Surface particle, atomic imaging

MARKS AND SMITH Atomic Imaging of Particle Surfaces... 

Based on TEM studies of supported metal catalysts, several workers have concluded that their catalysts were made of two-dimensional discs or rafts , where virtually all atoms are at the particle surface. However, sample tilting experiments in TEM have shown that great care should be exercised in the interpretation of TEM images of small particles (<2 nm in size), since phase contrast effects may dominate and variations in the particle contrast with specimen orientation can occur as a result of amplitude contrast effects (Treacy and Howie 1980). Sample tilting is therefore necessary to ensure correct interpretations of TEM images of metal-particle catalysts. This will be discussed further in the following sections. 

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... 

Electron microscopes use electrons (small particles found inside the atom) to produce images of tiny objects. The scanning tunelling microscope (STM) is the most sophisticated of this type of instrument and it can produce images of the surfaces of elements which show the individual atoms. 

The particle surface has the shape of a curved staircase formed by overlapped crystallites. In the case of both of the types of carbon black particles on the boundaries of crystallites, there can be a great number of hydrogen atoms, oxidized carbon atoms, and broken carbon bonds. Atom force microscopy (AFM) images of carbon black (Tanahashi et al. 1990, Donnet 1994) showed that crystallites are presented on the surface and they have the form of rectangles, but their arrangement can be random. It is evident that the differences in the carbon black particle structure observed by various techniques for diverse materials reflect the diversity of synthesized carbonaceous materials... 

Atomistic MD simulations use the laws of classical physics and therefore can only approximate full quantum-mechanical reality. The most straightforward type of MD simulation tracks the motion of a fixed number N of atoms, constrained to move inside a fixed volume V. Because any today computer can operate only with a drastically smaller number of particles than in a real maaoscopic system, where N is 10, for emulating a large sample of matter, all the particles are placed in a box - a basic cell - which usually is a cube with a specified edge L To eliminate the influence of surface effects, one can use periodic boundary conditions in which the basic cell is surrounded by identical translated images. Any particle is free to ctoss the box boundary, and when this happens, an identical particle enters the box from the opposite side, with the same velocity vector. One can picture a periodic system either as extending infinitely far in all directions, or as a finite system in which opposite sides are artificially coupled together (see Box 5). 

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