Imaging atom-probe

Imaging atom-probes, although severely limited in mass resolution, are very useful where one seeks information about the spatial distribution of chemical species on the emitter surface as well as in the bulk. They find many applications in studies of metallurgical problems,55 in studies of chemisorptions and surface reactions56 and oxidation of metals,57 etc., as will be discussed in later chapters. In using imaging atom-probes, it is important to select the system carefully and intelligently so that mass overlap of different elements can be avoided. 

The Chevron channel plate ion detector assembly of an imaging atom-probe can also be replaced by a position sensitive particle detector combined with a data processor, as reported by Cerezo etal.5s (A position sensitive detector was used earlier for the purpose of field ion image recording and processing.59) With such a detector both the chemical identity and the spatial origin on the emitter surface can be found for each field evaporated ion. This position sensitive atom-probe can be used to study the spatial distribution of different ion species on the emitter surface as well as inside the bulk of the emitter with a spatial resolution nearly comparable to the FIM. For such a purpose, one carries out the field evaporation at an extremely slow rate so that no more than one ion is detected from the entire field ion emitter surface in each pulsed field evaporation. From the flight time of the ion its chemical species is identified, and from the location of the detector where the ion is detected the spatial origin of the ion is located. With a fast data processor, a two-dimensional distribution of chemical species on the tip surface can be 

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Fig. 3.13 Schematic of an imaging atom-probe which uses the pulsed-laser...

Fig. 3.14 A ToF spectrum of a high purity silicon whisker in hydrogen image gas, obtained with a pulsed-laser imaging atom-probe. Below 80 K, silicon is an excellent insulator. However, the tip surface can be easily field evaporated with the stimulation of laser pulses.

The reactivity of a surface depends on many factors. These include the adsorption energies of chemical species and their dissociation behavior, their diffusion on the surface, the adatom-adatom interactions, the active sites where a chemical reaction can occur, and the desorption behavior of a new chemical species formed on the surface. The site specificity depends on at least three factors the atomic configuration of the surface, the electronic structures of the surface, and the localized surface field. In atom-probe experiments, the desorption sites can be revealed by a timegated image of an imaging atom-probe as well as by an aiming study with a probe-hole atom-probe, the electronic structure effect of a chemical reaction can be investigated by the emitter material specificity, and the surface field can be modified by the applied field. 

Field ion microscopy is one of the older techniques to involve ions and yet it remains the only means available for viewing individual atoms on a surface directly. Since an extensive literature exists, the subject will not be considered here except to mention some recent papers on surface diffusion and atomic interactions, cluster formation, the imaging atom-probe microscope, and reviews in the fields of metallurgy and surface chemistry. ... 

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