Interplay of Chemical and Electronic Structure on the Single-Molecule Level in 2D Polymerization

Single layers of covalently linked organic materials in the form of two-dimensional (2D) polymers constitute structures complementary to inorganic 2D materials. The electronic properties of 2D polymers may be manipulated through a deliberate choice of the organic precursors. Here we address the changes in electronic CO structure-from precursor molecule to oligomer by scanning tunneling spectroscopy and ultraviolet photoelectron spectroscopy. For this purpose, we introduce the polymerization reaction of 1,3,5-tris(4-carboxyphenyl)benzene via decarboxylation on Cu(111), which is thoroughly characterized by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. We present a comprehensive study of a contamination-free on-surface coupling scheme and study how dehydrogenation, decarboxylation, and polymerization affect the electronic structure on the molecular level.

Growth and characterization of specific self-assembled supramolecular architectures at crystal surfaces

Marta Canas Ventura

In the framework of this thesis self-assembled supramolecular architectures of several molecular species on crystal metal surfaces are characterized. These investigations combine results obtained by means of scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), X-Ray Photoelectron Spectroscopy (XPS), and Near-Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS). The interest in controlling the formation of specific arrangements of supramolecular ensembles at surfaces is motivated to a great extent by the prospect to design molecular devices and nanomaterials with customized properties. The goal is a "spontaneous" creation of targeted, well-ordered molecular architectures, which shall be achieved by taking advantage of the molecule's propensity to self-assemble. Control means, in this context, predictability. This term address the accuracy of the prediction of the structures resulting from molecular self-assembly for systems exhibiting different characteristics. Here, system stands for a particular combination of a single-crystal surface and adsorbed molecular species. In a first part, the focus is given to the impact of a Pd(111) surface exhibiting high chemical activity. Due to surface induced deprotonation two resulting chemical states are found for the adsorbed terephthalic acid species. Deprotonation of the carboxyl moieties is observed to affect the topology of the molecular assemblies even though hydrogen bonds (H-bonds) are responsible for the stabilization of assemblies of both, intact and deprotonated molecular species. Therefore, the selective fabrication of a single specific molecular structure becomes a challenge when the surface is prone to induce chemical reactions. Secondly, a molecular species exhibiting conformational flexibility is studied on a largely inert Au(111) surface. It is observed that the characteristic conformational flexibility is also active on the surface, and the molecule is found to adopt different conformations depending on the locally most favorable bonding motif. In particular, phenyl groups may undergo a rotation such that one-dimensional chains can be stabilized by means of intermolecular H-bonds. This phenomenon is very interesting from an adsorbate-substrate interaction point of view, but it limits the predictability of supramolecular structures formed upon self-assembly. In a third step, a comparative study of the self-assembly of three closely related molecular species highlights the impact of concurrent competing intermolecular interactions. Molecules exhibiting an increasing number of possible interaction channels are found to assemble into an increasing number of distinct supramolecular architectures. The most flexible molecular species allowing for different hydrogen-bonding patterns as well as dipolar coupling self-assembles into various structurally complex architectures. Structural predictability, however, is low since the formation of one or the other assembly appears to depend on small variations in sample preparation conditions and on poorly controllable local surface properties. Finally, a system where the resulting structures fully agree with predictions evidences the possibility to control the formation of targeted specific supramolecular architectures. The formation of well ordered bimolecular ribbons and wires over extended length scales is accomplished by the rational choice of both, complementary building blocks for the selective formation of three-fold H-bonds and a nanostructured surface to guide the molecular self-assembly. Furthermore, the impact of H-bond formation on the electronic structure of self-assembled molecular building blocks is probed by STS measurements. The experimental data reveal shifts in the electronic levels depending on the H-bond configuration and on the local characteristics of the underlying surface. In order to improve the precision of such a study, single molecules, homo- and heteromolecular dimers as well as trimers are successfully isolated and assembled by molecular manipulation with the STM tip. Further STS studies on such artificially assembled structures are suggested.

EPFL2008

Scanning Tunneling Luminescence of Pentacene Nanocrystals

Alexander Kabakchiev

Organic semiconductors are promising materials for future electronic and electroluminescence applications. A detailed understanding of organic layers and nano-sized crystals down to single molecules can address fundamental questions of contacting organic semiconductors at the nanometer limit and obtaining luminescence from them. In this thesis, electroluminescence spectra from pentacene, a policyclic hydrocarbon (acene), are discussed. The luminescence is induced by the current from the tip of a scanning tunneling microscope (STM). Pentacene is an organic semiconductor which gained a lot of attention in technology because of its electronic properties suitable for applications in thin film devices. Moreover, recent fundamental studies employed pentacene as a standard to demonstrate sub-molecular resolution by scanning probe techniques. This work reports the first observation of light emission from nanometer-sized pentacene crystals grown on an ultrathin insulating layer on noble metal surfaces. Different STM-techniques are combined to characterize the individual systems studied with respect to topography, crystal structure, electronic properties and work function changes. The initial step of the project was the implementation of an optical system for light collection from the tunnel-junction of a low-temperature STM. In the set-up, a novel approach based on the use of in situ adjustable lenses has been realized. Three lenses placed in the vicinity of the tip-apex allow light collection into three independent channels which can be used for versatile optical analysis. An important part in the characterization of the organic system was to clarify the interaction between adsorbed molecules and substrates. We investigated individual pentacene molecules on different ultrathin insulator-metal systems. With the STM tip in tunnel contact, the molecules are situated in a double barrier junction formed by the insulating layer on one side and the vacuum gap on the other side. The metal surface and the STM-tip form the electrodes. The electronic properties of pentacene in this configuration have been characterized. Insulator-metal-systems, which provide a good electronic decoupling for pentacene from the metal, have been chosen as substrates for the growth of pentacene nanocrystals. Using the sub-molecular resolution of the STM we resolved the structure of the top-layer of the pentacene nanocrystals and found that the crystal phase agrees with the pentacene bulk structure. Moreover, the comparison of charge injection barriers between individual molecules and nanocrystals of pentacene indicates a significant change of electronic properties after the formation of ordered structure from the individual building blocks. Optical spectroscopy of the emitted light reveals an excitonic emission from the nanocrystals, which is in very good agreement with photoemission spectra of macroscopic crystals. Although a highly localized current-injection by the STM tip is used for excitation, it can be concluded that the source of light emission is delocalized. In contrast to luminescence measurements reported for other organic materials in STM, the excitation is not localized on the individual molecule, into which a charge is injected by the STM-tip. Our study indicates the importance of inter-molecular coupling leading to a high mobility of the excited state which has to be considered in the framework of STM-induced luminescence. A further result is the observation of the Stark shift of pentacene luminescence, that has so far not been measured at large electric fields of the order of 1V/nm. The evaluation of the Stark shift provides information on dipole and polarizability change during the decay of an exciton. The measured large dipole change of the lowest singlet exciton in pentacene indicates a mixing of the Frenkel exciton (FE) with a small contribution of charge-transfer (CT). Finally, we developed a set-up for photon correlation measurements in the STM. The properties of photon statistics is a means which can unambiguously prove that the luminescence is due to a single-photon source, i.e. a light source which emits no more than one photon at a time. A typical example of such a source is the emission from a single molecule. We performed photon correlations measurements of the luminescence from pentacene and C60 nanocrystals. While the excitonic bulk emission of pentacene is not expected to represent a single photon emitter, we also did not observe anticorrelations in the case of C60 indicating the importance of the local environment in the STM-junction, e.g. the short distance to the metal electrodes.

EPFL2010

Supramolecular assembly, chirality, and electronic properties of rubrene studied by STM and STS

This thesis presents the first experimental results of a scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) investigation of rubrene at the supramolecular, molecular and submolecular level. Based on its semiconducting and fluorescent properties, this molecule is of particular interest in view of the emerging fields of molecular electronics and optoelectronics which could one day replace the conventional technology relying on semiconductors such as silicon and gallium arsenide. The goal is the substitution of these inorganic materials by cheap and flexible layers of semiconducting organic molecules for a new class of diodes and transistors, as well as the realization of electronic switches based on individual molecules. One fundamental approach is to take advantage of the molecular self-assembly behavior which results in the creation of well-ordered supramolecular structures. The investigations of the self-assembly of rubrene adsorbed on metal surfaces (Au(111), Au(100), Ag(111), and Ag(100)), which were carried out within the framework of this thesis, show a surprising diversity of supramolecular structures. Amongst other shapes, the molecules organize themselves into geometries of perfect hexagonal and pentagonal symmetry and create multifaceted patterns on the surface. A fascinating peculiarity consists in the spontaneous construction of nested structures which are built up by a hierarchical self-assembly of individual molecules into pentagonal supermolecules which form in a second step perfect supramolecular decagons. The geometric shape of rubrene is characterized by a structural asymmetry leading to the existence of two mirror imaged versions of the molecule which are not superimposable to each other, such as for instance our left and right hand or the helical DNA. The aspect of chirality is crucial for basic processes in living systems and calls for a fundamental understanding of the interaction mechanisms occurring between chiral molecules. The experiments on rubrene reveal that the intermolecular bonding differentiates between the two chiral types of the molecule (chiral recognition), yielding the self-organization into homochiral structures. These assemblies exhibit a geometry which is again chiral, demonstrating a propagation of chirality throughout the three stages of the supramolecular hierarchy. The semiconducting behavior of rubrene is furthermore probed by STS measurements detecting the energetic positions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The experimental data uncover that different adsorption conformations exhibit characteristic HOMO energies and reveal adsorption conformations of rubrene which preserve the intrinsic electronic structure of the free molecule. Furthermore, a switching of the molecular conformation and the electronic structure of one rubrene conformer is induced with the STM.

EPFL2006