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Alkali-Halide Thin Films : Growth, Structure and Reactivity upon Electron Irradiation

Abstract : NaCl and KCl are wide band gap materials that are increasingly used as thin films in surface science and single-molecule spectroscopy studies to electronically decouple organic molecules from a metal substrate. In addition, the reactivity of bulk NaCl and KCl crystals under electron irradiation has been widely studied since the year 1970 s. In this dissertation, we report a detailed investigation on the structures of two different thin films of NaCl and KCl grown on the Ag(001) substrate by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) in ultrathin vacuum condition. In order to obtain high-quality thin films and to control the properties of these films on the metal surface (film thickness, domain size, domain orientation, etc), we study in detail the film growth, especially as a function of the growth parameters. In addition, the apparent height measurements by STM show that the apparent thickness and the STM contrast of these two insulating films are bias dependent. We report on an electron-induced modification of alkali halides in the ultrathin film regime. The reaction kinetics and products of the modifications are investigated in the case of NaCl films grown on Ag(001). Their structural and chemical modification upon irradiation with electrons of energy 52–60 eV and 3 keV is studied using LEED and AES, respectively. The irradiation effects on the film geometry and thickness (ranging from between two and five atomic layers) are examined using STM. We observe that Cl depletion follows different reaction kinetics, as compared to previous studies on thick NaCl films and bulk crystals. Na atoms produced from NaCl dissociation diffuse to bare areas of the Ag(001) surface, where they form Na-Ag superstructures that are known for the Na/Ag(001) system. The modification of the film is shown to proceed through two processes, which are interpreted as a fast disordering of the film with removal of NaCl from the island edges and a slow decrease of the structural order in the NaCl with formation of holes due to Cl depletion. The kinetics of the Na-Ag superstructure growth is explained by the limited diffusion on the irradiated surface, due to aggregation of disordered NaCl molecules at the substrate step edges. We have also investigated the deposition of PTCDA molecules on the metallic substrate Ag(001) and on the KCl/Ag(001) system using STM. We obtain a monolayer of PTCDA molecules on Ag(001) that is arranged in a square unit cell. On KCl films, both individual PTCDA molecules and a densely packed structure are found.
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Submitted on : Thursday, October 1, 2020 - 1:06:35 AM
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Ala Husseen. Alkali-Halide Thin Films : Growth, Structure and Reactivity upon Electron Irradiation. Atomic Physics [physics.atom-ph]. Université Paris Saclay (COmUE), 2018. English. ⟨NNT : 2018SACLS081⟩. ⟨tel-02954356⟩



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