Advanced Analytical Chemistry

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• Chapter 6 Electron Spectroscopy
• Analytical Chemistry of Surfaces
• 1.1 Surface 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.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• 1.2 Spectroscopic Techniques
• Table 6-1 Comparison of classical and modern
  methods for surface characterization

Classical methods Classical methods Modern Spectroscopic Modern Spectroscopic
Adsorption Adsorption Elemental analysis Elemental analysis
Surface areas Chemical information Chemical information
Pore size distribution Oxidation state
Surface roughness Surface roughness Functional groups
Photoelectric work function Photoelectric work function Quantitative analysis Quantitative analysis
Microscopy Microscopy Elemental ratios
Reflectivity Reflectivity Oxidation ratios
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• Classical methods
• Descriptive, provide little qualitative or
  chemical information.
• Modem methods
• Spectroscopic, provide chemical information.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

Not included in the list photons-in and
photons-out processes generate the well-known
techniques of infrared and Raman spectroscopy
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• 1.3 Experimental parameters
• Sampling depth
• Table 6.3 Penetration depth of particles

Particles Energy (eV) Depth (Å)
Photon 1000 10,000
Electron 1000 20
Ions 1000 10
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.
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron 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 gt conductors

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron 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.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron 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.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• Electron Spectroscopy

Spectroscopic Techniques Measures
Optical Spectroscopy Intensity of photons as a function of the energy of the photons
Mass Spectroscopy Intensity of ions as a function of the m/z ratio
Electron Spectroscopy Power of the electron beam produced by incident beams (h?, electron etc.)
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• 2.1 X-ray photoelectron spectroscopy (XPS)
• 2.1.1 Principles of XPS
• It is also called electron spectroscopy for
  chemical analysis (ESCA)

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron 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 Eb hv Ek w Eb is binding
energy. Ek 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. Ek hv Eb You
measure Ek!
L
K
The binding energy of an electron is
characteristic of the atom and orbital from which
the electron was emitted
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

Surface oxidation
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
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• 2.1.2. Applications
• Qualitative analysis elemental composition
• Chemical shifts and oxidation states
• Chemical shifts and structure

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• 2.2 Auger electron spectroscopy
• 2.2.1 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

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• (2) Auger electron emission

• (i) A ? A hvf (x-ray)
• X-ray fluorescence. Note that the energy of
  fluorescence radiation is independent of the
  excitation energy.
• 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 Eb.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• The kinetic energy of the Auger electron
• Ek (Eb Eb) Eb
• Ek kinetic energy of the Auger electron
• Eb Eb the energy released in relaxation
  of the excited ion
• Eb the energy required to remove the
  second electron from its orbit (binding energy
  of the second electron).

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• 2.2.2 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 hvf (x-ray)
• B. Auger electron emission
• A ? A e-A(Auger) (discrete energy Auger)

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 6 Electron Spectroscopy

• 2.2.3 Applications
• 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.