Beam Damage Effects Research Articles

In situ cyclic telescoping of multi-walled carbon nanotubes in a transmission electron microscope

In this work, we study the cyclic telescoping of multi-walled carbon nanotubes (MWCNTs) inside a high-resolution transmission electron microscope. The nanotubes are observed in real time, while simultaneously measuring their electrical properties. Experiments are conducted by first ‘sharpening’ a MWCNT by approaching it with a piezo-controlled tungsten tip and applying a single voltage pulse. This results in controlled and localized peeling of the outer walls, giving rise to a telescope structure. The exposed core is then contacted with the tungsten tip and reproducibly pulled out then re-inserted, while measuring conductance behaviour at each movement step. Results demonstrate that an electrical current can be maintained for multiple in-and-out cycles, and for telescoping distances of up to 650 nm. The systems display complex conductance behaviour, with most MWCNT telescopes demonstrating diminishing conduction (and current) as a function of the number of cycles, while for some these properties improve. Control experiments have been carried out to investigate the effect of current annealing and beam damage on whole MWCNTs. One potential application for these structures is use as flexible electrical contacts, which allow for relative motion of the two electrodes, while maintaining electrical conduction.

Site- and Symmetry-Resolved Resonant X-ray Emission Study of a Highly Ordered PTCDA Thin Film

The complex electronic structure of ordered 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA) multilayers on Ag(111) is investigated by angle-dependent resonant X-ray emission spectroscopy at the carbon K edge. Beam damage effects were characterized and special care was taken to avoid their influence on the spectra. Appropriate selection of excitation energies and detection geometries allow us to record element-, atom-, and symmetry-specific X-ray emission spectra by making use of dipole selection rules. This specificity, especially the ability to distinguish between σ and π orbitals, gives unprecedented experimental insights into the electronic structure of this large organic molecule. The experimental results are compared with calculated spectra of the isolated molecule, as derived from density functional theory. This allows a detailed discussion of the spectra and separation of the contributions from nonequivalent carbon atoms.

Electron Probe Microanalysis of REE in Eudialyte Group Minerals: Challenges and Solutions.

Accurate quantification of the chemical composition of eudialyte group minerals (EGM) with the electron probe microanalyzer is complicated by both mineralogical and X-ray-specific challenges. These include structural and chemical variability, mutual interferences of X-ray lines, in particular of the rare earth elements, diffusive volatility of light anions and cations, and instability of EGM under the electron beam. A novel analytical approach has been developed to overcome these analytical challenges. The effect of diffusive volatility and beam damage is shown to be minimal when a square of 20×20 µm is scanned with a beam diameter of 6 µm at the fastest possible speed, while measuring elements critical to electron beam exposure early in the measurement sequence. Appropriate reference materials are selected for calibration considering their volatile content and composition, and supplementary offline overlap correction is performed using individual calibration factors. Preliminary results indicate good agreement with data from laser ablation inductively coupled plasma mass spectrometry demonstrating that a quantitative mineral chemical analysis of EGM by electron probe microanalysis is possible once all the parameters mentioned above are accounted for.

Assessing Spatial Resolution of Cathodo-Luminescence Imaging and Spectroscopy at the Nanoscale for Inorganic Phosphor Powders

It is now possible to easily collect CL images (using a photomultiplier tube using total or filtered light) and emission spectra from individual nanometer sized particles using the commercially available Gatan Vulcan attachment for a transmission electron microscope (TEM). In our case, at Brunel University London, we have fitted this to a JEOL 2100F field emission TEM. This work demonstrates the spatial resolution that has been achieved when imaging various phosphor nano materials and discusses the limitations of the technique. It also demonstrates the uniformity of excitation/emission from various phosphors, and hence the effectiveness of the synthetic method used to manufacture the phosphor can be seen. Conventionally, photoluminescence (PL) and cathodoluminescence (CL) spectra are collected from bulk phosphor samples and are the average of all of the particles in the sample. This is true of most samples made either in the laboratory for research or commercial materials prepared by industrial companies. Cathodoluminescence spectra have been collected from nanometer-sized crystals of quantum dots (spherical and rods) and various oxide and oxysulfide phosphors. The latter phosphors have been implemented into CL applications on a vast scale whereas the quantum dot materials are not designed for CL application and therefore great care is required when imaging these materials to minimize the effect of beam damage. When observing mixtures of phosphors (Gd2O2S:Tb3+ and Y2O3:Eu3+) it is apparent that when the electron beam is positioned onto one of these two materials and the spectrum observed, then the other phosphor emission spectrum can be observed if the particles are nearby. However when observing quantum dot materials the spatial resolution is only limited by the particle size. A tentative explanation for this effect is given. In this work we compare the spectra of nanometer sized phosphor crystals/particles in order to understand any variation in the emission spectra between the phosphor grains and to understand the effect of temperature on their luminescent properties. Our results will be presented that could only have been found by using CL spectra obtained directly from single nanoparticles. In conclusion: 1) The effect of different TEM machine parameters will be discussed 2) The CL spatial resolution will be demonstrated Figure 1. Left) HAADF STEM of nanometre-sized single crystals of Y2O3Eu3+ and Gd2O2S:Tb3+; The green line spectrum image shows where 10 emission spectra were recorded at equally spaced intervals. Right) Extracted spectra taken from opposite ends on the line scan show the characteristic CL emission spectrum for both the Eu3+ and Tb3+ are observed in both spectra. Figure 1

Trace Element Analyses by EMP: Pb-in-Monazite and New Multipoint Background Method

University of Oregon, CAMCOR, Eugene (OR), USA Precision in electron microprobe analysis is primarily a matter of counting statistics, and involves voltage, beam current, counting time, and geometric efficiency. Besides potential issues with dead-time correction and beam damage, counting statistics do not significantly affect the accuracy of major and minor element analyses. Accuracy chiefly depends on standards and matrix correction. Additional challenges with both accuracy and high precision appear when measuring trace elements [1]: 1) Analytical precision requires the use of high intensity monochromators, optimized PHA settings, high beam current, higher voltage, and/or lengthy count time. These latter requirements can lead to beam damage, surface contamination, and internal charge effects. 2) Characterization and measurement of the background become crucial for accurate results when the peak-to-background ratio approaches 1. 3) Accurate results require correction or avoidance of peak and background interferences, even for some 2

Reversed scan direction reduces electron beam damage in EBSD maps

The deleterious effects of electron beam damage on high-resolution electron backscatter diffraction (EBSD) maps of undeformed quartz are significantly reduced by scanning in the direction opposite to that dictated by widely used EBSD acquisition software. Higher quality electron backscatter patterns are produced when the electron beam moves progressively down the sample (the apparent 'up' direction in the resulting maps) for all step sizes where beam damage affects EBSD map quality (≤ ∼0.4 μm in this study). The relative improvement associated with downward scanning increases as step size is reduced. A comparison of high-resolution maps made in experimentally deformed quartz demonstrates that downward scanning reduces by a factor of ∼2 the lower limit in step size relative to maps scanned in the conventional direction. The electron beam damages quartz at its point of entry, forming ∼0.1-μm diameter bumps visible in Scanning electron microscope (SEM) images. Downward scanning produces better results because it minimizes the flux of electrons through these loci of damaged crystal.

Low-flux electron diffraction study for the intercellular lipid organization on a human corneocyte

Human skin stratum corneum (SC) structures were investigated by electron diffraction (ED) with a very low-flux electron beam with the help of high-sensitivity detectors, the imaging plate and the CCD camera. This low-flux electron diffraction (LFED) method made it possible to minimize the unfavorable effect of electron beam damage and to give a reliable diffraction pattern from a small selected area (0.2μm2) on a corneocyte. Dependence of the 2-dimensional ED pattern on the size of the selected area showed that orientational correlation between lipid packing domains can persist over the area much larger than their domain size. The LFED method also allowed us to trace the detailed structural change induced by the electron beam damage. The ED diffraction peak for the lattice constant of about 4.1nm decayed in three steps. The detailed analysis of these three steps suggested that a different type of orthorhombic structure exists interacted with the well-described hexagonal and orthorhombic structures, in the process of decay resulting from electron beam damage.

Trace analysis in EPMA

Trace element micro-analysis has evolved steadily since the early days of EPMA, yet remains an extraordinarily challenging subject. The enhanced capabilities of modern instrumentation, including the use of spectrometers with high X-ray collection efficiencies, high brightness electron sources, and improved stability all contribute to our ability to improve detection limits and analytical spatial resolution. Along with much improved software for data acquisition and analysis, recent progress in EPMA has made the trace realm more accessible than ever. High count precision can be obtained in order to easily bring analytical sensitivity into the single ppm range, but accuracy remains the greatest struggle. With the exception of the calibration, all sources of error encountered in major element analysis are magnified in trace analysis, and other sources become apparent where high spatial resolution is needed. Beam damage and charge effects are difficult problems in high sensitivity, high spatial resolution analysis, particularly in the analysis of insulators. Software can minimize some of the resulting effects on count rates during acquisition in order to improve accuracy, and analysts can empirically evaluate the conditions of analysis (count time, voltage, current, etc.) to try to minimize these effects. Trace analysis is fundamentally an exercise in background characterisation, and the acquisition and evaluation of background is a subject of developing methodology. Background curvature and interferences can result in considerable inaccuracy, but can be dealt with via detailed quantitative wavelength scanning or multi-point spectral acquisitions which allow proper regression of the background shape. In the absence of excellent quality trace element secondary standards of similar matrix to unknowns, blank testing and consistency standards can be used to test at least some aspects of the methods employed. Ultimately, the analyst must rely on accuracy evolving from application of the most rigorous protocols.

Electron Energy-Loss Safe-Dose Limits for Manganese Valence Measurements in Environmentally Relevant Manganese Oxides

Manganese (Mn) oxides are among the strongest mineral oxidants in the environment and impose significant influence on mobility and bioavailability of redox-active substances, such as arsenic, chromium, and pharmaceutical products, through oxidation processes. Oxidizing potentials of Mn oxides are determined by Mn valence states (2+, 3+, 4+). In this study, the effects of beam damage during electron energy-loss spectroscopy (EELS) in the transmission electron microscope have been investigated to determine the "safe dose" of electrons. Time series analyses determined the safe dose fluence (electrons/nm(2)) for todorokite (10(6) e/nm(2)), acid birnessite (10(5)), triclinic birnessite (10(4)), randomly stacked birnessite (10(3)), and δ-MnO(2) (<10(3)) at 200 kV. The results show that meaningful estimates of the mean Mn valence can be acquired by EELS if proper care is taken.

Liquid/Solid Interface of Ultrathin Ionic Liquid Films: [C1C1Im][Tf2N] and [C8C1Im][Tf2N] on Au(111)

Ultrathin films of two imidazolium-based ionic liquids (IL), [C(1)C(1)Im][Tf(2)N] (= 1,3-dimethylimidazolium bis(trifluoromethyl)imide) and [C(8)C(1)Im][Tf(2)N] (= 1-methyl-3-octylimidazolium bis(trifluoromethyl)imide) were prepared on a Au(111) single-crystal surface by physical vapor deposition in ultrahigh vacuum. The adsorption behavior, orientation, and growth were monitored via angle-resolved X-ray photoelectron spectroscopy (ARXPS). Coverage-dependent chemical shifts of the IL-derived core levels indicate that for both ILs the first layer is formed from anions and cations directly in contact with the Au surface in a checkerboard arrangement and that for [C(8)C(1)Im][Tf(2)N] a reorientation of the alkyl chain with increasing coverage is found. For both ILs, geometry models of the first adsorption layer are proposed. For higher coverages, both ILs grow in a layer-by-layer fashion up to thicknesses of at least 9 nm (>10 ML). Moreover, beam damage effects are discussed, which are mainly related to the decomposition of [Tf(2)N](-) anions directly adsorbed at the gold surface.

Investigation of viability of plant tissue in the environmental scanning electron microscopy

The advantages of environmental scanning electron microscopy (ESEM) make it a suitable technique for studying plant tissue in its native state. There have been few studies on the effects of ESEM environment and beam damage on the viability of plant tissue. A simple plant tissue, Allium cepa (onion) upper epidermal tissue was taken as the model for study. The change of moisture content of samples was studied at different relative humidities. Working with the electron beam on, viability tests were conducted for samples after exposure in the ESEM under different operating conditions to investigate the effect of electron beam dose on the viability of samples. The results suggested that without the electron beam, the ESEM chamber itself can prevent the loss of initial moisture if its relative humidity is maintained above 90%. With the electron beam on, the viability of Allium cepa (onion) cells depends both on the beam accelerating voltage and the electron dose/unit area hitting the sample. The dose can be controlled by several of the ESEM instrumental parameters. The detailed process of beam damage on cuticle-down and cuticle-up samples was investigated and compared. The results indicate that cuticular adhesion to the cell wall is relatively weak, but highly resistant to electron beam damage. Systematic study on the effect of ESEM operation parameters has been done. Results qualitatively support the intuitive expectations, but demonstrate quantitatively that Allium cepa epidermal cells are able to be kept in a hydrated and viable state under relevant operation condition inside ESEM, providing a basis for further in situ experiments on plant tissues.

High-resolution, high-transmission soft x-ray spectrometer for the study of biological samples

We present a variable line-space grating spectrometer for soft x-rays that covers the photon energy range between 130 and 650 eV. The optical design is based on the Hettrick-Underwood principle and tailored to synchrotron-based studies of radiation-sensitive biological samples. The spectrometer is able to record the entire spectral range in one shot, i.e., without any mechanical motion, at a resolving power of 1200 or better. Despite its slitless design, such a resolving power can be achieved for a source spot as large as (30 x 3000) microm2, which is important for keeping beam damage effects in radiation-sensitive samples low. The high spectrometer efficiency allows recording of comprehensive two-dimensional resonant inelastic soft x-ray scattering (RIXS) maps with good statistics within several minutes. This is exemplarily demonstrated for a RIXS map of highly oriented pyrolytic graphite, which was taken within 10 min.

Cortactin Adopts a Globular Conformation and Bundles Actin into Sheets

Cortactin is a filamentous actin-binding protein that plays a pivotal role in translating environmental signals into coordinated rearrangement of the cytoskeleton. The dynamic reorganization of actin in the cytoskeleton drives processes including changes in cell morphology, cell migration, and phagocytosis. In general, structural proteins of the cytoskeleton bind in the N-terminal region of cortactin and regulatory proteins in the C-terminal region. Previous structural studies have reported an extended conformation for cortactin. It is therefore unclear how cortactin facilitates cross-talk between structural proteins and their regulators. In the study presented here, circular dichroism, chemical cross-linking, and small angle x-ray scattering are used to demonstrate that cortactin adopts a globular conformation, thereby bringing distant parts of the molecule into close proximity. In addition, the actin bundling activity of cortactin is characterized, showing that fully polymerized actin filaments are bundled into sheet-like structures. We present a low resolution structure that suggests how the various domains of cortactin interact to coordinate its array of binding partners at sites of actin branching.

A liquid flow cell to study the electronic structure of liquids with soft X-rays

We describe the design of a temperature-controlled flow-through liquid cell dedicated to the study of liquids with soft X-rays. The cell can be operated with internal ambient pressure and is mounted on a standard vacuum manipulator, making it compatible to the ultra-high vacuum environment required in synchrotron beamlines. The liquid is separated from the vacuum by a thin membrane, allowing the use of soft X-ray photon-in–photon-out techniques such as X-ray emission and fluorescence-yield X-ray absorption spectroscopy to study the electronic structure of liquids and liquid–solid interfaces. Special care was taken for a rapid and effective flow of the liquid inside the cell in order to minimize local heating and beam damage effects. To illustrate the capabilities, oxygen K X-ray emission spectra of D 2O and H 2O are presented and briefly discussed together with possible problems that may arise from X-ray-induced oxide formation at the membrane–liquid interface.

Monitoring x-ray beam damage on lipid films by an integrated Brewster angle microscope/x-ray diffractometer

We describe an integrated Brewster angle microscope (BAM), Langmuir trough, and grazing incidence x-ray diffraction assembly. The integration of these three techniques allows for the direct observation of radiative beam damage to a lipid monolayer at the air-water interface. Although beam damage has been seen in x-ray measurements, it has not been directly observed in situ at the micron scale. Using this integrated assembly, we examined the effects of radiative beam damage on Langmuir monolayers of 1,2-dimyristoyl-sn-glycero-3-[phospho-L-serine] (DMPS), 1:1 DMPS:1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, and 1:1 DMPS:1,2-dioleoyl-sn-glycero-3-phosphocholine held at a constant surface pressure. For constant surface pressure experiments, we observed a marked decrease in the surface area of the film upon exposure to the beam due to photodissociation. For a condensed lipid film, a change in refractive index of the film was observed post-beam-exposure, indicating areas of damage. For DMPS in an oxygenated environment, the Bragg peak intensity decreased with beam exposure. In mixed monolayer systems, with saturated and unsaturated lipids, an increase in the number of small saturated lipid domains was seen as the unsaturated lipid was preferentially damaged and lost from the monolayer. We show that BAM is a highly effective technique for in situ observation of the effects of radiative damage at the air/water interface during a synchrotron experiment.

Nuclear microscopy measurement of copper in atherosclerosis – Sensitivity and limitations to spatial resolution

Nuclear microscopy studies at the Centre for Ion Beam Applications have indicated a link between iron, zinc and the development of atherosclerosis. In the present study, we have extended this study to copper, since copper is also capable of inducing free radical mediated damage. As copper in biological tissue occurs at the parts per million level and therefore close to the detection limits for PIXE analysis, we have substantially increased the beam intensity and scanning time to obtain adequate statistics and analytical sensitivity. The experiments were conducted on male New Zealand White rabbits, weighing approximately 2.5 kg and fed on high cholesterol diets for 8 weeks. Unlike iron concentrations, which were observed to be increased in the atherosclerotic lesion, our experiments show that copper is depleted in the lesion (1.9 ppm) compared with the adjacent artery wall (4.1 ppm). The concentration of copper present in the lesion is also much less than iron (approximately 30 times less on average), indicating that iron, rather than copper, is more likely to induce atherosclerosis through free radical mediated damage, purely on the basis of greatly reduced concentrations. To obtain adequate statistics for copper, a 2.1 MeV proton beam focused to 2 μm with a relatively high beam current (from about 800 pA to 1 nA) was scanned over the sample for approximately 4 h. The effects of beam damage due to these high beam currents have been investigated and it was found that the tissue shrinks 10% each in the X and Y direction contributing to total area shrinkage of 20%. Despite tissue shrinkage problems, our results indicate that even with 1 nA beam currents, 4 h scan times and scan sizes of around 1 × 1 mm 2, we can still extract accurate trace elemental concentrations.

Understanding and preventing beam damage effects in partially processed high-k gate stacks

Electron beam damage effects on a partially and a fully processed HfO2 gate stack on silicon substrates are investigated, and their origins and prevention are discussed. Growth of silica between the silicon and hafnia layers is observed for the partially processed sample but is not seen for the fully processed wafer. Two sources of oxygen are found to react with the substrate to form silica. One is from the glue used in sample preparation. The oxygen from this source can be prevented from diffusing to the substrate by putting a gold barrier layer between the stack and the glue. The other source seems to come from the amorphous HfO2 layer. Using a cooling rod sufficiently slows the diffusion rate so that growth is no longer observed.

Neutron and X-ray scattering studies of cholera toxin interactions with lipid monolayers at the air–liquid interface

Using neutron/X-ray reflectivity and X-ray grazing incidence diffraction (GID), we have characterized the structure of mixed DPPE:GM 1 lipid monolayers before and during the binding of cholera toxin (CTAB 5) or its B subunit (CTB 5). Structural parameters such as the density and thickness of the lipid layer, extension of the GM 1 oligosaccharide headgroup, and orientation and position of the protein upon binding are reported. Both CTAB 5 and CTB 5 were measured to have ∼50% coverage when bound to the lipid monolayer. X-ray GID experiments show that both the lipid monolayer and the cholera toxin layer are crystalline. The effects of X-ray beam damage have been assessed and the monolayer/toxin structure does not change with time after protein binding has saturated.

Thermal stability of beam sensitive atmospheric aerosol particles in electron probe microanalysis at liquid nitrogen temperature

Thin-window electron probe X-ray microanalysis offers new analytical possibilities for low- Z detection (i.e. elements with low atomic number; such as C, N and O). However, the quantitative analysis of individual particles raises some practical questions concerning the technique. From the analytical point of view, beam damage is one of the most important problems due to its big impact on the analysis of individual atmospheric particles. The dependence of the beam-damage effect on the type of collection substrate was studied using standard aerosol particles. Different metallic substrates were rigorously tested in relation to the beam damage effects to different kinds of beam sensitive particles. Ammonium sulfate, ammonium nitrate, sodium nitrate as well as sulfuric acid droplets were analyzed using a liquid nitrogen cooled sample stage on different metallic substrates such as Be, Al, Si and Ag. The obtained results confirm that the use of Be as a collection surface offers some advantages in order to minimize the damage to beam sensitive particles, as suggested in earlier research.

Conditions for imaging emulsions in the environmental scanning electron microscope.

Environmental scanning electron microscopy (ESEM) is a technique capable of imaging volatile and/or insulating samples in their natural state, without prior specimen preparation. It is thus a powerful potential tool for the study of the structure and dynamics of emulsions and other complex liquid systems, at a resolution greater than that obtainable by conventional optical microscopy. We present images of a variety of liquid systems containing micron-scale and smaller features. The morphology of these systems may be clearly discerned. The contrast observed between the liquid phases was consistent with the model proposed by Stokes et al. (1998). The limits of resolution were determined by sample motion and by beam damage effects; under optimum conditions, resolution of a few tens of nanometers was obtained. This compares favourably with conventional and confocal optical microscopy. In some samples, thin films (solid or liquid) could be observed at the liquid/gas interface. Some of these films were so thin that they did not completely obscure the underlying structure of the bulk sample.