Transcript Techniques for probing solid surfaces

Techniques for probing solid
surfaces
What type of atoms are present and at what
concentration?
Where are they located and what are the bond lengths
and angles?
How strong is the surface bond and does it affect
reactivity?
Surface sensitivity and specificity
Inelastic Mean Free Path
• a measure of the average distance travelled by an electron
through a solid before it is inelastically scattered
P(d) = exp ( - d / l )
Virtually all ( > 95% ) of the electrons detected come from
within 3 IMFPs of the surface
IMFP
The Inelastic Mean Free Path (IMFP) in metals is typically
less than :
•
10 Angstroms ( 1 nm ) for electron energies in the range
15 < E/eV < 350
•
20 Angstroms ( 2 nm ) for electron energies in the range
10 > E/eV > 1400
i.e. the IMFP of low energy electrons corresponds to only a
few atomic layers
P = exp ( - t / l )
I = Io exp ( - t / l )
How can we change the degree of
surface sensitivity ?
collect photoelectrons at a more grazing emission angle
x = d cos q
Photoelectron Spectroscopy
• photo-ionisation with energy-dispersive analysis of the
emitted photoelectrons to study the composition and
electronic state of the surface region of a sample
X-ray Photoelectron Spectroscopy - using soft x-ray (200-2000 eV)
(XPS)
radiation to examine core-levels.
Ultraviolet Photoelectron
Spectroscopy
(UPS)
- using vacuum UV (10-45 eV)
radiation to examine valence
levels.
E=hn
where
h - Planck constant ( 6.62 x
10-34 J s )
n - frequency (Hz) of the
radiation
XPS
A + hn  A+ + eE(A) + hn = E(A+ ) + E(e-)
electron energy is kinetic
energy (KE)
KE = hn - ( E(A+ ) - E(A) )
term in brackets is the binding
energy (BE) of the electron
KE = hn - BE
KE = hn - BE - f s
Koopman’s Theorem: BE can be equated with the negative
orbital energy of the emitted electron
XPS Instrument
Requirements for a
photoemission experiment
(XPS or UPS) are:
a source of fixed-energy
radiation
an electron energy analyser
a high vacuum environment
Mg Ka radiation : hn = 1253.6 eV
Al Ka radiation : hn = 1486.6 eV
XPS applications
Characteristic
binding energy
associated with
each core
atomic orbital
XPS of Pd





valence band (4d,5s) emission occurs at BE 0 - 8 eV
4p and 4s levels give weak peaks at 54 and 88 eV respectively
most intense peak, 335 eV, 3d levels of the Pd atoms
3p and 3s levels give peaks at 534/561 eV and 673 eV
respectively.
remaining peak is not an XPS peak - it is an Auger peak from
x-ray induced Auger emission at a kinetic energy of 330 eV
Spin Orbit Coupling
photo-ionisation leads to a (3d)9 configuration for Pd
since the d-orbitals ( l = 2) have non-zero orbital angular momentum, there will be
coupling between the unpaired spin and orbital angular momenta
XPS
Chemical Shifts
The exact binding energy of an electron depends not only upon
the level from which photoemission is occurring, but also upon :
the formal oxidation state of the atom
the local chemical and physical environment
Atoms of a higher positive oxidation state exhibit a higher binding energy due
to the extra coulombic interaction between the photo-emitted electron and the
ion core.
XPS Databases
NIST X-ray Photoelectron Spectroscopy
Database
http://srdata.nist.gov/xps/
XPS international Databases
http://www.xpsdata.com/
For example periodic table of BE’s
Angle Dependent Studies
Auger Electron Spectroscopy
Atomic ionisation (by removal of a core
electron usually by high energy
electrons)
The ionised atom that remains is in a highly
excited state and will rapidly relax back
to a lower energy state by one of two
routes :
X-ray fluorescence , or
Auger emission
AES
KE = ( EK - EL1 ) - EL23
AES
• KE of Auger electron is independent of initial radiation energy
• Auger processes dominate for elements of low atomic number (vs
XRF) and all elements other than H and He give Auger spectra
Because electron sources can
be focused to spot sizes of
sub-micrometer dimensions
(see SEM 3rd year notes),
Scanning Auger Microscopy is
used to evaluate elemental
concentrations across surfaces
Depth Profiling
• AES is surface
sensitive (focus
to ~1mm2)
• controlled surface
etching of
analysed region
by exposure to
ion flux
Depth Profiling