Bonding and Charge Transfer in Metal-Organic Coordination Networks on Au(111) with Strong Acceptor Molecules

The geometric and electronic structure of two structurally similar metal organic networks grown on the Au(111) surface is investigated by scanning tunnelling microscopy (STM) and spectroscopy (STS) combined with density functional theory (DFT) calculations. The networks are composed of (i) F4TCNQ (C12F4N4, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane) molecules and Au adatoms segregated from the pristine metal surface, and (ii) TCNQ (C12H4N4, 7,7,8,8-tetracyanoquinodimethane) and codeposited Mn atoms. In both cases, the strong electron acceptor character of the molecules results in metal to-ligand charge transfer to the lowest unoccupied molecular orbital (LUMO). The amount of electrons donated from the 4-fold coordinated Mn atoms to TCNQ is higher compared to the 2-fold coordinated Au adatoms to F4TCNQ This behavior is reflected in the appearance of distinct spectral features in STS data in the energy region close to the Fermi level resulting from the intricate interplay between surface states, adatom states, and molecular orbitals. These observations are consistent with a picture in which the LUMO of the TCNQ acceptor molecule hybridizes with Mn and Au substrate metal states becoming practically filled, while the LUMO of F4TCNQ is only partially filled despite being the stronger electron acceptor. Our results reveal the importance of the type of bonding between the strong acceptor and the metal center (Au or Mn) as well as its coordination in the determination of the charge transfer to the adlayer, which is important for its electronic properties.

Design of Surface-Supported Bio-inspired Networks for CO2 and O2 Activation at Room Temperature

Daniel Eduardo Hurtado Salinas

The structure of a biological system defines its function. Therefore, nature has developed sophisticated systems that catalyze chemical reactions that are vital for life on earth. For instance, the photosynthesis is a biological process that is partially regulated by the metallo-enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase), whose function is to catalyze the capture and transfer of CO2 into organic molecules. This carboxylation reaction occurs at the active site, where a magnesium ion (Mg2+) is bound to organic molecules with carboxylate (COO-) and amine (NH2) functional groups. In the last decades, supramolecular chemistry has proven to be a valid approach to replicate the structural characteristics of these natural metal-organic systems. Metal atoms and organic molecules are used as building blocks to form two-dimensional (2D) metal-organic networks (MONs) on metal surfaces, through a self-assembly process that occurs spontaneously. In these systems the molecules bind to the metal centers and control their oxidation states that, as found in metallo-enzymes, is crucial to perform a catalytic task. Therefore, these supramolecular assemblies show promising features to be explored for heterogeneous catalysis. The main objective of this thesis is to reverse the structure-function equation, that is: instead of finding specific functions of well-known 2D structures for future applications, 2D structures are designed to reproduce the specific functions of well-known bio-systems. Thus, inspired in the structure of the active site of RuBisCO, 2D Mg-based supramolecular assemblies are rationally designed on Cu(100) or Mg(0001) surfaces at room temperature (RT) as a platform for CO2 adsorption. This work is the first study of self-assembly of alkaline earth metal atoms (Mg) with organic molecules ((1,4-benzenedicarboxylic acid (TPA) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT)) on metal surfaces. The first part of this thesis investigates the self-assembly of TAPT molecules. Scanning Tunneling Microscopy (STM) and High Resolution X-ray Photoemission Spectroscopy (HR-XPS) demonstrate that the molecules typically semi-deprotonate to form MONs with the adatoms from the host substrates. The exposure of these structures to CO2 results in the carboxylation of the amino groups. The second part proves that the COOH moieties of TPA molecules fully-deprotonate to form homo-molecular structures. The incorporation of Mg adatoms result in ionic MONs, which reproduce the carboxylate environment of the active site of RuBiSCO. STM, XPS and Density Functional Theory (DFT) analyze the structural changes, the chemical reactions and charge transfer during either the CO2 or O2 adsorption on the Mg2+ centers. The last part focusses on the co-deposition of both molecules to form 2D metallic-hetero-molecular networks with Mg2+ centers. The role of the Mg2+ centers as active sites for molecular adsorption and model systems for gas storage is studied. STM reveals that CO2 partially modifies the network towards a configuration that resembles the active site of RuBisCO. The formation of 3D structures to enhance CO2 adsorption is also evaluated.

EPFL2018

Monitoring Charge Carrier Dynamics at Atomic Length Scales by STM-induced Luminescence

Christoph Grosse

The goal of this thesis is the combination of the high spatial resolution of scanning tunneling microscopy (STM) with the high temporal resolution of optical spectroscopy, by monitoring the light emitted from the tunnel junction. Two complementary techniques are presented, which allow (a) access to the local photon statistics and (b) following the luminescence response to nanosecond voltage pulses at atomic length scales. The versatility of these methods is demonstrated by using two different model systems: pure C60 films on Ag(111) and Au(111) substrates, as well as single fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) molecules adsorbed on them. These two systems reveal the possibility to investigate both the local charge carrier dynamics of mesoscopic systems as well as individually and selectively addressable quantum systems. Furthermore, these methods are not limited to pure emission processes such as the recombination of electron hole pairs; rather, it is also possible to study the dynamics of non-emitting processes via the excitation and radiation of surface plasmon polaritons (SPPs) from the tunnel junction. To achieve high time resolutions in the luminescence response of an investigated system, a further technique is developed that permits a quantitative mapping of the voltage at the tunnel junction with millivolt and nanosecond accuracy. Knowing the precise shape of the voltage pulses arriving at the tunnel junction offers compensation for ubiquitous pulse distortions arising from reflections and attenuations of the pulses on their way to the tunnel junction, thus enabling pulse rise and falling times of a few nanoseconds. Investigations of the electronic structure of pure C60 films show that the electronic states of C60 multilayers experience a band bending in STM due to the electric field between the STM tip and the metal substrate. As the band bending is sufficiently strong to align the lowest unoccupied molecular orbitals with the Fermi energy of the substrate, electrons can be injected by the substrate. At structural defects, these electrons can recombine with holes in the highest occupied molecular orbitals injected by the STM tip. At such defects, local deviations from the highly ordered structure of the C60 film result in localized electronic states within the band gap. These states act as traps for the injected electrons and holes and increase their lifetime such that they can recombine with each other. The underlying injection dynamics are accessible by the luminescence time response of these defects to nanosecond voltage pulses. Measurements of the second-order intensity correlation function prove that these defects act as single photon sources with exciton lifetimes of

EPFL2015

Hierarchical Assembly and Reticulation of Two-Dimensional Mn- and Ni-TCNQ(x) (x=1, 2,4) Coordination Structures on a Metal Surface

Nasiba Abdurakhmanova, Klaus Kern, Sebastian Stepanow, Tzu-Chun Tseng

We report a scanning tunneling microscopy (STM) investigation of the self-assembly of two-dimensional metal-organic coordination networks comprised of Mn or Ni atoms (M) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules on a Ag(100) surface. The growth is dominated by the coupling of the cyano ligands to the substrate, and this interaction is reduced by lateral coordination of the molecules to metal adatoms. Initially mononuclear M-TCNQ(4) complexes form at room temperature, which hierarchically assemble into reticulated M-TCNQ(2) networks at sufficiently high metal concentrations. M-TCNQ(1) networks with fully Coordinated molecules can be obtained at elevated substrate temperatures and sufficient metal concentration. A commensurate a and an incommensurate beta phase can be distinguished that are unrelated to the antedecent M-TCNQ(4) complexes. The coordination of the cyano groups to metal adatoms alters the balance between lateral interactions and coupling of the molecules to the substrate. This effect is accompanied by distinct changes in the apparent heights of metal centers in the STM data indicating changes in their electronic configuration.