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Spectroscopic Techniques for Probing Solid Surfaces

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What types of atoms are present at a surface and what is their surface concentration? ... Because alkali metals exhibit low IE's, they tend to transfer electron charge ... – PowerPoint PPT presentation

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Title: Spectroscopic Techniques for Probing Solid Surfaces


1
Spectroscopic Techniques for Probing Solid
Surfaces
  • Part II of Surface Primer

2
Characterization of a Surface
  • To fully characterize a surface, we need to know
  • What types of atoms are present at a surface and
    what is their surface concentration?
  • Where are the atoms/molecules located on a
    surface?
  • How strong is the bonding of adsorbate atoms and
    how does the nature of the surface bond influence
    surface reactivity?

3
LEED (low energy electron diffraction)
  • Incident e-s, which are elastically back
    scattered from a surface, are analyzed from
    20-100eV.
  • The e-s possess de Broglie wavelengths of the
    same order of magnitude as the interatomic
    spacing between atoms/molecules at surfaces.
  • undergo diffraction if the atoms in the surface
    are arranged periodically

4
LEED patterns
  • From the position of the diffracted beams, the 2D
    periodicity of the surface unit cell may be
    deduced as well as variations in the unit cell
    size induced by adsorption.
  • From the variation of spot intensities with beam
    energy, the complete surface geometry, including
    bond lengths and angles, can be obtained.

Si(111) 7 X 7 LEED Pattern
5
Work Function
  • The work function (?) is the minimum energy
    required to remove an electron from the Fermi
    level of a solid to a sufficient distance outside
    the surface such that it no longer feels
    long-range Coulomb interactions with the positive
    hole left on the surface.
  • Fermi E of a solid is bulk property
  • electrostatic attraction between atomic nuclei
    and valence elections

6
Work Function Contd.
  • Atomically clean single crystal surfaces of
    differing geometric structure have different work
    functions.
  • A surface does not present an infinite potential
    energy barrier to the e-s within a solid.
  • The e- wavefunctions may have a non-zero
    amplitude just outside (within 10 Å) of the
    surface.

7
Dipole Layer
  • Electron wavefunctions are exponentially damped
    as they penetrate outside the surface and give
    rise to e- overspill.
  • The excess (-) charge is balanced by a
    corresponding excess () charge at the solid
    surface
  • Formation of a dipole layer
  • The greater the overspill the larger the
    surface dipole
  • Extent of overspill is a function of surface
    geometry

8
Adsorption
  • Adsorption induces changes in the work function
  • modifications of the surface dipolar layer
  • particularly if significant charge transfer
    occurs between the adsorbate and surface
  • measurements of ?F yield critical information on
    the degree of charge reorganization upon
    adsorption
  • ?? ?adsorbate covered - ?clean

9
Electropositive Adsorbates
  • Because alkali metals exhibit low IEs, they tend
    to transfer electron charge from their outer
    valence shell to the substrate.
  • This process ceases when the Fermi level of the
    substrate e-s highest occupied electron state
    in the broadened valence level of the adsorbate.
  • A dipole layer is formed in which net () charge
    resides on the adsorbate
  • The dipole layer is in the opposite direction to
    the dipole layer at a clean surface, so a
    lowering in the work function is expected.

10
Electronegative Adsorbates
  • These possess an unfilled affinity level that is
    situated largely or entirely below the highest
    occupied state of the substrate.
  • charge transfer occurs from substrate to
    adsorbate
  • The dipole layer has an outermost (-) charge,
    which is in the same direction as the dipole on a
    clean surface, so there is a work function
    increase upon adsorption of EN elements.

11
STM
  • An atomically sharp tip is brought within a few
    nm. of a conducting surface and a small potential
    difference is applied between tip and sample.
  • If the tip has a positive bias relative to the
    sample, an energetic incentive is provided for
    e-s from the surface to flow to the tip.
  • The tip is mounted on a piezoelectic tube
    scanner, which expands or contracts when a
    voltage is applied across it.

12
Tunnelling
  • In an energy level diagram for a tip close to a
    conducting surface, for a finite barrier (like a
    real metal), the wavefunction of an e- in the
    Fermi level penetrates beyond the sample such
    that e- density drops to 0 at distances of nm.
    from the surface.
  • The e- can tunnel to the tip where it will lower
    its E due to the positive potential applied to
    the tip.
  • The positive potential shifts the electronic E
    levels on the tip to lower E, thus facilitating
    e- transfer into unoccupied states of lower E.

13
Work Function and STM
  • Tunnelling current (I) depends exponentially on
    the sample to tip gap (W) and the sample work
    function (F)
  • I(W) Cexp(-WF(1/2))
  • The tunnelling current will increase in areas
    where protrusions exist because of a lowering in
    the gap distance.

14
Modes of Scanning in STM
  • Constant height mode the tip is scanned in the
    xy-plane of the surface while remaining
    stationary in the z-direction. Image is produced
    consisting of tunnelling current variations as a
    function of position.
  • Good for flat surfaces because rapid scanning is
    possible

15
Modes of Scanning in STM
  • Constant current mode the value of W (tip gap)
    is fixed by movement of the tip in the
    z-direction, while scanning in the xy-plane. A
    plot of the z-piezo electric voltage vs. lateral
    position is created.
  • Good for rough surfaces to avoid tip-surface
    collisions

16
HOPG Surface Geometry
  • STM current depends on lateral variation of e-
    density of the sample, which is dependent on
    surface geometry.
  • In a sample of HOPG, there are atoms (B) with no
    neighbors directly below and atoms (A) with an
    atom directly below.
  • These atoms have different local bonding
    environments and therefore e- density will vary,
    thus creating a different tunnelling current.

17
Atomic Force Microscopy
  • Allows for imaging of non-conducting surfaces
  • A tip, usually silicon nitride (diameter of 1-20
    nm.), is mounted on a cantilever with a force
    constant between 0.001 and 0.2 Nm-1
  • The cantilever has a natural resonant vibration
    frequency as far removed from those experienced
    in the building as possible.
  • Resonant frequency (?)
  • ? 1/2pvK/m
  • To obtain high v, cantilevers are made with very
    low masses (1 µg.) with low force constants.

18
AFM Contd.
  • Contact Mode The tip is scanned at tip-sample
    separation corresponding to a chemical bonding
    length of the tip/sample combination. This leads
    to the cantilever being attracted (e- overlap) or
    repelled (closed electronic shells) as it scans
    the surface.

19
AFM Contd.
  • Non-contact (tapping) mode The tip vibrates
    close to its resonance frequency. Variations in
    the sample-tip forces alter the resonance
    frequency, and the shift is used to measure the
    magnitude of the forces in action.
  • Topographic images of surface force vs. lateral
    position
  • Good for delicate samples
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