Quantification of Spectra

Basics of Quantification

A Tutorial in Quantification of XPS Spectra

Raw peak areas from XPS spectra require a series of corrections before quantification in terms of %Atom Concentrations can be obtained and interpreted with any degree of certainty. The following video is intended to provide an overview of how understandable quantification results can be obtain by XPS.

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Quantification of High Resolution Spectra as a means of Normalising the Display of Spectra

Quantification of XPS spectra in terms of atomic concentration is performed as a means of normalising peak intensities in an attempt to remove instrumental and sample artefacts when making a comparison of samples. These normalisation steps typically presented as tables of numerical values can also be applied to the spectra so data envelopes can be displayed overlaid using a common intensity scale. The following video illustrates processing steps required to transform spectra to a background-subtracted form of photoemission peaks which are scaled appropriately for the display of both C 1s and O 1s spectra.

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Peak Area, Counts per Second, Energy Step-Size and Quantification of XPS Spectra

Peak Area in XPS is measured using units of CPSeV. These units are designed to allow data collection with appropriate dwell-times and energy step-sizes for a given sample. Low intensity or low concentrations of a material require longer dwell-times while intense broad peaks can be measured with shorter dwell-times and fewer energy increments by performing larger energy steps. The aim in varying these basic acquisition parameters is to maintain similar signal to noise for all peaks used in a quantification calculation.

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Propagating a Peak Model from one VAMAS block to a Set of VAMAS blocks

A peak model from a C 1s spectrum designed for a spectrum in one VAMAS file is propagated or copied to a set of VAMAS blocks in another VAMAS file. The propagation dialog is used to move a region, four component peaks, fit these components to the target data and display the results via an annotation table. Propagation requires a designated C 1s spectrum for which a peak model etc. has been prepared and the specification of a list of C 1s spectra defined by the current selection in the right-hand pane of the experiment frame window.

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Propagating a Peak Model from one VAMAS file to VAMAS blocks in different VAMAS files

The Select menu and Window menuís Cascade option are used to support the propagation of a peak model from one VAMAS file to equivalent spectra saved in separate VAMAS files. The Select menu is used to make appropriate selections of VAMAS blocks within all open VAMAS files in preparation for copying a peak model prepared on a C 1s spectrum to all C 1s spectra in multiple VAMAS files open in the current CasaXPS session. Energy calibration is used to align the VAMAS blocks prior to copying and auto-fitting the peak model in all VAMAS files. Once the C 1s spectra are fitted with component peaks, a text report is generated from these components and moved through the clipboard to a spreadsheet program.

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Quantification by Auger Electron Spectroscopy

Quantification of an Auger survey spectrum is performed using numerical differentiation. Relative sensitivity factors and their relationship to the differentiated spectrum are explained using a platinum survey spectrum.

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Chemical State Determination

Quantification of Poly (tetrafluoroethylene) PTFE and Transmission Correction

PTFE is an ideal material for examining the influence of XPS signal in response to differences in kinetic energy resulting from photoemission peaks. PTFE stoichiometry for carbon and fluorine is expected to be 1 : 2. Further fluorine has two well formed peaks in the form of F 1s and F 2s separated by more than 650 eV and traceable RSFs are available in the form of computed and published Scofield cross-sections which can be corrected for escape depth using a formula published by Martin Seah. The video illustrates the influence of transmission on quantification by comparing the ratio for F 1s to F 2s measured from PTFE using a number of different instruments. The discussion is used to explain how RSFs are used and the importance to quantification by XPS of transmission correction. The calculator is used to both calculate an estimate for the transmission correction for a particular acquisition mode and to add the calculated transmission function to VAMAS blocks, and in so doing obtain the correct stoichiometry for PTFE.

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Quantification of Nylon-6

Quantification of Nylon-6 is examined using an evolving sequence of spectra measured from Nylon-6 after sputtering the surface with an argon cluster source ion gun. The video is in three parts: Part 1) examines the sample by creating a peak model for C 1s intensities measured as high energy resolution spectra. Synthetic peaks are assigned to chemically shifted C 1s photoemission lines. Part 2) extends the analysis by considering how Nylon-6 evolves after Ar cluster cleaning cycles. Difference spectra are used to construct three spectral forms (one of which is a good approximation to the expected XPS of Nylon-6) suitable for reconstructing the sequence of sputtered Nylon-6 in a linear least squares sense. Part 3) examines the properties of the instrument used to collect these Nylon-6 data and explains why it could be possible that these O 1s, N 1s and C 1s spectra yield the expected stoichiometry for the material.

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Characterisation of sp2 and sp3 Carbon

C KLL Auger peaks from diamond and graphite are used to illustrate how to compute the D-Parameter in CasaXPS. Auger peaks are differentiated using synthetically derived curves rather than simply performing a Savitzky Golay differentiation directly applied to raw data. The Poly Regression background type in CasaXPS is applied to the data to obtain a smooth approximation to the data. An option on the Test Data property page of the Spectrum Processing dialog window is used to differentiate the polynomial approximation to the Auger peaks and the SP2SP3 background type applied to the differentiated data envelope measures the D-Parameter for a boron-doped diamond and graphite sample.

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Using Synthetic Components to Construct RuO2 (4+) Ru 3d Doublet Lineshape

A lineshape is constructed from data acquired from a standard RuO2 (4+) material. A synthetic peak model is created to allow for C 1s contamination to the standard RuO2 sample. A lineshape characteristic of Ru 3d doublet is extracted used the Test Data property page of the Spectrum Processing dialog window to sum Ru 3d doublet component peaks only. Component peaks are identified using the Comp Index field on the Components property page of the Quantification Parameters dialog window.

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A Molybdenum Oxide Peak Model

An as received MoO2 standard sample exhibits evidence of Mo6+, Mo5+ and the expected Mo4+ oxidation states. A peak model for Mo 3d is loaded from a library and the mixed oxidation states within the sample are used to assess the validity of the library peak model. The approach adopted is to compare intensities from O 1s and Mo 3d and verify the O 1s contains signal which can be entirely allocated based on the stoichiometry of the Mo oxides predicted by the peak model when fitted to the current data. The video illustrates the concepts of relative sensitivity, instrument transmission, lineshapes derived from data, combining VAMAS blocks to form a single VAMAS block, Standard Report configuration and a peak modelwith area constraints derived from RSFs and transmission ratio between O 1s and Mo 3d photoemission peaks.

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XPS Analysis of Poly (Ether Ether Ketone) PEEK

PEEK includes two oxygen atoms singly bonded to carbon and one oxygen atom doubly bonded to carbon. As a consequence PEEK provides an example of CO bonds where it is clear which component peaks within a peak model must correspond between O 1s and C 1s spectra. The analysis of PEEK is performed to illustrate how a survey spectrum provides supporting information to enable a detailed chemical state interpretation for both carbon and oxygen via high resolution narrow scan spectra. Chemistry is established in terms of chemical shifted peaks and also by considering relative intensities for component peaks assigned to C-O and C=O in both O 1s and C 1s spectra.

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Using a Combination of Survey, Narrow Scan Core Level and Valence Band Spectra to Examine Chemical State

Oxidation states of Cu are examined by first constructing component spectra based on Cu 2p, Cu Auger and O 1s data. Further information about chemical state is extracted from valence band spectra. These videos illustrate many techniques for data analysis including the use of a spreadsheet to generate strings suitable for use in the spectrum calculator of CasaXPS, noise reduction based on a PCA approach, difference spectra and more. Fundamental to these videos is the manipulation of spectra using an approach aimed at simplifying spectral forms to recognisable shapes.

Part I: Analysis using Cu 2p, Cu Auger and O 1s Data

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Part II: Analysis of Valence Band Spectra by Vector Techniques

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Quantification of XPS Data

Quantification of Thermo Avantage Spectra

Scofield cross-sections, transmission correction and a new option for selecting from a range of escape depth correction algorithms is introduced using gold spectra measured from a Thermo K-Alpha XPS instrument. Three survey spectra acquired from an as-received gold sample and two measured from ion-beam-cleaned gold are converted from the Avantage avg ASCII export data format and then quantified using two different escape depth corrections to peak areas for five separate gold photoemission peaks.

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Adventitious Carbon and Quantification by XPS

Many samples analysed by XPS have been exposed to air and as a consequence the surface is potentially contaminated by a thin film over the surface of unwanted material. XPS is a surface sensitive technique precisely because photoemission signal is attenuated by inelastic scattering within the solid and a film of material other than the material of interest attenuates signal as a function of kinetic energy for photoemission peaks, thus introducing a systematic error into atomic concentration calculations. The following video illustrates an option which corrects for a systematic attenuation of photoemission by an overlayer material.

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Escape Depth Correction and Quantification using Scofield Cross-Sections

Scofield cross-sections are often used as part of a traceable quantification procedure for XPS spectra. These Scofield cross-sections represent the probability for photoemission from an individual electronic state within atoms and are used to scale photoelectron peak area to account for differences in peak intensity due to the physics of the scattering of electrons by photons of a given energy. In addition to scaling by these photoionisation cross-sections, photo-emitted electrons within solid state materials are subject to inelastic scattering before escaping the surface en route to the detection system. These inelastic scattering events attenuate the signal for a given photoelectron peak, where the rate of attenuation varies with kinetic energy of these photoelectrons. For homogeneous materials, peak intensities are adjusted for escape depth by using a kinetic energy dependent correction factor which can be entered on the Regions property page of the Quantification Parameters dialog window. The following video illustrates how escape depth correction is applied to spectra measured from a Theta Probe, however the principles discussed are applicable to all XPS instruments for which transmission correction is available.

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Monte Carlo Error Analysis applied to a Peak Model for O 1s Spectra

Stability of a peak model with respect to noise within the measurement process can be assessed using Monte Carlo methods. The Monte Carlo approach involves constructing synthetic spectra with simulated noise. The default Monte Carlo option within CasaXPS assumes data are pulse counted and therefore noise scales as the square root of the counts per bin. Unfortunately, not all data are presented as counts per bin and some spectra are altered by pre-processing performed by the acquisition system even though the underlying detection system is logically event counting. The Calculator property page includes an option to re-scale data to simulate Poisson behaviour suitable for Monte Carlo analysis of a peak model. These ideas and options are discussed using O 1s spectra from a polymer sample.

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LA Lineshape and Fitting Sulphur Peaks in WS2

This video introduces the LA lineshape by considering S 2s photoemission as an example of a mostly Lorentzian peak shape and follows up by a comparison with the narrow doublet peak shapes from S 2p.

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Exploring the Difference between Peak Fitting using Chi Square and RMS as the Figure of Merit

Given are the measured data intensities corresponding to energies with individual standard deviationsand are intensities calculated from a synthetic peak model evaluated at the set of energies , then the Figure of Merit used to optimise parameters within the synthetic envelope are either chi square ( ) or root mean square (RMS). These figures of merit are calculated as follows.

Fitting of peaks to data is explored using exact data envelopes created to illustrate the nature of optimisation and fitting peaks to data. The consequences for optimising using compared to RMS are examined and the importance of designing the most appropriate lineshape for a given photoemission peak is illustrated.

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Chi Square and Evaluating the Quality of Peak Models

Chi square figure of merit is discussed using examples for lineshapes and parameter constraints to illustrate the relationship between a good value for the chi square and the information available from a peak model when fitted with a set of synthetic component peaks. A simulated C 1s spectrum provides the basis for which the quality of a peak model, fitted to data of known origin, can be assess as a valid outcome to the optimisation step.

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Transmission Correction

Graphene Oxide is used to illustrate how transmission correction to photoemission intensities allows favourable comparison between quantification tables compiled using data acquired via different energy resolution instrumental settings. A peak model for a C 1s spectrum shows how a traceable quantification approach allows the relationship between carbon and oxygen to be investigated. Peak model area constraints are discussed and used to partition the C 1s signal into carbon bonded to oxygen and other forms of carbon signal. Quantification based on both regions and components is illustrated via the Quantification property page of the Annotation dialog window.

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Transmission for an XPS instrument is determined by the lens and aperture arrangement for an instrument. In particular as a spectrum is recorded the kinetic energy for the photoelectrons forces adjustments to lens voltages which in turn alters the efficiency for an instrument as a function of kinetic energy. It is therefore important to understand the consequences of adjusting lens voltages and to this end a feature in CasaXPS is used to assist the tuning of lenses by systematically selecting an image of a circular aperture representing an image of best focus. The video shows how a set of images representing the selected area aperture as a function of lens voltage is ordered to identify the best voltage as part of a tuning procedure.

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Multiple Detector Response to Energy Dispersion in a Hemispherical Analyser

The geometric interaction of HSA and multiple channel detection is examined using Au 4f 7/2 photoemission peak and a 1D delay line detector separated into 140 logical signal streams. The essential processing steps used to combine 140 spectra collected from 140 detector channels are illustrated by performing energy calibration to align signal corresponding to Au 4f 7/2 and a re-binning procedure performed to ensure the best possible lineshape for a single spectrum constructed from these multiple data streams.

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2D Detectors and Non-Energy Dispersive Direction in a Hemispherical Analyser

When a spectrum is acquired using a 2D detector, signal is spread over the detector where the meaning for the signal depends on the physical spatial location of counting events with respect to the active area of the detector. Electrons entering the HSA following a radial direction with respect to the spherical geometry are dispersed in energy. Events recorded at right-angles to these energy dispersive directions provide information about the location within the entrance slit from which an electron originates. The transfer lens system determines what meaning can be assigned to these trajectories arriving off-centre on a 2D detector. If an image of the sample is formed at the entrance slit then spatial information is preserved by the HSA for electrons arriving at the 2D detector in the direction orthogonal to the energy dispersive direction. An alternative mode is to limit the spatial information arriving at the entrance slit and use the transfer lens system to vary signal entering the HSA as a function of the angle at the sample from which an electron originates. The 2D detector under these conditions provides angle resolved XPS without tilting the sample.

These modes of operation are explored using data from a ScientaOmicron instrument, where data are converted from binary files with pxt file extension to VAMAS format. Each spectrum is maintained as multiple detector-slices representing data recorded by a 2D detector in the non-energy dispersive direction.

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Quantification based on Regions with Error Estimates

Lithium sulphate is quantified using regions and survey data. Monte Carlo error estimates are calculated for %Atomic Concentrations and the difference between chi square and RMS figure of merit when fitting peak models to data is explained giving an example of where RMS is the best figure of merit for optimisation in a specific situation.

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Standard Report

Configuration of Region Standard Report

The format for a Standard Report is modified using the configuration file RegionQuantTable.txt by adding keywords to the configuration file obtained from tables in the Orange Manual Book page 337-338. Configuration files for CasaXPS are maintained within the directory in which CasaXPS.exe is located. The shortcut icon is used to navigate to the CasaXPS.DEF directory containing the appropriate RegionQuantTable.txt configuration file and new entries are added to the file via NotePad to obtain a new set of columns in a quantification report generated from region information on a survey spectrum.

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Spectra, Heterogeneous Samples and Quantification by XPS

Large area analysis involves measuring spectra where the composition is expected to be an average over the analysis area, which in the case of heterogeneous samples may include a range of chemically different zones. This video looks at how the composition of a sample may change from one location to another and investigates the influence of instrumental electron optics on data measured from samples with spatially separated chemical zones.

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Thin Film Quantification using TAGs

Two videos are used to explain first, why quantification by XPS of samples depends on the in-depth distribution of materials and second, how understanding the layer structure in a sample allows quantification to be performed. Quantification by Tags is used to combine information from survey and high resolution spectra thus providing insight into the composition for a thin film. Carbon 1s high resolution spectra are fitted using lineshapes from data. Lineshapes are computed using data measured from standard PVEE and PS polymers and applied to a thin film of PVEE on PS.

Video 1: Layer structures within samples and how in-depth distribution affects quantification by XPS.

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Video 2: Quantification of a surface film using survey and high resolution data.

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Quantification using TAGs

Well characterised operating modes yielding good signal-to-noise but low resolution survey data are combined with less well characterised high resolution but low count-rate narrow scan data to obtain a quantification report which removes the need for transmission correction for all operating modes of an instrument. Quantification using TAGs allows high resolution narrow scan spectra provide chemical state information in the form of a peak model which is used to proportion elemental %atom concentration calculated from survey data. The resulting quantification table presents %atom concentration values in terms of both elemental and chemical state with the accuracy offered by the survey acquisition mode.

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Effective RSFs and Quantification by TAGs

Overlapping peaks from different elements complicates quantification via survey data due to the mixing of signal from more than one photoemission line with different relative sensitivity factors. An example of K 2p and C 1s photoemission signal is used to illustrate how Effective RSFs calculated from high resolution data can be used to obtain quantification from a low resolution good signal-to-noise survey spectrum. Quantification using TAGs is demonstrated for a simple case where the K 2p doublet is easily resolved from C 1s signal via high resolution data, but within a pass energy 160 spectrum these same photoemission lines appear as a poorly defined data envelope.

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Custom Report

Quantification of XPS Survey Spectra containing Copper and Aluminium

Measuring aluminium using XPS survey spectra when copper is present in the sample is an issue because the two aluminium peaks available for quantification of the aluminium signal, namely Al 2p and Al 2s, overlap with the Cu 3p and Cu 3s peak positions. While at first sight the need for high resolution spectra from the Al 2p region might seem the only solution the problem can be addressed via survey spectra using the ideas described by Hazell SIA (2002). Apart from solving the problem for materials with both Al and Cu, the solution illustrates how the custom report in CasaXPS can be used to manipulate peak intensities leading to an indirect quantification for a sample.

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††††††††††† Chemical State and Peak Offsets

The Custom Report on the Quantification Parameters dialog window is used to calculate the difference in energy between an O 1s photoemission peak and a step in the inelastic background associated with the O 1s signal from Nylon-6. The techniques illustrated in the video have general applications. For example, offsets between Auger and photoelectron peaks are referred to as the Auger Parameter and are used to infer chemical state.Similarly differences between O 1s and Ru 3d5/2 peaks were used to study band gap properties of doped TiO2.

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Introduction to Creating a Depth Profile using both Regions and Components

A simple depth profile measured from an aluminium oxide layer on metallic aluminium is used to illustrate how a depth profile VAMAS file is created from a sequence of acquisitions separated by etch cycles. A region defined on O 1s spectra and a set of component peaks used to model the evolution of metal and oxide signal within an Al 2p spectral envelope provide the raw signal from which the profile plotted as a functions of etch time is created as a new VAMAS file. Quantification of the O 1s and Al 2p spectra is performed using the Custom Report on the Quantification Parameters dialog window of CasaXPS.

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Calculating Proportion of Signal for C-N and C-H in Ionic Liquids using Peak Models and the Custom Report

C1s high resolution narrow scan XP spectra measured from ionic liquid samples are used to illustrate how to display component peaks from complex peak models, and how to gather and tabulate signal using the Custom Report by making use of formulae to compare the calculated CN:CH ratio with the expected ratio for a range of [CnC1]+[TfO]- ionic liquids.

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Introduction to Sputter Depth Profile Creation and Display Options

An SiO2/TiO2 multilayer sample is quantified using the Custom Report to create depth profile from a VAMAS file containing narrow scan spectra measured following a sequence of sputter cycles. These data are used to illustrate the basics of creating and displaying a profile experiment.

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Element Library

Doublet Peak Specification

A simple Si 2p doublet is used to illustrate editing the Element Library within CasaXPS. The library is configured to include and entry which permits the simultaneous creation to two components with constraints appropriate for doublet XPS peaks.

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JEOL Auger Element Library

A depth profile performed on a JEOL Auger instrument is quantified using differentiated spectra and peak-to-peak intensities scaled using appropriate RSFs loaded from JEOL element tables in ASCII format. The video illustrates how to load an element library file, how to selectively propagate quantification regions and a number of display options useful to profile data.

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Hiden SNMS Profiles

An SNMS depth profile is calibrated in terms of intensity and depth using the Dynamic SIMS dialog window on the SIMS toolbar of CasaXPS. The video illustrates how to load a library file containing computed relative sensitivity factors appropriate for a given set of measurement conditions used to measure a profile from a steel sample. RSFs from the standard profile are copied to the data measured from the steel sample, a sputter rate is calculated and the depth profile from the steel sample is calibrated and displayed.

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