Biomedical subjects

Nigel D Browning

Publications and source records attributed to Nigel D Browning.

11 recordsLinked to original sources

PubMed2006

Comet 81P/Wild 2 under a microscope.

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.

Journal Article↗

PubMed2006

Practical considerations for high spatial and temporal resolution dynamic transmission electron microscopy.

Although recent years have seen significant advances in the spatial resolution possible in the transmission electron microscope (TEM), the temporal resolution of most microscopes is limited to video rate at best. This lack of temporal resolution means that our understanding of dynamic processes in materials is extremely limited. High temporal resolution in the TEM can be achieved, however, by replacing the normal thermionic or field emission source with a photoemission source. In this case the temporal resolution is limited only by the ability to create a short pulse of photoexcited electrons in the source, and this can be as short as a few femtoseconds. The operation of the photo-emission source and the control of the subsequent pulse of electrons (containing as many as 5 x 10(7) electrons) create significant challenges for a standard microscope column that is designed to operate with a single electron in the column at any one time. In this paper, the generation and control of electron pulses in the TEM to obtain a temporal resolution <10(-6)s will be described and the effect of the pulse duration and current density on the spatial resolution of the instrument will be examined. The potential of these levels of temporal and spatial resolution for the study of dynamic materials processes will also be discussed.

Journal Article↗

PubMed2006

Quantification of the size-dependent energy gap of individual CdSe quantum dots by valence electron energy-loss spectroscopy.

Valence electron energy-loss spectroscopy (VEELS) performed in a monochromated scanning transmission electron microscope was used to measure the energy gaps of individual quantum dots (QDs). The gap energies of a series of CdSe QDs measured by VEELS reveal the expected quantum confinement effect; the gap energy increases with decreasing particle size. However, the values derived from these first VEELS measurements of single QDs are larger than the values commonly measured by optical spectroscopy. As standard optical methods lack the spatial resolution to probe individual nanoparticles, the particle-size distribution influences the optical response. It is suggested that the impact of the particle-size distribution accounts for the discrepancy between the energy-gap values derived from VEELS of single QDs and from optical methods of ensembles of QDs.

Journal Article↗

PubMed2006

Atomic scale defect analysis in the scanning transmission electron microscope.

Z-contrast imaging and electron energy loss spectroscopy in the scanning transmission electron microscope provide the ability to investigate the structure-composition-property relationship at individual defects on the atomic scale. In this article, the main principles behind the techniques will be described. The application of these methods to the analysis of individual dislocations in GaN will also be discussed. In this case, the atomic scale methods indicate that many of the structural and electronic properties of dislocations are modified by the presence of impurities, such as oxygen.

Gallium↗

PubMed2005

Experimental and theoretical improvements on understanding of the O K-edge of TeO2.

Using an electron monochromator attached to an electron microscope, high energy-resolution electron energy-loss spectra collected from TeO2 have revealed new features in the Oxygen K-edge. Using density-functional theory in the local density and the generalized gradient approximation, we find that core-hole strength of 1.3 gives an excellent fit to our high-resolution experimental data. This indicates that screening is not weak in this oxide, as normally assumed, and that neither the ground state nor a full core-hole model is adequate in quantitative reproduction of the O K-edge in the TeO2 system.

Journal Article↗

PubMed2005

Intact and fragmented triosmium clusters on MgO: characterization by X-ray absorption spectroscopy and high-resolution transmission electron microscopy.

Oxidative fragmentation of the clusters Os(3)(CO)(12) adsorbed on MgO powder was investigated by X-ray absorption spectroscopy and scanning transmission electron microscopy (STEM). Exposure of the clusters to air leads to their fragmentation, oxidation of the osmium, and formation of ensembles consisting of three Os atoms. X-ray absorption near-edge spectra demonstrate the oxidative nature of the fragmentation process. Extended X-ray absorption fine structure (EXAFS) spectra indicate an average Os-Os distance of 3.33 Angstrom and an Os-Os coordination number of 2, consistent with the formation of ensembles of three Os atoms on the support. STEM images confirm the presence of such trinuclear ensembles, and the diameters of the observed scattering centers (6.0 Angstrom) match that indicated by the EXAFS results.

Journal Article↗

PubMed2005

Valence electron energy-loss spectroscopy in monochromated scanning transmission electron microscopy.

With the development of monochromators for (scanning) transmission electron microscopes, valence electron energy-loss spectroscopy (VEELS) is developing into a unique technique to study the band structure and optical properties of nanoscale materials. This article discusses practical aspects of spatially resolved VEELS performed in scanning transmission mode and the alignments necessary to achieve the current optimum performance of approximately 0.15 eV energy resolution with an electron probe size of approximately 1 nm. In particular, a collection of basic concepts concerning the acquisition process, the optimization of the energy resolution, the spatial resolution and the data processing are provided. A brief study of planar defects in a Y(1)Ba(2)Cu(3)O(7-)(delta) high-temperature superconductor illustrates these concepts and shows what kind of information can be accessed by VEELS.

Journal Article↗

PubMed2005

Analysis of extraterrestrial particles using monochromated electron energy-loss spectroscopy.

A monochromated (scanning) transmission electron microscope was used to analyze individual sub-micron grains within interplanetary dust particles (IDP). Using low-loss and core-loss electron energy-loss spectroscopy, we analyzed fluid and gas inclusions within vesiculated alumosilicate grains. It is shown that nanometer-sized vesicles contain predominantly molecular oxygen (O(2)) beside a small fraction of H(2)O. Low-loss spectra reveal the Schumann-Runge continuum peaking at 8.6 eV and absorption bands reflecting vibrational excitation states of O(2) molecules between the first (12.1 eV) and second (16.1 eV) ionization energy. The presence of oxygen gas is supported by the corresponding oxygen K-edge fine structure. The valence state of Fe in iron-oxide within the IDP was also studied. Low-loss spectra provide qualitative information about the oxidation state of iron consistent with the Fe(2+)/Fe(3+) ratio quantitatively derived from the Fe L(2,3) edge.

Journal Article↗

PubMed2005

Atomic and electronic structure of mixed and partial dislocations in GaN.

Here we present a detailed study of mixed dislocations in GaN, in which the complexities of the atomic arrangement in the cores have been imaged directly for the first time using an aberration corrected scanning transmission electron microscope. In addition to being present as a full-core structure, the mixed dislocation is observed to dissociate into partial dislocations separated by a stacking fault only a few unit cells in length. The generation of this stacking fault appears to be impurity driven and its presence is consistent with theoretical predictions for dislocation dissociation in materials with hexagonal crystal symmetry.

Journal Article↗

PubMed2004

Pyromorphite growth on lead-sulfide surfaces.

Electrochemical Scanning Tunneling Microscopy (EC-STM) and electron microscopies have been used to follow the nucleation and growth of approximately 10-15 nm pyromorphite (Pb5(PO4)3Cl,OH) particles on a galena (PbS) substrate under oxidative conditions. The particle sizes and crystal morphologies are found to be strongly affected by solution and oxidation potential, and in the earliest stages the particles are generally sufficiently small to be mobilized in a soil. It is clear that the particles grow epitaxially under these conditions, based on observations of the particles' adherence to the surface during imaging, their preferred crystallographic orientation, their growth along surface features on the galena, and commensurate atomic structures. Through cyclic voltammetry, we show that the presence of phosphate also partially passivates the surface of the galena to oxidation. We propose two possibilities for the mechanism of passivation, one is that pyromorphite nucleation inhibits the retreat of steps, and the second is that adsorbed phosphate stabilizes a lead-terminated surface structure by coordinating lead and slowing its dissolution.

Electrochemistry↗

PubMed2002

Atomic scale characterization of vacancy ordering in oxygen conducting membranes.

This article presents a comprehensive investigation of (La, Sr)FeO3 by correlated atomic resolution annular dark field imaging and electron energy loss spectroscopy. Here, the ability of these techniques to characterize point defect formation and phase transitions under reducing conditions in situ in the scanning transmission electron microscope is evaluated and the influence of oxygen vacancies on the structure-property relationships is discussed. In particular, the evolution of the Ruddlesden-Popper, Brownmillerite, and Aurivillius phases can be associated directly with the ionic and electronic conductivity of the bulk material under different thermodynamic conditions. These results lead naturally to an atomistic defect chemistry model to explain the high temperature ionic and electronic conductivity in this and other perovskite materials.

Electric Conductivity↗