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Principles And Applications Of Ion Scattering Spectrometry: Surface Chemical And Structural Analysis

-Introductory, theoretical, and experimental aspects of ion scattering -General features and structural analysis -The recent technique of scattering and recoiling imaging spectrometry -Examples of structural analysis -Ion-surface charge exchange phenomena -Hyperthermal ion-surface interactions

Principles And Applications Of Ion Scattering Spectrometry: Surface Chemical And Structural Analysis

"...useful monograph...recommended...a good reference book that is strong on the principles and limited to one part of the 'applications' of ion scattering spectrometry." (Applied Spectroscopy, Vol. 57, No.8, August 2003)

Low-energy ion scattering spectroscopy (LEIS), sometimes referred to simply as ion scattering spectroscopy (ISS), is a surface-sensitive analytical technique used to characterize the chemical and structural makeup of materials. LEIS involves directing a stream of charged particles known as ions at a surface and making observations of the positions, velocities, and energies of the ions that have interacted with the surface. Data that is thus collected can be used to deduce information about the material such as the relative positions of atoms in a surface lattice and the elemental identity of those atoms. LEIS is closely related to both medium-energy ion scattering (MEIS) and high-energy ion scattering (HEIS, known in practice as Rutherford backscattering spectroscopy, or RBS), differing primarily in the energy range of the ion beam used to probe the surface. While much of the information collected using LEIS can be obtained using other surface science techniques, LEIS is unique in its sensitivity to both structure and composition of surfaces. Additionally, LEIS is one of a very few surface-sensitive techniques capable of directly observing hydrogen atoms, an aspect that may make it an increasingly more important technique as the hydrogen economy is being explored.

CHEM E 467 Biochemical Engineering (3)Application of basic chemical engineering principles to biochemical and biological process industries such as fermentation, enzyme technology, and biological waste treatment. Rapid overview of relevant microbiology, biochemistry, and molecular genetics. Design and analysis of biological reactors and product recovery operations. Prerequisite: CHEM E 340; either CHEM 223, CHEM 237, or CHEM 335. Offered: jointly with BIOEN 467; W.View course details in MyPlan: CHEM E 467

CHEM E 515 Experimental Methods in Chemical Engineering Research (3)Lecture and laboratory studies in current research methods of chemical engineering. Includes surface science, biochemical engineering, collodial chemistry, light scattering, and nanoscience techniques. Offered: A.View course details in MyPlan: CHEM E 515

  • Show caption HideParticle beam interaction using ToF-SIMS. Incident particles bombard the surface liberating single ions (+/-) and molecular compounds. DetailsToF-SIMS uses a focused, pulsed particle beam (typically Cs or Ga) to dislodge chemical species on a materials surface. Particles produced closer to the site of impact tend to be dissociated ions (positive or negative). Secondary particles generated farther from the impact site tend to be molecular compounds, typically fragments of much larger organic macromolecules. The particles are then accelerated into a flight path on their way towards a detector. Because it is possible to measure the "time-of-flight" of the particles from the time of impact to detector on a scale of nano-seconds, it is possible to produce a mass resolution as fine as 0.00X atomic mass units (i.e. one part in a thousand of the mass of a proton). Under typical operating conditions, the results of ToF-SIMS analysis include: a mass spectrum that surveys all atomic masses over a range of 0-10,000 amu,

  • the rastered beam produces maps of any mass of interest on a sub-micron scale, and

  • depth profiles are produced by removal of surface layers by sputtering under the ion beam.

ToF-SIMS is also referred to as "static" SIMS because a low primary ion current is used to "tickle" the sample surface to liberate ions, molecules and molecular clusters for analysis. In contrast, "dynamic" SIMS is the method of choice for quantitative analysis because a higher primary ion current results in a faster sputtering rate and produces a much higher ion yield. Thus, dynamic SIMS creates better counting statistics for trace elements. Organic compounds are effectively destroyed by "dynamic" SIMS, and no diagnostic information is obtained.

  • Surveys of all masses on material surfaces; these may include single ions (positive or negative), individual isotopes, and molecular compounds;

  • Elemental and chemical mapping on a sub-micron scale;

  • High mass resolution, to distinguish species of similar nominal mass (mass resolution is at least 0.00x amu);

  • High sensitivity for trace elements or compounds, on the order of ppm to ppb for most species;

  • Surface analysis of insulating and conducting samples;

  • Depth profiling (in the near surface environment, on the order of individual atomic layers to 10s of nanometers);

  • Non-destructive analysis;

  • Retrospective analysis, for post-data acquisition analysis and interpretation of stored images and spectra.

The most commonly used surface spectroscopic techniques are x-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), Auger electron spectroscopy (AES), and ion scattering spectroscopy (ISS). XPS and UPS are similar techniques and can be grouped under photoemission spectroscopy (PES). All four techniques are used widely for the study of solid surfaces both in fundamental scientific studies and in applied studies of polymers, ceramics, heterogeneous catalysts, metals and alloys, semiconductors, nanoparticles, biomaterials, etc. They can provide information about composition, chemical state, electronic structure, and geometrical structure. Detailed reviews have been presented previously [1,2].

Since absolute quantification of XPS, AES, and ISS is not easy, another approach is to use relative quantification, that is, to compare spectra from related surfaces. This could take the form of preparing a number of samples in which just one parameter was varied systematically, and then collecting and comparing spectra. Even small differences in the spectra would then be significant and interpretable. A similar approach would be to subject a sample to various treatments in UHV, and again collect and compare spectra. The treatments could be oxidation, reduction, ion sputtering, annealing, etc., and the sample might oscillate between two distinct states as a result of these treatments. In a study of TiO2(001) [28], sequential reduction, oxidation, and sputtering was applied, with analysis after each treatment by XPS, AES, and ISS, and it was found that the chemical state at the surface changed systematically and reproducibly with the various treatments, in a cyclical manner. The study demonstrated that very small changes in features or lineshapes in spectra can be significant.

Hydrogen is an element that is virtually impossible to detect with these methods. This is unfortunate because surface hydrogen is often present and can determine the chemical behavior of a surface. Surface hydrogen has been observed directly using ISS [29,30] but only at extremely small scattering angles that are not accessible with most ISS systems. Hydrogen has no core-level electrons so it cannot be observed directly with either XPS or AES, but it can sometimes be observed indirectly by XPS because of its presence as part of a surface group. For example, hydroxyl groups on oxide surfaces yield O 1s peaks at higher BEs than the oxide O 1s peaks. If it is necessary to establish that hydrogen, or an H-containing molecule, is definitely present, then either one of the SIMS family of techniques, or electron stimulated desorption (ESD), must be used.

The ability to obtain spectra from samples subjected to in-situ high pressures is not without sacrifice. There will be a consequent pressure-dependent loss in source intensity at the sample surface due to the x-ray transmission properties of the introduced gas. In addition, photoelectrons ejected from the sample undergo scattering off the gas molecules so that the reduction in signal depends on both the nature of the gas and its pressure. Since the scattering cross sections of the photoelectrons are energy dependent, the overall transmission function of the spectrometer changes, which affects data analysis. Correction of spectra for charging encountered with semiconductor or insulator samples is more complicated because the surface charging is also influenced by both gas composition and pressure. The highest pressure that has been reported when using a conventional x-ray source is about 100 Pa [70].

0139 Chemistry of Complex Interfaces, Catalysts, and Devices. Application of scientific principles to understand the preparation, characterization of and mechanistic operation of important interfacial chemical processes. Topics include controlled surface preparation and characterization, self-assembly, lithography, and molecular beam, chemical vapor, and atomic layer deposition methods and their application to single molecule studies, heterogeneous catalysis, self-assembly, two-dimensional device operation, and design and function in nanoscience. Characterization methods include optical and electron-based surface spectroscopies, beam and desorption-based methods, scanning probe and other surface microscopies.


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