Spectroscopie de réflectivité différentielle sur des couches moléculaires: aspects expérimentaux

Doctora

PRINCIPLES OF KELVIN PROBE FORCE MICROSCOPY AND APPLICATIONS PRINCIPLES OF KELVIN PROBE FORCE MICROSCOPY AND APPLICATIONS

Doctora

Understanding the atomic-scale contrast in Kelvin Probe Force Microscopy

A numerical analysis of the origin of the atomic-scale contrast in Kelvin probe force microscopy (KPFM) is presented. Atomistic simulations of the tip-sample interaction force field have been combined with a non-contact Atomic Force Microscope/KPFM simulator. The implementation mimics recent experimental results on the (001) surface of a bulk alkali halide crystal for which simultaneous atomic-scale topographical and Contact Potential Difference (CPD) contrasts were reported. The local CPD does reflect the periodicity of the ionic crystal, but not the magnitude of its Madelung surface potential. The imaging mechanism relies on the induced polarization of the ions at the tip-surface interface owing to the modulation of the applied bias voltage. Our findings are in excellent agreement with previous theoretical expectations and experimental observations

Microscopie de force dynamique: éléments (Deuxième partie)

DEACe cours en deux parties (deuxième partie ici) introduit les deux modes dynamiques utilisés en microscopie à force atomique (AFM): mode de modulation d'amplitude (AM-AFM), plus connu sous le nom de "Tappling" et mode de modulation de fréquence (FM-AFM), également connu sous le nom de non-contact AFM. Les aspects pratiques et théoriques propres à chaque mode sont discutés et illustrés d'un point de vue expérimental. Concernant le non-contact AFM, nous détaillons point par point l'électronique de contrôle de l'instrument et illustrons son fonctionnement à partir des résultats d'un code numérique qui reprend très précisément une électronique de contrôle existante

Microscopie de force dynamique: éléments (Première partie)

DEACe cours en deux parties (première partie ici) introduit les deux modes dynamiques utilisés en microscopie à force atomique (AFM): mode de modulation d'amplitude (AM-AFM), plus connu sous le nom de "Tappling" et mode de modulation de fréquence (FM-AFM), également connu sous le nom de non-contact AFM. Les aspects pratiques et théoriques propres à chaque mode sont discutés et illustrés d'un point de vue expérimental. Concernant le non-contact AFM, nous détaillons point par point l'électronique de contrôle de l'instrument et illustrons son fonctionnement à partir des résultats d'un code numérique qui reprend très précisément une électronique de contrôle existante

DNA properties investigated by dynamic force microscopy

In this work, we show that by varying the experimental conditions, the driving amplitude, a dynamic force microscope allows DNA properties to be selectively imaged. The substrate on which the DNA is fixed is a silica surface grafted with silanes molecules ended with amine groups. Use of small oscillation amplitudes favors the attractive interaction between the tip and the sample, while use of large amplitudes renders the contribution of the attractive interaction negligible. Particularly, at small amplitudes, the images show that the attractive interaction is strongly enhanced along the DNA. This enhancement is found to be amenable with a model considering a narrow strip of randomly oriented dipoles on each side of the molecule. This work should provide new insights on the DNA interaction and conformational changes with localized charges

On the relevance of the atomic-scale contact potential difference by Amplitude modulation- and Frequency modulation-Kelvin probe force microscopy

International audienceThe influence of short-range electrostatic forces on the measured local Contact Potential Difference (CPD) by means of Amplitude Modulation- and Frequency Modulation-Kelvin Probe Force Microscopy (AM- and FM-KPFM) is discussed on the base of numeric and analytic descriptions of both methods. The goal of this work is to help interpreting recent experimental results reporting atomically-resolved CPD images, in particular on bulk insulating samples. The discussion is carried out on the base of spectroscopic curves. The expression of the bias-dependent electrostatic force derives from a previous work and is estimated between a tip with simple geometry and the (001) facet of a perfect alkali halide single crystal. The force, with a short-range character, scales as a second-order polynomial function of the bias voltage. It is stated that the linear term is responsible for the occurrence of the atomic-scale CPD contrast, while the quadratic one, involving the sample polarisation, accounts for the detected signal by the KPFM methods. Nevertheless, analytic and numeric approaches stress the influence of the linear term on the measured CPD which intrinsically hinders the possibility to perform quantitative CPD measurements, but also makes the measured ``pseudo-CPD" strongly deviating from the surface potential. Hence, in the short-range regime, AM- or FM-KPFM measurements neither reflect the CPD nor the local surface potential, but rather an effective value which is convoluted by the geometric parameters of the tip, the so-called local CPD. At last, the influence of long-range, capacitive, electrostatic forces is discussed in conjunction with the short-range ones. This allows us to draw conclusions regarding the distance dependence of the local CPD which then exhibits a resonant behavior as a function of the tip-surface separation. This phenomenon is expected to play a role in the KPFM imaging process