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Xps simplified 2 polymers with speaker notes
XPS can be used to characterize polymers at the surface. It provides both elemental and chemical information about the top 10 nm of a sample. Elemental identification and quantification is possible from survey spectra. High resolution region spectra can identify functional groups and chemical environments through chemical shifts. XPS can map the distribution of elements and chemical states across a sample surface. Depth profiling with cluster ion beams maintains chemical information during sputtering and allows buried layers and interfaces to be analyzed. XPS was used to characterize a fluoropolymer coating on PET and PTFE, identifying the chemical structure and confirming it matched the expected structure from the plasma deposition process.
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Xps simplified 2 polymers with speaker notes
- 1. XPS Simplified 2. Characterizingpolymers with X-ray Photoelectron Spectroscopy (XPS)
- 2. 2 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
- 3. 3 Why are weinterested in the surface of polymers? • 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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- 5. 5 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
- 6. 6 XPS instrumentation • UHVSystem • 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
- 7. 7 XPS instrumentation H– H+ Photoelectrons Detector KE =EP KE < EP KE > EP EBE = 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
- 8. 8 XPS instrumentation Hemispherical analyser Detector Ion gun Floodgun X-ray source Mono crystal 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
- 9. 9 The problem withanalysing insulators Spectrum of an insulator without charge compensation Spectrum of an insulator with charge compensation 020040060080010001200 Counts/s Binding Energy (eV) 020040060080010001200 Counts/s Binding Energy (eV)
- 10. 10 +- The problem withanalysing 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.
- 11. 11 +- The problem withanalysing 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
- 12. 12 How does thecharge 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
- 13. 13 Sample handling • Samplesneed to be handled carefully to prevent contamination from fingerprints, gloves, tools etc • Samples also need to be vacuum compatible
- 14. 14 XP spectra –survey spectra Elemental 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% 020040060080010001200 Binding energy / eV Poly(ethylene terephthalate), PET C1s O1s Elemental quantification of PETsample Element At% C 71 O 29 C Auger O Auger O2s
- 15. 15 Nylon elemental analysis •NB Spectra offset for clarity 01002003004005006007008009001000110012001300 Counts/s Binding Energy (eV) Nylon 6-12 Nylon 6-9 Nylon unknown C1s N1s O1s O KLL N KLL C KLL Atomic % C N O Unknown 76 12 12 Nylon(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 R2 Unknown ??? 6.4 2 Nylon(6,9) 9 9.0 5 Nylon(6,12) 12 12.2 8
- 16. 16 XP spectra –region spectra Elemental identification • Chemical state quantification • Chemical environment • Functional groups Poly(ethylene terephthalate), PET n O OO O CC CC π-> π* shake-up 280282284286288290292294 Counts/s Binding Energy (eV) C1s Scan Binding Energy (eV) O1s Scan Counts/s π-> π* shake-up 528530532534536538540542
- 17. 17 C1s chemical shifts 280282284286288290292294296298 C1sScan - PE C-C 285 284286287288289290291292293 Binding Energy (eV) C-C Counts/s Binding Energy (eV)
- 18. 18 C1s chemical shifts 285284286287288289290291292293 Binding Energy (eV) C-CC-N C=O 280282284286288290292294296298 C1s Scan – Nylon 6,9 C-C C-N C=O Counts/s Binding Energy (eV)
- 19. 19 C1s chemical shifts 280282284286288290292294296298 C1sScan - Polycarbonate C1s(O-(C=O)-O) C1s(shake-up) C=C C-C C-O O-(C=O)-O 285 284286287288289290291292293 Binding Energy (eV) C-C C=C C-NC-O C=O Counts/s Binding Energy (eV)
- 20. 20 C1s chemical shifts 280282284286288290292294296298 C1sScan - PVF *C-CF C-F (C-C) 285 284286287288289290291292293 Binding Energy (eV) C-C C=C C-NC-O *C-CFC-F C=O Counts/s Binding Energy (eV)
- 21. 21 C1s chemical shifts 280282284286288290292294296298 Counts/s BindingEnergy (eV) C1s Scan - PTFE CF2 285 284286287288289290291292293 Binding Energy (eV) C-C C=C C-NC-O *C-CFC-F CF2 CF3 C=O
- 22. 22 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 Counts/s Binding Energy (eV) C1s polyethylene polypropylene
- 23. 23 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 band polyethylene polypropylene 010203040 Counts/s Binding Energy (eV)
- 24. 24 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 envelope Raw data 2:1 ratio of PE:PP valence band spectra
- 25. Application examples 1. Mapping 2.Depth profiling
- 26. 26 Mapping X-ray spot Stage Movement Thesample 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.
- 27. 27 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
- 28. 28 A Map atEach Binding Energy • 10 of the 128 possible maps in the C 1s region Binding Energy
- 29. 29 Chemical State Maps 284.7eV Hydrocarbon 291 eV Fluorocarbon Overlay
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- 31. 31 280282284286288290292294296298 Binding Energy (eV) PURE substrate factor PURE fluorocarbon factor PUREC1s 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.
- 32. 32 Thickness Map • Substratecan 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
- 33. 33 Depth Profiling • XPShas 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
- 34. 34 Profiling oforganic 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
- 35. 35 280284288292296300 Binding Energy (eV) MonatomicAr+ 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
- 36. 36 Soft profilingof 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
- 37. 37 Fluoropolymer coatingon 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 process280282284286288290292294296 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
- 38. 38 Fluoropolymer coating onPET • 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 Atomicpercent(%) 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)
- 39. 39 280284288292296300 Binding Energy (eV) C1s 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=O C-O p-p* shake-up
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Editor's Notes
- #2 This is the 2nd in a series of webinars designed to describe the important contribution that Surface analysis can make to the characterisation of modern materials.
- #3 In this webinar we will first discuss why it is important to be able to characterise the surfaces of polymers. Then I’ll give a brief overview of how X-ray photoelectron spectroscopy can be a key technique in performing surface analysis. The second section will discuss XPS in more detail, with a short introduction to the theoretical principals, the instrumentation required for XPS, the types of information that can be obtained, and a run through how the analysis is actually done – from loading the samples to collecting a spectrum.The final section will go through the kinds of data that can be collected and how it can be used. We’ll see how you can get both elemental and chemical composition information for the surface, and look at a couple of examples of how the XPS can be used.
- #4 So, why are we interested in the surface? The surface of a solid is the point which interacts with the external environment and other materials. Therefore the modification of surfaces can be used in a wide variety of applications to alter the performance and behaviour of a material.As the demand for high performance materials increases, so does the importance of surface engineering. Questions such as “how do you protect the surface?”, “how do layers interact?”, or, perhaps more frequently during development, “why doesn’t it work?” can all be investigated using surface analysis techniquesFrom mundane applications such as coatings on non-stick cookware to the development of polymer-based electronics and bio-sensitive surfaces, XPS plays a fundamental role as a characterisation tool.
- #5 So what can XPS characterise, and how does it relate to today’s topic, the characterisation of polymers. The main areas would be:Elemental and chemical identification and quantification – establishing what is present at the surface of the sample, and how much of it is there.Following on from that, contaminants, both organic and inorganic can be identified.The uniformity of the surface can be investigated. This could be to identify features or patterns, but also includes the measurement of the thickness of ultra-thin filmsFinally, interfacial chemistry can be probed, by alternately removing material from the surface and measuring what remains.We’ll see how this all works in a moment, but first a quick primer on the theory and instrumentation.
- #6 This will be familiar to those of you that attended XPS Simplified #1 (those of you that didn’t, it is available to watch on demand from the Thermo Scientific Website!).First off the physics. XPS is electron spectroscopy and is often referred to as ESCA, electron spectroscopy for chemical analysis.XPS relies on the detection of electrons ejected from the surface of a material. These electrons are generated as a result of irradiation of the surface with X-rays. This is known as the photoelectric effect, discovered by Hertz in the 19th century, and explained by Einstein in a 1905 paper.The photoelectrons have a characteristic kinetic energy, which is related to the binding energy it had within the atom as shown in the equation, where the other term is the energy of the X-ray photon. The binding energy is characteristic of the element, orbital and chemical environment of the atom, and so by measuring the kinetic energy using XPS we can learn a great deal about our surface.Because of the strong interaction of electrons with solid materials, only electrons generated near the surface can escape without losing too much energy. This is the reason for the high degree of surface sensitivity of XPS.The effect of this is that all the information contained within the data is from the top 1-10nm, depending on the materials being analysed.To be able to extend the technique to thicker and multilayered samples we use XPS in combination with an Argon ion milling source.
- #14 The next part of the webinar will focus on how we get the data from the samples. Samples need to be carefully handled before XPS analysis. A fingerprint can put microns of material on a sample, which can obscure the real focus of the experiment completely. Even gloves can deposit material onto samples, so it is important to ensure that they are handled away from areas of interest.******* VIDEO CLIP NEXT ************
- #16 Here we have survey scans from 3 samples. I’ve offset them in the X-direction so that you can see the differences in the peak heights. Two of the samples are known samples of Nylon; (6.9) and (6,12). The numbers relate to the two starting materials for the co-polymer, where the first number is the number of carbon atoms in the diamine starting block and the second is the number of atoms in the diacid. The other sample was unknown, but believed to be nylon.The samples were prepared by cutting to expose a fresh surface, and the procedure we saw in the video was followed.The survey from the unknown sample is very similar to the other two (in red and green) but the peak ratios are different. By looking at the quantification, and assuming that the first number in the co-polymer is 6 (not unreasonable as nylon 6,x co-polymers tend to be more common than other diamine starting blocks), we can calculate that the unknown material is likely to be Nylon (6,6). Therefore R2 is 2.
- #18 Starting off simply with polyethylene, we just have 1 peak for the saturated C-C bond. This comes in at 285 eV.
- #19 If we return to nylon, we can see the C1s spectrum for nylon (6,9) here. We can see the peaks for the C-N and C=O bonds in addition to the C-C peak. As we would expect the C-N and C=O peaks have similar areas, as there are 2 N atoms and 2 O atoms per repeating unit.
- #20 If we look at a sample with an aromatic ring, we can see two things. One is that the unsaturated C=C bond appears at a lower binding energy than the C-C bond in polyethylene (this sample has both types of bonding).The other feature is the “shake-up” satellite, which is seen at 292 eV here. These features are the result of kinetic energy losses due to excitation of other electronic transitions, here the pi-pi* transition in the aromatic ring structure. They can be very useful for identifying the presence of particular functional groups, particularly aromatic rings.It is also worth noting the big range available for the C=O functional group. This sample shows a peak at the upper limit of binding energy, due to the bonding of the C atom to 3 oxygen atoms.
- #21 Finally we can look at a couple of fluoropolymer examples. Here we have PVF, and so we can see two main peaks, and a small one due to some hydrocarbon contamination. The lower be peak (pink) appears at a slightly higher BE than the standard C-C energy due to a processes of “secondary shifting”, where a nearby functional group bonded to a neighbour affects an atom. These effects are most significant with more electronegative functional groups such as fluorine.
- #22 The second fluoropolymer is PTFE. We can see the single peak for the CF2 group. CF3 would appear at an even higher binding energy as seen on the chart.
- #23 So far we’ve looked at how we can use chemical shifts to distinguish between different samples, but what if there isn’t anything like that to go on? For example, here we have the C1s spectra for polyethylene and polypropylene. They look very similar, and so it would be very difficult to distinguish them – and if we have a mixture, then the quantification would be very tough.So we need a different approach, and for this we move away from looking at the core-level electrons
- #24 Instead we look at the valence band electrons at the very low end of the binding energy scale. Although the structure of the peaks is quite complex, resulting from a convolution of the bonding orbitals, they can be used as a fingerprint for particular molecules. As we can see here, now we can clearly see a difference between PE and PP.This is sort of analogous to how FT-IR can be used to fingerprint particular molecules. With XPS, this can be used for quantification too, to a certain extent.
- #25 The raw data here corresponds to a surface mixture of PE and PP. By fitting data from our reference samples, we can establish the ratio of PE:PP in the sample, which turn out to be 2:1 of polyethylene to polypropylene.
- #26 We’ll finish by having a look at two examples.
- #27 We can extend the analysis to look at the whole surface by mapping, to create surface chemical images. In this mode of operation, the sample is divided into a grid, and spectra are collected in each grid square. By analysing all the spectra, atomic concentration images can be created to show such things as surface homogeneity, or the location of contamination that cannot be seen optically.
- #32 By using statistical analysis such as PCA, we can extract the “pure” spectra for each component. This now allows us to fit the data set for each molecule, and use that to calculate the thickness of the deposited layer
- #34 Depth profiling is a very common experiment, but less so for polymers due to the chemical damage that monatomic ion beams can cause. In this mode of operation, cycles of ion beam etching are interleaved with spectroscopy. The data can then be analysed as before to generate an atomic concentration profile, which shows the variation in the chemistry with depth into the surface. Depths of up to a few microns can be investigated using this approach.
- #37 So we can use this technology to investigate another fluoropolymer coating, this time on polyester. This is the type of coating used for coating fabrics to limit the build-up of dirt and act as a liquid repellent layer.
- #38 The deposited layer looks like this at the top surface. It closely matches the expected composition, with some surface contamination.We can then profile through it to investigate the layer
- #39 We can look at all the components as we fo deeper into the sample. The layer is just over 10 nm thick (based on using the 50% value for the F1s signal).Most of the components for each layer follow each other, but the two C=O functional groups change slightly differently, suggesting that there may be an interaction there between the two layers.
- #40 Looking at the spectra at the end, we can see they very closely resemble the PET spectra from earlier on. Again, using the cluster source we are maintaining the surface chemistry, which in turn ensures that we can measure it correctly with XPS.
- #41 XPS delivers chemical state analysis for surfaces. Analysts can investigate a wide range of surface problems on polymersincludingComposition identification and quantification,Surface coating integrity,Thickness measurements,Application of surface treatments,This unique, surface specific information is complementary to other bulk sensitive techniques in the laboratory, enabling complete characterisation of materials.
- #42 Thanks for joining us for the webinar today, we hope you found it interesting and now in the remaining time we’ll be happy to answer your questions.