XPS 050652

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Surface spectroscopic techniquesXPS

http://www.lasurface.comhttp://www.chem.qmul.ac.uk/surfaces/http://www.chem.qmul.ac.uk/surfaces/scc

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Surface spectroscopic techniquesEle

ctron

spec

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opies

Ion sp

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ies

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Surface spectroscopic techniques

Depth resolution

Spatial resolution

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Beam in Beam out

Surface

Principle of electron and ion-beam spectroscopy

Analyzed by spectrometer

Beam in Beam outXPS Soft X-rays (200-2000 eV) Core-level PhotoelectronsUPS Vacuum UV radiation (10-45 eV) Valence PhotoelectronsAES Electrons Auger electronsSIMS Ions Secondary ions

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X-ray photoelectron spectroscopy (XPS)

using soft x-rays (with a photon energy of 200-2000 eV) to examine core-levels

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Outline• Introduction (XPS basic principles)• Quantification.• Wide scan data (low resolution)• Narrow scan data (high resolution)• Chemical state analysis.• Sputter depth profiles.• Imaging• Applications in material sciences

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General Introduction

Photoemission was first detected by Hertz in 1887, and explainedBy Einstein in 1905.

Photoemission process

Photon energy E = hν

Einstein explained that experiment and show that light behaves like particles

photoelectrons

Light Photonshν

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• Photo-ionization the surface by monoenergetic soft X-rays (wavelength 0.1—1 nm)

• Photoelectrons are ejected from the surface

A + hν → A+ + e-

Qualitative analysis

• Identification of the elements in the sample can be made directlyfrom the kinetic energies (KE) of these ejected photoelectrons.

Quantitative analysis• The relative concentrations of elements can be determined

from the photoelectron intensities.

Basic principles a photon energy of 200-2000 eV

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(KE)

A + hν → A+ + e-

Photo-ionization (Photoelectron formation)

Conservation of energy

E(A) + hν = E(A+) + E(e-)

E(e-) = hν - [ E(A+) - E(A) ]

KE = hν - BE BE = hν - KEBinding energy (BE) allows

identification of elements

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Line widthEnergy (hν)RadiationAnode

0.85 eV1486.6 eVK(alpha)Al0.7 eV1253.6 eVK(alpha)Mg

Widely used X-ray sources

BE = hν - KE

known detected

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Palladium (Metal)

1s22s22p63s23p64s23d104p65s24d8

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wide cell/41

x 104

0

2

4

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CPS

1400 1200 1000 800 600 400 200 0Binding Energy (eV)

O 1s

C 1s

O KLLC KLL

OOHO

HOH2C

HO

OH

OO

HOH2C

HO

OH

OO

HOH2C

HO

OH

Cellulose

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The deeper-ejected electrons- recaptured or trapped in the solid- loss energy during the travel to the surface, resulting in spectrum background

Surface sensitivity of XPS

to Detector

Only electron escaped from atoms at first few layers (top 1-10 nm) show specific photoelectron peak. (No energy loss)

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Qualitative • Detection of all elements from 3Li to 103Lr (Except H and He)

“Electron spectroscopy for chemical analysis” or ESCA• Detection limits that range form 0.1 to 1.0 atom%• Chemical state analysis (Include Bonding and oxidation state)

Analysis capabilities

• Relative elemental composition of the surface (depth 1-10 nm)Semi-Quantitative

• None destructive (some damage to X-ray beam sensitive materials)• Conducting and insulating materials.• Spatial resolution for surface mapping from > 10 mm• Depth profiling capabilities (1 µm)

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Estimated Analysis Time

§ Requires 2-4 hours vacuum to pump down before analysis§ Typical operating pressure is 10-9 to 10-11 Torr.§ Could require overnight to pump down

§ Qualitative can be performed in 5 to 10 minutes § Quantitative analysis requires 1 hour to several hours depending

on the information desired

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Identification of elementsElectrons from all orbitals of an atom with BE < X-ray energy can be excited

Spin-Orbit Splitting

Characteristic photoelectron for elements

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Note: The far e- from nucleus, the less BE

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Typical XPS spectraXPS Spectroscopic mode – Wide scan (Survey scan)

Clean Ag

(High pass energy Ex. 160eV)Doublets are present in the non-s levelsas a result of spin-orbital coupling.

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Secondary Electron Emission (Auger Electron)l Low Energy “hole” producedl High Energy electron fills hole and energy is emittedl Secondary Electron can absorb excess energy

and enter vacuum (Called “Auger Electron”)

Photoemission process Auger effect

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XPS Spectroscopic mode – Region scan (High resolution) (Low pass energy Ex. 20 eV or 40 eV)

Ag 3d region spectrum

Area of 3d3/2 = 3Area of 3d5/2 5

Spin-Orbit Splitting

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XPS of the Ag 3d region of Ag based catalyst on γ-Al2O3

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F 1s

F KLL Auger

C 1sF 2s

F 1s

C C

F

F F

Fn

XPS Spectroscopic mode – Wide scan (Survey scan)PTFE

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100

FI

FI

% atomic ]A[ A

A

×

∑

=

C C

F

F F

Fn

PTFE

Semi-quantification

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XPS Spectroscopic mode – Region scan (High resolution)PTFE

C C

F

F F

Fn

C 1s F 1s

No doublet peaks from 1s photoelectrons

C-F C-F291.5 eV 689 eV

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XPS of Poly(ethylene terephthalate)

Three different C atoms

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Chemical State Identification

• Since BE of a photoelectron is sensitive to the chemical

surroundings of the atom → there is a chemical shift in BE.• XPS provides a tool to identify individual chemical states

of an element of interest.

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Relationship between oxidation state and BE

+e

Less d more attraction more BEd

Li < Be < B < C < N < O < FBE (eV) : 60 120 180 285 400 530 689

More positive charge more attraction more BE

Pt < Pt2+ < Pt4+

BE at 4f7/2 (eV) : 71-72 72-73 76

In case of same atom

In case of different atom

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For example, the C 1s BE is observed to increase as the number of O atoms bonded to C increases. As O is high EN atom, the attached C is charged positively

(C-C) < (C-O) < (O-C=O) < (O-(C=O)-O)

Potential in organic substances

285 286-287 288-289 291-291.5BE C 1s (eV)

In case of same atom

More positive charge more attraction more BE

(C-C) < (C-N) < (C-O) < (C-F)285 285-6 286-7 291-3BE C 1s (eV)

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Fitting peaks

C1 : C2 : C3 = 3 : 1 : 1

Gaussian Curve

288 eV

286 eV

285 eV

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Oxidation States of Titanium (Ti 3d)

Ti0

Ti4+

Note: two spin orbit components exhibit the same chemical shift (~ 4.6 eV)

~ 4.6 eV

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Use Reference data to identify chemical state information• http://www.lasurface.com• www.NIST.gov• XPS handbook• Areas under Gaussian curve proportional to ratio of chemical species present

High Resolution XPS and Fitting Peak

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Examples of BE (C) in polymers

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Examples of BE (C) in polymers

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XPS compositional depth profiling

1. Non-destructive depth profiling method

2. Depth profiling by erosion with inert gas ions (Destructive method)

Angle-resolved XPS (ARXPS)

Sputtering the surface within the spectrometer and use normal mode of XPS

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Id = I0 exp(-d/λsinθ)

(nm)

Beer-Lambert relationship

d

where Id = intensity of emitted photoelectron as a function of depth (d)I0 = intensity of emitted photoelectron from an infinity thick substrateλ = inelastic mean free path of electron (2-5 nm)θ = electron take-off angle relative to the surface

Depth and sensitivity

Normal mode θ = 90o

Id = I0 exp(-d/λ)

more d less I1λ

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Depth and sensitivityId = I0 exp(-d/λ)

P = Id/I0 = exp(-d/λ)

If θ = 90o

P = probability of emitted electron reaches the surface and being analyzed

Surface-sensitiveλ = 2-5 nm

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d = [ln (I/Io)] λsinθ

Angle-resolved XPS

d ∝ sinθ

Id = I0 exp(-d/λsinθ)

θ

e

θ

e e

3λ 3λ 3λ

Analysis Depth (d) = 3λsinθ

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3λ 0.8λ2.1λ 0.26λIf λ= 5 nm

15 nm 10.5 nm 4 nm 1.3 nmAnalysis depth(> 95% signal)

Sampling depth as a function of electron take-off angle

θ

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ARXPS of Thiol treatedGaAs(100)

R-SHR-SH

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AR-XPS of a Silicon Wafer with a Native Oxide

Si 2p3/2 (metal) BE ~ 99.0 eVSi 2p3/2 (SiO2) BE ~ 103.0 eV

Si

SiO2

TOA

More surface sensitive, more SiO2 is detected.

Si0SiO2

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Depth profiling by sputtering (Destructive method)

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Depth profiling by sputtering (Destructive method)

XPS depth profileof SiO2 on Si

Wide scan – quantitative analysis

If sputter rate is known, thickness of the surface layers can be calculated.

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Depth profiling by sputtering (Destructive method)

XPS depth profile of SiO2on Si

Si = 99.3 eVSiO2 = 103.3 eV

Sputter time

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XPS- Instrumentation

http://en.wikipedia.org/wiki/Image:System2.gif

1. Vacuum system2. Sample preparation3. X-ray source4. Electron energy analyzer5. Detection system

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Sample preparation§ Solid samples are preferable. (Films, powder (pressed))§ Stable in the UHV chamber§ Sample can be conducting or insulating.

Mounting the sample

ClipsDouble sided adhesive tape

Single barSpecial holder stubs/cups are available for powder samples.

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X-ray sources for XPS

< 5000 eV

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X-ray sources for XPSFormation of X-ray

The energy of X-ray (Photon) mustbe high enough (at least 900 eV)to excite the electron in K-shell of all elements.

Most popular anode materials are Al and Mg

Bombard metal target by high energy electrons –then photon with known energy are emitted

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Al KαMg Kα

Photons with known energy

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Monochromatic X-ray

X-ray with a specific energy

Monochromation

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X-ray Sources

2.1 eV5417K(alpha)Cr2.0 eV4510.9K(alpha)Ti2.6 eV2984.3L(alpha)Ag1.6 eV2042.4L(alpha)Zr

0.85 eV1486.6K(alpha)Al0.7 eV1253.6K(alpha)Mg

Line WidthEnergy (eV)RadiationMaterial

http://www.thermo.com/eThermo/CDA/Products/Product_Detail/0,1075,15955-158-X-1-13080,00.html

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Electron energy analyzer1. Double Cylindrical Mirror Analyzer (CMA)

Low resolution dE/E ~1%Outer - Negative potentialInner – GroundOnly e- of a fixed energy can be passed and detected

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2. Hemispherical Mirror Analyzer

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l A potential difference between the cylinders or hemispheres allow only electrons with specific kinetic energies to make it to the electron detector.

l Varying potentials measure different kinetic energiesl Computer calculates binding energy

High resolution dE/E < 0.5 %

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Kratos Ultra (UK)

www.latrobe.edu.au

Sample Mounting

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Applications of XPS in Material Science

Metallurgy and Corrosion science1. Interaction of a metal surface with its environment and

the formation of passive layer (Oxide film)2. The breakdown of the surface film by a localized phenomenon

such as pitting.3. Elemental distributions on mineral particle surfaces4. Depth profiles of corroded materials

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MetallurgyStainless steel

Surface Cr rich Cr-oxide

Bulk Cr 18%, Ni 8%, Fe 72%

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http://www.eaglabs.com/techniques/analytical_techniques/xps_esca.php

WW Oxide

Identification of Pitting corrosion

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Corrosion Problems on Steel Wires

a significantly higher concentration of iron in the corroded areas coupled with lower levels of calcium and sodium which are attributed to the drawing lubricants(Ca and Na stearates).

XPS images of a corroded stainless steel wire

Fe

Cl

Ca

C

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Applications of XPS in Material ScienceMicroelectronics and Semiconductor Materials

XPS depth profiling is used to confirm the layer uniformity

Semiconductor distributed Bragg Reflector Stack

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• Location of ppm/ppb levels of elements in the microstructure • Characterisation of ceramic compositional change with depth • Analysis of nm thick surface layers

CeramicsApplications of XPS in Material Science

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Thin film coatings on glass

Architectural glass, lenses, mirrors, and many other products are coated to provide specific optical properties. These coatings are often a stack of thin layers that can be characterized using Depth-profiling XPS to verify the composition of the layers, detect contaminants, and estimate layer thickness.

Fig. 1: XPS sputter depth profile of an architectural glass coating

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Example – Vanadium PhosphousOxide Catalyst Sample

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Polymer – surface modification

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C60 sputter depth profiling of polymer films and surfacesRecently C60 sputtering has been shown to be very effective for removing surface contaminants and sputtering through polymer films while causing minimal chemical damage.

Fig. 2: A C60 sputter depth profile through a wax layer on polyurethane showsthe ability to sputter through organic and polymer materials without causing significant chemical damage to the materials.