Summary of ISO/TC 201 Technical Report 23173—Surface chemical analysis—Electron spectroscopies—Measurement of the thickness and composition of nanoparticle coatings
Abstract:including X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and synchrotron techniques, can be employed to calculate the coating thicknesses and compositions of nanoparticles. The document has been developed to review and outline the current state-of-the-art for such measurements. Such analyses of core-shell nanoparticles are common within the literature, however the methods employed are varied; the relative advantages and disadvantages of these methods, and the optimal usage of each ma… Show more
“…For nanoparticle samples, it is also necessary to consider contributions from coating at the sides and beneath the core—therefore, each system must be modelled completely, with expected intensities summed across every point on the surface of the particle to generative expected intensity ratios. This process has been described in more detail in an ISO technical report on nanoparticle coating analysis [ 29 , 30 ]. Fig.…”
Core–shell nanoparticles have attracted much attention in recent years due to their unique properties and their increasing importance in many technological and consumer products. However, the chemistry of nanoparticles is still rarely investigated in comparison to their size and morphology. In this review, the possibilities, limits, and challenges of X-ray photoelectron spectroscopy (XPS) for obtaining more insights into the composition, thickness, and homogeneity of nanoparticle coatings are discussed with four examples: CdSe/CdS quantum dots with a thick coating and a small core; NaYF4-based upconverting nanoparticles with a large Yb-doped core and a thin Er-doped coating; and two types of polymer nanoparticles with a poly(tetrafluoroethylene) core with either a poly(methyl methacrylate) or polystyrene coating. Different approaches for calculating the thickness of the coating are presented, like a simple numerical modelling or a more complex simulation of the photoelectron peaks. Additionally, modelling of the XPS background for the investigation of coating is discussed. Furthermore, the new possibilities to measure with varying excitation energies or with hard-energy X-ray sources (hard-energy X-ray photoelectron spectroscopy) are described. A discussion about the sources of uncertainty for the determination of the thickness of the coating completes this review.
“…For nanoparticle samples, it is also necessary to consider contributions from coating at the sides and beneath the core—therefore, each system must be modelled completely, with expected intensities summed across every point on the surface of the particle to generative expected intensity ratios. This process has been described in more detail in an ISO technical report on nanoparticle coating analysis [ 29 , 30 ]. Fig.…”
Core–shell nanoparticles have attracted much attention in recent years due to their unique properties and their increasing importance in many technological and consumer products. However, the chemistry of nanoparticles is still rarely investigated in comparison to their size and morphology. In this review, the possibilities, limits, and challenges of X-ray photoelectron spectroscopy (XPS) for obtaining more insights into the composition, thickness, and homogeneity of nanoparticle coatings are discussed with four examples: CdSe/CdS quantum dots with a thick coating and a small core; NaYF4-based upconverting nanoparticles with a large Yb-doped core and a thin Er-doped coating; and two types of polymer nanoparticles with a poly(tetrafluoroethylene) core with either a poly(methyl methacrylate) or polystyrene coating. Different approaches for calculating the thickness of the coating are presented, like a simple numerical modelling or a more complex simulation of the photoelectron peaks. Additionally, modelling of the XPS background for the investigation of coating is discussed. Furthermore, the new possibilities to measure with varying excitation energies or with hard-energy X-ray sources (hard-energy X-ray photoelectron spectroscopy) are described. A discussion about the sources of uncertainty for the determination of the thickness of the coating completes this review.
sessa (Simulation of Electron Spectra for Surface Analysis) is a software that was frequently used by the late Charles Fadley, since it provides a convenient means to simulate peak intensities as well as entire spectral regions for photoelectron spectroscopy. X-ray photoelectron spectra can be simulated for several types of nanostructures. sessa can also be utilized in more complex cases, e.g., if the nondipolar terms in the photoelectric ionization cross section need to be taken into account, a typical situation encountered in spectroscopy using synchrotron radiation. The software was initially released in 2005 as a National Institute of Standards and Technology Standard Reference Database. Here, we describe two new features that have recently been added to the newest version (sessa V2.2) of the software, i.e., simulation of surface excitations and an effective approach to account for the energy dependence of the interaction characteristics of emitted photoelectrons. Furthermore, we illustrate some functionalities of sessa by presenting several applications. These include overlayer measurements to determine the effective electron attenuation length, quantitative analysis of impurities in multilayer materials, analysis of ionic liquids, the influence of nondipolar effects for photon energies above a few keV, and analysis of nanoparticles by means of photoelectron spectroscopy.
Electron beam techniques are indispensable tools for the analysis of surfaces in fundamental as well as applied fields of science and technology. Significant improvements have been made in the past decades in the quantitative understanding of electron spectra, particularly with respect to the near-surface transport of signal electrons. The concept of partial intensities is a simple approach providing physical insight into transport of electrons in solids, a numerically convenient means for spectrum modelling and an essential ingredient for spectrum interpretation. The energy-dissipation process of energetic electrons in solids is discussed from the perspective of the partial intensities, offering a unified model for any type of electron beam technique, be it in the quasi-elastic (QE) or the continuous slowing down (CSD) regime. Examples are given for modelling and analysis of electron spectra such as X-ray photoelectron spectra (XPS), Elastic peak electron spectra (EPES) and electron energy-loss spectra (EELS). The physical model as well as the quantities for electron beam interaction with solids are reviewed, and methods for spectrum modelling and analysis are presented. The examples considered demonstrate the high level of accuracy nowadays attainable for characterisation of surfaces and nanostructures employing techniques using medium energy electrons (>100 eV). This is in contrast to the low energy regime, where a number of problems prevent a similar level of understanding. The topic of low energy electrons (LEEs) is rapidly gaining importance since in this energy range the involved electrons not only act as signal carriers, but also actively participate in electrochemical processes of importance in fields ranging from nanotechnology to life sciences. Issues of future importance in the field of LEEs are discussed and recent developments such as the 2-dimensional electron cascade in the scanning field electron microscope are highlighted.
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