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Introduction to X-Ray Photoelectron Spectroscopy, Lecture notes of Chemical Instrumentation and Analysis

Sri Venkateswara UniversityChemical Instrumentation and Analysis

X-Ray Photoelectron Spectroscopy - Basics and Applications

Typology: Lecture notes

2018/2019

Uploaded on 04/18/2019

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X-ray Photoelectron Spectroscopy
April 18, 2019 01
Dr. B. SREEDHAR
Senior Principal Scientist
Professor, AcSIR
Inorganic and Physical Chemistry Division
Indian Institute of Chemical Technology
(Council of Scientific & Industrial Research)
Hyderabad –500 607
sreedharb@iict.res.in
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Download Introduction to X-Ray Photoelectron Spectroscopy and more Lecture notes Chemical Instrumentation and Analysis in PDF only on Docsity!

X-ray Photoelectron Spectroscopy

April 18, 2019 01 Dr. B. SREEDHAR Senior Principal Scientist Professor, AcSIR Inorganic and Physical Chemistry Division Indian Institute of Chemical Technology (Council of Scientific & Industrial Research) Hyderabad – 500 607 sreedharb@iict.res.in

e

A

+ e

A

Sample

Inputs Outputs

Spectroscopy

γ e

γ Sample

Photons In

Infrared Visible Ultraviolet X-Rays

Photons Out

Fourier Transform Infrared Spectroscopy Raman Visible Ultraviolet X-Ray Fluorescence X-Ray Diffraction

Electrons Out

XPS, X-ray Photoelectron Spectroscopy Probe: Photons April 18, 2019 4

A

A

Sample

Ions In

Ions Out

SIMS, Secondary Ion Mass Spectrometry ToF SIMS, Time of Flight SIMS, ICP MS, Inductively Coupled Plasma Mass Spectrometry

Probe: Ions

April 18, 2019 5

  • Introduction (XPS basic principles)
  • Quantification.
  • Energy resolution and count rates.
  • Wide scan data (low energy resolution)
  • Narrow scan data (high energy resolution)
  • Chemical state analysis.
  • Sensitivity.
  • Sputter depth profiles.
  • Line scans.
  • Maps. 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.

Information Derived from XPS Photoelectrons out X-rays in d = 3l

  • X-ray photoelectron spectroscopy works by irradiating a sample material with monoenergetic soft x-rays causing electrons to be ejected.
  • Identification of the elements in the sample can be made directly from the kinetic energies of these ejected photoelectrons.
  • The relative concentrations of elements can be determined from the photoelectron intensities.

X-ray Photoelectron Spectroscopy

Small Area Detection

X-ray Beam X-ray penetration depth ~1 μ m. Electrons can be excited in this entire volume. X-ray excitation area ~1x1 cm^2_. Electrons are emitted from this entire area Electrons are extracted only from a narrow solid angle. 1 mm_^2 10 nm April 18, 2019 13

Chemical State Information

  • The binding energy of an electron is dependent on the atomic orbital the electron occupies and the chemical environment of the atom.
  • The variation of binding energy of a specific photoemission peak provides information on the chemical state of the atom or ion. Core level electron, high binding energy Valence electron, low binding energy

L L K Auger Electron

Principle of XPS

XPS involves irradiation of a sample under vacuum by x-rays

of known energy which causes photo-ejection of electrons

from atoms near the surface

The emitted electrons have a kinetic energy Ek, which is

given by

Ek = hn - Eb - F

where hn = photon energy

(Planck’s constant = 6. 62 x 10

  • 34

Jsec)

Eb = Binding energy relative to Fermi level

F = spectrometer work function

Spectrum of Eb Þ Spectrum of Ek

Surface analysis by XPS requires irradiating a solid in an Ultra-high Vacuum (UHV) chamber with monoenergetic soft X-rays and analyzing the energies of the emitted electrons. ■ Remove adsorbed gases from the sample. ■ Eliminate adsorption of contaminants on the sample. ■ Prevent arcing and high voltage breakdown. ■ Increase the mean free path for electrons, ions and photons. Degree of Vacuum 10 10 10 10 10 2

_- 1

  • 4
  • 8
  • 11 Low Vacuum Medium Vacuum High Vacuum Ultra-High Vacuum Pressure Torr_ April 18, 2019 19