ISO/TR 23173:2021
(Main)Surface chemical analysis — Electron spectroscopies — Measurement of the thickness and composition of nanoparticle coatings
Surface chemical analysis — Electron spectroscopies — Measurement of the thickness and composition of nanoparticle coatings
This document provides a description of methods by which the coating thickness and chemical composition of "core-shell" nanoparticles (including some variant and non-ideal morphologies) can be determined using electron spectroscopy techniques. It identifies the assumptions, challenges, and uncertainties associated with each method. It also describes protocols and issues for the general analysis of nanoparticle samples using electron spectroscopies, specifically in relation to their importance for measurements of coating thicknesses. This document focuses on the use of electron spectroscopy techniques, specifically X-ray photoelectron spectroscopy, Auger electron spectroscopy, and synchrotron-based methods. These cannot provide all of the information necessary for accurate analysis and therefore some additional analytical methods are outlined in the context of their ability to aid in the interpretation of electron spectroscopy data.
Analyse chimique des surfaces — Spectroscopies d'électrons — Mesurage de l'épaisseur et de la composition des revêtements de nanoparticules
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TECHNICAL ISO/TR
REPORT 23173
First edition
2021-06
Surface chemical analysis — Electron
spectroscopies — Measurement of
the thickness and composition of
nanoparticle coatings
Analyse chimique des surfaces — Spectroscopies d'électrons —
Mesurage de l'épaisseur et de la composition des revêtements de
nanoparticules
Reference number
ISO/TR 23173:2021(E)
©
ISO 2021
---------------------- Page: 1 ----------------------
ISO/TR 23173:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 23173:2021(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 1
5 Overview . 3
6 X-ray photoelectron spectroscopy. 4
6.1 General . 4
6.2 Coating thickness measurement. 4
6.3 Nanoparticle coating thickness . 5
6.4 Numerical methods . 6
6.5 Descriptive formulae. 9
6.6 Modelling and simulation software .11
6.7 Method comparisons .12
6.8 Inelastic background analysis .15
6.9 Elemental composition .16
6.10 Variable excitation energy XPS .18
6.10.1 General.18
6.10.2 Qualitative depth-profiling . .19
6.10.3 Quantitative depth-profiling .22
6.11 Near-ambient-pressure XPS (NAP-XPS) .23
6.11.1 General.23
6.11.2 Internal structure of bimetallic NP catalysts .24
6.11.3 Measurement of NP's in liquid suspension .24
7 Auger electron spectroscopy .26
7.1 General .26
7.2 Coating thickness measurement.26
7.2.1 General.26
7.2.2 Destructive depth-profiling .27
7.2.3 Non-destructive depth-profiling .27
7.2.4 Elemental composition . .27
7.2.5 Imaging and line scans.28
8 Complementary analysis .30
9 Deviations from ideality .33
9.1 General .33
9.2 Multilayered coatings .33
9.3 Other non-ideal cases .35
Annex A (informative) Example script for modelling of XPS data from nanoparticles .40
Bibliography .42
© ISO 2021 – All rights reserved iii
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ISO/TR 23173:2021(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 201, Surface chemical analysis,
Subcommittee SC 7, Electron spectroscopies.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 23173:2021(E)
Introduction
Recently, there has been increasing development and use of nanoparticles in a wide range of application
[1]–[7]
areas, including catalysis, medicine, energy, optoelectronics and cosmetics . In particular,
nanoparticles having some form of coating layer, which is present either by design or due to incidental
[8]–
processes such as contamination or oxidation, are among the most commonly studied and utilised
[11]
. An essential part of the characterisation of nanoparticles is the measurement of the surface
properties because a large proportion of the material is at a surface or interface. In the case of coated
nanoparticles, the thickness and composition of the coating has a significant role determining its
functional properties and defines the interaction of the particle with its environment. Many applications
require nanoparticles to have coatings that are specifically designed in order to achieve a desired
level of performance. Measurement of surface composition and coating thickness of nanoparticles
is a challenge to which electron spectroscopies are well suited, due to high surface sensitivity, well-
understood physical principles and accessibility. Such measurements can have a significant dependence
on sample format and condition; sample handling and provenance of nanoparticle samples for surface
[12]
chemical analysis are addressed in ISO 20579 . A general introduction to the challenges of surface
[13]
chemical analysis of nanostructured materials is provided in ISO/TR 14187 .
© ISO 2021 – All rights reserved v
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TECHNICAL REPORT ISO/TR 23173:2021(E)
Surface chemical analysis — Electron spectroscopies
— Measurement of the thickness and composition of
nanoparticle coatings
1 Scope
This document provides a description of methods by which the coating thickness and chemical
composition of "core-shell" nanoparticles (including some variant and non-ideal morphologies) can
be determined using electron spectroscopy techniques. It identifies the assumptions, challenges,
and uncertainties associated with each method. It also describes protocols and issues for the general
analysis of nanoparticle samples using electron spectroscopies, specifically in relation to their
importance for measurements of coating thicknesses.
This document focuses on the use of electron spectroscopy techniques, specifically X-ray photoelectron
spectroscopy, Auger electron spectroscopy, and synchrotron-based methods. These cannot provide all
of the information necessary for accurate analysis and therefore some additional analytical methods
are outlined in the context of their ability to aid in the interpretation of electron spectroscopy data.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 18115-1, Surface chemical analysis — Vocabulary — Part 1: General terms and terms used in
spectroscopy
ISO 18115-2, Surface chemical analysis — Vocabulary — Part 2: Terms used in scanning-probe microscopy
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18115-1 and ISO 18115-2
apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Symbols and abbreviated terms
X subscripts denote the material of the overlayer
Y subscripts denote the material of the core
x subscripts denote a specific photoelectron peak from material X
y subscripts denote a specific photoelectron peak from material Y
I
intensity of electrons arising from a peak, i
i
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ISO/TR 23173:2021(E)
I intensity of electrons from peak i arising from pure material I
iI,
a
vertical thickness of the overlayer material at a given position
b
vertical thickness of the core material at a given position
effective attenuation length of electrons from peak i travelling through material J
L
iJ,
R
nanoparticle core radius
T
thickness of the overlayer
d
horizontal displacement of a specific line of material
angle between the central vertical axis of the particle and the point of the particle's surface
θ
which is at displacement x
normalised intensity ratio of the intensities of peaks x and y
A
xy,
γ
dimensionless scaling factor
T estimated overlayer thickness for a large sphere
∞
T
estimated overlayer thickness for infinitesimally small particles
0
T
estimated overlayer thickness for a nanoparticle
NP
AES Auger electron spectroscopy
AFM atomic force microscopy
CSNP core-shell nanoparticle
EAL effective attenuation length
EDX energy dispersive X-ray analysis
ICP-AES inductively coupled plasma atomic emission spectroscopy
IMFP inelastic mean free path
KE kinetic energy
MPA mercaptopropanoic acid
NAP-XPS near-ambient-pressure x-ray photoelectron spectroscopy
NP nanoparticle
SAM scanning Auger microscopy
SANS small angle neutron scattering
SAXS small-angle X-ray scattering
SEM scanning electron microscopy
TEM transmission electron microscopy
2 © ISO 2021 – All rights reserved
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ISO/TR 23173:2021(E)
TOP trioctylphosphine
UHV ultra-high vacuum
XPS X-ray photoelectron spectroscopy
5 Overview
The methods described in this document are listed by clause and outlined in Table 1. The primary use
detailed is the determination of the thickness of a nanoparticle coating from electron spectroscopy
data, for which three main methods are described, with a specific example given for each. Methods for
coating thickness determination that are described in detail include the use of descriptive formulae
for calculation of coating thicknesses from X-ray photoelectron spectroscopy (XPS) peak intensities;
numerical modelling of XPS intensities from nanoparticles, and general structure and layer thickness
determination by the use of in-depth simulation software. Interpretation of sample composition from
electron spectroscopy data for layered samples is discussed. Rudimentary analysis of the inelastic
background in XPS data is described, alongside the relevant considerations for interpreting inelastic
backgrounds from nanoparticle samples. Discussions of the use and potential benefits of synchrotron-
XPS, near-ambient-pressure XPS (NAP-XPS), and Auger electron spectroscopy (AES) are included, along
with any related issues and considerations. For all methods of analysis, additional characterisation
is required before confident estimates of coating thickness or composition can be made. Therefore, a
range of measurement techniques which are complementary to electron spectroscopy analysis, the
benefits they provide, and any relevant concerns or disadvantages are outlined. A number of alternate
morphologies and deviations from a uniform concentric core-shell structure are described. The effects
these structural variations have on data from such samples are identified, and methods for their
interpretation and analysis are discussed.
Table 1 — Summary of methods and analyses outlined in this document for the measurement of
the thickness and composition of nanoparticle coatings
Clause Details
The use of simple numerical modelling to generate estimated XPS
peak intensities from nanoparticles of a defined morphology. A
6.4 Numerical methods method by which such modelling can be performed is provided,
alongside a simple MathWorks® MATLAB script for performing
such calculations.
The use of methods for calculation of overlayer thicknesses using
empirical or semi-empirical formulae derived from theory or mod-
6.5 Descriptive formulae elling. Typically, these are methods whereby measured data can
be input directly into a set of equations in order to derive a single
calculated coating thickness value.
The use of electron spectroscopy modelling and simulation software.
6.6 Modelling and simulation software SESSA is described in detail as an example, and comparisons between
it and the other methods herein are summarised, with examples.
Overview of the analysis of the inelastic background signal in
6.8 Inelastic background analysis XPS for planar overlayers, and the potential application of this for
coated nanoparticles.
Overview of the extraction of elemental compositions from electron
6.9 Elemental composition spectroscopy data for coated nanoparticles And the challenges
posed by systems with internal structure.
The use of variable-photon-energy XPS (e.g. utilising a synchrotron
6.10 Variable excitation energy XPS light source) for depth profiling of nanoparticles. The capabilities
and applications of such methods are described, with examples.
An outline of the use of NAPXPS to coated nanoparticle systems,
specifically regarding the potential differences between samples
6.11 Near-ambient-pressure XPS (NAPXPS)
analysed in ultra-high vacuum conditions compared to those in an
environment relevant to their application.
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ISO/TR 23173:2021(E)
Table 1 (continued)
Clause Details
A summary of Auger electron spectroscopy for the analysis of
nanoparticles, including destructive and non-destructive depth
Clause 7 Auger Electron Spectroscopy (AES)
profiling, imaging, and line-scans of individual particles. Several
examples of use are summarised.
A list of supporting measurement techniques which provide infor-
mation that can be useful when analysing electron spectroscopy
Clause 8 Complementary techniques
data from nanoparticles. The benefits and disadvantages of each
suggested technique are outlined.
A summary of how nanoparticle systems might deviate from the ide-
Clause 9 Deviations from ideality alised model of a uniform, concentric, spherical coated nanoparticle,
and the effects of such deviations on electron spectroscopy data.
6 X-ray photoelectron spectroscopy
6.1 General
XPS provides quantitative information of the surface composition of a sample by the collection of
photoelectrons emitted under exposure to an x-ray beam. The information depth of XPS is limited by
the attenuation of the electrons through the sample, which itself is determined by both the properties
of the sample material, and the kinetic energy of the emitted electrons. Lab-based instruments
typically use either aluminium or magnesium K x-rays at a photon energy of 1 486,6 eV or 1 253,6 eV,
α
respectively, this corresponds to a maximum information depth for the elastic photoelectron peaks of
approximately 10 nm. More recently, lab-based instruments with higher energy X-ray sources have also
been developed, with correspondingly larger information depths due to the higher kinetic energies of
the photoelectrons.
Due to this high surface sensitivity, XPS is an inherently nanoscale technique in terms of depth of
analysis and is thus suited to the analysis and characterisation of nano-objects. It is commonly used
to provide quantitative information on the relative concentrations of elements within the surface of a
sample under the assumption of homogeneity, however with a proper understanding of the underlying
theory and appropriate methodology, greater information on the surface structure of samples can be
extracted.
In most lab-based XPS instruments, the analysis area under standard operating conditions is on the
2 2
order of 0,01 mm to 1 mm with some instruments possessing lens-based area-limiting or micro-
,
-4 2
focussed x-ray beams that allow analysis areas down to 10 mm ; thus for samples of nanomaterials
XPS typically serves as a population measurement technique, where the measured intensities are an
average of the material within the analysis area.
Given the high surface sensitivity of XPS, it is also of crucial importance that samples be prepared,
handled, and cleaned with appropriate procedures. The presence of contaminants within a sample
can drastically influence the results of any measurements made. This is of especial importance
for nanomaterial samples, which may often require more careful preparation, or be susceptible to
additional sources of contamination. ISO 20579-4 discusses the issues relating to the handling of nano-
[12]
objects prior to surface analysis .
6.2 Coating thickness measurement
For flat, uniform surfaces measurement of overlayer thickness using XPS has been understood for some
[14]
time. A formula for the calculation of oxide overlayer thicknesses was developed in the 1970s . More
[15]
recently, ISO 14701 ,dealing with the measurement of silicon oxide thickness using XPS, has been
[16]
published. For reporting on measurements of overlayer thicknesses using XPS, ISO 13424 describes
the information to be included. For cases where the overlayer and substrate peaks to be quantified are
[17]
not of similar kinetic energy, a graphical method known as the "Thickogram" was developed. In any
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ISO/TR 23173:2021(E)
calculation of an overlayer thickness, it is necessary that the peak areas corresponding to the overlayer
and substrate materials are identifiable and measurable.
For samples which are not flat, such methods will be in error due to the effects of sample geometry
on the path-length of electrons through the overlayer. Under the assumption of a uniform overlayer
thickness, a sample with a flat surface oriented for normal emission to the detector presents the
shortest possible direct path for electrons through the overlayer. For a conformal, uniform overlayer
any topography therefore increases this path length, in a manner equivalent to tilting the sample.
Analytical methods to determine the "effective average tilt" of the sample which results from the
[18]
topography have been developed if the topography is either known or can be measured, for example
by atomic force microscopy (AFM). For generic morphologies such as spheres and cylinders, a simpler
[19]
method using the concept of "topofactors" has been shown . In methods of this type, a calculation
is made treating the sample as if flat, and then the relevant "topofactor" is applied to correct for the
[19]–[21]
known topography .
Such methods for measurement of overlayers on topographic samples apply only in the case where the
topography is on the macroscopic scale – that is, they cannot account for topography on the scale of the
electron IMFP’s within the material. At this length scale the volumetric contribution of the coating to the
XPS data becomes significant and the assumption of a continuous underlying substrate becomes invalid.
For nanoparticle samples, the presence of overlayer material on the sides and underside of the particles,
and potentially even particles beneath the outermost layer, can contribute to the measurement result.
6.3 Nanoparticle coating thickness
Several methods for the determination of nanoparticle coating thicknesses from XPS data are
available. These can broadly be categorised into three types: simple numerical modelling, empirically
determined formulae and the use of more rigorous simulation software. When any analysis of XPS data
from nanoparticles is considered however, there are several assumptions which are typically made.
Hereafter, particles conforming to these assumptions are described as "ideal" core-shell particles.
— The analysis area is assumed to be representative of the whole sample, exhibiting no macroscopic
variation. In situations where this is not the case, multiple non-overlapping analysis areas can be
used to assess the effect of any variation.
— Unless specifically accounted for, the nanoparticles are assumed to be randomly deposited, with
[22]
no large-scale ordering . This assumption is not necessary if the analysis method requires, or is
capable of modelling, particles in a specific distribution.
— All of the measured XPS peak intensities are assumed to arise from the nanoparticles, with no
[10]
significant contribution from the substrate or contaminants .
— The core material and coating are each assumed to be uniform in density, i.e. possess no gaps,
density gradients, or similar. It follows from this assumption that the boundary between the core
and coating materials is abrupt, with no mixing layer.
— The core and coating are assumed to form a pair of concentric spheres.
— All the particles in the analysed population are identical in both chemical and physical structure.
— There is no significant contribution to the signal from particles below the outermost layer, i.e. the
electron path lengths do not exceed the particle size.
Depending on the analysis method selected, some of these assumptions might not be necessary, or
deviations can be accounted for. This is particularly true for more advanced simulation methods,
as these can be capable of accounting for many possible structural variations. Because there are a
large number of possible structural variations which are indistinguishable directly from XPS data, it
is important that deviations from the typical assumed case are understood and characterised using
relevant analytical techniques. In some cases, variation in the XPS measurements taken can be used
to corroborate or disprove these assumptions; for example the use of multiple separate analysis areas
to judge sample homogeneity. Sample rotation (with respect to the analyser) may be used to identify
© ISO 2021 – All rights reserved 5
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ISO/TR 23173:2021(E)
the presence, or lack, of signal arising from the substrate, or to indicate structural discrepancies; for
spherical, randomly deposited particles, angle-dependant XPS should not observe differences in the
relative signal observed from the core and shell. Any discrepancy would therefore be due either to
signal from the substrate, or structural deviations.
In most realistic scenarios, many of these assumptions are invalid to some degree. The effects of
deviations from the assumed morphology are discussed in Clause 9.
6.4 Numerical methods
In general, a numerical modelling approach involves writing a simple script or program to calculate
relative XPS intensities for the core and overlayer materials arising from a nanoparticle. By per
...
TECHNICAL ISO/TR
REPORT 23173
First edition
Surface chemical analysis — Electron
spectroscopies — Measurement of
the thickness and composition of
nanoparticle coatings
PROOF/ÉPREUVE
Reference number
ISO/TR 23173:2021(E)
©
ISO 2021
---------------------- Page: 1 ----------------------
ISO/TR 23173:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 23173:2021(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 1
5 Overview . 3
6 X-ray photoelectron spectroscopy. 4
6.1 General . 4
6.2 Coating thickness measurement. 4
6.3 Nanoparticle coating thickness . 5
6.4 Numerical methods . 6
6.5 Descriptive formulae. 9
6.6 Modelling and simulation software .11
6.7 Method comparisons .12
6.8 Background analysis .15
6.9 Elemental composition .16
6.10 Variable excitation energy XPS .18
6.10.1 General.18
6.10.2 Qualitative depth-profiling . .19
6.10.3 Quantitative depth-profiling .21
6.11 Near-ambient-pressure XPS (NAP-XPS) .22
6.11.1 General.22
6.11.2 Internal structure of bimetallic NP catalysts .23
6.11.3 Measurement of NP's in liquid suspension .23
7 Auger electron spectroscopy .25
7.1 General .25
7.2 Coating thickness measurement.25
7.2.1 General.25
7.2.2 Destructive depth-profiling .26
7.2.3 Non-destructive depth-profiling .26
7.2.4 Elemental composition . .26
7.2.5 Imaging and line scans.27
8 Complementary analysis .29
9 Deviations from ideality .31
9.1 General .31
9.2 Multilayered coatings .32
9.3 Other non-ideal cases .33
Annex A (informative) Example script for modelling of XPS data from nanoparticles .38
Bibliography .40
© ISO 2021 – All rights reserved PROOF/ÉPREUVE iii
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ISO/TR 23173:2021(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 201, Surface chemical analysis,
Subcommittee SC 7, Electron spectroscopies.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 23173:2021(E)
Introduction
Recently, there has been increasing development and use of nanoparticles in a wide range of application
[1]–[7]
areas, including catalysis, medicine, energy, optoelectronics and cosmetics . In particular,
nanoparticles having some form of coating layer, which is present either by design or due to incidental
[8]–
processes such as contamination or oxidation, are among the most commonly studied and utilised
[11]
. An essential part of the characterisation of nanoparticles is the measurement of the surface
properties because a large proportion of the material is at a surface or interface. In the case of coated
nanoparticles, the thickness and composition of the coating has a significant role determining its
functional properties and defines the interaction of the particle with its environment. Many applications
require nanoparticles to have coatings that are specifically designed in order to achieve a desired
level of performance. Measurement of surface composition and coating thickness of nanoparticles
is a challenge to which electron spectroscopies are well suited, due to high surface sensitivity, well-
understood physical principles and accessibility. Such measurements can have a significant dependence
on sample format and condition; sample handling and provenance of nanoparticle samples for surface
[12]
chemical analysis are addressed in ISO 20579 . A general introduction to the challenges of surface
[13]
chemical analysis of nanostructured materials is provided in ISO/TR 14187 .
© ISO 2021 – All rights reserved PROOF/ÉPREUVE v
---------------------- Page: 5 ----------------------
TECHNICAL REPORT ISO/TR 23173:2021(E)
Surface chemical analysis — Electron spectroscopies
— Measurement of the thickness and composition of
nanoparticle coatings
1 Scope
This document provides a description of methods by which the coating thickness and chemical
composition of "core-shell" nanoparticles (including some variant and non-ideal morphologies) can
be determined using electron spectroscopy techniques. It identifies the assumptions, challenges,
and uncertainties associated with each method. It also describes protocols and issues for the general
analysis of nanoparticle samples using electron spectroscopies, specifically in relation to their
importance for measurements of coating thicknesses.
This document focuses on the use of electron spectroscopy techniques, specifically X-ray photoelectron
spectroscopy, Auger electron spectroscopy, and synchrotron-based methods. These cannot provide all
of the information necessary for accurate analysis and therefore some additional analytical methods
are outlined in the context of their ability to aid in the interpretation of electron spectroscopy data.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 18115-1, Surface chemical analysis — Vocabulary — Part 1: General terms and terms used in
spectroscopy
ISO 18115-2, Surface chemical analysis — Vocabulary — Part 2: Terms used in scanning-probe microscopy
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18115-1 and ISO 18115-2
apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Symbols and abbreviated terms
X subscripts denote the material of the overlayer
Y subscripts denote the material of the core
x subscripts denote a specific photoelectron peak from material X
y subscripts denote a specific photoelectron peak from material Y
I
intensity of electrons arising from a peak, i
i
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ISO/TR 23173:2021(E)
I intensity of electrons from peak i arising from pure material I
iI,
a
vertical thickness of the overlayer material at a given position
b
vertical thickness of the core material at a given position
effective attenuation length of electrons from peak i travelling through material J
L
iJ,
R
nanoparticle core radius
T
thickness of the overlayer
d
horizontal displacement of a specific line of material
angle between the central vertical axis of the particle and the point of the particle's surface
θ
which is at displacement x
normalised intensity ratio of the intensities of peaks x and y
A
xy,
γ
dimensionless scaling factor
T estimated overlayer thickness for a large sphere
∞
T
estimated overlayer thickness for infinitesimally small particles
0
T
estimated overlayer thickness for a nanoparticle
NP
AES Auger electron spectroscopy
AFM atomic force microscopy
CSNP core-shell nanoparticle
EAL effective attenuation length
EDX energy dispersive X-ray analysis
ICP-AES inductively coupled plasma atomic emission spectroscopy
IMFP inelastic mean free path
KE kinetic energy
MPA mercaptopropanoic acid
NAP-XPS near-ambient-pressure x-ray photoelectron spectroscopy
NP nanoparticle
SAM scanning Auger microscopy
SANS small angle neutron scattering
SAXS small-angle X-ray scattering
SEM scanning electron microscopy
TEM transmission electron microscopy
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TOP trioctylphosphine
UHV ultra-high vacuum
XPS X-ray photoelectron spectroscopy
5 Overview
The methods described in this document are listed by clause and outlined in Table 1. The primary use
detailed is the determination of the thickness of a nanoparticle coating from electron spectroscopy
data, for which three main methods are described, with a specific example given for each. Methods for
coating thickness determination that are described in detail include the use of descriptive formulae
for calculation of coating thicknesses from X-ray photoelectron spectroscopy (XPS) peak intensities;
numerical modelling of XPS intensities from nanoparticles, and general structure and layer thickness
determination by the use of in-depth simulation software. Interpretation of sample composition from
electron spectroscopy data for layered samples is discussed. Rudimentary analysis of the inelastic
background in XPS data is described, alongside the relevant considerations for interpreting inelastic
backgrounds from nanoparticle samples. Discussions of the use and potential benefits of synchrotron-
XPS, near-ambient-pressure XPS (NAP-XPS), and Auger electron spectroscopy (AES) are included, along
with any related issues and considerations. For all methods of analysis, additional characterisation
is required before confident estimates of coating thickness or composition can be made. Therefore, a
range of measurement techniques which are complementary to electron spectroscopy analysis, the
benefits they provide, and any relevant concerns or disadvantages are outlined. A number of alternate
morphologies and deviations from a uniform concentric core-shell structure are described. The effects
these structural variations have on data from such samples are identified, and methods for their
interpretation and analysis are discussed.
Table 1 — Summary of methods and analyses outlined in this document for the measurement of
the thickness and composition of nanoparticle coatings
Clause Details
The use of simple numerical modelling to generate estimated XPS
peak intensities from nanoparticles of a defined morphology. A
6.4 Numerical modelling of XPS data method by which such modelling can be performed is provided,
alongside a simple MathWorks® MATLAB script for performing
such calculations.
The use of methods for calculation of overlayer thicknesses using
empirical or semi-empirical formulae derived from theory or mod-
6.5 Descriptive formulae of XPS data elling. Typically, these are methods whereby measured data can
be input directly into a set of equations in order to derive a single
calculated coating thickness value.
The use of electron spectroscopy modelling and simulation software.
6.6 Modelling and simulation software SESSA is described in detail as an example, and comparisons between
it and the other methods herein are summarised, with examples.
Overview of the analysis of the inelastic background signal in
6.8 Background analysis XPS for planar overlayers, and the potential application of this for
coated nanoparticles.
Overview of the extraction of elemental compositions from electron
6.9 Elemental composition spectroscopy data for coated nanoparticles And the challenges
posed by systems with internal structure.
The use of variable-photon-energy XPS (e.g. utilising a synchrotron
6.10 Variable excitation energy XPS light source) for depth profiling of nanoparticles. The capabilities
and applications of such methods are described, with examples.
An outline of the use of NAPXPS to coated nanoparticle systems,
specifically regarding the potential differences between samples
6.11 Near-ambient-pressure XPS (NAPXPS)
analysed in ultra-high vacuum conditions compared to those in an
environment relevant to their application.
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Table 1 (continued)
Clause Details
A summary of Auger electron spectroscopy for the analysis of
nanoparticles, including destructive and non-destructive depth
Clause 7 Auger Electron Spectroscopy (AES)
profiling, imaging, and line-scans of individual particles. Several
examples of use are summarised.
A list of supporting measurement techniques which provide infor-
mation that can be useful when analysing electron spectroscopy
Clause 8 Complementary techniques
data from nanoparticles. The benefits and disadvantages of each
suggested technique are outlined.
A summary of how nanoparticle systems might deviate from the ide-
Clause 9 Deviations from ideality alised model of a uniform, concentric, spherical coated nanoparticle,
and the effects of such deviations on electron spectroscopy data.
6 X-ray photoelectron spectroscopy
6.1 General
XPS provides quantitative information of the surface composition of a sample by the collection of
photoelectrons emitted under exposure to an x-ray beam. The information depth of XPS is limited by
the attenuation of the electrons through the sample, which itself is determined by both the properties
of the sample material, and the kinetic energy of the emitted electrons. Lab-based instruments
typically use either aluminium or magnesium K x-rays at a photon energy of 1 486,6 eV or 1 253,6 eV,
α
respectively, this corresponds to a maximum information depth for the elastic photoelectron peaks of
approximately 10 nm. More recently, lab-based instruments with higher energy X-ray sources have also
been developed, with correspondingly larger information depths due to the higher kinetic energies of
the photoelectrons.
Due to this high surface sensitivity, XPS is an inherently nanoscale technique in terms of depth of
analysis and is thus suited to the analysis and characterisation of nano-objects. It is commonly used
to provide quantitative information on the relative concentrations of elements within the surface of a
sample under the assumption of homogeneity, however with a proper understanding of the underlying
theory and appropriate methodology, greater information on the surface structure of samples can be
extracted.
In most lab-based XPS instruments, the analysis area under standard operating conditions is on the
2 2
order of 0.01 mm to 1 mm with some instruments possessing lens-based area-limiting or micro-
,
-4 2
focussed x-ray beams that allow analysis areas down to 10 mm ; thus for samples of nanomaterials
XPS typically serves as a population measurement technique, where the measured intensities are an
average of the material within the analysis area.
Given the high surface sensitivity of XPS, it is also of crucial importance that samples be prepared,
handled, and cleaned with appropriate procedures. The presence of contaminants within a sample
can drastically influence the results of any measurements made. This is of especial importance
for nanomaterial samples, which may often require more careful preparation, or be susceptible to
additional sources of contamination. ISO 20579-4 discusses the issues relating to the handling of nano-
[12]
objects prior to surface analysis .
6.2 Coating thickness measurement
For flat, uniform surfaces measurement of overlayer thickness, using XPS has been understood for some
[14]
time. A formula for the calculation of oxide overlayer thicknesses was developed in the 1970s . More
[15]
recently, ISO 14701 ,dealing with the measurement of silicon oxide thickness using XPS, has been
[16]
published. For reporting on measurements of overlayer thicknesses using XPS, ISO 13424 describes
the information to be included. For cases where the overlayer and substrate peaks to be quantified are
[17]
not of similar kinetic energy, a graphical method known as the "Thickogram" was developed. In any
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calculation of an overlayer thickness, it is necessary that the peak areas corresponding to the overlayer
and substrate materials are identifiable and measurable.
For samples which are not flat, such methods will be in error due to the effects of sample geometry
on the path-length of electrons through the overlayer. Under the assumption of a uniform overlayer
thickness, a sample with a flat surface oriented for normal emission to the detector presents the
shortest possible direct path for electrons through the overlayer. For a conformal, uniform overlayer
any topography therefore increases this path length, in a manner equivalent to tilting the sample.
Analytical methods to determine the "effective average tilt" of the sample which results from the
[18]
topography have been developed if the topography is either known or can be measured, for example
by atomic force microscopy (AFM). For generic morphologies such as spheres and cylinders, a simpler
[19]
method using the concept of "topofactors" has been shown . In methods of this type, a calculation
is made treating the sample as if flat, and then the relevant "topofactor" is applied to correct for the
[19]–[21]
known topography .
Such methods for measurement of overlayers on topographic samples apply only in the case where
the topography is on the macro-scale – that is, they cannot account for topography on the scale of the
electron IMFP’s within the material. At this length scale the volumetric contribution of the coating to the
XPS data becomes significant and the assumption of a continuous underlying substrate becomes invalid.
For nanoparticle samples, the presence of overlayer material on the sides and underside of the particles,
and potentially even particles beneath the outermost layer, can contribute to the measurement result.
6.3 Nanoparticle coating thickness
Several methods for the determination of nanoparticle coating thicknesses from XPS data are
available. These can broadly be categorised into three types: simple numerical modelling, empirically
determined formulae and the use of more rigorous simulation software. When any analysis of XPS data
from nanoparticles is considered however, there are several assumptions which are typically made.
Hereafter, particles conforming to these assumptions are described as "ideal" core-shell particles.
— The analysis area is assumed to be representative of the whole sample, exhibiting no macroscopic
variation. In situations where this is not the case, multiple non-overlapping analysis areas can be
used to assess the effect of any variation.
— Unless specifically accounted for, the nanoparticles are assumed to be randomly deposited, with
[22]
no large-scale ordering . This assumption is not necessary if the analysis method requires, or is
capable of modelling, particles in a specific distribution.
— All of the measured XPS peak intensities are assumed to arise from the nanoparticles, with no
[10]
significant contribution from the substrate or contaminants .
— The core material and coating are each assumed to be uniform in density, i.e. possess no gaps,
density gradients, or similar. It follows from this assumption that the boundary between the core
and coating materials is abrupt, with no mixing layer.
— The core and coating are assumed to form a pair of concentric spheres.
— All the particles in the analysed population are identical in both chemical and physical structure.
— There is no significant contribution to the signal from particles below the outermost layer, i.e. the
electron path lengths do not exceed the particle size.
Depending on the analysis method selected, some of these assumptions might not be necessary, or
deviations can be accounted for. This is particularly true for more advanced simulation methods,
as these can be capable of accounting for many possible structural variations. Because there are a
large number of possible structural variations which are indistinguishable directly from XPS data, it
is important that deviations from the typical assumed case are understood and characterised using
relevant analytical techniques. In some cases, variation in the XPS measurements taken can be used
to corroborate or disprove these assumptions; for example the use of multiple separate analysis areas
to judge sample homogeneity. Sample rotation (with respect to the analyser) may be used to identify
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the presence, or lack, of signal arising from the substrate, or to indicate structural discrepancies; for
spherical, randomly deposited particles, angle-dependant XPS should not observe differences in the
relative signal observed from the core and shell. Any discrepancy would therefore be due either to
signal from the substrate, or structural deviations.
In most realistic scenarios, many of these assumptions are invalid to some degree. The effects of
deviations from the assumed morphology are discussed in Clause 9.
6.4 Numerical methods
In general, a numerical modelling approach involves writing a simple script or program to calculate
relative XPS intensities for the core and overlayer materials arising from a nanoparticle. By
...
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