Dispersion of Ni2+ ions via acetate precursor in the preparation of NaNiPO4 nanoparticles: effect of acetate vs. nitrate on the capacitive energy storage properties.

The influence of the precursors on the dispersion of Ni2+ ions and the presence of several other functional groups was investigated in the preparation of sodium nickel phosphate (NaNiPO4) cathode for a supercapacitor study. The dispersion of nickel phases, in the form of nanosheets, is influenced by the type of precursors used in the synthesis. XPS based spectroscopic information on the surface functional groups on NaNiPO4 show differences between the precursors (i.e.) acetate- and nitrate-derived materials. The benefits of using acetate as an alternative to nitrate are explored by using the NaNiPO4 nanoparticles as a cathode for supercapacitor applications. The acetate-derived material exhibits improved electrochemical properties possessing both redox behaviour and double-layer capacitance. The results indicate that the metal acetates are homogenously distributed. Acetate functionalization resulted in an improved capacitance of 90 F g-1 compared with that obtained from the nitrate precursor derived material (58 F g-1). Capacitance retention and high rate capability were also a feature of the acetate-derived material. The sodium nickel phosphate cathode material has provided useful insights on the precursor chemistry in storing renewable energy have been reported for the first time.

In situ characterisation of nanostructured multiphase thermoelectric materials at elevated temperatures.

Multiphase thermoelectric materials have recently attracted considerable attention due to the high thermoelectric efficiencies which can be achieved in these compounds compared to their single-phase counterparts. However, there is very little known on the structural evolution of these phases as a function of temperature. In this work we performed an in situ high temperature structural characterisation of recently reported high efficiency p-type multiphase (PbTe)0.65(PbS)0.25(PbSe)0.1 compounds by hot stage transmission electron microscopy and high-resolution neutron powder diffraction. We observed the microstructural evolution of precipitates and determined the lattice parameters of phases as a function of temperature for materials, which have been heavily and lightly doped with sodium. The role of the sodium is to optimize the concentration of charge carriers. It has been shown to distribute heterogeneously between the phases in multiphase compounds. The dissolution of secondary phases is found to occur at elevated temperatures. Although sodium concentration produces no significant differences between the lattice constants of the phases and the dissolution sequence of precipitates, it affects quite significantly the kinetics of precipitation. The heavily doped samples reach structural thermodynamic equilibrium more quickly than the lightly doped compound. These results are a step forward in designing high performance multiphase thermoelectric materials.

Development of an ellipse fitting method with which to analyse selected area electron diffraction patterns.

A software method has been developed which uses ellipse fitting to analyse electron diffraction patterns from polycrystalline materials. The method, which requires minimal user input, can determine the pattern centre and the diameter of diffraction rings with sub-pixel precision. This enables accurate crystallographic information to be obtained in a rapid and consistent manner. Since the method fits ellipses, it can detect, quantify and correct any elliptical distortion introduced by the imaging system. Distortion information derived from polycrystalline patterns as a function of camera length can be subsequently recalled and applied to single crystal patterns, resulting in improved precision and accuracy. The method has been implemented as a plugin for the DigitalMicrograph software by Gatan, and is a freely available via the internet.

Contamination mitigation strategies for scanning transmission electron microscopy.

Modern scanning transmission electron microscopy (STEM) enables imaging and microanalysis at very high magnification. In the case of aberration-corrected STEM, atomic resolution is readily achieved. However, the electron fluxes used may be up to three orders of magnitude greater than those typically employed in conventional STEM. Since specimen contamination often increases with electron flux, specimen cleanliness is a critical factor in obtaining meaningful data when carrying out high magnification STEM. A range of different specimen cleaning methods have been applied to a variety of specimen types. The contamination rate has been measured quantitatively to assess the effectiveness of cleaning. The methods studied include: baking, cooling, plasma cleaning, beam showering and UV/ozone exposure. Of the methods tested, beam showering is rapid, experimentally convenient and very effective on a wide range of specimens. Oxidative plasma cleaning is also very effective and can be applied to specimens on carbon support films, albeit with some care. For electron beam-sensitive materials, cooling may be the method of choice. In most cases, preliminary removal of the bulk of the contamination by methods such as baking or plasma cleaning, followed by beam showering, where necessary, can result in a contamination-free specimen suitable for extended atomic scale imaging and analysis.

RDFTools: a software tool for quantifying short-range ordering in amorphous materials.

A software package for computing radial distribution functions and other pair correlation functions from electron diffraction patterns of disordered solids is presented. The package, called RDFTools, is freely available via the internet and allows rapid in situ measurements of such quantities as interatomic nearest neighbor distances, average bond angles and coordination numbers. The software runs under DigitalMicrograph™ (Pleasanton, California, Gatan), a very widely used program in transmission electron microscopy. All implemented algorithms have been designed to compute diffraction integrals and data-processing averages in a fast and efficient manner to enable quick processing of publication ready, quantitative pair distribution function information. In the development of RDFTools, significant attention was paid to provide a robust and intuitive user-interface for deriving reliable semiquantitative information. For example, RDFTools enables accurate pair separation distances to be revealed upon immediate interrogation at the microscope; even for potentially thick specimens and/or regions of unknown elemental composition.

Fabrication of surface magnetic nanoclusters using low energy ion implantation and electron beam annealing.

Magnetic nanoclusters have novel applications as magnetic sensors, spintronic and biomedical devices, as well as applications in more traditional materials such as high-density magnetic storage media and high performance permanent magnets. We describe a new synthesis protocol which combines the advantages of ion implantation and electron beam annealing (EBA) to produce surface iron nanoclusters. We compare the structure, composition and magnetic properties of iron nanoclusters fabricated by low dose 15 keV Fe implantation into SiO(2) followed by 1000 °C EBA or furnace annealing. Atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM) images together with superconducting quantum interference device (SQUID) magnetometry measurements show that only EBA leads to the rapid formation of surface crystalline Fe spherical nanoclusters, showing magnetic moments per Fe atom comparable to that of bulk bcc Fe and superparamagnetic properties. We propose a fabrication mechanism which includes e-beam enhanced desorption of SiO(2). This method has potential for fabricating nanoscale magnetic sensors integrated in microelectronic devices.

Iron-monosulfide oxidation in natural sediments: resolving microbially mediated S transformations using XANES, electron microscopy, and selective extractions.

Iron-monosulfide oxidation and associated S transformations in a natural sediment were examined by combining selective extractions, electron microscopy and S K-edge X-ray absorption near-edge structure (XANES) spectroscopy, The sediment examined in this study was collected from a waterway receiving acid-sulfate soil drainage. It contained a high acid-volatile sulfide content (1031 micromol g(-1)), reflecting an abundance of iron-monosulfide. The iron-monosulfide speciation in the initial sediment sample was dominated by nanocrystalline mackinawite (tetragonal FeS). At near-neutral pH and an 02 partial pressure of approximately 0.2 atm, the mackinawite was found to oxidize rapidly, with a half-time of 29 +/- 2 min. This oxidation rate did not differ significantly (P < 0.05) between abiotic versus biotic conditions, demonstrating that oxidation of nanocrystalline mackinawite was not microbially mediated. The extraction results suggested that elemental S (S8(0)) was a key intermediate S oxidation product Transmission electron microscopy showed the S8(0) to be amorphous nanoglobules, 100-200 nm in diameter. The quantitative importance of S8(0) was confirmed by linear combination XANES spectroscopy, after accounting for the inherent effect of the nanoscale S8(0) particle-size on the corresponding XANES spectrum. Both the selective extraction and XANES data showed that oxidation of S8(0) to SO4(2-) was mediated by microbial activity. In addition to directly revealing important S transformations, the XANES results support the accuracy of the selective extraction scheme employed here.

Microstructural and compositional analysis of strontium-doped lead zirconate titanate thin films on gold-coated silicon substrates.

This article discusses the results of transmission electron microscopy (TEM)-based characterization of strontium-doped lead zirconate titanate (PSZT) thin films. The thin films were deposited by radio frequency magnetron sputtering at 300 degrees C on gold-coated silicon substrates, which used a 15 nm titanium adhesion layer between the 150 nm thick gold film and (100) silicon. The TEM analysis was carried out using a combination of high-resolution imaging, energy filtered imaging, energy dispersive X-ray (EDX) analysis, and hollow cone illumination. At the interface between the PSZT films and gold, an amorphous silicon-rich layer (about 4 nm thick) was observed, with the film composition remaining uniform otherwise. The films were found to be polycrystalline with a columnar structure perpendicular to the substrate. Interdiffusion between the bottom metal layers and silicon was observed and was confirmed using secondary ion mass spectrometry. This occurs due to the temperature of deposition (300 degrees C) being close to the eutectic point of gold and silicon (363 degrees C). The diffused regions in silicon were composed primarily of gold (analyzed by EDX) and were bounded by (111) silicon planes, highlighted by the triangular diffused regions observed in the two-dimensional TEM image.

Nanocolumnar Preferentially Oriented PSZT Thin Films Deposited on Thermally Grown Silicon Dioxide.

We report the first instance of deposition of preferentially oriented, nanocrystalline, and nanocolumnar strontium-doped lead zirconate titanate (PSZT) ferroelectric thin films directly on thermal silicon dioxide. No intermediate seed or activation layers were used between PSZT and silicon dioxide. The deposited thin films have been characterised using a combination of diffraction and microscopy techniques.

Microstructural investigation of nickel silicide thin films and the silicide-silicon interface using transmission electron microscopy.

This article discusses the results of transmission electron microscopy (TEM)-based investigation of nickel silicide (NiSi) thin films grown on silicon. Nickel silicide is currently used as the CMOS technology standard for local interconnects and in electrical contacts. Films were characterized with a range of TEM-based techniques along with glancing angle X-ray diffraction. The nickel silicide thin films were formed by vacuum annealing thin films of nickel (50 nm) deposited on (100) silicon. The cross-sectional samples indicated a final silicide thickness of about 110 nm. This investigation studied and reports on three aspects of the thermally formed thin films: the uniformity in composition of the film using jump ratio maps; the nature of the interface using high resolution imaging; and the crystalline orientation of the thin films using selected-area electron diffraction (SAED). The analysis highlighted uniform composition in the thin films, which was also substantiated by spectroscopy techniques; an interface exhibiting the desired abrupt transition from silicide to silicon; and desired and preferential crystalline orientation corresponding to stoichiometric NiSi, supported by glancing angle X-ray diffraction results.

DiffTools: electron diffraction software tools for DigitalMicrograph.

A suite of software tools (DiffTools) has been developed for DigitalMicrograph--a software platform widely used in transmission electron microscopy (TEM) laboratories world-wide. These tools include capabilities for calibration, center determination, rotational averaging with automatic peak location, pattern spot spacing and interspot angle determination, rotation and inset of a pattern into an image, pattern intensity inversion and enhancement, and calculation of basic crystallographic data. The implementation and application of these tools to experimental diffraction patterns is illustrated and measurement of d-spacings with an accuracy of 0.63% or better is demonstrated. These tools, which are freely available via the Internet, enable users to rapidly and efficiently process selected area electron diffraction patterns. This can be performed entirely within the DigitalMicrograph environment using familiar menu-based commands and user-friendly dialog-based scripts.

Circular Hough transform diffraction analysis: a software tool for automated measurement of selected area electron diffraction patterns within Digital Micrograph.

A software tool (script and plugin) for computing circular Hough transforms (CHT) in Digital Micrograph has been developed, for the purpose of automated analysis of selected area electron diffraction patterns (SADPs) of polycrystalline materials. The CHT enables the diffraction pattern centre to be determined with sub-pixel accuracy, regardless of the exposure condition of the transmitted beam or if a beam stop is present. Radii of the diffraction rings can also be accurately measured with sub-pixel precision. If the pattern is calibrated against a known camera length, then d-spacings with an accuracy of better than 1% can be obtained. These measurements require no a priori knowledge of the pattern and very limited user interaction. The accuracy of the CHT is degraded by distortion introduced by the projector lens, and this should be minimised prior to pattern acquisition. A number of optimisations in the CHT software enable rapid processing of patterns; a typical analysis of a 1k x 1k image taking just a few minutes. The CHT tool appears robust and is even able to accurately measure SADPs with very incomplete diffraction rings due to texture effects. This software tool is freely downloadable via the Internet.

Plasmon imaging: an efficient TEM-based method for locating noble metal particles dispersed on oxide catalysts at very low densities.

We have used transmission electron microscopy to study catalysts comprising nanoparticulate gold dispersed on a highly porous nanoparticulate TiO2 (anatase) support. The similarity of the morphology of the two phases, and the low number density of gold particles (1 in 65,000) makes this challenging. Diffraction contrast imaging could not differentiate the two phases, since TiO2 oriented at strong Bragg conditions, produced similar contrast to the Bragg/mass-thickness contrast of the gold. Mass-thickness contrast imaging allowed gold to be differentiated from TiO2 only in the thinnest regions, where the mass-thickness of TiO2 was low. Plasmon imaging, using an energy loss of 24 eV and an energy window width of 5 eV, was very effective at locating gold. Both the TiO2 and impregnating resin produced a strong plasmon signal, while the much weaker signal from the gold made it appear dark. This permitted the gold particles to be readily located, irrespective of whether they were located in the thin or thick regions of the TiO2 support.

Determination of mean free path for energy loss and surface oxide film thickness using convergent beam electron diffraction and thickness mapping: a case study using Si and P91 steel.

Determining transmission electron microscope specimen thickness is an essential prerequisite for carrying out quantitative microscopy. The convergent beam electron diffraction method is highly accurate but provides information only on the small region being probed and is only applicable to crystalline phases. Thickness mapping with an energy filter is rapid, maps an entire field of view and can be applied to both crystalline and amorphous phases. However, the thickness map is defined in terms of the mean free path for energy loss (lambda), which must be known in order to determine the thickness. Convergent beam electron diffraction and thickness mapping methods were used to determine lambda for two materials, Si and P91 steel. These represent best- and worst-case scenario materials, respectively, for this type of investigation, owing to their radically different microstructures. The effects of collection angle and the importance of dynamical diffraction contrast are also examined. By minimizing diffraction contrast effects in thickness maps, reasonably accurate (+/-15%) values of lambda were obtained for P91 and accuracies of +/-5% were obtained for Si. The correlation between the convergent beam electron diffraction-derived thickness and the log intensity ratios from thickness maps also permits estimation of the thickness of amorphous layers on the upper and lower surfaces of transmission electron microscope specimens. These estimates were evaluated for both Si and P91 using cross-sectional transmission electron microscopy and were found to be quite accurate.

Scripting-customized microscopy tools for Digital Micrograph.

Software is an integral part of all electron microscopy systems, encompassing hardware control, data acquisition and processing. It is unlikely that any one software system will meet all the requirements of experienced users. However, if the software supports custom scripting, then users are well placed to address any shortcomings by writing their own software. In this paper, we highlight the scripting capability within Gatan Inc.'s Digital Micrograph (DM) software, a widely used program for TEM imaging and EELS spectroscopy. We show how scripting can greatly extend the capabilities of the DM software, in tasks ranging in complexity from simple image manipulation through to full-blown microscope/imaging filter control and data acquisition. Scripting enables customized software tools to be developed to meet individual experimental needs, something which no software manufacturer could ever hope to do on a commercial basis. In essence, scripting allows the microscopist to drive the software rather than the software drive the microscopist. To foster an increased awareness and interest in DM scripting we have developed a web-based archive for DM scripts, which is freely accessible via the internet.