Theoretical Study On The Structure And Formation Mechanism Of Interfacial Molecular Self-Assembly

Self-assembly is a ubiquitous phenomenon in nature.As a practical strategy for the preparation of nanomaterials by "bottom-up" approach.molecular self-assembly is widely used in materials science,biomedicine,nanofabrication and drug design.Molecular self-assembly at interfaces can not only create new materials,but also has methodological significance.In this thesis.the simulation strategy suitable for revealing the complex structure and behavior of interfacial molecular self-assembly was studied by combining the advantages and characteristics of density functional theory(DFT).molecular mechanics(MM)and molecular dynamics(MD).With the aid of scanning tunneling microscopy(STM)observation data,self-assembly systems containing 5-(benzyloxy)-isophthalic acid derivative(BIC),Oligo(p-phenylenevinylene)derivatives(AS-OPV)and meso-diphenyltetrathia[22]-annulene[2,1,2,1](DPTTA)molecules were studied separately through the combined simulation method we proposed.In addition,the effects of environment factors including solvent,temperature,substrate and molecular density on self-assembled structure were investigated,and the self-assembly structure and formation mechanism were revealed and predicted.The main research contents and results are as follows:Firstly,based on simulation calculation and the STM experimental data as an aid,a set of combined simulation method that can accurately identify the self-assembled atomic structure and formation mechanism was proposed.DFT calculation was used to obtain accurate structural parameters,and took its advantage of calculating electronic property to realize the simulation and prediction of STM image.With the ability of MM quickly calculating the large and complex structure,self-assembly model was optimized,and the force field suitable for research system was determined.Cooperating with MD to obtain the dynamic information of system,the evolution of ordered structure was simulated,and the stability of self-assembled structure was studied.Meanwhile,with the help of STM experimental data,the self-assembled model was established and the reliability of simulation result was verified.The combined simulation method was elaborated with the example of S-OA/BIC/HOPG system,ie the self-assembly of BIC and(S)-(-)-2-octanol(S-OA)molecules on highly oriented pyrolytic graphite(HOPG)surface.Secondly,the effect of solvent was studied following the combined simulation method.The self-assembly of BIC on HOPG surface were simulated under chiral(R)-(-)-2-octanol(R-OA)and achiral 2-octanol(OA)conditions.Combining the simulation results of configuration,energy and hydrogen bond characteristic parameter,it revealed that different types of solvents can induce the homochiral or racemic self-assembled structure.The formation rule of chiral configuration was based on the orientation of the methyl group attached to the chiral carbon atom of solvent relative to the substrate.Different orientations result in the difference of steric hindrance between solvent molecules and substrate,causing the different strength of hydrogen bonding interaction in the chiral configuration.The simulation results were consistent with the experimental STM observations.confirming the applicability and reliability of the combined simulation method.Thirdly,the study of temperature effect was based on the self-assembly of AS-OPV molecules on HOPG surface.According to the combined simulation method,the atomic structure of AS-OPV/HOPG self-assembled system was accurately identified,and the self-assembly behavior from 248 K to 573 K was predicted.By the simulation of configuration evolution and the calculation of weak interaction energy,the critical temperature that AS-OPV/HOPG self-assembly transformed into disordered structure was predicted to be 398 K.The STM images below the critical temperature were also simulated.The analysis of structure,weak interaction and electronic properties revealed that Van der Waals forces played a leading role in AS-OPV/HOPG self-assembly.The reason for the disordered structure above the critical temperature was that the alkoxy branch of the AS-OPV molecule was distorted,resulting in a weakening of the van der Waals interaction.Finally,the effects of substrate and molecular density on the self-assembly of DPTTA molecules were investigated.The combined simulation method was used to deterrmine the structure and evolution of DPTTA molecules on Au and HOPG substrates under different molecular densities.The calculation results of weak interaction,adsorption energy and electronic property revealed that the different configurations of DPTTA molecules on Au and HOPG substrates were due to the difference in molecule-substrate interaction intensity.Based on the construction of different molecular density models,structural evolution simulation and adsorption energy calculations,the stability of models were evaluated and the critical molecular density of DPTTA/Au self-assembly was determined.The DPTTA/Au self-assembled structure was thus identified and its STM image was simulated.In summary,using the flexible and reliable combined simulation method proposed in this study,the atomic structure and behavior of interfacial molecular self-assembly under different assembly environments were investigated.The combined simulation method successfully applied and verified in(S/R-)OA/BIC/HOPG,AS-OPV/HOPG and DPTTA/Au self-assembly systems.The causes and rules of the above self-assembly process were revealed.The critical transition temperature and molecular density of the self-assembled structure were predicted and the STM images were also simulated.This research provided a convenient and effective way to study the structure and transformation behavior of interfacial molecular self-assembly mainly based on theoretical calculation.