Biomedical subjects

P D Nellist

Publications and source records attributed to P D Nellist.

7 recordsLinked to original sources

PubMed2004

Direct sub-angstrom imaging of a crystal lattice.

Despite the use of electrons with wavelengths of just a few picometers, spatial resolution in a transmission electron microscope (TEM) has been limited by spherical aberration to typically around 0.15 nanometer. Individual atomic columns in a crystalline lattice can therefore only be imaged for a few low-order orientations, limiting the range of defects that can be imaged at atomic resolution. The recent development of spherical aberration correctors for transmission electron microscopy allows this limit to be overcome. We present direct images from an aberration-corrected scanning TEM that resolve a lattice in which the atomic columns are separated by less than 0.1 nanometer.

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PubMed2004

Spectroscopic imaging of single atoms within a bulk solid.

The ability to localize, identify, and measure the electronic environment of individual atoms will provide fundamental insights into many issues in materials science, physics, and nanotechnology. We demonstrate, using an aberration-corrected scanning transmission electron microscope, the spectroscopic imaging of single La atoms inside CaTiO3. Dynamical simulations confirm that the spectroscopic information is spatially confined around the scattering atom. Furthermore, we show how the depth of the atom within the crystal may be estimated.

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PubMed2003

Towards sub-0.5 A electron beams.

In the 4 years since the previous meeting in the SALSA series, aberration correction has progressed from a promising concept to a powerful research tool. We summarize the factors that have enabled 100-120kV scanning transmission electron microscopes to achieve sub-A resolution, and to increase the current available in an atom-sized probe by a factor of 10 and more. Once C(s) is corrected, fifth-order spherical aberration (C(5)) and chromatic aberration (C(c)) pose new limits on resolution. We describe a quadrupole/octupole corrector of a new design, which will correct all fifth-order aberrations while introducing less than 0.2mm of additional C(c). Coupled to an optimized STEM column, the new corrector promises to lead to routine sub-A electron probes at 100kV, and to sub-0.5A probes at higher operating voltages.

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PubMed2003

Comparison of simulation methods for electronic structure calculations with experimental electron energy-loss spectra.

The electronic structure of hexagonal GaN is studied using two simulation techniques in order to develop a method to interpret the fine-structure of an experimental nitrogen K-edge electron energy loss spectrum obtained using a scanning transmission electron microscope. The application of these simulation methods to the bulk spectrum is a necessary first step in developing a fundamental understanding of the effect of changes in the electronic structure on the properties of defects. It is found here that both of the techniques used, multiple scattering (MS) and density functional theory (DFT), produce excellent agreement with the experimental bulk spectrum. The MS method is limited in accuracy but efficient in time, while the DFT method is more accurate but time consuming. Through the combination of these methods, experimental energy loss spectra can be readily understood, and a means to unravel the complexities of the electronic structure can be determined.

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PubMed2001

Progress in aberration-corrected scanning transmission electron microscopy.

A new corrector of spherical aberration (C(S)) for a dedicated scanning transmission electron microscope (STEM) is described and its results are presented. The corrector uses strong octupoles and increases C(C) by only 0.2 mm relative to the uncorrected microscope. Its overall stability is greatly improved compared to our previous design. It has achieved a point-to-point resolution of 1.23 A in high-angle annular dark field images at 100 kV. It has also increased the current available in a 1.3 A-sized probe by about a factor of ten compared to existing STEMs. Its operation is greatly assisted by newly developed autotuning software which measures all the aberration coefficients up to fifth order in less than one minute. We conclude by discussing the present limits of aberration-corrected STEM, and likely future developments.

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PubMed2001

On the origin of transverse incoherence in Z-contrast STEM.

We use a Bloch wave approach to further investigate the origins of the incoherent nature of Z-contrast imaging using an ADF detector in a STEM. We discuss how, although at high angles the collected electrons will be mostly thermally scattered in addition to the elastic scattering, it is not the thermal scattering that destroys the coherence, rather the combination of the large detector with the high-angle elastic scattering. This incoherent nature of the elastic scattering arises through the filtering of the 1s-type Bloch states by the detector geometry. We show that it is this filtering that renders an atomic column an independent scatterer insensitive to the configuration of neighbouring columns. It also makes the image contrast insensitive to the effects of beam spreading onto neighbouring columns as the probe propagates through the crystal. We also discuss the implications of this for previous calculations of the intensity of Z-contrast images.

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PubMed2000

Z-contrast Imaging in an Aberration-corrected Scanning Transmission Electron Microscope.

We show that in the limit of a large objective (probe-forming) aperture, relevant to a spherical aberration corrected microscope, the Z-contrast image of a zone-axis crystal becomes an image of the 1s Bloch states. The limiting resolution is therefore the width of the Bloch states, which may be greater than that of the free probe. Nevertheless, enormous gains in image quality are expected from the improved contrast and signal-to-noise ratio. We present an analytical channeling model for the thickness dependence of the Z-contrast image in a zone-axis crystal, and show that, at large thicknesses, columnar intensities become proportional to the mean square atomic number, Z(2).

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