Electron Interaction Volume Research Articles

Use quantum mechanical computational methods to investigate interactions between imidacloprid and boron nitride nanotubes

The study investigates the interaction between a boron nitride nanotube (BNNT) and an imidacloprid (IM) molecule. The structure of the BNNT is detailed, measuring 9.964 Å in diameter and 18.186 Å in length. Six interaction geometries (BNNT_IM_1 to BNNT_IM_6) between BNNT and IM are investigated, with BNNT_IM_6 showing the most thermodynamically stable interaction. The electronic interaction energies are calculated, and BNNT_IM_6 presents the lowest value (−1.89 eV), indicating a favorable interaction. Quantum theory of atoms in molecules (QTAIM) and analysis of noncovalent interactions (NCI) support the findings. BNNT_IM_6, with IM inside the nanotube, shows the highest electron sharing, explaining its superior stability. The chlorine-BNNT interaction was revealed to be a determining attractive component. The attraction/repulsion indices highlight BNNT_IM_1 as having the most attractive interaction, driven mainly by oxygen-BNNT and chlorine-BNNT interactions. The investigation emphasizes the importance of considering multiple factors, including electron density and interaction volume, in evaluating the strength of BNNT-IM interactions. This study model opens a proposal to assess the impact of functional groups, from a physicochemical perspective, of a molecule interacting with a nanotube surface.

Design and optimization of a conical electrostatic objective lens of a low-voltage scanning electron microscope for surface imaging and analysis in ultra-high-vacuum environment

Low-voltage scanning electron microscopy (LV-SEM) with landing energies below 5 keV has been widely used due to its advantages in mitigating the damage and charging effects to a specimen and enhancing surface information due to small interaction volume of electrons inside a specimen. Additionally, for elemental analysis of the surfaces of bulk specimens with Auger electron spectroscopy (AES) or electron energy loss spectroscopy (EELS), ultra-high-vacuum (UHV) environment is essential to maintain clean surfaces without the absorption of gas molecules during the electron beam irradiation for the acquisition of spectral data. In this study, we propose the optimal design and condition of a conical Electrostatic Objective Lens (EOL) for a UHV LV-SEM to achieve the high spatial resolution and secondary electron (SE) detection efficiency. The EOL is composed of only the three electrodes (retarding, focusing and booster electrodes) and the insulators, which is suitable for maintaining a UHV environment with less out-gassing. The cone angle of the EOL is determined as 60° to integrate a spectrometer in the UHV LV-SEM and in a large size and a higher tilt angle of the sample. Through the optimization with the simulations, the EOL achieves the minimized spherical and chromatic aberration coefficients of 0.05 and 0.03 mm at the sample side, respectively, at the landing energy of 50 eV and the shortest working distance (WD) of 1 mm for high-resolution imaging. In addition, the probe diameter of the optimized EOL is 2.3 nm at 1 keV and 5.7 nm at 50 eV with a WD of 1 mm and a probe current of 10 pA, which are comparable to previously studied compound objective lenses with magnetic and electrostatic lenses. Using a longer WD of 4 mm for analysis, the probe diameter was 5.4 nm at 1 keV and the SE detection efficiency was 83.3 % owing to the separated scintillator detector structure from the booster electrode.These results imply that the optimized EOL has the potential to be applied to a high-performance UHV LV-SEM for the surface imaging and analysis with a simple system configuration.

Low-voltage electron scattering in advanced extreme ultraviolet masks

To limit the shadowing effect, improve the process stability, and enhance the optical performance of extreme ultraviolet (EUV) masks, several absorbers have been proposed in previous studies. We investigated the effects of some of these absorbers on electron scattering events through Monte Carlo simulations in which the mask throughput was considered at 5 keV. A two-layer structure consisting of a resist and an absorber substrate, rather than a full-mask structure, was used to eliminate the influence of electron scattering on the resist. The effects of electron interaction volume, ray tracing, and scattering dependency on penetration depth, backscattering coefficient, lateral radius, and absorbed energy distribution were analyzed for absorber materials of conventional argon fluoride and advanced EUV masks. The results of the proposed method for electron scattering analysis and prediction exhibited greater agreement with the simulation results than those of relevant conventional methods.

Direct optical-structure correlation in atomically thin dichalcogenides and heterostructures

Atomically thin transition metal dichalcogenides (TMDs) have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios, which can be further modulated by making more complex TMD heterostructures. However, resolution limits of conventional optical methods do not allow for direct optical-structural correlation measurements in these materials, particularly of buried interfaces in TMD heterostructures. Here we use, for the first time, electron beam induced cathodoluminescence in a scanning transmission electron microscope (CL-STEM) to measure optical properties of monolayer TMDs (WS2, MoS2 and WSSe alloy) encapsulated between layers of hBN. We observe dark areas resulting from localized (~ 100 nm) imperfect interfaces and monolayer folding, which shows that the intimate contact between layers in this application-relevant heterostructure is required for proper inter layer coupling. We also realize a suitable imaging method that minimizes electron-beam induced changes and provides measurement of intrinsic properties. To overcome the limitation of small electron interaction volume in TMD monolayer (and hence low photon yield), we find that encapsulation of TMD monolayers with hBN and subsequent annealing is important. CL-STEM offers to be a powerful method to directly measure structure-optical correspondence in lateral or vertical heterostructures and alloys.

Solving the iron quantification problem in low-kV EPMA: An essential step toward improved analytical spatial resolution in electron probe microanalysis—Olivines

AbstractThe relatively recent entry of field emission electron microprobes into the field of microanalysis provides another tool for the study of small features of interest (e.g., mineral and melt inclusions, ex-solution lamellae, grain boundary phases, high-pressure experimental charges). However, the critical limitation for accurate quantitative analysis of these submicrometer- to micrometer-sized features is the relationship between electron beam potential and electron scattering within the sample. To achieve submicrometer analytical volumes from which X-rays are generated, the beam accelerating voltage must be reduced from 15–20 to ≤10 kV (often 5 to 7 kV) to reduce the electron interaction volume from ~3 to ~0.5 μm in common geological materials. At these low voltages, critical Kα X-ray lines of transition elements such as Fe are no longer generated, so L X-ray lines must be used. However, applying the necessary matrix corrections to these L lines is complicated by bonding and chemical peak shifts for soft X-ray transitions such as those producing the FeLα X-ray line. It is therefore extremely challenging to produce accurate values for Fe concentration with this approach. Two solutions have been suggested, both with limitations. We introduce here a new, simple, and accurate solution to this problem, using the common mineral olivine as an example. We also introduce, for the first time, olivine results from a new analytical device, the Extended Range Soft X-ray Emission Spectrometer.

Quantitative Measurement of Iron-Silicides by EPMA Using the Fe Lα and Lβ X-ray Lines: A New Twist to an Old Approach.

The recent availability of Schottky-type field emission electron microprobes provides incentive to consider analyzing micrometer-sized features. Yet, to quantify sub-micrometer-sized features, the electron interaction volume must be reduced by decreasing accelerating voltage. However, the K lines of the transition elements (e.g., Fe) then cannot be excited, and the L lines must be used. The Fe Lα1,2 line is the most intense of the L series but bonding effects change its atomic parameters because it involves a valence band electron transition. For successful traditional electron probe microanalysis, the mass absorption coefficient (MAC) must be accurately known, but the MAC of Fe Lα1,2 radiation by Fe atoms varies from one Fe-compound to another and is not well known. We show that the conventional method of measuring the MAC by an electron probe cannot be used in close proximity to absorption edges, making its accurate determination impossible. Fortunately, we demonstrate, using a set of Fe-silicide compounds, that it is possible to derive an accurate calibration curve, for a given accelerating voltage and takeoff angle, which can be used to quantify Fe in Fe-silicide compounds. The calibration curve can be applied to any spectrometer without calibration and gives accurate quantification results.

High Spatial Resolution EDS Mapping of Nanoparticles at Low Accelerating Voltage

Abstract In conventional energy-dispersive spectroscopy/scanning-electron microscopy (EDS/SEM) analysis, the pear-shaped interaction volume of incident electrons in the bulk sample determines spatial resolution and the actual scope of EDS analysis is usually a few cubic microns. Lowering the accelerating voltage can reduce effectively the interaction volume of the electron beam. In this study, Monte Carlo (MC) simulation was used to analyze effectively the actual interaction between low-energy incident electrons and nanoparticles. MC x ray, a new Monte Carlo program, was used to further simulate EDS mapping of Ni nanoparticles on the top of an Al2O3 matrix at an accelerating voltage of 3 kV, and the theoretical spatial resolution was calculated to be approximately 10 nm. In addition, EDS mapping images of 17-nm Ni nanoparticles were successfully acquired at high spatial resolution similar to the theoretical value by adjusting the operating conditions for SEM-EDS analysis, including the use of immersion objective lens, the removal of electron trap and collimator, and the reduction of working distance as well as detector distance.

Measurements of the quantitative lateral analytical resolution at sputtered gold-layers with the FEG-EPMA JEOL JXA-8530F

In this paper we present measurements of the quantitative lateral analytical resolution of the field emission (FEG) electron microprobe JEOL JXA-8530F. In particular, the minimum thicknesses of gold layers that are suitable for an accurate quantitative analysis are determined for several measurement parameters. Both, low voltage analyses at acceleration voltages from 15 kV to 5 kV and low overvoltage analyses at 13 and 15 kV are regarded. For both methods a minimum analytical resolution well below 170 nm is obtained while the X-ray signal is 5 times lower for the low overvoltage method. Special care is taken for the preparation of the test specimen. A Ag-layer reduces fluorescence effects and the cleaved surfaces are polished by a gentle focussed ion beam. The porosity of the gold layers is regarded by a separate measurement of the density (17.6 g/cm3). The results coincide well with Monte Carlo simulations of the source volume of generated X-rays if the reduced density of the gold layers and experimental determined values of the beam diameter are used. Additionally the results are used to investigate methods to estimate the width of the source volume of emitted X-rays by the superposition of the beam diameter and theoretical predictions of the width of the electron interaction volume. The linear sum of both gives a quite good agreement.

Application of Transmitted Kikuchi Diffraction in Studying Nano-oxide and Ultrafine Metallic Grains.

Transmitted Kikuchi diffraction (TKD) is an emerging SEM-based technique that enables investigation of highly refined grain structures. It offers higher spatial resolution by utilizing conventional electron backscattered diffraction equipment on electron-transparent samples. A successful attempt has been made to reveal nano-oxide grain structures as well as ultrafine severely deformed metallic grains. The effect of electron beam current was studied. Higher beam currents enhance pattern contrast and intensity. Lower detector exposure times could be employed to accelerate the acquisition time and minimize drift and carbon contamination. However, higher beam currents increase the electron interaction volume and compromise the spatial resolution. Lastly, TKD results were compared to orientation mapping results in TEM (ASTAR). Results indicate that a combination of TKD and EDS is a capable tool to characterize nano-oxide grains such as Al2O3 and Cr2O3 with similar crystal structures.

Strategies for Low Accelerating Voltage X-ray Microanalysis of Sub-Micrometer Features with the FE-EPMA

X-ray microanalyses of sub-micrometer features require that a lower accelerating voltage be used. This reduction in the accelerating voltage reduces the penetrating distance of the beam electrons and thereby reduces the analytical volume. Two general strategies have been proposed for achieving the smallest analytical volume. The first approach involves significantly reducing the accelerating voltage to minimize the electron interaction volume. For many element systems, the optimum accelerating voltage is in the 5-8kV range, assuming a field emission electron gun is used for the analysis. A lower kV will reduce the penetrating depth of the electrons further. However with lower kV, the diameter of the electron beam becomes a critical factor in determining the analytical area, and with a lower kV, the beam diameter becomes larger. Mostly this comes from the higher required beam current used due to the lower X-ray production rate at the lower kV. This low-kV strategy also typically requires a different set of X-ray lines be used, since some of the more commonly used X-ray lines are not generated at these lower accelerating voltages. However, it can produce a very small analytical volume (Fig. 1a).

Confocal filtering in cathodoluminescence microscopy of nanostructures

Cathodoluminescence (CL) microscopy allows optical characterization of nanostructures at high spatial resolution. At the nanoscale, a main challenge of the technique is related to the background CL generated within the sample substrate. Here, we implement confocal detection of the CL signal to minimize the background contribution to the measurement. Nano-phosphors were used as point sources to evaluate the filtering capabilities of our confocal CL system, obtaining an axial intensity profile with 2.7 μm full width at half maximum for the central peak, in good correspondence with theoretical expectations. Considering the electron interaction volume, we found that the confocal filter becomes effective for electron energies above 20 keV, when using a 25 μm pinhole (0.86 Airy units). To illustrate our approach, we present confocal CL imaging of gold nanowires and triangular shaped plates deposited on an indium-tin oxide covered glass substrate, comparing the images with those obtained in standard unfiltered CL detection. The results show that confocal CL microscopy is a valuable tool for the investigation of nanostructures on highly cathodoluminescent substrates, widely used in biological and optical applications.

Five-Parameter Grain Boundary Inclination Recovery with EBSD and Interaction Volume Models

While electron backscatter diffraction (EBSD) patterns are often used to present two-dimensional information about a material microstructure, they are in fact a product of the three-dimensional electron interaction volume. Consequently, 3D spatial information exists in EBSD images, which is generally not accessed. Specifically, the inclination of the grain boundary plane may be observed in EBSD patterns taken near grain boundaries. If, at the same time, the shape of an electron interaction volume in the material is known, a grain boundary plane normal direction can be obtained from a sequence of EBSD images taken stepwise in a line crossing the grain boundary. Here, these two principles are used for demonstrating the determination of grain boundary normal vectors from EBSD images. Coherent twin boundaries and focused ion beam serial scan data are used for validation. Results indicate a mean error for this approach of 3 deg with a standard deviation of 3.8 deg.

Manipulation of Dopant Distribution in Yttrium-Doped Ceria Particles

Yttrium is one of the most widely used dopants (e.g., in yttrium-doped ceria (YDC) and yttria-stabilized zirconia) for improving performance of solid electrolytes utilized in solid oxide fuel cells. Oxygen ion conductivity is known to be a function of yttrium concentration in the electrolytes and thus it is important to further understand the factors that influence such concentration distribution. In this study, we examine the change in the distribution of yttrium concentration after annealing in YDC particles. The geometry and morphology of YDC particles, synthesized by spray pyrolysis, were characterized using transmission electron microscopy, and the distribution of yttrium was investigated by X-ray energy-dispersive spectroscopy (XEDS) considering electron interaction volume and geometry of the particle in the XEDS profile analysis. In addition, relative concentrations of Ce(lll) ions, which are thought to be proportional to the concentrations of the oxygen vacancies in the YDC particles, were examined using X-ray photoelectron spectroscopy to correlate with yttrium concentration distributions. It was found that in the as-prepared hollow spherical particles the concentration of yttrium increases linearly from the inner surface towards the outer surface. After annealing the distribution of yttrium becomes non-linear and the dopant is found to migrate to the inner and outer surfaces of the particle. The decrease of the concentration of Ce(lll) ions upon annealing followed by the formation of the dopant concentration gradient is likely due to the increase of the size of the crystallites in the particles.

Thin membranes versus bulk substrates: investigation of proximity effects in focused electron beam-induced processing

The resolution of focused electron beam induced processing techniques is limited by electron scattering processes. General wisdom holds that using a membrane, this can be effectively improved due to a cutoff of the electron interaction volume and thus diminished proximity effects. Recently, we demonstrated that in contrast to the expectation, proximity effects can be indeed larger on a 200 nm silicon nitride membrane than on the respective bulk substrate, due to charging-induced surface activation. Herein, we expand these investigations on proximity effects in electron beam-induced surface activation to other substrates and to electron beam-induced deposition followed by autocatalytic growth.

Pattern Center Determination in Electron Backscatter Diffraction Microscopy

The pattern center of an electron backscatter diffraction (EBSD) image indicates the relative position of the image with reference to the interaction volume of the sample. As interest grows in high-resolution EBSD techniques, accurate knowledge of this position is essential for precise interpretation of the EBSD features. In a typical EBSD framework, Kikuchi bands are recorded on a phosphor screen. If the flat phosphor were instead shaped as a sphere, with its center at the specimen's electron interaction volume, then the incident backscattered electrons would form Kikuchi bands on that sphere with parallel band edges centered on great circles. In this article, the authors present a method of pattern center (PC) refinement that maps bands from the planar phosphor onto a virtual spherical screen and measures the deviation of bands from a great circle and from possessing parallel edges. Potential sources of noise and error, as well as methods for reducing these, are discussed. Finally, results are presented on the application of the PC algorithm to two types of simulated EBSD patterns and two experimental setups, and the resolution of the method is discussed.

Stereology of backscatter electron images of etched surfaces for characterization of particle size distributions and volume fractions: Estimation of imaging bias via Monte Carlo simulations

Abstract On metallic specimens in which a secondary phase has been selectively removed by a chemical etchant, the use of backscatter electron (BSE) imaging yields images that are more readily segmented with image processing algorithms than other modes of imaging in the scanning electron microscope. The contrast mechanisms in this imaging mode, however, produce a bias in the observation of particle sizes and volume fractions due to the effects of the electron interaction volume in the specimen. This stereological bias is quantified using Monte Carlo (MC) simulation of backscatter images. It is observed that the overprojection of features with centroids residing beneath the plane of polish is largely canceled out by the reduced segmentation size of features with centroids residing above the plane of polish.

Towards ultrahigh resolution EBSD by low accelerating voltage

By using a low SEM accelerating voltage in the range of 5–7·5 kV with an electron backscatter diffraction system, the interaction volume of electrons with matter is markedly decreased, and the spatial resolution is improved. The authors present measurements which prove this relationship. Electron backscatter diffraction at low acceleration voltage, however, is not a straightforward technique with the current equipment and a standard set-up. The authors show that an optimised working distance and dedicated sample preparation are essential to obtain improved spatial resolution for detailed nanoscale microstructure investigations.

Methodology for processing backscattered electron images. Application to Aguada archaeological paints

Scanning electron microscopy is a powerful technique in several fields of science and technology. In particular it is an important complement in the characterization of materials for which X-ray analysis is not possible. Such is the case of thin paint layers on ceramic pots, in which, even for low incident energies, the electron interaction volume can be greater than the paint thickness—in addition to the problem arising from similar compositions. With the aim of complementing other common techniques used in compositional materials characterization, in this work, an image-processing software has been developed, which implements a new methodology for the treatment of backscattered electron (BSE) images in order to bring to evidence small mean atomic number contrasts, usually imperceptible to human eye. The program was used to study black and white pigments of ceramic pieces belonging to the Ambato style of “Aguada” culture (Catamarca province, Argentina, IV–XII centuries AD). Although the BSE images acquired for these samples showed no apparent contrast between sherd and black and white pigments, through image-processing algorithms using different space filters, chemical contrast between regions has been brought to evidence with a minor detail loss. This has been accomplished by applying a smoothing filter, after which the main routine for contrast enhancement reveals details in the grey-level region of interest; finally, a filter for edge enhancement permits to recover some details lost in the previous steps, achieving satisfactory results for the painted sherd samples analyzed. In order to validate the mean atomic number differences found between each pigment and the ceramic body, X-ray diffraction diagrams have been refined with Rietveld method using the software DIFFRACplus Topas ®, arriving to mineralogical differences which agree with the results obtained. As a consequence of this study, the program developed has proven to be a suitable tool for routine analysis of samples with slight chemical contrast.

Nanoscale yttrium distribution in yttrium-doped ceria powder

The microstructure and geometry of yttrium-doped ceria particles, synthesized by spray pyrolysis (SP), were characterized by transmission electron microscopy, and the distribution of yttrium was investigated by energy-dispersive X-ray spectroscopy (EDX). The yttrium distribution was investigated by considering an electron interaction volume and geometry of the particle in an EDX profile analysis. Two models, linear and non-linear, for the yttrium concentration gradient were tested against the experimental EDX data for hollow particles. The results showed that the linear concentration distribution of yttrium fits the EDX profiles appreciably better than the non-linear concentration gradient model. This suggests that yttrium distribution is mainly controlled by the different diffusion rates of the precursors from the centre to the surface of the particle, which could be related to their different solubilities, during the heating process in SP.

Understanding the Kinetics and Nanoscale Morphology of Electron‐Beam‐Induced Deposition via a Three‐Dimensional Monte Carlo Simulation: The Effects of the Precursor Molecule and the Deposited Material

The electron-beam-induced deposition of silicon oxide from tetraethyorthosilicate and tungsten from tungsten hexafluoride is simulated via a Monte Carlo simulation. Pseudo one-dimensional nanopillars are grown using comparable electron-beam parameters and a comparison of the vertical and lateral growth rate and the pillar morphology is correlated to the precursor and deposited material parameters. The primary and secondary electrons (type I) are found to dominate the vertical growth rate and the lateral growth rate is dominated by forward and secondary electrons (type II). The resolution and morphology of the nanopillars are affected by the effective electron interaction volume and the resultant surface coverage of the precursor species in the effective electron interaction region. Finally, the simulated results are compared to previously reported experimental results.