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