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Oral, sans d'actes; International audience; We studied by non-contact AFM (nc-AFM) the formation of molecular self-assemblies on the passivated surface of boron-doped Silicon B-Si(111)-(√3×√3)R30. The investigated molecules (1-(4’cyanophenyl)-2,5-bis(decyloxy)-4-(4’-iodophenyl)benzene) possess aliphatic chains attached to a triphenyl core ended with two possible different terminations (either iodine or cyano group). The use of a passivated semiconductor substrate enables creating regular and extended structures without significant change in electronic properties of molecules [1]. Scanning tunneling microscopy and nc-AFM imaging have been performed using a low-temperature (AFM/STM (JT AFM/STM,SPECS) operated at T=4K with high stiffness Kolibri sensors (k=540 kN/m, f0=1 MHz). The growth of a periodic molecular network is observed, formed by parallel lines made by molecule aromatic cores and interdigitated aliphatic chains placed between adjacent rows. We obtain submolecular resolution in the constant height ∆f images without intentional tip functionalization, but only by conditioning the tip on the Si surface [2]. Kelvin Probe Force Microscopy (KPFM) images of single molecules and molecular assemblies with sub-elementary charge sensitivity [3] and submolecular resolution will be shown. They attest of the dipolar character of asymmetric molecules and are consistent with the formation of dipole-driven molecular arrays.

The progress of non-contact Atomic Force Microscopy (nc-AFM) in the last decade enabled studying the structural and electronic properties of molecules at the submolecular scale. The process of tip functionalization relying in attaching single atoms or molecules to the tip apex demonstrated an enhanced resolution in both STM and AFM images, such that the chemical structures of many different molecules deposited on metallic surfaces have been resolved. However, few studies have been presented on a semiconductor surface so far. In this work, we study the formation of molecular self-assemblies on the passivated surface of boron doped silicon Si(111)-(√3x√3)R30°-B by combined STM/nc-AFM with Kelvin Probe Force Microscopy (KPFM). The experiments have been performed on the Joule-Thomson Scanning Probe Microscope (SPECS), working in ultra high vacuum at temperature of 4 K, using a high stiffness sensor (Kolibri, k=540 kN/m, f0=1 MHz). The investigated molecules are: 1-(4’cyanophenyl)-2,5-bis(decyloxy)-4-(4’-iodophenyl)benzene (CDB-I), which possess two aliphatic chains attached to a triphenyl core ended with two different terminations (either iodine or cyano group) and non-polar 1,4 bis(4'cyanophenyl)-2,5bis(decyloxy)benzene (CDB) molecule, with two identical cyano group terminations. The first main objective of this work was to verify the sensitivity of the Kolibri sensor in the charge detection. KPFM measurements of boron doped silicon defects are presented, showing different charge states for dangling bond (DB), silicon vacancy and buried boron dopant defects. A positive charge state is found for the DB, in accordance with previous STM studies. The surface potential of this defect constitutes a reference value of a single charge on the surface. The second objective of this thesis was to obtain submolecular resolution in topography and electrical imaging on molecules, without intentional tip functionalization. A submolecular contrast is observed in the frequency shift images of single molecules with identification of the three-phenyl core and details of the aliphatic chains. Moreover, a high resolution is obtained in the surface potential images from KPFM measurements as well. Depending on type of the adsorption of molecule on the surface, two different KPFM contrasts are distinguished. Lastly, the comparison of organization in the assemblies is done for both CDB and CDB-I molecules. A similar organization is found for both molecules in nc-AFM images. The asymmetry of the nc-AFM topography and KPFM map of CDB-I molecules indicates the dipolar organization along a given assembly row.; Le progrès important de la microscopie à force atomique sans contact (nc-AFM) au cours de la dernière décennie a permis d'étudier les propriétés structurelles et électroniques des molécules à l'échelle sub-moléculaire. Le processus de fonctionnalisation de la pointe, qui consiste à attacher des atomes ou des molécules uniques à l'apex de la pointe, a démontré une résolution exaltée des images STM et AFM, de sorte que les structures chimiques de nombreuses molécules différentes déposées sur des surfaces métalliques ont été résolues. Cependant, peu d'études ont été présentées jusqu'à présent sur des surfaces de semiconducteurs. Dans cette thèse, nous étudions la formation d'auto-assemblages moléculaires sur la surface passivée du silicium Si(111)-(√3x√3)R30°-B dopée au bore par STM/nc-AFM combiné avec Kelvin Probe Force Microscopy (KPFM). Les expériences ont été réalisées sur un microscope AFM/STM Joule-Thomson (SPECS), travaillant sous ultra-vide à une température de 4 K, en utilisant un capteur de force à haute rigidité (Kolibri, k=540 kN/m, f0=1 MHz). Les molécules étudiées sont : 1-(4'cyanophényl)-2,5-bis(décyloxy)-2,5-bis(décyloxy)-4-(4'-iodophényl)benzène (CDB-I), qui possèdent deux chaînes aliphatiques fixées à un noyau triphényle avec deux terminaisons différentes (soit iode, soit cyano) et la molécule nonpolaire 1,4 bis(4'cyanophényle)-2,5bis(décyloxy)benzène (CDB), avec deux terminaisons cyano identiques. Le premier objectif principal de ce travail était de vérifier la sensibilité de la sonde Kolibri dans la détection de charge. Les mesures KPFM portent sur les défauts du substrat dopé au bore, montrant les différents états de charge pour la liaison pendante (DB), la lacune de silicium et le dopant de bore enterré. L'état de charge positif est trouvé pour la DB, conformément aux études antérieures de la STM. Le potentiel de surface du défaut constitue la valeur de référence d'une seule charge sur la surface. Le deuxième objectif de cette thèse était d'obtenir une résolution sub-moléculaire en topographie et en imagerie KPFM sur les molécules, sans fonctionnalisation intentionnelle de la pointe. Un contraste sub-moléculaire est observé dans les images de décalage de fréquence de molécules individuelles, permettant l’identification du noyau triphényle et des détails des chaînes aliphatiques. De plus, la haute résolution est également obtenue dans les images de potentiel de surface des mesures KPFM. Selon le type d'adsorption de la molécule en surface, on distingue deux contrastes KPFM différents. Enfin, la comparaison de l'organisation dans les assemblages est faite pour les molécules CDB et CDB-I. Malgré l'organisation similaire des deux molécules dans les images nc-AFM, une différence de potentiel de surface est observée pour les deux terminaisons de la molécule CDB-I, indiquant l'organisation dipolaire le long des rangées de molécules des auto-assemblages.

National audience; We studied by non-contact AFM (nc-AFM) the formation of molecular self-assemblies on the passivated surface of boron doped silicon B-Si(111)-(V3xV3) R30°. The investigated molecule (1-(4’cyanophenyl)-2,5-bis(decyloxy)-4-(4’-iodophenyl)benzene) possesses two aliphatic chains attached to a triphenyl core ended with two different terminations (either iodine or cyano group). The use of a passivated semiconductor substrate enables creating regular and extended structures without significant change in electronic properties of molecules. Scanning tunneling microscopy and nc-AFM imaging have been performed using a low-temperature (AFM/STM (JT AFM/STM, SPECS) operated at T=4K with high stiffness Kolibri sensors (k=540 kN/m, f0=1 MHz). The growth of a periodic molecular network is observed, formed by parallel lines made by molecule aromatic cores and interdigitated aliphatic chains placed between adjacent rows (see Figure 1). We obtain submolecular resolution in the constant height f images without intentional tip functionalization, but only by conditioning the tip on the silicon surface [4]. We will discuss the high-resolution AFM imaging, as well as the conformation of the molecules in the observed assemblies (e.g. changes in the interdigitated aliphatic chains ordering), and in particular the role of the molecule dipoles. Kelvin probe spectroscopy images with sub-molecular resolution will be presented.

International audience; We present a new concept for the fabrication of up to 1µm long polymer fibres on insulating alkali-halide substrates [1]. The novelty of our approach is that first, the chosen dimaleimide molecules (fig. a) perform a chain-like (and not a step-like) growth via a radical polymerization, second, that the polymerization is initiated from a two-dimensional gas phase rather than from an intermediate self-assembled structure, and third, that the cations of the chosen alkali-halide substrate interact with the molecule’s oxygen atoms and facilitate a defect-free propagation of the polymer fibres along a preferred axis of the substrate. We discuss the different initiation mechanisms of the polymer reaction at room-temperature that can either be spontaneous (fig. b), or induced by UV-light (fig. d). Once initiated, the chain-like polymerization proceeds easily and is only stopped by either defects on the surface or exhaustion of the precursor molecules. The propagation of this reaction on the surface, the involved reaction barriers as well as the structure and stability of the formed 1D polymer fibres (fig. c) are discussed by means of Density Functional Theory calculations, Climbing Image Nudged Elastic Band Calculations, and noncontact Atomic Force Microscopy imaging and manipulation at room temperature.