TCNQ-based Supramolecular Architectures at Metal Surfaces

Molecular self-assembly at metal surfaces has been recognized as an efficient strategy to create supramolecular nanoarchitectures with promising functionalities. The focus of this thesis lies on the strong electron accepting molecule 7,7,8,8-tetracyanoquinodimethane (TCNQ). The self-assembly of TCNQ and metal-TCNQ coordination networks on metal surfaces were investigated. The structural formation is directed by non-covalent interactions. By controlling fabrication parameters, surface coordination nanostructures with different chemical composition or molecular packing have been synthesized and characterized by scanning tunneling microscopy (STM). The electronic and magnetic properties of the surface organic or metal-organic nanostructures were studied by a combination of x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS). The thesis is organized into three parts. In the first part, the self-assembly of TCNQ molecules on Cu(100) and Ag(100) surfaces will be discussed. The two substrates provide different chemical environments as exhibited by the surface mobility of TCNQ. Charge transfer from either Cu(100) or Ag(100) substrate induces conformational bending of the -C≡N cyano groups of TCNQ, which consequently facilitates the bonding of molecules to surfaces via nitrogen lone pairs. As shown by density functional theory (DFT) calculations, on Cu(100), the substrate-cyano interaction leads to local substrate reconstructions at the proximity of cyano groups. These charge-transfer induced structural modifications at the TCNQ/Cu(100) interface and the electrostatic interaction between molecules determine the self-assembly of TCNQ into long-range ordered domains. The adsorption and self-assembly of TCNQ on Ag(100) is presumably governed by a similar mechanism as concluded from the molecular packing and charge transfer phenomenon. In the second part of this thesis, the fabrication of MTCNQx (M = Mn, Fe, Co, and Ni) coordination complexes on metal surfaces is presented. The molecular adsorption geometry in all structures indicates the participation of the cyano-substrate interaction in determining the final structures, except the x = 1 case in which all cyano groups are coordinated to metal adatoms. The competition between the coordination of TCNQ to metal adatom and TCNQ to substrate is shown by the growth phenomena on the Cu(100) and Ag(100) substrates. The weaker substrate interaction provided by the Ag(100) substrate results in multiple ordered phases (x = 1, 2, or 4), in contrast to the single ordered x = 2 phase on Cu(100). The MTCNQ2 networks synthesized on both substrates are isostructural apart from a rotation of the network orientation to adapt the substrate periodicities. This highlights the importance of the substrate lattice in guiding the growth of supramolecular networks as well as the robust intra-network interaction to obtain a stable structure. The inherent chemical reactivity of Mn, Fe, Co and Ni atoms, which can be further modified upon adsorption due to substrate hybridization, strongly influences the resultant coordination structures. In addition to metal coordination, hydrogen bonding also plays a role in the stabilization of metal-TCNQ networks. Adjusting the substrate temperature has been an important and effective method to tune the interplay between adsorbate-adsorbate and adsorbate-substrate interactions. These experiments provide a systematic investigation on the formation of metal-TCNQ coordination structures on metal surfaces. In the last part of this thesis, the electronic and magnetic properties of the MTCNQx complexes on metal surfaces will be discussed. XPS measurements and DFT calculations show that both the substrate and metal coordination atoms contribute to the electron donation to TCNQ molecules. The magnetic properties of the metal coordination centers were studied in x-ray magnetic circular dichroism (XMCD) measurements. The Mn centers are found to be paramagnetic from 300 K to 8 K. The Mn-coordinated networks represent ordered arrays of magnetic atoms carrying a high spin moment of S = 5/2. In the case of Ni-coordinated networks, the magnetism of Ni centers is induced by TCNQ coordination and dehybridization from the substrate. These results show the interesting physical phenomena emerging in two-dimensional metal-organic coordination systems.


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