Analytical Three-Dimensional Field Ion Microscopy of an Amorphous Glass FeBSi.

Three-dimensional field ion microscopy is a powerful technique to analyze material at a truly atomic scale. Most previous studies have been made on pure, crystalline materials such as tungsten or iron. In this article, we study more complex materials, and we present the first images of an amorphous sample, showing the capability to visualize the compositional fluctuations compatible with theoretical medium order in a metallic glass (FeBSi), which is extremely challenging to observe directly using other microscopy techniques. The intensity of the spots of the atoms at the moment of field evaporation in a field ion micrograph can be used as a proxy for identifying the elemental identity of the imaged atoms. By exploiting the elemental identification and positioning information from field ion images, we show the capability of this technique to provide imaging of recrystallized phases in the annealed sample with a superior spatial resolution compared with atom probe tomography.

Development of Wide Field of View Three-Dimensional Field Ion Microscopy and High-Fidelity Reconstruction Algorithms to the Study of Defects in Nuclear Materials.

This article presents a fast and highly efficient algorithm developed to reconstruct a three-dimensional (3D) volume with a high spatial precision from a set of field ion microscopy (FIM) images, and specific tools developed to characterize crystallographic lattice and defects. A set of FIM digital images and image processing algorithms allow the construction of a 3D reconstruction of the sample at the atomic scale. The capability of the 3D FIM to resolve the crystallographic lattice and the finest defects in metals opens a new way to analyze materials. This spatial precision was quantified on tungsten, analyzed at different analyzing conditions. A specific data mining tool, based on Fourier transforms, was also developed to characterize lattice distortions in the reconstructed volumes. This tool has been used in simulated and experimental volumes to successfully locate and characterize defects such as dislocations and grain boundaries.

Enhancing Element Identification by Expectation-Maximization Method in Atom Probe Tomography.

This paper describes an alternative way to assign elemental identity to atoms collected by atom probe tomography (APT). This method is based on Bayesian assignation of label through the expectation-maximization method (well known in data analysis). Assuming the correct shape of mass over charge peaks in mass spectra, the probability of each atom to be labeled as a given element is determined, and is used to enhance data visualization and composition mapping in APT analyses. The method is particularly efficient for small count experiments with a low signal to noise ratio, and can be used on small subsets of analyzed volumes, and is complementary to single-ion decomposition methods. Based on the selected model and experimental examples, it is shown that the method enhances our ability to observe and extract information from the raw dataset. The experimental case of the superimposition of the Si peak and N peak in a steel is presented.

Optimizing Atom Probe Analysis with Synchronous Laser Pulsing and Voltage Pulsing.

Atom probe has been developed for investigating materials at the atomic scale and in three dimensions by using either high-voltage (HV) pulses or laser pulses to trigger the field evaporation of surface atoms. In this paper, we propose an atom probe setup with pulsed evaporation achieved by simultaneous application of both methods. This provides a simple way to improve mass resolution without degrading the intrinsic spatial resolution of the instrument. The basic principle of this setup is the combination of both modes, but with a precise control of the delay (at a femtosecond timescale) between voltage and laser pulses. A home-made voltage pulse generator and an air-to-vacuum transmission system are discussed. The shape of the HV pulse presented at the sample apex is experimentally measured. Optimizing the delay between the voltage and the laser pulse improves the mass spectrum quality.