Conjugated organic nanostructures that are flexible and featuring unique electronic properties are promising candidates to build functional molecular devices. However, the fabrication of a single conjugated organic layer remains a great challenge to the researchers. In this regard, the investigation of the on-surface synthesis process as well as the strategy of synthesizing extended conjugated organic monolayer are likely important. This thesis mainly studies the fabrication of low-dimensional conjugated organic and metal-organic nanostructures through on-surface synthesis on Au(111), Cu(111) and Ag(111) substrates. Scanning Tunneling Microscopy is utilized as an experimental tool to study the structures and assist to understand the on-surface reaction process at a single-molec...[ Read more ]

Conjugated organic nanostructures that are flexible and featuring unique electronic properties are promising candidates to build functional molecular devices. However, the fabrication of a single conjugated organic layer remains a great challenge to the researchers. In this regard, the investigation of the on-surface synthesis process as well as the strategy of synthesizing extended conjugated organic monolayer are likely important. This thesis mainly studies the fabrication of low-dimensional conjugated organic and metal-organic nanostructures through on-surface synthesis on Au(111), Cu(111) and Ag(111) substrates. Scanning Tunneling Microscopy is utilized as an experimental tool to study the structures and assist to understand the on-surface reaction process at a single-molecular level.

The thesis is composed of five projects as below:

In the first project, I used porous supramolecular structures as templates to make two-dimensional (2D) superlattices of Bi nanoclusters on a Au(111) surface. The Bi atoms were confined in the supramolecular pores and formed nanoclusters of a critical size that is defined by the pore size. These nanoclusters were arranged as a 2D superlattice dictated by the structure of the supramolecular templates. The nanocluster size and superlattice periodicity can be adjusted by appropriately designing the supramolecular structures.

In the second project, I synthesized a single layer of two-dimensional metal-organic framework on a Cu(111) substrate using benzenehexol molecules. The structure of the framework was determined to be Cu₃(C₆O₆) which is stabilized by O-Cu-O bonding motifs combing techniques of scanning tunneling microscopy, X-ray Photoelectron Spectroscopy and Density-Functional Theory calculation. I find that the O-Cu-O bonds offer efficient charge delocalization, giving rise to highly dispersive electronic bands with an effective mass of 0.45 m_e at the bottom of the lowest conduction band. The free-standing single layer is metallic, whereas upon adsorption on Cu(111) it becomes an n-doped semiconductor with a band gap of 1.5 eV.

In the third project, I studied Sonogashira cross coupling between terminal alkynes and aryl halides at room temperature on Au(111) surface assisted by templates and Pd catalysis. The homo coupling of aryl halides was sufficiently suppressed through forming a robust metal-organic templates. By using the highly active catalyst Pd, the Sonogashira cross-coupling reaction has been effectively promoted to afford a yield of ~67%.

In the fourth project, I synthesized four- and six-member cyclic products via sequential multi-step aryl-aryl coupling reactions of 2,3,6,7,10,11-hexabromotriphenylene molecules on a Au(111) surface. The final products as well as the organo-gold intermediate structures were identified using Scanning Tunneling Microscopy and Density-Functional Theory simulation. By adjusting reaction temperature and post-annealing temperature, the yields of the four-member and six-member cyclic products were enhanced/suppressed. This work exemplifies intricate kinetically-controlled on-surface synthesis when multiple reactions of different reaction order are involved.

In the fifth project, I investigated on-surface [2+2] cycloaddition reaction of 2,3,6,7,10,11-hexabromotriphenylene on Ag(111), Cu(111), and Cu-dosed Au(111) surfaces. The reactions on the four surfaces exhibit distinctive behavior: On Ag(111), the derbominated odb groups bond with Ag trimers to form extended organo-silver networks that are an intermediate of the [2+2] cycloaddition; On Cu(111), similar organo-copper networks are formed, which is very stable and does not undergo [2+2] cycloaddition up to 290°C; On Au(111), no ogano-gold structure is observed; On Cu-dosed Au(111), the debrominated odb groups bond with Cu dimers to form an intermediate of the [2+2] cycloaddition. This systematic study sheds light on the pathway of the [2+2] cycloaddition reactions taking place on different surfaces.

In summary, these studies may contribute to the fabrication and characterization of low-dimensional conjugated framework.