| Organic molecular thin films have been widely studied in the fields of optoelectronics and superconductivity.With the development of experimental techniques,it has been possible to study the growth process of thin films at the atomic-molecular level.Theoretical calculations show that triphenyl bismuth(TPB)molecular crystals exhibit Kagome lattice morphology and are a kind of two-dimensional organic topological insulator with quantum edge states.Two-dimensional organic topological insulators with robust conducting edge states are not affected by elastic backscattering and electron localization,and have potential applications in quantum information and spintronics.It has been shown that superconductivity occurs when TPB is doped with potassium at a low temperature of 3.5 ～ 7.2 K.However,the epitaxial growth of TPB molecular films on different substrates using Scanning Tunneling Microscope(STM)has not been reported in the literature yet,so a suitable substrate needs to be selected to probe the self-assembly behavior of TPB molecules needs to be investigated by selecting suitable substrates.10,10’-bromo-9,9’-bianthryl(DBBA)molecule has received much attention as a chiral precursor for the preparation of graphene nanoribbons.The deposition of DBBA molecules on the surface of precious metals such as gold,silver and copper can make the DBBA molecules self-assemble to form graphene nanoribbons after dehalogenation by high temperature annealing at about 680 K.However,no report has been seen on the preparation of graphene nanoribbons by depositing DBBA molecules on the surface of Cd(0001),so Cd(0001)was chosen as the substrate for depositing DBBA molecules in this experiment.In this thesis,TPB and DBBA molecules grown on the surface of Cd(0001)were selected as the objects of study,and the nucleation,growth and adsorption induced substrate reconfiguration of the two molecules were investigated by STM and Density Functional Theory(DFT).(1)Adsorption of TPB molecules on the Cd(0001)surface.At low coverage,TPB molecules first coalesced into multimers,and with the increase of deposition,TPB molecules started to grow along the step edges.When the coverage increases to 0.3 ML,TPB molecules adsorbed on the Cd(0001)surface form not only two-dimensional selfassembled submonolayer islands,but also a mixed phase consisting of TPB molecules and Cd atoms.The oblique lattice of the two-dimensional self-assembled submonolayer islands corresponds to the substrate forming a 4 × √13 reconstruction;in the mixed phase composed of TPB molecules and Cd atoms,the grooves formed by the articulated chains of Cd atoms are separated by the two-dimensional self-assembled TPB structural domains.When the coverage increases to 0.8 ML,the mixed phase consisting of the original TPB molecule and Cd atoms forms a structural domain consisting of Cd atom chains only.Among them,the Cd atom chain corresponds to the substrate as a 2 × 2 higher order metric phase,while in the mixed phase,the Cd atom chain exhibits higher order metric only when it is restricted by the self-assembled structure of the TPB molecule.The occurrence of large-scale surface rearrangements and long-range large-scale transport of substrate atoms can be attributed to molecule-substrate interactions,which are observed for the first time on hexagonal dense-row metal surfaces.When the coverage is increased to 1 ML,TPB molecules form monolayer films of hexagonal dense-row structure on the Cd(0001)surface,corresponding to a substrate of 4 × 4 high order metric phase.DFT calculations show that the adsorption energy of TPB molecules on the defective Cd(0001)surface is larger than that on the perfect Cd(0001)surface.Therefore,the formation mechanism of Cd recombination can be attributed to the strong TPB-Cd interactions.(2)Adsorption of DBBA molecules on the Cd(0001)surface.By depositing molecules at different coverages,it was found that only at low coverages,DBBA molecules adsorbed freely as single molecules or clusters and extended along the step edges of Cd(0001).With the increase of molecular coverage,the molecules coalesced into dimers and formed further critical islands and islets at the step edges.When the molecular coverage increased to 0.6 ML,the DBBA molecules formed a two-dimensional monolayer nanoribbon structure on the surface of Cd(0001),each nanoribbon was composed of multiple bimolecular chains,and the visible height of the monolayer nanoribbon was around 0.35 nm.At a coverage of 1.5 ML,bilayer and trilayer DBBA molecular nanoribbons appeared on the surface of Cd(0001).In addition,we found that when the DBBA molecules were adsorbed on the Cd(0001)surface,the originally smooth Cd(0001)step edges were "eroded" after the deposition of DBBA molecules.Due to the chemical reaction between halogenated elements and metal Cd under high temperature conditions,the substrate Cd atoms undergo long-distance mass transport,and the eroded part migrates to the table surface near the Cd(0001)step edge under stress,forming irregular islands.Thus,the deposition of halogenated organic molecules onto the surface of metal substrates results in changes in the substrate surface morphology in addition to the molecular self-assembly that occurs.Understanding the process and mechanism of these special reactions is very important for understanding the change of substrate surface morphology,and also provides new ideas for further understanding the adsorption of halogenated organic molecules on the metal surface. |
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