Photo-switching of organic monolayers on silicon surfaces

Abstract : The design of “smart” surfaces responsive to external stimuli (light, electromagnetic field, chemical environment…) is attracting considerable interest because of their potential for a wide range of applications. Within this context we are studying the photoswitching properties of a monolayer of organic photochromes immobilized onto silicon surfaces.Fulgimide groups are anchored through covalent linkage atop of functionalized alkyl monolayers grafted on oxide free Si(111) surfaces. The monolayers composition at the photo stationary states PSS-UV and PSS-Vis is determined from quantitative analysis of the infrared band intensity characteristic of open (E,Z) and closed (C) isomers. The UV-Vis surface photocommutation is monitored by in-situ real time FTIR measurements during UV-Vis illumination. Time dependence studies of photocommutation evidence decreasing quantum efficiency during the commutation. This decrease in quantum efficiency only weakly depends on fulgimide density and is not observed in solution. However, PC measurements as a function of photon flux enabled determining a PC cross section (σ) of the majority of switching molecules. Polarization dependent photocommutation studies show the strong dependence of σ with respect to the local electric field of the isomerization-exciting light.Analytical models and Monte Carlo simulations based on nearest neighbor interactions are performed to gain deeper insight in the experimental observations. These simulations qualitatively explain the temperature dependence of the commutation kinetics, decreasing quantum efficiency and weak surface density dependence of the photocommutation.It has been shown in this thesis that σ depends on the local electric field. Similar to the Surface Enhanced Raman Spectroscopy the local electric field at surfaces is increased due to the plasmon of gold nanoparticles. The plasmon of the gold nanoparticle monolayer and thereby the enhancement of the electric field depends on the wavelength of the external irradiation. Exploitation of this effect improves the photo switching kinetics significantly depending on the wavelength of the irradiation. This wavelength dependent amplification of the switching kinetics is explained by the same wavelength dependent enhancement of the electric field.
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Stefan Klaes. Photo-switching of organic monolayers on silicon surfaces. Materials Science [cond-mat.mtrl-sci]. Université Paris-Saclay, 2017. English. ⟨NNT : 2017SACLX071⟩. ⟨tel-01680118⟩

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