Transcript Slide 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
Lecture 5—chemical shift
1
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
2
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
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Chemical Shift in Au compounds vs. bulk elemental gold
PHI handbook
Binding energies and particle size
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eEkin
hv
Ekin
Evacuum
Evacuum
Φspectrometer
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)
5
eEkin
hv
Ekin
Evacuum
Evacuum
Φspectrometer
EF
Changes in EB result from :
EB
1. Changes in oxidation state of
the atom (initial state effect)
mainly
2. Changes in response of the
system to the core hole final
state:
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
6
7
Primarily an initial state effect
8
ΔEb = kΔqi + ΔVij
Vij often similar in different atoms of
same material, so Δvij is typically 9
negligible
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
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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
Binding energies and particle size
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e-
O
C
EN = 3.5
EN = 2.5
C
C
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
e-
Ti
C
Ti donates charge to C,
binding energy shifts
to smaller values
relative to 285 eV
EN = 1.5
Binding energies and particle size
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Thus, a higher oxidation state (usually) yields a higher binding energy!
13
Electron withdrawing groups shift core levels to higher binding energy
Binding energies and particle size
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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
Vacuum
Atomic O
= Ni
= Si
Bulk
NiSi (Schematic, not real
structure)
Binding energies and particle size
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Pauling Electronegativities, Ground State
Si = 1.8
Ni-O or Si-O formation  shift of Ni or Si to higher BE
O = 3.5
Ni = 1.8
Question: Ni-Si Ni-Ni. Which way should BE move
(think).
Binding energies and particle size
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SiO2
Si
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)
Exposure to
atomic O
Ni (2p) shifts to lower
BE. Why?
17
O + O2
Si SiO2
PtSi Pt1+ySi
NiSi  Ni1+x Si
(A)
Si transport
and oxidation
Preferential Si
oxidation, Si flux
creates metal-rich
substrates
O + O2
Pt silicate formation
Pt1+y Si
Ni1+x Si
(B)
Si transport
kinetically inhibited,
metal oxidation
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
19
Different Boron Environments in orthocarborane
derived films (B10C2HX and B10C2HX:Y)
B-B-H
C2-B-H
C-B-H
RC-B
Rc=Ring carbon
C2-B
CB-B
B-B-H
C2-B-H
C-B-H
B2-B
Figure 3
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
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Attenuation:
hv
I = I0
Clean surface of a film or single crystal
hv
eI = I0exp(-d/λ)
d
Issues:
film or single crystal with
overlayer of thickness d
1. Average coverage
2. Calculating λ
3. Relative vs. Absolute intensities
Binding energies and particle size
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Monolayer
Surface coverage = Θ1
d = d1
Bare surface
Coverage = 1-(Θ1+Θ2)
Bilayer
Surface coverage = Θ2
d = d2
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
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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 position
2. Changes in x-ray flux
IB/IA will remain constant
Binding energies and particle size
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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/Ep2([βln(γE)-(C/E)+(D/E2])
Binding energies and particle size
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Element
ρ(g
cm-3)
Nv
E
Eg
(ener (Band
gy)
Gap
M
Ep
Sulfur
6
2.07
32
152
Co
O in MgO th-C
9
8.9
58.9
765
6
2.25 12.01 722.6
17.94
0 2308
33.58
1 5444
30.53
0 4293
C in C th--C
4
2.25 12.01
265
Co Thr--C
9
2.25
765
0
C in C th--Co
4
O Thru C
6
β
γ
U
C
D
(Ep)2 ln(γE) (C/E)
(D/E2 [βln(γE)Ep2([βln(γE)- IMFP=E/Ep2([βln(γE)
2
) (C/E)+(D/E ] (C/E)+(D/E2])
-(C/E)+(D/E2])
0.026 0.1327 0.3881 1.616 45.326 321.9 3.0046 0.010 0.001 0.07190997
8203 5418 4372 789 6107 264 2437 637 962
8
0.013 0.0640 1.3599 0.732 25.112 1127. 3.8913 0.000 4.29E 0.05338727
9544 2335 9767 402 0484 982 6834 957 -05
6
0.005 0.1273 1.1241 0.947 30.018 932.3 4.5219 0.001 5.75E 0.02472347
7446 3333 1749 053 3562 43 0886 311 -05
9
23.14972023
6.56595408
60.21989045
12.7034439
23.05076374
31.34820209
24.93 0.012 0.1273 0.7494 1.288 37.812 621.5 3.5187 0.004 0.000
0 1146 6928 3333 1166 035 2375 62 8287 861 538 0.04034127
25.07460164
10.56846301
16.88 0.030 0.1273 0.3438 1.657 46.248 285.1 4.5789 0.002 7.9E68 7302 3333 1964 124 5516 64 2887 166 05 0.13862427
39.5306522
19.3520713
2.25 12.01 1201
24.93 0.012 0.1273 0.7494 1.288 37.812 621.5 5.0299 0.001 2.62E 0.06279824
0 1146 6928 3333 1166 035 2375 62 6286 072 -05
3
39.03300374
30.76883368
2.25 12.01
30.53 0.005 0.1273 1.1241 0.947 30.018 932.3 4.1536 0.001 0.000 0.02208713
0 4293 7446 3333 1749 053 3562 43 6114 894 12
5
20.59278693
24.2803464
48.27 0.005 0.1273 2.8102 0.587 5.0541 2330. 4.3882 0.000 -1.3E- 0.02373862
0 8956 6184 3333 9373 367 0956 858 5884 93 05
2 -55.33134684
-11.42571139
58.9
500
Ni Thru C
15
2.25 12.01 632.2
Mg thru C
2
2.25 12.01
1435.
5
17.62 0.028 0.1273 0.3747 1.629 45.606 310.7 5.2083 0.001 2.21E 0.14668249
0 8982 3767 3333 0583 018 1187 81 2154 135 -05
4
45.58613442
31.48983827
Fe thru C
6
2.25 12.01 775.1
30.53 0.005 0.1273 1.1241 0.947 30.018 932.3 4.5920 0.001
0.02520763
0 4293 7446 3333 1749 053 3562 43 4509 222 5E-05
3
23.50216098
32.979946
Mg thru MgO
8
3.58
1435.
5
24.36 0.017 0.1009 0.7160 1.318 38.506 593.8 4.9761 0.000 1.87E 0.08430378
0 9634 1225 4664 3453 409 4818 79 0525 918 -05
2
50.06624913
28.67201009
40
Binding energies and particle size
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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.
d
overlayer
substrate
overlayer
A(Ini*IcS)
B(Ic*IniS)
C
0
7660
0
1
7374.851
2454.299
2
7100.317
4835.711
3
6836.003
7146.401
4
6581.528
9388.467
5
6336.526
11563.95
6
6100.644
13674.83
7
5873.543
15723.01
8
5654.897
17710.37
9
5444.389
19638.71
10
5241.718
21509.78
11
5046.591
23325.29
12
4858.729
25086.88
13
4677.859
26796.15
14
4503.722
28454.67
15
4336.068
30063.92
16
4174.655
31625.39
17
4019.251
33140.48
18
3869.631
34610.58
19
3725.581
36037.02
20
3586.894
37421.1
21
3453.369
38764.08
22
3324.815
40067.17
23
3201.047
41331.56
24
3081.885
42558.4
25
2967.16
43748.81
26
2856.706
44903.87
27
2750.363
46024.62
28
2647.978
47112.09
29
2549.406
48167.26
30
2454.502
49191.1
31
2363.132
50184.53
32
2275.162
51148.46
substrate
Si
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
7660
82642.42
Binding energies and particle size
=D6*EXP(-A6/26.36)
=E6*(1-EXP(-A6/33.17))
=Area under the curve1915/0.25
=Area under the curve 54544/0.6
29
15000
13000
11000
9000
7000
Series1
Series2
5000
3000
1000
-1 -0.5 0
-1000
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
Binding energies and particle size
6.5
7
7.5
8
30
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)
θ
θ = 0  normal emission
θ=89  grazing emission
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)!
Take-off angle variations in XPS:
Sampling Depth (d)
normal emission (θ = 0)
d ~ λ (inelastic mean free path)
λ
θ
λcosθ
increased take-off angle:
d~ λ cosθ (reduced sampling depth)
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
Si
λ
SiO2
λcosθ
Si
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.
Ta sample
holders
Binding energies and particle size
36
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.
Binding energies and particle size
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Multiplet splitting in Cu
2p3/2
2p1/2
Binding energies and particle size
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Auger Spectroscopy: Final State Effects
XPS initial State
XPS Final State
hv or e-
Auger Initial State
Binding energies and particle size
Auger Final State
39
Kinetic Energy of Auger Electron:
This transition is denoted as (KLL)
e-
detector
eInitial state
Final State
L2,3 (2p)
L1 (2s)
K (1s)
L2,3 (2p)
L1 (2s)
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
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L2,3 (2p)
L1 (2s)
K (1s)
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
Binding energies and particle size
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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
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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
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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
Binding energies and particle size
Note corresponding
change in Cu(2p3/2)
binding energy.
44
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
45
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
46
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
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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
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N(E)
KE
Peak-to-peak height
Binding energies and particle size
49