Jury:

• Thierry MELIN, Director of Research, University of Lille (Thesis supervisor)
• Benjamin GREVIN, Director of Research, University of Grenoble (Rapporteur)
• Mariusz ZDROJEK, Professor, University of Warsaw, Faculty of Physics (Rapporteur)
• Laurent NONY, Senior Lecturer, Aix-Marseille University (Examiner)
• Jérôme LAGOUTE, Research Fellow, Université Paris Diderot (Examiner)
• Sylvie GODEY, Research Engineer, University of Lille (Examiner)
• Frank PALMINO, Professor, Université de Franche Comté (Examiner)
• Xiaonan SUN, Research Engineer, Université Paris Diderot (Examiner)

Summary:

Significant advances in non-contact atomic force microscopy (nc-AFM) over the last decade have made it possible to study the structural and electronic properties of molecules at the sub-molecular scale. The process of tip functionalization, which involves attaching single atoms or molecules to the tip apex, has demonstrated exalted resolution of STM and AFM images, so that the chemical structures of many different molecules deposited on metal surfaces have been resolved. However, few studies have been presented to date on semiconductor surfaces.

In this thesis, we study the formation of molecular self-assemblies on the passivated surface of boron-doped silicon Si(111)-(√3x√3)R30°-B by STM/nc-AFM combined with Kelvin Probe Force Microscopy (KPFM). The experiments were carried out on a Joule-Thomson AFM/STM microscope (SPECS), working in ultra-high vacuum at a temperature of 4 K, using a high-rigidity force transducer (Kolibri, k=540 kN/m, f0=1 MHz). The molecules studied were : 1-(4'cyanophenyl)-2,5-bis(decyloxy)-2,5-bis(decyloxy)-4-(4′-iodophenyl)benzene (CDB-I), which have two aliphatic chains attached to a triphenyl ring with two different terminations (either iodine or cyano) and the non-polar molecule 1,4 bis(4'cyanophenyl)-2,5bis(decyloxy)benzene (CDB), with two identical cyano terminations.

The first main objective of this work was to verify the sensitivity of the Kolibri probe in charge detection. The KPFM measurements focus on defects in the boron-doped substrate, showing the different charge states for the dangling bond (DB), the silicon gap and the buried boron dopant. The positive charge state is found for the DB, consistent with previous STM studies. The surface potential of the defect is the reference value of a single charge on the surface.

The second objective of this thesis was to obtain sub-molecular resolution in topography and KPFM imaging of molecules, without intentional tip functionalization. Sub-molecular contrast is observed in frequency shift images of individual molecules, allowing identification of the triphenyl ring and details of aliphatic chains. In addition, high resolution is also achieved in the surface potential images of the KPFM measurements. Depending on the type of adsorption of the molecule on the surface, two different KPFM contrasts can be distinguished. Finally, the organisation in the assemblies is compared for the CDB and CDB-I molecules. Despite the similar organisation of the two molecules in the nc-AFM images, a difference in surface potential is observed for the two termini of the CDB-I molecule, indicating dipolar organisation along the rows of molecules in the self-assemblies.

Abstract:

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.