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Atom/Ion Sources
Published in Peter E. J. Flewitt, Robert K. Wild, Physical Methods for Materials Characterisation, 2017
Peter E. J. Flewitt, Robert K. Wild
Ion scattering spectroscopy is a technique where a specimen surface is bombarded with ions of a particular species. As the energy and momentum of these ions increase, there is a likelihood that these ions will penetrate further into the surface, knocking atoms from their normal lattice positions that are then ejected from the material. At all energies, there will be a proportion of incident ions scattered from the surface but the fraction decreases with increasing energy, and it is these ions scattered from the matrix that form the basis of SIMS (Benninghoven et al. 1987). The technique has gained widespread acceptance as a tool for studying surfaces because it has high sensitivity, can give chemical state information and in recent years has been used to produce ion images with high spatial resolution. When a beam of ions is incident on the surface of a material, the considerable momentum is sufficient to break the atom bonds and translate surface atoms a considerable distance into the bulk. The various processes are summarised in Figure 6.13. Here, the displaced atoms may cause other atoms and atom clusters to be dislodged from their normal positions and a fraction of these will be ejected from the surface and into the containing vacuum. The bulk of the atoms and atom clusters are ejected in the neutral state, but some will be ionised while others will have a negative charge. At the same time, electrons are emitted from the surface. The ionised atoms and particles are detected by SIMS while the neutral atoms are collected using the technique of SNMS to be described in the next section. Both the electrons and the ions emitted from the surface can be used to obtain images. A schematic of the basic equipment required to obtain SIMS information is shown in Figure 6.14 where ions from a suitable source are arranged to be incident on the surface being studied. The ejected ions and ion clusters are first separated using a filter, usually an electrostatic deflection system to remove unwanted neutrals, electrons and ions of the opposite polarity to that being detected. They are then detected using either magnetic sector analysers, quadrupole mass spectrometers or TOF mass spectrometers. Since most of the ion guns can be used with any combination of analyser and detector, ion guns and analysers will be discussed separately.
Study on emission and particulate matter characteristics from diesel engine fueled with n-pentanol/Fischer–Tropsch diesel
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2020
Lihua Ye, Yefan Shi, Tianyu Liu, Qingming Zheng, Muhammad Muzamal Ashfaq, Chengwei Sun, Aiping Shi, Muhammad Ajmal
DXR laser raman spectrometer is produced by Thermo Fisher Company of the USA. DXR is equipped with three excitation wavelengths of 532 nm, 633 nm, and 780 nm and the Raman shift of each wavelength can be measured not less than 100–3000 cm−1. The spectral repeatability is less than or equal to±0.2 cm−1; the system light-passing efficiency is greater than or equal to 30%; the silicon third-order peak signal-to-noise ratio is better than 10:1, and the fourth-order peak can be observed; the spectral resolution is less than 2 cm−1. The ESCALAB250Xi X-ray photoelectron spectrometer was manufactured by Thermo Fisher Scientific of the USA. The ESCALAB 250Xi X-ray photoelectron spectrometer was equipped with dual anode Al/Mg target X-ray source, reflected electron energy loss spectroscopy, ion scattering spectroscopy, argon ion cluster ion gun, heating/cooling stage, and sample breaking table. It can detect the component information of the surface depth of a sample by 1–10 nm and can analyze the elements with an atomic content greater than 0.1% except helium and hydrogen.
Understanding the fundamentals of TiO2 surfaces
Published in Surface Engineering, 2022
Pan et al. [85] have studied the reactivity of stoichiometric rutile (110) surfaces to molecular oxygen using X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LEIS). With its use of isotopically labelled 18O, it is possible using LEIS to differentiate between adsorbed oxygen and lattice oxygen [34,85,86]. Oxygen adsorption was not observed at room temperature (RT) on a stoichiometric rutile (110) surface [5]. Furthermore, density functional theory (DFT) calculations have predicted that molecular oxygen physisorption on stoichiometric rutile (110) is a strongly endothermic reaction and thus unfavourable [57,90,91]. In addition, the oxidation leads to loss of the surface stoichiometry [57,91,92].
New perspectives on the nature and imaging of active site in small metallic particles: II. Electronic effects
Published in Chemical Engineering Communications, 2021
We will now illustrate these features with a few relevant examples, and will consider both size-selective clusters on model (single crystal) supports, and technical supports. An experimental study of the cluster size effects on electronic structure of size-selective clusters, Pdn/TiO2 (110) model support, was recently reported by Kaden et al. (2009). The observed shifts in B.E. were corrected for final-state effects (with an exponent of -0.2), which typically occur due to the core hole screening (charge screening due to presence of a core hole). With this correction, the core-level B.E. shifts were observed to correlate strongly with number of cluster atoms, at cluster morphologies which were planar, i.e., a monolayer of Pd atoms (<16 atoms). This correlation is shown in Figure 4(a). In addition, He+ ion scattering spectroscopy (ISS) was used to further shed light on the effect of cluster size and morphology on the activity of CO oxidation. The kinetic rates (molecules of CO2/surface Pd atom. s) were also observed to follow the same trends in core-level B.E. shifts (and cluster size) for the smallest clusters (Figure 4(b)). However, for the clusters with 16, 20, and 25 atoms, the onset to a metallic character (Pd0 from Pd+ state) was found to coincide with the transition from a planar shape to a 2-D and 3-D morphology. These findings are consistent with other recent reports which indicate similar effects of cluster size/morphology on the electronic structure and ionic character (Lightstone et al. 2008; Mathes et al. 2004; Vajda et al. 2009). It also appears that the smallest clusters (typically <10 atoms) have the most dominant influence on the surface reactivity, which is often correlated to their under-coordinated sites and ionic character.