Xps simplified 2 polymers with speaker notes

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





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





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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+-


-

• 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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+-


• 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

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sπ-> π*shake-up

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C1s chemical shifts

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C1s Scan - PE

C-C

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Binding Energy (eV)

C-C

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Binding Energy (eV)

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C1s chemical shifts

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Binding Energy (eV)

C-CC-N

C=O

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C1s Scan – Nylon 6,9

C-CC-NC=O

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Binding Energy (eV)

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C1s chemical shifts

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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

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Binding Energy (eV)

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C1s chemical shifts

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C1s Scan - PVF

*C-CFC-F(C-C)

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Binding Energy (eV)

C-C

C=C

C-NC-O *C-CFC-F

C=O

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Binding Energy (eV)

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C1s chemical shifts

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Binding Energy (eV)

C1s Scan - PTFE

CF2

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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

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Binding Energy (eV)

C1spolyethylenepolypropylene

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• 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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• 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

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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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

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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


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• 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.

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40

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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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Binding Energy (eV)

C 1s PET spectrum after profiling C-C

C=O

C-O

p-p* shake-up

Indicates intact aromatic rings


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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

http://www.thermoscientifc.com/surfaceanalysis

Xps simplified 2 polymers with speaker notes

Documents

xps of polymers

surface of polymers

photoelectrons analyser

electron energies detector

xps instrumentation

sample ejected photoelectrons

cluster ion sources

ion beam depth profiling