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XPS
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Surface Analysis Methods X-‐ray Photoelectron Spectroscopy
Outline • Photoelectric Effect
• Instrumenta8on • X-‐ray source • Electron energy analyzer • Ar ion gun • Neutralizer • Vacuum system
• XPS Examples
Photoelectric Effect
Light striking a surface causes electron excita8on Einstein, Nobel Prize 1921
Photoelectric effect as an analy8cal tool Kai Siegbahn, Nobel Prize 1981
X-‐ray hv 1253.6 eV
e-‐ e-‐ Ek = kinetic energy of electrons
e-‐
Eb
core electrons
Ev
valence electrons
hv
Ek = hv-‐B.E.-‐Φ
410 408 406 404 402 400 398
3000
4000
5000
6000
Counts
Binding Energy, eV
-‐NO2 -‐NH2
High Res Scan
1000 800 600 400 200 0 0
5000
10000
15000
Counts
Binding Energy, eV
F1S
O1S
N1S
C1S Survey Scan
B.E. = hv-‐Ek-‐Φ
X-‐ray Photoelectron Spectroscopy
Photoemission Photoemission can be thought of as three steps (a) Photon absorp8on and ioniza8on (ini8al state effects)
(b) Response of atom and crea8on of photoelectron (final state effects)
(c) Transport of electron to the surface (extrinsic effects)
X-‐ray hv 1253.6 eV
e-‐ e-‐
e-‐ e-‐
10 nm
inelastic scattering
Core level electrons that have lost KE due to collisions within sample 600 400 200 0 0
5000
10000
15000
Counts
Binding Energy, eV
Photoelectron Escape Depth
Photoelectrons Auger electrons
X-‐rays
Auger process and x-‐ray photon emission • Low atomic number elements, the most probable transi8ons occur when a K-‐level electron is ejected by the primary beam, an L-‐level electron drops into the vacancy, and another L-‐level electron is ejected (KLL)
• Higher atomic number elements have LMM and MNN transi8ons that are more probable than KLL.
Chemical Effects in XPS Chemical ShiV: The change in binding energy of a core electron of an element due to a change in the chemical bonding environment of that element
Withdrawal of electron charge Addi8on of electron charge
Increased B.E. Decreased B.E.
XPS Schematic
• X-‐ray source • Electron energy analyzer • Ar ion gun • Neutralizer • Vacuum system
XPS -‐ source X-‐ray anode like SAXS
• Al, Mg or Ag radia8on
Can also use synchrotron radia8on
Spatial Resolution Two ways to create spa8ally resolved photoelectron map of a surface
• Focus your X-‐rays (difficult) • Focus your photoelectrons (easier)
Focusing Electrons Much like electron microscopy, apertures and lenses are used to focus electrons
Imaging XPS By focusing the photoelectrons you can scan across a sample much like SEM
• Can generate XPS images
Alterna8vely, can use a 2D detector to image all photoelectrons simultaneously, like SAXS
In prac8ce both methods can be used
Charge Neutralization Photoelectric process is a net oxida8on, thus a build up of posi8ve charge occurs For a conduc8ve sample this is negated by grounding Insula8ng samples however cannot dissipate the charge Sample charging results in aberra8ons in the spectra Electron gun can be used to alleviate charging problem
XPS Detectors
Delay-‐line detector similar to SAXS or use a photomul8plier tube (limited lateral resolu8on)
Example: PET Poly-‐ethylene-‐terephthalate
-‐(-‐O-‐C-‐ -‐C-‐O-‐CH2-‐CH2-‐)-‐ = =
O O n
2 2 2 3
1
3 2
1
1
Spin Orbital Splitting Spin-‐orbital spli`ng
• For p, d, f … orbitals two peaks are observed
• The separa8on between peaks is similar for all compounds of a given element
• Peak area ra8os are the same for a given orbital
Quanti\ication Must account for inelas8cally scabered electrons when considering baseline Transmission Factor – a func8on of detec8on efficiency of an electron in the analyzer which is a func8on of the electron energy Orbital Cross-‐Sec8on – another correc8on based on the ioniza8on probabili8es calculated from scabering theory In prac8ce the transmission factor and orbital cross sec8on are automa8cally computed by the analysis soVware
Depth Effects
d d
Si elemental
Si oxide
Sample with different depths of oxide layers Note large chemical shiV difference between SiO2 and elemental Si
Sampling Depth
• Electrons in a solid have a finite mean free path λ
• For typical X-‐ray energies (Al, Mg) the depth sampled is < 10 nm
• Thus XPS is a surface sensi8ve technique
• However, we may want to look deeper or shallower, how do we achieve this?
Angle Resolved XPS (ARXPS) To look at shallower slices we can rotate the sample at an angle to the detector
Example: Al film
Depth Pro\iling To go deeper into the sample we can use an ion gun to etch away the top layers
Typically performed with Ar+ ions, energies of 0.5-‐5 kV
Example of a depth profile of a glass with a metal alloy in the centre
XPS Imaging Photoresist
Si oxide
Si metal
Image by scanning at a given B.E. for each element
XPS Imaging
Can record spectra at selected posi8ons
XPS Imaging Line profile generated showing the loca8on of C, Siox and Simet
Energy Losses in XPS Plasmon Loss – In some materials there is a probability of loss of a specific amount of energy due to interac8on between the photoelectron and other electrons
b – bulk plasmon s – surface plasmon
Some electrons loose energy more than once
Energy Losses in XPS Shake up peaks – For some materials the photoelectric process leads to forma8on of an ion in an excited state instead of the ground state.
Example: Iden8fica8on of Cu (II) by shake up peaks
X-‐ray induced Auger Spectroscopy (XAES) • Talked briefly about the Auger process earlier • Since it is simply an electron our detector records the events
XAES Some8mes photoelectron peak is insufficient for oxida8on state iden8fica8on
XPS Spectra of Cu2O and Cu metal
Spectra of Cu LMM peak demonstrates difference
Wagner Plot Wagner introduced a new parameter to XPS analysis α’, the Auger parameter
α’ = K.E.Auger + B.E.Photoelectron
