Evaluation of correlated studies using liquid cell and cryo-transmission electron microscopy: Hydration of calcium sulphate and the phase transformation pathways of bassanite to gypsum.

Insight into the nucleation, growth and phase transformations of calcium sulphate could improve the performance of construction materials, reduce scaling in industrial processes and aid understanding of its formation in the natural environment. Recent studies have suggested that the calcium sulphate pseudo polymorph, gypsum (CaSO4 ·2H2 O) can form in aqueous solution via a bassanite (CaSO4 ·0.5H2 O) intermediate. Some in situ experimental work has also suggested that the transformation of bassanite to gypsum can occur through an oriented assembly mechanism. In this work, we have exploited liquid cell transmission electron microscopy (LCTEM) to study the transformation of bassanite to gypsum in an undersaturated aqueous solution of calcium sulphate. This was benchmarked against cryogenic TEM (cryo-TEM) studies to validate internally the data obtained from the two microscopy techniques. When coupled with Raman spectroscopy, the real-time data generated by LCTEM, and structural data obtained from cryo-TEM show that bassanite can transform to gypsum via more than one pathway, the predominant one being dissolution/reprecipitation. Comparisons between LCTEM and cryo-TEM also show that the transformation is slower within the confined region of the liquid cell as compared to a bulk solution. This work highlights the important role of a correlated microscopy approach for the study of dynamic processes such as crystallisation from solution if we are to extract true mechanistic understanding.

High-resolution imaging of organic pharmaceutical crystals by transmission electron microscopy and scanning moiré fringes.

Formulation processing of organic crystalline compounds can have a significant effect on drug properties, such as dissolution rate or tablet strength/hardness. Transmission electron microscopy (TEM) has the potential to resolve the atomic lattice of these crystalline compounds and, for example, identify the defect density on a particular crystal face, provided that the sensitivity of these crystals to irradiation by high-energy electrons can be overcome. Here, we acquire high-resolution (HR) lattice images of the compound furosemide using two different methods: low-dose HRTEM and bright-field (BF) scanning TEM (STEM) scanning moiré fringes (SMFs). Before acquiring HRTEM images of furosemide, a model system of crocidolite (asbestos) was used to determine the electron flux/fluence limits of low-dose HR imaging for our scintillator-based, complementary metal-oxide semiconductor (CMOS) electron camera by testing a variety of electron flux and total electron fluence regimes. An electron flux of 10 e- /(Å2 s) and total fluence of 10 e- /Å2 was shown to provide sufficient contrast and signal-to-noise ratio to resolve 0.30 nm lattice spacings in crocidolite at 300 kV. These parameters were then used to image furosemide which has a critical electron fluence for damage of ≥10 e- /Å2 at 300 kV. The resulting HRTEM image of a furosemide crystal shows only a small portion of the total crystal exhibiting lattice fringes, likely due to irradiation damage during acquisition close to the compound's critical fluence. BF-STEM SMF images of furosemide were acquired at a lower electron fluence (1.8 e- /Å2 ), while still indirectly resolving HR details of the (001) lattice. Several different SMFs were observed with minor variations in the size and angle, suggesting strain due to defects within the crystal. Overall BF-STEM SMFs appear to be more useful than BF-STEM or HRTEM (with a CMOS camera) for imaging the crystal lattice of very beam-sensitive materials since a lower electron fluence is required to reveal the lattice. BF-STEM SMFs may thus prove useful in improving the understanding of crystallization pathways in organic compounds, degradation in pharmaceutical formulations and the effect of defects on the dissolution rate of different crystal faces. Further work is, however, required to quantitatively determine properties such as the defect density or the amount of relative strain from a BF-STEM SMF image.

Application of automated electron microscopy imaging and machine learning to characterise and quantify nanoparticle dispersion in aqueous media.

For many nanoparticle applications it is important to understand dispersion in liquids. For nanomedicinal and nanotoxicological research this is complicated by the often complex nature of the biological dispersant and ultimately this leads to severe limitations in the analysis of the nanoparticle dispersion by light scattering techniques. Here we present an alternative analysis and associated workflow which utilises electron microscopy. The need to collect large, statistically relevant datasets by imaging vacuum dried, plunge frozen aliquots of suspension was accomplished by developing an automated STEM imaging protocol implemented in an SEM fitted with a transmission detector. Automated analysis of images of agglomerates was achieved by machine learning using two free open-source software tools: CellProfiler and ilastik. The specific results and overall workflow described enable accurate nanoparticle agglomerate analysis of particles suspended in aqueous media containing other potential confounding components such as salts, vitamins and proteins. LAY DESCRIPTION: In order to further advance studies in both nanomedicine and nanotoxicology, we need to continue to understand the dispersion of nanoparticles in biological fluids. These biological environments often contain a number of components such as salts, vitamins and proteins which can lead to difficulties when using traditional techniques to monitor dispersion. Here we present an alternative analysis which utilises electron microscopy. In order to use this approach statistically relevant large image datasets were collected from appropriately prepared samples of nanoparticle suspensions by implementing an automated imaging protocol. Automated analysis of these images was achieved through machine learning using two readily accessible freeware; CellProfiler and ilastik. The workflow presented enables accurate nanoparticle dispersion analysis of particles suspended in more complex biological media.

Universal geometric frustration in pyrochlores.

Materials with the pyrochlore/fluorite structure have diverse technological applications, from magnetism to nuclear waste disposal. Here we report the observation of structural instability present in the pyrochlores A2Zr2O6O' (A = Pr, La) and Yb2Ti2O6O', that exists despite ideal stoichiometry, ideal cation-ordering, the absence of lone pair effects, and a lack of magnetic order. Though these materials appear to have good long-range order, local structure probes find displacements, of the order of 0.01 nm, within the pyrochlore framework. The pattern of displacements of the A2O' sublattice mimics the entropically-driven fluxional motions characteristic of and well-known in the silica mineral ß-cristobalite. The universality of such displacements within the pyrochlore structure adds to the known structural diversity and explains the extreme sensitivity to composition found in quantum spin ices and the lack of ferroelectric behavior in pyrochlores.

Interfacial Origin of the Magnetisation Suppression of Thin Film Yttrium Iron Garnet.

Yttrium iron garnet has a very high Verdet constant, is transparent in the infrared and is an insulating ferrimagnet leading to its use in optical and magneto-optical applications. Its high Q-factor has been exploited to make resonators and filters in microwave devices, but it also has the lowest magnetic damping of any known material. In this article we describe the structural and magnetic properties of single crystal thin-film YIG where the temperature dependence of the magnetisation reveals a decrease in the low temperature region. In order to understand this complex material we bring a large number of structural and magnetic techniques to bear on the same samples. Through a comprehensive analysis we show that at the substrate -YIG interface, an interdiffusion zone of only 4-6 nm exists. Due to the interdiffusion of Y from the YIG and Gd from the substrate, an addition magnetic layer is formed at the interface whose properties are crucially important in samples with a thickness of YIG less than 200 nm.

Robust theoretical modelling of core ionisation edges for quantitative electron energy loss spectroscopy of B- and N-doped graphene.

Electron energy loss spectroscopy (EELS) is a powerful tool for understanding the chemical structure of materials down to the atomic level, but challenges remain in accurately and quantitatively modelling the response. We compare comprehensive theoretical density functional theory (DFT) calculations of 1s core-level EEL K-edge spectra of pure, B-doped and N-doped graphene with and without a core-hole to previously published atomic-resolution experimental electron microscopy data. The ground state approximation is found in this specific system to perform consistently better than the frozen core-hole approximation. The impact of including or excluding a core-hole on the resultant theoretical band structures, densities of states, electron densities and EEL spectra were all thoroughly examined and compared. It is concluded that the frozen core-hole approximation exaggerates the effects of the core-hole in graphene and should be discarded in favour of the ground state approximation. These results are interpreted as an indicator of the overriding need for theorists to embrace many-body effects in the pursuit of accuracy in theoretical spectroscopy instead of a system-tailored approach whose approximations are selected empirically.

Long spin lifetime and large barrier polarisation in single electron transport through a CoFe nanoparticle.

We have investigated single electron spin transport in individual single crystal bcc Co30Fe70 nanoparticles using scanning tunnelling microscopy with a standard tungsten tip. Particles were deposited using a gas-aggregation nanoparticle source and individually addressed as asymmetric double tunnel junctions with both a vacuum and a MgO tunnel barrier. Spectroscopy measurements on the particles show a Coulomb staircase that is correlated with the measured particle size. Field emission tunnelling effects are incorporated into standard single electron theory to model the data. This formalism allows spin-dependent parameters to be determined even though the tip is not spin-polarised. The barrier spin polarisation is very high, in excess of 84%. By variation of the resistance, several orders of magnitude of the system timescale are probed, enabling us to determine the spin relaxation time on the island. It is found to be close to 10 µs, a value much longer than previously reported.

Visualizing surface plasmons with photons, photoelectrons, and electrons.

Both photons and electrons may be used to excite surface plasmon polaritons, the collective charge density fluctuations at the surface of metal nanostructures. By virtue of their nanoscopic and dissipative nature, a detailed characterization of surface plasmon (SP) eigenmodes in real space-time ultimately requires joint nanometer spatial and femtosecond temporal resolution. The latter realization has driven significant developments in the past few years, aimed at interrogating both localized and propagating SP modes. In this mini-review, we briefly highlight different techniques employed by our own groups to visualize the enhanced electric fields associated with SPs. Specifically, we discuss recent hyperspectral optical microscopy, tip-enhanced Raman nano-spectroscopy, nonlinear photoemission electron microscopy, as well as correlated scanning transmission electron microscopy-electron energy loss spectroscopy measurements targeting prototypical plasmonic nanostructures and constructs. Through selected practical examples from our own laboratories, we examine the information content in multidimensional images recorded by taking advantage of each of the aforementioned techniques. In effect, we illustrate how SPs can be visualized at the ultimate limits of space and time.

Microscopy of nanoparticulate dispersions.

We present a critical review of the common methods for determining the dispersion state of nanoparticulate samples particularly in liquid media, including the determination of particle size and morphology; particle size distributions and polydispersity and equilibrium particle structure and chemistry. We highlight the potential contributions of both scanning probe and electron microscopies in this analysis which is of benefit in understanding nanoparticulate formulations and their behaviour applied across a very wide range of technologies and industry sectors.

Fabrication and characterization of gold nano-wires templated on virus-like arrays of tobacco mosaic virus coat proteins.

The rod-shaped plant virus tobacco mosaic virus (TMV) is widely used as a nano-fabrication template, and chimeric peptide expression on its major coat protein has extended its potential applications. Here we describe a simple bacterial expression system for production and rapid purification of recombinant chimeric TMV coat protein carrying C-terminal peptide tags. These proteins do not bind TMV RNA or form disks at pH 7. However, they retain the ability to self-assemble into virus-like arrays at acidic pH. C-terminal peptide tags in such arrays are exposed on the protein surface, allowing interaction with target species. We have utilized a C-terminal His-tag to create virus coat protein-templated nano-rods able to bind gold nanoparticles uniformly. These can be transformed into gold nano-wires by deposition of additional gold atoms from solution, followed by thermal annealing. The resistivity of a typical annealed wire created by this approach is significantly less than values reported for other nano-wires made using different bio-templates. This expression construct is therefore a useful additional tool for the creation of chimeric TMV-like nano-rods for bio-templating.

Bench scale production of pure nanocrystalline molybdenum nitride through solid-gas phase reduction.

Solid-Gas phase reduction is an attractive method to produce high surface area, mesoporous powders, as well as pure product. Disadvantage of this method is low amount of product. Increasing the amount of precursor in quartz reactor causes the accumulation of reaction products and results in hydrothermal sintering. Therefore, the diameter of reactor was increased, and it allowed to increase the precursor and also to prevent the hydrothermal sintering. The products were investigated by XRD, BET, SEM and TEM. The surface area of mesoporous powder, from 7 g loaded precursor, was 112 m2/g and crystallite size was about 10 nm. Increasing the precursor up to 30 g, under controlled program, resulted in lower surface area, but still above 105 m2/g. The pore volume was 72.36 cm3/g, quite suitable for catalytic applications.

Fantastic improvement in quality and quantity of carbon nanotubes synthesized on Al2O3-SiO2 supports by N2 pretreatment.

The raw materials, condition and the method of preparing the catalysts play an important role in the growth of high quality Carbon Nanotubes by Catalytic Chemical Vapor Deposition method. In this work, the efficiency of Carbon Nanotubes growth was increased by a simple controlled preheating of the catalyst in N2 atmosphere. Supports were prepared by mixing alumina powder with tetraethyl orthosilicate (TEOS) by a chemical method at low temperature. Afterwards, the supports were impregnated with iron. The dried and ground catalyst was heated in N2 atmosphere at 500 degrees C for 1 hour followed by cooling down to room temperature. Methane was passed over the prepared catalyst bed at 900 degrees C. Supports, supported catalysts and Carbon Nanotubes samples have been characterized by Transmission Electron Microscopy, Scanning Electron Microscopy, Gas Adsorption/Desorption Analysis, X-Ray Diffraction and Raman Spectroscopy. Scanning Electron Microscopy images of the nanotubes showed a drastic increase in the growth rate, length and straightness of the Nanotubes in comparison to the growth without preheating and even preheating in air atmosphere. Raman Spectroscopy of the samples and Transmission Electron Microscopy pictures showed bundles, mostly equi-diameter Single Wall Nanotubes. In fact, the growth rate, length, and purity of the Nanotubes, also the homogeneity of the tubes improved. The conclusion can be made with the help of proposed theory of nucleation and growth of Nanotubes based on comparative results of the characterizations with and without preheat-treatment. It seems that the preheat-treatment in N2 affected the catalyst structure and its interaction with support as well as distribution of the catalyst particles on the support. These changes in return affect the quality and quantity of final production.

Characterization of dentine structure in three dimensions using FIB-SEM.

All biological tissues are three dimensional and contain structures that span a range of length scales from nanometres through to hundreds of millimetres. These are not ideally suited to current three-dimensional characterization techniques such as X-ray or transmission electron tomography. Such detailed morphological analysis is critical to understanding the structural features relevant to tissue function and designing therapeutic strategies intended to address structural deficiencies encountered in pathological states. We show that use of focused ion beam milling combined with scanning electron microscopy can provide three-dimensional information at nanometre resolution from biologically relevant volumes of material, in this case dentine.

Study on the magnetorheological properties of maghemite-kerosene ferrofluid.

As the ferrofluids are synthesized in a controlled atmosphere to Prevent oxidation of the magnetite phase; most reports of rheological properties have been derived from magnetite based ferrofluids. In this paper a ferrofluid based on iron oxide was synthesized by co-precipitation with air. Lauric acid was used to coat magnetic nanoparticles in the kerosene. The microstructural features of the ferrofluid and the variation with time and temperature of its rheologic and magnetic properties were investigated. The results indicated that the magnetic particles had an average size of 10.6 nm consisting of maghemite as the major phase. Viscosity of ferrofluid showed considerable variation with time and temperature. It was specified that the time dependency of the magnetoviscousity is related to particle size and rearrangement of nanoparticles of product is independent from the magnetic field. Moreover at low shear rates (< 0.1 s(-1)) the interaction of nanoparticles is related to the van der waals forces which cause the increase of the viscosity with time. The temperature effect showed that the magnetoviscosity decreases considerably above 45 degrees C.

A study on the effects of three types of deflocculants and the increase in the pH on the rheological behavior of nano carbon suspensions.

In this study, the rheological behavior of carbon black suspension with different amounts of three deflocculants (Dispex G40, Dispex N40 and CMC) has been investigated. As the rheological behavior of carbon black is markedly dependent on pH, the effect of the increase in pH has been studied. Dispex G40 and Sodium carboxymethycellulose (CMC), are effective in promoting complete deflocculation of carbon black suspensions with 20, 30 and 40 wt% solids concentrations for Dispex G40, and 40 wt% for CMC. 40 wt% suspensions containing Dispex N40 exhibit a yield stress at all levels of deflocculant investigated. However, a minimum in the extrapolated yield stress and apparent viscosity curve indicates that there is an optimum deflocculation level. The addition of alkali enhances the deflocculating power of CMC and Dispex G40, reducing any existing yield stress to zero. However, it would appear that Dispex N40 is desorbed when the pH is increased as rapid coagulation of the system is apparent, arising from a reduction in the repulsive forces.

Electronic property investigations of single-walled carbon nanotube bundles in situ within a transmission electron microscope: an evaluation.

We have evaluated a combined transmission electron microscopy (TEM)-scanning tunnelling microscopy (STM) (hence TEM-STM) sample holder for the investigation of the mechanical and electrical properties of individual bundles of single-walled carbon nanotubes (SWCNTs) together with their simultaneous observation, analysis and mechanical modification in the TEM. Current-voltage (I-V) measurements from bundles of SWCNTs were observed to change as the bundles were deformed both reversibly and irreversibly, although the observed behaviour was somewhat complex. Electron energy loss spectroscopy (EELS) analysis revealed measurable changes in the bonding of the carbon atoms within the graphene layers upon bundle deformation, with measurable changes in the pi*/(pi*+sigma*) peak ratios observed at the carbon K-edge. Reversible deformation of the bundles was consistent with the sensitivity of sigma bonding to deviations from nonplanarity, whereas irreversible deformation was consistent with the introduction of nonhexagonal defects into the graphene sheets.

Hydrogarnet: a host phase for Cr(VI) in chromite ore processing residue (COPR) and other high pH wastes.

For understanding both the environmental behavior and developing remediation treatments for chromium ore processing residue (COPR) it is important to identify all the potentially soluble sources of Cr(VI). Hydrogarnet has been identified as a major phase in COPR and it has been previously speculated that it has a capacity to host Cr(VI). Here we provide direct evidence of this capacity by demonstrating the incorporation of Cr(VI) into laboratory synthesized hydrogarnet. Electron microscopy and energy dispersive X-ray microanalysis show incorporation of approximately 17000-22000 mg Cr(VI) kg(-1) hydrogarnet. X-ray powder diffraction data show that peak intensities are altered by chromium substitution and that chromium substituted hydrogarnets have a smaller unit cell than the pure Ca-Al end member. This is consistent with substitution of hydroxyl tetrahedra by smaller chromate tetrahedra. Electron energy loss spectroscopy confirms the tetrahedral coordination and hexavalent oxidation state of chromium in the hydrogarnets. The maximum amount of hexavalent chromium that can be introduced synthetically corresponds to a replacement of about one out of every eight hydroxyl tetrahedral per unit cell by a CrO4(2-) tetrahedra and tallies closely with the amount of chromium measured in hydrogarnets from COPR. Chromium-bearing hydrogarnet is the most abundant crystalline phase in millions of tons of COPR contaminating land around Glasgow, Scotland, and was recently identified in COPR from sites in North America. Calculations based on its abundance and its Cr(VI) content indicate that hydrogarnet can host as much as 50% of the Cr(VI) found in some COPR samples.

HREM of the [111] surfaces of iron oxide nanoparticles.

Mixed phase Fe3O4-gamma-Fe2O3 (magnetite-maghemite) iron oxide nanoparticles have been fabricated by colloidal routes. HRTEM/HRSTEM images of the nanoparticles show the presence of [111] facets that terminate with enhanced contrast, which is shown to be caused by the presence of additional cations at the edges of the nanoparticles. HRTEM images were taken on a FEI CM200 FEGTEM, a JEOL 3100 with a LaB6 source, and a double aberration corrected JEOL-JEM 2200FS FEGTEM. The enhanced contrast effect was observed on the [111] surface atomic layers resolved using each machine. HRSTEM images, taken on an aberration corrected STEM, resolved enhanced contrast at specific surface sites. Exit wave reconstruction was also carried out on focal series taken on a double aberration corrected JEOL-JEM 2200FS and showed similar highly resolved enhanced contrast at specific surface cation sites. It is apparent that additional cations are occupying the [111] terminating layers of these nanoparticle surfaces. The use of different microscopes and techniques in this paper provides strong evidence that the enhanced contrast is a real effect and not an effect caused by microscope aberrations.

Electron beam damage studies of synthetic 6-line ferrihydrite and ferritin molecule cores within a human liver biopsy.

In order to achieve an accurate understanding of the crystal structure of 6-line ferrihydrite (6LFh) and ferritin molecule cores within a human liver biopsy using transmission electron microscopy (TEM), electron beam damage should be considered. For the case of 6LFh, the electron energy loss near-edge structure (ELNES) of core ionisation edges in the electron energy loss spectrum (EELS) combined with multiple linear least-square (MLLS) fitting of reference spectra together with analysis of selected area electron diffraction (SAED) patterns suggests that the iron in 6LFh is solely octahedrally coordinated Fe3+. With increasing electron dose, an increasing percentage of this octahedrally coordinated Fe3+ migrates to tetrahedral sites. When the dose exceeds 3 x 10(8) electrons/nm2, Fe2+ is found to be present in the material. This method also indicates that the iron in ferritin molecule cores within a human liver biopsy is the same as in 6LFh, entirely Fe3+ in octahedral coordination with oxygen. Again the percentage of octahedrally coordinated Fe3+ decreases as the accumulated electron dose increases and Fe2+ is produced in the liver biopsies when the electron dose exceeds 10(6)electrons/nm2.

Quantitative valence plasmon mapping in the TEM: viewing physical properties at the nanoscale.

Using a series of graphitising carbons heat treated at different temperatures, the peak position of the bulk (pi+sigma) plasmon was measured using electron energy loss spectroscopy and observed to shift between 22 and 27eV. Experimental data is presented and discussed showing the effects of the collection conditions and sample orientation upon the observed spectra. We present an empirical technique by which quantitative energy filtered transmission electron microscopy (EFTEM) maps with two energy windows selected in the plasmon region can be readily acquired and processed, the results of which may be interpreted as graphitisation maps and subsequently physical property maps. An experimentally established resolution of approximately 1.6nm makes this technique a very useful tool with which to examine nanoscale properties in microstructural regions of interest in TEM specimens such as fibre/matrix interfaces within carbon-carbon composites, multi-walled carbon nanotubes and graphitic inclusions in carbon steels. Also presented is data demonstrating the unsuitability of pi(*)-related chemical EFTEM maps in both the low-loss region and at the carbon K ionisation edge for mapping bonding in such highly anisotropic media due to the strong orientation dependence of the intensity of the transitions involved. This is followed by suggestions for wider application of the plasmon mapping technique within systems other than those based upon carbon.