Ultra-thin dielectric layers on metal substrates studied by scanning tunneling microscopy and scanning tunneling spectroscopy

In this thesis, ultra-thin dielectric layers on metal substrates are studied mainly by scanning tunneling microscopy and scanning tunneling spectroscopy. In chapter 2, field emission resonances, i.e. bound states in the potential well between sample and tip with an applied voltage, are used to determine the local work function on a sample with patches of different structural composition. To do so, a simple theoretical model is developed and used to simulate the peak positions of the measured dI/dV spectra. In the present case the sample is a Ag(100) surface covered by up to three monolayers of NaCl. The presented method is also applicable to other systems suitable for STM measurements. Additionally, the surface potential is locally probed in the dipole layer region between domains of different work function. In chapter 3, new molecular structures on silver surfaces are presented, that are formed by a mixture of gases. This mixture contains the small molecules H2, H2O, CO, CO2, and N2. No final conclusion can be drawn on the chemical nature of these structures due to the impossibility of the scanning tunneling microscope to determine chemical species. But already the presence of these structures indicates that there is a multitude of unknown interactions, that are possible between a transition metal surface and even smallest molecules. In chapter 4, the moiré pattern is described that a BC3 film creates on a NbB2(0001) surface. As the growth of the BC3 sheet is incommensurate, a moiré pattern is formed that modulates the apparent height of the atoms on the STM images, i.e. the local density of states. The BC3 layer exhibits a completely different behavior than the closely related graphite layer, because it is transparent for the tunnel current. In the present case, the BC3 layer is visible on the STM images only by its moiré effect.

Local work function changes determined by field emission resonances: NaCl/Ag(100)

François Patthey, Marina Pivetta, Wolf-Dieter Schneider

Electrons trapped in field emission resonances (FERs) in front of a Ag(100) surface covered with ultrathin NaCl islands are probed by scanning tunneling spectroscopy. The eigenstates in the potential well formed between the tip and the sample are calculated within a one-dimensional model. This approach permits to locally determine a work function reduction of 1.3 eV in going from the bare substrate to NaCl islands of up to 3 ML. Spatial mapping of the FERs across a NaCl island edge at typical distances of 1 nm from the surface yields a lateral resolution for the surface potential changes of 1 nm.

2007

Single-molecule-level characterization of supramolecular systems at surfaces

Dietmar Payer

This thesis deals with the self-organization of individual building blocks, specifically organic molecules and metal atoms, into supramolecular structures on metal surfaces under ultra high vacuum conditions (UHV). Self-organization of supramolecular systems is a promising candidate for a bottom-up approach in the fabrication of complex structures with specific properties. This process is steered by interactions between functional groups of the individual building blocks and is of apparent interest to gain a detailed understanding of the influence and the behavior of these functional groups enabling intermolecular binding. In the first part of the thesis, we investigate hydrogen bonded structures on metal surfaces. Of especial interest is the possibility of the formation of ionic hydrogen bonds, a special class of hydrogen bond which connects a charged donor (acceptor) and a neutral acceptor (donor) and is important in biological systems. By combined Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) measurement we show the deprotonation of carboxylic functions in the molecules, forming carboxylate groups involved in intermolecular binding. With our complementary experimental and theoretical examinations we clearly show that the carboxylate groups are indeed still charged although adsorbed onto a conducting metal surface and that the observed structures can be computationally reproduced by using these charges in classical molecular dynamic calculations. Our investigations clearly reveal that all requirements for the formation of ionic hydrogen bonds in these samples at metal surfaces under solvent free conditions are fulfilled. In the second part, the confinement of surface state electrons in self-assembled hydrogen-bonded structures is investigated. The examined structure contains two-dimensional cavities, with different inner diameters in the periodic arrangement and in domain boundaries. Inside the cavities we detect a spatially localized modification in the local density of states (LDOS) of the Ag(111) surface state electrons. The energy of the peak correlates with the cavity area as predicted by a simple "particle in a box" model. This clearly shows that the surface state electrons form a confined state inside the cavities. Furthermore we examine self-assembled periodic structures as templates for binding single macrocyclic molecules. The macrocyclic molecules can be deposited by sublimation, as STM-topographs with submolecular resolution reflect the shape of the intact molecules and reveal a coverage dependent aggregation behavior. We show that it is possible to fabricate a structure in which at most one macrocyclic molecule per pore can be found in a defined binding position. In the last part of the thesis we investigate the influence of individual metal atoms on the chemical activity and the self-assembly process of molecules on metal surfaces. First, we demonstrate the high chemical reactivity of these metal atoms. In our experiments we address the chemical reactivity of static active sites, like kinks and step edges, and of dynamic active sites, like the metal atoms, separately. Doing this we show that a non-negligible amount of the chemical reactivity of a metal surface is due to this dynamic adatom gas. Furthermore the influence of metal atoms on the structure formation of organic molecules is investigated. We show that two-dimensional structures based on a complex formation between metal centers and ligands can be formed and that the coordination number is independent of the substrate symmetry. Furthermore, we show the possibility that on metal surfaces one can prepare three-fold coordinated Fe-centers, a coordination number which is rarely seen in three dimensional chemistry. In the last part we study catenanes, which consist of two interlocked macrocyclic molecules. Main interest is on the confirmation of a structural change of the catenanes adsorbed on a metal substrate. For our experiments we use the fact that the catenanes are designed to "trap" a Cu-atom by complex formation. As this reaction is associated with a structural change and a change in intermolecular interaction, we can verify a reaction of catenanes adsorbed onto a surface with Cu adatoms and therefore a structural alteration by simply imaging the formed structures. Our measurements show the possibility of depositing large molecules like catenanes by sublimation and of their potential for use as molecular machines adsorbed onto metal surfaces.

EPFL2007

Scanning tunneling spectroscopy at the single atom scale

Markus Ternes

This thesis reports measurements at the single atom scale by using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Different sample systems where analyzed with normal conducting and superconducting tips. Chapter 2 presents the theoretical aspects which have to be taken into account for a detailed analysis and a consistent interpretation of the STS measurements. In chapter 3 the creation of a hexagonally ordered superlattice of single Ce adatoms on Ag(111) is reported and understood within a scattering model of the surface state electrons with the adatoms. Furthermore, the change in the local density of states of the surface state in ordered and slightly disordered superlattices is measured and theoretically explained within a tight-binding model which allows to understand the creation and stability of the superlattice by an energy gain of the participating surface-state electrons. Because Ce atoms have a non-vanishing magnetic moment which is expected to interact with the continuous states of the supporting surface leading to a Kondo resonance, chapter 4 presents measurements on single Ce adatoms on different Ag surfaces. This chapter shows the difficulties to interpret the obtained data. For instance, bistable Ce adatoms are detected on Ag(100) which show drastical changes in their apparent height and spectral signature depending on the tunneling conditions. The possible physical processes behind these phenomena are discussed. While the results presented in the first chapters were obtained with a normal conducting tip, chapter 5 intensively discusses the opportunities superconducting tips offer in low-temperature STS measurements. Novel insight in and thorough understanding of Andreev reflection processes are obtained by using the unique possibility of having different superconducting gaps in the tip and the sample. Detailed analyses of the supercurrent at low tunneling resistances reveal tunneling currents which are not described within the standard resistivity shunted junction model, and are presumably due to self-induced tunneling or due to an additional quasiparticle tunneling channel which only exist in asymmetric junctions. Furthermore, the influence of single magnetic Co atoms inbetween the superconducting tunnel junction on the obtained spectrum is discussed.