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