac-BINAP, a model crystallizable organic semiconductor. First, micro-aperture shadow masks were used to confine the deposited rac-BINAP volume and geometry before thermal annealing. By reducing the available nucleation volume, aperture confinement biased crystallization toward isolated single-grain deposits. Single-grain yield was found to depend on aperture size and geometry, with 200 micron square apertures giving the highest yield of 30%, close to the theoretical maximum in an intermediate confinement regime. Then, aperture-confined single-grain rac-BINAP deposits were used as crystalline seeds for solid-phase homoepitaxy. An amorphous rac-BINAP overlayer was deposited onto the crystalline seed and then crystallized by thermal annealing. Under optimized conditions, a 100 nanometer overlayer on a 150 nanometer seed crystallized laterally from the seed, producing a single-domain or nearly single-domain region over hundreds of microns. Growth-front measurements showed that the propagation velocity decreased with time, suggesting that the process may be influenced by mass transport and evolving film morphology rather than purely interface-limited growth.