Electron Attenuation Length Research Articles

Applications of the National Institute of Standards and Technology (NIST) database for the simulation of electron spectra for surface analysis for quantitative x-ray photoelectron spectroscopy of nanostructures

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.

Monolayer attenuation length of low-energy electrons in Gd and Tb

The electron attenuation length is a material property, which defines the probing depth of all electron spectroscopies. For kinetic energies below 50 eV, very few data exist in the literature, although this energy range is vital for electron spectroscopy using low-energy electrons, e.g., for studying the valence electronic band structure. Extremely short attenuation lengths are expected for the transition metals gadolinium and terbium. To determine the electron attenuation length in these materials, we conducted inverse-photoemission overlayer experiments with ultrathin Gd and Tb films on W(110). For 10-eV electrons, we find an attenuation length of about one monolayer in Gd and even less in Tb, i.e., two thirds (or more) of the electrons are stopped by only one monolayer. These results imply extremely short information depths for electron spectroscopies with low energies on the lanthanides Gd and Tb.

Thickness measurement of nm HfO2 films

Main textA pilot study for the thickness measurement of HfO2 films was performed by the Surface Analysis Working Group (SAWG) of the Consultative Committee for Amount of Substance (CCQM). The aim of this pilot study was to ensure the equivalency in the measurement capability of national metrology institutes for the thickness measurement of HfO2 films. In this pilot study, the thicknesses of six HfO2 films with nominal thickness from 1 nm to 4 nm were measured by X-ray Photoelectron Spectroscopy (XPS), X-ray Reflectometry(XRR), X-ray Fluorescence Analysis (XRF), Transmission Electron Spectroscopy (TEM), Spectroscopic Ellipsometry (SE) and Rutherford Backscattering Spectrometry (RBS). The reference thicknesses were determined by mutual calibration of a zero-offset method (Medium Energy Ion Scattering Spectroscopy (MEIS) of KRISS) and a method traceable to the length unit (the average thicknesses of three XRR data except the thinnest film). These reference thicknesses are traceable to the length unit because they are based on the traceability of XRR. For the thickness measurement by XPS, the effective attenuation length of Hf 4f electrons was determined. In the cases of XRR and TEM, the offset values were determined from a linear fitting between the reference thicknesses and the individual data by XRR and TEM. The amount of substance of HfO2, expressed as thickness of HfO2 films (in both linear and areal density units), was found to be a good subject for a CCQM key comparison.To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCQM.

Experimental determination of electron attenuation lengths in complex materials by means of epitaxial film growth: Advantages and challenges

Accurate electron attenuation lengths are of critical importance in using electron spectroscopic methods to quantitatively characterize complex materials. Here, the authors show that analysis of core-level and valence-band x-ray photoelectron spectra excited with monochromatic AlKα x-rays from the substrate and measured as a function of film thickness can be used to determine electron attenuation lengths in epitaxial SrTiO3 films on Ge(001). Closely lattice-matched epitaxial heterojunctions are ideal systems for determining attenuation lengths provided the films grow in a layer-by-layer fashion, leading to atomically flat surfaces, and the buried interfaces are atomically abrupt. In principle, either the rate of attenuation of substrate peak intensities or the rate of increase of film peak intensities can be used for this purpose. However, the authors find that structural nonuniformities in the films reduce the accuracy of electron attenuation lengths determined from photoelectrons that originate within the films. A more reliable source of information is found in photoelectrons from the substrate which traverse the film. By using the energy dependence of calculated electron attenuation lengths from the NIST database in combination with Ge 3d core and Ge-derived valence-band intensities, the authors determine electron attenuation length as a function of kinetic energy for SrTiO3.

Determination of effective attenuation length of slow electrons in polymer films

Slow electrons (with energy below 10 eV) play an important role in nature and technology. For instance, they are believed to initiate solubility change in extreme ultraviolet resists. Depending on their mobility, such secondary electrons can lead to image blur and degradation of patterning resolution. Hence, it is important to characterize the transport of slow electrons by measuring parameters such as the effective attenuation length (EAL). We present a technique that allows for prompt characterization of EAL in polymer films. In this experiment, slow electrons are generated in a substrate upon absorption of x-ray photons. The attenuation of electron flux by a polymer film is measured as a function of film thickness, allowing for the determination of EAL for slow electrons. We illustrate this method with poly(hydroxy styrene) and poly(methyl metacrylate) films. Furthermore, we propose an improvement for this technique that would enable the measurement of EAL as a function of electron kinetic energy.

Enhanced near-Infrared Photoresponse from Nanoscale Ag-Au Alloyed Films

Alloying of metals provides a vast parameter space for tuning of material, chemical, and mechanical properties, impacting disciplines ranging from photonics and catalysis to aerospace. From an optical point-of-view, pure thin metal films yield enhanced light absorption due to their cavity effects. However, an ideal metal–semiconductor photodetector requires not only high absorption, but also long hot carrier attenuation lengths in order to efficiently collect excited carriers. Here we demonstrate that Ag-Au alloys provide an ideal model system for controlling the optical and electrical responses in nanoscale thin metal films for hot carrier photodetectors with improved performance. While pure Ag and Au have long hot carrier attenuation lengths >20 nm, their optical absorption is insufficient for high efficiency devices. Instead, we find that alloying Ag and Au enhances the absorption by ∼50% while maintaining attenuation lengths >15 nm, currently limited by grain boundary scattering, although the electron attenuation length of pure Au outperforms pure Ag as well as all of the alloys investigated here. Further, our density functional theory analysis shows that the addition of small amounts of Au to the Ag lattice significantly enhances the hot hole generation rate. Combined, these findings suggest a route to high efficiency hot carrier devices based on metallic alloying with potential applications ranging from photodetectors and sensors to improved catalytic materials.

Effective attenuation lengths of low energy electrons in MgO thin films

The effective attenuation length (EAL) of electrons in MgO films has been measured in the (5.5–28) eV energy range by the over-layer method and correlated with the band structure of the material. As expected, the EAL is found to increase when the electron energy is decreased, but the obtained values are smaller than those predicted by the universal curve. The comparison of the experimental results with calculated optical properties available in the literature suggests that, for energies lower than 20 eV, the relevant scattering mechanisms are described by the imaginary part of the dielectric function, accounting in particular for the steep increase of the EAL for energies smaller than the insulator band gap.

Effective attenuation length of keV photoelectrons in silicon measured by transmission through thin membranes

We use 9 nm and 15 nm thin membranes for determining the effective attenuation length of photoelectrons in silicon. One side of silicon membranes was covered with a thin film of aluminium and exposed to X-rays with energies from 3 to 8 keV. We recorded Al 1s and 2s photoelectrons that were (a) emitted from the Al film directly and (b) transmitted through the membranes. With the help of the ratio of both yields, we obtained values for the effective attenuation length (EAL) of electrons with kinetic energies up to 7.9 keV in silicon. The experimentally determined EAL values are smaller than obtained from different predictive equations. Using a power law fit EAL(k,p)=kEkinp to the experimental and predicted EAL values we find that mainly different is the pre-factor of the power law, k, while the exponent, p, i.e. the dependence on kinetic energy Ekin is represented well. Our study underlines the feasibility of using membranes for investigating surfaces under (near) ambient pressure conditions by photoelectron spectroscopy and points out the advantages of employing hard X-rays.

Versatile technique for assessing thickness of 2D layered materials by XPS

X-ray photoelectron spectroscopy (XPS) has been utilized as a versatile method for thickness characterization of various two-dimensional (2D) films. Accurate thickness can be measured simultaneously while acquiring XPS data for chemical characterization of 2D films having thickness up to approximately 10 nm. For validating the developed technique, thicknesses of few-layer graphene (FLG), MoS2 and amorphous boron nitride (a-BN) layer, produced by microwave plasma chemical vapor deposition (MPCVD), plasma enhanced chemical vapor deposition (PECVD), and pulsed laser deposition (PLD) respectively, were accurately measured. The intensity ratio between photoemission peaks recorded for the films (C 1s, Mo 3d, B 1s) and the substrates (Cu 2p, Al 2p, Si 2p) is the primary input parameter for thickness calculation, in addition to the atomic densities of the substrate and the film, and the corresponding electron attenuation length (EAL). The XPS data was used with a proposed model for thickness calculations, which was verified by cross-sectional transmission electron microscope (TEM) measurement of thickness for all the films. The XPS method determines thickness values averaged over an analysis area which is orders of magnitude larger than the typical area in cross-sectional TEM imaging, hence provides an advanced approach for thickness measurement over large areas of 2D materials. The study confirms that the versatile XPS method allows rapid and reliable assessment of the 2D material thickness and this method can facilitate in tailoring growth conditions for producing very thin 2D materials effectively over a large area. Furthermore, the XPS measurement for a typical 2D material is non-destructive and does not require special sample preparation. Therefore, after XPS analysis, exactly the same sample can undergo further processing or utilization.

Low-energy photoelectron transmission through aerosol overlayers.

The transmission of low-energy (<1.8 eV) photoelectrons through the shell of core-shell aerosol particles is studied for liquid squalane, squalene, and di-ethyl-hexyl-sebacate shells. The photoelectrons are exclusively formed in the core of the particles by two-photon ionization. The total photoelectron yield recorded as a function of shell thicknesses (1-80 nm) shows a bi-exponential attenuation. For all substances, the damping parameter for shell thicknesses below 15 nm lies around 8 to 9 nm and is tentatively assigned to the electron attenuation length at electron kinetic energies of ≲1 eV. The significantly larger damping parameters for thick shells (>20 nm) are presumably a consequence of distorted core-shell structures. A first comparison of aerosol and traditional thin film overlayer methods is provided.

Ionization potential and electron attenuation length of titanium dioxide deposited by atomic layer deposition determined by photoelectron spectroscopy in air

Photoelectron emission spectroscopy in air (PESA) has been used to investigate titanium dioxide (TiO2) deposited by atomic layer deposition (ALD). A procedure has been developed to unambiguously determine the photoemission threshold energy (also referred to as the “ionization potential”) of TiO2 thin films, avoiding inherent artifacts due to photoelectron emission from the substrate, which supplies misleading results. This has been achieved using PESA measurements performed as a function of TiO2 film thickness on two substrates with different work functions. We find that proper measurements of the photoemission threshold energy (including work function and ionization potential) of thin films by PESA require the use of films much thicker than their electron attenuation length (EAL). A photoemission threshold energy of 5.0±0.2eV is obtained for TiO2 and has been attributed to a trap level due to oxygen vacancies, which lie within the band gap of the TiO2. The analysis of the photoemission decay with film thickness also provides a method for determining a “practical” (or effective) EAL at excitation energy slightly above the photoemission threshold energy of the material. We extract an EAL for the deposited TiO2 of 0.65±0.02nm (at 0.5eV). The procedure can also be adopted for determining the thickness of extremely thin films, provided their thickness is smaller than their EAL.

Thickness Measurement of Oxide and Carbonaceous Layers on a 28Si Sphere by XPS

Surface layer characterization is crucial for the high-accuracy determination of the Avogadro constant using 28Si enriched spheres to redefine the kilogram. In this paper, we measured the thickness of the oxide layer (OL) and carbonaceous layer (CL) on a 28Si-enriched sphere by X-ray photoelectron spectroscopy (XPS). A mechanism for the rotation of the sphere was integrated into the XPS system to measure the thickness at different points on the surface of the sphere. A SI-traceable X-ray reflectometry system was used to determine an attenuation length for the Si 2p electrons in SiO2, which was an important parameter for the calculation of the oxide thickness on the Si sphere by XPS. The average thickness and uncertainty budget of the OL and CL were evaluated.

Low energy electron attenuation lengths in core-shell nanoparticles.

A velocity map imaging spectrometer is used to measure photoemission from free core-shell nanoparticles, where a salt core is coated with a liquid hydrocarbon shell (i.e. squalane). By varying the radial thickness of the hydrocarbon shell, electron attenuation lengths (EALs) are determined by measuring the decay in photoemission intensity from the salt core. In squalane, electrons with kinetic energy (KE) above 2 eV are found to have EALs of 3-5 nm, whereas electrons with smaller KE (<2 eV) have significantly larger EALs of >15 nm. These results (in the context of other energy-resolved EAL measurements) suggest that the energy dependent behavior of low energy electrons is similar in dielectrics when KE > 2 eV. At this energy the EALs do not appear to exhibit strong energy dependence. However, at very low KE (<2 eV), the EALs diverge and appear to be extremely material dependent.

A Technique for Calculation of Shell Thicknesses for Core-Shell-Shell Nanoparticles from XPS Data.

This paper extends a straightforward technique for the calculation of shell thicknesses in core-shell nanoparticles to the case of core-shell-shell nanoparticles using X-ray Photoelectron Spectroscopy (XPS) data. This method can be applied by XPS analysts and does not require any numerical simulation or advanced knowledge, although iteration is required in the case where both shell thicknesses are unknown. The standard deviation in the calculated thicknesses vs simulated values is typically less than 10%, which is the uncertainty of the electron attenuation lengths used in XPS analysis.

Towards monomaterial p-n junctions: Single-step fabrication of tin oxide films and their non-destructive characterisation by angle-dependent X-ray photoelectron spectroscopy

The application of a non-destructive method for characterization of electronic structure of an ultra-thin SnO1&amp;lt;x&amp;lt;2 layer synthesized by spin coating on Si wafers was demonstrated. Utilizing angle dependent XPS, we quantified stoichiometry changes inside the SnO1&amp;lt;x&amp;lt;2 layers of thickness comparable with the electron attenuation length. The O/Sn concentration varied from 1.25 near the SnOx surface to 1.10 near the substrate/overlayer interface. Deviations from ideal stoichiometry are caused by defects, and defect levels affect the band structure of the SnOx layers. By investigation of the valence band region, followed by main core level position tracking, changes of electronic parameters like energy levels shift were identified. The results indicated a downward energy levels shift by 0.45 eV in SnOx layers at the SiO2/SnOx interface. In combination with the detected upward energy levels shift in the substrate's electronic structure, these results suggest a negative charge displacement across the SiO2 layer. As a consequence, there is a significant carrier concentration gradient in the layer, from a nearly insulating oxide at the SnOx surface to a semiconducting one at the bottom of the SnOx film. The results showed that the application of a simple and cost-effective method allows tuning the materials' properties towards the one-step fabrication of materials with ambipolar doping.

Quantitative determination of ligand densities on nanomaterials by X-ray photoelectron spectroscopy.

X-ray photoelectron spectroscopy (XPS) is a nearly universal method for quantitative characterization of both organic and inorganic layers on surfaces. When applied to nanoparticles, the analysis is complicated by the strong curvature of the surface and by the fact that the electron attenuation length can be comparable to the diameter of the nanoparticles, making it necessary to explicitly include the shape of the nanoparticle to achieve quantitative analysis. We describe a combined experimental and computational analysis of XPS data for molecular ligands on gold nanoparticles. The analysis includes scattering in both Au core and organic shells and is valid even for nanoparticles having diameters comparable to the electron attenuation length (EAL). To test this model, we show experimentally how varying particle diameter from 1.3 to 6.3 nm leads to a change in the measured AC/AAu peak area ratio, changing by a factor of 15. By analyzing the data in a simple computational model, we demonstrate that ligand densities can be obtained, and, moreover, that the actual ligand densities for these nanoparticles are a constant value of 3.9 ± 0.2 molecules nm(-2). This model can be easily extended to a wide range of core-shell nanoparticles, providing a simple pathway to extend XPS quantitative analysis to a broader range of nanomaterials.

Growth of single and multi-layer graphene on Ir(1 0 0)

We report on the high temperature chemical vapor deposition of ethylene on Ir(100) and the resulting development of single and multi-layer graphene films. By employing X-ray photoemission electron spectromicroscopy, low energy electron microscopy and related microprobe methods, we investigate nucleation and growth of graphene as a function of the concentration of the chemisorbed carbon lattice gas. Further, we characterize the morphology and crystal structure of graphene as a function of temperature, revealing subtle changes in bonding occurring upon cooling from growth to room temperature. We also identify conditions to grow multi-layer flakes. Their thickness, unambiguously determined through the analysis of the intensity of the Ir 4f and C 1s emission, is correlated to the electron reflectivity at very low kinetic energy. The effective attenuation length of electrons in few-layer graphene is estimated to be 4.4 and 8.4Å at kinetic energies of 116 and 338eV, respectively.

Distance Dependence of Exciton Dissociation at a Phthalocyanine–C60 Interface

Exciton dissociation at donor–acceptor (DA) interfaces is critical for the operation of organic photovoltaic (OPV) devices, yet a detailed physical understanding of this process is lacking. This work examines an important aspect of this process, namely the dependence of the exciton dissociation rate on distance from the DA interface. Time-resolved two-photon photoemission (TR-2PPE) measurements were performed on bilayer H2Pc\C60 heterojunctions fabricated using organic molecular beam epitaxy (MBE) with varying H2Pc thickness. In the measurements, the dynamics of the H2Pc S1 exciton population created with a 1.55 eV pump pulse were monitored via photoemission with a delayed UV probe pulse. The depth sensitivity of TR-2PPE, due to the short electron attenuation length, provides the means to follow excited state dynamics as a function of H2Pc thickness. Analysis of the S1 population decay as a function of H2Pc thickness revealed that the electron transfer rate for the first H2Pc layer, adjacent to C60, is kCT = (2.3 ± 0.4) × 1012 s–1. Exciton dissociation is reduced by a factor of at least 10 for the second H2Pc layer and beyond.

Stability of Graphene Oxide Film to Electron Beam Irradiation and Possible Thickness Dependence of Electron Attenuation

Abstract Graphene oxide (GO), one of the best transparent substrates for electron microscopy of biological substances, is known to be not very stable to exposure to electron beams (e-beams). We present a method of the preparation of GO film highly resistive to e-beams by controlling layer-by-layer thickness and quantitatively examined the stability of GO film thus prepared. Scanning transmission electron microscopic measurements were engaged in these films with 10-kV acceleration-voltage. A simple method is proposed to classify the layer structure of GO. As an application of the method, we determined the electron attenuation length through GO film in nm scale.

Comparative surface studies of oxygen passivated FeCo nanoparticles and thin films

AbstractWe present results on the comparative studies of the surfaces of oxygen passivated FeCo nanoparticles (NPs) and thin films prepared by inert gas condensation and pulse laser deposition, respectively. Morphology and structure of the NPs and thin films are investigated by transmission electron microscopy (TEM) and X‐ray diffraction. Due to the larger surface area, thicker oxide over‐layer is observed for particles in comparison with thin films. X‐ray photoelectron spectroscopy (XPS) spectra strongly depended on the morphology and oxide over layer thicknesses for nanostructures. This leads to observation of clearly different background and is explained in term of different cationic distribution in the overlayer, thicknesses, and morphology (enhanced surface area) of the nanostructures. TEM analyses showed an oxide over‐layer on the NP of approximately 2.6 nm, which is consistent with the theoretical model based on electron attenuation lengths. However, in the case of thin films a much thinner oxide over‐layer of about 1.3 nm was observed. As expected, XPS spectra of oxygen passivated particles show oxide peaks with weak metal peaks compared to oxygen passivated thin films which show significantly high zero‐valent metal peaks.