Investigating On-surface Self-assembled Nanostructures Of Functional Organic Molecules Via Scanning Tunneling Microscopy

In the field of science,the"bottom-up"surface synthesis technology has become one of the important methods to fabricate low-dimensional materials.The surface synthesis can be classified into two main fields:molecule on-surface self-assembly and molecule surface reaction.On-surface self-assembly is the spontaneous construction of building blocks on the surface into order structures controlled by non-covalent intermolecular interactions.Surface reaction is two-dimensional confined reactions that occur on surfaces.Surface reaction uses specific precursors to synthesize various compounds and two-dimensional structures through various chemical processes such as dimerization,polymer reaction,addition reaction and cyclization.Organicπ-conjugated compounds have interesting structures,optical and electronic properties.The adsorption and electronic structures ofπ-conjugated molecules on surfaces have been studied extensively.On the other hand,specific functional groups can affect self-assembly and surface reactions.Melamine is a triazine compound containing amino groups.Melamine is one of the common precursors for the construction of self-assembled structures and the preparation of covalent structures on the surface.Terminal alkynes have found a broad range of applications in the field of surface synthesis.Terminal alkynes can form not only C-C covalent structures,but also metal–organic compounds with metal atoms.In the thesis,melamine,ethynyl-phenanthrene(EP)and ethynyl-iodophenanthrene(EIP)molecules are used as precursor molecules.We have studied the adsorption of these three molecules on Ag(111)and Au(111)by scanning tunneling microscopy and spectroscopy(STM/STS),and attempted to synthesize covalent net or polymer chains from them on the surface.And we have studied the electronic state properties of the two-dimensional ordered structure,especially the molecular state information.The main parts of thesis include:1.We studied the magic number clusters of melamine by depositing a small amount of melamine on Ag(111).They were a set of ordered and chiral self-assembled clusters of melamine on Ag(111).The clusters were possibly stabilized by intermolecular interaction and adsorption-substrate interaction.And the stable clusters were composed of molecules with magic number.Our study was expected to have some significance for the growth of large scale ordered chiral clusters.2.The large amount melamine molecules were deposited on Ag(111)surface.We systematically investigated the adsorption as well as possible surface-induced transformations of the precures on Ag(111)in a wide annealing temperature range in UHV,employing STM/STS.A new phase were studied by STM and STS.Employing tiling analysis,γphase can be expressed with the(6~3)Archimedean tiling.And this architecture exhibits hierarchic orders.The hexagonal-core arrays induce a chiral honeycomb electronic lattice on the Ag(111),which could potentially invoke designer Dirac fermions as well as flat band.3.We studied the local density of state of the(3.4.6.4)Archimedean tiling and the role of iodine atoms in the system by experiments and theoretical calculations.In the experiment,the(3.4.6.4)Archimedean tessellation composed of the local density of electron states(LDOS)distribution was obtained at 250 m V,which we speculated was due to the contribution of molecular orbitals.Iodine atoms as guest atoms interact with the substrate.And iodine atoms interact with the basic building block bis(phenanthren-2-yleythnyl)silver(BPE-Ag)molecule.We analyzed the adsorption position of iodine atom,the type of hydrogen bond in the tessellation and the charge density difference plots of this structure with iodine.4.STM and STS were used to study the structural characteristics and LDOS properties of BPE-Ag molecular dimers,and the LDOS information of three complex supramolecular semi-regular Archimedean tiling architectures.Experiments show the effect of BPE-Ag molecular dimer on the electronic states of three structures.And the Archimedean tessellations composed of LDOS distribution were obtained at the high bias voltage.It was guessed that this was due to the contribution of superatom molecular orbitals.5.Room-temperature adsorption of EP and EIP precursors on Au(111)were investigated.EP did not react on Au(111),and they formed the organized chiral EP molecular trimers via hydrogen bonds.RT deposited EIP molecules are partially deiodinated,leading to terminal alkyne activation and concomitant covalent linking between precursors.The experiment and theoretical calculation evidences that a possible radical-promoted alkyne activation which facilitates a polymerization process in a mild temperature regime.And the strong radical-substrate bond increases the stability of deiodinated species at the beginning of the reaction.