Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy Chapter 6 Electron Spectroscopy 1.Analytical Chemistry of Surfaces 1.1Surface characterization Definition of a solid surface: A surface is generally considered to be the boundary layer of one phase at its interface with another. The surface most frequently encountered in chemistry are at solid-gas or liquid-solid interfaces. Usually the surface is considered as being part of the solid. The surface is usually considered to be more than one atomic layer deep.

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy 1.2Spectroscopic Techniques Table 6-1Comparison of “classical” and “modern” methods for surface characterization Classical methodsModern Spectroscopic AdsorptionElemental analysis Surface areas Chemical information Pore size distributionOxidation state Surface roughness Functional groups Photoelectric work functionQuantitative analysis Microscopy Elemental ratios Reflectivity Oxidation ratios

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy Classical methods: Descriptive, provide little qualitative or chemical information. Modem methods Spectroscopic, provide chemical information.

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy

Not included in the list: photons-in and photons-out processes generate the well-known techniques of infrared and Raman spectroscopy

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy 1.3Experimental parameters  Sampling depth Table 6.3 Penetration depth of particles ParticlesEnergy (eV)Depth (Å) Photon100010,000 Electron Ions Generally either the beam-in or the beam-out must involve electrons or ions. Photon-in and photon-out techniques will not normally be surface sensitive. The only surface-sensitive techniques involving photon emission are infrared and Raman spectroscopy.

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy  Sample charge Bombarding a surface with charged particles or having charged particles emitted from a surface, charging of the sample may occur. This is a significant problem for all spectroscopic techniques involving electrons or ions. Some of the problems caused by sample charging are –Distortion of spectra –Shift of peak location –Movement of surface Extent of problem: Insulators » semiconductors > conductors

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy Compensation methods: –Surface conductivity: the surface conductivity of most samples is much greater than the bulk conductivity. Therefore, even though samples tend to build up charge on the surface, frequently there is sufficient surface conduction that the charge builds quickly to a steady state value and does not change. –Stray electrons: In ESCA there are frequently stray electrons in the vicinity of the sample, which can help to reduce sample charging. –Flood-gun: if a surface tends to build up a positive charge, a stream of low energy (thermal) electrons can be used to neutralized the positive charge.

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy  Surface contamination –Adsorption of components of the atmosphere, such as water, oxygen, and carbon dioxide, even in a vacuum condition. –Cleaning methods, such as baking the sample at high temperature, sputtering with a beam of inert gas ions from an electron gun, mechanical scraping……, can be used to clean the surface for analysis.

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy 2.Electron Spectroscopy Spectroscopic TechniquesMeasures Optical SpectroscopyIntensity of photons as a function of the energy of the photons Mass SpectroscopyIntensity of ions as a function of the m/z ratio Electron SpectroscopyPower of the electron beam produced by incident beams (hν, electron etc.)

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy 2.1X-ray photoelectron spectroscopy (XPS) Principles of XPS It is also called electron spectroscopy for chemical analysis (ESCA)

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy A + hv  A +* + e - Since photons with a monochromatic X- ray beam of known energy hv are used, electrons produced are having discrete binding energy E b = hv – E k – w E b is binding energy. E k is kinetic energy of the emitted electron, w is the work function of the spectrometer, a factor that corrects for the electrostatic environment in which the electron is formed and measured. E k = hv – E b You measure E k ! The binding energy of an electron is characteristic of the atom and orbital from which the electron was emitted L K

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy The huge background caused by inelastic collisions between ejected electrons and the solid sample. NOT every electron ejected can be measured at its binding energy Surface oxidation

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy Applications  Qualitative analysis: elemental composition  Chemical shifts and oxidation states  Chemical shifts and structure

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy

2.2 Auger electron spectroscopy Principles of AES Two steps involved (1) Electron ionization: Formation of electronically excited A +* is brought about by exposing the analyte to a beam of electrons, or X-ray. With X-ray A + hv  A +* + e - While with electrons A + e - i  A +* + e - i ’ + e - A

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy (2)Auger electron emission: (i) A +*  A + + hv f (x-ray) X-ray fluorescence. Note that the energy of fluorescence radiation is independent of the excitation energy. (ii)A +*  A ++ + e - A (Auger) Note that the energy of the Auger electron is independent of the energy of the photon or electron that originally created the vacancy in energy level E b.

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy The kinetic energy of the Auger electron: E k = (E b – E b ’) – E b ’ E k :kinetic energy of the Auger electron E b – E b ’:the energy released in relaxation of the excited ion E b ’:the energy required to remove the second electron from its orbit (binding energy of the second electron).

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy XPS and AES Table 6.2 Fundamental Physical Processes in Electron Spectroscopy Primary Processes A.Photoionization A + hv  A +* + e - (discrete energy – ESCA/XPS) B.Electron Ionization A + e - i  A +* + e - i ’ + e - A (not discrete energy) Secondary Processes A.Photon emission A +*  A + + hv f (x-ray) B.Auger electron emission A +*  A ++ + e - A (Auger) (discrete energy Auger)

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 6Electron Spectroscopy 2.2.3Applications  Qualitative analysis of solid surfaces: Because of the low energy of Auger electrons, an Auger spectrum is likely to reflect the "true" surface composition of a solid.  Depth profile of the surfaces The surface can be etched away using Auger ion sputtering and then followed by either XPS or AES with the latter more common.