On-surface synthesis of two-dimensional conducting metal-organic frameworks

Thesis

This thesis focuses on the synthesis of two-dimensional metal-organic frameworks (MOFs) using on-surface methods on metal substrates. Scanning tunnelling microscopy (STM) and density-functional theory (DFT) calculations are used as the experimental and theoretical tools to study the structural and electronic properties of the frameworks.

The thesis starts with the study of hexahydroxytriphenylene (HHTP) molecules on a Au(111) substrate. Following vapour deposition from a Knudsen cell, HHTP molecules self-assemble into a near close-packed phase with two alternating molecular rows. After reacting with transition metals (Ni/Co), HHTP forms a dense structure on the surface. The metal centres adopt a three-fold coordination geometry, connecting all the molecules into a network.

Changing the ligand to hexaaminotriphenylene (HATP), single-layer MOFs with a honeycomb structure can be synthesised when metals (Ni/Co) are evaporated onto the surface containing HATP molecules. The Ni-HATP MOF has a shape of large islands, while the Co-HATP domains are long ribbons. A bimetallic MOF can also be obtained by co-depositing Ni and Co atoms. Different metal centres can be separately visualised in STM images and identified with the aid of DFT modelling.

Using a smaller ligand, hexahydroxybenzene (HHB), nonporous metal-organic coordination polymers are synthesised on the Au(111) substrate. Ni- and Co-HHB polymers have an identical structure in which each molecule is bonded with six metal atoms. The only difference is that Ni atoms are visible in STM images, while Co atoms are not evident because of their lower height.

When mixing HHTP and HHB, two phases of self-assemblies are found on the Au(111) surface. The α-phase has an HHTP : HHB ratio of 2 : 1, and the β-phase has a ratio of 6 : 1. Depositing Ni onto the α-phase mixture can create a dual-ligand MOF, where each Ni atom coordinates with one HHTP and one HHB. Besides, HHB has a stronger coordination ability than HHTP and HATP, causing ligand substitution reactions on the Au(111) surface.

Authors: Shu, S

• DOI: 10.5287/ora-wrnkyrvbd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: STM; on-surface synthesis; metal-organic frameworks
• Subjects: Nanostructured materials