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

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Lecture 5—chemical shift 1 Quantization and depth effects, XPS and Auger I.XPS: The Chemical Shift II.Mean free path, overlayer attenuation, etc. III.Auger spectroscopy, final state effects

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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. - PowerPoint PPT Presentation

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

Page 1: Quantization and depth effects, XPS and Auger XPS:  The Chemical Shift

Lecture 5—chemical shift 1

Quantization and depth effects, XPS and Auger

I.XPS: The Chemical Shift

II.Mean free path, overlayer attenuation, etc.

III.Auger spectroscopy, final state effects

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

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ΔEChemical Shift

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

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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

3Binding energies and particle size

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Binding energies and particle size 4

Chemical Shift in Au compounds vs. bulk elemental gold

PHI handbook

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EF

Evacuum

EB

hvEkin

e-

Φspectrometer

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)

Evacuum

Ekin

5

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EF

Evacuum

EB

hvEkin

e-

Φspectrometer

Evacuum

Ekin

Changes in EB result from :

1.Changes in oxidation state of the atom (initial state effect)

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

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

6

mainly

sometimes

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Primarily an initial state effect

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ΔEb = kΔqi + ΔVij Vij often similar in different atoms of same material, so Δvij is typically negligible

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ΔEb = kΔqi + ΔVij

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

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.

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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

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CO

e-

EN = 2.5EN = 3.5

CTi

EN = 1.5

e-

CC

O withdraws valence charge from C: C(1s) shifts to higher BE relative to elemental C (diamond) at 285.0 eV

Elemental C: binding energy = 285.0 eV

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

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

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Electron withdrawing groups shift core levels to higher binding energy

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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

= Ni

= Si

NiSi (Schematic, not real structure)

Bulk

VacuumAtomic O

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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).

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SiSiO2

Exposure to atomic O

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 peak appears (shift to higher BE)

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

Page 18: Quantization and depth effects, XPS and Auger XPS:  The Chemical Shift

PtSi Pt1+ySi

NiSi Ni1+x SiSi transport and oxidation

Pt1+y Si

Ni1+x Si

(B)

Si transport kinetically inhibited, metal oxidation

Si SiO2

Pt silicate formation

(A)

Preferential Si oxidation, Si flux creates metal-rich substrates

O + O2

O + O2

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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

Page 20: Quantization and depth effects, XPS and Auger XPS:  The Chemical Shift

C2-B-H

C-B-H

B-B-H

RC-B

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

Rc=Ring carbon

Page 21: Quantization and depth effects, XPS and Auger XPS:  The Chemical Shift

Figure 3

B2-B

CB-B

C2-B

C2-B-H

C-B-H

B-B-H

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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

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Attenuation:

Clean surface of a film or single crystal

hv

e-

I = I0

d

film or single crystal with overlayer of thickness d

I = I0exp(-d/λ)hv

Issues:

1.Average coverage

2.Calculating λ

3.Relative vs. Absolute intensities

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MonolayerSurface coverage = Θ1

d = d1

BilayerSurface coverage = Θ2

d = d2

Bare surfaceCoverage = 1-(Θ1+Θ2)

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!

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Consider 2 cases:

1.dave < 1 ML (0<Θ<1)

2.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:

1.Small changes in sample position2.Changes in x-ray flux IB/IA will remain constant

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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/Ep

2([βln(γE)-(C/E)+(D/E2])

Page 27: Quantization and depth effects, XPS and Auger XPS:  The Chemical Shift

Binding energies and particle size 27

Element Nvρ ( g cm-3) M

E (energ

y)

Eg (Band Gap

Ep β γ U C D (Ep)2 ln(γE) (C/E) (D/E2) [βln(γE)-(C/E)+(D/E2]

Ep2([βln(γE)-

(C/E)+(D/E2])IMFP=E/Ep

2([βln(γE)-(C/E)+(D/E2])

Sulfur 6 2.07 32 152 017.942

3080.0268

2030.1327

54180.3881

43721.616

78945.326

6107321.9

2643.0046

24370.010

6370.001

9620.07190997

8 23.14972023 6.56595408

Co 9 8.9 58.9 765 133.585

4440.0139

5440.0640

23351.3599

97670.732

40225.112

04841127.

9823.8913

68340.000

9574.29E

-050.05338727

6 60.21989045 12.7034439O in MgO th--C 6 2.25 12.01 722.6 0

30.534293

0.0057446

0.12733333

1.12411749

0.947053

30.0183562

932.343

4.52190886

0.001311

5.75E-05

0.024723479 23.05076374 31.34820209

C in C th--C 4 2.25 12.01 265 024.931

1460.0126

9280.1273

33330.7494

11661.288

03537.812

2375621.5

623.5187

82870.004

8610.000

538 0.04034127 25.07460164 10.56846301

Co Thr--C 9 2.25 58.9 765 016.886

80.0307

3020.1273

33330.3438

19641.657

12446.248

5516285.1

644.5789

28870.002

1667.9E-

05 0.13862427 39.5306522 19.3520713

C in C th--Co 4 2.25 12.01 1201 024.931

1460.0126

9280.1273

33330.7494

11661.288

03537.812

2375621.5

625.0299

62860.001

0722.62E

-050.06279824

3 39.03300374 30.76883368

O Thru C 6 2.25 12.01 500 030.534

2930.0057

4460.1273

33331.1241

17490.947

05330.018

3562932.3

434.1536

61140.001

8940.000

120.02208713

5 20.59278693 24.2803464

Ni Thru C 15 2.25 12.01 632.2 048.278

956

-0.0056

1840.1273

33332.8102

9373

-0.587

367

-5.0541

09562330.

8584.3882

5884

-0.000

93-1.3E-

05

-0.02373862

2 -55.33134684 -11.42571139

Mg thru C 2 2.25 12.011435.

5 017.628

9820.0283

7670.1273

33330.3747

05831.629

01845.606

1187310.7

815.2083

21540.001

1352.21E

-050.14668249

4 45.58613442 31.48983827

Fe thru C 6 2.25 12.01 775.1 030.534

2930.0057

4460.1273

33331.1241

17490.947

05330.018

3562932.3

434.5920

45090.001

222 5E-050.02520763

3 23.50216098 32.979946

Mg thru MgO 8 3.58 401435.

5 024.369

6340.0171

2250.1009

46640.7160

34531.318

40938.506

4818593.8

794.9761

05250.000

9181.87E

-050.08430378

2 50.06624913 28.67201009

Page 28: Quantization and depth effects, XPS and Auger XPS:  The Chemical Shift

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

Column Term used

1 Valence electrons of the element (O)

2 Density of the over layer (Carbon)

3 Mass of the over layer

4 Kinetic Energy of the element(O)

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

Page 29: Quantization and depth effects, XPS and Auger XPS:  The Chemical Shift

Binding energies and particle size 29

overlayer substrate overlayer substrated A(Ini*IcS) B(Ic*IniS) C Si

0 7660 0 7660 82642.421 7374.851 2454.299 7660 82642.422 7100.317 4835.711 7660 82642.423 6836.003 7146.401 7660 82642.424 6581.528 9388.467 7660 82642.425 6336.526 11563.95 7660 82642.426 6100.644 13674.83 7660 82642.427 5873.543 15723.01 7660 82642.428 5654.897 17710.37 7660 82642.429 5444.389 19638.71 7660 82642.42

10 5241.718 21509.78 7660 82642.4211 5046.591 23325.29 7660 82642.4212 4858.729 25086.88 7660 82642.4213 4677.859 26796.15 7660 82642.4214 4503.722 28454.67 7660 82642.4215 4336.068 30063.92 7660 82642.4216 4174.655 31625.39 7660 82642.4217 4019.251 33140.48 7660 82642.4218 3869.631 34610.58 7660 82642.4219 3725.581 36037.02 7660 82642.4220 3586.894 37421.1 7660 82642.4221 3453.369 38764.08 7660 82642.4222 3324.815 40067.17 7660 82642.4223 3201.047 41331.56 7660 82642.4224 3081.885 42558.4 7660 82642.4225 2967.16 43748.81 7660 82642.4226 2856.706 44903.87 7660 82642.4227 2750.363 46024.62 7660 82642.4228 2647.978 47112.09 7660 82642.4229 2549.406 48167.26 7660 82642.4230 2454.502 49191.1 7660 82642.4231 2363.132 50184.53 7660 82642.4232 2275.162 51148.46 7660 82642.42

=D6*EXP(-A6/26.36)

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

=Area under the curve1915/0.25

=Area under the curve 54544/0.66

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Binding energies and particle size 30

Page 31: Quantization and depth effects, XPS and Auger XPS:  The Chemical Shift

Take-off angle variations in XPS:Definition

θ

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 = 0 normal emission θ=89 grazing emission

Page 32: 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)!

Page 33: 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

λλcosθ

θ increased take-off angle: d~ λ cosθ (reduced sampling depth)

Page 34: 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

λ

SiO2

Si

Si

SiO2

λcosθ

SiO2

Si

Page 35: Quantization and depth effects, XPS and Auger XPS:  The Chemical Shift

In Dragon and other systems:

Si

SiO2

Ta sample holders

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.

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Multiplet Splitting:

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

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

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

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Binding energies and particle size 38

2p1/2

2p3/2

Multiplet splitting in Cu

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Binding energies and particle size 39

Auger Spectroscopy: Final State Effects

hv or e-

XPS initial State XPS Final State

Auger Initial StateAuger Final State

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Binding energies and particle size 40

Kinetic Energy of Auger Electron:This transition is denoted as (KLL)

K (1s)

L1 (2s)

L2,3 (2p)

e-

K (1s)

L1 (2s)

L2,3 (2p)

Initial stateFinal State

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?

e- detector

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Binding energies and particle size 41

K (1s)

L1 (2s)

L2,3 (2p)

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

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Binding energies and particle size 42

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

R

Heat in UHV

Agglomeration, should see shift in Auger peak as Ueff decreases

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Binding energies and particle size 43

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

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Binding energies and particle size 44

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

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

Cu(LVV) shift with increasing Cu coverage

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Auger in derivative vs. integral mode

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

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Binding energies and particle size 46

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”

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Binding energies and particle size 47

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.

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Binding energies and particle size 48

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.

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Binding energies and particle size 49

N(E)

KE

Peak-to-peak height