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XPS Simplified
2. Characterizing polymers with X-ray Photoelectron Spectroscopy (XPS)
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Webinar overview
• Introduction• Why are we interested in surfaces?• How XPS assist with surface
problems?• What is XPS?
• Theory• Instrumentation• The analysis process
• What can we learn about polymers with XPS?
• Elemental information• Chemical information• Application examples
• Summary
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Why are we interested in the surface of polymers?
• Electrical• OLEDs• Organic PV Cells• Contact
resistance• Multilayer
superconductors
• Physical• Plasma-
treatment• Scratch
resistance
• Low friction coatings
• Composite materials
• Biological• Implant
acceptance• Plasma treatment• Cell promotion
• Chemical activity• Adhesion• Coatings
• The surface of a solid is the point where it interacts with it’s environment.• Many properties can all depend on the first few atomic layers of a material.
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XPS of polymers
By using XPS, analysts can investigate a wide range of surface problems on polymer systems, including:
Chemical identificationMeasuring quantified chemical information
Contaminant identificationChecking component cleanliness after manufacturing
Interfacial chemistryUsing depth profiling to identify layer chemistry at interfaces
Surface homogenietyCreating chemical images of the surface to determine film uniformityIdentifying surface features
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What is XPS?
• Through the photoelectric effect, core electrons are ejected from the surface irradiated with the X-ray beam.
• These have a characteristic kinetic energy depending on the element, orbital and chemical state of the atom
• Layers up to ~10 nm thick can be probed directly.
• Thicker layers can be analysed by ion beam depth profiling
EBE = hn - EKE
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XPS instrumentation
• UHV System• Allows longer photoelectron path length• Ultra-high vacuum keeps surfaces clean
• Electron analyser• Lens system to collect photoelectrons• Analyser to filter electron energies• Detector to count electrons
• X-ray source• Typically Al Ka radiation• Monochromated using quartz crystal
• Low-energy electron flood gun• Analysis of insulating samples
• Ion gun• Sample cleaning• Depth profiling• For polymers, cluster ion sources may be
required
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XPS instrumentation
H–
H+
Photoelectrons
Detector
KE = EP
KE < EP
KE > EPEBE = hn - EKE
• UHV System• Allows longer photoelectron path length• Ultra-high vacuum keeps surfaces clean
• Electron analyser• Lens system to collect photoelectrons• Analyser to filter electron energies• Detector to count electrons
• X-ray source• Typically Al Ka radiation• Monochromated using quartz crystal
• Low-energy electron flood gun• Analysis of insulating samples
• Ion gun• Sample cleaning• Depth profiling• For polymers, cluster ion sources may be
required
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XPS instrumentation
Hemispherical analyser
Detector
Ion gun
Flood gun
X-ray source
Monocrystal
Electron transfer lens
• UHV System• Ultra-high vacuum keeps surfaces clean• Allows longer photoelectron path length
• Electron analyser• Lens system to collect photoelectrons• Analyser to filter electron energies• Detector to count electrons
• X-ray source• Typically Al Ka radiation• Monochromated using quartz crystal
• Low-energy electron flood gun• Analysis of insulating samples
• Ion gun• Sample cleaning• Depth profiling• For polymers, cluster ion sources may be
required
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The problem with analysing insulators
Spectrum of an insulator without charge compensation
Spectrum of an insulator with charge compensation
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Binding Energy (eV)
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sBinding Energy (eV)
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+-
The problem with analysing insulators
-
• No problem with conductors!
• X-rays irradiate the surface of the sample
• Ejected photoelectrons leave “core holes” of positive charge.
• In a conductor these are replaced by e- conducted thorough the sample from ground.
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+-
The problem with analysing insulators
• With insulators charging of the surface occurs
• X-rays irradiate the surface of the sample
• Ejected photoelectrons leave “core holes” of positive charge.
• There is no path to replace the photoelectrons, and so the surface charges
+ + ++
+ + + ++
X
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How does the charge compensation system work?
• A beam of low energy electrons is directed at the analysis position and surrounding area
• This neutralises the positive charge that builds up due to the loss of photoelectrons
• An excess of electrons is supplied to ensure that small fluctuations do not affect performance
+-
-
X
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Sample handling
• Samples need to be handled carefully to prevent contamination from fingerprints, gloves, tools etc
• Samples also need to be vacuum compatible
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XP spectra – survey spectraElemental identification
• Elemental identification• Which elements are present?• Can detect all elements except for H
• Elemental quantification• How much of an element is present?• Detection limit >0.05% for most elements• Allows determination of stoichiometry• Peak area converted using “sensitivity
factors” to give At%
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Binding energy / eV
Poly(ethylene terephthalate), PET
C1sO1s
Elemental quantification of PETsample
Element At%
C 71
O 29C Auger
O Auger
O2s
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Nylon elemental analysis
• NB Spectra offset for clarity
01002003004005006007008009001000110012001300
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Binding Energy (eV)
Nylon 6-12
Nylon 6-9
Nylon unknown
C1s
N1s
O1s
O KLLN KLL
C KLL
Atomic %C N O
Unknown 76 12 12Nylon(6,9) 79 11 11
Nylon(6,12) 82 9 9
• R2 is a C4 unit in each case
• R1 can be calculated, based on the measured At%
Expect Calc R2Unknown ??? 6.4 2
Nylon(6,9) 9 9.0 5
Nylon(6,12) 12 12.2 8
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XP spectra – region spectraElemental identification
• Chemical state quantification• Chemical environment • Functional groups
Poly(ethylene terephthalate), PET
n
O
OO
O CC CC
π-> π*shake-up
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Binding Energy (eV)
C1s Scan
Binding Energy (eV)
O1s Scan
Co
un
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sπ-> π*shake-up
528530532534536538540542
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C1s chemical shifts
280282284286288290292294296298
C1s Scan - PE
C-C
285 284286287288289290291292293
Binding Energy (eV)
C-C
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Binding Energy (eV)
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C1s chemical shifts
285 284286287288289290291292293
Binding Energy (eV)
C-CC-N
C=O
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C1s Scan – Nylon 6,9
C-CC-NC=O
Co
un
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s
Binding Energy (eV)
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C1s chemical shifts
280282284286288290292294296298
C1s Scan - PolycarbonateC
1s (
O-(
C=
O)-
O)
C1s
(sh
ake-
up)
C=CC-CC-OO-(C=O)-O
285 284286287288289290291292293
Binding Energy (eV)
C-C
C=C
C-NC-O
C=O
Co
un
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s
Binding Energy (eV)
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C1s chemical shifts
280282284286288290292294296298
C1s Scan - PVF
*C-CFC-F(C-C)
285 284286287288289290291292293
Binding Energy (eV)
C-C
C=C
C-NC-O *C-CFC-F
C=O
Co
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s
Binding Energy (eV)
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C1s chemical shifts
280282284286288290292294296298
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Binding Energy (eV)
C1s Scan - PTFE
CF2
285 284286287288289290291292293
Binding Energy (eV)
C-C
C=C
C-NC-O *C-CFC-F
CF2
CF3
C=O
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Polyethylene & polypropylene
• Poly-alkenes (or olefins) tends to have the same C1s spectra
• This makes them difficult to differentiate from one another using the core level spectra
280282284286288290292294296298
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Binding Energy (eV)
C1spolyethylenepolypropylene
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Polyethylene & polypropylene
• By looking the valence band photoelectrons, we can easily differentiate between the PE and PP samples
• The valence band can act as a ‘fingerprint’ – an additional check for determining the chemical make-up of the sample
Valence bandpolyethylenepolypropylene
010203040
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Binding Energy (eV)
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Polyethylene & polypropylene
• Based on valence band analysis, a surface mixture of PE and PP can be quantified.
• The raw data was least-squares-fit using the two reference valence band shapes
• The fit used a 2:1 ratio of PE:PP valence band spectra, indicating that the surface was composed of the polymers in that ratio
0246810121416182022
Binding Energy (eV)
Valence band fitting
PP
PE
Fit envelopeRaw data
2:1 ratio of PE:PP valence band
spectra
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Application examples
1. Mapping
2. Depth profiling
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Mapping
X-rayspot
Stage Movement
The sample is divided into a grid.
A spectrum is acquired at each grid point.
The X-ray spot position is fixed, so that the sample is scanned underneath it.
The X-ray spot size should normally be comparable to the grid cell size (i.e. the step size between points).
The spectra are processed into quantitative maps.
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Chemical State Mapping
• Sample Preparation• Plasma patterned fluorocarbon on substrate• Grid laid on substrate during plasma polymerisation• Grid removed after deposition
Substrate
Grid
Plasma Containing Fluorocarbon Monomer
Patterned Fluorocarbon
Polymer
We would like to thank Plasso Technology Ltd., UK (www.plasso.com) for supplying the sample analysed in this work.
Substrate = Silicon coated with an acrylic acid plasma
polymer
• Analytical Conditions• Monochromator spot size = 30 µm• C 1s and F 1s collected in ‘Snapshot’ mode• 128 channels used for each region• Image step size 10 µm• Imaged area 660 x 930 µm
• Complete spectrum at each pixel
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A Map at Each Binding Energy
• 10 of the 128 possible maps in the C 1s region
Binding Energy
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Chemical State Maps
284.7 eVHydrocarbon
291 eVFluorocarbon
Overlay
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Selected area spectra
Substrate
Fluorocarbon
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280282284286288290292294296298
Binding Energy (eV)
PURE substrate
factor
PURE fluorocarbon
factor
PURE C1s spectral factors identified by PCA
Principal Component Analysis
• PCA can identify pure component spectra which can be used to reconstruct dataset - even if the pure components are never measured in isolation (such as the fluorocarbon here, which is always present with the substrate).
• PCA is not restricted to images, but can be used for depth profiles and other multi-level data sets.
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Thickness Map
• Substrate can be seen in the regions covered by fluorocarbon so the overlayer must be thin
• Use of the ‘Single Overlayer Thickness Calculator’ in Avantage produces a thickness map
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Depth Profiling
• XPS has a limited analysis depth • Signals are observed from less than 10 nm into the sample
• Many features of interest lie deeper than this• Layers of up to a few µm thickness are common
• There may be buried layers• The interfaces between these layers are often of interest
• How can we access the deeper layers?• By progressively removing material from the surface• Ion beam depth profiling is the most common method• Data collected after each etch period
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Profiling of organic samples Many polymers cannot be
sputtered with monoatomic argon
Chemical information is destroyed & composition is modified
Argon clusters can be used to successfully profile organic multilayer samples
Chemical and compositional information is maintained
Depth profiling polymers
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280284288292296300
Binding Energy (eV)
Monatomic Ar+ damaged PMMA
C-O and O-C=O functionality is mostly destroyed after only 10 sec. Ar+ sputtering
Monatomic v cluster profiling
• Many polymers cannot be sputtered with monoatomic argon• Chemical information is destroyed & composition is modified• C1s spectra shown for ion beam etched polymethylmethacrylate
Ar cluster cleaned PMMA
C-O and O-C=O functionality is maintained during sputtering
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Soft profiling of fluoropolymer plasma coating• Statement of problem and XPS analysis solution
• Chemical reaction leading to fluoropolymer coating
Conventional plasmas fragment the monomer structure
• It is proposed that a novel plasma method retains monomer structure
Improves liquid repellent properties of a range of materials Surface of PET, for example, can be modified from slightly
hydrophillic to significantly hydrophobic using this coating
• XPS/soft profiling of fluoropolymer coatings to evaluate if this is true
Textile fluoropolymer coating for improved liquid repellent properties
Fluoropolymer coating on PET
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Fluoropolymer coating on PTFE• Surface composition with XPS
• Elemental & chemical analysis
Measured surface elemental & chemical composition matches expected “non-fragmented” polymer formula closely
Consistent with suggestion that monomer does not significantly fragment during novel plasma process
280282284286288290292294296
Binding Energy (eV)
CF3
CF2
CF
C-C
C-CF
Ccoating before profiling
FC=O
Element/chemical state
Expected At%
Measured At%
F 53 55 O 6 6
CCF3 3 3 CCF2 22 20 CC=O 3 3
Other 13 13
Fluoropolymer coating on PET
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Fluoropolymer coating on PET
• Chemical state profile• Convert etch scale to depth
based on known performance of ion source on standard materials
• Use peak deconvoluted spectra to generate profile
• Appears that there is some interaction between the PET C=O group and the FC=O fluoropolymer group.
0
10
20
30
40
50
60
0 20 40 60 80
Ato
mic
pe
rce
nt
(%)
Etch Time (nm)
Atomic Percent Profile
C1s (C-C)
C1s (C-F)C1s (FC=O)C1s (CF2)C1s (CF3)
C1s (C-O)C1s (O-C=O)
F1s
O1s (C=O)O1s (C-O)
C1s (C-CF)
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280284288292296300
Binding Energy (eV)
C 1s PET spectrum after profiling C-C
C=O
C-O
p-p* shake-up
Indicates intact aromatic rings
Fluoropolymer coating on PET
524526528530532534536538540542
Binding Energy (eV)
O 1s PET spectrum after profiling
C=OC-O
p-p* shake-up
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Summary
XPS delivers chemical state analysis for surfaces. Analysts can investigate a wide range of surface problems on polymers and plastics:
Composition identification and quantification
Chemical identification
Coating thickness measurements
Application of surface treatments
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Further information
• www.thermoscientifc.com/surfaceanalysis