Growing two-dimensional honeycomb metal-organic frameworks

Abstract

This thesis describes a study on two-dimensional metal-organic frameworks with a honeycomb geometry on weakly interacting metal surfaces. The aim of the study was to gain insight into the growth of these networks and characterisation of their electronic structure using a combination of scanning tunneling microscopy, atomic force microscopy and scanning tunneling spectroscopy. Three different diphenylacetylene molecules were deposited on weakly interacting metal surfaces: bis(para-benzoic acid)acetylene (BPBA), bis(para- benzonitrile)acetylene (BPNA) and bis(para-pyridyl)acetylene (BPPA). BPBA and BPNA were co-deposited with cobalt atoms on Au(111) in an attempt to grow metal-organic frameworks. The experiments of BPBA, BPNA and BPPA on Au(111) resulted in a wide range of different surface structures, both close packed and open. It was found that surface coverage plays a large role in the distribution of these structures; samples with a low surface coverage generally yield more open structures, whereas samples with a high surface coverage mainly show close packed crystals. Samples of BPBA show open crystals with honeycomb lattice and square lattice. Structures for their intermolecular junctions have been proposed, but no definite structures have been identified. BPNA on Au(111) only yields close packed crystals and a few open structures. Samples of BPNA on Ag(111) show only close packed crystals at low surface coverage and mainly weave crystals at high surface coverage. BPPA on Au(111) yields only close packed crystals at high surface coverage. At low surface coverages, windmill and tetramer crystals are observed as well as a variety of threefold coordinated open structures. The experiments of BPBA and cobalt on Au(111) did not lead to the controlled growth of honeycomb structures; instead, the honeycomb crystals observed on earlier samples without cobalt were no longer found. Samples of BPNA and cobalt on Au(111) did yield small honeycomb structures. STS experiments found an electronic state at +2.0 V that is delocalised across the entire network. AFM experiments helped to determine that the observed three- and fourfold structures are coordinated by metal atoms.