Surface-Confined Metal?Organic Nanostructures from Co-Directed Assembly of Linear Terphenyl-dicarbonitrile Linkers on Ag(111)

A detailed structural analysis of the surface supported self-assembly of terphenyl-4,4′′-dicarbonitrile molecules (NC−Ph3−CN) linked by Co adatoms on Ag(111) reveals different surface patterns depending on the constraints applied to the system. Without constraints, i.e., sufficient mobility and absence of space limitations at the surface, extended regular honeycomb nanomeshes are formed. On the basis of high-resolution scanning tunneling microscopy images, an atomistic model is derived showing the crystallographic orientation of the molecules and a commensurate alignment of the honeycomb networks, which exist in two rotational domains on the Ag(111) atomic lattice. For Co deficiency, an additional star-like Co-directed motif has been identified, and fully disordered networks are present if space limitations are imposed. In these cases, nodal motifs exist showing between 3- and 6-fold coordination of Co centers.

Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling

Frédéric Rossel

This thesis reports on the optical properties of fullerene molecules forming nanoscale crystals grown on ultrathin NaCl films on Au(111) investigated by Scanning Tunneling Microscope (STM)-induced light emission spectroscopy. In this regard, an existing optical setup has been improved. The new instrument facilitates the alignment of the detection lens with the tunnel junction. STM images of the investigated samples show that, upon adsorption on a NaCl covered Au(111) surface, C60 and C70 crystallize into multilayer islands with a hexagonally arranged close-packed top layer in which electron-poor regions on one molecule face electron-rich regions on the adjacent molecule. Luminescence spectra from these nanocrystals induced by electrons tunneling through a double barrier STM junction are presented. Their bias dependence as well as their similarity with laser-induced photoluminescence spectra indicate a hot electron injection mechanism followed by a radiative decay associated with the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap of the molecules. Local fluorescence and phosphorescence of C70 molecules are identified, leading to unambiguous chemical recognition on the nanoscale. Moreover, the molecular luminescence is demonstrated to be selectively enhanced by localized surface plasmons in the STM tip-sample gap, the tunnel junction acting as an optical antenna. Finally, the results of a scanning tunneling microscopy and scanning tunneling spectroscopy investigations of self-assembled islands of Azure A molecules on a Au(111) substrate are presented. Although no final conclusion can be drawn on the molecular arrangement owing to the impossibility to recognize specific parts of the molecule, the presence of these structures is found to modify the Au(111) herringbone reconstruction.

EPFL2009

Does the Surface Matter? Hydrogen-Bonded Chain Formation of an Oxalic Amide Derivative in a Two- and Three-Dimensional Environment

Harald Brune, Marta Canas Ventura, Sylvain Clair, Klaus Kern, Uta Schlickum

We report on a multi-technique investigation of the supramolecular organisation of N,N-diphenyl oxalic amide under differently dimensioned environments, namely three-dimensional (3D) in the bulk crystal, and in two dimensions on the Ag(111) surface as well as on the reconstructed Au(111) surface. With the help of Xray structure analysis and scanning tunneling microscopy (STM) we find that the molecules organize in hydrogen-bonded chains with the bonding motif qualitatively changed by the surface confinement. In two dimensions, the chains exhibit enantiomorphic order even though they consist of a racemic mixture of chiral entities. By a combination of the STM data with near-edge X-ray absorption fine-structure spectroscopy, we show that the conformation of the molecule adapts such that the local registry of the functional group with the substrate is optimized while avoiding steric hindrance of the phenyl groups. In the low coverage case, the length of the chains is limited by the Au(111) reconstruction lines restricting the molecules into fcc stacked areas. A kinetic Monte Carlo simulated annealing is used to explain the selective assembly in the fcc stacked regions.

2008

Scanning tunneling microscopy on large bio-molecular systems on surfaces

Sebastian Koslowski

The ever growing demand for the development of new technologies and materials has led to extensive studies inspired by biological functional complexes. Peptides with their outstanding ability to efficiently self-assemble can express a broad spectrum of intriguing functionalities. A key question in mimicking their assembly and function is the precise understanding of the specific interactions between the contained amino acids on the level of a sub-molecular length scale. An excellent tool to probe samples at these length scales is available with scanning tunneling microscopy (STM) and its operation under the controlled conditions of ultra-high vacuum (UHV) and low temperature. Within this thesis it is demonstrated how high-resolution studies by STMin combination with a controlled sample preparation by electrospray ion beam deposition (ES-IBD) under UHV conditions allows for the structural determination of peptides on surfaces. Furthermore the results presented here contribute to the development of a method capable of directly identifying individual amino acids within a peptide sequence. The structural and electronic properties of molecules on surfaces crucially depend on their interaction with the underlying substrate. Implementing a thin dielectric layer on the metallic surface, electronically decouples molecules from the substrate and enables an unperturbed observation of the molecular electronic structure. In Chapter 2 the properties of hexagonal boron nitride (h-BN) on Rh(111) as decoupling layer are assessed on behalf of the structural and electronic properties of the molecular model system pentacene adsorbed on it. In a second part of this chapter the discovery and the characterization of a new phase of h-BN/Rh(111) is described. In Chapter 3, we gained insight into the properties of amino acids on metal surfaces by utilizing the capability of the STM to probe the structure and the electronic characteristics in high resolution imaging and scanning tunneling spectroscopy (STS). As a second important aspect of this chapter, the modification of STM tips with amino acids is investigated as a method to enhance the structural and electronic resolution. An experimental protocol allowing to adhere amino acids on the STM tip be could developed. Using functional STM tips in STS experiments on amino acids enabled the observation of specific molecular resonances. An obstacle in investigating large bio-polymers, such as natural peptides, is their high structural complexity and conformational freedom. Therefore the utilization of custom designed synthetic sequences tailored towards a specific property is a good approach. In Chapter 4 studies performed on two synthetic peptide sequences deposited by ES-IBD on an Au(111) surface are discussed. The first sequence assembled in ordered two-dimensional networks. A folded gas-phase conformation could be utilized to rationalize the observed structures in the networks. Subsequently it was shown that the self-assembly behavior of the peptide could be steered towards chain-like assemblies by modifying the sequence at the peptide C-terminal. Using specific amino acid functionalized STM tips a sensitivity towards an amino acid of the same type in the peptide sequence could be observed. Thereby a partial sequencing of the synthetic peptide was enabled.

EPFL2017

Scanning tunneling microscopy on large bio-molecular systems on surfaces

Sebastian Koslowski

EPFL2017