Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure...
Transcript of Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure...
Energy Spectroscopy
Different probes are possible:Auger -> electrons (2 –
10 keV)XPS (or ESCA) -> X rays
(0.2 –
2 keV)(x-rays photoelectron spectroscopy)UPS
-> Ultraviolet photons (10 –
50 eV)(UV photoelectron spectroscopy)
Energy spectral analysis of the out coming particles
Out coming particles: electrons
Electromagnetic spectrum
E = hν
= hc/λc = 3 108
m/sh = 6.6 10-34
Js
Excitation by means of a probe
Energy spectral analysis of the in coming particles -> XASor
The Auger process
Ekin
– E(L3
) = E(L1
) –
E(K)
E(Li
) and E(K) depend on the atomic structure ->Ekin
does not depend on Ei
chemical sensitivity
Ekin
excitation relaxationradioactive Auger
Ejected electron Vacuum level
Ejected electron
KL1
L3
Auger spectroscopy is based upon a singleelectron in -
electron out process.
N.B.: the sample must be a conductor and must be connected to ground to avoid charging
electron gunchanneltrongrids
sample
electrons
Auger experimental setup
Electron energy: in 1-
10 keVout 10 –
2000 eV
Auger spectrum: number of emitted electrons as a function of their kinetics energy
CMA = Cylindrical Mirror Analyzer
MgO
Auger spectrumDerivative modeCounting mode
The monotone background is due to multi-scattered electrons
Auger transitions for the different chemical elements
Thickness sensibilityThe thickness of the investigated surface depends:1)
Electron energy2)
Probability of the Auger transition3)
Atomic scattering cross section4)
Abortion of the Auger electrons
Electron beam intensity at depth z: J(z) = I(0) r-z/d
r = layer attenuation factor; d = atomic layer thickness
Auger electrons (detected at surface) coming from depth z:I(z) = I(0) r-z/d
s-z/d
= I(0) exp(-z/d ln(rs)) = I(0) exp(-z/λ)s = attenuation factor for the Auger electrons;
λ = d/ln(rs) electron mean free path
Total Auger electron current measured at surface:I = ∫0
z
I(z) dz
= I(0) λ
(1-exp(-z/λ))
λ = electron mean free path
1/λ
= 1/λι
+ 1/λAuger ∼
1/λAuger λι
>> λAuger
Depth sensibility (D): 95% of the signal coming from a film with infinite thickness -> I(D) = 0.95 I(0) λ
(1-exp(-D/λ)) -> D = -
λ ln(0.05) ∼ 3 λ
surface sensibility is given by the reduced mean free path of the out coming electrons
Continuous film of thickness h
B
A
h
IB
= I(0) λB
(1-exp(-h/ λB
(EB
)))
J(h) = I(0) (λι
> 10 ML) electrons impinging on A
I(h) = I(0) λA
Auger electrons from A moving through B =>
IA
= I(0) λA
exp(-h/ λB
(EA
))
h < 4 –
5 MLEB
= Energy of the Auger electron generated in BEA
= Energy of the Auger electron generated in A
B
A
h
θ
–> fraction of the surface covered by the film
IB
= I(0) λB
θ
(1-exp(-h/ λB
(EB
)))
IA
= I(0) λA
[(1-θ) + θ
exp(-h/ λB
(EA
))]
Discontinuous film of thickness h
n < h < n+1 layers ->IA and IB
proportional to h
Growth of Ni films on 1ML Co/Pt(111)
848
Alloying during annealing of 2 ML Ni/1 ML Co/Pt(111)
Co53
increases and Ni102
decreases -> Ni-Co alloying on top of Pt(111)
Co53
and Ni102
decrease while Pt237
increases -> Ni-Co alloying with the Pt surface
Co53
-> λ
= 3.9 ÅCo656
-> λ
= 11.4 ÅNi102
-> λ
= 4.6 ÅNi848
-> λ
= 13.2 Å
Co656
and Ni848
decrease while Pt237
increases -> Ni-Co alloying with Pt bulk
C. S. Shern
et al. Phys. Rev. B 70, 214438 (2004)
Exciting particle -> photonsEmitted particle -> electrons
XPS and UPS
X ray photons (0.2 –2 keV) -> to investigate core levelsUV photons (10 -
45 eV) -> to investigate valence levels
Photoelectron spectroscopy is based upon a single photon in/electron out process.The energy of a photon is given by the Einstein relation :
E = h ν
h -
Planck constant ( 6.62 x 10-34
J s )ν −
frequency
(Hz) of the radiation
EkinEkin
Ws
= work function
Free atom Atom in a solid
Ebond Ebond
Evacuum
Ekin
= hν
- Ebond Ekin
= hν
– Ws
- Ebond
Experimental Details
1) source of fixed-energy radiation (an x-ray source for XPS or, typically, a He discharge lamp for UPS)
2) electron energy analyzer (which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy)
3) high vacuum environment (to enable the emitted photoelectrons to be analyzed without interference from gas phase collisions)
Detectors: CMA or hemispherical analyzer
Vin
rin
rout
Vout
e-
Only the electrons satisfying the relation: Vout
– Vin
= Ee
(rout
/rin
- rin
/ rout
) move through the analyzer
X-rays: electron beam impinging at energies of 10-50 keV
on an anode excites the core electron of the anode -> during the relaxation photons are emitted
Mg Kα
E = 1253.6 eV
ΔE = 0.7 eVAl Kα
E = 1486.6 eV
ΔE = 0.9 eV
Laboratory photon sources
He I
E = 21.2 eV
ΔE = 0.01 eVHe II
E = 40.8 eV
ΔE = 0.01 eVNe I
E = 16.9 eV
ΔE = 0.01 eV
UV-rays: discharge in a lamp containing rare gas at low pressure (0.1 mbar) -> during the relaxation photons are emitted
Photon penetration depth > 1-10 μm -> the surface sensibility is given by the reduced mean free path of the out coming electrons
Sensibility to Auger transitionTo distinguish between photo-electrons and Auger-electrons is sufficient to take two spectra at different energies:XPS -> the energy of the out coming electron depends on hνAuger -> the energy of the out coming electron depends on the core transition
XPS Spectrum
2 ML MgO/Fe
Energy (eV)
XPS Spectrum
UPS Spectrum
Fe 3d
Shift of the 3d Fe peak following MgO
deposition -> Fe oxidation
Bonding energy (eV)
Inte
nsity
(arb
. un.
)
d -
band
He2
40.8 eV
MgO
is an insulator with a gap of 8 eVFe electronic structure -> 3d6
4s2
He I UP spectra of the pure Fe film and after its exposure to various oxygen doses at 300 K.
Sicot
et al. Phys. Rev. B 68, 184406 (2003)K. Ruhrnschopf
et al. Surf. Sci. 374, 269 (1997)
O2
dose
ARUPS: Angle Resolved UPS
Measure of the dispersion relation (energy vs
wave vector) of surface states i.e. the band structure of a surface . Three-step model:1) The electron is excited from an initial to a final state within the crystal;2) The electron travels through the solid towards the surface;3) The electron crosses the surface and is emitted into the vacuum with a certain kinetic energy.Measurement of the dispersion curve requires a determination of the wave vector of the emitted photoelectrons. The wave vector has a component both parallel and perpendicular to the surface, so that the kinetic energy of the photoelectron should be written:
Measuring the photoelectron intensity as a function of E and θ
one gets the dispersion relation
Relationship between the k||outside
(value of the photoelectron in vacuum outside
the crystal)
and the value of k||inside
(of the electron in the solid)
Momentum conservation: the photon momentum is negligible and thus the electron’s final momentum must equal its initial momentum in the solid.
However, the photoelectron must, after traveling through the solid,
traverse the surface into the vacuum. The surface represents a scattering potential with only 2D translational symmetry. As for LEED, the electron will be scattered by a reciprocal lattice vector of the
surface. Thus, the relationship between the photoelectron’s wave vector in the solid and in vacuum is:
k||outside = k||
inside + Gswhere Gs is a reciprocal lattice vector of the surface. Note that, the component of wave vector perpendicular to the surface is not conserved. Thus, from a measurement of the energy and emission angle of the
photoelectron the value of k||
within the solid can be determined.
Elastic scattering
-> Ekin
= hν
- Ws
- Ebond
pe
= (2mE)1/2
-> ~ 4 10-25
for 1 eV
electronphv
= E/c -> ~ 3 10-28 for 1 eV
photon
UPS Spectrum
UV photons (10 -
45 eV) -> to investigate valence levels
Note: You can measure the wave vector k and the energy at the same time (band structure)
With
STM only
access
to the DOS (density
of state as a function
of energy, but no information on k)
Surface State
The surface states are localized at surface -> k⊥
~ 0
Es
= EF
– E0
+ 1/2m* (ħk)2
m* -> effective mass of the surface state electron
Conduction electrons behave likes a 2D gas of free electrons
Surface states on Au(788)// to the steps ⊥
to the steps
k⊥
= nπ/LEn
= EF
– E0
+ n2/2m* (ħπ/L)2
One dimensional quantum well of size L perpendicularly to the steps
L = terrace size = 3.8 nm
A. Mugarza
et al., Phys. Rev. Lett. 87, 107601 (2001)
Ψk
(r) ≈
exp(ik//
y) cos(k⊥
x)
Carbon based material
Electronic structure:1s2 2s2 2p2
Diamond:sp3 bonding
Graphite:sp2 bonding
GrapheneGraphene
is an atomic-scale honeycomb lattice made of carbon atoms.
Free electron gas:E ∝
k2
graphene: E ∝
k
Nature Materials 6, 183–191 (2007)