421-821-chapter-21

421-821-chapter-21 - Surface Analysis A Introduction Solid...

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Surface Analysis A.) Introduction : Solid Surface: the boundary layer between a solid and either a vacuum, a gas or a liquid - differs substantially in its physical properties and chemical composition relative to the interior of the solid and average composition of the bulk sample. - not just the top layer of atoms, but a transition layer with non-uniform composition that varies from the outer layer to the bulk. - generally a very tiny fraction of the total solid Operational Definition: volume of a solid that a particular measurement techniques samples - several surface techniques may sample different surface Increasing magnification of the surface of a pin with the presence of bacteria
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B.) Spectroscopic Surface Methods : 1. Provide chemical information of the composition of a surface (few . to ~ 10s . thick) From source (primary beam) To Spectrometer (secondary beam) Sample - Results from scattering, sputtering or emission. - Further studied by other spectroscopic techniques - Photons, electrons, ions or neutral molecules. - Need to limit to surface analysis Penetration depth of 1-keV electrons is ~ 25 S , where a photon beam of same energy is ~ 10 4 S
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Method and Common Acronym Primary Beam Secondary Beam X-ray Photoelectron Spectroscopy (XPS), or Electron Spectroscopy for Chemical Analysis (ESCA) X-ray photons Electrons Auger Electron Spectroscopy (AES) Electrons, or X-ray photons Electrons Ultraviolet Photoelectron Spectroscopy (UPS) UV photons Electrons Secondary Ion Mass Spectrometry (SIMS) Ions Ions Laser Microbe Mass Spectrometry (LMMS) Photons Ions Electron Microbe (EM) Electrons X-ray photons Some Common Types of Spectroscopic Methods for Analysis of Surfaces Methods are differentiated by the nature of the primary and secondary beams
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2. Sampling Surfaces and Contamination - focus beam on a single surface - mapping the surface by scanning the beam across the surface in a raster pattern can be linear or two-dimensional observe changes in secondary beam as a function of position - depth profiling etch a hole in the surface by sputtering a finer primary beam is used to generate a secondary beam from the hole observe changes in secondary beam as a function of depth Depth Profiling Raster Pattern (data is obtained from primary beam centered in each grid)
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- contamination of the surface from the atmosphere or the primary beam - need to clean surface in vacuum chamber S bake sample at high temperatures sputtering the sample with a beam of inert gas ions from electron gun mechanical scraping or polishing the surface with an abrasive ultrasonic washing in various solvents bathing the sample in a reducing atmosphere to remove oxides Surface contamination
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3. Electron Spectroscopy - signal from sample is a beam of electrons (secondary beam) - determine the power of the beam as a function of the energy of the electrons (h ν ) - determine identification of all elements (except H and He) oxidation state of element type of species element is bonded to electronic structure of molecules
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