Formation and nanoscale characterization of 2D molecular self-assembly and metal-organic nanostructures on surfaces by scanning tunneling microscopy

Abstract

There is an increasing interest for two-dimensional (2D) assemblies of organic molecules on surfaces with a wide range of potential applications including organic electronic devices. Due to the large surface area of porous networks that molecular assemblies can form, a significant number of studies have been dedicated to examining their electronic properties as well as potential applications as storage devices where a targeted molecule or atom is held within a pore of the framework through adsorption. The goal of this project was to create 2D molecular assemblies made of molecules with carboxyl groups, as well as utilizing them to explore their interaction with metal atoms towards creating metal-organic nanostructures. The metal-carboxyl coordination bond is both planar and of sufficient strength to create stable frameworks.2 The experiments were carried out through a bottom-up epitaxy deposition on Ag(111) and Au(111) under Ultra-High Vacuum (UHV) conditions. Co-adsorption of organic molecules and metal atoms and subsequent annealing was the driving force for the interaction between the two entities to form coordination bonding. The two studied precursors were 4,4',4'',4'''-(Pyrene-1,3,6,8-tetrayl)tetrabenzoic acid (PTTB) and 1,3,5-Tris(4-carboxyphenyl)benzene (TCPB) with three and four carboxyl groups, and D2h and D3h symmetry, respectively, to form SAMNs. The deposition time and post-annealing temperature were the factors identified in the three PTTB self-assembled structure formed on Ag(111). Their carboxyl functional groups acted as participating sites in coordination bonding with deposited niobium metal atoms. The introduction of metal atoms was achieved using an electron beam evaporator, with a AgCu alloy allowing for the co-deposition of Ag and Cu atoms, and a solid Nb rod used as a source of niobium atoms. Scanning Tunneling Microscopy (STM) was the primary nanoscale characterization technique to analyze the overall surface coverage, network structure and unit cell parameters, and bonding mechanism of the resulting frameworks from the deposition of these materials on single crystal surfaces.