Single-molecule Chirality And Chiral Self-assembly Induced By Molecular-substrate Interaction

In recent years,with the further research of photodiode,solar thin film cell,field effect transistor and other technical fields,more and more attention has been paid to the microstructure of thin film surface.Thin film materials composed of phthalocyanine compounds have been widely used in gas sensors,organic thin-film transistors and new energy batteries due to their advantages of stable performance and high average carrier drift velocity.Naphthalocyanine（H₂Pc）molecules are a very special class of phthalocyanine molecules,in which two hydrogen atoms replace the metal atoms（such as cobalt,iron,copper,manganese,etc.）in the center of the phthalocyanine（TMPC）molecules.Due to its quadruple symmetry and stable chemical properties,the electron,optical and magnetic properties of H₂Pc molecule can be adjusted.Therefore,H₂Pc molecule has become a research hotspot in the fields of biosensors,molecular magnetism and photochemistry,and has been widely used in molecular devices.Chirality exists widely in nature and has fundamental significance in physics,chemistry,life,medicine and materials science.The chirality of phthalocyanine has been studied,including chiral amplification,chiral inversion,chiral recognition and enantiomer separation.STM is an important detection and characterization technology in the field of atomic and microscopic research.It can not only obtain the morphology characteristics of the sample at the nanoscale,but also has the function of STS,which can explore the electron distribution morphology on the sample surface,and can realize the chiral recognition,chiral separation and direct observation research of chiral application.In this paper,the chirality and chiral self-assembly induced by the interaction between naphthalocyanine（H₂Pc）and Cd（0001）substrate with quantum well effect have been studied by low temperature tunneling microscopy（LT-STM）.The whole experiment was carried out at the liquid nitrogen temperature（78 K）.Under the condition of extremely low coverage,we observed the H₂Pc monolecule on the Cd（0001）substrate.It can be found that the molecular morphology and molecular structure model were not completely consistent,because of the force between the molecule and the substrate,the molecular leaf flap was arranged along the substrate crystal direction.Further increasing the coverage of H₂Pc molecules,the bimolecular,molecular chain and multi-molecular cluster of H₂Pc molecules can be observed successively.Careful observation can be found that the intermolecular force increases with the increase of molecular coverage,making part of the molecules and the substrate radially form an angle of 8°.With the further increase of molecular coverage,the second layer of molecules showed a"slanting"arrangement.Especially,when the coverage was 0.8 m L,we observed that M-type H₂Pc molecule had obvious chiral characteristics,which was manifested by the obvious bending phenomenon of the four phthalocyanine groups of H₂Pc molecule.The chiral morphology characteristics will change with the difference of scanning bias,and the chiral characteristics will gradually weaken or even disappear at high bias,showing a bias dependence.According to DFT calculation,The occurrence of chirality and bias dependence is mainly due to the asymmetric charge transfer between the H₂Pc molecule and the substrate Cd（0001）.More surprisingly,when the coverage was 1.2 ML,H₂Pc molecules did not immediately form a second layer,but instead formed a triangular structure of H₂Pc monolayers,which was completely different from the quadruple symmetry of H₂Pc molecules and consistent with the phenomenon observed on Ag（110）and Ag（111）.At present,there are no reports that H₂Pc molecules exhibit triplet symmetric properties on Au（111）and Ag（111）metal substrates.What is more surprising is that the chiral structure is also found in the triangular structure,which we speculate is mainly due to the asymmetric charge transfer between the molecular lobes and the adjacent molecular lobes.