Comparative Study On Self-Assembled Dicyanoanthracene Molecular Films Grown On Cd(0001) And Bi(111)

Molecular self-assembly is the spontaneous combination of molecules into stable and well-defined aggregates connected by non-covalent bonds under equilibrium conditions.Molecular self-assembly is considered to be an effective way to construct twodimensional nanostructures with controllable interface properties and is widely used in materials,molecular photonics,sensors and other fields.The interface structure and monolayer formed between organic molecules and solid surfaces play a key role in the morphology,quality,transport and optical properties of the films in the later period.In the process of self-assembly,the interaction between molecules and substrates plays a very important role in the growth of organic thin films.Molecules have different performances on different substrates.For metal substrates and semi-metal substrates,the interaction between them and molecular thin films will be obviously different due to different electronic structures.In this thesis,the adsorption and self-assembly growth of dicyanoanthracene(DCA)molecules grown on the surface of metal Cd(0001)and semi-metal Bi(111)were studied by ultrahigh vacuum-low temperature-scanning tunneling microscope(UHV-LT-STM)and molecular beam epitaxy(MBE).It was found that when a small amount of DCA molecules were deposited on the surface of low temperature(100 K)Cd(0001),the existence of a single DCA molecule was observed.When the deposition amount was increased to 0.3 ML,the three-dimensional self-assembly structure of DCA island was observed under the coverage of sub-monolayer,and the molecular heights of the first layer,the second layer and the third layer were different,and the height increased with the increase of the number of layers.The stability of DCA double-layer islands observed is poor,and the STM tip will destroy the island morphology and the number of molecules it contains during scanning.After raising the substrate temperature to room temperature(300 K)and increasing the coverage to 0.4 ML,many DCA islands adsorbed on the edge of Cd steps are observed,and the island morphology is different,and the existence of multi-layer islands is not observed.After increasing the coverage to 0.7 ML,it was observed that the size of DCA monolayer islands increased significantly,and molecular islands appeared not only at the edge of the step,but also in the central area of the substrate mesa.After the coverage is increased to 1 ML,the DCA molecular monolayer film covering the whole substrate surface can be observed,and its monolayer has an obliquely symmetric lattice,in which one crystal phase is parallel to the basal vector direction of the substrate.According to the measurement,DCA molecules are arranged in 4×√13 reconstruction phase.After increasing the coverage,no multilayer molecular thin film structure was observed.The increase of coverage will lead to the destruction of molecular thin films,and the structure will change from order to disorder,and the disordered region will become larger with the increase of coverage.After a small amount of DCA molecules were deposited on Bi(111)at low temperature(100 K),molecular clusters were observed,but no single DCA molecules were observed.The molecular clusters were orderly in structure and had a deviation angle of 6 from the substrate.When the coverage is increased to 0.2 ML,it is observed that the molecules form a larger sub-monolayer island,and the DCA molecules are arranged in an oblique structure,which is not congruent with the substrate,thus the existence of Moire fringes is observed,and its transverse period is about 2.6 nm,which is consistent with the calculated Moire fringe period.After increasing the coverage to 1 ML,the molecular thin film covered the whole surface of the substrate,but there was no second molecular island.In addition,three domains with different molecular orientations were observed,in which the deflection angle of the molecular lattice of domain II and III was +6°,while that of domain I was-6°.When DCA molecules were deposited at room temperature(300 K),the traces of DCA molecules could not be observed by STM scanning.When annealing low-temperature samples,molecules will escape from the sample surface at room temperature and above.