Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure...

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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/λ c = 3 10 8 m/s h = 6.6 10 -34 Js Excitation by means of a probe Energy spectral analysis of the in coming particles -> XAS or

Transcript of Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure...

Page 1: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 2: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 3: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 4: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

MgO

Auger spectrumDerivative modeCounting mode

The monotone background is due to multi-scattered electrons

Auger transitions for the different chemical elements

Page 5: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 6: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 7: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

Growth of Ni films on 1ML Co/Pt(111)

848

Page 8: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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)

Page 9: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 10: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 11: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 12: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 13: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 14: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 15: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 16: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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)

Page 17: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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

Page 18: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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)

Page 19: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

Carbon based material

Electronic structure:1s2 2s2 2p2

Diamond:sp3 bonding

Graphite:sp2 bonding

Page 20: Energy Spectroscopy - EPFLSurf. Sci. 374, 269 (1997) O. 2. dose. ARUPS: Angle Resolved UPS. Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band

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)