Structure Modulation Of Molecular Self Assembly

Multicomponent organic films have increasing applications in photovoltaictechnologies and other electronic devices. These applications mainly depend on thenanoscale morphology and electronic structure of the heterojunctions. A comprehensiveunderstanding of the self-assembly process of organic molecules on surface as well as thepathway to control it will contribute to the fabrication and optimization of themolecular-based nanostructures for applications. This thesis reports a detailedinvestigation of these important aspects by using STM, such as template effects on surfacepatterns and structure control in molecular film heterojunctions, by selecting C₆₀/AnCAand C₆₀/SrTiO₃as model systems.At first, surface self-assembly process of AnCA on Ag （111） was investigated usingSTM. Depending on the molecular surface density, four spontaneously formed and oneannealed AnCA ordered phases were observed, namely Phase Ⅰ^Ⅴ. Phase I is a parallelbelt phase, Phase Ⅱ is a zigzag double-belt phase, Phase Ⅲ and Ⅳ are two simpler dimerphases, and Phase Ⅴ is a kagome phase. This structural diversity stems from a complicatedcompetition of different interactions of AnCA molecules on metal surface, includingintermolecular and molecular-substrate interactions, as well as the steric demand fromhigh molecular surface density.Two AnCA self-assembly structures on Ag （111）, Phase Ⅲ and Ⅳ, were employedto investigate the template effect on sequentially deposited C₆₀molecules using STM. Theinitial AnCA structures execute strong modulations on C₆₀growth. For AnCA Phase Ⅲtemplate, C₆₀molecules can assemble into chains and small trigonal islands at lower C₆₀coverage, and maintain separated domains from AnCA with tens of nanometer in domainsize at higher coverage. For AnCA Phase Ⅳ template, an epitaxial adlayer structure of C₆₀dimers can be realized on top of AnCA, forming a nice interface between these twomolecular species. These distinct C₆₀growth modes are closely related to the structuralstability of the AnCA templates.By choosing different sample annealing conditions and Ar+sputtering, distinct nanostructured SrTiO₃（001） surfaces were fabricated. The surface patterns wereinvestigated with STM, and the observed nanostructures were assigned. Sequential C₆₀deposition onto these nanostructured templates reveals distinct growth modes, includingdiscrete small C₆₀islands on the c（4×2） reconstruction surface, parallel one-dimensionalC₆₀chains on （6×2） dilines, C₆₀double chains on （8×2） trilines, epitaxial C₆₀closepacked adlayers over （11×2） tetralines, and two-dimensional ordered C₆₀dimer arrays on（7×6） waffles. Our observations demonstrate the effectiveness of nanostructured oxidesurfaces as templates in controlling molecular assembly.By employing C₆₀/AnCA and C₆₀/SrTiO₃as model systems, I investigated thecontrolled formation of heterojunction architecture, measured the orientation andseparation of donor-acceptor molecules along the domain boundaries. These systematicalworks shed lights on the optimization of molecular electronic devices from a fundamentalmicroscopic perspective.