Microscopie électronique à balayage

thumb|right|Premier microscope électronique à balayage par M von Ardenne thumb|right|Microscope électronique à balayage JEOL JSM-6340F thumb|upright=1.5|Principe de fonctionnement du Microscope Électronique à Balayage La microscopie électronique à balayage (MEB) ou scanning electron microscope (SEM) en anglais est une technique de microscopie électronique capable de produire des images en haute résolution de la surface d’un échantillon en utilisant le principe des interactions électrons-matière. S'appuyant sur les travaux de Max Knoll et Manfred von Ardenne dans les années 1930, la MEB consiste en un faisceau d’électrons balayant la surface de l’échantillon à analyser qui, en réponse, réémet certaines particules. Ces particules sont analysées par différents détecteurs qui permettent de reconstruire une image en trois dimensions de la surface. Les travaux menés dans les années 1960 dans le laboratoire de Charles Oatley à l’université de Cambridge ont grandement contribué au développ

Focused electron- and ion-beam induced processes

Vinzenz Friedli

Focused electron- and ion-beam induced processing are well established techniques for local deposition and etching that rely on decomposition of precursor molecules by irradiation. These high-resolution nanostructuring techniques have various applications in nanoscience including attach-and-release procedures in nanomanipulation and fabrication of sensors (magnetic, optical and thermal) for scanning probe microscopy. However, a complete physical and chemical understanding of the process is hampered by the lack of suitable means to monitor and to access the numerous interrelated and time-varying process parameters (deposition and etch rate, yield, molecule flux and adsorption/desorption). This thesis is a first attempt to fill this gap. It is based on experimental and simulative approaches for the determination of process conditions and mechanical properties of deposited materials: Mass and force sensors: The use of tools merging micromechanical cantilever sensors and scanning electron microscopy was demonstrated for in situ monitoring and analysis. A cantilever-based resonant mass sensing setup was developed and used for real-time mass measurements. A noise level at the femtogram-scale was achieved by tracking the resonance frequency of a temperature stabilized piezoresistive cantilever using phase-locking. With this technique the surface coverage and residence time of (CH3)3PtCpCH3 molecules, the mass deposition rate, the yield, and the material density of corresponding deposits were measured. In situ cantilever-based static force sensing and mechanical modal vibration analysis were employed to investigate the Young's modulus and density of individual high aspect ratio deposits from the precursor Cu(hfac)2. Precursor supply simulations and experiments: A prerequisite to understand and quantify irradiative precursor chemistry is the knowledge of the local flux of molecules impinging on the substrate. Therefore, Monte Carlo simulations of flux distributions were developed and gas flows injected into a vacuum chamber were analyzed experimentally for the precursors Co2(CO)8, (hfac)CuVTMS, and [(PF3)2RhCl]2. The process parameters extracted from the mentioned approaches are valuable input for numerical focused electron- and ion-beam induced process models (Monte Carlo, continuum). We evaluated the precursor surface diffusion coefficient and the electron impact dissociation cross-section by relating deposit shapes to a continuum model.

EPFL2008

Synthesis of carbon nanotubes and characterization of nanotubes as atomic force microscopy tips

Yanki Keles

This thesis describes the synthesis of carbon nanotubes (CNTs) by chemical vapor deposition (CVD) and the evaluation of them as tips of atomic force microscope (AFM) probes. Both arc-discharge and CVD grown nanotubes were investigated as AFM tips. Two methods were used to produce nanotube tipped AFM probes: (i) nanotubes were directly grown on the apex of AFM probes by CVD and (ii) pre-grown nanotubes were mechanically attached onto the apex of AFM probes. The first part of the thesis describes experiments that sought to control the growth of carbon nanotubes by chemical vapor deposition. The effects of various parameters e.g. catalyst, the carbon source, the addition of hydrogen and the growth temperature, on carbon nanotube growth were investigated. Ethylene was found to lead to better quality nanotubes than the acetylene. Grown nanotubes' quality was found to improve upon flow of hydrogen during the synthesis. Nanotubes synthesized on 3 nm iron film at 840°C using ethylene and hydrogen led to utilizable carbon nanotube tipped AFM probes. The second part of the thesis focuses on the mechanical production of nanotube tipped AFM probes in high resolution electron microscopes using nanomanipulators. Scanning electron microscopy (SEM) was found to be more convenient than transmission electron microscopy (TEM) to carry out the nanomanipulation process. Both arc-discharge and CVD grown nanotubes were successfully attached onto the AFM probes using a custom made nanomanipulator. Nanotubes were glued onto the probes by focused electron beam (FEB) induced amorphous carbon deposits. FEB irradiation deposited amorphous carbon performed more reliable attachment on CVD nanotubes than the arc-discharge nanotubes. The last part of the thesis describes tapping mode AFM measurements done using nanotube tipped AFM probes. Different nanotube tipped probes were investigated as AFM tips for scanning colloidal gold particles with different diameters and gratings from Digital Instruments™ AFMs. High resolution AFM images were obtained by arc-discharge nanotube tips. CVD grown nanotubes were shown to be more resilient to tip crashes than the arc-discharge grown nanotubes. However, all the AFM images acquired from the scans by CVD nanotubes had the same kind of ringing artifact, in good agreement with a previously reported data about the artifacts of the CVD nanotube tipped AFM probes. Open ends of the CVD nanotubes, lack of crystallinity and the defects in their structure are suggested to be the reason for these artifacts.

EPFL2006

Experimental evaluation and modeling of fatigue of a 316L austenitic stainless steel in high-temperature water and air environments

Wen Chen

Austenitic stainless steels is used in many components of nuclear power plants, particularly in the pipes of cooling systems. Owing to power transients and to start-ups and shutdowns, these components are subjected to thermo-mechanical loadings (low-cycle fatigue LCF & high cycle fatigue HCF) and flow-induced vibration (HCF). The corresponding fatigue design curves were established with solid smooth specimens tested in air at room temperature under strain-control. However, these curves do not consider the influence of LWR water environments that were shown to reduce fatigue life. US NRC Regulatory Guide 1.207 or other national equivalents (JSME Code in Japan) then were established taking strain rate, dissolved oxygen and temperature into consideration. Besides, a significant number of issues have been recently identified, which may have an impact on fatigue life but have not been sufficiently investigated, e.g., the potential negative effects of mean stress, mean strain, surface finish, long-term static hold, specimen geometry, multiaxial stress state were sensitivities not explicitly addressed. This study was launched to assess the effect of mean stress on fatigue behavior of a 316L austenitic steel in boiling water reactor environment with hydrogen water chemistry (BWR/HWC) at 288°C. Load-controlled fatigue tests were selected as the easiest experimental technique to impose a pre-defined mean stress. The tests were carried out on hollow specimens at different stress amplitudes and mean stresses in BWR/HWC environment and in air at 288°C, the latter serving as reference environment to evaluate the life reduction induced by the BWR/HWC medium. Few tests in strain-controlled were performed to derive a consistent way to represent the data obtained with the two control modes together. The test results reported in the form of stress-life curves showed that, in LCF regime (< 1e5 cycles), positive and negative mean stresses increase and BWR/HWC environment decreases fatigue life. In the HCF regime, negative mean stress is always beneficial for fatigue life but positive mean stress decreases fatigue life in BWR/HWC. The beneficial effect of mean stress is attributed to its enhanced cyclic hardening on material, which leads to smaller strain amplitude at given stress amplitude. The load-controlled data were converted into strain-life presentation by considering the average strain amplitude over the whole loading cycles. Additionally, a modified Smith-Watson-Topper mean stress correction method was successfully used to correlate the results with and without mean stress. Finally we showed that strain energy density criteria are good alternatives to correlate all data. Crack growth rates were determined from the striation spacing on the fracture surfaces with high resolution scanning electron microscopy (HRSEM); crack initiation sites were also studied with HRSEM, and the microstructures were observed with transmission electron microscopy and electron channeling contrast imaging. From the striation spacing measurement, we concluded that BWR/HWC environment essentially reduces the number cycles needed to initiate a physical crack (crack depth < 50 microns) but modifies slightly the crack growth rate, which correlates well with a strain intensity factor and J-integral method. Life predictions were also done by a well-trained artificial neuron network that considers mechanical, environmental and material properties factors.

EPFL2020