The performance evaluation of direct detection electron energy-loss spectroscopy at 200 kV and 80 kV accelerating voltages.

Direct electron detectors (DeDs) have been widely used for imaging studies because of their higher beam sensitivity, lower noise, improved pixel resolution, etc. However, there have been limited studies related to the performance in spectroscopic applications for the direct electron detection. Hereby, taking the advantage of the DeD installed on a high-performance electron energy-loss spectrometer, we systematically studied the performance of a DeD (Gatan's K2 IS) fitted on an aberration-corrected transmission electron microscope (TEM) equipped with an electron monochromator. Using SrTiO3 as the model system, the point spread function in the zero-loss region of the spectrum and the performance for core loss spectroscopy have been investigated under both 200 kV and 80 kV operating conditions. We demonstrate that the K2 detector can achieve an overall better performance at 200 kV than a charge coupled device (CCD) detector. At 80 kV, the K2 DeD is still better than a CCD, except for the relative broad tails of the zero-loss peak. The signal-to-noise ratio is very close for DeD and CCD under 80 kV. Based on our data obtained at different operating voltages, it is clear that DeD will benefit the microscopy community and boost the development of cutting-edge materials science studies by pushing the frontiers in electron energy-loss spectroscopy.

Direct Detection Electron Energy-Loss Spectroscopy: A Method to Push the Limits of Resolution and Sensitivity.

In many cases, electron counting with direct detection sensors offers improved resolution, lower noise, and higher pixel density compared to conventional, indirect detection sensors for electron microscopy applications. Direct detection technology has previously been utilized, with great success, for imaging and diffraction, but potential advantages for spectroscopy remain unexplored. Here we compare the performance of a direct detection sensor operated in counting mode and an indirect detection sensor (scintillator/fiber-optic/CCD) for electron energy-loss spectroscopy. Clear improvements in measured detective quantum efficiency and combined energy resolution/energy field-of-view are offered by counting mode direct detection, showing promise for efficient spectrum imaging, low-dose mapping of beam-sensitive specimens, trace element analysis, and time-resolved spectroscopy. Despite the limited counting rate imposed by the readout electronics, we show that both core-loss and low-loss spectral acquisition are practical. These developments will benefit biologists, chemists, physicists, and materials scientists alike.

Interface-induced nonswitchable domains in ferroelectric thin films.

Engineering domains in ferroelectric thin films is crucial for realizing technological applications including non-volatile data storage and solar energy harvesting. Size and shape of domains strongly depend on the electrical and mechanical boundary conditions. Here we report the origin of nonswitchable polarization under external bias that leads to energetically unfavourable head-to-head domain walls in as-grown epitaxial PbZr(0.2)Ti(0.8)O3 thin films. By mapping electrostatic potentials and electric fields using off-axis electron holography and electron-beam-induced current with in situ electrical biasing in a transmission electron microscope, we show that electronic band bending across film/substrate interfaces locks local polarization direction and further produces unidirectional biasing fields, inducing nonswitchable domains near the interface. Presence of oxygen vacancies near the film surface, as revealed by electron-energy loss spectroscopy, stabilizes the charged domain walls. The formation of charged domain walls and nonswitchable domains reported in this study can be an origin for imprint and retention loss in ferroelectric thin films.

Probing the chemical structure in diamond-based materials using combined low-loss and core-loss electron energy-loss spectroscopy.

We report the analysis of the changes in local carbon structure and chemistry caused by the self-implantation of carbon into diamond via electron energy-loss spectroscopy (EELS) plasmon energy shifts and core-edge fine structure fingerprinting. These two very different EELS energy and intensity ranges of the spectrum can be acquired under identical experimental conditions and nearly simultaneously using specially designed deflectors and energy offset devices known as "DualEELS." In this way, it is possible to take full advantage of the unique and complementary information that is present in the low- and core-loss regions of the EELS spectrum. We find that self-implanted carbon under the implantation conditions used for the material investigated in this paper creates an amorphous region with significant sp 2 content that varies across the interface.

Review of recent advances in spectrum imaging and its extension to reciprocal space.

Using examples from various domains of science, this review covers some recent developments in spectrum imaging (SI) using scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Advanced multi-dimensional acquisition methods allow the acquisition of STEM-EELS data with other complementary data such as energy dispersive X-ray spectroscopy (EDS), cathodoluminescence and even combining them with reciprocal space analysis through a new method called diffraction imaging. This method allows real and reciprocal space information to be mixed to get a more complete description of the electron-sample interaction. The developments in SI data analysis such as multiple linear least-squares fitting, non-linear least-squares fitting and multivariate analysis allow for a robust extraction not only of each elemental distribution but also of each chemical phase in a sample through an intuitive computer-assisted method.

Four-dimensional STEM-EELS: enabling nano-scale chemical tomography.

Advances in electron-based instrumentation have enabled the acquisition of multidimensional data sets for exploring the unique structure-property relationship of nanomaterials. In this manuscript, we report a technique for directly probing and analyzing the three-dimensional (3D) electronic structure of a material at the nano-scale. This technique, referred to here as 4D STEM-EELS, utilizes a rotation holder and pillar-shaped samples to allow STEM mode high-angle annular dark-field (HAADF) and EELS spectrum images to be recorded over a complete 180 degrees rotation to minimize artifacts. The end result is a four-dimensional data set, containing two spatial dimensions, rotation angle and energy-loss information I(x, y, theta, DeltaE), which can then be processed to extract any EELS signal as a rotation or "tilt-series" map. If the extracted properties satisfy the linear projection criteria, these maps can then be used for tomographic reconstruction to yield volumetric maps of the corresponding properties. Hence by combining STEM HAADF and energy-loss information from such a series of spectrum images, it is possible to map not only the microstructure, but also the elemental, physical and chemical state information of a material in three dimensions. Two examples are reported here to demonstrate the potential of this technique. To illustrate chemical tomography, 4D STEM-EELS was used to directly probe the 3D electronic structure of a W-to-Si contact from a semiconductor device. Core-loss data were used to reconstruct and render the composition of the W-to-Si contact in three dimensions. The fine structure of the 99eV Si edge was analyzed with MLLS fitting to map the variations in Si bonding in 3D. To illustrate the direct probing of intrinsic material anisotropy, 4D STEM-EELS was used to probe a ZnO thin film. Subtle but systematic changes in low-loss structure were observed as a function of electron-beam orientation with respect to the ZnO crystallographic axes. Together these examples illustrate how the 4D STEM-EELS technique reported here can be used to probe the elemental, physical and chemical state information of a material in three dimensions and extend our knowledge of nano-scale structures.

Fast, completely reversible li insertion in vanadium pentoxide nanoribbons.

Layered-structure nanoribbons with efficient electron transport and short lithium ion insertion lengths are promising candidates for Li battery applications. Here we studied at the single nanostructure level the chemical, structural, and electrical transformations of V2O5 nanoribbons. We found that transformation of V2O5 into the omega-Li3V2O5 phase depends not only on the width but also the thickness of the nanoribbons. Transformation can take place within 10 s in thin nanoribbons, suggesting a Li diffusion constant 3 orders of magnitude faster than in bulk materials, resulting in a significant increase in battery power density (360 C power rate). For the first time, complete delithiation of omega-Li3V2O5 back to the single-crystalline, pristine V2O5 nanoribbon was observed, indicating a 30% higher energy density. These new observations are attributed to the ability of facile strain relaxation and phase transformation at the nanoscale. In addition, efficient electronic transport can be maintained to charge a Li3V2O5 nanoribbon within less than 5 s. These exciting nanosize effects can be exploited to fabricate high-performance Li batteries for applications in electric and hybrid electric vehicles.

Metal core bonding motifs of monodisperse icosahedral Au13 and larger Au monolayer-protected clusters as revealed by X-ray absorption spectroscopy and transmission electron microscopy.

The atomic metal core structures of the subnanometer clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) were characterized using advanced methods of electron microscopy and X-ray absorption spectroscopy. The number of gold atoms in the cores of these two clusters was determined quantitatively using high-angle annular dark field scanning transmission electron microscopy. Multiple-scattering-path analyses of extended X-ray absorption fine structure (EXAFS) spectra suggest that the Au metal cores of each of these complexes adopt an icosahedral structure with a relaxation of the icosahedral strain. Data from microscopy and spectroscopy studies extended to larger thiolate-protected gold clusters showing a broader distribution in nanoparticle core sizes (183 +/- 116 Au atoms) reveal a bulklike fcc structure. These results further support a model for the monolayer-protected clusters (MPCs) in which the thiolate ligands bond preferentially at 3-fold atomic sites on the nanoparticle surface, establishing an average composition for the MPC of Au180[S(CH2)11CH3]40. Results from EXAFS measurements of a gold(I) dodecanethiolate polymer are presented that offer an alternative explanation for observations in previous reports that were interpreted as indicating Au MPC structures consisting of a Au core, Au2S shell, and thiolate monolayer.

Sub-nanometer Au monolayer-protected clusters exhibiting molecule-like electronic behavior: quantitative high-angle annular dark-field scanning transmission electron microscopy and electrochemical characterization of clusters with precise atomic stoichiometry.

The synthesis and characterization of the clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) are described. These mixed-ligand, sub-nanometer clusters, prepared via exchange of dodecanethiol onto phosphine-halide gold clusters, show enhanced stability relative to the parent. The characterization of these clusters features the precise determination of the number of gold atoms in the cluster cores using high-angle annular dark-field scanning transmission electron microscopy, allowing the assignment of 13 gold atoms (+/-3 atoms) to the composition of both cluster molecules. Electrochemical and optical measurements reveal discrete molecular orbital levels and apparent energy gaps of 1.6-1.7 eV for the two cluster molecules. The electrochemical measurements further indicate that the Au13[PPh3]4[S(CH2)11CH3]2Cl2 cluster undergoes an overall two-electron reduction. The electrochemical and spectroscopic properties of the two Au13 cluster molecules are compared with those of a secondary synthetic product, which proved to be larger Au thiolate-derivatized monolayer-protected clusters with an average core of Au180. The latter shows behavior fully consistent with the adoption of metallic-like properties.

Physical evidence that yeast frataxin is an iron storage protein.

Frataxin is a conserved mitochondrial protein required for iron homeostasis. We showed previously that in the presence of ferrous iron recombinant yeast frataxin (mYfh1p) assembles into a regular multimer of approximately 1.1 MDa storing approximately 3000 iron atoms. Here, we further demonstrate that mYfh1p and iron form a stable hydrophilic complex that can be detected by either protein or iron staining on nondenaturing polyacrylamide gels, and by either interference or absorbance measurements at sedimentation equilibrium. The molecular mass of this complex has been refined to 840 kDa corresponding to 48 protein subunits and 2400 iron atoms. Solution density measurements have determined a partial specific volume of 0.58 cm(3)/g, consistent with the amino acid composition of mYfh1p and the presence of 50 Fe-O equivalents per subunit. By dynamic light scattering, we show that the complex has a radius of approximately 11 nm and assembles within 2 min at 30 degrees C when ferrous iron, not ferric iron or other divalent cations, is added to mYfh1p monomer at pH between 6 and 8. Iron-rich granules with diameter of 2-4 nm are detected in the complex by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. These findings support the hypothesis that frataxin is an iron storage protein, which could explain the mitochondrial iron accumulation and oxidative damage associated with frataxin defects in yeast, mouse, and humans.