In the rapidly developing electronics industry, it has become increasingly necessary to explore materials that are cheap, flexible and versatile which have led to significant research efforts towards organic molecular thin films. Understanding and control of heterointerface between highly ordered organic molecular thin films with extended π systems and inorganic materials are therefore of critical importance for the development of modern organic electronics. Organic molecules are unique compared to their inorganic atomic counterparts as their properties can be tuned drastically through chemical functionalization, offering versatility, though their extended shape and weak intermolecular interactions bring significant challenges to the control of both the growth and the electronic structures of molecular thin films. This is further complicated by interaction between organic molecules and the underlying substrate which can lead to interfacial effects such as charge transfer, chemical interaction and electrostatic screening, all of which can significantly impact device performance and/or the characteristic of the organic thin film. This dissertation will first focus on a systematic review of the growth and electronic structure of organic molecular thin films, particularly on weakly interacting substrates. The self-assembly process and how long-range ordered organic molecular thin films are established will be discussed. We will also discuss how the electronic structures of thin films are impacted by the molecule’s local electrostatic environment and its interaction with the substrate, within the context of controlling interfacial energy level alignment between organic semiconductors and electrodes in electronic devices. Employing scanning tunneling microscopy and spectroscopy, experimental studies focusing on characterizing the growth and electronic structure of organic molecules on weakly interacting substrates were carried out and discussed. Studies focusing on the electronic structure of zinc phthalocyanine (ZnPc) and its fluorinated counterpart F16ZnPc were carried out on the deactivated Si(111)-B surface and h-BN/Cu(111). We show that interfacial charge transfer occurs between the deactivated Si(111)-B substrate and the F16ZnPc monolayer, which gives rise to a pronounced spatial variation of the occupied molecular state across the molecular assembly attributed to the inhomogeneous electrostatic screening of the intra-orbital Coulomb interaction in molecular adsorbates arising from the substrate boron distribution in the deactivated Si(111)-B substrate. To circumvent this inhomogeneous effect, the donor-acceptor molecular pair was studied on weakly interacting hexagonal boron nitride (h-BN)/Cu(111) which possesses a periodic electronic corrugation. We show that the formation of the lateral heterostructure drastically increases the charge transfer between F16ZnPc molecules and the substrate, which is attributed to the greater electrostatic stability of the heterostructure compared to that of the pure phase. This study highlights the importance of the substrate, even a weakly interacting one, such as h-BN/metal, can still perturb the intermolecular charge transfer and thereby the heterostructure behaviors via interfacial processes. The focus of a secondary study was to initiate preliminary experimentation towards understanding the substrate’s influence on the exotic properties of a class of organic-based systems known as charge transfer complexes (CTC). By utilizing the unique modulation properties of various weakly-interacting substrates, control of the properties of CTCs could be attained allowing for a better understanding of their fundamental physical mechanism to be developed and a new class of thin-film CTCs which will be highly relevant towards organic electronics to be developed.
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