Donor and acceptor molecules on metal surfaces: supramolecular self-assembly, metal-organic coordination networks, and charge-transfer complexes

Resumen

This thesis presents a dissertation on supramolecular self-assembly, coordination networks and charge transfer complexes involving four molecules: Tetracyanoethylene (TCNE), Tetracyanoquinodimethane (TCNQ), Tetrathiafulvalene (TTF), and Dicyano-p-quinonediimine (DCNQI) on a noble metal surface. Different systems were analyzed by variable temperature scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and the results were compared with theoretical calculation within the density functional theory (DFT). In the first part of this thesis, the adsorption of the individual molecules (TCNE, TCNQ, and TTF) on a Ag(111) surface was studied. The growth of TCNE on Ag(111) results in the formation of two different Ag-TCNE coordination networks, depending on the substrate temperature, where the silver adatoms originate from the step etching of the silver substrate. According to DFT calculations and in agreement with XPS measurements, the TCNE molecules are negatively charged ( 1 e), taking charge from the substrate and from the silver adatom (the charge on the silver adatoms is 0.45 e). The adsorption of TCNQ on Ag(111) shows the formation of three different phases, where the molecules are bonded together by hydrogen bonds, but with a strong influence of the interaction of the cyano groups with the silver substrate. Actually, in some phases the participation of silver adatoms cannot be completely ruled out. There is also a strong charge transfer from the silver to the TCNQ molecule ( 1 e). On the other hand, for a bilayer of TCNQ molecules on Ag(111), the second layer is much more decoupled from the substrate, and the self-assembly is driven exclusively by the formation of hydrogen bonds between the molecules, in a similar behavior to that reported on graphene and Au(111) substrates. The adsorption of the electron donor TTF on Ag(111) was also studied. After room temperature deposition no isolated molecules or islands could be imaged due to a high molecular diffusivity. Annealing the sample at 350 K results in the formation of well-ordered island where the molecules are slightly tilted with respect to the surface. DFT calculations and XPS measurements show that in this case the charge transfer, although very small ( 0.1 e), takes place in the opposite direction, the molecule remaining positively charged. In the second part of the thesis, mixtures of donor/acceptor molecules (TCNQ-TTF, TCNE-TTF) have been studied on a Ag(111) substrate. If in bulk phase there is only a single TCNQ-TTF charge transfer complex, the metal surface allows us to expand the variety of such Donor-Acceptor (D-A) networks. We show that these systems exhibit various structural phases, depending on the stoichiometry, each leading to different levels of charge transfer. In particular, both TCNE and TCNQ, being strong electron acceptors, hold in every case a negative charge close to 1 e. On the contrary, the charge on the TTF molecule, being positive, seems to increase with the TCNQ content. Thus, by controlling the stoichiometry ratio in these complexes we can tune both the structural and the electronic properties (for example, the work-function) of a D-A system. Finally, in the last part of the thesis the temperature controlled irreversible transition between the two isomeric forms (trans and cis) of the strong electron acceptor DCNQI, both on Cu(100) and Ag(111) surfaces is reported. The experiments and DFT calculations show that the isomerization barrier is lower than in gas phase or solution due to the fact that charge transfer from the substrate modifies the bond configuration of the molecule. In addition, an Fe-DCNQI coordination network was studied by mixing Fe atoms and DCQNI molecules on Ag(111). After annealing at 380 K, one-dimensional (1-D) network has been observed where one Fe atom is connected to 4 DCNQI, forming chains that assemble together by hydrogen bonds. The electronic structure of this network reveals that the iron atom changes from the metallic state to the oxidized state. In summary, since charge transfer at the metal organic interface plays an important role in the efficiency of many organic optoelectronic devices, we have studied the effect of charge transfer in model donor, acceptor and donor-acceptor systems on a Ag(111), Cu(100) and Au(111).