Quantization and depth effects, XPS and Auger
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Quantization and depth effects, XPS and Auger XPS: The Chemical Shift Mean free path, overlayer attenuation, etc. Auger spectroscopy, final state effects. The XPS Chemical Shift: Shifts in Core level Binding Energies with Chemical State. Δ E Chemical Shift.

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Quantization and depth effects, XPS and Auger XPS: The Chemical Shift

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Quantization and depth effects xps and auger xps the chemical shift

  • Quantization and depth effects, XPS and Auger

  • XPS: The Chemical Shift

  • Mean free path, overlayer attenuation, etc.

  • Auger spectroscopy, final state effects

Lecture 5—chemical shift


Quantization and depth effects xps and auger xps the chemical shift

The XPS Chemical Shift: Shifts in Core level Binding Energies with Chemical State

ΔEChemical Shift

In part fromC. Smart, et al., Univ. Hong Kong and UWO


Quantization and depth effects xps and auger xps the chemical shift

The binding energy is defined as:

Eb = hv –Ek –Φ

Where hv= photon energy

Ek = kinetic energy of the photoelectron

Φ = work function of the spectrometer

Specifically, the CHEMICAL SHIFT is ΔEb

That is the change in Eb relative to some chemical standard

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Chemical Shift in Au compounds vs. bulk elemental gold

PHI handbook

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

e-

hv

Ekin

Ekin

Evacuum

Φspectrometer

Evacuum

EF

EB

Because the electron emitted from the solid has to impact on the analyzer/dectector to be counted, the relationship Ekin and EB has to include the work function term of the detector (typically, 4-5 eV):

Ekin = hv-EB – Φspectrometer

We only need the work function term for the spectrometer, not the sample, because (for a conducting sample) the two Fermi levels are coupled.

Obviously, electrically insulating samples present problems (Charging)


Quantization and depth effects xps and auger xps the chemical shift

e-

hv

Ekin

Ekin

Evacuum

Φspectrometer

Evacuum

EF

  • Changes in EB result from :

  • Changes in oxidation state of the atom (initial state effect)

  • Changes in response of the system to the core hole final state:

EB

mainly

sometimes

ΔEB = ΔE(in.state) – ΔR + other effects (e.g., band bending)

where ΔR = changes in the relaxation response of the system to the final state core hole (see M.K. Bahl, et al., Phys. Rev. B 21 (1980) 1344


Quantization and depth effects xps and auger xps the chemical shift

Primarily an initial state effect


Quantization and depth effects xps and auger xps the chemical shift

ΔEb = kΔqi + ΔVij

Vij often similar in different atoms of same material, so Δvij is typically negligible


Quantization and depth effects xps and auger xps the chemical shift

Initial state term, often similar for diff. atoms in same molecule

ΔEb = kΔqi + ΔVij

In principle, can be obtained from ground state Mulliken Charge Density calculations

Valence charge is removed or added to an atom by interaction with surrounding atoms.

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

  • Chemical shift is dominated by changes in ground state valence charge density:

  • Changes in valence charge density dominated by nearest-neighbor interactions

  • Qualitative interpretation on basis of differences in ground state electronegativities

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

e-

O withdraws valence charge from C:

C(1s) shifts to higher BE relative to elemental C (diamond) at 285.0 eV

O

C

C

C

C

EN = 3.5

EN = 2.5

Elemental C: binding energy = 285.0 eV

Ti

e-

Ti donates charge to C, binding energy shifts to smaller values relative to 285 eV

EN = 1.5

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Thus, a higher oxidation state (usually) yields a higher binding energy!


Quantization and depth effects xps and auger xps the chemical shift

Electron withdrawing groups shift core levels to higher binding energy

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Binding energy shifts can be used to follow the course of surface reactions for complex materials:

e.g., atomic O /(Pt)NiSi (e.g., Manandhar, et al., Appl. Surf. Sci. 254(2008) 7486

Vacuum

Atomic O

= Ni

= Si

Bulk

NiSi (Schematic, not real structure)

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Pauling Electronegativities, Ground State

Si = 1.8

O = 3.5

Ni = 1.8

Ni-O or Si-O formation  shift of Ni or Si to higher BE

Question: Ni-Si Ni-Ni. Which way should BE move (think).

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

XPS binding energy shifts for Pt-doped NiSi as a function of exposure to atomic O at room temp.

(Manadhar, et al., Appl. Surf. Sci. 254 (2008) 7486

SiO2

Si

SiO2 peak appears (shift to higher BE)

Ni (2p) shifts to lower BE. Why?

Exposure to atomic O


Quantization and depth effects xps and auger xps the chemical shift

O + O2

Si SiO2

(A)

Preferential Si oxidation, Si flux creates metal-rich substrates

PtSi Pt1+ySi

Si transport and oxidation

NiSi  Ni1+x Si

O + O2

Pt silicate formation

(B)

Si transport kinetically inhibited, metal oxidation

Pt1+y Si

Ni1+xSi


Quantization and depth effects xps and auger xps the chemical shift

How do we estimate q, Δq?

This is usually done with Mulliken atomic charge densities, originally obtained by LCAO methods:

ΨMO = caΦa + cbΦb Φa(b) atomic orbital on atom a (b)

 Ψ2 = caca*ΦaΦa* + [cross terms] + cbcb*ΦbΦb*

Atomic charge on atom a

Atomic charge on atom b

Overlap charge


Quantization and depth effects xps and auger xps the chemical shift

Different Boron Environments in orthocarborane derived films (B10C2HX and B10C2HX:Y)

B-B-H

C2-B-H

RC-B

C-B-H

Rc=Ring carbon


Quantization and depth effects xps and auger xps the chemical shift

C2-B

B-B-H

C2-B-H

CB-B

C-B-H

B2-B

Figure 3


Quantization and depth effects xps and auger xps the chemical shift

  • Chemical Shifts: Final Note

  • Calculating ground state atomic charge populations with DFT:

  • Minimal basis sets give best results (LCAO-MO)

  • Such basis sets are not best for lowest energy/geometric optimization

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Attenuation:

hv

I = I0

Clean surface of a film or single crystal

e-

hv

I = I0exp(-d/λ)

d

  • Issues:

  • Average coverage

  • Calculating λ

  • Relative vs. Absolute intensities

film or single crystal with overlayer of thickness d

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Bilayer

Surface coverage = Θ2

d = d2

Bare surface

Coverage = 1-(Θ1+Θ2)

Monolayer

Surface coverage = Θ1

d = d1

We can only measure a total intensity from a macroscopic area of the surface:

I = [1-(Θ1+Θ2)] I0 + Θ1I0 exp[-d1/λ] + Θ2 1I0 exp[-d2/λ]

= I0exp[-dave/λ]

 we can only determine average coverage with XPS!

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

  • Consider 2 cases:

  • dave < 1 ML (0<Θ<1)

  • dave> 1 ML (Θ> 1)

  • We need to look at the RATIO of Isubstrate (IB) and Ioverlayer (IA)

  • Why? Absolute intensity of IB can be impacted by:

  • Small changes in sample position

  • Changes in x-ray flux

  • IB/IA will remain constant

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Calculation of the overlayer coverage

First, we need to calculate the IMFP of the electrons of the substrate through the overlayer and the IMFP of the electrons in the overlayer.

The formula to calculate the IMFP is (NIST):

IMFP=E/Ep2([βln(γE)-(C/E)+(D/E2])

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Binding energies and particle size


Terms used in the excel sheet example carbon through mgo

Terms used in the excel sheet (example Carbon through MgO)

After you insert all the four columns, the IMFP is calculated on its own.


Quantization and depth effects xps and auger xps the chemical shift

=D6*EXP(-A6/26.36)

=E6*(1-EXP(-A6/33.17))

=Area under the curve1915/0.25

=Area under the curve 54544/0.66

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Take-off angle variations in XPS:

Definition

Take off angle (θ) is the angle between the surface normal and the axis of the analyzer. (Some people use 90-θ)

Surface normal

θ

θ = 0  normal emission

θ=89  grazing emission


Quantization and depth effects xps and auger xps the chemical shift

Take-off angle variations in XPS:

Intensity vs. θ

Intensity of a photoemission peak goes as

I ~ I cosθ

Therefore, intensities of adsorbates and other species are NOT enhanced at grazing emission (large θ)!


Quantization and depth effects xps and auger xps the chemical shift

Take-off angle variations in XPS:

Sampling Depth (d)

normal emission (θ = 0)

d ~ λ (inelastic mean free path)

λ

λ

θ

increased take-off angle:

d~ λcosθ (reduced sampling depth)

λcosθ


Quantization and depth effects xps and auger xps the chemical shift

d~ λ cosθ:

Effective sampling depth (d) decreases as θ increases

Relative intensities of surface species enhanced relative to those of subsurface:

Si

SiO2

SiO2

SiO2

λ

Si

Si

SiO2

Si

λcosθ


Quantization and depth effects xps and auger xps the chemical shift

In Dragon and other systems:

Arrangement of sample holder may cause increased signal from Ta or other extraneous materials. These should be monitored.

However, enhancement of SiO2 relative to Si will remain the same.

SiO2

Si

Ta sample holders


Quantization and depth effects xps and auger xps the chemical shift

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

  • Multiplet Splitting:

  • Valence electrons give rise to different spin states (crystal field, etc.  Cu 2p 3/2 vs. ½ states

  • Formation of a core hole shell yields an unpaired electron left in the shell

  • Coupling between the core electron spin and valence spins gives rise to final states with different total angular momentum.

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Multiplet splitting in Cu

2p3/2

2p1/2

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Auger Spectroscopy: Final State Effects

XPS initial State

XPS Final State

hv or e-

Auger Final State

Auger Initial State

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Kinetic Energy of Auger Electron:

This transition is denoted as (KLL)

e-

detector

e-

Initial state

Final State

L2,3 (2p)

L2,3 (2p)

L1 (2s)

L1 (2s)

K (1s)

K (1s)

KEAuger = EK - EL1 – EL2,3 - Ueff ~ EK – EL-EL - Ueff

Note: Auger transitions are broad, and small changes in BE (EL1 vs. EL2,3 ) sometimes don’t matter that much (sloppy notation)

What is Ueff?

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

L2,3 (2p)

L1 (2s)

Ueff is the coulombic interaction of the final state holes, as screened by the final state response of the system:

e.g., Jennison, Kelber and Rye “Auger Final States in Covalent Systems”, Phys. Rev. B. 25 (1982) 1384

K (1s)

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

For a typical metal, the final state holes are often delocalized (completely screened), and Ueff ~ 0 eV.

However, for adsorbed molecules, or nanoparticles, the holes are constrained in proximity to each other. Ueff can be large, as large as 10 eV or more.

Nanoparticle, Ueff ~ 1/R

Agglomeration, should see shift in Auger peak as Ueff decreases

R

Heat in UHV

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

KE(LVV) = EL –EV – EV – Ueff as particle size increases, Ueff decreases

Note shift in Cu(LVV) Auger as nanoparticles on surface agglomerate

J. Tong, et al. Appl. Surf. Sci. 187 (2002) 253

Cu/Si:O:C:H

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Similar effects in Auger KE are seen for agglomeration during Cu deposition at room temp. (Tong et al.)

Cu(LVV) shift with increasing Cu coverage

Note corresponding change in Cu(2p3/2) binding energy.

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Auger in derivative vs. integral mode

When doing XPS, x-ray excited Auger spectra are acquired along with photoemission lines

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Auger spectra, though broad, can give information on the chemical state (esp. if the XPS BE shift is small as in Cu(0) vs. Cu(I)

Above spectra are presented in the N(E) vs. E mode—or “integral mode”

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

  • However, in some cases Auger spectroscopy is used simply to monitor surface cleanliness, elemental composition, etc. This often involves using electron stimulated Auger (no photoemission lines).

  • Auger spectra are typically broad, and on a rising background. Presenting spectra in the differential mode (dN(E)/dE) eliminates the background.

  • Peak-to-peak height (rather than peak area) is proportional to total signal intensity, and the background issue is eliminated. Except in certain cases, however, (e.g., C(KVV)) most chemical bonding info is lost.

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

Auger (derivative mode) of graphene growth on Co3O4(111)/Co(0001) (Zhou, et al., JPCM 24 (2012) 072201

Homework: explain the data on the right.

Binding energies and particle size


Quantization and depth effects xps and auger xps the chemical shift

N(E)

KE

Peak-to-peak height

Binding energies and particle size


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