David R. G. Mitchell

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. Microsc. Res. Tech., 2008.

Transmission electron microscopy studies of atomic layer deposition TiO2 films grown on silicon

Transmission electron microscopy techniques have been used to characterise atomic layer deposition TiO films grown on 2 silicon substrates after RCA and HF treatment. The influence of deposition temperature (250–350 8C) and substrate type on the film microstructure have been determined. The major influence of substrate type is to control nucleation of crystallisation. HF treated silicon, which was devoid of the native oxide layer, promoted a crystalline, island growth mode. The nucleation of crystalline particles at the onset of deposition resulted in films with very fine grain sizes (f20 nm). The RCA treated silicon, which was coated with amorphous native oxide, caused the growth of an initially amorphous TiO film, which crystallised once 2 a critical film thickness had been exceeded. The major influence of temperature on the films grown on RCA treated silicon was to control nucleation of crystallisation within the amorphous layers, resulting in grain size refinement at higher deposition temperatures. Under the processing conditions used, other than the transient amorphous films formed on RCA treated silicon, anatase was the only phase formed. No evidence for preferred orientation was found. 2003 Elsevier Science B.V. All rights reserved.

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.

Atomic layer deposition of TiO2 and Al2O3 thin films and nanolaminates

We have been developing our capability with atomic layer deposition (ALD), to understand the influence of deposition parameters on the nature of TiO2 and Al2O3 films (high and low refractive index respectively), and multilayer stacks thereof (nanolaminates). These stacks have potential applications as anti-reflection coatings and optical filters. This paper will explore the evolution of structure in our films as a function of deposition parameters including temperature and substrate surface chemistry. A broad range of techniques have been applied to the study of these films, including cross-sectional transmission electron microscopy, spectroscopic ellipsometry and secondary-ion mass spectrometry. These have enabled a wealth of microstructural and compositional information on the films to be acquired, such as accurate film thickness, composition, crystallization sequence and orientation with respect to the substrate. The ALD method is shown to produce single-layer films and multilayer stacks with exceptional uniformity and flatness, and in the case of stacks, chemically abrupt interfaces. We are currently extending this technology to the coating of polymeric substrates.

Characterisation of PI3 and RF plasma nitrided austenitic stainless steels using plan and cross-sectional TEM techniques

Abstract Type 304 and 316 austenitic stainless steels treated with plasma immersion ion implantation (PI 3 ™) and RF plasma nitriding have been studied with transmission electron microscopy (TEM). A range of TEM specimen preparation techniques have been used to enable characterisation of the nature of the treated surface layers. Plan view TEM specimens, prepared by thinning from the back, highlighted the removal by sputtering of original surface features in steels during PI 3 processing, while the less energetic RF plasma nitriding retained the original surface features. All treatments resulted in the formation of an austenite phase heavily expanded by the high levels of interstitial nitrogen. Decomposition of this expanded austenite into CrN and α Fe occurred at higher treatment temperatures, and was found to nucleate at the grain boundaries preferentially. Conventional cross-sectional TEM specimens also showed the importance of the surface as a nucleation site for this transformation. The transformation process and the products of expanded austenite decomposition were the same regardless of the treatment process used. The expanded austenite layer was aligned with the underlying steel, with no orientational difference across the interface observed. Focused ion beam milling has the ability to consistently section treatment depths of several micrometers, although further work is necessary to produce through-section specimens thin enough for EELS and EFTEM analysis.

Modulation of Photocatalytic Properties by Strain in 2D BiOBr Nanosheets

BiOBr nanosheets with highly reactive {001} facets exposed were selectively synthesized by a facile hydrothermal method. The inner strain in the BiOBr nanosheets has been tuned continuously by the pH value. The photocatalytic performance of BiOBr in dye degradation can be manipulated by the strain effect. The low-strain BiOBr nanosheets show improved photocatalytic activity. Density functional calculations suggest that strain can modify the band structure and symmetry in BiOBr. The enhanced photocatalytic activity in low-strain BiOBr nanosheets is due to improved charge separation attributable to a highly dispersive band structure with an indirect band gap.

Mineral-Biochar Composites: Molecular Structure and Porosity.

Dramatic changes in molecular structure, degradation pathway, and porosity of biochar are observed at pyrolysis temperatures ranging from 250 to 550 °C when bamboo biomass is pretreated by iron-sulfate-clay slurries (iron-clay biochar), as compared to untreated bamboo biochar. Electron microscopy analysis of the biochar reveals the infusion of mineral species into the pores of the biochar and the formation of mineral nanostructures. Quantitative (13)C nuclear magnetic resonance (NMR) spectroscopy shows that the presence of the iron clay prevents degradation of the cellulosic fraction at pyrolysis temperatures of 250 °C, whereas at higher temperatures (350-550 °C), the clay promotes biomass degradation, resulting in an increase in both the concentrations of condensed aromatic, acidic, and phenolic carbon species. The porosity of the biochar, as measured by NMR cryoporosimetry, is altered by the iron-clay pretreatment. In the presence of the clay, at lower pyrolysis temperatures, the biochar develops a higher pore volume, while at higher temperature, the presence of clay causes a reduction in the biochar pore volume. The most dramatic reduction in pore volume is observed in the kaolinite-infiltrated biochar at 550 °C, which is attributed to the blocking of the mesopores (2-50 nm pore) by the nonporous metakaolinite formed from kaolinite.

Lowering N2O emissions from soils using eucalypt biochar: the importance of redox reactions

Agricultural soils are the primary anthropogenic source of atmospheric nitrous oxide (N2O), contributing to global warming and depletion of stratospheric ozone. Biochar addition has shown potential to lower soil N2O emission, with the mechanisms remaining unclear. We incubated eucalypt biochar (550 °C) – 0, 1 and 5% (w/w) in Ferralsol at 3 water regimes (12, 39 and 54% WFPS) – in a soil column, following gamma irradiation. After N2O was injected at the base of the soil column, in the 0% biochar control 100% of expected injected N2O was released into headspace, declining to 67% in the 5% amendment. In a 100% biochar column at 6% WFPS, only 16% of the expected N2O was observed. X-ray photoelectron spectroscopy identified changes in surface functional groups suggesting interactions between N2O and the biochar surfaces. We have shown increases in -O-C = N /pyridine pyrrole/NH3, suggesting reactions between N2O and the carbon (C) matrix upon exposure to N2O. With increasing rates of biochar application, higher pH adjusted redox potentials were observed at the lower water contents. Evidence suggests that biochar has taken part in redox reactions reducing N2O to dinitrogen (N2), in addition to adsorption of N2O.

Nb-substitution and Cs+ ion-exchange in the titanosilicate sitinakite

Cesium-133 MAS NMR and X-ray powder diffraction have been used to investigate Cs+ ion-exchange in synthetic samples of the titanosilicate mineral, sitinakite. The effect of incorporating Nb using Nb(V) ethoxide as the Nb source into the sitinakite framework and its influence on Cs+ ion-exchange was also studied. Refinements of X-ray powder data show that substitution of niobium for titanium in the sitinakite framework occurs to a limited extent using the synthetic method employed. Niobium substitution is also supported by 29Si MAS NMR spectra. Any niobium that was not substituted into the sitinakite structure was present in a distinct Nb-rich oxide/hydroxide phase. NMR studies reveal that in the undoped samples with low Cs loadings, at least four types of Cs+ are present, while at higher levels of Cs+ loadings, an additional two sites are populated. Similar sites appear to be occupied in the Nb-substituted samples but the relative occupancies are strongly modulated relative to the undoped samples. The multiple Cs environments are attributed to Cs+ ions occupying similar structural sites in the titanosilicate channels but experiencing different hydration water coordination environments.

Thermoelectric performance of n-Type (PbTe)0.75(PbS)0.15(PbSe)0.1 composites

Lead chalcogenides (PbQ, Q = Te, Se, S) have proved to possess high thermoelectric efficiency for both n-type and p-type compounds. Recent success in tuning of electronic band structure, including manipulating the band gap, multiple bands, or introducing resonant states, has led to a significant improvement in the thermoelectric performance of p-type lead chalcogenides compared to the n-type ones. Here, the n-type quaternary composites of (PbTe)0.75(PbS)0.15(PbSe)0.1 are studied to evaluate the effects of nanostructuring on lattice thermal conductivity, carrier mobility, and effective mass variation. The results are compared with the similar ternary systems of (PbTe)(1-x)(PbSe)x, (PbSe)(1-x)(PbS)x, and (PbS)(1-x)(PbTe)x. The reduction in the lattice thermal conductivity owing to phonon scattering at the defects and interfaces was found to be compensated by reduced carrier mobility. This results in a maximum figure of merit, zT, of ∼1.1 at 800 K similar to the performance of the single phase alloys of PbTe, PbSe, and (PbTe)(1-x)(PbSe)x.