Etude par microscopie à force atomique en mode non contact et par microscopie à sonde de Kelvin de dérivés du Triphénylène sur KBr(001) dans l'ultra vide

L'imagerie de molécules adsorbées sur des surfaces isolantes est l'un des défis actuels pour les nanosciences et l'électronique moléculaire. L'instrument de choix pour imager une surface isolante et obtenir la résolution atomique est le microscope à force atomique en mode non contact (NC-AFM). Plus récemment, la microscopie Kelvin (KPFM), qui permet de caractériser certaines des propriétés électriques de surface est devenue une technique complémentaire au NC-AFM. Cette thèse s'inscrit dans un programme de recherche visant à imager des molécules uniques sur des surfaces d'isolants massifs, l'objectif ultime étant de connecter la molécule à des électrodes métalliques afin de construire un dispositif à molécule unique. Une difficulté importante pour y parvenir est d'immobiliser la molécule. En effet, à température ambiante, la barrière de diffusion de la plupart des molécules sur surface isolante est trop faible et leur vitesse de diffusion trop élevée pour qu'elles puissent être imagées. La surface de KBr(001) a été choisie pour ces expériences, pour la simplicité de sa préparation et la relative facilité à obtenir la résolution atomique en NC-AFM. Les molécules imagées dans cette thèse ont été conçues et synthétisée au laboratoire dans le but d'interagir fortement avec la surface par interaction électrostatique. Elles sont basées sur un cœur polyaromatique tryphénylène plan équipé de six groupes polaires. La molécule d'hexamethoxytriphénylène équipée de 6 groupes -O-CH3, a été imagée en NC-AFM pour différents taux de couverture. Pour les taux de couverture importants, les images obtenues seront comparées à la structure cristalline connue du cristal moléculaire correspondant. La molécule d'hexacyanopropyloxytriphénylène, équipée de 6 groupes -O-C3H6CN a été imagée en NC-AFM et en KPFM pour différents taux de couvertures. Les contrastes Kelvin obtenus pour différentes monocouches ou îlots sur la surface de KBr(001) seront présentés et discutés. Enfin, des images démontrant la manipulation contrôlée de la molécule d'hexacyanopropyloxytriphénylène sur la surface de KBr(001) seront présentées.Imaging adsorbed molecules on insulating surfaces is one of the actual challenge for nanosciences and molecular electronic. An instrument of choice to obtain atomic resolution on insulating surfaces is the atomic force microscope working in the non contact mode (NC AFM). Recently, Kelvin probe force microscopy (KPFM) a technique able to measure electric properties of surfaces has been implemented to the NC AFM. This thesis belong to a research program aimed to image single molecule adsorbed on insulating surfaces . The final goal of his project is to connect the molecule to metallic electrodes, in order to built an electronic device based on a single molecule. A major difficulty is to immobilize the molecule. At room temperature, the diffusion barrier is indeed to high to allow molecule imaging. The KBr(001) surface has been chosen for the experiments for it's ease of preparation and the easiness to get atomic resolution in NC-AFM. Molecules imaged during this thesis were designed and synthesized in the laboratory in order to interact strongly with the surface by electrostatic interactions. They are based on a plane polyaromatic triphenylene core and equipped with six polar groups. The hexamethoxytriphenylene molecule equipped with six -O-CH3 groups were imaged in NC- AFM for different coverage rates. For high coverage, images are compared to the known crystal structures of the corresponding molecular crystal. The hexacyanopropyloxytriphenylene molecule equipped with six -O-C3H6CN groups was imaged in NC-AFM and KPFM for different coverage rates. Kelvin contrasts for different monolayers or KBr surface were obtained and are presented and discussed. Finally images of the controlled manipulation of the hexacyanopropyloxytriphenylene molecule on the KBr(001) surface are presented

Influence of electrospray deposition on C_60 molecular assemblies

Maintaining clean conditions for samples during all steps of preparation and investigation is important for scanning probe studies at the atomic or molecular level. For large or fragile organic molecules, where sublimation cannot be used, high-vacuum electrospray deposition is a good alternative. However, because this method requires the introduction into vacuum of the molecules from solution, clean conditions are more difficult to be maintained. Additionally, because the presence of solvent on the surface cannot be fully eliminated, one has to take care of its possible influence. Here, we compare the high-vacuum electrospray deposition method to thermal evaporation for the preparation of C; 60; on different surfaces and compare, for sub-monolayer coverages, the influence of the deposition method on the formation of molecular assemblies. Whereas the island location is the main difference for metal surfaces, we observe for alkali halide and metal oxide substrates that the high-vacuum electrospray method can yield single isolated molecules accompanied by surface modifications

Low Friction at the Nanoscale of Hydrogenated Fullerene-Like Carbon Films

Friction force microscopy experiments at the nanometer scale are applied to study low friction of hydrogenated fullerene-like carbon films. The measured friction coefficients indicate that lower hydrogen concentration during preparation is beneficial to enter the low friction regime, especially in combination with only methane as precursor. Furthermore, two regions are found with distinct friction coefficients and surface roughnesses related to different surface structures. One is rich in amorphous carbon and the other is rich in fullerene-like carbon, dispersed on the same surface. Transmission electron microscopy and Raman spectroscopy images verify this observation of the two separated structures, especially with the extracted fullerene-like structures in the wear debris from macro friction experiments. It is speculated that hydrogen may tend to impair the growth of fullerene-like carbon and is therefore detrimental for lubricity

Synthesis of Giant Dendritic Polyphenylenes with 366 and 546 Carbon Atoms and Their High-vacuum Electrospray Deposition

Dendritic polyphenylenes (PPs) can serve as precursors of nanographenes (NGs) if their structures represent 2D projections without overlapping benzene rings. Here, we report the synthesis of two giant dendritic PPs fulfilling this criteria with 366 and 546 carbon atoms by applying a "layer-by-layer" extension strategy. Although our initial attempts on their cyclodehydrogenation toward the corresponding NGs in solution were unsuccessful, we achieved their deposition on metal substrates under ultrahigh vacuum through the electrospray technique. Scanning probe microscopy imaging provides valuable information on the possible thermally induced partial planarization of such giant dendritic PPs on a metal surface

Reconstruction of a 2D layer of KBr on Ir(111) and electromechanical alteration by graphene

A novel reconstruction of a two-dimensional layer of KBr on an Ir(111) surface is observed by high-resolution noncontact atomic force microscopy and verified by density functional theory (DFT). The observed KBr structure is oriented along the main directions of the Ir(111) surface, but forms a characteristic double-line pattern. Comprehensive calculations by DFT, taking into account the observed periodicities, resulted in a new low-energy reconstruction. However, it is fully relaxed into a common cubic structure when a monolayer of graphene is located between substrate and KBr. By using Kelvin probe force microscopy, the work functions of the reconstructed and the cubic configuration of KBr were measured and indicate, in accordance with the DFT calculations, a difference of nearly 900 meV. The difference is due to the strong interaction and local charge displacement of the K; +; /Br; -; ions and the Ir(111) surface, which are reduced by the decoupling effect of graphene, thus yielding different electrical and mechanical properties of the top KBr layer

2D KBr/Graphene Heterostructures-Influence on Work Function and Friction

The intercalation of graphene is an effective approach to modify the electronic properties of two-dimensional heterostructures for attractive phenomena and applications. In this work, we characterize the growth and surface properties of ionic KBr layers altered by graphene using ultra-high vacuum atomic force microscopy at room temperature. We observed a strong rippling of the KBr islands on Ir(111), which is induced by a specific layer reconstruction but disappears when graphene is introduced in between. The latter causes a consistent change in both the work function and the frictional forces measured by Kelvin probe force microscopy and frictional force microscopy, respectively. Systematic density functional theory calculations of the different systems show that the change in work function is induced by the formation of a surface dipole moment while the friction force is dominated by adhesion forces

Anchoring of a dye precursor on NiO(001) studied by non-contact atomic force microscopy

The properties of metal oxides, such as charge transport mechanisms or optoelectronic characteristics, can be modified by functionalization with organic molecules. This kind of organic/inorganic surface is nowadays highly regarded, in particular, for the design of hybrid devices such as dye sensitized solar cells. However, a key parameter for optimized interfaces is not only the choice of the  compounds but also the properties of adsorption. Here, we investigated the deposition of an organic dye precursor molecule on a NiO(001) single crystal surface by means of non-contact atomic force microscopy at room temperature. Depending on the coverage, single molecules, groups of  adsorbates with random or recognizable shapes, or islands of closely packed molecules were identified. Single molecules and self assemblies are resolved with sub-molecular resolution showing that they are lying flat on the surface in a trans-conformation. Within the limits of our Kelvin probe microscopy setup a charge transfer from NiO to the molecular layer of 0.3 electrons per molecules was observed only in the areas, where the molecules are closed packed

Observation of robust superlubricity of MoS₂ on Au(111) in ultrahigh vacuum

The structural and superlubric properties of single layer MoS2 on Au(1 1 1) forming moiré superlattice structures have been investigated by means of ultrahigh vacuum atomic force microscope with bimodal and contact modes. We synthesize epitaxial monolayer MoS2 flakes on the Au(1 1 1) surface in ultrahigh vacuum. Using friction force microscopy, atomic friction measurements indicate a superlubric regime between the tip apex and the moiré corrugated MoS2 surface in which the friction force remains at an ultralow value and is independent from normal load. Superlubricity conditions are observed for different loads and velocities which indicates the absence of out-of-plane deformations. We find that the MoS2 layer including the moiré superlattice modulation originating from the natural misfit between MoS2 and the Au(1 1 1) substrate is relatively rigid. We also demonstrate a low friction coefficient of the MoS2 surface crossing a single Au(1 1 1) step. Our results open up a new avenue for minimizing friction in nanoscale electronic devices and other dry rigid contacts used in aerospace lubrication

Velocity Dependence of Moiré Friction

Friction force microscopy experiments on moiré superstructures of graphene-coated platinum surfaces demonstrate that in addition to atomic stick–slip dynamics, a new dominant energy dissipation route emerges. The underlying mechanism, revealed by atomistic molecular dynamics simulations, is related to moiré ridge elastic deformations and subsequent relaxation due to the action of the pushing tip. The measured frictional velocity dependence displays two distinct regimes: (i) at low velocities, the friction force is small and nearly constant; and (ii) above some threshold, friction increases logarithmically with velocity. The threshold velocity, separating the two frictional regimes, decreases with increasing normal load and moiré superstructure period. Based on the measurements and simulation results, a phenomenological model is derived, allowing us to calculate friction under a wide range of room temperature experimental conditions (sliding velocities of 1–104 nm/s and a broad range of normal loads) and providing excellent agreement with experimental observations