Investigation On The Assembling Of Oligothiophene-Graphene/Graphite Interface Structures By STM

For the photoelectric devices, the interfacial structure plays a crucial role in the performance. Due to its extraordinary conductivity and transparency, graphene is considered as an excelent material as transparent conducting electrode. Thus, investigations on the controlling and tuning of graphene-organic semiconductor interfacial structure have both fundamental and applicantion significance. To date, most efforts with respect to the assembly at the semiconductor-graphene interface focus on graphene prepared by epitaxial growth by Scanning tunneling Microscopy(abbreviated STM) under ultrahigh vacuum condition, hence suffering from much limit. Graphene prepared by chemical vapor deposition(abbreviated CVD) is facile to transfer to other substrates and convenient for applying. Few reports are published on how the microcosmic structures of CVD graphene such as grain boundary, step, ripple and wrinkle affect the assembly of organic molecules on graphene. In this graduation thesis, we investigate the assembling and tuning of oligothiophene- graphene/graphite interfacial structures, and the corresponding influencing factors on them by STM under ambient conditions. The interfacial structures are built and tuned based on the molecular non-covalent interactions and reversible imine covalent bond. The investigations contain the effect of functional groups and defect-like structures of graphene on the interfacial structure, as well as preparation, assembling and reaction process of 1D/2D Schiff base polymers from oligothiophene. The main contents are as follows:Firstly, functional groups, solvent, and substrate effects on the assembly of oligothiophene on graphene are studied by STM under ambient conditions. We synthesized four oligothiophenes substituted by β-alkyl group and with different number of aldehyde/carboxyl functional groups in α-position of each molecule backbone, by which the assembling structures can be tuned at the oligothiophene-graphene interface. The results reveal that diverse hydrogen bonds constructed by end-substituted functional groups contribute to the various organizations of oligothiophenes on graphene, changing from the lamella with single molecular line into different lamella with double molecular lines, and network structure. Significant solvent and substrate effects have also been confirmed by comparing the assembled structures of oligothiophenes dissolved into tetrahydrofuran, 1,2,4-trichlorobenzene, and octanoic acid on both graphene and graphite. The results demonstrate octanoic acid molecules co-adsorb with QT molecules on substrates because the interactions between QT molecules are very weak. QTDA assemble into different structures on HOPG from that on graphene. The assembled structures of other molecules are mostly similar in the different solvents or on HOPG/graphene substrates.The defect and defect-like structures of graphene were illustrated to change the distribution of density states of graphene and generate new defect related states. These alterations may affect the interaction between graphene and organic semiconductors assembled on it. Therefore, we have studied how the microcosmic structures on CVD graphene affect the assembly of organic molecules under ambient conditions. We found that the supralattice structures have neglectable influence on the assembling behavior of oligothiophenes. Oligothiophenes can stride over the underlying steps and assemble into continuous structures similar to that on the surface of terrace. However, the ripples and wrinkles with different curvatures can alter the molecule-substrate interaction in different degree. The larger the curvatures of the ripples or wrinkles are, the harder the molecules assemble on their surface. The increased local curvature on the wrinkles can provide strong enough perturbation to break the six-fold symmetry of graphene, which is responsible for the weakening of the interaction between oligothiophene and graphene.In addition, one-dimensional/zero dimensional polymers/oligomers are prepared on HOPG with on-surface Schiff base reactions between oligothiophene and aromatic diamines, the reaction process and the assembling structures of the products are studied by STM under ambient conditions. QTD/QTA with two/one terminal formyl groups and PDA/TPDA are used as monomers for the surface reaction. We adopt the premixing method to prepare the one-dimensional polymer and oligomer. To investigate the evolvement process from monomers to resultant polymers and the process of reactions, we have controlled the ratio of precursors and reactive temperature, and obtained the high resolution STM images of the coexistence of monomer and one-dimensional polymer. Two possible reactive ways are proposed according to the STM images. The results suggest that preferential adsorption and assembly of QTD than PDA/TPDA has a great influence on the on-surface Schiff base reaction. The difference in the length of the diamine backbones determines the dominant pathway during the reaction process. Evaporation method is also applied for the surface-confined synthesis of one dimensional imine polymer. Meanwhile, XPS and DFT calculation are also used to confirm the surface reaction and the imine bond formation in the products.Lastly, two-dimensional polymers are prepared by the Schiff base reaction of QTD with TAPP on HOPG with the same method, and the assembled structures and controlling factors of the product topology are investigated by STM under ambient conditions. The geometrical and electronic structures of the two-dimensional materials are investigated with state-of-the-art DFT calculations. Both liner-pattern and network structures are obtained on HOPG by controlling the ratio of precursors in the pre-mixing solution under ambient conditions. STM reveals that the flexible QTD backbones adopt various conformations in the two-dimensional materials, which is responsible for the distorted polygons of the products. And when the ratio of aldehyde to amino group changed from 1:4 to 1:1.5, the structures composed of mainly compact linear polymers are observed. The preferential adsorption of QTD plays a dominant role in changing the morphology of two-dimensional polymer. The geometrical and electronic structures of these two-dimensional materials are investigated with state-of-the-art DFT calculations. The result reveals that COFTAPP-QTD is planar squares and it is a two-dimensional organic semiconductor with band gap around 1 e V.