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

摘要

This thesis presents a dissertation on supramolecular self-assembly, coordinationnetworks and charge transfer complexes of four molecules: Dicyanop-quinonediimine (DCNQI), Tetracyanoethylene (TCNE), Tetracyanoquinodimethane(TCNQ) and Tetrathiafulvalene (TTF) at a noble metal surface.Different systems were analyzed by variable temperature scanning tunnelingmicroscopy (STM), X-ray photoelectron spectroscopy (XPS) and the resultswere compared with theoretical 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 differentAg-TCNE coordination networks, depending on the substrate temperature,where the silver adatoms originate from the step etching of thesilver substrate. According to DFT calculations and in agreement with XPSmeasurements, the TCNE molecules are negatively charged ( 1 e), takingcharge from the substrate and from the silver adatom (the charge on thesilver adatoms is 0.45 e).The adsorption of TCNQ on Ag(111) shows the formation of three differentphases, where the molecules are bonded together by hydrogen bonds, butwith a strong influence of the interaction of the cyano groups with the silversubstrate. Actually, in one of the phases the participation of silver adatomscannot be completely ruled out. There is also a strong charge transfer fromthe silver to the TCNQ molecule ( 1 e). On the other hand, for a bilayer ofTCNQ molecules on Ag(111), the second layer is much more decoupled fromthe substrate, and the self-assembly is driven exclusively by the formation ofhydrogen bonds between the molecules, in a similar behavior to that reportedon graphene and Au(111) substrates.The adsorption of the electron donor TTF on Ag(111) was studied, afterroom temperature deposition. No isolated or islands could be imaged due toa high molecular diffusivity. Annealing the sample at 350 K results in theformation of well-ordered island where the molecules are slightly tilted withrespect to the surface. DFT calculations and XPS measurements show thatin this case the charge transfer, although very small ( 0:1 e), takes place inthe opposite direction, the molecule remaining positively chargedIn the second part of the thesis, mixtures of donor/acceptor molecules(TCNQ-TTF, TCNE-TTF) have been studied on a Ag(111) substrate. Themetal surface allows us to expands the variety of such Donor-Acceptor (DA)networks. We show that these systems exhibit various structural phases,depending on the stoichiometry, each leading to different levels of chargetransfer. In particular, both TCNE and TCNQ, both being strong electronacceptors, hold in every case a negative charge close to 1 e. On the contrary,the charge on the TTF molecules, being positive, seems to increase with theTCNQ content. Thus by controlling the stoichiometry ratio in these complexes,we can tune both the structural and the electronic properties (forexample, the work-function) of a D-A system.Finally, in the last part of the thesis on the temperature controlled irreversibletransition between the two isomeric forms (trans and cis) of thestrong electron acceptor DCNQI, both on Cu(100) and Ag(111) surfaces isreported. The experiments and DFT calculations show that the isomerizationbarrier is lower than in gas phase or solution due to the fact thatcharge transfer from the substrate has modified the bond configuration ofthe molecule. In addition, an Fe-DCNQI coordination network was studiedby mixing Fe atoms and DCQNI molecules on Ag(111). After annealing at380 K, one-dimensional (1-D) network has been observed where one Fe atomis connected to 4 DCNQI, forming chains that assemble together by hydrogenbonds. The electronic structure of this network reveals that the iron atomchanges from the metallic state to the oxidized state.In summary, since charge transfer at the metal organic interface playsan important role in the efficiency of many organic optoelectronic devices,we have studied the effect of charge transfer in model donor, acceptor anddonor-acceptor systems on a Ag(111), Cu(100) and Au(111).