Centre d’Élaboration de Matériaux et d’Etudes Structurales (UPR 8011)


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Thibault Ardhuin PhD defense

"The charge process of molecules deposed on insulator substrate and its dependence with its thickness"

PhD defense on Monday, September 24th at 2:30pm
CEMES conference room

 

 

Jury members

 

  • M. Jérôme LAGOUTE (Université de Paris Diderot) Rapporteur
  • M. Frank PALMINO (UFC/IUT de Belfort Montbéliard) Rapporteur
  • M. Pierre MALLET (Université de Grenoble Alpes) Examinateur
  • Mme. Laurence MASSON (Université Aix Marseille) Examinatrice
  • M. Arjan BERGER (Université Paul Sabatier) Examinateur
  • M. Philippe DUMAS (Université Aix Marseille) Examinateur
  • M. Sébastien GAUTHIER (CEMES CNRS) Directeur de thèse
  • M. Olivier GUILLERMET (CEMES CNRS) Co-Directeur de thèse

 

 

 Abstract

In recent years, new techniques have emerged to control the charge of individual nano-objects (atom, molecule, metal aggregate or semiconductor, etc.) deposited on insulating substrates. This achievement has been made possible by the refinement of Tunneling Microscopy (STM) and Atomic Force (AFM) methods. By combining these tools, the precursors succeeded in controlling the state of charge of a gold atom deposited on a NaCl (001) bilayer on a Cu (111) substrate. Subsequently, this type of manipulation has been extended to molecular systems, in particular at the CEMES with Cu(dbm)2.

This subject is part of the continuity of these studies. The objective was to analyze the impact of the increase of the thickness of the insulating film on the charge mechanisms. This problem requires a quantification of the state of the system charge as well as a measurement of the insulation thickness. In this work, we have been able to study KBr and NaCl films deposited on Cu(111) and Ag(111) surfaces.

For these studies, whether in tunnel current (STM) or force gradient (NC-AFM), the control of the tip state is essential. When working on an insulating substrate, the tip tends to collect contaminants that change their electronic properties. However, to charge a system in a reproducible way, we must imperatively control the metallicity of the apex. This control requires a frequent re-preparation of the tip on a metal surface, difficult to find in the case of a thick film. To overcome this scarcity, we have implemented a deposition mask allowing a control of the gradient of the thickness of the insulating film while preserving clean metal zones. This allowed us to carry out our measurements with a better controlled state of the tip.

The instability of the tip state has also led us to perform Z (V) regulated current spectroscopies. By controlling this current, it is then possible to minimize the interaction between the tip and the insulating film, thus making the tip last longer. These Z (V) spectroscopies also make it possible to increase the measurement voltage until reaching the field emission regime. We have observed a variation of the modulation of the field emission resonances (FER) amplitude as a function of the thickness of the insulating film.

Numerical modeling by finite differences method has been developed to understand this phenomenon. Preliminary results show that the exploitation of this property could be the basis of an experimental method by allowing the measurement of the thickness of the insulating film.

We then studied the adsorption of the hexa-peri-hexabenzocoronene (HBC) molecule on NaCl or KBr films deposited on substrates of Cu (111) and Ag (111). It was then possible to characterize the molecular resonances (MERs) of isolated HBCs and molecular assemblies constructed by manipulation with the STM tip on these two substrates in both spectroscopy and imaging.

In order to characterize the state of charge of the HBC molecules, we carried out Δf (V) spectroscopies by NC-AFM, which make it possible to characterize the electrostatic force between the tip and the surface. From a thickness of 6 monolayers of KBr, we noted the occurrence of a shift in the force curve at the HBC MER voltages (in positive and negative polarities), signature of charge accumulation.

A next step in the continuity of this work would be to combine molecular assemblies with the charging process. It would then be possible to modulate the residence time of the electron by playing not only on the thickness of the insulating film but also on the intermolecular couplings by adapting the structure of the assemblies and thus to study different situations where the charge can or not explore the assembly before being captured by the substrate.