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Ming H. Cheng
John C. Hemminger group
August 3, 2010
1
Outline
• Introduction of XPS
• Pyrite thin films on the glass Na migration to the surface
Ar sputtering effect
• Pyrite thin films on the quartz Removal of oxidation layers on the surface
Time-dependent oxidation study
• Pyrite thin films on the Si substrate Synchrotron radiation (Advance Light Source in Berkeley National Labs)
Surface chemical states of pyrite thin films on the Si substrate
2
E kin = hν-EB-Φs
Ekin : The kinetic energy of ejected photoelectrons; EB : Binding Energyhν : The energy of the incident X-ray photons. Φs : Work function term (almost constant)
Conduction band
Valence band
Vacuum level
Fermi level
E kin
Φs
EB
1S
2S
2P1/2
2P3/2
Incident X-rayhν
Photoelectron (1s)Free electron level
The principle of X-ray photoelectron spectroscopy (XPS)
XPS energy level diagram
e-
DetectorX-ray
Ultra-high vacuum (<10-9 torr) is required.
Photoelectron effect
3
Pyrite thin films on the glass
700 710 720 730 740
20000
25000
30000
35000
40000
Iron
oxide
Fe2p1/2
Fe2p3/2
Fe2p
Binding Energy (eV)
Iron
oxide
Inte
ns
ity
155 160 165 170 1754000
6000
8000
10000
12000
14000
16000
18000
20000
Inte
ns
ity
Binding Energy (eV)
S2p
Sulfate (SO-2
4)
Sulfide (FeS2)
1060 1064 1068 1072 1076 1080
60000
65000
70000
75000
Inte
ns
ity
Na1s
Binding Energy (eV)
• The pyrite has been oxidized.
• Na exists on the surface of the pyrite thin film.
4
As-deposited VS Sputtered
• Sputtering condition: 5*10-5 torr Ar, 2 KV (beam energy), 20mA (emission current), 5 mins.
• Na is removed by Ar sputtering which indicates Na only exists on the surface.
• The broadening of Fe 2p and S2p peaks after Ar sputtering is attributed to sulfur preferential sputtering.
1060 1064 1068 1072 1076 1080
60000
65000
70000
75000
Inte
ns
ity
Binding Energy (eV)
As-deposited
sputteringNa1s
155 160 165 170 175
5000
10000
15000
20000
Inte
ns
ity
Binding Energy (eV)
As-deposited
Sputtering
Sulfate
S2p
700 710 720 730 740
16000
24000
32000
40000
48000
56000
Inte
ns
ity
Binding Energy (eV)
FeS2
As-deposited
Sputtering
FeS +FeS2
Iron oxide
Fe2p
5
Pyrite thin films on the quartz
520 525 530 535 540 545
3000
4000
5000
6000
7000
8000
9000
As-depoisted
Acid-etched
B.E. (eV)
Inte
ns
ity
O1s
155 160 165 170 175 180 185
3000
6000
9000
12000
15000
18000
21000
24000
As-deposited pyrite
Acid-etched pyrite
Sulfate
Sulfide
B.E. (eV)
Inte
ns
ity
S2p
700 710 720 730 740
6000
8000
10000
12000
14000
16000
18000
20000
22000
24000
26000
28000
As-deposited pyrite
Acid-etched pyrite
B.E. (eV)
Inte
ns
ity
(F
e2
p)
Fe2p
• The composition of mixed acid used for removing the oxide layer is: 0.1% HF, 0.1% Acetic acid, and 0.1% Nitric acid
• The oxide layer can be removed by this mixed acid.
6
Time-dependent oxidation study of acid-
etched pyrite thin films on the quartz
700 710 720 730 7405000
10000
15000
20000
25000
30000
35000
40000
Inte
ns
ity
(F
e2
p)
B.E. (eV)
0 hours
4 hours
11 hours
31 hours
50 hours
66 hours
76 hours
222 hours
155 160 165 170 175 180 185
5000
10000
15000
20000
25000
30000
35000
Sulfide
Inte
ns
ity
(S
2p
)
B.E. (eV)
0 hours
4 hours
11 hours
31 hours
50 hours
66 hours
76 hours
222 hours
Sulfate
520 525 530 535 540 5454000
8000
12000
16000
20000
24000
Inte
ns
ity
(O
1s
)
0 hours
4 hours
11 hours
31 hours
50 hours
75 hours
222 hours
B.E. (eV)
• The oxide layer grows with respect to exposure air time.
7
8
0 50 100 150 200 250
0
5
10
15
20
25
30
35
Exposure Air Time (Hour)
Ox
ida
tio
n p
erc
en
tag
e
Oxidation percentage:
A (sulfate) / A(sulfide + sulfate)
A is peak area
•This oxidation study is still in
progress, since the oxide layer is
NOT saturated.
Time-dependent oxidation study of
acid-etched pyrite thin films on quartz
Pyrite thin films on the Si (in Irvine)
700 710 720 730 740 750
4000
8000
12000
16000
20000
24000
Inte
ns
ity
BE (eV)
As-deposited
Acid-etched
Exposed air 5 mins
after dipping acid
Fe2p
150 155 160 165 170 175 180 185 190
2000
4000
6000
8000
10000
Inte
ns
ity
S2p
BE (eV)
As-deposited
Acid-etched
Exposed air 5 mins
after dipping acid
525 530 535 540 5452000
4000
6000
8000
10000
12000
14000
Inte
ns
ity
BE (eV)
As-deposited
Exposed air 5 mins
after dipping acid
Acid-etched
O1s
• After dipping the acid solution, the oxide layer was removed.
• The acid-etched pyrite keeps unoxidized after exposing to air 5 mins.
9
Depth profile experiment and inelastic mean free path (IMFP)
10J. Phys. Chem. Ref. Data, Vol. 28, No. 1, 1999
e-e-
Detector
Synchrotron Light (various hν)
Z
Depth profile experiment
Photoelectrons with different kinetic energies come from different depth of the sample.
e-
Detector
X-ray (constant hν )
E kin = hν-EB-Φs
Photoelectron effect
159 160 161 162 163 164 165 166 167
0
100000
200000
300000
400000
500000
(1S, 2Fe)
Raw
Fitting
S2-
2
S2-
2
S2-
(1S, 3Fe)
B.E. (eV)
, Fully coordinated sulfur
,lower-coordinated sulfur
S 2p (KE = 200eV)
S
Acid-etched pyrite thin films on Si (in Berkeley)
11
700 705 710 715 720 725 730 735
10000
15000
20000
25000
30000
35000
40000
45000
B.E. (eV)
Inte
ns
ity
Fe 2p (KE = 200eV)
Acid-etched pyrite thin films on Si (in Berkeley)
12
155 160 165 170 1750
10000
20000
30000
40000
50000
60000
B.E. (eV)
Sulfate
Acid-etched pyrite thin films on Si after exposing to air 4 hours (in Berkeley)
S 2p (KE = 200eV)• The pyrite has been oxidized even though it only has been exposed to air for 4 hours.• The component, S-2, is the mostliable to oxidize.
13
700 705 710 715 720 725 730 735
10000
12000
14000
16000
18000
20000
22000
24000
B.E. (eV)
Iron
oxide
Acid-etched pyrite thin films on Si after exposing to air 4 hours (in Berkeley)
Fe 2p (KE = 200eV)
14
Pyrite thin films on the Si substrate
The pyrite nanowires were formed on Si substrate instead of forming pyrite thin films.
15
Setup of Ambient Pressure XPS
Adjustable experimental conditions:• Adding H2O vapor up to a few torr.
• Lower down temperature (~-20 degrees) for the sample (change humidity)
• Rise up temperature (~1000 degrees) for the sample.
16Salmeron et al, Surface Science Reports 2008, 63, 169.
Bluhm et al, Journal of Electron Spectroscopy and Related Phenomena 2006, 150, 86
Conclusion
17
• Pyrite thin films on the glass Na migrates to the surface of the pyrite thin films on the glass.
Ar sputtering is NOT a good way to remove the oxide layer due to sulfur preferential sputtering.
• Pyrite thin films on the quartz The oxidation layers on the surface can be removed by the mixed
acid but we still don’t know when the oxide layer will be saturated.
• Pyrite thin films on the Si substrate Using Synchrotron radiation, three sulfur surface chemical states
can been revealed and the component, S-2, is the most liable to oxidize.
Prof. John Hemminger and the rest of our group.
Beamline scientists in Lawrence Berkeley National Labs.
Funding Sources
Acknowledgements
18