X-Ray Photoelectron Spectroscopy (XPS)

Nanoparticle Surface Characterization by X-Ray Photoelectron Spectroscopy

What Leads to Unsual Properties?

Any solid or liquid has 2 components:

•surface

•bulk

Shrinking Down to the Nanoscale

5%

10%

50%

100%

Properties of surface atoms/molecules now

affect the overall properties.

C60

XPS Background XPS technique is based on Einstein’s idea about the

photoelectric effect, developed around 1905

The concept of photons was used to describe the ejection of electrons from a surface when photons were impinged upon it

During the mid 1960’s Dr. Siegbahn and his research

group developed the XPS technique.

In 1981, Dr. Siegbahn was awarded the Nobel Prize in Physics for the development of the XPS technique

X-Rays Irradiate the sample surface, hitting the core electrons (e-) of the

atoms.

The X-Rays penetrate the sample to a depth on the order of a micrometer.

Useful e- signal is obtained only from a depth of around 10 to 100 Å on the surface.

The X-Ray source produces photons with certain energies: MgK photon with an energy of 1253.6 eV AlK photon with an energy of 1486.6 eV

Normally, the sample will be radiated with photons of a single energy (MgK or AlK). This is known as a monoenergetic X-Ray beam.

X-ray Photoelectron SpectroscopySmall Area Detection

X-ray BeamX-ray Beam

X-ray penetration X-ray penetration depth ~1depth ~1m.m.Electrons can be Electrons can be excited in this excited in this entire volume.entire volume.

X-ray excitation area ~1x1 cmX-ray excitation area ~1x1 cm22. Electrons . Electrons are emitted from this entire areaare emitted from this entire area

Electrons are extracted Electrons are extracted only from a narrow solid only from a narrow solid angle.angle.

1 mm1 mm22

10 nm10 nm

XPS spectral lines are XPS spectral lines are identified by the shell from identified by the shell from which the electron was which the electron was ejected (1s, 2s, 2p, etc.).ejected (1s, 2s, 2p, etc.).

The ejected photoelectron has The ejected photoelectron has kinetic energy:kinetic energy:

KE=hv-BE-KE=hv-BE- Following this process, the Following this process, the

atom will release energy by atom will release energy by the emission of an Auger the emission of an Auger Electron.Electron.

Conduction BandConduction Band

Valence BandValence Band

L2,L3L2,L3

L1L1

KK

FermiFermiLevelLevel

Free Free Electron Electron LevelLevel

Incident X-rayIncident X-rayEjected PhotoelectronEjected Photoelectron

1s1s

2s2s

2p2p

The Photoelectric Process

L electron falls to fill core level L electron falls to fill core level vacancy (step 1).vacancy (step 1).

KLL Auger electron emitted to KLL Auger electron emitted to conserve energy released in conserve energy released in step 1.step 1.

The kinetic energy of the The kinetic energy of the emitted Auger electron is: emitted Auger electron is:

KE=E(K)-E(L2)-E(L3).KE=E(K)-E(L2)-E(L3).

Conduction BandConduction Band

Valence BandValence Band

L2,L3L2,L3

L1L1

KK

FermiFermiLevelLevel

Free Free Electron Electron LevelLevel

Emitted Auger ElectronEmitted Auger Electron

1s1s

2s2s

2p2p

Auger Relation of Core Hole

Instrumentation: How are measurements made?

Essential components: Sample: usually 1 cm2

X-ray source: Al: 1486.6 eV; Mg 1256.6 eV

Electron Energy Analyzer: 100 mm radius concentric hemispherical analyzer; vary voltages to vary pass energy.

Detector: electron multiplier (channeltron)

Electronics, Computer Note: All in ultrahigh vacuum

(<10-8 Torr) (<10-11 atm) State-of-the-art small spot

ESCA: 10 mm spot size.

5 4 . 7

X-rayX-raySourceSource

ElectronElectronOpticsOptics

Hemispherical Energy AnalyzerHemispherical Energy Analyzer

Position Position Sensitive Sensitive

Detector (PSD)Detector (PSD)

Inner SphereInner Sphere

SampSamplele

ComputeComputer Systemr System

Analyzer Analyzer ControlControl

Multi-Channel Multi-Channel Plate Electron Plate Electron MultiplierMultiplierResistive Resistive Anode Anode EncoderEncoder

Lenses for Lenses for Energy Energy Adjustment Adjustment (Retardation)(Retardation)

Lenses for Lenses for Analysis Area Analysis Area DefinitionDefinition

Position Position Address Address ConverterConverter

XPS Energy Scale

The XPS instrument measures The XPS instrument measures the kinetic energy of all collected the kinetic energy of all collected electrons. The electron signal includes electrons. The electron signal includes contributions from both photoelectron contributions from both photoelectron and Auger electron lines.and Auger electron lines.

Why the Core Electrons? An electron near the Fermi level is far from the nucleus,

moving in different directions all over the place, and will not carry information about any single atom. Fermi level is the highest energy level occupied by an

electron in a neutral solid at absolute 0 temperature. Electron binding energy (BE) is calculated with respect to the

Fermi level.

The core e-s are local close to the nucleus and have binding energies characteristic of their particular element.

The core e-s have a higher probability of matching the energies of AlK and MgK.

Core e-

Valence e-

Atom

Binding Energy (BE)

These electrons are attracted to the

proton with certain binding energy x

This is the point with 0 energy of attraction

between the electron and the nucleus. At this point the electron is free from

the atom.

The Binding Energy (BE) is characteristic of the core electrons for each element. The BE is determined by the attraction of the electrons to the nucleus. If an electron with energy x is pulled away from the nucleus, the attraction between the electron and the nucleus decreases and the BE decreases. Eventually, there will be a point when the electron will be free of the nucleus.

0

x

p+

B.E.

Energy Levels

Vacumm Level

Fermi Level

Lowest state of energy

BE

Ø, which is the work function

At absolute 0 Kelvin the electrons fill from the lowest energy states up. When the electrons occupy up to this level the neutral solid is in its “ground state.”

XPS Instrument

XPS is also known as ESCA (Electron Spectroscopy for Chemical Analysis).

The technique is widely used because it is very simple to use and the data is easily analyzed.

XPS works by irradiating atoms of a surface of any solid material with X-Ray photons, causing the ejection of electrons.

University of Texas at El Paso, Physics DepartmentFront view of the Phi 560 XPS/AES/SIMS UHV System

XPS Instrument

The XPS is controlled by using a computer system.

The computer system will control the X-Ray type and prepare the instrument for analysis.

University of Texas at El Paso, Physics DepartmentFront view of the Phi 560 XPS/AES/SIMS UHV System and

the computer system that controls the XPS.

XPS Instrument

The instrument uses different pump systems to reach the goal of an Ultra High Vacuum (UHV) environment.

The Ultra High Vacuum environment will prevent contamination of the surface and aid an accurate analysis of the sample.University of Texas at El Paso, Physics Department

Side view of the Phi 560 XPS/AES/SIMS UHV System

XPS Instrument

X-Ray Source

Ion Source

SIMS Analyzer

Sample introductionChamber

Sample Introduction Chamber

The sample will be introduced through a chamber that is in contact with the outside environment

It will be closed and pumped to low vacuum.

After the first chamber is at low vacuum the sample will be introduced into the second chamber in which a UHV environment exists.

First ChamberSecond Chamber UHV

Diagram of the Side View of XPS System

X-Ray source

Ion source

Axial Electron Gun

Detector

CMA

sample

SIMS Analyzer

Sample introduction Chamber

Sample Holder

Ion PumpRoughing Pump Slits

X-ray Photoelectron Spectrometer

5 4 . 7

Magnetic ShieldShieldOuter SphereOuter Sphere

Position ComputerPosition Computer

X-rayX-raySourceSource

ElectronElectronOpticsOptics

Hemispherical Energy AnalyzerHemispherical Energy Analyzer

Position Sensitive Position Sensitive Detector (PSD)Detector (PSD)

Inner SphereInner Sphere

SampleSample

Computer Computer SystemSystem

Analyzer ControlAnalyzer Control

Multi-Channel Multi-Channel Plate Electron Plate Electron MultiplierMultiplierResistive Anode Resistive Anode EncoderEncoder

Lenses for Energy Lenses for Energy Adjustment Adjustment (Retardation)(Retardation)

Lenses for Analysis Lenses for Analysis Area DefinitionArea Definition

Position Address Position Address ConverterConverter

How Does XPS Technology Work?

A monoenergetic x-ray beam emits photoelectrons from the surface of the sample.

The X-Rays either of two energies:

Al Ka (1486.6eV) Mg Ka (1253.6 eV)

The x-ray photons penetrate about a micrometer of the sample

The XPS spectrum contains information only about the top 10 - 100 Ǻ of the sample.

Ultrahigh vacuum environment to eliminate excessive surface contamination.

Cylindrical Mirror Analyzer (CMA) measures the KE of emitted e-s.

The spectrum plotted by the computer from the analyzer signal.

The binding energies can be determined from the peak positions and the elements present in the sample identified.

Why Does XPS Need UHV? Contamination of surface

XPS is a surface sensitive technique.• Contaminates will produce an XPS signal and lead to incorrect

analysis of the surface of composition.

The pressure of the vacuum system is < 10-9 Torr

Removing contamination To remove the contamination the sample surface is bombarded

with argon ions (Ar+ = 3KeV). heat and oxygen can be used to remove hydrocarbons

The XPS technique could cause damage to the surface, but it is negligible.

X-Rays on the Surface

Atoms layers

e- top layer

e- lower layer with collisions

e- lower layer but no collisions

X-Rays

Outer surface

Inner surface

X-Rays on the Surface The X-Rays will penetrate to the core e- of the atoms in the

sample.

Some e-s are going to be released without any problem giving the Kinetic Energies (KE) characteristic of their elements.

Other e-s will come from inner layers and collide with other e-s of upper layers These e- will be lower in lower energy. They will contribute to the noise signal of the spectrum.

X-Rays and the ElectronsX-RayElectron without collision

Electron with collision

The noise signal comes from the electrons that collide with other electrons of different layers. The collisions cause a decrease in energy of the electron and it no longer will contribute to the characteristic energy of the element.

X-Rays and Auger Electrons

When the core electron leaves a vacancy an electron of higher energy will move down to occupy the vacancy while releasing energy by: photons Auger electrons

Each Auger electron carries a characteristic energy that can be measured.

Two Ways to Produce Auger Electrons

1. The X-Ray source can irradiate and remove the e- from the core level causing the e- to leave the atom

2. A higher level e- will occupy the vacancy. 3. The energy released is given to a third higher

level e-. 4. This is the Auger electron that leaves the atom.

The axial e- gun can irradiate and remove the core e- by collision. Once the core vacancy is created, the Auger electron process occurs the same way.

Auger Electron

Free e-

e- Vacancy

e- of high energy that will occupy the vacancy of the core levele- released to

analyze1

1, 2, 3 and 4 are the order of steps in which the e-s will move in the atom when hit by the e- gun.

e- gun

23

4

Auger Electron Spectroscopy (AES)

Atom layers

e- released fromthe top layer

Outer surface

Inner surfaceElectron beam from the e- gun

Cylindrical Mirror Analyzer (CMA)

Slit

Detector

Electron Pathway through the CMA

0 V

+V

0 V 0 V

0 V

+V

+V

+V

X-RaysSource

SampleHolder

Equation KE=hv-BE-Ø

KE Kinetic Energy (measure in the XPS spectrometer)

hv photon energy from the X-Ray source (controlled)

Ø spectrometer work function. It is a few eV, it gets more complicated because the materials in the instrument will affect it. Found by calibration.

BE is the unknown variable

The equation will calculate the energy needed to get an e- out from the surface of the solid.

Knowing KE, hv and Ø the BE can be calculated.

KE versus BE

E E E

KE can be plotted depending on BE

Each peak represents the amount of e-s at a certain energy that is characteristic of some element.

1000 eV 0 eV

BE increase from right to left

KE increase from left to right Binding energy

# o

f ele

ctr

on

s

(eV)

Interpreting XPS Spectrum: Background The X-Ray will hit the e-s

in the bulk (inner e- layers) of the sample

e- will collide with other e- from top layers, decreasing its energy to contribute to the noise, at lower kinetic energy than the peak .

The background noise increases with BE because the SUM of all noise is taken from the beginning of the analysis. Binding energy

# o

f ele

ctr

on

sN1

N2

N3

N4

Ntot= N1 + N2 + N3 + N4

N = noise

XPS Spectrum The XPS peaks are sharp.

In a XPS graph it is possible to see Auger electron peaks.

The Auger peaks are usually wider peaks in a XPS spectrum.

XPS Spectrum

O 1s

O becauseof Mg source

CAl

Al

O 2s

O Auger

Sample and graphic provided by William Durrer, Ph.D.Department of Physics at the Univertsity of Texas at El Paso

Aluminum foil

Auger Spectrum

Characteristic of Auger graphsThe graph goes up as KE increases.

Sample and graphic provided by William Durrer, Ph.D.Department of Physics at the Univertsity of Texas at El Paso

300 295 290 285 280 275

0

200

400

600

800

1000

1200

Co

un

ts /

s

C(1s) Binding Energy (eV)

Peak Position Area % C1s Carbon- ID

1 285.50 1457.1 52% Pristine C60

2 287.45 489.75 18% Mono-oxidized C

3 289.73 836.67 30% Di-oxidized C

Identification of XPS Peaks

The plot has characteristic peaks for each element found in the surface of the sample.

There are tables with the KE and BE already assigned to each element.

After the spectrum is plotted you can look for the designated value of the peak energy from the graph and find the element present on the surface.

X-rays vs. e- Beam

X-Rays Hit all sample area simultaneously

permitting data acquisition that will give an idea of the average composition of the whole surface.

Electron Beam It can be focused on a particular area

of the sample to determine the composition of selected areas of the sample surface.

XPS Technology Consider as non-

destructive because it produces soft

x-rays to induce photoelectron emission from the sample surface

Provide information about surface layers or thin film structures

Applications in the industry: Polymer surface Catalyst Corrosion Adhesion Semiconductors Dielectric materials Electronics packaging Magnetic media Thin film coatings