Electron Beam Radiation Damage Research Articles

Molecular replacement for small-molecule crystal structure determination from X-ray and electron diffraction data with reduced resolution.

The resolution of 3D electron diffraction (ED) data of small-molecule crystals is often relatively poor, due to either electron-beam radiation damage during data collection or poor crystallinity of the material. Direct methods, used as standard for crystal structure determination, are not applicable when the data resolution falls below the commonly accepted limit of 1.2 Å. Therefore an evaluation was carried out of the performance of molecular replacement (MR) procedures, regularly used for protein structure determination, for structure analysis of small-molecule crystal structures from 3D ED data. In the course of this study, two crystal structures of Bi-3812, a highly potent inhibitor of the oncogenic transcription factor BCL6, were determined: the structure of α-Bi-3812 was determined from single-crystal X-ray data, the structure of β-Bi-3812 from 3D ED data, using direct methods in both cases. These data were subsequently used for MR with different data types, varying the data resolution limit (1, 1.5 and 2 Å) and by using search models consisting of connected or disconnected fragments of BI-3812. MR was successful with 3D ED data at 2 Å resolution using a search model that represented 74% of the complete molecule.

Low energy electron microscopy at cryogenic temperatures

We describe a cryogenic sample chamber for low energy electron microscopy (LEEM), and present first experimental results. Modifications to our IBM/SPECS aberration-corrected LEEM instrument are presented first. These include incorporation of mechanisms for cooling the sample and its surroundings, and reduction of various sources of heat load. Using both liquid nitrogen and liquid helium, we have reached sample temperatures down to about 15 K. We also present first results for low-temperature LEEM, obtained on a three-monolayer pentacene film. Specifically, we observe a reduction of the electron beam irradiation damage cross-section at 15 eV by more than a factor of five upon cooling from 300 K down to 52 K. We also observe changes in the LEEM-IV spectra of the sample upon cooling, and discuss possible causes.

Cryogenic Electron Microscopy Methodologies as Analytical Tools for the Study of Self-Assembled Pharmaceutics.

Many pharmaceutics are aqueous dispersions of small or large molecules, often self-assembled in complexes from a few to hundreds of molecules. In many cases, the dispersing liquid is non-aqueous. Many pharmaceutical preparations are very viscous. The efficacy of those dispersions is in many cases a function of the nanostructure of those complexes or aggregates. To study the nanostructure of those systems, one needs electron microscopy, the only way to obtain nanostructural information by recording direct images whose interpretation is not model-dependent. However, these methodologies are complicated by the need to make liquid systems compatible with high vacuum in electron microscopes. There are also issues related to the interaction of the electron beam with the specimen such as micrograph contrast, electron beam radiation damage, and artifacts associated with specimen preparation. In this article, which is focused on the state of the art of imaging self-assembled complexes, we briefly describe cryogenic temperature transmission electron microscopy (cryo-TEM) and cryogenic temperature scanning electron microcopy (cryo-SEM). We present the principles of these methodologies, give examples of their applications as analytical tools for pharmaceutics, and list their limitations and ways to avoid pitfalls in their application.

STEM-EELS investigation of c-Si/a-AlOx interface for solar cell applications

In this article, STEM-EELS methodology is described to investigate the composition of sensitive crystalline Silicon/amorphous aluminum oxide (c-Si/a-AlOx) interface of an a AlOx/amorphous hydrogenated silicon nitride (a-AlOx/a-SiNx:H) passivation stack of a c-Si solar cell. In this stack, a-AlOx has the distinctive characteristic to provide both chemical and field effect passivation, which need further research to be more controlled in order to improve solar cell efficiency. a-AlOx is known to be unstable under the electron-beam, so we first present a detailed study on the electron-beam radiation damage to c-Si/a-AlOx interface. This interface can indeed undergo several electron-beam irradiation damage like sputtering, knock-on or radiolysis if precautions are not taken. Radiolysis damage has been found to be the dominant radiation damage. Thus, several STEM-EELS acquisition parameters like acceleration voltage, electron dose and scan orientation were taken into account and modified to limit this radiolysis damage. Once the irradiation was limited, STEM-EELS investigation was conduct using DualEELS on the Si and Al L2,3 and OK edge fines structures. The interface was found to be composed of a-SiOx and non-stoichiometric aluminum silicate with a predominance of tetrahedrally coordinated Al in its first layer.

Simulation study of electron beam irradiation damage to ZnO and TiO<sub>2</sub>

Wurtzite ZnO and rutile TiO<sub>2</sub> have important application value in solar cells, photocatalysts, self-cleaning coatings, etc. In addition, ZnO and TiO<sub>2</sub> are crucial basic materials for the development of semiconductor spintronics devices due to room temperature ferromagnetism in the state of defects or doped specific elements. Many studies indicate that the magnetic, optical, and electrical properties of ZnO and TiO<sub>2</sub> are affected by intrinsic defects (such as vacancies, interstitial atoms, etc.). Electron irradiation has the incomparable advantages over other particle beam irradiation, the defects produced by electron beam irradiation are mainly independent vacancy-interstitial atom pairs (Frenkel pairs), and there are no new doping elements introduced into the material during the irradiation by electron beam with energy of several MeV, that is, electron irradiation is a relatively “pure” particle irradiation method. On the one hand, since the displacement threshold energy values of different atoms are different from each other, the type of defect during electron irradiation can be controlled by the energy of the electron beam. On the other hand, the electron fluence can determine the concentration of defects. Therefore, various defects of different concentrations can be generated by electron irradiation, thereby studying the influences of related defects on the magnetic, optical, and electrical properties of ZnO and TiO<sub>2</sub>. However, simulation calculations related to electron beam irradiation damage are relatively scarce. Therefore, in this work, the electron beam irradiation damage is taken as a research topic and the related theoretical simulation calculations are carried out, which lays a theoretical foundation for subsequent experimental researches. The size and the distribution of radiation damage (dpa) caused by point source electrons and that by plane source electrons with different energy values in ZnO and TiO<sub>2</sub> are simulated and calculated through the MCNP5 program combined with the MCCM algorithm. The calculation results show that O atoms and Zn atoms can be dislocated when the electron energy values are greater than 0.31 MeV and 0.87 MeV in ZnO, respectively; while in TiO<sub>2</sub>, O atoms and Ti atoms can be dislocated when the electron beam energy values are greater than 0.12 MeV and 0.84 MeV, respectively. The dpa caused by point source electrons is mainly distributed in the longitudinal direction, and attenuates quickly in the lateral direction; on the contrary, the dpa caused by plane source electrons first increases and then decreases with the augment of the electron incidence depth, and the unevenness of the dpa distribution becomes more serious with the increase of the electron energy. Therefore, for each of ZnO and TiO<sub>2</sub>, the dpa will be relatively even distribution when the thickness of the sample is about 0.25 mm. Furthermore, the calculation results of the electron energy deposition show that the size of the energy deposition area is closely related to the electron beam energy. At the same time, with the increase of the electron beam energy, the position where the maximum energy deposition appears gradually moves to the inside of the sample, and the entire energy deposition area has a tendency to lean forward.

Transition Metals Ion Behavior of Cathode Materials for Li Ion Battery By Electron Beam Irradiation in Scanning Transmission Electron Microscopy

Investigating the structural properties of Li-ion batteries (LIBs) and, if possible, determining the overall structure is important in understanding the physical and chemical properties. Understanding the structural properties and the electrochemical properties of the arranged atoms of cathode materials is therefore essential for new battery material design and system optimization. Particularly, cathode materials such as LiNixMnyCo1-x-yO2 (NMC), lithium-rich Li(LiyNix-yMnxCo1-2x)O2, lithium-rich / manganese-rich xLi2MnO3 (1-x)LiMO2 are analyzed by the scanning transmission electron microscopy (STEM) techniques to identify structural characteristics, leading to the conversion of the layer structure to the spinel and/or salt structure, leading to increased impedance, voltage reduction and capacity of the LIBs. The atomic behavior of Li ion in the cathode materials that determines LiBs performance is hardly characterized by transmission electron microscopy (TEM) owing to its weak electron-scattering power. In this sense, annular bright-field (ABF) STEM, in which the contrast has a low scaling rate with the atomic number, has been proven to be a robust technique for simultaneous imaging of light and heavy elements. However, it is very important to understand the damage mechanism of cathode materials by electron-beam irradiation in STEM and to develop this technique. In this study, we investigated the effect of electron-beam irradiation damage on the surface reaction layer and LiNi0.6Co0.2Mn0.2O2 material during STEM data acquisition. This electron-beam irradiation damage process is similar to the lattice reconstruction and chemical evolution caused by the charge-discharge cycle. Furthermore, based on the phase transition phenomenon by the electron-beam irradiation, it is examined whether the electron beam irradiation damage in the lattice structure substituted by Al and Ti affects the phase transition phenomenon.

Electron beam damage of epoxy resin films studied by scanning transmission X-ray spectromicroscopy

Focused ion beam coupled with scanning electron microscopy (FIB-SEM) is a popular technique for advanced electron microscopy with applications such as, high-precision site-specific lamella sample preparation for transmission electron microscopy (TEM) and slice-and-view FIB 3-dimensional tomography. Damage caused by the electron imaging component of FIB-SEM may be compounded with damage from the ions during the ion milling process. There are known strategies for mitigating damage from ions and electrons (cryo-SEM, dose-control, voltage control), but the electron damage on common embedding resins for EM has not been explored in detail beyond their resistance to shape-change. The relationship between beam parameters and damage mechanisms remains unclear. Since we are relying on the physical, chemical and thermal stability of embedded samples during ion-beam milling, it is important to distinguish electron beam damage from ion beam damage. Scanning transmission X-ray microscopy (STXM) has been used for analyzing the electron beam radiation damage on polymer films by characterizing the chemical bonding changes. In this paper, we focus on the effect of beam voltage and electron dose on electron beam damage to epoxy resin thin films. Irradiated areas on polymer thin films were characterized by near edge X-ray absorption fine structure (NEXAFS) in STXM. We found that, even when using low current and voltage, the electron beam can still cause noticeable chemical changes within the polymer film. The degree of electron beam damage depends not only on the beam energy, but also on the amount of inelastic scattering occurring within the material, as determined by the sample thickness.

Electron-beam radiation induced degradation of silicon nitride and its impact to semiconductor failure analysis by TEM

By in-situ transmission electron microscopy (TEM), we performed a detailed study on the electron-beam radiation damage to nanostructured silicon nitride thin-film process layers in a typical semiconductor NVM device. It was found that high-dose electron-beam radiation at 200 kV led to rapid degradation of silicon nitride process layers, i.e. thin-downing of nanostructured silicon nitride, inter-diffusion of O and N, the formation of bubble-like defects and segregation of N at neighbouring interfaces. Further detailed analysis revealed that radiation-induced modification in the microstructure and chemical composition of silicon nitride layers could be ascribed to the electron radiation induced knock-on damage and ionization damage. The radiation enhanced diffusion (RED) accounted for the continuous thin-down of the nitride process layer and the formation of bubble-like defects in thick nitride spacer process layers. The work well demonstrated the electron-beam sensitivity of nanostructured silicon nitride materials in the semiconductor devices, and thus may give useful information about electron-dose control during TEM failure analysis of the semiconductor devices containing nanostructured silicon nitride process layers.

Effect of electron beam irradiation in TEM on the microstructure and composition of nanoprecipitates in Al-Mg-Si alloys

The evolution of nanoprecipitates β″ to disordered precipitates under electron beam irradiation in transmission electron microscope (TEM) has been studied. High-resolution TEM images show that β″ precipitates became disordered and diminished gradually under the 200 keV or 300 keV electron beam but displayed no significant changes under 80 keV. Under the same energy, the electron beam irradiation damage to the β″ was found as a function of electron dose. Knock-on damage and diffusion rather than radiolysis or heating effect are the main reasons for the disordering of β″. The vacancies caused by knock-on damage facilitated outward diffusion of solutes which lowered their solute concentration inside the nanoprecipitates. The occasional growth of β″ precipitates was induced by the outward diffusion of solute atoms but was dissolved soon under the 200 keV electron beam irradiation. 80 keV electron beam can evaporate hydrocarbon from the specimen and compensate for the resolution loss caused by the reduction of acceleration voltage. Our work validates that electron beam irradiation is one of the reasons for disordering in nanoprecipitates in Al alloys, and suggests that a careful check at low energies down to 80 keV should be performed during TEM studies.

Effect of Electron Beam Irradiation on Polymers

Electron energy loss spectroscopy (EELS) in combination with transmission electron microscopy (TEM) is widely used for chemical state analysis of variety of chemical compounds. High beam sensitivity of substances like polymers hinders the possibility of exploring in-depth analysis provided through the high spatially resolved EELS spectroscopy. In this study, the electron beam irradiation damage on polymers were analyzed with varying dose of electron beams. The stability of the polymers under electron beam exposure depends on the chemical structure on the polymers. In this study the polymers with and without phenyl groups namely Polycarbonate, Polyethylene terephthalate, Polystyrene, Styrene Maleic Anhydride and Polymethylmethacrylate are selected for the comparative degradation study. Effect of varying the electron dose on the stability of polymers were monitored by recording the low-loss EELS spectrum in π to π* transition and (π+σ) to (π+σ)* transition region.

Advanced Transmission Electron Microscopy for Li-Ion Battery Cathodes

The design and improvement of the cathode materials for Li-ion batteries requires the detailed knowledge on the crystal structure at different charge/discharge states and comprehensive understanding of the processes occurring at the nanoscale or even atomic scale level, as many electrode materials demonstrate highly inhomogeneous non-equilibrium behavior. Advanced transmission electron microscopy (TEM) is by far the most suitable and direct tool to view materials down to atomic scale. Recent progress in the electron diffraction methods, related to implication of quantitative electron diffraction tomography, and in the aberration-corrected scanning TEM imaging will be illustrated here with the examples of atomic structure investigation of various cathode materials. Precession electron diffraction and electron diffraction tomography provide quantitative diffraction data with substantially suppressed dynamical effects, enabling reliable structure solution and refinement. In contrast to X-ray and neutron diffraction, the electron diffraction experiments require extremely small quantity of material, which is typically less than 1 mm3, making this method applicable to virtually all samples extracted from electrochemical cells. Electron diffraction patterns can be taken at a very low incoming electron dose, enabling investigation of the materials sensitive to the electron beam irradiation damage, such as polyanion and mixed-anion Li-ion battery cathodes, particularly in their charged state. The capability of quantitative electron diffraction in ab initio structure solution, locating Li atoms and refining the occupancy of the Li positions will be demonstrated using the Li2CoPO4F, Li2FePO4F and LiMn0.5Fe0.5PO4 cathode materials [1]. Aberration-corrected scanning transmission electron microscopy (STEM) techniques deliver the information on the local structural state with sub-angstrom resolution. High angle annular dark field STEM (HAADF-STEM) imaging provides clear visualization of the cation positions, whereas annular bright field STEM (ABF-STEM) shows the location of the “light” elements, such as O and Li. HAADF-STEM method has been applied to investigate the capacity and voltage fading in the layered rock-salt type oxides, which are determined to large degree by the cumulative local structure changes upon continuous electrochemical cycling. A comparative HAADF-STEM study of the layered oxides in the Li-Ru-Ti-O and Li-Ru-Sn-O systems at different stages (pristine, fully charged, discharged and cycled over different number of times) allowed establishing the cation migration pathways during charge-discharge processes and identifying the cation traps responsible for the degradation of the electrochemical performance [2]. Clear visualization of the structure modifications in the oxygen sublattice upon Li extraction is delivered by the ABF-STEM method enabling direct observation of O-O peroxo dimers in Li0.5IrO3 and O vacancy formation in LixFe0.56TeO6 [3,4] and helping to establish the fundamental relation between the anionic redox process and the evolution of the O-O bonding in layered oxides. The figure shows [001] HAADF-STEM and ABF-STEM images of Li0.5IrO3 and the enlarged ABF-STEM image demonstrating the alternating O-O pairs with long and short (marked with dumbbells) projected distances. These O-O dimers arise from twisting the opposite triangular faces of the IrO6 octahedra (shown in yellow). 1. J.Hadermann, A.M.Abakumov, S.Turner, Z.Hafideddine, N.R.Khasanova, E.V.Antipov, G.Van Tendeloo, Chem. Mater., 23, 3540 (2011). 2. M.Sathiya, A.M.Abakumov, D.Foix, G.Rousse, K.Ramesha, M.Saubanère, M.L.Doublet, H.Vezin, C.P.Laisa, A.S.Prakash, D.Gonbeau, G.Van Tendeloo, J-M. Tarascon, Nature Mater., 14, 230 (2015). 3. E.McCalla, A.S.Prakash, E.Berg, M.Saubanere, A.M.Abakumov, D.Foix, B.Klobes, M.-T.Sougrati, G.Rousse, F.Lepoivre, S.Mariyappan, M.-L.Doublet, D.Gonbeau, P.Novak, G.Van Tendeloo, R.P.Hermann, J.-M.Tarascon, J. Electrochem. Soc., 162, A1341 (2015). 4. E.McCalla, A.M.Abakumov, M.Saubanère, D.Foix, E.J.Berg, G.Rousse, M.-L.Doublet, D.Gonbeau, P.Novák, G.Van Tendeloo, R.Dominko, J.-M.Tarascon, Science, 350, 1516 (2015). Figure 1

Quantification of the boron speciation in alkali borosilicate glasses by electron energy loss spectroscopy.

Transmission electron microscopy and related analytical techniques have been widely used to study the microstructure of different materials. However, few research works have been performed in the field of glasses, possibly due to the electron-beam irradiation damage. In this paper, we have developed a method based on electron energy loss spectroscopy (EELS) data acquisition and analyses, which enables determination of the boron speciation in a series of ternary alkali borosilicate glasses with constant molar ratios. A script for the fast acquisition of EELS has been designed, from which the fraction of BO4 tetrahedra can be obtained by fitting the experimental data with linear combinations of the reference spectra. The BO4 fractions (N4) obtained by EELS are consistent with those from 11B MAS NMR spectra, suggesting that EELS can be an alternative and convenient way to determine the N4 fraction in glasses. In addition, the boron speciation of a CeO2 doped potassium borosilicate glass has been analyzed by using the time-resolved EELS spectra. The results clearly demonstrate that the BO4 to BO3 transformation induced by the electron beam irradiation can be efficiently suppressed by doping CeO2 to the borosilicate glasses.

Observation of Fine Distribution of Minor Dopants in an Erbium-Doped Fiber Core using a Sample Thinning Technique for Field Emission Electron Probe Microanalysis.

To observe the fine distribution of minor aluminum and germanium dopants in the erbium-doped fiber (EDF) core of an optical amplifier, a sample thinning technique was applied for field emission electron probe microanalysis (FE-EPMA) together with wavelength-dispersive X-ray spectrometry. This technique significantly improved the spatial resolution without much degradation of the minimum detection limit for FE-EPMA. As such, this enabled us to observe the distribution of minor dopants in EDF. Moreover, we propose a very simple sample preparation to prevent electron-beam radiation damage, a problem involved with FE-EPMA of low-conductivity materials such as SiO2 glass, which is the main component of EDF.

In situ cleavage prepared bilayer graphene device and its large magnetoresistance

A Zn-based in situ cleavage method is applied to fabricate few-layer graphene devices. This approach avoids unintentional electron-beam irradiation damage, in addition to creating a series of clean graphene devices with well controlled layer numbers. As prepared bilayer graphene surprisingly achieves a magnetoresistance ratio as high as 350% at 1.9 K and 100% at room temperature. For this reason, in situ cleavage method/electron-beam lithographic could provide a more effective way to fabricate modern day carbon electronic components.

Catalyst-Layer Ionomer Imaging of Fuel Cells

Investigation of membrane/electrode assembly (MEA) microstructure has become an essential step to optimize the MEA manufacturing processes or to study the degradation of the different MEA components. Transmission electron microscopy (TEM) is a tool of choice as it provides direct imaging of the different components. TEM is then widely used for analyzing catalyst nanoparticle structures and distribution, as well as their chemical composition. The carbon support can also be imaged; for example, the degree of graphitization can be investigated. However, the ionomer network inside the electrode is more difficult to be imaged due to the fact that the ionomer mainly forms an ultrathin layer surrounding the carbon support. Moreover, these two components, having similar density, present no difference in contrast. The presence of the ultrathin ionomer layer can only be revealed in some favorable zones on high resolution TEM images1. The entire ionomer network inside the catalyst layer has never been imaged, even though this microstructural parameter plays a crucial role in ionic conduction through the catalyst layer. In this work, we show how the advanced TEM techniques of electron tomography and X-ray energy dispersion spectroscopy (EDS) elemental mapping provide new possibilities for imaging this ionomer network. Electron tomography in HAADF-STEM (high annular dark field – scanning transmission electron microscopy) mode was successfully used to image the 3D morphology of the ultrathin ionomer layer surrounding the carbon particles2. For this experiment, the ionomer contrast was enhanced by selectively staining the ionic domains with Cs+ions. In order to avoid high contrast of the Pt nanoparticles (compared to the ionomer contrast), a model active layer consisting of ionomer and carbon black without Pt nanoparticles was made using usual electrode manufacturing process. The influence of the ionomer/carbon black ratio introduced in the ink was studied by preparing two samples with two different ionomer/carbon black ratios equal to 0.5 and 0.2 w/w. Figure 1 shows the 3D-rendered volume extracted from the reconstructed tomogram for the two samples. The 3D repartition of the ultrathin ionomer layer (in blue) surrounding the carbon black (in grey) is clearly revealed. The quantitative tomogram data analyses showed that doubling the amount of ionomer in the catalyst layer does not change the mean thickness of the ionomer layer, measured around 7 nm, but leads to a twofold increase in its degree of carbon particle coverage. These electron tomography analyzes are of great interest for finding the optimum manufacturing process that will lead to the maximum carbon coverage without increasing the ionomer layer thickness. Indeed, the optimized ionomer network structure have to ensure both ionic contact with a maximum of Pt nanoparticles and the connectivity of the ionic conduction paths to the membrane without inhibiting the gas diffusivity. On the other hand, recent developments of high performance EDS detectors offer new possibilities to acquire chemical elemental maps in a very short time (few seconds or minutes). Fluorine being one of the main elements of the ionomer, ionomer distribution within the catalyst layer can be visualized by acquiring fluorine EDS elemental maps in a MEA ultramicrotomed TEM sample (Figure 2). One limitation of this technique is that a high EDS signal requires a high electron dose, especially when high spatial resolution is required. Unfortunately, the ionomer is highly sensitive to electron beam radiation damage and a continuous F loss happens during the TEM observation. Measurement of the F loss under different acquisition conditions has shown that F loss can be minimized by controlling the electron dose and by cooling the specimen during analysis3. Using these conditions, it is now possible to obtain quantitative data on the active layer ionomer content that in some electrodes can reveal distribution heterogeneity. But more importantly, these analyses allowing a quantitative comparison of the ionomer content in different electrodes offer the possibility to study the active layer ionomer evolution after MEA ageing tests. 1K. L. More, R. Borup, K. S. Reeves, ECS Trans 3, 717-733 (2006). 2 M. Lopez-Haro, L. Guetaz, T. Printemps, A. Morin, S. Escribano, P.H. Jouneau, P. Bayle-Guillemaud, F. Chandezon, G. Gebel, Nat Commun, 5, 5229 (2014). 3 D. A. Cullen, R. Koestner, R. S. Kukreja, Z. Y. Liu, S. Minko, O. Trotsenko, A. Tokarev, L. Guetaz, H. M. Meyer, C. M. Parish, K. L. More, J. Electrochem. Soc., 161 (10) F1111-F1117 (2014). Figure 1

High-Resolution Mapping of the PFSA Polymer Distribution in PEFC Electrode Layers

Perfluorosulfonic acid (PFSA) ionomer is a key component within the electrode layers of polymer electrolyte fuel cells (PEFCs) since this solid electrolyte provides an efficient proton transport path to the active catalyst sites. Both the average loading on a 100-nm scale as well as the actual film thickness distribution on a 1-nm scale of the PFSA ionomer in the porous layer structure are critical to electrode performance so quantitative measurements of both properties are highly desired.Scanning transmission electron microscopy (STEM) is one useful tool to map this PFSA distribution in the electrode layer [1]. However, it has long been recognized that fluorinated compounds are sensitive to electron beam radiation damage [2,3]. By maintaining an electron dose below 1x104 e-/nm2 at 100-nm lateral resolution and cooling the specimen below -100ºC, the total fluorine mass loss for Nafion® ionomer is held below 10% a/a whereby the electron beam damage threshold for a typical Pt/C electrode layer is improved by 2-3 orders-of-magnitude at this sample temperature. Atomic force microscopy (AFM) [4] is a complementary tool suitable to map the Nafion® coverage on a 1-nm length scale on the electrode surface. The adhesion of the AFM tip to Nafion® in air was measured to be very low as the fluorocarbon backbone was preferentially exposed at polymer-air interface. In this paper, we present ionomer mapping measurements of two common PEFC electrode constructions. Figure 1 shows Pt/C/Nafion® layers at 8 μm dry thickness that are coated from the same electrode ink onto either a decal substrate (CCD) or directly onto the gas diffusion media. The other electrode layer comprises a PtCoMn nanostructured thin film (NSTF) electrode [5] that is overcoated with Nafion® ionomer using a preferred adsorption scheme to a targeted 2.0 nm dry thickness. Figure 2 shows that a significantly different through-layer ionomer/carbon (I/C w/w) profile is measured by energy-dispersive X-ray spectroscopy (EDS) for the CCD and CCDM electrode layers that can be ascribed to partial permeation of the electrode ink solution into the porous gas diffusion media substrate. In addition, the AFM adhesion measurement in Figure 3 shows a linear increase in ionomer coverage on the NSTF electrode surface to the adsorption limit (at 2 nm dry thickness). ORNL Research sponsored by (1) the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and (2) the Shared Research Equipment (ShaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy.

Membrane Tethering and Nucleotide-dependent Conformational Changes Drive Mitochondrial Genome Maintenance (Mgm1) Protein-mediated Membrane Fusion

Cellular membrane remodeling events such as mitochondrial dynamics, vesicle budding, and cell division rely on the large GTPases of the dynamin superfamily. Dynamins have long been characterized as fission molecules; however, how they mediate membrane fusion is largely unknown. Here we have characterized by cryo-electron microscopy and in vitro liposome fusion assays how the mitochondrial dynamin Mgm1 may mediate membrane fusion. Using cryo-EM, we first demonstrate that the Mgm1 complex is able to tether opposing membranes to a gap of ∼15 nm, the size of mitochondrial cristae folds. We further show that the Mgm1 oligomer undergoes a dramatic GTP-dependent conformational change suggesting that s-Mgm1 interactions could overcome repelling forces at fusion sites and that ultrastructural changes could promote the fusion of opposing membranes. Together our findings provide mechanistic details of the two known in vivo functions of Mgm1, membrane fusion and cristae maintenance, and more generally shed light onto how dynamins may function as fusion proteins.

Dip-Pen Nanolithography of Electrical Contacts to Single Graphene Flakes

This study evaluates an alternative to electron-beam lithography for fabricating nanoscale graphene devices. Dip-pen nanolithography is used for defining monolayer graphene flakes and for patterning of gold electrodes through writing of an alkylthiol on thin films of gold evaporated onto graphene flakes. A wet gold etching step was used to form the individual devices. The sheet resistances of these monolayer graphene devices are comparable to reported literature values. This alternative technique for making electrical contact to 2D nanostructures provides a platform for fundamental studies of nanomaterial properties. The merits of using dip-pen nanolithography include lack of electron-beam irradiation damage and targeted patterning of individual devices with imaging and writing conducted in the same instrument under ambient conditions.

The electron beam irradiation damage on nanomaterials synthesized by hydrothermal and thermal evaporation methods—An example of ZnS nanostructures

Using transmission electron microscopy-related techniques, we have compared the degradation behaviors of several different types of ZnS nanostructures, including the ZnS nanosheets synthesized by hydrothermal method (with different oxygen impurity concentration) and ZnS nanobelts grown using thermal evaporation. We have identified that displacement damage, sputtering, and oxidation mechanisms exist during the electron irradiation process. While oxidation of the nanostructure is always observed, displacement damage appears to be the dominant mechanism contributing to the final structural collapse of ZnS nanosheets (synthesized via hydrothermal methods), but sputtering mechanism becomes critical in changing the surface roughness of the ZnS nanobelts (grown by thermal evaporation). The specific damage mechanisms of these nanomaterials disclose that different synthesis process results in different structure quality (particularly impurity related interior defects) of the ZnS nanostructures, which determines their specific degradation behaviors under the electron beam irradiation.

Investigation of the hydroxyapatite obtained as hydrolysis product of α-tricalcium phosphate by transmission electron microscopy

Alpha tricalcium phosphate (α-TCP) is widely used as a reactant in many calcium phosphate cements, where the setting reaction occurs through its hydrolysis to a calcium deficient hydroxyapatite (CDHA), at room or body temperature. Transmission electron diffraction studies of cement single CDHA crystals produced at 37 °C have revealed the systematic presence of unexpected diffraction spots other than those assigned to the archetypal P63/m apatite. Three possible causes can explain the existence of these extra spots: electron beam irradiation damage (inducing phase transformation to monoclinic apatite), presence of octacalcium phosphate (OCP) and presence of monoclinic apatite in the hydrolyzed product. Determination of the occurrence of any of these phases would have great implications in the properties of the hydrolyzed product. However, the identification of their corresponding electron diffraction patterns is not straightforward due to coincidental spots under basal zone axis orientation. In the present work, off-zone axis electron diffraction in the transmission electron microscope, TEM, has been used for phase identification, clearly indicating the presence of the monoclinic apatite phase. These findings could be a strong asset reinforcing the possibility of the monoclinic phase as a constituent of hard bone tissue.