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