X-Ray Photoelectron Spectroscopy XPS Mark Engelhard

1 Physical Electronics Quantera XPS High Energy Resolution Focused X-ray Beam Capability

EMSL XPS InstrumentationXPS High XPS glove box Glove pressure interface box cell XPS Tube Glove box furnace

UHV transfer

Catalysis reaction and processing chamber with inert atmosphere glove box connected to a PHI Quantera Scanning X-ray Microprobe system. • High pressure reactor with heating up to 800°C • Vacuum/Atmosphere tube furnace with heating up to 1K°C

• Currently six gases including: H2, O2, He, N2, NO and CO. • Gas with liquid vapor exposure capabilities. • Pfeiffer OMNI star gas analysis system. 2 PHI 5000 VersaProbe XPS

PHI VersaProbe monochromatic Al Kα non-monochromatic UPS capability C60 ion gun Side chamber

3 Kratos Axis DLD X-ray Photoelectron Spectrometer

Frozen sample

Liquid nitrogen cooling station

 Sample preparation in glove box  Frozen from room temperature to liquid nitrogen temperature within 10 minutes  Maintained at liquid nitrogen temperature during XPS measurements

4 History (XPS)

X-ray Photoelectron Spectroscopy (XPS) was developed in the mid-1960s by Kai Siegbahn and his research group at the University of Uppsala, Sweden. The technique was first known by the acronym ESCA (Electron Spectroscopy for Chemical Analysis). The variation of photopeak energy with chemistry allowed the development of this surface sensitive chemical analysis method.

The advent of commercial manufacturing of surface analysis equipment in the early 1970s enabled the placement of equipment in laboratories throughout the world. In 1981, Siegbahn was awarded the Nobel Prize for Physics for his work with XPS.

5 Outline

• Introduction to XPS (basic principles) • Quantification • Energy resolution and count rates • Wide scan data (low energy resolution spectra) • Narrow scan data (high energy resolution spectra) • Chemical state analysis • Detection sensitivity • Depth profiles, Line scans, and Elemental Maps • Sample neutralization • Sample holders • Useful web sites and references

6 Introduction to XPS

7 Published with permission from Rina Beltran, Marketing Specialist with Charles Evans & Associates Introduction to XPS

• X-ray photoelectron spectroscopy works by irradiating a sample material with monoenergetic soft x-rays causing electrons to be ejected. • Identification of the elements in the sample can be made directly from the kinetic energies of these ejected photoelectrons. • The relative concentrations of elements can be determined from the photoelectron intensities. • An important advantage of XPS is its ability to obtain information on chemical states from the variations in binding energies, or chemical shifts, of the photoelectron lines.

8 Introduction to XPS -- Analysis capabilities

► Elements detected from Li to U ► Quantitative ► Chemical state identification ► Valence band electronic structure ► Conducting and insulating materials ► Surface sensitivity from 5 to 75 angstroms ► Detection limits that range form 0.01 to 0.5 atom percent ► Chemical/Chemical state distributions ● Mapping (x,y) with <10 µm resolution ● Depth (z) - Sputter depth profiling - Angle dependent depth profiling - Layer information from electron energy loss

9 Introduction to XPS -- XPS emission process

An incoming photon causes the ejection of the photoelectron Photoelectron

Efermi level

3/2

2p 1/2 Binding Energy (BE) 2s

1s KE = hν - BE - eΦ Kinetic Energy = hν (Al Kα 1486.7 eV) – Binding Energy

10 Introduction to XPS -- Basic principles

The relationship governing the interaction of a photon with a core level is:

KE = hν - BE - eΦ KE Kinetic Energy of ejected photoelectron hν characteristic energy of X-ray photon BE Binding Energy of of the atomic orbital from which the electron originates. eΦ spectrometer work function

11 Introduction to XPS -- Hemispherical Analyzer

Energy Analyzer

Input lens

Al Kα (1486.7 eV) Photon source Microchannel plate with anode electrodes or multiple channel electron multipliers

Escaping photoelectrons

Sample

12 Introduction to XPS -- Surface sensitivity

X-rays Escaping photoelectrons

13 Introduction to XPS -- Surface sensitivity

X-ray Photoelectron Spectroscopy Auger Electron Spectroscopy Secondary Ion Mass Spectrometry Surface Enhanced Raman Spectroscopy Ultraviolet Photoelectron Spectroscopy Low Energy Electron Diffraction

Surface to volume ratio

A 3 nm iron particle has 50% atoms on the surface A 10 nm particle has 20% on the surface A 30 nm particle has 5% on the surface

14 Introduction to XPS -- Analysis objective

Composition Ax By Cz ∆

Surface Layer (d) thickness

Bulk

• Composition (Ax By Cz) • (d) Thickness (depth resolution) • ∆ Lateral resolution (spatial resolution)

15 Quantification

• Quantitative data can be obtained from peak heights or peak areas. • The following building blocks are used to provide accurate quantification: - A standardized set of sensitivity factors - The transmission function of the spectrometer - Corrections for geometric asymmetry related to the angle between the X-ray source and the analyzer input lens.

16 Count rate as function of energy resolution

10,000,000 95W, HP mode 45°

1,000,000 20W,100um, 75° 20W, 100um, 45° Counts 100,000

5/2 Ag3d 10,000

1,000 0 40 80 120 160 200

Pass Energy (eV)

17 XPS of Poly(ethylene terephthalate)

3 3 2 2 ( ) O C 1 C O CH2CH2 n O O

Atom % Atomic % O1s -

C = 71 CH = 63% 1 O = 29 C-O = 20% CH C1s -

O=C-O = 17%

c/s c/s 3 O=C-O 2 C-O

1000 800 600 400 200 0 300 295 290 285 280 Binding energy (eV) Binding energy (eV) Quantitative elemental information Chemical state information

18 Wide scan data (low energy resolution spectra) -- Ag photoelectron and Auger lines

Ag 3d5 Ag -

3d Ag 3d3 Ag - Auger

c/s

3p Ag 3p3 Ag

Ag MNN -

Ag 3p1 Ag 4d -

Ag MNN -

Ag MNN 3s -

Ag 3s Ag 4s -

Ag 4d Ag -

Ag 4p Ag Ag 4s Ag - -

1200 1000 800 600 400 200 0 Binding Energy (eV)

19 Narrow scan data (high energy resolution spectra) -- Ag 3d region -- Ag catalyst on γ-Al2O3

1.2 AT% Ag 3d5/2

0.9 AT% Ag 3d3//2

0.7 AT% c/s

0.5 AT%

0.3 AT%

382 378 374 370 366 362 Binding Energy (eV)

20 XPS spectra – P type absorber exposed to KCN prior to deposition of an N type partner

Se 3d5/2 Se

KCN treated

c/s

Se2O3

66 64 62 60 58 56 54 52 50 Binding Energy (eV)

Work in collaboration with Peter Eschbach from Washington State University Ph.D. Thesis by Peter Eschbach, “Investigation of Buffer Layers in Copper Indium Gallium Selenium Solar Cells” (2002)

21 High energy resolution spectra -- Oxidized and reduced CeO2

4+ 2 Ce 3d5/2 4f

3+ 1 Ce 3d5/2 4f

3+ 2 Ce 3d5/2 4f

4+ 1 Ce 3d5/2 4f NormalizedIntensity

895 890 885 880 875 Binding Energy (eV)

M. A. Henderson, C. L. Perkins, M. H. Engelhard, S. Thevuthasan, and C.H.F. Peden, “Redox Properties of Water on the Oxidized and Reduced Surfaces of CeO2(111)” Surface Science, 526:1-2 (2003) 1-18. 22 Ti 2p XPS spectra for Ti and TiO2

2p 2p3/2 1/2 Ti metal peak @ 454 eV

475 470 465 460 455

2p3/2 2p1/2 +3 TiO2 with some reduced Ti

475 470 465 460 455

2p3/2 2p 1/2 TiO2 peak @ 458.8 eV

475 470 465 460 455 Binding Energy (eV)

23 High energy resolution spectra -- SnO2 doped hematite

Sn 3d 5/2 1% by mole 0.3 atom %

c/s 0.1% by mole 0.02 atom %

490 489 488 487 486 485 484 Binding Energy (eV) Work in collaboration with Barbara Balko & Kathleen Clarkson from Lewis & Clark College “The Effect of Doping with Ti(IV) and SN(IV) on Oxygen Reduction at Hematite Electrodes” J. of Electrochemical Society, (2001) 24 148 XPS depth profile – SiO2 on Si

100 SiO /Si 90 2 interface

80 O 1s 70

50 40 Si 2p

Silicon Oxide 30

Atomic Concentration (%) 20 Silicon (substrate) 10

-10 0 10 20 30 40 50 60 70 Sputter Time (min)

25 XPS depth profile – SiO2 on Si

Si2p

Si = 99.3 eV

SiO2 = 103.3 eV SiO2

c/s

110 108 106 104 102 100 98 96 94 Binding Energy (eV)

26 XPS depth profile -- Multilayer Ni/Cr/CrO/Ni/Cr on Si sample

100 Si 2p 90 Ni 2p3 Cr 2p3 Ni 2p3 Cr 2p3 80 70 Cr 2p3 Nickel (30 nm) 60 Chromium (32 nm) 50 O1s Chromium Oxide (32 nm) Nickel (30 nm) 40 Chromium (30 nm) 30 (%) Atomic Concentration Silicon (substrate) 20 10

0 0 10 20 30 40 50 Sputter Time (min)

27 XPS line scan – Patterned polymer film using 20 um diameter Al Kα X-ray beam

100

90 80 XPS line scan 70 C1s 60

50 O1s 40

30 Si2p 20 N1s 10 0 0 50 100 150 200 250 300 Distance (µm) 100 µm Secondary X-ray image

Work in collaboration with Mingdi Yan and Michele Bartlett from Portland State University “Micro/Nanowell Arrays Fabricated from Covalently Immobilized Polymer Thin Films on Flat Substarte” Nano Letters V2, N4 (2002)

28 XPS Elemental Map – ITO circular patterns

InSn 20 µm

20 µm

29 Charge neutralization for insulating samples

X-ray beam

------+ + + + ------

Photoemitted electrons leave local positive charges on the surface

30 Charge neutralization for insulating samples

X-ray beam

------+ - - + ------

Low-energy electrons from a cold cathode flood gun alleviates most positive charges

31 Charge neutralization for insulating samples

X-ray beam

Flooding the surface with both positive ions and negative electrons provide a uniform surface potential

32 Examples of sample holders

TiO2 and SrTiO3 crystals Self-assembled monolayers on Au/Si

255 PHOTO 255

16.5 mm 17 10 75 mm High temperature sample platen Standard sample platen

33 Examples of sample handling

High temperature sample platen

sample vacuum transfer chamber

34 Examples of sample holders

35 Useful web sites

• PNNL EMSL: www.emsl.gov • AVS Science & Technology Society: www.avs.org • AVS Surface Science Spectra: www.avs.org/literature.sss.aspx • Evans Analytical Group: www.cea.com • NIST X-ray Photoelectron Spectroscopy Database: www.srdata.nist.gov/sps/ • NIST Electron Inelastic-Mean-Free-Path Database: www.nist.gov/srd/nist71.htm • QUASES-IMFP-TPP2M QUASES-Tougaard Inc.: www.quases.com • Surfaces & Interfaces Section, National Physical Lab. www.npl.co.uk/npl/cmmt/sis • XPS MultiQuant www.chemres.hu/aki/XMQpages/XMQhome.htm • ASTM International: www.astm.org

36 Useful XPS references

1. Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, edited by D. Briggs and J.T. Grant. IM Publications and SurfaceSpectra Limited (2003).

2. Practical Surface Analysis (second edition) V1, edited by D. Briggs and M. P. Seah. John Wiley and Sons Ltd. (1990).

3. J.F. Moulder, W.F. Stickle, P.E. Sobol and K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy. Physical Electronics Inc., Eden Prairie (1995).

4. G. Beamson and D. Briggs, High Resolution XPS of Organic Polymers-The Scientia ESCA 300 Database. John Wiley & Sons, Chichester (1992).

5. B.V. Crist, Handbooks of Monochroamtic XPS Spectra, 5 Volume Series. XPS International, Kawasaki (1997).

6. Encyclopedia of Materials Characterization, edited by C.R. Brundle, C.A. Evans, S. Wilson. Butterworth-Heinemann (1992).

7. N. Ideo, Y. Iijima, N. Niimura, M. Sigematsu, T. Tazawa, S. Matsumoto, K. Kojima, Y. Nagasawa, Handbook of X-ray Photoelectron Spectroscopy. JEOL, Tokyo, Japan (1991).