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Lecture 18. Chemical: XPS

Lecture 18. Chemical: XPS. X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) is a widely used technique to investigate the chemical composition of surfaces. XPS is used to measure:

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Lecture 18. Chemical: XPS

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  1. Lecture 18.Chemical: XPS

  2. X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) is a widely used technique to investigate the chemical composition of surfaces.

  3. XPS is used to measure: • elemental composition of the surface (top 1–10 nm usually) • empirical formula of pure materials • elements that contaminate a surface • chemical or electronic state of each element in the surface • uniformity of elemental composition across the top surface (aka, line profiling or mapping) • uniformity of elemental composition as a function of ion beam etching (aka, depth profiling)

  4. Components of an XPS system The main components of a commercially made XPS system include: • A source of X-rays • An ultra-high vacuum (UHV) stainless steel chamber with UHV pumps • An electron collection lens • An electron energy analyzer • Mu-metal magnetic field shielding • An electron detector system • A moderate vacuum sample introduction chamber • Sample mounts • A sample stage • A set of stage manipulators

  5. XPS Energy Scale The XPS instrument measures the kinetic energy of all collected electrons. The electron signal includes contributions from both photoelectron and Auger electron lines.

  6. Quantitative Analysis by XPS For a Homogeneous sample: • I = NsDJLlAT • where: N = atoms/cm3 • s = photoelectric cross-section, cm2 • D = detector efficiency • J = X-ray flux, photon/cm2-sec • L = orbital symmetry factor • l = inelastic electron mean-free path, cm • A = analysis area, cm2 • T = analyzer transmission efficiency

  7. Quantitative Analysis by XPS • N = I/sDJLlAT • Let denominator = elemental sensitivity factor, S • N = I / S • Can describe Relative Concentration of observed elements as a number fraction by: • Cx = Nx / SNi • Cx = Ix/Sx / S Ii/Si • The values of S are based on empirical data.

  8. X-ray Photoelectron Spectrometer

  9. XPS Spectrum • The XPS peaks are sharp. • In a XPS graph it is possible to see Auger electron peaks. • The Auger peaks are usually wider peaks in a XPS spectrum. • Aluminum foil is used as an example on the next slide.

  10. XPS Spectrum

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