XRay Photoelectron Spectroscopy of Interfaces

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X-Ray Photoelectron Spectroscopy of Interfaces

• ChE5535
• ALEXANDER COUZIS

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What Is XPS (ESCA)?

• XPS is an abbreviation for X-ray Photoelectron
  Spectroscopy
• ESCA is an acronym for Electron Spectroscopy for
  Chemical Analysis

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What is XPS ?

• Photoelectrons
• When light strikes an atom an electron may be
  ejected if the energy of the light is high
  enough. The energy in the light is determined by
  its wavelength or frequency (short wavelength
  high energy and high frequency high energy)
  X-rays have high energy. When X-rays strike a
  solid electrons are always ejected from the
  near-surface region of the solid.

• An XPS instrument has two main components
• An X-ray source, preferably monochromatic(The
  exciting photon is a characteristic soft x-ray
  from a suitable metal, Al (1486.6eV) Mg K
  (1253.6eV) being the most common.)
• An electron energy analyser, usually a spherical
  sector analyzer

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XPS Principles

• If we consider a single atom with just one x-ray
  photon on the way, the total energy is hvEi,
  where hv is the photon energy and Ei the energy
  of the atom in its initial state.
• Following the absorption of the photon and the
  emission of the photoelectron, the total energy
  is now KEEf, where KE is the electron kinetic
  energy and Ef the final state energy of the atom
  (now an ion).
• Because total energy is conserved
• hvEi KEEf
• or
• hv-KE Ef-Ei BE
• where we call the difference between the photon
  energy (which we know) and the electron energy
  (which we measure), the binding energy of the
  orbital from which the electron was expelled. We
  can see that the binding energy is determined by
  the difference between the total energies of the
  initial-state atom and the final-state ion.
• It is roughly equal to the Hartree-Fock energy of
  the electron orbital and so peaks in the
  photoelectron spectrum can be identified with
  specific atoms and hence, a surface compositional
  analysis performed.

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What is XPS ?

• If we measure the energy of the ejected
  photoelectrons we can calculate its Binding
  Energy which is the energy required to remove the
  electron from its atom. From the binding energy
  we can learn some important facts about the
  sample under investigation
• The elements from which it is made
• The relative quantity of each element
• The chemical state of the elements present
• Modern XPS instruments can also produce images or
  maps showing the distribution of the elements or
  their chemical states over the surface. A good
  instrument would have a spatial resolution of a
  few microns.

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Components of an XPS Instrument

• The measurements must be made in ultra-high
  vacuum (uhv), for two reasons
• To allow the photoelectrons to travel from the
  surface of the sample to the detector without
  striking a gas atom
• If a clean surface is prepared for analysis, it
  would become contaminated if it were not under
  uhv.
• Other, optional, items may also be present on an
  XPS spectrometer
• A low energy electron flood gun which must be
  used to prevent insulating samples from becoming
  charged during analysis
• An ion source which is used both to clean a
  surface prior to analysis and to erode the
  surface of the sample so that concentration depth
  profiles can be measured.

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XPS Spectra
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XPS Spectra
Chemical bonding will clearly have an effect on
both the initial state energy of the atom and the
final state energy of the ion created by emission
of the photoelectron. The changes brought about
in the initial state energy by bond formation are
well-understood and can, in principle, be
calculated by quantum chemical methods. They are
basically due to the redistribution of electrons
as the constituent atoms of a molecule or crystal
come together in the solid state and will depend
principally upon the electro-negativities of the
atoms involved. The creation of the ion by
photoemission will cause a further redistribution
of the electrons surrounding the target atom and
this will have an impact on the final-state
energy. This process, called electronic
relaxation, has both an intra-atomic and an
extra-atomic component, and will be dominated by
the polarizabilities of the atoms involved. So
the presence of chemical bonding (and hence,
neighboring atoms) will cause binding energy
shifts, that can be used to extract information
of a chemical nature (such as atomic oxidation
state) from the sample surface. For this reason,
XPS is also known as Electron Spectroscopy for
Chemical Analysis (ESCA).
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XPS Spectra
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XPS Spectra
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Depth Profiling
An important characteristic of the XP experiment
is its surface dependence. Although X-rays
penetrate to a depth of several
micrometers, ejected photoelectrons generally
come from only the first several nanometers of
material. Thus, XPS is very much a surface
technique, much more so than X-ray fluorescence.
This aspect of XPS necessitates great care in
experimental design, as the surface may be
contaminated, non-uniform, or unrepresentative.
At the same time, surface phenomena may be
addressed explicitly. Composition also may be
studied as a function of distance from the
surface through the use of ion sputtering or
etching, whereby a stream of ions, usually Ar,
is used to remove a defined surface layer.
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Depth Profiling
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Thiol SAMs
Chemisorption is Epitaxial.
Long Alkyl Chain Dialkyldisulfides
Long Alkyl Chain Thiol
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Structure of Thiol SAMs
Au-S distance is 1.905Å Hollow sites are 4.99 Å
apart Gold atoms are 2.884Å apart
Thiol SAM on Au (111)
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Tilt Structure of Thiols
110o
110o
ODD of Cs
Even of Cs
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Thiols on Gold

• Short alkyl chain thiols are adsorbed onto gold
  at a higher rate than long alkyl chain thiols in
  a diffusion-controlled processes. Furthermore, we
  also studied the effect of the surface conditions
  of gold before 1-octadecanethiol (ODT)-SAM
  formation using the XPS

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Thiol Monolayers On Gold
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Thiol Monolayers On Gold